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Biological resistance of chemically modified aspen composites
International Biodeterioration & Biodegradation 43 (1999) 181±187
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Biological resistance of chemically modi®ed aspen composites M.C. Timar a, A. Pitman b,*, M.D. Mihai a a
``Transilvania'' University of Brasov, Romania Buckinghamshire Chilterns University College, High Wycombe, UK
b
Received 9 October 1998; received in revised form 18 December 1998; accepted 10 March 1999
Abstract Aspen wood (Populus tremula L.) was chemically modi®ed by a two-step procedure consisting of esteri®cation with maleic anhydride (MA) and subsequent oligoesteri®cation with MA and glycidyl methacrylate (GMA) or allyl glycidyl ether (AGE). This chemical modi®cation procedure was carried out on solid wood, veneers and sawdust. The modi®ed wood showed thermoplastic properties and could be thermally formed by hot-pressing. As a result, solid wood and the veneer samples had smooth, glossy surfaces, while a plastic-like material was produced on thermally forming the modi®ed sawdust. The biological resistance of chemically modi®ed and thermally formed samples was assessed by determination of weight loss following exposure to a decay fungus in a laboratory test and by a weathering test. Modi®ed samples exposed to the white rot Coriolus versicolor for 12 weeks in the laboratory were more resistant to decay, with weight losses signi®cantly lower than for the corresponding control samples. Solvents and thermal treatments employed in the chemical modi®cation process had no signi®cant eect on decay resistance of Aspen veneers. However, hot-pressing signi®cantly improved decay resistance in unmodi®ed wood samples by limiting hyphal colonisation of the wood structure. A microscopic comparison of chemically modi®ed and unmodi®ed wood samples was conducted to examine extent of fungal colonisation and decay. Chemical modi®cation was also shown to enhance the weathering resistance of aspen wood to discoloration and surface erosion by UV and rainwater and to stain from mould fungi. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Man has utilised wood for a wide range of end uses because of its availability, ease of working and its mechanical properties (Panshin and De Zeeuw, 1980; Dinwoodie, 1989). Wood has been shown to be colonised and decayed by organisms including bacteria, fungi, insects and marine wood borers as part of the natural carbon cycle (Panshin and De Zeeuw, 1980). However, fungal decay and dis®gurement of wood in service is deleterious (Eaton and Hale, 1993). It has been recognised that the service life of a timber structure may be extended, either by employing a timber species which is naturally resistant to decay, or
* Corresponding author at: Buckinghamshire Chilterns University College, High Wycombe Campus, Queen Alexandra Road, Bucks HP11 2JZ, UK. Tel.: 01494-522-141; fax: 01494-524-392.
by treating less durable species with preservatives (Eaton and Hale, 1993; Blanchette, 1995). More recently, research has demonstrated that chemical modi®cation enhances a number of wood properties including its resistance to fungal decay, colonisation by mould fungi and attack by marine wood borers and insects. Enhancement of biological resistance by chemical modi®cation is believed to result from alteration of the chemical structure of the cell wall polymers making them unidenti®able for microbial decay enzymes, and by reducing moisture absorption by the wood cell walls by partial substitution of hydroxyl groups in the wood polymers (Militz et al., 1997). However, the degree of bioprotection achieved and the spectrum of activity is reportedly dependant on the type of modi®cation and the wood species (Takahashi, 1996; Forster et al., 1997). Extensive research has been conducted to determine the biological resistance of esteri®ed wood treated with acetic anhydride (most often) and other anhydrides. It
0964-8305/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 4 - 8 3 0 5 ( 9 9 ) 0 0 0 5 2 - 9
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M.C. Timar et al. / International Biodeterioration & Biodegradation 43 (1999) 181±187
Fig. 1. Esteri®cation process.
was found that acetylated wood exhibited good resistance to brown rot, white rot and soft rot decay (Beckers et al., 1994; Takahashi, 1996; Forster et al., 1997; Suttie et al., 1997), whilst resistance to colonisation by lower fungi including moulds and stains proved unsatisfactory (Beckers et al., 1994). Other studies also showed enhanced resistance of acetylated wood to attack by subterranean termites (Imamura and Nishimoto, 1986) and marine borers (Johnson and Rowell, 1988). Wood esteri®ed with dierent alkyl anhydrides (with up to six carbon atoms in the molecule) exhibited comparable bioresistances to fungi and insects, although there were indications that smaller reactant molecules produced better products (Suttie et al., 1997). Improved resistance to biodeterioration was also demonstrated for other chemical modi®cation processes including epoxydation (Rowell et al., 1979), reaction with isocyanates (Ellis and Rowell, 1984; Forster et al., 1997; Suttie et al., 1997), cross-linking with aldehydes (Yusuf et al., 1994, 1995) and oligoesteri®cation (Matsuda, 1993). The current paper reports on the eect of a two-step chemical modi®cation treatment: esteri®cation with maleic anhydride and subsequent oligoesteri®cation with maleic anhydride and an epoxide, followed by hot-pressing, on the decay resistance of aspen exposed to Coriolus versicolor under laboratory conditions, and to mould fungi under weathering conditions. The research is an extension of a previous study, which showed this treatment reduced mould growth on wood products manufactured by hot-pressing oligoesteri®ed aspen sawdust (Timar et al., 1997). Aspen was selected for use in this study because it
has a low natural durability (BS EN 350-2: 1994), is readily permeable (facilitating chemical impregnation) and has previously been employed in chemical modi®cation studies (Rowell et al., 1990; Clemons et al., 1992; Rowell et al., 1993; Chow et al., 1996). However, little information is available on the biological resistance of this species when chemically modi®ed (Beckers et al., 1994) and no previous study has investigated biological resistance of oligoesteri®ed aspen. 2. Materials and methods 2.1. Chemical modi®cation and sample preparation Aspen sawdust (diameter 0.3±0.5 mm) or veneers (30 30 2 mm) were dried at 103228C to constant weight prior to chemical modi®cation. Sawdust and veneers were esteri®ed with maleic anhydride (MA) to produce esteri®ed wood (WE), as a mixture of the monoester MM (principal component) and the diester DM (secondary component), as presented in Fig. 1. Esteri®ed samples were then treated with a mixture of an epoxide (allyl glycidyl ether (AGE) or glycidyl methacrylate (GMA) and MA. The most probable structure of the resulting oligoesteri®ed wood is shown in Fig. 2. The reaction conditions employed for the chemical modi®cation procedure are summarised in Table 1. After the reaction, samples were washed with acetone to remove unreacted chemicals, dried under vacuum at 208C and then oven dried at 103 2 28C to constant weight. An approximate weight percent gain (WPGa) which is directly correlated to the degree of
Fig. 2. Oligoesteri®cation process.
