Effects of a High and Low Relative Humidity Cycle on Properties of Southern Yellow Pine Particleboards

Among the three types of boards tested (Urea with wax, phenolic with and without wax), after one complete cycle (65 percent, 30 percent, 65 percent, 90 percent, 65 percent RH), the phenolic board with wax exhibited the highest retention of flexural properties, while the phenolic board without wax exhibited the highest retention of internal bond strength and the best thickness stability. Flexural properties, plate shear modulus, and internal bond strength of all particleboards increased slightly when conditions changed from 65 percent to 30 percent RH, but the properties decreased considerably when conditions changed from 65 percent to 90 percent RH and failed to recover completely with reconditioning to 65 percent RH. The percentage reductions of MOE, MOR, plate shear modulus, internal bond strength caused by exposure to one complete cycle were 20, 16, 16, and 4 percent respectively. It is of practical importance that reductions of bending stiffness (EI) and maximum bending load at 90 percent RH are not as much as the reported reductions of MOE and MOR. This is because moisture has a weakening effect and also increases the thickness of the material. No hygroscopic hysteresis developed during sub-cycling from 65 percent to 30 percent and back to 65 percent RH. However, a 2 percent M.C. hysteresis developed when boards were subjected to humidity changes from 65 percent to 90 percent and back to 65 percent RH. A similar phenomenon, namely irreversible thickness swelling, took place between the same changes of humidity. The irreversible swelling was 2.4 percent for urea board with wax, 1.2 percent for phenolic board without wax. Linear changes, due to exposure at 30 percent RH and 90 percent RH, were about 0.2 percent based on the dimensions at 65 percent RH.

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