Abstract
The radial and tangential swelling as well as the fully dried density of low-density wood materials densified via the Thermo-Vibro-Mechanic® method were evaluated in response to applying wood stain and preservative. The samples obtained from Uludağ fir (Abies bornmüelleriana Mattf.) and black poplar (Populus nigra L.) in the radial and tangential direction were pre-treated with wood stain and preservative before undergoing Thermo-Vibro-Mechanic® densification. Thermo-Vibro-Mechanic® densification was performed at three different temperatures (100 °C ± 3 °C, 120 °C ± 3 °C, and 140 °C ± 3 °C), three different vibration pressures (0.60 MPa, 1.00 MPa, and 1.40 MPa), and three different vibration times (20 s, 60 s, and 100 s). Afterwards, changes in the fully dried density and swelling amounts in the radial and tangential directions of the samples were determined. The fully dried density increased by 15.4% to 38% and the radial and tangential swelling amounts increased by 73.2% to 242.6%, when the densified samples were compared to the control samples. In general, the fully dried density and swelling values increased depending on the Thermo-Vibro-Mechanic® densification parameters; higher values were found as the compression ratio and total application time increased.
Download PDF
Full Article
Effects of Thermo-Vibro-Mechanic® Densification on the Density and Swelling of Pre-Treated Uludağ Fir and Black Poplar Wood
Mehmet Budakçı,a,* Süleyman Şenol,b and Mustafa Korkmaz a
The radial and tangential swelling as well as the fully dried density of low-density wood materials densified via the Thermo-Vibro-Mechanic® method were evaluated in response to applying wood stain and preservative. The samples obtained from Uludağ fir (Abies bornmüelleriana Mattf.) and black poplar (Populus nigra L.) in the radial and tangential direction were pre-treated with wood stain and preservative before undergoing Thermo-Vibro-Mechanic® densification. Thermo-Vibro-Mechanic® densification was performed at three different temperatures (100 °C ± 3 °C, 120 °C ± 3 °C, and 140 °C ± 3 °C), three different vibration pressures (0.60 MPa, 1.00 MPa, and 1.40 MPa), and three different vibration times (20 s, 60 s, and 100 s). Afterwards, changes in the fully dried density and swelling amounts in the radial and tangential directions of the samples were determined. The fully dried density increased by 15.4% to 38% and the radial and tangential swelling amounts increased by 73.2% to 242.6%, when the densified samples were compared to the control samples. In general, the fully dried density and swelling values increased depending on the Thermo-Vibro-Mechanic® densification parameters; higher values were found as the compression ratio and total application time increased.
Keywords: Thermo-Vibro-Mechanic® (TVM) densification; Wood stain; Wood preservative; Density; Swelling
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
Most of the mechanical properties of wood materials are related to their density (Blomberg and Persson 2004; Kamke 2006; Kutnar and Šernek 2007; Rautkari 2012; Budakçı et al. 2016; Pelit et al. 2017, Şenol and Budakçı 2019). Generally, high-density wood species are preferable for many engineering structures and applications due to their high mechanical strength. However, high-density wood resources are limited and usually expensive (Fang et al. 2019). Densification treatments make it possible to increase the density of low or moderate density woods, as well as obtaining a specific strength in densified woods that is higher than most structural metals and alloys. These characteristics make it a low-cost, high-performance, and lightweight alternative compared to other structural materials (Song et al. 2018; Fang et al. 2019). Since the densification of wood drastically improves its mechanical properties and hardness, multiple studies have been carried out to determine the optimum parameters for proper densification (Blomberg and Persson 2004; Rautkari et al. 2008, 2009; Fu et al. 2016, 2017; Li et al. 2017; Sandberg et al. 2017; Şenol et al. 2017; Şenol and Budakçı 2018; Cruz et al. 2018; Şenol and Budakçı 2019). With the densification process, low-density woods are converted to high density woods, therefore making them commercially high-value products. In addition, high-density woods can be made more durable via densification (Blomberg et al. 2005; Kutnar and Šernek 2007; Ulker et al. 2012; Şenol and Budakçı 2019).
