DURABLE WOOD ADHESIVES BASED ON CARBOHYDRATES A. W. CHRISTIANSEN Forest Products Laboratory One Gifford Pinchot Drive Madison, WI 53705

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1 DURABLE WOOD ADHESIVES BASED ON CARBOHYDRATES A. W. CHRISTIANSEN Forest Products Laboratory One Gifford Pinchot Drive Madison, WI Introduction Adhesives presently used for exterior-type, wood-based products depend upon petroleum for their starting materials. With the increasing energy needs of an expanding population and the inevitable decline in oil and gas supplies in the long term, the need for alternative adhesive systems from renewable resources is self-evident. The purpose of this investigation was to explore the potential of carbohydrates as constituents in water-resistant adhesives. Many renewable materials have been investigated over past years for constituents usable in adhesives, but none of the systems developed is suitable for exterior exposure. The largest volume of renewable material is from plants. These plants produce lignocellulosic material that becomes partially available as wastes or residues from food or wood processing. Although there is a wide variety of chemicals that are produced by plants, their basic chemical constituents are hemicellulose, cellulose, lignin, and extractives. From among these, we chose the carbohydrates or sugars from the hemicellulose and cellulose portions to evaluate as wood-adhesive constituents, Their basic chemical structure is well known, and a wealth of information is available about their reactions and reaction mechanisms. Although carbohydrates contain large numbers of hydroxyl groups that would possibly lend water sensitivity to reaction products, I felt that a polymerization system might be found to overcome this effect. Background Carbohydrates have been investigated in the past as coreactants with phenolic resins and as the sole ingredient in the adhesive. Meigs (17-19) carried out some of the earliest work with phenol-carbohydrate combinations. He was searching for a process to produce solid, fusible, thermoset molding compounds. The reactions used both basic and acid catalysis and often included reactions with aniline or aliphatic amines. In other examples, Chang and Kononenko (5) developed an adhesive system, claimed to be suitable for exterior plywood applications, by reacting sucrose in a phenolformaldehyde resin preparation under alkaline conditions. More recently Gibbons and Wondolowski (11) reacted carbohydrates with phenol and urea, or a diamine, in the presence of an acid catalyst. The fusible resin, which was used as a molding compound, was cured to the thermoset stage by heating with hexamethylenetetramine. Later, Gibbons and Wondolowski (12) produced a soluble, liquid resin in which the carbohydrate-urea-phenol reaction product was neutralized, reacted further with formaldehyde, and formulated as an alkaline system for potential use as a plywood adhesive. Other investigators have used acidic conditions only. Mudde (20) described a system that depended upon the acidic conversion of starch to 5-hydroxymethyl-2-furaldehyde (HMF) for condensing with phenol in a novolac resin. Turner et al. (28) investigated carbohydrates, not involving phenol as a reactant, and produced a water-resistant adhesive. Turner et al. degraded pentose or hexose sugars by acids, the products reacting with such materials as formaldehyde, furfuryl alcohol, polyvinyl alcohol, or amines to produce an adhesive suitable for particleboard production. As another example, Stofko (25-27) proposed systems that used a variety of carbohydrate sources and reactions under acidic conditions with different modifiers to yield adhesives that produced water resistant bondlines. All the adhesives I investigated depended upon the degradation of carbohydrates under acid catalysis. Resin formation takes place between the degradation products themselves, or with intermediates formed during the degradation, or between degradation products and other monomers such as phenol and urea. Taking glucose as an example of a carbohydrate which can form a resin, the following mechanism, proposed by Anet (3) and modified by Feather and Harris (8), illustrates the mechanism for its degradation to 5-(hydroxymethyl)-2-furaldehyde (Structure IV): In: Christianse, Alfred W.; Gillespie, Robert; Myers, George E.; River, Bryan H., eds. Wood adheeivee in 1985: Status and needs. 211 Proceedings of a conference; 1985 May 14-16; Madison, WI. Madison, WI: Forest Products Rasearch Society; 1936:

