Abstract
Wood is a renewable and environmental friendly material, but its low dimensional stability characteristics limit its applications. In this study, wax mixed with dimethyl silicone oil was used to enhance the dimensional stability under heat treatment. Samples were heated at 120 °C under 3 impregnation conditions (wax, wax + 20% dimethyl silicone oil, and wax + 40% dimethyl silicone oil) for 3 and 6 h respectively. After treatment the effects of combination pretreatment on wood weight gain percentage (WPG), tangential, radial and volume swelling coefficients (TS, RS, VS), distribution of impregnation liquid, and the types of functional groups of African Padauk (Pterocarpus soyauxii) were evaluated. The results showed that impregnation improved the dimensional stability of wood to a certain extent; moreover, the addition of dimethyl silicone oil improved the modification effect. Furthermore, the VS reduced to 0.66 (±0.28)% in the treatment of wax + 20% dimethyl silicone oil for 6 h. The impregnation liquid mainly adhered to the walls of vessels and ray cells. The hydroxyl absorption intensity of the impregnated groups was lower than that of the control group.
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Effect of Wax and Dimethyl Silicone Oil Pretreatment on Wood Hygroscopicity, Chemical Components, and Dimensional Stability
Jing Qian, Zhengbin He,* Jinpeng Li, Zhenyu Wang, Lijie Qu, and Songlin Yi *
Wood is a renewable and environmental friendly material, but its low dimensional stability characteristics limit its applications. In this study, wax mixed with dimethyl silicone oil was used to enhance the dimensional stability under heat treatment. Samples were heated at 120 °C under 3 impregnation conditions (wax, wax + 20% dimethyl silicone oil, and wax + 40% dimethyl silicone oil) for 3 and 6 h respectively. After treatment the effects of combination pretreatment on wood weight gain percentage (WPG), tangential, radial and volume swelling coefficients (TS, RS, VS), distribution of impregnation liquid, and the types of functional groups of African Padauk (Pterocarpus soyauxii) were evaluated. The results showed that impregnation improved the dimensional stability of wood to a certain extent; moreover, the addition of dimethyl silicone oil improved the modification effect. Furthermore, the VS reduced to 0.66 (±0.28)% in the treatment of wax + 20% dimethyl silicone oil for 6 h. The impregnation liquid mainly adhered to the walls of vessels and ray cells. The hydroxyl absorption intensity of the impregnated groups was lower than that of the control group.
Keywords: Dimensional stability; FTIR; SEM; WPG; Wood modification
Contact information: Beijing Key Laboratory of Wood Materials and Engineering, College of Materials Science and Technology, Beijing Forestry University, No. 35, Qinghua East Road, Haidian District, 100083, Beijing, P.R. China;
*Corresponding authors: hzbbjfu@126.com; ysonglin@126.com
INTRODUCTION
Wood is a renewable and environmentally friendly material. It provides numerous advantages, rendering it applicable in many aspects of human life. Specifically, many hardwood materials are judged as satisfactory due to their excellent properties (such as color, texture, etc.). However, low dimensional stability limits their wide application. Since wood shrinks and swells with changes of temperature and humidity, wood modification is a key step before wood products are put into use (Sun et al. 2010; Li et al. 2012). Wood modification alters the material to overcome or ameliorate its disadvantages. Hill (2006) further subdivided wood modification methods based on Norimoto and Gril (1993) into four types: chemical modification, thermal modification, surface modification, and impregnation modification.
Wax modification is an impregnation method. Wax has many advantages, such as cost-effectiveness, abundance, and low toxicity (Chau et al. 2015). The wax impregnation of wood is thought to have originated from the Chinese Ming dynasty, while the use of wax has a history spanning more than 3,000 years in China.
