THE EFFECT OF PAPER STRUCTURE ON BEHAVIOR IN A CALENDER NIP

Cross-sections for scanning electron microscopy were prepared from an uncalendered eastern Canadian TMP newsprint and from the same paper ca lendered under two sets of conditions. As well, a bleached, filled kra ft sheet was hand calendered and sectioned. Paper structure (in terms of variations in local thickness and local apparent density) and fibre properties (in terms of coarseness and fibril angle) as determined fr om analysis of the SEM photographs were related to the stress-strain b ehaviour of paper measured in a calender nip, Calendering imposes high er pressures on locally dense or thick spots in the sheet, causing gre ater stresses in those locations, while adjacent bulky or thin areas a re not as highly stressed. In areas where local stress is high, local strain is also high and the local void fraction in the sheet may appro ach zero. in these high-stress areas, some coarse, brittle fibres frac ture while more flexible, ductile fibres deform without damage. The ro le that the fracturing of fibres during calendering may have on paper strength reduction is discussed.