Abstract
The physical and mechanical properties of thermally modified wood impregnated with linseed oil were investigated to find the relationship between the treatment and the acoustic properties. The samples were impregnated with linseed oil (LO). Heat treatment was performed in an oven at four different temperatures, namely 160, 180, 210, and 240 °C. Statistically, the physical, mechanical, and acoustic properties of the treated wood were significantly intensified as compared to the control samples. The physical properties (water absorption) of the treated wood decreased by 72% as compared to the control group. However, while the increase in both MOR and MOE at 240 °C was 13%, the increase in CS at 240 °C was 7%. As a result of the heat treatment, the porosity increased by 33% as compared to the control group. At 240 °C the maximum SAC value was observed to be 0.71 at 5000 Hz and at 4500 Hz 0.78 and 0.80, respectively. The highest STL value, which was 69.9 dB, was observed at 1540 Hz and 3600 Hz. As a result of improved heat transfer, impregnating samples with LO before thermal modifications was observed to increase the efficiency of thermal modification.
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Combined Effects of Linseed Oil and Heat Treatment on the Properties of Cypress and Maple Wood Part 1: Water Absorption, Mechanical Properties, and Sound Absorption Capacity
Ali Ihsan Kaya *
The physical and mechanical properties of thermally modified wood impregnated with linseed oil were investigated to find the relationship between the treatment and the acoustic properties. The samples were impregnated with linseed oil (LO). Heat treatment was performed in an oven at four different temperatures, namely 160, 180, 210, and 240 °C. Statistically, the physical, mechanical, and acoustic properties of the treated wood were significantly intensified as compared to the control samples. The physical properties (water absorption) of the treated wood decreased by 72% as compared to the control group. However, while the increase in both MOR and MOE at 240 °C was 13%, the increase in CS at 240 °C was 7%. As a result of the heat treatment, the porosity increased by 33% as compared to the control group. At 240 °C the maximum SAC value was observed to be 0.71 at 5000 Hz and at 4500 Hz 0.78 and 0.80, respectively. The highest STL value, which was 69.9 dB, was observed at 1540 Hz and 3600 Hz. As a result of improved heat transfer, impregnating samples with LO before thermal modifications was observed to increase the efficiency of thermal modification.
DOI: 10.15376/biores.18.2.2940-2963
Keywords: Impregnation; Heat treatment; Linseed oil; Water resistance; Mechanical properties; Sound absorption capacity
Contact information: Department of Material Science, Technical Sciences Vocational School, Burdur Mehmet Akif Ersoy University, Bahcelıevler, Burdur 15100, Turkey;
* Corresponding author: aliihsankaya21@gmail.com
INTRODUCTION
Noise has a negative effect on the comfort of people living in urban environments (Zannin et al. 2002; Szeremeta and Zannin 2009; Hunashal and Patil 2012). The source of noise can be indoors or outdoors (Tang and Wang 2007; Wang and Kang 2011; Yang and Kang 2022). Building sound barriers on the walls between the interiors and near the sound source in the outer spaces are some solutions. Technically, sound barriers are sound insulation materials that absorb sound perfectly (Jang et al. 2020). Sound absorption is based on the principle that the sound is reduced by the friction formed on the surfaces of the pores as it passes through the porous materials and by conversion of sound energy into heat energy (Kang et al. 2011; Wang et al. 2014; Kang et al. 2020).
Mineral-based materials (i.e., rock wool and glass wool) and synthetic materials are widely used to prevent noise. Mineral-based sound insulation materials may pose a risk to human health because they contain mineral particles that can become airborne (Berardi and Iannace 2017). Synthetic materials may cause environmental pollution during their production and disposal (Yang et al. 2020; Gliscinska et al. 2021). Synthetic materials turn into micro plastics and pose risk especially to marine life and humans (Almroth et al. 2018). Therefore, research has mainly focused on natural, environmentally sensitive, sound insulation materials.
Wood can be used as a sound insulation material due to its porous structures. When a sound wave enters the pores in the wood, the energy of sound decreases, and sound is absorbed (Wang et al. 2014). Studies have been carried out to improve the acoustic properties of wood. Low-pressure steam application (Kang et al. 2010), delignification process (Kang et al. 2008), underwater shock application (Itoh et al. 1998), microwave treatment (Wang et al. 2014), and heat treatment (Chung et al. 2017) are research strategies that target at improving sound absorption capacity.
Wood is an eco-friendly traditional biomaterial that can be used in many different areas (i.e., construction, furniture, and chemical industries) (Rowell et al. 2012). Its mechanical resistance and shock resistance are very high as compared to its specific gravity (Popescu and Popescu 2013). Its chemical structure is complex and its main components are cellulose, hemicellulose, and lignin. These main components are present in different proportions in the cell structure, which renders wood heterogeneous (Korkut and Hiziroglu 2014). The presence of many hydroxyl groups in the chemical structure of wood adversely affects its strength performance, dimensional stability, and biological structure, and this can limit the long-term use of wood.
Resistance of wood against outer atmospheric conditions is low. Wood often cannot meet the performance expected from it in the determined service period (Kasemsiri et al. 2012; Li et al. 2015; Okon et al. 2017). Modification methods can overcome such limitations. Researchers have developed new methods to counterbalance these disadvantages of wood. Although wood modification methods such as furfurylation, HT, condensation, and impregnation are reported to improve the elasticity, strength, shock absorption properties of wood partially, the moisture and-water resistance properties are reported to be improved significantly (Pfriem et al. 2007; Dietrich et al. 2014; Bucur 2016; Krüger et al. 2018).
The most common commercial treatment methods are impregnation and HT. The impregnation method, which entails injection of chemicals into the body of the wood, was observed to improve its hydrophobicity and resistance to biological pests and this also extends its lifetime. At the same time, it increases both the dimension and the number of its pores. Different impregnation materials can be used depending on whether it is intended for indoor or outdoor use (Hebisch et al. 2020). The CCA salts, creosote oil, coal oil, and other oily compounds are exploited for outdoor use and especially on woods in constant contact with water, roadside sound barriers, bridges, railway sleepers, power transmission poles, and construction molds (Animpong et al. 2017; Unnisa and Hassanpour 2017; Belchinskaya et al. 2020, 2021).
Since linseed oil impregnation is natural and non-toxic, it is especially preferred for impregnation of wooden materials used indoors and outdoors. LO is a hydrophobic and inexpensive product (Humar and Lesar 2013). Water absorption is decreased and biological resistance is provided by closing the pores of the impregnated wood with a thin film layer (Ulvcrona et al. 2006). Linseed oil creates resistance to hydrophilicity directly without chemical bonding between wood and oil (Humar and Lesar 2013).
Heat treatment is a type of wood modification that was developed via experimental studies and was then commercialized. Studies have focused on the effect of HT on mechanical properties, dimensional stability, and color of wood (Sailer et al. 2000).
