Evaluation of Fire Retardancy of Wood-Mineral Composed Veneers Made with Sugi and Konara Using the Colloidal Silica-Boric Acid System
Takashi GOTO and Takeshi FURUNO
Abstract:Wood-mineral composed (WMC) veneers were manufactured with Japanese cedar (Sugi: Cryptomeria japonica) and Japanese oak (konara: Quercus serrata) using the colloidal silica-boric acid system for the purpose of enhancing the fire retardancy of veneers. The evaluation of fire retardancy was experimented by two burning tests with oxygen index (OI) and microburner methods. In addition, the residual char produced after the OI-burning test was observed by electron probe X-ray microanalysis (EPMA). Weight percent gains (WPGs) for sugi (sapwood and heartwood) and konara veneers were about 50% and 14%, respectively. Ratios of fixation of unleached inorganic substances in WMC veneers were about 80% for sugi and 62% for konara. OI Values of untreated veneers were 22% regardless of wood species and the presence of leaching procedure, whereas WMC veneers of sugi and konara showed OI values of about 57% and 33%, respectively. The burning time and the percent weight of residual char were increased by combination of wood and silicate. As a result of EPMA observation, carbonized cell walls of untreated veneers became thinner, showing an abnormal shrinkage of tracheids and vessels. In the case of WMC veneers of sugi and konara, silica bodies filled up the lumina of tracheids and vessels or sometimes coated the lumen surfaces, and the abnormal shrinkage of cells and the thinning of cell walls hardly occurred. Also, the distribution of silica in the carbonized walls of tracheids, vessel, and wood fibers was found interestingly, giving a fire retardant effect to WMC veneers together with boron compound. The correlation between WPG and OI was recognized and consequently the evaluation of fire retardancy with WPG would be possible. In the burning test of WMC veneers with a microburner, the increase in temperature on the upper side of specimen was slow. The ignition of WMC sugi veneer was not recognized in 30 minutes and the carbonized area on the under side of specimen was small. When WMC sugi veneer was directly exposed to fire and OI values estimating from the linear regression equation were more than 60%, the fire retardancy could be evaluated to be better. Key Words:Sugi veneer, Konara veneer, Wood-mineral composed veneer, Colloidal silica, Boric acid, Weight percent gain, Oxygen index, EPMA observation, Fire retardancy