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Thermal treatment has been used for many years for the modification of wood. This method applies thermal treatment to wood materials in order to improve the relation between wood and water. In this study, changes in the chemical composition (CC) and moisture content (MC) of Uludag Fir (Abies bornmulleriana Mattf.) wood during thermal treatment were investigated. For this study, three different temperatures (170, 190, and 210 oC) and three different durations (4, 8, and 12 hours) were used. After thermal treatment, degradations in the CC and MC of the samples were determined. The data obtained were analyzed using variance analysis, and Tukey's test was used to determine the changes in the CC and MC of Uludag Fir wood during thermal treatment. The results showed that thermally treated wood permanently changed several of its chemical structures and that the changes were mainly caused by thermal degradation of wood polymers, such as cellulose and hemicelluloses. Also it was found that decreasing of cellulose and hemicelluloses ratio had a favorable effect on the interaction of the wood with moisture.
Since ancient times, wood has been used for manufacturing various materials utilized in daily life. In the last century, the development of new products, such as plastics, have led to a decrease in the development of wood technologies even though, unlike plastic, wood is a biologically sustainable material. Wood is a complex material that consists mainly of cellulose, hemicelluloses, lignin, and extractive compounds. Cellulose represents the crystalline part of wood, while the structures of hemicelluloses and lignin are amorphous. The main mechanical function of hemicelluloses and lignin is to buttress the cellulose fibrils (Wikberg and Maunu 2004). Cellulose is the main polymeric component of the walls of the wood cells. Cellulose consists of anhydro-D-glucopyranose units linked by Î²-1-4-glycosidic bonds to form linear, polymeric chains. Cellulose forms partially crystalline microfibrils that are oriented helically around the cell axis. Hemicelluloses are branched heteropolysaccharides that consist of different monosaccharide units. Lignin is a complex heterogeneous macromolecule. Normal softwood lignin is predominantly built of coniferyl alcohol units. In wood, the structures of hemicelluloses and lignin are amorphous.
The modification of the structure of wood during heat treatment inhibits the absorption of water, thereby reducing the possibility of decay and water absorption of wood. When wood absorbs moisture from its surroundings, the moisture can break hydrogen bonds between cellulose and hemicelluloses chain (Hinterstoisser et al. 2003). Thus it significantly reduces the ability of the water to penetrate into the wood (Homan et al. 2000). Thermally treated wood becomes dimensionally more stable compared to untreated wood. Elimination of hydroxyl groups also reduces the number of potential anchor-points for fungi (Poncsak et al. 2006). During the thermal treatment process, many volatile organic compounds, such as alcohols, resins, terpenes, formic acid, and acetic acid are produced and released from the wood (Manninen et al. 2002; Graf et al. 2003). A significant decrease in the content of hemicelluloses has also been reported in the literature (Bekhta and Niemz 2003).
The hemicelluloses degrade first (between 160 and 260 oC), since their low molecular weight and their branched structure facilitate faster degradation compared to the other components present in wood (Fengel and Wegener 1989). The chemical modifications of wood structure that occur at high temperature are accompanied by several favorable changes in physical structure, e.g., reduced shrinkage and swelling, low equilibrium moisture content, better decay resistance, enhanced weather resistance, and a decorative, dark colour (Santos 2000; Kaygin et al. 2009a and 2009b). The reduction in water absorption causes the swelling and shrinking of wood to decrease, leading to improved dimensional stability (Gunduz et al. 2008a).
Heat treatment of wood is primarily used to increase the durability, reduce the hygroscopicity, and improve the dimensional stability of wood. Beside these desirable changes, heat treatment also can cause unfavorable effects, such as diminished strength, surface hardness, and toughness (Aydemir 2007; Gunduz et al. 2007 and 2008; Boonstra et al. 2006; Aydemir and Gunduz 2009).
Bekhta and Niemz (2003) reported that both bending strength and modulus of elasticity decreased when the treatment temperature was more than 100 oC, and a 50% decrease in bending strength was observed for spruce wood treated at 200 oC. When wood is heated, its chemical and physical properties undergo permanent changes, and its structure is reformed. The observed changes in properties occur mainly due to the degradation of hemicelluloses. The changes continue as the temperature is increased during the heating process.
In earlier work, we conducted heat treatment investigations of the physical, mechanical, and technological properties of various wood species (Aydemir 2007; Gunduz et al. 2008a and 2008b; Kaygin et al. 2008; Gunduz et al. 2009a, 2009b, 2009c, and 2009d). Uludag Fir is an endemic species in Turkey. It grows in the Bursa region in the western part of Turkey. It has a low density, and it is very commonly used to create cheap wood products. However, the changes induced in the structure of Uludag Fir wood by thermal treatment have not been reported extensively elsewhere. Therefore, our objective is to determine the changes in chemical structure caused by the heat treatment of Uludag Fir (Abies bornmulleriana Mattf.) wood. After thermal treatment, potential enhanced use of Uludag fir will be determined.
