Abstract
A densification method is proposed and developed to improve the mechanical properties of Uludağ fir (Abies bornmüelleriana Mattf.) and black poplar (Populus nigra L.) woods. The method, called Thermo-Vibro-Mechanic® densification, is derived from the hypothesis that the vibration added to the traditional thermo-mechanical densification process can cause the wood cell walls to interlock with each other at the micro-level via the friction effect. In addition, it aims to remove the cell cavities under lower pressure compared to other densification methods via the shaking effect. To test this hypothesis, the samples, obtained in both the radial and tangential directions, were pre-treated with wood stain and preservative before undergoing the densification process. Thermo-Vibro-Mechanic® densification was performed at varying temperatures (100, 120, and 140 °C), pressures (0.60, 1.00, and 1.40 MPa), and durations (20, 60, and 100 s). The changes in the values of the bending strength, modulus of elasticity, and compression strength parallel to the grain in the radial and tangential directions were determined accordingly. The results showed that the Thermo-Vibro-Mechanic® densification process increased the bending strength and modulus of elasticity values up to 50%, while the compression strength reached 67% higher than the untreated wood.
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Thermo-Vibro-Mechanic® (TVM) Wood Densification Method: Mechanical Properties
Mehmet Budakçı,a,* Süleyman Şenol,b and Mustafa Korkmaz a
A densification method is proposed and developed to improve the mechanical properties of Uludağ fir (Abies bornmüelleriana Mattf.) and black poplar (Populus nigra L.) woods. The method, called Thermo-Vibro-Mechanic® densification, is derived from the hypothesis that the vibration added to the traditional thermo-mechanical densification process can cause the wood cell walls to interlock with each other at the micro-level via the friction effect. In addition, it aims to remove the cell cavities under lower pressure compared to other densification methods via the shaking effect. To test this hypothesis, the samples, obtained in both the radial and tangential directions, were pre-treated with wood stain and preservative before undergoing the densification process. Thermo-Vibro-Mechanic® densification was performed at varying temperatures (100, 120, and 140 °C), pressures (0.60, 1.00, and 1.40 MPa), and durations (20, 60, and 100 s). The changes in the values of the bending strength, modulus of elasticity, and compression strength parallel to the grain in the radial and tangential directions were determined accordingly. The results showed that the Thermo-Vibro-Mechanic® densification process increased the bending strength and modulus of elasticity values up to 50%, while the compression strength reached 67% higher than the untreated wood.
DOI: 10.15376/biores.17.1.1606-1626
Keywords: Wood modification; Thermo-Vibro-Mechanic® densification; Mechanical properties
Contact information: a: Department of Wood Products Industrial Engineering, Faculty of Forestry, Düzce University, Düzce 81060 Turkey; b: Deceased; *Corresponding author: mehmetbudakci@duzce.edu.tr
INTRODUCTION
The use of wood materials in different areas and for different purposes is becoming popular with the developments in woodworking technology. Thanks to wood modification methods, certain wood can be used even in processes where its use was not possible until a few decades ago. Wood modification, which is defined as changing the structure of the wood material while applying new features, is a general term used for chemical, mechanical, thermal, etc. processes applied to wood. It includes many methods e.g., heat treatment, impregnation, densification, and surface modification (Hill 2006; Boonstra 2008; Petrič 2013).
According to a database containing more than 16,000 tree species spread around the world, the average global wood density is approximately 0.61 g/cm3 (Zanne et al. 2009). Wood shows very high resistance against static and dynamic loads, although the wood material typically is lighter in comparison to many building materials. Wood materials, which have a very high specific resistance (strength-to-weight ratio) compared to other materials, often provide better results than titanium or steel in terms of the specific tensile strength (Stalnaker and Harris 1989; Boyer et al. 1994).
Density is one of the most important parameters of a wood material, as it has a direct effect on its strength and durability (Evans and Ilic 2001; Downes et al. 2002; Blomberg et al. 2005; Kamke 2006; Kutnar and Sernek 2007). Therefore, dense wood types are preferred in industrial applications that require high strength. In general, fast-growing species tend to have a lower density than slow-growing ones (Udayakumar and Sekar 2017). Although slow-growing species show better mechanical properties, their commercial production requires a longer time (Black et al. 2008). However, many studies are carried out on the usage of timber obtained from low density and fast-growing tree species on an industrial scale (Wu et al. 2010; Han et al. 2015; Dong et al. 2016). The densification process, which is a mechanical modification method, is the frequently preferred method for increasing the density of low-density wood materials (Blomberg and Persson 2004; Blomberg et al. 2005; Kutnar and Sernek 2007; Pelit et al. 2015; Şenol and Budakçı 2016). In this regard, densification not only enables commercial use of fast-growing low-density trees, but it also contributes to the sustainability of slow-growing high-density species.