The type of material, temperature, softening or plasticizing period, densification method, and amount of pressure are the most important variables in the densification of wood materials. Each of these parameters affects the resistance properties of the wood after the densification process. A different application of these parameters can increase the strength properties of densified wood materials up to 100%. (Ulker et al. 2012; Gao et al. 2019).
The densification of wood materials via compression is based on the principle of collapsing the cell wall, thus reducing the void volume. (Kutnar et al. 2009; Budakçı et al. 2016). However, breaks and cracks can occur in the cell wall of densified wood material under standard room conditions. The natural elastic structure of the wood plays an important role in the densification process via compression. Lignin, which gives the wood its rigidity, exhibits an elastic property when the wood temperature is greater than the glass-transition temperature. Therefore, the densification process above the glass transition temperature can be performed without major deformations in amorphous polymers or cracks in cell walls. Compression properties mostly depend on the density, moisture content, cell wall volume, and compressing direction of the wood. The biggest problem with the densification process is that the compressed woods tend to return to their original dimensions, due to the spring-back effect, when exposed to moisture or water (Seborg et al. 1956; Kollmann et al. 1975; Kultikova 1999; Morsing 2000; Blomberg et al. 2006; Rautkari 2012). However, this problem can be eliminated by using heat and steam during the densification process (Kutnar and Šernek 2007; Rautkari et al. 2010; Li et al. 2017; Şenol and Budakçı 2019).
The densification of wood without the use of chemicals has been known for a long time. However, due to the plasticization and insufficient dimensional stability of the final products, it has not been widely adopted by industry. Many types of densified wood products have been produced at various times around the world. In addition, due to the increase in environmental awareness in the last quarter-century, there have been restrictions on the usage of environmentally harmful preservatives. This has led to the development of new environmentally friendly methods that preserve the wood against biological degradation and increase its dimensional stability (Korkut and Kocaefe 2009; Şenol and Budakçı 2016). Examples of these methods are as follows: densification using temperature and pressure in an open system, i.e., thermo-mechanical (TM); densification using temperature, pressure, and steam in a closed system, i.e., thermo-hygro-mechanical (THM); densification using temperature and pressure after pre-softening with steam, i.e., viscoelastic-thermal-compression (VTC); and densification using temperature, pressure, and vibration, i.e., thermo-vibro-mechanic® (TVM), which is a new application method (Şenol and Budakçı 2016). The hypothesis behind the TVM densification process is that it is advantageous to reduce the long, cylindrical void spaces, called lumens, in wood cells by heat treatment combined with compression. Unlike other densification techniques, this technique aims to mechanically bond the opposite inner faces of the cell wall on the axis of the densification direction to each other by a vibration effect.
The main aim of the study was to determine the radial and tangential swelling as well as the fully dried density of TVM densification applied pre-treated low-density wood materials. Also, the goal was to obtain more durable wood by improving the strength properties of low-density wood species via the TVM densification process, which is an environmentally friendly new modification method and an alternative to densification and thermal modification processes. For this purpose, a new and special TVM densification press was designed and manufactured. The samples were prepared from Uludağ fir (Abies Bornmülleriana Mattf.) and black poplar (Populus nigra L.) in the radial and tangential direction. They were pre-treated with wood stain and wood preservative before being densified via the TVM method. Lastly, the changes caused by TVM densification in terms of the values of the fully dried density, according to TS standard 2472 (1976), and the swelling in radial and tangential directions, according to TS standard 4084 (1983), were determined. The results of the experiments were analyzed and interpreted.
