Chemical analysis of densification, drying, and heat treatment of Scots pine (Pinus sylvestris L.) through a hot-pressing process

Li, L., Wang, X., and Wu, F. (2016). "Chemical analysis of densification, drying, and heat treatment of Scots pine (Pinus sylvestris L.) through a hot-pressing process," BioRes. 11(2), 3856-3874.

Abstract

This study investigated a new potential hot-pressing method for wood modification, in which densification, drying, and heat-treatment were carried out in sequence. The effects of heat treatment on the chemical components of wood were evaluated. The specimens were treated at different temperatures (180 to 220 °C) for 2 to 5 h. Holocellulose, α-cellulose, and lignin were extracted from the treated and untreated milled wood. The changes in these components were analyzed by thermogravimetry (TG) and Fourier-transform infrared spectroscopy (FTIR). Due to its amorphous structure, most hemicelluloses were degraded when it was exposed to 220 °C for 3 h and to 200 °C for 5 h. Conversely, the lignin contents increased continuously throughout the treatment due to the loss of polysaccharides and the formation of cross-links. Because of the crystallinity, α-cellulose degradation was slight. According to the analysis of functional groups, FTIR showed treated wood was more hydrophobic than the untreated one.

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Chemical Analysis of Densification, Drying, and Heat Treatment of Scots pine (Pinus sylvestris L.) through a Hot-Pressing Process

Lili Li, Ximing Wang,* and Feiyu Wu

Keywords: Hot-pressing; Heat treatment; Chemical changes

Contact information: College of Material Science and Art Design, Inner Mongolia Agricultural University, Hohhot010018, China;

* Corresponding author: wangximing@imau.edu.cn; w_ximing@263.net

INTRODUCTION

Wood is a renewable natural resource with many attractive characteristics. However, it is also prone to cracking, dimensional changes, and rotting, which considerably limit its outdoor applications (Boonstra and Tjeerdsma 2006; Poletto et al. 2012; Lacic et al. 2014). For these reasons, various chemical, physical, and biological methods have been used to modify wood so that its properties meet industrial requirements (Akgül et al. 2006; Boonstra et al. 2007; Azadfallah et al. 2008; Adewopo and Patterson 2011; Hill et al. 2011; Kutnar et al. 2011; Yin et al. 2011; Brischke et al. 2014; Lacic et al. 2014). Although chemical modifications work quickly, they release a considerable amount of toxic volatiles into the environment (Willems 2014). Therefore, there remains a need for improvements in chemical modification. Biological modifications are more environmentally friendly but also have undesirable aspects, such as long production cycles and limited applications (Wang 2012). Consequently, physical modifications are gaining attention, but it is challenging to simultaneously improve the dimensional stability, decay resistance, and mechanical properties of wood through physical modification.

Currently, compressed wood and thermal modified wood are the most common physically modified materials. Low-quality wood is transformed into compressed wood through hot pressing, which improves the density, mechanical strength, and hardness (Ellis and Steiner 2002;Jung and Lee 2002; Kutnar et al. 2011; Kutnar and Kamke 2012; Rassam et al. 2012; Candan et al. 2013; Rautkari et al. 2013). Compression is an effective way to improve softwood properties and has expanded timber utilization (Inoue et al. 2005; Diaz-vaz et al. 2007). However, elastic recovery results in deformed compressed wood when it is exposed to a humid or moist atmosphere (Fang et al. 2011; Laine et al. 2013; Rautkari et al. 2013), and the densification did not improve the durability (Kutnar et al. 2011), which negatively impacts its industrial applications. Compression ratio plays a key role on the cupping of surface compressed wood: the higher the compression ratio, the easier the wood cupping (Belt et al. 2013). Some researchers considered the measures such as the saturated steam treatment and high temperature treatment, forming cross-linking of chemical components, or the release of inner chemical stresses had an effective impact on the deformation fixation of the compressed wood (Tang 2004; Kutnar and Kamke 2012; Laine et al. 2013).

