Abstract
The objective of this study was to investigate the color and chemical changes of thermo-vacuum treated larch wood. Specimens were heat treated in a vacuum at various temperatures for 4 h. The color parameters of untreated and heat-treated samples were measured using the CIELab color system. Changes in the chemistry of larch wood were investigated using X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR) spectroscopy, and UV-Vis spectroscopy. The results showed that the color of larch wood became darker at higher treatment temperatures. The O/C ratio decreased with an increase in treatment temperature. The C1 peaks showed that there was an elevated content of lignin and extractives on the wood surface after heat treatment. There were more free radicals for thermally treated wood samples, as demonstrated by the increase in the intensities of the ESR signals. Some groups of lignin that contribute to the darkness of the wood increased after thermo-vacuum treatment. Changes in surface chemistry may be the cause of discolorations that occurred during thermo-vacuum treatment.
Download PDF
Full Article
Effect of Thermo-Vacuum Treatment on the Color and Chemistry of Larch Wood
Zhe Wang, Bailing Sun, and Junliang Liu*
The objective of this study was to investigate the color and chemical changes of thermo-vacuum treated larch wood. Specimens were heat treated in a vacuum at various temperatures for 4 h. The color parameters of untreated and heat-treated samples were measured using the CIELab color system. Changes in the chemistry of larch wood were investigated using X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR) spectroscopy, and UV-Vis spectroscopy. The results showed that the color of larch wood became darker at higher treatment temperatures. The O/C ratio decreased with an increase in treatment temperature. The C1 peaks showed that there was an elevated content of lignin and extractives on the wood surface after heat treatment. There were more free radicals for thermally treated wood samples, as demonstrated by the increase in the intensities of the ESR signals. Some groups of lignin that contribute to the darkness of the wood increased after thermo-vacuum treatment. Changes in surface chemistry may be the cause of discolorations that occurred during thermo-vacuum treatment.
Keywords: Larch wood; Thermo-vacuum treatment; Color; Surface chemistry
Contact information: Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing 100091, P. R. China; *Corresponding author: liujunliang@caf.ac.cn
INTRODUCTION
Heat treatment is a well-developed and innovative method for improving the properties of wood without using any chemicals (Esteves et al. 2008; Akyildiz et al. 2009; Esteves and Pereira 2009; Dubey et al. 2012; Huang et al. 2012). Although the study of heat treatment was started by Stamm (1946), it has been systematically studied and industrialized only in the last dozen years. There are several different industrial heat treatment processes: “Thermowood” in Finland by the use of steam (Viitanen et al. 1994), Plato wood in the Netherlands with a combination of steam and heated air (Tjeerdsma et al. 1998), retification or bois perdure in France (Dirol and Guyonnet 1993), oil heat treatment in Germany (Rapp 2001), and the Termovuolo process by applying vacuum (Allegretti et al. 2012; Ferrari et al. 2013). Changes in color and surface chemistry caused by heat treatment have been intensely investigated (Bryne et al. 2010; Matsuo et al. 2011; Tuong and Li 2011).
The color of wood is determined by chemical components, including cellulose, hemicelluloses, lignin, and extractives. The darkening and reddening of wood as a result of heat treatment has been studied by previous researchers (Johansson and Morén 2006; Aksoy et al. 2011). The discoloration mechanism is complex: Sundqvist and Morén (2002) attribute the formation of color to the degradation of extractives, hemicelluloses, and lignin by hydrothermal treatment. The results obtained from Sehlstedt-Persson (2003) indicate that the formation of color could result from the hydrolysis of hemicelluloses by the heat treatment of extractive free wood. The cleavage of ether linkages or the condensation of lignin (Wikberg and Maunu 2004), and the formation of oxidation products of phenolic substances (Bekhta and Niemz 2003) can all contribute to color changes.
The investigation of the surface chemical structure of some wood species has been carried out with X-ray photoelectron spectroscopy (XPS) before and after heat treatment (Inari et al. 2006; Gerardin et al.2007). Electron spin resonance (ESR) is the instrument of choice for studying several aspects of wood chemistry (Solala et al. 2012; Xue et al. 2012), especially the formation of free radicals in wood. Sivonen et al. (2002) studied the formation and stability of free radicals in pine formed by thermal treatment. The intensities of the ESR signals from thermally treated wood samples increase with increasing thermal treatment temperature. The occurrence of condensation reactions and the formation of other aromatic structures have been studied by UV analyses during heat treatment (Boonstra and Tjeerdsma 2006; Chen et al. 2012a).
