In: Proceedings of the IUFRO division 5 forest products subject group S.5.03: Wood protection; 1987 May 16-17; Honey Harbour, ON, Can.; 1988:

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1 In: Proceedings of the IUFRO division 5 forest products subject group S.5.03: Wood protection; 1987 May 16-17; Honey Harbour, ON, Can.; 1988: WOOD MODIFICATION IN THE PROTECTION OF WOOD COMPOSITES Roger M. Rowell, Glenn R. Esenther, and John A. Youngquist Forest Products Laboratory, 1 Forest Service U.S. Department of Agriculture Madison, Wisconsin Darrel D. Nicholas Forest Products Utilization Laboratory Mississippi State University Mississippi State, Mississippi Thomas Nilsson Department of Forest Products The Swedish University of Agriculture Sciences Uppsala, Sweden Yuji Imamura Wood Research Institute Kyoto University Uji, Kyoto, Japan Waltrant Kerner-Gang Bundesanstalt fur Materialprufung Berlin, West Germany Lucien Trong Preservation Unit Centre Technique Forestier Tropical Kourou, French Guyana. Gerard Deon Preservation Division Centre Technique Forestier Tropical Nogent-sur-Marne, France Abstract Aspen flakeboards were made from control and acetylated flakes at various levels of acetyl content using a phenol-formaldehyde or isocyanate adhesive. These boards were subjected to laboratory termite and soil-block decay tests, and weight loss measured. Fungal cellar tests were done using

2 240 brown-, white-, and soft-rot fungi and tunneling bacteria. Strength losses were determined by static bending tests after brown-rot fungal attack and by bending creep tests during brown- and white-rot fungal attack. In general, flakeboards made from acetylated flakes above about 16 percent acetyl weight gain showed little or no weight loss in termite tests and in fungal tests. These boards also showed little strength loss during or after fungal attack. Résumé Des panneaux de tremble ont été fabriqués à partir de flocons témoins et de flocons acétylés présentant diverses concentrations d'acétyle et d'une colle à base de phénol-formaldéhyde ou d'isocyanate. Ces panneaux one été soumis à des essais de résistance aux termites et à des essais sur sol de résistance à la pourtiture réalisés en laboratoire, puis on a mesuré la perte de poids. Des essais ont été menés avec des champignons de cave : pourriture brune et blanche, carie spongieuse et bactéries creusane des galeries dans le bois. On a mesuré les pertes de résistance en procédant à des épreuves de flexion statique après une attaque par les champignons de la pourriture brune et à des épreuves de fluage dû à la flexion pendant une attaque par des champignons de la pourriture brune et blanche. En général, les panneaux de flocons acétylés présentant une augmentation de poids en acétyle supérieure à 16 pour cent environ ont subi une perte de poids négligeable ou n'en ont subi termites et aux champignons. sucune au cours des essais de résistance aux En outre, aucune perte de résistance n'a été relevée pendant ou après les attaques. Introduction The chemical and physical properties of a material are, for the most part, a result of the chemistry of the components in the material. If we desire to change the properties of that material, we can manipulate the chemistry of the components. In the case of a polymer blend in, for example, a textile, we modify, substitute, or eliminate one or more of the polymers in the composite in order to improve properties. While we practice this in textiles (i.e. wrinkle resistance, permanent press, color fastness, fire retardancy, stain resistance, etc.), we do not apply the same thinking to wood. Wood is, however, a three-dimensional polymeric composite made

3 241 up of cellulose, hemicelluloses, and lignin. These polymers make up the cell wall and are responsible for most of the chemical and physical properties exhibited by wood. Wood is a preferred building material because it is economical, renewable, widely available, strong, aesthetically pleasing, and low in process energy. It has, however, several disadvantageous properties such as biodegradability, flammability, changing dimensions with varying moisture contents, and degradability due to ultraviolet light, acids, and bases. These properties of wood are all the result of chemical reactions involving degradative environmental agents. Because all of these degradative effects are chemical in nature, it should be possible to eliminate or greatly reduce the rate of degradation by changing the basic chemistry of the cell wall polymers. By chemically modifying the carbohydrate polymers (cellulose and hemicelluloses), for example, the highly specific enzymatic reactions involved in biodegradation cannot take place because the chemical configuration and molecular conformation of the substrate have been altered. Modifying the cell wall polymers to make them more hydrophobic or bulking them with bonded chemicals can also help protect wood from biodegradation by lowering the equilibrium moisture content. In developing the basic techniques, various chemical reaction systems were studied, looking for reagents that would react quickly with hydroxyl groups on cellulose, hemicellulose, and lignin. We looked for a chemical reaction system that was capable of reacting with wood hydroxyls under neutral or mildly alkaline conditions at temperatures below 120 C. The system was to be simple, capable of swelling the wood structure to facilitate penetration, yield stable chemical bonds, and the treated wood must still possess the desirable properties of untreated wood while improving one or more undesirable properties.

