Abstract
Fourier-transform infrared spectroscopy (FTIR), energy dissipation spectrometry, dimensional stability, and compressive strength tests were conducted parallel to the fibers on various heat-treated trees, i.e., narrow-leaved ash (Fraxinus angustifolia Vahl), aspen (Populus tremula L.), Oriental spruce (Picea orientalis (L.) Link.), and Uludağ fir (Abies bornmüelleriana Mattf.), which grow naturally in forests in Turkey. Panels made from these trees were first heat treated via the ThermoWood® method, producing ThermoWood® panels. Then, FTIR, as well as energy dissipation spectrometry analysis, dimensional stability, and compressive strength tests were performed on test samples prepared from the panels. The FTIR data showed that the hemicelluloses were degraded in the ThermoWood® test samples and the proportion of cellulose increased. The energy dissipation spectrometry results showed that the amount of carbon increased, the amount of oxygen decreased, while the amount of hydrogen remained approximately the same in the ThermoWood® panels compared to the control samples. It was determined that the proportion of silicon increased in the narrow-leaved ash panels. In addition, among the physical properties, the amount of shrinkage and swelling decreased in all the tested ThermoWood® panels compared to the control samples, whereas the compressive strength values, which are considered a mechanical property, increased.
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Chemical, Hygroscopic, and Mechanical Properties of Various Wood Species Heat Treated via the ThermoWood® Method
Ayhan Aytin,a,* Nevzat Çakıcıer,b and Turgay Birtürk c
Fourier-transform infrared spectroscopy (FTIR), energy dissipation spectrometry, dimensional stability, and compressive strength tests were conducted parallel to the fibers on various heat-treated trees, i.e., narrow-leaved ash (Fraxinus angustifolia Vahl), aspen (Populus tremula L.), Oriental spruce (Picea orientalis (L.) Link.), and Uludağ fir (Abies bornmüelleriana Mattf.), which grow naturally in forests in Turkey. Panels made from these trees were first heat treated via the ThermoWood® method, producing ThermoWood® panels. Then, FTIR, as well as energy dissipation spectrometry analysis, dimensional stability, and compressive strength tests were performed on test samples prepared from the panels. The FTIR data showed that the hemicelluloses were degraded in the ThermoWood® test samples and the proportion of cellulose increased. The energy dissipation spectrometry results showed that the amount of carbon increased, the amount of oxygen decreased, while the amount of hydrogen remained approximately the same in the ThermoWood® panels compared to the control samples. It was determined that the proportion of silicon increased in the narrow-leaved ash panels. In addition, among the physical properties, the amount of shrinkage and swelling decreased in all the tested ThermoWood® panels compared to the control samples, whereas the compressive strength values, which are considered a mechanical property, increased.
DOI: 10.15376/biores.17.1.785-801
Keywords: ThermoWood®; Fourier-transform infrared spectroscopy; Compressive strength; Energy dissipation spectrometer; Dimensional stability
Contact information: a: Interior Design, Department of Design, Duzce Vocational School, Duzce University, Duzce 81100 Turkey; b: Faculty of Forestry, Forest Industrial Engineering Department, Duzce University, Duzce 81100 Turkey; c: Faculty of Forestry, Forest Engineering Department, Duzce University, Duzce 81100 Turkey; *Corresponding author: ayhanaytin@duzce.edu.tr
INTRODUCTION
Since the 1990s, heat treatment has been accepted as an environmentally friendly production method for wood materials and has begun to be used more frequently in the market. With this application, the dimensional stability of wood materials is improved and resistance to biological degradation is ensured. Furthermore, the wood color is darkened, and it becomes possible to obtain a more homogeneous color. Thus, the positive changes that occur with heat treatment, particularly in the physical properties of the wood materials, bring new opportunities in the use of wood materials (Aytin 2013; Shen et al. 2016).
Wood has a complex structure, ranging from low molecular compounds to polymeric compounds. In order to understand the numerous changes that occur in the physical and mechanical structure of wood during the heat treatment process, it is necessary to know the chemical composition of the wood, as well as the basic characteristics and the physical properties of the primary components that make up its structure (Korkut and Kocaefe 2009). In heat-treated wood, the cell wall components exhibit different behavior from each other under the effect of temperature (Sundqvist 2004). For example, while the change in lignins and hemicelluloses begins to be seen at a temperature of approximately 120 °C, cellulose begins to be affected at higher temperature values (ThermoWood 2003).
