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Modification of wood with Si compounds to limit boron leaching from treated wood and to increase termite and decay resistance
International Biodeterioration & Biodegradation 63 (2009) 187–190
Contents lists available at ScienceDirect
International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod
Modification of wood with Si compounds to limit boron leaching from treated wood and to increase termite and decay resistance S. Nami Kartal a, *, Tsuyoshi Yoshimura b, Yuji Imamura b a b
Department of Forest Biology and Wood Protection Technology, Forestry Faculty, Istanbul University, Bahcekoy 34473, Istanbul, Turkey Research Institute for Sustainable Humanosphere (RISH), Kyoto University, Uji, 611-0011 Kyoto, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 June 2008 Received in revised form 28 August 2008 Accepted 29 August 2008 Available online 4 November 2008
In this study, we tested tetraethoxysilane and methyltriethoxysilane as modifying silicon-based compounds for their potential to limit boron leachability from modified wood and to increase biological durability of the wood against fungi and termites. Both the silane compounds were used in silane state where acidified ethanol was added and stirred at ambient temperature for 30 min. We used two different processes for preservative treatments: double treatment and single treatment. In double treatment, the specimens from sugi wood were first treated with boric acid at 1% concentration and subsequently treated with the silanes. In single treatment, boric acid was mixed with the silane compounds in the silane state yielding 1% boric acid concentration. Subsequent to the treatments, wood specimens were subjected to laboratory leaching tests, and leachates were analyzed for boron content with an inductively coupled plasma (ICP) spectrometry. ICP analyses showed that silane treatments were able to limit boron leaching from treated wood by about 40% in all cases for each silane compound. Wood specimens were then subjected to laboratory termite and decay resistance tests using the subterranean termites, Coptotermes formosanus, and the wood decaying fungi, Fomitopsis palustris and Trametes versicolor. Termite and fungal decay resistance tests revealed that resistance of modified wood with the silane and boron compounds increased when compared to untreated and boron-only treated wood specimens. More indepth studies on the mechanisms of interactions between the silicon compounds, boron elements and wood components are in progress. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Wood modification Boron Silicon compounds Silane Termite resistance Fungal resistance
1. Introduction Numerous trials have been conducted with several chemical compounds to bring modification in wood so as to improve its several characteristics (Rowell, 1983; Militz et al., 1997; Norimoto, 2001; Donnath et al., 2004). Many types of silicon compounds have been applied for this purpose. Silanes have long been used as modification agents in a number of applications, including in hydrophobication of ceramics, scratch-resistant surfaces, soil proofing and anti-graffiti coatings, and as adhesion promoters between organic and inorganic materials (Mai et al., 2003; Donnath et al., 2004; Hill et al., 2004). The silanol groups of silanes can react with hydroxyl groups of cell wall polymers forming a covalent bond between the silicon compound and cell wall polymers; however, Si–O–C bonds are weak and susceptible to hydrolysis. Alkylalkoxysilanes bear at least one alkyl group which remains after the sol– gel process, because in contrast to Si–O bonds the Si–C bonds are * Corresponding author. Tel.: þ90 212 226 1100/þ90 532 352 88 99 (mobile); fax: þ90 212 226 1113. E-mail address: [email protected] (S.N. Kartal). 0964-8305/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ibiod.2008.08.006
stable against hydrolysis (Brinker and Scherer, 1990; Sebe and Brook, 2001; Donnath et al., 2004). In addition to improved wood properties, such modification may have the potential to limit preservative release from treated wood and to increase biological resistance against wood-degrading fungi and insects. Kartal et al. (2004) found that acryl-silicon-type resin emulsion and boron-treated wood specimens resulted in about 54% boron release after a 10-day leaching process; however, nearly all boron was leached out from boron-only treated specimens. Similar results were obtained when wood specimens were treated with boron-containing quaternary ammonia compound (DBF) and the same emulsion in surface treatments (Kartal et al., 2004). A study by Lin and Chen (2006) showed that a series of cleavable water-soluble silicon surfactants prepared by the reaction of a hydroxyl-terminated polyester and an organopolysiloxane bear siloxane as the functional group of water repellency effect. Since several silicon compounds may have the ability to increase water repellency and reaction with cell wall components, such compounds can be used to limit boron release from boron-treated wood. In this study, we tested tetraethoxysilane and methyltriethoxysilane as modifying Si compounds for their potential to
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S.N. Kartal et al. / International Biodeterioration & Biodegradation 63 (2009) 187–190