The amount of densification to be applied to the wood material is determined by the type of wood, as well as where and for what purpose the product will be used for (Kutnar and Sernek 2007). The density and strength properties of a densified wood vary according to the densification parameters, e.g., temperature, softening and plasticizing period, and pressure. The proper selection of these parameters ensures a product with up to 2 times the density compared to the undensified one (Ülker et al. 2012; Ulker and Hiziroglu 2017; Gao et al. 2019).
A wood material without any modification has a sponge-like structure (Kollmann and Côté 1968). During the densification process, thanks to this porous structure, the cell wall is collapsed into the cell cavities (Kutnar et al. 2009; Şenol and Budakçı 2016). The densification process at low temperatures is not effective, since it requires a greater amount of energy and causes damage to the wood fibers, resulting in a considerable decrease in the mechanical properties (Fang et al. 2012; Popescu et al. 2014). For this reason, the densification temperature typically is close to the glass transition temperature (Tg), which reduces the stiffness of the material and provides better results (Wolcott 1989; Arruda and Del Menezzi 2013; Laine et al. 2014; Tu et al. 2014). As the pressing time increases, the wood material is exposed to the temperature for a longer time and thus reaches higher compression ratios compared to those exposed to the same temperature for a shorter period of time (Inoue et al. 1993). Similarly, higher compression ratios can be achieved as the press pressure increases (Cloutier et al. 2008).
Kutnar et al. (2008) reported that the viscoelastic thermal compression (VTC) process increases the bending strength of hybrid poplar. They also reported that the bending properties increased linearly as the amount of densification was increased. Skyba et al. (2009) indicated that thermo-hydro-mechanical (THM) densification exerted a positive effect on the bending properties of spruce and beech woods.
Thermo-Vibro-Mechanical® (TVM) densification, as a new and environmentally friendly wood densification method, can be defined as the process of the densification of a material under a certain temperature, pressure, and frequency of vibration. It requires less energy compared to other modification methods. This method can be described as a surface densification method since it is especially effective on the outer parts of the wood. It was also reported that there was a serious change in the surface properties of the materials condensed with TVM, including its density, hardness, glossiness, abrasion, and swelling resistance (Şenol and Budakçı 2016; Şenol et al. 2017; Budakçı et al. 2021). In this study, the bending strength (BS), modulus of elasticity (MOE), and compression strength (CS) in the direction parallel to the fiber of Uludağ fir and black poplar specimens densified in the radial and tangential directions via the TVM method were investigated.
EXPERIMENTAL
Materials
In the study, Uludağ fir (Abies bornmüelleriana Mattf.) and black poplar (Populus nigra L.) woods, which are widely used in the furniture industry, were analyzed. It was taken into consideration that the wood, procured in the form of logs from a local forest management office in the Kütahya province of Turkey, would be robust with no growth defects or decay. Flawless wood pieces were cut into samples with dimensions of 360 (length) mm × 60 (width) mm × 21 (thickness) mm from sapwood, according to Turkish standard TS 2470 (1976). These samples were subjected to technical drying at a temperature of 20 °C ± 2 °C with a relative humidity of 65% ± 3% to achieve an air-dried moisture content of 12%, as per TS standard 2471 (1976). Conditioned samples were cut to a draft size according to the standards of the applied tests and then sanded with 100-grit sandpaper via a calibrated sanding machine (Version 1100 Melkuç Machine Company, Ankara, Turkey).
Afterwards, prior to TVM densification, the samples were impregnated with Akzo Nobel Kemipol brand-unicolor light walnut color (Catalog color-H 108 8001) aniline-based wood stain and Dewilux Dewitex 129-0174-52 brand colorless alkyd resin-based wood preservative using a 15 s dipping method. Since the wood stain was mixed with distilled water (85% solution), the wood preservative was applied without modification. The samples were conditioned again to eliminate any differences in humidity occurring after these processes. A total of 6084 samples were prepared for 507 different groups, which each contained 12 independent samples.
Thermo-Vibro-Mechanic® (TVM) Densification
The TVM densification process was carried out by applying constant linear vibration at a 3 mm amplitude and a 100 Hz frequency at three different temperatures (100 °C ± 3 °C, 120 °C ± 3 °C, and 140 °C ± 3 °C), three different pressures (0.60 MPa, 1.00 MPa, and 1.40 MPa) and three pressing duration levels (20 s, 60 s, and 100 s) as a result of preliminary trials and experiments. In addition, other studies in the field of wood densification were used in parameter selection (Rautkari et al. 2008, 2009). For this process, the samples were placed on the TVM press, which was specifically designed and manufactured within the scope of the research. They were first kept under positive low-pressure conditions (0.2 MPa) so that both surfaces were in contact with the press table. The samples remained in this position until their internal temperature reached the target temperature via checking with a digital thermometer (Fig. 1). Then, the target pressure was applied together, which included the vibration.
