Strength and durability of alkylresorcinolic adhesives from Estonian oil shale kerogen

Bryan H. River

have developed a series of exterior-type adhesives based Abstract on Estonian kerogen. We received some samples of Estonian scientists have developed adhesives from these adhesives during a recent technologicalexchange the kerogen in Estonian oil shale. The adhesives, based with the Soviet Union. (The exchange was with the on a series of alkylresorcinols extracted from the ker­ Ministry of Building Construction, Moscow, U.S.S.R.) ogen, have distinct economic and environmental advan­ Our objective has been to compare them with re­ tages over resorcinol-based resins for wood laminating. sorcinolic adhesives. A previous report (4) summarized In this study we evaluated their unaged shear strength the literature on the Estonian kerogen-based adhesives and wood failure in bonded hard maple specimens and and compared four of them with eight phenolic and the change in these properties after elevated tempera­ resorcinolic resins by these laboratory tests: ture aging in constant wet and dry exposures. Based on 1. Infrared spectral analysis; these studies, the adhesives are quite comparable in strength, although small but statistically significant 2. Nuclear magnetic resonance spectral analysis differences are detected in both unaged and aged speci­ and; mens. They should have comparable service lives in 3. Differential scanning calorimetry. constant service conditions. The major difference be­ This report comparesthe shear strength, wood fail­ tween the adhesives is the low wood failure in speci­ ure, and durability in wet and dry exposures of two mens bonded with alkylresorcinolic adhesives. Low Estonian kerogen-based adhesives and two resorcinolic wood failure suggests the alkylresorcinolic adhesives adhesives. may be sensitive to cyclic swelling and shrinking. In­ Methyl and dimethyl resorcinols or alkyl­ creasing adhesive failure may not be a problem with resorcinols are the important constituents in Estonian softwood lumber or with different bonding conditions, kerogen for resin synthesis (14). They are obtained by but further exploration will be required before alkyl­ dry distilling the kerogen between 270°C and 290°C. resorcinolic adhesives could be accepted as equal sub­ The principal alkylresorcinols in the distillate, shown stitutes for resorcinolic adhesives in structural lami­ in Figure 1A, are as follows (5, 12, 14): nates for exterior use. 1. 5-methyl resorcinol (36%-56% of total distillate); 2. 4,5-dimethylresorcinol (6%-15% of total Upheavals in the oil industry during the 1970s distillate); have had lasting effects on the adhesives industry. 3. 2,5-dimethylresorcinol (11%-12% of total Periods of uncertain price and availability of petro­ distillate). chemicalsrocked the industry. Both suppliers and users were hard hit. Now supplies are once more abundant and prices have dropped, but the market’s stability is uncertain. Adhesives suppliers and users maintain a The author is a Research Forest Products Technologist, strong interest in developing systems equivalent to USDA Forest Serv., Forest Prod. Lab., One Gifford Pinchot Dr., Madison, WI 53705. E. Arnold Okkonen, Physical Science phenolic and resorcinolic resins from nonpetroleum Technician at the FPL, assisted in the experimental work. Al­ sources. fred W. Christiansen, Chemical Engineer, and George E. My­ One possible source is kerogen from oil shale. The ers, Research Chemist, both of the FPL staff, contributed suggestions and helped interpret the results. This paper was kerogen from Estonian oil shale is reportedly uniquely received for publication in May 1985. suited to the synthesis of adhesive resins (14). In fact, © Forest Products Research Society 1986. scientists at the Tallin Technical University in Estonia Forest Prod. J. 36(4):25-34.

