Polymer-Bound Benzoin Ether Photoinitiators

Transcription

1 Journal of Photopolymer Science and Technology Volume 3, Number 2 (1990) Polymer-Bound Benzoin Ether Photoinitiators Kwang-Duk Ahn*, Ick-Char Kwon, and Hyang-Sook Choi Functional Polymers Lab., Korea Institute of Science and Technology, P. 0. Box 131, Cheongryang, Seoul , Korea ABSTRACT : For improving the storage stability and the compatibility of benzoin alkyl ether (BAE) photoinitiators in photocurable formulations, the polymer-bound (or oligomeric) BAE photoinitiators were designed and their photoreactions were studied. Three kinds of functional BAE, a-(z-carboxyethyl) benzoin alkyl ether ( BAE-CA), a -methylolbenzoin alkyl ether ( BAE-OH), and newly synthesized a-(2-cyanatoethyl)benzoin alkyl ether (BAE-NCO) were used for making the polymeric photoinitiators. The functional BAE's were reacted with appropriate reactive oligomers and obtained three kinds of polymerbound BAE photoinitiators such as epoxy modified BAE (ER-BAE), PTMG-bound BAE (PTMG-BAE) and PTMGurethane modified BAE ( PTMG-U-BAE). They have BAE photoinitiator moieties in both chain-ends as well as identical chain structures with the photosensitive oligomers. The photoinitiators were used to make photocurable f ormurlations with photosensitive epoxy acrylate (EA), urethane acrylate (PTMG-UA) or diluents such as trimethylolpropane triacrylate (TMPTA) and 1, 6-hexanediol diacrylate (HDDA). The formulations were UV irradiated in thin film and the extent of photocuring was evaluated in terms of the degree of residual unsaturation (DRUS) by measuring the decrease in the intensity of IR absorption at 1405 cm-1. The polymer-bound BAE photoinitiators in all the formulations exhibited higher photopolymerization efficiencies than the conventional BAE photoinitiators in the similar formulations. Received Accepted May 13, 1990 May

2 I Photopolym. Sci. Technol., Vol. 3, No.2,1990 INTRODUCTION Benzoin ethers are the important class of commercial photoinitiators (PI) utilized in UV-curing formulations. Benzoin alkyl ethers (BAE) undergo photocleavage to produce benzoyl (B) and benzyl ether (E) radicals. The role of two kinds of the radicals B and E is of controversy in photopolymerization, but both radicals are known to be usually effective in photocuring under high concentrations of acrylates and methacrylates [11. Benzoin ether photoinitiators exhibit poor storage stability (or shelf life) in the presence of reactive monomers, resulting in premature polymerization even in dark storage. This instability has been attributed to the benzylic hydrogen of BAE molecules, which is readily abstracted by adventitious radicals such as peroxy radicals. Therefore various radical photoinitiators, which do not possess a benzylic hydrogen, are developed and commercialized. Upon substitution of the benzylic hydrogen atom the BAE photoinitiators exhibit improved storage stability [2]. One of the unavoidable problems is the incompatibility of the photoinitiators in a viscous formulation which contains ingredients such as multifunctional monomers and photosensitive oligomers. In addtion, being relatively low molecular weight compounds, the photoinitiators are apt to bleed out of a system in which they are incorporated both before and after curing. The incompatibility of the photoinitiators in a UV-curing formulation leads to insufficient curing, resulting in adverse effects on the physical properties of the cured system. They remain as undesirable small molecular inclusions in the finished products. The use of polymeric PI's is expected to eliminate those problems encountered in using conventional PI's such as volatility, odor and mixing properties. In a recent work, a polymeric photoinitiator of 2-hydroxy-2-methylpropiophenone even exhibits to be very efficient in acrylate systems in which it displays high photochemical efficiency and low odor photolysis products [3]. In this regard, for improving both the storage stability and the compatibility of BAE photoinitiators in photocurable formulations, the polymer-bound (or oligomeric) PI's were designed [4]. Utilizing the reactive benzylic hydrogen of BAE, some useful functional groups such as carboxylic acid, methylol, or isocyanate were introduced. Upon reactions with the substituents at the a-position of BAE, epoxy- and urethane-type polymer-bound benzoin ethers were prepared, and their photopolymerization ability with some multifunctional acrylates including photosensitive oligomers were investigated previously [5]. The polymer-bound BAE photoinitiators, which have similar or identical chain structures with the photosensitive oligomers and no benzylic hydrogen, are expected to have better storage stability and higher reactivity in photopolymerization by attaining the improved compatability in the formulations. RESULTS AN DISCUSSIONS The following benzoin alkyl ethers (BAE-X) with functional substituents at a-position were prepared benzoin alkyl ethers (BAE-NCO) as shown in Scheme I. Two BE-NCO with methyl and isobutyl ether groups, in other word, a-(2-isocyanatoethyl)benzoin methyl ether (IEBME) and a-(2-isocyanatoethyl)benzoin isobutyl ether (IEBIBE), were synthesized by a procedure of the Curtius rearrangement [8] as described in Scheme II. Two kinds of BAE-CA with methyl or isobutyl ether groups were reacted with methyl chloroformate. Then the mixture was reacted with sodium azide to make acyl azide. utilizing the reactive benzylic hydrogen of BAE photoinitiators (PT). They are a-(2-carboxyethyl )benzoin alkyl ethers (BAE-CA) [6], a-methylolbenzoin alkyl ethers (BAE-OH) [7] and a-(2-isocyanatoethyl)- The mixture of acyl azide was obtained as 138