M.C. Timar et al. / International Biodeterioration & Biodegradation 43 (1999) 181±187
183
Table 1 Chemical modi®cation procedure for aspen samplesa Form of wood material Sawdust
Reagents MA TEA DMF MA TEA DMF MA TEA DMF MA TEA DMF
Veneers
Esteri®cation T(8C)
29.0% 1.3% 69.7% 32.0% 1.0% 67.0% 32.0% 1.0% 67.0% 32.0% 1.0% 67.0%
Time (h)
120
1
120
1.5
120
1.5
120
1.5
Oligoesteri®cation Reagents T(8C) MA GMA DMF
MA AGE DMF MA GMA DMF
±
Time (h) 7
Code of sample
18.4% 35.1% 46.5%
90±95
17.4% 18.8% 63.8% 12.0% 24.5% 63.5%
120
1.5
WO1
120
1.5
WO2
±
WOE ±
WE
a Note: MA, maleic anhydride; GMA, glycidyl methacrylate; AGE, allyl glycidyl ether. DMF (N,N-dimethylformamide) was used as a solvent in the two steps of chemical modi®cation. TEA (triethanolamine) was employed as a catalyst in the esteri®cation step.
chemical modi®cation was calculated for the modi®ed sawdust and veneers according to Eq. (1). WPGa
Mra ÿ M0 100 % M0
1
where: Mra represents the weight of the chemically modi®ed sample after washing with acetone and oven drying to constant weight; M0 represents the weight of the oven dried, unmodi®ed sample. Chemically modi®ed and unmodi®ed sawdust was formed into test discs (22 mm diameter and 2 mm thick) by hot-pressing in an aluminium alloy mould at 160±1708C and 45 MPa for 30 min. Tough, glossy, water-resistant plastic-like composites resulted from the oligoesteri®ed wood. In the case of the veneers, reference (control) samples were prepared according to methods outlined in Table 2. To study the eect of the hot-pressing on the decay resistance of veneers, six unmodi®ed and chemically Table 2 Preparation of veneer reference (control) samplesa Type of reference sample Unmodi®ed wood Control sample C1 Control sample C2
modi®ed samples were hot-pressed at 1308C and 10 MPa for 30 min perpendicular to their faces. 2.2. Biological tests Two tests were undertaken to assess biological resistance of the modi®ed aspen wood and composites: a laboratory decay test and a natural weathering test. The resistance of experimental and reference (control) veneers and sawdust samples to decay by C. versicolor (Linnaeus) Quelet was determined using a small block test. All samples, previously oven dried at 1002 58C to constant weight, were sterilised with g-radiation and exposed to C. versicolor on 2% malt extract agar in Petri dishes, inoculated 3±4 days prior to the test. Samples were supported on sterile plastic mesh to prevent contact with the agar and incubated at 268C and 65% relative humidity for 12 weeks. After this period, hyphae adhering to sample surfaces were removed and samples dried at 103 2 28C to constant weight. The weight losses (WL) for individual samples and mean percentage weight losses for each sample set were determined using Eq. (2). WL 100
Treatment before decay test
Code
None ÐImmersion in DMF 5 min/208C ÐThermal treatment 90 min/1208C ÐAcetone washing, drying ÐImmersion in DMF 5 min/208C ÐThermal treatment 90 min/1208C ÐImmersion in DMF 5 min/208C ÐThermal treatment 90 min/1208C ÐAcetone washing, drying
W C1 C2
a Note: C1 and C2 controls determine the eects of solvents and thermal treatment on decay resistance.
m0 ÿ mf % m0
2
where: m0 mass of oven-dried sample prior to the test; mf mass of oven-dried sample after the test. Fungal colonisation and decay of experimental and reference (control) veneers was also examined using scanning electron microscopy (SEM). Sub-samples (2 mm3) were sectioned from unexposed unpressed veneers, unexposed pressed veneers and exposed unpressed veneers using disposable microtome blades (Agar Scienti®c). Samples were dried over phosphorus pentoxide for 48 hours, mounted on aluminium stubs using silver dag (Agar Scienti®c), gold coated in a
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Table 3 Percentage weight losses of aspen veneers exposed to Coriolus versicolor for 12 weeksa Type of sample
WPGa (%)
Average weight loss (%)
Standard deviation
Number of replicates
Unmodi®ed wood W Control C1 Control C2 Esteri®ed WE Oligoesteri®ed WO1 Oligoesteri®ed WO2
0.0 0.0 0.0 13.6 16.0 17.9
61.8 58.5 51.4 20.6 17.8 17.1
4.8 12.7 10.2 4.3 2.7 4.3
5 6 5 5 6 7
a
Note: WPGa, approximate percentage weight increase of samples due to chemical modi®cation.
