Preliminary Report on a Proposed New Method of Estimating the Service Life of Exterior Grade Plywood

A test is being made of a proposed new method of estimating the service life of plywood. It is based upon the rate of reduction in strength of a bond with time in service and the correlation between the rates of strength reduction produced by accelerated weathering and service exposure. Previous investigations showed that the durability of a bond is associated with the wood–its chemical properties and resistance to fatigue and decay. The strength of weatherproof bonds is reduced by repeated boiling and drying. Nonweatherproof bonds suffer more rapid reduction in strength than do weatherproof bonds. In the reported project, five factors are being considered as variables: 1) three weathering systems–boiling and drying, a test fence exposure of panels protected with three coats of paint, and a similar exposure of unprotected panels; 2) seven degrees of weathering severity followed by mechanical tests on the glue line; 3) four phenolic resin adhesive strengths from “weaker” to “stronger-than-the-wood;” 4) ten trees; and 5) two glue lines, one with loose-to-loose grain and the other tight-to-loose grain. Only one-third of the testing is currently completed–the boiling and drying accelerated weathering phase. The rest will be done at intervals over the next 20 years. It is expected that the results will show: a) the strength of “”weatherproof” bonds is reduced by weathering just as it is by repeated boiling and drying: b) nonweatherproof bonds suffer a much more rapid reduction in strength than do weatherproof bonds, and c) the rates of reduction in strength caused by repeated boiling and drying can be used to predict the service life of plywood bonds with greater accuracy than has previously been possible.

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