BIO-BASED REACTIVE DILUENTS AND THIOL-ENE PHOTOPOLYMERIZATION FOR ENVIRONMENTALLY BENIGN COATINGS. A Dissertation. Presented to

Transcription

1 BIO-BASED REACTIVE DILUENTS AND THIOL-ENE PHOTOPOLYMERIZATION FOR ENVIRONMENTALLY BENIGN COATINGS A Dissertation Presented to The Graduate Faculty of the University of Akron In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Kosin Wutticharoenwong December, 2007

2 BIO-BASED REACTIVE DILUENTS AND THIOL-ENE PHOTOPOLYMERIZATION FOR ENVIRONMENTALLY BENIGN COATINGS Kosin Wutticharoenwong Dissertation Approved: Accepted: Advisor Dr. Mark D. Soucek Department Chair Dr. Sadhan C. Jana Committee Member Dr. Kevin Cavicchi Dean of the College Dr. Stephen Cheng Committee Member Dr. Kyonsuku Min Cakmak Dean of the Graduate School Dr. George R. Newkome Committee Member Dr. George Chase Date Committee Member Dr. Wiley J. Youngs ii

3 ABSTRACT Tung oil was used as diene for modification via a Diels-Alder reaction with acrylate dienophiles. Tung oil was modified with three different acrylate molecules: 3- methacryloxypropyl trimethoxysilane (MAS), 2,2,2-trifluoroethyl methacrylate (TFM) and triallyl ether acrylate (TAEA) at atmospheric pressure. The modified tung oils were characterized using 1 H NMR, 13 C NMR, and FT-IR. The molecular weight and distribution were characterized using GPC, and MALDI-TOF. The effects of new acrylate modified tung oils on the properties of alkyd-based coatings film were investigated including hardness, solvent resistance, flexibility, gloss, impact resistance, contact angle, tensile, and thermo-mechanical properties. The viscosity can be reduced to the application viscosity of the alkyd by the reactive diluents. Drying time study showed that drying time can be altered by types and level of diluent added. All the results revealed that modified tung oils can be used as volatile organic compound (VOC) compliant in alkyd systems. Synthetic methods were developed to prepare thiols and mercaptopropionate thiols for thiol-ene photopolymerization. New thiols were characterized by 1 H NMR, 13 C NMR, Fourier transform infrared spectroscopy (FT-IR), and elemental analysis (C, H, S, O). Photopolymerization kinetics of the thiols with representative alkenes were investigated by time resolved Fourier transform infrared spectroscopy (FT-IR) with and iii

4 without photoinitiator. It was proposed that steric hinderance of thiol structures lower the initial reaction rate of photopolymerization. An experiment was planned according to ternary mixture design to investigate the influence the three thiols on the thermomechanical and coatings properties of thiol-ene photopolymerizable materials. It was found that the steric and rigidity of the double cycloaliphatic structure of thiol enhanced the tensile strength, tensile modulus, glass transition temperature (T ) and pencil hardness with loss of crosslink density of the film. The result suggested that steric and rigidity of thiols structures played an important role in both kinetics and material properties on thiolene photopolymerization. A thiol colloid oligomer was prepared vas sol-gel method from mercaptopropyltrimethoxysilane (MPTS). Thiol colloid oligomer was also characterized using 1 H NMR, 29 Si NMR, FT-IR, GPC, and MALDI-TOF mass spectroscopy. Thiol colloid oligomer g was attempted to photopolymerize with triallyl ether modified tung oil (TAETO) as a bio-based alkene via thiol-ene photopolymerization. However, kinetics results via timeresolved FT-IR indicated that MPTS colloid oligomer did not affect the initial reaction rate and final conversion of MPTS/TAETO system due to the immiscibility between TAETO and MPTS oligomer. iv

5 DEDICATION To my parents and my sisters for their exceptional love and support v

6 ACKNOWLEDGEMENTS I would like to express my special thank to my advisor, Dr. Mark D. Soucek, for his guidance, support, and motivation throughout my doctoral work. I would like to also thank to my committee members, Dr. Kevin Cavicchi, Dr. Kyonsuku Min Cakmak, Dr. George Chase, and Dr. Wiley J. Youngs for their contributions. I would like to thank Dr. Jun Hu for allowing me to use the time-resolved FT-IR equipment in Department of Chemistry. I would like to also thank Mr. Jay Hawkins, Mr. Chris Harding from Waterlox Coatings Inc. for their financial support and allowing me to have a chance to work with tung oil, Mr. Steven Johnson from Advanced coatings international for my internship experiences. I would like to thank to the faculty and stuff in the Department of Polymer Engineering and my colleagues in Soucek s research groups for their help and support. Finally, I would like to express my sincere gratitude to my parents Chamroen and Rattana Wutticharoenwong. Without their unconditional love and support, this work would not have been possible. vi

7 TABLE OF CONTENTS Page LIST OF TABLES... xii LIST OF FIGURES.....xiii CHAPTER I. INTRODUCTION..1 II. LITERATURES REVIEW: BIO-BASED REACTIVE DILUENTS Historical Solvents Drying oils Diels-Alder Reactions Sol-gel Chemistry Fluorinated polymers Alkyds resins High Solid Coatings Previous Studies of Reactive Diluents III. SYNTHESIS AND CHARACTERIZATION OF ACRYLIC MODIFIED TUNG OILS Abstract Introduction vii

8 3.3 Materials Instruments and Characterization Synthesis of acrylic modified tung oils Result and discussion Conclusions IV. EVALUATION OF MODIFIED TUNG OIL AS A REACTIVE DILUENT ON COATINGS PROPERTIES IN ALKYD SYSTEMS Abstract Introduction Materials Synthesis of long-oil alkyd resin Coating Formulation and Film preparation Instruments and Characterization Result and Discussion Conclusions...70 V. THERMO-MECHANICAL PROPERTIES OF ALKYD/ACRYLIC MODIFIED TUNG OIL COATINGS Abstract Introduction Materials Synthesis of long-oil alkyd resin Coating Formulation and Film preparation Instruments and Characterization...77 viii

9 5.7 Result and discussion Conclusions...87 VI. LITERATURES REVIEW: THIOL-ENE PHOTOPOLYMERIZATION Historical Polymerization mechanism of thiol-ene systems Application of thiol-ene photopolymerization Experimental design VII. SYNTHESIS AND CHARACTERIZATION OF THIOLS Abstract Introduction Materials Instrument and Characterizations Synthesis of thiols Result and discussion Conclusions..123 VIII. REACTION KINETICS OF THIOLS FOR THIOL-ENE PHOTOPOLYMERIZATION Abstract Introduction Materials Instrument and Characterizations Evaluation of photopolymerization Result and discussion ix

10 8.7 Conclusions IX. EVALUATION OF NEW 3-MERCAPTOPROPIONATE THIOLS FOR THIOL- ENE PHOTOPOLYMERIZATION COATINGS USING EXPERIMENTAL DESIGN Abstract Introduction Materials Instrumentation and testing protocol Formulations and Film Formation Results Discussion Conclusions X. THIOL-ENE UV-CURING ORGANIC-INORGANIC HYBRID BASED ON MODIFIED TUNG OIL Abstract Introduction Materials Instruments Preparation of mercaptopropyltrimethoxysilane (MPTS) colloid Evaluation of photopolymerization Result and discussion Conclusion X. CONCLUSIONS x

11 REFERENCES xi

12 LIST OF TABLES Table Page 2-1 Fats and oils used in the drying oil industries in the United States Typical Fatty acid compositions of vegetable oils Formulations of different coatings systems used for testing Drying time study experiment Coatings properties of neat alkyd resin and diluent/alkyd mixtures Coatings properties of neat alkyd resin and diluent/alkyd mixtures (Cont.) Formulations of different coatings systems used for testing Viscoelastic properties of the alkyd and alkyd/diluent cured films Summary of initial reaction rate and conversion at 60 sec (without PI) Summary of initial reaction rate and conversion at 60 sec (with PI) Initial rate of photopolymerization and double bond conversion Pseudo formulation matrix for simplex centroid design of experiments Tensile and viscoelastic properties General coatings properties The peak assignment of MPTS oligomer in MALDI-TOF mass spectra..190 xii

13 LIST OF FIGURES Figure Page 2-1 General chemical structure of triglyceride Depiction of the sol-gel reactions Acid catalyzed Hydrolysis mechanism of alkoxysilane Base catalyzed hydrolysis mechanism of alkoxysilane H NMR spectra of raw tung oil C NMR of tung oil FT-IR spectrum of tung oil MALDI-TOF mass spectra of tung oil Reaction of siloxane functionalized tung oil (SFTO) H NMR spectra of siloxane functionalized tung oil (SFTO) C NMR of siloxane functionalized tung oil (SFTO) FT-IR spectra of Siloxane functionalized tung oil (SFTO) MALDI-TOF spectra of siloxane functionalized tung oil (SFTO) Reaction of fluorine functionalized tung oil (FTO) H NMR of fluorine functionalized tung oil (FTO) C NMR of fluorine functionalized tung oil (FTO) FT-IR spectra of fluorine functionalized tung oil (FTO) MALDI-TOF spectra of fluorine functionalized tung oil (FTO)...48 xiii

14 3-14 Reaction scheme of triallyl ether functionalized tung oil (TAETO) H NMR of triallyl ether functionalized tung oil (TAETO) C NMR of triallyl ether functionalized tung oil (TAETO) FT-IR spectra of a) Tung oil, b) Triallyl ether functionalized tung oil (TAETO) MALDI-TOF spectra of Triallyl ether functionalized tung oil (TAETO) Reaction of a soya-based alkyd synthesized via the monoglyceride process Viscosity behavior of neat alkyd and alkyd/diluent mixtures at a shear rate of 2.2 s Contact angle of diluents/alkyd coatings Water contact with 20 %wt FTO/alkyd Reaction of a soya-based alkyd synthesized by the monoglyceride process Tensile Strength (MPa) with increased loading of siloxane modified tung oil (Tung-Si), fluorine modified tung oil (Tung-F), and allyl ether modified tung oil (Tung-AE) Elongation at break as a function of increased loading of siloxane modified tung oil (Tung-Si), fluorine modified tung oil (Tung-F), and allyl ether modified tung oil (Tung-AE) Tensile Modulus (MPa) as a function of increased loading of siloxane modified tung oil (Tung-Si), fluorine modified tung oil (Tung-F), and allyl ether modified tung oil (Tung-AE) Modulus (E ) as a function of temperature of the alkyd and a) siloxane modified tung oil (Tung Si), b) fluorine modified tung oil (Tung F), and c) allyl ether modified tung oil (Tung AE).alkyd/diluent cured films Tan δ as a function of temperature of the alkyd and a) siloxane modified tung oil (Tung-Si), b) fluorine modified tung oil (Tung-F), and c) allyl ether modified tung oil (Tung-AE) alkyd/diluent cured films Chemical structure of the thiols: (a) trans-1,4,bis(mercaptomethyl) cyclohexane (CHDMT), (b) 1,4-bis(mercaptomethyl)benzene (BDMT), (c) 1,6-hexane bis(3- mercaptopropionate) (HD-SH), (d) trans-1,4-cyclohexane dimethyl bis(3- xiv

15 mercaptopropionate) (CHDM-SH), (e) 4,4 -isopropylidenedicyclohexane bis(3- mercaptopropionate) (HBPA-SH) The reactions for synthesis of 1,4-benzenedimethanethiol (BDMT) The reactions for synthesis of tran-1,4-bis(mercaptomethyl)cyclohexane (CHDMT) Synthesis of the thiols FT-IR spectra of 1,6-Hexane bis(3-mercaptopropionate) (HD-SH) Proton NMR of 1,6-Hexane bis(3-mercaptopropionate) (HD-SH) Carbon NMR of 1,6-Hexane bis(3-mercaptopropionate) (HD-SH) FT-IR spectra of trans-1,4-cyclohexanedimethyl bis(3-mercaptopropionate) Proton NMR of trans-1,4-cyclohexanedimethyl bis(3-mercaptopropionate) Carbon NMR of trans-1,4-cyclohexanedimethyl bis(3-mercaptopropionate) FT-IR spectra of 4,4 -Isopropylidenedicyclohexane bis(3-mercaptopropionate) Proton NMR spectra of 4,4 -Isopropylidenedicyclohexane bis(3-mercapto - propionate) Carbon NMR spectra of 4,4 -Isopropylidenedicyclohexane bis(3- mercaptopropionate) Thiol-ene photopolymerization reaction Time-resolved infrared spectroscopy experiment set up Chemical structure of the thiols: a) trans-1,4,bis(mercaptomethyl) cyclohexane (CHDMT), b) 1,4-bis(mercaptomethyl)benzene (BDMT), c) 1,8-octanedithiol (ODT) d) 1,6-hexane bis(3-mercaptopropionate) (HD-SH), e) trans-1,4- cyclohexane dimethyl bis(3-mercaptopropionate) (CHDM-SH), f) 4,4 - isopropylidenedicyclohexane bis(3-mercaptopropionate) (HBPA-SH) Chemical structure of the alkenes: a) di(ethylene glycol)divinyl ether (DVE) b) trimethylolpropane diallyl ether (DAE), c) 1,6-hexanediol dimethacrylate (HDMA), d) 1,3,5-Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO).131 xv

16 8-5 IR-derived conversion of vinyl ether as a function of time for di(ethylene glycol)divinyl ether (DVE) with ( ) tran-1,4-bis(mercaptomethyl)cyclohexane (CHDMT), ( )1,4-benzenedimethanethiol (BDMT), and ( )1,8-octanedithiol (ODT). No photo-initiator was added IR-derived conversion of allyl ether as a function of time for trimethylolpropane diallyl ether (DAE) with ( ) tran-1,4-bis(mercaptomethyl)cyclohexane (CHDMT), ( ) 1,4-benzenedimethanethiol (BDMT), and ( )1,8-octanedithiol (ODT). No photo-initiator was added IR-derived conversion of acrylate as a function of time for 1,6-hexanediol dimethacrylate (HDMA) with ( ) tran-1,4-bis(mercaptomethyl)cyclohexane (CHDMT), ( ) 1,4-benzenedimethanethiol (BDMT), and ( )1,8-octanedithiol (ODT). No photo-initiator was added IR-derived conversion of alkene as a function of time for trans-1,4- bis(mercaptomethyl)cyclohexane (CHDMT) with ( ) di(ethylene glycol)divinyl ether (DVE), ( ) trimethylolpropane diallyl ether (DAE), and ( ) 1,6-Hexanediol dimethacrylate (HDMA), No photo-initiator was added IR-derived conversion of alkene as a function of time for 1,4- benzenedimethanethiol (BDMT) with ( ) di(ethylene glycol)divinyl ether (DVE), ( ) trimethylolpropane diallyl ether (DAE), and ( )1,6-Hexanediol dimethacrylate (HDMA), No photo-initiator was added IR-derived conversion of alkene as a function of time for 1,8-octanedithiol (ODT) with ( ) di(ethylene glycol)divinyl ether (DVE), ( ) trimethylolpropane diallyl ether DAE, and ( )1,6-Hexanediol dimethacrylate (HDMA), No photo-initiator was added IR-derived conversion of (a) vinyl ether and (b) thiol as function of time for DVE with ( ) tran-1,4-bis(mercaptomethyl)cyclohexane (CHDMT), ( ) 1,4- benzenedimethanethiol (BDMT), and ( ) ODT, 1% photo-initiator added; thiol:ene stoichiometric ratio of 1: IR-derived conversion of (a) allyl ether and (b) thiol as function of time for DAE with ( ) tran-1,4-bis(mercaptomethyl)cyclohexane (CHDMT), ( ) 1,4- benzenedimethanethiol (BDMT), and ( )1,8-octanedithiol (ODT), 1% photoinitiator added; thiol:ene stoichiometric ratio of 1: IR-derived conversion of (a) acrylate and (b) thiol as function of time for HDMA with ( ) tran-1,4-bis(mercaptomethyl)cyclohexane (CHDMT), ( ) 1,4- benzenedimethanethiol (BDMT), and ( )1,8-octanedithiol (ODT), 1% photoinitiator added; thiol:ene stoichiometric ratio of 1: xvi

17 8-14 Time-resolved FT-IR conversion of thiol (dot) and alkene (solid) as a function of time for the mixture of 1,6-Hexane bis(3-mercaptopropionate) (HD-SH) and 1,3,5- Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO) without photoinitiator Time-resolved FT-IR conversion of thiol (dot) and alkene (solid) as a function of time for the mixture of trans-1,4-cyclohexanedimethyl bis(3-mercaptopropionate) (CHDM-SH) and 1,3,5-Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO) without photoinitiator Time-resolved FT-IR conversion of thiol (dot) and alkene (solid) as a function of time for the mixture of 4,4 -Isopropylidenedicyclohexane bis(3- mercaptopropionate) (HBPA-SH) and 1,3,5-Triallyl-1,3,5-triazine- 2,4,6(1H,3H,5H)-trione (TATATO) without photoinitiator Time-resolved FT-IR conversion of thiol (dot) and alkene (solid) as a function of time for the mixture of 1,6-Hexane bis(3-mercaptopropionate) (HD-SH) and 1,3,5- Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO) with 1 % photoinitiator Time-resolved FT-IR conversion of thiol (dot) and alkene (solid) as a function of time for the mixture of trans-1,4-cyclohexanedimethyl bis(3-mercaptopropionate) (CHDM-SH) and 1,3,5-Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO) with 1 % photoinitiator Time-resolved FT-IR conversion of thiol (dot) and alkene (solid) as a function of time for the mixture of 4,4 -Isopropylidenedicyclohexane bis(3- mercaptopropionate) (HBPA-SH) and 1,3,5-Triallyl-1,3,5-triazine- 2,4,6(1H,3H,5H)-trione (TATATO) with 1 % photoinitiator Chemical formula of 2,2-dimethly-2-hydroxyacetophenone (Darocure 1173) Simplex centroid design of experiment containing HD-SH, CHDM-SH and HBPA-SH Three-dimensional surface plot for glass transition temperature Contour Plot of tensile strength model (A) and standard error (B) Contour Plot of tensile modulus model (A) and standard error (B) Contour Plot of elongation-to-break model (A) and standard error (B) Contour Plot of glass transition temperature model (A) and standard error (B) xvii

18 9-8 Contour Plot of XLD (v e ) model (A) and standard error (B) Contour Plot of gloss 20 o model (A) and standard error (B) Contour Plot of gloss 60 o model (A) and standard error (B) Contour Plot of hardness model (A) and standard error (B) Hydrolysis and condensation reactions of thiol oligomer H NMR spectra of MPTS: (a) MPTS monomer, (b) MPTS oligomer Si NMR spectra of MPTS: (a) MPTS monomer, (b) MPTS oligomer Comparison of FT-IR spectra: (a) MPTS monomer, (b) MPTS oligomer MALDI-TOF mass spectra of thiol oligomer FT-IR derived allyl ether conversion as a function of time for the MPTS/TAETO systems. 1 wt% photo-initiator added xviii

19 CHAPTER I INTRODUCTION The present price of crude oil has alerted many people to the fact that oil will not remain an endless cheap source of energy and chemicals. In addition, environmental issues have become an important concern for many countries. Polymer and coatings industries rely mainly on petrochemical products derived from crude oil, which are predicted to become scarce very soon. However, bio-based materials derived from natural products are an unlimited alternative for the polymer and coatings industries. In the coatings industry, drying oils and alkyds have been used for decades as renewable natural binders for various coating applications, including varnish and paint. Unfortunately, in order to formulate them into a proper product for coatings, drying oils and alkyds require the incorporation of volatile organic compounds (VOCs) to satisfy the coatings application, especially the viscosity application. In addition, drying oils and alkyds-based materials are generally inferior in material properties compared to those of petrochemical derivative products.[1] The United States Environmental Protection Agency (EPA) specified the limit of VOCs usage in varnish and paint formulations, which has created enormous pressure on the varnish and paint industries to reduce solvent emissions. One possible solution to resolve this regulation problem is to develop a reactive diluent which would function as a 1

20 solvent in the formulation of the coating, but which, during the cure process, is converted to an integral part of the film. Another solution for the VOC restriction problem is to employ a high technology UV-curable system which is solvent-free, and low in energy consumption. Bio-based UV-curable materials such as epoxy soybean oil acrylate,[2] and epoxynorbornane linseed oils[3] have already been developed. In addition, a tung oilbased material was reported as a radiation curable-composition in 1978.[4] There was also an attempt to employ cationic copolymerization for tung oil.[5] In this research, the work was divided into two interconnected focus areas that relate to the reduction and/or replacement of the VOCs used in coatings formulations. The two primary topics were the development of bio-based reactive diluents and new thiols for thiol-ene photopolymerization. In this study, specifically, the work is listed as follows: 1. synthesis and characterization of acrylic modified tung oils; 2. evaluation of modified tung oil as a reactive diluent on coatings properties in alkyd systems; 3. thermo-mechanical properties of alkyd/acrylic modified tung oil coatings; 4. synthesis and characterization of thiols; 5. reaction kinetics of thiols for thiol-ene photopolymerization; 6. evaluation of new 3-mercaptopropionate for thiol-ene photopolymerization coatings using experimental design; and 7. thiol-ene UV-curable based on modified tung oils. 2

21 The first focus area was presented in Chapters III, IV, and V. Development of new bio-based reactive diluents was presented in Chapter III. Chemical spectroscopic methods were employed to identify the chemical structure of the reactive diluents. Evaluation of coatings and thermo-mechanical properties of alkyd/modified tung oil systems (Chapter IV and V) were demonstrated with the objective of employing acrylic modified tung oil as reactive diluents. The effect of modified tung oils on important coatings formulation properties such as viscosity, drying time, tensile modulus, tensile strength, elongation-at-break, crosslink density, glass transition temperature, hardness, flexibility, and solvent resistance was investigated as a function of reactive diluents. The present study established the groundwork for development of novel bio-based reactive diluents to replace the VOCs in coatings. The second focus area was presented in Chapters VII, VIII, IX, and X. Chapter VII concentrated on the synthesis and characterization of new thiol monomers for thiolene photopolymerization. The kinetics of thiol-ene photopolymerization of the new thiols was investigated via a spectroscopy technique (Chapter VIII). This knowledge established the structure-reactivity relationship of thiols and alkenes. Design of experiment technique was employed to study the structure-properties relationship of new thiols based on thiol-ene photopolymerization (Chapter IX). Statistical techniques provided convenient investigation and fundamental knowledge of the effect of the thiol structure on thiol-ene photopolymerization materials. Finally, thiol-ene photopolymerization was attempted to photocure with bio-based materials in Chapter X. Thiol colloid oligomer was prepared and used as a multi-functional thiol to 3

22 photopolymerize with modified tung oil to illustrate the potential of utilizing thiol-ene photopolymerization in bio-based system. Ultimately, once these materials were synthesized and optimized into the coatings formulation, addition of value to the renewable bio-based material, tung oil, was created; in addition, study of new thiols in thiol-ene photochemistry provided the design flexibility for coating formulators. Consequently, a number of industrial applications in thiol-ene photopolymerization are available. The success of this work had a large impact on the coatings industry, particularly in the field of solvent-borne coatings. Moreover, this work resolved the problem of meeting strict VOCs regulation which has been a difficulty for industrial companies. 4

23 CHAPTER II LITERATURE REVIEW: BIO-BASED REACTIVE DILUENTS 2.1 Historical Organic coatings are complex mixtures of chemical substances. Broad categories of coating compositions are 1) binders, 2) volatile components, 3) pigment, and 4) additives.[1] Most coatings contain volatile material that evaporates during application and film formation. Such volatile components are added to reduce viscosity and help with leveling of coating application. Unfortunately, almost all solvents are classified by the U.S. Environmental Protection Agency (EPA) as photochemically reactive volatile organic compounds (VOCs), and their use has been regulated since the 1970s to reduce air pollution. In 1990, the U.S. Congress listed certain common solvents as hazardous air pollutants (HAPs), limiting their use. Binders are the major component that form a continuous film and adhere to the substrate (the surface being coated). Binders are polymeric materials which are prepared and incorporated into the coating; therefore, the binder, to a large extent, dictates the properties of the final film. There are several organic coating binders available for various applications and end uses such as polyester resin, formaldehyde-based resin, silicon resin, epoxy resin, acrylic resin, polyurethane resin, alkyds and drying oils. Drying oil and alkyd are among the oldest resins used as binders. 5

24 1975[6] Table 2-1: Fats and oils used in the drying oil industries in the United States, Production (10 6 lb) Average Average Average Product Linseed oil Castor oil Fish oil Soybean oil Tung oil Oiticica oil Tall oil Other primary fats and oils Other secondary fats and oils Total Drying oil was a common binder for paints and varnishes in the 19 th and early 20 th centuries; however, the use of drying oil has decreased due to several factors. During the decade from 1950 to 1960, factory consumption of oils such as linseed oil and tung oil in paint products in the United States averaged about 1 billion lb/year (Table 2-1). One crucial reason for the decline in their use is that there are other new resins or binders such as alkyd resins, epoxy esters, and uralkyds which may give different and/or better properties to the coatings. Since the film forming properties of drying oils are closely related to their degree of polymerization of unsaturation, thermal modification of drying oils, bodied oils and blown oils were reported to improve application and performance 6

25 characteristics of the coating film.[1, 6-8] Modern requirements for protective coatings are extremely different from conventional coatings; for instance, modern assembly line methods of manufacture produce a requirement of quick-drying finishes. Modern paint and varnish industries have therefore introduced synthetic resins as replacement for natural resins. Unfortunately, most synthetics resins are derived from petroleum, whose price has become very high with the worldwide demand for more and more energy. On the other hand, vegetable oils, which are a renewable resource, have shown much better price reliability; accordingly, many forecasters predict the demand for petroleum for energy will bring back the consumption of drying oils in coatings. 2.2 Solvents The action of a solvent in the production of a varnish or paint is essentially no different from what takes place when sugar is added to a cup of tea. The water disperses the sugar molecules and a solution is formed. Similarly, when linseed oil is added to hexane, the triglycerides of which the oil consists are dispersed or dissolved and a more or less viscous solution results. A solvent can be defined as a liquid which disperses a solid or semi-solid substance so as to yield a useful solution. A variety of organic compounds and mixtures are used as solvents. Organic compounds can be classified in three broad categories: weak hydrogen-bonding, hydrogen-bond acceptor, and hydrogenbond donor-acceptor solvents. Weak hydrogen-bonding solvents are aliphatic and aromatic hydrocarbons; commercial aliphatic solvents are mixtures of straight chain, branched chain and alicyclic hydrocarbons. They vary in volatility and solvency. Mineral spirits are slow evaporating aliphatic hydrocarbons. Aliphatic solvents have the advantage of lower costs compared to 7

26 aromatic solvents. However, aromatic solvents dissolve a broader range of resins. Use of benzene is prohibited due to toxicity. Toluene, xylene and high flash aromatic naphthas have been widely used; nevertheless, they are listed as HAPs. Hydrogen-bond acceptor solvents are esters and ketones. Ketones are generally less expensive than esters with corresponding vapor pressures. Use of methyl ethyl ketone (MEK) and methyl isobutyl ketone (MIBK) is being reduced because they are on the HAP list. Use of acetone is increasing because it has been delisted and is no longer included as VOC compound by the EPA. Alcohols are used as strong hydrogen-bond donor-acceptor solvents. The most widely used volatile alcohols are methyl, ethyl, isopropyl, n-butyl, sec-butyl, and isobutyl alcohols. Most latex paints contain a slow evaporating, water soluble solvent, such as ethylene or propylene glycol that does not dissolve in the polymer particles Properties of Solvents The evaluation of a liquid as a solvent for varnishes and lacquers requires knowledge of its physical properties, of which the following are important: 1) vapor pressure, boiling point, rate of evaporation and distillation range; 2) solvent power and miscibility with other liquids; 3) viscosity and the viscosity of solutions; 4) inflammability, flash point, and explosive limits; and 5) toxicity The pressure of a vapor in equilibrium with its non-vapor phases is vapor pressure. As the temperature is raised the molecules become more and more agitated and an increasing number escape and return to the surface. An important factor controlling 8

27 the vapor pressure of a liquid is its latent heat of evaporation. This expression is mainly the energy required to overcome the attraction exerted on any particular molecule by its neighbors. These attractions must of course be overcome if the molecule is to escape and contribute to the vapor pressure. If the temperature of a liquid is raised so high that the resulting vapor pressure just exceeds the external pressure, the liquid will boil freely. This means that the molecules leave the liquid surface continuously and thus fill the space created by those molecules which have escaped into the ambient space. This behavior gives rise to a boiling point which is defined as the temperature at which the vapor pressure of the heated liquid equals the external pressure. The rate of evaporation of a liquid is technically important because this affects the rate of deposition of a film from solution, and this factor in turn controls the structure of the film. If the evaporation rate is too rapid, the film may lack homogeneity and moisture may be simultaneously deposited from the ambient atmosphere. The solvent power of two liquids, e.g., water and alcohol, for a crystalline substance, e.g., sugar, can be readily compared in terms of the solubility of sugar in water and alcohol at a stated temperature. Three methods have been developed for comparing the solvent powers of liquids: first, use of Kauri resin solutions in butyl alcohol; second, dilution ratio; third, viscosity of solutions. Each method has its sphere of usefulness. The first method is of interest in connection with varnish manufacturing. The second method is important in connection with the dissolving of nitrocellulose. The third method has wide applications in technical practice. 9