Except for the control group, half of the samples were densified in the tangential direction, while the other half were densified in the radial direction, in order to observe the effect of the direction of densification on the mechanical properties. At the end of the TVM densification process, the samples were removed from the TVM densification press, and they were cooled to a temperature of 60 °C at a pressure of 0.5 MPa to eliminate any spring-back effect. The samples were then kept in a conditioning cabinet (Fig. 2) at a temperature of 20 °C ± 2 °C with a relative humidity of 65% ± 3% until a constant weight was achieved. After the densification process, the thickness of the samples was approximately 20 mm.
Fig. 1. The TVM densification press and working principle
Fig. 2. Air conditioning and measuring the samples
Methods
Mechanical tests
Mechanical tests were performed to determine the bending strength (BS), modulus of elasticity (MOE), and compression strength (CS) of the prepared samples (as shown in Fig. 3). The BS and MOE values of the samples were determined using a UTEST 7012 (UTEST Material Testing Equipment, Ankara, Turkey) (at 50 kN) test device in accordance with TS standard 2474 (1976) and TS standard 2478 (1976), respectively. They were calculated according to Eqs. 1 and 2, respectively,
MOE (N/mm2) = PL3 / 4bh3 f
BS (N/mm2) = 3Pmax L / 2bh2
where P is the difference between
the mean of the lower and upper limits of the force (N), b and h are the width and height of the sample (mm), respectively, f is the displacement at the point of fracture (mm), L is the span between the bearings (mm), and Pmax is the fracture force (N).
The CS tests were conducted according to ISO standard 13061-17 (2017), while the CS was calculated according to Eq. 3,
CS (N/mm2) = Pmax / bh
where Pmax is the maximum force (N) applied to the specimen, and b and h are the width and height (mm) of the samples, respectively.
Fig. 3. Testing the bending strength and compression strength
Statistical analysis
The SPSS 22 statistical package program (IBM Corp., Armonk, NY) was used to analyze the data gathered in this study. The effects of the wood type, sectional direction, surface process, densification factors, and the interactions of these factors of each sample on the BS, MOE, and CS were determined via a multivariate analysis of variance (ANOVA) test. Comparisons were applied using Duncan’s multiple range test (DMRT) and least significant difference (LSD) critical values, while the factors causing the differences were examined as well.
RESULTS AND DISCUSSION
Bending Strength (BS)
The arithmetic means were obtained to determine the effect of the TVM densification process on the BS in terms of the wood type, sectional direction, surface process, and densification factors. Afterwards, an ANOVA test was performed to determine the factors causing the difference. The test results are provided in Table 1.
According to the ANOVA, all factors and their mutual interactions were determined to be significant (p-value less than or equal to 0.05) with respect to the BS. Table 2 lists the DMRT comparison results with respect to the wood type, sectional direction, surface process, and densification factors using the LSD critical value.
Table 1. Results of the ANOVA of the Bending Strength (BS) Values
According to Table 2, the BS value was highest in the black poplar samples (80.4 N/mm2) while it was lowest in the fir (79.4 N/mm2). It was highest in the tangential direction (81.3 N/mm2) and lowest in the radial direction (78.4 N/mm2) with respect to the sectional direction. In addition, it was highest in the wood preservative applied samples (83.2 N/mm2) while it was lowest in the aniline dye applied samples (78.90 N/mm2) with respect to the surface process. It was highest in the samples where TVM densification was applied at a temperature of 140 °C, a pressure of 1.4 MPa, and duration of 100 s (87.5 N/mm2), while it was lowest in samples without densification (control) (71.9 N/mm2) with respect to the densification.
Regarding the wood type factor, higher BS values were obtained in the poplar samples compared to the fir samples. After undergoing the TVM densification process, the BS increased up to 41% in the fir samples and 30% in the poplar samples compared to the control samples. The higher BS increase in the poplar samples may have resulted from the low density, diffuse-porous, and coarse-textured structure of this material. For this reason, densification with compression is considered to be more appropriate.
Table 2. The DMRT Comparison Results Considering the Bending Strength (BS) With Respect to the Wood Type, Sectional Direction, Surface Process, and Densification Factor (N/mm2)
With respect to the sectional direction factor, higher BS values were obtained in the tangential direction compared to the radial direction. After undergoing the TVM densification process, the BS increased up to 38.1% in the tangential direction and 22.3% in the radial direction compared to the control samples. Previous studies indicated that different results were obtained from the radial and tangential compression of wood depending on its anisotropic structure. It was also reported that the probable reason for the samples in the tangential direction having a higher BS value is the spring and summerwood layers, which have different densities before compression but have similar densities after densification (Blomberg et al. 2005; Marttila et al. 2016).
The highest BS with regards to the surface process factor was obtained in the samples treated with wood preservatives. After undergoing the TVM densification process, the BS increased up to 36% in samples treated with wood preservative, 31.4% in untreated (but densified) samples, and 26% in samples treated with aniline dye when compared to the undensified and untreated control samples. This increase may be due to the plasticizer property of the oil alkyd used as the main binder resin in wood preservative. Previous studies stated that oil-based impregnates have a flexible structure and can adapt to dimensional changes in wood (Budakçı and Togay 2002).
