放湿過程の木材の誘電的性質について

Abstract

放湿中の木材 (ブナ, 半径方向, 30℃) の誘電的性質をY. E. W. 製のQメータを用いて, 周波数50×10_3 - 10, 000×10_3c/sの範囲で測定し, これと水分平衡時の木材のそれとを比較・考察した。その結果, 含水率7 - 13％では, 誘電損失率は水分平衡時におけるよりも, 放湿時における方が幾分大きいことが見出された。またそれ以下の低い含水率範囲では, 両状態における誘電損失率の差異は明らかでなかった。他方, 誘電率は水分の平衡・非平衡に関係なく, 同一含水率ではつねにほぼ一定値を示した。これらのことから, 放湿過程において生ずる脱着点の-OH基は水分子の脱着と同時に再結合することなく, 暫時不安定な状態にあると推測される。これはまた放湿中の木材の力学的緩和からえられる知見とよく一致する。しかしながら, このような水分非平衡にともなって出現するとみられる, 一種の緩和時間の存在とその大きさは, 測定周波数領域の拡大と測定精度の向上によって, 将来さらに明らかにされなければならないことがらである。

A Q-meter (YEW. QM-101) was used to measure the dielectric constant and dielectric loss of Buna (Fagus crenata Blume) in the radial direction of the grain at frequencies of 5×105c/s~1×107c/s and a temperature of 30℃. We compared the dielectric properties of the wood during the desorption of water with those in the equilibrium states of moisture. The results indicated that the dielectric loss of the former was greater than that of the latter in relatively high moisture contents (7.2%~12.9%). However, in the extremely and relatively low moisture contents (0.1%~4.4%) there were no differences between the two. On the other hand dielectric constants in both cases were proved to be nearly the same. In such a dielectric substance as wood which consists of many different structural units and various constituents dielectric properties of the material result both from electrical heterogeneity and the orientation of electric dipoles. In spite of this we assumed that the dielectric properties of wood with regard to the desorption of water result mainly from the orientation of hydrophile groups, particularly hydroxyl groups in wood. Then it comes that high values of dielectric loss of wood during the desorption of bound water may be caused by the increase of the degree of freedom of the orientation of hydroxyl groups. Because the re-conjugation of unpaired hydroxyl groups produced by the desorption of water molecules would require a finite time in general, and the time for extremely low moisture contents may be shorter than in high moisture contents. This would be also surmised from the mechanical behavior of wood during desorption which gives a deformation far greater than in the states of equilibrium moisture content.