Surface characterization and biodegradability of sodium hydroxide-treated Moso bamboo substrates

Alkali solution immersion is widely used as a pretreatment process during bamboo and wood product manufacturing. In this study, a surface analysis on bamboo chips pre-treated with sodium hydroxide (NaOH) at room temperature was performed. The influence of this treatment on the bamboo surface color, microstructure, surface chemical composition, and biological resistance was evaluated. The surface color rapidly became dark at 5% NaOH and became rough at 20% NaOH. After the alkali treatment, an absorption peak occurred near wavelengths of 390-410 nm in the Kubelka-Munk curve, owing to the presence of chromophore groups derived from lignin. Fourier transform infrared spectroscopy revealed that the conjugate structure of lignin-phenols changed after proton transfer in a hydroxyl radical solution. During the treatment, lignin and hemicellulose in the bamboo degraded to some extent, but cellulose was less affected. X-ray photoelectron spectroscopy demonstrated that the oxygen-carbon ratio and the oxygenated to unoxygenated carbon ratio increased with the treatment, indicating a decrease in the amount of extractives and lignin. X-ray diffraction results showed that the area of the crystallization zone increased rapidly at 5% NaOH and then decreased. Furthermore, upon reaching the crystalline region, the NaOH solution led to swelling of the cell wall, thereby increasing the accessibility of fungi and, hence, the susceptibility of the bamboo to fungi infection. The bamboo weight loss of decay after the alkaline treatment was larger than that of the control and was a maximum at 10% NaOH.