FOREST PRODUCTS JOURNAL Vol. 36, No. 4 25 905 shear tool (2). We used the same shear specimen and test to evaluate durability by measuring changes in the strength and wood failure of specimens after they had been aged at elevated temperatures for various lengths of time. We also conducted wet and dry aging experi­ ments. The resulting data provide the following types of information about the adhesive or its bonded joints: 1. Strength and wood failure; 2. Strength and wood failure after exposure to heat and moisture; 3. Degradation rates in wet and dry environments; 4. Estimated failure time in wet and dry environments. Experimental design Materials Adhesives. – The Ministry of Building Con­ struction, Moscow, U.S.S.R., supplied three adhesives Figure 1. – A) Three alkylresorcinols comprising 62 percent (DFK-14R, FR-100, and FRF-50) as part of a tech­ of the fraction dry distilled from Estonian oil shale kerogen nological exchange. Two of these, DFK-14R and between 270° and 300°C. B) Hydroxymethylation of the 5 FR-100, are based on alkylresorcinol. The third, methyl resorcinol (complexed with acetone) by formalin. FRF-50, is a conventional phenol-resorcinolic adhesive. We included one commercial U.S. phenol-resorcinolic adhesive, 5-PRF, for comparison in this study. Some comparative properties of these adhesives are sum­ Adhesives are formed by hydroxymethylation of the dry marized in Table 1. distillate with formalin (Fig. 1B) in the ratio of 1.7-2.2 Adherends. – We selected hard maple (Acer sac­ moles of formaldehyde to 1 mole of the distillate (aver­ charum Marsh.) lumber according to the recommen­ age mole weight 140 to 150) and subsequent poly­ dations in ASTM test method D 905 (2). condensation of the hydroxymethylated alkyl­ resorcinols to form alkylresorcinol novolak adhesives. Preparation Adding a complexing agent such as acetone or cap­ Adhesives. – The adhesives were prepared accord­ rolactam to the distillate before adding the formalde­ ing to the adhesive suppliers' instructions. hyde slows the hydroxymethylation reaction to a Adherends. – The adherends were prepared as manageable rate (5). Even so, the alkylresorcinol mix­ follows: ture reacts with formalin eight times faster than does 1. Saw rough 1-inch lumber into twenty 127- by resorcinol (5, 14). 356-mm (5 by 14 in.) blanks; Aside from exterior performance and independence 2. Condition blanks to equilibrium moisture con­ from petroleum, these adhesives have several reported tent (EMC) at 27°C and 65 percent relative humidity advantages (8, 12, 13): (RH); 1. More energy efficient (due to faster cure); 3. Resaw each blank on a bandsaw through the 2. Less toxic (due to lower free phenol and formal­ thickness to form two adherends approximately 11 by dehyde contents); 127 by 356 mm (7/16 by 5 by 14 in.); 3. Cheaper (due to the use of natural resorcinols) 4. Surface each adherend just before bonding with a than resorcinol adhesives derived from petroleum. freshly sharpened three-blade hand-fed jointer; In spite of these reported advantages, after 10 years of 5. Reduce adherend thickness to 7.9 mm (5/16 in.) use in the U.S.S.R. and 6 in Japan, these adhesives have by abrasive planing on the opposite side; received little attention in the West. The United States 6. Cut adherends to 280-mm (11 in.) length; has large oil shale deposits, and although the kerogen is 7. Pair adherends randomly for bonding; reported less suitable for adhesive feedstocks,1 the po­ tential should not be overlooked. 8. Recondition adherends at 27°C and 65 percent RH not more than 24 hours before bonding. Approach Panels. -Theadhesive spread rate, assembly time, We measured bond strength by a compression shear bonding pressure, temperature, and cure time recom­ test employing a reduced version of the standard ASTM mended by the adhesive supplier and the conditions we D 905 block shear specimen and the standard ASTM D used are given in Table 2. The procedure was to: 1. Spread the adhesive on the adherends' jointed surface with a grooved rubber roll spreader, applying equal amounts to each adherend; 1Estonian oil shale kerogen has a high resin content and many 2. Cure the adhesive; phenolic compounds while U.S. shale kerogen is reported to have lower resin contents and very low content of phenolic 3. Recondition at 27°C and 65 percent RH; compounds (11).

26 APRIL 1986 TABLE 1 – Adhesive characteristics. Mix Mixed ratiod adhesive Resin resin/ Adhesive Type solidsa pHb Viscosityc hardener Reactivitye Potlifed (cps at (min at (%) 25°C) 20°C) FRF-50 Standard phenol- resorcinol- formaldehyde used for laminating in U. S. S.R. 65 9.1 365 100/13.5 4 210 DFK-14R Phenol-alkyl- resorcinol(U.S.S.R.) 30%-40% phenol substitution 58 91 411 100/13 5 2 150 FR-100 Alkylresorcinol (U.S.S.R.) 60 89 171 100/13.5 1 60 5-PRF Phenol-resorcinol- formaldehyde used for laminating in U.S. 55 96 3,038 100/25/25ff 3 180 aDetermined by heating 1 to 2 g of resin + hardener in an aluminum dish at 105°C until a constant weight was attained (± 0 001 g) (A. W. Christiansen, personal communication, 1983) bElectrode calibrated at pH 9.18 (A. W. Christiansen, personal communication, 1983). cDetermined with a Brookfield viscometer (A. W. Christiansen, personal communication, 1983) dInformation from product data sheets eRank based on exotherm peak temperature detected by differential scanning calorimetry with heating at 10°C/min and upon calculated cure rate at 20°C (4). fResin/hardener/added water

TABLE 2. -Bonding conditions. Adhesive Spread Assembly Pressure Temperature Time (kg/m2) (min.) (MPa) (°C) (hr.) FRF-50 0.35-0.60a <150 0.5-1.0 23 12-16 0.44b 60 1.2 23 18 DFK-14R 0.35-0.60 40-50 0.5-1.0 23 12-16 0.44 30 1.2 23 18 FR-100 0.35-0.60 30-40 0.5-1.0 23 10-12 0.44 30 1.2 23 18 5-PRF 0.25-0.34 60 0.5-1.0 23 10-12 0.29 45 1.2 23 18 aTop number = recommended. bBottom number = actually used.