Larch is one of the most important plantation tree species in northeast China. Larch wood has a high usage potential for solid wood products used for the construction, decoration, and furniture industries because of its high hardness, beautiful texture, and strong decay resistance (Li 2006). Consumers often like the darker tonality of wood due to the better decorative function. In order to meet the needs of customers, heat treatment has been used as a way to make wood darken and redden (Bekhta and Niemz 2003; Esteves and Pereira et al. 2009). Thermo-vacuum treatment has been reported as a method of darkening the wood color by some researchers (Allegretti et al. 2012; Ferrari et al. 2013). The color of alder birch wood is darkened by thermo-vacuum treatment and the discoloration mechanisms are analyzed using wet chemical analysis and FTIR (Yang et al. 2015). The discoloration mechanisms of heat-treated wood under vacuum are still not clear; therefore further studies still need to be done to elucidate the discoloration mechanisms. For this purpose, larch wood was heat treated under vacuum and the wood color and chemical changes were studied. The aim of this study was to investigate the color and chemical changes of thermo-vacuum treated larch wood and better understand the discoloration mechanisms by using XPS, ESR, and UV-Vis Spectra.
EXPERIMENTAL
Materials
Larch wood was obtained from Qingyuan County in Liaoning Province, China. Larch wood was sawn and processed with a planer to obtain specimens with a length of 400 mm (longitudinal), a width of 40 mm (radial), and a thickness of 40 mm (tangential). The specimens were free of macroscopic defects such as knots and splits.
Methods
Prior to heat treatment, all specimens were oven-dried at 103 ± 2 °C to 0% moisture content and then placed in a vacuum pressure furnace. Heat treatment was conducted at temperatures of 160, 180, 200, 220, and 240 °C for 4 h under the absolute pressure range from 0.01 to 0.05 MPa with reference to atmospheric pressure. At the end of each heat treatment, the furnace was turned off and the wood was kept inside until the temperatures reached 40 °C; then, the wood was removed and stored. The untreated specimens were used as control test samples.
Color values were measured by the CIELab system with a Minolta spectrophotometer (model CM-2600d, Japan) before and after heat treatment. The sensor head was 8 mm in diameter. Measurements were made using a D65 illuminant and a 10° standard observer. 10 samples were treated at each temperature and color was measured at 3 locations on each sample, and the mean value was calculated. The three coordinates of CIELab represent the lightness (L*=0 indicates pure black and L*=100 indicates pure white), its position between red and green (a* negative values indicate green, while positive values indicate red), and its position between yellow and blue (b* negative values indicate blue and positive values indicate yellow). The total color difference (ΔE*) was calculated according to the following equation,
(1)
where ΔL*, Δa*, and Δb* are the color changes between the untreated and heat-treated samples.
The results were analyzed using one-way ANOVA (P = 0.05) from the SPSS statistical software program, and significant differences were determined by the Tukey multiple comparison test (α = 0.05).
Wood samples for the XPS study were collected from the same radial surface of samples used for color change measurements and processed with sizes of 5 mm× 5 mm× 1 mm (L × R × T) in the laboratory. X-ray photoelectron spectroscopy was performed on a Kratos Axis Ultra spectrometer (Kratos Analytical Ltd., UK) using a monochromatic Al Kα X-ray source (λ = 1486.7 eV) with a power of 225 W. Spectra were analyzed using CasaXPS software (2.3.12Dev7, UK). A high-resolution spectrum of the C1s region from 280 to 300 eV was collected at a pass energy of 20 eV and resolution of 0.05 eV/step, for quantitative measurements of binding energy and atomic concentration. Chemical bond analysis of the carbon was accomplished by fitting the C1s peak and de-convoluting it into three sub-peaks.
