Effect of pre-mechanical compression on free water removal, drying collapses and associated internal voids of oil palm wood

Abstract

To prevent defects caused by drying collapse, transverse pre-mechanical compression with lateral restraint of green oil palm wood (OPW) before kiln-drying was performed. OPW specimens were divided according to specific gravity into low density (LD 0.22 ± 0.05) and medium-density (MD 0.33 ± 0.02) groups. Effects of compressive strain (0–0.65) and strain rate (0.01–1.0/min) on moisture removal and wood recovery after the compression and on collapse and internal void formations after kiln-drying were investigated. Results indicate that pre-mechanical compression linearly removes moisture out of wood and linearly generates permanent thickness strain that is independent of the strain rate used. The pre-compression reduces the true cross-sectional strain in the kiln-dried LD OPW and MD OPW and the internal voids caused by drying collapse in the LD OPW. The collapse-induced voids, not observed in all kiln-dried MD OPW, disappear when compressive strain > 0.45 is applied to the LD OPW. Microstructural examination reveals that directional folding of parenchyma cells caused by the compression reduces intercellular micro-voids randomly generated during drying by the non-directional collapse of parenchyma cells caused by capillary force. However, above the compressive strains of 0.35 and 0.25 in the LD OPW and MD OPW, interfacial micro-cracks, generated at the boundaries between the parenchyma cells and the fiber bundles, link to form shear bands at the angle of approximately 30–40° to the compression axis. After kiln-drying, these shear bands expand into shear-induced voids. Finally, mechanical properties of the kiln-dried LD OPW pre-compressed at a 0.45 strain without collapse-induced voids are comparable to those that have been air-dried.