Phenolic adhesive bonds to aspen veneers treated with amino-resin fire retardants

Transcription

1 Phenolic adhesive bonds to aspen veneers treated with amino-resin fire retardants Charles B. Vick Abstract Structural failures of fire-retardant-treated wood may be caused by hydrolysis of the wood by acidic fire retardants. Amino-resin fire retardants are acidic, but they are believed to minimize the risk of failure from acid hydrolysis. The purpose of this study was to determine if a phenolic-based adhesive bonding system could produce strong and highly durable bonds to aspen veneers treated with urea/dicyandiamide/phosphoric acid/formaldehyde (UDPF) fire retardant, which is an amino-resin fire retardant, and with UDPF fire retardant combined with didecyldi methylammonium chloride (DDAC) preservative. The aspen veneers were pressure treated with appropriate molar ratios of UDPF at total retentions of 6.0 and 9.0 lb./ft. 3 (96 and 144 kg/m 3 ). To improve adhesion, water solutions of Na 2 CO 3 and NaOH were applied as surface treatments before bonding. Integrity of the bonds was determined from wet wood failure and shear strength. Two of the three phenol-formaldehyde (PF) adhesives tested developed bonds of high wet wood failure and shear strength on aspen veneers treated with an amino-resin fire retardant -butonly when a surface treatment was used. When 0.6 lb./ft. 3 (9.6 kg/m 3 ) of DDAC was combined with UDPF at the same retention levels, adhesion of the PF adhesives was poor and the two alkaline surface treatments were ineffective at improving adhesion. Concentration of nonpolar alkyl groups of DDAC at the surface of the wood interfered with penetration of polar PF adhesives. percent increase from 1984 to 1985 (19). Perhaps the main reason for this abrupt halt in growth is the great number of structural failures that have occurred in fire-retardant-treated plywood roof decking and framing lumber in multifamily dwellings and light commercial construction. These failures have been caused by acidic fire retardants that hydrolyze the wood. The most common fire-retarding chemicals used for wood are inorganic salts based on phosphorous (primarily mono- and di-ammonium phosphate salts), nitrogen, and boron. Other inorganic salts include zinc chloride, ammonium sulfate, borax, and boric acid. All except borax are acidic. Acid salts release acid to decrease flammable volatiles and increase char: this makes acid salts effective fire retardants (18). Some salts release acid at temperatures lower than fire conditions, and these salts are the most destructive in hydrolyzing wood. Hydrolysis is accelerated by increasing acidity, temperature, and moisture conditions. Furthermore, inorganic-salt fire retardants generally are hygroscopic, readily leach from wood, corrode metal fasteners, and are quite difficult to bond with phenolic-based adhesives. Another class of fire retardants, known as organicresin salt or amino-resin fire retardants, has been in the process of development during the last 30 years. As early as 1964, Arsenault (7) made flakeboards with high strength and low water absorption from flakes treated with an equimolar solution of dicyandiamide and orthophosphoric acid, using phenolic and urea In 1988, ft. 3 ( m 3 ) of fire-retardant treated wood were produced in the United States, which was a 3.6 percent decrease from 1987 (20). Production had increased only 2.0 percent from 1986 to Growth in production appears to have halted and possibly has begun to decline, compared with the 21 percent increase from 1985 to 1986 and the 53 The author is a Research Forest Products Technologist, USDA Forest Serv., Forest Products Lab., One Gifford Pinchot Dr., Madison, WI The use of trade or firm names in this publication is for reader convenience and does not imply endorsement by the USDA of any product or service. This paper was received for publication in February Forest Products Society Forest Prod. J. 44(1): FOREST PRODUCTS JOURNAL Vol. 44, No. 1 33