4. Store panels for 3 to 4 months before cutting into specimens; 5. Lay out specimen cutting pattern and mark each specimen with a code for the adhesive, panel number, and specimen number; Figure 2. – Reduced block shear specimen used for deter­ 6. Cut individual specimens from the panels. mining the adhesives shear strength and resistance to aging (17). Ten panels were prepared with each adhesive. Specimens. – Twenty-eight (four rows of seven) reduced-size ASTM D 905 shear block specimens (17) mens for each time-temperature combination within were cut from each panel. The specimen is 16 mm (5/8 the group (Table 3). in.) thick, with a 645-mm2 (1 in.2) bond area, and 6.4-mm (1/4 in.) offsets at each end (Fig. 2). The finished 1. Dry conditioning treatment (for unaged dry specimens were reconditioned to EMC at 27°C and 65 strength): percent RH and stored at those conditions until use. a. Condition the 20 specimens to EMC at 27°C and 30 percent RH; Specimens were drawn at random from the total lot b. Store at 27°C and 30 percent RH until test. of 280 (of each adhesive) and assigned to 1 of 4 groups: unaged dry strength, unaged wet strength, dry aging, or 2. Wet conditioning treatment (for unaged wet strength): wet aging. a. Soak 20 specimens in tap water at atmo­ Conditioning and aging treatments spheric pressure for 18 hours; Each unaged strength group was assigned 20 b. Soak 1/2 hour under 685-mm (27 in.) Hg vac­ specimens. Each aging group was assigned 20 speci- uum;

FOREST PRODUCTS JOURNAL Vol. 36, No. 4 27 TABLE 3. – Aging intervals. 5. Conduct an analysis of variance of strength Dry exposure Wet exposure differences due to adhesive, temperature, and time aging temperature aging temperature effects within the dry and the wet aging treatments; Test 120°C 135°C 150°C 65°C 75°C 85°C 6. Test the significance of strength differences ob­ served among adhesives at each individual aging Aging temperature/time using Tukey's studentized range test period 1 46 9 2 28 9 3 (16); (Est. 10% loss) 7. Calculate the least squares fit of the regression Aging of log strength on aging time for each adhesive at each period 2 93 18 4 57 18 6 moisture-temperature aging treatment (Appendix, (Est. 20% loss) Equation [1]); 8. Calculate the estimated failure time for each adhesive at each aging temperature in both the wet and c. Release vacuum but keep specimens in water; dry aging environments (Appendix, Equation [2]); d. Soak 1/2 hour under 450 to 510 kPa (65 to 75 9. Calculate the least squares fit of the regression psi); of log failure time on reciprocal temperature for both the e. Release pressure; wet and dry aging environments (Appendix, Equation f. Store in water at 27°C until test. [3]). 3. Dry aging treatment: Results and discussion a. Condition all specimens to EMC at 27°C and In the following discussion, the individual alkyl­ 30 percent RH; resorcinolic and phenol-alkylresorcinolic adhesives are b. Stabilize three ovens at aging temperatures of frequently referred to collectively as alkylresorcinolic 120°, 135°, and 150°C; or simply as alkyl adhesives and the phenol-resorcinolic c. Place 2 groups of 20 specimens in each oven; adhesives collectively as resorcinolic adhesives. d. Age each group for a preselected time (see times in Table 3); Strength of unaged e. Recondition specimensto EMC at 27°C and 30 specimens (Tables 4 and 5) percent RH; The four adhesives when tested dry exceed the 14.5 f. Store at 27°C and 30 percent RH until test. MPa strength requirement for hard maple joints in the 4. Wet aging treatment: U.S. standard for structural glued-laminated timber a. Presoak all specimensby the wet conditioning (1). The four adhesives are comparable in strength to treatment; each other and to similar specimens of solid maple and b. Stabilize three water baths at 65°, 75°, and hot-press phenolic bonded maple tested in an earlier 85°C; study (15). Their strengths greatly exceed 16 MPa 2 C. Place 2 groups of 20 specimens in each water (2,330 psi), the strength of solid hard maple (7). When bath; soaked and tested wet, they lose 46 to 55 percent of their d. Age each group for a preselected time (see dry strength on the average. These levels compare Table 3); favorably to 55 and 63 percent losses for solid maple and e. Remove from water bath, cool to room tem­ hot-pressed phenolic bonded maple specimens tested perature in water; under the same conditions by Millett and Gillespie (15). f. Store in water at 27°C until test. In spite of the relative uniformity of the strengths in Table 4, small but statistically significant differences Testing and data analysis between adhesives are detected by Tukey's studentized The specimens were tested in the apparatus and range test (16). according to the procedure described in ASTM method D The test indicates that 905 (2) except for use of a crosshead speed of 5 mm/min. 1. Adhesive 5-PRF (phenol-resorcinol) is sig­ (0.2 in./min.). The load at failure was recorded and the nificantly stronger than adhesive FRF-50 (phenol­ percentage of wood failure of each specimen was esti­ resorcinol) and FR-100 (alkyl-resorcinol) in the dry test mated. Wet specimens were air-dried before wood fail­ and; ure estimation. 2. Adhesives FRF-50 and 5-PRF (both resorcinolic) The data were used to: are significantly stronger than DFK-14R and FR-100 1. Calculate the strengths of unaged specimens as (both alkyl-resorcinolic) in the wet test. the average shear stress at failure of unaged specimens; Soviet researchers report shear strengths in the range 2. Test the significance of differences observed in of 12 to 15 MPa for beech and oak specimens (com­ the strengths of unaged specimens of the four adhesives by Tukey’s studentized range test (16); 3. Calculate the strengths of aged specimens as the average shear stress at failure of aged specimens; 2The bonded specimens’ strengths are higher than the species' strength due to differences in the size and shape of the test 4. Calculate the average wood failure percentages specimens and differences in the amount of offset in the shear of each adhesive's specimens in each conditioning and tool. In an unpublished study, we found the increase could be aging treatment; as great as 100% because of these factors.

