Microscopic Identification of Undercured Glue Bonds in Plywood

An hypothesis that undercured plywood glue bonds could be recognized under low power magnification was tested. Plywood specimens containing a wide range of bond qualities and representing resinous and non-resinous softwoods as well as hardwoods with hard and soft grain and a range of densities were studied in detail. All panels were glued with the standard hot-press phenolic resin commercial plywood adhesive. Bond qualities were determined by the British Standards knife test. Visual examination of gluelines was made on specimens prepared from microtome sections of panel edges viewed under a stereomicroscope at low power. Analysis of the data indicated that the degree of cure of hot-press phenolic plywood bonds can be estimated within limits. The quantity of percentage of glue which has migrated into the lathe checks is an indication of poor bond, with the greater migration resulting in greater undercuring of the glue when press times are constant. The correlation between discontinuities in the glueline due to cohesion failures and poor bond quality was good, indicating lack of strength to resist stresses resulting from release of platen pressure. The most useful single indicator was described as the condition of lathe checks partially filled with glue, and particularly those portions of the checks which are more nearly parallel to the glueline. Glue-filled portions of checks that are bonded give evidence that the main glueline was adequately cured. When these checks are not bonded, this is evidence that the glue was not sufficiently cured to withstand tensile forces exerted on the lathe checks by the spring-back of the veneers.

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