Mechanical performance and cellulose microfibrils in wood with high S2 microfibril angles

Corewood and compression wood, both with high S2 microfibril angles, are the worst parts of the tree most in need of improvement in wood quality. This study focuses on the characteristics of cellulosic reinforcement in wood with high S2 microfibril angles lying between 35° and 60°, as well as the probable influence of these characteristics on wood longitudinal mechanical properties. Stiffness and tensile strength were measured on 221 thin radiata pine wood strips. The results indicate that at high angles, the average values of wood stiffness and tensile strength are low with relatively large variations. The variations of stiffness and strength are weakly correlated with the variation of S2 microfibril angles, regardless of wood type. Further study involved examining the characteristics of cellulose microfibrils within the S2 layer using dual axis electron tomography for samples with the same microfibril angle but differing mechanical performance. From the fact that the severe compression wood possessed fewer cellulose microfibrils and had abundant dislocations, and that at many kinking points these cellulose microfibrils were broken down into shorter dislocation segments, it was concluded that the characteristics of cellulose microfibril reinforcement are responsible for the inferior stiffness and tensile strength in the severe compression wood compared with normal corewood with the same high S2 microfibril angle.