Shrinkage stress of wood during drying under superheated steam above 100°C

In order to reduce the internal stress of wood during drying, an attempt was made to dry wood with superheated steam above 100°C. Sugi (Cryptomeria japonica D. Don) wood specimens were dried with radial restraint using superheated steam at up to 180°C, and the resulting restrained shrinkage stress was examined. A load cell capable of measuring shrinkage stress inside an autoclave was developed and the shrinkage stress was measured at temperatures from 80 to 180°C and a relative humidity of 0% to 80%. It was found that the shrinkage stress decreased dramatically with drying above 140°C and above 60% relative humidity. It was also found that the shrinkage stress above 140°C occurred even at 100% RH and was ascribed to the structural change in cell-wall components.