The nature of strength reduction when newsprint is calendered.

Application: Understanding how calendering reduces tensile strength in newprint will help us find ways to diminish the strength loss.

Calendering is known to reduce the tensile strength of paper, but the nature of the damage has not been extensively studied. The likelihood that calendering causes damage to the component fibers of paper or that it causes disruption of the bonds between fibers has been discussed in the relevant literature for various types of calendering technology. However, these hypotheses have not been researchers measured the reduction in strength of newsprint calendered to the commercially required density by calenders of various types.

The elements of tensile strength attributed to the "fiber and "bonding" components have been separated with a simple equation that reflects their paralled nature. This equation is sufficient to demonstrate whether or not a change in measured fiber can account for a change in paper tensile strength. Some decrease in the strength of fibers was measured, but the overwhelming contribution to the loss of strength from calendering was found to be caused by a decrease in bonding strength.

This finding is not in keeping with comments in the published research literature. Apparently, little fiber damage is sustained when low-density papers are calendered on a pilot paper machine heavily enough to achieve commercial densities.

Burnett, now process development engineer at UPM-Kymmenne's Shotton Paper Company, Flintshire, UK, worked on this study while a student at UMIST; I'Anson is lecturer in paper science, Department of Paper Science, UMIST, P.O. Box 88, Manchester, M60, 1QD, UK. Email I'Anson at [e-mail].