Properties of Steam-Pressed Particleboard

Thin southern pine flakes were formed into panels about 14 in. square and 0.6 in. thick in a laboratory press with vented platens. Saturated steam at gauge pressures of 100 psi or 125 psi was applied to the mat during the pressing operation for periods of 1, 2 or 3 minutes. The total time in the press was about 1 1/2 minutes longer than the steaming period. Control boards were made in the same press without steam injection and with a press time of 7 minutes. The furnish was bonded with a liquid phenol-formaldehyde adhesive, the resin solids content (based on the oven-dry weight of the flakes) being 6 percent. Steaming for 2 or 3 minutes, but not for 1 minute, appeared to provide satisfactory cure of the resin. When removed from the press, the moisture content of the boards was, on average, about 14 percent and 17 percent for steam pressures of 100 psi and 125 psi respectively, compared with 8 percent for the controls. The increase was caused by condensation of the steam. Steaming did not affect the linear expansion, but it did reduce the thickness swelling considerably. After being soaked in water for 24 hours, the thickness swelling of the steamed boards was between 50 percent and 64 percent that of the controls and, when dried, the springback was less than one-third that of the controls. Boards made with steam at 100 psi pressure had better mechanical properties than those made with steam at 125 psi, but the effect was not significant except for modulus of rupture. The steamed boards generally had lower mechanical properties than the controls, probably due to the effect of the increased moisture content on the curing of the resin and to some hydrolysis of the hemicellulose. The internal bond strength was low for both steamed and control boards, and this indicates that a higher resin content may have been desirable for these thin flakes with their large surface areas.

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