THE EFFECT OF MOLAR RATIO AND ph DURING SYNTHESIS ON THE PERFORMANCE OF PHENOL-MELAMINE-FORMALDEHYDE AVTAR SINGH SIDHU

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1 THE EFFECT OF MOLAR RATIO AND ph DURING SYNTHESIS ON THE PERFORMANCE OF PHENOL-MELAMINE-FORMALDEHYDE ADHESIVES by AVTAR SINGH SIDHU B.Sc, The University of British Columbia, 1988 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF REQUIREMENT FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES Department of Wood Science We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA April 1998 Avtar Singh Sidhu, 1998

2 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of The University of British Columbia Vancouver, Canada Date DE-6 (2788)

3 ABSTRACT In North America, almost all exterior grade plywood, particleboard, or flakeboard is made with phenol formaldehyde (PF) resins. Formulations are available which can satisfy a wide range of working and performance property demands of the users. PF resins suffer from a few disadvantages which include the distinct dark brown color in the cured glueline and the relatively high temperature (120 C at glueline) required for curing. Modification of PF resins with several different chemical groups has been tried to reduce or remove the drawbacks" associated with PF resin. One particular modification technique involves the addition of melamine into the PF resin system. Although modification using melamine has been carried out in Europe and in Japan, where the melamine is cheaper than phenol, no such attempt has been made in North America, where the price of melamine is higher than that of phenol. Even though melamine is an expensive chemical compared to phenol, its advantages may lie in the lower cure temperatures and shorter press cycles that are required during hot pressing operations. In this study, an array of phenol-melamine-formaldehyde (PMF) resins were synthesized by varying the formaldehyde/phenol and formaldehyde/melamine ratios at ph 7.5 and 9.0. Melamine formaldehyde (MF) and PF resins were also synthesized for comparison. The structure of all these resins, as well as commercial MF and commercial PF resins, was characterized using Fourier Transform Infrared (FTIR) spectroscopy, Proton Nuclear Magnetic Resonance ( 1 H-NMR) spectroscopy, Gel Permeation Chromatography (GPC) and Differential Scanning Calorimetry (DSC). The bond performance of these resins was evaluated by producing 3-ply plywood panels ii

4 and testing these panels for shear strength and wood failure under dry, wet and boiled conditions. Better bond performance was observed for PMF resins that were synthesized at ph 7.5 than the ones synthesized at ph 9.0. Very reasonable wood failure results were obtained for these resins and also for MF resins (synthesized and.commercial) at lower press temperatures(120 C) and lower press times (3 min.) compared with the PF resin. The existence of co-condensation in PMF resins was confirmed with the IR and NMR analysis. The majority of the co-condensation in resins prepared at ph 7.5 was by way of methylene bridges. The bond performance was attributed to the presence of melamine in the system and not to the level of coplymerization that occured between phenol and melamine. These resins and also the MF resins were of much lower molecular weight (<1,500) in comparison with the PF (>20,000) resins. DSC data of PMF resins cooked at ph 7.5 showed that these resins exhibit two exotherms, the first exotherm (150 C) corresponding to the condensation reactions which take place during curing while the second exotherm (220 C) possibly corresponding to the elimination of formaldehyde from the dimethylene ether links to form methylene cross-links. iii

5 TABLE OF CONTENTS Page ABSTRACT ii TABLE OF CONTENTS.' iv LIST OF FIGURES vii LIST OF TABLES xi ACKNOWLEDGEMENT xii DEDICATION ' xiii QUOTATION xiv 1. INTRODUCTION 1 2. LITERATURE REVIEW Resin Chemistry Melamine Formaldehyde Synthesis : Synthesis of Phenol Melamine Formaldehyde Resin Resin Curing Resin Characterization Instrumental Techniques Bond Performance METHODOLOGY Resins ' : Laboratory Resins Phenol Melamine Formaldehyde Resin Synthesis Phenol Formaldehyde Resin Synthesis Melamine Formaldehyde Resin Synthesis 38 iv

6 3.1.2 Commercial Resins Resin Analysis Fourier Transform Infrared Absorption Spectroscopy Differential Scanning Calorimetry Gel Permeation Chromatography Nuclear Magnetic Resonance Spectroscopy Wood Bonding Study Gluing Procedure Sample Preparation and Testing Results and Discussion Resin Synthesis : PMF and MF Resins PF Resins Resin Properties ' Fourier Transform Infrared Spectroscopy Synthesized and Commercial PF and MF resins PMF Resins Proton Nuclear Magnetic Resonance Spectroscopy Gel Permeation Chromatography Differential Scanning Calorimetry Wood Bonding Results Dry Specimens Soaked Specimens Boiled Specimens 117

7 4.3.4 Summary of Wood Bonding SUMMARY CONCLUSION LITERATURE CITED 127 APPENDIX I 132 APPENDIX II 135 vi

8 LIST OF FIGURES Figure 1. Resonance structures of melamine Page 7 2. Additional resonance structures of melamine 8 3. Resonance structures of melamine as a result of tautomerism 8 4. Reaction of melamine with formaldehyde Reaction mechanism for methylol melamine formation Proposed structure of phenol melamine formaldehyde resin Reaction products from the reaction between methylol phenol and methylol melamine Formation of methylene and ether linkages Formation of methylene linkage during the reaction of hexamethylolmelamine Structure of melamine formaldehyde resin as proposed by Koehlerand Frey (1943), Structure of 2,4,6-trimethylolmelamine at low ph Mechanism for the polymer growth at ph 6.3 according to Sato and Naito (1973) i Alternative mechanism for polymer growth,according to Sato and Naito (1973) 'H-NMR spectra of PFM, PF, MF, and PF+MF Models for the polymeric structures of the PMF resin Resin kettle for preparation of resins Gel Permeation Chromatography System Veneer Orientation in Panel Dimensions of the tension shear specimens 46 vii