Wood samples for the ESR and UV-Vis study were ground into powder and passed through a 100-mesh screen, but not a 120-mesh screen. A total of 0.02 g of powder from untreated and treated wood was used to study the intensities of the free radicals. ESR measurements were carried out on a JEOL JES-FA 200 ESR spectrometer (Japan; sweep width 15 mT, microwave frequency 9.06 GHz, microwave power 0.998 mW, modulation amplitude 0.1 mT, time constant 0.3 s, sweep time 4 min). Intensities of the free radical signals were measured from the amplitude of the free radical signal. Spectral measurements in the range 240 to 800 nm were performed on a Cary 5000 UV-Vis spectrophotometer (Varian, USA).
RESULTS AND DISCUSSION
Color Changes of the Treated Wood
The color change data of heat-treated larch wood are presented in Table 1. According to the results, the color of thermo-vacuum treated wood became increasingly darker, compared with the original color, with increased treatment temperature. Similar results have been reported by Matsuo et al. (2011). Compared with the untreated samples, the lightness values (L*) decreased by 13.2%, 22.4%, 22.5%, 28.2%, and 43.8% with increasing temperature of heat treatment, respectively. Similar results have been reported by Allegretti et al. (2012) for spruce and fir wood by thermo-vacuum treatment. Both a* and b* values gradually increased as the increasing treatment temperature. The former increased from 7.9 to 17.0 and the latter increased from 28.8 to 39.0. Similar results have been obtained in previous studies (Mottonen and Karki 2008; Gunduz et al. 2010). In this study, ΔE* values increased from 13.3 to 35.8 with the increase of treatment temperature from 160 °C to 240 °C. There is a similar trend with the results studied by Ferrari et al. (2013) for some European softwood and hardwood species using thermo-vacuum treatment. The mechanism of color change is complex: Sehlstedt-Persson (2003) indicates that the formation of color could result from the hydrolysis of hemicelluloses by the heat treatment of extractive free wood. The results obtained by McDonald et al. (1997) show that the Amadori-Maillard reaction results in the discoloration of wood. Sundqvist and Moren (2002) evaluate the effects of wood polymers and extractives on the color during hydrothermal treatment and show the extractives involve in the color formation of hydrothermal treated wood.
Table 1. Color Change Values of Heat-treated Wood
Standard deviations are shown in parentheses.
Means followed by the same letter are not significantly different by Tukey test at 0.05 probability level.
XPS Analysis
The main reason for performing XPS analysis was to obtain information concerning the chemical environment and atomic concentrations present in untreated and heat-treated larch wood. Figure 1 shows a typical XPS survey spectrum of larch wood before and after thermo-vacuum treatment, and Table 2 lists the experimental atomic composition as determined from the XPS spectra analysis and the calculated oxygen to carbon (O/C) ratio for all samples. The XPS spectra revealed that carbon and oxygen were the major elements, and their peaks appeared at about 284 and 532 eV, respectively. A small amount of calcium and silicon was detected on the surface from their characteristic emission peaks at 347 and 98 eV, respectively. Table 2 shows some increase in the percentage of carbon atoms and at the same time some decrease in the percentage of oxygen atoms for each heat-treated wood sample in comparison with the control sample.
Table 2. Relative Amount of Atoms and O/C Ratio Determined by XPS for Heat-treated and Untreated Wood
Fig. 1. XPS survey spectra of untreated and heat-treated larch wood: (a) control; (b) heat-treated at 160 °C for 4 h; (c) heat-treated at 200 °C for 4 h; and (d) heat-treated at 240 °C for 4 h
Using the total area of peaks and the respective photoemission cross-sections, a quantitative determination of oxygen to carbon (O/C) ratios for heat-treated and control samples is presented in Table 2. The results showed that O/C ratios were reduced from 17.2% to 10.4% with the increase of heat treatment temperature. The O/C atomic ratio of lignin is 0.33, extractives is 0.1, and cellulose is 0.83 (Kamdem et al. 1991). The decrease in O/C ratios might result from the formation of volatile by-products and an increase in lignin proportion on wood surface (Sernek et al. 2004; Nguila et al. 2006; Kocaefe et al. 2013). There was a high carbon content in untreated and heat-treated wood samples, which could indicate that the lignin and extractives (or their degradation products) content was higher on the wood surface, especially extractives. The XPS results obtained by Bryne et al. (2010) show that the high carbon content on wood surface is due to the migration of low-oxygenated species (presumably extractives) to the surface. In addition, the high carbon content might result from the new products with lower oxygen content due to the dehydration of wood components (Tuong and Li 2011).
