Evaluation of the Surface Bonding Strength of Paper.

Tests for measuring the surface bonding strength of paper, which relates to the resistance of paper to picking in the printing operation, are discussed, and a new method is described. Tests in which a transverse force is applied to blocks cemented to the paper would seem to have merit, in principle, but are not accurate because the cementing operation modifies the paper and because localized stress concentration during the application of the load introduces a hidden error; moreover, the stress concentration involved does not simulate that existing in a nip of a rotary press. The friction tape and Scotch Tape tests, in the hands of experts, are useful for the subjective evaluation of the surface bonding strength of paper, but fail to provide a numerical measure of this property. There is almost no fundamental basis for the wax pick test, which is widely used. In the hands of expert operators of good judgment, this test is useful as a guide, but results obtained with the test correlate only crudely with actual performance, and in some cases the results can be very misleading. In consideration of these difficulties with existing methods, an apparatus for the measurement of the viscosity-velocity product (VVP) has been developed. In the new method, the stress which ruptures the paper surface arises in high rate of shear of a viscous film, which is quickly applied to and removed from the paper under carefully controlled conditions that closely simulate the disruptive action occurring in the operation of a rotary press. Apparatus for the development of the method, and new equipment currently under design, are described.