limit boron leachability from chemically modified wood and to increase biological durability of the wood against fungi and termites. 2. Materials and methods Wood specimens, 20 (radial) by 20 (tangential) by 10 (longitudinal) mm, were cut from sapwood portions of sugi wood (Cryptomeria japonica D. Don). Before treatment, all wood specimens were conditioned at 20 C and 65% relative humidity (RH) for 2 weeks. The specimens were free of knots and visible concentration of resins, and showed no visible evidence of infection by mold, stain, or wooddegrading fungi. As modifying agents, tetraethoxysilane (Si-I) and methyltriethoxysilane (Si-II) were used in silane state where acidified ethanol (1 mol) was added to the silanes and stirred at ambient temperature for 30 min as described by Donnath et al. (2004). As catalyst, 2.1 and 0.9 mg l1 HCl (37%) was added to tetraethoxysilane and methyltriethoxysilane compounds, respectively (Donnath et al., 2004). There were two different processes for preservative treatments: double treatments and single treatments. In double treatments, wood specimens were first treated with boric acid (BA) at 1% concentration. The specimens were then reconditioned at 20 C and 65% RH for one day and treated with the silane compounds in silane state. In single treatments, BA was mixed with the silane compounds in silane state yielding 1% BA concentration. In both treatments, treatment cycle consisted of a 40-min vacuum (88 KPa absolute pressure) in a treatment desiccator. After all treatments, the specimens were blotted dry and reweighed to determine the uptake chemical retention. All treated specimens were then reconditioned at 20 C and 65% RH for one day and then dried at 105 C for one day before leaching process. The leaching process was conducted according to Japanese Industrial Standard (JIS) K 1571 (JIS, 2004) the process involved immersing wood specimens in deionized water, stirring with a magnetic stirrer (400–450 rpm) at 27 C for 8 h followed by drying at 60 C for 16 h. This cycle was repeated 10 times. After each leaching cycle, the water was renewed with fresh deionized water to a ratio of 10 volumes of water to 1 volume of wood. The sample preparation for boron analyses was similar to the American Wood Preservers’ Association (AWPA) A2-98 standard method (AWPA, 1999). The specimens were ground to pass through a 40-mesh screen in the Wiley mill, oven-dried, and 1.5 g of ground wood was weighed to the nearest 0.001 g into a 250 ml flask. For each treatment group, two specimens were ground and analyzed. One hundred milliliters of deionized water was added to the flask containing the ground wood. The flask was placed in a water bath at 90–95 C for 60 min with agitation every 15 min. After cooling, the contents in the flask were filtered through Whatman #4 filter paper, rinsed 3 times with 20 ml of hot deionized water, and diluted to 200 ml in a volumetric flask. Leachates sampled from the leaching cycles for 10 days and extracts from the treated wood were analyzed with an inductively coupled plasma (ICP) spectrometry (ICP-S 1000iii Shimadzu Co Ltd, Japan). The percentage reduction of boron in the specimens was calculated based on the initial amount of boron in the specimens. Untreated and treated specimens were exposed to the subterranean termites, Coptotermes formosanus Shiraki, according to the JIS K 1571 standard method (JIS, 2004). An acrylic cylinder (80 mm in diameter, 60 mm in height) whose lower end was sealed with a 5 mm thick hard plaster (GC New Plastone, Dental Stone, GC Dental Industrial Corp., Tokyo, Japan) was used as a container. A test specimen was placed at the centre of the plaster bottom of the test container. A total of 150 worker termites collected from a laboratory colony of RISH, Kyoto University were introduced into each test container together with 15 termite soldiers. Five wood specimens per treatment were assayed against the termites. The assembled containers were set on damp cotton pads to supply water to the specimens and kept at 28 C and >85% RH in darkness for three weeks. The mass losses of the specimens due to termite attack were calculated based on the differences in the initial and final ovendry (60 C, 3 days) weights of the specimens after cleaning off the debris from the termite attack. No reference samples to account for leaching of chemicals were used in the tests, therefore leaching from the specimens affecting mass loss other than termite feeding was not accounted. Decay resistance test was conducted according to the JIS K 1571 standard method (JIS, 2004) using the brown-rot fungus, Fomitopsis (Tyromyces) palustris (Berkeley et Curtis) Murrill (FFPRI 0507) and the white-rot fungus, Trametes versicolor (L. Ex Fr.) Quel. (FFPRI 1030). After the oven-dried weights were determined, the specimens were sterilized with gaseous ethylene oxide. Three wood specimens per treatment were placed in a glass jar on the surface of 250 g quartz sand wetted with 80 ml nutrient solution and inoculated with liquid fungal cultures. Liquid fungal cultures were prepar inoculating 1000 ml liquid medium which contained 40 g glucose, 3 g peptone, 15 g malt extract and 1000 ml distilled water. The medium was shaken at 26 C for 10 days at 100 rpm. The nutrient solution used for wetting the quartz sand contained 40 g glucose, 3 g peptone, 15 g malt extract and 1000 ml distilled water for the white-rot fungus and 20 g glucose, 1.5 g peptone, 7.5 g malt extract, and 1000 ml distilled water for the brown-rot fungus. The test jars were then incubated at 27 C for 12 weeks. Nine replicates were tested for each decay fungus. The extent of the fungal attack was expressed as the percentage of mass loss.