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4 J. Photopolym. Sci. Technol., Vol. 3, No.2,1990 jelly-like matter and used in the subsequent step without separation. The acyl azide was subjected to thermolysis in ref luxing toluene to give the rearranged product BAE-NCO. Yellowish viscous IEBME was obtained in a yield of 60 % based on starting BAE-CA and solidified refrigerator. In the case of IEBIBE, the yield was 63 % and it was waxy materials. Through microdistillation in high vacuum it was possible to obtain pure IEBME. The chemical strucures of IEBME and IEBIBE were confirmed with infrared, in a proton NMR and mass spectral analyses. In an infrared spectrum IEBME has strong absorptions at 2250 cm 1 for isocyanate, 1680 cm-1 for benzoyl C=0 and 1100 cm- for ether as shown in Fig. 1. The NMR spectrum in Fig. 2 clearly shows protons corresponding to aromatic, methoxy and two kinds of methylene groups. In the subsequent reactions for applying as a polymer-bound (or oligomeric) photoinitiator IEBIBE was usually used because BIBE is more useful in photocuring. The polymer-bound BAE pliotoinitiators were prepared by reacting BAE derivatives (BAE-X) with appropriate reactive oligomers such as epoxy resin (ER), PTMG-diisocyanate and PTMG as shown in Scheme III. (i) Epoxy modified BAE (ER-BAE) through the reaction of BAE-CA and epoxy resin (ii) PTMG-bound BAE (PTMG-BAE) through the reaction of of BAE-NCO (i.e., (ER). IEBIBE) and PTMG with mol, wt. (iii) PTMG-urethane modified BAE ( PTMG-U-BAE) through the reaction of BAE-0H and PTMG-diisocyanate. PTMG-diisocyanate was prepared by a reaction of PTMG with two equivalents of a diisocyanate, MDI or TDI. Viscous PTMG-urethane diacrylate reacting PTMG-diisocyanate with 2-hydroxyethyl acrylate (HEA). ( PTMG-UA), a photosensitive oligomer, was prepared in this work by Three kinds of polymer-bound PT's, ER-BAE, PTMG-BAE and PTMG-U-BAE were mixed with photosensitive epoxy acrylate (EA), urethane acrylate ( PTMG-UA) or diluents such as trimethylolpropane triacrylate ( TMPTA) and 1, 6-hexanediol diacrylate ( HDDA) with or without solvents. The photocurable formulations investigated in this work are summarized as follows: (a) polymer-bound PI + EA (b) polymer-bound PI + EA + HDDA (c) polymer-bound PI + PTMG-UA (d) polymer-bound PI + PTMG-UA + HDDA or TMPTA (e) polymer-bound PI + HDDA or TMPTA (f) BAE + various acrylates A formulation containing a polymeric PI and acrylates was casted to make thin film on a sodium chloride plate. The film was irradiated with an UV exposure equipment mounted with a high pressure mercury lamp. The photosensitivities of the formulations are related to the relative disappearance of the acrylate functionalities for the given exposure time. The concentrations of acrylate groups were calculated by comparing the IR absorption intensities. The extent of photocuring (or photopolymerization) was evaluated by measuring the decrease in the intensity of IR absorption of the sample film at 1405 cm-1 due to methylene in-plane bending of acrylate double bonds. Thus the extent of photopolymerization was calculuated in terms of 'the degree of residual unsaturation' (DRUS). DRUS is the ratio of the concentration of acrylate groups of a formulation before and after UV irradiation for the given time and it means the ratio of the contents of unreacted double bonds after irradiation. The change in IR absorption spectra of a formulation (e) containing TMPTA and 4 equivalent % of PTMGbound benzoin methyl ether ( PTMG-BME) is shown in Fig. 3. and the remarkable reduction in absorption intensities was observed at 1405 cm' due to the photopolymerization of acrylate groups. A formulation (c) containing a photosensitive oligomer PTMG-UA and 4 equivalent % of PTMG-BME also showed the similar 140

