Effect of thermo-mechanical refining pressure on the properties of wood fibers as measured by nanoindentation and atomic force microscopy

Refined wood fibers of a 54-year-old loblolly pine (Pinus taeda L.) mature wood were investigated by nanoindentation and atomic force microscopy (AFM). The effect of steam pressure, in the range of 2–18 bar, during thermo-mechanical refining was investigated and the nano-mechanical properties and nano- or micro-level damages of the cell wall were evaluated. The results indicate that refining pressure has important effects on the physical and mechanical properties of refined fibers. No obvious damage was observed in the cell walls at pressures between 2 and 4 bar. Nano-cracks (most less than 500 nm in width) were found in fibers at pressures in the range of 6–12 bar, and micro-cracks (more than 5 μm in width) were found in fibers subjected to pressures of 14 and 18 bar. The damages caused at higher pressures were more severe in layers close to the lumen than on the fiber surfaces. Under special circumstances, the S3 layer was heavily damaged. The natural shape of the cross sectional dimensions of the cell walls was not changed at lower pressures (2 and 4 bar), but, as pressure was increased, the fibers tended to collapse. At pressures around 18 bar, the lumina were augmented again. The nano-mechanical properties in terms of elastic modulus and hardness were obviously decreased, while nanoindentation creep increased with refining pressure.