1. INTRODUCTION
The development of community forests has been one of the solutions for overcoming the shortage of raw material for wood industries in Indonesia. These forests areusually planted with fast-growing species, such as jabon (Anthocephalus cadamba). Jabon naturally grows not only in Indonesia, but also in Australia, China, India, Malaysia, Papua New Guinea, the Philippines, Singapore, and Vietnam (Orwa et al., 2009). Jabon wood is widely used as wood packaging but it has numerous other potential applications, including raw material plywood, pulp and paper, building construction, and flooring. It has several drawbacks, however, namely low density (0.3 g/cm3) (Lestari et al., 2018), poor strength, and poor durability, which are primarily having a large proportion of juvenile wood. The large proportion of juvenile wood in jabon caused it can be harvested at 5,6, and 7 year-old (Martawijaya, 2005). In a previous study, Rahayu et al. (2014) found that 5 year-old jabon consists of 100% juvenile. Because the quality of juvenile wood is much lower than that of mature wood, it is not recommended for structural purposes (Bowyer et al., 2007). However, its quality could be improved through technological modifications of its structure.
One method of wood modification is impregnation, a process through which a molecular compound is penetratedinto the wood structure and there by increases the density and hardness of the wood. Impregnation of organic and inorganic compounds into wood structure by soaking or using vacuum pressure tube can be performed to improve physical properties of wood (Hadi et al., 2018; Hartono et al., 2016; Oh and Park, 2015). Impregnation research on fast-growing wood species in Indonesia has been widely performed, using styrene (Darma et al., 2002; Jasni et al., 2004) and methyl methacrylate (MMA) (Wardani et al., 2012; Hadi et al., 2013, 2015), wood vinegar and an animal adhesive (Arsyad et al., 2019), but little research has been done on impregnation with monoethylene glycol (MEG) and SiO2 nanoparticles (nano SiO2). MEG is a hygroscopic substance and can absorb twice its weight in water at 100% relative humidity (Budavari, 1989). Based on its perfect water-soluble characteristics and its relatively low cost, we selected MEG for use in this study. SiO2 nanoparticles (Nano SiO2) are single particles of silica dioxide, an inorganic metal oxide, with a diameter less than 100 nm. Nano SiO2 has been used as a polymer to increase the strength and improve the properties of modified wood (Dong et al., 2014). Nano particles resumably produce uniform distribution and penetration in wood, as well as providing low-viscosity properties (Fufa and Hovde, 2010). Impregnation of poplar (Populus spp.) wood with furfuryl alcohol (FA) and nano SiO2 was shown to effectively improve the properties of the wood (Dong et al., 2014). Further, Rahayu et al. (2019) found that impregnation of the wood of another fast-growing species, sengon (Falcataria moluccana), by MEG and nano SiO2 enhanced its dimensional stability. In this study, we used MEG and nano SiO2 impregnation technology to improve the quality of jabon wood.
The objectives of our research were to determine the effects of MEG and nano SiO2 impregnation treatment on the dimensional stability and density of jabon wood to identify the characteristics of impregnated jabon wood.
2. MATERIALS and METHODS
Five-year-old jabon wood was originated from the community forest in Bogor, West Java, Indonesia. The wood was cut to sample sizes of 2 cm × 2 cm × 2 cm based on British Standard (BS) 373 (BS, 1957) for testing, which assessed weight percent gain (WPG), antiswelling efficiency (ASE), water uptake (WU), bulking effect (BE), and density. The materials used in this study were MEG and nano SiO2 (diameter 15 ± 5 nm; Anhui Elite Industrial Co, Ltd, China).
The impregnation process began with drying samples at a temperature of 103 ± 2°C. Three treatment solutions were used in this study: water-treated as untreated; 50% MEG (vol/vol) in an aqueous solution; and 0.5% MEGSiO2, which contained 0.5% (wt/vol) SiO2 nano-particles in 50% MEG solution (Table 1). A sonicator (Cole Palmer) was used to mix MEG and nano-SiO2 at 40% amplitude for 60 min. The impregnation process was initiated with a vacuum of 0.8 atm for 60 minutes and followed by pressure at 2.5 bar for 120 minutes. The sample was wrapped with aluminum foil, kept at room temperature for 12 hours, and then dried in an oven at 100°C for 12 hours. The polymerization process was then carried out. The sample was then immersed in water for 24 hours for ASE and WU test, followed by drying in an oven at 103 ± 2°C until it reached constant weight. The impregnation method is adopted from Dong et al. (2014) and Rahayu et al. (2019). Five samples were used to test each treatment.
| Treatment | MEG (mL) | Water (mL) | Nano SiO2 (g) |
|---|---|---|---|
| Untreated | 0 | 1000 | 0 |
| MEG | 500 | 500 | 0 |
| 0.5% MEGSiO2 | 500 | 500 | 5 |
We used the formulas for calculating WPG, WU, BE (Hill, 2006), and ASE (Rowell and Ellis, 1978):
Where,
W0 = the initial oven-dried weight of wood sample
W1 = the oven-dried wood sample weight after treatment
W2 = the weight of the wood sample after immersion in water for 24 h
V0 = the initial oven-dried volume of wood sample
V1 = the oven-dried wood sample volume after treatment
Su = the volume shrinkage of untreated wood sample
St = the volume shrinkage of treated wood sample
The density (Bowyer et al., 2007), ρ, was calculated after treatment, using the WPG and BE values, and evaluation of treatment-related changes used the untreated density as the baseline.