At the densification factor level, an increase varying from 13% to 52% was found in the BS values compared to the control samples. The highest BS values were obtained from samples under the following conditions: high temperatures (140 °C), high pressure (1.4 MPa), and longer durations (60 s and 100 s). Previous studies emphasized that the density increases as the compression ratio increases, which results in the increase of some of the mechanical properties of the wood (Tabarsa and Chui 1997; Blomberg et al. 2005; Ülker et al. 2012; Pelit 2014; Marttila et al. 2016)
The DMRT comparison and interaction of the results performed with respect to the level of wood type, sectional direction, surface process, and densification factors are provided in Figs. 4 and 5 to illustrate the results of single comparisons.
Fig. 4. The DMRT comparison results for the BS values with respect to the wood type, sectional direction, surface process, and densification interaction level
Fig. 5. The DMRT comparison results for the BS values with respect to the wood type, sectional direction, surface process, and densification interaction level
The results in Figs. 4 and 5 indicate that the highest BS value (103.9 N/mm2) was obtained from the tangential direction poplar samples that were treated with wood preservative and underwent TVM densification at a pressure of 1.4 MPa, a temperature of 140 °C, and a duration of 100 s.
The lowest value (60.1 N/mm2) was obtained from the undensified radial direction black poplar samples that were treated with aniline dye. The BS increased depending on the TVM densification parameters, and higher values were determined as the compression ratio and application time increased.
Modulus of Elasticity (MOE)
In order to determine the effect of the TVM densification process on the MOE in terms of the wood type, sectional direction, surface process, and densification factors, the arithmetic means were obtained. Afterwards, an ANOVA test was performed to determine the possible factors causing the difference. The test results are provided in Table 3.
According to the ANOVA, all factors and their mutual interactions were significant (p-value less than or equal to 0.05) with respect to the MOE. Table 4 shows the DMRT comparison results with respect to the wood type, sectional direction, surface process, and densification factors using the LSD critical value.
The results in Table 4 show that the MOE value was highest in the black poplar samples (9440 N/mm2), while it was lowest in the fir (9380 N/mm2) with respect to the wood type. It was highest in the tangential direction (9780 N/mm2) and lowest in the radial direction (9040 N/mm2) with respect to the sectional direction. In addition, it was highest in the wood preservative applied samples (9720 N/mm2), while it was lowest (9140 N/mm2) in the aniline dye applied ones with respect to the surface process.
Table 3. The ANOVA Results of the Modulus of Elasticity (MOE) Values
The MOE was also the highest in the samples where TVM densification was applied at a temperature of 140 °C, a pressure of 1.4 MPa, and a duration of 100 s (10200 N/mm2) while it was lowest in samples without densification, i.e., the control, (8660 N/mm2) with respect to the densification.
Table 4. The DMRT Comparison Results Considering the Modulus of Elasticity (MOE) With Respect to the Wood Type, Sectional Direction, Surface Process, and Densification Factor (N/mm2)
Regarding the wood type factor, higher MOE values were obtained in the poplar samples compared to the fir samples. After undergoing the TVM densification process, the MOE increased up to 34% in fir samples and 25% in poplar samples when compared to the control samples.
From the sectional direction factor standpoint, higher MOE values were obtained in the tangential direction compared to the radial direction. After undergoing the TVM densification process, the MOE increased up to 28.2% in the tangential direction and 17.6% in the radial direction when compared to the control samples. Previous studies stated that the density of a wood material provides an idea about its properties and usage possibilities. It was also indicated that the increase in density may have a positive effect on the MOE (Kollmann and Côté 1968; Marttila et al. 2016).
The highest MOE with respect to the surface process factor was obtained in the samples treated with wood preservatives. After undergoing the TVM densification process, the MOE increased up to 32.9% in samples treated with wood preservative, 22.3% in untreated (but densified) samples, and 22.5% in samples treated with aniline dye when compared to the undensified and untreated control samples. The results were in parallel with the findings of the BS tests. At the densification factor level, an increase varying from 11% to 50% was obtained for the MOE values in comparison to the control samples depending on TVM densification parameters. The highest MOE values were attained from samples under the following conditions: high temperature (140 °C), high pressure (1.4 MPa), and longer compression time (60 s and 100 s). A positive linear relationship between the densification ratio and the density value with the MOE has been reported in the literature (Kurtoğlu 1984; Tabarsa and Chui 1997; Marttila et al. 2016). In addition, the compression ratio has been declared as the most important factor affecting the MOE after the densification process and the compression temperature has been stated as not important (Lamason and Gong 2007; Pelit 2014).
