Influence of straining during wet pressing and drying on strength properties of paper

During multi-cylinder drying on a paper machine, the paper web is under tension in the machine direction, but restraint in the cross direction is not controlled. The objective of this study was to determine the effects of multi-stage straining during wet pressing and drying on the tensile properties of dried paper. Oriented SC paper samples were prepared on a pilot paper machine with varying levels of draw between the press rolls and subsequently dried in a laboratory tensile tester and strained at three stages during drying. The results showed that increased straining generally led, almost linearly, to decreased strain at break of the dried paper. Tensile stiffness was increased considerably (i.e. 15-20%) by straining at the press section and during drying. The web solids content during the draws significantly influenced the development of tensile stiffness. The efficiency factor, previously used to describe changes in inter- and intra-fiber effects on the shape of the tension-strain curve, was found to correlate with the draw history and strain at break of dried paper. Although a certain level of draw is needed to ensure wet web runnability, straining has a detrimental effect on a number of dry paper web properties. Straining strategy improvements can be made by controlling the solids content at the straining phases.