木材の湿潤性に関する研究 : (第1報) 木材の湿潤性測定法

Abstract

木材の加工処理のうち, 接着, 塗装, 注入などの溶液が木材表面上に拡がる容易さ, すなわち, 湿潤性は基礎的に重要な特性の一つである。一般に, 湿潤性測定法には, 直接的方法 (接触角測定法, 湿潤熱測定法) と間接的方法 (毛管上昇法) とがある。本報は, ヒノキ材一水系について, 両測定法の基礎的な検討を加えた。その結果, 1) 毛管上昇法について, ガラス管内径の差異は, 水が上昇した高さ (以後, 毛管上昇高という) に影響を及ぼさず, 木粉のガラス管内における充てん度は, 極端に粗な場合をのぞいて毛管上昇高にほとんど影響しない。木粉粒度については, 毛管上昇高は60 - 80mesh＞40 - 60mesh＞20 - 40mesh の順になり, 温度の上昇にともなって, 上昇高が高くなることが明らかになった。2) 接触角測定法について, 滴下水滴の経時変化が大きく, 他の極性物質の挙動と類似していること, 拡張ぬれの影響で断面の種類, 材面での測定方向さらに材面仕上げの程度などによって接触角に差が認められることが明らかになった。3) これらの結果を考慮して, それぞれの測定方法を規制し, スギ, ブナ, ホワイトラワン, レッドラワンの湿潤性を検討した。その結果, 湿潤性はスギ (辺材) ＞ヒノキ (辺材) ＞スギ (心材) ＞ブナ＞ホワイトラワン＞レッドラワンの順になり, しかも, 供試針葉樹材は広葉樹材よりも明らかに湿潤性が高く, スギについて検討した結果, 辺材は心材よりも, 心材の早材は晩材よりも湿潤性がやや高い。

In such processes as adhesion, surface coating, impregnation wettability has a significant meaning. However few studies have been made on the wettability of wood. It is well known that the successful adhesion and surface coating are accomplished with the aid of the wettability of wood which depends on many factors, but this dependence has not been clearly understood yet. The present paper is concerned with establishing the measurement method of wettability of wood, factors affecting wettability of wood, and differences in wettability among the species of wood. Wood flours prepared from Hinoki (Chamaecyparis obtusa ENDL.), Sugi (Cryptomeria japonica D. DON.), Buna (Fagus crenata BLUME), White lauan (Pentacme contorta M. et R.) and Red lauan (Shorea negrosensis FOXW.) were used for these experiments, and water was used as a wetting liquid. I. Factors affecting the wettability of Hinoki are at first investigated by the two method, the capillary rise method and the contact angle method. a) The capillary rise method: The apparatus consisted of glass tubes about 600mm high with various inside diameters and a screen of 100 mesh at each bottom (Fig. 2). Wood flour was filled uniformly in the glass tubes up to the height of approximately 500mm. The tube was then immersed in water to a depth of 15mm, and the height of capillary rise of water through the wood flour was measured at appropriate intervals. Wettability of the sample is expressed by the height of capillary rise of water after 72 hours in centimeters. The wettability difined above was affected with many factors except the inside diameter of the glass tubes (Table 1). The height of capillary rise increased with the fineness of wood flour (Table 1). It also depended slightly on the degree of filling wood flour (Table 2). For this reason it was necessary to standardise the size distribution and the degree of filling of wood flour used. An increase in temperature reduces the surface tension of water and consequently increases the height of capillary rise (Table 3). b) The contact angle method: The contact angle between the solid-liquid interface and liquid-air surface was measured by a goniometer eyepiece (Fig. 3). Both the three section of wood and the direction of the grain affected the wettability (Table 4). It was also found that the contact angle changed with the lapse of time after dropping of water on the surface of wood (Table 6). II. As for Sugi, there were some differences in wettability between spring and summer wood, and between sap and heart wood. The spring or sap wood was more wettable than summer or heart wood, respectively. These were confirmed by both the capillary rise and the contact angle method (Table 5. 6). III. The height of capillary rise gave the following order of decreasing in wettability: 1) Sugi (sap wood), 2) Hinoki (sap wood), 3) Sugi (heart wood), 4) Buna (sap wood), 5) White lauan and 6) Red lauan (Fig. 8). The same order was also obtained by the contact angle method applied on the planed surfaces of wood (Fig 8). As a rule conifers are admittedly more wettable than hardwood and tropical woods. In all species examined the wettability of the planed surfaces of wood was higher than that of the sawn surfaces. IV. It seems that the capillary rise method does not need any comlicated apparatus, and that the method gives reporoducible values of wettability. But the method can not distinguish the value of one section of wood from that of others, and is a time-consuming method. On the other hand, the contact angle method is a quick and simple method to conduct, although the values obtained indicate a considerable variation and are affected by the time observed.