For wood materials, the effects of heat treatment studies such as reduction in the emission of volatile organic compounds of wood (Manninen et al. 2002), increase in dimensional stability of wood material (Kamdem et al. 2000; Papp and Sailer 2001; Tejada et al. 1997), improvement in material durability (Hanger et al. 2002), and improvement of color uniformity (Sailer et al. 2000) were investigated. The data obtained in studies targeting at investigating the effect that heat treatment causes significant changes in the chemical composition of wood have shown that hemicellulose is decomposed (Tjeerdsma et al. 1998), the volatile components in wood decreased (Garrote et al. 2001; Kamdem et al. 2002), and the cellulose crystallinity increased (Bhuiyan et al. 2000), especially when the amorphous carbohydrates decompose. Wood acidity increases with heat treatment.
In studies focused on how HT affected acoustic properties (Kubojima et al. 1998), such as vibrational performance of wood (Schwarze et al. 2008), the temperature was in the range 120 to 200 °C and the process was carried out for a period between 0.5 and 16.0 h in the presence of N2 or air. They researched acoustic performance of Norway spruce and American sycamore. In another study, effect of HT on acoustic properties of Chinese fir the study was conducted at 120 °C, 140 °C, 160 °C, 180 °C, 200 °C, and 220 °C, and the holding time of was 2 h (Zhu et al. 2016). The acoustic performance was studied on cellulose crystals (Ma 2005). However, a very limited number of studies were carried on sound absorption performance of wood.
Due to the heat treatment, the disadvantages related to the use of wood material (water absorption, change in size, and color, decay, etc.) are minimized, and the use of such heat-treated wood material has become widespread. However, combination of this modification method with other modifications has not been extensively studied. On a study performed to this end, the effect of resistance properties of heat-treated wood was investigated relying on the strong correlation between the effect of impregnation of wood with an alkaline buffering medium such as borate and its acidity and thermal degradation (Awoyemi and Westermark 2005). Salman et al. (2014) used boric acid in combination with HT. Irbe et al. (2020) applied the modification methods thermo-hydro treatment and copper impregnation processes together to increase the resistance of wood against rot fungi and found that the combined use of these two processes together against brown rot was more effective than using only one of these treatment procedures.
The present study was targeted at investigating the effect of impregnation on two different heat-treated woods, namely the Mediterranean cypress (Cupressus sempervirens L.) and field maple (Acer campestre L.) wood. The pretreatment of impregnation was performed with a natural solution LO, and the HT was performed at four different temperatures (160, 180, 210, and 240 °C). The treatment methods used in this study alone were previously reported to improve the properties of wood (Temiz et al. 2013; Chen et al. 2020; Yermán et al. 2021; Perdoch et al. 2022). The main motivation for this study was to improve the humidity and water resistance, dimensional stability, mechanical resistance, sound absorption, and reflection properties of heat-treated impregnated wood.
EXPERIMENTAL
Materials and Methods
Mediterranean cypress (Cupressus sempervirens L.) and field maple (Acer campestre L.) wood samples were harvested randomly from the natural forest in Burdur city, Turkey. The information regarding the collected samples is given in Table 1. The samples were prepared to have regular fibers, no knots or cracks, and to be as perfect as possible. The samples were prepared by using sapwood lumber. The samples were cut into 600 ×120 × 20 mm3 (longitudinal, tangential, radial) dimensions before HT. The samples were kept in the air conditioning room at 20 ± 2 °C with relative humidity of 65% ±5 until they attained a relative humidity of 12%. The dimensions of the samples, each of which were prepared in 30 for the experiments, were 30 × 20 ×20 mm3 (longitudinal × radial × tangential), 300 × 20 × 20 mm3 (longitudinal × radial × tangential), 100 × 20 mm2 (diameter × thickness), respectively, and they had 29 × 20 mm2 (diameter × thickness).
Table 1.Basic Information on Tree Samples Collected
The natural LO used in the study was obtained from the BTC chemistry company (BTC Europe GmbH NY 8001-26-1, Germany) with 100% purity. Its technical properties are given in Table 2.
Table 2. Physical Properties of Linseed Oil
Impregnation of Wood Samples with Linseed Oil
The wood samples, which were preconditioned before the process, were immersed into containers filled with linseed oil (LO) at 130 °C and were kept in the containers for 60 min. Then they were placed and kept in another container filled with oil at 30 °C for 60 min (cooling container). The method applied for the wood samples was the hot-cold bath method (Esteves et al. 2014). In this method, the impregnating oil penetrated into the voids, taking advantage of vacuum that forms during the warming and cooling process without applying pressure (Lahtela and Kärki 2016). Therefore, this is an effective method by which to form a protective layer inside the internal surface as well. In the next step the wood samples were dried by keeping them at room temperature for 7 days, and then they were dried in the oven 60 ± 10 °C for 24 h. During this process, the temperature was adjusted to keep the oil leakage under control. Sorption of the impregnated oil was evaluated as per weight (WPG). Specimens that were not impregnated were kept as the control group.
Heat Treatment (HT)
Heat treatment was performed using a temperature-controlled electrical laboratory type oven (Supertherm HT16/16, Nabertherm GmbH, Lilienthal Germany). The samples from the two different tree families – namely Mediterranean cypress (Cupressus sempervirens L.) and field maple (Acer campestre L.) – were heat-treated at atmospheric pressure under oxygen-free conditions. The heat treatment of wood samples was carried out in three main steps (Fig. 1). In the first step, water was removed from the wood starting with the ambient temperature 20 °C and rapidly heating up to 40 °C, followed by a slower increase in temperature in the range of 40 to 130 °C. This step was applied in the same way for each sample. In the second step of heat treatment, the targeted temperature (160, 180, 200, or 240 °C) was reached starting from 130 °C. When the targeted temperature was reached, the temperature was kept constant for 3 hours. In the third step, the samples were cooled gradually until the temperature of the samples reached room temperature.
Fig. 1. Heat treatment steps
Table 3.Treatment Groups
At the end of this process, the lid of the device was opened after the samples were cooled down to room temperature. After heat treatment was applied, the test specimens were kept in an air-conditioned cabinet until they reached constant weight under the conditions of 65±5% relative humidity at 20±2 °C. In this case, heat treatment was applied to the untreated (Untreated) and the reference (impregnated pre-treated) groups. Nine different treatment groups were used for comparisons (Table 3).
Measuring Weight Gain in Percentage (WPG)
Weight Gain in Percentage (WPG) was determined after impregnation process. WPG was determined according to Eq. 1,
WPG = ([m1–m2]/m2) x 100 (%) (1)
where m1 and m2 are the dry weight (g) after and before impregnation, respectively.
Density Determination (DD) and Mechanical Properties (MP)
Density was determined according to ISO13061-2 standard (2014) via the stereometric method, which relies on measuring the densities, volumes, and weight of the samples. Weight of each sample was determined by using analytical balance (with 0.001 g accuracy). To find the volume, all dimensions were determined with a digital caliper with 0.01 mm precision. The densities were calculated via Eq. 2,
do = mo/vo (2)
where do is the density of the oven-dried sample (g/cm³); mo is the weight of the oven dried sample (g) and vo is the volume of the oven dried sample (cm³).
