Evaluation of Mechanical Properties of Wood, Changed by Structural Changes in Alkali Treatment and Liquid Impregnation

One of the increasingly serious environmental problems is plastic waste disposal. In addition to direct solutions such as reducing the amount of plastic used, there are indirect solutions such as using less environmentally hazardous materials as substitutes for plastic materials. Familiar examples include the widespread use of paper bags and straws. In this research, we focused on wood, whose main component is cellulose, as a natural polymeric material with the potential to have the same properties as light and strong plastic. In order to increase its versatility and make it an excellent substitute for plastic, we aimed to investigate novel mechanical properties, such as controlled flexibility, by treating it with alkaline solutions and impregnating it with various liquids.<br/>Commercially available balsa wood was used as the experiment material. Alkaline mixture solutions of NaOH 2.5 mol/L and Na₂SO₃ 0.4 mol/L were prepared, and the balsa wood was immersed in the prepared alkaline mixture solutions on a hot stirrer set at 212 °F . Duration times of alkali treatment were 0 h (untreated) and 6 h. The alkali-treated balsa wood was washed with pure water and freeze-dried. Structural observation and mechanical strength measurements were performed on these two types of samples.<br/>Scanning electron microscopy (SEM) was used to observe the cross-sectional structure in each cellulose fiber direction. In particular, cross-sectional observation in the vertical fiber direction showed that the cell wall thickness increased in the alkali-treated samples. This is thought that the cellulose fibers unraveled and swelled by dissolving some of the hemicellulose and lignin that bind the cellulose fibers together. It is expected that this structural change affects the novel mechanical properties.<br/>In the compression test, dry samples before liquid impregnation and liquid-impregnated samples of pure water, ethanol, ethylene glycol, and silicone oil of various kinematic viscosities up to equilibrium were prepared. All measurements were performed in the direction parallel to the cellulose fiber. The results showed that strength varied significantly depending on the alkali treatment and the liquid impregnation. The samples without alkali treatment showed no significant difference in strength depending on the type of impregnating liquid. On the other hand, the alkali-treated samples were softer in water and ethylene glycol and harder in ethanol. This led us to believe that the ratio of hydrophilic to hydrophobic groups was a factor in determining whether the liquid acted as a cushion between the cellulose fibers. With silicone oil, the smaller the viscosity, the stiffer the sample. This is expected to be due to the degree of penetration of the liquid, i.e., the degree to which the molecular chains of the silicone fluid entangle with the loosened cellulose fibers.<br/>In the bending test and the falling-ball impact test, two types of samples were prepared, one in a dry state before liquid impregnation and the other in a liquid-impregnated state after absorption of pure water. Measurements were performed in the direction parallel and vartical to the cellulose fiber. As a result, it was found that the tenacity against bending and the tendency of the force to receive impact differed depending on the fiber direction. We believe that this is due to the effect of structural changes identified by SEM.<br/>As described above, it is possible to control the physical properties of wood using a relatively simple method, and it is hoped that this research will contribute to the development of highly functional sustainable materials that can replace plastics in the future.