Fig. 6. The DMRT comparison results for the MOE values with respect to the wood type, sectional direction, surface process, and densification interaction level
Fig. 7. The DMRT comparison results for the MOE values with respect to the wood type, sectional direction, surface process, and densification interaction level
The DMRT comparison and interaction of the results performed with respect to the wood type, sectional direction, surface process, and densification factors are provided in Figs. 6 and 7 to illustrate the results of single comparisons.
According to Figs. 6 and 7, the highest MOE value (12,600 N/mm2) was obtained from the tangential direction poplar samples that were treated with wood preservative and underwent TVM densification at 1.4 MPa and 140 °C for 100 s.
The lowest value (6130 N/mm2) was obtained from the radial direction untreated fir samples that underwent TVM densification at a pressure of 0.6 MPa, a temperature of 100 °C, and a duration of 20 s. The MOE increased depending on the TVM densification parameters while higher values were obtained as the compression ratio and application time increased.
Compression Strength (CS)
The arithmetic means were obtained to examine the effect of the TVM densification process on the CS in terms of the wood type, sectional direction, surface process, and densification factors. Afterwards, an ANOVA test was performed to determine the factors causing the difference. The test results are shown in Table 5.
According to the ANOVA, all factors and their mutual interactions were significant (p-value less than or equal to 0.05) with respect to the CS. Table 6 shows the DMRT comparison results with respect to the wood type, sectional direction, surface process, and densification factors using the LSD critical value.
Table 5. ANOVA Results of the Compression Strength (CS) Values
According to Table 6, the CS values obtained were the highest in the fir samples (43.7 N/mm2) and lowest in the poplar (41.1 N/mm2) with respect to the wood type. It was highest in the tangential direction (43.5 N/mm2) and lowest in the radial direction (41.2 N/mm2) with respect to the sectional direction. When the surface process was taken into account, the CS was highest in the wood preservative applied samples (43.0 N/mm2) and lowest in the aniline dye applied samples (41.5 N/mm2). In addition, it was highest in the samples where TVM densification was applied at a temperature of 140 °C, a pressure of 1.4 MPa, and a duration of 100 s (46.8 N/mm2), while it was lowest in samples without densification (control) (37.198 N/mm2).
Table 6. The DMRT Comparison Results Considering the Compression Strength (CS) With Respect to the Wood Type, Sectional Direction, Surface Process, and Densification Factor (N/mm2)
Regarding the wood type factor, higher CS values were obtained from the fir samples compared to the poplar samples. After undergoing the TVM densification process, the CS increased up to 29.4% in poplar samples and 23.0% in fir samples compared to the control samples.
Such a result may depend on the fact that fir is a coniferous species, meaning that the regions of the summer wood and spring wood in the annual rings are distinctly different. It has been reported in the literature that the amount of latewood positively affects the hardness, abrasion, and resistance properties of the wood (Kutnar and Sernek 2007; Lamason and Gong 2007; Rautkari et al. 2013).
With respect to the sectional direction factor, higher CS values were obtained in the tangential direction compared to the radial direction. After undergoing the TVM densification process, the CS increased up to 33.0% in the tangential direction and 23.5% in the radial direction when compared to the control samples. In addition, it has been stated in the literature that the strength properties of densified wood material are improved compared to untreated wood and this increase is generally proportional to the increase in density (Morsing 2000; Pelit 2014).
The highest CS value with respect to the surface process factor was obtained from the samples treated with wood preservatives. After undergoing the TVM densification process, the CS increased up to 35% in samples treated with wood preservative, 33.5% in untreated (but densified) samples, and 28.2% in samples treated with aniline dye compared to the undensified and untreated control samples. Previous studies have stated that the amount of oil retention and the density values are parallel. In addition, the increment in density considerably increases the CS as well as other strength properties (Bozkurt and Erdin, 1997; Kollmann and Côté 1968). The results of Fourier-transform infrared spectrophotometry (FTIR) analysis showed that only a small part of the wood compositions was degraded, with most of their structure being preserved. The oil absorption of wood may also have increased the lateral stability by filling the cell gaps and increasing the cell wall thickness.
At the densification factor level, an increase varying from 15% to 67% was observed in the CS values when compared to the control samples. The highest CS values were obtained from samples under the following conditions: high temperature (140 °C), high pressure (1.4 MPa), and longer duration (100 s). The results also showed that the CS increased depending on the TVM densification parameters; higher values were determined as the compression ratio and application time increased. It has been stated in the literature that the increase in the strength values of the wood after the densification process is due to the decrease in the void volume of the wood material as well as the increase in the load-bearing cell wall per unit volume (Ülker et al. 2012). Additionally, it has been reported that the increment in density increases the CS parallel to the fibers, the BS in radial and tangential directions, and the Brinell hardness values (Blomberg et al. 2005).
The DMRT comparison and interaction of the results performed with respect to the wood type, sectional direction, surface process, and densification factors are provided in Figs. 8 and 9 to illustrate the results of single comparisons.
