Effect of Matrix Crystallization on Vickers Hardness of Cellulose Fiber / Poly(lactic acid) Composites

Plastic materials have some environmental problems such as marine pollution caused by microplastics. Therefore, composites of poly(lactic acid) (PLA) and cellulose nanofiber (CNF) have been attracted attention because the CNF/PLA composite has degradability in the natural environment. However, the mechanical properties of the CNF/PLA composites are low. This is due to the low compatibility between CNF and PLA, and to obtain interfacial delamination. In this study, shrinkage of matrix due to crystallization in CNF/PLA composite was focused on as a method of physical interface treatment. The CNF/PLA composite was annealed, and the mechanical properties of this annealed CNF/PLA composites were investigated by tensile test and Vickers hardness test. Tensile test results showed that the elastic modulus of the annealed CNF/PLA composite increased slightly due to crystallization, but the strength decreased significantly. This decrease in strength may be due to crystal coarsening, interface delamination, and residual stress. On the other hands, Vickers hardness of CNF/PLA composite increased with annealing. And, matrix crystallinity of CNF/PLA composite also increased. However, this increase of hardness was small considering the shrinkage of the matrix due to crystallization. This might be due to the relaxation of residual stress during annealing.