EPMA Observation of Wood-Mineral Composites Using the Silicate-Boron Compound System and Estimation of Fire Resistance by Oxygen Index Method
Takeshi FURUNO, Takashi GOTO and Sadanobu KATOH
Abstract:The distribution of silicic acid gel and boron compounds in wood-mineral composites made using the silicate-boron compound system was examined by electron probe X-ray microanalysis (EPMA). The fire resistance of the composites was also evaluated by applying both oxygen index and percent weight of residual char. The composites were produced by diffusion-penetrating the water glass-boron compound system (double treatment) and the colloidal silica solution system in the sapwood specimens of Sugi (Cryptomeria japonica). For comparison, the composites were also prepared by impregnating boron compounds only into the wood(single treatment). In the single treatment, most of boron compounds in the composites was located in the cell walls as obvious from boron X-ray maps. However, boron compounds themselves were almost all leached out by the leaching procedure, showing no fixation of them in the composites. On the other hand, in the double treatment having the fixation of 54-70% after leaching, silicic acid gel was located mostly in the cell lumina and it was very small in quantity in the cell walls as seen from silicon X-ray maps whereas boron compounds existed in the cell walls besides the lumina even after leaching. In the colloidal silica solution system having the high fixation of 86-94%, most of silicic acid gel was deposited in the cell lumina as an insoluble solid body and it was also very little in the cell walls. In addition, the distribution of boric acid was clearly observed not only in the lumina but also in the walls even after leaching. As a result of fire-resisting test, the oxygen index of the composites trended to increase with an increase in weight percent gain (WPG) and there was a high correlation between the two. The percent weight of residual char was highly correlated to WPG independently of leaching and it increased lineally with an increase in WPG. It may be possible that the percent weight of residual char becomes one way for estimating the fire resistance of the composites because of a high correlation with the oxygen index after leaching. Key Words:Wood-mineral composite, Colloidal silica, Water glass, Boron compounds, Silicic acid gel, EPMA observation, Cell wall, Oxygen Index, Residual char, Fire resistance