Effect of humidity on physical and mechanical properties of New Zealand wood composites

The effects of a range of moisture contents (MCs) induced by different relative humidities have been studied for tempered hardboard, urea-formaldehyde-bonded flooring particleboard, and medium density fiberboard (MDF) commercially available in New Zealand. MCs, dimensional change., and mechanical properties were recorded after the MCs were close to equilibrium. There were significant differences between the control humidity and the other humidities pooled for modulus of elasticity (MOE), modulus of rupture (MOR), internal bond strength, and compressibility. However, for tempered hardboard, controls were only different from the other humidities for MOE and compressibility. The effects of MC on physical dimensions, MOR, and MOE were similar to those reported by other researchers for particleboard and hardboard, but no literature could be found to compare with MDF mechanical properties. Some implications of this study for current New Zealand building practices are discussed. In particular, at a high humidity of 95 percent, which may be found in a damp crawl space, the MOE of flooring particleboard is lowered by 47 percent to 63 percent.

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