EXPERIMENTAL
Materials
Preparation of the wooden materials
Uludağ fir (Abies bornmüelleriana Mattf.) and black poplar (Populus nigra L.), which are widely used in the forest products industry in Turkey, were the preferred species used for the preparation of the samples. It was taken into consideration that the wood, procured in the form of logs from Forest Management in the Kütahya province of Turkey, would be robust with no growth defects or decay. Flawless (as much as possible free from defects such as knots, rot, burl tissue, coarse grain, cracks etc.) wood pieces were cut with dimensions of 360 (length) × 60 (width) × 21 (thickness) mm from sapwood according to TS standard 2470 (1976). These timber samples were subjected to technical drying to achieve an air-dried moisture of 12%. Samples at the specified air-dried moisture percentage 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. Afterwards, prior to TVM densification, the samples were impregnated with Akzo Nobel Kemipol brand-unicolor open walnut (Catalog color-H 108 8001, AkzoNobel Kemipol AS, Kemalpaşa, Turkey) color aniline-based wood stain and Dewilux Dewitex 129-0174-52 brand colorless alkyd resin-based wood preservative (DYO Boya, İzmir, Turkey) using a 15 s dipping method. After the wood stain was mixed with 85% distilled water, the wood preservative was applied at the packaging viscosity. The samples were conditioned again at a temperature of 20 °C ± 2 °C and a relative humidity of 65% ± 3% in order to achieve equilibrium moisture content, as per TS standard 2471 (1976).
TVM densification
The TVM densification process was conducted with constant linear vibration at 100 Hz frequency and 3 mm amplitude at three different temperatures (100 ± 3 °C, 120 ± 3 °C, and 140 ± 3 °C), three different pressures (0.60 MPa, 1.00 MPa, and 1.40 MPa), and three different vibration times (20 s, 60 s, and 100 s). For this process, the samples placed on the TVM density press table, especially designed and manufactured within the scope of the research, were first kept under low positive pressure conditions (0.2 MPa) so that both surfaces were in contact with the press table. The samples remained in this position until the internal temperature of the samples reached the target temperature values via checking with a digital thermometer (as shown in Fig. 1).
Fig. 1. TVM densification press and working principle (Şenol 2018)
At the end of the TVM densification process, the samples were removed from the TVM density press and cooled to a temperature of 60 °C on a different press (plating press) at a pressure of 0.5 MPa to eliminate any spring-back effect. Then, the samples were kept in the climate chamber at a temperature of 20 °C ± 2 °C temperature and a relative humidity of 65% ± 3%, according to TS standard 2471 (1976), before undergoing testing and they reached a constant weight (Fig. 2). Half of the samples (n=2016) were densified in the tangential and the other half in the radial direction. The final dimensions of samples were 20 mm in width (tangential), 20 mm in height (radial), and 30 mm in length (longitudinal) for both tests. A total of 4032 samples were prepared for 168 different groups, each consisting of 12 samples. Each group was created using different and independent samples.
Fig 2. Fully dried samples and measuring the samples
Methods
Fully dried density
The density measurements in fully dried conditions were made in accordance with TS standard 2472 (1976) procedures. The air-dry samples were kept in an oven at a temperature of 103 °C ± 2 °C until they reached a constant weight. Then, the masses of the samples were determined using an analytical balance with an accuracy of ± 0.01 g, and their dimensions were measured with a digital caliper with a precision of ± 0.01 mm. Lastly, their volume (V) was determined according to Eq. 1,
(1)
where δ0 is the fully dried density (g/cm3), M0 is the fully dried mass (g), and V0 is the fully dried volume (cm3).
Radial and tangential swelling
The determination of the swelling rates of the samples were performed in accordance with TS standard 4084 (1983). Initially, the samples were kept at a temperature of 103 °C ± 2 °C until they reached a constant size and mass. Subsequently, the radial and tangential thicknesses of the samples were determined from their midpoints with a digital caliper with a precision of ± 0.01 mm. Thereafter, samples were placed in a glass aquarium containing distilled water at a temperature of 20 ± 2 °C. A wire mesh was placed over the aquarium so that the samples were completely submerged in water. The position of the samples was changed via stirring with a rod at regular intervals (Fig. 3). When the dimensions of the samples became stable, they were taken out of the aquarium and any excess water was gently wiped off. Finally, the radial and tangential thickness were measured again from the first measurement points. The measurements were made in the direction in which the densification was made (Fig 4).
