GLUING FIRE-RETARDANT- TREATED DOUGLAS-FIR AND WESTERN HEMLOCK

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1 U.S. DEPARTMENT OF AGRICULTURE FOREST PRODUCTS LABORATORY In Cooperation with the University of Wisconsin FOREST SERVICE MADISON, WIS. U.S.D.A. FOREST SERVICE RESEARCH NOTE FPL-0160 NOVERMBER 1968 GLUING FIRE-RETARDANT- TREATED DOUGLAS-FIR AND WESTERN HEMLOCK

2 Abstract Ammonium sulfate, zinc chloride, boric acid, and sodium dichromate--components of the American Wood-Preservers Association Type D fire retardant--were found to affect the rate of gelation of a resorcinol-resin adhesive. Boric acid retarded the rate of gelation, whereas the other components of the fire-retardant mixture increased the rate of gelation. In block-shear evaluations, the specimens made with resorcinol-resin glues from joints of either treated Douglas-fir or western hemlock yielded lower quality joints than did similar untreated specimens.

3 GLUING FIRE-RETARDANT-TREATED DOUGLAS-FIR AND WESTERN HEMLOCK By R E. SCHAEFFER, Chemist 1 Forest Products Laboratory, Forest Service U.S. Department of Agriculture Introduction It has been shown that wood treated with ammonium salts cannot be bonded as readily with resorcinol-resin adhesives as untreated wood of the same species under similar conditions (3, 8, 9, 11). 2 However, acceptable glued products incorporating ammonium-salt-treated wood have been made with resorcinol adhesives for several years. This is very often accomplished by using a resorcinol-resin adhesive with higher than normal amounts of formaldehyde hardener and by curing the glue bonds at elevated temperature. A recent study at the Forest Products Laboratory has indicated some of the chemical interactions of ammonium salts and a resorcinol adhesive that interfere with normal gel and cure of the adhesive (10). Chemical interaction of the salts and the adhesive is one of the causes of lower joint strength, of less wood failure, and of higher delamination in the glued treated wood as compared to the glued untreated wood of the same species (11). Several compounds other than ammonium salts are used as fire retardants: of these, borax, boric acid, and zinc chloride are some of the most used fireretardant compounds. Wood treated with some of these compounds in combination with ammonium salts has been reported to yield lower joint strength when glued with resorcinol adhesives (3). 1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 2 Underlined numbers in parentheses refer to Literature Cited at the end of this report. FPL-0160

4 This is a report of preliminary studies at the Forest Products Laboratory on the effect of other fire retardants, such as zinc chloride and boric acid alone and in combination with ammonium sulfate, on the rate of increase in viscosity of a resorcinol-resin adhesive. The effect of these fire retardants on the chemistry of phenolic resins has been discussed (6). Included here also are the results of gluing Douglas-fir and western hemlock treated with a mixture of these fire retardants. Effect of Selected Chemicals on Viscosity of a Resorcinol-Resin Adhesive The effect of zinc chloride, boric acid, and sodium dichromate, separately and in combination with ammonium sulfate, on the viscosity of a resorcinolresin adhesive was investigated by a method similar to that used in a previous investigation (10). In these investigations on the effect of these chemicals on viscosity of the adhesive, a commercial resorcinol-resin adhesive was used. This adhesive is normally used with a paraformaldehyde hardener system. Formalin was substituted for paraformaldehyde in this work because paraformaldehyde does not lend itself well to the study of the rate of increase in viscosity of an adhesive with respect to time. Paraformaldehyde itself must dissociate before the formaldehyde becomes available for reaction with the resorcinol resin. Procedure The amount of fire retardant added to the resorcinol-resin adhesive for determining the change in viscosity with time was estimated from a gluing situation generally encountered for ammonium salt-treated southern pine. The amount of the fire-retardant chemicals (as supplied by the treater) in the Douglas-fir and western hemlock used in this work was one-half of that found in the southern yellow pine of the previous study (10). For comparative purposes, however, the level of chemicals used in the previous work was used here for investigating the effects of these chemicals on the change in viscosity of the adhesives. The amount selected was based on the calculated number of milliequivalents of ammonium ion in a piece of lumber treated to a level of 6 pounds per cubic foot with a salt, such as ammonium sulfate. It is assumed that the salt in the wood available for interaction with the adhesive is the amount that is present in the wood to a depth of one-thousandth of an inch. FPL

