DENSITY-RELATED PROPERTIES IN 2-LAYER STRATIFICATION.

Abstract.

This article describes results of 2-layer stratification of through air dried, creped and uncreped (flat) sheets on a slow speed (40 fpm) pilot paper machine, using two different, "strong and weak" model systems, one consisting of refined and unrefined NSWK the other NSWK and Eucalyptus. The study dealt with the effects of BW, composition, and layer orientation on creped and uncreped (i.e. flat) strength, density, bulk, and water absorptive capacity. Surprisingly, stratification showed no clear benefit in improving density-related properties; stratified sheets had close to the same density, bulk, and water absorptive capacity as homogeneous sheets of similar strength and composition. However, compared to the homogeneous controls, stratification altered the strength of the creped web. The direction of the change depended on the orientation of the layers; the drier side layer lost 30% more, while the air side 30% less than the unstratified homogeneous control.

These findings would most likely apply to the more popular 3-layer stratification. Furthermore, they imply that stratification would not improve any other density-related tissue product properties, like, for instance, sheet flexibility and bulk softness. However, the findings do not contradict the well-known overall softness and economic benefits of 3-layer stratification. The article also shows how to compare products of different Basis Weight using corrected Breaking Length and corrected Bulk/BW, since these properties are not BW-independent.