High temperature treatment as a physical modification is achieved through the heat treatment of wood under high temperatures (160 to 250 °C) with lumber pyrolysis in shielded gas environments (Inari et al. 2006; Boonstra et al. 2007; Korkut et al. 2008; Kocaefe et al. 2013). This method effectively enhances the dimensional stability by relieving internal stress and improves the decay resistance (Park et al. 2014). Aydemir et al. (2011) used FTIR to investigate heat-treated hornbeam and uludag fir woods, which had lower absorbed water and swelling than the control; treated wood at 210 °C for 12 h presented the best dimensional stability. Navickas and Albrektas (2013) studied dimensional changes in oak and found that specimens exposed to 220 °C obtained the least amount of moisture in both a humid atmosphere and water; this result indicated that dimensional stability was improved significantly through heating. Dubey et al.(2012) reported that treated wood became darker and the dimensional stability and fungal resistance were better by the immersion treatment in heating oil. Durability of heat-treated wood exposed for 12 weeks laboratory soil block was improved (Kamden et al. 2002). In addition, with the increasing contact angle, heat-treated wood wettability decreased, indicating the hydrophobic increasement (Cademartori et al. 2013). The quality of heat-treated wood is affected by the duration of treatment, temperature, and wood species (Yildiz et al. 2006). Without any added chemicals or toxic components, thermal wood is considered an environmentally friendly material. However, heat treatment usually causes some mass loss, which reduces mechanical strength. Ratnasingam and Ioras (2013) found heat-treated rubberwood furniture components and joints were easier to subject to fatigue failure than the control samples. In thermally treated wild pear wood, bending and compression strength decreased by about 7.5% after 2 h at 160 °C (Gunduz et al. 2009). Kačíková et al. (2013) researched the MOR, MOE, and the masses of holocellulose, cellulose, and hemicellulose reduced in thermal treated Norway spruce wood. Therefore, heat-treated wood has disadvantages for some industrial applications.

Compressed wood has higher density, better mechanical properties, and better abrasion resistance, whereas heat-treated wood has better dimensional stability and decay resistance. If these characteristics could be combined through multi- modifications, the desirable features could be integrated. This study investigated a promising method that achieves densification, drying, and heat treatment through hot-pressing (Wu and Wang 2015).

To elucidate structural changes within the modified timber, the analyses were primarily focused on mass loss and changes in the main extractive components as determined by thermogravimetry (TG) analysis and other standard methods. Changes in the functional groups of holocellulose, α-cellulose, lignin, and other components were studied by Fourier transform infrared spectroscopy (FTIR).

EXPERIMENTAL

Materials

Scots pine (Pinus sylvestris L.) from Russia Far East was flat-sawn into rectangular specimens with dimensions of 450 mm (longitudinal)×150 mm (tangential)×30 mm (radial), and its original oven dry density was 0.441g/cm³. The number of specimens for each experiment is given in Table 1, and the samples were taken from the same part of one tree. To ensure that plastic deformation occurred, all specimens had an initial moisture content of approximately 30%. The moisture contents of samples were tested by moisture meters (KT-50B, KLONTESER Corporation, Italy), which was checked by weighing method. After hot-pressing, the control specimen and one specimen from each of the 7 heat-treated groups was ground into powder (40-to 60-mesh), and the holocellulose, α-cellulose, and lignin extracted from these specimens were used for thermogravimetric and chemical analysis. The extracted holocellulose, α-cellulose, lignin, and whole wood from each group were ground into 100- to 200-mesh powder for FTIR analysis.