The mechanical tests were performed with a Shimadzu AGS-X (10 kN) universal test device. The bending elasticity modulus test (MPa) was used to determine the stiffness of the wood by using a four point system according to the ISO 3349 (1975) standard. The distance between the support points was 320 mm, and the loading points were at an 80 mm distance. The static bending resistance test (MPa) represents the maximum weight that wood can momentarily support before breaking when the ends of the test sample are placed on a support and when the sample is gradually loaded from two points according to the ISO 3133 (1975) standard. The compression strength (||) (MPa) ISO 3787 (1976) standard represents the maximum weight applied before the sample breaks.
The changes in the mechanical properties and density of the wood samples before treatment (control and reference groups) and after treatment at different temperatures were calculated according to Eq. 3,
Δd,MOR,MOE,CS = ([SC,R–S1]/SC,R) x 100 (%) (3)
where SC,R indicates the values before the treatment and S1 indicates the values after the treatment, respectively.
Water Absorption (WA)
The samples were conditioned until they attained constant weight. The samples were immersed in a container filled with water at 20 °C and then they were removed out of the water in periodic time intervals of 2 h, 24 h (1 d), 48 h (2 d), 72 h (3 d), 96 h (4 d), 120 h (5 d), 168 h (7 d), 336 h (14d ), 504 h (21 d), and 720 h (30 d), and they were dried with a dry paper before the measurements (Poncsak et al. 2011). The weight measurements were performed. Water absorption was determined according to Eq. 4,
WA = ([w1–w2]/w2) x 100 (%) (4)
where w1 and w2 are the weight (g) after and before immersion, respectively.
Porosimetry (P)
Mercury inlet porosimetry (MIP)
Mercury intrusion porosimetry (MIP, AutoporeTM IV 9500 Automated Mercury Porosimeter, Micrometrics Instrument Corp., USA) was used to characterize the porosity, pore dimensions, and pore size distributions of the samples. The samples with the dimensions 8×6×6 mm (L×T×R) were dried in an oven at 103 ± 2°C before the tests. The pressure capacity of the equipment used was 200 MPa. The process parameters were as follows: mercury with surface tension of 0.485 g/cm2 and the density lies between 13.5325 and 13.5379 g.mL-1. The contact angle between the mercury and the wood (forward-backward) was 130°, and the balance period between the high and low pressure was 10 s.
Scanning Electron Microscope Imaging (SEM)
The changes in the microstructures of the samples used in this study were investigated via a Hitachi S-4800 Surface Emission Scanning Electron Microscopy (SEM, Japan). The samples were cut in the tangential and cross-section directions with 5×5 mm2 dimension. The acceleration voltage in this study was 10 kV. The samples were mounted onto the aluminum studs via double sided tape and were gold plated via spraying. Then the microstructures of the tangential and cross-sections were analyzed.
Sound Absorption Coefficient (SAC)
The sound absorption capacity (α) of the wood samples was tested according to ISO standard 10534-2 (1998) method by using impedance tubes of Type SW 422 and Type SW 477 (BSWA Technology Co., Ltd., China). The air void between the sample and the steel support in the sound absorption measurements was 10 mm. The sound absorption coefficients of the 1/3-octave frequencies were measured by using a big tube (SW 422, Φ 100 mm) in the frequency range between 63 and 1600 Hz. The sound frequency absorption coefficients in the frequency range between 1600 and 6300 Hz were measured by using a small tube (SW 477, Φ29 mm).
Sound Transmission Loss (STL)
Sound transmission loss in the frequency range of 50 to 6400 Hz was measured by using ASTM standard E2611-Type SW 422 and Type SW 477 impedance tubes (BSWA Technology Co., Ltd., China). An impedances tube of 29 mm diameter was used for measuring sound. The transmission loss was measured in the frequency range of 50 to 6400 Hz at ambient conditions of 20 °C and 1031 hPa.
Statistical Analysis
Statistical differences in the mean values of the parameters investigated in this work were estimated by using two-way analysis of variance (ANOVA) to investigate the effects of different types of impregnating oil and HT (temperature) on physical-mechanical and acoustic properties. When significant differences were detected by ANOVA, Duncan’s test was used to evaluate the relationship between applied temperature and impregnation. All statistical analyses were performed via SPSS® 20.0 for Windows®. The mean values were accepted to be considerably important when p ≤ 0.05.
Measuring Weight Gain as a Percentage (WPG)
Table 4 presents the results of the Mediterranean cypress and field maple wood samples impregnated with linseed oil (WPG). It can be seen that after impregnation, 39.5 % of the linseed oil was retained inside the wood. This is considered to be a high value for treatment. Temiz et al. (2013) reported similar results in their study in which LO was used as impregnating agent and following impregnation WPG was found to be 40.4%. This indicates that LO highly penetrated the wooden structure. Lahtela et al. (2016) treated Scots pine (Pinus sylvestris) wood with melamine, and the average WPG was found to be 38.71%, which also indicates strong penetration into the wood structure.
Table 4. WPG Values in Various Linseed Oil Impregnation Treatments
Density Determination and Mechanical Properties
Sample densities before and after modification are shown in Table 5. As presented in Table 5 and Fig. 2, when the Mediterranean cypress and field maple wood samples are compared with the control group (Un-treated) for the non-impregnated samples, as the temperature increased, the mechanical properties such as the modulus of rupture (MOR-bending strength) and modulus of elasticity (MOE), as well as the modulus of rupture (MOR-bending strength) and modulus of elasticity (MOE) values, decreased, while the compression strength (CS) increased, as it is generally accepted (Korkut et al. 2008; Ateş et al. 2009; Militz and Altgen 2014).
Fig. 2. Percentage of change in the mechanical properties of the non-impregnated samples as a function of HT
Table 5. Properties Before and After Impregnation and HT
For the Mediterranean cypress wood samples, the maximum decrease in MOR was observed at 240 °C and was 27%. The decrease in MOE at the same temperature was 23%. The increase in the CS value at 160 °C was 9%. The increase in the CS tended to decrease as the temperature increased, and it was not more than 2% at 210 °C. The decrease in MOR for the field maple wood samples at 240 °C was 23% and the MOE was 18%. The increase in the CS at 160 °C was 6%. The reaction of the Mediterranean cypress wood to heat was more pronounced than that of the field maple wood.
In a similar way, when decrease in density was observed for the Mediterranean cypress wood, it decreased from 0.574 to 0.471 g/cm3 at 240 °C, and the loss in density was 18%. The loss in density for the field maple wood was 30%. The main reason for this decrease was the low moisture content due to removal of water in the structure and the degradation that starts in the wood and especially in the hemicellulose components (Ateş et al. 2009; Boonstra et al. 2007). In Table 5 and Fig.3, Mediterranean cypress and field maple wood samples were compared with the reference group impregnated with linseed oil (LO) (impregnated with LO but not heat treated). It was seen that as the temperature increased the mechanical properties such as MOR and MOE increased but CS did not change. For the Mediterranean cypress wood samples, the maximum increase in MOR was observed at 240 °C, and it was 8%. The increase in MOE was 4% at 240 °C. For the field maple wood samples, the increase in both MOR and MOE was 13% at 240 °C, while the increase in CS at 240 °C was 7%.