2 Scheme I 1 Glucose, after intermolecular rearrangement to form the 1,2-enediol (I) loses 3 moles of water successively to finally form the aldehydesubstituted furan ring of HMF (IV). The fact that resins or polymers can be formed from carbohydrates under acidic conditions has been attributed to the formation of 5-(hydroxymethyl)- 2-furaldehyde (IV) from hexoses, or 2-furaldehyde (furfural) from pentoses. These compounds form resins quite readily under acid conditions through the loss of water by a series of complex reactions which have not been clarified. Consequently, the process of forming resins from sugars may involve the loss of large amounts of water. Approach After various adhesive systems described in the literature (26,28) were evaluated in a preliminary survey phase of this investigation, I selected the carbohydrate-phenol-ureaformaldehyde system (12) for more detailed investigation, This system l Glucose exists predominantly as a glucopyranose ring structure. The open chain structure is shown here for simplification. was studied to test its potential for producing strong, durable wood bonds and to provide understanding to help control the reactions and possibly reduce the amount of phenol required for resin synthesis. As this was an exploratory investigation of resin systems, there were few replications and no effort to optimize reaction or bonding conditions. The procedures used were selected to evaluate small quantities of starting materials. Consequently, small two-ply parallel-grain panels were bonded, and specimen size was reduced to a minimum consistent with the collection of reliable data. Yellow birch was selected for the veneer, because its uniform grain structure and high shear strength relative to that of softwoods means that the adhesive is being more directly tested. Both 2-hour boil and vacuum pressure-soak exposures were selected to provide a measure of water resistance and an indication of any undercure during bonding. Carbohydrates were chosen to cover a range of sugar (saccharides) starting materials: glucose, an aldohexose (6-carbon aldehyde sugar); fructose, an isomer of glucose and a ketohexose (6-carbon ketone sugar); xylose, an aldopentose (a 5-carbon aldehyde sugar) and a major component of hardwood hemicelluloses; sucrose, a disaccharide (two-sugar molecule) composed of equal parts of glucose and fructose; corn syrup, a partially hydrolyzed polysaccharide of glucose units; and methyl glucoside, the ether of glucose in which a methyoxy group replaces the hydroxy group on the anomeric carbon (C1) of glucose. The number of moles of carbohydrate was calculated from the basic monosaccharide that would be produced by complete hydrolysis of the di- or polysaccharides used. The mole ratios of phenol-to-carbohydrate-to-urea-toformaldehyde were varied over rather broad limits to explore the range of reactant concentrations that might prove useful. For most experimental adhesives, panels were prepared using several combinations of bonding time and temperature to estimate pressing conditions that would yield well-bonded panels İn another phase of the work, samples taken during syntheses were analyzed to try to determine the chemical nature of the reaction products, to give some insight to what reactions might be occurring. 212

3 Part I. Adhesive Formulation and Bonding Experiments Materials The carbohydrates and other chemicals used in the syntheses included a-d-glucose (2% β -anomer), D-xylose (approximately 98%), corn syrup (82.8% nonvolatile), D-fructose (98%), methyl glucoside (a mixture of α and β -anomers), sucrose (commercially purified granulated sugar), phenol (reagent grade), urea, formaldehyde (37%, with 5% methanol), sulfuric acid (5.0 N), and calcium hydroxide. The phenol-formaldehyde resin used as a control adhesive was a commercial resin reported by the manufacturer to have 43.1 ± 0.1 percent nonvolatile, a ph of 12.2 ± 0.3, a viscosity of Pascalsecond ( cp) at 25 C, and a specific gravity of ± at 25 C. The wood adherends were clear, flat-grained, rotary-peeled yellow birch veneer with moisture content equilibrated at 27 C (80 F) and 30-percent relative humidity (about 6% moisture on an ovendry basis). The pieces were 3.2 mm (1/8 in) thick and 150 mm by 150 mm (6 in x 6 in) in area. Methods Resin synthesis procedure. The carbohydrate, phenol, and urea were placed-in a 0.5-liter resin kettle fitted with a thermometer, a stirrer, and a side-arm condenser. The mixture was heated to 90 C while stirring, and the sulfuric acid catalyst was added. In most cases, the amount of sulfuric acid used to degrade the