5 Boric acid (considering only the first ionization potential), zinc chloride, and sodium dichromate (considered as an oxidizing agent) were added to separate portions of the adhesive in a ratio of 25 milliequivalents of chemical to 150 grams of mixed adhesive. In a typical determination, liquid resorcinol resin (120 grams) was weighed in a 300-milliliter tall-form beaker and cooled to approximately 74 F. with an ice-water bath. Formalin solution (30 grams) was added with mechanical stirring, and the run was timed after this addition The stirring was accomplished with an air-motor-driven Hershberg stirrer (4). The temperature of the mixture rose to 78 F. in approximately 1 minute and was then maintained at 80 ±2 F. After 15 minutes, the fire-retardant chemical (25 milliequivalents dissolved in 25 milliliters of water) was added to the adhesive during continued stirring. In the control run, distilled water (25 milliliters) was added to dilute the adhesive as much as in the test runs. The temperature was maintained at 80" ±2 F. by raising and lowering the water bath or substituting an ice-water bath depending on the temperature rise generated by the heat of reaction. After 5 minutes of stirring and manipulation of water and ice-water baths, the temperature was held within the desired range with a single controlledtemperature water bath. Stirring was discontinued, and the stirrer was replaced by a Brookfield Synchro-Lectric Viscometer. Viscosity readings were taken at various intervals until the viscosity reached 5,000 centipoises or 165 minutes had passed--whichever occurred first. The viscometer was run continuously for each determination, and consequently provided only minimal stirring action. In each determination, an equilibrium ph measurement was made on a duplicate mixture of adhesive and additives. Results and Discussion The effects of certain fire-retardant chemicals on the change in viscosity and ph of a resorcinol adhesive are shown in figure 1. The changes in ph caused by addition of each fire-retardant chemical to the resorcinol-resin adhesive were not sufficient to account for the resulting changes in the rate of gelation. Ammonium sulfate.--in an earlier report (10) it has been shown that the addition of ammonium sulfate at the 25-milliequivalent level caused a decrease in ph from 7.2 to 5.5 and a rapid increase in viscosity. The effects of ammonium salt fire-retardant chemicals on the ph and gelation rate of resorcinol-resin FPL

6 Figure I.--Relation of change in viscosity and ph to time for a resorcinol-resin adhesive with a fire-retardant mixture and its component compounds. The reagents, dissolved in 25 grams of water, were added to 150 grams of mixed adhesive and maintained at 80 F. ±2 F. (Two curves are shown for the fire-retardant mixture.) M FPL

7 adhesives were attributed to ammonium ion-formaldehyde interaction. A mechanism of ammonium ion-formaldehyde interaction is discussed in the earlier report. Adding one-half as much ammonium sulfate (12.5 milliequivalents) resulted in a moderate decrease in ph to 6.5, but the rate of increase in viscosity was greater than that of the control mixture. The increase in viscosity at both levels of ammonium salt addition took place at ph values that would normally suppress the rate of increase in viscosity. Boric acid. --Boric acid is classified as a very weak acid in aqueous solutions (k' A1 = 5.8 x ) (7). However, the addition of boric acid (25 milliequivalents) caused a slight reduction in ph of the adhesive from 7.2 to 6.5 and suppressed the viscosity change throughout the 165 minutes of observation. The ph of the adhesive mixture alone is not a sufficient explanation for suppressing the change in viscosity (5). It has been reported (12) that boric acid forms chelate structures with phenolic compounds such as catechol and salicyclic acid. If such an interaction occurred between boric acid and resorcinol resin to interfere with the normal condensation reaction with formaldehyde, a reduced rate of viscosity increase might be expected. Zinc chloride.--the curve describing the increase in viscosity for the resorcinol adhesive in the presence of zinc chloride shows a period of about 30 minutes with little change, followed by a moderate rise in viscosity for another 30 minutes, and a subsequent rapid approach to gelation. It is recognized that the gel time of a typical resorcinol resin reaches a maximum at a ph of about 3.5, and falls sharply at higher and lower ph values (5). Considering only the ph of the adhesive in this run (6.4), it would be expected that a curve more closely related to that from the boric acid run would result. The gelation curve for the resorcinol adhesive in the presence of zinc ion may not be unusual. Bender (2) showed that the phenol-formaldehyde condensation reaction produced the more reactive 2,2' isomer more frequently in the presence of zinc ion than in acid-catalyzed condensations. Thus, an influence of zinc ion on the mechanism of the phenol-formaldehyde condensation has been demonstrated. Zinc ions might be expected to influence the mechanism of formaldehyde condensation with the resorcinol ring in much the same manner. The curve describing the change of viscosity of the resorcinol adhesive in the presence of zinc ion (fig. 1) has more meaning if it is assumed that the zinc FPL