sputter coater (Polaron) and examined in a Cambridge Stereoscan 250 SEM. For the weathering test, modi®ed solid wood and corresponding control samples (100 50 5) mm were exposed between July±November 1996 at Brasov, Romania. Samples were supported 2 m from the ground at 458 facing south west. The supporting frames were constructed so that the samples would be exposed to maximum amount of sunlight and rain. Exposed samples were periodically examined visually and microscopically to determine changes in colour and surface ®nish and to asses the extent of biodeterioration. The weathering test compared the behaviour of unmodi®ed and chemically modi®ed wood and examined the in¯uence of hot-pressing on weathering resistance. For this reason one half of the unmodi®ed and chemically modi®ed samples were hot-pressed on their faces at 1508C and 4 MPa for 30 min (P coded samples). Samples with the highest gloss ®nish after hot pressing were chosen for the weathering test. These were oligoesteri®ed wood samples of type WO1 based on MA and AGE (see Table 1). All samples were conditioned at 50±55% relative humidity and 20± 228C for 15 days prior to exposure.
3. Results and discussion 3.1. Decay test The results of the decay test for unpressed chemically modi®ed and reference veneers have been summarised in Table 3. Statistical analysis of this data (ttest assuming unequal variances), showed there were no signi®cant dierences between weight losses for the reference samples (W, C1, C2), indicating solvents and thermal treatment did not in¯uence decay resistance of aspen. In addition, there were no signi®cant dierences between weight losses for the chemically modi®ed samples (WE, WO1, WO2), although these chemical modi®cations signi®cantly enhanced decay resistance to C. versicolor. Fungal colonisation of chemically modi®ed and reference samples was found to dier. The surfaces of all reference samples were colonised within one week of exposure. In comparison, colonisation of modi®ed samples took place more slowly and the mycelium did not adhere well to the sample surface forming a thin ®lm at the end of the test. The SEM study showed reference samples to be
Fig. 3. Scanning electron micrographs showing transverse sections through reference (control) samples (a) and chemically modi®ed samples (b) following exposure to the decay fungus. Note intact ®bre walls (arrowed) and vessel lumens (V) free from hyphae for modi®ed samples (b) (scale bar 50 mm). Note ®bre wall thinning (arrowed) and hyphae in vessel lumens (V) of unmodi®ed samples (a)(scale bar 50 mm).
M.C. Timar et al. / International Biodeterioration & Biodegradation 43 (1999) 181±187
185
Fig. 4. The eect of hot-pressing on decay resistance of chemically modi®ed and unmodi®ed aspen.
heavily colonised by the fungus, with hyphae penetrating throughout the sample section. Moreover, ®bre wall thinning was observed. In contrast, little colonisation and no structural changes in wood cell walls characterised chemically modi®ed samples (Fig. 3). However, more research is required to explain why weight losses of between 17±20% were registered for chemically modi®ed samples following the test. A partial de-esteri®cation in a long-term exposure under high humidity conditions might be considered. Results from the test to examine the eect of hotpressing on decay resistance are presented in Fig. 4. Although the average weight losses were lower for the hot-pressed veneers in all sample types, statistical analysis of the data showed these dierences to be signi®cant only in the unmodi®ed and esteri®ed wood specimens. This increase in decay resistance of unmodi®ed hotpressed veneers probably resulted from compression of samples and subsequent vessel collapse, reducing hyphal access via this pathway. Compression of the modi®ed wood did not have such a marked eect on decay resistance and access was not the main factor in¯uencing decay in modi®ed specimens. The study therefore demonstrates the improved decay resistance of chemically modi®ed wood products resulted from the chemical modi®cation. Moreover, the chemical modi®cation induced a reduced hygroscopicity which
probably in¯uenced colonisation and subsequent decay by fungi. Equilibrium moisture contents of samples conditioned at 208C and 65% r.h. was between 5.7± 6.3% for chemically modi®ed samples, compared to 9.5±10.3% for unmodi®ed and control samples. Table 4 compares the decay resistance between oligoesteri®ed wood and the unmodi®ed wood in tests carried out on veneers and composites based on hotpressed sawdust. From the results, compared to the unmodi®ed veneers (unpressed and hot-pressed), higher weight losses were recorded in the unmodi®ed sawdust samples. This probably resulted from better hyphal penetration of a material with a larger surface area to volume ratio, since the unmodi®ed sawdust samples swelled to twice their initial thickness when introduced into the plates. Lower weight losses recorded for the plastic-like Table 4 Results of the decay test carried out on dierent types of aspen wood products Unmodi®ed wood Type of product
WL (%)
St. dev.
Oligoesteri®ed wood (MA/GMA) WL (%) St. dev.