Figure 2 shows the types and amounts of carbon-oxygen bonds on C1s spectra of treated and control samples. According to the relevant literature, the assignment of deconvoluted peaks C1s for lignocellulosic materials, which correspond to the four types of carbon atoms, is well-established: C1, C2, C3, and C4 (Kamdem et al. 2001; Nzokou and Kamdem 2005). C1 corresponds to carbon only bonded to other carbon atoms and/or hydrogen atoms, which arises mainly from lignin and wood extractives. C2 has one link to a single non-carbonyl oxygen atom, which has been shown to be mainly derived from cellulose. C3 bonds to one carbonyl oxygen or two non-carbonyl oxygen atoms (Kamdem et al. 1991). As for C4, it was neglected here because it was low in content.
Fig. 2. C1s spectra of untreated and heat-treated larch wood: (a) control; (b) heat-treated at 160 °C for 4 h; (c) heat-treated at 200 °C for 4 h; and (d) heat-treated at 240 °C for 4 h
Table 3 shows the variation of peak area contributions of the C1, C2, and C3 components before and after thermo-vacuum treatment. C1 were the most important contributions for the surfaces of untreated and heat-treated samples. The C1 peak area increased by 4.5%, 12.4%, and 9.3% with the increasing treatment temperature, respectively. The C2 peak area decreased by 13.9%, 51.7%, and 41.7% with the increasing of treatment temperature, respectively. Nishimiya et al. (1998) attributed C1 and C2 to the aromatic carbon and aliphatic carbon of wood, respectively. The increase in the C1 peak area indicates higher concentration of lignin-derived aromatic carbon due to the partial degradation of hemicelluloses. The decrease in the C2 peak area indicates the reduction of aliphatic carbon of wood. As stated above, the results may be related to changes in the surface extractives or to the changes in chemical functional groups by the degradation of wood components during heat treatment (Weiland and Guyonnet 2003; Hakkou et al. 2005).
Table 3. C1s Division Peak Percentage Content of Wood Surface Due to Heat Treatment
ESR Analysis
The ESR spectra of untreated and heat-treated larch wood are shown in Fig. 3. The signals grew with increasing temperature of heat treatment. The intensities of the ESR signals of thermo-vacuum treated wood samples were higher than those of untreated samples, indicating that more free radicals were formed during heat treatment, which was in agreement with previous results (Sivonen et al. 2002; Petric et al. 2004). The g-value of the free radical peaks was 2.003, attributable to the phenoxy free radicals generated from lignin (Sivonen et al. 2002; Kuzina et al. 2004; Baur and Easteal 2014). The further reaction of phenoxy free radicals may result in the formation of quinones that are responsible for darkening the wood (Petric et al. 2004; Evans 2012).
Fig. 3. ESR spectra of free radicals for different treatment temperatures: (a) control; (b) heat-treated at 160 °C for 4 h; (c) heat-treated at 200 °C for 4 h; (d) heat-treated at 240 °C for 4 h
UV-Vis Spectra Analysis
The UV-Vis diffuse reflectance spectra obtained from the untreated and heat-treated samples are shown in Fig. 4. There was no obvious difference in the absorptions of the UV region. The absorptions at 280 and 350 nm increased slightly after heat treatment, which was attributed to β-5, the unsaturated Cα = Cβ bond, and the β-C = O structure of lignin and the conjugated carbonyl group in the side chains of lignin and double bonds, respectively (Chen et al. 2012b). The absorptions after heat treatment in the visible region (400 to 700 nm) were obviously increased, which indicated that some coloring reactions could occur during heat treatment. The absorptions in the 400 to 500 nm region could be attributed to quinones, which were formed from the reaction of the aromatic hydroxyl groups of lignin during heat treatment (Chen et al. 2012b).
Fig. 4. UV-Vis diffuse reflectance spectra of untreated and heat-treated larch wood: (a) control; (b) heat-treated at 160 °C for 4 h; (c) heat-treated at 200 °C for 4 h; and (d) heat-treated at 240 °C for 4 h
CONCLUSIONS
- It was found that thermo-vacuum treatment darkened larch wood. The values of L* decreased and the chromaticity indices a* and b* increased after heat treatment.