8 ion causes an increase in the concentration of these more reactive resin units during the initial 30-minute period, followed by a rapid change in viscosity as these resin units become cross-linked. Sodium dichromate.--the concentration of sodium dichromate used in this investigation was 18 times greater than normally encountered when it is used in a fire-retardant mixture. The dichromate usually functions as a preservative, and as such is present in the fire-retardant mixture to the extent of only about 5 percent of the total weight of the mixture. Even in the high concentration used here, this strong oxidizing agent does not increase the viscosity of the adhesive as rapidly as does 25 milliequivalents of ammonium ion without changing the ph, but it does have a significant effect in causing a more rapid viscosity increase than for the control. AWPA Type D fire retardant.--the duplicate curves describing the effect of this fire-retardant mixture show a reasonable degree of precision in the method for determining change in viscosity of the resorcinol adhesive. The curve for the mixture shows the increase in viscosity of the adhesive in the presence of the several constituent chemicals shown in figure 1. The concentration of American Wood-Preservers' Association (AWPA) Type D fire retardant (1) added to the resorcinol-resin adhesive was equivalent to that amount of similar fire retardant present in wood treated to a level of 8.5 pounds per cubic foot. It is assumed that the salt in the wood available for interaction with adhesive is the amount that is present in the wood to a depth of onethousandth of an inch. At this level of addition, the ammonium sulfate in the fire retardant (composed of 35 percent ammonium sulfate, 35 percent zinc chloride, 25 percent boric acid, and 5 percent sodium dichromate) was present in the resorcinol adhesive at a concentration of 12.5 milliequivalents of ammonium ion. This served as a basis of comparison with the previous work on ammonium salt type fire retardants (10). Even at this unrealistically high concentration of fire retardant, the increase in viscosity of the resorcinol adhesive was less than the rate of increase in viscosity caused by the presence of 25 milliequivalents of ammonium sulfate. However, the rate of increase in viscosity of the adhesive in the presence of the fire-retardant mixture was greater than that which occurred in the presence of an equivalent amount of ammonium ion (12.5 milliequivalents). Ammonium sulfate, zinc chloride, and sodium dichromate increase the normal rate of gel of a resorcinol-resin adhesive, whereas boric acid retards the rate of gel. Boric acid may serve as a chelating agent, as already suggested, or it FPL

9 may serve to retard the effects of zinc chloride and ammonium sulfate through its known ability to form complexes with ammonia and with heavier elements, such as zinc (7). Effect of Fire-Retardant Chemicals on Gluing Treated Douglas-fir and Western Hemlock The purpose of this phase of the study was to bond Douglas-fir and western hemlock that had been treated with the AWPA Type D fire-retardant mixture. Joints were made with two resorcinol-resin-type adhesives at either of two closed assembly times, 20 or 60 minutes, and were cured at either 80 F. or 150 F. Procedure Joints for block-shear specimens (American Society for Testing and Materials (ASTM) Standard Method D ) were prepared from nominal 1-inch Douglasfir and western hemlock lumber treated with the fire-retardant mixture composed of ammonium sulfate, zinc chloride, boric acid, and sodium dichromate. Matching specimens of untreated Douglas-fir and western hemlock were also prepared to provide a basis for comparison. Wood Douglas-fir and western hemlock lumber (30 pieces of each species, nominal 1 inch by 6 inches by 4 feet) was treated commercially with the fire-retardant mixture. Matching boards of untreated Douglas-fir and western hemlock were also supplied by the treater. The average salt retention of the treated lumber, as reported by the supplier, was approximately 3.4 pounds per cubic foot. The treated and untreated lumber was cut into blocks 3/4 inch by 2-1/2 inches by 12 inches. These blocks were conditioned at 80 F. and 65 percent relative humidity before gluing. The surfaces to be bonded were lightly surfaced on a jointer before the adhesive was spread. FPL