3. Results and discussion Fig. 1 shows the amount of boron released (in ppm) from the specimens during a 10-day leaching course. In BA-only treated specimens, 212 ppm boron was released in total, and only 189 ppm boron was released on the first day of leaching. Both Si-I and Si-II treated specimens showed lesser boron release during the course as compared to BA-only treated specimens. Fig. 2 shows the percentage of boron release from the specimens at the end of the leaching course. Nearly all boron was leached out from BA-only treated wood specimens; however, about 40% of total boron remained in the specimens treated with either Si-I or Si-II compounds. It should be noted that original BA retention in the specimens (i.e., before leaching) was nearly 7 kg m3. Lesser boron leachability in Si-I and Si-II treated specimens has likely resulted from smaller pore size and spaces within the cell wall, leading to reduced adsorption of water. Similar results were obtained by Donnath et al. (2004), who treated wood specimens with various types of alkoxysilanes. They suggested that the chemical modification of wood requires the reaction of chemicals with cell wall components to achieve mechanical fixation of the compounds in wood. In our study, much of boron present in the wood lumina may have been released from the treated specimens, since there was likely no reaction between boron and silicon element precipitated in the lumina. Our assumptions were that some parts of boron remained in the cell wall and that the silicon emulsion remained in the wood after the leaching process. Fig. 3 shows a scheme for the sol–gel deposition of alkoxysilane on the wood surface, causing a covalent bond with the wood surface and resulting in hydrophobication (Tshabalala et al., 2003). According to Sebe et al. (2004), silicon-based monomers undergo selective bond cleavage and recombination to form macromolecular layers based on Si–O– Si and Si–O–C linkages. Covalent grafting of silicon polymers is considered another potential reaction of wood hydrophobication. Saka et al. (2001) have suggested that silane compounds react with the cell wall via the sol–gel process for the deposition of silicon in wood. They have also observed that the silicon compounds in the lumina of wood did not make any contribution to decreased water absorption (Saka et al., 1992). Sol–gel polycondensation networks of alkoxysilanes deposited within the wood cell wall enhanced water resistance in wood (Tshabalala et al., 2003). The resistance of organosilane to leaching by water relies on either bond formation with the cell wall polymers or entanglement of the silane polymer within the cell wall (Hill, 2006). The formation of a three-dimensional silicate network is not generally completed after curing of silicon compounds. Apparently, non-hydrolysed ethoxy groups and non-condensed hydroxyl (silanol) groups may still present within
Fig. 1. Amount of boron (ppm) released from the wood specimens during the leaching course.
S.N. Kartal et al. / International Biodeterioration & Biodegradation 63 (2009) 187–190
Fig. 2. Amount of boron released from the wood specimens and boric acid retention in the specimens after 10-day leaching course. Each figure is the average of 3 specimens. The same letters (for retention capital letters) in each column indicate that there is no statistical difference between the specimens according to Duncan’s Multiply Range Test (p 0.05).
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Fig. 4. Mass losses in the specimens exposed to the termites for 3 weeks. Filled bars show unleached specimens. Each figure is the average of 5 specimens. The same letters in each column indicate that there is no statistical difference between the specimens according to Duncan’s Multiply Range Test (p 0.05).
the silicon-network. These groups allow the sol–gel reaction to continue after curing. The sol–gel process also proceeds by hydrolysis of ethoxy groups due to moisture uptake (Brinker and Scherer, 1990; Donnath et al., 2004). Wood moisture content before silicon treatments might have effect on the reactions between silane and water as well as with wood; however, in the recent study, only wood specimens dried at 20 C and 65% RH for 2 weeks were used. Fig. 4 shows mass losses in the specimens after a 3-week exposure to termites. All unleached specimens showed perfect protection against termites; however, mass losses in control specimens and BA-only treated and leached specimens were nearly 30%. More interesting results were obtained in Si-I and Si-II treated specimens, where mass losses were considerably lower than that of BA-only and untreated control specimens. As mentioned earlier, about 40% of total boron remained in these specimens, which, along with the silicon compound, increased resistance. In our study, nearly 2.5 kg m3 BA retention in the specimens was achieved after leaching. For commercial use against termites, a retention in excess of 4.5 kg m3 boric acid equivalent (BAE) is recommended. However, in the UK, a minimal cross-sectional retention of 1.8 kg m3 BAE is recommended because termite attacks are
Fig. 3. Deposition of alkoxysilane on wood and hydrophobication – covalent bond formation in silanol groups of alkoxysilane, and hydrophobication on wood surface (Tshabalala et al., 2003).
Fig. 5. Mass losses in the specimens exposed to F. palustris for 12 weeks. Filled bars show unleached specimens. Each figure is the average of 9 specimens. The same letters in each column indicate that there is no statistical difference between the specimens according to Duncan’s Multiply Range Test (p 0.05).
