Effect of supercritical CO2 dewatering followed by oven-drying of softwood and hardwood timbers

Supercritical CO₂ dewatering of green wood creates timber with unique properties due to the removal of sap directly from cell lumens as a result of cycling between supercritical and gas phases. The susceptibility of 22 softwoods and hardwoods to shrinkage, collapse and checking during dewatering and oven-drying was investigated. The results were compared to green control specimens that were directly oven-dried. Dewatering efficiency was highly variable amongst species and was highest (93–94%) for the permeable sapwood of four softwoods and lowest for the impermeable heartwood of two hardwoods and two softwoods (4–27%). In general, there was less collapse after dewatering followed by oven-drying than after oven-drying alone, more so in hardwoods than softwoods. Six species (a softwood and five hardwoods) displayed strong collapse. Checking was more prevalent after CO₂ dewatering and oven-drying than after oven-drying alone. The supercritical dewatering treatment alone did not induce collapse or internal checking; however, for collapse-prone timber either collapse or internal checking, or a combination of both, was induced on subsequent oven-drying.