5 n J. Photopolym. Sci. Technol., Vol. 3, No.2, 1990 Scheme III BAE-CH2CH2000H + CH2-CHCH2-0-BPA-0-CH2CH-CHI --+ BAE-CA Epoxy Resin BAE -CH2CH2000-CH2CHCH2-0-BPA-0-CH2CHCHz OCOCH2CH= BAE BAE-CH2CH2-N=C=O + HO 4(cH2)4 O ]-H 141

6 I Photopolym. Sci. Technol., vol. 3, No.2, 1990 Fig.l. IR spectrum of IEBME (NaCI plate smear). Fig.2. Proton NMR spectrum of IEBME (CDCI3 ). 142

7 J. Photopolym. Sci. Technol., Vol. 3, No.2, 1990 Fig. 3. IR spectra of photopolymerization of TMPTA by PTMG-BME (4 eq. %) o. uncured ( ), cured (5 sec,---), extensively cured film (15 min, ---). Fig. 4. The extent (4, eq. Z). of photopolymerization of TMPTA by two TMPTA/BIBE ( o ) ; TMPTA/PTMG-BAE ( p photoinitiators 143

8 I Photopolym. Sci. Technol., Vol. 3, No.2, 1990 IR spectral change depending on the curing time. In fact, in the latter case the concentration of initial acrylate groups was lower than that of the former formulation. From these IR spectral change at 1405 cm-1 DRUS values in percent were calculated for comparing the photopolymerization efficiencies of the various polymer-bound PT's. The photoinitiating efficiencies of those polymer-bound PT's in the formulations from (a) to (e) were compared with those of the simple PI, for instance, benzoin isobutyl ether (BIBE) of the formulations (f) in the light of the calculated DRUS values. As two representative examples of those formulations, a polymer-bound PI, PTMG-BAE was mixed with a diluent monomer TMPTA or a photosensitive oligomer PTMCrUA and the photopolymerization rates were compared. In Fig. 4, two formulations with PTMG-BAE and a simple PI, BIBE with 4 equivalent % in TMPTA show big difference in photopolymerization efficiency. The polymer-bound PI, PTMG--BAE exhibits the remarkable increse in photosensitivity in a diluent TMPTA. In two similar formulations with a photosensitive oligomer PTMG-UA, the polymer-bound PI, PTMG-BAE also gave better photosensitivity as shown in Fig. 5. When compared with a simple photoinitiator, the urethane-type polymeric PT, PTMG-U-BAE also revealed the enhanced photoinitiating efficiency in a formulation containing a photosensitive oligomer PTMG-UA and a diluent HDDA in 6:4 mole ratio as shown in Fig. 6. The polymer-bound PI's in all the formulations from (a) to (e) showed greater photosensitivity than a simple BAE photoinitiator, BIBE in the formulations (f) even in the initial exposure time. All the formulations also appeared to be solidified in the short exposure time less than 5 sec. The extent of photocuring was, however, found to be different in every formulations even in the longer exposure time. CONCLUSIONS Three kinds of functional benzoin alkyl ethers, BAE-CA, BAE-OH and newly synthesized BAE-NCO