9 20. "Sure Grip" Wedge Grips for tension shear test Divisioned plexiglass for wood failure determinations Hydrophobe solids and water tolerance profile for the PMF and MF resin cooks, IR spectra for (a) synthesized and (b) commercial PF resins IR spectra for (a) synthesized and (b) commercial MF resins 57 25a,b. IR spectra for PMF resins prepared at ph c. IR spectra for PMF resins prepared at ph IR spectra for PMF resins prepared at ph NMR spectra for synthesized MF resin NMR spectra for commercial MF resin NMR spectra for synthesized PF resin NMR spectra for commercial PF resin NMR spectra for synthesized MF resin 69 i 32. NMR spectra for synthesized MF resin after addition of D NMR spectra for Cook #1 PMF resin NMR spectra for Cook #2 PMF resin NMR spectra for Cook #3 PMF resin : NMR spectra for Cook #5 PMF resin NMR spectra for Cook #6 PMF resin GPC calibration curve using poystyrene standards (a) Molecular weight distribution and (b) GPC detector response for synthesized PF resin GPC spectra of synthesized and commercial PF resins GPC spectra of synthesized and commercial MF resins (a) Symmetric and (b) Asymmetric trimethylolomelamines GPC spectra of synthesized PMF resins prepared at ph viii

10 44. GPC spectra of synthesized PMF resins prepared at ph Typical DSC spectra for PF resins showing (a) a single peak and (b) two overlapping peaks The peak temperatures and the amount of heat evolved for (a) MF and (b) PF resins during different advancement stages of synthesis 93 47a,b. The peak temperatures and the amount of heat evolved for (a) Cook #1 and (b) Cook #2 during different advancement stages of synthesis 94 47c. The peak temperatures and the amount of heat evolved for Cook #3 during different advancement stages of synthesis 95 48a,b. The peak temperatures and the amount of heat evolved for Cook #5 and Cook #6 during different advancement stages of synthesis DSC spectra for (a) synthesized PF and (b) commercial PF resins 99 50, DSC spectra for (a) synthesized MF and (b) commercial MF resins a,b. DSC spectra for (a) Cook #1 and (b) Cook #2 resin c. DSC spectrum for Cook #3 resin a,b DSC spectra for (a) Cook #5 and (b) Cook #6 resins Pressure and temperature profiles for the 3 minute and 5 minute press times for panels pressed at 120 C Pressure and temperature profiles for the 3 minute and 5 minute press times for panels pressed at 150 C : Two-way interactions (resin x time and time x temperature) for shear strength of dry specimens Two-way interactions (resin x temperature) for wood failure of dry specimens : Two-way interactions (resin x time and resin x temperature) for shear strength of wet specimens Two-way interactions (resin x time and resin x temperature) for wood failure of wet specimens Two-way interactions (resin x time) and three-way interactions (resin x time x temperature) for shear strength of boiled specimens Two-way interactions (resin x time) and three-way interactions (resin x time x temperature) for wood failure of boiled specimens 119 ix

11 A1. Graph of the main effects of all resins from the ANOVA tables, (a) shear strength (b) wood failure 138 A2. Non-transformed average shear strength values for panels pressed at 120 C for 3 and 5 minutes ; 139 A3. Non-transformed average shear strength values for panels pressed at 150 C for 3 and 5 minutes 140 A4. Non-transformed average wood failure values for panels pressed at 120 C for 3 and 5 minutes 141 A5. Non-transformed average wood failure values for panels pressed at 150 C for 3 and 5 minutes 142 I

12 LIST OF TABLES Table Page 1. The selected mole ratios and ph conditions for PMF resins Reactants for the synthesis of PF resin Reactants for the synthesis of melamine formaldehyde resin Resin solids, viscosity and free formaldehyde for all synthesized resins Proportion of functional groups (relative to one another) present in all the resins ' Molecular Weights (Mn and Mw) and the polydispersity index for all resins Analysis of variance of shear strengths for dry specimens.: Analysis of variance of wood failure for dry specimens Analysis of variance of shear strengths for wet specimens Analysis of variance of wood failure for wet specimens Analysis of variance of shear strengths for boiled specimens Analysis of variance of wood failure for boiled specimens 110 A1. Non-transformed average shear strength values for panels pressed at 120 C for 3 and 5 minutes 133 A2. Non-transformed average wood failure values for panels pressed at 120 C for 3 and 5 minutes. 134 A3. Non-transformed average shear strength values for panels pressed at 150 C for 3 and 5 minutes 135 A3. Non-transformed average wood failure values for panels pressed at 150 C for 3 and 5 minutes 136 xi

13 Acknowledgements I would like to thank my late supervisor, Dr. Paul Steiner for giving me the opportunity to pursue my graduate studies while I was an employee in the Wood Science Department at the University of British Columbia. I also like to express my gratitude to Dr. Simon Ellis for offering me direction and guidance as well as kind criticism throughout the latter part of my M.Sc. program. A special note of thanks goes to Dr. David Barrett for allowing me to continue my M.Sc. program after Dr. Paul Steiner's death in May of I would also like to thank Dr. Frank Lam for his support and encouragement. I want to also acknowledge Dr. Laszlo Paszner, James T. White, Dr. Gary Troughton, Axel Anderson and Dr. Sergey Shevchenko for their help and their very insightful suggestions. I am very grateful to Martin Feng from Bordon Company Limited, Sammy Edwards from Melamine Chemicals Inc.,Tom Holloway from Neste Resins Corp., and Dr. Bunichiro Tomita for their very kind help in supplying the commercial resins. Most of all, I am very grateful to my wife, Karen, for her support and understanding without whom my graduate studies would not have been possible. xii