- The results of the XPS spectral analysis show that the O/C ratios decreased after heat treatment. The increase in the C1 peak area indicates higher concentration of lignin-derived aromatic carbon due to the partial degradation of hemicelluloses.
- The results of the ESR study of the thermo-vacuum treatment of wood suggest that phenoxy free radicals are generated during heat treatment. The signals grow with increasing temperature of heat treatment. The reaction of phenoxy free radicals might be one cause of wood discoloration during heat treatment.
- The formations of degradation and condensation products were demonstrated by the increasing absorption of the UV-Vis spectra after heat treatment.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China Grant No. 31370558, “Thermal effect and mechanism of wood under vacuum heat treatment.”
REFERENCES CITED
Aksoy, A., Deveci, M., Baysal, E., and Toker, H. (2011). “Color and gloss changes of Scots pine after heat modification,” Wood Research 56(3), 329-336.
Akyildiz, M. H., Ates, S., and Özdemir, H. (2009). “Technological and chemical properties of heat-treated Anatolian black pine wood,” African Journal of Biotechnology 8(11), 2565-2572. DOI: 10.5897/AJB09.306
Allegretti, O., Brunetti, M., Cuccui, I., Ferrari, S., Nocetti, M., and Terziev, N. (2012). “Thermo-vacuum modification of spruce (Picea abies Karst.) and fir (Abies alba Mill.) wood,” BioResources7(3), 3656-3669.
Baur, S. I., and Easteal, A. J. (2014). “ESR studies on the free radical generation in wood by irradiation with selected sources from UV to IR wavelength regions,” Holzforschung 68(7), 775-780. DOI: 10.1515/hf-2013-0185
Bekhta, P., and Niemz, P. (2003). “Effect of high temperature on the change in colour, dimensional stability and mechanical properties of spruce wood,” Holzforschung 57(5), 539-546. DOI: 10.1515/HF.2003.080
Boonstra, M. J., and Tjeerdsma, B. (2006). “Chemical analysis of heat treated softwoods,” Holz als Roh- und Werkstoff 64(3), 204-211. DOI: 10.1007/s00107-005-0078-4
Bryne, L. E., Lausmaa, J., Ernstsson, M., Englund, F., and Walinder, M. E. P. (2010). “Ageing of modified wood. Part 2: Determination of surface composition of acetylated, furfurylated, and thermally modified wood by XPS and ToF-SIMS,” Holzforschung 64(3), 305-313. DOI: 10.1515/hf.2010.062
Chen, Y., Fan, Y. M., Gao, J. M., and Li, H. K. (2012a). “Coloring characteristics of in situ lignin during heat treatment,” Wood Science and Technology 46(1), 33-40. DOI: 10.1007/s00226-010-0388-5
Chen, Y., Fan, Y. M., Gao, J. M., and Stark, N. M. (2012b). “The effect of heat treatment on the chemical and color change of black locust (Robinia pseudoacacia) wood flour,” BioResources 7(1), 1157-1170. DOI: 10.15376/biores.7.1.1157-1170
Dirol, D., and Guyonnet, R. (1993). “Durability by rectification process,” Technical Report No. IRG/WP 93-40015, International Research Group on Wood Protection, Stockholm, Sweden.
Dubey, M. K., Pang, S. S., and Walker, J. (2012). “Changes in chemistry, color, dimensional stability and fungal resistance of Pinus radiata D. Don wood with oil heat-treatment,” Holzforschung 66(1), 49-57. DOI: 10.1515/HF.2011.117
Esteves, B. M., Domingos, I. J., and Pereira, H. M. (2008). “Pine wood modification by heat treatment in air,” BioResources 3(1), 142-154. DOI: 10.15376/biores.3.1.142-154
Esteves, B. M., and Pereira, H. M. (2009). “Wood modification by heat treatment: A review,” BioResources 4(1), 370-404.
Evans, P. D. (2012). “Weathering of wood and wood composites,” in: Handbook of Wood Chemistry and Wood Composites, R. M. Rowell (ed.), CRC Press, Boca Raton, FL, pp. 151-216.
Ferrari, S., Cuccui, I., and Allegretti, O. (2013). “Thermo-vacuum modification of some European softwood and hardwood species treated at different conditions,” BioResources 8(1), 1100-1109.