Wax impregnation is usually conducted at elevated temperature and concentration, either in a vacuum or under pressure. Based on existing research, the wax application of wood can be divided into two categories: wax impregnation (Scholz et al. 2010b; Chau et al. 2015; Li et al. 2015; Wang et al. 2015; Yang et al. 2017) and wax impregnation combined with other treatments (Partansky 1959; Wang et al. 2016; Humar et al. 2016; Liao et al. 2016). Wax was used as a water repellent to reduce hygroscopicity and water absorption (Feist and Mraz 1978; Ghosh et al. 2009; Xie et al. 2013), thereby improving the dimensional stability of wood for long-term use. The rate of hygroscopicity and water absorption could be reduced after wax fills the cell cavity (Papadopoulos and Pougioula 2010).
Wax impregnation could also improve the termite resistance of wood; moreover, the effect was found to be related to the type and the proportion of wax (Scholz et al. 2010b). Wax once was applied to the surface of wood to improve the resistant of wood decay fungi, although it could not improve resistance to blue stain fungi (Lesar and Humar 2011). In addition, different from chemical modifications (Dunningham et al. 1992), all the evaluated mechanical properties of the wax-treated wood in previous studies were improved (Hill 2006; Scholz et al. 2010a; Möttönen et al. 2015). As for combined treatments, they had a synergistic effect that can better improve the properties of the wood. The combination of wax impregnation and thermal modification could not only improve the hydrophobicity, dimensional stability, and the resistance against fungal decay of wood considerably, but also reduce the uptake of liquid water and water vapor (Wang et al. 2015; Humar et al. 2016). The wax and copper azole emulsion compound systems could lower the relaxation, reduce the water absorption, and improve the shrinkage and swelling as well (Liao et al. 2016; Wang et al. 2016). The cutting qualities were improved by impregnating the wood with wax and polyethylene glycol (Partansky 1959).
Moreover, the wood quality could be improved by heat treatment with a temperature below 260 °C (Sidorova 2008). Oil heat treatment (OHT) was shown to transfer heat in the wood effectively and uniformly and had already in use in Germany (Boskou 2011). Vegetable oil is one of the heat transfer media because of their boiling point exceeding 260 °C. Under anaerobic conditions, a certain amount of vegetable oil was absorbed into wood, which can improve wood performance (Cheng et al. 2013). Besides, dimethyl silicone oil was applied to heat treatment as the most common and well-studied silicone oil (Noll 1968; Weigenand et al. 2007). Okon et al. (2017) was the first to use dimethyl silicone oil for heat treatment of Chinese fir. Analysis of the physical and chemical properties of Chinese fir samples showed that its dimensional stability was enhanced. In addition, dimethyl silicone oil treated wood had greater resistance to soft rot and showed lower weight loss and loss of dynamic MOE than untreated wood (Ghosh et al. 2008; Weigenand et al.2008; Ghosh 2009).
As previously stated, few impregnation tests have been performed directly with melted wax at 120 °C under atmospheric pressure. No previous studies have used dimethyl silicone oil in the combined treatment with wax to modify P. soyauxii. This study evaluated the improvement in the dimensional stability of P. soyauxii with different dimethyl silicone oil to wax ratios. The distribution of the impregnation liquid was observed by scanning electron microscope (SEM). Moreover, Fourier-transform infrared spectroscopy (FTIR) analysis of wood samples was carried out before and after the impregnation modification.
EXPERIMENTAL
Materials
African Padauk (Pterocarpus soyauxii) (4.80% moisture content) with an air-dry density of 0.63 g/cm3 were collected from YiJiuXuan company, Xianyou, China. Heartwood was selected as the test material and cut into specimens of 20 mm (L) × 20 mm (R) × 20 mm (T). The specimens were free of knots and lacked visible evidence of infection by mold, stain, or fungi. 85# microcrystalline wax with a melting point of 82 °C to 87 °C was used. Dimethyl silicone oil, a colorless (or light yellow), tasteless, and high-transparency liquid with a thermal conductivity of 0.134 to 0.159 W/(M*K), can be used long-term at -50 °C to 200 °C.
