Wood Adhesives proceedings of a Symposium Sponsored by. USDA Forest Service Forest Products Laboratory. and. The Forest Products Society

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1 Wood Adhesives 1995 proceedings of a Symposium Sponsored by USDA Forest Service Forest Products Laboratory and The Forest Products Society EDITORS Alfred W. Christiansen and Anthony H. Conner Proceedings No Forest Products Society 2801 Marshall Court Madison, WI phone: fax:

2 PREFACE The manuscripts in this book are based on presentations from the symposium Wood Adhesives 1995 held in Portland, Oregon, June 29-30, This is the sixth in a series of symposia organized by the Wood Adhesives Science and Technology Work Unit at the Forest Products Laboratory and held every five years. The symposia are dedicated to the exchange of information and ideas about research on wood adhesives, adhesion to wood, and new bonded wood products. We hope that these proceedings will further stimulate the exchange of needs, ideas, and information on research and development of wood adhesives among researchers, producers, and consumers, The symposium was organized into sessions: The Customer in Global Markets, Surface Chemistry and Modifications for Enhanced Adhesion, Greening of Bonded Wood Products VOCs, and New Developments in Conventional and Renewable Adhesive Systems. In addition to the oral presentations devoted to these topics, a number of presentations were given in a poster format. Both the oral and the poster presentations have been included in these proceedings. With the exception of the papers from the first session, because the subjects did not lend themselves to independent assessment, the papers were reviewed before acceptance into the proceedings. The Planning Committee thanks all the authors for their work in preparing their oral presentations and posters; the success of the symposium was due to their thorough preparations and excellent presentations. We also greatly appreciate the scientists who reviewed the papers, thus providing guidance in achieving the quality seen in this book. We thank Art Brauner, Susan Rutter, Julie Lang, Ann Messing, and Doris Robertson of the Forest Products Society for making the site arrangements, and Susan Stamm for final editing and preparation of this typeset copy of the proceedings, We thank the Session Chairs who guided these sessions: Michael Hoag, Alan L, Lambuth (now deceased), Richard W. Hemingway, and Cynthia D. West. PLANNING COMMITTEE Anthony H. Conner, Co-Chair AIfred W. Christiansen, Co-Chair Melissa G.D. Baumann Douglas J. Gardner Lawrence Gollob Chung-Yun Hse Paul M. Smith Henry N. Spelter Charles B. Vick

3 Modification of Urea-Formaldehyde Resin Adhesives: A Computational Study Quan Tang, Thomas Elder, and Anthony H. Conner Abstract In studies on the reduction of formaldehyde emission from wood products bonded with urea-formaldehyde (UF) resins, the polymer has been modified by the incorporation of furfural, furfuryl alcohol and 2,5-furandimethanol in lieu of formaldehyde. Presumably these compounds are chemically similar to formaldehyde in their reactivity toward urea but have substantially lower vapor pressures. Therefore, these compounds can be used as a partial or total replacement for formaldehyde, potentially lowering formaldehyde emissions. The objective of the current study is to examine the structures and initial reactions of the urea-formaldehyde, urea-furfural, urea-furfuryl alcohol, and urea-2,5-furandimethanol polymerization, by the application of computational chemical methods. Background Urea-formaldehyde (UF) resins are thermosetting materials used primarily as adhesives in plywood, fiberboard, particleboard, and furniture manufacture. As a wood adhesive, urea-formaldehyde resins are advantageous because they are inexpensive, have good processing and curing properties, and are resistant to fungi and termites (9). Greater utilization of urea-formaldehyde resins is impeded, however, because of the emission of formaldehyde which may result in potential health hazards and environmental pollution (4). It is believed that formaldehyde emission is due, to a considerable degree, to the free formaldehyde formed during condensation of the resin and to hydrolytic degradation of UF polymer, especially under warm conditions, high humidity, and high acidity (12). Ongoing research is directed toward the prevention or reduction of formaldehyde emission, either by chemical or physical methods (2,6-9,11,13,14). The most common and best proven method for reducing emissions from UF resins consists of using low F/U molar ratios (7, 10) during formulation of the resin. The current project proposes to reduce formaldehyde emission by the replacement of formaldehyde with compounds which modify the UF polymers. Such alternatives include furfural, furfuryl alcohol, and 2,5-furandimethanol which presumably are chemically similar to formaldehyde in their reactivity toward urea but have substantially lower vapor pressures than formaldehyde (19). In this paper, computational methods will be used to examine the structures and initial steps in the formation of urea-formaldehyde, urea-furfural, urea-furfuryl alcohol, and urea-2,5-furandimethanol polymers. Basic Chemistry The reaction between urea and formaldehyde is very complex, resulting in linear polymers, branched polymers, and tridimensional networks, in the cured resin (9). In the first stage of UF resin formation, methylolation, formaldehyde reacts with urea under neutral or slightly alkaline conditions (ph 7-8) to produce mono-, di-, tri-, and tetramethylol ureas (19). Because of problems associated with formaldehyde emission, it is now common practice to use resins with lowered levels of formaldehyde. Therefore, only monomethylolurea and small amounts of dimethylolurea will be formed at this stage. Modifications of UF resin with furfural and furfuryl alcohol have long been the subjects of studies (1,5,15,19), but no reports have been found on the details of these reactions. However, due to the aldehydic character of furfural and the alcoholic characters of furfuryl alcohol and 2,5-furandimethanol, the reactions of these three compounds with urea are assumed to be similar to those occurring between urea and formaldehyde. Calculations All calculations have been performed using SYBYL, Version 6.1, under license from Tripes Associates*, installed on a Silicon Graphics-Indigo Workstation, housed in the School of Forestry at Auburn University All structures were developed with SYBYL using default bond lengths, angles, and dihedral angles. These structures were pre-optimized using the Tripes 5.2 forcefield (3) and conjugate gradient minimization. The resultant structures were next submitted for molecular orbital calculations using MOPAC (17). The PM3 Hamiltonian Quan Tang, Graduate Research Assistant, Thomas Elder, Associate Professor, School of Forestry, Auburn University, Auburn, Alabama, and Anthony H. Conner, Project Leader, USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin. * The use of trade names is for information only and is not intended to be an endorsement by the USDA Forest Products Laboratory of any product or service to the exclusion of others which maybe suitable. 235