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residues of boron compounds after the leaching process. Due to effect of wood moisture content on modification and curing reactions, usage of wood specimens conditioned at different temperatures and RHs might be useful to obtain better understanding of silicon-based modifications. Acknowledgments
Fig. 6. Mass losses in the specimens exposed to T. versicolor for 12 weeks. Filled bars show unleached specimens. Each figure is the average of 9 specimens. The same letters in each column indicate that there is no statistical difference between the specimens according to Duncan’s Multiply Range Test (p 0.05).
considerably less. In the United States, lumber receiving commercial treatment with borates is treated with DOT (disodium octoborate tetrahydrate), in that borate retention of 2.7 kg m3 BAE is required to control decay, beetles, and native termites, and 4.5 kg m3 BAE is required to control Formosan subterranean termites (Lloyd, 1997). Figs. 5 and 6 show the mass losses in specimens exposed to 12week fungal resistance tests followed by a 10-day leaching process. Mass losses in untreated control specimens due to fungi F. palustris and T. versicolor exceeded 30%. BA-only treated specimens also showed similar mass losses due to high boron release from the specimens during the leaching process. However, unleached specimens of this treatment group showed no mass losses. Mass losses in Si-I and Si-II treatments were less than 5%, suggesting that boron residues in the specimens after leaching helped protect the wood against the fungi tested. European standards usually require 0.76, 0.59, 0.32, and 0.30 kg m3 DOT retention levels for protection against T. versicolor, Gloeophyllum trabeum, Coniophora puteana, and Poria placenta, respectively (Lloyd, 1997). Na-borate retention levels of 0.8– 1.1 kg m3 are expected to protect wood against Neolentinus lepideus, C. puteana, and P. placenta, but higher Na-borate retention levels are needed against T. versicolor and G. trabeum (Abbot et al., 2000). According to Tsunoda (2001), the toxic threshold values for sugi sapwood specimens treated with BA were 0.8 kg m3, 0.9– 1.8 kg m3, and 0.9–1.8 kg m3 BAE against T. versicolor, F. palustris, and the subterranean termites, C. formosanus, respectively. A study by Akbulut et al. (2004) showed that 0.1% BAE borax treatment in the medium density fiberboard (MDF) specimens decreased the mass loss to less than 3% against F. palustris and T. versicolor. 4. Conclusions In this study, we evaluated the effects of two silane compounds, tetraethoxysilane and methyltriethoxysilane, on boron leaching and decay and termite resistance of wood under laboratory conditions. Silane treatments helped the wood specimens retain about 2.5 kg m3 BA after the leaching process. Several previous studies have shown that termite resistance usually requires a retention level of more than 1 kg m3 BAE in wood; however, a retention level of less than 1 kg m3 is needed to protect against fungal wood decay. In our study, the specimens treated with BA and silane compounds and subjected to leaching showed good resistance against fungi and subterranean termites as a result of the
This paper was partly presented at the 11th International Conference of Durability of Building Materials and Components (DBMC11), 11–14 May 2008, Istanbul, Turkey. The first author acknowledges the Turkish Academy of Sciences (TUBA) for the grant given under the program to reward successful young scientists (TUBA-GEBIP), and RISH and National University Corporation, Kyoto University, Japan. The present work was also supported by ¨ P-1758. the Research Fund of Istanbul University; Project No: YO References Abbot, W., Woodward, B., West, M., 2000. Efficacy of copper borax preservative against wood decay. In: Proceedings of 97th Annual Meeting of the American Wood-Preservers’ Association (AWPA). Minneapolis Marriott City Center Minneapolis, Minnesota, May 20–23, 2000. Volume 97. American Wood-Preservers’ Association, Granbury, Texas. Akbulut, T., Kartal, S.N., Green, F., 2004. Properties of medium density fiberboard (MDF) treated with N0 -N-(1,8-naphthalyl) hydroxylamine (NHA–Na), borax and boric acid. Forest Products Journal 54 (10), 59–64. American Wood Preservers’ Association (AWPA), 1999. Standard Methods for Analysis of Waterborne Preservatives and Fire-retardant Formulations. AWPA A2-98. AWPA, Birmingham, AL. Brinker, C.F., Scherer, G.W., 1990. Sol–Gel Science. Academic, San Diego. Donnath, S., Militz, H., Mai, C., 2004. Wood modification with alkoxysilanes. Wood Science and Technology 38, 555–566. Hill, C.A.S., Farahani, M.R.M., Hale, M.D.C., 2004. The use of alkoxysilane coupling agents for wood preservation. Holzforschung 58, 316–325. Hill, C., 2006. Wood Modification. Chemical, Thermal and Other Processes. Wiley Series in Renewable Resources. John Wiley & Sons, Ltd., USA, ISBN 0-47002172-1. JIS, 2004. JIS K 1571. Test Methods for Determining the Effectiveness of Wood Preservatives and their Performance Requirements. Japanese Standard Association (in Japanese). Kartal, S.N., Yoshimura, T., Imamura, Y., 2004. Improvement of boron leachability from disodium octaborate tetrahydrate (DOT)-treated wood by in situ copolymerization of allyl glycidyl ether (AGE) with methyl methacrylate (MMA). International