Fig. 3. Soaking samples in distilled water
Fig. 4. Preparing and testing of samples; (Mt: measuring of samples densificated in tangential direction; Mr: Measuring of samples densificated in radial direction)
The swelling ratio in the compression direction (radial and tangential) was calculated according to Eq. 2,
(2)
where αk is the swelling ratio of the compression (%), L0 is the fully dried dimensions (mm), and LR is the swollen dimension (mm).
Statistical analysis
The SPSS 22 statistical package program (IBM Corp., Armonk, NY) was used to evaluate the data. Multivariate analysis of variance (ANOVA) tests determined the effects of the wood type, sectional direction, surface process, densification factors, and the interactions of these factors with the density and swelling values at a significance level of 0.05. Comparisons were made using Duncan’s multiple range test (DMRT) and least significant difference (LSD) critical values, and the factors causing the differences were examined.
RESULTS AND DISCUSSION
Fully Dried Density
The arithmetic means were obtained to determine the effect of the TVM densification process on the fully dried density values in terms of the wood type, sectional direction, surface process, and densification factors. Multiple variance analysis (ANOVA) was performed to determine which factor caused the difference, and the results are shown in Table 1.
Table 1. Results of the ANOVA of the Fully Dried Density Values
According to the ANOVA results, the CD, ABC, ACD, BCD, and ABCD interactions were insignificant and had no effect on fully dried density value. On the other hand, the other factors and interactions were significant with respect to the fully dried density values (p-value was less than or equal to 0.05). Table 2 shows Duncan’s multiple range test (DMRT) performed for the wood type, sectional direction, surface process, and densification factors using the LSD critical value.
Table 2. DMRT Comparison Results for the Wood Type, Sectional Direction, Surface Process, and Densification Factors (g/cm3)
According to Table 2, the fully dried density value was highest in fir samples (0.434 g/cm3) and lowest in the poplar samples (0.43 g/cm3) at the wood type level. It was highest in the tangential direction (0.44 g/cm3) and lowest in the radial direction (0.425 g/cm3) at the sectional direction level. It was highest in the wood preservative applied samples (0.436 g/cm3) and lowest in the aniline dye applied samples (0.43 g/cm3) at the surface process level. With respect to densification level, it was highest in the samples where TVM densification was applied at 140 °C, 1.4 MPa, and 100 s (0.47 g/cm3), while lowest in samples without densification (control) (0.386 g/cm3).
Regarding the wood type factor, higher fully dried density values were obtained in the fir samples compared to the poplar samples. After undergoing the TVM densification process, the density increased up to 18.5% in fir samples and 25.5% in poplar samples compared to the control samples. The higher density increase in poplar samples may have resulted from the low density, diffuse-porous, and coarse-textured structure of this material. Thus, it was more appropriate for densification with compression. Previous studies on the compressibility of wood indicated that compressibility is dependent on the anatomical properties of the wood material, e.g., density, late wood ratio, cell wall volume, and the compression direction (Kutnar and Šernek 2007). In addition, it was stated that the density increase obtained via compression was dependent on the spring-back effect, densification method, and the amount of compression, as well as the properties of wood. (Rautkari 2012; Pelit et al. 2015). In different studies, it has been reported that as the compression amount of wood increases, the density increase becomes more evident (Blomberg 2006; Rautkari et al. 2010; Laine 2014). In this respect, the present study is compatible with previous studies.