2 TABLE 1. Chemical and physlcal properties of adhesives and resins. Adhesive and resin Adhesive and resin property SP5300D a HB37 a SPC52CF a Resin molecular weight, weight-average (Mw) 9,742 Resin molecular weight, number-average (Mn) Resin peak temperature ( F) b 7, Resin heat of cure (cal./g) c Adhesive gel time (sec.) 125 Adhesive viscosity (cps) e 10,320 a Borden Chemical Co., Columbus, Ohio. b C = ( F-32)/1.8. c 1 cal./g = J/kg. d Adhesive mixture was too viscous to be measured with Sunshine gel timer. e 1 cps = Pa. s. f Adhesive mixture was too viscous to be measured with available viscometer. 9,462 7, ,780 8,408 6, d f -- resins. According to Arsenault, the treatments equaled Minalith and Pyresote in fire resistance at a retention of 6.0 lb./ft. 3 (96 kg/m 3 ). Other claims for the organic-resin salt were that it caused no darkening of the wood in the hot-press, resisted water leaching, was nonhygroscopic, and minimized hydrolysis of the wood by forming an acid-salt buffer. Juneja and others (10-14) extensively researched melamine- and urea-based fire retardants at the Eastern Forest Products Laboratory in Ottawa, Canada. Holmes and Knispel (15) and LeVan and Holmes (17) investigated the effectiveness of some leach-resistant amino-resin fire retardants in wood shingles weathered outdoors for 10 years. Now, commercial aminoresin fire retardants based on varying proportions of urea, dicyandiamide, phosphoric acid, and formaldehyde (UDPF) are available for treating wood. One such fire retardant also contains preservative and is manufactured in Australia under the trade name Pyrogard H. It has been found to be an effective fire retardant with its efficacy dependent on the total retention of chemical (1). Research has been underway at the USDA Forest Service, Forest Products Laboratory in Madison, Wis., to develop a combined fire retardant and preservative that could be impregnated into wood in a one-step process (16). Relative to most inorganic-salt fire retardants, this new treatment would not hydrolyze wood readily and would be less hygroscopic and more resistant to leaching. Most importantly, it would resist biodeterioration in severe service environments yet be an effective fire retardant. The combined fire retardant and preservative is based on appropriate molar ratios of UDPF in a water mixture with quaternary ammonium compounds. The efficacy of these chemicals in wood is being evaluated in fire- and decay-resistance tests. High-value panel products could be manufactured that would resist fire and biodeterioration if veneers, flakes, and fibers could be treated with chemicals that would not hydrolyze the wood and then could be bonded without those chemicals interfering with adhesion. However, severe service environments require the strongest and most durable structural adhesives, in particular the phenol-formaldehyde (PF) resin adhesives used to manufacture exterior grades of plywood, flakeboard, and hardboard. The purpose of this study was to determine if a phenolic-based adhesive bonding system could produce strong and highly durable bonds in aspen veneers treated to 6.0 and 9.0 1b./ft. 3 (96 and 144 kg/m 3) with UDPF fire retardant and to 6.6 and 9.6 lb./ft. 3 (106 and 154 kg/m 3 ) with combined UDPF fire retardant and didecyldimethylammonium chloride (DDAC) preservative. Experimental materials Adhesives Three commercial resole-type PF resins were selected for bonding aspen veneers treated with UDPF and UDPF/DDAC fire retardants. Trade names, molecular weights, and curing characteristics of the three resins, as well as viscosities and gel times of the adhesive mixtures, are shown in Table 1. All three resins were mixed with the same ingredients in the same proportions by this formula: Parts by Adhesive ingredient weight Water 25.4 Co-Cob extender (S.W. Dehyco, Memphis, Tenn.) 13.6 Glu-X wheat flour (Robertson Corp., Brownstown, Ind.) 11.0 Resin (first addition) 35.0 NaOH (50% concentration) 6.0 Resin (second addition) Veneers The veneers were rotary-cut, 1/4-inch- (6.3-mm-) thick aspen sapwood. They were dried to a moisture content of 4 to 6 percent before being treated with fire retardants. After treatment and heat-curing of the chemicals, the wood was conditioned to an equilibrium moisture content (EMC)near 4 percent at 94 F (35 C) and 20 percent relative humidity (RH). The untreated veneers were conditioned to the same EMC before bonding. 34 JANUARY 1994

3 Experimental treatment Phenolic adhesives Fire retardants with retentions Surface treatments a 1 lb./ft. 3 = kg/m 3. TABLE 2. Experimental design. Level of treatment SP5300D HB37 SPC52CF Untreated UDPF (6.0 lb./ft. 3 ) a UDPF (9.0 lb./ft. 3 ) UDPF/DDAC (6.6 lb./ft. 3 ) UDPF/DDAC (9.6 lb./ft. 3 ) None 3 N NaOH 3 N Na2CO3 Fire-retardant chemicals, treatment, and curing A 50 percent concentrate of UDPF fire retardant (without preservative) was prepared from a mixture of the following proportions (weight basis) of ingredients: Parts by Ingredient of UDPF weight Formaldehyde, 37 percent CH 2 O Urea 3.18 Dicyandiamide Phosphoric acid, 85 percent H 3 PO 4 Distilled water The concentrate was adjusted with water to produce retentions of 6.0 and 9.0 lb./ft. 3 of UDPF. An 80 percent concentrate of DDAC was blended with UDPF and water to produce a retention of 0.6 lb./ft. 3 (9.6 