The changes in the wood material structure due to heat treatment and the results of these changes are worth researching and examining for various reasons, especially in order to benefit from the material more rationally. Numerous studies have been conducted on the chemical structure of wood materials and the effect of heat treatment on the chemical structure, as well as the reflection of this effect on the physical properties and mechanical resistance values, which are important for determining its usage. One of these studies reported that the heat treatment of Eastern beech wood (Fagus orientalis L.) at 150, 175, and 200 °C for 1, 3, and 5 h increased the weight and density loss and improved the dimensional stability (DS) at high temperatures (Perçin et al. 2016). However, it was reported that there was a small increase in the bending strength and elastic modulus in terms of the bending and hardness values. In addition, an increase was observed in the compressive strength (CS) resistance following treatment at a temperature of 200 °C, except for a treatment time of 5 h, and the mechanical resistance values decreased as the heat treatment temperature increased). Another study examining the bamboo tree found that the cellulose and lignin content increased due to heat treatment and the hemicellulose content decreased (Wang et al. 2020). It was observed that the hygroscopicity decreased, and the strength and flexibility increased (at temperatures of 180 and 200 °C and different moisture contents) in heat-treated ash wood (Fraxinus excelsior L.) (Roszyk et al. 2020). Considering the results that improved the moisture properties and equilibrium moisture content of laminated strand lumber (LSL) and laminated veneer lumber (LVL), which were heat treated at different temperatures, it was stated that it might be possible to obtain products with the desired properties in terms of strength (Aroa et al. 2016). Based on the results of Fourier-transform infrared (FTIR) spectroscopy performed after the heat treatment of oak wood via the ThermoWood® method at a temperature of 160, 180, and 210 °C, Kubovský et al. (2020) stated that hemicelluloses were less stable than celluloses. They emphasized that thermal modification causes chemical changes in the primary components of wood materials, and it is important to consider the changes in terms of mechanical strength, especially in wood structure design. However, Özgenç et al. (2017) determined the chemical changes in scotch pine (Pinus sylvestris L.), Eastern beech (Fagus orientalis L.), and Oriental picea (Picea orientalis L.) trees after heat treatment at different temperatures using attenuated total reflection (ATR) FTIR and FT Raman spectrometry. The findings revealed that the effects on the carbohydrate and lignin peaks varied across the tree species. Based on the spectral changes that occurred after the heat treatment, it was emphasized that the hemicelluloses degraded, the free hydroxyl groups decreased, and the crystalline cellulose proportion increased due to the degradation of amorphous cellulose at high temperatures. The FTIR analysis of beech (Fagus sylvatica L.) wood after heat treatment via the ThermoWood® method revealed that the changes in the chemical structure were related (Timar et al. 2016).
When the previous studies are examined, it can be seen that the changes in the chemical structure of heat-treated wood and the physical and mechanical properties that determine its areas of use are related. It is known that the physical, mechanical, chemical, and anatomical properties of tree species that grow in different geographical regions can change. For this reason, it is important to investigate the post-heat treatment properties of tree species that grow naturally in different climatic regions, even for the same tree species.
This study aimed to examine the changes in two hardwoods and two softwoods species growing naturally in Turkey after heat treatment. To this end, the chemical structure changes were investigated using FTIR spectroscopy, and elemental analysis was performed via energy dissipation spectrometry (EDS). In addition, the compressive strength parallel to the fibers and the dimensional stability values were determined.
EXPERIMENTAL
Materials
The tree species used in this study were selected among the species that have an important sectoral use and potential, according to TS standard 4176 (1984). Tree species and their geographic growing areas are shown in Fig. 1 and Table 1. The map was obtained from the Internet, and the tree species were marked on the places where they were taken (Google 2020).
