Effect of Molding Temperature on the Strength and Dimensional Stability of Hardboards from Fiberized Water-Soaked Douglas-Fir Chips

Strong hardboard was made from stock prepared by fiberizing water-soaked Douglas-fir chips. The hardboard was made without resorting to steam-cooking, the incorporation of synthetic resins, or prolonged heat treatment of the board. The drying and heat-treating were achieved in one short operation in a hot press by means of high molding temperatures. Since the temperature of exothermic decomposition is not appreciable below 270?C., it appears that molding temperatures in excess of 240?C. could be used. Under such conditions the molding could be less than 10 minutes. The increases in flexural strength and in stability in thickness (swelling and recovery) were comparable to those obtained in the commercial 4- to 5-hour kiln-drying procedure. The flexural strength increase was, however, considerably below that secured by means of tempering. Raising the maximum temperature from 180?C to 240?C. for a 5-minute molding period increased the flexural strength 32 percent; stiffness, 19 percent; water resistance 33 percent; and decreased swelling and recovery 27 percent upon humidification. Molding for periods at maximum temperatures of 25 to 75 minutes gave further improvements in stiffness, water resistance, and thickness stability, but little or no improvement in flexural strength and an additional decrease in toughness. Linear expansion was unaffected by the temperature or time of molding.

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