Effect of supercritical CO2 pre-treatment and kiln-drying of fresh green Pinus radiata sapwood on kiln brown stain and drying stress

High pressure (20 MPa), cyclic, supercritical carbon dioxide (scCO2) treatments can reduce the moisture content of green Pinus radiata sapwood from 150–200% to 35–40%. Such treatments can be used as a dewatering pre-treatment before the kiln-drying of timber. Kiln-drying can utilise various temperature and humidity schedules, targeting around 10% moisture content, with a final stress-relieving steam-conditioning step. After scCO2 treatment and kiln-drying of samples, kiln brown stain was evaluated using the CIE L*a*b* colour space while drying stress was assessed by stress-cup measurements. The most significant results of scCO2 pre-treatment of Pinus radiata sapwood followed by kiln-drying plus steam-conditioning were as follows: Drying from green (36 h from a moisture content (MC) of 164%) using a conventional temperature schedule (90 °C/60 °C) took 2–5 times longer than kiln-drying scCO2 pre-treated boards (37.5% MC) to a target of 10% MC. Colour measurements proved that kiln brown stain does not occur. The use of a steam-conditioning step in reducing internal drying stresses was important irrespective of whether or not there was a scCO2 pre-treatment step. Over all drying schedule combinations, internal drying stress of both green and scCO2 pre-treated timber was similar after kiln-drying plus steam-conditioning. However, using only 90 °C/60 °C schedule data, with steam-conditioning, drying stresses were lower using kiln-drying without the scCO2 pre-treatment. This was surprising since the scCO2 step reduced the moisture content to around 37.5% without significant moisture gradients and so a secondary kiln-drying to 10% moisture content could have been expected to yield lower internal stress levels by preventing large moisture gradients to develop during drying. This result confirms the efficacy of the steam-conditioning step following standard kiln-drying. The colour data demonstrating the prevention of kiln brown stain using kiln-drying schedules offers a path to increasing timber quality for interior applications.