Table 1. Hot-Pressing Conditions

Hot-Pressing Process

Before the hot-pressing experiments, two perforated plates were placed inside the hot press to assist in air and heat circulation. To avoid or minimize crushing during compression, the specimens were compressed to the target thickness (15 mm) in three stages via the position-control procedure, i.e., wood was compressed to a thickness of 25 mm and held for 300 s, to 20 mm and held for 300 s, and finally, to 15 mm. The samples were dried under 2.5 MPa pressure at 160 °C for approximately1 h, and the press was opened every 10 min for 20 s to circulate air and heat. This process was repeated 5 times. Heat treatment experiments were then performed in the designated temperatures and durations (Table 1). Treated specimens were adjusted to a constant temperature (20±0.5 °C) and relative humidity (65±2%) in a temperature humidity chamber.

Thermogravimetric Analysis

The degradation of timber mainly depends on the temperature of heat treatment. Heat causes mass loss and, consequently, diminished mechanical properties, which reduces the value of wood. Thus, experiments were modified to include a safe temperature range that would avoid as much mass loss as possible. The thermal behaviors of holocellulose, α-cellulose, and lignin extracted from Pinus sylvestris were investigated by a simultaneous thermal analyzer (STA; 409 PC, Netzsch Corporation, Germany). Approximately 9 mg of each sample was placed in an Al₂O₃crucible and heated from room temperature to approximately 700 °C at a rate of 10 °C/min and with N₂ flowing at 20 mL/min. The changes in mass loss (TG) and mass loss rate (DTG) were simultaneously measured by the STA.

Chemical Changes of Main Wood Components

Because heat treatment changes the composition of wood, the holocellulose, α-cellulose, hemicellulose, lignin, and extractive contents of treated and untreated specimens were determined. To determine the holocellulose and extractives contents, 100 g of wood powder (40- to 60-mesh) was extracted for 6 h with benzene-alcohol solution (2:1) in a 150-mLSoxhlet extractor (GB/T 2677.6-94), dried thoroughly at 60 °C, and mixed with 0.5 mL of glacial acetic acid, 0.6 g of sodium chlorite, and acetone (GB/T 2677.10-95 1995). The α-cellulose content was determined using the nitric acid-ethanol technique (GB/T 744-89 1989). Hemicellulose was determined using Eq. 1:

Hemicellulose content = (holocellulose content)- (α-cellulose content) (1)

The Klason lignin content was determined by the sulfuric acid hydrolysis method (GB/T 2677.8-94 1994).

FTIR Analysis

Fourier-transform infrared spectroscopy was used to qualitatively and quantitatively examine functional groups in the treated and untreated specimens as well as the extracted holocellulose, α-cellulose, and lignin from each group. A total of 2 mg of each specimen was evenly mixed with 60 mg of KBr and then compressed into a thin film. A film of pure KBr was used for the background spectra, and all spectrograms were obtained from 4000 cm^-1 to 400 cm^-1 by a TENSOR 27 spectrometer (Bruker Corporation, Germany) with a resolution of 4 cm^-1 and with an accumulation of 16 scans. The baseline was corrected at the approximate wavenumbers of 3750 cm^-1, 1850 cm^-1, 833 cm^-1, and 400 cm^-1. Because heat treatment did not dramatically change the peak profiles in the spectra, a quantitative assessment was performed after normalization to the absorption band at1030 cm^-1(C=O stretching vibrations).

RESULTS AND DISCUSSION

Thermogravimetric Analysis

To explore the pyrolysis mechanism, the changes in holocellulose, α-cellulose, and lignin were analyzed in the untreated specimens (Fig. 1, Table 2). The derivative thermogravimetric curves represented three chemical components that went through a two-step degradation—the reduction of water and components content—from room temperature to approximately 700 °C. α-Cellulose and hemicellulose had similar thermal degradation behaviors in a TG-DTG analysis of Gympie messmate wood (Cademartori et al. 2013).

