Improved Particleboard Through Better Resin Efficiency

Dried, screened, Douglas-fir flakes were obtained from a flakeboard manufacturer and experimental boards were prepared following commercial practice. Variables investigated were: method of spraying (air-spray, airless spray, and sonic spray); percentage of solids in resin (65, 58.5, 53, and 42 percent for a urea resin and 42 and 31 percent for a phenol resin, based on ovendry weight); temperature of resin during spraying (70, 105, and 140?F.); degree of atomization (fine or coarse); and spraying–blending time for the resins. The following factors were held constant for all boards: Douglas-fir; particle type, mill-run flakes; board size, 18 by 18 inches; board thickness, 3/4 inch, controlled by stops and sanded to 5/8 inch; density, 40 pounds per cubic foot; and closing pressure, 300 psi. The moisture content of the mat was 12 percent in preliminary tests, 10 percent in succeeding tests with urea resin, and 8 percent with phenolic resin. A wax emulsion, supplied at 50 percent solids, was added. After manufacture, boards were conditioned at 70?F. and 65 percent relative humidity for 2 weeks, sanded, and conditioned for another week; then strips for testing were cut and conditioned until testing. In testing. ASTM Standard D 1037-63T was followed. Measured variables were: modulus of rupture and modulus of elasticity based on a 15-inch span bending specimen, specific gravity, moisture content, specimen volume, internal bond, and stability measurements on a 3- by 6-inch piece taken from the bending specimens. The following conclusions were derived from this project: The use of dye in resin was an important aid in judging the relative sizes of resin droplet, as well as the distribution of resin. Dye had no noticeable effects on properties. The most important factor in strength and stability of the boards was the distribution of resin. Fine atomization of resin and a suitable period of application were the most important factors controlling optimum distribution of resin. Increases averaged about 8 percent and 19 percent for modulus of rupture and 25 percent and 42 percent for internal bond, with fine atomization of urea-formaldehyde and phenol-formaldehyde resins. Good distribution was more effective in improving quality, with phenol-formaldehyde at 4 percent resin content than it was with urea-formaldehyde resin at 6 percent resin content. Uniform distribution cf resin produced high-quality boards regardless of the sprayer. Solids content of resin and the temperature of application had only slight effects on strength or stability of boards. Generally, best results were obtained at high content of solids and normal room temperature. Viscosity, which is directly related to solids content and temperature, had only slight effects on properties, even though drastic changes were made in reducing viscosity.

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