MATERIALS AND METHODS
The Uludag Fir (Abies bornmulleriana Mattf.) wood samples used during this study were obtained from Bartin, Turkey. They had initial moisture contents that ranged from 11 to 13% before they were air dried. The dimensions of the samples were 20 x 20 x 30 mm. The test samples were thermally treated in an oven in which the temperature could be controlled to Â± 1 oC. The treatments were conducted at three different temperatures (170, 190, and 210 oC) and for three different periods of time (4, 8, and 12 h) at atmospheric pressure in the presence of air. Before chemical analysis, thermal-treated and non-treated (control) wood samples were ground in a Wiley mill to a homogenous meal. This grinding was done according to TAPPI T257 cm-02. For the isolation of thermal-treated and non-treated wood extracts, extractives were dissolved in selected solvents, and the Soxhlet extraction process, which is highly efficient for this purpose because of alternate siphoning and filling of the extractor body with fresh solvent, was used. The material to be extracted was placed in a tall, glass extraction thimble, and the thimble was placed in a round flask. As a result, wood material was isolated from the extractive materials, such as terpenes, stilbenes, flavonoids, and volatile compounds.
Chemical analyses of the wood components were performed on non-treated and thermally treated Uludag fir wood. The data obtained in the chemical analyses of the extracted wood particles were Î±-cellulose content (TAPPI T212 om-02, 2002), holocelluloses content (Wise and John, 1952), lignin content (TAPPI T222 om-02, 2002), cold and hot water solubility, and 1% NaOH solubility (TAPPI T207 cm-99, 1999). To determine the moisture content of the samples, the specimens were conditioned until their weights reached equilibrium; then they were weighed, and their equilibrium moisture contents (EMC) were calculated according to ISO 3130.
RESULTS AND DISCUSSION
Wood can be degraded due to many reasons (such as fungal activity, insects, and high humidity). Therefore, new methods have been developed to slow or stop the degradation process. One of these methods is the thermal treatment process. This study was conducted to determine the changes in the chemical structure of thermally treated Uludag fir wood. The strength properties of cellulosic materials are intrinsically related to the interactions between the cellulose molecules. Therefore, it is important to determine how the chemical structure of Uludag fir wood was affected by thermal treatment.
All statistical calculations for data obtained during the tests were based on the 95% confidence level. ANOVA and Tukey's Multiple Range Tests show that all differences were significant. Table 1 shows the influence of thermal treatment at different temperatures and durations on chemical properties, such as moisture content (%), extractive content (%), holocelluloses, Î± - cellulose, lignin, cold water solubility, hot water solubility, and 1% NaOH solubility as compared to control specimens. Uludag fir wood contains some volatile materials that evaporate during thermal treatment. However, thermal treatment had different effects on extractive mass content. According to Table 1, at first, the extractive mass content increased as treatment temperature and duration increased, but then it decreased because most of it had already been removed due to the high temperature of the wood.
Table 1. Changes in the Structure of Uludag Fir Wood during Thermal Treatment
170 Â°C/4 h
170 Â°C/8 h
170 Â°C/12 h
190 Â°C/4 h
190 Â°C/8 h
190 Â°C/12 h
210 Â°C/4 h
210 Â°C/8 h
210 Â°C/12 h
Comparisons were done between the each control and test. Thirty replicates were used in each test. All data in Variance and one-way ANOVA tests were done at a confidence level of p < 0.05 (95%).
Thermal treatment results a decrease in the mass of the wood. The decrease of the mass is caused by: 1) lowering the moisture content; 2), evaporation of extractives; and 3) degradation of wood components, especially the hemicelluloses, accompanied by evaporation of the degradation products. Also, the decrease of various properties, such as mechanical properties such as bending strength, compression strength and hardness, is mainly caused by the degradation of wood components (cellulose and especially the hemicelluloses), and this degradation is the reason the mass decreases during heat treatment (Boonstra et al. 2006).
Uludag fir wood contains some volatile compounds that evaporate during thermal treatment. However, thermal treatment has a different effect on extractive mass content. It was determined that the maximum and minimum ratios of extractive mass content occurred when the treatment conditions were 170 Â°C for 4 hours and 190 Â°C for 4 hours. During thermal treatment, it was found that basic components (hemicellulose, lignin, and cellulose) of UludaÄŸ fir wood were changed at different ratios. As the treatment temperatures were increased, it was shown that groups of hemicellulose and cellulose began to degrade, and their ratios began to decrease. But lignin started to degrade at a temperature of 206 oC; therefore the lignin amount was not significantly changed during thermal treatment under 200 oC. In contrast, the lignin ratio was increased because additional lignin became available when some branched structure components were disconnected from cellulose and hemicelluloses. It also was determined that the increase in lignin occurred parallel to the decreasing cellulose and hemicellulose ratios (Fig. 1). The maximum increase in the lignin ratio of 34.8% occurred at 200 oC for 8 hours, while the lowest increase in the lignin ratio of only 6.4% occurred at 170 oC for 4 hours.
