Mechanical behaviour of chemically modified Norway spruce (Picea abies L. Karst.): Experimental mechanical studies on spruce wood after methacrylation and in situ polymerization of styrene

Chemical modification of wood mainly aims at improving dimensional stability, resistance to biodeterioration and surface degradation. In some cases, it is specifically targeted at adapting or improving mechanical performance. However, a general understanding of the effects of chemical modification on the mechanical properties of wood, which would facilitate more efficient modification strategies, is missing. Here, a combined experimental and simulation study is provided to gain a more general understanding of the mechanical behaviour of chemically modified wood. In the first part of this study, the mechanical properties of chemically modified Norway spruce are studied experimentally. In Mora Mendez et al. (Wood Sci Technol 2019), simulations of different types of chemical modifications will be presented using a multi-scale model and the outcome will be compared with the obtained experimental data. Chemical modification was based on a two-step modification process. The first step involved methacrylation of the OH-groups in the cell wall. In the second step, in situ polymerization of styrene was induced in the methacrylated samples, which resulted in a partial cell wall and lumen filling. Tensile stiffness and rolling shear stiffness were analysed for methacrylated and polymerized samples. Whereas only small changes in mechanical properties were found for methacrylated samples, the polymerization process led to pronounced increases in elastic modulus and shear stiffness because of weight percent gains of 60-95%. Yet, the specific stiffness was lowered, as the density increase was disproportionate to the stiffness increase. Moreover, a pronounced improvement in rolling shear modulus (G(RT)) by a factor of 4.5 was obtained for the in situ polymerized specimen.