Veneers Hot-pressed veneers Hot-pressed sawdust
61.8 51.8 76.7
4.8 1.5 5.8
17.1 13.5 10.9
4.3 1.9 0.4
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M.C. Timar et al. / International Biodeterioration & Biodegradation 43 (1999) 181±187
Table 5 Results of weathering test on aspen solid wood Type of wood
Code of sample
Unmodi®ed
W
Unmodi®ed hot-pressed
W-P
Oligoesteri®ed
WO1
Oligoesteri®ed hot-pressed
WO1-P
Initial aspect
Aspect after weathering
White, smooth, annual rings dicult to observe White, smooth, semi-glossy annual rings dicult to observe Yellowish brown, annual rings clearly observed Light brown, glossy, annual rings clearly observed
composites resulting from hot-pressing of oligoesterifed sawdust may be explained by the higher degree of chemical modi®cation achieved for these samples (WPGa=91.6%) and, possibly, by the higher density of these products (1.34 g/cm3) compared with veneers. The surfaces of these products were hardly colonised by the fungus, so that after 12 weeks, slight colonisation adjacent to the disc edges was observed. 3.2. Weathering test Table 5 presents the results from the visual examination of samples prior to and following the weathering test. It was noted that glossy and aesthetically appealing surfaces were obtained by chemical modi®cation followed by hot-pressing of aspen wood. The SEM study showed this to be due to a thin outer layer formed during hot-pressing, which resulted in partial melting of the oligoesteri®ed wood with thermoplastic properties. Secondly, compared to the unmodi®ed wood, little change occurred in the aspect (colour, roughness) of the modi®ed samples when weathered, indicating improved weathering resistance following chemical modi®cation. Examination of unmodi®ed reference samples W and W-P under the microscope showed biodeterioration during weathering. Dark surface staining following exposure resulted from mould growth, and hot pressing before exposure failed to reduce this growth. In contrast, no biodegradation was observed on the chemically modi®ed wood samples either pressed or unpressed following exposure, indicating modi®cation improved resistance to colonisation by mould fungi, as previously observed in a laboratory test (Timar et al., 1997). Little lightening of sample surfaces was observed in the chemically modi®ed samples following exposure. This may result from UV absorption by the double bonds in the MA and AGE molecules in the oligoesteri®ed wood. A medium UV resistance of modi®ed wood containing allyl groups (allylated wood) was reported by Kiguchi (1996). Improved UV resistance in the chemically modi®ed samples also resulted in
Colour change to grey roughened, frequent dark stains Colour change to grey, roughened, frequent dark stains Little lightening, little change in roughness Little darkening, no important changes in gloss and roughness
reduced sample roughness following exposure. Improved UV resistance results in less erosion of UV degraded lignin products by rainwater and therefore less surface roughening, as previously reported (Plackett et al., 1992, 1996). To conclude, chemical modi®cation by esteri®cation with maleic anhydride and subsequent oligoesteri®cation with maleic anhydride and glycidyl methacrylate or allyl glycidyl ether, signi®cantly improved decay resistance of aspen to the white rot fungus C. versicolor. This was shown for veneers and hot-pressed sawdust discs. Solvents and temperature used in the chemical modi®cation procedures had no signi®cant eect on decay resistance of aspen. Hot-pressing was shown to signi®cantly improve decay resistance of unmodi®ed wood samples by limiting hyphal penetration through the wood structure. Secondly, the chemical modi®cation achieved in this research enhanced the weathering resistance of aspen wood, including resistance to colonisation by mould fungi and to discoloration and surface erosion by UV radiation and rainwater. References Beckers, E.P.J., Militz, H., Stevens, M., 1994. Resistance of acetylated wood to Basidiomycetes, soft rot and blue stain, International Research Group on Wood Preservation Document No. IRG/WP 94-40021. Blanchette, R.A., 1995. Degradation of the lignocellulosic complex in wood. Canadian Journal of Botany 73 (Suppl.), S999±S1010. BS EN 350-2, 1994. Durability of wood and wood-based productsÐ Natural durability of solid wood. British Standards Institution. Chow, P., Bao, Z., Youngquist, J., Rowell, R., Muehl, J., Krzysik, A., 1996. Properties of hardboards made from acetylated aspen and southern pine. Wood and Fiber Science 28 (2), 252±258. Clemons, C., Young, R.A., Rowell, R.M., 1992. Moisture sorption properties of composite boards from esteri®ed aspen ®ber. Wood and Fiber Science 24 (3), 353±362. Dinwoodie, J.M., 1989. Wood: Nature's Cellular, Polymeric Fibre Composite. Institute of Metals, London, p. 138. Eaton, R.A., Hale, M.D.C., 1993. Wood Decay, Pests and Protection. Chapman and Hall, London, p. 546. Ellis, W.D., Rowell, R.M., 1984. Reaction of isocyanates with southern pine wood to improve dimensional stability and decay resistance. Wood and Fiber Science 16 (3), 349±356. Forster, S.C., Hale, M.D., Williams, G., 1997. Ecacy of anhydrides
M.C. Timar et al. / International Biodeterioration & Biodegradation 43 (1999) 181±187 as wood protecting chemicals, International Research Group on Wood Preservation Document No. IRG/WP 97-30162. Imamura, Y., Nishimoto, K., 1986. Resistance of acetylated wood to attack by subterranean termites. Wood research 72, 37±44. Johnson, B.R., Rowell, R.M., 1988. Resistance of chemically modi®ed wood to marine borers. Material und Organismen 23 (2), 147±156. Kiguchi, M., 1996. Surface modi®cation and activation of wood. In: Hon, D.N-S. (Ed.), Chemical Modi®cation of Lignocellulosic Materials. Marcel Dekker Inc, New York, pp. 197±226. Matsuda, H., 1993. Preparation and properties of oligoesteri®ed wood blocks based on anhydride and epoxide. Wood Science and Technology 27, 23±34. Militz, H., Beckers, E.P.J., Homan, W.J., 1997. Modi®cation of solid wood; research and practical potential, International Research Group on Wood Preservation Document No. IRG/WP/97-40098. Panshin, A.J., de Zeeuw, C., 1980. Text-book of Wood Technology. McGraw-Hill Inc, New York, p. 722. Plackett, D.V., Dunningham, E.A., Singh, A.P., 1992. Weathering of chemically modi®ed woodÐaccelerated weathering of acetylated radiata pine. Holz als Roh- und Werksto 50, 135±140. Plackett, D.V., Dunningham, E.A., Singh, A.P., 1996. Weathering of chemically modi®ed wood. In: Hon, S. (Ed.), Chemical Modi®cation of Lignocellulosic Materials. Marcell Dekker Inc, New York, pp. 277±293. Rowell, R.M., Hart, S.V., Esentgher, G.R., 1979. Resistance of alkylene-oxide modi®ed southern pine to attack by subterranean termites. Wood Science and Technology 11 (4), 271±274.
