Nanoindentation of juvenile and mature loblolly pine (Pinus taeda L.) wood fibers as affected by thermomechanical refining pressure

The effects of thermomechanical refining pressures, varying from 2 to 18 bars, on the cell-wall properties of refined wood fibers of a 54-year-old loblolly pine (Pinus taeda L.) with reference to both juvenile (JW) and mature wood (MW) were investigated using nanoindentation and atomic force microscopy. The results of this study indicate that refining pressure plays a significant role in the physical damage sustained by refined wood fibers. No obvious damage was observed in the cell walls of MW fibers refined at 2 and 4 bar. Nanocracks (< 500 nm in width) were found in fibers refined at pressures in the range of 2-12 bar for JW and 6-12 bar for MW, and micro cracks (> 3,000 nm in width) were found in both MW and JW fibers subjected to a refining pressure of 14 and 18 bar. The micro damage to the fibers refined at higher pressures was more severe inside the lumen than on the surface of the fibers, and the lumen or S-3 layer was significantly damaged. The elastic modulus, hardness, and creep resistance of MW fibers were higher than those of the JW fibers subjected to the same refining-pressure conditions. The elastic modulus and hardness decreased, whereas nanoindentation creep increased, with increasing refining pressure. This study also suggests that lower refining pressures (< 4 bar) and higher pressures (> 14 bar for MW and > 12 bar for JW) should be avoided in the manufacture of fiberboards and wood fiber-polymer composites, because of the lower aspect ratio of the fiber bundles, shorter length of the fibers and fines, and severe damages to the fiber cell walls.