Gerardin, P., Petric, M., Petrissans, M., Lambert, J., and Ehrhrardt, J. J. (2007). “Evolution of wood surface free energy after heat treatment,” Polymer Degradation and Stability 92(4), 653-657. DOI: 10.1016/j.polymdegradstab.2007.01.016
Gunduz, G., Aydemir, D., and Korkut, S. (2010). “The effect of heat treatment on some mechanical properties and color changes of uludag fir wood,” Drying Technology 28(2), 249-255. DOI: 10.1080/07373930903530162
Hakkou, M., Petrissans, M., Zoulalian, A., and Gerardin, P. (2005). “Investigation of wood wettability changes during heat treatment on the basis of chemical analysis,” Polymer Degradation and Stability89(1), 1-5. DOI: 10.1016/j.polymdegradstab.2004.10.017
Huang, X. A., Kocaefe, D., Kocaefe, Y., Boluk, Y., and Pichette, A. (2012). “Study of the degradation behavior of heat-treated jack pine (Pinus banksiana) under artificial sunlight irradiation,” Polymer Degradation and Stability 97(7), 1197-1214. DOI: 10.1016/j.polymdegradstab.2012.03.022
Inari, G. N., Petrissans, M., Lambert, J., Ehrhardt, J. J., and Gérardin, P. (2006). “XPS characterization of wood chemical composition after heat-treatment,” Surface and Interface Analysis 38(10), 1336-1342. DOI: 10.1002/sia.2455
Johansson, D., and Morén, T. (2006). “The potential of colour measurement for strength prediction of thermally treated wood,” Holz als Roh- und Werkstoff 64(2), 104-110. DOI: 10.1007/s00107-005-0082-8
Kamdem, D. P., Riedl, B., Adnot, A., and Kaliaguine, S. (1991). “ESCA spectroscopy of poly(methyl methacrylate) grafted onto wood fibers,” Journal of Applied Polymer Science 43(10), 1901-1912. DOI: 10.1002/app.1991.070431015
Kamdem, P., Zhang, J., and Adnot, A. (2001). “Identification of cupric and cuprous copper in copper naphthenate-treated wood by X-ray photoelectron spectroscopy,” Holzforschung 55(1), 16-20. DOI: 10.1515/HFSG.2001.16
Kocaefe, D., Huang, X. A., Kocaefe, Y., and Boluk, Y. (2013). “Quantitative characterization of chemical degradation of heat-treated wood surfaces during artificial weathering using XPS,” Surface and Interface Analysis 45(2), 639-649. DOI: 10.1002/sia.5104
Kuzina, S. I., Brezgunov, A. Y., Dubinskij, A. A., and Mikhailov, A. I. (2004). “Free radicals in the photolysis and radiolysis of polymers: IV. Radicals in γ- and UV-irradiated wood and lignin,” High Energy Chemistry 38(5), 298-305. DOI: 10.1023/b:hiec.0000041340.45217.bd
Li, J. (2006). The Study of Wood Protection, Science Press, Beijing, China.
McDonald, A., Fernandez, M., and Kreber, B. (1997). “Chemical and UV-VIS spectroscopic study on kiln brown stain formation in Radiata pine,” in: 9th International Symposium of Wood and Pulping Chemistry, Montreal, QC., June 9-12, 70, 1-5.
Mottonen, V., and Karki, T. (2008). “Color changes of birch wood during high-temperature drying,” Drying Technology 26(9), 1125-1128. DOI: 10.1080/07373930802266116
Nguila, I. G., Petrissans, M., Lambert, J., Ehrhardt, J. J., and Gerardin, P. (2006). “XPS characterization of wood chemical composition after heat treatment,” Surface and Interface Analysis38(10), 1336-1342. DOI: 10.1002/sia.2455
Nishimiya, K., Hata, T., Imamura, Y., and Ishihara, S. (1998). “Analysis of chemical structure of wood charcoal by X-ray photoelectron spectroscopy,” Journal of Wood Science 44(1), 56-61. DOI: 10.1007/BF00521875
Nzokou, P., and Kamdem, D. P. (2005). “X-ray photoelectron spectroscopy study of red oak (Quercus rubra), black cherry (Prunus serotina) and red pine (Pinus resinosa) extracted wood surfaces,” Surface and Interface Analysis 37(8), 689-694. DOI: 10.1002/sia.2064
Petric, M., Kricej, B., Humar, H., Pavlic, M., and Tomazic, M. (2004). “Patination of cherry wood and spruce wood with ethanolamine and surface finishes,” Surface Coatings International Part B, Coatings Transactions 87(B3), 95-201. DOI: 10.1007/BF02699635
Rapp, A. (2001). “Review on heat treatments of wood,” COST ACTION E22-Environmental optimisation of wood protection, Antibes, France.