The results in Figs. 8 and 9 show that the highest CS value (52.7 N/mm2) was obtained from the tangential direction untreated poplar samples that underwent TVM densification at a pressure of 1.4 MPa, a temperature of 140 °C, and a duration of 100 s. The lowest value (29.5 N/mm2) was obtained from undensified radial direction black poplar samples that were treated with aniline dye. The CS increased depending on the TVM densification parameters, while higher values were obtained as the compression ratio and application time increased.
Fig. 8. The DMRT comparison results for the CS values with respect to the wood type, sectional direction, surface process, and densification interaction level
Fig. 9. The DMRT comparison results for the CS values with respect to the wood type, sectional direction, surface process, and densification interaction level
CONCLUSIONS
- This study developed an innovative and environmentally friendly new method called Thermo-Vibro-Mechanic® (TVM) densification as an alternative to densification and thermal modification processes to obtain wood material with high performance by improving the resistance characteristics of low-density wood types.
- Densification with TVM increased the bending strength (BS) and modulus of elasticity (MOE) values up to 50% and the CS value up to 67%.
- Generally, the highest mechanical properties were obtained in the samples that underwent TVM densification at the highest pressure (1.4 MPa), temperature (140 °C), and duration (100 s).
- It was observed that the TVM densification process caused a noticeable improvement in the mechanical properties of fast-growing and low-density woods. The study has particular value in the TVM process. It is a new method and provides an alternative contribution to the use of fast-growing low-density species with relatively low commercial value.
- Although densification via the TVM method especially affected the parts of the samples near the surface, it also caused a notable improvement in the mechanical properties. In this respect, it can be claimed that the mechanical properties were improved by consuming less energy compared to methods that completely densified the wood.
ACKNOWLEDGMENTS
This research has been written in memory of Dr. Süleyman Şenol, who passed away on May 10, 2019. The study was supported by Project No. 115O138 within the scope of the “TÜBİTAK-3001 Start R&D Projects Support Program”. In addition, the trademark Thermo-Vibro-Mechanic® (TVM) was registered on July 01, 2019 with Patent No. 2018/120796 by the Turkish Patent and Trademark Authority. The Thermo-Vibro-Mechanic® (TVM) density press patent registration process has also been continued with Application No. 2019/01054.
REFERENCES CITED
Arruda, L. M., and Menezzi, C. H. D. (2013). “Effect of thermomechanical treatment on physical properties of wood veneers,” International Wood Products Journal 4(4), 217-224. DOI: 10.1179/2042645312Y.0000000022
Black, B. A., Colbert, J. J., and Pederson, N. (2008). “Relationships between radial growth rates and lifespan within North American tree species,” Écoscience 15(3), 349-357. DOI: 10.2980/15-3-3149
Blomberg, J., and Persson, B. (2004). “Plastic deformation in small clear pieces of Scots pine (Pinus sylvestris) during densification with the CaLignum process,” Journal of Wood Science 50(4), 307-314. DOI: 10.1007/s10086-003-0566-2
Blomberg, J., Persson, B., and Blomberg, A. (2005). “Effects of semi-isostatic densification of wood on the variation in strength properties with density,” Wood Science and Technology 39(5), 339-350. DOI: 10.1007/s00226-005-0290-8
Boonstra, M. J. (2008). A Two-stage Thermal Modification of Wood, Ph.D. Dissertation Ghent University, Ghent, Belgium.
Boyer, R., Welsch, G., and Collings, E. W. (1994). Materials Properties Handbook: Titanium Alloys, ASM International, Novelty, OH.
Bozkurt, Y., and Erdin, N. (1997). Wood Technology, Istanbul University Publication, Istanbul, Turkey.
Budakçı, M., and Togay, A. (2002). “The color changing effect of outdoor conditions on synthetic varnish and a diffusion paint applied scotch pine,” The Journal of the Industrial Arts Education Faculty of Gazi University 11(10), 55-68.
Budakçı, M., Şenol, S., and Korkmaz, M. (2021). “Effects of Thermo-Vibro-Mechanic® densification on the density and swelling of pre-treated Uludağ fir and black poplar wood,” BioResources 16(1), 1581-1599. DOI: 10.15376/biores.16.1.1581-1599
Cloutier, A., Fang, Ch., Mariotti, N., Koubaa, A., and Blanchet, P. (2008). “Densification of wood veneers under the effect of heat, steam and pressure,” Proceedings of the 51th International Convention of Society of Wood Science and Technology, November 10-12, Concepción, Chile, pp. 1-9.