Fig. 1. The areas where the tree species used in this study grow
Table 1. Production Areas Where the Trees in the Study were Obtained
Methods
Heat treatment application
Tree samples prepared with dimensions of 26 mm x 100 mm x 600 mm by giving dimensional tolerances from trial trees were subjected to heat treatment at a temperature of 190 °C for 1 h (TW1) and 212 °C for 1 h (TW2) using the ThermoWood® method at the Novawood Forest Products Factory (Gerede/Bolu). The ThermoWood® trees obtained were kept in an air-conditioning room with control samples at an average temperature of 20 °C ± 2 °C and a relative humidity of 60% ± 5% until they reached a constant weight.
Determining the amount of shrinkage and swelling
The amount of shrinkage was determined in longitudinal, radial, tangential, and volumetric terms in accordance with the principles of TS standard 4083 (1984) and TS standard 4085 (1983). According to TS CEN/TS standard 15679 (2010), a total of 120 shrinkage samples, 30 samples for each tree species (10 control samples, 10 samples of 190°C ThermoWood®, and 10 samples of 212°C ThermoWood®), were prepared and kept in water until there was no change in their size. Then, the dimensions of the samples were measured with a precision digital caliper (up to ± 0.001 mm), and their dimensions when they were moist were determined. The samples were kept under normal atmospheric conditions for two weeks and then dried in a drying cabinet at a temperature of 103 °C ± 2 °C until their weight remained constant. The samples were left to cool in a desiccator with CaCl2 and the exact dimensions when they were dry were found by measuring again at the first measurement points, and calculated using Eq. 1,
β = (Ry – R0)/Ry × 100 (%) (1)
where β is the amount of shrinkage (%), Ry is the wet measurement (mm), and R0 is the full dry measurement (mm).
The amount of swelling was determined in longitudinal, radial, tangential, and volumetric terms in accordance with the principles of TS standard 4084 (1984) and TS standard 4086 (1983). According to TS CEN/TS standard 15679 (2010), a total of 120 shrinkage samples, 30 samples for each tree species (10 control samples, 10 samples of 190 °C ThermoWood®, and 10 samples of 212 °C ThermoWood®), were prepared. They were dried in a drying cabinet at a temperature of 103 °C ± 2 °C until their weight remained constant. Then, after cooling in a desiccator containing CaCl2, the lengths between the points marked on the two opposite sections were measured with a precision digital caliper (up to ± 0.001 mm). The samples were kept in water until there was no change in their size. Their dimensions at humid conditions were measured with a ± 0.001 mm precision digital caliper. The swelling percentages (α) for the dimensions at moist and fully dry conditions were determined as a % using Eq. 2,
α = (Ry – R0)/R0 × 100 (%) (2)
where α is the amount of swelling (%), Ry is the wet measurement (mm), and R0 is the full dry measurement (mm).
Determining the compressive strength parallel to the fibers
The compressive strength parallel to the fibers was determined according to the principles of TS standard 2595 (1977). A total of 120 test samples were prepared, 10 for each variation, with a cross-section of 20 mm x 20 mm x 30 mm. Before the experiment, the cross-sectional dimensions of the test samples were measured with a caliper that can measure with an accuracy of ± 0.01 mm. Their weights were measured on a 0.01 g precision scale, and then were placed on a universal testing machine with the force direction parallel to the fiber. The universal testing machine was operated at a speed of 6 mm/min to ensure that the breakage occurred 1.5 min to 2 min after the loading time. The compressive strength parallel to the fibers was calculated using Eq. 3,
σw = Ρmax / Α (N/mm2) (3)
where σw is the CS (N/mm2), Pmax is the maximum force at break (N), and A is the cross-sectional area of the sample (mm2).
After testing the pressure parallel to the fibers, the humidity values were determined according to TS standard 2471 (1976). The following equation was used to set the values of the CS to 12% for the control samples with humidity values different from 12%, as shown in Eq. 4,
σW12 = σR [1+α(R-12)] (N/mm2) (4)
where σW12 is the CS for air-dry fibers (N/mm2), R is the moisture (%), σR is the CS of the measured fibers (N/mm2), and α is the correction factor (0.05).
