Durability of Exterior Siding Plywood with Southeast Asian Hardwood Veneers

Gluing difficulties were experienced when keruing and kapur veneers were used in lieu of meranti for face veneers over Douglas-fir in exterior siding plywood. Kapur-faced plywood consistently is not durable in exterior plywood bonded with phenolic adhesives, and this plywood is not able to pass standard tests for exterior gluebonds. The poor performance is attributed to extractives that coat the veneer surface and interfere with adhesive bond development both by causing the adhesive to gel or thicken prematurely and by preventing its complete cure. Thus, kapur veneer is no longer used in exterior plywood. Keruing-faced plywood occasionally is not durable in exterior plywood bonded with phenolic adhesives, although such plywood generally passes standard tests for exterior gluebonds. These marginal gluebonds may result from poor wettability of the veneer coupled with high stress during cyclic changes of MC of these dense veneers. Available evidence does not clearly indicate cause for the glueline failures that occasionally occur. The use of keruing veneer in exterior plywood has, therefore, been limited by most manufacturers to those panels designed for use in applications less demanding than siding.

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