Hardboard Stiffness and Tensile Strength Over a Moisture and Temperature Range Simulating Exterior Use

The effects of a range of moisture contents (MCs) from zero to 20 percent and of temperatures from -15? to ?C on the tensile properties of a wet-process heat treated hardboard have been investigated at equilibrium conditions. The procedure involves the simple and rapid measurement of preconditioned samples in a temperature controlled inert oil which does not affect the strength properties. The tensile strength is constant or increases and the modulus of elasticity decreases with increasing MC up to about 5 percent, but above this level both decrease significantly with a further increase in MC. The effect of moisture on bending stiffness is smaller since the board thickness increases with increasing MC. At all MCs, both the tensile strength and the modulus of elasticity fall off with increasing temperature. The elongation at rupture shows a corresponding increase with increasing MC and temperature. The tensile energy absorption increases with increasing MC up to about 14 percent. The effects on the modulus of elasticity are discussed in terms of thermal softening and of water as a softener for the amorphous cellulose and hemicellulose polymer. The glass transition temperatures obtained are in agreement with the theoretical approach by Kaelble and its application to paper materials.

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