Fourier transform infrared (FTIR) spectroscopy analysis
Fourier transform infrared (FTIR) spectroscopy was recorded with an ATR Attached Shimadzu IR Prestige 21 spectrometer. Spectrum analysis was performed between 400 and 4000 nm, with 64 scans (Demirci 2018). The ATR Attached Shimadzu IR Prestige 21 spectrometer is shown in Fig. 2.
Fig. 2. The ATR Attached Shimadzu IR Prestige 21 spectrometer
Energy dissipation spectrometry (EDS) analysis
The elemental analysis was carried out with a Thermo Scientific Flash 2000 instrument. The device is designed for the unattended and fully automatic determination of CHNS and oxygen in any sample (Thermo Scientific 2021).
Statistical evaluation
The SPSS package program (IBM, SPSS 15.0 for Windows, Armonk, NY) was used for the statistical evaluation of the data. An analysis of variance was used to determine whether the factors were effective for the results obtained, and Duncan’s test was used to determine the size of the difference on the factors that were found to be significant.
RESULTS AND DISCUSSION
Results Regarding the Amount of Shrinkage and Swelling
The basic analysis of variance (BVA), statistics, and Duncan test results regarding shrinkage values according to tree species are given in Table 2.
Table 2. Shrinkage Values for Each Tree Species (Basic Analysis of Variance (BVA) Results, Statistics, and Duncan’s Test Results)
Table 3. Swelling Values for Each Tree Species (Basic Analysis of Variance (BVA) Results and Statistics)
The highest volumetric shrinkage value was determined as 12.61 in the UF control samples, and the lowest volumetric shrinkage value was 6.23, from the PT 212 °C ThermoWood® samples. In all ThermoWood® samples, the amount of volumetric shrinkage in the control samples decreased. The maximum decrease in the volumetric shrinkage was 50.47% in the PT 212 °C ThermoWood® test samples, and the lowest decrease was 28.14% in the OP 212 °C samples.
The BVA results and statistics regarding the swelling values of each tree species are given in Table 3. The highest volumetric swelling value was 16.33, which was in the PT control samples, and the lowest value was 5.20, which was in the PT 212 °C ThermoWood® samples. The volumetric swelling amount increased in all the ThermoWood® samples compared to the control samples. The maximum increase in volumetric swelling was 68.15% in the PT 212 °C ThermoWood® samples, and the lowest was in the OP 212°C samples, with an increase of 33.51%.
The shrinkage and swelling results obtained in the study are compatible with the results found in previous literature. Boonstra (2008) states that dimensional stability increases as a result of decreasing shrinkage and swelling in heat-treated wood material. Bak and Nemeth (2012) found that the dimensional stability of poplar wood, which they heat treated at different temperatures ranging from 160 to 200 °C, considerably improved. Li et al. (2011) reached similar results in their study, in which they investigated the effect of high temperature on some physical properties of Douglas fir wood by applying heat treatment at a temperature of 160, 180, 200, and 220 °C for 1, 2, 3, or 4 h. In their study with Eastern beech, Kol et al. (2017) found that the amount of shrinkage was higher in the heat-treated samples compared to the control samples as the heat treatment temperature increased. The results of the study show that the shrinkage difference of 14.31% at a temperature of 170 °C compared to the control samples increased to 54.28% at a temperature of 212 °C. Şahin and Güler (2018) stated in their study on ash wood that the dimensional stability value could be improved up to 58.4% by increasing the heat treatment temperature to 210 °C. İçel et al. (2015) revealed in their study that up to a 58% improvement in the dimensional stability in European spruce and 52% in Scotch pine can be achieved with heat treatment.
Findings regarding compressive strength (CS) parallel to the fibers
The BVA results and statistics regarding the CS values for the PT, FA, OP, and UF samples are given in Table 4. Out of all the ThermoWood® products in all tested tree species, the highest percentage increase was 17.37% and 21.1% in PT and UF, respectively, when compared to the control samples. In the narrow leaf ash tree and OP samples, the CS value was determined as 7.83 and 4.42, respectively, compared to the control samples.