Table 2. Determined Values during Thermal Analysis of Wood Components

Fig. 1. Thermal analysis of components from the control specimen. (a) and (b) represent TG and DTG curves, respectively

The thermograms of holocellulose and α-cellulose showed that this peak (in the DTG curves) corresponded to water evaporation, which resulted in about 3.6% mass loss (ML₁) (i.e., free water, absorbed water, and crystal water). The maximum rate of mass loss (V_m1=0.68 %/min) corresponded to 81 °C (T_m1) for the two chemical components. The second process showed the onset pyrolytic temperature of holocellulose was near 203 °C, whereas for α-cellulose, the depolymerization reaction occurred at a higher temperature (251 °C). However, their end temperatures occurred at about 380 °C. Figures 1(a) and (b) indicated holocellulose mass lost 62.12%, and the maximum rate of mass loss (V_m2) reached 13.03%/min at 334 °C (T_m2); α-cellulose lost 75.16% and its V_m2 was 37.12%/min at 340 °C. According to Cademartori et al.(2013), hemicelluloses and cellulose degrade rapidly from 225 to 320 °C and from 310 to 400 °C, respectively. Thus, the hemicellulose decomposition temperature was lower than that of α-cellulose in the second region.

Lignin also degraded gradually over a wide temperature range (ambient temperature to 700 °C). The two peaks that represented V_m1(0.68%/min) and V_m2(1.87%/min) of the maximum mass loss rate corresponded to T_m1(81°C) and T_m2(391°C). The first stage was the same with the one of holocellulose and α-cellulose (Table 2) representing the disappearance of water content (Manara et al. 2014). Lignin in the second stage was gradually decomposed from 200 to 520 °C, which corresponded to V_m2(1.87%/min) and T_m2(395 °C).

The measured solid residues of holocellulose, α-cellulose, and lignin were 8.0%, 0%, and 42.03%, respectively, at about 700 °C, suggesting that the holocellulose residue mainly consisted of hemicellulose and the main components of wood residue were hemicelluloses and lignin.

Holocellulose contains hemicellulose and cellulose. α-Cellulose is composed of glucose without branches, with a high degree of crystallization and well-organized microfibrils (Zhang et al. 2014). Hemicellulose has a low molecular weight, lacks crystallization, and contains various saccharides, aliphatic side chains and hydrophilic groups. The thermostability of α-Cellulose is superior to that of hemicellulose. The lignin structure contains many benzene rings with various side chains, and it includes more functional groups, which cause deterioration to occur over a wide range of temperatures (Kim et al. 2014). Figure 1 shows that severe degradation occurred at temperatures above 250 °C. Therefore, the temperature range of 180 to 220 °C in this study did not dramatically affect the strength of the treated specimens, where the mass losses of holocellulose, α-cellulose, and lignin were 4.3 to 5.3%, 3.7 to 4.3%, and 4.0 to 4.9% respectively.

Chemical Changes in Main Wood Components

After heat-treatment, the color of specimens became darker and the changes in dimensions were reduced relative to the control. These effects were closely related to changes in the internal structures within main wood components. Therefore, the components extracted from the untreated and treated wood were primarily studied. There were obvious changes in the holocellulose, hemicelluloses, α-cellulose, lignin, and extractives contents after the thermal modification (Tables 3 and 4). Both holocellulose and hemicellulose decreased with increased temperature and duration of treatment. The percentage change in hemicellulose was very dependent on the temperature. Hemicellulose is more susceptible to heating than the cellulose and lignin in wood because it is composed of a variety of sugars, such as xylose, galactose, and arabinose, that dissipate during deacetylation reactions during heating, forming aldehydes including furfural and hydroxymethylfurfural (Yildiz et al. 2006; Esteves et al. 2008; Esteves and Pereira 2009). The decrease in hemicellulose—a highly hydrophilic polymer—reduces the free hydroxyl groups. Because the transformation of carbohydrates blocks fungi growth and reproduction (Dubey et al.2012), heat-modified wood has better dimensional stability and decay resistance.