kg/m 3 ) of DDAC and retentions of 6.6 and 9.6 lb./ft. 3 of combined UDPF/DDAC. The full-cell treating procedure was used for impregnating aspen veneers to target retentions. With veneers stacked on spacers and weighted, a vacuum near 25 in. Hg (84 kpa) was maintained for 30 minutes. Then, veneers were soaked for 30 minutes at atmospheric pressure. After draining and stickering, the treated veneers were air-dried overnight at an air-flow rate near 200 ft./min. (61 m/min.). The fire retardants were cured in the veneers in an oven maintained at 180 F (82 C) for 6 hours. Finally, the treated veneers were conditioned to an EMC of 4 percent. Surface treatments Water solutions of Na 2CO 3 and NaOH were prepared in a 3 N concentration. They were brushed on both bonding surfaces at approximately 0.03 lb./ft. 2 (0.15 kg/m 2 ). Afterwards, surface-treated veneers were air-dried for about 2 hours, then dried in an oven for about 2 hours at 160 F (71 C). Finally, they were conditioned to an EMC near 4 percent the same as all other treated and untreated veneers. Experimental methods Experimental design These experiments were designed to determine if PF resins could be used directly or with modification to bond UDPF- and UDPF/DDAC-treated aspen veneers. Experimental combinations of three adhesive systems, as they were applied to five fire-retardant treatments with three surface treatments, are listed in Table 2. The integrity of these bonds was determined from wet wood failure and shear strength measured after specimens had been subjected to a vacuum-pressure soak (VPS) in water, which is an American Plywood Association procedure. After one cycle of VPS, this procedure was shown to be a simpler, faster, and more sensitive method for detecting interference with adhesion on preservative-treated veneers than were measurements of dry shear strength and wood failure after multiple cycles of VPS and drying (26). This experiment was divided into two statistical experiments, one for wet wood failure and the other for wet shear strength. Both were completely randomized models with treatments arranged factorially for 3 adhesive systems, 5 fire-retardant treatments, and 3 surface treatments, to yield 45 treatment combinations. Four replicates (test panels) were prepared for each treatment combination, and from each panel, 6 specimens were tested for a total of 24 specimens per treatment combination. Separate analyses of variance (ANOVAs) were conducted for wood failure and shear strength. Because percentages of wood failure were not normally distributed, using the arcsine transformation was necessary to fulfill the normality requirements of the experimental model (22). The Ryan-Einot-Gabriel-Welsch multiple-range F-test was used to detect significant differences between various treatment combinations (27). Specimen type Small, compression-loaded block-shear specimens with 1.0-in. 2 (6.45-cm 2 ) shear area were substituted for tension-loaded, lap-shear specimens that are typically used. Stress analysis has shown that tension perpendicular to the surface, not shear, is the primary force causing failure in tension-loaded, lap-shear specimens. This is caused by eccentricities in the direction of pull as the specimen, particularly a wet one, rotates with the notches. This effect produces failure at lower stress levels in lap-shear specimens than it does in block-shear specimens (23). Furthermore, strength and wood failure values tend to be more variable in tension-loaded lap-shear specimens than in comparably sized, compression-loaded block-shear specimens. Specimen preparation Test specimens were cut from test panels that were prepared by laminating two, 1/4-inch- (6.3-mm-) thick veneers with their grain directions parallel. The panels measured 4-1/2 by 5-1/8 inches (11.4 by 13 cm). All veneers were bonded alike whether untreated, treated, or surface treated with alkaline solutions. FOREST PRODUCTS JOURNAL Vol. 44, No. 1 35

4 Adhesive was spread on one surface only at a rate near lb./ft. 2 (0.220 kg/m 2 ). Closed assembly time was 20 minutes under a 10-pound (4.5-kg) deadweight. The panels were hot-pressed at 175 lb./in. 2 (120 N/cm 2 ) for 4.5 minutes. One panel was pressed with each press opening. The press temperature was set at 300 F (149 C), which was 50 F (10 C) lower than the normal temperature where these phenolic resins develop better bonds on aspen. The higher temperature caused the wood to darken, which is visual evidence that the acidic fire retardant is beginning to hydrolyze the wood. Darkening of the treated wood occurred to the same degree whether the wood was treated with UDPF or UDPF/DDAC. Hot-pressing