28 When a liquid is caused to move, opposition or resistance to the motion is set up between adjacent layers of the liquid. This internal friction is called viscosity. In ordinary practice viscosity is involved when a liquid is described as being thick or thin. Viscosity is of importance in the varnish, lacquer and paint industry for the following reasons: the viscosity of a coating affects the flow of the material after application; viscosity and consistency data provide tests for controlling such finished products; the suspension of pigments is influenced to some extent by the viscosity of the medium; the stability of emulsion paints is influenced by the viscosity of the continuous phase; and the solvent power of a liquid is closely associated with viscosity data (the greater the solvent power the lower the viscosity of its solutions) Toxic Hazards Toxic risks of volatile solvents depend on the extent of exposure and toxicity data. All solvents are toxic at some level of exposure. Clearly, no one should ingest them. The hazard from skin contact can be controlled by wearing protective clothing. The highest risk always comes from inhalation. Three general types of toxicity data are important. Acute toxicity data indicates the level of intake in single doses that can be injurious or lethal; this kind of information can be particularly important in cases of accidental ingestion or spillage. The second type of toxicity data concerns the level of exposure that is safe when people are going to be exposed eight hours a day for long period of time. This kind of data is used, for example, to set the upper concentration limit for a solvent spray booth. The third type is risk of exposure to low level of materials over a period of years that can increase health risks such as cancer. Extensive tables of all three types of data are available. 10

29 In formulating coatings, it is also necessary to consider the clientele that will use them. While coatings sold to retail consumers are carefully labeled to include application cautions, one must assume that many people will not read the labels. When selling to a large corporation, it is reasonable to assume that the Material Safety Data Sheets will be read and appropriate practices will be established. In 1990, the U.S. Congress listed hazardous air pollutants (HAP) for which use is to be reduced.[9] Among those of importance in the coatings field are methyl ethyl ketone, methyl isobutyl ketone, n- hexane, toluene, xylene, methyl alcohol, ethylene glycol, and ethers of ethylene glycol. The U.S. EPA s Hazardous Air Pollutants Strategic Implementation Plan describes regulatory efforts.[10] The first step was a voluntary program aimed at reducing emission of 17 chemicals, including MEK, MIBK, toluene, and xylene, by 50% (of 1995 levels) by Mandatory HAP limits are included in EPA s Unified Air Toxics Regulations, issued for all major categories of coating users in Compliance is required within three years of the issue date. A group of producers has petitioned for removal of 2-butoxy ethanol, MEK and MIBK from the HAP list. The present HAP list motivates replacement of listed solvents with solvents not on the HAP list. Such substitutions may do little or nothing to improve air quality if the unlisted solvent is as hazardous as the listed one.[1] Atmospheric Photochemical Effects Volatile organic compounds (VOCs) have been recognized to cause serious problems in air pollution. In Europe, solvents are classified by their photochemical ozone creation potential (POCP). Three important end effects of VOC emissions into the atmosphere are: formation of eye irritants, particulates, and toxic oxidants, especially 11

30 ozone. While all of these factors are important, the most critical for coatings is ozone. Even though ozone is a naturally occurring component of the atmosphere, it is toxic to plants and animals. With the rapid growth of VOC emissions from man-made sources since 1900, ozone levels on many days of the year in many parts of the world, especially in and around cities, have exceeded the levels that many plants can withstand and have endangered human health, especially for susceptible individuals. The largest source of man-made VOC emissions is transportation: auto and truck tailpipe emissions, along with fuel leakage during distribution. The second largest source is coatings; in 1995, coatings and adhesives accounted for 11.6% of man-made VOC emissions.[11] Photochemical reactions in the atmosphere are complex and dependent on many variables in addition to the amount and structure of VOCs, especially the concentrations of various nitrogen oxides. The principle pathways leading to the generation of ozone are by way of hydrogen abstraction from the VOC compounds. In early investigations of the effect of VOC on air pollution, organic compounds were divided by rabbit eye irritation tests into photochemical active compounds of high and low reactivity. It was proposed that if the emission of the highly reactive compounds could be limited, the less reactive ones could dissipate and avoid high local concentration of pollutants. This led to the establishment of a definition of photochemically reactive solvents in Rule 66 of the Los Angeles Air Control District. After some year of experience, it was realized that most organic compounds are photoreactive and that the extent of dissipation in the atmosphere after local emission had been overestimated. The coatings industry objected to having to use different solvent combinations in different parts of the country. This situation led to 12

31 the conclusion that it would be best to limit the emission of almost all organic compounds into the atmosphere Determination of VOC Experimental determination of VOC is not straightforward. The amount of VOC released depends on conditions under which the coating is used. Times, temperature, film thickness, air flow over the surface, and, in some cases, the amount of catalyst are among the variables that affect the results obtained. Methods for determination of VOC and other useful information were collected by Brezinski in 1993.[12] While these methods remained current as of 1998, improvements are needed. In solvent-borne coatings, VOC is calculated by the following equation in which NVW is the weight of solids in the coating, determined under specified conditions, and ρ 1 is the density of the coating in g/ml. The factor 10 serves to adjust the VOC units to grams of solvent per liter of coating: VOC = 10(100-NVW)ρ 1 (2-1) An alternative to VOC analysis is to calculate VOC based on the formulation of the coating. This calculation requires knowledge of the solvent content of all coating components and assumptions about what fraction of the solvents are actually emitted and about how much additional VOC is produced by chemical reactions such as crosslinking. Even so, calculated VOC values may be more reliable than measured values in many cases. Some regulators would prefer that the units be kilograms per liter of applied coating solid. This may be a desirable objective, but requires determination of the 13

32 thickness and density of the applied cured film. ASTM Method D-2697 includes a procedure for using Archimedes liquid volume displacement principle to determine the density of the film cured on a metal disk for 60 minutes at 110 o C.[13] 2.3 Drying Oils Oil naturally occurs in animal, vegetable and mineral materials. Triglycerides, trimesters of glycerol, and fatty acids make up the largest proportion of the constituents in oils. Natural oils can be categorized into three types based on degree of unsaturation: drying oils such as linseed oil and tung oil, semi-drying oils such as soybean oil, and nondrying oils. Drying oils can be sub-divided into yellowing and non-yellowing oils. Drying oils are obtained mostly from the seeds of vegetables. The properties of drying oils depend largely on their chemical constituents, mainly the previously mentioned triglycerides Composition of Natural Oils The major component of natural oils is triglyceride. Also, vegetable oils contain varying amounts of non-glyceride components, which may include impurities such as phosphatides (or phospholipids), sterols, tocopherol, and coloring matter.[14] The triglycerides of naturally fatty acids are colorless. Oils are mixtures of triglycerides with different fatty acids distributed among the triglyceride molecules. O CH 2 CH CH 2 O O O C O C O C R 1 R 2 R 3 R = Fatty acid component Figure 2-1: General chemical structure of triglyceride 14

33 Fatty acids play an important role in oil properties. Fatty acids can be classified into saturated and unsaturated types. They are low in density, and nearly all are insoluble in water. Examples of common fatty acids found in vegetable oils are stearic acid, palmatic acid, oleic acid, linoleic acid, linolenic acid, pinolenic acid, ricinoleic acid and α-eleostearic acid. Typical fatty acid compositions of vegetable oils are shown in Table 2-2. In order to classify oils as drying oils, semi-drying oils or non-drying oils, we can use iodine value, that is, grams of iodine required to saturate the double bonds of 100g of an oil. An iodine value greater than 140 indicates a drying oil; iodine values between 125 and 140 indicate a semi-drying oil; and a non-drying oil is indicated by an iodine value less than 125.[15] Table 2-2: Typical Fatty acid compositions of vegetable oils Fatty acids Oil Saturated Oleic Linoleic Linolenic Other Linseed Safflower Soybean Sunflower, MN Sunflower, TX Tung a Tall oil fatty acid b Tall oil fatty acid c Castor Coconut a: alpha-eleostearic acid b: North American origin c: European origin 15

34 2.3.2 Tung Oil[16] Tung oil is obtained by expressing the kernels (which contain 50 percent or more oil) of the nuts of the tung tree, of which there are two main varieties, Aleurites fordii and A. montana. The tree is indigenous to China, but is now cultivated in many parts of the world, especially fordii in parts of the United States and South America, and small quantities of montana in Nyasaland. The oil is often called Chinese wood oil, Chinawood oil or simply wood oil. It s a clear pale yellow to yellowish brown with a characteristic odor, and it is more viscous than most other oils. The oil differs markedly in several respects from other oils, especially in its rapid thickening and gelation when heated, its rapid drying to a frosted film, and the possibility of visible isomerization. These properties are associated with the high content of triply conjugated α-eleostearic acid, which means that tung oil contains the triglyceride of α-eleostearic acid and other triglycerides containing not more than one other fatty acid. Compared with others tung oil has a more uniform composition and contains some simple triglycerides (in contrast with composite glycerides containing three fatty acids) Autooxidation and Drying Mechanisms After paint, varnish or oil coating material is applied to a substrate surface, the film gradually changes from liquid into solid form. Even though part of the drying mechanism is the evaporation of a volatile solvent, the key process of the transformation of liquid to film is polymerization via autooxidation. Autooxidation reactions that take place during drying are complex. Recently, modern analytical instrumentation has made it possible to reveal this complex problem.[17-19] 16

35 The following equations illustrate some of many reactions that occur during autooxidative drying reactions to form crosslink coating films. Initiation step: (2-2) In the initiation step, naturally present hydroperoxides decompose through the hemolytic cleavage of the peroxide to produce free radicals. This step can be accelerated by heating and incorporation of a drier. Propagation reaction occurs through the abstraction of hydrogens on methylene groups between double bonds, which are particularly susceptible to abstraction, forming a stable resonance free radical. The free radical resulting from the propagation step reacts with oxygen to give predominantly a conjugated peroxy free radical. The peroxy free radical can abstract hydrogen from other methylene groups between double bonds to form further free radicals; hence, a chain reaction is established, resulting in autooxidation. Propagation steps/oxygen uptake: RO or OH + CH CH CH 2 CH CH (2-3) CH CH CH CH CH + ROH (2-4) O O 2 O CH CH CH CH CH (2-5) Terminal steps: R + R R R (2-6) RO + R R O R (2-7) RO + RO R O O R (2-8) 17

36 Finally, termination reactions result from a combination of radicals and create carbon-carbon, ether and peroxy linkages and eventually form a polymer film Modified Drying Oil Drying oil can be treated or modified by various methods to get certain properties needed for coatings. Bodied oils are used in the manufacture of varnishes, enamels, printing inks, and lithographic varnishes.[6] The process is to heat the oil to o for nonconjugated oils and to o C for conjugated oils under an inert atmosphere or vacuum to avoid oxidation with air. Bodied oil has high viscosity and better performance characteristics.[1] Blown oils or oxidized oils are prepared by passing air through oils at elevated temperatures ranging from 40 to 150 o C. The process of blowing drying oil has been studied by Taylor.[20] Blowing oils are darker in color and higher in free fatty acid content than bodied oils of equivalent viscosity. Blowing oil also promotes pigment wetting due to its high surface activity. Dehydrated castor oil is a conjugated oil which dries relatively rapidly at room temperature. Kraft first dehydrated castor oil in 1877.[21] The acid catalytic method[22] of dehydrating castor oil is to heat it under a vacuum at a temperature range of 230 to 280 o C in the presence of a catalyst until water is no longer evolved. The process for creating maleated oils is to react maleic anhydride with conjugated oil or nonconjugated oil via Diels-Alder reaction.[1] The product of these reactions reacts with polyols to give moderate molecular weight derivatives that dry faster than an unmodified one. 2.4 Diels-Alder Reactions The Diels-Alder cycloaddition is the best-known organic reaction that is widely used to construct, in a region- and stereo-controlled way, a six-membered ring with up to 18

37 four stereogenic centers.[23] With the potential of forming carbon-carbon, carbonheteroatom and heteroatom-heteroatom bonds, the reaction is a versatile synthetic tool for constructing simple and complex molecules.[24, 25] Otto Diels and Kurt Alder received the Nobel Prize in chemistry in 1950 in recognition of the importance of this reaction to synthetic organic chemistry. In a Diels-Alder reaction, a conjugated diene reacts with a compound containing a carbon-carbon double bond. The latter compound is called a dienophile because it loves a diene. The general structure of the Diels-Alder Reaction is showed in equation 1-8. CH 2 CH CH CH 2 + CH 2 CH R R (2-9) This reaction may not look like any other reaction; however, it is a simple 1,4-addition of an electrophile and a nucleophile to a conjugated diene. Diels-Alder is a concerted reaction in that the addition of the electrophile and the nucleophile occurs in a single step. More precisely, the Diels-Alder reaction is a [4+2] cycloaddition reaction because, of the six π electrons involved in the cyclic transition state, four come from the conjugated diene and two come from the dienophile. The reaction, in essence, converts two π bonds into two σ bonds.[26] From 1928, when Otto Diels and Kurt Alder[27] made their extraordinary discovery, until 1960, when Yates and Eaton[28] reported the acceleration of the Diels- Alder cycloaddition by Lewis acid catalysts such as AlCl 3, ZnCl 3 and SnCl 4, these reactions were essentially carried out under thermal conditions owing to the simplicity of the thermal process. 19

38 For coating applications, researchers at Cargill chemically modified linseed oil with a Diels-Alder reaction of cyclopentadiene at high temperatures and pressure to form norbornene groups on the chain known as Dilulin.[29] Typically, only 2-5% of the double bonds are modified via these processes. Soucek et al. have prepared norbornene linseed oil with a Diels-Alder reaction of dicyclopentadiene with linseed oil at 250 o C at pressure 0.76 to 0.9 MPa. They reported high modification of double bonds through this process.[3] Tung oil contains approximately 80% α-eleostearic fatty acid, which is a major source of conjugate double bonds.[1] In order to create a cyclic structure using a Diels-Alder reaction, dienophile can be reacted with tung oil under certain conditions. It has been reported that a Diels-Alder product can be obtained from tung oil.[4] Acrylate monomer contains π-bond as well as an electron withdrawing group of ester, which means acrylate groups can be considered dienophiles for the Diels-Alder reaction. 2.5 Sol-gel Chemistry Sol-gel is the process of forming an inorganic network gel through a colloidal suspension (sol). Metal alkoxides are generally used as the precursors to synthesize sol colloids due to their reactivity with water. Tetraethoxysilane (TEOS) is the most widely used sol-gel process. Alkoxysilanes undergo hydrolysis and condensation reactions with water according to sol-gel chemistry as depicted in Figure 2-2. The properties of the inorganic network depend on the ph, catalyst, concentration of alkoxysilane, and temperature of the reaction. Therefore, by controlling these variables, it is possible to obtain a wide variety of materials. 20

39 Figure 2-2: Depiction of the sol-gel reactions Under acidic conditions, a hydrolysis reaction occurs in two steps according to S N 2 type mechanism depicted as in Figure 2-3.[30] First, an alkoxide group is protonated rapidly. Thus, electron density is withdrawn away from the silicon atom which makes it more susceptible to water. A silanol group forms after the release of the alcohol from the transition state. Base catalyzed hydrolysis reactions of alkoxysilanes are much slower compared to acid catalyzed reactions. The hydroxyl anion attaches directly to the silicon atom of the alkoxysilane according to the S N 2 mechanism as shown in Figure 2-4. In this mechanism, the base alkoxide group repels the hydroxyl anion and slows down the reaction. Once the first hydrolysis reaction occurs, subsequent hydrolysis of the remaining alkoxides becomes easier. 21

40 Figure 2-3: Acid catalyzed Hydrolysis mechanism of alkoxysilane Figure 2-4: Base catalyzed hydrolysis mechanism of alkoxysilane Condensation of alkoxysilanes with silanols or another alkoxysilane produces dimers, trimers and cyclic siloxanes. The rate of condensation is proportional to the (H + ) and (OH - ) concentration below ph 2 and between ph 2 and 6, respectively.[30] Solubility of the silica particles is low at ph values lower than 2. Thus, gel networks are formed mostly from very small particles in ph below 2. Through combining various organic and inorganic constituents in different preparation and processing methods, very versatile materials can be produced for optical, structural, and coating applications. Schmidt et al. introduced Ormosils, which were prepared by a sol-gel technique using silicon alkoxides having non-hydrolyzable substituents. In those materials, the inorganic component is the continuous phase and organic polymers are added for gas permeability, flexibility and toughness.[31-34] Combining forces between the two phases were hydrogen bonds, and there was no physical covalent bond formation between inorganic and organic phases. Wikes et al.[35] 22

41 pioneered the second kind of inorganic-organic hybrid materials (Ceramers) having covalent bonds between the two phases. In these materials, the organic part was the continuous matrix. Low molecular weight organic polymers were modified with trialkoxysilanes to provide covalent bonds between the polymer and growing inorganic network.[36] Most of the research in inorganic/organic hybrids is focused on the inorganic modification of organic polymers with a coupling agent. Organofunctional silanes have an alkoxysilane group and organic functional group and have been utilized as coupling agents for a variety of organic binders, including polyurethane and unsaturated polyesters.[1, 37, 38] Hydrolysis and condensation reactions form crosslinks or, in the case of sol-gel precursors, form a silicon oxide network. It has been shown by Soucek and co-workers that alkoxysilanes play the critical role of the compatibilizer and the coupling agent for polyurethane/teos based inorganic/organic hybrids.[39, 40] They reported that the alkoxysilane groups functioned as a nucleation site for silicon-oxocluster growth, thus providing a template for uniform dispersion of the silicon-oxonanophase. 2.6 Fluorinated Polymers Fluorinated polymers are well-known as high value-added materials due to their outstanding properties. Fluorinated polymers exhibit low intermolecular and intramolecular interactions; these characteristics lead to low cohesive energy and, therefore, to low surface energy. Moreover, they also show high thermostability and chemical innertness, low refractive index and friction coefficient, good hydrophobicity and lipophobicity, valuable electrical properties, and low dielectric constants and 23

42 dissipation factors.[41] Polytetrafluoroethylene (PTFE) has the greatest exterior durability and heat resistance of any polymer used in coating. However, PTFE is insoluble in solvents, and its fusion temperature is so high that its coating uses are limited to applications in which the substrate can withstand high temperatures.[42] After application, the polymer particles are sintered at temperatures as high as 425 o C. Like all highly fluorinated compounds, PTFE has such a low surface free energy that it is not wet by either water or oils; hence, it provides a nonstick cooking surface. Fluorinated copolymer with functional groups such as hydroxy groups can be crosslinked after application. Copolymer of vinylidene fluoride (VDF) with a hydroxy-functional monomer cross-linked with a polyisocyanate give coatings with superior wet adhesion and corrosion as compared with poly(vinylidene fluoride) PVDF homopolymer.[43] Perfluoroalkyl acrylate esters have been copolymerized with hydroxyethyl methacrylate (HEMA) to form solvent-soluble resins; however, the monomers are expensive. 2.7 Alkyd Resins Alkyd resin is the reaction product of polyhydric alcohol and polybasic acids. Alkyds, which can be considered a particular type of polyester, were first proposed by Kienle in There are two reasons that alkyd resins have become important for coatings. Firstly, alkyds are ultimately versatile and allow choices of structure combination to give tailor-made properties. Secondly, the success of alkyds is due to the relatively low cost. Alkyds can be classification by various criteria. One classification is by oil length based on the ratio of monobasic fatty acid to dibasic acid utilized during synthesis. 24

43 Classification based oil length is only correct when glycerol is used as the polyol. The oil length of an alkyd is calculated by the expression: Oil Length = (1.04*Weight of fatty acids)/(weight of alkyd Water evolved)*100 The amount of oil is defined as the triglyceride equivalent to the amount of fatty acids in the alkyd. The 1.04 factor converts the weight of fatty acids to the corresponding weight of triglyceride oil. Alkyds with oil lengths greater than 60 are long oil alkyds; those with oil length from 40 to 60, medium oil alkyds, and those with oil lengths less than 40, short oil alkyds.[1] Another classification of alkyd resins is based on whether they are oxidizing or non-oxidizing type alkyds. Oxidizing alkyds are comprised of one or more polyols, one or more dibasic acids, and fatty acid derived from one or more drying or semi-drying oils. Non-oxidizing alkyds are used as polymeric plasticizers or as hydroxyfunctional resins and are crosslinked with melamine-formaldehyde (MF) resins or ureaformaldehyde (UF) resins. The last classification is modified or unmodified alkyds. Modified alkyds contain other monomers in addition to polyols, polybasic acids, and fatty acids, such as styrenated alkyds and silicone alkyds.[1] High Solids Oxidizing Alkyds Environmental issues have become a crucial problem that pushed the government to implement regulations to minimize VOC emissions. As a result, efforts have been made to increase solid content in alkyd resin coatings. Aliphatic hydrocarbon solvents promote intermolecular hydrogen bonding, especially between carboxylic acids, but also between hydroxyl groups, thereby increasing viscosity. Use of at least some hydrogenbond acceptor solvent such as an ester, or hydrogen-bond acceptor-donor solvent such as an alcohol, give a significant reduction in viscosity at equal solids.[1] 25

44 One approach to increasing the solids of alkyd resins is to decrease molecular weight. This approach can be done by decreasing the dibasic acid to polyol ratio and going to longer oil alkyds. However, increasing oil length results in lower functionality and a lower ratio of aromatic to aliphatic chains; therefore, the time for drying is increased. Increasing the concentration of driers accelerates drying, but also accelerates yellowing and embrittlement. Another approach to increasing solid content is by making resin with a narrower molecular weight distribution. For instance, one can add a transesterification catalyst near the end of the alkyd cook; this gives more uniform molecular weight and a lower viscosity product. Reactive diluents are one important approach to producing high solids alkyds. Reactive diluent can be used in place of part of the solvent. The key point of the reactive diluent is to have a low molecular weight and viscosity component in the alkyd resin which reacts with the alkyd during drying. Three types of reactive diluent have been used in high solid alkyds: polyfunctional methacrylate monomers such as trimethylolpropane trimethacrylate, dicyclopentadienyloxyethyl methacrylate,[44] and mixed acrylic and drying oil fatty acid amides of hexa(aminomethoxymethyl)melamine.[45] Using optimized resin and, in some cases, reactive diluents, good quality air dry and baking alkyd coatings can be formulated with VOC levels of 280 to 350 g/l of coatings. 2.8 High-Solids Coatings High-solids liquid coatings have turned out to be the most significant development in the industrial finishing market. Practical high-solids liquid coatings can be formulated at more than 70% solid by weight. This ratio of solids to solvent is 26

45 presently acceptable in several areas. In the past, attainment of a level of 77% solid by weight in a conventional solution coating would have prevented practical application. A group of low molecular weight resins has been developed that overcome these viscosity restraints. These oligomers can be formulated with allowed amounts of solvent and appropriate crosslinkers to produce sprayable, thermosetting industrial coatings that range from 70 to 80% by weight of solids for application at room temperature. High-solids liquid coatings have been slow in development. The breakthrough is that the finished coatings compare favorably in gloss, mechanical properties, weatherability, and especially sagging properties with conventional thermosetting coatings Factors Controlling Solids Content Possibilities in the synthesis: The chemical reactions leading to the formation of macromolecules and oligomers, like all chemical reactions, are governed by the law of statistics, so that the resulting molecules must have a statistical distribution of molecular sizes. Flory presented the well-known and classic most probable distribution of difunctional monomers (Flory, 1936). The actual parameters to be controlled can be average chain size, chain unit structure, and chain size distribution. Average chain size: It is apparent that the average size of the linear polycondensates, or of any macromolecules formed by functional-group polymerization, is dictated by the stoichiometry and extent of reaction. Chain addition reaction, such as free radical polymerization, result in the real problem of controlling the average molecular weight. Controlled chain-transfer reactions offer a practical means of controlling the average 27

46 molecular weight. Using organic sulphur compounds as a chain-transfer agent during the polymerization of dienes prevents the formation of gel and improves the processibility. Control of molecular-weight distribution: To increase the solids, polydispersity of resins can be decreased; however, this may narrow the breadth of the Tg transition range, which may adversely affect the mechanical properties of films. This problem can be overcome by blending resins of different compositions to be similar enough to be compatible. Solubility and interaction parameters: Hydrocarbon solvents (mineral spirits, toluene) are regarded as conventional solvents for hydrocarbon resins, and oxygenated solvents (esters, ketones) as conventional solvents for oxygenated resins (cellulosics, polyvinyl acetate). The concept of like dissolves like can be used to estimate solubility behavior, and hence it is natural that the coating engineer should cast about for more sophisticated techniques for specifying likeness. A three-component parameter comprising dispersion (non-polar), polar, and hydrogen-bonding forces is used to specify likeness. Every solvent and polymer is uniquely characterized by these three quantities. The more closely materials match each other in these three categories, the better is the solvency or compatibility. Such is the essential idea of the solubility parameter system. 2.9 Previous Studies of Reactive Diluents A key to developing high-solid varnish and paint formulations is the development of a reactive diluent which can function as a solvent in the formulation of the coating, but which during the cure process is converted to an integral part of the film. The key characteristic of such materials are low volatility, low toxicity, low odor, and a solubility 28

47 parameter. Satoru et al.[46] studied reactive diluents for air-dried alkyd paint. A number of approaches to identification of a suitable reactive-diluent have been described. The chemistry of the vinyl cyclic acetals and their air-drying reactions have been studied by Hochberg.[47] Therefore, the vinyl dioxolane system developed by the DuPont Company became very interesting. Another attractive route to reactive diluents investigated by ICI[48] should be found in properly designed vinyl monomers, since a whole body of knowledge exists for converting solvent-like vinyl monomers to polymers by a freeradical polymerization process. Unfortunately, the commercial monomers are not fully satisfactory for various reasons including toxicity and high volatility. Bruson[49] synthesized dicyclopentyl methacrylate from the addition of methacrylic acid to dicyclopentadiene and noted that this ester was converted to a hard insoluble film at ambient temperature. Later on, Donald et al. studied the dicyclopentyl methacrylate as a reactive diluent for high-solid alkyd coatings.[44] Zabel et al.[50] studied design and incorporation of reactive diluents for air-drying high-solid alkyd paints. Zabel et al. described some requirement and design aspects for reactive diluents and also introduced a new class of reactive diluents based on fumarates and succinates of octadienol (OFS) and reported the cure and performance of OFS diluents. Muizebelt et al.[51] studied the crosslink mechanisms of high-solids alkyd resins in the presence of reactive diluents via NMR and mass spectroscopy employing model compounds. They found that allyl ether groups appear to react fastest whereas allyl esters show generally little reactivity. 29

48 CHAPTER III SYNTHESIS AND CHARACTERIZATION OF ACRYLIC MODIFIED TUNG OILS 3.1 Abstract Tung oil was used as diene for modification via a Diels-Alder reaction with acrylate dienophiles. Tung oil was modified with three different acrylate molecules: 3- methacryloxypropyl trimethoxysilane (MAS), 2,2,2-trifluoroethyl methacrylate (TFM) and triallyl ether acrylate (TAEA) at atmospheric pressure. A free-radical inhibitor (phenothiazine) was added to prohibit free-radical reaction. The modified tung oils were characterized using 1 H NMR, 13 C NMR, and FT-IR. The molecular weight and distribution were characterized using GPC, and MALDI-TOF. The reaction could be carried-out under pressure or at atmospheric pressure. The spectroscopy techniques showed evidence for a Diels-Alder reaction for each of the acrylic modified tung oil. The siloxane modified tung oil, fluorine modified tung oil, and triallyl ether modified tung oil exhibited the viscosity of 1000, 417, and 80 cp, respectively. 3.2 Introduction Environmental concern has become one of the most important topics in the coatings industry in the last three decades,[1] researchers are continually attempting to develop greener coatings systems.[1, 52, 53] Seed-oil based materials are an attractive choice, as the seed oils are biodegradable and readily available from renewable resources. 30

49 Seed oils are triglycerides of fatty acids. Seed oils such as linseed oil, soybean oil, or tung oil can form a cross-linked polymeric film when exposed to air, and are often used in the manufacture of coatings binders including most importantly alkyds.[1] Seed oils are classified into different categories (non-drying, semi-drying, and drying) based on the number of unsaturation sites located in their fatty acid side chains. The higher the number of unsaturation sites, the more readily a film is formed when exposed to the atmosphere. The process by which a seed oil based film is formed is commonly referred to as autoxidative curing. [54-57] Oxidation of the drying oil begins when molecular oxygen attacks an active center on a fatty acid chain, followed by the homolytic cleavage of the peroxide to produce free radicals. Hydrogen is abstracted from the methylene group between carbon-carbon double bonds followed by isomerization in a double bond position to form a conjugated structure. Termination results in carboncarbon, ether, and peroxy linkages which create an interlocking network. Tung oil is unique because it is one of the very few conjugated seed oil. Tung oil is derived from the nuts of Aleurites fordii; it is obtained from the kernels of the nuts and classified as a drying oil. Typical fatty acid compositions of tung oil are: 5% saturated acid, 8% oleic acid, 4% linoleic acid, 3% linolenic acid and 80% α-eleostearic acid.[1, 7] Due to the unique drying speed and excellent water resistance, tung oil is very valuable in modern manufacturing of varnish and related materials. Oxidation of conjugated tung oil catalyzed by metal driers has been investigated.[58] It has been shown that besides radical recombination, crosslinking reactions also occur through direct addition of free radicals to the conjugated double bonds, frequently leading to higher molecular weight oligomers.[51, 59] Recently, Larock et. al.[5, 60] has reported cationic and thermal 31