In their study with Eastern beech (Fagus orientalis Lipsky) wood, Perçin et al. (2016) revealed that the compressive strength increased with heat treatment. Özçifçi et al. (2009) and Sefil (2010) found that compressive strength parallel to the fibers increased as the heat treatment temperature increased. In another study, it is stated that the strength in wood material is related to the covalent and hydrogen polymer bonds (Çalıova 2011). The increase in the amount of bound water and the weakening of the hydrogen bonds between the polymeric compounds of the cell wall decrease the strength properties. However, it is also emphasized that the increase in compressive strength parallel to the fibers is due to the decrease in the amount of equilibrium humidity in heat treated trees. Thus, it is stated that heat treated wood is less hygroscopic, and since the maximum amount of bound water is reduced, heat treatment can positively contribute to its strength properties (Aytin 2013).
Table 4. Basic Analysis of Variance (BVA) Results and Statistics Regarding the Compressive Strength Values
Fourier-transform infrared (FTIR) spectroscopy and energy dissipation spectrometer (EDS) analysis
The FTIR analyses for the PT are given in Fig. 3, and the frequency reactions and EDS results of the functional groups are given in Table 5.
Fig. 3. FTIR analysis of the PT samples
Table 5. Reactions in the Aspen (PT) Bond Structure and the Energy Dissipation Spectrometry (EDS) Results
The changes between the peaks of the wood carbohydrates and lignins in the neighborhoods of 1730 and 650 cm-1, which are the characteristic peaks of the wood structure in the ThermoWood® PT samples (as shown in the spectrum in Fig. 3). The presence of peaks at 1105, 1030, and 892 cm-1, which are the primary lignin peaks according to the spectrum, indicated that the cellulose structure has started to deteriorate in the ThermoWood® samples. However, the peaks observed at wavelengths of 1028 and 3077 cm-1, representing hemicelluloses, indicated that the intensity of the peak decreased, and the structure was less converted to a cellulose structure.
The FTIR analyses for the FA samples are given in Fig. 4, and the frequency reactions and EDS results of the functional groups are given in Table 6.
Fig. 4. FTIR analysis of the FA samples
The changes in the lignin peaks and wood carbohydrate peaks between 1800 cm-1 and 650 cm-1, which are the characteristic peaks of the wood structure in the ThermoWood® FA samples, are given in the spectrum in Fig. 4. According to the spectrum, the presence of peaks at 1105, 1030, and 892 cm-1, which are the primary lignin peaks, shows that the cellulose structure deteriorated in the TW1 and TW2 samples. However, the peaks observed at wavelengths of 1028 and 3077 cm-1 are the peaks representing the hemi-cellulose found in the wood structure.
Table 6. Reactions in the Narrow-leaved Ash (FA) Bond Structure and the Energy Dissipation Spectrometry (EDS) Results
Fig. 5. FTIR analysis of the OP samples
The FTIR analysis for the OP samples is given in Fig. 5, and the frequency reactions and the EDS results of the functional groups are given in Table 7. According to the spectrum of the wood structure of the ThermoWood® PO samples, the presence of the primary peaks of lignins, i.e., 1105, 1030, and 892 cm-1, indicated that the TW1 and TW2 cellulose structures of the samples had started to deteriorate. However, the peaks observed at wavelengths of 1028 and 3077 cm-1 are the peaks that representing the hemicellulose in the wood structure. In addition, it was observed that the intensity of the peaks decreased at TW2, but the intensity of the peaks of the structure increased at TW1 and the structure started to transform into a cellulose structure.
Table 7. Reactions in the Oriental Picea (OP) Bond Structure and the Energy Dissipation Spectrometry (EDS) Results
Fig. 6. FTIR analysis of the UF samples
The FTIR analyses for the UF samples are given in Fig. 6, and the frequency reactions and the EDS results of the functional groups are given in Table 8. Considering the spectrum of the wood structure in the ThermoWood® UF samples, the primary peaks of lignins, i.e., 1105, 1030, and 892 cm-1, show that the cellulose structure deteriorated in TW1 and TW2. However, it was observed that the peaks observed at wavelengths of 1028 and 3077 cm-1 are the peaks representing the hemicellulose found in the wood structure. In addition, it was found that the intensity of the peaks of the structure in the TW1 and TW2 samples decreased, and that the structure turned into a lower amount of cellulose.