Morphology of jabon wood was examined by SEM coupled with EDX and XRD to assess the characteristics of the wood. The capability of MEG and nano SiO2 to penetrate inside cell wall was analyzed using SEM (Zeiss Evo 50 Type, Germany). Tangential sections of both treated and untreated samples (0.5 × 0.5 × 0.5 cm3) were mounted on conductive adhesives, sputtercoated with gold, and observed by SEM at an accelerating voltage of 15 kV, with a chamber pressure between 10 and 400 Pa, under (low vacuum mode) and a working distance of 8 mm. The impregnated samples were also analyzed by SEM coupled with EDX (Zeiss Evo 50 Type, Germany) using a spectra mode to analyze the chemical content in untreated and treated jabon wood.
The degree of crystallinity in untreated and treated jabon wood was analyzed by XRD. Jabon samples were milled with a Willey mill into a 40–60 mesh powder, and the crystallinity of each sample was evaluated by XRD with an XRD-7000 diffractometer (Shimadzu Co., Ltd., Japan). The measurements were conducted with Cu Kα radiation with a graphite monochromator at a constant voltage of 40 kV and an electric current of 30 mA over a 2θ scanning range of 5°–40° at a scanning speed of 2° min−1.
3. RESULTS and DISCUSSION
The impregnation treatment with MEG and nano SiO2 had a significant effect on WPG, ASE, BE, WU, and density as shown in Table 2. The ANOVA results indicated that impregnation had a statistically significant effect on the WPG, ASE, BE, WU, and density (p < 0.05) (Table 3). The results of Duncan’s multiple comparison test (Table 2) confirm that the WPG, ASE, BE, WU, and densityvalues of the treated samples were significantly different from those of the water- and MEG-treated sample. The WPG, ASE, BE, and density values of samples treated with 0.5% MEGSiO2 were greater than those of samples treated with water or MEG. However, the 0.5% MEGSiO2 samples had lower WU values than samples treated with water or MEG. These findings are similar to those of Dong et al. (2014), who found that the WPG of poplar treated with FA and nano SiO2 was greater than that of wood treated with FA treated alone. Similar results was reported by Hartono et al. (2016), who concluded that higher WPG Oil Palm trunk after impregnation with phenol formaldehyde (PF) resulted in the increase of dimensional stability as shown by an increase of density. Wood density has a decisive effect on the physical and dynamic properties of wood (Lee and Lee, 2018; Zhang et al., 2018). In accordance with Hill (2006), the findings suggest that impregnation of wood with a polymer causes to penetrate and strengthen cell walls. The strengtheningis believed to occur through the polymer filling the void space within the cell wall.
| Source | F-value | ||||
|---|---|---|---|---|---|
| WPG | ASE | WU | BE | Density | |
| Treatment | 109.79** | 88.00** | 116.16** | 44.28** | 26.36** |
The increasing ASE values were followed by the increasing WPG values. The highest ASE value of 67.2% and the highest WPG value of 56.7% were achieved with 0.5% MEGSiO2 treatment. The higher the ASE value, the less the wood is able to adsorb water vaporfrom the surrounding environment. Further, the addition of SiO2 nano can replace the bound water that exists within the cell wall. This result is in line with a report by Deka et al. (2007), who concluded that ASE values increased as WPG rose in Norway Spruce after impregnation with melamine formaldehyde (MF), it might be due MF resin is blocking the OHgroup within the cellulose chain.
The WU values of the samples treated with 0.5% MEGSiO2 were the lowest among all treatments. This outcome shows that the ability of wood to adsorb water decreases with the addition of MEG and nano SiO2. The decrease of water absorption indicates the reduction of wood hygroscopicity (Priadi et al., 2019). Hill’s(2006) suggestionthat if the cell wall is filled by a polymer then it will become stronger. A negative correlation was observed between WPG and WU, and as WPG increased, WU decreased. This finding may have been due to the MEG and MEGSiO2 filling the cell wall, minimizing the availability of space for water molecules.