50 copolymerization of tung oil with aromatic comonomers such as styrene and divinylbenzene to obtain a variety of new polymer ranging from rubbery to tough and rigid plastics. Tung oil modified polyol has been cationically ultraviolet cured and formed smooth films by Thames et. al.[61], the fatty acid chain has reported to improve the flexibility and impact resistance. The classical Diels-Alder mechanism is a cycloaddition between a conjugated diene and a dienophile. A dienophile is a molecule which contains at least one π-bond. The Diels-Alder reaction is the most widely used and best-known for assembling sixmembered rings with up to four stereogenic centers. The main limitation of this reaction is the slow rate of reaction. Higher temperature, catalysis by a Lewis-acid, or high pressure reactions are common solutions for increasing the reaction rate.[25, 62-64] It has been reported that Diels-Alder products can be obtained from tung oil.[4] This is possible because tung oil contains approximately 80% of α-eleostearic fatty acid, which is a major source of conjugated double bonds.[1] However, in order to create a cyclic structure by a Diels-Alder mechanism, a dienophile must be reacted with tung oil under certain conditions. Acrylate monomer contains π-bonds as well as an electron withdrawing group located on the ester. This makes an acrylate group a good choice as a dienophile for the Diels-Alder reaction. Acrylate monomers had been used to form 6-member ring structures by Diels- Alder reaction.[65, 66] Alkoxysilane had been used to form hybrid inorganic-organic coatings through the sol-gel process. This process involves in situ polycondensation of metal or silicon alkoxide with an organic polymer matrix.[67-69] The inorganic microdomain is formed by hydrolysis, condensation, and gelation of the metal or silicon 32

51 alkoxide in solution. Fluorinated polymers exhibit low intermolecular and intramolecular interactions. These characteristics lead to low cohesive energy, which in turn provides low surface energy. Moreover, fluorinated materials show high thermal stability, chemical inertness, low refractive index and friction coefficient, good hydrophobicity and lipophobicity, valuable electrical properties, and low dielectric constants and dissipation factors.[41] For coatings application, research at Cargill chemically modified linseed oil with Diels-Alder reaction of cyclopentadiene at high temperature and pressure to form norbornene groups on the chain known as Dilulin.[70] Typically, only 2-5 % of the double bonds are modified via these processes. Soucek et al. have been prepared norbornene linseed oil by Diels-Alder reaction of dicyclopentadiene with linseed oil at 250 o C at pressure 0.76 to 0.9 MPa. In this study, high modification of double bonds through this process has been reported.[3] Fatty acid methyl ester and its derivatives were also proposed as reactive diluents in several coatings system.[71-73] Even though fatty acid derivatives have low viscosity, a draw back of this diluent is that there was always unreacted fatty acid derivative in the coatings due to the low reactivity. As a result, these reactive diluents affect negatively to the film properties. The purpose of this study was to develop a new class of materials to replace the need for organic solvents that compatible with alkyd-based coatings system. In this section, three reactive diluents were synthesized based on tung oil. Tung oil was modified with three different acrylate molecules: 3-methacryloxypropyl trimethoxysilane, 2,2,2- trifluoroethyl methacrylate and triallyl ether acrylate via a Diels-Alder reaction mechanism. Structures of the modified oils were characterized by various spectroscopy 33

52 methods including: 1 H NMR, 13 C NMR, FT-IR, gel permeation chromatography (GPC), and MALDI-TOF mass spectroscopy. 3.3 Materials Tung oil was obtained from Waterlox Coatings Inc. The 3-methacryloxypropyl trimethoxysilane was purchased from Gelest Inc. The phenothiazine, pentaerythritol allyl ether (PETAE), acrylic acid (AA), p-toluenesulfonic acid and 2,2,2-trifluoroethyl methacrylate were purchased from Aldrich chemical company. All materials were used as received. 3.4 Instruments and Characterization 1 H NMR and 13 C NMR were recorded on Mercury-300 spectrometer (Varian) in CDCl 3 solvent. Fourier transform infrared (FTIR) spectroscopy was performed on an ATI Mattson Genesis FT-IR spectrometer by the casting of thin liquid samples on KBr plates. The mass spectra were acquired by using a Bruker REFLEX III time-of-flight (TOF) mass spectrometer (Bruker Daltonics, Billerica, MA) equipped with a nitrogen laser (337nm), a single-stage pulsed ion extraction ion source, a two-stage grid-less reflector, and the two dual microchannel plate detectors for detection in linear and reflection mode. MS spectra were measured with reflection mode, with the ion source and reflector lens potentials keep at 20 kev and 22.5 kev, respectively. Triglyceride, Dithranol and Sodium trifluoroacetate (NaTFA) were dissolved in tetrahydrofuran (THF) at a concentration of 20 mg/ml, 10 mg/ml and 10 mg/ml, respectively. These solution were mixed in the ratio of triglyceride:dithranol:natfa was 2:10:1 and 0.5 µl of the mixture was applied on the MALDI sample target. The attenuation of the nitrogen laser was adjusted to get the maximize sensitivity without causing the fragmentation of 34

53 triglyceride. The mass scale was calibrated by using 6 peaks of polymethylmethacrylate standard (PMMA) that molecular weight ~ A water system was used for gel permeation chromatography (GPC) with a HR4, HT2, HR1, HR0.5 Styragel, and 500A o Ultrastyragel columns connected in series. Tetrahydrofurun was applied as the mobile phase and delivered at a rate of 1.0 ml/min. Viscosity measurement was accomplished by Laboratory Brookfield viscometer (DVII+ PRO Digital) at room temperature. Spindle SC4-25 was used at 100 RPM rotating speed (shear rate of 100 s -1 ). Approximately g of samples was prepared in a 20 ml vial and mixed on roll mill for 30 min. 3.5 Synthesis of acrylic modified tung oils Synthesis of Siloxane functionalized tung oil (SFTO) Tung oil (80 g), 3-methacryloxypropyltrimethoxysilane (27.2 g, mol) and phenothiazine (1.6 g, mol) were added into 250 ml three-neck round bottom flask equipped with mechanical stirrer, reflux condenser and temperature controller. The mixture was mixed for 15 min under argon purge. Then the temperature was increased to 180 o C and maintained for 3 h. The result product was cooled to room temperature and characterized by 1 H NMR, 13 C NMR, FT-IR, and MALDI-TOF MS. 1 H NMR (300 MHz, CDCl 3 ) δ(ppm): 0.67 (H-2 ), 0.89 (H-1, H-22), 1.06 (H-10), 1.30 (H-2, H-12, H-13, H-14, H-15, H-16, H-22, H-23, H-24, H-32, H-33, H-34, H-35), 1.60 (H-17, H-36), 1.73 (H-3 ), 2.08 (H-3, H-6, H-31), 2,31 (H-18, H-37), 2.76 (H-28), 3.57 (H-1 ), 4.04 (H-4 ), (H-19, H-20), (H-7, H-8), (H-26, H-27, H-29, H-30); 13 C NMR (300 MHz, CDCl 3 ) δ(ppm): 5.53 (C-2 ), (C-1, C-3 ), (C-2, C-10), (C-17, C-38), ,68 (C-3, C-4, C-12, C-13, C-14, C-15, C-16, C-26, C-27, C-34, C-35, C-36, C-37), (C-30), (C-9), (C-11), 35

54 50.72 (C-1 ), (C-19, C-20), (C-4 ), (C-21), (C-29, C-31), , (C-7, C-8), (C-4, C-5, C-28, C-29, C-31, C-32), (C- 22), (C-23). Synthesis of fluorine functionalized tung oil (FTO) Tung oil (60g), 2,2,2-trifluoroethyl methacrylate (TFM) (13.78 g, mol) and phenothiazine (1.0 g, mol) were added into 250 ml three-neck round bottom flask equipped with mechanical stirrer, reflux condenser and temperature controller. The mixture was mixed for 15 min. under argon purge. Then the temperature was raised to 150 o C and kept for 2.5 h. The result product was cooled to room temperature and characterized by 1 H NMR, 13 C NMR, FT-IR, and MALDI-TOF MS. 1 H NMR (300 MHz, CDCl 3 ) δ(ppm): 0.89 (H-1, H-22), 1.06 (H-10), 1.30 (H-2, H-12, H-13, H-14, H-15, H-16, H-22, H-23, H-24, H-32, H-33, H-34, H-35), 1.60 (H-17, H-36), 2.08 (H-3, H-6, H-31), 2,31 (H-18, H-37), 2.76 (H-28), (H-19, H-20), 4.48 (H-1 ), (H-7, H-8), (H-26, H-27, H-29, H-30); 13 C NMR (300 MHz, CDCl 3 ) δ(ppm): (C-1), (C-2, C-10), (C-17, C-38), ,68 (C-3, C-4, C-12, C-13, C-14, C-15, C-16, C-26, C-27, C-34, C-35, C-36, C-37), (C- 30), (C-9), (C-11), (C-19, C-20), (C-2 ), (C-21), (C-1 ), (C-29, C-31), , (C-7, C-8), (C-4, C-5, C-28, C-29, C-31, C-32), (C-22), (C-23). Synthesis of triallyl ether functionalized tung oil (TAETO) Triallyl ether functionalized tung oil has been prepared by two-step reactions. First step, preparation of triallyl ether acrylate was accomplished by esterification. Pentaerythritol allyl ether (PETAE) (100 g mol), acrylic acid (AA) (56.16 g,

55 mol ), phenothiazine (1 g, mol) as an inhibitor, and p-tsa (0.67 g, mol) as an acid catalyst, and dichloromethane (200 ml) were added in a three-neck round bottom flask and heated to reflux temperature (45 o C). The reaction was maintained at reflux temperature for 3 h to ensure completed reaction after which dichloromethane was removed from the mixture solution in vacuo. For extraction diethyl ether anhydrous (200 ml) was added, and 5 wt% sodium carbonate solution (200 ml) was added to the system slowly. The organic layer was wash with deionized water (3 x 200 ml) and dried with MgSO 4 anhydrous (50 g). Solvent was removed in vacuo to give triallyl ether acrylate. 1 H NMR (300 MHz, CDCl 3 ) δ(ppm): 3.96 (t, 2H, (R 1 ) 3 -CH 2 -COO-R 2 ), 3.94 (t, 6H, R 3 -O- CH 2 -R 4 ), 3.49 (s, 6H, R 5 -CH 2 -O-CH 2 -R 4 ). 13 C NMR (300 MHz, CDCl 3 ) δ(ppm): 44.57, 63.93, 65.71, 68.97, 70.70, 72.26,116.34, , , , In the second step, triallyl ether acrylate was reacted with tung oil via Diels-Alder reaction. Tung oil (80 g) and triallyl ether acrylate (TAEA) (28 g, 0.09 mol) were added into 250 ml. Three-neck round bottom flask equipped with mechanical stirrer, reflux condenser and temperature controller. The mixture was mixed for 15 min under argon purge. Then, the temperature was increased to 120 o C and maintained for 2 h. The result product was cooled to room temperature and characterized by 1 H NMR, 13 C NMR, FT- IR, and MALDI-TOF MS. 1 H NMR (300 MHz, CDCl 3 ) δ(ppm): 0.89 (H-1, H-22), 1.30 (H-2, H-12, H-13, H- 14, H-15, H-16, H-22, H-23, H-24, H-32, H-33, H-34, H-35), 1.60 (H-17, H-36), 2.08 (H- 3, H-6, H-31), 2,31 (H-18, H-37), 2.76 (H-28), 3.48 (H-3 ), 3.73 (H-4 ), 3.95 (H-5 ), (H-19, H-20), (H-1 ), (H-7, H-8), (H-26, H-27, H-29, H-30, H-2 ); 13 C NMR (300 MHz, CDCl 3 ) δ(ppm): (C-1), (C-2, C-10), 37

56 25.01 (C-17, C-38), ,68 (C-3, C-4, C-12, C-13, C-14, C-15, C-16, C-26, C-27, C-34, C-35, C-36, C-37), (C-30), (C-9), (C-6 ), (C-11), (C-19, C-20), (C-5 ), (C-3 ), (C-21), (C-4 ), (C-1 ), (C-29, C-31), , (C-7, C-8), (C-4, C-5, C-28, C-29, C- 31, C-32), (C-22), (C-23). 3.6 Result and discussion An ideal reactive diluent should exhibit low viscosity, good compatibility with the system, proper reactivity, low volatility, and no toxicity. Tung oil is a good source of conjugated double bond (α-eleostearic acid); hence, Diels-Alder reaction was utilized as a route in the preparation of the modified tung oil reactive diluent. A siloxane group was chosen to incorporate into the tung oil molecules due to excellent interfacial properties and the ability to moisture curing of siloxane group. Under acidic conditions, siloxane groups can react with water to afford silanol groups which can then condense to form siloxane crosslink.[74] A fluorinated group was also chosen to react with tung oil owing to the exceptional properties of fluorinated material such as high thermal stability, chemical inertness, and low surface energy.[41] A triallyl ether group was introduced into tung oil molecules in order to increase the number of functionality and reactivity for an autoxidative cure.[50, 51] Synthesis and Characterization For comparison purpose, tung oil was characterized by 1 H NMR, 13 C NMR, FT- IR and MALDI-TOF Mass spectrometer. 1 H NMR and 13 C NMR of tung oil are shown in Figure 3-1 and 3-2, respectively. Tung oil contains 5% saturated fatty acid (steric acid (18:0), palmitic (16:0)), 8% oleic acid (18:1), 4% linoleic acid (18:2), 3% Linolenic 38

57 (18:3) and 80% conjugated fatty acid (α-eleostearic acid).[1] The majority of the double bonds that are present in tung oil are conjugated CH=CH, appearing at δ ppm. Non-conjugated double bond cis-ch=ch appears at δ ~5.4 ppm. The resonances for the rest of the chemical structure are assigned in Figure 3-1. The assigned resonance of 13 C NMR of tung oil shows in Figure 3-2, the resonance at δ ppm is correspond to the unsatuaration carbon, the resonance at δ 173 ppm is the carbonyl groups in tung oil. FT-IR and mass spectra of the tung oil is shown in Figure 3-3 and 3-4, respectively. The absorption bands at 990 and 965 cm -1 represent conjugated double bond in tung oil. The absorption at 738 cm -1 is attributed to cis-c=ch bending. The mass spectrum exhibited a peak at 895 m/z is indicative of the triglyceride structure of tung oil. Figure 3-1: 1 H NMR spectra of raw tung oil. 39

58 Figure 3-2: 13 C NMR of Tung oil Figure 3-3: FT-IR spectrum of Tung oil 40

59 Figure 3-4: MALDI-TOF mass spectra of Tung oil. Siloxane functionalized tung oil (SFTO) was prepared via Diels-Alder reaction of 3-methacryloxypropyltrimethoxysilane (MAS) and tung oil with a free radical inhibitor. Siloxane was added at 50 mol% of conjugated double bond presented in tung oil. The synthetic diagram is presented in Figure 3-5. The pendant alkoxysilane group provided the capability of moisture cure via sol-gel chemistry.[68, 69] The identification of SFTO was investigated by 1 H NMR, 13 C NMR, FT-IR, MALDI-TOF mass spectrometer and Gel permeation chromatography (GPC). 1 H NMR and 13 C NMR spectrum of siloxane functionalized tung oil is shown in Figure 3-6 and 3-7, respectively. O O Si O Diels-Alder Reaction O O O O O O O O O O O SiO O,180 o C, 3 h O O Si O O O O O O O O O Figure 3-5: Reaction of siloxane functionalized tung oil 41

60 Figure 3-6: 1 H NMR spectra of Siloxane functionalized tung oil (SFTO) Figure 3-6 shows the 1 H NMR of siloxane functionalized tung oil (SFTO). Diels- Alder reaction between 3-methacryloxypropyltrimethoxysilane (MAS) and tung oil took place due to the reduction of the resonance of conjugated double at δ ppm. New resonances appeared at δ ppm due to the cyclohexene ring created from Diels- Alder reaction. Methyl proton ( CH 3 ) from methacrylate group shifted from δ 1.94 to 1.06 ppm indicated new formation of a Diels-Alder product. The rest of the resonances were assigned to the structure of siloxane functionalized tung oil in Figure 3-6. (See experimental section) 42

61 Figure 3-7: 13 C NMR of Siloxane functionalized tung oil (SFTO) Figure 3-7 shows 13 C NMR of siloxane functionalized tung oil. The assigned 13 C NMR spectrum of siloxane functionalized tung oil shown in Figure 3-7; comparing to 13 C NMR spectrum of raw tung oil (Figure 3-2), a new carbonyl resonance appeared at δ 177 ppm corresponds to the addition of 3-methacryloxypropyltrimethoxysilane (MAS) to the tung oil backbone. A new double bond formed from the Diels-Alder reaction showed a new resonance at δ 126 and 128 ppm. Resonance at δ ~42 and 45 ppm confirmed the sixmember ring structure created by Diels-Alder reaction. FT-IR spectra of siloxane functionalized tung oil is shown in Figure 3-8. As expected, the conjugated double bond absorption bands at 990 and 965 cm -1 for the SFTO reduced which is the indicative of a cycloaddition onto the α-eleostearic pendant group. 43

62 Figure 3-8: FT-IR spectra of Siloxane functionalized tung oil (SFTO) Structure of the siloxane functionalized tung oil was also identified by MALDI- TOF mass spectrometer. The spectra of siloxane functionalized tung oil are shown in Figure 3-9. The peak at 895 m/z indicated the presence of tung oil. The peak at 1,143, 1,391 and 1,640 m/z indicated mono, di and tri-addition of 3- methacryloxypropyltrimethoxysilane (MAS), respectively. The small peak at 1,768 m/z showed a dimer of tung oil and the peak at 2,016, 2,264 and 2,512 m/z indicated mono, di and tri-addition of MAS to a dimer of tung oil. Gel permeation chromatography (GPC) was also used to determine the polydispersity index (PDI). The GPC result showed that SFTO has the PDI of

63 Figure 3-9: MALDI-TOF spectra of siloxane functionalized tung oil (SFTO) Synthesis of fluorine functionalized tung oil (FTO) was accomplished by Diels- Alder reaction of 2,2,2-trifluoroethyl methacrylate (TFM) and tung oil with a free radical inhibitor. Fluoromethacrylate was added at 20 mol% of conjugated double bond presented in tung oil. The synthetic diagram is presented in Figure The fluoropendant group provided the surface active properties such as hydrophobicity and solvent resistance to the material. Figure 3-11 and 3-12 show the 1 H NMR and 13 C NMR spectra of FTO, respectively. The 1 H NMR and 13 C NMR spectra of fluorine functionalized tung oil were consistent with a Diels-Alder reaction. The 1 H NMR showed partial reduction of the resonance of conjugated double bond at δ ppm, and new resonances appeared at δ ppm. Also, the protons of methyl group from methacrylate shifted from δ 1.94 to 1.06 ppm. The 13 C NMR shows a new carbonyl resonance group appeared at δ 177 ppm. 45

64 Diels-Alder Reaction F F C O O F O O O O O O F O O C F F,150 o C, 2.5 h F F C F O O O O O O O O Figure 3-10: Reaction of fluorine functionalized tung oil. Figure 3-11: 1 H NMR of fluorine functionalized tung oil The FT-IR spectrum of fluorinated tung oil (FTO) is shown in Figure The FT-IR spectrum of FTO showed the reduction of absorption bands at 990 and 965 cm -1 (conjugated double bond) which similarly happened in SFTO spectra. MALDI-TOF mass spectrometry was used to determine the molecular structure of FTO as shown Figure The mass spectrum of FTO exhibited a peak at 895 m/z which indicated the presence 46

65 of tung oil. The mass clusters at 1,063, 1,231, 1,399 m/z indicated that there was mono, di, and tri-addition of trifluoromethacrylate, respectively. Mass spectra also showed a small trace of tung oil dimer at 1,768 m/z. The GPC result showed the PDI of Figure 3-12: 13 C NMR of fluorine functionalized tung oil 47

66 Figure 3-13: FT-IR spectra of fluorine functionalized tung oil (FTO) Figure 3-14: MALDI-TOF spectra of fluorine functionalized tung oil (FTO). Synthesis of triallyl ether functionalized tung oil (TAETO) was prepared by two step reactions. First, esterification of PETAE and acrylic acid was accomplished with p- toluenesulfonic acid as catalyst and phenothiazine as free-radical inhibitor to produce triallyl ether acrylate. Stoichiometric ratio of 2 : 1 (acrylic acid : PETAE) was used to 48

67 ensure the complete reaction. The synthetic route of the first step is shown in Figure The product, triallyl ether acrylate was characterized by 1 H NMR and 13 C NMR. Second step was the Diels-Alder reaction between triallyl ether acrylate (TAEA). Stoichiometric mole ratio of 1 : 1 (TAEA : conjugated double bond) presented in tung oil was used in the reaction. The allyl ether pendent groups provided crosslinkable site to the tung oil molecule. The 1 H NMR and 13 C NMR spectra of triallyl functionalized tung oil are shown in Figure 3-16 and 3-17, respectively. The spectrum was consistent with SFTO and FTO. The 1 H NMR showed partial reduction of the resonance of conjugated double bond at δ ppm, a new resonance appeared at δ ppm. The protons of methyl group from methacrylate shift from δ 1.94 to 1.06 ppm. The 13 C NMR showed a new carbonyl resonance group appeared at δ 177 ppm. Figure 3-15: Reaction scheme of triallyl ether functionalized tung oil 49

68 The FT-IR spectrum of triallyl ether functionalized tung oil is shown in Figure The FT-IR spectrum showed the reduction of bands at 990 and 965 cm -1 (conjugated double bond) which was similar to the SFTO and FTO spectra. MALDI-TOF mass spectrometry was used to determine the molecular structure of TAETO. Figure 3-19 illustrated the mass spectrum of TAETO. The peak at 1,205, 1,515, and 1,825 m/z indicated mono, di and tri-addition of triallyl ether acrylate respectively. Mass spectra also showed trace of mono-addition of triallyl ether acrylate to tung oil dimer at 2,091 m/z. The GPC result showed the PDI of Figure 3-16: 1 H NMR of triallyl ether functionalized tung oil 50

69 Figure 3-17: 13 C NMR of triallyl ether functionalized tung oil Figure 3-18: FT-IR spectra of a) Tung oil, b) Triallyl ether functionalized tung oil (TAETO) 51

70 Figure 3-19: MALDI-TOF spectra of Triallyl ether functionalized tung oil (TAETO). Three modified tung oil that showed in this article were prepared in the same manner; however, each specific reaction were not as reactive as the others. Since the reactions were carried out at atmospheric pressure, the SFTO had more capability to carry out at higher temperature due to high boiling point of MAS. As a result, the SFTO showed fully mono, di, and tri-addition to the tung oil. The FTO has relatively lower in boiling point; therefore, the reactivity of the reaction was lower. The TAETO had some limitation over the others modified tung oil. The reaction mostly gelled at very high temperature or long reaction time in TAETO reaction because triallyl ether acrylate contains substantial amount of unsaturation. To avoid gelation, high pressure reaction could be an alternative pathway to prepare acrylic modified tung oil at low temperature 52

71 since there were several studies showed that Diels-Alder reaction can be induced accelerated by pressure.[75-77] The ability to modified tung oil with acrylate/methacrylate monomer was beneficial in that it provided a whole new pathway of using the Diels-Alder reaction to create new materials, films or other potential applications to the bio-based material (tung oil). The capability to modify or tailor the pendent groups introduced a new crosslink mechanism to tung oil other than traditional autoxidation reaction and also solved miscibility issues by making the groups similar in structure and/or polarity to a specific type of resin or solvent. A number of approaches were reported to identify a suitable reactive diluent for alkyd or air-drying paint systems.[44, 46, 78] However, all requirements for a good reactive diluent were difficult to be achieved. Reactive diluents derived from natural oil such as linseed oil generally has high viscosity, but they are compatible with alkyd based system and toxicity was not an issue. As presented here, the viscosity of siloxane modified tung oil, fluorine modified tung oil, and triallyl modified tung oil were 1,000, 417, and 80 cp, respectively. Although tung oil based reactive diluents as well exhibited relatively high viscosity than other reactive diluents, tung oil could dissolve and react quickly to form crosslink networks with air-drying alkyd system with low toxicity; also low cost is another advantage of tung oil. 3.7 Conclusion Diels-Alder reaction was employed to functionalize tung oil. Three different acrylate molecules: 3-methacryloxypropyl trimethoxysilane (MAS), 2,2,2-Trifluoroethyl methacrylate (TFM) and triallyl ether acrylate (TAEA) showed the capability to 53

72 functionalize tung oil via Diels-Alder reaction at elevate temperature and atmospheric pressure. Each individual property and reactivity of acrylate monomers such as boiling point dictated the reaction condition; therefore, affected the final functional yield. The reactive diluents were compatible with alkyd system and effectively reduced the viscosity of the system. 54

73 CHAPTER IV EVALUATION OF MODIFIED TUNG OIL AS A REACTIVE DILUENT ON COATINGS PROPERTIES IN ALKYD SYSTEMS 4.1 Abstract The effects of new acrylate modified tung oils on the coatings properties of alkydbased coatings were investigated. A long oil alkyd resin was synthesized and used as a model alkyd in the formulation. The three new tung oil derivatives were mixed with alkyd resins at 10, 20, and 30 wt %. Viscosity, drying time, and coatings properties of the formulations were studied as a function of reactive diluents. The effects on coatings properties were evaluated by solvent resistant, impact resistant, pencil hardness, flexibility, gloss, crosshatch adhesion, and contact angle measurement. The reactive diluents did not show any significant effect on flexibility that could be observed with the testing equipments that were used here. Hardness was increased in comparison to that of alkyd system when the diluents were introduced into the formulation. Solvent resistance of the systems varied according to the amount and type of diluents that was used. Drying time study showed that drying time could be altered by type and level of diluent added. Introduction of particular modified tung oil increased contact angle of the coatings. 55

74 4.2 Introduction During the past decade, paint and coating industries have been forced to decrease the use of organic solvents in coating formulations by the implementation of the 1990 Clean Air Act (CAA).[79] Volatile Organic compound (VOC) emissions from coatings formulation not only contributed to global warming, but also to low level ozone creation leading to the risk of human and animal health. Consequently, researchers are continually attempting to develop greener coatings systems.[1, 52, 53] Powder coatings and UVcurable systems were introduced to eliminate the VOCs content used in coatings formulation;[71, 80] however, these technologies have not applied to wide variety of application. Another interesting solution was to introduce reactive diluents as proposed in several reports.[50, 51, 72, 78] Reactive diluents are materials that function as an organic solvent in the coatings formulation, but are integrated into the film during the curing process. An ideal reactive diluent exhibits excellent solvency behavior such as lowering viscosity, good compatibility, low volatility, and low toxicity. In addition, if the reactive diluent could be derived from a bio-based resource, it would have a greater positive impact on the environment by introducing more renewable materials into the final film. As a result, seed-oil based materials are an attractive choice for reactive diluents, since seed oils are biodegradable and readily available from renewable resources. Seed oils are triglycerides of fatty acids. Seed oils such as linseed oil, soybean oil, or tung oil can form a networked polymer film when exposed to air and are often used in the manufacture of coatings binders. 56

75 Alkyds have the advantages of low cost and with bio-based monoglyceride. Typical alkyd resins incorporate linseed, soybean, safflower oil, and other drying oils into their chemical makeup. Alkyd based coatings have several advantages including high gloss, good color/gloss retention, good heat, and solvent resistance, with an autoxidative crosslinking mechanism. [1] However, alkyds do require the use of organic solvents to reach the desired application viscosity, which generates a problem with respect to VOCs. Tung oil based material has been reported as radiocurable compositions in 1978 [4] and a study to employ cationic copolymerization of tung oil was also conducted [5]. The reason tung oil is a popular choice for the foundation of new materials is the difference in chemical makeup from most other materials in its class. Tung oil is derived from the nuts of Aleurites fordii. It is obtained from the kernels of the nuts and classified as a drying oil. Typical fatty acid compositions of tung oil are: 5% saturated acid, 8% oleic acid, 4% linoleic acid, 3% linolenic acid and 80% α-eleostearic acid.[1, 7] Due to the unique drying speed and excellent water resistance, tung oil is very valuable in modern manufacturing of varnish and related materials. Oxidation of conjugated tung oil catalyzed by metal driers has been investigated.[58] It has been shown that besides radical recombination, crosslinking reactions also occur through direct addition of free radicals to the conjugated double bonds, frequently leading to higher molecular weight oligomers.[51, 59] Several attractive monomer choices for modification of tung oil include different acrylate monomers with a special focus on fluorinated monomer and inorganic materials such as alkoxysilanes. By incorporating alkoxysilanes into these new materials, hybrid inorganic-organic coatings can be formed through the sol-gel process. This process 57