carbohydrate was held constant at 40 milliequivalents (meq) per mole of monosaccharide. The temperature was allowed to rise as high as 150 C during 4 hours or until the desired amount of condensate had been collected (e.g., 64 ml condensate per mole of glucose was often the goal). Heating was discontinued until the temperature fell to 90 C at which time the collected condensate was returned to the reaction mixture. The ph was raised to near 7 by the addition of 6 meq of finely powdered calcium hydroxide per meq of sulfuric acid. After the temperature had fallen to 70 C, the desired amount of formalin was added and the mixture was again heated with stirring for 1 hour at 80 C. After cooling to room temperature, the ph and viscosity were measured. The viscosity was adjusted to a minimum of 0.3 Pa s (300 cp) by further heating at 80 C if necessary. A typical resin with molar ratios of phenol to carbohydrate to urea to formaldehyde (P/C/U/F) of 1/1/0.5/2.5 might use 18 (weight) percent phenol, 34 percent glucose, 6 percent urea, 2 percent 5N sulfuric acid, 2 percent calcium hydroxide, and 38 percent of formalin (itself a 37 pet aqueous solution). Adhesive formulation. Adhesive formulation consisted of adjusting the ph to between 10 and 12 and-the viscosity to between 0.5 and 2.5 Pa s (500 and 2500 cp). The PH adjustment was made with sodium hydroxide (6.6% of liquid resin weight) and sodium carbonate (2.2% of liquid resin weight). The viscosity adjustment was made by further heating at 80 C or by water dilution, whichever need was indicated. Panel preparation. Within 2 days of formulation, generally, each adhesive was brushed on each of the mating veneer surfaces at a rate of kg/m 2 ( lb/thousand square feet of single glueline, or lb/msgl). After a 15-minute open assembly time, the panels were assembled with their grain direction parallel. After a 10-minute closed assembly time, the panels were pressed at 1.21 MPa (175 psi) for either 5 or 10 minutes at a selected temperature between 145 and 180 C. At least one 2-ply panel was prepared with the phenolic resin as a control during each experiment. Specimen preparation and testing. Each panel yielded 14 specimens 62 mm (2.4 in) long in the grain direction and 18 mm (0.71 in) wide. Prior to cutting out the specimens, saw kerfs were cut to the bondline across both faces of the panels to produce an overlap region 10 mm (0.39 in) long in the center of each specimen, Figure 1. This provided a specimen shear test area of 180 mm 2 (0.280 in 2 ). For each bonded panel, specimens were randomly assigned to three conditioning groups of four specimens each, usually, leaving two as possible replacements. One group was conditioned at 27 C (80 F), 30-percent relative humidity for at least 16 hours before being tested for shear strength as the dry specimens. A second group was conditioned as above and then subjected to a vacuum-pressure-soak (VPS) 213

4 cycle. This consisted of one-half hour submersion in tap water under a vacuum of 740 mm (29 in) of mercury followed by one-half hour pressure of 0.41 MPa (60 psi). These specimens were tested for shear strength in the wet condition. The third group was conditioned at 27 C (80 F) and 30-percent relative humidity before submerging in water, boiling for 2 hours, cooling in water, and testing for shear strength in the wet condition. A substantial increase of strength for this treatment over the VPS treatment would be taken as an indication of undercure during bonding. There were only a few experiments in which replicate panels were pressed and assembled under the same conditions for one resin. From these, several specimens from each panel were tested after conditioning at the various regimes. In general, the panel-topanel variation was larger than the within-panel variation in these few experiments, Therefore, for the great majority of trial resins presented (for which there is no estimate of panel-to-panel variation) statistical tests on the data would not be meaningful, Results and Discussion Strength values for the best formulations for the glucose- and xylose-based adhesives showed which press conditions were sufficient to confidently reach the plateau of maximum strength, for dry and wet tests. For the glucose-based adhesives these conditions were found to be 5 minutes at 155 C, and for the xylose-based adhesives to be 5 minutes at 160 to 170 C. Strength values in Tables 1-4 are for panels that were bonded at these sufficient press conditions, where possible. Mean values for shear strengths (dry and 2-hour boiled) of panels bonded with