4 242 Several chemical systems have been explored for chemical modification of wood. 2 Three classes of reactive chemicals were studied in this work: epoxides, 3 isocyanates, 4 and anhydrides. 5 Reactions of these with wood are fast and complete, and stable chemical bonds are formed. All three reaction systems swell wood and penetrate well. Most of the early research on these three reaction systems was applied to solid wood. However, there are three major limitations in the chemical modification of solid wood. One is the need for dry wood to minimize reagent hydrolysis. Drying wood to less than 3 percent moisture content is very expensive and can cause damage to the structure because of drying defects. The second concern is the problem of penetration of the reacting chemical into, and chemical recovery from solid wood that is greater than 1 to 2 cm thick. The third concern is the complexity of many modifying systems that have been devised. Multicomponent systems create problems in chemical recovery. These concerns, along with the great potential seen in the growing future of reconstituted wood products, led to a new research program starting in 1981 on chemical modification of wood composites. The greatest potential for application of the technology developed in wood modification is in composite products, in which standard operating procedures call for near-dry wood materials and small particle size for good chemical penetration and chemical recovery. Chemical modification could greatly improve the properties of composite products. Dimensional instability, for example, especially in the thickness direction, is a greater problem in composite products such as flakeboards, particleboards, and fiberboard than it is in solid wood products. In these products not

5 243 only normal swelling occurs (reversible swelling), but also swelling caused by the release of residual compressive stresses imparted to the board during the pressing process (irreversible swelling). Chemical modification could also result in biological resistance in composite products, based on substrate blocking and cell wall moisture content lowering rather than toxicity. The experimental results from the epoxide, isocyanate, and anhydride studies showed that while epoxides and isocyanates have many advantages over other reaction systems, they tend to use toxic, flammable, and expensive chemicals which, in some cases, give inconsistent reaction weight gains. The anhydride systems, especially acetic anhydride, are more consistent and much less toxic, flammable, and expensive. All of the procedures, however, developed over the years to acetylate wood have been complicated reaction schemes using catalyst and/or organic cosolvent, and have required a long reaction time. Because of the complexities of the reaction/recovery systems and the length of reaction time, these acetylation schemes have not gone beyond laboratory technology. We developed a new procedure to acetylate wood particles which eliminates both catalyst and organic cosolvent, reduces reaction time required to give the desired level of acetylation, and simplifies chemical recovery after reaction. 6 We have applied this new acetylation procedure to various types of lignocellulosic materials. The purpose of the present research was to acetylate aspen flakes, use them to make flakeboards, and test both control and acetylated flakeboards in a variety of biological tests. The biological tests were

6 244 conducted by a nine-person team in seven different laboratories in six countries. They consisted of termite tests, weight loss tests with brownand white-rot fungi, fungal cellar tests with brown-, white-, and soft-rot fungi and tunneling bacteria, and strength loss tests with brown- and white-rot fungi. Experimental Reaction of Flakes and Board Production Ovendry aspen flakes (all from one commercial batch) were acetylated using the new dip procedure described earlier. 6 Flakes with weight gains of 7 to 18 percent (based on the original ovendry weight) were produced. Control and acetylated flakes were made into flakeboards (60 x 70 x 1.25 cm) using the computer controlled laboratory press previously described. Each board was made with a density of approximately 640 kg/m 3 using either phenol-formaldehyde resin (5%) or isocyanate resin (3% based on ovendry weight of control or acetylated flakes). The outer 3 cm were cut from each edge of each flakeboard to remove weak, low-density material so all tested boards would have approximately the same density, then the boards were cut into various specimen sizes for biological tests. Termite Tests Reticulitermes flavipes (Kollar), subterranean termites, were collected at Janesville, Wisconsin, and maintained in a 20-gallon metal container prior to use. In a preliminary test, approximately 87 percent