With respect to the sectional direction factor, higher fully dried density values were obtained in the tangential direction compared to the radial direction. After undergoing the TVM densification process, the density increased up to 23% in the radial direction and 20% in the tangential direction when compared to the control samples. In previous studies, it has been indicated that different results are obtained from the radial and tangential compression of wood due to its anisotropic structure. In addition, it is reported that radial direction samples can be less densified due to the higher rate of late wood, which can be less compressed due to its low porous structure (Blomberg et al. 2005; Marttila et al. 2016).
The highest fully dried density in regard to the surface process factor was obtained in the samples treated with wood preservative. After undergoing the TVM densification process, the fully dried density increased up to 24.3% in samples treated with wood preservative, 22.45% in untreated (but densified) samples, and 21.4% in samples treated with aniline dye compared to the undensified and untreated control samples. Previous studies stated that oil-based impregnates fill the radial rays and tracheid lumens, and the amount of oil early wood can absorb is greater than late wood due to its highly porous structure. However, it was stated that with an increase in weight, higher oil intake was observed in late wood (Olsson et al. 2001; Tomak 2011). It was also reported that there is a positive correlation between density and the amount of oil absorbed by the wood, which is more evident in early wood than in late wood (Ulvcrona et al. 2006; Tomak 2011).
At the densification factor level, an increase ranging from 15.4% to 38% was found in the fully dried density values compared to the control samples. The highest fully dried density values were obtained from samples under the following conditions: high temperatures (140 °C), high pressure (1.4 MPa), and longer compression times (100 s). In previous studies, it was emphasized that as the compression ratio increases, the density increases; this increase depends on the characteristics of the wood, the amount of compression, and the densification method (Blomberg et al. 2005; Gong and Lamason 2007; Ünsal and Candan 2008; Rautkari 2012; Rautkari et al. 2013; Pelit et al. 2015; Budakçı et al. 2016). Therefore, the present study is compatible with previous studies.
The DMRT comparison and interaction of the results performed at the level of wood type, sectional direction, surface process, and densification factors are shown in Figs. 5 and 6, in order to illustrate the results of single comparisons together.
Fig 5. The DMRT comparison results of the fully dried density values for Uludağ fir at the wood type, sectional direction, surface process, and densification interaction levels (g/cm3)
Fig. 6. The DMRT comparison results of the fully dried density values for black poplar at the wood type, sectional direction, surface process, and densification interaction levels (g/cm3)
According to Figs. 5 and 6, the highest fully dried density value (0.489 g/cm3) was found in the tangential direction poplar samples that were treated with wood preservative and underwent TVM densification at 1.4 MPa, 140 °C, and a duration of 100 s. The lowest value (0.352 g/cm3) was found in the untreated and undensified tangential direction black poplar samples.
Radial and Tangential Swelling
The arithmetic means obtained to determine the effect of the TVM densification process on the swelling ratio in the radial and tangential direction were different according to wood type, sectional direction, surface process, and densification factors. An ANOVA test was performed to determine which factor caused the difference, and the results are shown in Table 3.
Table 3. ANOVA Results of Swelling in the Radial and Tangential Direction
According to the results of the ANOVA, the wood type, sectional direction, surface process, and densification factors and their mutual interactions were significant (p-value was less than or equal to 0.05) with respect to the swelling ratio in the radial and tangential direction. Table 4 shows the DMRT comparison performed at the levels of wood type, sectional direction, surface process, and densification factors using the LSD critical value.
According to Table 4, the swelling ratio was the highest in the fir wood samples (7.88%) and the lowest in the poplar wood samples (7.189%) at the wood type level; it was highest in the tangential direction (9.6%) and lowest in the radial direction (5.463%) at the sectional direction level; it was highest in the untreated (control) samples (7.914%) and lowest in the wood preservative applied samples (7.219%) at the surface process level; and it was highest in the wood preservative-treated samples that underwent TVM densification at 1.4 MPa, 140 °C, and for 100 s (10.14% and 10.104%), while lowest in samples without densification (control) (4.81%) at the densification level.
Table 4. DMRT Comparison Results for the Wood Type, Sectional Direction, Surface Process, and Densification Factors (%)