trial resins at the various time and temperature combinations are collected in the Appendix. Results of bonding experiments using adhesives from glucose-based resins, as described under Methods, are shown in Table 1. These results can be compared with those from panels bonded with the commercial control phenolic adhesive; control values are the mean shear strength from 23 different panels produced at different times. The shear strength results from the glucose-based adhesives varied widely, ranging from values equal to those for the phenolic specimens to no strength at all in the wet condition. As the mole ratio of phenol to carbohydrate (P/C) was reduced from 1/1 to 0.5/1 both dry and wet shear strengths fell drastically, although the change made in other variables obscured the certainty of this effect. The most encouraging results were obtained with P/C/U/F mole ratios of 1.0/1.0/0.25/3.6 and removal of the volume equivalent of 3.6 moles of water as condensate of reaction (experiment 3); the shear strength results matched those for the phenolic-bonded specimens. One attempt to synthesize a resin without urea resulted in a resin which was extremely viscous by the end of the acidic stage. On cooling, this resin hardened to a solid, glassy mass. The shear strength results with carbohydrates that yield hexose sugars, such as glucose or fructose, upon hydrolysis are shown in Table 2. In most cases, these gave high shear strengths, often comparable to strengths of the phenolic-bonded specimens. The fructose-based adhesive yielded high dry- and wet-shear values. Methyl glucoside, corn syrup, and sucrose required hydrolysis to monosaccharides prior to reaction with urea and phenol. The hydrolysis required much additional time and energy to carry out, in contrast to the use of simple hexose sugars. The panels bonded with adhesives based upon xylose (a pentose sugar) also had high dry and wet shear strengths (Table 3). The best results were obtained in experiment 16, which involved a P/C/U/F ratio of l/1/0.5/ 2.5 with only 2.4 moles of water per mole of fructose eliminated during the dehydration reaction. Experiment 17 demonstrated that the resin could be stored for at least a week following synthesis and still provide shear properties equivalent to those obtained when first formulated. The lower strength values for experiment 17 compared with experiment 16 are attributed to lower formaldehyde use. One synthesis was attempted using a carbohydrate which cannot be hydrolyzed to an aldose or ketose sugar. Sorbitol (or D-glucitol), a sugar alcohol, exists only in a noncyclic form and has no carbonyl character. Sorbitol, in an acid-catalyzed reaction mixture with urea and phenol, showed only minimal dehydration. In syntheses with aldose- and ketosebased carbohydrates, it was necessary to produce two moles of product water per mole of monosaccharide to yield resins with good bond strength. 214

5 Sugar alcohols were not investigated The amount of formaldehyde required to produce high shear strength appears to be at least 2.O moles per mole of phenol. In the tables the formaldehyde content is shown as based upon the moles of monosaccharide used. When the P/C ratio was 1/1, the F/P ratio was the same as the F/C ratio. In the case of experiments 4-9 (Table 1) a calculated F/P ratio would in no case be less than 2/1. effect of formaldehyde-to-phenol ratio with glucose as the carbohydrate is shown in Figure 2. The highest wet shear strength values were reached only after the formaldehyde content had been increased to 2.0 moles per mole of phenol (Table 4). Wood failure was estimated in some cases with both the phenolic and the carbohydrate-based adhesives, But because of the parallel-grain laminated hardwood and the very small area of bond being tested, high variability among specimen replicates characterized the wood failure estimates, The only conclusion that seemed reasonable was that the experimental adhesive could develop wood-tearing bonds similar to those produced by a conventional phenolic adhesive. Savings resulting from phenol replacement are difficult to calculate with precision because of the everchanging formulations used in the industry and the uncertainties about the actual reactions that take place during resin formation and adhesive cure. Assumptions need to be made such as the elimination of a mole of water for every mole of formaldehyde used, the formation of sodium phenate to the full extent of phenol content, and equivalent spread rates and waste factors. Calculations based upon resin formation according to Redfern (23) and adhesive formulation for Douglas-fir according to Jarvi (15) resulted in a value of 50 pounds of phenol required for 100 pounds of cured adhesive solids in a conventional phenolic resin. For resin 12 (Table 2), which had a P/C/U/F molar ratio of 1/1/0.5/2.54, there would be a considerable saving in phenol for a softwood plywood application. It required only 26.2 pounds of phenol to produce 100 pounds (unfilled) of adhesive solids. This would be a savings of about 48 percent of the phenol required for softwood plywood production. Some of this savings would be required to pay for the urea and carbohydrate used. Adhesive formulations for southern pine, such as reported by Sellers (24), use extenders and fillers to replace some of the resin solids. For resin 12, addition of 10 parts of walnut shell flour to 100 parts of formulated adhesive did not reduce bond strengths below those of filled or unfilled control phenolic resins. The dry wood failure values were increased to near 100 percent, but the VPS and 2-hour boil wood failures were low compared to those for phenolic controls. These bonding experiments demonstrated that a variety of different carbohydrates could be reacted with phenol, urea, and formaldehyde to produce wood bonds that, when tested wet or dry, were within the range provided by the conventional phenolic adhesive. In general, it appeared that at least two moles of water per mole of monosaccharide needed to be eliminated during the dehydration reaction for high shear strength to develop. For exterior durability, at least two moles of aldehyde per mole of phenol are needed during the neutral synthesis stage. Part 2. Reaction Mechanisms The reaction mechanisms involved in the formation of the trial resins and the cured-adhesive solids described above are at best highly speculative. If only the carbohydrate provided the material for resin formation, the mechanism (Scheme I) proposed by Anet (3) would appear to have some applicability. When starting from glucose under acid catalysis, three dehydration steps result in the formation of 5-(hydroxymethyl)-2- furaldehyde (IV). But following this, rearrangements and the addition of 2 moles of water could result ultimately in the formation of formic acid (V) and levulinic acid (VI), in a mechanism such as that of Scheme II, adapted from Feather and Harris (8). After reaction has been in progress for awhile, therefore, the reaction mixture might easily contain a wide variety of molecular species including glucose, 5-(hydroxymethyl)-2- furaldehyde, and levulinic acid. Efforts were made to detect the presence of furan rings in the reaction mixture after various reaction times in attempts to obtain evidence in support of the above hypothesis. 215

6 Scheme II When carbohydrates were reacted with phenol and urea in acid medium, the same series of reactions referred to above could be expected plus a wide variety of additional reactions. Any of the dehydration or hydration intermediates mentioned above might react with either phenol or urea and, at the same time, mono- and/or diglycosyl ureas (VII and VIII in Scheme III) would be expected to form (13). The formation of glycosyl ureas usually requires at least a small amount of water in the reaction mixture (4,21). The conditions used in this study, however, would be expected to force dehydration reactions through the removal of the water of reaction as soon as it was formed. Efforts were made to detect the types of amide linkages that would be expected of glycosyl urea formation. Analytical Methods Samples taken during the synthesis were analyzed one of two ways using high performance liquid chromatography (HPLC). Samples containing phenol were analyzed on a Bio-Rad HPX87H + (acid) column, in N phosphoric acid, with an ultraviolet absorbance detector. Samples from which phenol had been sublimed away were analyzed on a series of two Bio-Rad HPX87P (lead ion) sugars analysis columns, in distilled water, with a differential refractometer detector. Samples were filtered before injection. A Nicolet 6000 Fourier transform infrared spectrometer recorded the infrared spectra. Samples from reaction mixtures were dispersed in water, spread on KBr crystals, and quickly dried in a vacuum desiccator for 2 hours. For spectral subtractions one subtracts a reference absorbance spectrum from a trial absorbance spectrum. In some cases a spectrum of a sample from the initial heated, but uncatalyzed, mixture was used as a reference spectrum for all subsequent sample spectra; these difference spectra showed how far reaction had progressed after catalyst was added. In other cases, the reference spectrum for each sample was that of the sample taken just previous to it; thus, these difference spectra showed only differences between consecutive samples. Pure component spectra of the carbohydrates, phenol, urea, 5-(hydroxymethyl)-2-furaldehyde (HMF), and water (between AgC1 crystals) were used to identify individual infrared peak positions of the reaction mixture. 