Permanent fixation of transversely compressed wood by heating and its mechanism

The permanent fixation of transversely compressed wood by heating has been presumed to be due to the release of stresses stored in the cell wall polym ers by their decomposition. It was shown, however, that the permanent fixat ion by steaming was the result not only of decomposition but also of the fo rmation of some cohesive structures that were not permanent. To clarify whe ther or not such impermanent structures are formed during heating, the stre ss relaxation of radial compression and the recovery of deformation (strain recovery) in compressed samples of sugi (Cyptomeria japonica) wood were ex amined. Samples were compressed at temperatures from 160 degreesC to 200 de greesC for various periods of up to 24 hrs. The compressed samples were imm ersed in boiling water and then in swelling liquids, i.e., pyridine, dimeth yl sulfoxide (DMSO) or 4% aqueous solution of sodium hydroxide (NaOH soluti on), and their strain recovery was measured. The relationship between the r esidual stress at the end of the relaxation measurements and the strain rec overy in boiling water could be expressed by a single curve regardless of h eating time and temperature. All the specimens showed greater recovery in t he above swelling liquids than in boiling water. In particular, marked reco very was observed in the NaOH solution, even when little recovery was recog nized in Boiling water for the specimen showing significant weight loss. Fr om these findings, it was concluded that permanent fixation in the heat tre atment is associated less with the decomposition of cell wall polymers but is dominated by the formation of some cohesive structures as in steam treat ment.