Relationship of z-tensile strength with in-plane strength properties of paper

Several methods are available for the measurement of strength of paper in the thickness direction. However, the values obtained from these methods are highly correlated with each other and also with the in-plane strength measurements. The relationship between z-direction tensile strength and in-plane tensile index of handsheets made at varying extents of pulp beating, types of strength aids and doses of strength aids has been studied. It has been noticed that the z-direction tensile strength of paper can be expressed in terms of in-plane tensile index by a single relationship whether the tensile index is changed by refining, or by adding strength aids, or by increasing the dose of the strength aid. In practice, an appropriate value of tensile index may be specified to ensure adequate performance of the paper even when it is subjected to stresses in z-direction.