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Bioresistance of poplar wood compressed by combined hydro-thermo-mechanical wood modification (CHTM): Soft rot and brown-rot
International Biodeterioration & Biodegradation 65 (2011) 866e870
Contents lists available at ScienceDirect
International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod
Bioresistance of poplar wood compressed by combined hydro-thermo-mechanical wood modification (CHTM): Soft rot and brown-rot Laya Khademi Bami, Behbood Mohebby* Dep. of Wood and Paper Sciences, Faculty of Natural Resources, Tarbiat Modares University, P.O. Box 46414-356, Noor, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 February 2011 Received in revised form 8 March 2011 Accepted 8 March 2011 Available online 23 July 2011
This work studied fungal bioresistance of combined hydro-thermo-mechanically modified (CHTM) poplar wood. The CHTM technique, introduced by Mohebby et al. (2009), is a combination of two wood modification techniquesehydrothermal wood modification and densification of wood. Blocks of poplar wood were initially treated hydrothermally at temperatures of 120, 150, and 180 C for holding times of 0, 30, and 90 min. Afterwards, the treated blocks were compressed by a hot press (160 and 180 C) for 20 min with a compression set of 60%. After the CHTM-treated blocks were dried, small specimens were cut for soft-rot and brown-rot decay tests according to ENV 807 and EN 113. Mass losses as well as metabolic moisture contents were determined in the decayed samples. Results revealed that the combination of wood modification techniques showed fungal suppression. It was also found that the hydrothermal treatment step could significantly reduce fungal attack in comparison with densification. Reduction of the mass losses was associated with the hydrothermal treatment temperature. Also, the level of metabolic moisture content was correlated with the mass losses for both fungi. Any reduction of the mass loss decreased the moisture content in the wood. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Bioresistance Combined hydro-thermo-mechanical wood modification (CHTM) Hydrothermal Densification
1. Introduction In comparison with other materials, wood as an engineering material has extraordinary properties that facilitate its wide use in various industries and for various purposes, such as furniture, construction, decoration, and composites. It has excellent working properties, which make it a preferred material: It is easy to work with, and has good machinability, a beautiful appearance, and good strength versus its light weight. However, due to some other less desirable characteristics, i.e., susceptibility to UV degradation, anisotropy, heterogeneous mechanical properties, hygroscopicity, and biodegradation, its applications are faced with restrictions. It is believed that all properties of wood are dependant on its structure and chemistry. Therefore, changing the basic chemistry as well as the structural nature of the wood, using new techniques of wood modification, might help to solve those problems. One of the new techniques is compression or densification of the wood, which is known as mechanical wood modification. Basically, in this technique, the wood is deformed under mechanical forces and its density is increased after compression (Kollman et al., 1975;
* Corresponding author. Tel.: þ98 9111255972; fax: þ98 1226253499. E-mail addresses: [email protected] (L. Khademi Bami), mohebbyb@ modares.ac.ir (B. Mohebby). 0964-8305/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ibiod.2011.03.011
Jennings, 2003). The resulting products have different names, such as compreg wood, lingostone, lignofol, and staypak (Kollman et al., 1975). In spite of its superior strength properties, when the compressed wood is exposed to wet conditions or soaked in water, it returns to its initial dimensions. This property is known as springback or set recovery (Dwainto et al., 1997; Ito et al., 1998a, b; Kultikova, 1999; Navi and Girardet, 2000; Heger et al., 2004; Kamke, 2006). The disadvantages of densification have motivated researchers to try to solve this problem, with different techniques having been proposed to reduce springback in densified wood (Dwainto et al., 1997; Ito et al., 1998a, b; Kultikova, 1999; Navi and Girardet, 2000; Heger et al., 2004; Kamke, 2006). Regarding successes in reducing springback, reports have indicated poor bioresistance of densified wood against microorganisms (Welzbacher et al., 2005). A new wood densification technique was introduced by Mohebby et al. (2009) in which a combination of two different wood modification techniques is proposed in order to achieve better results in terms of mechanical and physical properties. This technique is called combined hydro-thermo-mechanical wood modification (CHTM). The researchers believed that wood can achieve advantages of both techniques at the same time. As it is known, wood becomes hydrophobic due to the hydrothermal modification (Tjeerdsma and Militz, 2005; Mohebby and Sanaei, 2005; Rapp, 2001) and also achieves bioresistance (Hakkou et al., 2006). Furthermore,
L. Khademi Bami, B. Mohebby / International Biodeterioration & Biodegradation 65 (2011) 866e870
application of the forces increases the mechanical properties of the densified wood (Dwainto et al., 1997; Ito et al., 1998a, b; Kultikova, 1999; Navi and Girardet, 2000; Heger et al., 2004; Kamke, 2006). In the previous work, study of the bioresistance of the CHTMtreated wood was proposed by Mohebby et al. (2009). Therefore, the current research has aimed to study bioresistance of poplar wood treated by the CHTM process against soft-rot and brown-rot decay.