Dong, Y., Qin, Y., Wang, K., Yan, Y., Zhang, S., Li, J., and Zhang, S. (2016). “Assessment of the performance of furfurylated wood and acetylated wood: Comparison among four fast-growing wood species,” BioResources 11(2), 3679-3690. DOI: 10.15376/biores.11.2.3679-3690
Downes, G. M., Nyakuengama, J. G., Evans, R., Northway, R., Blakemore, P., Dickson, R. L., and Lausberg, M. (2002). “Relationship between wood density, microfibril angle and stiffness in thinned and fertilized Pinus radiata,” IAWA Journal 23(3), 253-265. DOI: 10.1163/22941932-90000302
Evans, R., and Ilic, J. (2001). “Rapid prediction of wood stiffness from microfibril angle and density,” Forest Products Journal 51(3), 53-57.
Fang, C.-H., Mariotti, N., Cloutier, A., Koubaa, A., and Blanchet, P. (2012). “Densification of wood veneers by compression combined with heat and steam,” European Journal of Wood and Wood Products 70, 155-163. DOI: 10.1007/s00107-011-0524-4
Gao, Z., Huang, R., Chang, J., Li, R., and Wu, Y. (2019). “Effects of pressurized superheated-steam heat treatment on set recovery and mechanical properties of surface-compressed wood,” BioResources 10(3), 1718-1730. DOI: 10.15376/biores.14.1.1718-1730
Han, X., Miao, X., Zheng, X., Xing, L., and Pu, J. (2015). “Chemical modification by impregnation of poplar wood with functional composite modifier,” BioResources 10(3), 5203-5214. DOI: 10.15376/biores.10.3.5203-5214
Hill, C. A. S. (2006). Wood Modification Chemical, Thermal and Other Processes, John Wiley & Sons, Ltd., Hoboken, NJ.
Inoue, M., Norimoto, M., Tanahashi, M., and Rowell, R. M. (1993). “Steam or heat fixation of compressed wood,” Wood Fiber Sci. 25(3), 224-235.
ISO 13061-17 (2017). “Physical and mechanical properties of wood — Test methods for small clear wood specimens – Part 17: Determination of ultimate stress in compression parallel to grain,” International Organization for Standardization, Geneva, Switzerland.
Kamke, F. A. (2006). “Densified radiata pine for structural composites,” Maderas. Ciencia y Tecnologia 8(2), 83-92. DOI: 10.4067/S0718-221X2006000200002
Kollmann, F. F. P., and Côté Jr., W. A. (1968). Principles of Wood Sciences and Technology, Vol. I. Solid Wood, Springer Verlag, Berlin, Germany.
Kurtoğlu, A. (1984). “Wood material-weight relations,” Journal of the Faculty of Forestry Istanbul University 34(1), 150-163.
Kutnar, A., and Sernek, M. (2007). “Densification of wood,” Zbornik Gozdarstva in Lesarstva 82(2007), 53-62.
Kutnar, A., Kamke, F. A., and Sernek, M. (2008). “The mechanical properties of densified VTC wood relevant for structural composites,” Holz als Roh- und Werkstoff 66(6), 439-446. DOI: 10.1007/s00107-008-0259-z
Kutnar, A., Kamke, F. A., and Sernek, M. (2009). “Density profile and morphology of viscoelastic thermal compressed wood,” Wood Science and Technology 43(1-2), 1-12. DOI: 10.1007/s00226-008-0198-1
Laine, K., Segerholm, K., Wålinder, M., Rautkari, L., Ormondroyd, G., Mark, H., and D., J. (2014). “Micromorphological studies of surface densified wood,” Journal of Materials Science 49(5), 2027-2034. DOI: 10.1007/s10853-013-7890-8
Lamason, C., and Gong, M. (2007). “Optimization of pressing parameters for mechanically surface-densified aspen,” Forest Products Journal 57(10), 64-68.
Marttila, J., Möttönen, V., Bütün, Y., and Heräjärvi, H. (2016). “Bending properties of tangentially and radially sawn European aspen and silver birch wood after industrial scale thermo-mechanical modification,” in: Proceedings of the 2nd Conference on Engineered Wood Products Based on Poplar/Willow Wood, 8-10 September, León, Spain, pp. 113-124.
Morsing, N. (2000). Densification of Wood: The Influence of Hydrothermal Treatment on Compression of Beech Perpendicular to the Grain, Ph.D. Dissertation, Technical University of Denmark, Copenhagen, Denmark.
Pelit, H. (2014). “The Effects of Densification and Heat Treatment on Finishing Process with Some Technological Properties of Eastern Beech and Scots Pine,” Ph.D. Dissertation, Gazi University, Ankara, Turkey.
