Z-direction Properties: the Effects of Yield and Refining

The z-direction (ZD) elastic properties of paper have received little attention in the past because of measurement difficulties. This paper describes the effects of wet pressing, refining, and yield on three ZD elastic properties, C33 , C4 4 , and C55. The elastic parameters were measured using ultrasonic methods on an unbleached kraft oak pulp. The ZD elastic parameters were very sensitive to wet pressing pressure. Increasing the level of refining or decreasing pulp yield produced increases in C3 3, C4 4, or C5 5, which were greater than would be expected by wet pressing alone to the same density. A plausible explanation for this behavior is that the refining and yield changes also significantly change the ZD stiffness and shear stiffness of the fiber cell wall.