Enhanced Durability of One-Part Polyurethane Bonds to Wood Due to the Use of HMR Primer Alfred W. Christiansen Chemical Engineer Charles B. Vick Research Scientist E. Arnold Okkonen Physical Science Technician, USDA Forest Service, Forest Products Laboratory, Madison, WI Abstract Four commercial one-part polyurethane adhesives, along with a resorcinol-formaldehyde adhesive to represent a standard of performance, were subjected to a series of industry-accepted tests that assessed varying levels of bond strength and durability. The polyurethanes performed much like one another. In terms of dry shear strength and wood failure, wet shear strength, and resistance to deformation under static load, they performed at least as well as the resorcinol. Wet wood failure in shear tests for the polyurethane adhesives, however, was far below the levels observed for resorcinol-formaldehyde bonded wood. In moderately severe and severe cyclic delamination tests, the one-part polyurethanes had poor resistance to delamination. A recently discovered hydroxymethylated resorcinol (HMR) coupling agent, which physicochemically couples all thermosetting wood adhesives to wood, dramatically increased wet wood failure and resistance to delamination to levels comparable with resorcinol adhesives. Thus, by priming wood surfaces with HMR coupling agent prior to bonding, one-part polyurethane adhesives can meet the strength and durability requirements of the most rigorous of tests, including ASTM D2559, which is the standard for delamination. Introduction Water-based, polar adhesives such as phenolics, resorcinolics, and melamines perform quite well as structural adhesives on wood. Recently, one-part polyurethane adhesives have emerged onto the general pur- pose adhesive consumer market. Polyurethanes are well known for their excellent adhesion, flexibility, high cohesive strength, low-temperature performance, and amenable curing speeds. One-part polyurethanes are based on urethane prepolymers made by reacting an excess of methylene diphenyl diisocyanate with a polyol such that a small amount of isocyanate functional group remains. The basic reaction is: HO-R-OH + OCN-R -OCN HO-R-OOC-NH-R -OCN (urethane group) R = aliphatic groups, polyesters, polyethers R = aromatic groups Free isocyanate groups in the adhesive react with moisture on substrate surfaces to complete the cure. Isocyanate reacts with water, with evolution of carbon dioxide: R-NCO + H-OH R-NH 2 + CO 2 (gas) and then proceeds through an intermediate step to form urea linkages: R -NCO + R-NH2 R -NH-CO-NH-R (substituted urea) Since these new wood adhesives were introduced, the USDA Forest Service, Forest Products Laboratory (FPL), Madison, WI, has received many inquiries concerning the strength and water resistance of one-part polyurethane bonds to wood. Some commercial literature characterizes these adhesives as waterproof, which is not a legally defined Adhesives Technical Interest Group Session 489
term. The term waterproof might indicate to potential users that such bonds would exhibit acceptable resistance to moisture in any exterior exposure. For example, experience at FPL with epoxy adhesives has shown that adhesives that are generally thought of as waterproof in nonwood applications can have problems in forming durable bonds to wood. Epoxy-wood bonds exposed to the severe stresses of repeated soaking in water and drying failed to meet requirements for structural wood adhesives intended for exterior exposure (8). Experience at FPL with other isocyanate or moisture-curing polyurethane construction adhesives could not be used to credibly predict performance of this new class of polyurethane adhesives on wood, particularly in laminates exposed to wet-use conditions. Thus, a study was initiated at FPL to investigate the resistance of one-part polyurethane bonds in wood laminates to accelerated aging conditions and evaluate the exterior durability of these bonds. Part 1-Durability of the Bond of One-Part Polyurethanes to Wood Experimental Four commercial one-part polyurethane adhesives were obtained directly from U.S. suppliers. All four adhesives were applied and cured in accordance with the manufacturer s instructions. A resorcinol-formaldehyde (RF) adhesive, supplied by Indspec Chemical Corporation (now part of Borden Chemicals, Springfield, OR) and identified as Penacolite G-1131-A resin and G-1131-B