7 245 of the termites placed in containers with control blocks survived over a 4-week period, indicating that the termites would be acceptable for further experimental usage. Termite tests were run as previously described with minor modifications. 8 An untreated paper pulp block (0.3 x 0.2 x 2.5 cm in size, about 0.5 g) was placed in a cylindrical, clear plastic container (inside dimensions, 5-cm diameter and 3.5 cm deep). The paper pulp sheet is used as a nutritive supplement. It was wetted with 1.5 ml of distilled water. Aspen flakeboard specimens (2.5 x 2.5 x 1 cm) made from control and acetylated flakes with acetyl weight gains of from 8.7 to 17.6 percent were dipped in distilled water for 10 seconds prior to placement in the test container. The specimens were placed on the paper pulp sheet with the 2.5- by 2.5-cm face in contact with the sand. To each container 1 g of termites (mixed caste forms) was added. The number of termites in various caste forms (workers, nymphs, and soldiers) were counted in two out of every three replicate containers. The termite groups consisted mainly of the worker form, the form that actually attacks wood. The average number of termites in 1-g groups was 323 (89.5% workers, 9% nymphs, and 1.5% soldiers). The units, with lids, were stored in a 25 C incubator. After 2 weeks, an additional 0.5 ml water was added to each container. After 4 weeks, the test was stopped. Specimens were brushed free of debris, ovendried at 105 C overnight, weighed, and weight loss determined. The surviving termites were weighed and final termite biomass determined.

8 246 Soil-Block Tests The first soil-block test was run according to specifications outlined in American Society for Testing and Materials (ASTM) D Flakeboard specimens (2 x 2 x 1.25 cm) made from control flakes and from flakes with acetyl weight gains of 7.3 to 17.9 percent were sterilized and placed in test with the brown-rot fungus Gloeophyllum trabeum (Pers.:Fr.) murr. (Madison 617). Half of the specimens were leached according to ASTM Standard D 1413 for waterborne preservatives. Weight loss due to water leaching was also determined. Specimens were removed from the soil-block test after 12 weeks. Extent of fungal attack was determined by weight loss. The second soil-block test was run according to specifications outlined in Japan Wood Preserving Association (JWPA) Standard No Unleached flakeboard specimens (2.5 x 2.5 x 1.25 cm) made from control flakes and from flakes with an acetyl weight gain of 17 percent, were sterilized and placed in glass jars containing the brown-rot fungus Tyromyces palustris (Berk. et Curt.) murr. or the white-rot fungus Trametes versicolor (L.:Fr.) Pilát. Specimens were removed from the soil-block test after 12 weeks. Extent of fungal attack was determined by weight loss. A third soil-block test was run according to specifications outlined in BAM laboratory method Unleached flakeboard specimens (5 x 5 x 1.25 cm), made from control flakes and from flakes with an acetyl weight gain of 17 percent, were sterilized and placed in plastic beakers containing the white-rot fungus Trametes versicolor (L.:Fr.) Pilát

9 247 (Stam CTB 863 A). Specimens were removed from the test after 16 weeks. Extent of fungal attack was determined by weight loss. Fungal Cellar Tests 12 Fungal cellar tests were run as previously described. Flakeboard specimens (5 x 2.5 x 1.25 cm), made from control flakes and from flakes with acetyl weight gains of 7.3 to 17.9 percent, were incubated at approximately 25 C in moist unsterile soil. At 1-month intervals, each specimen was removed from test and inspected. It was determined that brown-, white-, and soft-rot fungi and tunneling bacteria were among the micro-organisms present in the soil. Inspections were carried out for 6 months. Degree of board swelling was noted. During and after the test was completed, sections were cut for microscopic examination. Static Bending Tests Static bending tests after exposure to a decay fungus were run according to Centre Technique Forestier Tropical (CTFT) Method Flakeboard specimens (19 x 1.5 x 1.25 cm), made from control flakes and from flakes with an acetyl weight gain of 17 percent, were exposed to the brown-rot fungi Gloeophyllum trabeum in a vermiculite and malt medium for 12 weeks. At the end of the fungal exposure period, the flakeboards were removed from test, mycelium adhering was removed, and they were conditioned at 65 percent relative humidity, 20 C for 4 weeks. The flakeboards were then subjected to the static bending test as described by the French Standard NF B