13 C-NMR spectrograms were collected on a Bruker WM-250 FT-NMR spectrometer at MHz, The spectra were obtained at ambient temperature with broad band decoupling. The chemical shifts were measured relative to internal DMSO-d 6, the solvent, and converted to values relative to tetramethylsilane. Scheme III Results and Discussion (ML ) FTIR spectroscopy. Spectroscopic and chromatographic techniques were used in attempts to identify intermediate products in samples taken from a reaction mixture at various times. Interferences from the reactants prevented identification of the presence of 5-(hydroxymethyl)-2-furaldehyde (HMF) by infrared spectroscopy, even by using spectral subtractions. Not much pure HMF is expected to exist at 216

7 any time because of its high reactivity under acid catalysis. HMF's strong infrared absorption peak is at 1680 cm -1, but urea s strongest absorption region covers the range from 1600 to 1680 cm -1. One of HMF s two medium strength absorption peaks is at 1020 cm -1, where glucose also has its strongest absorption. HMF s other medium strength absorption peak is at 1520 cm -1, near the end of a strong phenol absorption doublet at 1475 to 1510 cm -1. Samples taken only minutes after the acid was added in experiments 3 or 6 (Table 1) show that the mixtures absorb fairly strongly in the whole region from 970 to 1720 cm -1. Constantly shifting ratios of reactants and products make it difficult to identify or quantify minor components, HPLC. Samples for HPLC taken minuts before acid addition in several syntheses show peaks for initial ingredients, Where phenol is absent at the beginning, there were also very small peaks for isomers (e.g., fructose) or saccharide-urea condensation products (e.g., monoand diglucosyl ureas). After acid addition, sample chromatograms also show peaks for fructose, glucosyl ureas and several unknown reaction products, eluting both before and after the glucose peak. In the early stages, the glucosyl urea peaks intensify as the glucose peak diminishes, but later the glucosyl urea peaks also diminish. In both the acids and sugars columns a sample of pure HMF will elute at relative retention times well-separated from other peaks seen in the syntheses, but only rarely were even minor HMF peaks observed in synthesis samples. Urea, which gave low, broad peaks, disappeared before the acidic synthesis had proceeded for an hour, whereas phenol appeared to remain little diminished throughout the whole acidic synthesis. NMR spectroscopy. HMF is more reactive than either 2-furaldehyde or furfuryl alcohol, both of which-form resins by themselves or with phenol under acidic conditions (6). Gibbons and Wondolowski (11,12) postulated that during their synthesis glucose degrades to HMF. Then the aldehyde group of HMF reacts with urea to form disubstituted urea compounds containing furan rings from the HMF. Next, the hydroxymethyl groups attached to the furan rings react with available phenol. However, samples taken from our syntheses using either glucose or xylose showed no 13 C NMR signals attributable to furan rings, especially in the 105 to 125 ppm region where the less reactive C3 or C4 carbons of the furan rings produce signals (7,10,14). In the region from 115 to 158 ppm, where the original phenol produces signals, no new signals or shifts arise during the acid-catalyzed step, indicating that phenol essentially acted only as a solvent during this step. New signals do appear in this region after formaldehyde was reacted into the mixture. The changes in the spectral region from 60 to 100 ppm are complicated. To unravel at least some of the early changes, reference was made to papers on spectra of the initial monosaccharides (1,22) run in water. Because DMSO-d 6 had proved to be a better overall solvent for the range of resin samples taken during a synthesis, spectra of pure monosaccharide samples in DMSO-d 6 were analyzed by carbon-13 NMR spectroscopy. Samples of mono- and diglucosyl ureas were synthesized according to published procedures (4,21), and their spectra were recorded. The spectra for α glucose, β -glucose, fructose, and a mixture of