28 APRIL 1986 TABLE 4. – Initial strength and wood failure of specimens tested dry and TABLE 5 – Initial strength and wood failure of specimens tested wet, differencesa among adhesive's specimen's strengths. differencesa among adhesives, and comparisons to dry strengths Adhesive Adhesive Hot-press Solid Hot-press Solid PRF-5 DFK-14R FR-100 FRF-50 phenolicb mapleb PRF-5 DFK-14R FR-100 FRF-50 phenolicb mapleb Avg. shear 25.5 23.7 23.2 21.9 27.5 21.2 Avg. shear 11.9 11.8 10.8 10.4 10.3 12.1 strength (MPa) strength (MPa) Wood failure (%) 44 0 10 80 69 100 Wood failure (%) 100 85 10 23 57 100 a Reduction from Average strengths not underscored by the same line are significantly dry strength(%) 53 46 54 55 63 56 different from each other 99% of the time when compared by Tukey’s studentized range test (16). aAverage strengths not underscored by the same line are significantly bValues from Millett and Gillespie who used the same species, specimen, and different from each other when compared by Tukey’s studentized range test test (15). (16). bValues from Millett and Gillespie who used the same species, specimen, and test (15).

parable in density to maple) bonded with adhesives 3. The bonding conditions might not have been FRF-50, DFK-14R, and FR-100 (5, 9). The strengths optimal for maple; they report are in the same range as the wet strengths in 4. Maple might be incompatible with alkyl this study. But because the moisture content (MC) in adhesives. their tests is unspecified, I assume they are dry First, the adhesives received by the Laboratory strengths. The difference in the results is caused by the were delayed several months in shipping. One of the five larger size of the Soviet specimen (50 mm square as adhesives received, a very reactive acid curing phenolic, compared to 25.4 mm square) and possibly by differ­ was too viscous to use, so it seems possible the others ences in the test fixtures for the reasons given in foot­ may not have been in prime condition. note 2. Second, microscopic examination after testing re­ Wood failure of vealed the presence of small cavities widely distributed unaged specimens in the cured alkyl adhesives bondlines. The cavities are The amount of specimen wood failure, in contrast to more frequent and often larger than observed in bond- strength, differed greatly between adhesives in this lines of the phenol-resorcinolic adhesives even though study (Table 4). Specimens bonded with phenol­ all the adhesives were mixed similarly. Some cavities resorcinolic adhesive, FRF-50, exceed the 60 percent appear filled with a tan powder that could be undis­ wood failure requirement of the U.S. standard for wet- persed hardener; others are empty. The cavities might use adhesives for glued-laminated timber (1). Speci­ also have been formed by air entrained during mixing or mens of the others did not. But the most striking result by entrapped gaseous reaction products. In any case, the is the virtual absence of wood failure in the specimens of greater incidence of voids in the bondlines of the alkyl the alkyl adhesives tested dry (Table 4). This result is adhesives would certainly reduce the adhesives' ap­ puzzling because the strengths of specimens of the four parent strength and the tendency for failure to occur in adhesives are comparable and because the strengths of the wood. the two phenol-resorcinolic adhesive bonded specimens Third, adhesives for structural laminates are (both with high wood failure) bracket the strengths of usually formulated for softwood species. Although the alkyl adhesive bonded specimens (both with low maple is an acceptable wood for qualifying adhesives for wood failure). structural glued-laminated wood, lower density woods Soviet reports (5, 9) do not give actual wood failure are normally used and so formulations tend to favor values of hardwood specimens. They do mention the softwood bonding. It is often difficult to achieve high occurrence of low or inconsistent wood failure in alkyl wood failure percentages in hard maple with com­ adhesive joints of dense hardwoods. A Japanese re­ mercial room-temperature cured phenol-resorcinolic searcher reports that wood failure percentage in hard­ laminating adhesives without carefully optimizing the wood joints bonded with alkyl adhesives is normally bonding conditions. The final possibility is that maple inferior to the percentage expected in resorcinolic ad­ wood and the alkyl adhesives are chemically hesive bonded joints but that it becomes comparable incompatible. upon heating for 6 hours at 80°C (12). However, neither Regardless of which factor caused it, further study the Soviet nor the Japanese reports suggest that the of this behavior is needed. It is important because low wood failure percentages of hardwood joints observed by wood failure of otherwise strong joints often indicates the researchers were as low as the percentages revealed poor resistance to cyclic swelling and shrinking in this study. Some other factor must be involved, such stresses, a serious deficiency for an exterior-type struc­ as: tural adhesive (6). In support, one Soviet report shows 1. The adhesive may have been damaged or beyond 30 percent more strength loss by an alkylresorcinolic its useful storage life; adhesive (FR-100) than by a phenol-resorcinolic ad­ 2. The adhesive components may have been inad­ hesive (FRF-50) after exposure to 40 swell-shrink cycles equately mixed; (3).

