Abstract
This study aimed to determine the gloss and hardness values of low-density wood materials densified using the Thermo-Vibro-Mechanic® (TVM) method after pretreatment with wood stain and wood preservative. This was carried out with a TVM density press that was designed and produced with the support of project 115O138 of the Scientific and Technological Research Council of Turkey (TUBITAK). The samples obtained from Uludağ fir (Abies bornmüelleriana Mattf.) and black poplar (Populus nigra L.) were pretreated with wood stain and wood preservative prior to the TVM densification process. The TVM densification operation was conducted at three different temperatures, three different vibration pressures, and three different vibration times. After the TVM densification process, changes in the gloss (ISO 2813 2014) and Brinell hardness values (TS 2479 1976) of the samples were determined. According to the results, the TVM densification method increased the gloss value of the Uludağ fir and black poplar wood pretreated with the wood preservative by 175% and 1390%, respectively, and increased the Brinell hardness value by 63% and 150%, respectively.
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Effect of Thermo-Vibro-Mechanic® Densification Process on the Gloss and Hardness Values of Some Wood Materials
Süleyman Şenol a and Mehmet Budakçı b,*
This study aimed to determine the gloss and hardness values of low-density wood materials densified using the Thermo-Vibro-Mechanic® (TVM) method after pretreatment with wood stain and wood preservative. This was carried out with a TVM density press that was designed and produced with the support of project 115O138 of the Scientific and Technological Research Council of Turkey (TUBITAK). The samples obtained from Uludağ fir (Abies bornmüelleriana Mattf.) and black poplar (Populus nigra L.) were pretreated with wood stain and wood preservative prior to the TVM densification process. The TVM densification operation was conducted at three different temperatures, three different vibration pressures, and three different vibration times. After the TVM densification process, changes in the gloss (ISO 2813 2014) and Brinell hardness values (TS 2479 1976) of the samples were determined. According to the results, the TVM densification method increased the gloss value of the Uludağ fir and black poplar wood pretreated with the wood preservative by 175% and 1390%, respectively, and increased the Brinell hardness value by 63% and 150%, respectively.
Keywords: Thermo-Vibro-Mechanic® (TVM) densification; Wood stain; Wood preservative; Gloss; Hardness;
Contact information: a: Department of Interior Architecture, Faculty of Architecture, Kütahya Dumlupinar University, Kütahya – Turkey; b: Department of Wood Products Industrial Engineering, Faculty of Technology, Düzce University; 81060, Düzce, Turkey;
* Corresponding author: mehmetbudakci@duzce.edu.tr
INTRODUCTION
In recent years, wood material has become an industrial material with increasing application thanks to technological developments. Human population growth and the rise in new wood applications increases the need for wood material and therefore causes the material with these superior properties to gradually decrease in availability. This suggests the necessity of a more efficient use of existing resources, such as the recycling of residual materials or the use of various types of commercially undesirable wood species with low-resistance properties in the industry through various modifications and producing various materials (Pelit 2014; Şenol et al. 2017).
The term “wood modification” is defined as the change or improvement of the negative properties of wood (Şenol and Budakçı 2016; Sandberg et al. 2017). Wood material with increased density can be an alternative to other structural materials (Blomberg and Persson 2004; Blomberg et al. 2005; Kutnar and Šernek 2007; Pelit et al. 2015; Şenol and Budakçı 2016). Most of the mechanical properties of wood material are related to density (Blomberg and Persson 2004; Kamke 2006; Kutnar and Šernek 2007; Rautkari 2012; Pelit et al. 2014; Budakçı et al. 2016; Pelit et al. 2018). Because densification of wood material increases the mechanical properties and hardness of the wood material, many experiments have been conducted to develop a suitable process (Blomberg and Persson 2004; Rautkari et al. 2011; Fang et al. 2012; Gaff and Gašparík 2013; Laine et al. 2014; Fu et al. 2017; Li et al. 2017; Şenol et al. 2017; Şenol and Budakçı 2018; Song et al. 2018).
