Objective Bamboo, due to its excellent mechanical properties, has become an ideal raw material for the “bamboo-plastic substitution” strategy. However, its special gradient structure and high rigidity lead to cracking and rebound during processing, severely restricting its high-value utilization. Addressing the issues of low efficiency and poor results in traditional softening techniques, this study aims to construct a green and efficient “deep eutectic solvent (DES)-high-temperature steam” synergistic softening system, reveal its multiple softening mechanisms, and provide a new approach for solving the plastic processing of bamboo.

Method This study selected moso bamboo as the object and used three typical DES systems: lactic acid/choline chloride (n(LA) : n(ChCl) = 2 : 1), zinc chloride/choline chloride (n(ZnCl₂) : n(ChCl) = 1 : 1), and lactic acid/zinc chloride (n(LA) : n(ZnCl₂) = 2 : 1) for pretreatment, combined with high-temperature steam to achieve bamboo softening. The softening effect of bamboo was evaluated through short-term compression ratio and elastic modulus. Comprehensive utilization of dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), carbon-13 solid-state nuclear magnetic resonance (¹³C NMR), thermogravimetric analysis (TG/DTG), X-ray diffraction (XRD), and small-angle X-ray scattering (SAXS) analysis methods was used to analyze the plasticizing mechanism from multiple dimensions, including glass transition temperature, microstructure, chemical bonding and hydrogen bonding network, thermal stability, crystallinity and crystal structure, microfibril angle distribution, and fiber orientation.

Result (1) Considering the effects of softening time, temperature, and concentration, it was found that the optimal treatment process was using a 70% LA/ZnCl₂ system at 130 ℃ steam for 1 hour. Under these conditions, the short-term compression ratio of bamboo increased by about 165%; the elastic modulus decreased by about 53%, and the softening effect was significantly better than that of pure steam treatment and other DES systems. (2) Acidic DES and high-temperature steam synergistically promoted the hydrolysis of hemicellulose and the breaking of β-O-4 ether bonds in lignin, partially deconstructing the LCC network that restricts molecular motion, and promoted softening at the molecular level. (3) Zn²⁺ deeply penetrates and disrupts the structure of the cellulose crystalline region, destroying the hydrogen bond network between molecular chains through coordination, resulting in a 3.60% decrease in crystallinity and a 2.90% decrease in microfibril orientation, further enhancing the softening effect.

Conclusion The LA/ZnCl₂ high-temperature steam system achieves efficient plasticization of bamboo cell walls through a dual pathway of “chemical degradation of the matrix” and “physical deconstruction of crystals”, providing a new technical solution for the preparation of high-performance bamboo-based recombinant materials and the realization of complex plastic deformation processing of bamboo.