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Rowell, R.M., Simonson, R., Tillman, E., 1990. Acetyl balance for the acetylation of wood particles by a simpli®ed procedure. Holzforschung 44, 263±269. Rowell, R.M., Cleary, B.A., Rowell, J.S., Clemons, C., Young, R., 1993. Results of chemical modi®cation of lignocellulosic ®bers for use in composites. In: Wolcott, M.P. (Ed.), Wood and Fiber Polymer Composites: Fundamental Concepts, Processes, and Material Options. Forest Products Society, pp. 121±127. Suttie, E.D., Hill, C.A.S., Jones, D., Orsler, R.J., 1997. Assessing the bioresistance conferred to solid wood by chemical modi®cation, International Research Group on Wood Preservation Document No. IRG/WP 97-40099. Takahashi, M., 1996. Biological properties of chemically modi®ed wood. In: Hon, S. (Ed.), Chemical modi®cation of lignocellulosic materials. Marcell Dekker Inc, New York, pp. 331±359. Timar, M.C., Mihai, M.D., Baciu, G., 1997. About the water and biological resistance of some new chemically modi®ed wood composites, International Research Group on Wood Preservation Document No. IRG/WP 97-40077. Yusuf, S., Imamura, Y., Takahashi, M., Minato, K., 1994. Biological resistance of aldehyde treated wood, International Research Group on Wood Preservation Document No. IRG/WP 94-40018. Yusuf, S., Imamura, Y., Takahashi, M., Minato, K., 1995. Physical and biological properties of Albizzia waferboard modi®ed with cross-linking agents, International Research Group on Wood Preservation Document No. IRG/WP 95-40018.
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Biological resistance of chemically modi®ed aspen composites M.C. Timar a, A. Pitman b,*, M.D. Mihai a a
``Transilvania'' University of Brasov, Romania Buckinghamshire Chilterns University College, High Wycombe, UK
b
Received 9 October 1998; received in revised form 18 December 1998; accepted 10 March 1999
Abstract Aspen wood (Populus tremula L.) was chemically modi®ed by a two-step procedure consisting of esteri®cation with maleic anhydride (MA) and subsequent oligoesteri®cation with MA and glycidyl methacrylate (GMA) or allyl glycidyl ether (AGE). This chemical modi®cation procedure was carried out on solid wood, veneers and sawdust. The modi®ed wood showed thermoplastic properties and could be thermally formed by hot-pressing. As a result, solid wood and the veneer samples had smooth, glossy surfaces, while a plastic-like material was produced on thermally forming the modi®ed sawdust. The biological resistance of chemically modi®ed and thermally formed samples was assessed by determination of weight loss following exposure to a decay fungus in a laboratory test and by a weathering test. Modi®ed samples exposed to the white rot Coriolus versicolor for 12 weeks in the laboratory were more resistant to decay, with weight losses signi®cantly lower than for the corresponding control samples. Solvents and thermal treatments employed in the chemical modi®cation process had no signi®cant eect on decay resistance of Aspen veneers. However, hot-pressing signi®cantly improved decay resistance in unmodi®ed wood samples by limiting hyphal colonisation of the wood structure. A microscopic comparison of chemically modi®ed and unmodi®ed wood samples was conducted to examine extent of fungal colonisation and decay. Chemical modi®cation was also shown to enhance the weathering resistance of aspen wood to discoloration and surface erosion by UV and rainwater and to stain from mould fungi. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Man has utilised wood for a wide range of end uses because of its availability, ease of working and its mechanical properties (Panshin and De Zeeuw, 1980; Dinwoodie, 1989). Wood has been shown to be colonised and decayed by organisms including bacteria, fungi, insects and marine wood borers as part of the natural carbon cycle (Panshin and De Zeeuw, 1980). However, fungal decay and dis®gurement of wood in service is deleterious (Eaton and Hale, 1993). It has been recognised that the service life of a timber structure may be extended, either by employing a timber species which is naturally resistant to decay, or
* Corresponding author at: Buckinghamshire Chilterns University College, High Wycombe Campus, Queen Alexandra Road, Bucks HP11 2JZ, UK. Tel.: 01494-522-141; fax: 01494-524-392.
by treating less durable species with preservatives (Eaton and Hale, 1993; Blanchette, 1995). More recently, research has demonstrated that chemical modi®cation enhances a number of wood properties including its resistance to fungal decay, colonisation by mould fungi and attack by marine wood borers and insects. Enhancement of biological resistance by chemical modi®cation is believed to result from alteration of the chemical structure of the cell wall polymers making them unidenti®able for microbial decay enzymes, and by reducing moisture absorption by the wood cell walls by partial substitution of hydroxyl groups in the wood polymers (Militz et al., 1997). However, the degree of bioprotection achieved and the spectrum of activity is reportedly dependant on the type of modi®cation and the wood species (Takahashi, 1996; Forster et al., 1997). Extensive research has been conducted to determine the biological resistance of esteri®ed wood treated with acetic anhydride (most often) and other anhydrides. It
0964-8305/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 4 - 8 3 0 5 ( 9 9 ) 0 0 0 5 2 - 9
182
M.C. Timar et al. / International Biodeterioration & Biodegradation 43 (1999) 181±187
Fig. 1. Esteri®cation process.