Monitoring the loss in density, it was observed that for the Mediterranean cypress wood the loss was 3%. While HT caused a decrease in the impregnated samples, it was observed that the MOR, MOE, and the CS values of the samples that were thermally processed with linseed oil treatment increased by ~10%. This can be considered as evidence of the positive effect of natural linseed oil. Other studies reported a similar increase in MOR and MOE upon impregnation with wax (Humar et al. 2017).
Fig. 3. Percentage of change in the mechanical properties of the samples impregnated with LO as a function of HT
Water Absorption of Wood (WA)
Absorption of water by wood is important in buildings and industrial applications. Depending on the area of use (i.e., indoor or outdoor), wood material is generally subject to water, water vapor, and water leakage. This affects dimensional stability and durability of wood. Wood is hygroscopic in nature and therefore it is rendered more hydrophobic through treatment. Impregnation and HT are widely used for this purpose.
Water absorption properties of the non-impregnated but heat-treated Mediterranean cypress and the field maple wood samples are presented in Fig. 4. In the first 300 hours of the water absorption test, it was observed that water absorption ratio of the non-impregnated samples was similar. As the treatment temperature increased, water absorption decreased. It was observed that at 240 °C the lowest absorption was 39% for the field maple wood. This data became stable after 400 hours. In a similar way after 350 h stability was achieved for the Mediterranean cypress wood. This situation can be explained by the degradation and decomposition of hemicellulose, which is hydrophilic component of wood, in both tree species, and by the fact that the extractive agents become mobile at high temperatures and evaporate to a certain extent and the remaining part migrates to the surface, covers the surface, and decomposes. Metsä-Kortelainen and Viitanen (2012) have achieved similar results in their study. In another study, Nuopponen et al. (2003) achieved this effect through softening lignin due to the mobility of the extractive agents. The water absorption properties of the wood, which were obtained from the Mediterranean cypress and the field maple trees, impregnated with linseed oil and the reference (Ref. LO) wood samples after HT are presented in Fig. 5.
Fig. 4. Water sorption data of the Mediterranean cypress and field maple wood after HT (heat treatment)
Fig. 5. Water sorption data of the Mediterranean cypress and field maple wood impregnated with LO and after HT (heat treated)
The data in Fig. 5 show that after 240 °C treatment, water absorption from the Mediterranean cypress wood at the end of Day 30 (720 h) was 36%. At the same temperature and in the same time interval water absorption was 80% for the reference group. This result shows that a 55% improvement in water absorption was found to be 65% for the control group. The finding of a similar result for field maple wood shows that a 50% improvement in water absorption was found to be 72% for the control group. According to the results due to the effect of HT, LO – an impregnating agent – provided considerable input, and thus high hydroscopic property was attained. This finding shows the importance of impregnation with linseed oil. Meyer et al. (2016), Humar et al. (2017), and Fojutowski et al. (2009) have achieved similar results.
Porosimetry
Wood material is a porous polymer in nature. The wood pore structure directly affects its density, permeability, dimensional stability, durability, impregnability, absorption, and acoustic properties, etc. (Hill and Papadopoulos 2001). The pore size, amount, and pore type of wood change with the type of the wood, the height of the tree, the meteorological conditions of the place where the tree is located, and the anatomical structure of the wood (Ona et al. 1999; Almeida and Hernández 2007).
The porosimetry results of the wood samples presented in Table 6 indicate that the porosity was 62.2% for the non-impregnated Mediterranean cypress wood at 240 °C before impregnation and 65.3% after HT. It was observed that the porosity increased by 4.9% for the Mediterranean cypress wood with temperature. The change in porosity was 58.1% and 64.8%, respectively, for the non-impregnated field maple wood. Porosity was observed to increase with the temperature systematically with the field maple wood, and the ratio of change at 240 °C was 11.5%. Porosity increased as a function of temperature in HT, and porosity and dimensional stability increased due to the degradation of hemicellulose followed by decrease in the number of the hydroxyl groups.
Table 6 shows 59.3% and 78.7% increase in LO impregnated Mediterranean cypress wood as compared to the pre-treated wood. After the treatment with LO impregnating agent, the porosity increased by 32.6%. The porosity before and after impregnation of the field maple wood with LO were 55.2% and 74.8%, respectively. Impregnation of the field maple wood with LO caused a 35.3% increase in porosity. When HT was applied on LO-impregnated Mediterranean cypress at 240 °C, the porosity increased for both of the tree types. The wood samples with the highest porosity were filled with the impregnating oils the most. Porosity was observed to increase after impregnation because, in the course of impregnation, LO fills the air voids in the cell walls in the wood and this has adverse effect on porosity.
Table 6. Porosity of the Treated and Untreated Trees
Scanning Electron Microscope Imaging (SEM) Studies
In Figs. 6 and 7, the SEM micrographs of the Mediterranean cypress and the field maple wood, which depict the effect of thermal process after impregnation and HT, are presented. When the internal surface images of the cell walls of the wood samples given in Fig. 6 are investigated. As can be seen in Fig. 6 (a to d), no deformation was observed in the cell structure.
Fig. 6. SEM micrographs of Internal surface of the cell wall in Mediterranean cypress and field maple woods untreated (control) (a and d), impregnated (LO) (b and e), impregnated and heat treated at 240 °C (c and f)
In Fig. 6 (b-e) it can be seen in the images that LO filled the cell voids after impregnation. The extent of absorption in this image is in conformity with the result of the WPG results. The average WPG of the impregnating agents LO was found to be 37.8%. The micro-cracks that form on the surface after the HT are indicative of the deformation caused by the heat. Under the influence of heat treatment, the shape of the cell changed and the cell wall became rough. The intercellular bonds were broken by the effect of heat. Both the micro-cracks and the bubbles caused by boiling of LO can be seen in Fig. 6 (c-f).
In a similar way, in Fig. 7 different images obtained due to the properties of the impregnating agent can be seen. The empty cell lumen observed in Fig. 7 (a-d) indicates that no treatment was given. Figure 7 (b-e) shows the diffusion of the impregnating agent, which has expanded upon heating, into the internal pores of the wood. The voids in the cells are partially filled with the impregnating oils and they act as a barrier against water and humidity. Due to the differences in pressure, partial damage and collapses in the cell walls were observed after impregnation. In Fig. 7 (c-f), the damage, which was caused by the migration of the impregnating agent from inside out and its entrapment inside the voids caused by the mobility due to HT, can be seen. This supports the finding that porosity increases with increase in the temperature of wood. It is observed that hemicelluloses decomposed and lignin was plasticized, and thus structural changes took place in the cell and the voids increased due to the physical and chemical change caused.
Fig. 7. SEM micrographs of cross-sectional surface of the cell wall in Mediterranean cypress and field maple woods untreated (control) (a and d), impregnated (LO) (b and e), impregnated and heat treated at 240 °C (c and f)
Sound Absorption Coefficient (SAC)
In Fig. 8 a-d are given the SAC values measured in the 100 to 6300 Hz frequency range before (control) and after impregnation of the Mediterranean cypress wood and the field maple wood samples with LO. In general an increase was observed in sound absorption coefficient due to the effect of impregnation. In Fig. 8a it can be seen that for the Mediterranean cypress wood samples the average sound absorption coefficients were 0.19 and 0.28 before impregnation (control) and after impregnation with LO and in the tangential direction the SAC were 0.12 and 0.17 (Fig. 8c).