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Table 1 Summarized results of ANOVA for mass losses due to fungal decay in CHTMetreated poplar wood. Main effect
Interaction effect
Htb Tpc Th Ht Th Tp Ht Tp Th Ht Tp Mass loss due to Tha Brown rot decay 0.00d 0.817ns 0.245ns 0.790ns 0.438ns 0.808ns 0.897ns Soft rot decay 0.00d 0.249ns 0.928ns 0.368ns 0.872ns 0.037e 0.063ns ns
Insignificant. Th: Hydrothermal treatment temperature. Ht: Holding time. c Tp: Press temperature. d Significant at above 99% of confidence level. e Significant at 95% of confidence level. a
b
2. Materials and methods 2.1. Combined hydro-thermo-mechanical wood modification (CHTM) Wood blocks of 50 50 500 mm were cut from fresh sawn poplar wood (Populus deltoides Clone 79/51) and soaked in water for 24 h prior to treatment. Afterwards, the blocks were placed in a stainless steel cylinder containing preheated water (100 C) and treated at temperatures of 120, 150, and 180 C for holding times of 0, 30, and 90 min (Holding time refers to the time when the blocks remained at a constant temperature after achieving the target treatment temperature.) The hydrothermally treated wood blocks were immediately placed in a hot press and then compressed in the radial direction for 20 min with a compression set of 60% and pressure of 80 bar at press temperatures of 160 and 180 C. This process was introduced by Mohebby et al. (2009) as combined hydro-thermo-mechanical wood modification (CHTM). At least 10 blocks were treated for each treatment.
MC ¼
Ml ¼
Ww Wd 100 Wd
(1)
Wi Wd 100 Wi
(2)
where Ml is mass loss, as a percentage; MC: moisture content (%); Wi: dry weight before decay (in grams); Ww: wet weight after decay (grams); Wd: dry weight after decay (grams). Results of the mass losses due to soft-rot and the brown-rot decay were statistically analyzed based on a full factorial design and the means were grouped according to Duncan’s multiple range test (DMRT). 3. Results
2.2. Soft-rot trial
3.1. Brown rot
Mini-stakes with dimensions of 10 10 100 mm were cut from the treated blocks, and oven-dried at 103 2 C for 24 h to determine dry weights. The mini-stakes were exposed to John Innes II soil according to ENV 807 and incubated for 150 days at a relative humidity of 65 5% and a temperature of 25 C. At the end of the incubation period, the samples were weighed and ovendried to determine the moisture contents as well as the dry mass losses due to soft-rot decay according to Eqs. 1 and 2. At least 10 mini-stakes were used for each treatment (Mohebby and Militz, 2002).
Summarized results of the analysis of variance (ANOVA) are shown for the main and interaction effects of the applied factors in Table 1. According to the results, only the hydrothermal treatment temperature significantly affected the brown-rot decay in the CHTM-treated poplar wood. As is indicated in Table 2, mass losses due to brownrot decay are grouped into three different subsets. The low mass losses were determined for the treated samples at 150 and 180 C. The weight loss due to the G. trabeum fungus is shown in Fig. 1. Results revealed that the CHTM treatment reduced the mass loss of the wood. Any increase of the hydrothermal treatment temperature was more effective in prevention of brown-rot decay than that of the holding time and the press temperature. However, extending the holding time caused more reduction of the mass loss than the press temperature. According to the results, the first step of the CHTM treatment, the hydrothermal modification, had a major inhibitory effect on the brown rot in comparison with the second step, the mechanical modification. The inhibitory effect of the hydrothermal treatment of the poplar wood was remarkable at a temperature of 180 C. The mass loss of the treated samples at 120 C was significantly higher than that of the untreated wood.
2.3. Brown-rot trial Small specimens with dimensions of 5 5 20 mm were cut from the treated blocks and then oven-dried for 24 h at 103 2 C to determine their dry weights prior to the fungal test. The miniblocks were exposed to the brown-rot fungus Gloeophyllum trabeum, in an agar, glucose, malt-extract (ME) medium and incubated for 120 days in a relative humidity of 65 5% at an ambient temperature of 25 C (Dorado et al., 2000; Mohebby, 2003). After incubation, the mini-blocks were weighed, oven-dried, and then reweighed to determine the moisture contents in the decayed specimens as well as the dry mass loss according to Eqs. 1 and 2. This test was adapted to EN 113 with the exception of the sample sizes. At least 10 mini-blocks were used for each treatment.
3.2. Soft rot According to the results of the ANOVA, only the hydrothermal treatment temperature, as the main effect, as well as the interaction
Table 2 Summarized results for the Duncan multiple range test for means of mass losses due to fungal decay in CHTMetreated poplar wood. Holding time (min)
Hydrothermal treatment temperature ( C) Subsetsa Mass loss due to a
Brown rot decay Soft rot decay
1 180, 150 180
2 Untreated 150
3 120 Untreated, 120
1 90, 3, 0 90, 30, 0
Means of mass losses are grouped from the lowest to the highest amounts in different subsets for each treatment factor.