4 and complete geometry optimization, with the MMOK keyword which accounts for the known underestimation of rotational barriers about peptide bonds and gives flat nitrogens (16), were used. Results and Discussion Urea-Formaldehyde Resins Urea The atomic charges and heat of formation calculated for urea are shown in Figure 1. It can be seen that the hydrogens differ with respect to their atomic charges, leading one to ask if these differences might be translated into differential reactivity and subsequently to changes in the overall polymer structure. The observed differences might, however, be obviated if the H-N-C-O bonds were capable of free rotation. This question was evaluated by performing a reaction path calculation in which one of the H-C-N-O bonds was changed in 30 increments through 360, while allowing the rest of the structure to optimize. The results from the reaction path calculations are illustrated in Figure 2. From the data in Figure 2, a rotational barrier of approximately 20 kcal/mole is observed, corresponding to reported experimental values of 13.5 to 20 kcal/mole (18). Furthermore, cyclical variations are observed in the charges on the hydrogens (Figure 2), supporting the idea that the hydrogens are non-equivalent and should, therefore, be treated as separate reaction sites, Formaldehyde The heat of formation and atomic charges of formaldehyde are as shown in Figure 1. Formation of monomethylolurea The results from the overall reaction of urea and formaldehyde to form monomethylolurea are as indicated in Figure 3. The reaction at either hydrogen of urea is exothermic, with the substitution at position 2 giving a slightly more stable product (V). While the larger partial positive charge at hydrogen-1 of urea might be interpreted as indicating greater reactivity, the sterically preferred reaction at position 2 appears to offer a slight thermodynamic advantage. Urea-Furfural Resins Furfural The structure, heat of formation, and atomic charges of furfural are as reported in Figure 1. Formation of monofurfurylohrea Figure 3 shows the results calculated by PM3-MMOK with full geometry optimization for the formation of monofurfurylolurea (equations 2 and 6). As was the case with formaldehyde, furfural may react with urea at either Figure 1. Results-from semi-empirical PM3 calculations for reactants. 236

5 hydrogen, forming monofurfurylolurea (II and VI). As shown in the Figure, both reactions are exothermic, with reaction at hydrogen-2 slightly favored over position 1. These results are consistent with those for the monomethylolureas, and the differential stability is probably due to steric considerations. Furthermore, the stability increase upon reaction with furfural is less than that observed with formaldehyde, again probably due to the larger steric bulk of furfural. Urea-Furfuryl Alcohol Resins Furfuryl alcohol Figure 1 also shows the results from molecular orbital calculations for furfuryl alcohol (2-furanmethanol). Figure 2. Energy and atomic charges of urea vs torsional angles. 237

6 results for formaldehyde and furfural indicating lowered stabilization with reaction. Urea-2,5-Furandimethanol Resins 2,5-furandimethanol Lastly Figure 1 shows the results for 2,5-furandi- methanol. This compound differs from furfuryl alcohol by the presence of a second methanol group on the furan ring. Formation of monofurfurylurea Furfuryl alcohol, in contrast to formaldehyde and furfural, will react with one molecule of urea resulting in monofurfurylurea and water in an exothermic reaction (Figure 3 equations 3 and 7). Again, furfuryl alcohol preferentially attacks urea through H(2) forming a more stable product (VII) which is sterically preferred over H(1) of urea (III). While consistently exothermic, the overall heat of reaction continues to decrease, from the Figure 3. Formation of monomethylolureu, monofurylolurea, monofurfurylureu, and 5-methanol-monofurfurylurea. 238

7 Formation of 2-methanol-monofurfurylurea As was the case with furfuryl alcohol, 2,5-furandimethanol reacts with one molecule of urea through one of the two hydroxyl groups, leading to 5-methanol-monofurfurylurea and water (Figure 3, equations 4 and 8). Again, substitution of furfuryl alcohol at H(2) of urea is favored, resulting in a product (VIII) which is kcal/mole more stable than that (IV) formed through H(1) of urea. The overall heats of reaction again indicate that both reactions are exothermic, and the trend, in terms of relative heats of reaction, show a further reduction in stabilization when compared to the previous reactions. As shown in Figure 3, the heats of reactions of the urea with formaldehyde, furfural, furfuryl alcohol, and 2,5-furandimethanol are all negative, indicating that these reactions are thermodynamically favorable at this point of polymerization. Reactions at H(2) of urea are uniformly more exothermic than at H(1) of urea. As indicated by the heats of reaction, the reaction of urea with formaldehyde results in the largest increase in stability, followed by furfural, furfuryl alcohol, and 2,5- furandimethanol. This trend is probably due to the steric hindrance that arises from the increasing bulk of the reactants. Conclusions Based on these results, the following conclusions can be drawn: 1. The hydrogens of urea differ from each other in terms of chemical reactivity 2. The reactions of formaldehyde, furfural, furfuryl alcohol, and 2,5-furandimethanol with urea are exothermic and therefore thermodynamically favorable. 3. It has been found that it is preferable for the reactions to occur through H(2) rather than H(1) of urea since it results in a more stable product. 239