湿度制御した温風処理による漆仕上げ材の表面ひずみの測定

摘要

近年，木造建築物や木質美術工芸品に対する害虫駆除法として，湿度制御した温風処理がヨーロッパを中心に普及しつつある。湿度制御した温風処理とは，断熱環境に置いた処理対象物に対して，木材の水分平衡状態を可能な限り保ちながら緩やかに温湿度を上昇させ，材の内部温度を害虫の耐性に応じて約55から60℃に到達させ，その状態で1時間から5時間保持することで，木材中に生息している昆虫の体を構成しているたんぱく質を変性させ，卵，幼虫，蛹，成虫のすべての齢を駆除する方法である。%Humidity controlled warm air treatment is a method to disinfest insects in wood and other organic materials used for historical buildings and/or art works. The infested object is warmed with humidity controlled air until its temperature reaches higher than 55℃ and kept in this condition for several hours. In the present study, a 300 mm cube block of keyaki (Zelkova serrata), one side surface of which is uncovered and the others covered with Japanese lacquer "urushi" layer was treated according to a pre-set program of the warm air treatment in an environmental chamber. Temperature and strain on the surfaces with and without urushi layer were measured using thermocouples and strain gauges, respectively. The measured strains were compared with theoretically calculated strains, based on the linear coefficient of expansion and the shrinkage rate for a change in moisture content for wood. Tensile or compression strains were observed perpendicular or parallel to wood grain, respectively. Absolute values of measured strains increased with increase in temperature and diminished as temperature decreased. Maximum tensile strains were 1200-1600 με for both wood and urushi surfaces, and these values were four or five times larger than the maximum absolute values of compression strains. These maximum strains were approximately one-third of annual strain changes measured on parts in traditional wooden buildings. The measured strain was divided into two components caused by temperature change and by moisture content change. The former was the value of strain gauge attached on the surface and covered with aluminum foil for moisture isolation. The latter was obtained by substituting the temperature-caused strain from the strain without foil. The temperature-caused strain was as similar as the theoretically calculated strain. The MC change-caused strain was significantly smaller than the theoretically calculated strain, even n the humidity condition was set to change the MC of 1.5%. The study suggests that the strain caused by MC change could be kept at smaller levels for both surfaces when temperature and humidity are controlled appropriately.