10 248 Bending Creep Tests Bending creep tests during exposure to a brown- or white-rot fungus were run as previously described. 14 Flakeboard specimens (35 x 5 x 1.25 cm), made from control flakes or flakes with an acetyl weight gain of 17 percent, were placed in the test chamber. Mycelial fragments of the brown-rot fungus Tyromyces palustris or of the white-rot fungus Trametes versicolor were spread on the bottom tension surfaces of the sterilized flakeboards. A tray filled with sterilized water was set at the bottom of the chamber to keep the specimen in a moist condition. Load was applied at the center of each specimen outside of the decay chamber. The load applied to each test specimen was that amount to cause 1-mm initial deflection. Deflection of the board at the center of the span length (300 mm) was measured regularly with an electronic dial-gauge as a criterion to determine performance. The bending creep test was carried out until the flakeboard broke under load or 100 days for the brown-rot fungus or 250 days for the white-rot fungus. The test apparatus was maintained in a conditioned room at 26 C, which was suitable for the incubation of the fungus. After failure, or 100 days (brown-rot fungus) or 250 days (white-rot fungus), ovendry weight loss was determined on each flakeboard. Results and Discussion Aspen flakes react with acetic anhydride to give an acetyl weight gain of about 17 percent in 2 hours at 120 C. Since the flakes are thin, anhydride penetration takes place quickly, and no pressure is required in

11 249 the system. Since acetylated flakes are more hydrophobic and less wettable than control flakes, the adhesives do not penetrate the acetylated flakes. 15 Because of this, acetylated flakeboards have a lower internal bond strength than control flakeboards. 7 Termite Tests Termite tests were run on flakeboards made from control flakes and flakes acetylated to acetyl weight gain of 8.7 to 17.6 percent using 5 percent phenol-formaldehyde adhesive (Table 1). During the 4-week test, control boards lost 0.12 to 0.16 g. In earlier tests, solid aspen control blocks lost 0.14 to 0.19 g during the same test period, showing that the phenolic adhesive did not have any adverse effects on the termites ability to attack the flakeboards. As the acetyl weight gain increases, specimen weight loss decreases. Final termite biomass remains about the same, except in specimens at the highest level of acetyl weight gain. At this level there is a slightly reduced termite biomass. In the flakeboards at 13.6, 16.3, and 17.6 acetyl weight gain, there were a few dead termites at the end of the 4-week test. Even in the highest acetyl weight gain boards, weight loss due to termite attack is not completely stopped. This may be attributed to the severity of the test, however since termites can live on acetic acid and decompose cellulose to mainly acetic acid, perhaps it is not surprising that acetylated wood is not completely resistant to termite attack.

12 250 Soil-Block Tests Three soil-block tests were done in three laboratories. Table 2 shows the results of a 12-week ASTM Standard D 1413 test using 5 percent phenol-formaldehyde adhesive and the brown-rot fungus G. trabeum. Nonleached control flakeboards lost only 28.5 percent weight during the test while water-leached control boards (2 wk in distilled water) lost 44.1 percent. Earlier results on solid aspen control blocks averaged 40 percent weight loss, showing that the toxic effects of the adhesive on fungal attack could be overcome by water leaching. 16 Even though the weight loss due to water leaching is low (about 2%), the leachate must contain chemicals which are toxic to the fungi. Less weight loss is observed as the level of acetylation increases, which may be due to the removal of soluble wood materials during the acetylation process. Table 3 shows the results of a 12-week soil block using JWPA Standard on flakeboards made from either 5 percent phenolformaldehyde or 3 percent isocyanate adhesive. The toxic effect of the phenolic adhesive can be seen in the low (2%) weight loss in nonleached control boards exposed to T. palustris. The isocyanate adhesive is not very toxic to this fungi as the nonleached control boards made with isocyanate adhesive lost 30.4 percent weight. Control boards made from both adhesives are attacked to about the same extent by T. versicolor, losing between 35 and 40 percent weight during the test. The phenolic adhesive does not offer the same protection to this white-rot fungus as it does to the brown-rot fungus.

