Effects of thermal modification on the mechanical properties of the wood cell wall of soft wood: behavior of S2 cellulose microfibrils under tensile loading

This report examines the effects of thermal modification on the mechanical properties of soft wood, at the cell wall level, to clarify how micro-level behavior (cellulose) relates with macro-level behavior (bulk wood). Both level behaviors were simultaneously measured in thermally modified 5-mm-thick specimens containing annual rings under tensile loading, using synchrotron radiation XRD. Thermal modifications were performed under six sets of conditions: 2 temperature × 3 mass loss (ML). Maximum strain, maximum load, stiffness, and strain energy were obtained at both levels based on corresponding load–strain curves. Data showed that thermal modification tended to reduce the mechanical performance of both, with two exceptions: maximum cellulose strain was increased when wood had been modified at 150 °C, and cellulose stiffness was increased when wood had been modified at 180 °C (except for the ML-18% condition). The extent of those reductions differed between bulk wood and cellulose. The mechanical behavior modified at 150 °C suggested that properties of bulk wood are also influenced by binding elements that connect cellulose microfibrils with the surrounding matrix. When expressed in terms of cellulose-to-bulk (C/B) ratios, different properties were likewise affected by temperature and ML. The tendencies exhibited by C/B ratios of maximum strain and strain energy differed greatly by modification temperature. The C/B ratios of maximum load and stiffness showed similar trends with increasing ML under both temperature conditions although those changes differed in magnitude. Moreover, it seems that not only ML, but also the thermal modification speed, can affect the mechanical behavior of the cellulose microfibrils.