Biodeterioration and Biodegradation 53 (2), 111–117. Lin, L.H., Chen, K.M., 2006. Surface activity and water repellency properties of cleavable-modified silicone surfactants. Colloids and Surfaces A: Physicochemical and Engineering Aspects 275, 99–106. Lloyd, J.D., 1997. International status of borate preservative systems. In: Proceedings of the Second International Conference on Wood Protection with Diffusible Preservatives and Pesticides. Proceedings No. 7284. Forest Products Society, Madison, WI, pp. 45–54. Mai, C., Donath, S., Militz, H., 2003. Modification of wood with silicon compounds. In: Proceedings of the First European Conference on Wood Modification. Gent, Belgium, pp. 239–251. Militz, H., Beckers, E.P.J., Homan, W.J., 1997. Modification of Solid Wood: Research and Practical Potential. IRG International Research Group on Wood Preservation, Stockholm. Document No. IRG/WP 40098. Norimoto, M., 2001. Chemical modification of wood. In: Dns, Hon, Shiraishi, N. (Eds.), Wood and Cellulosic Chemistry, second ed. Dekker, New York, pp. 573–598. Rowell, R.M., 1983. Chemical modification of wood. Forest Products Abstracts 6, 363–382. Saka, S., Sasaki, M., Tanahashi, M., 1992. Wood-inorganic composites prepared by sol–gel processing. 1. Wood-inorganic composites with porous structure. Mokuzai Gakkaishi 38, 1043–1049. Saka, S., Miyafuji, H., Tanno, F., 2001. Wood-inorganic composites prepared by the sol–gel process. Journal of Sol–Gel Science and Technology 20, 213–217. Sebe, G., Brook, M.A., 2001. Hydrophobization of wood surfaces: covalent grafting of silicone polymers. Wood Science and Technology 35 (3), 269–282. Sebe, G., Tingaut, P., Tchiama, R.S., Petraud, M., Grelier, S., De Jeso, B., 2004. Chemical reaction of maritime pine sapwood (Pinus pinaster Soland) with alkoxysilane molecules: a study of chemical pathways. Holzforschung 58, 511–518. Tshabalala, M.A., Kingshott, P., Vanlandingham, M.R., Plackett, D., 2003. Surface chemistry and moisture sorption properties of wood coated with multifunctional alkoxysilanes by sol–gel process. Journal of Applied Polymer Science 88, 2828–2841. Tsunoda, K., 2001. Preservative properties of vapor-boron-treated wood and woodbased composites. Journal of Wood Science 47, 149–153.
Contents lists available at ScienceDirect
International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod
Modification of wood with Si compounds to limit boron leaching from treated wood and to increase termite and decay resistance S. Nami Kartal a, *, Tsuyoshi Yoshimura b, Yuji Imamura b a b
Department of Forest Biology and Wood Protection Technology, Forestry Faculty, Istanbul University, Bahcekoy 34473, Istanbul, Turkey Research Institute for Sustainable Humanosphere (RISH), Kyoto University, Uji, 611-0011 Kyoto, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 June 2008 Received in revised form 28 August 2008 Accepted 29 August 2008 Available online 4 November 2008
In this study, we tested tetraethoxysilane and methyltriethoxysilane as modifying silicon-based compounds for their potential to limit boron leachability from modified wood and to increase biological durability of the wood against fungi and termites. Both the silane compounds were used in silane state where acidified ethanol was added and stirred at ambient temperature for 30 min. We used two different processes for preservative treatments: double treatment and single treatment. In double treatment, the specimens from sugi wood were first treated with boric acid at 1% concentration and subsequently treated with the silanes. In single treatment, boric acid was mixed with the silane compounds in the silane state yielding 1% boric acid concentration. Subsequent to the treatments, wood specimens were subjected to laboratory leaching tests, and leachates were analyzed for boron content with an inductively coupled plasma (ICP) spectrometry. ICP analyses showed that silane treatments were able to limit boron leaching from treated wood by about 40% in all cases for each silane compound. Wood specimens were then subjected to laboratory termite and decay resistance tests using the subterranean termites, Coptotermes formosanus, and the wood decaying fungi, Fomitopsis palustris and Trametes versicolor. Termite and fungal decay resistance tests revealed that resistance of modified wood with the silane and boron compounds increased when compared to untreated and boron-only treated wood specimens. More indepth studies on the mechanisms of interactions between the silicon compounds, boron elements and wood components are in progress. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Wood modification Boron Silicon compounds Silane Termite resistance Fungal resistance
1. Introduction Numerous trials have been conducted with several chemical compounds to bring modification in wood so as to improve its several characteristics (Rowell, 1983; Militz et al., 1997; Norimoto, 2001; Donnath et al., 2004). Many types of silicon compounds have been applied for this purpose. Silanes have long been used as modification agents in a number of applications, including in hydrophobication of ceramics, scratch-resistant surfaces, soil proofing and anti-graffiti coatings, and as adhesion promoters between organic and inorganic materials (Mai et al., 2003; Donnath et al., 2004; Hill et al., 2004). The silanol groups of silanes can react with hydroxyl groups of cell wall polymers forming a covalent bond between the silicon compound and cell wall polymers; however, Si–O–C bonds are weak and susceptible to hydrolysis. Alkylalkoxysilanes bear at least one alkyl group which remains after the sol– gel process, because in contrast to Si–O bonds the Si–C bonds are * Corresponding author. Tel.: þ90 212 226 1100/þ90 532 352 88 99 (mobile); fax: þ90 212 226 1113. E-mail address: [email protected] (S.N. Kartal). 0964-8305/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ibiod.2008.08.006
