热改性竹材细胞壁胶合界面微纳结构表征及温湿度响应机制研究

High-performance bamboo-based composite materials developed based on thermal modification technology have been fully recognized by the market. However, the strength faliure of bamboo-based composite generally occurred under the hygrothermal conditions when appllied in outdoor. The bonding interphase between bamboo cell-walls and adhesive molecules is the key factor to determine the performance of bamboo-based composites. This proposed project will investigate the characteristics of bonding interphase in thermal-modified bamboo-based composite and its responding mechanism to the temperature and humidity on cell-wall level by using advanced analytical methods with high resolution such as nanoindentation, atomic force microscope-based infrared spectroscopy, etc. The effect of thermal modification on the characteristics of bamboo cell-wall including microscopic pore structure, cell wall chemistry, and crystal structure, etc. will be investigated firstly to indicate the infiltration characteristics of adhesive molecules in cell-wall structure. And then the basic characteristics of bonding interphase including morphology, chemistry, and mechanics, etc. and the interaction between these characteristics will be quantitatively analyzed to build the micro-nanoscale interface system between bamboo cell-wall and adhesive. On the basis of these results above, the evolvement rule of the characteristics of bonding interphase in the changing temperature and humidity environment will be analyzed and then the internal relations among the interfacial properties, microscopic strain in bondline, and macroscopic strength of composites will be discussed. The multi-scale characterization method of the characteristics of bonding interphase is finally established to reveal the strength failure mechanism of bamboo-based composite. This project will promote the development of bamboo bonding theory system and provide a beneficial theoretical foundation for the interphase structural design and modification, and facilitating the development of the high quality and value-added bamboo-based products.

基于热改性技术开发的高性能竹基复合材料已得到市场充分认可，但其在户外使用过程中易受温湿环境影响出现强度失效问题。细胞壁作为竹材实质承载物质与胶粘剂树脂所构筑的界面是决定复合材料强度的关键因素。本项目拟以热改性竹材为研究对象，集成应用高分辨率微区化学/力学分析等技术研究热改性对竹材细胞壁孔隙结构、细胞壁层微区化学分子结构的影响，探索细胞壁结构特性变化对树脂渗透的作用机理；精确表征胶合界面的形貌、化学、力学等结构特征，分析界面特征之间的内在联系，阐明热改性竹材细胞壁界面形成机制；在此基础上，研究界面特征在温湿度作用下的演变规律，并分析界面特征的演变对胶层微观应变和宏观胶接性能的影响，建立界面性能多尺度评价方法，阐明热改性竹基复合材料强度失效机理。预期研究结果将丰富竹材胶合理论体系，为复合材料界面结构设计和调控改性提供理论依据，促进竹材的提质增效利用。

细胞壁作为竹材实质承载物质与胶粘剂树脂所构筑的界面是决定复合材料强度的关键因素。本项目以热改性竹材为研究对象，集成应用高分辨率微区化学/力学分析等技术研究热改性对竹材细胞壁孔隙结构、微区化学分子结构的影响，探索细胞壁结构特性变化对树脂渗透影响；精确表征胶合界面的形貌、化学、力学等结构特征；在此基础上，研究界面特征在温湿度作用下的演变规律，并分析界面特征的演变对胶层微观应变和宏观胶接性能的影响。研究结果表明：高温热处理过程中竹材中含有大量羟基的半纤维素相对含量减少、进而使得竹材表面润湿性下降；同时热作用下竹材中大孔结构增多、但细胞之间连通的纹孔发生坍塌皱缩，进而使得酚醛树脂胶粘剂在竹材表面的渗透性下降。拉曼光谱和纳米力学测试结果发现，胶粘剂分子可以渗透进入细胞壁层，进而形成纳米尺度的胶钉作用。然而，胶粘剂的有效渗透深度降低，使得胶粘剂分子与细胞壁物质形成的交联作用减少，无论在微米尺度还是纳米尺度上，胶粘剂与细胞壁之间的机械啮合作用减弱。因此，在拉伸剪切作用下，应变分布集中于胶层，竹材胶合界面出现层间滑移，进而使得竹材的胶合强度显著下降。环境温湿度对热处理竹材胶合界面具有显著影响，但影响规律不一致。温度作用下，胶合界面区域胶粘剂、竹材细胞以及混合区域材料的力学性能变化不协调，尤其混合区域材料力学强度显著增加，使得胶合界面传递、分散应力的能力下降，将对胶合性能产生不利影响；而在一定湿度作用下，界面区域中细胞壁和混合区域材料由于自身吸湿作用，力学性能有一定程度降低，与胶粘剂的力学性能之间更协调，使得胶合界面应变分布更均匀，应力传递更有效，有助于竹材胶合性能的提高。针对热改性竹材胶合不良的问题，本项目还初步探讨使用植酸-铁离子络合物对竹材表面进行修饰，取得的一定效果，为后续研究打下了良好的基础。本项目成果将丰富竹材胶合理论体系，为热处理竹材的界面调控和产品开发提供理论依据。