glucosyl ureas (Fig. 3) show how distinct the spectra for these compounds are. The spectrum for fructose contains signals from three identifiable forms. The glucosyl ureas, distinguishable by their slightly different frequency shifts for the glucose C1 and urea carbonyl carbons, show greater similarity to β -glucose than to α -glucose. Assignments of carbon-13 NMR signals (relative to TMS) for the β -monoglucosyl urea, by analogy with β -glucose, are ppm for the carbonyl (down 1.9 ppm from urea), 81.3 for C1, 72.9 for C2, 77.7 and 77.6 for C3 and C5, 70.3 for C4, and 61.2 for C6. The largest changes are occurring, as expected, for the C1, C2, and carbonyl carbons, and no change occurs for the more distant C4 and C6 carbons. For β -diglucosyl urea, the carbonyl signal is at ppm, C1 appears at 80.8 ppm (separate C1 peaks appear in a mixture of the mono- and diglucosyl ureas), and the others are the same as for monoglucosyl urea. A synthesis was performed to look at just the glucose and urea acidcatalyzed reactions, since phenol does not react during the acidic stage. Sulfolane was used as the solvent, because it is a low-volatility (b.p. 217

8 287 C), polar, non-protic liquid with resistance to acids, bases, and high temperature, and its carbon-13 NMR signals at 21.9 and 50.4 ppm do not interfere with signals of interest. Samples were immediately neutralized with potassium hydroxide. The first spectrum of samples taken from the reaction mixture at various times (Fig. 4) shows that before the acid was added much of the original α -glucose had already converted to β -glucose. Five minutes after acid was added, a complex spectrum showed signals for fructose isomers and for glucosyl ureas, in addition to the glucose isomers. At this point several yet unidentified, distinct signals were also noted, signals that had shown up as small impurities in the synthesis of glucosyl ureas earlier. One also notices the start of a broad, low background of signals in the region from about 60 to past 80 ppm. The third sample, taken after 150 minutes, gave a spectrum (here magnified for clarity) that shows as predominant peaks those unidentified peaks from the previous spectrum and also shows the broad, low background of signals. Known compounds in earlier mixtures no longer appear, and at this stage the mixture has become a viscous, resinous mass. The broad, low grouping of signals indicates a profusion of products, each at fairly low concentration, and the frequency range indicates that they are predominantly aliphatic. The fourth spectrum in this figure shows the two signals that pure HMF has between 50 and 110 ppm: the lower is due to the hydroxymethyl group and could disappear on reaction, but the upper signal is due to the much less reactive C4 carbon and should show (with only small shifts) in the products of the reaction sampled at 150 minutes if a reasonable amount of furan rings persisted. A portion of one acid-catalyzed synthesis of a glucose-based resin having P/C/U = 1/1/0.5 was dialyzed using a 3,500 molecular weight cutoff membrane and distilled water. Samples recovered from both the retentate and the permeate were vacuum dried and redispersed in DMSO-d 6 for carbon-13 NMR spectroscopy. The retained (high molecular weight) material had broad, low bands of signals from 0 to 50 and from 60 to 90 ppm, and there were at least two discernible peaks, at 129 and 115 ppm, attributable to phenol. The permeate had relatively low background signals, the four intense, sharp signals of phenol, the previously mentioned distinct, unidentified signals, and other slightly broadened, unidentified signals in the range of 60 to 80 ppm. Thus the broad, low bands of signals appear to be associated with the polymer portion of the mixture, which must have a complex structure consisting primarily of aliphatic groups, some carbons attached to oxygen or nitrogen (most of those with signals from 60 to 90 ppm) and others attached to just carbon and hydrogen (those with signals from O to 40 ppm). The dried retentate contained 8.94 weight percent nitrogen. There may be an analogy here to reaction of glucose with amino acids, or amines, to form Amadori compounds and Maillard polymers (2,9). The Maillard polymers have broad, low bands of signals in carbon-13 NMR spectra, primarily in the aliphatic region, and elemental analysis suggests amino acids are incorporated into the material (9). For a sample taken from the reaction mixture before acid catalyst is added there is only one signal above 158 ppm, that around ppm produced by urea s carbonyl group. Samples