Press temperature ( C) 2 e e
3 e e
1 180, 160 160, 180
2 e e
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L. Khademi Bami, B. Mohebby / International Biodeterioration & Biodegradation 65 (2011) 866e870
Fig. 1. Mass loss caused by brown-rot fungus (Gloeophyllum trabeum) in CHTM-treated poplar wood after 120 days.
between the holding time and the press temperature, significantly affected the soft-rot decay in the CHTM-treated poplar wood. Regarding the DMRT results, the low the mass loss due to the softrot decay was determined in the treated wood at 180 C and then for 150 C (Table 2). Any mass losses due to the soft-rot fungal attack are indicated in Fig. 2. According to the results, the CHTM treatment could reduce the soft-rot attack in the compressed poplar wood. As was noticed for the brown-rot decay, raising the hydrothermal treatment temperature decreased the mass losses due to soft-rot decay, except for the treatment temperature of 120 C, which was also the greatest one. However, treatment of the poplar wood at 180 C showed a greater inhibitory effect against soft-rot decay. As was the case for brown-rot decay, the hydrothermal treatment step was more effective against the soft-rot attack than was the mechanical treatment step. Extending the holding time as well as increasing the press temperature had no significant effects on the soft-rot attack. 3.3. Moisture content due to fungal activity Results indicated that the metabolic moisture contents of the decayed wood blocks were correlated with the mass losses for both
decay fungi in the CHTM-treated poplar wood (Figs. 3 and 4). Any increase of the mass losses could increase the moisture content in the wood blocks. It was also revealed that the CHTM-treated wood samples had lower moisture contents than that of the untreated one, with the exception of the samples treated at 120 C, which showed higher mass losses and the moisture contents. 4. Discussion According to the results, increasing the hydrothermal treatment temperature had inhibitory effects on the brown- and soft-rot decay fungi, with the exception of the treatment temperature of 120 C. The main reason, it should be expressed that the chemical alteration of the cell wall polymers due to the hydrothermal treatment is responsible for prevention of the fungal attacks. Another reason is that reduction in the number of the wood and the cell wall porosities prohibits penetration of the fungal hyphae and their degrading enzymes into the cell cavities or cell wall micropores. Most reports have indicated that the hydrothermal treatment of wood alters its chemistry, making it hydrophobic, and changes it to an unknown substrate for fungal enzymes, which makes attack by
Fig. 2. Mass loss caused by soft-rot fungus in CHTM treated poplar wood after 150 days of exposing to soil beds.
L. Khademi Bami, B. Mohebby / International Biodeterioration & Biodegradation 65 (2011) 866e870
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Crystallization of the cellulose (Yildiz and Gümüs¸kaya, 2007; Gonzales et al., 2005) could also be another reason to prohibit fungal attacks, especially of brown-rot fungi. These fungi prefer to attack the amorphous regions in the cellulose (Green and Highley, 1997). It could also be expressed that cross-linkings and condensation reactions in the lignin as well as the appearance of new chemical bonds in its structure during the hydrothermal treatment would cause changes in chemistry of the lignin (Tjeerdsma and Militz, 2005) to unknown or indistinguishable substrate for preoxidases as well as lignin peroxides. It is also likely that the cleaved mono-, di-, and oligosaccharides, and still nontoxic transformed compounds in the wood treated at 120 C encourage the fungal attack and result in greater mass losses. Fig. 3. Correlation between the moisture content and weight loss due to the brown-rot fungus Gloeophyllum trabeum.
fungal enzymes difficult. Raising the hydrothermal treatment temperature causes a series of chemical changes in the cell wall polymers (Garrote et al., 1999, 2001). Hemicelluloses are degraded at lower temperatures and transformed into polyoses, acetic acid, formic acid, furfural, etc. As the temperature rises, the occurrence of those compounds increases (Garrote et al., 1999, 2001; Tjeerdsma and Militz, 2005; Boonstra and Tjeerdsma, 2006). Since most of the transformed compounds, such as furfural, are toxic for fungi, those compounds prevent the microorganisms from being active in the wood (Kamdem et al., 1999), or they can even act as biocidal materials. As has been known, brown-rot fungal enzymes have larger sizes to penetrate into the cell wall micropores. Therefore, at initial stages of the attack, they cannot enter into the cell wall to degrade the substrate polymers (Koenig, 1974; Wilcox, 1993). To solve this problem, using Fenton’s reaction and sending small-sized oxygen peroxides as well as producing calcium oxalate crystals (Green and Highley, 1997) are the best solutions for initial dissolution of the polysaccharides to facilitate lignin degradation (Zabel and Morrell, 1992). The presence of oxygen is obligatory for Fenton’s reaction. So any oxygen limitation also prevents brown-rot decay (Kazemi et al., 1998, 1999). Decrease of the cell lumen as well as reduction in the cell wall micropores due to the compression process should have a preventive effect on the fungal enzymes’ activities. Therefore, the less the oxygen is expected in the compressed wood cell walls.
Fig. 4. Correlation between the moisture content and weight loss due to soft-rot fungal attack.