The α-cellulose content was relatively constant during the heating process, which is closely related its degree of crystallinity (Esteves et al. 2008). The amorphous α-cellulose became degraded, resulting in a small reduction in α-cellulose content and a relative increase in crystallinity compared with the control specimens; this change also increased the hydrophobicity. As the thermogravimetry analysis showed, the absorbed water was lost at low temperatures (<120 °C), whereas the amorphous region of α-cellulose began to degrade at 220 °C. Therefore, the small changes in α-cellulose did not lead to a noticeable decrease in mechanical strength. The diminished percentages of holocellulose were primarily due to the depolymerisation of hemicelluloses in the severe conditions of the heat treatment.

Tables 3 and 4 revealed remarkable increases in lignin during heat treatment. As noted previously (Boonstra and Tjeerdsma (2006); Yildiz et al. (2006), the mass loss of holocellulose played a key role in increasing the relative content of lignin. Due to its low rate of chemical decomposition, lignin is the most thermally stable component; it hinders polymer degradation. The formation of cross-links during lignin condensation also contributes to its increased representation (Esteves and Pereira 2009).

In this study, the extractives content followed a pattern similar to lignin. Most extractives degrade into volatiles when they are exposed to elevated temperatures for an extended duration. Esteves et al. (2008) proposed that although the original extractives became volatilized during the heat treatment, the measured extractive content first increased and then decreased. Because of the degradation of hemicellulose and lignin, new extractives were freshly generated. The results in Tables 3 and 4 support this hypothesis.

Table 3. Chemical Components in Wood Treated at Different Temperatures

Table 4. Chemical Components in Wood Treated for Different Durations

FTIR Analysis

During the heating process, α-cellulose, hemicellulose, and lignin of Pinus sylvestris underwent chemical changes that affected some physical and mechanical properties of the treated wood. To understand these changes, FTIR spectra were collected between 1800 and 800 cm^-1 for each heat-treated wood and its extracted holocellulose, α-cellulose, and lignin. However, several simultaneous chemical reactions prevented accurate analysis between treated and untreated wood by spectrometry. Nevertheless, some functional groups changed with increasing heating temperature and duration of treatment.

FTIR analysis of the whole wood of each specimen

The C=O stretching vibrations from 1700 to1750 cm^-1represent acetoxy groups in hemicellulose and carbonyl groups in lignin (Fig. 2). Boonstra and Tjeerdsma (2006), Kocaefe et al. (2008), and Dubey et al. (2012) found that the absorption intensity at 1740 cm^-1decreased during the heat treatment, demonstrating that hemicellulose is not stable and is easily degraded by a high temperature treatment. In contrast, this study revealed that the peak areas at 1732 cm^-1 increased with rising temperature, especially at 220 °C, and did not decrease (Fig. 2(a) and Table 5). A possible explanation is that softwood contains more lignin than hardwood, and the increased carbonyl groups were primarily due to the resultant condensation of lignin, which is confirmed by the spectra of lignin at1710 cm^-1(Fig. 5(a)).

Fig. 2. FTIR spectra of the whole wood at different temperatures (a) and durations (b)

With respect to the esterification reaction of the acid and hydroxyl radicals in wood, similar results were obtained by Kotilainen et al. (2000), Tjeerdsma and Militz (2005), and Esteves et al. (2013). During esterification, the absorption intensity at 1269 cm^-1 increases because of vibrating acetyl ester groups, which was observed in the specimens (Table 5). This result confirmed the esterification assumption at 1732 cm^-1(Esteves et al. 2013). Similarly, the IR absorption at 1732 cm^-1 increased slightly with increasing time (Fig. 2(b) and Table 6). A similar explanation to the above-mentioned can be given. The spectra clearly showed that the heating temperature had a much greater influence on the peak areas than the duration of heat treatment.

Table 5. Area Ratios of Whole Wood Peaks (Different Temperatures)

Table 6. Area Ratios of Whole Wood Peaks (Different Durations)

The band at 1652 cm^-1(absorption of water) decreased with increasing heating temperature in comparison to the control (Aydemir et al. 2011). In this study, the intensity change was not evident between 180 °C and 200 °C, but the peak disappeared entirely at 220 °C. At the same time, the extended duration of treatment slight diminished the absorption band.