stable against hydrolysis (Brinker and Scherer, 1990; Sebe and Brook, 2001; Donnath et al., 2004). In addition to improved wood properties, such modification may have the potential to limit preservative release from treated wood and to increase biological resistance against wood-degrading fungi and insects. Kartal et al. (2004) found that acryl-silicon-type resin emulsion and boron-treated wood specimens resulted in about 54% boron release after a 10-day leaching process; however, nearly all boron was leached out from boron-only treated specimens. Similar results were obtained when wood specimens were treated with boron-containing quaternary ammonia compound (DBF) and the same emulsion in surface treatments (Kartal et al., 2004). A study by Lin and Chen (2006) showed that a series of cleavable water-soluble silicon surfactants prepared by the reaction of a hydroxyl-terminated polyester and an organopolysiloxane bear siloxane as the functional group of water repellency effect. Since several silicon compounds may have the ability to increase water repellency and reaction with cell wall components, such compounds can be used to limit boron release from boron-treated wood. In this study, we tested tetraethoxysilane and methyltriethoxysilane as modifying Si compounds for their potential to
188
S.N. Kartal et al. / International Biodeterioration & Biodegradation 63 (2009) 187–190
limit boron leachability from chemically modified wood and to increase biological durability of the wood against fungi and termites. 2. Materials and methods Wood specimens, 20 (radial) by 20 (tangential) by 10 (longitudinal) mm, were cut from sapwood portions of sugi wood (Cryptomeria japonica D. Don). Before treatment, all wood specimens were conditioned at 20 C and 65% relative humidity (RH) for 2 weeks. The specimens were free of knots and visible concentration of resins, and showed no visible evidence of infection by mold, stain, or wooddegrading fungi. As modifying agents, tetraethoxysilane (Si-I) and methyltriethoxysilane (Si-II) were used in silane state where acidified ethanol (1 mol) was added to the silanes and stirred at ambient temperature for 30 min as described by Donnath et al. (2004). As catalyst, 2.1 and 0.9 mg l1 HCl (37%) was added to tetraethoxysilane and methyltriethoxysilane compounds, respectively (Donnath et al., 2004). There were two different processes for preservative treatments: double treatments and single treatments. In double treatments, wood specimens were first treated with boric acid (BA) at 1% concentration. The specimens were then reconditioned at 20 C and 65% RH for one day and treated with the silane compounds in silane state. In single treatments, BA was mixed with the silane compounds in silane state yielding 1% BA concentration. In both treatments, treatment cycle consisted of a 40-min vacuum (88 KPa absolute pressure) in a treatment desiccator. After all treatments, the specimens were blotted dry and reweighed to determine the uptake chemical retention. All treated specimens were then reconditioned at 20 C and 65% RH for one day and then dried at 105 C for one day before leaching process. The leaching process was conducted according to Japanese Industrial Standard (JIS) K 1571 (JIS, 2004) the process involved immersing wood specimens in deionized water, stirring with a magnetic stirrer (400–450 rpm) at 27 C for 8 h followed by drying at 60 C for 16 h. This cycle was repeated 10 times. After each leaching cycle, the water was renewed with fresh deionized water to a ratio of 10 volumes of water to 1 volume of wood. The sample preparation for boron analyses was similar to the American Wood Preservers’ Association (AWPA) A2-98 standard method (AWPA, 1999). The specimens were ground to pass through a 40-mesh screen in the Wiley mill, oven-dried, and 1.5 g of ground wood was weighed to the nearest 0.001 g into a 250 ml flask. For each treatment group, two specimens were ground and analyzed. One hundred milliliters of deionized water was added to the flask containing the ground wood. The flask was placed in a water bath at 90–95 C for 60 min with agitation every 15 min. After cooling, the contents in the flask were filtered through Whatman #4 filter paper, rinsed 3 times with 20 ml of hot deionized water, and diluted to 200 ml in a volumetric flask. Leachates sampled from the leaching cycles for 10 days and extracts from the treated wood were analyzed with an inductively coupled plasma (ICP) spectrometry (ICP-S 1000iii Shimadzu Co Ltd, Japan). The percentage reduction of boron in the specimens was calculated based on the initial amount of boron in the specimens. Untreated and treated specimens were exposed to the subterranean termites, Coptotermes formosanus Shiraki, according to the JIS