13 251 Only One level of acetyl weight gain was used (17%) since this level had been found to be effective in earlier experiments (Table 2). Flakeboards made from the acetylated flakes were very resistant to attack by T. versicolor. A small amount of attack was observed in the acetylated boards using isocyanate adhesive with T. palustris. No attack was seen in the acetylated boards made with phenolic adhesive with T. palustris. In the third soil-block test, a longer test period was used (16 wk) with T. versicolor in the BAM 1984 method (Table 3). The fungus attacks both control boards made from either adhesive about the same, and boards made from 17 percent acetyl weight gain were resistant to attack. Fungal Cellar Tests Control boards using a phenolic adhesive were completely destroyed in 6 months in the fungal cellar (Table 4). Similar results were observed in earlier results on solid aspen wood, showing that the adhesive does not 16 interfere with the test. Flakeboards made from acetylated flakes are resistant to attack at acetyl weight gains above 13.6 percent. It was observed that no biological attack occurred before swelling of the specimens. In specimens that were degraded, tunneling bacteria were the first organisms to attack. Specimens which were heavily attacked were degraded by tunneling bacteria and brown- and soft-rot fungi. Static Bending Tests Weight loss resulting from fungal attack is the method most used to determine the effectiveness of a preservative treatment to protect wood from decaying. In some cases, especially for brown-rot attack, strength

14 252 loss may be a more important measure of attack since it is known that for solid wood large strength losses occur at very low wood weight loss. 17 One test that has been developed to determine strength loss after brown-rot fungus attack is the CTFT method. 13 The decay fungus is allowed to attack the center of a specimen for 12 weeks followed by a static bending test. The results of this test are shown in Table 5. Control flakeboards made with both adhesives fail under a very small load after fungal exposure. Flakeboards made from acetylated flakes using an isocyanate adhesive show no strength loss after fungal attack and 15 to 33 percent strength loss using phenol-formaldehyde adhesive. Bending Creep Tests Another test determines strength losses which occur during fungal attack. In this test both a white- and brown-rot fungus were used. Table 6 shows the load required to cause a 1-mm deflection before fungus exposure, time to failure, and weight loss which occurred at failure. In bending creep tests, after fungal exposure, phenol-formaldehydebonded control flakeboards failed in an average of 71 days with T. palustris and 212 days with T. versicolor (Table 6). At failure, weight loss averaged 7.1 percent for T. palustris and 31.6 for T. versicolor. Isocyanate-bonded control flakeboards failed in an average of 20 days with T. palustris and 118 days with T. versicolor, with an average weight loss at failure of 5.5 percent and 34.4 percent, respectively.

15 253 Very little or no weight loss occurred with both fungi in flakeboards made using either adhesive with acetylated flakes (Table 6). None of these specimens failed during the test period. Deflection-time curves for flakeboards are shown in Figure 1. There is an initial increase of deflection for both control and acetylated flakeboards, then a stable zone, and, finally, for control boards a steep slope to failure. Mycelium fully covered the surfaces of isocyanate control flakeboards within 1 week, but mycelial development was significantly slower in phenolformaldehyde control flakeboards. Both isocyanate and phenol-formaldehydebonded acetylated flakeboards showed surface mycelium colonization during the test time, but the fungus did not attack the acetylated flakes, so little strength was lost. Since creep in wood is dependent on moisture content, it is important to determine the creep due to the moisture present in the wood separately from the creep due to strength losses associated with decay. In developing the test procedure for acetylated flakeboards 18 two sets of control specimens were used. One set had aqueous inoculum of either fungus applied to bottom (tension) surface of the test specimens while a second set had an equal amount of sterile water applied to the bottom side. The wet sterile controls showed less than a 5-mm deflection during the 100-day test for T. palustris and 250 days for T. versicolor, while the controls with decay fungus failed within the test periods. This showed that only a small part of the deflection observed was associated with creep due to moisture.

