Surface Modification of Balsa Wood for Functional Oil/Water Separation
| Meeting | VIWC2026 |
| Topic | OS3: Water Environment and Mining |
| Author | Huong T.T. Tong |
| Organization | tongthithanhhuong@humg.edu.vn |
| ID | 2022.1672450199 |
Content
- Introduction
The emphasis of research in the literature has focused on surface wettability, an inherent attribute of wood surfaces that demonstrates the final wetting/dewetting behavior when liquids like water or oil are brought into contact with them.
Wood is widely available, economical, readily processed, environmentally friendly, and easily manufactured. Scientists are motivated to create porous materials by using wood as a hygroscopic and anisotropic template because of its unique structure and mechanical characteristics. Wood is a biological substance having cells that resemble honeycombs, a high stiffness to weight ratio, and a high value for hierarchical organization. In particular, it consists of parallel hollow tubes with layers of nanocomposite materials made of oriented cellulose microfibrils in a hydrated matrix of lignin and hemicellulose on the cell walls of the wood (Burgert et al., 2015, Zhu et al., 2016) In the wood structure, matrix components like lignin and hemicellulose can be partially removed to create nanoporosity in the wood cell walls, increasing the porosity of wood templates and facilitating the penetration of chemical compounds into the wood cell walls. After pretreatment step, resin-impregnated wood-based products having pores have been created.
In the study, we describe how balsa wood was used as a template to create robust, mesoporous, and hydrophobic 3D wood structures for selective oil/water separation. By removing lignin in the first step, a highly porous delignified wood template with excellent hydrophilic performance is produced. In a subsequent process, the delignified wood template is impregnated with epoxy in acetone. Delignified epoxy-wood biocomposites with excellent oil absorption capacity and hydrophobicity/oleophilicity are successfully created after curing. The delignified wood template and delignified wood/epoxy biocomposite are studied in terms of their chemical composition, morphology, mechanical characteristics, and selective wettability.
- Experimental
Balsa wood was purchased from San Ho Timber Pte Ltd, Singapore. The wood sample was cut into size of 30 × 30 × 2 mm3 (longitudinal × radial × tangential). Sodium hydroxide (NaOH), sodium sulfite (Na2SO3), hydro peroxide (H2O2) and acetone were purchased from Xilong Scientific Co., Ltd., China. Acrylic epoxy resin and hardener epoxy resin were provided by JDiction.
2.2 Preparation of Delignified Wood Template.
The balsa samples were chemically extracted with NaOH/Na2SO3 solution at 110°C for 6 h. Samples after removing lignin with NaOH/Na2SO3 continued to removing lignin with H2O2 (9%). After boiling the wood sample with alkaline solution, the solution in the bottom flask was removed then replace it by H2O2 (9%). Continue boiling the wood with H2O2 in 30 minutes at 100oC.
2.3 Preparation of Delignified Wood/Epoxy Biocomposite.
The porous delignified wood template was impregnated with epoxy/acetone solution (the ratios of epoxy/acetone were 2:8, 3:7, and 4:6, v/v). The epoxy solution was impregnated into the wood template by using 0.3 bar vacuum for 30 min. After that, these impregnated samples were polymerized by increasing temperature step by step in an oven to 90 °C for 12 h. The biocomposite samples were thoroughly washed with acetone twice to remove extra epoxy.
2.4 Characterizations.
The surface morphology and microstructure of the samples were characterized using a scanning electron microscope Quanta 450 FEI. Changes in the surface chemistry of the wood before and after the treatment were analyzed using Fourier transform infrared spectroscopy (IRAffinity-1, Shimadzu). The FTIR spectra were recorded using the KBr pellet method, ranging from 4000 to 400 cm-1 at a 4 cm-1 resolution for 32 scans. Water contact angles were measured using a Ramé-hart Contact Angle Goniometer model 250. Wettability and absorption capability of water are reported in the supporting information.
- Results and Discussion
3.1 Fourier Transform Infrared (FTIR) Characterization
The FTIR spectra of the natural and modified wood samples are showed in Fig. 1. According to the spectral results, lignin was removed from the wood sample. The aromatic ring structure remained on the lignin-type sample (1509 cm-1). However at 1238 cm-1, in the delignified samples, no signal of valence fluctuations appeared. This is the signal for C – O in a G_Ring (guaiacyl unit) C=O stretching structure. Therefore, it can be assumed that some of the lignin has been removed from the sample, but not completely. The absorption peaks epoxy resin include the symmetric stretching vibration of the C – H group on CH3 at a wavelength of 2965 cm−1, the antisymmetric stretching vibration of the C – H group on CH3 at 2865, 1606, and 1508 cm−1. Besides, it include the asymmetric vibration band of the benzene ring skeleton at 1455 cm−1, the scissor swing vibration of CH3 at 734 cm−1, the stretching vibration of aliphatic ether C – O – C at 1241 cm−1, the out of plane deformation of para- substituted benzene ring of = CH at 830 cm−1. It can be seen from the figure that the infrared spectrum of the transparent sample obtained by impregnation of epoxy resin has not only the characteristic absorption peak of delignified wood, but also the characteristic peak of epoxy resin.
Figure 1. FTIR spectra of the Balsa wood sample and modified wood samples
Therefore, from the molecular point of view, the wood – epoxy composite sample contained the molecular groups in the delignified wood template and epoxy resin, and the epoxy resin was successfully polymerized with the delignified wood template.