Sehlstedt-Persson, M. (2003). “Colour responses to heat treatment of extractives and sap from pine and spruce,” 8th International IUFRO Wood Drying Conference, Brasov, Romania, August 24-29, pp. 459-464.
Sernek, M., Kamke, F. A., and Glasser, W. G. (2004). “Comparative analysis of inactivated wood surface,” Holzforschung 58(1), 22-31. DOI: 10.1515/HF.2004.004
Sivonen, H., Maunu, S. L., Sundholm, F., Jämsä, S., and Viitaniemi, P. (2002). “Magnetic resonance studies of thermally modified wood,” Holzforschung 56(6), 648-654. DOI: 10.1515/HF.2002.098
Solala, I., Volperts, A., Andersone, A., Dizhbite, T., Mironova-Ulmane, N., Vehniäinen, A., Pere, J., and Vuorinen, T. (2012). “Mechanoradical formation and its effects on birch kraft pulp during the preparation of nanofibrillated cellulose with Masuko refining,” Holzforschung 66(4), 477-483. DOI: 10.1515/hf.2011.183
Stamm, A., Burr, H., and Kline, A. (1946). “Staybwood—Heat-stabilized-wood,” Industrial & Engineering Chemistry 38(6), 630-634. DOI: 10.1021/ie50438a027
Sundqvist, B., and Moren, T. (2002). “The influence of wood polymers and extractives on wood colour induced by hydrothermal treatment,” Holz als Roh- und Werkstoff 60(5), 375-376. DOI: 10.1007/s00107-002-0320-2
Tjeerdsma, B., Boonstra, M., Pizzi, A., Tekely, P., and Militz, H. (1998). “Characterisation of thermally modified wood: Molecular reasons for wood performance improvement,” Holz als Roh- und Werkstoff 56(3), 149-153. DOI: 10.1007/s001070050287
Tuong, V. M., and Li, J. (2011). “Changes caused by heat treatment in chemical composition and some physical properties of acacia hybrid sapwood,” Holzforschung 65(1), 67-72. DOI: 10.1515/hf.2010.118
Viitanen, H., Jamsa, S., Paajanen, L., Nurmi, A., and Viitaniemi, P. (1994). “The effect of heat treatment on the properties of spruce,” Technical Report IRG/WP 94-40032, International Research Group on Wood Protection, Stockholm, Sweden.
Weiland, J. J., and Guyonnet, R. (2003). “Study of chemical modifications and fungi degradation of thermally modified wood using DRIFT spectroscopy,” Holz als Roh- und Werkstoff 61(3), 216-220. DOI: 10.1007/s00107-003-0364-y
Wikberg, H., and Maunu, S. L. (2004). “Characterisation of thermally modified hard- and softwoods by 13C CPMAS NMR,” Carbohydrate Polymers 58(4), 461-466. DOI: 10.1016/j.carbpol.2004.08.008
Xue, W., Kennepohl, P., and Ruddick, J. N. R. (2012). “Investigation of copper solubilization and reaction in micronized copper treated wood by electron paramagnetic resonance (EPR) spectroscopy,” Holzforschung 66(7), 889-895. DOI: 10.1515/hf.2011.188
Yang, Y., Zhan, T. Y., Lu, J. X., and Jiang, J. H. (2015). “Influences of thermo-vacuum treatment on colors and chemical compositions of alder birch wood,” BioResources 10(4), 7936-7945.
Article submitted: October 14, 2015; Peer review completed: December 19, 2015; Revised version received and accepted: January 4, 2016; Published: January 26, 2016.
DOI: 10.15376/biores.11.1.2349-2360