木材における接着層の熱劣化に関する電子顕微鏡的観察

Abstract

木材における接着層の熱劣化について, 走査型電子顕微鏡を用いて研究した。ユリア樹脂接着剤を用いて接着したカバ材のT型試験片 (板目接着) を, 60, 80, 100, 120℃の各温度でそれぞれ50時間処理した後, 接着力試験を行なった。接着力試験によって破断した表面にCとAuを蒸着して観察し, 無処理のものと比較した。走査型電子顕微鏡は, 接着力試験によって破断した表面の観察にはきわめて有用である。しかしながら, 破断が被着木材と接着剤層との界面またはその近くでおこる場合には, 観察像の解釈に注意を要する。劣化処理温度が60, 80℃では破断は一般に接着剤層と被着木材細胞膜の界面近くで行なわれるが, 80℃では接着剤層内での破断も現われる。ところが処理温度が100, 120℃では破断はともに接着剤層内で行なわれる。なお, 120℃処理の破断面は100℃のものにくらべて滑らかである。

Scanning electron microscopy was employed to examine the effects of thermal deterioration of the glue line in wood. The T-shaped glued specimens of birch wood were made with ureaformaldehyde resin extended with wheat flour. The specimens were first heattreated for 50hrs. at selected temperatures such as 60, 80, 100 and 120℃, and shear-tested for determining the bond strength of the glue line of the specimens (Fig. 1). Fracture surfaces of the shear-tested specimens were coated with C and Au, and the characteristics of the surfaces of the specimens were examined using the JSM U-3 scanning electron microscope. It was found that the scanning electron microscope provides useful information about the characteristics of fracture surfaces of glued wood specimens. However, in the case of examination of the regions where the fracture of the specimens occurred close to the interface of wood surface and glue line, consideration should be paid to the differentiation of the wood structure from its replica on the fracture surfaces (Plates 5 and 6). The effect of treatment temperature on the appearance of the fracture surfaces was observed at 60, 80, 100 and 120℃ under scanning electron microscopy (Cf. Plates 5, 6 and 7). Fracture surfaces exposed at 60 and 80℃ reveal mainly the interface between wood substance and glue line (Plates 8 and 9). On the contrary, the surfaces produced at 100 and 120℃ are the fracture surfaces within the glue layer (Plates 10 and 11). Furthermore the treatment at 120℃ produces smoother surfaces. It was indicated from the observations that thermal deterioration of the glue line made with urea-formaldehyde resin in wood progresses with increasing temperature of treatment.