were reacted with appropriate reactive oligomers such as epoxy resin, PTMG-diisocyanate and PTMG to make polymer-bound (or oligomeric) photoinitiators., ER-BAE, PTMG-BAE and PTMG-U-BE, which have BAE moieties in both chain-ends. The extent of photocuring by the photoinitiators was calculated in terms of the degree of residual unsaturation (DRUS) for various formulations. Upon comparing DRUS values in all the formulations it was found that the polymer-bound BAE photoinitiators showed higher photopolymerization efficiency than the simple BAE photoinitiators in photocuring. The higher photosensitivity of the polymerbound PT's is ascribable to their polymeric or oligomeic natures. The macroradicals, which are produced by photolysis of polymeric PT's, may be alive longer because of retardment of radical recombination [3] and they are amid more homogeneous state in the systems through better mixing. Thus the produced radicals consequently favor the initiation of polymerization being able to attack the acrylate double bonds. They could bring better storage stability and compatibility in the formulations as well as improved mechanical properties in the cured coatings. EXPERIMENTAL Materials Benzoin methyl ether (BME) and benzoin isobutyl ether (BIBE) were purchased from Aldrich chemical Co. and Wako chemical Co., respectively. Poly(tetramethylene ether) glycol (PTMG) of mot. wt produced by Ouaker Oats was purified by drying in vacuum for 3 hr at 60 C. Trimethylolpropane triacrylate (TMPTA), 1, 6-hexanediol diacrylate ( HDDA), and bisphenol A-epoxy diacrylate (EA) were kindly donated by Korea Chemical Co. a-methylolbenzoin alkyl ether (BAE-OH) [7] and a-(2-carboxyethyl)benzoin alkyl ether (BAE-CA) [6] were prepared according to the known procedures. The preparation of PTMG-urethane diacrylate 144 (PTMA-UA) was described elsewhere [5].

9 J. Photopolym. Sci. Technol., Vol. 3, No.2,1990 Polymer-Bound Photoinitiators - ER-BAE and PTMG-U-BAE [5J: Epoxy modified benzoin alkyl ether (ER-BAE) was prepared by the reaction of BAE-CA and bisphenol A- epoxy resin (ER). PTMG-urethane modified BAE ( PTMG-U-BAE) was prepared by reacting with BAE-OH (Scheme III). a-(z-isocyanatoethyl)benzoin Alkyl Ether (BAE-NCO) Two kinds of BAE-NCO, a-(z-isocyanatoethyl)benzoin PTMG-diisocyanate methyl ether (IEBME) and a-(z-isocyanatoethyl) benzoin isobutyl ether (IEBIBE) were prepared by a modified procedure of the Curtius rearrangement [8] (Scheme II). Fig. 5. The extent (4 eq. %) of photocuring of PTMG-UA by two different photoinitiators PTMG-UA/BIBE ( o ) ; PTMG-UA/PTMG-BAE ( D Fig. 6. The extent of photocuring photoinitiators (4 eq. %) of PTMG-UA and :BIBE(0); HDDA (6:4 PTMG-U-BAE by mol) by D ), two 145