76 involves in situ polycondensation of metal or silicon alkoxide with an organic polymer matrix.[67-69] The inorganic microdomain is formed by hydrolysis, condensation, and gelation of the metal or silicon alkoxide in solution; while the organic polymer matrix of the drying oil forms cross-linking structure by an autooxidative process. Fluorinated polymers exhibit low intermolecular and intramolecular interactions. These characteristics lead to low cohesive energy, which in turn provides low surface energy. Moreover, fluorinated materials show high thermal stability, chemical inertness, low refractive index and friction coefficient, good hydrophobicity and lipophobicity, valuable electrical properties, and low dielectric constants and dissipation factors.[41] In this work, the effect of the new reactive diluents on the curing and coatings properties of alkyd resin formulations was studied. Three modified tung oil materials were formulated with a long oil alkyd. Viscosity, drying time and coatings properties were investigated as a function of the reactive diluent percent and reactive diluent type. The overall goal of this research was to develop a volatile organic compound (VOC) free coatings alkyd-based system. 4.3 Materials Refined soybean oil used for alkyd synthesis was acquired from Cargill Inc., while lithium hydroxide, phthalic anhydride (PA), and glycerol were all purchased from Aldrich. Tung oil was obtained from Waterlox Coatings Inc., methacryloxypropyl trimethoxysilane was purchased from Gelest Inc, phenothiazine, pentaerythritol allyl ether (PETAE), acrylic acid (AA), p-toluenesulfonic acid and 2,2,2-Trifluoroethyl methacrylate were purchased from Aldrich chemical company. Driers used in coatings formulations, 5 wt% cobalt Hydro-Cure II, 12 wt% Zirconium Hydro-Cure, and 5 wt% 58

77 calcium Hydro-Cure, were obtained from OMG Group. All materials were used as received. 4.4 Synthesis of long-oil alkyd resin The alkyd resin was prepared by the monoglyceride process. The reaction was conducted in a 500 ml four-neck round bottom flask equipped with an inert gas inlet, thermometer, reflux condenser, and a mechanical agitator. The transesterification step involved soybean oil (200 g) and an excess of glycerol (44.75 g, 0.48 mol). These two materials were purged with argon gas for ~15 min. The mixture was then heated to 120 C and lithium hydroxide catalyst was introduced into the reaction mixture ( wt% of polyol). The temperature was gradually increased to 240 o C. After approximately 1 h, a small aliquot was removed and mixed in three parts 95 % ethanol. This was repeated until the resulting solution was clear. The reaction mixture from step 1 was cooled to around 100 C and a Dean-Stark trap was introduced to the reaction set up. The reaction mixture was then charged with phthalic anhydride (PA) (71.65 g, 0.48 mol) and 100 ml of xylene for use as a reflux solvent. The mixture was then slowly heated to 220 C. After every hour of reaction, a sample was removed and the acid number was determined. The reaction was stopped once an acid number of below 10 was achieved. Acid number determination was based on ASTM D with 1 M KOH and phenolphthalein indicator. The product was then cooled to room temperature and stored under argon atmosphere. Figure 4-1 shows the reaction path for a soybean oil based alkyd synthesized by the monoglyceride process. 59

78 Figure: 4-1 Reaction of a soya-based alkyd synthesized via the monoglyceride process. 60

79 4.5 Coating Formulation and Film preparation Each modified tung oil was formulated with the alkyd resin at three different levels (10 wt%, 20 wt% and 30 wt% based on total formulation), 2 wt% metal drier package (10 wt% Cobalt Hydro- Cure II, 80 wt% Zirconium Hydro-Cem, 10 wt% Calcium Hydro-Cem) and 1 wt% wetting agent. Formulations of each mixture can be found in Table 4-1. The contents were introduced into a sealed vial and mixed on a roller mill for 2 h. A draw down bar was used to cast films on clean glass panels (6 mils wet film) and on aluminum panels (3 mils wet film). The wet films were cured in the oven at 120 o C for 2 h, followed by a second cure at 160 o C for 3 h. The films were kept at room temperature for 7 days before any tests were performed to ensure a through cure was achieved. Table 4-1: Formulations of different coatings systems used for testing Samples Soy Bean LOA Diluent Drier (2%) Wetting (1%) (g) (g) (g) (g) Tung Si Tung Si Tung Si Tung Si Tung F Tung F Tung F Tung F Tung AE Tung AE Tung AE Tung AE Total Weight 10 gram 4.6 Instruments and Characterization All experiments were conducted according to ASTM standards. Viscosity measurement was accomplished by Laboratory Brookfield viscometer (DVII+ PRO 61

80 Digital) at room temperature. Spindle SC4-25 was used at 100 RPM rotating speed (shear rate of 100 s -1 ). Approximately g of samples were prepared in a 20 ml. vial and mixed on roll mill for 30 min. Drying time study was evaluated according to ASTM standard (D ). The coatings (1 mil) were applied to a glass panel and a circular dry time recorder was immediately placed on the wet film. Tack-free time and dry-hard time were observed. All other film properties were found according to the corresponding ASTM standard, and the appropriate equipment was utilized when specified. Contact angle of the coating films were measured via the sessile drop method using deionized water with a Rame-Hart contact angle goniometer, model Coatings samples (3 mil) were prepared on aluminum panels for the contact angle measurement. 4.7 Result and Discussion The overall objective of this study was to incorporate the modified tung oil diluents into an alkyd system and determine the effect of new reactive diluents on formulation and final coatings properties of alkyd coatings. In order to achieve this, three modified tung oil reactive diluents were prepared according to previous section (Chapter III)[81] and a soybean-based long-oil alkyd resin was synthesized and formulated with different levels of the diluents. The main advantage of these new systems was the reduction/elimination of organic solvents in the formulation. The low molecular weight of the modified tung oil diluents can function as an organic solvent in the formulation, yet enhance the coatings properties of the films by adding additional film networking sites to the system. It is well known that tung oil based materials cure faster than most other oilbased (soybean, linseed, and sunflower) materials due to their high level of conjugated 62

81 double bonds present in the fatty acid chains. Through incorporation of the modified tung oil diluents, the curing kinetics of the films could be altered due to the rapid curing mechanism of tung oil. The difference of these curing mechanisms although it might be small, could affect the overall coatings properties. The extent of the effect was studied by varying the loading and type of diluents Viscosity The synthesized materials were anticipated to reduce the viscosity of the alkyd on account of the compatibility with the alkyd resin, the lower molecular weight, and additional flexibility compared to an alkyd chain. The viscosities as a function of amount and type of reactive diluents were investigated and compared to alkyd resin. The results for this experiment are presented in Figure 4-2. The viscosity of the particular alkyd resin used for all runs was found to be 1600 cp. As expected, all three reactive diluents effectively reduced the viscosity of the neat material (up to 48% for SFTO, 60% for FTO and 70% for TAETO). The siloxane functionalized material showed the lowest viscosity reduction, followed by the fluorine modified product and the triallyl ether functionalized tung oil. The difference in viscosity can be attributed to the varying levels of cycloaddition at the conjugation sites of tung oil and oligomerization that is obtained for each of the modification reactions. It is apparent that using of these reactive diluents in small quantities is sufficient to reach application viscosities of alkyd resins, and eliminate the need for organic solvents in alkyd-based coatings. 63

82 SFTO FTO TAETO Viscosity (cp) % Diluents Figure 4-2: Viscosity behavior of neat alkyd and alkyd/diluent mixtures at a shear rate of 2.2 s Drying time study Incorporation of tung oil reactive diluent altered the crosslink mechanism; therefore, investigation of drying time is also crucial for diluent/alkyd system. Drying time experiment was performed under ambient condition (25 o C, RH = of 50 +/-5 %). The result of the drying time study is shown in Table 4-2. Tack-free time of alkoxysilane and fluorine modified diluent with alkyd was delayed compared to neat alkyd system. The amount of diluent added into the system affected the drying mechanism to be slower. Dry-hard time of alkoxysilane and fluorine modified diluent with alkyd were more than 24 h except the 10 weight percent of alkoxysilane (Tung Si-10). However, the triallyl ether modified diluent (Tung AE) improved tack-free time from 6 to 5 h at 10 weight 64

83 percent added to alkyd system. At 20 wt% of triallyl ether modified diluent, tack-free time of the system was equal to the alkyd. Tack-free time was increased with the amount of triallyl ether modified diluent. It was interesting that dry-hard time of alkyd system was improved greatly by triallyl ether modified diluent. Dry-hard time of triallyl ether modified diluent with alkyd systems were around 12 h comparing to 20 h of alkyd system without reactive diluent. The improvement of drying time by triallyl ether modified diluent could be explained by the higher functionality and low viscosity of the system. Low viscosity provided mobility to the fatty acid chain to react; therefore, the curing time was faster. Table 4-2: Drying time study experiment Sample Tack free time (h) Dry Hard Time (h) Neat Alkyd 6 20 Tung Si Tung Si-20 8 >24 Tung Si-30 9 >24 Tung F-10 7 >24 Tung F-20 8 >24 Tung F >24 Tung AE Tung AE Tung AE General film properties Films were prepared with the neat alkyd and alkyd/diluent mixtures with varying levels of each diluent. Metal driers and a wetting agent were incorporated into the formulations to enhance the autoxidative curing process and eliminate surface wrinkling of the final films. Typical coatings properties were evaluated to determine the effects of 65

84 each diluent on an alkyd-based coating. The results of the different tests are presented in Table 4-3. In addition to reducing viscosity, the diluents were expected to enhance certain properties of the alkyd films. Since the diluents contain additional sites for network formation, the crosslink density was expected to increase with the addition of reactive diluent. Solvent resistance, hardness, and glass transition temperature are all related to the crosslink density of the film. Therefore, these properties were also expected to improve with the addition of diluent. When evaluating the film properties, it was also important to note if any of the diluents produced a negative effect on the film properties. The fluorine modified tung oil was not expected to enhance the performance as much as other reactive diluents due to the lower crosslink site. The main function of this material was to improve the surface tension of the films. From the test results presented in Table 4-3 and 4-4, several trends can be noted and discussed about the behavior of the films. The pencil hardness of the films also improved as predicted for the reactive diluents and remained the same for systems containing the fluorinated material. Solvent resistance, on the other hand, showed unexpected results. The MEK double rub test shows that coatings with SFTO reduced the solvent resistance and incorporation of FTO and TAETO did not significantly improve the solvent resistance of the coatings. It is important to note that the thickness of the coating also affect the result of the solvent resistance test, in which the neat alkyd films were thicker than those films that contain the diluents. FTO generally exhibited relatively higher solvent resistance than SFTO and TAETO. This result may attribute to the excellent chemical resistance of fluorinated groups in FTO. The remaining properties, 66

85 flexibility and impact resistance, were not affected by the incorporation of the diluents as predicted in compliance with the limits of our testing equipment. Additional coatings properties such as the gloss and crosshatch adhesion appeared to be independent the diluent content. (see Table 4-4.) Table 4-3: Coatings properties of neat alkyd resin and diluent/alkyd mixtures Sample Film Thickness MEK Double Rubs Impact Resistance Pencil Hardness Mandrel Bend Flexibility Cylindrical Bend Flexibility (micron) (Ib/in) (% Elongation) (mm) Neat Alkyd >40 2B >32% 2 Tung S >40 B >32% 2 Tung S >40 HB >32% 2 Tung S >40 HB >32% 2 Tung F >40 2B >32% 2 Tung F >40 2B >32% 2 Tung F >40 2B >32% 2 Tung AE >40 HB >32% 2 Tung AE >40 HB >32% 2 Tung AE >40 HB >32% 2 Tung S-x: Alkyd with x wt% alkoxysilane modified tung oil Tung F-x : Alkyd with x-wt% fluorine modified tung oil Tung AE-x : Alkyd with x-wt% allyl ether modified tung oil Contact angle In order to investigate the surface properties of coatings film, water contact angle measurements were performed. Each contact angle result is shown in Figure 4-3. The contact angles were taken 8-10 readings per sample. The contact angle of alkyd film was 88 o. Introduction of SFTO into the formulation increased the contact angle of the film to 93 o, 97 o, and 100 o at 10 wt%, 20 wt%, 30 wt% SFTO, respectively. Similarly, FTO 67

86 increased the contact angle of alkyd film. The contact angle of 10 wt%, 20 wt%, and 30 wt% FTO in alkyd were 96 o, 100 o, and 103 o, respectively. As expected, the hydrophobicity of alkyd film was increased by introducing fluorinated or alkoxysilane groups into the film. An example of water contact with 20 wt% FTO/alkyd coatings is shown in Figure 4-4. Table 4-4: Coatings properties of neat alkyd resin and diluent/alkyd mixtures (Cont.) Sample Film Thickness (micron) Viscosity 20º Gloss 60º Gloss Crosshatch Adhesion (cp) Neat Alkyd B Tung S B Tung S B Tung S B Tung F B Tung F B Tung F B Tung AE B Tung AE B Tung AE B 68

87 Contact Angle (Degree) Tung Si Tung F Alkyd 50 Alkyd 10% 20% 30% Figure 4-3: Contact angle of diluents/alkyd coatings. Figure 4-4: Water contact with 20 %wt FTO/alkyd Overall, modified tung oil diluents generally provided a good film and coatings properties. The primary goal of this study was to replace the organic solvent used in the alkyd-based coatings. The reduction of the viscosity without damaging the coatings properties indicated the accomplishment of this goal. Regarding to the film properties, the 69

88 addition of diluents did not show any adverse effect on coatings properties. On the other hand, the hardness of the film was increased with the incorporation of alkoxysilane modified tung oil and allyl ether modified tung oil. However, fluorine modified tung oil generally showed relatively higher solvent resistance than the other diluents. Contact angle of the alkyd films was increased with the incorporation of both siloxane modified tung oil and fluorine modified tung oil. With respect to the result, it has been proposed that the modified tung oil diluents incorporated into the film formation mechanism and generally improved the film properties. 4.8 Conclusion Incorporation of the diluents into alkyd-based coatings formulations has shown that the new functionalized tung oil derivatives acted effectively as reactive diluents. The viscosity of the neat alkyd was significantly decreased with increasing load of the diluents. In addition, the new materials added additional crosslinking sites for the overall system; as a result, improved hardness, and coatings properties were achieved with the addition of modified tung oils. 70

89 CHAPTER V THERMO-MECHANICAL PROPERTIES OF ALKYD/ACRYLIC MODIFIED TUNG OIL COATINGS 5.1 Abstract New acrylate modified tung oils were prepared to use as reactive diluents in alkyd-based system. Tung oil was modified with three different acrylate monomers: 3- methacryloxypropyl trimethoxysilane, 2,2,2-Trifluoroethyl methacrylate and triallyl ether acrylate. New tung oil reactive diluents were mixed with the same alkyd resin as a function of weight percent. To evaluate the effect on the physical properties, tensile properties were studied using a stress-stain experiment. The results showed that through the addition of the reactive diluents, the tensile strength and tensile modulus were improved compared to the values of the neat alkyd system. However, addition of diluents showed slightly reduction in the elongation-at-break over the neat alkyd. Dynamic mechanical analysis (DMA) was also performed on the resulting films. The results provided tools for determination of crosslink density and glass transition temperatures of the films. Both crosslink density and glass transition temperature were increased over that of the alkyd system when the diluents were introduced into the formulation. 71

90 5.2 Introduction Environmental concern has become one of the most important topics in the coatings industry in the last three decades.[1, 53] The regulatory pressure, higher cost of solvents and energy have reflected the coating technologies by pushing them toward high solid and low volatile organic content (VOC) coatings. Researchers are continually attempting to develop greener coatings systems.[1, 52, 53] In order to acquire the application viscosity in high solid coatings, polymers generally comprise of low molecular weight and narrow molecular weight distribution were employed which leading to undesired film properties.[1] Alkyd resin is the reaction product of dibasic acid, polyols, and mono-functional fatty acid. Alkyd can be considered as a particular type of polyester. Because of the push to develop new materials using renewable resources, alkyds are an attractive binder, as they are derived from plant and vegetable oils. Typical alkyd resins incorporate linseed oil, soybean oil, safflower oil, and other drying oils into their chemical makeup. Two reasons that alkyd resin becomes an important resin for coatings. First, alkyds are ultimately versatile and allow choices of structure combination to give tailor-made properties. Second, the success of alkyd is the relatively low cost. In addition, alkyd based coatings have several advantages including high gloss, good color/gloss retention, good heat and solvent resistance, and an autooxidative crosslinking mechanism.[1] Recently, alkyd usage has been declining in the United States due to volatile organic content (VOC) regulation and replacement with higher performance polyester/isocyanate type systems. However, the overall market of alkyd is still high due to the environmentally friendly attributes compare to isocyanate counterparts. However, they do 72

91 require the use of organic solvents to reach the desired application viscosity, which cause a problem in a time where environmental restrictions are more severe. One attractive approach to achieve high solid coatings is through the use of reactive diluents which can function as a solvent in the coatings formulation; then during the curing, reactive diluents participate into the crosslink reaction and become part of the film. Tung oil has been used in efforts for new materials. Tung oil based material has been reported as radiocurable compositions in 1978 [4] and a study to employ cationic copolymerization of tung oil was also conducted [5]. The reason tung oil is a popular choice for the foundation of new materials is the difference in chemical makeup from most other materials in its class. Tung oil is derived from the nuts of Aleurites fordii. It is obtained from the kernels of the nuts and classified as a drying oil. Typical fatty acid compositions of tung oil are: 5% saturated acid, 8% oleic acid, 4% linoleic acid, 3% linolenic acid and 80% α- eleostearic acid.[1, 7] Due to the unique drying speed and excellent water resistance, tung oil is very valuable in modern manufacturing of varnish and related materials. Oxidation of conjugated tung oil catalyzed by metal driers has been investigated.[58] It has been shown that besides radical recombination, crosslinking reactions also occurred through direct addition of free radicals to the conjugated double bonds, frequently leading to higher molecular weight oligomers.[51, 59] Several attractive monomer choices for modification of tung oil included different acrylate monomers with a special focus on fluorinated monomer and inorganic materials such as alkoxysilane. By incorporating alkoxysilane into these new materials, hybrid inorganic-organic coatings can be formed through the sol-gel process. This process involves in situ 73

92 polycondensation of metal or silicon alkoxide with an organic polymer matrix. [67-69] The inorganic microdomain is formed by hydrolysis, condensation, and gelation of the metal or silicon alkoxide in solution; while the organic polymer matrix of the drying oil forms cross-linking structure by an autoxidative process. Fluorinated polymers exhibit low intermolecular and intramolecular interactions. These characteristics lead to low cohesive energy, which in turn provides low surface energy. Moreover, fluorinated materials show high thermal stability, chemical inertness, low refractive index and friction coefficient, good hydrophobicity and lipophobicity, valuable electrical properties, and low dielectric constants and dissipation factors.[41] The objective of this study was to evaluate new acrylate modified tung oil materials as reactive diluents to replace the need for organic solvents in alkyd-based coatings and enhance the film properties of traditional alkyd resins. In this chapter, three acrylate modified tung oil based reactive diluents have been prepared according to previous chapter (Chapter III).[82] The modified tung oil materials were formulated with a neat long oil alkyd and other additives in order to form crosslinked films. The thermomechanical properties such as tensile properties, T g, and crosslink density were investigated as a function of reactive diluent weight percent and type. 5.3 Materials Refined soybean oil used for alkyd synthesis was acquired from Cargill Inc., while lithium hydroxide, phthalic anhydride (PA), and glycerol were all purchased from Aldrich. Tung oil was obtained from Waterlox Coatings Inc., 3-methacryloxypropyl trimethoxysilane was purchased from Gelest Inc, phenothiazine, pentaerythritol allyl ether (PETAE), acrylic acid (AA), p-toluenesulfonic acid and 2,2,2-trifluoroethyl 74

93 methacrylate were purchased from Aldrich chemical company. Driers used in coatings formulations, 5 wt% cobalt Hydro-Cure II, 12 wt% Zirconium Hydro-Cure, and 5 wt% calcium Hydro-Cure, were obtained from OMG Group. All materials were used as received. 5.4 Synthesis of long-oil alkyd resin The alkyd resin was prepared by the monoglyceride process. The reaction was conducted in a 500 ml four-neck round bottom flask equipped with an inert gas inlet, thermometer, reflux condenser, and a mechanical agitator. The transesterification step involved soybean oil (200 g) and an excess of glycerol (44.75 g, 0.48 mol). These two materials were purged with argon gas for ~15 min. The mixture was then heated to 120 C and lithium hydroxide catalyst was introduced into the reaction mixture ( wt% of polyol). The temperature was gradually increased to 240 o C. After approximately 1 h, a small aliquot was removed and mixed in three parts 95 % ethanol. This was repeated until the resulting solution was clear. The reaction mixture from step 1 was cooled to around 100 C and a Dean-Stark trap was introduced to the reaction set up. The reaction mixture was then charged with phthalic anhydride (PA) (71.65 g, 0.48 mol) and 100 ml of xylene for use as a reflux solvent. The mixture was then slowly heated to 220 C. After every hour of reaction, a sample was removed and the acid number was determined. The reaction was stopped once an acid number of below 10 was achieved. Acid number determination was based on ASTM D with 1 M KOH and phenolphthalein indicator. The product was then cooled to room temperature and stored under argon atmosphere. Figure 4-1 shows the reaction path for a soybean oil based alkyd synthesized by the monoglyceride process. 75

94 Figure 5-1: Reaction of a soya-based alkyd synthesized by the monoglyceride process 5.5 Coating Formulation and Film preparation Each diluent was formulated with the alkyd resin at three different levels (10 wt%, 20 wt% and 30 wt% based on total formulation), 2 wt% metal drier package (10 76

95 wt% Cobalt Hydro- Cure II, 80 wt% Zirconium Hydro-Cem, 10 wt% Calcium Hydro- Cem) and 1 wt% wetting agent. Formulations of each mixture can be found in Table 5-1. The contents were introduced into a sealed vial and mixed on a roller mill for 2 h. A draw down bar was used to cast films on clean glass panels (6 mils wet film) and on aluminum panels (3 mils wet film). The wet films were cured in the oven at 120 o C for 2 h, followed by a second cure at 160 o C for 3 h. The films were kept at room temperature for 7 days before any tests were performed to ensure a through cure was achieved. Table 5-1: Formulations of different coatings systems used for testing Samples Soybean LOA Diluent Drier (2%) Wetting (1%) (g) (g) (g) (g) Tung Si Tung Si Tung Si Tung Si Tung F Tung F Tung F Tung F Tung AE Tung AE Tung AE Tung AE Total Weight 10 gram 5.6 Instruments and Characterization The viscoelastic properties of the films were investigated using a Perkin-Elmer Rheometric Scientific dynamic mechanical thermal analyzer (DMTA) at a frequency of 1 Hz and a heating rate of 4 C/min over a range of -50 to 250 C. The gap distance was set at 3 mm for rectangular specimens (10 mm wide, 20 mm long, and 0.10 mm thick). The maximum of the tan delta was used to determine the glass transition temperature, while the crosslink density was determined by utilizing the lowest E point value at least 50 o C 77

96 beyond the point at which the T g was found. The tensile properties of the films were evaluated using an Instron Universal Tester. The dimensions of the films for tensile testing were 0.05 mm in thickness, mm wide, with an initial length of 10 mm. A crosshead speed of 10 mm/min with a load cell of 100 N was applied to determine elongation-at-break, tensile modulus, and tensile strength of each system. For each film, between five and eight samples were tested. The data obtained are reported as the mean of the data set, error, and standard deviation. 5.7 Result and discussion The objective of this study was to incorporate the acrylate modified tung oil diluents into an alkyd system and determine the effects on overall film performance. In order to achieve this, a soya-based long-oil alkyd resin was synthesized and formulated with different levels of the diluents. Tensile properties along with viscoelastic properties were evaluated. The main advantage of these new systems was the reduction/elimination of organic solvents in the formulation. The low molecular weight of the modified tung oil diluents functioned as an organic solvent in the formulation, while also enhanced the physical properties of the films by adding additional film networking sites to the system. It is well known that tung oil based materials cure faster than most other oil-based (soybean, linseed, and sunflower) materials due to their high level of conjugated double bonds present in the fatty acid chains. Therefore, physical and viscoelastic properties can be influenced by incorporation of the modified tung oil diluents. The extent of the effect was studied by adjusting the load and type of diluents. Tensile modulus, tensile strength, and elongation-at-break were determined along with the viscoelastic properties including: modulus, crosslink density, and glass transition temperature. 78

97 5.7.1 Tensile Properties Tensile strength with increasing reactive diluent content for each of the different systems is shown in Figure 5-2. From the plot, general trends were observed for all films that contained both the siloxane modified diluent and the allyl ether modified diluent independent of functional group modification. Each of these diluents produced an increase in the tensile strength to the alkyd film when added to the formulation. In addition, it appeared that with each of these diluents there was a maximum increase in tensile strength at a certain loading level that was reached. Nonetheless, as the diluent amounts continued to increase, tensile strength was still greater than that of the neat alkyd films. As expected, the fluorine modified tung oil containing films showed no effect on the tensile strength of the films at any of the loading levels tested. The tensile strength maxima value can be explained by the competing curing mechanisms of the tung-oil based diluent and the soya-oil based alkyd resin. The tung oil reactive diluents have many reactive sites per chain and are smaller in molecular weight, while the soya-based alkyd resin has fewer crosslinking sites per molecule but less chain diffusivity. The autoxidative crosslinking can begin with the tung oil derived diluents, due to their higher mobility in comparison with the alkyd resins. However, a point in the curing mechanism where the tung oil reactive diluents build molecular weight similar to that of the alkyd can be reached. At this point, the curing rate is now then model that of a traditional soya-based alkyd resin more closely than that of tung oil due to the similarity in the diffusion rates of the two different materials. In addition, the reactive diluents are incorporated into the alkyd network as reactive diluents have similar reactive sites and curing mechanisms. Because of this phenomenon that is described, the more tung oil 79

98 based material that was used in the formulation, the faster the equilibrium were reached. Hence, a less pronounced effect on the curing mechanism could be seen, resulting in similar properties to that of the neat alkyd Silicone Fluorine Allyl Ether Tensile Strength (MPa) % 10% 20% 30% Weight % of Reactive Diluent in Formulation Figure 5-2: Tensile Strength (MPa) with increased loading of siloxane modified tung oil (Tung-Si), fluorine modified tung oil (Tung-F), and allyl ether modified tung oil (Tung-AE). When compared the results from the different reactive diluents, the siloxane modified tung oil enhanced the tensile strength the most, while the fluorinated diluent showed no changes. The allyl ether modified tung oil fell somewhere in between the two. These results were consistent based on previous work done by Soucek et al. on inorganic/organic hybrid materials that contained alkoxysilane, as alkoxysilane 80

99 functionality increased the strength of the coatings.[39] The reason for the higher tensile strength in the films that contain the siloxane modified diluents versus the allyl ether modified films was attributed to the additional crosslinking mechanism that was introduced by the alkoxysilane groups. As mentioned earlier, there was in situ polycondensation of the metal or silicon alkoxide with the organic polymer matrix. Consequently, increase of the crosslink density of the films that contain these diluents was observed.[67-69] The fluorine containing diluents, did not exhibit the same properties as the other two materials, due to the tendency of fluorine groups to migrate to the air/surface interface. When this occurred, fewer additional crosslinking sites were available during the curing mechanism. The elongation-at-break of the different systems was also measured. The values for each system are shown in Figure 5-3. Based on the results, there was slightly decreasing of the elongation-at-break by adding the diluents. Since alkyd films were so readily crosslinked and form strong networks, the elongation-at-break of these films was fairly low. Even though some flexibility is inherent in the fatty acid groups, increasing the crosslink density of the films did not show a dramatic effect on this specific property. 81

100 Silicone Fluorine Allyl Ether Elongation-at Break % 10% 20% 30% Weight % of Reactive Diluent in Formulation Figure 5-3: Elongation-at-break as a function of increased loading of siloxane modified tung oil (Tung-Si), fluorine modified tung oil (Tung-F), and allyl ether modified tung oil (Tung-AE) Silicone Fluorine Allyl Ether 45.0 Tensile Modulus (MPa) % 10% 20% 30% Weight % of Modified Tung Oil Diluents in Formulation Figure 5-4: Tensile Modulus (MPa) as a function of increased loading of siloxane modified tung oil (Tung-Si), fluorine modified tung oil (Tung-F), and allyl ether modified tung oil (Tung-AE). 82

101 Another important tensile property of coatings systems is the tensile modulus. The tensile moduli are shown as a function of increasing diluent amount for each system in Figure 5-4. Once again, the allyl ether and siloxane modified systems showed the most difference from that of the alkyd. This was expected since the tensile modulus is dependent crosslink density if the other variables were held constant. Based on earlier discussions, tensile modulus observed here was the result of the additional crosslinking mechanism that was afforded by modified tung oil diluents Viscoelastic Properties The viscoelastic properties of all systems are shown in Figures 5-5 and 5-6. All samples, regardless of functional group or loading level, exhibited the same overall trend with minor differences in temperatures and modulus (E ) values. In all the samples, the modulus displayed a small decreasing trend until the temperature reached about 45 C. The moduli of all samples then took a significant drop between 45 C and 100 C. This region where the modulus continued to fall was known as the α or rubbery plateau region. From this plot, the crosslink density and the glass transition temperature were found. The expression used to find the crosslink density is given in Equation 5-1, where ν e is the number of moles of elastically effective chains per cubic centimeter of the film, E min is the minimum storage modulus in the rubbery plateau in N/m 2, R is the gas constant (8.314 N*m/g mol * K), and T is the absolute temperature in Kelvin. E' min = 3ν ert (5-1) 83