14 To my sons, Kevier and Karmvier nephew, Harvier and my niece, Sukhjote

15 "Poor indeed is the student who does not become better than his teacher." (Cited by Pizzi, in Advanced Wood Adhesives Technology, 1994)

16 1. INTRODUCTION Phenol formaldehyde (PF) and urea formaldehyde (UF) are resins with a long standing history remaining very important polymers among synthetic resins today. In the wood composite industry, UF resins have been used successfully for interior applications whereas PF resins have been utilized for both interior and exterior applications. Both resins offer several advantages to the consumer/user. One big advantage offered by PF resins is durability whereby wood composites bonded with them can withstand severe outdoor conditions over extended periods of time. UF resins offer faster curing rates and competitive pricing. However, together with these advantages, there are some inherent disadvantages for both resins. Wood composites manufactured with UF resin tend to delaminate when exposed to high moisture or high humidity environments. PF resins cure at a relatively high temperature and require long press times. Furthermore, the PF glue-line exhibits an unattractive dark color. To overcome some of these disadvantages, there has been an attempt in the last two decades to incorporate melamine in the PF and UF systems (Maylor,1995). Melamine copolymer resins (so called, melamine-urea-formaldehyde (MUF), melamine-ureaphenol-formaldehyde (PMUF) and phenol-melamine-formaldehyde (PMF) binders have been developed which give improved moisture resistance, low formaldehyde emissions and lighter colored glue-lines. Melamine (2,4,6-triamino-1,3,5-triazine) was investigated by Liebig as early as 1834 (Hodgins, 1941). It was almost forgotten, however, because no use was found for it until the development of the melamine resins around In 1936, Ciba A. G. in Basel, Switzerland, applied for a patent for a process producing melamine from calcium cyanamide (Widmer, 1965). This process made melamine comparatively inexpensive l

17 and readily available. Since then, the chemistry of melamine has been extensively studied. Initially, the discovery of a cheap melamine synthesis threatened the use of UF adhesives. Since melamine offers six active hydrogen atoms for reaction with formaldehyde, it yields better cross-linking and thus better water-resistant adhesives. Thus, several resin producers, CIBA, Heubel (BP 455, 000 by W. Hentisch and R. Koehler 1936) and others shifted production to melamine-formaldehyde (MF) resins (Widmer, 1965). Initially, because of limited production capacity and relatively higher cost, the melamine resins were only used to fortify existing urea resin products and left the bulk plywood and particleboard markets to UF resins only. In fact, MF resin has established itself as a wood adhesive in Europe and Japan, but even as of today, has not yet reached a comparatively great importance in North America (Maylor, 1995). The MF resin consumption in North America is driven by the surface coatings and laminate markets, which account for approximately 71% of MF resin demand while wood adhesives account for only 6% of MF resin demand. In Japan, the wood adhesives market accounts for 58% of the demand for MF resins (Gorbaty et al., 1994). Initially, improved heat and water resistance imparted to molded and laminated products were the principal features noted upon the addition of melamine to PF and UF adhesives. With the advent of World War II, military requirements gave considerable impetus to the development and production of melamine resin materials (Widmer, 1965). Generally speaking, the period from 1940 to 1950 was very productive in the growth of amino resins. MF resins have been used in many thermosetting resin applications such as molding resins; adhesives (mainly for plywood and furniture); laminating resins for counter, cabinet, and table tops; textile resins to impart crease resistance, stiffness, shrinkage control, water repellency and fire retardance; wet- 2

18 strength resins for paper. MF resins have also been used in alkyd resin preparations to give baking enamels such as for automotive finishes. MF and UF resins fall in the category of aminoresin adhesives or aminoplastic adhesives. These are important members of the thermosetting class of synthetic resin products made by combining an aldehyde with a compound containing an amino (-NH 2 ) group (Wohnsiedler, 1952). The global amino resin capacity reached 18 billion pounds in 1993 and UF resin accounts for over 80% of this amount; MF resin accounts for most of the rest (Gorbaty et al, 1994). Therefore, urea and melamine resins are the most prominent members of the amino resin class. The less prominent members of the amino resin class are based on thiourea, aniline, ethylene urea and the guanamines. The advantages of aminoresin adhesives are their "(1) initial water solubility which renders them eminently suitable for bulk and relatively inexpensive production, (2) hardness after curing, (3) nonflammability after curing, (4) good thermal properties, (5) absence of color in cured polymers and (6) easy adaptability to a variety of curing conditions" (Pizzi, 1983). Urea-and melamine-formaldehyde condensation products have very much in common in their chemical behavior, production of the intermediates during curing, capacity to be cured to high molecular-weight condensation polymers and properties of the final products (which can be used, to a wide extent, in the same type of applications) (Maylor, 1995). In the manufacture of melamine-ureaformaldehyde (MUF) resins, urea and melamine together are reacted with formaldehyde, which results in the formation of addition products, such as methylol compounds. Further reaction, and the concurrent elimination of water leads to the formation of low molecular weight condensates which are still soluble in water. Higher molecular weight products that are insoluble and infusible, are obtained by further 3