Methods
Determination of initial moisture content
The air-dry samples were numbered, weighed, and recorded in accordance with GB/T 1931 (2009) (China) to determine moisture content.
Wax impregnation
A total of 105 specimens were selected before they were randomly classified into seven groups (A, B, C, D, E, F, and G), with 15 specimens numbered 1 to 15 in each group. The experimental design is presented in Table 1. A certain amount of dimethyl silicone oil was added into the fluid wax obtained by melting the solid wax in a steel tank (32 cm × 16 cm × 16 cm) at 120 °C. The test material was immediately dipped into the stirring-well mixture to prevent the liquid from curding. After impregnation, the wood samples were wiped to remove residual impregnation liquid and then cooled in a silica gel desiccator balanced at room temperature.
Table 1. Experimental Design of the Processes Conducted at 120 °C
Characterization experiments
Weight gain rate (WPG) was conducted on an AR124CN electronic balance (Ohaus Instruments Co., Shanghai, China) and calculated using Eq. 1,
(1)
where ΔG represents the WPG (%) of the specimen after wax impregnation relative to before treatment; G0 and G1 denote the weight of oven-dry samples before and after impregnation treatments, respectively.
All specimens underwent hygroscopicity testing at a constant temperature and in a humidity chamber (DHS-500, Beijing Yashilin Test Equipment Co., Beijing, China) at 20 °C and a relative humidity of 65% in accordance with the GB/T 1934.2 (2009) standard (China). After moisture absorption, the changes of tangential, radial, and volume (which is the product of tangential dimension, radial dimension and longitudinal dimension) swelling coefficients (marked by TS, RS, and VS, respectively) were determined by the following equation,
(2)
where S0 and S1 are the tangential dimensions (radial dimensions or volumes) of wood samples before and after impregnation, respectively.
At the same time, in order to observe the internal wax distribution, seven pieces of wood from six experimental groups and the control group were randomly selected. The tangential section was observed by scanning electron microscopy (SEM) (Hitachi S－3400N, Techcomp (China) Ltd, Beijing, China).
The Fourier transform infrared (FTIR) spectra of 4 samples selected randomly from group B, D, F, and G were obtained by infrared spectrometry (TENSOR27, Tianjin Optical Instrument Factory, Tianjin, China). Measurements of 4000 to 500 cm-1 were recorded. Prior to testing, all samples were prepared as 100 to 120 purpose wood flour and then dried at 103 ± 2 °C.
RESULTS AND DISCUSSION
Weight Gain Percentage
The WPG reflects the quantity of the impregnation liquid entering the wood. Figure 1 shows the WPG of P. soyauxii impregnated under different conditions. The WPG of wood treated for 6 h was higher than that treated for 3 h under all impregnation conditions. However, the growth rates in the latter 3 h of the 6 h impregnation groups were slower than that in the previous 3 h period. According to the experimental data, the calculated WPG growth rates of the latter 3 h period were 8.18% (group A, B), 29.9% (group C, D), and 42.16% (group E, F) of the growth rates obtained in the previous 3 h period. This may be due to the fact that the impregnation was completed basically at a certain point during the latter 3 h period, so that, the impregnation liquid could not further fill the cell pores as time went on.
Fig. 1. Weight gain rates of the impregnated groups. (A) wax 3 h, (B) wax 6 h, (C) wax+20% dimethyl silicone oil 3 h, (D) wax+20% dimethyl silicone oil 6 h, (E) wax+40% dimethyl silicone oil 3 h, (F) wax+40% dimethyl silicone oil 6 h
WPG showed a decreasing trend with an increase in the proportion of dimethyl silicone oil after impregnation for 3 h, while it increased along with the increasing proportion of dimethyl silicone oil when the impregnation time was 6 h. The WPG of group A without dimethyl silicone oil was 11.64(±1.67)%, which was increased to 13.96(±2.16)% in group F with 40% dimethyl silicone oil.