the wood at 300 F for 4.5 minutes reduced darkening of veneers to a light buff color. Specimen testing Twenty-four specimens representing each treatment combination were subjected to a single VPS. The saturation procedure is an industry standard for quality-control testing of exterior-grade plywood (2) and consists of the following sequence of events: 1) specimens are submerged in tap water at room temperature in a pressure vessel; 2) a vacuum of 27 in. Hg (0.1 MPa) is maintained for 30 minutes; 3) a pressure of 60 lb./in. 2 (0.4 MPA) is maintained for 30 minutes; and 4) specimens are submerged in water until tested. The water-saturated specimens were loaded in compression to failure at a constant rate of 0.10 in./min. (2.5 mm/min.) in a shearing tool as described in ASTM Method D 905 (3). The maximum load at failure was used to calculate shear strength in pounds per square inch of shear area. The percentage of wood failure was estimated on the sheared area to the nearest 5 percent. Adhesive and resin characterization Apparent resin weight-average molecular weights (M w) and number-average molecular weights (M n) were determined by gel permeation chromatography using polystyrene standards for calibration. Samples were analyzed on a Waters liquid chromatograph, using Polymer Laboratories Ltd. styrogel columns with pore sizes of 10 4 and 500 A (10 3 and 50 nm) connected in series. The solvent was dimethylformamide with 2 percent acetic acid. The flow rate was 1.00 ml (10-6 m 3 ) per minute. A Waters model 441 ultraviolet detector, operated with a 28-A (280-nm) filter, was used for detection. Apparent resin M w and M n are shown in Table 1. The reactivities of the phenolic resins, as indicated by resin peak temperature and heat of cure, were measured with a Perkin-Elmer DSC-2 differential scanning calorimeter, as shown in Table 1. Samples of 15 to 18 mg of phenolic resin were reacted in Perkin-Elmer large-volume capsules (LVC) made of stainless steel and fitted with Vitron O-rings. By sealing samples in LVCs that withstand vapor pressures up to 218 lb./in. 2 (1.5 MPa), the curing reactions proceeded exothermically. The scans ranged from 80' to 390 F (27 to 200 C). and the rate of scan was 50 F/min. (10 C/min.). The instrument was calibrated with indium. The gel times of adhesive mixtures were measured according to ASTM Method D 3056 (6). The temperature of the water bath was maintained at the boiling point with a Barnant temperature controller. Gel times are shown in Table 1. TABLE 3. Mean wet wood failure and shear strength of adhesive bonds. Adhesive SP5300D HE37 SPC52CF Surface Wood Shear Wood Shear Wood Shear Fire retardant treatment (retention) treatment failure strength failure strength failure strength (%) (lb./in. 2 ) a (%) (lb./in. 2 ) (%) (lb./in. 2 ) Untreated None NaOH Na2CO UDPF (6.0 lb./ft. 3 ) b None NaOH Na2CO UDPF (9.0 lb./ft. 3 ) None NaOH Na2CO UDPF/DDAC (6.6 lb./ft. 3 ) None NaOH Na2CO UDPF/DDAC (9.6 1b./ft. 3 ) None NaOH Na2CO a 1 lb./in. 2 = N/cm. 2 b 1 1b./ft. 3 = kg/m JANUARY 1994

5 The viscosities of adhesive mixtures were measured according to ASTM Method D 2556 (5) because of their shear-rate-dependent flow properties. A Brookfield Model DV-I digital viscometer with a No. 4 spindle rotating at 30 rpm was used to measure viscosities at 75 F (24 C). Viscosities are shown in Table 1. ph of fire retardants and treated wood The ph levels of treated and untreated woods were measured according to ASTM Method D 1583 (4). Material ph Untreated aspen 4.80 UDPF treating solution 2.00 UDPF/DDAC treating solution 2.00 UDPF-treated aspen 2.26 UDPF/DDAC-treated aspen 2.41 Solution concentrations were targeted for retentions of 6.0 lb./ft. 3 UDPF and 6.6 lb./ft. 3 of UDPF/DDAC. Even though this ASTM method is intended for measuring the ph of dry adhesive films, it is satisfactory for ph measurement of ground wood. The wood was ground with a Wiley mill, and particles that passed through a Tyler mesh 40 (0.42 mm) were used for ph measurements. Water was added to the wood particles in a weight ratio of 10 to 1 because of the highly absorbent nature of wood particles. An Orion Model SA520 ph meter with a flat-surface electrode was used for measurements. Results and discussion Means and ANOVAs of wet wood failure and shear strength Mean wet wood failure percentages and wet shear strengths for all 45 combinations of 3 adhesives, 5 fire-retardant treatments, and 3 surface treatments are shown in Table 3. Estimates of percentages of wood failure in wet shear specimens have proven to be a valid criterion for evaluating adhesion of PF adhesives to veneers either treated with chemicals (26) or untreated (3,9). The only industry-accepted standard for such a test is the 85 percent minimum wood-failure requirement for water-saturated, lap-shear plywood specimens, as specified in PS 1-83 (2). This standard does not contain wet shear strength requirements. Furthermore, such strength tests have not proven to be an acceptable measure of bond durability. Results from the ANOVAs of wet wood failure and shear strength showing statistical significance of adhesives, fire retardants, surface treatments, and their interactions are shown in Table 4. Adhesives were not significantly different from each other as determined by wet wood failure, nor did they interact significantly with either the fire retardants or surface treatments. However, they were involved in a significant three-way interaction and required subanalyses. Fire retardants and surface treatments were significant factors affecting both wood failure and shear strength. They were also involved in significant three-way interactions and required subanalyses. Effects of adhesives on bond integrity Three PF resins were selected for study from several in preliminary experiments, primarily because these adhesives performed better on UDPF-treated aspen than on untreated wood. Proportions of ingredients in the adhesive mixtures were also selected on this basis. For these reasons, all three adhesives produced higher wood failure percentages and shear strengths on the UDPF-treated aspen than on untreated aspen (Table 3). All tested adhesives, including those not selected for this study, tended to overpenetrate the untreated aspen, partly because their molecular weights were too low for porous aspen (Table 1) and because the cure rate was slowed by lowering the press temperature to 300 F from the optimum 350 F (177 C). Without surface treatment, no adhesive produced wood failure on fire-retardant-treated veneer that would meet the 85 percent minimum requirement of PS 1-83 (2). As a main effect, the three adhesives did not differ significantly from each other in producing wood failure (Table 4). In interactions though, the SP5300D resin produced significantly higher wood failure percentages than did other resins when the NaOH surface treatment was applied to UDPF-treated veneer, No other differences in wood failure were significant on UDPF-treated wood. Wood failure percentages were so far below accept- TABLE 4. Results of anova showing levels of significance of experimental factors and their interactions of wet wood failure and shear strength. Source of variation Degree of freedom Wet wood failure Wet shear strength Adhesive(A) 2 NS a *** b Fire-retardant treatment (F) 4 *** *** Surface treatment (S) 2 *** *** A F 8 NS *** A S 4 NS NS F S 8 A F S 16 Error 135 Total 179 a NS = not significant at the 0.05 level of probability. b *** = significance at the level of probability. c ** = significance at the 0.01 level of probability. *** *** *** ** c FOREST PRODUCTS JOURNAL Vol. 44, No. 1 37

6 able standards for the UDPF/DDAC-treated veneers, regardless of surface treatment, no effort was made to define contributions of individual adhesives. Adhesives differed from each other more significantly in wet shear strength than in wet wood failure, both as a main effect and in interactions (Table 4). The SP5300D resin generally produced the highest shear strengths on veneers treated with UDPF at the highest retention, with or without surface treatment. However, at the lowest retention of UDPF, shear strengths between adhesives were generally not different. Adhesive differences were not analyzed for UDPF/DDAC treatments because of generally poor adhesion, although the SP5300D resin produced shear strengths somewhat higher than those of the other resins. The SP5300D resin performed slightly better than did the other resins probably because of its higher molecular weight and optimum viscosity, which produced better flow and transfer at elevated temperatures (Table 1). The SPC52CF resin contained additives that produced a mixture of very high viscosity with less desirable spreading and flow characteristics. It also gelled rapidly. For these reasons, the SPC52CF resin penetrated less than either SP5300D or HB37. The HB37 was a wet-process hardboard resin that penetrated slightly more than did the SP5300D resin. The phs of UDPF- and UDPF/DDAC-treated veneers, 2.26 and 2.41, respectively, did not appear to cause premature gelation of the adhesives and thereby inhibit their wetting, flow, and penetration. When wood failures and shear strengths of the untreated veneers without surface treatments are compared with the UDPF-treated veneers without surface