The increase in the BE values suggests that adding MEGSiO2 via the impregnation process can improve wood dimension stability. The samples treated with 0.5% MEGSiO2 hadthe greatest BE values (6.3%). This finding indicated that the addition of MEG and Nano SiO2 as bulking agents is could be a reliable means of improving wood quality. The increasing BE values were accompanied by increasing WPG and density values. According to Hill (2006) polymerentering cell walls and strengthening can improve dimensional stability, and the extent of this effect likely depends on the amount of bulking agnt used. Bowyer et al. (2007) further pointed out that the higher the wood density, the greater the wood content in the cell wall and the cell wall can be make thicker via artificial means, such as thorugh the use of the bulking agents tested in this study.
Previous studies have highlighted that improvements in dimensional stability and mechanical properties in poplar wood were achieved by reducing the porosity of the wood by filling void space with urea formaldehyde and nano SiO2. These addition prevented moisture frompenetrating into wood (Jiang et al., 2013).
SEM analysis revealed the morphology of untreated jabon wood, usingmagnification of ×250 (Fig. 1a) and ×500 (Fig. 1b). Figures 1a and 1b show empty spaces in the pits and cell lumens. With the addition of MEG, jabon wood underwentmorphological changes, as shown in Figure 1c. MEG appears to partially cover the cell wall in a scattered pattern (Fig. 1d). The morphological differences between untreated jabon wood and treated jabon wood can be seen clearly.
Figs. 1e and 1f show that MEG and nano SiO2 penetrated into vessel walls and evenly adhered to and cover cell wall of jabon. The SEM analysis revealed the likely cause of the increases in the WPG, ASE, BE, and density values and the decrease in the WU value in the treated jabon wood. The images illustrate that the addition of nano SiO2 can increase the MEG distribution, allowing the MEG to more readily enter, scatter, and even cover pits and vessel walls. Overall, the 0.5% MEGSiO2 treatment was the most optimal treatment of the threepolymer treatments investigated.
The chemical composition of wood samples treated with water, MEG, and MEGSiO2 was determined by SEM-EDX, and the results are shown in Table 4. The untreated treatment and MEG resulted in no SiO2 content. In contrast, in the wood samples impregnated with 0.5% MEGSiO2 the silicon content of the wood samples reaching 3.03%. Thus,it can be concluded that there is silicon in the wood treated with MEGSiO2.
| Treatment | Carbon (wt. %) | Oxygen (wt. %) | Silicon (wt. %) |
|---|---|---|---|
| Untreated | 61.99 | 28.25 | 0 |
| MEG | 60.02 | 28.01 | 0 |
| 0.5% MEGSiO2 | 55.29 | 31.77 | 3.03 |
Fig. 2 shows the XRD spectra of the untreated jabon wood samples and the treated samples (MEG and 0.5% MEGSiO2). The XRD wave peaks associated with the untreated treatment, MEG, and 0.5% MEGSiO2 occurred respectivelyat an angle for 2θ of 22.14°, 21.76°, and 22.08°. Furthermore, cellulose peaks were also detected at an angle for 2θ = 15.78°, 15.84°, and 15.30° corresponding to the peak of cellulose in the JCPDS data base I012 2θ = 22.6° and I101 2θ = 17.13°.
Crystallinity is an important properties in polymers that exhibit bonds between molecular chains, resulting in a more orderly arrangement of molecules. In the Fig. 2 it can be seen that the peak of cellulose decreases with the addition of MEG and 0.5% MEGSiO2. It is suspected that MEG and 0.5% MEGSiO2 have an influence on the structure of the jabon wood. The addition of nano SiO2 used has semi-crystalline characteristics that can increase the cristallinity. Similar results were previously reported by Dirna (2019), who found that the degree of crystallinity of jabon wood was increased through MEG and silica treatment. Similar results were shown in Rahayu et al. (2019) study, that MEG and MEGSiO2 treatment can increased crystallinity of sengon wood.
The results of the XRD analysis show the effects of nano SiO2 addition onWPG, ASE, WU, BE, and the resulting density. There effects are supported by the appearance of silicone peaks in the diffraction field that correspond to the ICDC database. The results show that the addition of SiO2 nano increases the degree of crystallinity in the wood. High crystallinity indicates strong and rigid bonds between MEGSiO2 polymers.
4. CONCLUSION
MEG and nano SiO2 impregnation of the jabon wood had a significant effect on the resulting dimensional stability of the wood. Increases in WPG, ASE, BE, and density indicates that polymers can fill cell walls in wood, causing a decrease in WU value and a consequent reduction in the ability of jabon wood to adsorb water. Changes in jabon wood morphology with MEG treatment and MEGSiO2 treatment relative to untreateds were demonstrated by SEM analysis results showing that MEGSiO2 polymers can cover part of the pits in the wood vessel wall of jabon. This finding was reinforced by EDX results showing that the silicon content was increased due to the addition of nano SiO2. The XRD diffraction pattern indicated that MEGSiO2 treatment increased thedegree of crystallinity in wood samples. Overall, treatment with 0.5% MEGSiO2 led to the most improvement in the dimensional stability of 5-year-old jabon wood in this sample.
