Shrinkage stress of wood during drying under superheated steam and its mechanism

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In order to reduce the internal stress of wood during drying, an attempt was made to dry wood with superheated steam above 100℃. Sugi (Cryptomeria japonica D.Don) wood specimens were dried with radial restraint using superheated steam at up to 180℃ 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℃ and a relative humidity of 0 to 80%. It was found that the shrinkage stress decreased dramatically with drying above 140℃ and above 60 percent relative humidity. It was also found that the shrinkage stress above 140℃ occurred even at 100%RH and was ascribed to the structural change of cell wall components.