In Fig. 8b, the field maple wood samples the average sound absorption coefficients in the longitudinal direction before (control) and after impregnation with LO were found to be 0.23 and 0.31, and in the tangential direction after impregnation with LO the average sound absorption coefficients were found to be 0.14 and 0.17, respectively (Fig. 8d).
Figure 8b shows that the impregnating agents (LO) decreased the SACs to the 100 to 1000 Hz range. While the effect of impregnating agent could not be observed in the frequency range of 1000 to 2000 Hz, they were found to increase sound absorption by 10% in the 2000 to 6300 Hz frequency range.
When the data given in Figs. 8a and 8b were evaluated at low frequency bands, the impregnating agents were observed to stop the sound waves from entering the wood. Since the cell wall was full of the impregnating agent after the impregnation process, the sound waves could not reach this distance. Most of the low frequency sound waves were reflected back. However, as the frequency increased, the situation was reversed. When the frequency of the sound waves increased, they moved toward the cell wall but could not exit from there and they were trapped inside, because after the impregnation with the collapse of the cell plates inside and the contraction of the cell wall, the sound energy was rapidly converted into heat energy.
Fig. 8. The sound absorption coefficient (SAC) of the Mediterranean cypress and the field maple wood samples before impregnation (control) and after impregnation with LO, longitudinal direction (a-b), tangential direction (c-d)
The natural LO impregnating agents vibrate with high frequency sound waves and form resonance and synergistically contribute to absorption of sound inside the wood. Liu et al. (2008) achieved similar results in their work. About half of the effect of this situation in the longitudinal direction occurred in the tangential direction.
In Fig. 9a, the HT results of the LO impregnated Mediterranean cypress wood are given, and the SAC value, which peaked at 5000 Hz at 240 °C, was 0.71. At 240 °C, the average SAC was found to be 0.30.
Fig. 9. The SAC data of the Mediterranean cypress wood and the field maple wood samples after impregnation with LO and after HT, longitudinal direction (a-b), tangential direction (c-d)
The SAC values in the longitudinal direction of the heat-treated LO impregnated field maple wood samples are given in Fig. 9b. The sound absorption of LO impregnated samples was observed to increase with the increase in the temperature. The two peak points for the SAC values, which were 0.78 and 0.80, were observed at 2500 and 4500 Hz, respectively. The average SAC value was found to increase with the temperature in the treated samples. For the samples processed at 240 °C, the SAC was found to be 0.37 on average. For the heat-treated Mediterranean cypress wood, the highest peak for the SAC in the tangential direction, which was 0.51, was observed at 5200 Hz (Fig. 9c). The average SAC value for the LO impregnated samples and the heat-treated field maple wood samples in the tangential direction was determined to be 0.19. The results of the SAC measurements showed two peaks, which were 0.58 and 0.59, at 2200 Hz and 4450 Hz, respectively (Fig. 9d).
This was realized in different ways in the low and high frequency bands. In the low frequency band the increase in the SAC values was slow. The low frequency band (0 to 2000 Hz) was affected by the temperature increase in the wood. The increase in the high frequency band (2000 to 6000 Hz) was faster but occurred in a narrow band range. Increase in the temperature had prevented the collapse in the cell walls and the resulting porous structure might have prevented the sound waves. Then, continued sound absorption can be explained as being due to an increase in surface area.
Sound Transmission Loss (STL)
When sound waves hit the material surface, some of the sound energy generated on the surface is absorbed, some is reflected, and the rest is transmitted by the surface. Sound transmission loss (STL) indicates the sound blocking capacity of the material. From an architectural point of view, the sound blocking capacity of the material is considered as an indicator of the sound insulation of the material. Depending on the frequency, the high sound transmission loss means that the sound insulation effect is also high (Lee et al. 2011; Kim et al. 2015).
Fig. 10. The STL data of the Mediterranean cypress wood and the field maple wood, longitudinal direction (a), tangential direction (b)
Figure 10a shows the STL values of the Mediterranean cypress wood samples before (control) and after impregnation with LO in the frequency range 100 to 6300 Hz. The average STL of the Mediterranean cypress wood was found to be 48.6 dB. The average STL for the control (non-impregnated) group was 26 dB and the highest STL at 3500 Hz was 37.4 dB. In Fig. 9a, the highest average STL, which was 48.6 dB, was found with the group impregnated with LO. In this group, the highest STL value, which was 69.9 dB, was observed at 1540 Hz and 3600 Hz.
In Fig. 10b, the average STL values for the field maple wood was observed to be 37 dB and for the control group it was found to be 28 dB. In this group, the highest STL value, which was 64.7 dB, was observed at 640, 1100, and 4100 Hz.
When the average values were considered, it was observed that as porosity increased, the STL values for the Mediterranean cypress and field maple wood increased by 86% and 32%, respectively. Kang et al. (2019) obtained similar results in their study. They observed that for a 20 mm thick non-treated Paulownia tomentosa wood the average sound loss in the range 50 to 6400 Hz was 35 and 36.9 dB. Despite its low specific gravity and high porosity, the sound transmission loss for Paulownia tomentosa wood was high.
CONCLUSIONS
1. Compressive strength (CS) increased with increasing heat treatment temperatures. However, as the temperature increased, the compressive strength tended to decrease. Compressive strength (CS) increased with increasing heat treatment temperatures in impregnated heat treatment. The flexural strength (MOR) and flexural modulus of elasticity (MOE) decreased due to heat treatment. However, the flexural strength (MOR) flexural modulus of elasticity (MOE) increased due to impregnated heat treatment.
2. Density values decreased with increasing heat treatment temperature.
3. The percentage of average water absorption decreased significantly as the temperature increased. The lowest water absorption was observed for the samples impregnated at 240 °C. Water absorption was observed to decrease significantly as the temperature increased from 160 to 240 °C in heat treatment for each sample with and without impregnation.
4. Porosity decreased very little after the impregnation; in the HT, the increase in temperature had a positive affect (max. 35.3%).
5. For the wood samples, the average sound sorption coefficient in the longitudinal direction before impregnation (control) and after impregnation with LO were 0.19, and 0.28, respectively. In the tangential direction after impregnation with LO, it was found to be 0.12 and 0.17, respectively. The increase in temperature increased sound absorption in samples impregnated with LO. Two peak points were found as sound absorption coefficients which were 0.78 and 0.80 at 2500 Hz and 4500 Hz, respectively.
6. The highest STL was found at 640 Hz, 1100 Hz, and 64.7 dB at 4100 Hz in the group impregnated with LO.
7. The disadvantages that arise during use of wood samples were minimized via impregnation with LO and with TM. Thus, besides achieving ideal sound absorption it also attains properties of being durable, water-repellent, and dimensionally stable material during the desired service time. LO impregnated and the heat-treated wood can be used as natural, decorative, and environmentally sensitive wood material for sound insulation both indoors and as well as outdoor applications.
