Fracture Mechanics - A Tool for Evaluation the Effect of Reinforcement Pulps in Mechanical Pulps, 1999 International Paper Physics Conference Proceedings

Christer Fellers, Jenny Melander, Ulla-Britt

Web breaks in press rooms are one of the most unwanted causes of economical loss for a printer. The most common cause of web breaks is defects in the paper. To quantify paper runnability, a non-linear fracture mechanics model, based on the J-integral theory, was used. This model makes it possible to quantify the runnability of paper in printing presses in terms of critical force and critical elongation, two quantities that describe the ability of the web to resist crack propagation.

The aim of this study was to evaluate how the shape, treatment, and addition level of softwood chemical fibres affected the fracture properties of laboratory sheets produced from a mixture of TMP and chemical pulp. The proposed fracture mechanics model can be used to calculate runnability of paper in printing presses in terms of critical force and critical elongation when the paper contains a defect of given size. When the defect size increases, the critical force and critical elongation decrease dramatically. A positive effect on runnability was observed with only small additions of chemical pulp. For small additions of chemical pulp, differences in fibre shape and treatment had little effect on the runnability. At higher additions of chemical pulp, straight, beaten fibres were the best to get a high critical force, while curly fibres were the best to get a high critical elongation. The evaluation of reinforcement pulps from handsheets of 100 % sulphate pulp is not relevant for the reinforcement ability in combination with other fibres. The information necessary to evaluate the runnability of paper in printing presses can only be obtained from fracture mechanics theory and not from other standardised paper tests.