5. Conclusion This research showed that the combination of the hydrothermal wood modification technique with the mechanical compression process, introduced as CHTM by Mohebby et al. (2009), increases fungal bioresistance of the compressed wood. The optimal hydrothermic temperature to achieve better mechanical strength was reported to be 150 C in the previous paper; here, this temperature did indeed show an increased inhibition of biodeterioration. Therefore, it can be believed that this temperature provides optimized conditions for the CHTM process to achieve better wood properties in all aspects. The press temperature as well as the holding time has less effect on the bioresistance of the compressed wood. References Boonstra, M.J., Tjeerdsma, B., 2006. Chemical analysis of heat treated softwoods. Holz als Roh- und Werkstoff 64 (3), 204e211. Dorado, J., Claassen, F.W., Lenon, G., van Beek, T.A., Wijnberg, J.B.P.A., SierraAlvarez, R., 2000. Degradation and detoxification of softwood extractives by sap stain fungi. Bioresource Technology 71, 14e20. Dwainto, W., Inoue, M., Norimoto, M., 1997. Fixation of deformation of wood by heat treatment. Makuzai Gakkaishi 43 (4), 303e309. EN 113, 1980. Wood Preservatives-determination of the Toxic Values against Wood Destroying Basidiomycetes Cultured on Agar Medium. ENV 807, 1993. Wood Preservatives-determination of the Toxic Effectiveness against Soft Rotting Micro-fungi and Other Soil Inhabiting Micro Organisms. Garrote, G., DomõÂnguez, H., Parajo, J.C., 1999. Hydrothermal processing of lignocellulosic materials. Holz als Roh- und Werkstoff 57 (3), 191e202. Garrote, G., DomõÂnguez, H., Parajo, J.C., 2001. Study on the deacetylation of hemicelluloses during the hydrothermal processing of Eucalyptus wood. Holz als Roh- und Werkstoff 59 (1e2), 53e59. Gonzales, M., Breese, M.C., Hale, M.D.C., 2005. Studies on the relaxation of heat treated wood. The Second European Conference on Wood Modification. 6e7 Oct., Göttingen, Germany, 87e90. Green III, F., Highley, T.L., 1997. Mechanism of brown rot decay: paradigm or paradox. International Biodeterioration & Biodegradation 39 (2), 113e124. Hakkou, M., Petrissans, M., Ceradine, P., Zoulalian, A., 2006. Investigation of the reasons for fungal durability of heat treated beech wood. Polymer Degradation and Stability 91 (2), 393e397. Heger, F., Giroux, M., Welzbacher, C., Rapp, A.O., Navi, P., 2004. Mechanical and durability performance of THM-densified wood. Final Workshop Cost Action, Environmental Optimization of Wood Protection, Lisbon, Portugal, 22nde23rd March 2004, 1e10. Ito, Y., Tanahashi, M., Shigematsu, M., Shinoda, Y., Ohta, C., 1998a. Compressivemolding of wood by highepressure steametreatment: I. Development of compressive molded squares from thinnings. Holzforschung 52 (2), 211e216. Ito, Y., Tanahashi, M., Shigematsu, M., Shinoda, Y., 1998b. Compressive-molding of wood by highepressure steametreatment: II. Mechanism of permanent fixation. Holzforschung 52 (2), 217e221. Jennings, J.D., 2003. Investigation the surface energy and bond performance of compression densified wood. M.Sc. Thesis, Virginia Polymeric Institute and State University, p.147. Kamdem, D.P., Pizzi, A., Guyonnet, R., Jrmannaud, A., 1999. Durability of Heattreated Wood, Doc. No. IRG/WP/99e40145. The International Research Group on Wood Preservation, Stockholm. Kamke, F.A., 2006. Densified Radiata pine for structural composites. Maderas, Ciencia Y Tecnologia 8 (2), 83e92.
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Kazemi, S.M., Dickinson, D.J., Murphy, R.J., 1998. The Influence of Gaseous Oxygen Concentration on Fungal Growth Rates. Biomass Production and Wood Decay, IRG/ WP 98e10283. The International Research Group on Wood Preservation, Stockholm. Kazemi, S.M., Dickinson, D.J., Murphy, R.J., 1999. The Effect of Initial Moisture Content on Wood Decay at Different Levels of Gaseous Oxygen Concentration, IRG/WP 99e10316. The International Research Group on Wood Preservation, Stockholm. Koenig, J.W., 1974. Hydrogen peroxide and iron: a proposed system for decomposition of wood by brown rot basidiomycetes. Wood and Fiber 6 (1), 66e80. Kollman, F.P., Kuenzi, E.W., Stamm, A.J., 1975. Principle of Wood Science and Technology. In: Wood Based Materials, first ed., Vol. II. Springer-Verlag, New York, Heidelberg, Berlin, p. 703. Kultikova, E.V., 1999. Structure and properties relationships of densified wood. M.Sc. Thesis, Virginia Polytechnic Institute and State University, p.139. Mohebby, B., Militz, H., 2002. Soft Rot Decay in Acetylated Wood. IRG/WP 02e40231. The International Research Group on Wood Preservation, Stockholm. Mohebby, B., Sanaei, I., 2005. Influences of the Hydro-thermal Treatment on Physical Properties of Beech Wood (Fagus orientalis). IRG/WP 05e40303. The International Research Group on Wood Preservation, Stockholm. Mohebby, B., Sharifnia-Dizboni, H., Kazemi-Najafi, S., 2009. Combined hydrothermo-mechanical modification (CHTM) as an Innovation in Mechanical Wood Modification. Stockholm, Sweden, 353e362, April 27the29th.
