Evaluation and Optimisation of the In-Plane Tearing Strength of Paper

Web failure in the papermaking and printing operations is considered as an in-plane tearing mechanism and two aspects of this-in-plane edge tear and in-plane started tear-are discussed. A technique is proposed for the measurement of in-plane edge tearing strength and this is used to assess disc slitter performance and the potential of new approaches to slitting. It is shown that water jet slitting offers a possible alternative to conventional disc slitting, yielding edges with higher and more uniform edge tearing strength than those obtained from conventional disc slitting. The technique is used also to evaluate the influence of various edge defects on edge tearing strength. It is found that, whereas shives have no significant effect on the edge tearing strength of newsprint evaluated in this investigation, short edge cuts cause significant reductions in edge tearing strength. The strain concentration about web defects is examined using laser holographic interferometry and the observations are in qualitative agreement with those obtained with the edge tear tester. Whereas the edge tearing strength is a measure of the force required to initiate a tear in the edge of a paper web, the in-plane started tearing strength is an energy measure of the flaw-carrying ability of the web. A pendulum tester, designed to evaluate this parameter, is used to determine how the degree of bleaching, the extent of beating and the drying history of chemical pulp in conjunction with the drainage characteristics of groundwood affect the in-plane started tear characteristics of paper. The relationship between Elmendorf and in-plane started tear is also discussed.