REFERENCES CITED
Almeida, G., and Hernández, R. E. (2007). “Influence of the pore structure of wood on moisture desorption at high relative humidities,” Wood Mater Sci Eng. 2(1), 33-44. DOI: 10.1080/17480270701538383
Almroth, B. M. C., Aström, L., Roslund, S., Petersson, H., Johansson, M., and Persson, N. K. (2018). “Quantifying shedding of synthetic fibers from textiles, a source of microplastics released into the environment,” Environ. Sci. Pollut. 25(2), 1191-1199. DOI: 10.1007/s11356-017-0528-7
Animpong, M. A. B., Oduro, W. O., Koranteng, J., Ampomah-Benefo, K., Boafo-Mensah, G., Akufo-Kumi, K., Tottimeh, G. O., and Amoah, J. Y. (2017). “Coupling effect of waste automotive engine oil in the preparation of wood reinforced LDPE plastic composites for panels,” S. Afr. J. Chem. Eng. 24, 55-61. DOI: 10.1016/j.sajce.2017.01.004
ASTM E2611-09 (2009). “Standard test method for measurement of normal incident sound transmission of acoustical materials based on the transfer matrix method,” West Conshohocken, PA, USA.
Ates, S., Akyildiz, M. H., and Ozdemir, H. (2009). “Effects of heat treatment on Calabrian pine (Pinus brutia Ten.) wood,” BioResources 4(3), 1032-1043.
Awoyemi, L., and Westermark, U. (2005). “Effects of borate impregnation on the response of wood strength to heat treatment,” Wood Sci. Technol. 39(6), 484-491. DOI: 10.1007/s00226-005-0001-5
Belchinskaya, L., Zhuzhukin, K., Dmitrenkov, A., and Roessner, F. (2020). “Studying and imparting moisture absorption qualities of the new wood based bio-composite material,” In:IOP Conference Series: Earth and Environmental Science, IOP Publishing, Bristol, UK, p. 595. DOI: 10.1088/1755-1315/595/1/012053
Belchinskaya, L., Zhuzhukin, K. V., Ishchenko, T., and Platonov, A. (2021). “Impregnation of wood with waste engine oil to increase water-and bio-resistance,” Forests 12(12), 1762. DOI: 10.3390/f12121762
Berardi, U., and Iannace, G. (2017). “Predicting the sound absorption of natural materials: Best-fit inverse laws for the acoustic imped-ance and the propagation constant,” Appl. Acoust. 115, 131-138. DOI: 10.1016/j.apacoust.2016.08.012
Bhuiyan, M., Rabbani, T., Hirai, N., and Sobue, N. (2000). “Changes of crystallinity in wood cellulose by heat treatment under dried and moist conditions,” J. Wood Sci. 46(6), 431-436. DOI: 10.1007/BF00765800
Boonstra, M. J, Acker, J., Tjeerdsma, B. F, and Kegel, E. (2007). “Strength properties of thermally modified softwoods and its relation to polymeric structural wood constituents,” Ann. For. Sci. 64(7), 679-690.
Bucur, B. (2016). Handbook of Materials for String Musical Instruments, Springer International Publishing, Switzerland. DOI: 10.1007/978-3-319-32080-9-19
Chen, J., Wang, Y., Cao, J., and Wang, W. (2020). “Improved water repellency and dimensional stability of wood via impregnation with an epoxidized linseed oil and carnauba wax complex emulsion,” Forests 11(3), 271. DOI: 10.3390/f11030271
Chung, H., Park, Y., Yang, S.Y., Kim, H., Han, Y., Chang, Y.S., and Yeo, H. (2017). “Effect of heat treatment temperature and time on sound absorption coefficient of Larix kaempferi wood,” J. Wood Sci. 63(6), 575-579. DOI: 10.1007/s10086-017-1662-z
Dietrich, T., Buchelt, B., and Wagenführ, A. (2014). “Modified wood as substitute for ebony wood in musical instruments,” in: Proceedings of the 7th European Conference on Wood Modification, Lisbon, Portugal, pp. 73-82.
Esteves, B., Nunes, L., Domingos, I., and Pereira, H. (2014). “Improvement of termite resistance, dimensional stability and mechanical properties of pine wood by paraffin impregnation,” Eur. J. Wood Wood Prod. 72(5), 609-615. DOI: 10.1007/s00107-014-0823-7
Fojutowski, A., Noskowiak, A., and Kropacz, A. (2009). “Physical and mechanical properties and resistance to fungi of Scots pine and birch wood modified thermally and using natural oil,” Drewno 52, 43-62.
Gliscinska, E., Amezaga, J. P., Michalak, M., and Krucinska, I. (2021). “Green sound-absorbing composite materials of various structure and profiling,” Coatings 11(4), 407. DOI: 10.3390/coatings11040407
Hanger, J., Huber, H., Lackner, R., Wimmer, R., and Fellner, J. (2002). “Improving the natural durabilityof heat-treated spruce, pine and beech,” Holzforschung 54(5), 92-93.
Hebisch, R., Karmann, J., Schäferhenrich, A., Göen, T., Berger, M., Poppek, U., and Roitzsch, M. (2020). “Inhalation and dermal exposure of workers during timber impregnation with creosote and subsequent processing of impregnated,” WoodEnviron. Res. 181, 108877. DOI: 10.1016/j.envres.2019.108877
Hill, C. A., and Papadopoulos, A. N. (2001). “A review of methods used to determine the size of the cell wall microvoids of wood,” Journal-Institute of wood science 15(6), 337-345.
Humar, M., Kržišnik, D., Lesar, B., Thaler, N., Ugovšek, A., Zupančič, K., and Žlahtič, M. (2017). “Thermal modification of wax-impregnated wood to enhance its physical, mechanical, and biological properties,” Holzforschung 71(1), 57-64. DOI: 10.1515/hf-2016-0063
Humar, M., and Lesar, B. (2013). “Efficacy of linseed- and tung-oil-treated wood against wood-decay fungi and water uptake,” Int. Bio-deterior. Biodegradation 85, 223-227. DOI: 10.1016/j.ibiod.2013.07.011
Hunashal, R. B., and Patil, Y. G. (2012). “Assessment of noise pollution indices in the city of Kolhapur, India,” Procedia Soc. 37, 448-457. DOI: 10.1016/j.sbspro.2012.03.310
ISO 10534-2 (1998). “Acoustics – Determination of sound absorption coefficient and impedance in impedance tubes. Part 2,” International Organization of Standardization, Geneva, Switzerland.
ISO 13061–2 (2014). “Physical and mechanical properties of wood test methods for small clear wood specimens. Part 2: Determination of density for physical and mechanical tests,” International Organization for Standardization, Geneva
ISO 3133 (1975). “Wood – Determination of ultimate strength in static bending,” International Organization for Standardization, Switzerland.
ISO 3349 (1975). “Wood – Determination of modulus of elasticity in static bending,” International Organization for Standardization. Switzerland.