Mohebby, B, 2003. Biological attack of acetylated wood. Ph.D. Thesis, Göttingen University, Germany, p. 147. Navi, P., Girardet, F., 2000. Effects of thermo-hydro-mechanical treatment on the structure and properties of wood. Holzforschung 54 (3), 287e293. Rapp, A., 2001. Review on heat treatment of wood. In: proceedings of COST E22 special seminar, February 9th, Antibes, France. Tjeerdsma, B.F., Militz, H., 2005. Chemical changes in hydrothermal treated wood. FTIR analysis of combined hydro thermal and dry heat-treated wood. Holz als Roh- und Werkstoff 63 (2), 102e111. Welzbacher, C.R., Rapp, A.O., Haller, P., Wehsener, J., 2005. Biological and mechanical properties of densified and thermally modified Norway spruce. The Second European Conference on Wood Modification, October 6e7th, Göttingen, Germany. Wilcox, W.W., 1993. Comparison of SEM and LM for the diagnosis of early stage of brown rot wood decay. International Association of Wood Anatomists Journal 14, 219e226. Yildiz, S., Gümüs¸kaya, E., 2007. The effect of thermal modification on crystalline structure of cellulose in soft and hardwood. Building and Environment 42 (4), 62e67. Zabel, R.A., Morrell, J.J., 1992. Wood Microbiology: Decay and Its Prevention. Academic Press, San Diego, Calif, p. 47.
Contents lists available at ScienceDirect
International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod
Bioresistance of poplar wood compressed by combined hydro-thermo-mechanical wood modification (CHTM): Soft rot and brown-rot Laya Khademi Bami, Behbood Mohebby* Dep. of Wood and Paper Sciences, Faculty of Natural Resources, Tarbiat Modares University, P.O. Box 46414-356, Noor, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 February 2011 Received in revised form 8 March 2011 Accepted 8 March 2011 Available online 23 July 2011
This work studied fungal bioresistance of combined hydro-thermo-mechanically modified (CHTM) poplar wood. The CHTM technique, introduced by Mohebby et al. (2009), is a combination of two wood modification techniquesehydrothermal wood modification and densification of wood. Blocks of poplar wood were initially treated hydrothermally at temperatures of 120, 150, and 180 C for holding times of 0, 30, and 90 min. Afterwards, the treated blocks were compressed by a hot press (160 and 180 C) for 20 min with a compression set of 60%. After the CHTM-treated blocks were dried, small specimens were cut for soft-rot and brown-rot decay tests according to ENV 807 and EN 113. Mass losses as well as metabolic moisture contents were determined in the decayed samples. Results revealed that the combination of wood modification techniques showed fungal suppression. It was also found that the hydrothermal treatment step could significantly reduce fungal attack in comparison with densification. Reduction of the mass losses was associated with the hydrothermal treatment temperature. Also, the level of metabolic moisture content was correlated with the mass losses for both fungi. Any reduction of the mass loss decreased the moisture content in the wood. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Bioresistance Combined hydro-thermo-mechanical wood modification (CHTM) Hydrothermal Densification
1. Introduction In comparison with other materials, wood as an engineering material has extraordinary properties that facilitate its wide use in various industries and for various purposes, such as furniture, construction, decoration, and composites. It has excellent working properties, which make it a preferred material: It is easy to work with, and has good machinability, a beautiful appearance, and good strength versus its light weight. However, due to some other less desirable characteristics, i.e., susceptibility to UV degradation, anisotropy, heterogeneous mechanical properties, hygroscopicity, and biodegradation, its applications are faced with restrictions. It is believed that all properties of wood are dependant on its structure and chemistry. Therefore, changing the basic chemistry as well as the structural nature of the wood, using new techniques of wood modification, might help to solve those problems. One of the new techniques is compression or densification of the wood, which is known as mechanical wood modification. Basically, in this technique, the wood is deformed under mechanical forces and its density is increased after compression (Kollman et al., 1975;
* Corresponding author. Tel.: þ98 9111255972; fax: þ98 1226253499. E-mail addresses: [email protected] (L. Khademi Bami), mohebbyb@ modares.ac.ir (B. Mohebby). 0964-8305/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ibiod.2011.03.011
Jennings, 2003). The resulting products have different names, such as compreg wood, lingostone, lignofol, and staypak (Kollman et al., 1975). In spite of its superior strength properties, when the compressed wood is exposed to wet conditions or soaked in water, it returns to its initial dimensions. This property is known as springback or set recovery (Dwainto et al., 1997; Ito et al., 1998a, b; Kultikova, 1999; Navi and Girardet, 2000; Heger et al., 2004; Kamke, 2006). The disadvantages of densification have motivated researchers to try to solve this problem, with different techniques having been proposed to reduce springback in densified wood (Dwainto et al., 1997; Ito et al., 1998a, b; Kultikova, 1999; Navi and Girardet, 2000; Heger et al., 2004; Kamke, 2006). Regarding successes in reducing springback, reports have indicated poor bioresistance of densified wood against microorganisms (Welzbacher et al., 2005). A new wood densification technique was introduced by Mohebby et al. (2009) in which a combination of two different wood modification techniques is proposed in order to achieve better results in terms of mechanical and physical properties. This technique is called combined hydro-thermo-mechanical wood modification (CHTM). The researchers believed that wood can achieve advantages of both techniques at the same time. As it is known, wood becomes hydrophobic due to the hydrothermal modification (Tjeerdsma and Militz, 2005; Mohebby and Sanaei, 2005; Rapp, 2001) and also achieves bioresistance (Hakkou et al., 2006). Furthermore,
L. Khademi Bami, B. Mohebby / International Biodeterioration & Biodegradation 65 (2011) 866e870
application of the forces increases the mechanical properties of the densified wood (Dwainto et al., 1997; Ito et al., 1998a, b; Kultikova, 1999; Navi and Girardet, 2000; Heger et al., 2004; Kamke, 2006). In the previous work, study of the bioresistance of the CHTMtreated wood was proposed by Mohebby et al. (2009). Therefore, the current research has aimed to study bioresistance of poplar wood treated by the CHTM process against soft-rot and brown-rot decay.