hardener, was used as a standard of performance for comparison with polyurethane adhesives. Yellow birch and Douglas-fir lumber conditioned at 23 C and 50 percent relative humidity was used for specimens. Lumber was knife-planed 24 hours before bonding. It was planed to 8 mm for shear tests and 19 mm for delamination tests on lumber. All two- and six-ply laminates were prepared in the same manner. All laminates to be bonded with polyurethane were sprayed with a fine water mist before adhesive application, as recommended by the adhesive suppliers. The adhesive spread rate for polyurethanes was 0.098 kg/m 2 and for RF adhesive 0.146 kg/m 2. They were bonded under 0.69 MPa pressure for Douglas-fir and 1.03 MPa for yellow birch. In the shear tests, 25 specimens were used for each set of conditions. The specimens chosen for evaluating strength and wood failure values were small compression-loaded block-shear specimens with a shear area of 25.4 by 25.4 mm (1 in. 2 ) (6) (Fig. 1). Grain direction was parallel in both laminae and was parallel to the shearing stress in the specimens. For dry strength properties, ASTM Figure 1. Modified compression-loaded block-shear specimen with 645-mm 2 (1-in. 2 ) shear area. D905 was followed (2). For the water resistance tests, U.S. Product Standard PS-1-83, for softwood plywood, was followed (1). Three different types of PS-1-83 water resistance tests were used: Vacuum-Atmospheric Soak (VAS) test-specimens were soaked in water at 49 C while a vacuum of 50 kpa was maintained for 30 minutes. Water soaking was continued for 1 5 hours at atmospheric pressure, and samples were loaded to failure while still wet. The wood failure value must average 245 percent. Vacuum-Pressure Soak (VPS) test Specimens were soaked in cold water while a vacuum of 84 kpa was maintained for 30 minutes. This was immediately followed by pressure at 414 kpa for 30 minutes, with specimens still under water, and samples were loaded to failure while still wet. The wood failure value must average 285 percent. Boil-Dry-Boil (BDB) test Specimens were submerged in boiling water for 4 hours. They were dried for 20 hours at 63 C. They were boiled again for 4 hours and then cooled in tap water. Then, the samples were loaded to failure while still wet. The wood failure value must average ³85 percent. Results and Discussion The averaged shear strength and percentage wood failure results of four polyurethanes bonded to Douglas-fir (7) are shown in Figure 2. The results are averaged because of their general similarity. The shear strength was less for both the polyurethanes and RF specimens when bonds became wet, a reflection of the 490 FPS 1999 Annual Meeting
Figure 2. Shear strength and wood failure values for bonds of four (averaged) one-part polyurethanes and RF adhesive to Douglas-fir, decreased strength of wood. The wood failure values were much lower for the polyurethanes when the bonds became wet, not meeting any of the specification levels for VAS, VPS, or BDB, whereas the reference RF adhesive showed high wood failure values in all tests. The results for polyurethane bonds to yellow birch, a more dense wood that is generally harder to bond, showed similar but more drastic decreases in wood failure values (Fig. 3). The reference RF adhesive on yellow birch showed much higher wet wood failure than did the polyurethanes. Part 2-Effect of HMR Coupling Agent on Polyurethane Bonds to Wood Hydroxymethylated resorcinol (HMR) coupling agent substantially increased the durability of epoxy bonds to wood, which was noted when specimens were exposed to conditions simulating accelerated exposure to exterior conditions (8). Bonding of epoxy molecules with hydroxymethyl groups of HMR was hypothesized as being important to the increased durability of epoxy bonds to wood. Because the one-part polyurethanes contain isocyanate groups, which also easily react with hydroxymethyl groups, HMR coupling agent was used with the hope of improving the durability of one-part polyurethane bonds to wood. Experimental The formulation of HMR is as follows: Ingredients Parts by weight Water, deionized 90.43 Resorcinol, crystalline 3.34 Figure 3. Shear strength and wood failure values for bonds of four (averaged) one-part polyurethanes and RF adhesive to yellow birch. Formaldehyde, 37% aq. solution 3.79 Sodium hydroxide, 3 M 2.4 Total 100.00 The mixture is prepared as follows. Mix the