19 condensing the low molecular weight condensates and cross-linking. The main differences between these two amino resins are the better water and heat resistance, greater hardness, and the capacity to be cured more rapidly and under slightly basic conditions of melamine- compared to urea-formaldehyde resins. Some researchers believe that the disadvantage of these aminoplastic resins is their bond deterioration caused by water and moisture. This is a result of the hydrolysis of the aminoplastic or aminomethylenic bond (Pizzi, 1983). This seems to be true more for UF and MUF resins but not for the MF and PMF resins. The higher resistance of melamine-formaldehyde resins to moisture attack is due to the considerably lower solubility of melamine in water. It is important to note, that melamine dissolves in hot water only, whereas, urea dissolves in both hot and cold water. Therefore, ureaformaldehyde adhesives are used for interior applications only whereas MF and MUF resins can be employed successfully for rather severe outdoor conditions. Melamineformaldehyde adhesives have replaced urea-formaldehyde resins in wood gluing to only a certain extent. They produce high quality plywood because their adhesive joints are boil-proof (Pizzi, 1983). Phenol-modified melamine-urea-formaldehyde (PMUR) resins have been used for production of exterior grade particleboard and oriented strand board (OSB) in Germany and France (Clad and Schmidt-Hellerau, 1977). In the Asia-Pacific region, PMF resins have been used for the production of moisture resistant medium density fiberboard (MDF) (Maylor, 1995). In Japan, PMF resins, in combination with ureaformaldehyde resins, have been used for the manufacturing of concrete form plywood (Tamura etal., 1981). 4

20 Phenol-melamine-formaldehyde resins obtained by incorporating melamine in the PF resin system cure at a lower temperature than the PF resin alone, thus offering an advantage over phenolic resin itself from the aspect of saving energy in the production of plywood (Tamura, 1981). The copolymerization which occurs between the methylols of phenol and melamine might play a major part in the resin curing at lower temperatures and shorter press times. Aqueous phenol-melamine resins have good adhesion to softwood and resinous tropical hardwoods such as apitong. Many researchers uphold the good weather resistance of these recently developed phenol triazine resins. The PMF resins used in the manufacture of concrete form plywood have been traditionally made by first synthesizing PF and MF resins separately and then mixing the two in appropriate proportions prior to use. According to Higuchi et al. (1994), it is not possible to achieve copolymerization between melamine and phenol unless the resin is synthesized with both melamine and phenol in a single cook. While some work has been carried out by Higuchi et al. (1994) to elucidate the chemical structure of PMF resins, very little is known about the curing nature (i.e., speed of cure, temperature of cure) of these resins. There is also very little information available on how the bond performance is altered with change in formaldehyde/phenol and formaldehyde/melamine molar ratios; at what temperature the PMF resin can be optimally cured and what time frame is required to give optimal bond strength. The current study was undertaken to investigate the curing characteristics and bond performance of the phenol-melamine- formaldehyde resin systems. The first step involved synthesizing an array of PMF resins at several different molar ratios and under different ph conditions. Control resins (MF and PF resins) were also synthesized for comparison purposes. All resins were subsequently characterized by proton-nuclear 5

21 magnetic resonance ( 1 H-NMR) spectroscopy and fourier transform infrared (FTIR) spectrophotometry to identify the relative proportion of different functional groups present. The molecular weight distribution of each resin was determined by gel permeation chromatography (GPC) and the curing behavior of each adhesive was determined by differential scanning calorimetry (DSC). Finally, the bond performance of each resin was assessed by first bonding three veneers together (3-ply plywood) and then testing these panels for shear strength and wood failure after conditioning the samples at three different conditions: (1) The first set was conditioned to 50% relative humidity at 20 C. (2) The second set was vacuum-pressure soaked. (3) Finally, the third set was boiled, dried and then boiled again to evaluate the long-term performance of the bonded specimens. 6

22 2. LITERATURE REVIEW 2.1. Resin Chemistry In the field of organic chemistry melamine (2,4,6-triamino-1,3,5-triazine) enters into many reactions which are of considerable interest to the resin chemist. Hofmann (1874) first found that melamine produces resinous products when he was. working with tetra-phenylmelamine (Hodgins et. al., 1941). Chemically, the most important property of melamine is the capacity for combining with formaldehyde to give resins. To provide a background for this discussion, it is necessary to examine the structure of melamine more closely. Hughes (1941) made an intensive study of the structure of melamine in the crystalline state. According to this work the triazine ring is a resonance structure of delocalized electrons. Its resonance system extends to the lone pair of electrons of nitrogen atoms outside the ring (Figure 1). + H 2 N' Figure 1. Resonance structures of melamine. Other less symmetrical structures may also make contributions, one set being (Figure 2): 7

23 + +NH 2 NH 2 1 Jl Figure 2. Additional resonance structures of melamine. In view of the possibility of the occurrence of tautomerism in melamine, three additional structures have received consideration (Wohnsiedler, 1952) (Figure 3). NH 2 NH 2 I N NT if^ H Normal or amino form Diamino-imino form NH 2 NH NH HN^N^NJH H Amino-diimino form HIsrSsiH HN^N^NJH H Iso-form Figure 3. Resonance structures of melamine as a result of tautomerism. 8

24 Which structure best expresses the properties of melamine is a problem in itself. Studies using ultraviolet absorption spectroscopy have shown that melamine undergoes structural changes in acid versus neutral or alkaline solutions (Dixon et al., 1947). From the point of view of this study and the fact that the triazine ring is a very stable structure at high temperatures, the benzenoid structure is favored with probably the diamino-imino form existing as a second or exclusive structure in acid solutions. In neutral solutions, the melamine is believed to exist as the iso-form Melamine Formaldehyde Synthesis The basic primary reactions between melamine, urea, substituted melamine or substituted urea and formaldehyde are very similar (Pizzi, 1994). Formaldehyde addition to melamine occurs more easily and more completely than its addition to urea. Under a specified set of conditions the primary reaction (addition) results in the attachment of formaldehyde to the nitrogen of an amino group to form methylol compounds and these methylol compounds further undergo condensation reactions with the splitting off of water and/or formaldehyde to form higher molecular weight intermediates. Since each melamine molecule contains three amino groups, in each of which either or both of the hydrogen atoms may be substituted, it is clear that a good many methylol derivatives of melamine may appear. Methylol derivatives which may be produced range from monomethylol to the hexamethylol melamine (Figure 4). The nature of these products is determined by the conditions under which.the reactions between the formaldehyde and melamine take place. Conditions which are important are: the relative proportions of formaldehyde and melamii,e present, the ph of the aqueous formaldehyde, the temperature and finally the time of the reaction. 9

