基于纳米力学的超疏水木材表面破坏机制及棒珠结构增强机理的研究

Superhydrophobic modification is an effective way for wood to keep its dimension stable. However, superhydrophobic wood surfaces which are commonly built by accumulation of inorganic nanoparticles still exist such problems as poor mechanical strength and stability. That’s attributed to a lack of destroying theory and enhancement mechanism from microscopic scale and nanomechanical level on superhydrophobic wood surfaces, making it difficult to construct firm and durable superhydrophobic wood surfaces. In this project, it is tried to simulate and analyze the macroscopic destroying process of superhydrophobic wood surfaces with nanomechanics from microscopic view. Ligin-“biomass waste” is used as the main structural material. Lignin nanoparticle (LNP) could be prepared by self-assembly of lignin and LNP/CNC rod-bead is formed when LNP grow on nanocellulose crystal (CNC) which is used as skeleton. To make a comparation of the superhydrophobic wood which are prepared by treatment of adhesive on wood surfaces and then introduction of LNP or LNP/CNC, nano indentation/scratching technologies are used to simulate macroscopic mechanical strength test and to characterize the nanomechanical strength, revealing the destroying theory of the accumulation structure of inorganic nanoparticles and illustrating the influence of combination way of multi-layer interfaces with wood-rubber-micro/nano structure on its strength and stability from microscopic scale. Sequentially, enhancements and stablity mechanism of the microstructure on superhydrophobic wood surfaces is established, providing key theoretical and technical reference for breakthrough of the industrialization bottlenecks and good dimension stability of superhydrophobic wood.

超疏水改性是提高木材尺寸稳定性的有效途径，但目前以无机粒子堆积构建的疏水木材表面存在机械强度弱，稳定性差等问题，归因于尚未能从微观尺度和纳米力学层面揭示其表面结构的破坏机制和增强机理，导致难以制备稳固的超疏水木材。本项目基于纳米力学手段从微观角度模拟分析超疏水木材表面宏观破坏进程，采用“生物质废料”—木质素为结构材料，自组装形成木质素纳米微球（LNP）和以纳米纤维素晶须（CNC）为骨架复合形成LNP/CNC棒珠，对比用胶粘剂预处理再植入LNP和LNP/CNC棒珠构建的超疏水木材表面，利用纳米压/划痕技术模拟超疏水木材表面的宏观机械强度测试以及表征其纳米力学强度，从微观尺度揭示其表面结构的破坏机制，阐明木材-胶-微纳结构多层界面结合方式对其强度和稳定性的影响规律，从而建立棒珠式超疏水木材表面的增强和稳固机理，为突破超疏水木材产业化瓶颈及实现木材尺寸长久稳定提供关键理论和技术参考。

本项目以木质素为主要原材料制备木质素纳米微球（LNP）、水热处理的LNP（HLNP）和HLNP@Fe3O4复合粒子，和将磷酸化木质素（PL）与SiO2或CNC@SiO2复合制得SiO2@PL粒子或CNC@SiO2@PL棒珠结构。为改善超疏水木材表面机械强度弱、易老化等问题，以这些木质素基纳米粒子或棒珠结构为主体构建超疏水木材表面，研究主体材料类型、粗糙结构构建方式和疏水改性体系等对其浸润性能、抗紫外（UV）性能的影响，结合宏观砂磨测试和微观纳米压痕（NI）mapping评价方式，分析微观结构的构建方式和不同的胶粘剂对超疏水木材表面机械性能的影响规律。研究结果表明，LNP由木质素分子通过分子间作用力聚集而成，可以通过控制木质素浓度、水滴速度等来调控其粒径；在LNP基超疏水木材表面中，因木质素含大量的紫外吸收基团，表现出良好抗UV性能，可抗高强度的UV辐射达7h；不过由于LNP稳定性较弱，导致LNP基超疏水木材表面的抗砂磨性能较差，仅能抵抗19次砂磨循环。LNP经水热处理形成HLNP，可以在木质素分子内部和分子间形成共价-非共价键，改善其稳定性和强度，HLNP基超疏水木材表面的浸润性能和抗UV性能相较于LNP基表面变化不大，但其抗砂磨循环升至24次。HLNP/Fe3O4基超疏水木材表面的水接触角（WCA）为167°，滚动角（SA）为5°，在CNC和TiO2协同增强作用下，表现出优异的机械性能，抗砂磨循环达到87次；且Fe3O4的引入，也赋予了体系多功能性，当以聚氨酯海绵为基材时，还表现出良好的磁性性能和油水分离性能。在使用相同胶粘剂的情况下，CNC@SiO2@PL棒珠式表面的简化弹性模量（Er）和硬度(H)比SiO2@PL粒子堆积式的表面更高，这得益于CNC对SiO2/PL的稳定作用；不同性质的胶粘剂对超疏水木材表面机械强度具有重要影响，软胶面（PDMS）比硬胶面（环氧树脂）在砂磨过程中发生更大能量耗散，更有利于保护其微观结构。研究成果将为木质素的高值化应用通过理论支撑，为构建稳定的超疏水木材表面提供直接技术借鉴。