A New Fiber-Overlay Process for Plywood

The greatest shortcoming of softwood plywood is the poor quality of the surfaces even after sanding. Hardboard can be laminated to 3-ply plywood in order to improve surfaces but this product (Plyron) is dense, costly, and variable in thickness because sanding is not practiced. A new product similar in concept to Plyron is introduced, one that is made from phenolic-resin-bonded plywood. The fiber surface sheets and plywood glues are cured in one hot press operation to yield a sandable hardboard-faced product. The product is covered by U.S. Patent Number 3,380,031 and Canadian Patent Number 691,705. Wood fiber is made from chipped mill wastes by steaming and refining. The furnish is dispersed in an alkaline aqueous slurry into which is added phenolic resin at the rate of 7.5 percent of the ovendry fiber weight. The slurry is acidified and the resin precipitates on the fiber. The mats of wood fiber are formed in a deckle box or on a cylinder machine, dried to 15-22 percent moisture content, and assembled with gluespread veneers into a 5-ply assembly, 3 plies of veneer between the two mats. These are hot pressed at 175-225 psi. The resulting panel had a specific gravity of 0.70 which compares favorably to 0.75 for Plyron and 0.59 for Douglas-fir plywood. Bending strength across the face grain vs. with the grain was more balanced for the experimental product (7190 psi vs. 8310 psi) than for Plyron (5560 psi vs. 7230 psi), medium-density overlaid (one face) 3-ply plywood (2950 psi vs. 10,120 psi), and 3-ply plywood (2740 psi vs. 10,790 psi). Thickness swell after 24-hour water soak was about the same for the new product (4.9 percent), Plyron (5.l percent), and MDO plywood (4.4 percent), but much lower than the plywood (7.2 percent). Taber abrasion showed a similar pattern. In addition to the plywood mill, a $450,000 mat-making plant is required which can produce mats at a cost of $18-23/MSF. It is stated that the product would have use where Plyron and MDO plywood are used and in store fixtures and cabinetry.

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