treatments, both property values are invariably higher on the UDPF-treated veneers for all three adhesives (Table 3). Resins and adhesive formulations were selected to perform better on the UDPF-treated veneers than on untreated veneers. Even so, if gelation were to cause significant interference with adhesion, it should have been more evident in the comparisons just discussed. Mixing the acidic treating solutions (ph 2.00) with alkaline adhesives (ph 11.89) caused the adhesives to gel. Apparently, this did not occur in the bondlines; if it did, the effect on bondline integrity was minimal. Effects of fire retardants on bond integrity The fire-retardant treatments had strong effects on bond integrity, and they interacted with surface treatments and adhesives. Therefore, subanalyses were necessary to better define the fire-retardant effects (Table 4). Without surface treatments, however, no adhesive produced wood failure above 79 percent on any fire-retardant-treated veneer, including the untreated controls (Table 3). Wood failures resulting from the 9.0- and 6.0-lb./ft. 3 -retention UDPF treatments were always greater than those of untreated controls or UDPF/DDAC treatments. The 9.0-lb./ft. 3 retention produced the highest of the two wood failure percentages with the SP5300D and HB37 adhesives. Differences in wood failure between the 6.0- and 9.0 lb./ft. 3 -retention UDPF treatments were not significant, but wood failure percentages for both retentions were significantly higher than those of the untreated controls or the 6.6- and 9.6-lb./ft. 3 retentions of the UDPF/DDAC treatments. Generally, the shear strength tests confirmed results of the wood failure tests. Shear strengths were always highest on the UDPF-treated veneer with the 9.0-lb./ft. 3 retention and usually higher than the 6.0-lb./ft. 3 retention. Shear strength at both retentions of UDPF treatments was always significantly higher than that of the untreated control with the Na 2CO 3 and NaOH surface treatments. Furthermore, strengths in the UDPF-treated veneers were usually significantly higher than the UDPF/DDAC-treated veneers. It is quite apparent from the wide differences in wood failures and shear strengths that the addition of 0.6 lb./ft. 3 DDAC to the UDPF fire retardant caused drastic decreases in bond integrity, compared with veneers treated only with UDPF. Previous studies (26) had shown that treatments of aspen veneers with water solutions of DDAC at retentions of 0.2, 0.4. and 0.6 lb./ft. 3 (3.2, 6.4, and 9.6 kg/m 3 ) did not interfere with adhesion of a formulation similar to SP5300D. Mean wet wood failures were 89, 93, and 93 percent, respectively, at each of these retentions, and mean wet shear strengths were well above 600 lb./in. 2 (414 N/cm 2 ), as were those of the untreated controls. Several observations have been made to explain the differences in adhesion between UDPF/DDAC-treated veneers and DDAC-treated veneers. Surfaces of veneers treated with the mixture of UDPF/DDAC were much more wettable with water than surfaces of either UDPF-treated or untreated veneers. When the water-based PF adhesive was spread on the UDPF/DDAC-treated veneers, water from the adhesive mixture was absorbed into the veneer surface so quickly that within a few minutes the wet adhesive film began to dry, crack, and peel from the surface. Very little adhesive penetration took place -justwater penetration. The result was poor adhesion to the veneers containing the DDAC mixture. The DDAC is a cationic surfactant. It drastically decreases the surface tension of water, and in water solution, DDAC effectively penetrates the microstructure of wood. The polar ends of DDAC molecules adsorb onto the polar surfaces of wood. In so doing, the lyophilic alkyl groups orient themselves away from the wood surface. However, DDAC orients itself differently when a mixture of DDAC and UDPF is applied to wood. The polar ends of DDAC molecules are attracted to the polar UDPF molecules, and probably to the polar wood surface as well, so that the opposite nonpolar lyophilic ends orient themselves away from the surface of the liquid mixture. As the UDPF/DDAC solution dries in the wood, the nonpolar ends of DDAC molecules remain oriented from the wood. Thus, when polar water-based PF adhesive was spread on the now nonpolar, UDPF/DDAC-coated surfaces, the adhesive 38 JANUARY 1994