FOREST PRODUCTS JOURNAL Vol. 36, No. 4 29 Strength after aging at room temperature. A portion of each increase might The aging behavior is summarized in 24 log be attributed to thermal stress relief. But post-cure is strength-time and 24 wood failure-time curves. These probably the major factor because the lowest aging are compressed into two figures by plotting log strength temperature (120°C) is higher than the highest low versus aging interval (Fig. 3) and wood failure per­ temperature exotherm (116.5°C) of any of these ad­ centage versus aging interval (Fig. 4). The aging inter­ hesives (4). Furthermore, in recent unpublished work at vals in the figures are the aging times estimated to the FPL, the fracture toughness of a catalyzed, room- cause approximately 10 and 20 percent strength loss at temperature cured urea-formaldehyde adhesive, stored a given temperature and moisture condition. The inter­ at room temperature, changed in an orderly fashion for vals vary with the temperature and moisture condition as long as 9 months after bonding. (see Table 3). Post-cure is most pronounced in the phenol­ Based on previous experience with the aging be­ resorcinolic adhesive DFK-14 in the dry exposures (Fig. havior of adhesives (10, 15), we expected a straight line 3). It is curious that this adhesive should be so strongly or a monotonically decreasing log strength-time (or log affected in the dry exposure, for it is the second most strength-aging interval) relationship for fully cured reactive of the four adhesives at temperatures below phenolic and resorcinolic adhesives. We expected to 120°C (the lowest dry aging temperature), and the least observe the same behavior by the adhesives in this reactive at temperatures above 120°C (4). The sole ex­ study because the adhesives, although cured only at ception to the general post-curing behavior in dry aging room temperature, cured for over 3 months before the is the alkylresorcinol adhesive, FR-100, aged at 120°C. aging exposures began. But as Figure 3 shows, in some It behaves like the other adhesives at 135° and 150°C, dry exposures, specimen strength actually increases but in the first 120°C aging interval, it loses strength above the unaged strength during the first aging inter­ rapidly as if it does not post-cure. FR-100 is also the only val. In the remaining dry exposures (with the exception adhesive without a phenolic constituent and the only of FR-100 at 120°C), there is a net strength decrease in adhesive without an exotherm above 100°C (4). These the first aging interval, but there is also indication of factors may be responsible for its anomolous behavior at the strength increasing before decreasing. The in­ 120°C. creases can be visualized where they occur by extra­ The most striking features of the log strength- polating the line (the dashed lines in Fig. 3) between the aging interval plots for wet aging are the pronounced first and second aging intervals to its intercept with the increases in the strength of all four adhesives at zero aging interval (zero aging time). 65°C in the first aging interval, and the in­ The extrapolated strengths at zero aging time are 7 creases in the strength of the alkyl adhesives at to 20 percent above the measured strengths at zero 75°C in the second aging interval. aging time (unaged strengths), with the exception of In contrast to dry aging, strength increases in the FR-100 at 120°C. 65°C aging exposure are probably due to stress Strength increases such as these upon exposure to relief rather than post-cure. Sixty-five degrees Celsius elevated temperatures suggest post-curing reactions is well below the lowest low temperature exotherm have occurred in all four adhesives even after 3 months (76°C) (4) that might induce post-cure and the

Figure 3. – Plots show the log shear strength at zero aging time (initial shear strength) and after the first and second aging intervals in wet and dry exposures. The points where the dashed lines intersect the zero aging interval indicate the probable maxi­ mum strength of the specimens had they been fully cured (hot-pressed) be­ fore aging.

30 APRIL 1986 Figure 4. – Wood failure percentages of specimens at zero aging time and after the first and second aging inter­ vals in wet and dry exposures. The horizontal line at 60 percent rep­ resents the minimum wood failure re­ quirement for maple bondlines in structural glued-laminated timbers (1). Solid lines (—) are dry be­ havior and dashed lines (------) are wet behavior.