ingredients, adding either formaldehyde or catalyst last. Wait 3 to 4 hours for necessary reactions, but use the agent before 6 to 8 hours. Spread approximately 0.15 kg/m 2 of the HMR solution on the wood. Allow the surface to dry at room temperature for about 24 hr. before applying polyurethane adhesive. After the surface has dried, polyurethane adhesives can be applied. For the evaluation of HMR, only the two best polyurethanes (labeled C and D) from Part I of this paper were used. Three tests not discussed in Part 1 were used in this part. For the ASTM D2559 delamination exposures (3), specimens were made of six lumber laminae, bonded together and cut into three central test sections and two unused end sections (Fig. 4). Specimens containing either Douglas-fir or yellow birch lumber were used. Twelve sections of laminate were tested for each exposure condition. The exposures for this test are composed of three cycles: i. vacuum-water soak for 5 minutes, pressure-water soak for 1 hour, repeat those two steps, dry at 66C for 21 to 22 hours; ii. steam at 100C for 1.5 hour, pressure-water soak for 40 minutes, dry as before; and iii. repeat cycle 1. The second test was HPVA HP-1-1994 (5), a two-cycle boil test of the Hardwood Plywood and Veneer Asso- Adhesives Technical Interest Group Session 491
Table 1. Dry and wet shear strengths of polyurethanes C and D and RF adhesive bonds to Douglas-fir, with and without HMR primer. Shear strength (MPa) Polyurethane C Polyurethane D Resorcinolformaldehyde Test a Unprimed HMR-primed Unprimed HMR-primed (unprimed) Dry 11.38 13.33 11.85 12.58 9.27 VAS 5.90 6.12 6.00 6.20 6.52 VPS 6.25 6.10 6.37 6.44 6.63 BDB 5.13 4.85 4.77 5.15 5.09 a VAS = vacuum-atmospheric soak; VPS = vacuum pressure soak; BDB = boil-dry-boil. Table 2. Dry and wet shear strengths of polyurethanes C and D and RF adhesive bonds to yellow-birch, with and without HMR primer. Shear strength (MPa) Polyurethane C Polyurethane D Resorcinolformaldehyde Test a Unprimed HMR-primed Unprimed HMR-primed (unprimed) Dry 22.02 22.53 22.00 23.67 19.10 VAS 8.30 8.18 8.40 8.43 9.01 VPS 8.38 8.28 8.00 8.12 8.89 BDB 7.11 7.38 7.44 7.26 8.09 a VAS = vacuum-atmospheric soak; VPS = vacuum pressure soak; BDB = boil-dry-boil. Figure 4. Lumber laminate and scheme for obtaining specimen blocks for ASTM 02559 delamination test. ciation. It consists of the following steps. Samples were submerged in boiling water for 4 hours. They were dried for 20 hours at 63 C. They were boiled again for 4 hours. They were dried for 3 hours at 63 C, and then they were examined for delamination. Any delamination greater than 25.4 mm is a failure. Within a lot, 90 percent of specimens must pass. A third test used specimens and a loading apparatus described in ASTM D4680-92 (4). Specimens were tested for resistance to loading under conditions described in ASTM D2559-92, Section 16. In this test, some specimens are statically loaded at 414 kpa intervals up to 2,069 kpa. Some specimens were exposed to an environment of 71 C at ambient humidity, and some specimens to 27 C at 90 percent relative humidity. Although the specification requires the test be continued for only 7 days, in this work, it was continued for 60 days. Results and Discussion Polyurethane bonds when tested in the dry state were somewhat stronger than bonds of RF adhesive, on both Douglas-fir (Table 1) and yellow birch (Table 2). However, when tested after the VAS, VPS, or BDB cycles, the RF adhesive bonds were usually slightly stronger. Wood failure values in dry tests for polyurethanes C and D were as high for both unprimed and primed Douglas-fir as those for the RF adhesive (Fig. 5). The wood failure values for wet tests, however, were significantly higher for polyurethane bonds that were HMRprimed compared with bonds that were unprimed. Polyurethane bonds that were HMR-primed were slightly better than those of RF adhesives. For yellow birch, the results (Fig. 6) showed that wet wood failure 492 FPS 1999 Annual Meeting