Pelit, H., Sönmez, A., and Budakçı, M. (2015). “Effects of thermomechanical densification and heat treatment on density and Brinell hardness of Scots pine (Pinus sylvestris L.) and Eastern beech (Fagus orientalis L.),” BioResources 10(2), 3097-3111. DOI: 10.15376/biores.10.2.3097-3111
Petrič, M. (2013). “Surface modification of wood,” Reviews of Adhesion and Adhesives 1(2), 216-247. DOI: 10.7569/RAA.2013.097308
Popescu, M.-C., Lisa, G., Froidevaux, J., Navi, P., and Popescu, C.-M. (2014). “Evaluation of the thermal stability and set recovery of thermo-hydro-mechanically treated lime (Tilia cordata) wood,” Wood Science and Technol. 48, 85-97. DOI: 10.1007/s00226-013-0588-x
Rautkari, L., Properzi, M., Pichelin, F., and Hughes, M. (2008). “An innovative thermo densification method for wooden surfaces,” in: Proceedings of the 10th World Conference on Timber Engineering, Miyazaki, Japan.
Rautkari, L., Properzi, M., Pichelin, F., and Hughes, M. (2009). “Surface modification of wood using friction,” Wood Sci. Technol. 43(3-4), 291-299. DOI: 10.1007/s00226- 008-0227-0
Rautkari, L., Laine, K., Kutnar, A., Medved, S., and Hughes, M. (2013). “Hardness and density profile of surface densified and thermally modified Scots pine in relation to degree of densification,” Journal of Materials Science 48(6), 2370-2375. DOI: 10.1007/s10853-012-7019-5
Şenol, S., and Budakçı, M. (2016). “Mechanical wood modification methods,” Muğla Journal of Science and Technology 2(2), 53-59. DOI: 10.22531/muglajsci.283619
Şenol, S., Budakçı, M., and Korkmaz, M. (2017). “The effect of thermo-vibro-mechanical (TVM) densification process on density and abrasion resistance of some wood materials,” Journal of Advanced Technology Sciences 6(3), 263-275.
Skyba, O., Schwarze, F., and Niemz, P. (2009). “Physical and mechanical properties of thermo-hygro-mechanically (THM) densified wood,” Wood Research 54(2), 1-18.
Stalnaker, J. J., and Harris, E. C. (1989). “Selecting sawn-timber compression and tension members,” in: Structural Design in Wood, J. J. Stalnaker, and E. C. Harris (ed.), Springer, Boston, MA, pp. 129-151.
Tabarsa, T., and Chui, Y. H. (1997). “Effects of hot-pressing on properties of white spruce,” Forest Products Journal 47(5), 71-76.
TS 2470 (1976). “Wood – Sampling methods and general requirements for physical and mechanical tests,” Turkish Standards Institute, Ankara, Turkey.
TS 2471 (1976). “Wood – Determination of moisture content for physical and mechanical tests,” Turkish Standards Institute, Ankara, Turkey.
TS 2474 (1976). “Wood – Determination of ultimate strength in static bending,” Turkish Standards Institute, Ankara, Turkey.
TS 2478 (1976). Wood – Determination of modulus of elasticity in static bending,” Turkish Standards Institute, Ankara, Turkey.
Tu, D., Su, X., Zhang, T., Fan, W., and Zhou, Q. (2014). “Thermo-mechanical densification of Populus tomentosa var. tomentosa with low moisture content,” BioResources 9(3), 3846-3856. DOI: 10.15376/biores.9.3.3846-3856
Udayakumar, M., and Sekar, T. (2017). “Estimation of leaf area–wood density traits relationship in tropical dry evergreen forests of Southern Coromandel Coast, Peninsular India,” in: Wood is Good, K. K. Pandey, V. Ramakantha, S. S. Chauhan, and A. N. A. Kumar (ed.), Springer, Singapore, pp. 169-187.
Ulker, O., and Hiziroglu, S. (2017). “Some properties of densified Eastern redcedar as function of heat and pressure,” Materials 10(11), 1-14. DOI: 10.3390/ma10111275
Ülker, O., Imirzi, Ö., and Burdurlu, E. (2012). “The effect of densification temperature on some physical and mechanical properties of Scots pine (Pinus sylvestris L.),” BioResources 7(4), 5581-5592. DOI: 10.15376/biores.7.4.5581-5592
Wolcott, M. P. (1989). Modeling Viscoelastic Cellular Materials for the Pressing of Wood Composites, Ph.D. Dissertation, Virginia Polytechnic Institute and State University, Blacksburg, Virginia.
Wu, G., Lang, Q., Qu, P., Jiang, Y., and Pu, J. (2010). “Effect of chemical modification and hot-press drying on poplar wood,” BioResources 5(4), 2581-2590. DOI: 10.15376/biores.5.4.2581-2590
Zanne, A. E., Lopez-Gonzalez, G., Coomes, D. A., Ilic, J., Jansen, S., Lewis, S. L., Miller, R. B., Swenson, N. G., Wiemann, M. C., and Chave, J. (2009). “Data from: Towards a worldwide wood economics spectrum,” (https://datadryad.org/stash/ dataset/doi:10.5061/dryad.234), Accessed: 28 September 2021.
Article submitted: October 19, 2021; Peer review completed: December 11, 2021; Revised version received and accepted: December 16, 2021; Published: January 14, 2022.
DOI: 10.15376/biores.17.1.1606-1626