was found that acetylated wood exhibited good resistance to brown rot, white rot and soft rot decay (Beckers et al., 1994; Takahashi, 1996; Forster et al., 1997; Suttie et al., 1997), whilst resistance to colonisation by lower fungi including moulds and stains proved unsatisfactory (Beckers et al., 1994). Other studies also showed enhanced resistance of acetylated wood to attack by subterranean termites (Imamura and Nishimoto, 1986) and marine borers (Johnson and Rowell, 1988). Wood esteri®ed with dierent alkyl anhydrides (with up to six carbon atoms in the molecule) exhibited comparable bioresistances to fungi and insects, although there were indications that smaller reactant molecules produced better products (Suttie et al., 1997). Improved resistance to biodeterioration was also demonstrated for other chemical modi®cation processes including epoxydation (Rowell et al., 1979), reaction with isocyanates (Ellis and Rowell, 1984; Forster et al., 1997; Suttie et al., 1997), cross-linking with aldehydes (Yusuf et al., 1994, 1995) and oligoesteri®cation (Matsuda, 1993). The current paper reports on the eect of a two-step chemical modi®cation treatment: esteri®cation with maleic anhydride and subsequent oligoesteri®cation with maleic anhydride and an epoxide, followed by hot-pressing, on the decay resistance of aspen exposed to Coriolus versicolor under laboratory conditions, and to mould fungi under weathering conditions. The research is an extension of a previous study, which showed this treatment reduced mould growth on wood products manufactured by hot-pressing oligoesteri®ed aspen sawdust (Timar et al., 1997). Aspen was selected for use in this study because it
has a low natural durability (BS EN 350-2: 1994), is readily permeable (facilitating chemical impregnation) and has previously been employed in chemical modi®cation studies (Rowell et al., 1990; Clemons et al., 1992; Rowell et al., 1993; Chow et al., 1996). However, little information is available on the biological resistance of this species when chemically modi®ed (Beckers et al., 1994) and no previous study has investigated biological resistance of oligoesteri®ed aspen. 2. Materials and methods 2.1. Chemical modi®cation and sample preparation Aspen sawdust (diameter 0.3±0.5 mm) or veneers (30 30 2 mm) were dried at 103228C to constant weight prior to chemical modi®cation. Sawdust and veneers were esteri®ed with maleic anhydride (MA) to produce esteri®ed wood (WE), as a mixture of the monoester MM (principal component) and the diester DM (secondary component), as presented in Fig. 1. Esteri®ed samples were then treated with a mixture of an epoxide (allyl glycidyl ether (AGE) or glycidyl methacrylate (GMA) and MA. The most probable structure of the resulting oligoesteri®ed wood is shown in Fig. 2. The reaction conditions employed for the chemical modi®cation procedure are summarised in Table 1. After the reaction, samples were washed with acetone to remove unreacted chemicals, dried under vacuum at 208C and then oven dried at 103 2 28C to constant weight. An approximate weight percent gain (WPGa) which is directly correlated to the degree of
Fig. 2. Oligoesteri®cation process.
M.C. Timar et al. / International Biodeterioration & Biodegradation 43 (1999) 181±187
183
Table 1 Chemical modi®cation procedure for aspen samplesa Form of wood material Sawdust
Reagents MA TEA DMF MA TEA DMF MA TEA DMF MA TEA DMF
Veneers
Esteri®cation T(8C)
29.0% 1.3% 69.7% 32.0% 1.0% 67.0% 32.0% 1.0% 67.0% 32.0% 1.0% 67.0%
Time (h)
120
1
120
1.5
120
1.5
120
1.5
Oligoesteri®cation Reagents T(8C) MA GMA DMF
MA AGE DMF MA GMA DMF
±
Time (h) 7
Code of sample
18.4% 35.1% 46.5%
90±95
17.4% 18.8% 63.8% 12.0% 24.5% 63.5%
120
1.5
WO1
120
1.5
WO2
±
WOE ±
WE
a Note: MA, maleic anhydride; GMA, glycidyl methacrylate; AGE, allyl glycidyl ether. DMF (N,N-dimethylformamide) was used as a solvent in the two steps of chemical modi®cation. TEA (triethanolamine) was employed as a catalyst in the esteri®cation step.
chemical modi®cation was calculated for the modi®ed sawdust and veneers according to Eq. (1). WPGa
Mra ÿ M0 100 % M0
1
where: Mra represents the weight of the chemically modi®ed sample after washing with acetone and oven drying to constant weight; M0 represents the weight of the oven dried, unmodi®ed sample. Chemically modi®ed and unmodi®ed sawdust was formed into test discs (22 mm diameter and 2 mm thick) by hot-pressing in an aluminium alloy mould at 160±1708C and 45 MPa for 30 min. Tough, glossy, water-resistant plastic-like composites resulted from the oligoesteri®ed wood. In the case of the veneers, reference (control) samples were prepared according to methods outlined in Table 2. To study the eect of the hot-pressing on the decay resistance of veneers, six unmodi®ed and chemically Table 2 Preparation of veneer reference (control) samplesa Type of reference sample Unmodi®ed wood Control sample C1 Control sample C2
modi®ed samples were hot-pressed at 1308C and 10 MPa for 30 min perpendicular to their faces. 2.2. Biological tests Two tests were undertaken to assess biological resistance of the modi®ed aspen wood and composites: a laboratory decay test and a natural weathering test. The resistance of experimental and reference (control) veneers and sawdust samples to decay by C. versicolor (Linnaeus) Quelet was determined using a small block test. All samples, previously oven dried at 1002 58C to constant weight, were sterilised with g-radiation and exposed to C. versicolor on 2% malt extract agar in Petri dishes, inoculated 3±4 days prior to the test. Samples were supported on sterile plastic mesh to prevent contact with the agar and incubated at 268C and 65% relative humidity for 12 weeks. After this period, hyphae adhering to sample surfaces were removed and samples dried at 103 2 28C to constant weight. The weight losses (WL) for individual samples and mean percentage weight losses for each sample set were determined using Eq. (2). WL 100
Treatment before decay test
Code
None ÐImmersion in DMF 5 min/208C ÐThermal treatment 90 min/1208C ÐAcetone washing, drying ÐImmersion in DMF 5 min/208C ÐThermal treatment 90 min/1208C ÐImmersion in DMF 5 min/208C ÐThermal treatment 90 min/1208C ÐAcetone washing, drying
W C1 C2
a Note: C1 and C2 controls determine the eects of solvents and thermal treatment on decay resistance.