taken after acid addition show this signal disappearing, while another 1.9 ppm below appears, grows, and then slowly disappears also. This latter signal correlates with the position for the carbonyl carbon in monoglucosyl urea s spectrum. This analysis of synthesis samples shows that glucosyl ureas are early intermediates during the acid-catalyzed formation of aliphatic, nitrogen-containing polymers in these adhesive resins. Phenol does not react until formaldehyde is added to the system. The previously expected conversion of saccharides to furans does not seem to be a step in the resin-forming process. Summary and Conclusions Experiments have shown that a reaction system using carbohydrates as the major component can provide wood bonding strengths equal to the strength of bonds made with a conventional phenolic resin. Acid catalyzed dehydration of carbohydrates in the presence of urea occurs in the first stage. Phenol, which remains as an inactive medium in that stage, reacts in the subsequent neutral stage when formaldehyde is added into the reaction mixture. Alkaline catalyst is then added for the bonding reaction, The most-water resistant bonds were 218

9 formed when the phenol-to-carbohydrate mole ratio was at least 1/1, the ureato-carbohydrate mole ratio covered the range from 0.25/1 to 0.5/1, and a formaldehyde-to-phenol mole ratio was at least 2/1, with at least two moles of product water per mole of monosaccharide removed during resin formation. Six carbohydrates--glucose, fructose, sucrose, xylose, corn syrup, and methyl glucoside--all performed satisfactorily. This indicated that a wide variety of carbohydrate sources would be usable for adhesives if reaction and bonding conditions were optimized. Bonding conditions were not excessive and included a spread rate of about 0.39 kg/m 2 (80 lb/msgl), a 15-minute open assembly time, and a 10-minute closed assembly time with pressing for 5 minutes under 1.21 MPa (175 psi) pressure, Temperatures as low as 155 C were satisfactory with hexose sugars, while C was required for an adhesive based upon a pentose sugar. Fourier transform infrared (FTIR), 13 C nuclear magnetic resonance (NMR), and high performance liquid chromatography (HPLC) were used in attempts to elucidate reaction mechanisms. These methods were unable to detect any appreciable quantities of 5-hydroxymethyl-2-furaldehyde, which would be formed upon complete dehydration of hexose sugars, Glucosyl ureas were formed, but disappeared in the later stages of the cook. The polymeric products resulting from the acid-catalyzed stage comprise a multitude of predominantly aliphatic structures. The mechanisms of the reactions remain obscure. Recommendations The carbohydrate-phenol-ureaformaldehyde system needs further development before it can be considered a definite replacement for phenolic adhesives. Additional studies of the reaction mechanism should be conducted to optimize the adhesive system. Resin reaction conditions and adhesive formulation must be optimized for particular adhesive applications. Assembly and pressing condition tolerances must be established. Finally, larger size softwood panels bonded with this adhesive must be tested after outdoor exposure, and/or more rigorous accelerated aging experiments, to assure the long-term durability of this adhesive compared to phenolic resins. Acknowledgment The author expresses his appreciation to the following Forest Products Laboratory employees for their help in this study: Wesley L. Rork, Physical Science Technician, for synthesizing the resins and evaluating their bond strengths; to Martin F. Wesolowski, Chemist, and Lester C. Zank, Chemist, for FTIR and NMR spectroscopic analyses, and to Virgil H. Schwandt, Chemist, for the HPLC analyses. 219

10 Appendix The following table contains the strength data on panels bonded under a variety of pressing time and temperature conditions. Unless otherwise stated, the assembly times were 15 minutes open and 10 minutes closed, and the strengths of four specimens per panel were averaged to provide the panel strength, To reduce the amount of data displayed, only the strengths of dry panel specimens and 2-hour boiled specimens are presented. The VPS-treated panel strength values are often nearly the same as for 2-hour boiled panels, but the boiling is generally a more severe test. The within-panel standard deviations for trial resin panels bonded under sufficient press conditions seem no worse than those for the 23 phenolic control panels. 220

11 221

12

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