10 Adhesives Two commercial adhesive formulations, mixed as recommended by the adhesive manufacturer, were used: Adhesive A: A conventional straight resorcinol resin with paraformaldehyde hardener. Adhesive B: A resorcinol-type resin with a liquid hardener containing a high concentration of formaldehyde (about 54 percent formaldehyde). Gluing Conditions Three grams of adhesive were brushed on each of the mating surfaces to be bonded (equivalent to approximately 61 pounds per 1,000 square feet of single glueline). After closed assembly periods of either 20 or 60 minutes, the block assemblies were pressed at 150 pounds per square inch for 20 hours. The joints were cured at either 80 F. or 150 F., and subsequently conditioned at least 1 week at 80 F. and 65 percent relative humidity before testing. Shear Tests Sixteen joints were made for each combination of adhesives and gluing conditions. Of these sixteen joints, four joints were made from treated Douglas-fir and four from treated western hemlock. Four other joints were made from untreated Douglas-fir and four from untreated western hemlock. The blocks for making the joints were randomly selected from the stock of treated and untreated blocks for each species. Five standard shear specimens were cut from each joint as prescribed in ASTM Standard Method D ; they were tested in shear by this method. Results and Discussion The effect of the AWPA Type D fire-retardant mixture on the rate of increase in viscosity of the adhesive was apparent during the gluing of Douglas-fir treated with this mixture. The more rapid increase in viscosity of the adhesive resulted in less squeezeout from joints made of treated Douglas-fir; this phenomenon is illustrated in figure 2. The rapid increase in viscosity undoubtedly affected the wetting and the flow characteristics of the adhesive. FPL

11 FPL Figure 2.--Difference in squeezeout of a cured conventional resorcinol-resin glue with paraformaldehyde hardener in joints of Douglas-fir treated with AWPA Type D fireretardant formulation (joints I and 2) and untreated Douglas-fir (joints 3 and 4). (All joints were pressed at 150 pounds per square inch after a 20-minute closed assembly and were cured overnight at 80 F.) M

12 When the data in table 1 are examined, some apparent differences in joint strength and wood failure are observed between the untreated and treated specimens of both species at the several combinations of variables involving adhesive formulation, assembly time, and cure temperature. However, not all these differences were significantly different at the 95-percent level. In the subsequent discussion, significant differences mentioned refer to this 95-percent level. Joints made of either treated Douglas-fir or western hemlock were, on the whole, significantly lower in shear strength and had less wood failure than their untreated counterparts. Reduction in shear strength for joints made with treated western hemlock as compared with untreated western hemlock was particularly significant for those joints cured at 80 F. (table 1). However, no significant difference in shear strength existed between the joints of treated and untreated wood of the same species at any combination of adhesive and assembly time for those joints cured at 150 F. Joints made with adhesive B were found to yield significantly higher wood failure than those made with adhesive A with either treated Douglas-fir or western hemlock. Under all experimental conditions, the wood failure for joints made of treated Douglas-fir bonded with adhesive B was higher than 80 percent, whereas, for joints bonded with adhesive A, the wood failure was generally less than 80 percent. The minimum joint requirement for wood failure is 80 percent for the Commercial Standard CS for structural glued laminated timber (13). On the whole, wood failure and shear strength were higher for untreated and treated Douglas-fir than for untreated and treated western hemlock It appears that the particular fire retardant mixture used in this study interferes chemically with the proper cure of the adhesive. The effects of this interference, lower joint strength and wood failure, were minimized through the use of a resorcinol adhesive employing extra formaldehyde in the hardener and curing at an elevated temperature (150 F.). The extent to which this effect can be minimized for species that treat better than Douglas-fir and western hemlock, with subsequent higher salt retention, is a matter for future investigation. FPL