25 Figure 4. Reaction of melamine with formaldehyde. A typical reaction mechanism that takes place during the hydroxy-methylation reaction in alkaline media involves an attack of a nitrogen anion of melamine on the carbonyl carbon of formaldehyde (Figure 5). 10

26 Figure 5. Reaction mechanism for methylol melamine formation. Because melamine is less soluble in water than urea, the hydrophilic stage proceeds more rapidly in MF resin formation than in UF resin formation. Therefore, hydrophobic intermediates of the MF condensation appear early in the reaction. Another important difference between MF and UF is that the MF condensation and curing occurs not only under acid conditions but also under neutral or even slightly alkaline conditions (Pizzi, 1983). This characteristic is often an advantage. Curing capacity in a non-acid medium is a highly valued property in electrical applications and in applications in which corrosion is a problem. 11

27 Of the nine methylol products possible from the condensation of melamine and formaldehyde, the most stable and readily isolated is hexamethylolmelamine (Ho.dgins et al., 1941). This compound may be produced either by heating melamine with an excess of neutral formaldehyde to 90 C, or by allowing the melamine to react with neutral formaldehyde at room temperature over a period of 15 to 18 hours. Elemental analysis indicates that the product formed in both cases is the same and contains one molecule of water of crystallization per molecule of hexamethylolmelamine. Another methylol compound that has been prepared is a trimethylolmelamine (one methylol group on each amino group). This compound is obtained by reacting one mole of melamine with three moles of neutral or slightly alkaline formaldehyde at room temperature for 15 hours. The entire separation and purification of this material must be carried out with extreme rapidity and at very low temperatures to prevent further reaction. Elemental analysis shows that under these conditions the compound crystallizes from aqueous solutions with two molecules of water of crystallization. Because of this extreme reactivity,,the material has no definite melting point and is not stable on standing at room temperature. The progressive polymerization of the methylol melamine is very dependent upon the reaction ph. Small changes in acidity or alkalinity have greater effect on the reaction rate than in the case of the methylol ureas. Therefore, the control of the ph during the reaction of melamine with formaldehyde is of paramount importance. The technical grades of formaldehyde often contains % of formic acid. Even this small amount of formic acid cannot be allowed during resin synthesis due to the fact that it will often acts as a catalyst and renders the reaction uncontrollable. With poor ph 12

28 control, a soft gel like white precipitate will form which will quickly transform into a hard block of resin and is often difficult to remove from the reaction vessel. The optimum ph range for methylolation and further condensation to medium and high molecular weight resins is The temperature of the reaction should be well controlled in the range of C, otherwise, it may lead to viscous, hydrophobic MF resins with separation of the upper water layer. At ph 7, water-soluble, hydrophilic polymers are first formed. Under continued heating the resin solutions have less tolerance for dilution with water. In commercial practice, the reaction is carried only to the water-soluble stage and then inhibited by increasing the ph to about 10 and cooling. The solubility and viscosity of the reaction mixture can be repeatedly monitored by two methods, the water tolerance test or the hydrophobe test. The water tolerance is checked by adding water to a 10 ml sample of reactants until a definite cloud develops throughout the solution. The amount of water added to bring the solution to a cloud point is multiplied by 10 to give the water tolerance value (Ashland Chemical Co., standard quality control method). The hydrophobic solids are calculated in much the same way as is water tolerance. A 20 g sample of the reactants is weighed and to this sample water is added until a cloud point persists in the solution after shaking. The hydrophobe solids (H.S.) are calculated by the following formula (Gaylord, 1968). A*B H.S.(%) = - - A ~l~ A = weight of sample B = % theoretical solids C = milliliters of water required to reach cloud point. It is important to realize that as the molecular weight of the resin syrup increases, the water tolerance decreases and the hydrophobe solids increase. Therefore, by controlling the ph and viscosity, the melamine formaldehyde solution can be kept at the 13

29 desired condensation stage by arresting the reaction. Further condensation is prevented by cooling and by bringing the ph to the level which guarantees best stability as mentioned earlier. 2.3 Synthesis of Phenol-Melamine-Formaldehyde Resin. Preparation of synthetic resin from melamine and phenol can be carried out by either co-condensation, where melamine and phenol are mixed together and then reacted with formaldehyde, or by the process of combining two phenolic and melamine resins, each being prepared in separate cooks. When a mixture of melamine and phenol are condensed with formaldehyde, there are two kinds of "homopolymers" (phenolic and triazine type) existing in the three-dimensional structure of macromolecules of resulting products (Chen-Chun et. al., 1982) (Figure 6). Figure 6. Proposed structure of phenol melamine formaldehyde resin. 14

30 This reaction is usually carried out in the ph range of within a short time. When this reaction is carried out under alkaline conditions, the formaldehyde reacts with phenol and melamine separately to form various hydroxymethyl substitutes with different degrees of substitution. The reactivity of these substituted compounds is very high. Hydroxymethyl groups react not only with hydrogen atoms on the triazine ring and on the phenol ring, but also react with each other (Figure 7). OH Figure 7. Reaction products from the reaction between methylol phenol and methylol melamine. These prepolymers then undergo further dehydration. By means of methylene bonds and ether bonds, a three dimensional heteroaromatic complex structure is finally formed, as shown in Figure 6. 15