Figure 1. Increment in lignin ratio after heat treatment
During thermal treatment, lignin is the most stable component of wood; whereas various changes were observed even at temperatures below 200 oC. The determination of the lignin content in thermally treated wood gave evidence of an increase in non-hydrolysable residue with increasing temperature up to 200 oC. Regarding the degree and shape of fiber swelling as criteria of thermally caused changes, researchers have found no change in lignin up to 155 oC. Heating at 175 oC caused a lignin condensation that increased with temperatures up to 240 oC (Fengel and Wegener 1989).
In the heat treatment process, wood is heated to temperatures of 160 to 250 oC. The treatment temperature depends on the species being treated and the desired properties, but, usually, the treatment temperature is above 200 oC, which causes various changes in the structure of wood. Initially, hemicelluloses start to degrade, since they have the lowest molecular weight among the wood polymers. The degradation of the hemicelluloses results in a reduction of hydroxyl groups and the formation of O - acetyl groups. With the subsequent cross-link formation between the wood fibers, wood becomes more hydrophobic. Parallel to this status, cellulose starts to degrade during heating above 200 oC, and the lignin ratio increases because of the attachment of separated substances from hemicelluloses and cellulose. These changes can affect the physical, mechanical, technological, and biological properties of wood both positively and negatively. During high temperature treatment, the wood is heated slowly up to 200 - 230 °C in humid, inert gas. This treatment reduces the hydrophilic behavior of the wood by modifying the chemical structure of some of its components (Raimo et al. 1996; Gailliot 1998).
Wood structure changes have been identified in Uludag fir wood samples after they were thermally treated at different temperatures and durations. With an increase in the treatment temperature, the free hydroxyl groups in hemicelluloses and cellulose are parts to be removed and transformed into groups of different molecules. Thus, one observes reduced swelling and shrinkage of wood. Therefore, the wood absorbs only a small amount of water, and it reaches its equilibrium moisture content with low moisture levels. This change is shown in Fig. 2, which gives the relationship between cellulose and hemicelluloses and the moisture cycle.
Figure 2. Effect of the loss of holocelluloses and Î±-cellulose on "r" loss
According to Fig. 3, during thermal treatment, it was found that cold and hot water solubility (%) were increased under test conditions of 170 Â°C for 8 hours and 190 Â°C for 12 hours, but they were decreased when the test conditions were 210 Â°C for 4 and 8 hours. This reversal was not observed for solubility in a 1% NaOH solution, as the solubility of Uludag fir wood was observed to increase consistently as treatment temperatures and durations increased. It can be said that these changes in the solubility of Uludag fir wood occurred because of the thermal degradation of wood, particularly hemicellulose, because it has more hydroxyl groups.
Figure 4. Changes in the solubility of Uludag fir wood after thermal treatment
The increase in the concentration of the soluble fraction in the treated product is due to depolymerization of the components of the cell wall during thermal treatment. The water-soluble fraction increased to a greater extent than the non-polar fraction. This indicates a depolymerization of the carbohydrate, especially of the hemicelluloses. A higher treatment temperature during the hydro-thermolysis appears to have an effect on the concentration of the soluble fraction, due to the formation of furfural and/or some degradation products of the lignin wood component (Boonstra and Tjeerdsma 2006).
In this study, Uludag fir (Abies bornmulleriana Mattf.) wood was exposed to thermal treatment at 170, 190, and 210 oC for periods of 4, 8, and 12 hours. Since holocelluloses was the main polymer component in the wood, and its mass was reduced by 3.5 to 29.8% due to thermal degradation, and the mass of Î±-cellulose was reduced by 5 to 35%, depending on the treatment conditions. The moisture content of the wood decreased by 9.5 to 32% because of the degradation of the hydroxyl groups of Î±-cellulose and the hemicelluloses. The parts that were broken down from cellulose and hemicelluloses connected easily with lignin, causing an increase in the lignin ratio in the wood. Because lignin is a hydrophobic material, the moisture content was decreased. Because of the reasons mentioned above, thermally treated Uludag fir wood can be used in saunas, pool edges, wood siding, ship decks, and garden furniture. Holocelluloses, which is a connecting component in wood, breaks down during heat treatment. Therefore, heat treatment causes mass loss, and the resulting material can be significantly lighter in weight than the original wood, making it useful for isolation, the Parquet industry, and for decorative purposes.
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