10 I Photopolym. Sci. Technol., Vol. 3, No.2, 1990 In a three-neck flask were placed 10.0 g (34 mmol) of a-(2-carboxyethyl)benzoin methyl ether (BME-CA) in 200 ml acetone and the mixture was cooled in an ice-water bath. To the mixture were added 5.5 ml (38 mmol) of triethylamine dropwisely and stirred for 30 min. Then 3.6 g (39 mmol) of methyl chloroformate were dropwisely added and reacted for 2 hr at the cooled condition. Further reaction with 3.0 g (46 mmol) of sodium azide was performed for Z hr. From the cold reaction mixture precipitated triethylamine hydrochloride was filtered off and the filtrate was concentrated by evaporation. The residue was extracted with ethyl acetate 3 times from water saturated with sodium chloride and the ethyl acetate extract was dried over anhydrous magnesium sulfate. The viscous concentrate after evaporation of ethyl acetate was dissolved in 150 ml toluene and the solution was refluxed for 4 hr under nitrogen. Toluene was evaporated in vacuo and the yellowish residue was confirmed to be the desired IEBME. The yield of IEBME was 60 % based on BME-CA and solidified in a refrigerator. Microdistillation in high vacuum (0.05 mmllg) at 250 C gave high purity IEBME. IR (hg.), 2250 (N=C=O), 1680 (benzoyl C=O), 1100 cm-1 (ether); NMR (CDC13), a' (m, phenyl, 211), (m, phenyl, 811), (m,-c112-nco, 211), 3.3 (s,-0c113, 3H), ppm (m, CH2, 211); Mass (m/e), 190, 147, 133, 105, 91, 77 (no molecular ion). Waxy IEBIBE was prepared with the same procedure from the starting a-(2-carboxyethyl)benzoin isobutyl ether (GIBE-CA) in a yield of 63 %. Its chemical structure was confirmed with NMR and IR spectral analyses. PTMG-Bound BAE Photoinitiator ( PTMG-BAE) : A mixture of 1.18g (4.0 mmol) of IEBME, 2.0 g (2.0 mmol) of PTMG and catalytic amounts of dibutyltin dilaurate without solvent was reacted for 5 hr at 50 C. Completion of the reaction was checked by disappearance of isocyanate absorption at 2250 cm-1. UV Irradiation of Photocurable Formulations: The photocurable formulations were made by mixing the photoinitiators and photosensitive oligomers and/or diluents acrylates in inert solvents or without using solvents. were controlled The contents of the initiators to be in a range of 1 to 4 equivalent % of all reactive acrylate groups. The formulations were spincoated on salt plates and after evaporation of the solvent the thickness of the coating was around 5,um. The thin film on a salt plate was irradiated by using a high pressure mercury lamp (500 W) with a light intensity of 45 mw/cm2 measured on a power meter with a probe of 365 nm. The extent photocuring was estimated in terms of DRUS by measuring the decrease in the intensity of IR absorption at 1405 cm-1 for the given exposure time. Acknowledgment : We acknowledge the support of Korea Ministry of Science and Technology on this photosensitive polymer project. We also appreciate Mr. Sung Bum Kim, Chulhee Kim, and Kyo Jin Ihn for their preliminary experimental contribution. REFERENCES 1. S.P. Pappas "Photoinitiation of Radical Polymerization" in "UV Curing: Science and Technology", S.P. Pappas, Ed., Chapter 1, Technology Marketing Corp., Norwalk, CT, USA, (1983), p S.A. Pappas, J. Radiat. Curing, 14 (July 1987), J.P. Fouassier, D.J. Lougnot, G. Li Bassi, and C. Nicora, Polymer Comm., 30 (1989), C.J. Schmidle for Thiokol Corp., US Patent 4,200,762 (1980). 5. K.-D. Ahn, 1.-C. Kwon, and 11.-S. Choe, IUPAC Polymer Symposium Preprints, Korea, Seoul, Korea (June 1989), p H.-G. Heine and H. Rudolph, Liebigs Ann. Chem., 754 (1971), K.-D. Ahn, K.J. Ihn, and I.C. Kwon, J. Macromol. Sci. -Chem., A23 (1986), 355. of The Polymer Society of 8. B.J. Hazzard, "Organicum", Addison-Wesley Publ. Co., Inc., Reading, Mass., USA, (1973), p