13 Table 1.--Strength of joints of untreated and fire-retardant-treated 1 Douglas-flr and western hemlock lumber bonded with two resorcinol-resinadhesives 1 Both Douglas-fir and western hemlock lumber were commercially treated with a fire retardant containing zinc chloride, ammonium sulfate, boric acid, and sodium bichromate. Average retention of fire retardant was 3.4 pounds per cubic foot. 2 Press pressure, 150 p.s.i. 3 Each value is average of 20 specimens tested according to standard block-shear test (ASM D ). 4 A, with a paraformaldehyde hardener. 5 B,specilaIIy formuiated for giuing fire-retardant-treated wood. FPL

14 Conclusions When AWPA Type D fire retardant, a mixture of ammonium sulfate, zinc chloride, boric acid, and sodium dichromate, was added to resorcinol-resin adhesives, the acidity of the mixture increased, and the rate of change in viscosity also increased. Changes in ph caused by addition of individual components of the fire retardant mixture to similar adhesive mixes were not sufficient to account for the changes in rate of gelation of the adhesive. The rate of change in viscosity of the adhesive caused by the components of the fire-retardant mixture is thought to be directly related to the mechanism of chemical interaction of the fire-retardant component and of formaldehyde condensation with the resorcinol rings of the resin. Ammonium sulfate, zinc chloride, and sodium dichromate increase the rate of gelation, whereas boric acid retards the rate of gelation of the resorcinol-resin adhesive. Chemical interference of the fire-retardant salts was also apparent during the bonding of Douglas-fir and western hemlock treated with the fire-retardant mixture. Less adhesive was squeezed out of joints made from the treated Douglas-fir and western hemlock than from joints of untreated wood. Shear strength for all joints made with treated wood was lower than that for the untreated controls. The effects of the interference of the fire-retardant chemicals were minimized through the use of a resorcinol adhesive employing extra formaldehyde in the hardener and by curing at an elevated temperature (150 F.). FPL

15 Literature Cited ( 1) American Wood-Preservers' Association Standards for fire-retardant formulations. In AWPA Standard, P ( 2) Bender, H. L Structural control of cure of phenolics. Modern Plastics 30(5): 136, 138, 220, 222. ( 3) Bergin, E. G The gluability of fire-retardant-treated wood. Forest Products J. 13(12): ( 4) Fieser, L. F Experiments in organic chemistry. Second ed., part 2, 488 pp. Boston: D. C. Heath and Company. ( 5) Little, G. E., and Pepper, K. W The influence of ph on the setting of phenolic resins. British Plastics 19(221): ( 6) Martin, R W The chemistry of phenolic resins; the formation, structure and reactions of phenolic resins and related products. 298 pp. New York: John Wiley & Sons, Inc. ( 7) Moeller, Therald Inorganic chemistry, an advanced textbook. New York: John Wiley & Sons, Inc., pp ( 8) Raknes, E Gluing of wood pressure-treated with water-borne preservatives and flame retardants. J. Inst. Wood. Sci., 11: ( 9) Schaeffer, R. E Preliminary study of the gluing of ammonium-salt-treated wood with resorcinol-resin glues. U.S. Forest Serv. Res. Note FPL Forest Prod. Lab., Madison, Wis. 9 pp., illus. (10), Gillespie, R H., and Blomquist, R. F Chemical interaction of ammonium salt fire retardants and resorcinol-resin adhesives. Forest Products J. 16(5): (11) Gluing ammonium-salt-treated southern pine with resorcinol resin. U.S. Forest Serv. Res. Note FPL Forest Prod. Lab., Madison, Wis. 16 pp., illus. (12) Schafer, Harald Z. anorg. allgem. Chem., 250: (13) U.S. Department of Commerce U.S. Commercial Standard CS , Structural Glued Laminated Timber. FPL

16 The FOREST SERVICE of the U.S. DEPARTMENT OF AGRICULTURE is dedicated to the principle of multiple use management of the Nation s forest resources for sustained yields of wood, water, forage, wildlife, and recreation. Through forestry research, cooperation with the States and private forest owners, and management of the National Forests and National Grasslands, it strives--as directed by Congress -- to provide increasingly greater service to a growing Nation.