Strength and Rheological Properties of Particleboard as Affected By Moisture Content and Sorption

Specimens were prepared from three-layer commercially manufactured Douglas-fir particleboard of disintegrated, flaked particles. Urea- and phenol-formaldehyde-bonded boards having 0.65 nominal specific gravity values were used. Resin content was 8.0 percent for the urea and 6.0 percent for the phenolic boards. All boards had 12.5 percent of the total material in each of the surface layers, which were composed of thinner, disintegrated flake material. Fifteen specimens for each resin type 3/4-inch thick, 2-inches wide and 20-inches long were tested with center loading over a span length of 18 inches. The specimens were tested at 6, 12, and 18 percent moisture content to determine the effect of moisture content on MOR and MOE. Similar tests to determine the effect of cyclic shrinking and swelling on board strength were conducted on 3/8-inch thick material. Tests were made after one, two, or three cycles consisting of a 7-day exposure to 20 percent EMC conditions followed by 7 days at 6 percent EMC conditions. Long-term loading tests used 3/4-inch material 2-inches wide in flexural tests with center loading over 18-inch spans. Specimens were conditioned and tested at 6, 12, and 18 percent EMC to provide nine treatment combinations. Stress levels were maintained at 30 percent of MOR values as determined at 6 percent moisture content and were carried out over a 2 to 4 week period. It can be concluded from this study that; in addition to the direct effects of moisture content, the sorption process associated with changing moisture content has an independent effect on many physical and rheological properties of particleboard. Under some conditions, moisture content and sorption effects are more pronounced in particleboard than in whole wood. This suggests that resin bonds and possibly the modification of wood under high temperature and compressive forces play important roles in determining the overall performance and durability of the material. The following significant conclusions were reached: No major differences can he found between urea- and phenolic-bonded materials in regard to moisture and sorption effects. The direct effect of moisture content is very pronounced for both modulus of rupture and modulus of elasticity, with values at 5 percent moisture content being 25 to 50 percent greater than at 15 percent moisture content. Subjecting material to conditions causing alternating shrinking and swelling results in significant reductions of strength, stiffness, and specific gravity. Relative creep in particleboard, unlike that in whole wood, is greatly affected by moisture content level, with values for material of high moisture content being as much as 10 times greater than those for material of low moisture content.

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