31 In the preparation of this resin, the phenol imparts a light brown to red-brown color to the resin and to any articles subsequently molded from it, as well as imparting a brown color in gluelines when used as an adhesive in plywood manufacture. Upon exposure to light or air this brown color has a tendency to deepen. Several methods of obtaining colorless melamine modified phenol formaldehyde resins have been proposed. According to British Patent No. 1,057,400 (Ibigawa Electric Industry Company Limited, 1963), the polycondensations are carried out at a ph value of 6.0 to 7.5. U.S. Patent No. 3,321,551 (Knutson, 1967) proposes a multistage process where the^polycondensation of a phenol formaldehyde precondensate with melamine is carried out at a ph value of 6.9 to 7.8. Also, in this process, the use of strong alkalis such as sodium hydroxide or potassium hydroxide is not permitted since they are believed to be responsible for producing brown-colored solutions due to the oxidation of free phenol. Instead, neutralizing agents (carbonates selected from the group consisting of the carbonates of calcium, barium and magnesium) are employed. In U.S. Patent No. 4,229,557 (Feinauer et. al.,1979), the production of melamine-phenolformaldehyde resins, which are substantially white and resistant to yellowing, is carried out by the polycondensation of melamine with phenol and formaldehyde in aqueous basic reaction medium at a ph value between 8 and 11. This process comprises adding to the reaction mixture one or more water-soluble ammonium or alkali phosphates or ammonium or alkali borates in an amount of at least 0.05% by weight, based on the reaction mixture or the dry resin, and subsequently isolating the resulting resin by dehydration at a temperature of at least 70 C or heating the dry resin to at least 70 C. The reason for using this particular ph range is to improve the storage life of the resin. 16

32 2.4. Resin Curing During the curing process, the prepolymers formed during the addition and condensation reactions are transformed to highly branched, three-dimensional structures. An increase in molecular size by linear reaction and cross-linking leads to formation of macromolecules and the hardened or cured products become insoluble and infusible. These hardening, wetting or curing reactions are most easily accomplished by the application of heat and either acid or alkali. The methylol melamines readily undergo condensations which transform them from small molecules, simple chemical entities to very large molecular polymers. This change is of great importance, as the desirable properties of the melamine resins, such as insolubility, infusibility and ease of film formation, are associated exclusively with the highly polymerized state (Powers, 1947). The most active participants in the condensation process appear to be -NH 2 groups because the more of these get substituted, the less methylol melamine condensation occurs. For example, hexamethylolmelamine is much more stable than monomethylolmelamine (Moncrieff, 1947). The actual reactions which take place during polymerization are complex since many different reactions occur simultaneously. The final resinous polymeric products are characterized by the possession of both methylene and methylene ether linkages, which are probably derived from reactions of the type shown in Figure 8. 17

33 NH 2 NH 2 N ^ N r ( N ^ N, H 2 N l^^^j NHCHJOH + HjNr+-l^NJl NHChfeOH H 2 N N ^ N N ^ N NHCH2NH NHCHfeOH Methylene Linkage NH 2 NH 2 "l jfx - A A H 2 N \ 4? NHCH 2 OH L N + H!OCH 2 Nrf 1 ^IST NH 2 H 2 N NHCH 2 OCH 2 HN Jl^^J NH 2 Methylene Ether Linkage Figure 8. Formation of methylene and methylene ether linkages. It must be realized that for all methylolmelamines containing one or more unreplaced hydrogen atoms, the mechanism can be readily formulated as going through the methylene linkage. However, with hexamethylolmelamine, it is necessary to split off formaldehyde in order to make such a linkage possible, as shown in Figure 9. In the polycondensation of melamine formaldehyde, Gams (1941) attached particular importance to the ether linkage and assigned a minor role to the methylene bridge as a polymer-forming linkage (Wohnsiedler, 1952). This conclusion was drawn after an 18

34 introductory study of the possible resinifying linkages in a hexamethylolmelamine and several other condensates. During this introductory study, it was realized that there were two linkages which may exist between molecules of methylolmelaminemethylene linkage and the methylene ether linkage. Figure 9 shows that with hexamethylolmelamine, it is necessary to split off formaldehyde in order to make such a linkage possible. In any other methylolmelamine which contains one or more unreplaced hydrogen atoms, the mechanism can be readily formulated as going through the methylene linkage. + n(3ch 2 0) Figure 9. Resinification through (1) Methylene and (2) Methylene Ether Linkages. 19

35 With such a multiplicity of functional groups and with at least two main reactions which are possible, very large molecules will soon be built-up. Moreover, these large polymeric molecules possess the desired properties of water and chemical resistance, heat stability and film-forming ability. Based on the work by Koehler and Frey (1943) a somewhat simplified possibility for the constitution of a melamine resin in the final cured stage is shown in Figure 10 (Wohnsiedler, 1952). Besides some unreacted methylol groups and methylene groups, the presence of many ether bridges is emphasized. This is because in curing melamine formaldehyde resins at temperatures of up to 100 C, no substantial amounts of formaldehyde are liberated. Only small quantities are liberated during curing up to 150 C. In contrast, urea formaldehyde resins cured under the same conditions liberate a great deal of formaldehyde. 20