Table 8. Reactions in the Uludağ Fir (UF) Bond Structure and the Energy Dissipation Spectrometry (EDS) Results
The results of the elemental analysis revealed that the proportional amount of carbon increased in all ThermoWood® products compared to the control samples, and the highest increase was in UF, with a 10% increase. In contrast to the carbon content, the oxygen content decreased in all the ThermoWood® samples compared to the control samples, and it was revealed that the highest decrease was in the FA samples. It was emphasized that the changes in the physical properties were caused by the important changes in the structure and chemical composition of the wood material during the modification process.
According to Hill (2006), the heat treatment of wood considerably reduces its water adsorption. The presence and/or accessibility of free hydroxyl groups in wood carbohydrates play an important role in water adsorption and desorption. It is obvious that there is a decrease in the accessibility to the free hydroxyl groups as a result of the heat treatment application. A reason for this is the depolymerization of the carbohydrates, especially hemicellulose, which leads to a decrease in the total amount of free hydroxyl groups. Another reason is the increase in the relative ratio of crystalline cellulose, where hydroxyl groups cannot easily reach water molecules, and the cross-linking of the lignin network, which prevents the access of free hydroxyl groups to water.
According to the results of Niemz et al. (2010), in which some physical, mechanical, and chemical properties of various industrially important hard and soft woods were measured and evaluated after heat treatment in a N2 atmosphere, the chemical changes affecting the physical properties could be investigated and monitored by establishing correlations between the physical and chemical properties. It was also emphasized that the correlation between the hard wood samples was stronger than between the soft woods.
CONCLUSIONS
- Within the scope of the study, after the heat treatment of aspen (Populus tremula L.), narrow-leaved ash (Fraxinus angustifolia Vahl), Oriental spruce (Picea orientalis (L.) Link.), and Uludağ fir (Abies bornmüelleriana Mattf.) trees, which naturally grow in the forests of Turkey, the dimensional stability (as a physical property), compressive strength parallel to the fibers (as a mechanical property), and bond structure (as a chemical property) were investigated with Fourier transform infrared (FTIR) and elemental analysis and it has been understood that the results obtained are in accordance with the literature on the subject.
- The dimensional stability test results revealed that the amount of shrinkage and swelling decreased in all the heat-treated tree species compared to the control (COS) samples. The greatest change was found in the COS samples, while the lowest change as found in TW2 test samples (heated at 212 °C for 1 h). The volumetric shrinkage values of TW2 samples occurred in the Populus tremula (PT), Fraxinus angustifolia (FA), Picea orientalis (OP), and Abies bornmüelleriana (UF) trees, 50.48, 34.58, 28.10, and 56.46%, respectively, being lower than COS samples; the volumetric swelling values of TW2 samples occurred in the PT, FA, OP and UF trees, 68.15, 50.46, 33.52, and 56.85%, respectively, being lower than COS samples. The chemical analysis results clearly support the reasons underlying the increase in dimensional stability. Similarly, the conversion of the wood structure to a cellulose structure via heat treatment, the increase in free hydroxyl groups, and the increase in crystalline cellulose support the trend that indicates an increase in the compressive strength parallel to the fibers. The lowest change was found in the COS samples, while the greatest change was found in all the heat-treated test samples. According to compressive strength parallel to the fibers values of heat-treated test samples occurred in the PT, FA, OP and UF trees, 17.37, 7.38, 4.42, and 26.46%, respectively being highest than COS samples.
- The findings of the study are important in terms of optimizing the existing resources for the use of natural resources. Finding materials that can be evaluated by the heat treatment industry, which is an environmentally friendly production system, and the creation of alternatives are the other option. The scientific contribution of the results should be taken into account.
ACKNOWLEDGMENTS
This study was supported by the Düzce University, Scientific Research Project (No. BAP-2015.21.07.314). The authors thank the Düzce University Scientific Research Projects Coordinatorship for their support to project No. BAP-2015.21.07.314.
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Article submitted: October 26, 2021; Peer review completed: November 30, 2021; Revised version received and accepted: December 2, 2021; Published: December 8, 2021.
DOI: 10.15376/biores.17.1.785-801