K 1571 standard method (JIS, 2004). An acrylic cylinder (80 mm in diameter, 60 mm in height) whose lower end was sealed with a 5 mm thick hard plaster (GC New Plastone, Dental Stone, GC Dental Industrial Corp., Tokyo, Japan) was used as a container. A test specimen was placed at the centre of the plaster bottom of the test container. A total of 150 worker termites collected from a laboratory colony of RISH, Kyoto University were introduced into each test container together with 15 termite soldiers. Five wood specimens per treatment were assayed against the termites. The assembled containers were set on damp cotton pads to supply water to the specimens and kept at 28 C and >85% RH in darkness for three weeks. The mass losses of the specimens due to termite attack were calculated based on the differences in the initial and final ovendry (60 C, 3 days) weights of the specimens after cleaning off the debris from the termite attack. No reference samples to account for leaching of chemicals were used in the tests, therefore leaching from the specimens affecting mass loss other than termite feeding was not accounted. Decay resistance test was conducted according to the JIS K 1571 standard method (JIS, 2004) using the brown-rot fungus, Fomitopsis (Tyromyces) palustris (Berkeley et Curtis) Murrill (FFPRI 0507) and the white-rot fungus, Trametes versicolor (L. Ex Fr.) Quel. (FFPRI 1030). After the oven-dried weights were determined, the specimens were sterilized with gaseous ethylene oxide. Three wood specimens per treatment were placed in a glass jar on the surface of 250 g quartz sand wetted with 80 ml nutrient solution and inoculated with liquid fungal cultures. Liquid fungal cultures were prepar inoculating 1000 ml liquid medium which contained 40 g glucose, 3 g peptone, 15 g malt extract and 1000 ml distilled water. The medium was shaken at 26 C for 10 days at 100 rpm. The nutrient solution used for wetting the quartz sand contained 40 g glucose, 3 g peptone, 15 g malt extract and 1000 ml distilled water for the white-rot fungus and 20 g glucose, 1.5 g peptone, 7.5 g malt extract, and 1000 ml distilled water for the brown-rot fungus. The test jars were then incubated at 27 C for 12 weeks. Nine replicates were tested for each decay fungus. The extent of the fungal attack was expressed as the percentage of mass loss.
3. Results and discussion Fig. 1 shows the amount of boron released (in ppm) from the specimens during a 10-day leaching course. In BA-only treated specimens, 212 ppm boron was released in total, and only 189 ppm boron was released on the first day of leaching. Both Si-I and Si-II treated specimens showed lesser boron release during the course as compared to BA-only treated specimens. Fig. 2 shows the percentage of boron release from the specimens at the end of the leaching course. Nearly all boron was leached out from BA-only treated wood specimens; however, about 40% of total boron remained in the specimens treated with either Si-I or Si-II compounds. It should be noted that original BA retention in the specimens (i.e., before leaching) was nearly 7 kg m3. Lesser boron leachability in Si-I and Si-II treated specimens has likely resulted from smaller pore size and spaces within the cell wall, leading to reduced adsorption of water. Similar results were obtained by Donnath et al. (2004), who treated wood specimens with various types of alkoxysilanes. They suggested that the chemical modification of wood requires the reaction of chemicals with cell wall components to achieve mechanical fixation of the compounds in wood. In our study, much of boron present in the wood lumina may have been released from the treated specimens, since there was likely no reaction between boron and silicon element precipitated in the lumina. Our assumptions were that some parts of boron remained in the cell wall and that the silicon emulsion remained in the wood after the leaching process. Fig. 3 shows a scheme for the sol–gel deposition of alkoxysilane on the wood surface, causing a covalent bond with the wood surface and resulting in hydrophobication (Tshabalala et al., 2003). According to Sebe et al. (2004), silicon-based monomers undergo selective bond cleavage and recombination to form macromolecular layers based on Si–O– Si and Si–O–C linkages. Covalent grafting of silicon polymers is considered another potential reaction of wood hydrophobication. Saka et al. (2001) have suggested that silane compounds react with the cell wall via the sol–gel process for the deposition of silicon in wood. They have also observed that the silicon compounds in the lumina of wood did not make any contribution to decreased water absorption (Saka et al., 1992). Sol–gel polycondensation networks of alkoxysilanes deposited within the wood cell wall enhanced water resistance in wood (Tshabalala et al., 2003). The resistance of organosilane to leaching by water relies on either bond formation with the cell wall polymers or entanglement of the silane polymer within the cell wall (Hill, 2006). The formation of a three-dimensional silicate network is not generally completed after curing of silicon compounds. Apparently, non-hydrolysed ethoxy groups and non-condensed hydroxyl (silanol) groups may still present within
Fig. 1. Amount of boron (ppm) released from the wood specimens during the leaching course.
