Redrying Fire-Retardant-Treated Structural Plywood


  • Phil Woo Lee
  • E. L. Schaffer


Fire retardants, structural plywood, Douglas-fir, aspen, thickness shrinkage, kiln-drying, press-drying, borax-boric acid, chromated zinc chloride, minalith, pyresote


Exterior grades of Douglas-fir and aspen plywood were impregnated with interior fire-retardant chemicals and redried under low-, intermediate-, and high-temperature drying conditions. Fire-retardant treatments included borax-boric acid, chromated zinc chloride, minalith, pyresote, and a commercial formulation. Drying processes included kiln- and press-drying. Evaluated were drying rates and defects generated. The borax-boric acid and the commercial treatments redried at rates similar to water-treated controls. Other salt treatments were significantly slower drying and more defect-prone. Chromated zinc chloride treatment was consistently the slowest drying and most defect-prone. Press-drying was three times faster at an equivalent temperature level. However, thickness shrinkage doubled because of 50 lb/in.2 platen pressure.


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