36 CH 2 OH NH N JvJ._ JL X ^ Y 1 Nhf^N^NHCH 2 OCH 2 N ^ N N H N H C H 2 O C H 2 N H N H C H 2 - N ^ N V^J - N H y N ^ N H C H 2 N A N A N H C H 2 o C H 2 N H N S N N H C H ^ NHCH 2OCH 2 Nhr N N HCH 2OCH 2NH^ NY N / H N H C H 2 O C H 2 N H - N H C H 2 O C H 2 N H - Figure 10. Structure of melamine formaldehyde resin as proposed by Koehler and Frey (1943). The reaction mechanisms of the acid catalyzed condensation of methylolmelamine to form polymers and resins, have been elucidated by Sato and Naito (1973). As mentioned earlier, the melamine assumes different resonance ionic forms at different ph values. Methylolmelamines can readily be visualized as existing in similar charged or ionic forms. For example, at low ph values, a symmetrical trimethylolmelamine molecule should exist in the following ionic form (Figure 11): 21

37 HOCH 2 H NHCH 2 OH NH + CH 2 OH Figure 11. Structure of 2,4,6-trimethylolmelamine at low ph. It is believed that at ph values of approximately 6.3 some of the ions begin to form (Wohnsiedler, 1952). One of the possible mechanism for polymer growth is through the interaction of the ionic and molecular forms of trimethylolmelamine (Figure 12). The reaction takes place between a methylol group attached to an amine of one methylolmelamine and an amine hydrogen of another ring. With the formation of the dimer, a molecule of water is released. This process takes place to a major extent with charged and uncharged dimers coexisting. Coincident with the release of water, the triazine ring reverts to the aromatic structure. 22

38 N N HOHjCHN-C^ ^C-NHCHjOH K,N-C^ C-NHCrt,OH MH N^. ^N NHj NHCHjOH (HtScHjOHofMFj) (MCH 2 OHof MF 2 ) HO-v H HOHjCHN-Cj^ ^C-NH ^ <^N C^^C-NHC^OH.NH H H H N^ HCR.OH HOH/IHN-Cj^^C NHCHjNH C^^C NH CK.OH N^ NHCHjOH HOK^HN^-^^C NHCRjNH C^^C NHC^OH q < J.^N L ^N C tilhj NHCKjOH w + ^ Figure 12. Mechanism for the polymer growth at ph 6.3 according to Sato and Naito (1973). There are alternative mechanisms that are possible (Figure 13). With the participation of ether and azomethine-type structural mechanisms at least two other ionic mechanisms are possible (Wohnsiedler, 1952). 23

39 HOHjCHN-C-^ ~^C NHCH 2 OH HOHjCHN-Cj ^C-NHCH 2 OH C C + UH 2 + /1H 2 ( HMCHjOH of MF Z ) (HMCHjOH of MF 2 ) HOHfJHN-rjj ^C NH c - N <jj C~* N H C H 2 0 H N.,NH H H CH 7 N^,NH *~ H OH I' NK, + NH? HOH.CHN-C^^C NHCH 2 N ^C-NHC^OH II I I I I + HO N^^NH CH 2 N^^NH ^ Figure 13. Alternative mechanism for polymer growth according to Sato and Naito (1973). Phenol-melamine-formaldehyde curing behavior has been studied very extensively by Roh et. al. (1987a,b, 1989,1990,1991). In their early studies they used a two-step extraction with a 4% aqueous sodium hydroxide solution and formic acid to separate the three-dimensionally cross-linked fraction from soluble fractions. Reasonable relationships between cure time and the amount of the insoluble fraction, the degree of swelling and the nitrogen content were obtained. It was shown that the infrared and ultraviolet absorption spectroscopy methods, which were previously described for determining deqree of cure of phenolic resins, were not successful in PMF systems. Roh et al. (1987a), investigated the effects of ph and temperature on the curing behavior of PMF resin. It was found that under acidic conditions (ph ) the 24

40 condensation of melamine was dominant in the rapid-first-stage and the polymerization of phenol took place in the slower-second-stage of the curing reaction. Moreover, it was realized that when a long hot pressing time was applied to promote the slow stage in the curing of the PMF resin, the durability of plywood was greatly increased. Roh et al. (1989) determined, by varying the molar ratios of phenol to melamine (P/M), formaldehyde to melamine (F/M) and formaldehyde to phenol (F/P), that the cocondensation of phenol with cross-linked melamine occurs in the second stage of PMF synthesis. By the use of proton-nuclear magnetic resonance ( 1 H-NMR) spectroscopy, Roh et al. (1990) demonstrated that the formation of the insoluble fraction in PMF resin is caused mainly by the co-condensation of phenol with melamine and also that the curing of the PMF resin at ph 8.5 progresses mainly by way of the formation of a dimethylene ether linkage between phenol and melamine. Higuchi (1990) suggested that knowledge of the polymeric structure of adhesives is very important in understanding and predicting the performance of adhesives. Investigation of the curing behavior of phenol-melamine-formaldehyde as well as melamine formaldehyde resins was carried out in order to deduce polymeric structures of these resins. Even though, 1 H-NMR and 13 C-NMR techniques can be used to distinguish the bonds between phenolic nuclei and melamine residues, there are no means available to determine the amount of methylene and dimethylene ether bonds which might be formed between melamine and other components in cured resins. When PMF resins were cured under acid conditions, (ph 5.0), it was found that the condensation of the melamine fraction was predominate over that of the phenol fraction. Curing at ph 8.5, the rate of conversion of melamine fraction was about the 25

41 same as that of the phenol part. It was also deduced that the co-condensation between phenol and melamine occured at this latter ph. The 'H-NMR spectrum of the product of the reaction of methylolated phenol with melamine at ph 8.5 compared with that of a mixture of a PF resin and a MF resin was obtained as shown in Figure 14. The spectrum for PMF resin shows a peak at 4.3 ppm. This peak was assigned to methylene linkages formed between phenolic and the triazine nuclei ppm Figure H-NMR spectra of PFM, PF, MF, and PF+MF Note: PFM: Product of the reaction between methylolated phenol and melamine. PF: Phenol formaldehyde resin. MF: Melamine formaldehyde resin. PF+MF: Mixture of PF and MF (1:1). 26