Through densification, low-density wood materials are transformed into high-density and can be converted into commercially high-value products. High-density wood material types can also be made more resistant through densification (Blomberg et al. 2005; Kutnar and Šernek 2007; Ulker et al. 2012).
The material type, temperature, the softening or plasticizing period, the densification method, and the pressing pressure are the most important variables in the densification of wood material. By application of these variables, the mechanical properties of densified wood can be increased up to 100% (Ulker et al. 2012; Gao et al. 2019).
In the densification of wood material via compressing, densification is achieved by reducing the cell wall and reducing the pore space (Kutnar et al. 2009; Pelit 2014; Budakçı et al. 2016). Ruptures and cracks may occur in the cell wall of the compressed wood material under normal atmospheric conditions. The natural elastic structure of wood plays an important role in densification by compressing. If the wood temperature in densification is above the critical transition temperature, densification is achieved without major deformation and without cellular breaks in the amorphous polymers. The compression properties are mostly dependent on the density, humidity, cell wall volume of the wood, and the compression direction. The biggest problem encountered in densified wood materials by compression is that they tend to return to their initial dimensions due to spring-back when used in places exposed to contact with moisture or water (Seborg et al. 1956; Kollmann et al. 1975; Morsing and Hoffmeyer 1998; Kultikova 1999; Blomberg et al. 2006; Gong and Lamason 2007; Rautkari 2012; Pelit 2014). This can be resolved by a heat and vapor effect (Gong and Lamason 2007; Kutnar and Šernek 2007; Rautkari et al. 2010; Pelit 2014; Li et al. 2017).
Recently, heat treatment/thermal modification applications have started to eliminate the negative aspects of the densified wood. However, the temperature values above 150 °C used in these applications change the color and weight of the wood material and diminish its mechanical properties. This may restrict the use of wood material, especially as structural systems (columns, beams, flooring, etc.) (Bekhta and Niemz 2003; Esteves et al. 2007; Boonstra 2008; Korkut and Kocaefe 2009; Pelit 2014; Şenol et al. 2017; Şenol and Budakçı 2018).
In light of this information, this study aimed to obtain high quality wood material through improving the resistance properties of especially low-density wood types using the Thermo-Vibro-Mechanic® (TVM) densification process, a new and environmentally friendly modification method that provides an alternative to existing densification and modification processes. For this purpose, a special “TVM density press” was designed and produced with the support of Project No. 115O138 of the Scientific and Technological Research Council of Turkey (TUBITAK). The samples obtained as radial and tangential cross-sections from Uludağ fir (Abies bornmüelleriana Mattf.) and black poplar (Populus nigra L.) were pretreated with wood stain and wood preservative, and then these samples were subjected to the TVM densification process. Then, the resulting changes in the gloss and Brinell hardness values of the samples from the TVM densification process were determined.
EXPERIMENTAL
Materials
Uludag fir (Abies bornmüelleriana Mattf.) and black poplar (Populus nigra L.), which are widely used in the forest products industry in Turkey, were preferred for the preparation of the samples. It was taken into consideration that the wood, procured in the form of logs from the Forest Management in the Kütahya province of Turkey, would be robust with no growth defect or decay. The logs were cut into tangential and radial directions considering the dry weight and sample sizes, and transformed from fresh wood parts into rough-sized timbers with smooth fiber, without knots or cracks, and with no density or color differences (TS 2470 1976). These timbers were subjected to technical drying to achieve an air-dried moisture of 12%. Samples at air-dried moisture were cut at 360 × 60 × 21 mm and 360 × 110 × 11 mm size, and then sanded with 100-grit sandpaper via a calibrated sanding machine. Afterwards, they 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) prior to the TVM densification process using the 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 20 ± 2 °C and 65 ± 3% relative humidity to eliminate the differences in humidity values occurring after these processes as per the TS 2471 standard (1976) (Fig. 1).
