乳酸/氯化锌基低共熔溶剂 (DES) 协同高温蒸汽处理竹材的增塑机制

Plasticization mechanism of bamboo via lactic acid/zinc chloride deep eutectic solvent (DES) and high-temperature steam

摘要

【目的】竹材因其优异的力学性能成为“以竹代塑”战略的理想原料，然而其特殊梯度结构和高刚性导致加工时易发生开裂和回弹，严重制约了高值化利用。针对传统软化技术效率低、效果差的问题，本研究通过构建绿色高效的“低共熔溶剂（DES）−高温蒸汽”协同软化体系，揭示其多重软化机制，旨在为解决竹材塑性加工提供新途径。【方法】本研究选取毛竹为对象，选用乳酸/氯化胆碱（ n（LA）︰ n（ChCl） = 2︰1）、氯化锌/氯化胆碱（ n（ZnCl₂）︰ n（ChCl） = 1︰1）和乳酸/氯化锌（ n（LA）︰ n（ZnCl₂） = 2︰1）三种典型 DES 体系进行预处理，结合高温蒸汽实现竹材软化。通过短时压缩率和弹性模量评价竹材软化效果；综合利用动态热机械分析（DMA）、扫描电镜（SEM）、傅里叶红外光谱（FT-IR）、X 射线光电子能谱（XPS）、碳-13 固体核磁共振（¹³C NMR）、热重分析（TG/DTG）、X 射线衍射（XRD）、小角 X 射线散射（SAXS）等分析手段，从玻璃态转变温度、微观形貌、化学结合与氢键网络、热稳定性、结晶度与结晶结构、微纤丝角分布及纤维取向度等多维度解析增塑机制。【结果】（1）综合软化处理时间、温度及浓度对软化效果的影响，发现最优处理工艺为使用 70% LA/ZnCl₂ 体系在 130 ℃ 蒸汽下处理 1 h。该条件下竹材短时压缩率提高约 165%；弹性模量降低约 53%，软化效果明显优于单纯蒸汽处理及其他 DES 体系。（2）酸性 DES 与高温蒸汽协同促进半纤维素的水解及木质素中 β-O-4 醚键的断裂，部分解构束缚分子运动的 LCC 网络，从分子层面促进了软化。（3）Zn²⁺深入渗透破坏纤维素结晶区结构，通过配位作用破坏分子链间氢键网络，使结晶度下降 3.60%，引起微纤丝取向度下降 2.90%，进一步提高软化效果。【结论】LA/ZnCl₂ 高温蒸汽体系通过“化学降解基质”与“物理解构晶体”的双重路径，实现竹材细胞壁的高效增塑，为制备高性能竹质重组材料以及实现竹材复杂塑性变形加工提供了新的技术方案。

Abstract

[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.

关键词

竹材 / 细胞壁 / 低共熔溶剂 (DES) / 增塑机制 / 木质素−碳水化合物复合体 (LCC) / 微纤丝角 / 协同效应

Key words

bamboo / cell wall / deep eutectic solvent (DES) / plasticization mechanism / lignin-carbohydrate complex (LCC) / microfibril angle / synergistic effect

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基金资助

国家重点研发计划(2023YFD2200502)

北京市自然科学基金项目(6242021)