m0 ÿ mf % m0
2
where: m0 mass of oven-dried sample prior to the test; mf mass of oven-dried sample after the test. Fungal colonisation and decay of experimental and reference (control) veneers was also examined using scanning electron microscopy (SEM). Sub-samples (2 mm3) were sectioned from unexposed unpressed veneers, unexposed pressed veneers and exposed unpressed veneers using disposable microtome blades (Agar Scienti®c). Samples were dried over phosphorus pentoxide for 48 hours, mounted on aluminium stubs using silver dag (Agar Scienti®c), gold coated in a
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Table 3 Percentage weight losses of aspen veneers exposed to Coriolus versicolor for 12 weeksa Type of sample
WPGa (%)
Average weight loss (%)
Standard deviation
Number of replicates
Unmodi®ed wood W Control C1 Control C2 Esteri®ed WE Oligoesteri®ed WO1 Oligoesteri®ed WO2
0.0 0.0 0.0 13.6 16.0 17.9
61.8 58.5 51.4 20.6 17.8 17.1
4.8 12.7 10.2 4.3 2.7 4.3
5 6 5 5 6 7
a
Note: WPGa, approximate percentage weight increase of samples due to chemical modi®cation.
sputter coater (Polaron) and examined in a Cambridge Stereoscan 250 SEM. For the weathering test, modi®ed solid wood and corresponding control samples (100 50 5) mm were exposed between July±November 1996 at Brasov, Romania. Samples were supported 2 m from the ground at 458 facing south west. The supporting frames were constructed so that the samples would be exposed to maximum amount of sunlight and rain. Exposed samples were periodically examined visually and microscopically to determine changes in colour and surface ®nish and to asses the extent of biodeterioration. The weathering test compared the behaviour of unmodi®ed and chemically modi®ed wood and examined the in¯uence of hot-pressing on weathering resistance. For this reason one half of the unmodi®ed and chemically modi®ed samples were hot-pressed on their faces at 1508C and 4 MPa for 30 min (P coded samples). Samples with the highest gloss ®nish after hot pressing were chosen for the weathering test. These were oligoesteri®ed wood samples of type WO1 based on MA and AGE (see Table 1). All samples were conditioned at 50±55% relative humidity and 20± 228C for 15 days prior to exposure.
3. Results and discussion 3.1. Decay test The results of the decay test for unpressed chemically modi®ed and reference veneers have been summarised in Table 3. Statistical analysis of this data (ttest assuming unequal variances), showed there were no signi®cant dierences between weight losses for the reference samples (W, C1, C2), indicating solvents and thermal treatment did not in¯uence decay resistance of aspen. In addition, there were no signi®cant dierences between weight losses for the chemically modi®ed samples (WE, WO1, WO2), although these chemical modi®cations signi®cantly enhanced decay resistance to C. versicolor. Fungal colonisation of chemically modi®ed and reference samples was found to dier. The surfaces of all reference samples were colonised within one week of exposure. In comparison, colonisation of modi®ed samples took place more slowly and the mycelium did not adhere well to the sample surface forming a thin ®lm at the end of the test. The SEM study showed reference samples to be
Fig. 3. Scanning electron micrographs showing transverse sections through reference (control) samples (a) and chemically modi®ed samples (b) following exposure to the decay fungus. Note intact ®bre walls (arrowed) and vessel lumens (V) free from hyphae for modi®ed samples (b) (scale bar 50 mm). Note ®bre wall thinning (arrowed) and hyphae in vessel lumens (V) of unmodi®ed samples (a)(scale bar 50 mm).
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185
Fig. 4. The eect of hot-pressing on decay resistance of chemically modi®ed and unmodi®ed aspen.
heavily colonised by the fungus, with hyphae penetrating throughout the sample section. Moreover, ®bre wall thinning was observed. In contrast, little colonisation and no structural changes in wood cell walls characterised chemically modi®ed samples (Fig. 3). However, more research is required to explain why weight losses of between 17±20% were registered for chemically modi®ed samples following the test. A partial de-esteri®cation in a long-term exposure under high humidity conditions might be considered. Results from the test to examine the eect of hotpressing on decay resistance are presented in Fig. 4. Although the average weight losses were lower for the hot-pressed veneers in all sample types, statistical analysis of the data showed these dierences to be signi®cant only in the unmodi®ed and esteri®ed wood specimens. This increase in decay resistance of unmodi®ed hotpressed veneers probably resulted from compression of samples and subsequent vessel collapse, reducing hyphal access via this pathway. Compression of the modi®ed wood did not have such a marked eect on decay resistance and access was not the main factor in¯uencing decay in modi®ed specimens. The study therefore demonstrates the improved decay resistance of chemically modi®ed wood products resulted from the chemical modi®cation. Moreover, the chemical modi®cation induced a reduced hygroscopicity which
probably in¯uenced colonisation and subsequent decay by fungi. Equilibrium moisture contents of samples conditioned at 208C and 65% r.h. was between 5.7± 6.3% for chemically modi®ed samples, compared to 9.5±10.3% for unmodi®ed and control samples. Table 4 compares the decay resistance between oligoesteri®ed wood and the unmodi®ed wood in tests carried out on veneers and composites based on hotpressed sawdust. From the results, compared to the unmodi®ed veneers (unpressed and hot-pressed), higher weight losses were recorded in the unmodi®ed sawdust samples. This probably resulted from better hyphal penetration of a material with a larger surface area to volume ratio, since the unmodi®ed sawdust samples swelled to twice their initial thickness when introduced into the plates. Lower weight losses recorded for the plastic-like Table 4 Results of the decay test carried out on dierent types of aspen wood products Unmodi®ed wood Type of product
WL (%)
St. dev.
