INFLUENCE OF THE CALENDERING CONDITIONS ON THE STRUCTURE AND THE PROPERTIES OF WOODFREE PAPER - A COMPARISON BETWEEN SOFT NIP CALENDERING AND HARD NIP CALENDERING

An uncoated woodfree paper was calendered with a hard (steel/steel) ni p or a soft (polymer/steel) nip at line loads between 50 and 350 kN/m using a pilot calender. The paper passed the calender nip only once. T he calendering was performed at three machine speeds (50, 500 or 1000 m/min) and using different steel roll temperatures (40, 100 and 200 de grees C with the hard nip; 40 and 100 degrees C with the soft nip). Fo r a given thickness reduction resulting from the calendering, the pape r gloss increased with increasing roll temperature. The effect of cale ndering on a number of paper properties was in many cases found to be uniquely related to the thickness reduction resulting from the calende ring. Calendering had in general a negative effect on the mechanical ( stiffness and strength) properties of the paper. Measurements of the s urface profile implied that calendering with the hard nip compressed t he thicker regions of the paper (fibre flocs) more than the soft nip. The same conclusion was drawn from measurements of the local thickness of the paper. The local density was evaluated by image analysis and i t was noted that the variations in density in the plane of the paper i ncreased with increasing degree of compression of the structure. In ge neral, the paper structure became more uniform when calendering with t he soft nip than with the hard nip.