42 In the course of curing at ph 10.0, the conversion of the phenol fraction preceded that of the melamine part. However, in the late stages of the curing course, the rate of conversion of the melamine part exceeded that of the phenol fraction. From this study Higuchi et al. (1990) proposed the following models for the polymeric structures of the PMF resin (Figure 15). M M M M I I M M M M (PH 5) -M P M P I M I I M "M P P M (ph 8.5) I -P M P M - P I P p p M P I M (ph 10) I Figure 15. Models for the proposed polymeric structures of the PMF resin. Note: P,M and denote a phenolic nucleus, a melamine residue and a methylene bond (or a dimethylene ether bond), respectively. The methylol groups are not illustrated Resin Characterization In order to gain a deeper understanding of the performance of wood adhesives, it is necessary to gain a wider knowledge of their polymeric structures. In the cured state, thermosetting polymers are generally insoluble, infusible materials which are difficult to examine by many analytical techniques. Many of the desirable physical 27

43 properties of adhesives are the result of their highly cross-linked structure (Ellis, 1989). In this regard, it is often desirable to determine the structure and concentration of each component formed when all reactants react under a specified set of conditions. The nature and distribution of molecular species formed during the preparation and subsequent thermal cure of PMF resins depend on reaction conditions such as F/M/P ratio, the nature and amount of catalyst present, and the reaction time and temperature. Full characterization of the resin is difficult because the complex distribution of molecular products formed are essentially insoluble, intractable materials (King et al., 1974). Nonetheless, several spectroscopic methods have been utilized for the elucidation of the chemical structure and to study the cure characteristics of these resins Instrumental Techniques Koeda (1954) originally demonstrated the chromatographic separation of monomeric methylol melamines of various degrees of substitution by means of paper chromatography. Braun and Legradic (1974a) used a silylation technique whereby the different methylo! compounds were separated and characterized by gel permeation chromatography (GPC). Silylation involved the modification of the reaction product of melamine and formaldehyde by treatment with N-diethyl-trimethylsilyl-amine to yield materials that were soluble in organic solvents, such as dimethyl formamide (DMF) or tetrahydrofuran (THF). Anderson et al. (1970) described the characterization of monomeric etherified methylolated melamines using GPC and 1 H-NMR spectroscopy. GPC analysis permits a rapid and accurate determination of the monomer and polymer 28

44 content of the products. 1 H-NMR spectroscopy allows the qualitative identification of the etherifying alcohols and a quantitative estimation of the alkoxy groups present. Chiavarini et al. (1976) presented a method based upon 1 H-NMR and chemical analysis by which they claimed it is possible to determine the content of methylol, methylene and oxymethylene formaldehyde, moreover non-substituted, monosubstituted and disubstituted amino groups. The MF condensates were examined in dimethyl sulfoxide d 6 /CaCI 2 solvent phase without any chemical pretreatment. Tomita and Ono (1979) used Fourier Transform 13 C-NMR Spectroscopy to characterize random chemical structures of melamine formaldehyde resins, including methylated melamine formaldehyde resins and urea melamine formaldehyde resins. Tomita and Matsuzaki (1985) used 13 C-NMR spectroscopy to analyze the chemical reactions between phenol or urea with melamine and formaldehyde and also to determine the conditions to produce their co-condensation. It was found that in the case of co-condensation between urea and phenol, the synthetic method to react methylolphenols with an excessive amount of urea itself under acidic conditions was desirable. In the case between melamine and phenol the co-condensation was achieved by reacting methylolphenols with methylolmelamines under a ph below neutral. Braun and Ritzert (1984) investigated the condensation reaction between phenol, melamine and formaldehyde at different conditions (ph, molar ratio of phenol and melamine) using gel permeation chromatography, 13 C-NMR spectroscopy and thermogravimetric techniques. It was found that the reactivity of phenol in the reaction with formaldehyde was insignificant at low ph values. At alkaline conditions this 29

45 reactivity increased very quickly. Melamine showed an inverse behavior. Braun and Ritzert (1984) also described a method to determine the curing of phenol-melamineformaldehyde condensate. The curing time was determined by estimation of free formaldehyde and by thermogravimetric analysis. The extractive separation of phenol (as well as cresol) and melamine was characterized by IR spectroscopy. Tomita and Matsuzaki (1985) used 13 C-NMR spectroscopy and GPC to analyze the chemical reactions of phenol-melamine-formaldehyde and phenol melamine urea systems and to determine the conditions necessary to produce their co-condensation. Roh et al. (1990) studied the curing behavior of phenol-melamine-formaldehyde resin using 1 H-NMR spectroscopy. It was deduced that at ph 8.5 co-condensation between phenol and melamine took place and also that curing of the phenol-melamineformaldehyde resin at ph 8.5 progresses mainly by way of the formation of a dimethylene ether linkage between phenol and melamine Bond Performance Bond strength depends in part upon the polymerization process as the adhesive is transformed from the liquid to solid state. When adhesive is first applied to wood it wets and interacts with the surface. The degree of adhesive penetration into the wood during the wetting and pressing process often influences strength development and stress distribution in the cured glueline (Pizzi, 1983). Too little adhesive penetration can result in wood-adhesive interface failure in service. Too much adhesive penetration gives starved gluelines during pressing and results in poor initial bond quality. Ideally, the penetration should be sufficient to reinforce the interface while providing strength to bond the wood elements. Generally speaking, a glueline should be just as strong and 30