102 The glass transition temperature (T g ) was determined by reading the temperature at which the E value was a maximum in the rubbery plateau region. The values for E min, ν e, and T g can be found for all samples in Table 5-2. Table 5-2: Viscoelastic properties of the alkyd and alkyd/diluent cured films. E' (min) ν e T g N/m 2 (mol/cm 3 ) C Alkyd 1.86E Tung Si E Tung Si E Tung Si E Tung F E Tung F E Tung F E Tung AE E Tung AE E Tung AE E E+09 Alkyd (Control) Tung Si-10 Tung Si-20 Tung Si E+08 E' (N/m 2 ) 1.00E E Temperature (C) (a) 84

103 1.00E+09 Alkyd (Control) Tung F-10 Tung F-20 Tung F E+08 E' (N/m 2 ) 1.00E E Temperature (C) (b) 1.00E+09 Alkyd (Control) Tung AE-10 Tung AE-20 Tung AE E+08 E' (N/m 2 ) 1.00E E Temperature (C) (c) Figure 5-5: Modulus (E ) as a function of temperature of the alkyd and a) siloxane modified tung oil (Tung Si), b) fluorine modified tung oil (Tung F), and c) allyl ether modified tung oil (Tung AE).alkyd/diluent cured films. 85

104 1.40E E E+00 Alkyd (Control) Tung Si-10 Tung Si-20 Tung Si E-01 Tan δ 6.00E E E E Temperature (C) (a) 1.40E E E+00 Alkyd (Control) Tung F-10 Tung F-20 Tung F-30 Tan δ 8.00E E E E E Temperature (C) (b) 86

105 1.40E E E+00 Alkyd (Control) Tung AE-10 Tung AE-20 Tung AE E-01 Tan δ 6.00E E E E Temperature (C) (c) Figure 5-6: Tan δ as a function of temperature of the alkyd and a) siloxane modified tung oil (Tung-Si), b) fluorine modified tung oil (Tung-F), and c) allyl ether modified tung oil (Tung-AE) alkyd/diluent cured films. As expected, the crosslink density of the film increased with addition of all three diluents, except for the mixture that contains 30 wt% FTO. The glass transition temperatures of all cured films that contain the diluents were all significantly higher except for the formulation that contains 30 wt% FTO. There appeared to be a maximum level of incorporation of the FTO that was crossed, above which the effects on film properties were not beneficial to the system. The increase in crosslink density and glass transition temperature was due to the incorporation of the additional crosslinking site in the tung oil. 87

106 5.8 Conclusion Incorporation of the diluents into alkyd- based coatings formulations has shown that the new acrylate modified tung oil derivatives performed effectively as reactive diluents. In addition, the new materials included additional crosslinking sites for the overall system. Consequently, higher crosslink density, and increased cured film glass transition temperature with the addition of diluent were observed. The tensile strength and tensile modulus were improved in films where the alkoxysilane and allyl ether modified materials were incorporated compared to the values of the alkyd system. However, addition of diluents showed slightly changed in the elongation-at-break over the neat alkyd in any of the systems. 88

107 CHAPTER VI LITERATURE REVIEW: THIOL-ENE PHOTOPOLYMERIZATION 6.1 Historical Thiol-ene chemistry has a long history founded by Posner in 1905.[83] Posner reported the facile addition of thiols to olefins including styrene, cyclopentadiene, and several other aromatic olefins. While most of the additions were acid catalyzed, it was noted that the addition of thiophenol to styrene or 1-phenylbutadiene needed no catalysis. The reaction can be initiated by acid catalysis, by base catalysis and also by free radicals. Acid catalysis of the reaction with monosubstituted olefins gives the expected Markovnikov addition products[84], while free-radical initiated additions give the anti- Markovnikov product. Basic catalysis is more complicated and can give two products, depending on the structure of the olefin and the presence and absence of excess oxygen. A good Michael acceptor olefin and strong base will produce predominated 1,4- conjugated addition products.[85] On the other hand, if the olefin is not sufficiently activated to this type of addition (e.g., allylics, vinyl ethers, etc.), the incipient thiolate ion can be air-oxidized to a thiyl radical giving rise to the formation of the typical freeradical addition products.[86] Kharasch and his co-workers played a major role in elucidating the reaction mechanism. They found that molecular oxygen could initiate the addition process by the 89

108 oxidation of thiol to a thiyl radical. In this manner, oxygen was incorporated into the reaction products.[87] Von Braun discovered that certain unsaturated thiols such as propene-3-thiol and butene-4-thiol undergo an apparently spontaneous reaction that gives a non-volatile product. Consequently, thiol-ene reactions can possibly be utilized to prepare polymers. In actual practice, the use of this type of difunctional monomer to prepare a polymer would be expected to be somewhat limited due to the difficulty in inhibiting the spontaneous self-reaction. Coffman examined the reaction of diolefins such as butadiene, cyclopentadiene, and 3-vinyl-cyclohexene with dithiols such as ethane dithiol, decanedithiol and m- benzenedithiol, the details of which were described in a patent.[88] Coffman found that liquid polymeric sulfide could be obtained by either heating a mixture of diolefin and dithiol or by subjecting the mixtures to the light of a mercury lamp. Catalysis studies of diolefins and dithiols for preparing high-molecular-weight polymers have found that the reaction is readily initiated either by a peroxide catalyst or by UV light. A number of novel dithiols and diene components have been prepared and characterized. Polymerization of these materials gives a wide range of linear polyalkylene sulfide polymers with various molecular weights. It is apparent that linear systems are much more amenable to solution-based chemical analytical methods such as nuclear magnetic resonance (NMR) and IR spectroscopy techniques, and the kinetics of these soluble systems is much more straightforward. A great study of thiol-ene photopolymerization by Morgan, Ketley and their coworkers in the 1970s brought thiol-ene chemistry into popularity. Grace and Armstrong introduced the first large-scale uses of radiation curing in the United States with the 90

109 successes of thiol-ene photopolymerization. For various reasons, thiol-ene photocuring gave way to acrylate-based photocuring systems. The main reasons at the time were the odor issue of thiol-ene systems as well as the incorrect perception of rapid yellowing upon weathering. As the new low-cost acrylate multifunctional monomers and oligomers became available at prices that allowed for extensive application, the exploitation of thiol-ene photopolymerization was severely restricted. There have been several efforts to restart the use of thiol-ene photopolymerization in industrial applications since the late 1970s in several parts of the world, including Klemm and coworkers[89-97] in Germany, Jacobine, Woods and coworkers[24, ] in the United States, and Toh and coworkers[ ] in Australia. Recent efforts by Hoyle, Jonsson and coworkers, and Bowman, Cramer, and coworkers, encouraged by National Science Foundation funding from the University of Southern Mississippi and the University of Colorado, have allowed thiol-ene photopolymerization to be resurrected for new chemistry and applications. 6.2 Polymerization Mechanism of Thiol-ene Systems Thiol-ene photopolymerization systems consist of reactive compositions based on the stoichiometric reaction of multifunctional olefins (alkenes) and thiols (mercaptans), which polymerize on exposure to UV-light or electron beam (EB) radiation.[114] The polymerization can also be initiated by peroxides and with thermal initiators such as azoisobutyronitrile (AIBN) or benzopinacol. Thiol-ene polymerization proceeds by a step growth addition mechanism that is propagated by a free-radical chain-transfer process.[115] 91

110 Studies of the polymerization mechanism of thiol-ene systems have been relatively rare until recently because most of the prevalent thiol-ene systems are generally highly cross-linked, and direct analysis is fairly difficult. More recently, interest in the thiol-ene reaction has focused on systems that can be reacted in solution and can be directly analyzed. These studies tend to concentrate on either non-polymeric model systems such as monothiols reacting with monoenes or linear polymers derived from various dithiols and diolefins or diacetylenes.[116, 117] Thiol-ene polymerizations are initiated by the reaction of a thiol with either an initiator fragment of a Norrish type I initiator or an excited state of a Norrish type II initiator to yield a reactive initiating species, the thiyl radical. At this point, first proposed by Kharasch,[87] several different exothermic reactions occur in a sequence. The overall rate of polymerization is related to the cumulative rates of each of the individual steps. The initiation reaction is followed by the addition of thiyl radical to olefin, which gives a β-thiolether carbon radical. Reactivity of the thiyl radical (an electrophile) and the olefin (the nucleophile) and the relative energetics of each component will determine the kinetics of this step and the thermodynamic reversibility of this addition. This β- thiolether carbon radical may then generate a new initiating thiyl radical by abstraction (chain transfer) from thiol or may undergo oxygen addition and thiol abstraction. In the absence of oxygen, chain transfer by thiol to the carbon radical will regenerate thiyl and yield the simple addition product. Reaction of the carbon radical with oxygen followed by proton abstraction from thiol regenerates the thiyl radical and gives overall cooxidation. The overall thiol-ene polymerization reaction is represented in Figure

111 Figure 6.1: Thiol-ene photopolymerization reaction schemes Chain transfer to oxygen followed by chain transfer to thiol is the most wellknown feature of thiol-ene systems and represents a dramatic point of difference from acrylate polymerizations, which is that the thiol-ene reaction is not inhibited by dissolved or ambient oxygen. The reaction is not inhibited because thiol is an extremely effective free-radical chain-transfer agent and oxygen incorporated onto the oligomer chain as an incipient peroxy radical rapidly undergoes a chain-transfer reaction with thiol to give an alkylhydroperoxide and regenerates the propagating species, the thiyl radical. Termination in thiol-ene polymerizations has been generally represented by radical-radical combination reactions of the β-carbon radicals or thiyl radicals.[118] More recently, the latter reaction has been reported to be quite slow.[89] Termination processes in thiol-ene polymerization are still obscure. As in all step growth processes, the molecular weight of the reaction mixture of starting materials and growing oligomeric 93

112 species increase in an orderly and predictable way. Near the gel point, the molecular weight increases rapidly until an infinite value is reached.[119] Alkene Structure Morgan et al.[118] conducted an extensive investigation of the effect of thiol and alkene structures on the overall rate of the thiol-ene addition process. The most fascinating feature of thiol-ene photopolymerization is that almost any type of alkene can participate in the thiol-ene reaction. Original observations by Morgan et al.[118] elaborated the relationship between the chemical structures of the alkene and thiol and the reactivity. Based on results from Hoyle et al., [120] the order of the alkene reactivity can be seen to be the following: Norbornene > Vinyl ether > Propenyl > Alkene Vinyl ester > N-Vinyl amides > Allyl ether Allyl triazine > Allyl isocyanurate > Acrylate > Unsaturated ester > N-substituted maleimide > Acrylonitrile > Methacrylate > Styrene > Conjugated dienes. The ordering may change slightly with the thiol structure. Except for the first entry and the last three entries, the alkene reactivity decreases most with the decrease of electron density of the carbon-carbon double bond. Norbornene, methacrylate, styrene, and conjugated dienes are special cases. The rates of free-radical addition of thiols to norbornene are exceptionally rapid due to a combination of the significant relief of ring strain resulting from the addition of the thiyl radical across the double bond and the subsequent rapid hydrogen abstraction rate of a thiol hydrogen by the carbon-centered radical. The carbon-centered radicals that form when the thiyl radical is added to a methacrylate, styrene, or conjugated diene carbon-carbon bond are very stable and produce radicals (methacrylic, benzylic, or allylic) that have inherently low hydrogen-abstraction rate constants. By the way, in contrast to the conjugated dienes, 94

113 non-conjugated dienes undergo thiol-ene reactions quite rapidly according to the exact position of the alkenes. Cramer et al.[121] determined the rate determining step from the two propagation steps in Figure 6.1. The order of the reactions with respect to both thiol- and alkenefunctional groups were determined (see eq 6-1). Table 6-1 shows the respective values for components n and m, which relate the polymerization rate (R polymerization ) to the thiol and ene concentrations, respectively, and the ratios of the rate constant for propagation (k p ) to the rate constant for chain transfer (k ct ) for four alkenes that were evaluated.[121] The results for k p /k ct for the addition of 1-butanethiol to styrene and pentene are also given in Table 5-1.[122] The chemical structure and acronyms for the alkene in Table 5-1 are shown in Figure 6-2. R polymerization α [RSH] n [C=C] m (6-1) The respective k p and k ct values for norbornene and vinyl ether are approximately equal. It is indicative that neither the propagation step nor the chain-transfer step is ratedetermining. The k p values for the acrylate, allyl ether and pentene were reported to be at least an order of magnitude greater than k ct. This indicates that the chain-transfer hydrogen abstraction process is the slow step for thiol-acrylate, thiol/allyl ether, and thiol-alkene copolymerization. 95

114 Figure 6-2: Structures of alkenes used in rate-constant analysis.[121, 122] Table 6-1: Kinetic-Rate-Constant Ratio[121, 122] Alkene k p /k ct n m Acrylate (HDDA) Norbornene (N1) Vinyl ether (TEGDVE) Allyl ether (AE) Styrene 800, Pentene The reactivity of the alkene in the thiol-ene reaction is dependent on the extent of substitution of the alkene.[ ] Studies have shown that highly substituted alkenes are less reactive than singly substituted alkenes. Also, to carry out a rapid thiol-ene reaction, the multifunctional alkene must have the ene groups located at terminal positions. Johansson et al.[126, 127] suggested that the addition of the thiyl radical to cis- 96

115 ene bonds is reversible with efficient isomerization, and this leads to the less reactive trans structure. The trans-ene structure has been reported to react with the thiyl radical in an efficient addition step, although the reaction is reversible and still not as fast as that with terminal alkenes. The reversibility of thiyl radical addition to internal alkenes has been reported previously.[ ] Gelation of Thiol-ene Photopolymerization The crosslink network of multifunctional alkenes and thiols can be formed with mechanical properties dependent on the chemical structure of the parent thiols and alkenes, their functionality, and the extent of conversion of each functional group. In comparison with traditional acrylate systems, thiol-ene photopolymerization exhibits a marked behavior in its delayed onset of gel formation. Until the gel point, the medium is low-viscosity and consists primarily of low-molecular-weight species. The gel point for the curing of thiol-ene by light, through which a crosslinked network is formed and the system diffusion rates are reduced dramatically can be obtained as follows:[102] α = [1/r(f thiol -1)(f ene -1)] 1/2 (6-2) where r is the thiol-ene molar ratio based on functional groups, f thiol is the thiol functionality, and f ene is the alkene functionality. The gel point can be controlled by variation of the functionality of thiol and alkene. For example, gelation take place at about 33% conversion when both thiol and alkene groups are tetrafunctional. Rheological experiments by Chiou and coworkers[ ] on thiol-ene photopolymerization showed that the elastic modulus, which can be used to represent the buildup of a network during the photopolymerization of trimethylolpropane tris(3-mercaptopropionate) and trifunctional allyl ether, began to increase at about 65-66% conversion of thiol-functional 97

116 groups (at slightly lower conversions than the gel point, which was found to occur at ca. 71% thiol functional conversion). This result is different from other typical free-radical polymerizations of multifunctional monomers such as acrylates. The thiol-ene photopolymerization networks are formed at a much higher conversion than multifunctional acrylate monomer polymerization. A direct benefit of shrinkage occurring in the liquid phase is that the final crosslinked films ultimately have a higher conversion of functional groups, and there is less stress buildup in the network that is eventually formed than for crosslinked network formed by traditional acrylate monomers. 6.3 Application of Thiol-ene Photopolymerization Thiol-ene photopolymer has various applications in almost all of the areas that radiation-curable material can be employed to advantage. In many of these specialized applications thiol-ene compositions offer unique advantages, and there are well over 150 patents in the US alone that describe the insensitivity to oxygen inhibition of the curing process. Thiol-ene compositions are quite sensitive to lower UV-light intensities. The application areas for thiol-ene compositions would include UV-curable adhesives gasgeting and sealants, UV-curable coatings such as conformal coatings for the protection of printed circuit boards and electronic components, solder resists, flameretardant coatings, conductive inks, wire coatings and protective coatings for optical fibers. Thiol-ene photopolymers also find wide application in the production of photopolymer printing plates and other imaging applications Adhesive The use of thiol-ene photopolymers as UV-curable adhesives is well-established. Woods[134] wrote a comprehensive review of the use of thiol-ene chemistry in adhesives 98

117 before General thiols can be used as chain-transfer agents to limit and control the molecular chain growth of monomers used in formulating pressure-sensitive and laminating adhesives by reducing the molecular weight of free-radical polymerizing monomers and produce tacky polymers that can aid adhesion. The thiol-ene adhesives are especially important in optical assembly bonding, where the combination of the lack of oxygen inhibition, optical properties (refractive index), and the ability of these materials to be partially cured allowing realignment of the assembly before cure offer a distinct advantage. The adhesives find use in binding lenses, prisms and other compound optics as well as terminating or splicing optical fibers. Shrinkage on curing and residual stress in the bondline is reported to be quite low. Resistance to moisture ingress and thermal degradation is reported to be superior. Klemm and Sensfuss[90] described optical adhesives made from the copolymerization of divinyl ethers and diallyl ethers with multifunctional thiols. Also, Woods, Jacobine and coworkers[104, 106, 109, 134] demonstrated the use of mixtures of multifunctional norbornene and multifunctional thiols as laminating adhesives Conformal Coatings for Printing Circuit Boards Fabricated printed circuit boards containing mounted electrical components often require protection from the service environment in which they operate. This can be achieved by the use of a UV-curable conformal coating which coats the printed circuit board and encapsulates the electronic component. Compositions based on thiol-ene have also been described for the conformal coating of flexible circuitry. A high degree of flexibility is required in this application as the circuit board is usually rolled up on itself. The excellent adhesion of these compositions to the plastic circuit is obviously a key 99

118 parameter for success in this application. The adhesion of these compositions to the copper and lead exposed areas was reported as excellent; the solvent resistance of the coating is high. A significant economic advantage for the screen printed thiol-ene composition compared to polyimide or polyester laminate was noted.[135] Imaging Application The production of the photopolymer printing plates is one of the more significant applications for thiol-ene photopolymers. These compositions are generally based on a wide variety of urethane pre-polymers derived from diisocyanates, such as toluene diisocyanates, and polyols, such as polyethylene glycol and polypropylene glycol. Ene functionality is introduced by reaction of the urethane pre-polymer with hydroxyl functional acrylic materials such as hydroxypropyl acrylate. Standard thiols such as trimethylolpropane tris-mercaptopropionate are used at 5 to 10% weight levels, and the formulation is usually applied to a support layer as a thick film Pigmented Coatings Photopolymerization of pigmented systems require high level of photoinitiator due to the light blocking effect of the dispersed pigments. Thin coatings are usually practical for this system. The addition of multifunctional thiols to typical acrylate-based pigmented coatings can increase the polymerization rate dramatically.[136, 137] Roper et al.[137] reported the effect of 30% mol trithiol on the conversion rate of a typical acrylate, tripropyleneglycoldiacrylate (TPGDA) containing 5 wt% calcium lithol rubine (a typical pigment) in air; the homopolymerization of pigmented TPGDA in air showed little or no conversion because of oxygen inhibition whereas, once 30 mol% of trithiol was introduced, the polymerization rate increased significantly. 100

119 6.4 Experimental Design Research in engineering, science, and industry is empirical and makes extensive use of experimentation. Statistical methods can greatly increase the efficiency of these experiments and often strengthen the conclusions so obtained. The proper use of statistical techniques in experimentation requires that the experimenter keep the following point in mind. Statistical design of experiments refers to the process of planning the experiment so that the appropriate data can be analyzed by statistical methods, resulting in valid and objective conclusions. The statistical approach of experimental design is necessary if we wish to draw meaningful conclusions from the data. When the problem involves data that are subject to experimental errors, statistical methods are the only objective approach to analysis. Thus, there are two aspects to any experimental problem: the design of the experiment and the statistical analysis of the data. Three basic principles of experimental design are randomization, replication, and blocking. Randomizing means both the allocation of the experimental material and the order in which the individual runs or trials of the experiment are to be performed are randomly determined. Replication means an independent repeat of each factor combination. Replication has two important properties. First it allows the experimenter to obtain an estimate of the experimental error. Second, if the sample mean is used to estimate the true mean response for one of the factor levels in the experiment, replication permits the experimenter to obtain a more precise estimate of this parameter. Blocking is the design technique used to improve the precision with which comparisons among the factors of interest are made. Frequently, blocking is used to reduce or eliminate the 101

120 variability transmitted from nuisance factors (factors that may influence the experimental response but in which we are not directly interested). In general, the scientist is concerned with a product, process, or system involving a response y that depends on the controllable input variable ζ 1, ζ 2,,ζ k. The relationship is y = f (ζ 1, ζ 2,,ζ k ) + ε (6-3) where the form of the true response function f is unknown and perhaps very complicated, and ε is a term that represents other sources of variability not accounted for in f. Assuming a normal distribution with mean zero and variance σ 2, if the mean of ε is zero, then E(y) η = E[ f (ζ 1, ζ 2,,ζ k )] + E(ε) = f (ζ 1, ζ 2,,ζ k ) (6-4) It is more convenient to transform the natural variable into coded variables x 1,x 2,,x k, which are usually defined to be dimensionless with mean zero and the same spread or standard deviation. In term of the coded variables, the true response function is now written as η = f (x 1,x 2,,x k ) (6-5) Because the form of the true response function f is unknown, an approximation must be made. Usually, a low-order polynomial in some relatively small region of the independent variable space is appropriate. A first-order model is suitable when the experimenter is interested in approximating the true response surface over a relatively small region of the independent variable space in a location where there is little curvature in f. If there is an interaction between these variables, an interaction term can be added. 102

121 The interaction term will introduce curvature into the response function. A second-order model will be required in the case that the curvature in the true response surface is strong enough that the first-order model is inadequate. For the case of two variables, the secondorder model is η = β 0 + β 1 x 1 + β 2 x 2 + β 11 x β 22 x β 12 x 1 x 2 (6-6) The second-order model is widely used in response surface methodology since the second-order model is very flexible, makes it easy to estimate the parameters, and is quite practical. Response surface methodology (RSM) and linear regression analysis have a close connection. The β s are a set of unknown parameters. To estimate the values of the parameters, the data must be collected from the system. Regression analysis is a branch of statistical model building that uses these data to estimate the β s. In the mixture design of an experiment, the factors are the ingredients or components of a mixture, and the response is a function of the proportions of the ingredients. The amount of each ingredient is typically measured by weight, volume, mole ratio and so forth. In contrast to other designs, the levels chosen for any factor in mixture design are dependent on the other factors. Applications of mixture experiments are found in many areas. A common application is product formulation, in which a product is formed by mixing several ingredients together, such as the formulation of soaps, cake mixes, and beverages. When there are three components of the mixture, the constrained experimental region can be conveniently represented on trilinear coordinate paper as shown in Figure 6-3. Mixture design has been used in many subject fields and applications, including chemicals, geology, petroleum, foods, and tobaccos.[ ] Simplex centroid design has also been used to study the effect of mixture components on 103

122 coating formulation.[142, 143] Data obtained from such experiments can be analyzed by computer software using analysis of variance (ANOVA); an optimization also can be accomplished for individual systems. Figure 6.3: Trilinear coordinate system 104

123 CHAPTER VII SYNTHESIS AND CHARACTERIZATION OF THIOLS 7.1 Abstract Two groups of thiol were synthesized. Firstly, trans-1,4-bis(mercaptomethyl) cyclohexane (CHDMT) and 1,4-bis(mercaptomethyl)benzene (BDMT) were prepared. The CHDMT was synthesized via a two step process using potassium thioacetate and hydrochloric acid as reagents. The BDMT was synthesized by a one step process using 1,4-benzenedimethanebromine with thiourea and potassium hydroxide as reagents. Secondly, a simple esterification of 3-mercaptopropionic acid with various alcohols was employed as a general synthetic approach for preparing three mercaptopropionate thiols. A strong acid, p-toluenesulfonic acid (p-tsa) was used to catalyze the esterification reaction resulting in high yield of the thiols: 1) 1,6-hexane bis(3-mercaptopropionate) (HD-SH), 2) trans-1,4-cyclohexanedimethyl bis(3-mercaptopropionate) (CHDM-SH) and 3) 4,4 -isopropylidenedicyclohexane bis(3-mercaptopropionate) (HBPA-SH). The thiols were characterized by 1 H NMR, 13 C NMR, Fourier transform infrared spectroscopy (FT- IR), and elemental analysis (C, H, S, O). 7.2 Introduction Photopolymerization has been developed for many applications[1, 53] such as optical lenses, dental materials and coatings materials for decades due to the 105

124 characteristic of energy efficiency, environmental compatibility and rapid reaction at ambient temperature. Photopolymerization of reactive oligomers and diluents to form solid films has been used to meet strict governmental regulation against solvent emissions for coating applications.[1, 53] Photopolymerizable films are initiated either via a radical or a cationic crosslinking mechanism. Most UV-curing systems used are based on acrylate systems, since acrylate double bonds are more reactive than vinyl or allyl groups; and radical initiation is more prevalent than cationic.[144] However, acrylate-based systems have a distinctive disadvantage of oxygen sensitivity and yellowing/degradation of the photoinitiator added in the formulation.[53] One important advantage of thiol-ene photopolymerization is that this type of polymerization can be proceed in the presence of oxygen.[123] Conventional free radical polymerization in the presence of oxygen leads to a formation of peroxide which slows the curing mechanism down to a termination of the reactive free radical. Conversely, thiol-ene photopolymerization incorporates oxygen into the growing chain of polymer as peroxy radical. Peroxy radical can proceed chain transfer with a thiol to produce a new thiyl radical which can continue the polymerization reaction. On the other hand, thiol-ene polymerization reaction can quench the oxygen that presence in the reaction which contrast to acrylate based system which significantly inhibited by the presence of oxygen. Nevertheless, one of the current disadvantages of thiol-ene material is that hardness, high glass transition temperature and toughness is not easily obtainable. These characteristics are the result from the unique thiol-ene cross-linked network structure and the flexible thioether linkages. 106

125 One of the major challenges of developing thiol-ene photopolymerizable material is to overcome the low glass transition temperature of most thiol-ene materials.[145] Recently, thiol-acrylate photopolymerization has been reported to be tunable via the acrylate concentration. Narrow mechanical/thermal transition regions and the peak maxima in the tan delta can be controlled by the concentration of the acrylate in thiolacrylate systems.[146] However, the restrictions in the available of traditional reactive and rigid alkene/acrylate limit the potential for selectively altering physical, mechanical, and optical properties. A solution to this problem can be obtained by developing new thiols which can compliment the properties derived from the reactive alkene. Unfortunately, synthesis of new thiols for specifically thiol-ene photopolymerization has not been thus far reported. In this study, two difunctional thiols, trans-1,4-bis(mercaptomethyl)cyclohexane (CHDMT) and 1,4-bis(mercaptomethyl)benzene (BDMT) were prepared. Three difunctional 3-mercaptopropionate thiols, 1,6-hexane bis(3-mercaptopropionate) (HD- SH), trans-1,4-cyclohexanedimethyl bis(3-mercaptopropionate) (CHDM-SH), and 4,4 - isopropylidenedicyclohexane bis(3-mercaptopropionate) (HBPA-SH) were prepared via an esterification reaction. The thiols were characterized using 1 H NMR, 13 C NMR, FT-IR and elemental analysis (C, H, O, S). 7.3 Materials Trans-1,4-bis(hydroxymethyl)cyclohexane (CHDM) was purchased from Acros Organics. Triethylamine (TEA), dimethylaminopyridine (DMAP), para-toluenesulfonyl chloride (TSCl), potassium thioacetate (KSAc) 1,4-bis(bromomethyl)benzene, thiourea, 4,4 -Isopropylidenedicyclohexanol mixture of isomers (hydrogenated Bisphenol-A) 107

126 (HBPA), 1,6-hexanediol (HD), trans-1,4-bis(hydroxymethyl)cyclohexane (CHDM) 3- mercaptopropionic acid, p-toluenesulfonic acid (p-tsa), toluene (reagent grade), diethyl ether anhydrous, and magnesium sulfate was purchased from Aldrich Chemical company. All the chemicals were used as received. SH SH HS HS a b HS O O O O SH c HS O O O O SH d O SH O O SH O e Figure 7-1: Chemical structure of the thiols: (a) trans-1,4,bis(mercaptomethyl) cyclohexane (CHDMT), (b) 1,4-bis(mercaptomethyl)benzene (BDMT), (c) 1,6-hexane bis(3-mercaptopropionate) (HD-SH), (d) trans-1,4-cyclohexane 108

127 dimethyl bis(3-mercaptopropionate) (CHDM-SH), (e) 4,4 - isopropylidenedicyclohexane bis(3-mercaptopropionate) (HBPA-SH) 7.4 Instrument and Characterizations For Characterization, 1 H NMR and 13 C NMR spectra were recorded on a Mercury-300 MHz spectrometer (Varian) in CDCl 3 as solvent at 20 o C. Chemical shifts are given relative to a TMS internal standard. Fourier transform infrared spectroscopy (FT-IR) was performed on an ATI Mattson Genesis FT-IR spectrometer by the casting of thin liquid samples on KBr plates. Elemental analysis was performed via combustion at 990 deg C with the elemental analyzer for carbon and hydrogen. Sulfur is done via flask combustion and subsequent titration. All the elemental analysis was performed by Midwest Microlab LLC. 7.5 Synthesis of thiols 1,4-benzenedimethanethiol (BDMT) 1,4-benzenedimethanebromine (5 g, mol), (1), thiourea (8.65 g, mol) were dissolved in ethanol (150 ml). The solution was heated to the reflux temperature of ethanol and kept for 12 h. The solution was cooled to room temperature. Solution of 20 wt% KOH (50 ml) was added to solution and reflux for another 4 h. The result solution was extracted with diethyl ether (3 x 100 ml). The aqueous layer was acidified with hydrochloric acid and extracted again with diethyl ether (100 ml). Organic layer was dried by Na 2 SO 4 (20 g) and Solvent was removed by rotary evaporator. Crude product was further purified by recrystallization from hexane in the freezer at -20 o C to afford pure 1,4-benzenedimethanethiol (BDMT) (3) as white solid. The combined yield was 85%. 1 H NMR (300 MHz, CDCl 3 ) δ(ppm): 1.76 (t, 2H, R-SH), 3.75 (d, 4H, R-CH 2-109