3.2 Surface Morphology
SEM investigations allowed us to determine the microstructure morphology and the chemical structure of wood before and after the epoxy surface modification. Figure 2 shows the SEM images of the original and modified wood samples. The cross section of wood should have abundant micron-sized pores. After modification, the micron-scale pores in the middle lamella and cell wall corners were infiltrated with epoxy (Figure 2c). In addition, the pit membranes were blocked and covered with a thin layer of epoxy, resulting in a smooth surface of lumen cell walls, due to the evaporation of acetone and formation of a thin epoxy coating of the inner surface of the lumen.
The delignified wood/epoxy biocomposite was yellow due to infiltration of epoxy in the microstructure. High-magnification images reveal the nanoscale structure of the cell wall (Figure 2). As long as lignin is embedded in the cell walls, the layer of balsa wood appears nonporous (Figure 2a). After partial removal of lignin and hemicellulose, nanoscale pores (Figure 2b) were observed in the layer, leading to increase in porosity. The cell wall of delignified wood/epoxy biocomposite exhibited a nonporous cell wall structure (Figure 2c) due to the infiltration of epoxy in the nanopores in the cell walls.
For the delignified epoxy-wood composite, the interface between lumen and the cell wall was coated with a thin layer of hydrophobic epoxy polymer. Thus, the pits on the fiber walls were blocked with hydrophobic polymer. In conventional hydrophobic wood biocomposites, the hydrophobic polymers infiltrate not only the free space in the cell walls but also the lumen space so that water transport pathways are completely blocked. In the present work, the polymer is only infiltrated inside the cell wall. The delignified epoxy-wood biocomposite is thus highly porous and hydrophobic, which is different from traditional wood modification methods.
Figure 2. Scanning electron microscopy (SEM) images of the (a) balsa wood, (b) delignified wood template, and (c) epoxy-wood biocomposite.
3.3. Hydrophobicity of Wood
The aim of this study is to improve the hydrophobicity, and hence, anti-fouling property of wood by epoxy surface modification. The hydrophobicity could be represented by water contact angle (WCA). When a drop of water was placed on the surface of the longitudinal cross section, the water droplet was immediately absorbed into the samples of native wood and delignified wood template (Fig. 3). For the delignified epoxy-wood biocomposite, the initial contact angle was rather high (126°) and decreased slightly to 121° over a period of 5 min. This indicates that the delignified epoxy-wood biocomposite not only is hydrophobic but also has much lower rate of water absorption. In contrast, the highly porous delignified wood template is hydrophilic.
Figure 3. Water drop in delignified wood template
The epoxy concentration is also affect to hydrophobic of epoxy-wood biocomposite. As shown in Fig. 4, the WCA of epoxy-wood biocomposite increased significantly with the epoxy concentration. The WCAs of the original balsa wood at cross is only 0°, indicating that the unmodified wood is very hydrophilic. As the epoxy/acetone content in the modifier solution increases from 2/8, 3/7, and 4/6 (v/v), the WCAs of the modified woods at the cross sections increase ~ 126°. So it can be explained that increasing the amount of epoxy in the mixture has increased surface energy of the wood. After modification, there are epoxy chains with very low surface energy grafted onto the wood surface, which increased the wood WCA significantly. More importantly, it should be pointed out that the epoxy/acetone modification of the wood surface is highly efficient. As shown in Fig. 4, a good hydrophobicity could be realized when epoxy was covered in the surface.
Figure 4. Water contact angle on the surfaces of modified wood samples at different epoxy/acetone ratios.
> 3.4. Oil Absorption
A droplet of paraffinic oil that was coloured with Oil Red O and was found at the bottom of the water container served as a demonstration of the wood structures\' selective oil absorption. The natural balsa and the delignified wood template were unable to absorb the paraffinic oil droplet. These two hydrophilic wood structures, in contrast, instantly absorbed water, demonstrating their hydrophilicity. For oil/water separation, the material\'s physical composition is certainly critical. Delignified wood template fiber walls develop nanoporosity, which promotes capillary action and water absorption, making the delignified wood template hydrophilic.
The oil droplet was quickly absorbed by the delignified epoxy-wood biocomposite underwater. Additionally, it was observed that the epoxy-wood biocomposite removed paraffinic oil, which was dyed with Oil Red O, off the water\'s surface (Fig. 5). As a result, oil on the water surface is selectively absorbed by the epoxy-wood biocomposite. The highly directional pores in the epoxy-wood biocomposite also exhibit a directional oil absorption property.
Figure 5. Oil absorption of epoxy-wood biocomposite
The capacity to absorb oil is clearly demonstrated on three samples of delignified epoxy-wood biocomposite with various epoxy/acetone ratios of 4/6, 3/7, and 2/8 (v/v). Time required for samples to completely absorb 0.5ml of paraffin oil was determined. The epoxy-wood biocomposite sample was exposed to the oil on the water, and the absorption time was evaluated from that moment till the red paraffin oil color disappeared from the fluid.
Figure 6. Oil absorption time for wood-epoxy biocomposite
Through the figure, we can see that reducing the ratio of epoxy in the mixture epoxy/acetone will also reduce the oil absorption capacity of the wood sample. This is in agreement with the results of experiments evaluating the water absorption of wood samples, which show that high water absorption capacity reduces the oil absorption ability of wood. Therefore, the optimal epoxy/acetone ratio on epoxy-wood is 4/6 (v/v), as determined by the experiment\'s measurements of water permeability and oil absorption.
- Conclusions
Acknowledgments
This research has done with the financial support from the International Centre of Physics under Grant No. ICP.2022.17.
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