867
Table 1 Summarized results of ANOVA for mass losses due to fungal decay in CHTMetreated poplar wood. Main effect
Interaction effect
Htb Tpc Th Ht Th Tp Ht Tp Th Ht Tp Mass loss due to Tha Brown rot decay 0.00d 0.817ns 0.245ns 0.790ns 0.438ns 0.808ns 0.897ns Soft rot decay 0.00d 0.249ns 0.928ns 0.368ns 0.872ns 0.037e 0.063ns ns
Insignificant. Th: Hydrothermal treatment temperature. Ht: Holding time. c Tp: Press temperature. d Significant at above 99% of confidence level. e Significant at 95% of confidence level. a
b
2. Materials and methods 2.1. Combined hydro-thermo-mechanical wood modification (CHTM) Wood blocks of 50 50 500 mm were cut from fresh sawn poplar wood (Populus deltoides Clone 79/51) and soaked in water for 24 h prior to treatment. Afterwards, the blocks were placed in a stainless steel cylinder containing preheated water (100 C) and treated at temperatures of 120, 150, and 180 C for holding times of 0, 30, and 90 min (Holding time refers to the time when the blocks remained at a constant temperature after achieving the target treatment temperature.) The hydrothermally treated wood blocks were immediately placed in a hot press and then compressed in the radial direction for 20 min with a compression set of 60% and pressure of 80 bar at press temperatures of 160 and 180 C. This process was introduced by Mohebby et al. (2009) as combined hydro-thermo-mechanical wood modification (CHTM). At least 10 blocks were treated for each treatment.
MC ¼
Ml ¼
Ww Wd 100 Wd
(1)
Wi Wd 100 Wi
(2)
where Ml is mass loss, as a percentage; MC: moisture content (%); Wi: dry weight before decay (in grams); Ww: wet weight after decay (grams); Wd: dry weight after decay (grams). Results of the mass losses due to soft-rot and the brown-rot decay were statistically analyzed based on a full factorial design and the means were grouped according to Duncan’s multiple range test (DMRT). 3. Results
2.2. Soft-rot trial
3.1. Brown rot
Mini-stakes with dimensions of 10 10 100 mm were cut from the treated blocks, and oven-dried at 103 2 C for 24 h to determine dry weights. The mini-stakes were exposed to John Innes II soil according to ENV 807 and incubated for 150 days at a relative humidity of 65 5% and a temperature of 25 C. At the end of the incubation period, the samples were weighed and ovendried to determine the moisture contents as well as the dry mass losses due to soft-rot decay according to Eqs. 1 and 2. At least 10 mini-stakes were used for each treatment (Mohebby and Militz, 2002).
Summarized results of the analysis of variance (ANOVA) are shown for the main and interaction effects of the applied factors in Table 1. According to the results, only the hydrothermal treatment temperature significantly affected the brown-rot decay in the CHTM-treated poplar wood. As is indicated in Table 2, mass losses due to brownrot decay are grouped into three different subsets. The low mass losses were determined for the treated samples at 150 and 180 C. The weight loss due to the G. trabeum fungus is shown in Fig. 1. Results revealed that the CHTM treatment reduced the mass loss of the wood. Any increase of the hydrothermal treatment temperature was more effective in prevention of brown-rot decay than that of the holding time and the press temperature. However, extending the holding time caused more reduction of the mass loss than the press temperature. According to the results, the first step of the CHTM treatment, the hydrothermal modification, had a major inhibitory effect on the brown rot in comparison with the second step, the mechanical modification. The inhibitory effect of the hydrothermal treatment of the poplar wood was remarkable at a temperature of 180 C. The mass loss of the treated samples at 120 C was significantly higher than that of the untreated wood.