16 254 Summary and Future Research Even at the highest level of acetylation, flakeboards were not completely resistant to attack by subterranean termites. This may, in part, be due to the termites ability to digest acetic acid and perhaps acetate. Flakeboards made from acetylated flakes above 15 percent acetyl weight gain were very resistant to attack by brown-, white-, and soft-rot fungi, and tunneling bacteria as shown by soil-block and fungal cellar tests. In fungal cellar tests, tunneling bacteria were the first to attack lower acetyl-substituted flakes, and no biological attack took place before swelling of the wood. Acetylation not only prevents weight losses but also prevents strength losses which can occur with both brown- and white-rot fungi. This is more important from an engineering point of view than is weight loss. The mechanism of biological resistance in acetylated wood is not known, however, it is thought to be due to two factors: greatly decreased moisture sorption and substrate blocking. Flakeboards made from acetylated aspen flakes above 17 percent acetyl weight gain have a fiber saturation point about 80 percent lower than control flakes. 19 There may not be enough moisture at the site far the enzymes to hydrolyze the target linkages. A better understanding of exactly where the acetyl groups are located in the cell wall polymers could shed light on the substrate blocking theory. This is presently under further investigation.

17 255 The carbohydrate polymers are the most susceptible to biological attack, with the hemicelluloses the most accessible and hygroscopic of the cell wall polymers. If the first step in fungal degradation of wood is attacking the hemicelluloses, acetylation of this fraction may be the key to biological protection by chemical modification. Flakeboard stakes made from various levels of acetylated aspen flakes using phenol-formaldehyde or isocyanate adhesive are presently in field tests in Mississippi to determine the durability of the product as well as the stability of bonded acetyl groups with periodic acetyl analysis. The Mississippi site will test the boards in the presence of various types of brown-, white-, and soft-rot fungi, soil bacteria, and subterranean termites. Acetylated flakeboards are also in test in the ocean to determine their resistance to marine organisms. Acetylated flakes at various levels of bonded acetyl groups are also under test as various ph's and under high humidity where acetyl stability will be determined. It is important to find out if acetyl is lost over time due to hydrolysis. If this occurs, then it is only a matter of time before the acetyl concentration will be below the threshold level, and the board will start to fail. If acetyl is hydrolyzed, it will probably be in the most accessible regions of the cell wall polymers, which are the most susceptible to biological attack. Other chemical reaction systems are also under investigation. This research will hopefully result in the most efficient/economic procedure to chemically modify wood.

18 256 While it is probably not realistic to assume acetylated wood or any other nontoxic modification can replace broad-spectrum toxic preservatives for inground and marine applications, it is important to determine the limits of its biological resistance. This information will lead to a better understanding of the mechanisms of biological resistance through chemical modification and perhaps to the development of a nontoxic, leash resistant procedure that is durable for moderate decay hazard environments.

19 257 References

20 258

21 259 List of Figures Figure 1--Deflection-time curves of phenol-formaldehyde- (PF) and isocyanate- (Is) bonded flakeboards in bending creep tests under progressive fungal attack by T. palustris (upper) and T. versicolor (lower). = PF control; = PF acetylated; l = Is control; o = Is acetylated. (ML )

22 ML

23 261 Table 1--Weight loss and termite survival tests 1 after 4 weeks on aspen flakeboards 2 made from control and acetylated flakes

24 262 Table 2--Average weight loss in soil-block tests 1 for aspen flakeboards 2 made from control and acetylated flakes exposed for 12 weeks to Gloeophyllum trabeum

25 263 Table 3--Average weight loss in soil-block tests for aspen flakeboards made from control and acetylated 1 flakes exposed for 12 weeks 2 to Tyromyces palustris or Trametes versicolor and 16 weeks 3 to Trametes versicolor

26 264 Table 4--Fungal cellar tests 1 on aspen flakeboards 2 made from control and acetylated flakes

27 265 Table 5--Bending strength tests 1 of aspen flakeboards made from control and acetylated 2 flakes before and after 12-week exposure to Gloeophyllum trabeum

28 Table 6--Load and test duration in bending creep tests 1 on aspen flakeboards made from control and acetylated 2 flakes exposed to Tyromyces palustris or Trametes versicolor