128 SH), 7.28 (s, 4H, benzene ring); 13 C NMR (300 MHz, CDCl 3 ) δ: (C ring ), (R-C-CH 2 -SH), and (R-C-CH 2 -SH). FTIR (cm -1, KBr plate): 2567 (s, SH stretching). trans-1,4-bis(mercaptomethyl)cyclohexane (CHDMT) trans-1,4-bis(hydroxymethyl)cyclohexane (CHDM), (4), (10 g, mol), triethylamine (TEA), (20 ml) and dichloromethane (100 ml) were stirred and cooled to 0 o C and a small trace of 4-dimethylaminopyridine (DMAP), (5 mg, 0.04 mmol) was added under the flow of Nitrogen. Para-toluenesulfonyl chloride (TSCl), (33 g, mol) was mixed with 150 ml of dichloromethane and then slowly added to the cooled solution of CHDM and TEA. After stirring overnight, the solution was extracted with deionized water (3 x 300 ml)and dried with Na 2 SO 4 anhydrous (50 g). The solvent was removed by a rotary evaporator. Tosylated CHDM, (5), was further reacted with excess potassium thioacetate (KSAc), (24 g, mol) in ethanol (200 ml) without additional purification. The solution was stirred under refluxed temperature for 18 h. The solvent (ethanol) was removed by rotary evaporator. The crude product was dissolved in diethyl ether and extracted with DI water (3 x 300 ml). The crude product solution was then dried with Na 2 SO 4 anhydrous (50 g) and the solvent was by a rotary evaporator to afford the protected thiol acetate, (6). The thiol acetate, (6), was dissolved in methanol (200 ml) and 20 wt% HCl in methanol (15 ml) was added. The solution was stirred at reflux temperature for 18 h. The solvent was removed by rotary evaporator. Crude product was dissolved in diethyl ether and extracted with DI water (3 x 300 ml). After the solvent (diethyl ether) was removed by rotary evaporator, the crude product was further purified by column chromatography (silica gel ( mesh) eluent: ethyl acetate/hexane = 110

129 30/70 (v/v)) to afford CHDMT, (7). Combined yield of this method was 60%. 1 H NMR (300 MHz, CDCl 3 ) δ(ppm): 2.44 (m, 4H, R-CH 2 SH), 1.94 (m, 4H, R-H eq ), 1.63 (m, 2H, R 2 -CH-CH 2 -SH), 1.32 (m, 2H, R-SH), 0.99 (m, 4H, R-H ax ); 13 C NMR (300 MHz, CDCl 3 ) δ(ppm): (R 2 -CH-CH 2 SH), (C ring ) and (R-CH 2 SH); FTIR (cm -1, KBr plate): 2567 (s, SH stretching). 1,6-Hexane bis(3-mercaptopropionate) (HD-SH) 1,6-Hexanediol (10.78 g, mol), 3-mercaptopropionic acid (22.32 g, mol) and p-toluenesulfonic acid (0.2 g, 0.001mol) was dissolved in toluene (150 ml) and charged into a 500 ml round-bottom three-necked flask equipped with mechanical stirrer, Dean-Stark trap and reflux condenser. The mixture was purged with argon, and heated to reflux temperature ~110 o C. The mixture was kept at reflux temperature for 3 h to ensure completed reaction after which the toluene was removed from the mixture solution in vacuo. The resultant product was cooled to room temperature and dissolved into 150 ml of diethyl ether anhydrous. The reaction mixture was washed with 5 wt% sodium bicarbonate (3 x 200 ml), and then washed with DI water (3 x 200 ml). The organic phase was dried with magnesium sulfate anhydrous (50 g.). Solvent was removed in vacuo to give HD-SH as colorless oil. Yield, 23 g (85 %). 1 H NMR (CDCl 3 ): δ 1.38 ppm (m, 4H), 1.64 ppm (m, 6H), 2.63 ppm (t, 4H), 2.73 ppm (q, 4H), 4.09 ppm (t, 4H), 13 C NMR (CDCl 3 ): δ 19.68, 25.44, 28.37, 38.35, 64.43, ppm. FTIR (cm -1, KBr plate): 2596 (s, SH stretching). Elemental analysis: carbon wt%, hydrogen 9.51 wt%, oxygen 14.8 wt% and sulfur wt%. Calculated C, H, O, S content: carbon wt%, hydrogen 8.71 wt%, oxygen wt%, and sulfur wt%. 111

130 trans-1,4-cyclohexanedimethyl bis(3-mercaptopropionate) (CHDM-SH) trans-1,4-cyclohexanedimethanol (14.08 g, mol), 3-mercaptopropionic acid (23.84 g, mol) and p-toluenesulfonic acid (0.2 g, 0.001mol) was dissolved in toluene (150 ml) and charged into a 500 ml round-bottom three-necked flask equipped with mechanical stirrer, Dean-Stark trap and reflux condenser. The mixture was purged with argon, and heated to reflux temperature ~110 o C. The mixture was kept at reflux temperature for 3 h to ensure completed reaction after which the toluene was removed from the mixture solution in vacuo. The resultant product was cooled to room temperature and dissolved into 150 ml of diethyl ether anhydrous. The reaction mixture was washed with 5 wt% sodium bicarbonate (3 x 200 ml), and then washed with DI water (3 x 200 ml). Organic phase was dried with magnesium sulfate anhydrous (50 g.). Solvent was removed in vacuo to give CHDM-SH as colorless oil. Yield, 26 g (80 %). 1 H NMR (CDCl 3 ): δ 0.99 ppm (m, 4H), 1.63 ppm (m, 4H), 1.94 ppm (d, 4H), 2.63 ppm (m, 4H), 2.73 ppm (m, 4H), 3.93 ppm (d, 4H), 13 C NMR (CDCl 3 ): δ 19.58, 28.54, 36.73, 38.21, 69.20, ppm. FTIR (cm -1, KBr plate): 2596 (s, SH stretching). Elemental analysis: carbon wt%, hydrogen 7.29 wt%, oxygen wt% and sulfur wt%. Calculated C, H, O, S content: carbon wt%, hydrogen 7.55 wt%, oxygen wt%, and sulfur wt%. 4,4 -Isopropylidenedicyclohexane bis(3-mercaptopropionate) (HBPA-SH) 4,4 -Isopropylidenedicyclohexanol (25 g, mol), 3-mercaptopropionic acid (26.05 g, mol) and p-toluenesulfonic acid (0.2 g, mol) was dissolved in toluene (150 ml) and charged into a 500 ml round-bottom three-necked flask equipped with mechanical stirrer, Dean-Stark trap and reflux condenser. The mixture was purged 112

131 with argon, and heated to reflux temperature ~110 o C. The mixture was kept at reflux temperature for 10 h to ensure completed reaction after which the toluene was removed from the mixture solution in vacuo. The resultant product was cooled to room temperature and dissolved into 500 ml of diethyl ether anhydrous. The reaction mixture was washed with 5 wt% sodium bicarbonate (3 x 200 ml), and then washed with DI water (3 x 200 ml). The organic phase was dried with magnesium sulfate anhydrous (50 g.). Solvent was removed in vacuo to give HBPA-SH as colorless oil. Yield, 32 g (75 %). 1 H NMR (CDCl 3 ): δ 0.72 ppm (m, 6H), ppm (m, 4H), ppm (m, 4H), ppm (m, 2H), 1.59 ppm (m, 2H), ppm (m, 4H), ppm (m, 4H), ppm (m, 4H), ppm (m, 2H), 13 C NMR (CDCl 3 ): δ 20.0, 20.68, 21.27, 24.89, 30.76, 32.27, 36.72, 38.89, 43.01, 70.05, 74.13, ppm. FTIR (cm -1, KBr plate): 2596 (s, SH stretching). Elemental analysis: carbon wt%, hydrogen 7.53 wt%, oxygen wt% and sulfur wt%. Calculated C, H, O, S content: carbon wt%, hydrogen 7.53 wt%, oxygen wt%, and sulfur wt%. 7.6 Result and Discussion The objective of this study was to investigate a convenient synthetic route for preparation of thiols including a new 3-mercaptopropionate family for thiol-ene photopolymerization. Two thiols (1,4-benzenedimethylthiol, BDMT and trans-1,4- bis(mercaptomethyl)cyclohexane, CHDMT) were not commercial available, and thus were synthesized. A new family of 3-mercaptopropionate was synthesized via esterification of 3-mercaptopropionic acid and diols. The reaction was facile driven by the removal of water. The thiol chemical structures were chosen to synthesize due to the difference in the electronic effect in aromatic ring (Figure 7-1b) and steric effect of the 113

132 cyclohexyl ring (Figure 7-1a, 7-1d and 7-1e). An aliphatic 6-carbon chain (Figure 7-1c) provides flexibility, whereas a cyclohexyl ring (Figure 7-1a, 7-1d, and 7-1e) offer rigidity and higher glass transition temperature Synthesis and Characterization of thiols The synthesis of BDMT was accomplished at high yield by the two-step reaction with thiourea[147] resulted the 85% yield of (3) as shown in Figure 7-2. The 1 H NMR spectrum showed the resonance at δ 1.76 indicated indicating the presence of proton from the thiol groups. The resonance at δ 3.75 indicated the methylene protons next to the thiol groups and the resonance at δ 7.28 indicated the aromatic protons. 13 C NMR spectrum showed the resonance at δ indicating methylene carbons next to the thiol groups. The resonance at δ indicated the aromatic carbon atoms. The IR spectra of BDMT showed a clear absorption band at 2567 cm -1 due to the S-H stretching. Conversely, the CHDMT was not as facile of a reaction and another reaction pathway was chosen due to the steric hinderance of the cyclohexane ring. The CHDMT had to be prepared by three-step reactions via tosylate reaction then using potassium thioacetate and hydrochloric acid as reagents as shown in Figure 7-3. This synthesis pathway utilized the sulfonate ester leaving group to result a reasonable yield of (7) (60%). The 1 H NMR spectrum showed the resonance at δ 1.32 indicating the presence of the protons from the thiol groups. The resonance at δ 2.44 indicated the methylene protons next to the thiol groups. The 13 C NMR spectrum showed three resonances at δ , , and indicating the presence of methine carbons, methylene carbons in the cyclohexane ring and methylene carbons next to the thiol groups respectively. The IR spectra showed a clear absorption band at 2567 cm -1 due to the S-H 114

133 stretching. The spectroscopic characterizations of both thiols were matched with the previous synthesis pathway.[147] Br Thiourea S NH NH 2 20 wt% KOH SH EtOH, Reflux Br HN S HS NH Figure 7-2: The reactions for synthesis of 1,4-benzenedimethanethiol (BDMT) OH TsCl OTs KSAc SCOCH 3 HCl SH HO DMAP,TEA,CH 2 Cl 2 TsO EtOH, Reflux H 3 COCS MeOH, Reflux HS Figure 7-3: The reactions for synthesis of tran-1,4-bis(mercaptomethyl)cyclohexane (CHDMT) Synthesis and Characterization of 3-mercaptopropionate thiols Preparation of the thiols, 1,6-hexane bis(3-mercaptopropionate) (HD-SH), trans- 1,4-cyclohexanedimethyl bis(3-mercaptopropionate) (CHDM-SH) and 4,4 - Isopropylidenedicyclohexane bis(3-mercaptopropionate) (HBPA-SH) is shown in Figure 6-4. Esterification reaction of the diols and 3-mercaptopropionic acid was employed to produce the difunctional thiols. Water from the reaction was used to monitor the extent of 115

134 reaction. Functional stoichiometric ratio of 1 : 1.2 of hydroxy group and acid was used to minimize homopolymerization of 3-mercaptopropionic acid. Reaction temperature (110 o C) was selected by the solvent (toluene) used in the reaction; xylene could have also been used, but toluene was easier to remove after esterification. A diagram of the general synthesis the thiols is presented in Figure 7-4. The synthesis of the three thiols was accomplished at high yield (> 75%) by a single step with 3-mercaptopropionic acid. Not surprisingly, it has been found that steric hinderance reduced the reactivity toward esterification reaction. Therefore, the HBPA-SH demands a longer reaction time, and provides relatively lower yield compared to the other two thiols. The 1,6-hexane bis(3-mercaptopropionate) (HD-SH) was characterized through FT-IR, H 1 NMR and C 13 NMR. The FT-IR spectrum of HD-SH is shown in Figure 7-5. The SH functionality is present due to the absorption band at 2596 cm -1. The absorption band at 1728 cm -1 is the indicative of a carbonyl ester group. The H 1 NMR and C 13 NMR of HD-SH are shown in Figure 7-6 and 7-7, respectively. The 1 H NMR spectrum of HD-SH (Figure 6-6) showed the resonances at δ 1.38 and 1.64 ppm which represent methylene protons along the backbone chain and the S-H functionality. The resonance at δ 4.09 ppm indicates the methylene proton adjacent to the ester group on the backbone chain. Protons of the two methylene groups next to the thiol were observed at δ 2.64 and 2.73 ppm. The integration also confirmed with the number of protons on the molecule. The 13 C NMR spectrum of 1,6-Hexane bis(3- mercaptopropionate) (Figure 7-7) showed the resonances at δ and ppm which represent the four carbons on the backbone chain. The resonances at δ and ppm indicated the two carbon atoms adjacent to the thiol groups. The carbon atoms next 116

135 to the ester groups on the backbone chain appear at δ ppm and the carbonyl carbon atoms show at the resonance at δ ppm.[148] Figure 7-4: Synthesis of the thiols 117

136 % Transmittance SH O HS O HD-SH O O SH O Wavenumber (cm-1) Figure 7-5: FT-IR spectra of 1,6-Hexane bis(3-mercaptopropionate) (HD-SH) Figure 7-6: Proton NMR of 1,6-Hexane bis(3-mercaptopropionate) (HD-SH) 118

137 Figure 7-7: Carbon NMR of 1,6-Hexane bis(3-mercaptopropionate) (HD-SH) The identification of the trans-1,4-cyclohexanedimethyl bis(3- mercaptopropionate) (CHDM-SH) was investigated by FT-IR, 1 H NMR, and 13 C NMR. The FT-IR, H 1 NMR and C 13 NMR spectra of the CHDM-SH are shown in Figure 7-8, 7-9 and 7-10, respectively. The presence of the absorption at 2596 cm -1 is indicative of the existence of a thiol group, the absorption at 1728 cm -1 is the indicative of an ester carbonyl group. Both the 1 H NMR, and 13 C NMR spectra of trans-1,4- cyclohexanedimethyl bis(3-mercaptopropionate) (CHDM-SH) (Figure 6-9 and 6-10) was consistent with the structure of CHDM-SH. The resonances at δ 0.99 and 1.63 ppm represent the methylene protons on the cyclohexane ring, and the methine protons are observed at δ 1.94 ppm. The methylene protons adjacent to the ester groups appear at δ 3.93 ppm, the resonances at δ 2.63 and 2.73 ppm represent the protons of the two methylene groups next to the thiols. The carbon resonance at δ ppm is the four methylene carbons in the cyclohexane ring. The two methine carbons in the cyclohexane 119

138 ring are observed at δ ppm. The methylene carbons adjacent to the ester groups are observed at δ ppm, and the resonance at δ ppm is the ester carbonyl groups. The carbons of the two methylene groups next to the thiols are observed at δ and ppm. 100 % Transmittance HS O O -SH 20 O O CHDM-SH SH O Wavenumber (cm-1) Figure 7-8: FT-IR spectra of trans-1,4-cyclohexanedimethyl bis(3-mercaptopropionate) Figure 7-9: Proton NMR of trans-1,4-cyclohexanedimethyl bis(3-mercaptopropionate) 120

139 Figure 7-10: Carbon NMR of trans-1,4-cyclohexanedimethyl bis(3-mercaptopropionate) The FT-IR, 1 H NMR, and 13 C NMR for 4,4 -isopropylidenedicyclohexane bis(3- mercaptopropionate) (HD-SH) are shown in Figure 7-11, 7-12 and 7-13, respectively. The FT-IR spectrum (Figure 7-11) shows that the SH functionality at 2596 cm -1, and an absorption band at 1728 cm -1 is indicative of carbonyl groups in the ester. The absence of the hydroxyl absorption at 3500 cm -1 indicates that unreacted hydrogenated bisphenol-a was not present. Further analysis via 1 H NMR and 13 C NMR confirmed the formation of the ester. The methine proton resonance of HBPA at δ 3.52 ppm shifted upfield to δ 4.64 ppm for the HBPA-SH, and the resonance of the thiol group appears at δ 1.59 ppm. The methylene groups adjacent to the thiols are observed at δ 2.58 and 2.74 ppm. Protons in the cyclohexane ring generally are at δ 1.0 to 2.2 ppm due to the mixture of isomers of HBPA. A new 13 C resonance at represents the carbonyl carbon at the ester 121

140 linkage. The resonance of the carbon next to the ester linkage at δ ppm shifted from the ppm of the α-carbon of HBPA. 100 % Transmittance O -SH 20 SH O O HBPA-SH O SH O Wavenumber (cm-1) Figure 7-11: FT-IR spectra of 4,4 -Isopropylidenedicyclohexane bis(3- mercaptopropionate) Figure 7-12: Proton NMR spectra of 4,4 -Isopropylidenedicyclohexane bis(3- mercaptopropionate) 122

141 Figure 7-13: Carbon NMR spectra of 4,4 -Isopropylidenedicyclohexane bis(3- mercaptopropionate) The ability to easily synthesize a new mercaptopropionate thiols family monomer was beneficial for thin films or composites. The new thiol monomers provided more of a capability to tailor the glass transition temperature and, crosslink density of the polymer. Utilization of this synthesis pathway, the integrated knowledge of the cure kinetics and the ability to customize thiol monomers revealed a great potential in the development of thiol-ene photopolymerization materials. The thermo-mechanical, coatings properties, and the influences of the rigidity of the thiols on the hardness and glass transition temperature will be investigated for each thiol monomer and reported in the next chapter. 7.7 Conclusion A convenient method to synthesize mercaptopropionate thiol monomers from alcohols was developed. Three thiols (1,6-hexane bis(3-mercaptopropionate) (HD-SH), trans-1,4-cyclohexanedimethyl bis(3-mercaptopropionate) (CHDM-SH) and 4,4-123

142 isopropylidenedicyclohexane bis(3-mercaptopropionate) (HBPA-SH)) were prepared at high yield demonstrating the success as a general method. The steric and rigidity of the diol determined the reaction time and the final yield. 124

143 CHAPTER VIII REACTION KINETICS OF THIOLS FOR THIOL-ENE PHOTOPOLYMERIZATION 8.1 Abstract Photopolymerization kinetics of difunctional thiols with alkenes was studied. Three types of alkenes (divinyl ether, diallyl ether, and dimethacrylate) were reacted with trans-1,4-bis(mercaptomethyl)cyclohexane (CHDMT), 1,4-bis(mercaptomethyl)benzene (BDMT) or 1,8-octanedithiol (ODT). Three mercaptopropionate thiols: 1,6-hexane bis(3-mercaptopropionate) (HD-SH), trans-1,4-cyclohexanedimethyl bis(3- mercaptopropionate) (CHDM-SH) and 4,4 -isopropylidenedicyclohexane bis(3- mercaptopropionate) (HBPA-SH) were photopolymerized with with one representative alkene, 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO). The kinetics of the photopolymerization was investigated by the time-resolved infrared spectroscopy with and without photoinitiator. The steric hinderance of the cyclohexane (CHDMT) resulted in a lower rate of photopolymerization compared to BDMT and ODT. With photoinitiator, the relative initial reaction rates of the thiols were HD-SH>CHDM- SH>HBPA-SH, whereas without photoinitiator, the reaction rates of the thiols were HBPA-SH>HD-SH>CHDM-SH. Therefore, it was proposed that steric hinderance of the thiol structure decreased the initial rate of photopolymerization. The vinyl ether (alkene) exhibited the highest activity compared to allyl ether and acrylate which was attributed to 125

144 a high electron density of the alkene. Incorporation of photoinitiator increased reaction rate and final conversion of the system, particularly in the ODT system. 8.2 Introduction In 1905, Posner[83] reported that a thiol-ene photopolymerization proceeds by a free-radical step-growth polymerization manner. The initiation step involves the formation of a thiyl radical by hydrogen atom abstraction such as initiation by UV radiation,[149, 150] The resulting product is an anti-markovnikov addition. Thiyl radical generated can propagate or terminate. Since the reaction proceeds via a step-growth mechanism, the thiol and alkene will be consumed at the same rate. The overall thiol-ene photopolymerization reaction is represented in Figure 8-1. Recently, thiol-ene photopolymerization has been an attractive area over the past few years on account of the fast growing in the field thiol-ene photopolymerization reaction.[114, 144, 151] Thiolene photopolymerization exhibits a number of advantages over conventional UV-curable resins including an inherently rapid reaction rate, reduced oxygen inhibition, less film shrinkage, and better film-substrate adhesion properties.[83, ] Figure 8-1: Thiol-ene photopolymerization reaction 126

145 Hoyle et.al.[120] studied the influence of alkene structure of thiol-ene photopolymerization. It has been found that terminal double bonds react more rapidly with thiol than internal double bonds, and the substitution on the α-position of carbon atom to the terminal alkene has little influence on the reactivity. Reactivity of cyclic double bonds is directed by ring strain, stereoelectronic effects, and hydrogen abstractability. Photopolymerization of thiol-ene without initiator has been studied with different vinyl chemistries by Cramer et.al.[155] With a 254 nm initiating light source, the initiation rates were proportional to the concentration of thiol groups, since the initiation mechanism was attributed to the direct cleavage of thiol groups. However, with 365 nm initiating light source, the initiation rates were proportional to the double bond groups. Cramer et.al.[156] and L. Lecamp et.al.[157] also studied the thiol-ene photopolymerization of thiol and acrylate. Homopolymerization of acrylate is ~1.5 times greater than the rate of hydrogen abstraction from thiol. Consequently at a stoichiometric ratio, the polymerization stops due to the complete consumption of acrylate double bonds. Limited number of studies of structure reactivity of thiols have been reported.[118] Understandably, most of the synthetic attempts were directed toward the preparation of new alkenes. Most of the polymerization studies or model reaction studies[118] have used commercial available thiols. Morgan et.al.[118] have described the relative reactivity of a well-known thiol, methyl mercaptopropionate. The ability of the thiols to form cyclic hydrogen-bonded intermediates would increase the activity of the mercaptopropionate by decreasing the strength of the thiol S-H bond. 127

146 In this study, three thiols, CHDMT, BDMT and octanedithiol (ODT), were photopolymerized with three types of alkenes, diallyl ether, divinyl ether, and dimethacrylate. Also, three difunctional mercaptopropionate thiols, 1,6-hexane bis(3- mercaptopropionate) (HD-SH), trans-1,4-cyclohexanedimethyl bis(3- mercaptopropionate) (CHDM-SH), and 4,4 -Isopropylidenedicyclohexane bis(3- mercaptopropionate) (HBPA-SH) were photopolymerized with 1,3,5-triallyl-1,3,5- triazine-2,4,6(1h,3h,5h)-trione (TATATO) and kinetics of the photopolymerization was investigated based on thiol and alkene chemical structures via time-resolved infrared spectroscopy. The effect of photoinitiator on reaction kinetics was also studied. 8.3 Materials 1,8-Octanedithiol (ODT), di(ethylene glycol)divinyl ether (DVE), trimethylolpropane diallyl ether (DAE), 1,3,5-Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)- trione (TATATO) and magnesium sulfate was purchased from Aldrich Chemical company. 1,6-Hexanediol dimethacrylate (HDMA) was obtained from Sartomer Company, Inc. The photoinitiator 2,2-dimethyl-2-hydroxyacetophenone (Darocure 1173),and mixture of 2,4,6-trimethylbenzoyl-diphenyl-phosphineoxide (50 wt%) and 2- hydroxy-2-methyl-1-phenyl-propan-1-one (50 wt%) (Darocure 4265) was obtained from Ciba Geigy. All the chemicals were used as received. Chemical structure of the thiols and alkenes are shown in Figure 8-2 and Instrument and Characterizations Time-resolved infrared spectroscopy studies were performed on a Nicolet Nexus 870-FTIR spectroscopy equipped with a Lesco Super Spot MKII UV Source shown in Figure 8-2. Series scans were recorded 4 scans per spectra. The UV-radiation intensity 128

147 was ~5 mw/cm 2. Data acquisitions and spectra calculations were performed using Omnic FT-IR software (Nicolet). 8.5 Evaluation of photopolymerization Typical formulation was prepared by mixing selected thiols and di(ethylene glycol)divinyl ether (DVE), trimethylolpropane diallyl ether (DAE) and 1,6-hexanediol dimethacrylate (HDMA) with at approximately 1:1.2 stoichiometric ratio of thiol to ene and 1,3,5-Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO) at 1:1 stoichiometric ratio of thiol to ene. Formulations with photo-initiator were added 1% wt. of Darocure 4265 or Darocure 1173 based on total formulations. The samples for the time-resolved infrared spectroscopy (TRIR) measurement were cast on a KBr plate as a thin film (~25 μm). The UV-light was delivered through a flexible light guide. KBr plate was positioned at an angle of 45 o and at the distance of 4 cm to the end of the light guide to facilitate full KBr plate exposure to UV-light (the set up is shown in Figure 8-2) The UV-light intensity was measured with a microcure radiometer (MC-2, EIT, Inc.) and found to be ~5 mw/cm 2. The IR-spectra were collected at the rate of approximately 2 scan per second with 4 scan per spectrum. Data acquisition and spectral processing was performed with Omnic FT-IR software (Nicolet). The initial rate of photopolymerization reaction was calculated by the initial slope of conversion versus time plots. 129

148 UV light Source UV IR DTGS KBr Detector Sample KBr Figure 8-2: Time-resolved infrared spectroscopy experiment set up. SH SH HS HS a HS b SH c HS O O O O SH d HS O O O O SH e O SH O O SH O 130 f

149 Figure 8-3: Chemical structure of the thiols: (a) trans-1,4,bis(mercaptomethyl) cyclohexane (CHDMT), (b) 1,4-bis(mercaptomethyl)benzene (BDMT), (c) 1,8-octanedithiol (ODT) (d) 1,6-hexane bis(3-mercaptopropionate) (HD- SH), (e) trans-1,4-cyclohexane dimethyl bis(3-mercaptopropionate) (CHDM-SH), (f) 4,4 -isopropylidenedicyclohexane bis(3- mercaptopropionate) (HBPA-SH) O O O HO O O a b O O O O O N N O N O c d Figure 8-4: Chemical structure of the alkenes: (a) di(ethylene glycol)divinyl ether (DVE) (b) trimethylolpropane diallyl ether (DAE), (c) 1,6-hexanediol dimethacrylate (HDMA) (d) 1,3,5-Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO) 8.6 Result and Discussion Thiol-ene kinetics of simple alkenes and commercially available thiols has been studied extensively.[120, 155, 156, 158, 159] Three of thiols were chosen for comparison 131

150 due to the different in steric and rigidity of the chemical structure. Two of the thiols were not commercially available, and thus were synthesized. These two thiols have not been previously studied in photopolymerized thiol-ene systems. The electronic effect of the aromatic ring (Figure 8-3b) was compared to the steric effect of the cyclohexyl ring (Figure 8-3a). Similarly, the kinetics of cyclic (Figure 8-3a) and acyclic (Figure 8-3c) systems were also compared. Three alkenes (vinyl ether, allyl ether and methacrylate) were employed in other thiol-ene photopolymerization studies and used as a benchmark for comparison. Another set of experiment was to investigate the kinetics of a new family of 3- (mercaptopropionate) thiol for thiol-ene photopolymerization. Tri-functional allyl was chosen as the alkene to minimize the effect of homopolymerization of the alkene. The thiols were formulated with 1,3,5-Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO) and a free-radical photoinitiator. The chemical structures of the diols were chosen to reach the representative bulk properties; therefore, aliphatic chemical structure of HD-SH provided flexibility while cyclohexyl ring of CHDM-SH and HBPA-SH offered rigidity and high glass transition temperature. Steric effect of cyclohexyl ring of CHDM-SH and HBPA-SH on the kinetics was compared to aliphatic structure of HD- SH. The kinetics of the photopolymerization was investigated by monitoring the disappearance of infrared absorbance bands: the disappearance of the thiol group at 2567 cm -1, vinyl ether at 1629 cm -1, allyl ether at 930 cm -1, and methacrylate at 1619 cm -1. Experiments were performed at ambient temperature at UV-light intensity of ~5 mw/cm 2. Time-resolved infrared spectroscopy set up is shown in Figure 8-2. The 132