TABLE 6 – A comparison of adhesive joint strengths after aging Dry Wet Treatment Rank and differencea Treatment Rank and differencea 120°C/46 days 4b 2 1 3 65°C/28 days 4 1 2 3 120°C/93 days 4 2 1 3 65°C/57 days 4 1 2 3 135°C/9 days 4 2 3 1 75°C/9 days 4 1 2 3 135°C/18 days 4 2 1 3 75°C/18 days 2 3 4 1 150oC/2 days 2 4 3 1 85°C/3 days 4 1 2 3

150°C/4 days 4 2 3 1 85°C/6 days 4 2 1 3 aThe adhesives are arranged by decreasing strength from left to right Differences between adhesives are compared by Tukey’s studentized range test (16). Adhesives whose code numbers are not underscored by the same line are significantly different from each other 95% of the time. bAdhesive code: 1 = FRF-50; 2 = DFK-14R; 3 = FR-100; 4 = 5-PRF. hot, wet exposure would certainly encourage stress re­ wood failure in phenol-resorcinolic adhesive bonded lief. The increases in the second aging interval at specimens increases to the 90 to 100 percent level in the 75°C are unexpected and unexplained. They are first aging interval and remains generally constant in statistically significant and they occur in both alkyl the second interval. These levels are acceptable under adhesives. the U.S. specification for structural glued-laminated Differences in the strengths of specimens of the timbers in exterior-use conditions (1).3 Wood failure in adhesives after equal amounts of dry aging (compared the phenol-alkylresorcinolic specimens increases from in Table 6) are significant just as are the differences in values below 10 percent for unaged specimens up to the the strengths of unaged specimens. Adhesive 5-PRF is 40 to 55 percent range with aging (Fig. 4); but this is not significantly stronger than adhesives FRF-50 and enough to meet the specification requirement. The FR-100 after both aging periods in all the dry aging alkylresorcinolic adhesive, FR-100, fares less well. Al­ treatments. There are fewer significant differences in though the wood failure of its specimens increases to wet aging, but adhesive 5-PRF is significantly stronger almost 40 percent with wet aging, it decreases to almost than adhesive FR-100 in every aging period and treat­ zero with dry aging. This suggests that the adhesive is ment except one. These strength differences are small more sensitive to heat than the wood adherend or than (with the exception of adhesive FR-100 at the phenolic and resorcinolic adhesives within our 120oC), but the specimen to specimen variability experience. is also small and this leads to a test that is very sensitive to small differences. The point is that the specimens of the adhesives are really quite comparable in terms of their strength. Wood failure after aging Wood failure increases with aging in every case 3The specification actually requires a minimum of 60% wood except in FR-100 specimens (Fig. 4). The percentage of failure of dry unaged specimens. Aging is not required.

FOREST PRODUCTS JOURNAL Vol. 36, No. 4 31 Degradation rates because TABLE 7. – Degradation rate (k). of wet and dry aging Exposure Adhesivea degradation rate (MPa) The strength degradation rates by adhesive and DRY exposure and their significance are shown in Table 7. 120°C W 5-PRF A FR-100 W FRF-50 A DFK-14R The table also shows the predominant type of failure -0.020 -0.045 -0.050 -0.115b 135°C W FRF-50 W 5-PRF A DFK-14R A FR-100 (wood or adhesive) and indicates whether it changes - 0.299b - 0.390b - 0.425b - 0.534b during aging. 150°C W 5-PRF A FR-100 W FRF-50 A DFK-14R -0.946 - 1.34 - 1.70b -2.15’ The least durable adhesives degrade two to five WET times faster than the most durable. We were able to test 65°C -A FR-100 W 5-PRF -A DFK-14R W FRF-50 whether or not the rates are statistically significantly -0.015 0.028b -0.031b 75°C -A FR-100 -A DFK-14R W FRF-50 W 5-PRF different from zero, but unfortunately we could not test +0.121 -0.032 -0.064 the differences between the rates of individual ad­ 85°C -A FR-100 -A DFK-14R W 5-PRF W FRF-50 hesives. Nevertheless, useful conclusions can be drawn - 0.057 -0.136 -0.186b -0.208b from the significance and rank of the degradation rates aLetter preceding the adhesive name indicates the predominant location of and the location of failure. failure: W = wood; A = adhesive; -A = decreasing adhesive. bReject null hypothesis (k = 0) with less than 5% probability of error. In dry aging: cStrength increased, no degradation rate calculated. 1. Alkylresorcinolic bonded specimens fail pre­ dominately in the adhesive; 2. Resorcinolic bonded specimens fail pre­ dominately in the wood; In the present case, increasing wood failure of DFK-14R specimens in both wet and dry aging and of 3. Alkylresorcinolic bonded specimens have sig­ FR-100 specimens in wet aging (Fig. 4) suggests that nificant degradation rates (with two exceptions in six the adhesive, although still weaker at the end of our cases); experiments, may be degrading slower than the wood 4. Resorcinolic bonded specimens have in­ and may eventually become the stronger element. significant degradation rates (with three exceptions in six cases). Failure-time and temperature Therefore alkylresorcinolic adhesives are less resistant dependence in wet and dry aging to dry aging effects (thermolysis) than resorcinolic ad­ Failure in this study is defined as the loss of hesives or solid wood. strength to 75 percent of the unaged shear strength. The In wet aging: time to failure (or failure time) depends on the tempera- ture and moisture levels. The failure time at a par­ 1. Alkylresorcinolic bonded specimens fail pre­ ticular temperature and moisture condition can be es­ dominately in the adhesive, but failure in the wood timated from the degradation rate equations (Ap­ increases over time; pendix, Equations [2] and [3]) determined for those 2. Resorcinolic bonded specimens fail pre­ conditions. These are shown in Figure 5. dominately in the wood; The regression of estimated failure time on the 3. Alkylresorcinolic bonded specimens have in­ reciprocal of the aging temperature represents the tem­ significant degradation rates (with one exception in six perature dependence of the life of the bonded specimen cases); in a particular constant moisture environment (Fig. 5). 4. Resorcinolic bonded specimens have significant The slope of the line represents the sensitivity of the degradation rates (with two exceptions in six cases). failure time to temperature. Therefore alkylresorcinolic adhesives, although in­ In dry aging, the failure times of the alkylresorcino­ itially weaker than wood and resorcinolic adhesives, are lic bonded specimens are comparable to those of the more resistant to wet aging effects (hydrolysis) than resorcinolic adhesive bonded specimens, with the excep­ resorcinolic adhesives or solid wood. tion of adhesive 5-PRF at 120°C (Fig. 5). Fur­ There is a risk in these conclusions. Within the thermore, their failure times are comparable to the aging times of these experiments, the resorcinolic ad­ failure times observed by Millett and Gillespie (15) for hesives are always stronger than the wood they bond. solid maple and phenolic bonded hard maple joints (also The observed degradation rate of these specimens is the plotted in Fig. 5). We did not statistically test these degradation rate of the wood adherends in a given aging differences and we could not test the differences in the environment. The actual degradation rates of the re­ temperature sensitivity of individual specimens of the sorcinolic adhesives are hidden. Resorcinolic adhesives adhesives. However, in dry aging, specimens bonded may actually degrade faster than either the wood or the with one resorcinolic adhesive (FRF-50) appear to have alkyl adhesives, but within the aging times of this study the same temperature sensitivity as the solid maple and and past studies (10, 15), they are always stronger so we phenolic adhesive bonded maple specimens in Millett don’t know their actual rates. If they do degrade faster, and Gillespie’s earlier study. The alkyl adhesive bonded then after a long enough time (aging times longer than specimens (DFK-14R and FR-100) appear to be slightly the ones we used) the phenol-resorcinol adhesive would less temperature sensitive than the others. become weaker than the wood, its rates would become This result supports the conclusion that alkyl ad­ visible, and it is possible the adhesive might degrade to hesives are less resistant to thermolysis than wood, zero strength before the wood or the alkyl adhesives. phenolic, and resorcinolic adhesives. It suggests that