The bands at 1269 cm^-1, 1458 cm^-1, 1510cm^-1, and 1606 cm^-1were assigned to guaiacyl ring breathing with CO stretching, asymmetric bending in CH₃, aromatic skeletal vibrations, and C=O conjugated to aromatic rings, respectively (Yin et al. 2011; Huang et al. 2012; Meng et al. 2013). These patterns are characteristic of lignin and markedly increased with increasing temperature and duration of the treatment (Tables 5 and 6). However, the peaks were more affected by temperature than by the duration of treatment.

These results also implied that the relative increase in lignin content was a result of carbohydrate degradation (Kotilainen et al. 2000; Kocaefe et al. 2008). An increase at 1269 cm^-1was associated with the esterification of acid and hydroxyl that reduced the hydroxyl content and increased the hydrophobicity of the wood. Moreover, the absorption at 1510 cm^-1signaled the cleavage of the aliphatic side chains and the cross-linking formation by means of condensation polymerization in lignin, which leads to decreased hygroscopicity and improved dimensional stability of wood (Kocaefe et al. 2008). The behavior near 1606 cm^-1can be interpreted by the C=O increase that is linked to benzene rings (Yin et al. 2011). Similar results were observed for the duration of the treatment.

The characteristic peaks of cellulose are located at 1317 cm^-1, 1372 cm^-1, and 1427 cm^-1, which correspond to O-H in plane bending, C-H bending vibration, and CH₂ scissoring, respectively (Nelson and O’Connor 1964). Spectra alterations at 1317 cm^-1appeared to be affected by the water absorption inside cellulose (Nelson and O’Connor 1964). As previously explained, a decrease in adsorbed water at 1658 cm^-1involved the O-H loss at 1317 cm^-1 with increased heating temperature and duration of treatment. The increased area ratio at around 1372 cm^-1 may be raised by the modified conformation of the glycosidic linkage (Hakkou et al. 2005). The decrease in the 1427cm^-1band was attributed to the reduction of amorphous cellulose.

FTIR analysis of holocellulose from each sample

The absorption band at 1732 cm^-1became reduced slightly at a high temperature and an extended duration, which indicated the unconjugated carbonyl loss of hemicelluloses (Fig. 3(a) and (b); Tables 7 and 8). Similarly, the peak at 1643 cm^-1(absorbed water) had no remarkable distinction under mild conditions, while the obtained area ratio decreased sharply at 200 °C for 5 h and at 220 °C for 3 h. Thus, more moisture was lost under severe conditions.

The gradual reduction of CH₂ bending at 1429 cm^-1 (in cellulose) confirmed that cellulose has good thermal stability. Also, the absorption band at 1240 cm^-1was assigned to the C-O linkage in acetyl groups of xyloglucan (Popescu et al. 2011); compared with the control, it was diminished during the heat treatment. This result could be due to the hydrolysis of acetyl groups, which breaks acetyl bonds and forms carbonic acids during hemicellulose thermal degradation (Kocaefeet al. 2008). In contrast, the band at 1110 cm^-1corresponding to the C-O bond increased during the treatment process, which suggested that some alcohols and esters were regenerated viachemical reactions (Kacikova et al. 2013). In conclusion, the FTIR spectroscopy results were consistent with the chemical analysis.

Fig. 3. FTIR spectra of holocellulose of each specimen at different temperatures (a) and durations (b)

Table 7. Area Ratios of Holocellulose Peaks (Different Temperatures)

Table 8. Area Ratios of Holocellulose Peaks (Different Durations)

Fig. 4. FTIR spectra of α-cellulose of each specimen at different temperatures (a) and durations (b)

Table 9. Area Ratios of α-Cellulose Peaks (Different Temperatures)

Table 10. Area Ratios of α-Cellulose Peaks (Different Durations)

