Impregnation to Control Dimensional Stability of Particleboard and Fiberboard

Three experiments were made to determine the effect of impregnation on irreversible and reversible swell of reconstituted products and on retention of strength characteristics at thoroughly water-soaked conditions. In the first experiment Pinus virginiana Mill flakes were pressure impregnated with 10 percent phenol formaldehyde. Similar flakes were impregnated with 85 percent thermoplastic polymer by soaking. Control boards were produced by spraying with 7 percent urea-formaldehyde and 5 percent phenol formaldehyde. Three board densities were produced for each adhesive and impregnant at 200, 370 and 860 psi. Specimens of these boards were subjected to five relative humidity cycles from ovendry to 90 percent relative humidity. Average thickness change was used as the comparative criterion. In the second experiment, ponderosa pine flakes were sprayed with 25 percent solids content phenol formaldehyde and dried to 6 – 7 percent moisture content. Subsequently they were sprayed again with phenol formaldehyde at 4 percent resin solids and then pressed to 1/8-inch stops at 500 psi with varying press temperature and time. Dimensional change determinations of specimens were made after soaking in boiling water. In the third experiment, Douglas-fir whole wood fiber was sprayed with 25 percent solids content phenol-formaldehyde resin to provide 50, 25, and 15 percent resin solids based on the air-dry weight of the fiber. The impregnated fiber was allowed to dry to 5.l percent moisture content and then pressed to 1/8-inch stops at 800 psi. The results of this study showed that the impregnation of lignocellulosic fibers and particles prior to board manufacture reduced reversible swell by bulking or reduction in hygroscopicity (or both) and by reducing dispersion. Irreversible swell was likewise reduced or eliminated. Tensile strength was largely retained when irreversible swell was minimized.

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