Effects of Sorption on the Flexural Creep Behavior of Hardboard

Most plastic and fibrous materials behave in a viscoelastic manner in that they exhibit elastic as well as viscous strain properties. The flexural creep behavior of wet and dry process hardboards was greatly influenced by sorption. Adsorption produced far greater creep than that which developed during constant or desorption conditions. Increases in temperature and stress level during constant or changing sorption conditions produced an increase in creep. Dry process hardboard exhibited greater creep than wet process hardboard except under test conditions of low stress and moisture conditions. All test specimens were prepared from 0.25-inch thick wet and dry process hardboards which were equalized to a nominal 5 percent moisture content. The wet process board contained 95 percent aspen and 5 percent jack pine fiber. In addition it contained 2 percent wax size and 1 percent phenolic resin. The dry process board was smooth both sides and the wet process board was smooth one side with a screen back. Samples used in the adsorption and desorption tests were equalized to either a nominal 5 or 10 percent moisture content at 72?F. or 92?F. Total deflection, elastic response, plus creep for the boards are given in curves. The stress level was 30 percent of the MOR. Recoverable and irrecoverable creep are the result of the same molecular processes. Creep recovery is the result of restoring forces that were temporarily locked into the structure. These temporary forces are relieved when moisture movement creates hydrogen bond breakage.

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