32 APRIL 1986 caution about a possible change in the degradation mechanism and the possibility of significant experi­ mental error apply to this conclusion as well. Conclusions Two alkylresorcinolic adhesives are quite com­ parable to two resorcinolic adhesives in terms of the strength of bonded hard maple specimens. This com­ parability includes wet and dry unaged specimens, wet aged specimens and, with a few exceptions, dry aged specimens. The alkylresorcinolic adhesives are not comparable to the resorcinolic adhesives in terms of wood failure in any test. In addition, one alkylresorcinolic adhesive is more sensitive to dry heat than the other adhesives. The low wood failure in alkylresorcinolic bonded specimens may be related to the greater incidence and larger size of bondline microvoids in alkylresorcinolic bonded speci­ Figure 5. – Linear regression lines of the experimental times mens; but the true cause of the low wood failure is to failure at elevated temperatures in wet and dry exposures. unknown. The time at each temperature is the estimated time for a group of specimens to lose 25 percent of their estimated initial Alkylresorcinolic adhesives will probably perform strength (not their measured initial strength) determined from comparably with resorcinolic adhesives, in terms of log strength versus time experiments. LEGEND: ——— 0 strength under constant environmental conditions. But FRF-50 (phenol-resorcinol), — — + DFK-14R (phenol- the low wood failure in alkylresorcinolic adhesive alkylresorcinol), ------FR-100 (alkylresorcinol), ———— bonded specimens is of special concern because it casts – 5-PRF (phenol-resorcinol, ——— solid and hot-press doubt on the adhesives' ability to resist delamination phenolic bonded maple (5) ). under cyclic swelling and shrinking conditions. Further study of the cause and effects related to low wood failure is required before alkylresorcinolic adhesives can be they will have shorter failure times at normal service considered fully comparable with resorcinolic adhesives temperatures. The last conclusion assumes there will be in fully exposed exterior conditions. no change in the degradation mechanism and that no Literature cited significant experimentqal error has occurred. Of course, 1. AMERICAN NATIONAL STANDARDS INSTITUTE. 1983. American these are distinct possibilities. national standards for wood products-structural glued laminated timber. ANSI A190.1. New York, N.Y. In wet aging, the phenol-alkylresorcinolic adhesive 2. AMERICAN SOCIETY FOR TESTING AND MATERIALS. 1981. Standard bonded specimens (DFK-14R) have about the same fail­ test method for strength properties of adhesive bonds in shear by ure times as specimens of the two resorcinolic ad­ compression loading. Desig. D 905-49. Philadelphia, Pa. 3. BASKAKIN, E.N. 1981. Gluing wood with the adhesives FR-100 and hesives, while the alkylresorcinolic adhesive bonded FRF-50 using high frequency. Derevoobrabatyvaiushchaia Prom­ specimens (FR-100) have longer failure times. All the yshlennost’. 12:8-9. adhesives in this study have longer failure times than 4. CHRISTIANSEN, A.W. 1984. Reactivity and spectral comparisons of alkylresorcinol laminating resins with phenolic and resorcinolic the specimens in Millet and Gillespie’s study. These resins. Int. J. of Adhesion and Adhes. 4(3):109-119. longer failure times may be due to the compensating 5. CHRISTIANSON, P.G., K.R. KIISLER, I.A. STARKOPF, and A.F. effects of stress relief and possibly post-cure that would KEEZEL. 1980. Alkylresorcinol adhesives DFK for wood gluing. not be expected in Millett and Gillespie’s solid maple Derevoobrabatyvaiushchaia Promyshlennost’. 9:19-20. 6. ENGLAND, R.F. 1954. The cyclical exposure test as a tool in laminat­ and hot-press bonded maple specimens. ing quality control. J. of Forest Prod. Res. Soc. 4(1):61-64. The failure times of both alkyl adhesives are less 7. FOREST PRODUCTS LABORATORY. 1974. Wood Handbook. Agri. Handb. No. 72. USDA Forest Serv., Washington, D.C. temperature sensitive in wet aging than the re­ 8. FREIDIN, A.C. 1979. Decreased energy consumption adhesives ap­ sorcinolic adhesives. The specimens of the two re­ plied to glulam structures. Cent. Sci. Res. Inst., Inst. of Build. sorcinolic adhesives have the same temperature sen­ Struct. Rept. (unpub.) obtained as part of a technological exchange. 9. .1980. The properties of contemporary glues for wooden sitivity as the solid maple and phenolic adhesive bonded glued structures. Derevoobrabatyvaiushchaia Promyshlennost' maple joints in Millett and Gillespie’s study. This is 9:17-19. expected because wood failure is the dominant mode in 10. GILLESPIE, R.H., and B.H. RIVER. 1975. Durability of adhesives in both studies. The alkyl adhesives are less temperature plywood dry heat effects by rate-process analysis. Forest Prod. J. 25(7):26-31. sensitive than resorcinolic adhesives, phenolic ad­ 11. HORIOKA, S. 1977. DFK resin adhesive produced in Estonia; social hesives, and solid maple. As a result, the earlier con­ and technical background. Kobunshi Kako (Polymer Applications). clusion based on the degradation rates (that alkyl ad­ Spec. ed., July. High Polymer Pub. Co. Japan. 12. . 1978. Properties and applications of DFK resin: new hesives are more hydrolysis resistant) must be modified denatured material using alkylresorcinol as the raw material. when considering what their behavior will be at normal Kogyo Zaiyro (Engineering Materials) 26(5):85-90. service temperatures. Alkyl adhesives will be about as 13. KIISLER, K.P., P. CHRISTIANSON, and Y.A. STARKOPF. 1978. Prop­ resistant to hydrolysis and have about the same failure erties and applications of DFK adhesives. Kobunshi Kako (Polymer Applications). Sept. Japan. pp. 17-22. times as wood and resorcinolic bonded specimens at 14. , , J. TANNER, and J. STARKOPF. 1979. Synth­ normal service temperatures. Of course the previous esis of alkylresorcinol-formaldehyde DFK adhesives and applica­

