Effects of Equilibrium Moisture Content Changes on Hardboard Properties

Since hardboard is a wood-base material, it absorbs or loses moisture with changes in humidity; the changes in moisture content will affect physical and mechanical properties. To use hardboard properly where changes in properties will affect its performance, particularly in structural applications subject to exterior exposure, the relationship between moisture content changes and hardboard properties must be determined. This work on effects of changes in equilibrium moisture content (EMC) on strength and elastic properties of 1/4-inch-thick tempered hardboard was part of a cooperative program with the American Hardboard Association. The EMC of the hardboards at 30, 50, 65, 80, and 90 percent relative humidity averaged 4.0, 5.1, 6.9, 9.5, and 10.9 percent, respectively. Calculations of strength and elastic properties in tension, compression, bending, and shear at different relative humidities between ovendry and 90 percent at 80?F. were based on thickness at 65 percent relative humidity and expressed as a percent of strength or elasticity at 65 percent relative humidity (control values). With the exception of interlaminar shear modulus, the ranges of the average strength and elastic properties of the six hardboards expressed as a percent of the values at 65 percent relative humidity were, approximately, the following: 100 to 120 percent between ovendry and 50 percent relative humidity, 80 to almost 100 percent at 80 percent relative humidity, and 70 to 90 percent at 90 percent relative humidity. Interlaminar shear modulus was affected more than any other property by changes in EMC. It averaged only 70 percent of the control value at 80 percent relative humidity and 60 percent at 90 percent relative humidity.

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