Figure 5. Dry and wet wood failure values for bonds of polyurethanes C and D and for RF adhesive on unprimed and primed Douglas-fir. Figure 6. Dry and wet wood failure values for bonds of polyurethanes C and D and for RF adhesive on unprimed and primed yellow birch. Table 3. Two-cycle boil-dry delamination values of unprimed and HMR-primed wood with polyurethane C and RF adhesives. Delamination a (mm) Yellow birch Douglas-fir Polyurethane C RF Polyurethane C RF Specimen Unprimed HMR-primed Unprimed Unprimed HMR-primed Unprimed 1 31.8 0 0 6.4 0 0 2 11.7 0 0 11.4 0 0 3 0 0 0 0 0 0 4 0 0 0 0 0 0 5 19.3 0 0 29.2 0 0 a Specimens with more than 25.4 mm of continuous delamination fail the test. Ninety percent of the specimens must pass. values for the polyurethane bonds to unprimed wood were much lower than for the RF-bonded specimens. However, the polyurethane bonds to primed wood had very high wood failure values. Two-cycle boil-dry delamination tests of primed and unprimed wood with polyurethane C and with RF adhesive showed (Table 3) that priming increased delamination resistance for polyurethane bonds so that they were comparable with RF bonds. Polyurethane bonds to unprimed wood showed significant delamination. This assessment is reinforced by results of Douglas-fir laminates put through the ASTM D2559 test (Fig. 7). This showed that the HMR-primed specimens bonded with polyurethane C and D passed the 5 percent maximum delamination requirement. The unprimed polyurethane bonds showed poor resistance to delamination. Since the wet tests showed low wood failure and low resistance to delamination for unprimed polyurethane bonds, there was some doubt about the creep resistance i Figure 7. Delam nation of polyurethane bonds in unprimed and HMR-primed Douglas-fir lumber joints after the ASTM D2559 cyclic delamination test. Error bars indicate one standard deviation. Adhesives Technical Interest Group Session 493
of polyurethane under stress. However, the specimens that were put through ASTM D2559 tests for resistance to deformation under static loading for 60 days (instead of 7 days) showed no measurable creep at loads up to 2,069 kpa either at elevated temperature or at elevated humidity conditions. Concluding Remarks In dry environments, one-part polyurethanes provided strong bonds of high shear strength and wood failure values equivalent to RF adhesive. Polyurethane resistance to creep in thin bondlines was excellent. With no priming and in wet environments, the polyurethanes had high shear strengths but their wood failure values were much lower than for RF adhesive. In severe delamination tests, polyurethanes had far less durability than RF adhesive. With HMR primer applied to the wood, the polyurethanes had high shear strengths and the wood failure values in wet environments were equivalent to RF adhesives. In severe delamination tests, polyurethanes on HMR-primed wood were equivalent to the RF adhesive in durability. References 1. American Plywood Association. 1983. U.S. product standard PS 1-83 for construction and industrial plywood with typical APA trademarks. APA, Tacoma, WA. 2. American Society for Testing and Materials. 1997. Standard test method for strength properties of adhesive bonds in shear by compression loading. ASTM D905-94. ASTM, West Conshohocken, PA. 3. American Society for Testing and Materials. 1997. Standard specification for adhesives for structural laminated wood products for use under exterior (wet use) exposure conditions. ASTM D2559-92 (reapproved 1996). ASTM, West Conshohocken, PA. 4. American Society for Testing and Materials. 1997. Standard test method for creep and time to failure of adhesives in static shear by compression loading. ASTM D4680-92. ASTM, West Conshohocken, PA. 5. Hardwood Plywood and Veneer Association. 1995. American national standard for hardwood and decorative plywood. ANSI/HPVA HP-1 1994. HPVA, Reston, VA. 6. Strickler, M.D. 1968. Adhesive durability: Specimen designs for accelerated tests. Forest Prod. J. 18(9):84-90. 7. Vick, C.B. and E.A. Okkonen. 1998. Strength and durability of one-part polyurethane adhesive bonds to wood. Forest Prod. J. 48( 11/12): 71-76. 8. vick, C.B., K.H. Richter, and B.H. River. 1996. Hydroxymethylated resorcinol coupling agent and method for bonding wood. Inventors, USDA assigned. U.S. Patent 5,543,487. 494 FPS 1999 Annual Meeting
Wood Adhesives 2000 June 22-23,2000 Harveys Resort Hotel Casino South Lake Tahoe, Nevada Sponsored by The USDA Forest Service, Forest Products Laboratory in cooperation with the Forest Products Society the Adhesion Society International Union of Forestry Research Organizations (IUFRO) the Japan Wood Research Society Forest Products Society 2801 Marshall Court Madison, WI 53705-2295 phone: 608-23 1-1361 fax: 608-23 1-2 152 www.forestprod.org