Deterioration of wood-based boards subjected to outdoor exposure in Tsukuba
Various types of wood-based boards were analyzed for deterioration after being exposed to an outdoor environment for 5 years in Tsukuba, Japan. In phenol–formaldehyde resin bonded particleboard (PB(PF)) and aspen oriented strand board (OSB(aspen)), longer exposure caused a greater reduction in the modulus of rupture and internal bond strength, an increase in the coefficients of variation, and a decrease in 95 % lower tolerance limit at the 75 % confidence level (95TL). Nail-head pull-through and lateral nail resistance were also reduced by outdoor exposure, but their coefficients of variation and 95TL were not significantly affected. In contrast, methylene diphenyl diisocyanate bonded medium density fiberboard (MDF(MDI)) only showed a slight deterioration of these properties even after 5-year exposure, and the coefficients of variation and 95TL hardly changed. After 5-year exposure, the retention of shear load in one-plane at relative displacement of 1.0 mm was high in MDF(MDI) and OSB(aspen) at 93.5 and 78.5 %, respectively, but low in PB(PF) at 41.1 %. As with PB(PF), OSB(aspen) also showed a sharp decrease in the modulus of rupture and internal bond strength, but only slightly reduced shear load in one-plane.
Evaluation of the weathering intensity of wood-based panels under outdoor exposure
In this study, the deterioration of wood-based panels at eight sites in Japan was investigated using outdoor exposure tests. In particular, the modulus of rupture (MOR) retention and internal bond strength (IB) retention after 5-year exposures were compared among panels and sites. The deterioration of panels located in southern Japan was higher than that of panels in northern Japan. To quantify the regional differences, the deterioration rates were calculated; the values showed clear regional differences. The deterioration rate for areas that receive much rain in the summer was higher than the rates for other sites. To eliminate regional differences, we carried out an analysis in terms of the “weathering intensity,” a factor which combines weather conditions (precipitation and temperature). Panels for which deterioration progressed extensively during exposure periods showed a strong correlation between strength retention and the weathering intensity. The significance of these parameters is discussed.
屋外に1, 2, 3, 7および15年暴露した難燃合板の耐久性
Fire-retardant plywood panels of lauan (Shorea negrosensis Fox) were pressure treated with a phosphoric acid containing melamine-formaldehyde condensate as a leach-resistant fire retardant and with diammonium hydrogen phosphate, guanidine phosphate, minalith and pyresote as non-leach resistant fire retardants. Their fire retarding efficiency and bending strength were measured after 1, 2, 3, 7 and 15-year outdoor exposure at the experimental field of the Wood Research Institute, Kyoto University, Uji, Kyoto.The fire retarding efficiency and burning behaviors of the fire retardant plywood in comparison with those of the non-treated ones were evaluated while being subjected to a fire test under a high radiation furnace. The burning behaviors such as: the ignition time and time-temperature of the fire-exposed surface; the time at which the unexposed surface reached the critical temperature of 260°C; the afterflame; and the afterglow of the specimens, as indications of fire retarding efficiency, were measured. The relationships between the modulus of rupture and/or modulus of elasticity of the specimen plywood and the exposure period were also discussed.The fire retarding efficiency of the fire retardant plywood decreased proportionately with longer exposure time. The fire retardant effectiveness of the plywood treated with inorganic fire retardants, such as diammonium hydrogen phosphate, guanidine phosphate, pyresote and minalith, was almost negligible after a year of exposure. However, the fire retardant effectiveness of those treated with phosphoric acid containing melamine-formaldehyde condensate was slightly remarkable even after seven years of outdoor exposure.The durability of the fire retardants was in the order of pyresote, diammonium hydrogen phosphate, guanidine phosphate, minalith and phosphoric acid containing melamine-formaldehyde condensate. The percentage loss in the bending strength were 10-20%, 40-65% and 50-70% after 1, 7 and 15-year exposure period, respectively.