7 was effectively repelled and prevented from penetrating the treated pore structure. At the same time, the surface tension of the water in the adhesive mixture was drastically decreased by an apparently high concentration of DDAC at the surface so that water was rapidly absorbed into the wood. The result was poor penetration of the adhesive. When an essentially similar adhesive was spread on DDAC-treated aspen veneers at the same 0.6 lb./ft. 3 retention, no such rapid absorption of water from the adhesive occurred, nor was the adhesive prevented from penetrating (26). Apparently, the aspen veneer had ample surface area within its microstructure to provide adsorption sites for DDAC without drastically increasing its hydrophobicity and limiting penetration of the adhesive. In this study, relatively high loadings of UDPF at either 6.0 or 9.0 lb./ft. 3 were added to 0.6 lb./ft. 3 These chemicals also strongly adsorb to wood. Thus, it appears quite likely that fewer sites were available for adsorption of DDAC from the UDPF/DDAC mixture. Furthermore, this may also explain the apparently high concentration of DDAC at the treated wood surface and the resultant reduction in surface tension of water in the adhesive as well as the poor adhesion of the adhesive. Effects of surface treatments on bond integrity The surface treatment with Na 2CO 3 was quite effective in improving adhesion to UDPF-treated veneers at retentions of 6.0 and 9.0 lb./ft 3. This treatment produced wood failure percentages above the 85 percent minimum requirement of PS 1-83 (2) for all three adhesives (Table 3); it also produced wood failure percentages for all three adhesives that were significantly higher than those of the UDPF-treated veneers that had no surface treatment. Wet shear strength on the 6.0-lb./ft. 3 - retention UDPF treatment was significantly improved by the Na 2CO 3 treatment for all adhesives (Table 3). But on the 9.0-lb./ft. 3 -retention UDPF treatment, the Na 2CO 3 treatment produced no significant improvements in shear strength compared to those without surface treatment. Applying the Na 2 CO 3 treatment to veneers treated with the combined UDPF/DDAC treatment did not appear to improve adhesion at all, whether determined from wood failure or shear strength (Table 3). Wood failure percentages were far below the 85 percent requirement, and shear strengths were not consistently or statistically improved by Na 2CO 3 treatment. The Na 2CO 3 treatment consistently increased both wood failure and shear strength of the untreated aspen veneers, significantly so with the SP5300D adhesive. The NaOH surface treatment was also effective in improving adhesion to veneers treated with UDPF at both retentions but not as effective as the Na 2CO 3 treatment. Wood failure was above 85 percent with the SP5300D and HB37 adhesives but was slightly below this requirement with the SPC52CF adhesive (Table 3). Only with the SP5300D adhesive on UDPF-treated veneers was wood failure significantly increased by NaOH treatment. At every treatment combination, NaOH treatment increased wood failure on the UDPFtreated wood. Generally, wet shear strength of NaOHtreated wood was only slightly greater than that of wood without surface treatment. Generally, the NaOH surface treatment caused decreases in wood failure and shear strength values on the 6.6- and 9.6-lb./ft. 3 retentions of UDPF/DDACtreated veneers compared with those without surface treatment (Table 3). Similar decreases in shear strength were caused by the NaOH treatment on untreated veneers, while wood failure increased somewhat. The mechanisms by which surface treatments with water solutions of Na 2CO 3 and NaOH improved adhesion of the PF adhesives to UDPF-treated aspen have not been proven here or explained in the literature. However, the technique has been used successfully in several applications, and an explanation of the most probable mechanism is offered here. Truax (24) used surface treatments of approximately 10 percent NaOH to increase shear strengths and wood failures on several high-density species; Chen (8) used NaOH in an alcohol solution to improve adhesion to fire-retardant-treated plywood; and Vick (25) improved adhesion to chromated- copper -arsenate-treated southern pine with water solutions of NaOH. Microscopic observation revealed that the alkali surface treatments caused the wood fibers to swell and the adhesives to wet the surface more spontaneously and penetrate the fibers more deeply. Minor and Springer (21) recently demonstrated that pretreatment of hardwood chips with alkali increased the permeability of the wood and increased the rate of diffusion ofwater-soluble pulping chemicals so that homogeneity and overall rate of pulping were increased. They theorized the mechanism was chemical cleavage and saponification of esters with a preferential weakening of the intercellular region in the microstructure of the wood. In the case of the