ISO 3787 (1976). “Wood – Test methods – Determination of ultimate stress in compression parallel to grain,” International Organization for Standardization. Switzerland.
Irbe, I., Zommere, Z., and Kurnosova, N. (2020). “Alternative wood preservation method: Double treatment and its effectiveness against wood decay fungi,” Eur. J. Wood Prod. 78(6), 1233-1247. DOI:10.1007/s00107-020-01582-1.
Itoh, S., Nagano, S., and Fujita, M. (1998). “The improvement of the permeability of wood by underwater shock wave,” Met. Mater. 4(4), 843-846. DOI: 10.1007/BF03026410
Jang, E. S., Yuk, J. H., and Kang, C. W. (2020). “An experimental study on change of gas permeability depending on pore structures in three species (hinoki, Douglas fir, and hemlock) of softwood,” J. Wood Sci. 66(1), 1-12. DOI: 10.1186/s10086-020-01925-9
Kamdem, D. P., Pizzi, A., and Triboulot, M. C. (2000). “Heat-treated timber: Potentially toxic byproductspresence and extent of wood cell wall degradation,” Holz Roh- Werkst. 58(4), 253-257. DOI: 10.1007/s001070050420
Kang, C., Kang, W., and Chung, W. (2008). “Changes in anatomical features, air permeability and sound absorption capability of wood induced by delignification treatment,” J. Fac. Agric. 53(2), 479-483. DOI: 10.5109/12861
Kang, C. W., Kim, G. C., and Park, H. J . (2010). “Changes in permeability and sound absorption capability of yellow poplar wood by steam explosion treatment,” J. Fac. Agric. 55(2), 327-332. DOI: 10.5109/18849
Kang, C. W., Lee, Y. H., Kang, H. Y., Kang, W., Xu, H., and Chung, W. Y. (2011). “Radial variation of sound absorption capability in the cross sectional surface of yellow poplar wood,” Korean Wood Sci. Technol. 39(4), 326-332. DOI: 10.5658/WOOD.2011.39.4.326
Kang, C. W., Jang, E. S., Jang, S. S., Cho, J. I., and Kim, N. H. (2019). “Effect of heat treatment on the gas permeability, sound absorption coefficient, and sound transmission loss of Paulownia tomentosa wood,” J. Korean Wood Sci. Technol. 47(5), 644-654. DOI: 0.5658/WOOD.2019.47.5.644
Kang, C. W., Jang, E. S., Jang, S. S., Hasegawa, M., and Matsumura, J. (2020). “Studies of the relationship between sound absorption coefficient and air permeability of wood,” J. Fac. Agric. 65(2), 351-355. DOI: 10.5109/4103900
Kasemsiri, P., Hiziroglu, S., and Rimdusit, S. (2012). “Characterization of heat-treated Eastern redcedar (Juniperus virginiana L.),” J. Mater. Process. Technol. 212(6), 1324-1330. DOI: 10.1016/j.jmatprotec.2011.12.019
Kim, D. R., Kim, T. M., Kim, J. T., Park, G. H., and Ryu, L. O. (2015). “A study on sensitivity of transmission loss using scaled reverberation chamber,” Trans. Korean Soc. Noise Vib. Eng. 4, 937-943.
Kook, J. H., Jung, C. W., Yun, J. H., and Kim, J. S. (2007). “Experimental study on wall sound transmission loss at construction equipment machinery room,” Proceedings Korean Soc. Noise Vib. Eng. 3, 695-699.
Korkut, D. S., and Hiziroglu, S. (2014). “Experimental test of heat treatment effect on physical properties of red oak (Quercus falcateMichx.) and Southern pine (Pinus taeda L.),” Materials 7(11), 7314-7323. DOI: 10.3390/ma7117314
Korkut, S., Kök, M. S., Korkut, D. S., and Gürleyen, T. (2008). “The effects of heat treatment on technological properties in Red-bud maple (Acer trautvetteri Medw.) wood,” Bioresourec Technol. 99(6), 1538-1543. DOI: 10.1016/j.biortech.2007.04.021
Krüger, R., Zauer, M., and Wagenführ, A. (2018). “Physical properties of native and thermally treated European woods as potential alternative to Indian rosewood for the use in classical guitars,” Eur. J. Wood Wood Prod. 76(6), 1663-1668. DOI: 10.1007/s00107-018-1345-5
Kubojima, Y., Okano, T., and Ohta, M. (1998). “Vibrational properties of Sitka spruce heat-treated in nitrogen gas,” J. Wood Sci. 44(1), 73-77. DOI:10.1007/BF00521878
Lahtela, V., and Kärki, T. (2016). “Effects of impregnation and heat treatment on the physical and mechanical properties of Scots pine (Pinus sylvestris) wood,” Wood Mater. Sci. Eng. 11(4), 217-227. DOI: 10.1080/17480272.2014.971428
Lee, J. H., Kim, B. S., and Kim, K. J. (2011). “Study of sound transmission characteristics of using a scale reverberation chamber and vibro acoustic FEM,” Proceedings Korean Soc. Noise Vib. Eng. 10, 92-95.
Li, W., Wang, H., Ren, D., Yu, Y., and Yu, Y. (2015). “Wood modification with furfuryl alcohol catalysed by a new composite acidic catalyst,” Wood Sci. Technol. 49(4), 845-856. DOI: 10.1007/s00226-015-0721-0
Liu, M., Zhong, H., Ma, E., and Liu, R. (2018). “Resistance to fungal decay of paraffin wax emulsion/copper azole compound system treated wood,” Int. Biodeterior. 129, 61-66. DOI: 10.1016/j.ibiod.2018.01.005
Ma, L. N. (2005). On Relationship between Structure and Acoustic Properties of Wood, Master’s Thesis, Anhui Agricultural University, Hefei, China.
Manninen, A. M., Pasanen, P., and Holopainen, J. K. (2002). “Comparing the emissions between air-dried and heat-treated Scots pine wood,” Atmos Environ. 36(11), 1763-1768. DOI: 10.1016/S1352-2310(02)00152-8
Metsä-Kortelainen, S., and Viitanen, H. (2012). “Wettability of sapwood and heartwood of thermally modified Norway spruce and Scots pine,” Eur. J. Wood Wood Prod. 70(1), 135-139. DOI: 10.1007/s00107-011-0523-5
Meyer, L., Brischke, C., Treu, A., and Larsson-Brelid, P. (2016). “Critical moisture conditions for fungal decay of modified wood by basidiomycetes as detected by pile tests,” Holzforschung 70(4), 331-339. DOI:10.1515/hf-2015-0046
Militz, H., and Altgen, M. (2014). “Processes and properties of thermally modified wood manufactured in Europe,” in: Deterioration and Protection of Sustainable Biomaterials, ACS Symposium Series, T. P. Schultz, B. Goodell, and D. D. Nicholas (ed.), Vol. 1158, pp. 269-285. DOI: 10.1021/bk-2014-1158.ch016
Nuopponen, M., Vuorinen, T., Jämsä, S., and Viitaniemi, P. (2003). “The effects of a heat treatment on the behaviour of extractives in softwood studied by FTIR spectroscopic methods,” Wood Sci. Technol. 37(2), 109-115. DOI: 10.1007/s00226-003-0178-4
Okon, K.E., Lin, F., Chen, Y., and Huang, B. (2017). “Effect of silicone oil heat treatment on the chemical composition, cellulose crystalline structure and contact angle of Chinese parasol wood,” Carbohydr. Polym. 164, 179-185. DOI: 10.1016/j.carbpol.2017.01.076
Ona, T., Ito, K., Shibata, M., Ootake, Y., Ohshima, J., Yokota, S., and Sonoda, T. (1999). “In situ determination of proportion of cell types in wood by Fourier transform Raman spectroscopy,” Anal. Biochem. 268(1), 43-48. DOI: 10.1007/978-94-011-4479-7_242
Papp, A. O., and Sailer, M. (2001). “Oil-heat-treatment of wood-process and properties,” Drv. Ind. 52(2), 63-70.
