Measurement of Glue Line Viscosity

Describes in some detail an instrument that indicates the changes in viscosity of a glue line under hot pressing conditions. The conditions employed were chosen to simulate those used in hot press manufacture of plywood. A hot plate the temperature of which can be varied is attached to a vertical shaft driven by a motor at 1 rpm. A hardboard base 1/3 inch thick is fixed to the hot plate, providing a horizontal surface on which the test adhesive can be spread. A second hardboard disc is secured to an upper vertical shaft so the surface of the disc can be placed in contact with the spread adhesive. Varying vertical loads can be placed on the upper shaft to simulate pressing pressures. The upper shaft is restrained from rotation by a strain gage which measures the torque transferred to the upper shaft through the adhesive film. The adhesive film is cured by applying heat to the hot plate and its viscosity characteristics are indicated by the torque measured by the strain gage as the lower shaft is rotated. Results of testing interior-type heat curable (animal blood base) adhesives and exterior-type (various phenolic resin-extender mixtures) heat curable adhesives are shown. The yield point of the adhesive film is plotted against the heating time in minutes. These results are correlated with assembly times or plywood gluing results to illustrate the viscosity characteristics of various types of adhesives. The device was also used to indicate glue line viscosity characteristics of resin-impregnated decorative overlay papers by placing the resin-saturated dry papers between the hardboard surfaces. Phenolic and melamine saturating resins were studied in this way and their differences were illustrated by characteristic curves of load versus heating time.

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