2.3. Brown-rot trial Small specimens with dimensions of 5 5 20 mm were cut from the treated blocks and then oven-dried for 24 h at 103 2 C to determine their dry weights prior to the fungal test. The miniblocks were exposed to the brown-rot fungus Gloeophyllum trabeum, in an agar, glucose, malt-extract (ME) medium and incubated for 120 days in a relative humidity of 65 5% at an ambient temperature of 25 C (Dorado et al., 2000; Mohebby, 2003). After incubation, the mini-blocks were weighed, oven-dried, and then reweighed to determine the moisture contents in the decayed specimens as well as the dry mass loss according to Eqs. 1 and 2. This test was adapted to EN 113 with the exception of the sample sizes. At least 10 mini-blocks were used for each treatment.
3.2. Soft rot According to the results of the ANOVA, only the hydrothermal treatment temperature, as the main effect, as well as the interaction
Table 2 Summarized results for the Duncan multiple range test for means of mass losses due to fungal decay in CHTMetreated poplar wood. Holding time (min)
Hydrothermal treatment temperature ( C) Subsetsa Mass loss due to a
Brown rot decay Soft rot decay
1 180, 150 180
2 Untreated 150
3 120 Untreated, 120
1 90, 3, 0 90, 30, 0
Means of mass losses are grouped from the lowest to the highest amounts in different subsets for each treatment factor.
Press temperature ( C) 2 e e
3 e e
1 180, 160 160, 180
2 e e
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Fig. 1. Mass loss caused by brown-rot fungus (Gloeophyllum trabeum) in CHTM-treated poplar wood after 120 days.
between the holding time and the press temperature, significantly affected the soft-rot decay in the CHTM-treated poplar wood. Regarding the DMRT results, the low the mass loss due to the softrot decay was determined in the treated wood at 180 C and then for 150 C (Table 2). Any mass losses due to the soft-rot fungal attack are indicated in Fig. 2. According to the results, the CHTM treatment could reduce the soft-rot attack in the compressed poplar wood. As was noticed for the brown-rot decay, raising the hydrothermal treatment temperature decreased the mass losses due to soft-rot decay, except for the treatment temperature of 120 C, which was also the greatest one. However, treatment of the poplar wood at 180 C showed a greater inhibitory effect against soft-rot decay. As was the case for brown-rot decay, the hydrothermal treatment step was more effective against the soft-rot attack than was the mechanical treatment step. Extending the holding time as well as increasing the press temperature had no significant effects on the soft-rot attack. 3.3. Moisture content due to fungal activity Results indicated that the metabolic moisture contents of the decayed wood blocks were correlated with the mass losses for both
decay fungi in the CHTM-treated poplar wood (Figs. 3 and 4). Any increase of the mass losses could increase the moisture content in the wood blocks. It was also revealed that the CHTM-treated wood samples had lower moisture contents than that of the untreated one, with the exception of the samples treated at 120 C, which showed higher mass losses and the moisture contents. 4. Discussion According to the results, increasing the hydrothermal treatment temperature had inhibitory effects on the brown- and soft-rot decay fungi, with the exception of the treatment temperature of 120 C. The main reason, it should be expressed that the chemical alteration of the cell wall polymers due to the hydrothermal treatment is responsible for prevention of the fungal attacks. Another reason is that reduction in the number of the wood and the cell wall porosities prohibits penetration of the fungal hyphae and their degrading enzymes into the cell cavities or cell wall micropores. Most reports have indicated that the hydrothermal treatment of wood alters its chemistry, making it hydrophobic, and changes it to an unknown substrate for fungal enzymes, which makes attack by
Fig. 2. Mass loss caused by soft-rot fungus in CHTM treated poplar wood after 150 days of exposing to soil beds.
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Crystallization of the cellulose (Yildiz and Gümüs¸kaya, 2007; Gonzales et al., 2005) could also be another reason to prohibit fungal attacks, especially of brown-rot fungi. These fungi prefer to attack the amorphous regions in the cellulose (Green and Highley, 1997). It could also be expressed that cross-linkings and condensation reactions in the lignin as well as the appearance of new chemical bonds in its structure during the hydrothermal treatment would cause changes in chemistry of the lignin (Tjeerdsma and Militz, 2005) to unknown or indistinguishable substrate for preoxidases as well as lignin peroxides. It is also likely that the cleaved mono-, di-, and oligosaccharides, and still nontoxic transformed compounds in the wood treated at 120 C encourage the fungal attack and result in greater mass losses. Fig. 3. Correlation between the moisture content and weight loss due to the brown-rot fungus Gloeophyllum trabeum.