Perdoch, W., Depczyńska, E., Tomkowiak, K., Furgał, M., Kurczak, M., and Mazela, B. (2022). “The Impact of vinylotrimethox-ysilane-modified linseed oil on selected properties of impregnated wood,” Forests 13(8), article 1265. DOI: 10.3390/f13081265
Pfriem, A., Eichelberger, K., and Wagenführ, A. (2007). “Acoustic properties of thermally modified spruce for use for violins,” Journal of the Violin Society of America: VSA-Paper 21(1), 102-111.
Poncsak, S., Kocaefe, D., and Younsi, R. (2011). “Improvement of the heat treatment of Jack pine (Pinus banksiana) using ThermoWood technology,” Eur. J. Wood Wood Prod. 69(2), 281-286. DOI:10.1007/s00107-010-0426-x
Popescu, C. M., and Popescu, M. C. (2013). “A near infrared spectroscopic study of the structural modifications of Lime (Tilia cordata Mill.) wood during hydro-thermal treatment,” Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 115, 227-233. DOI: 10.1016/j.saa.2013.06.002
Rowell, R., Pettersen, R., and Tshabalala, M. (2012). Handbook of Wood Chemistry and Wood Composites, 2nd Ed., Taylor & Francis Group, Tokyo, Japan.
Sailer, M., Rapp, A. O., Leithoff, H., and Peek, R. D. (2000). “Upgrading of wood by application of an oil heat treatment,”Holz Roh Werkst. 58(1–2), 15-22.
Salman, S., Pétrissans, A., Thévenon, M.F., Dumarçay, S., Perrin, D., Pollier, B., and Gérardin, P. (2014). “Development of new wood treatments combining boron impregnation and thermo modification: Effect of additives on boron leachability,” Eur. J. Wood Wood Prod. 72(3), 355-365. DOI: 10.1007/s00107-014-0787-7
Schwarze, F. W., Spycher, M., and Fink, S. (2008). “Superior wood for violins–wood decay fungi as a substitute for cold climate,” New Phytol. 179(4), 1095-1104. DOI: 10.1111/j.1469-8137.2008.02524.x
Szeremeta, B., and Zannin, P. H. T. (2009). “Analysis and evaluation of soundscapes in public parks through interviews and measurement of noise,” Sci. Environ. 407, 6143-6149. DOI: 10.1016/j.scitotenv.2009.08.039
Tang, U. W., and Wang, Z. S. (2007). “Influences of urban form on traffic-induced noise and air pollution: Results from a modelling sys-tem,” Environ. Model Softw. 22(12), 1750-1764. DOI: 10.1016/j.envsoft.2007.02.003
Tejada, A., Okuyama, T., Yamamoto, H., and Yoshida, M. (1997). “Reduction of growth stresses in logby direct heat treatment: assess-ment of a commercial scale operation,” For. Prod. J. 47(9), 86-93.
Temiz, A., Kose, G., Panov, D., Terziev, N., Alma, M. H., Palanti, S., and Akbas, S. (2013). “Effect of bio‐oil and epoxidized linseed oil on physical, mechanical, and biological properties of treated wood,” J. Appl. Polym. 130(3), 1562-1569. DOI: 10.1002/app.39334
Tjeerdsma, B. F., Boonstra, M., Pizzi, A., Tekely, P., and Militz, H. (1998). “Characterisation of thermally modified wood: Molecular reasons for wood performance improvement,” Holz Roh Werkst. 56(3), 149-153. DOI: 10.1007/s001070050287
Ulvcrona, T., Lindberg, H., and Bergsten, U. (2006). “Impregnation of Norway spruce (Picea abies L. Karst.) wood by hydrophobic oil and dispersion patterns in different tissues,” Forestry 79(1), 123–134. DOI: 10.1093/forestry/cpi064
Unnisa, S. A., and Hassanpour, M. (2017). “Development circumstances of four recycling industries (used motor oil, acidic sludge, plastic wastes and blown bitumen) in the world,” Renew. Sustain. Energy Rev. 72, 605-624. DOI: 10.1016/j.rser.2017.01.109
Wang, B., and Kang, J. (2011). “Effects of urban morphology on the traffic noise distribution through noise mapping: A comparative study between UK and China,” Appl. Acoust. 72(8), 556-568. DOI: 10.1016/j.apacoust.2011.01.011
Wang, D., Peng, L., Zhu, G., Fu, F., Zhou, Y., and Song, B. (2014). “Improving the sound absorption capacity of wood by microwave treatment,” BioResources 9(4), 7504-7518.
Wang, X., Mao, D., Yu, W., and Jiang, Z. (2015). “Sound barriers from materials of in homogeneous impedance,”J. Acoust. Soc. 137(6), 3190-3197. DOI: 10.1121/1.4921279
Yang, T., Hu, L., Xiong, X., Petrů, M., Noman, M. T., Mishra, R., and Militký, J. (2020). “Sound absorption properties of natural fibers: A review,” Sustainability 12(20), 1- 27. DOI: 10.3390/su12208477
Yang, X., and Kang, J. (2022). “The effect of visual and acoustic factors on the sound preference for waterscapes in urban public spaces,” Appl. Acoust. 197, article 108945. DOI: 10.1016/j.apacoust.2022.108945
Yermán, L., Ottenhaus, L. M., Zhang, Y., and Morrell, J. J. (2021). “Effect of thermal modification of slash pine with linseed oil on water repellency and performance of mechanical connections,” Constr. Build. Mater. 305, article 124776. DOI: 10.1016/j.conbuildmat.2021.124776
Zannin, P. H. T., Diniz, F. B., and Barbosa, W. A. (2002). “Environmental noise pollution in the city of Curitiba (Brazil),” Appl. Acoust. 63(4), 351-358. DOI: 10.1016/S0003-682X(01)00052-4
Zhu, L., Liu, Y., and Liu, Z. (2016). “Effect of high-temperature heat treatment on the acoustic-vibration performance of Picea jezoensis,” BioResources 11(2), 4921-4934.
Article submitted: December 11, 2022; Peer review completed: December 31, 2022; Revised version received and accepted: February 13, 2023; Published: February 28, 2023.
DOI: 10.15376/biores.18.2.2940-2963