FOREST PRODUCTS JOURNAL Vol. 36, No. 4 33 tion areas. Presented at the Int. Symp. on Phenolic Resin Chem., = estimated shear strength at zero aging time

Weyerhaeuser Tech. Center, Tacoma, Wash., June 6-9. = degradation rate (log10 MPa/da) 15. MILLETT, M.A., and R.H. GILLESPIE. 1978. Precision of the rate- t = time (day) process method for predicting bondline durability. Rept. prepared 2. Time to failure at a given temperature and moisture by USDA Forest Serv., Forest Prod. Lab. for the U.S. Dept. of condition Housing and Urban Dev., Office of Policy Dev. and Res. Nat. Tech. Inf. Serv., PB80-12l866. [2] 16. STATISTICAL ANALYSIS SYSTEM. 1983.SAS Inst.,Inc.,Raleigh,N.C. where: = the time for joint strength (S) to degrade to 75 17. STRICKLER M.D. 1968. Adhesive durability: specimen designs for t(076) accelerated tests. Forest Prod. J. 18(9):84-90. percent of the initial (unaged) value cal­ culated from Equation [1] above. Appendix 3. Failure time-temperature dependence (Arrhenius Calculations relationship) [3] 1. Strength versus time regression at a given tempera­ where: ture and moisture condition A = the fitted regression constant [1] B = the fitted regression coefficient, the activation where: energy S = shear strength (MPa) T = temperature in degrees Kelvin (degrees C + 273)

34 APRIL 1986