Effect of Heat Treatment on the Surface of Douglas-Fir Veneer

A number of Douglas-fir plywood panels were subjected to severe drying conditions. The veneer was found to be susceptible to the development of inactivation when over-dried, but this susceptibility varied within a single tree. Veneer extracted with acetone or a combination of acetone and methanol benzene could no longer be inactivated by over-drying. Extraction with water had no apparent affect on the development of inactivation. The veneers of other species tested did not appear to be adversely affected by serious over-drying. There is strong evidence that the underlying cause of the development of an inactivated surface on Douglas-fir veneer is the presence of “extractive” material. When subjected to excessive heat, the extractive material causes changes in the veneer surface which in turn affects the reactivity of the glue and results in an undercured bond. Replacement of extracted material did not re-establish the conditions conducive to the formation of an inactivated surface.

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