fungal enzymes difficult. Raising the hydrothermal treatment temperature causes a series of chemical changes in the cell wall polymers (Garrote et al., 1999, 2001). Hemicelluloses are degraded at lower temperatures and transformed into polyoses, acetic acid, formic acid, furfural, etc. As the temperature rises, the occurrence of those compounds increases (Garrote et al., 1999, 2001; Tjeerdsma and Militz, 2005; Boonstra and Tjeerdsma, 2006). Since most of the transformed compounds, such as furfural, are toxic for fungi, those compounds prevent the microorganisms from being active in the wood (Kamdem et al., 1999), or they can even act as biocidal materials. As has been known, brown-rot fungal enzymes have larger sizes to penetrate into the cell wall micropores. Therefore, at initial stages of the attack, they cannot enter into the cell wall to degrade the substrate polymers (Koenig, 1974; Wilcox, 1993). To solve this problem, using Fenton’s reaction and sending small-sized oxygen peroxides as well as producing calcium oxalate crystals (Green and Highley, 1997) are the best solutions for initial dissolution of the polysaccharides to facilitate lignin degradation (Zabel and Morrell, 1992). The presence of oxygen is obligatory for Fenton’s reaction. So any oxygen limitation also prevents brown-rot decay (Kazemi et al., 1998, 1999). Decrease of the cell lumen as well as reduction in the cell wall micropores due to the compression process should have a preventive effect on the fungal enzymes’ activities. Therefore, the less the oxygen is expected in the compressed wood cell walls.
Fig. 4. Correlation between the moisture content and weight loss due to soft-rot fungal attack.
5. Conclusion This research showed that the combination of the hydrothermal wood modification technique with the mechanical compression process, introduced as CHTM by Mohebby et al. (2009), increases fungal bioresistance of the compressed wood. The optimal hydrothermic temperature to achieve better mechanical strength was reported to be 150 C in the previous paper; here, this temperature did indeed show an increased inhibition of biodeterioration. Therefore, it can be believed that this temperature provides optimized conditions for the CHTM process to achieve better wood properties in all aspects. The press temperature as well as the holding time has less effect on the bioresistance of the compressed wood. References Boonstra, M.J., Tjeerdsma, B., 2006. Chemical analysis of heat treated softwoods. Holz als Roh- und Werkstoff 64 (3), 204e211. Dorado, J., Claassen, F.W., Lenon, G., van Beek, T.A., Wijnberg, J.B.P.A., SierraAlvarez, R., 2000. Degradation and detoxification of softwood extractives by sap stain fungi. Bioresource Technology 71, 14e20. Dwainto, W., Inoue, M., Norimoto, M., 1997. Fixation of deformation of wood by heat treatment. Makuzai Gakkaishi 43 (4), 303e309. EN 113, 1980. Wood Preservatives-determination of the Toxic Values against Wood Destroying Basidiomycetes Cultured on Agar Medium. ENV 807, 1993. Wood Preservatives-determination of the Toxic Effectiveness against Soft Rotting Micro-fungi and Other Soil Inhabiting Micro Organisms. Garrote, G., DomõÂnguez, H., Parajo, J.C., 1999. Hydrothermal processing of lignocellulosic materials. Holz als Roh- und Werkstoff 57 (3), 191e202. Garrote, G., DomõÂnguez, H., Parajo, J.C., 2001. Study on the deacetylation of hemicelluloses during the hydrothermal processing of Eucalyptus wood. Holz als Roh- und Werkstoff 59 (1e2), 53e59. Gonzales, M., Breese, M.C., Hale, M.D.C., 2005. Studies on the relaxation of heat treated wood. The Second European Conference on Wood Modification. 6e7 Oct., Göttingen, Germany, 87e90. Green III, F., Highley, T.L., 1997. Mechanism of brown rot decay: paradigm or paradox. International Biodeterioration & Biodegradation 39 (2), 113e124. Hakkou, M., Petrissans, M., Ceradine, P., Zoulalian, A., 2006. Investigation of the reasons for fungal durability of heat treated beech wood. Polymer Degradation and Stability 91 (2), 393e397. Heger, F., Giroux, M., Welzbacher, C., Rapp, A.O., Navi, P., 2004. Mechanical and durability performance of THM-densified wood. Final Workshop Cost Action, Environmental Optimization of Wood Protection, Lisbon, Portugal, 22nde23rd March 2004, 1e10. Ito, Y., Tanahashi, M., Shigematsu, M., Shinoda, Y., Ohta, C., 1998a. Compressivemolding of wood by highepressure steametreatment: I. Development of compressive molded squares from thinnings. Holzforschung 52 (2), 211e216. Ito, Y., Tanahashi, M., Shigematsu, M., Shinoda, Y., 1998b. Compressive-molding of wood by highepressure steametreatment: II. Mechanism of permanent fixation. Holzforschung 52 (2), 217e221. Jennings, J.D., 2003. Investigation the surface energy and bond performance of compression densified wood. M.Sc. Thesis, Virginia Polymeric Institute and State University, p.147. Kamdem, D.P., Pizzi, A., Guyonnet, R., Jrmannaud, A., 1999. Durability of Heattreated Wood, Doc. No. IRG/WP/99e40145. The International Research Group on Wood Preservation, Stockholm. Kamke, F.A., 2006. Densified Radiata pine for structural composites. Maderas, Ciencia Y Tecnologia 8 (2), 83e92.
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