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Fig. 2. Amount of boron released from the wood specimens and boric acid retention in the specimens after 10-day leaching course. Each figure is the average of 3 specimens. The same letters (for retention capital letters) in each column indicate that there is no statistical difference between the specimens according to Duncan’s Multiply Range Test (p 0.05).
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Fig. 4. Mass losses in the specimens exposed to the termites for 3 weeks. Filled bars show unleached specimens. Each figure is the average of 5 specimens. The same letters in each column indicate that there is no statistical difference between the specimens according to Duncan’s Multiply Range Test (p 0.05).
the silicon-network. These groups allow the sol–gel reaction to continue after curing. The sol–gel process also proceeds by hydrolysis of ethoxy groups due to moisture uptake (Brinker and Scherer, 1990; Donnath et al., 2004). Wood moisture content before silicon treatments might have effect on the reactions between silane and water as well as with wood; however, in the recent study, only wood specimens dried at 20 C and 65% RH for 2 weeks were used. Fig. 4 shows mass losses in the specimens after a 3-week exposure to termites. All unleached specimens showed perfect protection against termites; however, mass losses in control specimens and BA-only treated and leached specimens were nearly 30%. More interesting results were obtained in Si-I and Si-II treated specimens, where mass losses were considerably lower than that of BA-only and untreated control specimens. As mentioned earlier, about 40% of total boron remained in these specimens, which, along with the silicon compound, increased resistance. In our study, nearly 2.5 kg m3 BA retention in the specimens was achieved after leaching. For commercial use against termites, a retention in excess of 4.5 kg m3 boric acid equivalent (BAE) is recommended. However, in the UK, a minimal cross-sectional retention of 1.8 kg m3 BAE is recommended because termite attacks are
Fig. 3. Deposition of alkoxysilane on wood and hydrophobication – covalent bond formation in silanol groups of alkoxysilane, and hydrophobication on wood surface (Tshabalala et al., 2003).
Fig. 5. Mass losses in the specimens exposed to F. palustris for 12 weeks. Filled bars show unleached specimens. Each figure is the average of 9 specimens. The same letters in each column indicate that there is no statistical difference between the specimens according to Duncan’s Multiply Range Test (p 0.05).
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residues of boron compounds after the leaching process. Due to effect of wood moisture content on modification and curing reactions, usage of wood specimens conditioned at different temperatures and RHs might be useful to obtain better understanding of silicon-based modifications. Acknowledgments
Fig. 6. Mass losses in the specimens exposed to T. versicolor for 12 weeks. Filled bars show unleached specimens. Each figure is the average of 9 specimens. The same letters in each column indicate that there is no statistical difference between the specimens according to Duncan’s Multiply Range Test (p 0.05).
considerably less. In the United States, lumber receiving commercial treatment with borates is treated with DOT (disodium octoborate tetrahydrate), in that borate retention of 2.7 kg m3 BAE is required to control decay, beetles, and native termites, and 4.5 kg m3 BAE is required to control Formosan subterranean termites (Lloyd, 1997). Figs. 5 and 6 show the mass losses in specimens exposed to 12week fungal resistance tests followed by a 10-day leaching process. Mass losses in untreated control specimens due to fungi F. palustris and T. versicolor exceeded 30%. BA-only treated specimens also showed similar mass losses due to high boron release from the specimens during the leaching process. However, unleached specimens of this treatment group showed no mass losses. Mass losses in Si-I and Si-II treatments were less than 5%, suggesting that boron residues in the specimens after leaching helped protect the wood against the fungi tested. European standards usually require 0.76, 0.59, 0.32, and 0.30 kg m3 DOT retention levels for protection against T. versicolor, Gloeophyllum trabeum, Coniophora puteana, and Poria placenta, respectively (Lloyd, 1997). Na-borate retention levels of 0.8– 1.1 kg m3 are expected to protect wood against Neolentinus lepideus, C. puteana, and P. placenta, but higher Na-borate retention levels are needed against T. versicolor and G. trabeum (Abbot et al., 2000). According to Tsunoda (2001), the toxic threshold values for sugi sapwood specimens treated with BA were 0.8 kg m3, 0.9– 1.8 kg m3, and 0.9–1.8 kg m3 BAE against T. versicolor, F. palustris, and the subterranean termites, C. formosanus, respectively. A study by Akbulut et al. (2004) showed that 0.1% BAE borax treatment in the medium density fiberboard (MDF) specimens decreased the mass loss to less than 3% against F. palustris and T. versicolor. 4. Conclusions In this study, we evaluated the effects of two silane compounds, tetraethoxysilane and methyltriethoxysilane, on boron leaching and decay and termite resistance of wood under laboratory conditions. Silane treatments helped the wood specimens retain about 2.5 kg m3 BA after the leaching process. Several previous studies have shown that termite resistance usually requires a retention level of more than 1 kg m3 BAE in wood; however, a retention level of less than 1 kg m3 is needed to protect against fungal wood decay. In our study, the specimens treated with BA and silane compounds and subjected to leaching showed good resistance against fungi and subterranean termites as a result of the
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