Effect of drying processes and sampling positions on the microstructure and physical–mechanical properties of NaOH-treated bamboo (Phyllostachys pubescens) slivers

Simple aqueous alkali treatment has proven effective in improving the mechanical properties and flexibility of bamboo slivers, yet the effects of drying methods on the structure and properties of alkali-treated bamboo slivers remain not fully explored. Here, we mainly examined the effects of three different drying processes, i.e., hot-air (50 °C for 8 h), microwave (600 W for 2 min), and freeze drying (− 50 °C for 72 h), on the appearance, microstructure, and physical and mechanical properties of bamboo slivers that were pretreated in the 5% NaOH solution for 2 h. The results showed that hot air- and microwave drying resulted in a similar structure of collapsed parenchyma cells, but many large gaps between the cells were generated by microwave drying. Freeze drying well preserved the original structure. According to statistical analysis, the increases in density, tensile strength, and tensile modulus of the hot air-dried bamboo slivers were more significant than that of microwave-dried and freeze-dried slivers. Compared with the untreated bamboo slivers, the density of hot air-dried outer- and inner bamboo slivers was increased by 28.5 and 49.0%, respectively; the tensile strength was increased by 55.7% and 99.4%, respectively; and the tensile modulus was increased by 76.8 and 36.9%, respectively. Regardless of the drying process, the flexibility of all bamboo slivers was considerably improved after drying. The outer bamboo slivers dried by microwave were found to have the highest flexibility, while hot air drying was found the most effective in improving the flexibility of the inner bamboo slivers. The excellent tensile and bending properties of hot air-dried bamboo slivers were mainly attributed to the fact that hot air drying was beneficial for forming more hydrogen bonds after the removal of lignin and hemicellulose in alkali treatment, resulting in tight aggregations of cellulose molecules. The dense connections between parenchyma cells induced by hot air drying allowed the formation of interlocking cell structures, leading to higher tensile strength and flexibility. It suggests that hot air drying, following the alkali pretreatment, is an effective drying method to modify bamboo slivers to be strong and flexible.