Dimensional stability
Figure 2 shows the tangential swelling coefficients (TS) of P. soyauxii in seven groups. The TS of six impregnated groups were less than that of the control group. Moreover, the TS was lower when the impregnation time was extended to 6 h than 3 h. This finding indicates that the tangential dimensional stability increased with time. When the impregnation time was 3 h, TS decreased to 2.28 (±0.3)% in group C with 20% dimethyl silicone oil, which was 0.26% less than that of the control group. The TS of the group B impregnated in wax for 6 h was reduced to 1.45 (±0.19)% which was nearly half of the control group. When adding 40% dimethyl silicone oil, tangential dimensional stability was significantly improved, as the TS values were 1.91 (±0.16)% for 3h and 1.77 (±0.27)% for 6 h. When impregnating for 3 h, the TS value became smaller as the proportion of dimethyl silicone oil increased. The slight tangential dimensional stability enhancement may be attributed to the good lubrication of dimethyl silicone oil so that wax can more easily enter into wood. The experiment shows that the lowest TS was obtained in group B rather than other groups with added dimethyl silicone oil. The possible reason is that the wood had a limited volume to accommodate impregnation liquid, part of the volume of wax was replaced by dimethyl silicone oil.
Fig. 2. The tangential swelling coefficients of the control group and the impregnated groups. (A) wax 3 h, (B) wax 6 h, (C) wax+20% dimethyl silicone oil 3 h, (D) wax+20% dimethyl silicone oil 6 h, (E) wax+40% dimethyl silicone oil 3 h, (F) wax+40% dimethyl silicone oil 6 h, (G) control group
Figure 3 shows seven groups’ radial swelling coefficients (RS) of P. soyauxii. The figure shows that the RS obtained by 6 h impregnation was lower than that for 3 h. In addition, the radial dimensional stability of the impregnated groups improved markedly. Radial dimensional stability increased mostly when the dimethyl silicone oil ratio was 20% impregnated for 6 h, and it reduced from 1.85 (±0.45)% to 1.23 (±0.14)% in accordance with the specific RS value.
Fig. 3. The radial swelling coefficients of the control group and the impregnated groups. (A) wax 3 h, (B) wax 6 h, (C) wax+20% dimethyl silicone oil 3 h, (D) wax+20% dimethyl silicone oil 6 h, (E) wax+40% dimethyl silicone oil 3 h, (F) wax+40% dimethyl silicone oil 6 h, (G) control group
Figure 4 shows that the volume swelling coefficients (VS) of the impregnated groups were reduced compared with the control group. When the impregnation time was 3 h, the VS of the groups increased gradually with the increase in the proportion of dimethyl silicone oil but decreased with the increase in impregnation time. Moreover, the VS was 1.65 (±0.52)% after impregnating with wax for 3h, which was the lowest of the 3 impregnation conditions. Under the treatment with wax + 20% dimethyl silicone oil for 6 h, the VS was as low as 0.66 (±0.28)%, which was 20% of the control group (3.85±0.26%).
Fig. 4. The volume swelling coefficients of the control group and the impregnated groups. (A) wax 3h, (B) wax 6h, (C) wax+20% dimethyl silicone oil 3h, (D) wax+20% dimethyl silicone oil 6h, (E) wax+40% dimethyl silicone oil 3h, (F) wax+40% dimethyl silicone oil 6h, (G) control group
Therefore, the WPG was the largest under the treatment with an impregnation time of 6 h in wax + 40% dimethyl silicone oil, the TS was the lowest under the wax treatment for 6 h, the RS and the VS were the lowest under the wax + 20% dimethyl silicone oil treatment for 6h. The likely reason is that dimethyl silicone oil has better lubricity than wax, thereby making wax enter into wood more easily. However, the ratio of dimethyl silicone oil to wax has a critical value, if the proportion of dimethyl silicone oil continues to increase upon extending the critical value, the proportion of the wax that enters the wood will be reduced even though the WPG increases.