151 conversion of double bond and thiol groups at a given time was calculated from the change in the absorption peak area as a function of time:[40] [ A 1 ] t [ A 1 ] xcm 0 xcm t Conversion(%) = x100% (7-1) [ A ] 1 xcm t0 where [A x cm-1 ] t and [A x cm-1 ] t0 were the IR absorbance at specified bands at the given time and starting time respectively Kinetics study of thiols There are many analytical techniques and tools that have been used to study the kinetics of thiol-ene photopolymerization including differential scanning calorimetry,[137] thin-film calorimetry,[ ] optical pyrometry,[163] rheology,[131, 132] and time-resolved infrared spectroscopy. Of these methods, kinetics analysis by infrared spectroscopy has the distinct advantage of providing accurate and quantitative information such as rate of polymerization and the cure extent. Time-resolved technique has been developed to allow conversion as a function of time to be obtained for polymerizations that proceed within a fraction of a second under irradiation. The kinetics of the thiol-ene photopolymerization was investigated by monitoring the disappearance infrared absorbance bands: the disappearance of the thiol group at 2596 cm -1 and the C-H stretching of the double bond at 3081 cm -1. Effect of the thiol structures Although the effect of chemical structure of an alkene on the reactivity of thiolene photopolymerization has been reported thoroughly in the literature,[120, 155, 157, 164, 165] the effect of chemical structure of the thiols on thiol-ene reaction has not been as well studied. The thiols were chosen for comparison, since the number of carbon 133

152 atoms was similar; however, the thiols are very different in steric and chain flexibility. The CHDMT has a steric cycloaliphatic structure whereas the BDMT has a planar aromatic structure. The ODT represents a linear aliphatic chain. The effect of cyclic (CHDMT and BDMT) and acyclic (ODT) thiol structures, aromatic (BDMT) and cycloaliphatic (CHDMT) thiol structure were investigated. Figure 8-5: IR-derived conversion of vinyl ether as a function of time for di(ethylene glycol)divinyl ether (DVE) with ( ) tran-1,4-bis(mercaptomethyl)cyclohexane (CHDMT), ( )1,4-benzenedimethanethiol (BDMT), and ( )1,8- octanedithiol (ODT). No photo-initiator was added. 134

153 Figure 8-6: IR-derived conversion of allyl ether as a function of time for trimethylolpropane diallyl ether (DAE) with ( ) tran-1,4- bis(mercaptomethyl)cyclohexane (CHDMT), ( ) 1,4-benzenedimethanethiol (BDMT), and ( )1,8-octanedithiol (ODT). No photo-initiator was added. Figure 8-7: IR-derived conversion of acrylate as a function of time for 1,6-hexanediol dimethacrylate (HDMA) with ( ) tran-1,4-bis(mercaptomethyl)cyclohexane 135

154 (CHDMT), ( ) 1,4-benzenedimethanethiol (BDMT), and ( )1,8- octanedithiol (ODT). No photo-initiator was added. Figures 8-5, 8-6, and 8-7 compared the conversions of alkene as a function of time for cyclohexane ring (CHDMT), benzene ring (BDMT), and linear (ODT) thiols showed that cyclohexane ring (CHDMT) structure exhibits the lowest reactivity compare to benzene rings (BDMT) structure and linear (ODT) structure. According to Morgan et.al.,[118] the CHDMT, BDMT and ODT should not form the cyclic hydrogen bond intermediate structure that promotes reactivity for thiol bond breaking. Therefore, it should not be expected that any of the three thiols would have a rapid photopolymerization reaction with alkenes. As expected, the photopolymerization of CHDMT with the alkenes without photo-initiator, resulted in a sluggish reaction with a conversion of less than 30%. The sluggish reactivity of CHDMT may come from the steric effect of the cyclohexane structure and the ring strain as compared to the other thiol molecules. Effect of alkene structures In order to investigate the effect of the alkene structure: vinyl ether, allyl ether, and methacrylate, the thiols were reacted with three different alkenes. The alkenes conversions versus time were shown in Figures 8-8, 8-9 and Reaction rate of the CHDMT with DVE, DAE, and HDMA were 1.290, 0.326, and sec -1, respectively. The CHDMT showed the most initial reaction rate with DVE without photoinitiator but comparatively the same initial reaction rate with DAE and HDMA. Reaction rate of BDMT with DVE, DAE and HDMA were 2.592, 0.796, and sec -1, respectively. The initial reaction rate of BDMT with DVE is 2 times higher with HDMA and about 3 136

155 times higher than with DAE. The reaction rate of ODT with DVE, DAE, and HDMA were 1.706, 1.362, and sec -1. The ODT showed the least initial reaction rate with HDMA and the highest initial reaction rate with DVE. Summary of the initial reaction rate and conversion at 60 sec for the system without photoinitiator is shown in table 8-1. Figure 8-8: IR-derived conversion of alkene as a function of time for trans-1,4- bis(mercaptomethyl)cyclohexane (CHDMT) with ( ) di(ethylene glycol)divinyl ether (DVE), ( ) trimethylolpropane diallyl ether (DAE), and ( ) 1,6-Hexanediol dimethacrylate (HDMA), No photo-initiator was added. 137

156 Figure 8-9: IR-derived conversion of alkene as a function of time for 1,4- benzenedimethanethiol (BDMT) with ( ) di(ethylene glycol)divinyl ether (DVE), ( ) trimethylolpropane diallyl ether (DAE), and ( )1,6-Hexanediol dimethacrylate (HDMA), No photo-initiator was added. Figure 8-10: IR-derived conversion of alkene as a function of time for 1,8-octanedithiol (ODT) with ( ) di(ethylene glycol)divinyl ether (DVE), ( ) trimethylolpropane diallyl ether DAE, and ( )1,6-Hexanediol dimethacrylate (HDMA), No photo-initiator was added. 138

157 Table 8-1: Summary of initial reaction rate and conversion at 60 sec (without PI) Initial reaction rate (without PI) (s -1 ) Conversion at 60 sec (without PI) (%) CHDMT BDMT ODT CHDMT BDMT ODT DVE DAE HDMA Comparing the reaction rate of vinyl ether, allyl ether, and methacrylate, it was found that vinyl ether has the highest reactivity compare to allyl ether and methacrylate. By reacting DVE with CHDMT, BDMT and ODT, conversion at 60 sec were around 35, 70, and 60%, respectively. This result agreed with other literature[120, 151, 155] that vinyl ether has high reactivity due to the proximity of an electron-donating ether group and this increases the electron density of the olefin π-system and increases its nucleophilicity relative to the electrophilic thiyl radical. Acrylic and allylic alkenes generally have lower reaction rate owing to the stability of carbon-centered radicals that form when the thiyl radical is added to these alkenes as a result the produce radical exhibits low hydrogen-abstraction rate. It was also observed that the ODT exhibited the slowest initial reaction rate with HDMA while the CHDMT and BDMT showed the lowest initial reaction rate with the DAE. The difference of this reactivity may due to the effect of cyclic and acyclic of thiol structure. It should be noted that not only the alkene group can influenced the reactivity of thiol-ene photopolymerization, but the chemical structure can also affect the reactivity as well. 139

158 Effect of Photoinitiator In order to investigate the effect of photoinitiator on this system, 1 wt% of photoinitiator was added into the formulations. Conversion of thiol and alkene was monitored as a function of time. Figure 8-11a shows vinyl ether conversion versus time of photoinitiator added. Comparing the reaction rate to the system without photoinitiator (see Figure 8-5), the initial rate of photopolymerization with photoinitiator increased dramatically for the ODT (1.706 to sec -1 ) and the conversion at 60 sec was almost double from the non-photoinitiator added system (54% to 94% conversion). The initial rate of photopolymerization of DVE and the BDMT also increased with addition of photoinitiator (2.592 to sec -1 ) however the conversion at 60 sec does not significantly change. The addition of photoinitiator did not have a significant effect on the reaction of DVE with the CHDMT. Thiol conversion versus time plot also showed in Figure 8-11b. Although the vinyl ether was excess in the systems, the thiol conversions were typically higher than vinyl ether conversion. This may be due to the terminal reaction by disulfide formation. The ODT showed the least disulfide formation with vinyl ether. Table 8-2: Summary of initial reaction rate and conversion at 60 sec (with PI) Initial reaction rate (with PI) Conversion at 60 sec (%) (with PI) CHDMT BDMT ODT CHDMT BDMT ODT DVE DAE HDMA

159 (a) (b) Figure 8-11: IR-derived conversion of (a) vinyl ether and (b) thiol as function of time for DVE with ( ) tran-1,4-bis(mercaptomethyl)cyclohexane (CHDMT), ( ) 1,4-benzenedimethanethiol (BDMT), and ( ) ODT, 1% photo-initiator added; thiol:ene stoichiometric ratio of 1:1.2. Figure 8-12a shows allyl ether conversion versus time of photoinitiator added system. Comparing the reaction rate to the system without photoinitiator (see Figure 8-6), addition of the photoinitiator in the reaction of DAE with the ODT and BDMT had a great effect on the rate of reaction (1.362 to sec -1 for ODT and to sec -1 for BDMT) whereas the rate of reaction of DAE with the CHDMT did not show a major change compared to the other two monomers. Conversion at 60 sec of DAE significantly improved with the ODT (47% to 93 % conversion) and BDMT (32% to 64% conversion). On the other hand, conversion at 60 sec of the reaction with the CHDMT did not have a significant effect. Thiol conversion versus time plot also showed in Figure 8-12b. Although the allyl ether was excess in the systems, the thiol conversions were typically higher than allyl ether conversion. This may be due to the terminal reaction by disulfide formation. The ODT again showed the least disulfide formation with allyl ether. 141

160 (a) (b) Figure 8-12: IR-derived conversion of (a) allyl ether and (b) thiol as function of time for DAE with ( ) tran-1,4-bis(mercaptomethyl)cyclohexane (CHDMT), ( ) 1,4-benzenedimethanethiol (BDMT), and ( )1,8-octanedithiol (ODT), 1% photo-initiator added; thiol:ene stoichiometric ratio of 1:1.2. Figure 8-13a shows methacrylate conversion versus time of photoinitiator added systems. In comparison with the system without photoinitiator (see Figure 8-7); unlike the previous cases, the reaction rate of all three monomers generally did not change when the photoinitiator was added. However, methacrylate conversion at 60 sec in the reaction of the CHDMT and HDMA was increased (25% to 70% conversion). Reaction of HDMA with the ODT and BDMT did not have a significant effect of photoinitiator added on the conversion at 60 sec. Thiol conversion versus time plot also showed in Figure 8-13b. The thiol conversions were generally corresponding with methacrylate conversion. Therefore, the disulfide formation was small. Summary of the initial reaction rate and conversion at 60 sec for the system with photoinitiator is shown in table

161 (a) (b) Figure 8-13: IR-derived conversion of (a) acrylate and (b) thiol as function of time for HDMA with ( ) tran-1,4-bis(mercaptomethyl)cyclohexane (CHDMT), ( ) 1,4-benzenedimethanethiol (BDMT), and ( )1,8-octanedithiol (ODT), 1% photo-initiator added; thiol:ene stoichiometric ratio of 1:1.2. The BDMT and ODT showed relatively higher reactivity than CHDMT. The reaction rates of three thiols with alkenes were slow compared to the mercaptopropionate derivative thiols (commercially available).[120, 164, 166] Morgan et.al.[118] described the ability to form cyclic hydrogen-bond intermediate increases the hydrogen abstraction rate from the thiols. The effect of alkene structure with different thiols agreed with other literature[120, 155, 158] that the proximity of an electron-supplying group in vinyl ether create such high reactivity in thiol-ene photopolymerization. Incorporation of free-radical initiator dramatically changed the rate of reaction particularly in the case of BDMT and ODT (see Figure 8-11 and 8-12). 143

162 8.6.2 Kinetic study of mercaptopropionate thiols The conversion versus time curves obtained from the samples HD-SH/TATATO, CHDM-SH/TATATO and HBPA-SH/TATATO without the photoinitiator are shown in Figure 8-14, 8-15 and 8-16, respectively. The calculated initial rate of photopolymerizations (R p ) and conversions are summarized in Table 8-3. It has been reported that, without photoinitiator, thiol-ene photopolymerization can be initiated via the direct cleavage of the thiol functional groups;[155] therefore, degradation and yellowing that generally are the consequence from the initiator molecule can be minimized. Of the three thiol molecules, without the photoinitiator at 180 s, HD-SH has the highest conversion at (87.58 %) and HBPA-SH has the lowest conversion at (65.50 %). The initial rates of photopolymerization (without photoinitiator) showed that HBPA- SH exhibits the highest rate of photopolymerization (0.92 s -1 ) while CHDM-SH showed slightly lower initial rate of photopolymerization (0.43 s -1 ) than that of HD-SH (0.54 s -1 ). It was surprising that HBPA-SH exhibited a high initial rate of photopolymerization in the absence of photoinitiator. This result may be due to the better chain transfer capability of HBPA-SH. The conversion at 180 s indicated that the rate of reaction was dependent on the structure of the thiol. The linear structure of the HD-SH provided the highest conversion. The rigidity of the cyclohexane ring (CHDM-SH) showed lower conversion at 180 s, and the HBPA-SH with the most rigid structure gave the lowest conversion at 180 s. The flexibility of the thiol molecules clearly assisted in the extent of final cure of the polymer. 144

163 Conversion (%) Thiol Ene Irradiation Time (s) Figure 8-14: Time-resolved FT-IR conversion of thiol (dot) and alkene (solid) as a function of time for the mixture of 1,6-Hexane bis(3-mercaptopropionate) (HD-SH) and 1,3,5-Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO) without photoinitiator Conversion (%) Thiol Ene Irradiation Time (s) Figure 8-15: Time-resolved FT-IR conversion of thiol (dot) and alkene (solid) as a function of time for the mixture of trans-1,4-cyclohexanedimethyl bis(3-145

164 mercaptopropionate) (CHDM-SH) and 1,3,5-Triallyl-1,3,5-triazine- 2,4,6(1H,3H,5H)-trione (TATATO) without photoinitiator Conversion (%) Thiol Ene Irradiation Time (s) Figure 8-16: Time-resolved FT-IR conversion of thiol (dot) and alkene (solid) as a function of time for the mixture of 4,4 -Isopropylidenedicyclohexane bis(3- mercaptopropionate) (HBPA-SH) and 1,3,5-Triallyl-1,3,5-triazine- 2,4,6(1H,3H,5H)-trione (TATATO) without photoinitiator Table 8-3: Initial rate of photopolymerization and double bond conversion Initial Rp (1/s) Conversion at 180 s (%) Sample with PI w/o PI with PI w/o PI HD-SH/TATATO CHDM-SH/TATATO HBPA-SH/TATATO R p : Rate of photopolymerization 146

165 Figure 8-17, 8-18 and 8-19 illustrate the thiol-ene photopolymerization kinetics of the HD-SH/TATATO, CHDM-SH/TATATO and HBPA-SH/TATATO formulations in the presence of photoinitiator, respectively. As observed in the absence of photoinitiator, the conversions at 180 s with photoinitiator followed the same dependency on structure. It should be noted that the addition of a photoinitiator did not have a significant effect on the conversion at 180 s. In comparison, the initial rate of photopolymerization was dramatically affected by the addition of the photoinitiator. For example, the initial rate of photopolymerization of HD-SH/TATATO exhibited ~ 100x increases (from 0.54 s -1 to s -1 ) with addition of photoinitiator. The initial rate of photopolymerization of CHDM-SH/TATATO and HBPA-SH/TATATO increased from 0.43 s -1 to s -1, and 0.92 s -1 to s -1, respectively with addition of a photoinitiator Conversion (%) Thiol Ene Irradiation time (s) Figure 8-17: Time-resolved FT-IR conversion of thiol (dot) and alkene (solid) as a function of time for the mixture of 1,6-Hexane bis(3-mercaptopropionate) 147

166 (HD-SH) and 1,3,5-Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO) with 1 % photoinitiator Conversion (%) Thiol Ene Irradiation Time (s) Figure 8-18: Time-resolved FT-IR conversion of thiol (dot) and alkene (solid) as a function of time for the mixture of trans-1,4-cyclohexanedimethyl bis(3- mercaptopropionate) (CHDM-SH) and 1,3,5-Triallyl-1,3,5-triazine- 2,4,6(1H,3H,5H)-trione (TATATO) with 1 % photoinitiator Conversion (%) Thiol Ene Irradiation Time (s) 148

167 Figure 8-19: Time-resolved FT-IR conversion of thiol (dot) and alkene (solid) as a function of time for the mixture of 4,4 -Isopropylidenedicyclohexane bis(3- mercaptopropionate) (HBPA-SH) and 1,3,5-Triallyl-1,3,5-triazine- 2,4,6(1H,3H,5H)-trione (TATATO) with 1 % photoinitiator From this study, it has been shown that the new thiols exhibited reactivity of photopolymerization with alkenes. These thiols have an inherently rigid chemical structure; as a result, the thiols can be imparted into films or neat resins to provide a high glass transition temperature or hardness to a given application.[53] In a broader perspective, thiols are very efficient chain-transfer agent for common monomers such as methyl methacrylate and styrene.[167, 168] The structure of thiols has been previously reported as chain-transfer agents resulted in variation of the stability of the resulting radicals. However, the steric hinderance and rigidity of the cyclohexane thiol had an effect upon the chain-transfer kinetics and certainly the end mechanical properties.[169] 8.7 Conclusion Time-resolved infrared spectroscopy was used to examine the kinetics behavior of thiol-ene photopolymerization. In comparison between thiols, the cyclohexane ring (CHDMT) results the least reactivity with the high electron density alkene (vinyl ether), electron withdrawing alkene (methacrylate) and intermediated electron density alkene (allyl ether) compared to the planar benzene ring (BDMT) and linear structure (ODT). Similarly, the rigidity of the mercaptopropionate thiols resulted in the lower initial rate of photopolymerization and lower final conversion. Therefore, it was proposed that the steric hinderance of the cyclohexane ring structure was the cause of the slow of the reaction. Of the three types of alkenes used, vinyl ether exhibited the highest reactivity 149

168 with all thiols. Incorporation of the photoinitiator to the system increased the reaction rate and the conversions, especially with linear thiol (ODT) which has the most reactivity in thiol-ene photopolymerization compared to CHDMT and BDMT. 150

169 CHAPTER IX EVALUATION OF NEW 3-MERCAPTOPROPIONATE THIOLS FOR THIOL-ENE PHOTOPOLYMERIZATION COATINGS USING EXPERIMENTAL DESIGN 9.1 Abstract Three 3-mercaptopropionate thiols, 1,6-Hexane bis(3-mercaptopropionate) (HD- SH), trans-1,4-cyclohexanedimethyl bis(3-mercaptopropionate) (CHDM-SH), and 4,4 - Isopropylidenedicyclohexane bis(3-mercaptopropionate) (HBPA-SH) were formulated with 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO) and a photoinitiator. The formulations were photopolymerized via thiol-ene photopolymerization. A ternary experimental design was employed to elucidate the influence of the three thiols on the thermo-mechanical and coatings properties of thiol-ene photopolymerizable materials. Tensile strength, tensile modulus, elongation-to-break, glass transition temperature (T ) and crosslink density (XLD) were investigated. Coating properties including pencil hardness, pull-off adhesion, MEK double rubs, and gloss were also investigated. Thiolene photopolymerizable materials containing HBPA-SH resulted in improved tensile strength, tensile modulus, T and pencil hardness with loss of crosslink density. This was g attributed to steric and rigidity of the double cycloaliphatic structure. The inclusion of CHDM-SH into the systems resulted in the synergistic effect on elongation-at-break and g 151

170 pull-off adhesion. The HD-SH generally resulted in a diminution of thermo-mechanical and coating properties, but improved the crosslink density. 9.2 Introduction Recently, thiol-acrylate photopolymerization has been found to be tunable by the concentration of acrylate used; The glass transition temperature and hardness were adjustable depended on the concentration of acrylate.[146] However, the present choices of available alkene/acrylate systems limit the potential for selectively altering physical, mechanical and optical properties. A solution to this problem can be obtained by developing new thiols. Unfortunately, synthetic attempts of new thiols for thiol-ene photopolymerization have only been recently reported.[81] Another approach to develop hard thiol-ene polymers is to utilize thiol and ene monomer with rigid structures. However, synthesis of new monomers is necessary due to the limited availability of rigid thiols and alkenes that are not homopolymerizable. Ternary systems of thiol-vinyl-vinyl were studied to compare to binary thiol-ene system. The extent of vinyl homopolymerization and the thiyl radical reactivity toward both vinyl groups determines the polymerization mechanism and network evolution of the ternary systems. Control of polymerization kinetics, cross-linked network structure, and mechanical properties can be achieved with the thiol-vinyl-vinyl ternary system. Evaluation of thiol-allyl ether-methacrylate ternary systems showed that the concentration and structure of the thiol significantly affect the polymerization process and network structure.[145] Investigation of film and coatings properties of thiol-ene photopolymerization can be relatively difficult due to the complexity and numbers of variables. A simple 152

171 experiment can take long times to understand complex systems. Design of experiments can elucidate and quantify how the interaction of two or more factors affects the system. It is also easier with the designed experiments to demonstrate nonlinear relationships. The object of the response surface methodology (RSM) is to form a mathematical model of the system by using statistical analysis of the experimental results. When nonlinearity is concerned, more than two level experiments are required to describe a response curve.[170] In a mixture experiment, a special type of response surface experiment, the factors are the ingredients or components of a mixture. The response is a function of the proportions of each ingredient. These proportional amounts of each ingredient are typically measured by weight, by volume, by mole ratio and so forth. Data obtained from the designed experiments can be analyzed using analysis of variance (ANOVA), a collection of statistical models which compare means by subdividing the overall observed variance into different parts. ANOVA shows whether model variance is significant when compared to experimental variances. In this study, thiol-ene photopolymerization materials were statistically investigated upon the variation of thiol chemical structures. Three UV-curable mercaptopropionate thiols: 1,6-Hexane bis(3-mercaptopropionate) (HD-SH), trans-1,4- Cyclohexanedimethyl bis(3-mercaptopropionate) (CHDM-SH), and 4,4 - Isopropylidenedicyclohexane bis(3-mercaptopropionate) (HBPA-SH) were statistically formulated with 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO) for the elucidation of dependent variables, and optimization of mechanical, thermal, and coatings properties. The mechanical and thermal properties were evaluated via stress-strain tensile experiments and dynamic mechanical analysis (DMA). Pencil hardness, pull-off 153

172 adhesion, MEK double rubs, and gloss test were performed to investigate the coatings properties. The variables were evaluated through experimental design via a simplex centroid model. 9.3 Materials 4,4 -Isopropylidenedicyclohexanol mixture of isomers (hydrogenated Bisphenol- A) (HBPA), 1,6-hexanediol (HD), trans-1,4-bis(hydroxymethyl)cyclohexane (CHDM) 3-mercaptopropionic acid, p-toluenesulfonic acid (p-tsa), toluene (reagent grade), diethyl ether anhydrous, 1,3,5-Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO), and magnesium sulfate was purchased from Aldrich Chemical company. The photoinitiator 2,2-dimethyl-2-hydroxyacetophenone (Darocure 1173) was obtained from Ciba Specialty. All the chemicals were used as received. The chemical structure of the photoinitiator is shown in Figure 9-1. O C CH 3 C OH CH 3 Figure 9-1: Chemical formula of 2,2-dimethly-2-hydroxyacetophenone (Darocure 1173) 9.4 Instrumentation and testing protocol Coatings were evaluated using the following tests: pencil hardness (D ), pull-off adhesion (ASTM D ), and gloss. The mechanical properties were evaluated under ambient conditions. The viscoelastic properties were investigated using a Perkin-Elmer Rheometric Scientific DMA in tension mode with a frequency of 1Hz and heating rate of 5 o C/min over a range of -100 to 150 o C, an average sample thickness of 1 mm. The test geometry was rectangular (10 x 5 x 1 mm). The glass transition temperature (T g ) and the crosslink density (M c or ν e ) were obtained from DMA analysis. The DMA 154

173 spectrum has viscoelastic properties plotted versus temperature. The storage modulus and loss modulus measures the elastic and viscous response, respectively, of the material. The apex of the tan delta curve provides the T g of the sample. The crosslink density (ν e ) is calculated from the storage modulus vs temperature plot using the following equation: M c or ν e = E min /3RT (9-1) where E min is minimum storage modulus, T (K) is temperature at minimum storage modulus, and R is the universal gas constant. The temperature and the minimum storage modulus data were obtained from the DMA spectrum. Tensile test were performed using Instron universal tester. Tensile properties, stress, strain and tensile modulus were measured. The test was carried out at a strain rate 30 mm/min. The samples had an average width and thickness of 1.0 and 1.2 mm, respectively. An average of eight samples was tested for each composition and then the average values were recorded. A simplex centroid mixture design was created and analyzed by software V.6 (Stat-Ease, Inc.). 9.5 Formulations and Film Formation The coatings formulations were statistically prepared according to the simplex centroid mixture with augment design as shown in Table 9-1 and depicted in Figure 9-2. The design mixture of three components has been chosen for the analysis and optimization of coatings formulations. The mixture in molar ratio of 1,6-Hexane bis(3- mercaptopropionate) (HD-SH), trans-1,4-cyclohexanedimethyl bis(3- mercaptopropionate) (CHDM-SH), and 4,4 -isopropylidenedicyclohexane bis(3-155

174 mercaptopropionate) (HBPA-SH) was combined with 1,3,5-Triallyl-1,3,5-triazine- 2,4,6(1H,3H,5H)-trione (TATATO) at 1:1 stoichiometric ratio (thiol : alkene). The photoinitiator was 1 wt% of the total formulation. Formulations were cast using a 150 μm (6 mil) draw down bar on aluminum panels for the coating test and formulations were prepared in 1 mm-thick glass mold to obtain the sample for the DMA and tensile test. Formulations were immediately placed in an ultraviolet processor (Fusion-system, medium-pressure mercury UV lamp at 5 fpm). After cure, the coatings were immediately evaluated for dryness to touch. All samples were kept for 7 days before testing. Table 9-1: Pseudo formulation matrix for simplex centroid design of experiments Run HD-SH CHDM-SH HBPA-SH

175 A: HD-SH (1, 0, 0) 1.0 (0.67, 0.17, 0.17) (0.5, 0.5, 0) (0.5, 0, 0.5) (0.33, 0.33, 0.33) (0.17, 0.67, 0.17) (0.17, 0.17, 0.67) (0, 0.5, 0.5) (0, 1, 0) (0, 0, 1) B: CHDM-SH 0.0 C: HBPA-SH Figure 9-2: Simplex centroid design of experiment containing HD-SH, CHDM-SH and HBPA-SH 9.6 Result In this study, three different chemical structures of mercaptopropionate thiol (HD- SH (linear), CHDM-SH (single cycloaliphatic), and HBPA-SH (double cycloaliphatic)) were chosen to evaluate the relationship of chemical structure and the material properties. 1,3,5-Triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TATATO) was chosen as a representative alkene; as a result, the effect of homopolymerization was minimized. Utilization of experimental design was beneficial for optimization and investigation of interaction between parameters. In the previous chapter, the chemical structure and UV- 157

176 curing reactivity of these three thiols was investigated.[81] The steric strain of cycloaliphatic structure caused the initial reaction rate and final conversion to be relatively lower than the linear structure. In this study, it can be anticipated that cycloaliphatic structure provides more hardness and higher glass transition temperature to the final cure film; although, steric strain lower the reactivity of the thiol. Table 9-2: Tensile and Viscoelastic Properties Run Tensile Strength Tensile modulus Elongation E' min Tg XLD (MPa) (MPa) (MPa) ( o C) (mol/m 3 ) Table 9-3: General Coatings Properties Run Gloss 20 o 60 o Hardness Adhesion Rub Gloss Pencil Pull-off MEK H 200 > H 200 > H 210 > H 250 > H 200 > H 210 > HB 100 > F 100 > HB 110 > H 250 >500 Double 158

177 A simplification of the three-dimensional model is shown in Figure 9-3. The minimum of three-dimensional model was at the two vertices and the maximum was at the other vertex. This model was an attempt to fit a mathematical model surface that connected the four points from the test results. The contours traced the surface of the three-dimensional model at a given value. Using this method, trends of properties can be quickly established. Locating peaks and valleys along the contour correlates to synergistic or antagonistic results. In addition, overlay of the plots enable rapid target formulation of certain properties by locating regions where the desired properties overlap.. Figure 9-3: Three-dimensional surface plot for glass transition temperature The ternary plot in Figure 9-4 (A) shows linearly improved tensile strength with increasing the molar concentration of HBPA-SH ranging from 1 to 5 MPa The tensile strength also increased with the molar concentration of CHDM-SH ranging from 1 to 2 159

178 MPa without HBPA-SH. The minimum tensile strength occurred with the pure HD-SH formulation. The data in Figure 9-4 was obtained from the statistical calculation. A linear model relationship between HD-SH, CHDM-SH and HBPA-SH was selected. The best fit mathematical model in term of actual component for the tensile strength is Tensile strength = x (HD-SH) x (CHDM-SH) x (HBPA-SH) (9-2) The model F-value of implied that the model was significant. There was only a 0.57 % chance that a F-value this large could occur due to the noise. Values of prob > F less than indicated that the model terms were significant. In this case, linear mixture component was significant model terms. Values greater than indicated the model terms was not significant. Other statistics of interest are listed as following: standard deviation = 1.07, R-squared = , mean = 2.56, adj R-squared = , PRESS = 25.19, adeq precision = The Adeq precision measures the signal-tonoise ratio. A ratio greater than 4 was desirable. The ratio in Figure 9-4 of indicated an adequate signal. This model can be used to navigate the design space. 160