fire-retardant treatments, the microstructure of the wood was completely coated with resinous UDPF that had been partially thermoset at 180 F. Apparently, the alkaline treatments not only enhanced the wettability of the treated wood but also weakened the coating and swelled the wood enough to increase adhesive wetting and penetration. However, on the fire-retardant treatments containing DDAC, neither alkaline surface treatment improved adhesion. Conclusions Two PF adhesives developed bonds of high integrity on aspen veneers treated with UDPF fire retardant at retentions of 6.0 and 9.0 lb./ft. 3, but only after veneer surfaces were treated with 3 N aqueous solutions of either Na 2CO 3 or NaOH before bonding. These surface treatments were not effective in improving adhesion to veneers treated with the combined UDPF fire retardant and DDAC preservative at retentions of 6.6 and 9.6 lb./ft. 3. Concentration of nonpolar alkyl groups of FOREST PRODUCTS JOURNAL Vol. 44, No. 1 39

8 DDAC at the wood surface interfered with penetration 13. and L.R. Richardson Versatile fire-retarof polar PF adhesives. dants from amino-resins. Forest Prod. J. 24(5): and J.K. Shields Increased fungal resistance Literature cited of wood treated with modified urea-based fire-retardant resins. Forest Prod. J. 23(5): Alexion, P.N., W.D. Gardner, P. Lind, and D. Butler Holmes, C.A. and R.O. Knispel Exterior weathering Efficacy of an amino resin fire retardant. Forest Prod. J. durability of some leach-resistant, fire-retardant treatments for 36(11/12):9-15. wood shingles: A five-year report. Res. Pap. FPL 403. USDA 2. American Plywood Association U.S. Product Standard Forest Serv., Forest Prod. Lab., Madison. Wis. 10 pp. PS 1-83 for construction and industrial plywood with typical 16. LeVan, S.L. and R.C. De Groot U.S. PatentApplication, APA grade-trademarks. APA, Tacoma, Wash. 35 pp. Serial No. 07/714,402, June 12, American Society for Testing and Materials Standard test 17. and C.A. Holmes Effectiveness of fire-retarmethod for strength properties of adhesive bonds in shear by dant treatments for shingles after 10 years of outdoor weathercompression loading. ASTM D In: Annual Book of ASTM ing. Res. Pap. FPL 474. USDA Forest Serv.. Forest Prod. Lab., Standards, Adhesives. ASTM. Philadelphia. Pa. pp. Madison. Wis. 15 pp and J.E. Winandy Effects of fire-retardant Standard test method for hydrogen ion treatments on wood strength: A review. Wood and Fiber Sci. concentration of dry adhesive films. ASTM D In: 22(1): Annual Book of ASTM Standards, Adhesives. ASTM, Philadelphia, Pa. pp Mickelwright, J.T Wood preservation statistics, A Standard test method for apparent viscosity report to the wood preserving industry in the United States. Proc. of the American Wood Preservers' Assoc. 85: of adhesives having shear-rate-dependent flow properties. ASTM D In: Annual Book ofastm Standards, Wood preservation statistics A report Adhesives. ASTM, Philadelphia. Pa. pp to the wood-preserving industry in the United States. Proc Standard test method for gel the of solventless American Wood Preservers' Assoc. 86: varnishes. ASTM D In: Annual Book ofastm Stand- 21. Minor. J.L. and E.L. Springer Improved penetration of ards, Electrical Insulation(11): D 2518-Latest. ASTM. pulping reagents into wood. In: Proc. 6th International Sympo- Philadelphia. Pa. pp sium on Wood and Pulping Chemistry 1991, USDA ForestServ., 7. Arsenault. R.D Fire-retardant particleboard from treated Forest Prod. Lab., Madison. Wis. 7 pp. flakes. Forest Prod. J. 14(1): Snedecor, G.W. and W.G. Cochran Statistical Methods. 8. Chen. C.M Gluing study of pyresote-treated, fire-retar- (6th ed.). Iowa State Univ. Press, Ames, Iowa. pp dant plywoods-part I. Forest Prod. J. 25(2): Strickler, M. D Adhesive durability: specimendesigns for 9. Gollob, L., R.L. Krahmer, J.D. Wellons, and A.W. Christiansen. accelerated tests. Forest Prod. J. 18(9): Relationship between chemical characteristics of phenol- 24. Truax, T.R The gluing of wood. Dept. Bull. No formaldehyde resins and adhesive performance. Forest Prod. J. USDA. Superintendent of Doc., Washington. D.C. 48 pp. 35(3): Vick. C.B Structural bonding of CCA-treated wood for 10. Juneja. S.C Melamine-dicyandiamide-base resins. Ca- foundation systems. Forest Prod. J. 30(9): nadian Patent 907, Compatibility of nonacidic waterborne pre Urea-based fire retardant. Canadian Patent servatives with phenol-formaldehyde adhesive. Forest Prod. J (2): Stable and leach-resistant fire retardants for 27. Welsch, R.E Stepwise multiple comparison procedures. wood. Forest Prod. J. 22(6): J. Am. Statistical Assoc. 72(359): Printed on recycled paper 40 JANUARY 1994