Fig. 1. Preparation of the samples
Methods
TVM densification process
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 vibration 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 specially designed and manufactured within the scope of the research were first kept under low pressure (2 kg/cm2) so that both surfaces contacted with the press table, and they remained in this position until the internal temperature of the samples reached the target temperature values by checking with a digital thermometer (Fig. 2).
Fig. 2. TVM density 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 60 °C at a different press (plating press) under 5 kg/cm2 pressure to eliminate any spring-back effect (Fig. 3).
Fig. 3. Cooling the samples under pressure after TVM density operation
Then, the samples were kept in the climate chamber at 20 ± 2 °C temperature and 65 ± 3% relative humidity according to TS 2471 (1976) before trials until they reached a constant weight. Afterwards, the samples were sized to have 6 replications according to the TS 2479 (1976) and TS EN ISO 2813 (2014) standards of the tests to be applied.
Brinell hardness
The Brinell hardness values of the samples (50 × 20 × 20 mm) were determined using a UTEST 7012 (50 KN) test device according to TS 2479 (2016) principles (UTEST Material Testing Equipment, Ankara, Turkey). After a 10-mm diameter (D) half-sphere (steel ball) at the tip of the force application arm of the device was calibrated, so as to be at the center of the sample surface, the load was applied for 30 s. The load was reduced to zero in 15 s, and then the diameter of the pit that the steel ball opened on the sample surface was measured using a magnifying glass and a digital caliper with a precision of (d) ± 0.01 mm (Fig. 4). The Brinell hardness value was calculated according to Eq. 1,
(1)
where HB is the Brinell hardness value (N/mm2), F is the applied force (N), d is the diameter of the pit that the steel ball opened on the sample surface (mm), and D is the diameter of the steel ball (mm).
Fig. 4. Brinell hardness experiment (a) and measuring the pit diameter (b)
Gloss
Gloss measurements of the samples (100 × 100 × 10 mm) were determined using the BYK-Gardner Spectro-Guide 45/0 device (BYK-Gardner, Geretsried, Germany). The gloss measurements were made according to ISO 2813 (2014) using a 60 ° angle. After TVM densification, measurements were made perpendicular and parallel to the fibers for each surface, and their arithmetic means were recorded as a single value (Fig. 5).
Fig. 5. Gloss measurement device
Scanning electron microscopy
To determine the effects of the wood type, sectional surface, surface process, and densification factors on the hardness and gloss performance, SEM images were taken of the cross-section (transverse) of wood materials. To obtain clearer images for this purpose, 4 × 4 × 4 mm sized samples in groups of twelve were coated with gold metal using the Denton Vacuum Desk V (Denton, Moorestown, USA). The coated specimens were placed on the FEI Quanta FEG 250 SEM (FEI Company, Hillsboro, OR, USA) in such a way that measurements were taken from the section edge. Microscopic images were taken using the high-vacuum method.
Statistical analysis
An SPSS 22 statistical package program (IBM Corp., Armonk, NY, USA) was used to evaluate the data. Multivariate analysis of variance (ANOVA) tests determined the effects of wood type, sectional surface, surface process, densification factors, and the interactions of these factors on the hardness and gloss values at the 0.05 significance level. Comparisons were made using the Duncan’s Multiple Range Test (DMRT) and least significant difference (LSD) critical values, and the factors causing the differences were examined.
RESULTS AND DISCUSSION
Brinell Hardness
The arithmetic means of the Brinell hardness values obtained to determine the effect of the TVM densification process on the Brinell hardness values of the samples were different according to wood type, section surface, surface process, and densification factors. An ANOVA was performed to determine which factor caused this difference, and the results are given in Table 1.
According to the results of the analysis of variance, wood type, sectional surface, surface process, and densification factors and the mutual interactions of these factors were significant (p ≤ 0.05) with respect to the Brinell hardness values. Table 2 shows the DMRT comparison performed on the level of wood type, sectional surface, surface process, and densification factors using the LSD critical value.
Table 1. Results of ANOVA of Brinell Hardness Values