179 (A) (B) Figure 9-4: Contour Plot of tensile strength model (A) and standard error (B) 161

180 In Figure 9-5, the ternary plots of tensile modulus results are shown. The plot shows improved of tensile modulus with increasing the concentration of HBPA-SH ranging from 10 to 90 MPa. The maximum of tensile modulus appeared at the apex of HBPA-SH. The HD-SH and CHDM-SH mixture showed no synergistic effect on tensile modulus. The data in Figure 9-5 was fit with a quadratic model. The model F-value of implied the model was significant. There was only a 0.49% chance that a model F- Value this large could occur due to noise. Values of prob > F less than indicated model terms were significant. In this case linear mixture components, AC, BC were significant model terms. Values greater than indicated the model terms were not significant. Other statistics of importance are listed as follows: standard deviation = 9.44, R-squared = , mean = 19.22, adj R-squared = , PRESS = , adeq precision = A ratio greater than 4 was desirable. The ratio in Figure 9-5 of indicated an adequate signal. This model can be used to navigate the design space. The final equation in terms of actual components is: Tensile Modulus = x (HD-SH) x (CHDM-SH) x (HBPA-SH) x (HD-SH) x (CHDM-SH) x (HD-SH) x (HBPA- SH) x (CHDM-SH) x (HBPA-SH) (9-3) As shown in Figure 9-6, elongation-to-break trend is clearly shown. The maximum of elongation was located between HBPA-SH and CHDM-SH (~180 %) and the minimum was on the HD-SH vertex (~18 %). The HBPA-SH quadrants also displayed a relatively higher elongation than the other two. The combination of HBPA- SH and CHDM-SH showed a strong synergistic effect on improving elongation-to-break. The elongation-to-break data in Figure 9-6 was fit with a special cubic model. The model 162

181 F-value of implied the model was significant. There was only a 1.72 % chance that a model F-Value this large could occur due to noise. Values of prob > F less than indicate model terms were significant. In this case, linear mixture components BC were significant model terms. Values greater than indicated the model terms were not significant. Other statistics of interest are the following: standard deviation = 0.13, R- squared = , mean = 0.77, adj R-squared = , PRESS = 1.35, adeq precision = A ratio greater than 4 was desirable. The ratio in Figure 9-6 of indicated an adequate signal. This model can be used to navigate the design space. The final equation in term of actual components is: Elongation to break = x (HD-SH) x (CHDM-SH) x (HBPA-SH) x (HD-SH) x (CHDM-SH) x (HD-SH) x (HBPA-SH) x (CHDM-SH) x (HBPA-SH) x (HD-SH) x (CHDM-SH) x (HBPA- SH) (9-4) (A) 163

182 (B) Figure 9-5: Contour Plot of tensile modulus model (A) and standard error (B) (A) 164

183 (B) Figure 9-6: Contour Plot of elongation-to-break model (A) and standard error (B) (A) 165

184 (B) Figure 9-7: Contour Plot of glass transition temperature model (A) and standard error (B) The glass transition temperature model is shown in Figure 9-7. The result showed clearly improvement of T g with increasing the concentration of HBPA-SH. The maximum value of T g appeared at the apex of HBPA-SH (~75 o C), while the minimum showed at the apex of CHDM-SH (~20 o C). The glass transition temperature data in Figure 9-7 was fit with a linear model. The model F-value of implied the model was significant. There was only a 0.45 % chance that a model F-Value this large could occur due to noise. Values of prob > F less than indicated model terms were significant. In this case linear mixture components was significant model terms. Values greater than indicated the model terms were not significant. Other statistics of interest are listed as following: standard deviation = 10.65, R-squared = , mean = 166

185 41.28, adj R-squared = , PRESS = , adeq precision = A ratio greater than 4 was desirable. The ratio in Figure 9-7 of indicated an adequate signal. This model can be used to navigate the design space. The final equation in terms of actual components is: Tg = x (HD-SH) x (CHDM-SH) x (HBPA- SH) (9-5) The crosslink density (XLD) model calculation is shown in Figure 9-8. There was a clear trend showing that XLD decreased with the concentration of HBPA-SH and reached the minimum at the apex of HBPA-SH (~440 mol/m 3 ). The maximum of XLD located at the vertex of HD-SH (~1,070 mol/m 3 ). It should be noted that the XLD of CHDM-SH showed slightly different from HD-SH at the same level of HBPA-SH. The XLD data in Figure 9-8 was fit with a linear model. The model F-value of 6.87 implied the model was significant. There was only a 2.23% chance that a model F-Value this large could occur due to noise. Values of prob > F less than indicated model terms were significant. In this case, linear mixture components were significant model terms. Values greater than indicated the model terms were not significant. Other important statistical information about the plot area was listed as following: standard deviation = , R-squared = , mean = , adj R-squared = , PRESS = 3.19x10 5, adeq precision = The ratio in Figure 9-8 of indicated an adequate signal. This model can be used to navigate the design space. The final equation in terms of actual components is: 167

186 XLD = x (HD-SH) x (CHDM-SH) x (HBPA-SH) (9-6) (A) 168

187 (B) Figure 9-8: Contour Plot of XLD (v e ) model (A) and standard error (B) The ternary plots in Figure 9-9 and 9-10 show thm model of 20 o gloss and 60 o gloss. 20 o Gloss showed the minimum at the local between HBPA-SH and CHDM-SH but the minimum was favored to the side of CHDM-SH. As the concentration of HD-SH increased, 20 o gloss was increased and reached the maximum. However, 20 o gloss slightly decreased when approaching to the apex of HBPA-SH. For 60 o gloss, the trend appeared to be in the same manner as 20 o gloss. 60 o Gloss showed low value at the apex of the three components. A synergistic effect between HBPA-SH and HD-SH created the maximum 60 o gloss. Some synergistic effect between CHDM-SH and HD-SH can be observed in the model. The data in Figure 9-9 was fit with special cubic model. The Model F-value of implied the model was significant. There was only a 0.94% chance that a model F- Value this large could occur due to noise. Values of prob > F less than indicated model terms were significant. In this case, linear mixture components, BC, ABC were significant model terms. Values greater than indicated the model terms were not significant. Other statistical important are: standard deviation = 4.06, R-squared = , mean = , adj R-squared = , PRESS = , adeq precision = Adeq precision measures the signal-to-noise ratio. A ratio greater than 4 was desirable. The ratio in Figure 9-9 of indicated an adequate signal. This model can be used to navigate the design space. The final equation in terms of actual components is: 20 o Gloss = x (HD-SH) x (CHDM-SH) x (HBPA-SH) x (HD-SH) x (CHDM-SH) x (HD-SH) x (HBPA-SH) 169

188 x (CHDM-SH) x (HBPA-SH) x (HD-SH) x (CHDM-SH) x (HBPA-SH) (9-7) The data in Figure 9-10 was fit with quadratic model. The model F-value of 3.02 implied the model was not significant relative to the noise. There was a % chance that a model F-value this large could occur due to noise. Values of "Prob > F" less than indicated model terms were significant. In this case linear mixture components, AC were significant model terms. Values greater than indicated the model terms were not significant. Other statistical important data are: standard deviation = 1.42, R- squared = , mean = , adj R-squared = , PRESS = , adeq precision = A ratio greater than 4 was desirable. The ratio in Figure 9-10 of indicates an adequate signal. This model can be used to navigate the design space. The final equation in terms of actual components is: 60 o Gloss = x (HD-SH) x (CHDM-SH) x (HBPA-SH) x (HD-SH) x (CHDM-SH) x (HD-SH) x (HBPA-SH) x (CHDM-SH) x (HBPA-SH) (9-8) 170

189 (A) (B) Figure 9-9: Contour Plot of gloss 20 o model (A) and standard error (B) 171

190 (A) (B) Figure 9-10: Contour Plot of gloss 60 o model (A) and standard error (B) 172

191 In Figure 9-11, the ternary plots representing of pencil hardness results are shown. Pencils 8B-B, HB, F, and H-9H were assigned a respective number from 1-19 from soft to hard graphite, respectively. There was a trend of increased hardness toward both CHDM-SH and HBPA-SH apexes in the range of a pencil hardness of 2H to 3H. There was also increase in hardness for the points located near the HBPA-SH vertex. The minimum hardness appeared at the apex of HD-SH. The data in Figure 9-11 was fit with linear model. The model F-value of implied the model was significant. There was only a 0.06% chance that a model F-Value this large could occur due to noise. Values of prob > F less than indicated model terms were significant. In this case, linear mixture components were significant model terms. Values greater than indicated the model terms were not significant. Other important statistical information about the plot area was listed follows: standard deviation = 0.55, R-squared = , mean = 11.20, adj R-squared = , PRESS = 3.63, adeq precision = A ratio greater than 4 was desirable. The ratio in Figure 9-11 of indicated an adequate signal. This model can be used to navigate the design space. The final equation in terms of actual components is: Hardness = x (HD-SH) x (CHDM-SH) x (HBPA-SH) (9-9) 173

192 (A) (B) 174

193 Figure 9-11: Contour Plot of hardness model (A) and standard error (B) Pull-off adhesion can be seen in Figure A synergistic effect between HBPA- SH and CHDM-SH gave the maximum adhesion between these two components and provided the adhesive strength up to 260 lb/in 2. The lowest value (100 lb/in 2 ) was located at the HD-SH apex. The pull-off adhesion data in Figure 9-12 was fit with quadratic model. The model F-value of implied the model is significant. There was only a 1.16% chance that a model F-Value this large could occur due to noise. Values of Prob > F less than indicated model terms were significant. In this case linear mixture components were significant model terms. Values greater than indicated the model terms were not significant. Other important statistical information about the plot area was listed follows: standard deviation = 20.06, R-squared = , mean = 183, adj R-squared = , PRESS = , adeq precision = A ratio greater than 4 was desirable. The ratio in Figure 9-12 of indicated an adequate signal. This model can be used to navigate the design space. The final equation in terms of actual components is: Adhesion = x (HD-SH) x (CHDM-SH) x (HBPA-SH) x (HD-SH) x (CHDM-SH) x (HD-SH) x (HBPA- SH) x (CHDM-SH) x (HBPA-SH) (9-10) 9.7 Discussion One of the objectives of this study was to investigate synergistic effects on the coatings properties of the three mercaptopropionate thiols, HD-SH, CHDM-SH, and HBPA-SH for thiol-ene photopolymerization by utilizing the statistical evaluation. It should be noted that the coatings properties of thiol-ene photopolymerization materials 175

194 have not been previously reported, so this study can be considered as a benchmarking study. Binary system of thiol-ene-ene (two alkenes) and thiol-ene/acrylates (two alkenes) has been studied in order to tailor the final properties of the thiol-ene materials,[145, 171] whereas in this study, a ternary system of thiols was used. Another objective was to reveal the effect of the particular mercaptopropionate thiol on thermal, mechanical, and coatings properties. Three statistical scenarios were found. The first category was no problem in error and fit with good usable design space meaning that the result predictive design space was dependable. Pencil hardness resided in this category. The second category was a problem in error and fit, yet having a usable design space. This was attributed to a large block effect which had some effect upon the overall model fit. Six tests, tensile strength, tensile modulus, elongation-to-break, crosslink density glass transition temperature and 20 o gloss fell into this category. The final category was where the mean was a better prediction than the statistical model, and this was apparent for pull-off adhesion and 60 o gloss. Due to the inherent variation of the chemical structure of the three thiols, HD-SH (linear), CHDM-SH (single-cycloaliphatic), HBPA-SH (double-cycloaliphatic), the combination of these thiols would expect to afford a wide range of mechanical and coatings properties. The glass transition temperature, tensile strength, tensile modulus as expected, increased with the concentration of HBPA-SH. Improvement of pencil hardness can also be anticipated with the amount of HBPA-SH incorporated into the system. The phenomena were attributed from the double-cycloaliphatic structure of HBPA-SH which providing the rigidity and toughness to the materials. On the other hand, crosslink density (XLD) reduced proportionally to the concentration of HBPA-SH. 176

195 Even though CHDM-SH (single-cycloaliphatic) exhibited some steric and rigidity, the HD-SH and CHDM-SH provided approximately the same XLD amount. The HBPA-SH was by far more sterically hindered and rigid than the other two thiols which was in agreement with the previous results on kinetics study.[81] So the flexible HD-SH caused higher reaction completion (ponderal effect) and resulted in higher crosslink density. Synergistic effect was found in the elongation-to-break, pull-off adhesion, 20 o gloss and 60 o gloss; hence, the optimum values were obtained from a binary or ternary system. Maximum elongation-to-break exhibited in the binary mixture of HBPA-SH and CHDM-SH. Rigidity and toughness of HBPA-SH combined with high crosslink density of CHDM-SH resulted in an optimum elongation-to-break. Pull-off adhesion revealed the synergistic effect of HBPA-SH and CHDM-SH as well. The best adhesion to aluminum substrate can be observed with the mixture of the two thiols, while HD-SH showed inferior adhesion to the substrate. Gloss 20 o and gloss 60 o showed a synergistic result with the maximum gloss at combination of three thiols with an advantage near HD-SH apex. In thiol-ene photopolymerization, there are a number of available thiols and alkenes which can be combined to give a unique material property. It is almost impossible to find the best combination of thiols and alkenes without a systematic method of study. By and large, the statistical design did reveal both synergistic and antagonistic relationships between variables and showed the ability to achieve the optimum and predictive results with the minimum set of experiments. Therefore, statistical design was beneficial in with respect to studying thiol-ene photopolymerization as end usage coating. 177

196 Thiol-ene photopolymerization has shown large potential in coatings application, but there was no previous literature that reported the coatings properties of thiol-ene photopolymerization coatings. In comparison to other coatings system such as oil-based ceramer coatings,[68] polyacrylate[172] and polyurethane,[173] thiol-ene photopolymerization coatings performed comparable hard film (pencil hardness), good solvent resistance and high gloss while providing more flexibility to the coating film. In UV-curing system, as mentioned earlier, thiol-ene UV-curable coatings possessed the distinct advantages including no oxygen inhibited, delayed gelation, low shrinkage, high conversion and uniform crosslink density. 9.8 Conclusions A statistical approach to evaluate the material properties was accomplished. The HBPA-SH rich formulation resulted in improved tensile strength, tensile modulus, glass transition temperature and pencil hardness. Crosslink density decreased with HBPA-SH content due to the steric and rigidity of the double cycloaliphatic structure. Synergistic effect of CHDM-SH and HBPA-SH resulted an improving of elongation-at-break and pull-off adhesion. The HD-SH improved the crosslink density due to the flexibility. 178

197 CHAPTER X THIOL-ENE UV-CURABLE ORGANIC-INORGANIC HYBRID BASED ON MODIFIED TUNG OIL 10.1 Abstract A thiol siloxane colloid oligomer was developed for UV-curable inorganic and organic hybrid films. The thiol siloxane colloid was prepared from mercaptopropyltrimethoxysilane (MPTS) via sol-gel method. The thiol siloxane colloid structure was characterized using 1 H NMR, 29 Si NMR, FT-IR, GPC, and MALDI-TOF mass spectrometry. Triallyl ether modified tung oil (TAETO) was prepared via the Diels-Alder reaction and chosen as a bio-based alkene for thiol-ene photopolymerization. A photoinitiator was added to the formulation and the kinetics of the thiol-ene photopolymerization was investigated by time-resolved FT-IR spectroscopy as a function of MPTS oligomer. The kinetics results indicated that MPTS colloid oligomer did not affected the initial reaction rate and the final conversion of MPTS oligomer/taeto system due to the immiscibility between TAETO and MPTS oligomer Introduction Sol-gel is the process of the formation of inorganic network gel through a colloidal suspension (sol). Metal alkoxide such as tetraethoxysilane (TEOS) is one of the most commonly used as the precursor to synthesize sol colloid due to their reactivity with 179

198 water. The sol-gel process can be either acid- or base-catalyzed. It has been shown that the type of catalyst, water concentration, media, and temperature affected the structure of the pre-ceramic-oxo-cluster.[174] Under acidic conditions, hydrolysis reaction occurs in two steps according to S N 2 type mechanism depicted as in Figure 2-3. Based catalyzed hydrolysis reactions of alkoxysilane are much slower compared to the acid catalyzed one and hydrolysis reaction also occurs according to S N 2 mechanism as shown in Figure 2-4. Organic-inorganic hybrid materials have become very attractive since the hybrids synergistically combine the advantageous properties of both materials. Hybrid materials are considered to be innovative advanced materials, and capable of new applications in many fields.[ ] The UV-curing systems, in particular, offer many advantages that are of great interest for industrial applications such as fast curing speed, low energy cost, and use of little or no solvents.[144, 178] Consequently, there has been a growing interest in radiation cured materials. Radiation cured materials also have the advantage of an ambient temperature cure which enable heat sensitive substrates to be cured. The need for rapid cure materials with improved properties stimulated researchers to develop novel functional siloxane monomers or oligomers which were suitable for curable organic and inorganic materials. Crivello et al.[179] have synthesized siloxane containing epoxy resins for coatings. Bassidale and Gentle[180] have hydrosilated allyland vinyl-functional molecules onto the silsesquioxane monomers. Yukio et al.[181] have reported the synthesis of poly(siloxyethylene glycol) for new functional materials. Soucek et al. have studied di- and tri- TEOS functionalized polyols,[182] and interaction with cycloaliphatic epoxide UV-curable films. 180

199 Recently, research has been devoted to develop hybrid materials based on a renewable resource as an organic phase suitable for radiation curing.[ ] Vegetable oil and its derivatives have been commercially used in coatings and inks industries as a major ingredient in formulations.[1, 61] Norbornene linseed oil (NLO) and epoxidized norbornene linseed oil (ENLO) can be photopolymerized and used in UV-curable coatings and inks;[136, 186, 187] however, without reactive dilution the NLO suffered from a sluggish reaction rate. Therefore, it is be very beneficial to develop a novel functional oligomer which enhances the polymerization rate of seed oil derivatives. There have been numerous reports on photo-initiated polymerization of thiols and alkenes,[24, 123, 156, 164] and it was well-known that thiols rapidly added to alkenes when initiated by UV-radiation. Advantages of using thiol-ene systems over conventional photo-curable systems included ability to polymerize rapidly in air, the antioxidant properties of the thiols-ether unit, reduction in the effect of shrinkage upon substrate adhesion, and the achievement of relatively high degree of conversion.[119, 166, 188] However, only acrylate, vinyl ether, and epoxide systems have been investigated to a great extent.[189, 190] Since seed oils and their derivatives are the renewable and environmental friendly materials, it is important and necessary to study photo-initiated polymerization of the thiol-seed oil systems. In our previous work (see chapter III and IV), a triallyl ether modified tung oil was synthesized. Their chemical structures were characterized using FT-IR, NMR ( 1 H, 13 C NMR), and MALDI-TOF mass spectrometry. The autoxidative polymerization was studied in triallyl ether modified tung oil/alkyd system. Thermo-mechanical and coatings properties were investigated. 181

200 In this study, a multifunctional thiol siloxane oligomer was prepared by the solgel method. The thiol siloxane colloid structure was characterized using 1 H NMR, 29 Si NMR, FT-IR, and MALDI-TOF mass spectrometry. The UV-curing reactivity of MPTS /TAETO systems was studied by time-resolved FT-IR spectroscopy Materials Tung oil was obtained from Waterlox Coatings Inc., Mercaptopropyl trimethoxysilane (MPTS) was purchased form Gelest Inc., ethanol (Absolute), hydrochloric acid (37%), phenothiazine, pentaerythritol allyl ether (PETAE), acrylic acid (AA), p-toluenesulfonic acid were purchased from Aldrich Chemical. The photoinitiator, Irgacure 1173, was obtained from Ciba Specialty Chemical Corp. All the chemicals were used as received Instruments 1 H NMR and 13 C NMR were recorded on Mercury-300 spectrometer (Varian) in CDCl 3 solvent with tetramethylsilane (TMS) as a reference. The 29 Si NMR spectra were record on a Gemini-400 (Varian) in CDCl 3 with tetramethylsilane (TMS) as a reference. Fourier transform infrared (FTIR) spectroscopy was performed on an ATI Mattson Genesis FT-IR spectrometer by the casting of thin liquid samples on KBr plates. The mass spectra were acquired by using a Bruker REFLEX III time-of-flight (TOF) mass spectrometer (Bruker Daltonics, Billerica, MA) equipped with a nitrogen laser (337 nm), a single-stage pulsed ion extraction ion source, a two-stage grid-less reflector, and the two dual microchannel plate detectors for detection in linear and reflection mode. MS spectra were measured with reflection mode, with the ion source and reflector lens potentials keep at 20 kev and 22.5 kev, respectively. Triglyceride, Dithranol and Sodium 182

201 trifluoroacetate (NaTFA) were dissolved in tetrahydrofuran (THF) at a concentration of 20 mg/ml, 10 mg/ml and 10 mg/ml, respectively. These solution were mixed in the ratio of triglyceride:dithranol:natfa was 2:10:1 and 0.5 µl of the mixture was applied on the MALDI sample target. The attenuation of the nitrogen laser was adjusted to get the maximize sensitivity without causing the fragmentation of triglyceride. The mass scale was calibrated by using 6 peaks of Poly methyl methacrylate standard (PMMA) that molecular weight ~ A water system was used for gel permeation chromatography (GPC) with a HR4, HT2, HR1, HR0.5 Styragel, and 500A o Ultrastyragel columns connected in series. Tetrahydrofurun was applied as the mobile phase and delivered at a rate of 1.0 ml/min. Molecular weight and its distribution were determined by gel-permeation chromatography (GPC). The analysis was carried out using a five-column set of HR4, HT2, HR1, and HR0.5 Styragel, and 500 o A UI-Trastyragel columns (Waters Corporation) used in as series. Tetrahydrofuran (THF) served as the mobile phase and was delivered at a rate of 1.0 ml/min Preparation of mercaptopropyltrimethoxysilane (MPTS) colloid Mercaptopropyltrimethoxysilane (MPTS) was hydrolyzed via reacting with water by sol-gel method. mercaptopropyltrimethoxysilane (MPTS) (39.2 g, 0.19 mol) was dissolved in ethanol (75 ml) in a round-bottomed flask (500 ml) which was equipped with reflux condenser. distilled water (1.8 g, 0.1 mol), hydrochloric acid (11.6 M, 1 ml), and ethanol (75 ml) were mixed and then added dropwise into the above MPTS solution under magnetic stirring. The ph of the solution was adjusted to 2 by the addition of hydrochloric acid solution. The mixture was allowed to react at ambient temperature for 1 183

202 h and then heated and maintained at a reflux temperature (~60 o C) for 30 h. Solvent and excess MPTS were then removed in vacuo. The resultant product was characterized by 1 H and 29 Si NMR, FT-IR, GPC, and MALDI-TOF mass spectroscopy. The reaction scheme is shown in Figure Evaluation of photopolymerization Typical formulation was prepared by mixing triallyl ether modified tung oil (TAETO) with 5, 15, and 25 wt% of thiols colloid (MPTS). Formulations with photoinitiator were added 1 wt% of Darocure 4265 based on total formulations weight. The samples for the time-resolved infrared spectroscopy (TRIR) measurement were cast on a KBr plate as a thin film (~25 μm). The UV-light was delivered through a flexible light guide. KBr plate was positioned at an angle of 45 o and at the distance of 4 cm to the end of the light guide to facilitate full KBr plate exposure to UV-light (the set up is shown in Figure 7-2) The UV-light intensity was measured with a microcure radiometer (MC-2, EIT, Inc.) and found to be ~5 mw/cm 2. The IR-spectra were collected at the rate of approximately 2 scan per second with 4 scan per spectrum. Data acquisition and spectral processing was performed with Omnic FT-IR software (Nicolet). The initial rate of photopolymerization reaction was calculated by the initial slope of conversion versus time plots Result and discussion The objective of this work was to prepare a multifunctional thiol colloid oligomer (MPTS) as a hyper-crosslinker. The triallyl ether modified tung oil (TAETO) was chosen as the alkene for the enhanced reactivity of the triallyl ether group and for being an agricultural based feed stock (renewable resources). The triallyl ether modified tung oil 184

203 was prepared according to the method in Chapter II. The ex situ approach of preparation of the inorganic phase was used to control the colloid preparation process. Investigations were carried out for general use in a photo-curable thiol-ene system, at 0, 5, 15, and 25 wt% of thiol siloxane to study the effect of thiol siloxane colloid on the photo-curing activity in MPTS oligomer/taeto systems Characterization of MPTS thiol siloxane colloid The multifunctional thiol siloxane colloid was prepared by the sol-gel methods in Figure The viscosity and molecular weight of the MPTS colloid can be controlled by adjusting reaction conditions. Extension of the reaction time increased the average molecular weight and viscosity of the colloid. The MPTS colloid showed an average molecular weight of 1,531 and polydispersity of 1.45 via GPC. OCH 3 OH HS Si OCH 3 OCH 3 HCl/H 2 O -CH 3 OH HS Si OH OH OH OCH 3 HS Si OCH 3 + OCH 3 HS OH Si OH OH HCl -H 2 O, -CH 3 OH HS HS Si O O Si O OCH 3 Si O Si O SH SH Figure 10-1: Hydrolysis and condensation reactions of thiol oligomer 1 H NMR spectra of MPTS monomer and subsequent condensation reactant products is showed in Figure The strong, sharp resonance at δ 3.55 ppm in Figure 10-2(a) represents the methoxyl hydrogens of the Si-(OCH 3 ) 3 group decreased significantly in the MPTS colloid Figure 10-2(b), which indicated almost complete 185

204 condensation of the OCH 3 groups. The resonances at δ 0.65, 1.60, and 2.43 ppm from hydrogen atoms labeled a, b, and c, respectively, were broaden and intensified with the formation of the MPTS colloid compared to the MPTS monomer. Moreover, the small weak peak at δ 1.2 ppm in the spectrum of MPTS monomer, representing the thiol groups SH, increased in intensity for the MPTS colloid, which indicated that the incorporation of multiple SH terminal groups during the production of the oligomer. The new peak appeared at δ 3.7 ppm from the hydrogen atoms labeled f (in Figure 10-2(b)), representing the appearance of an OH group in MPTS thiol colloid provided the evidence of SiOCH 3 to SiOH hydrolysis. All the changes in the 1 H NMR spectra suggested the formation of MPTS oligomer. The 29 Si NMR spectra of thiol siloxane (MPTS) monomer and oligomer is shown in Figure The resonance of MPTS monomer at δ ppm corresponds to the Si(OCH 3 ) 3 group. After hydrolysis and condensation, the resonance at δ ppm disappeared in oligomer spectrum indicating a quantitative consumption of the Si(OCH 3 ) 3 group. A strong new resonance at δ ppm, representing the formation of Si-O-Si bands, appeared. Numerous resonances of MPTS oligomer in the region of δ -55 to -67 ppm were also observed. Due to the formation of Si-O-Si, Si-OH and the polydispersity of MPTS oligomer, there were many different chemical microenvironments of the Si atoms, which resulted in the multiple, broad peaks around δ -60 ppm in the 29 Si NMR spectra of MPTS oligomer. 186

205 Figure 10-2: 1 H NMR spectra of MPTS: (a) MPTS monomer, (b) MPTS oligomer Figure 10-3: 29 Si NMR Spectra of MPTS: a) MPTS monomer, b) MPTS oligomer 187

206 Figure 10-4 shows the comparison of FT-IR spectra of the MPTS monomer with the MPTS oligomer. The absorption at 2839 and 2941 cm -1 were attributed to CH 3 stretching in the MPTS monomer (Figure 10-4(a)). The strong band at 1081 cm -1 was due to the Si-O-CH 3 stretching, and the characteristic stretching vibration for the thiol groups was observed at 2564 cm -1. After hydrolysis and condensation, the oligomer spectrum (Figure 9-4(b)) showed that the intensity of CH 3 stretching band was decreasing while a new band at the 2928 cm -1 was appearing which corresponded to the stretching vibration of C-H group. The broad band from 1067 to 1117 cm -1 was attributed to the Si-O stretching vibration of Si-O-Si network. The broad weak band from 3600 to 3200 cm -1 corresponded to an O-H stretching vibration which was indicative of the presence of Si- OH groups. 188

207 Figure 10-4: Comparison of FT-IR spectra: a) MPTS monomer, b) oligomer The structure of the thiol oligomer has been characterized by MALDI-TOF mass spectrometry. The full spectrum and a list of identified products are shown in Figure 10-5 and Table 10-1, respectively. Based on the observed masses, thiol oligomers contained cyclic structures. The non-cyclic portion of the oligomer might be linear or branched since linear and branched structures have exactly the same mass. For the identified products, the number of terminal hydroxyl units ranged from n = 0-10, while the number of terminal methoxy units ranged from m = 0-5. The sulfur atoms, each with two abundant isotopes, increased the difficulty of peak assignment. Obviously, other peaks were observed in the spectrum, but were not listed in the Table if the abundance was very low in comparison (<3 wt%). It should be noted that other ions were observed with high abundance in the spectrum which have yet to be identified. 189