FTIR analysis of α-cellulose of each specimen

While the spectra for whole wood had the typical lignin-associated peaks at1606 cm^-1, 1510 cm^-1, and 1458 cm^-1(Fig. 4(a) and(b)), these peaks were absent in the α-cellulose spectra. The content of absorbed water near 1644 cm^-1was lost throughout the entire treatment, but the change was very small (Tables 9 and 10). This effect could be attributed to the crystalline structure of cellulose, which hinders moisture migration. When exposed to high temperature and longer duration, the band at 1372 cm^-1decreased considerably, which further verified the decomposition of amorphous cellulose. The absorption band at 1161 cm^-1was assigned to the antisymmetric bridge C-O-C stretching of β-(1-4)-glucosidic bond (Oh et al. 2005); it had no obvious change. Therefore, FT-IR of α-cellulose agreed with its chemical analysis (Tables 3 and 4).

Fig. 5. FTIR spectra of lignin of each specimen at different temperatures (a) and durations (b)

Table 11. Area Ratios of Lignin Peaks (Different Temperatures)

Table 12. Area Ratios of Lignin Peaks (Different Durations)

FTIR analysis of lignin from each specimen

In the FTIR spectra of lignin, the absorption intensities at 1714 cm^-1 and 1602 cm^-1were assigned to carbonyl groups and C=O stretching in the aromatic skeleton (Nada et al. 1998; Ke et al.2011). These peaks increased with increasing temperature and duration of treatment compared with the untreated wood. Additionally, the vibrations of aromatic ring were located at 1509 cm^-1and 1426 cm^-1. The spectra at 1457 cm^-1represented the C-H deformation in CH₂, CH₃, and benzene ring, and the peaks at 1257 cm^-1 and 1214 cm^-1were characteristic of C-O from the guaiacyl group and aromatic skeleton, respectively (Manara et al. 2014). Notably, these peaks often increased during the heat modification (Fig. 5(a) and (b); Tables 11 and 12), especially under severe conditions (220 °C for 3h and 200 °C for 5h), which indicated that the relative percentage of lignin in the treated specimens was indeed higher than in the untreated specimens. This result was attributed to the degradation of other polymers and volatile extractions (Nuopponen et al. 2004). Hemicellulose and α-cellulose are more hydrophilic than lignin due to their different chemical structures. Therefore, increased hydrophobic lignin improves the dimensional stability of the treated wood because the thermal treatment cross-links lignin and initiates esterification. Cross-linking reaction in the lignin network is more important to the eventual decrease in water absorption than esterification. Furthermore, the peaks located at 1140 cm^-1 and 1084 cm^-1, which were related to C-H deformation in the plane of guaiacyl aromatic nucleus and C-O deformation and ether bond in the secondary alcohols, respectively, also demonstrated distinct peak shapes. Therefore, the thermal modification method by hot-pressing is worth further investigation.

CONCLUSIONS

Lignin was shown to be the most thermally stable component, with a maximum rate of mass loss of 1.87% per min vs. holocellulose (13.03% per min) and α-cellulose (37.12% per min). From 180 °C to 220 °C, approximately 5% of holocellulose, cellulose, and lignin were lost. Hence, the effects of thermal modification on mechanical properties were ignored.
During thermal modification, the holocellulose and hemicellulose contents changed considerably due to polysaccharide degradation and acetyl group hydrolysis. In contrast, lignin content tended to increase with increasing severity of the heat treatment, which primarily resulted from the reduction of holocellulose and hemicellulose in spite of the occurrence of condensation.
FTIR analysis confirmed that the new thermal modification method effectively changed the internal structure and chemical components of Pinus sylvestris. The dimensional stability and decay resistance were improved by the reduction of hydrophilic groups.

ACKNOWLEDGMENTS

The authors gratefully acknowledge financial support from the International Science and Technology Cooperation Program of China (2013DFA32000) and Inner Mongolia Agricultural University.

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Article submitted: October 29, 2015; Peer review completed: January 31, 2016; Revised version received and accepted: February 26, 2016; Published: March 11, 2016.

DOI: 10.15376/biores.11.2.3856-3874