Oligoesteri®ed wood (MA/GMA) WL (%) St. dev.
Veneers Hot-pressed veneers Hot-pressed sawdust
61.8 51.8 76.7
4.8 1.5 5.8
17.1 13.5 10.9
4.3 1.9 0.4
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M.C. Timar et al. / International Biodeterioration & Biodegradation 43 (1999) 181±187
Table 5 Results of weathering test on aspen solid wood Type of wood
Code of sample
Unmodi®ed
W
Unmodi®ed hot-pressed
W-P
Oligoesteri®ed
WO1
Oligoesteri®ed hot-pressed
WO1-P
Initial aspect
Aspect after weathering
White, smooth, annual rings dicult to observe White, smooth, semi-glossy annual rings dicult to observe Yellowish brown, annual rings clearly observed Light brown, glossy, annual rings clearly observed
composites resulting from hot-pressing of oligoesterifed sawdust may be explained by the higher degree of chemical modi®cation achieved for these samples (WPGa=91.6%) and, possibly, by the higher density of these products (1.34 g/cm3) compared with veneers. The surfaces of these products were hardly colonised by the fungus, so that after 12 weeks, slight colonisation adjacent to the disc edges was observed. 3.2. Weathering test Table 5 presents the results from the visual examination of samples prior to and following the weathering test. It was noted that glossy and aesthetically appealing surfaces were obtained by chemical modi®cation followed by hot-pressing of aspen wood. The SEM study showed this to be due to a thin outer layer formed during hot-pressing, which resulted in partial melting of the oligoesteri®ed wood with thermoplastic properties. Secondly, compared to the unmodi®ed wood, little change occurred in the aspect (colour, roughness) of the modi®ed samples when weathered, indicating improved weathering resistance following chemical modi®cation. Examination of unmodi®ed reference samples W and W-P under the microscope showed biodeterioration during weathering. Dark surface staining following exposure resulted from mould growth, and hot pressing before exposure failed to reduce this growth. In contrast, no biodegradation was observed on the chemically modi®ed wood samples either pressed or unpressed following exposure, indicating modi®cation improved resistance to colonisation by mould fungi, as previously observed in a laboratory test (Timar et al., 1997). Little lightening of sample surfaces was observed in the chemically modi®ed samples following exposure. This may result from UV absorption by the double bonds in the MA and AGE molecules in the oligoesteri®ed wood. A medium UV resistance of modi®ed wood containing allyl groups (allylated wood) was reported by Kiguchi (1996). Improved UV resistance in the chemically modi®ed samples also resulted in
Colour change to grey roughened, frequent dark stains Colour change to grey, roughened, frequent dark stains Little lightening, little change in roughness Little darkening, no important changes in gloss and roughness
reduced sample roughness following exposure. Improved UV resistance results in less erosion of UV degraded lignin products by rainwater and therefore less surface roughening, as previously reported (Plackett et al., 1992, 1996). To conclude, chemical modi®cation by esteri®cation with maleic anhydride and subsequent oligoesteri®cation with maleic anhydride and glycidyl methacrylate or allyl glycidyl ether, signi®cantly improved decay resistance of aspen to the white rot fungus C. versicolor. This was shown for veneers and hot-pressed sawdust discs. Solvents and temperature used in the chemical modi®cation procedures had no signi®cant eect on decay resistance of aspen. Hot-pressing was shown to signi®cantly improve decay resistance of unmodi®ed wood samples by limiting hyphal penetration through the wood structure. Secondly, the chemical modi®cation achieved in this research enhanced the weathering resistance of aspen wood, including resistance to colonisation by mould fungi and to discoloration and surface erosion by UV radiation and rainwater. References Beckers, E.P.J., Militz, H., Stevens, M., 1994. Resistance of acetylated wood to Basidiomycetes, soft rot and blue stain, International Research Group on Wood Preservation Document No. IRG/WP 94-40021. Blanchette, R.A., 1995. Degradation of the lignocellulosic complex in wood. Canadian Journal of Botany 73 (Suppl.), S999±S1010. BS EN 350-2, 1994. Durability of wood and wood-based productsÐ Natural durability of solid wood. British Standards Institution. Chow, P., Bao, Z., Youngquist, J., Rowell, R., Muehl, J., Krzysik, A., 1996. Properties of hardboards made from acetylated aspen and southern pine. Wood and Fiber Science 28 (2), 252±258. Clemons, C., Young, R.A., Rowell, R.M., 1992. Moisture sorption properties of composite boards from esteri®ed aspen ®ber. Wood and Fiber Science 24 (3), 353±362. Dinwoodie, J.M., 1989. Wood: Nature's Cellular, Polymeric Fibre Composite. Institute of Metals, London, p. 138. Eaton, R.A., Hale, M.D.C., 1993. Wood Decay, Pests and Protection. Chapman and Hall, London, p. 546. Ellis, W.D., Rowell, R.M., 1984. Reaction of isocyanates with southern pine wood to improve dimensional stability and decay resistance. Wood and Fiber Science 16 (3), 349±356. Forster, S.C., Hale, M.D., Williams, G., 1997. Ecacy of anhydrides
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