2001 Volume 2
Web breaks in pressrooms have been modelled in terms of web strength variations and tension variations to identify principal factors controlling the pressroom performance. The web strength variations and their scaling law (the size dependence) have been formulated based on an extreme statistics approach, and the distribution parameters have been determined for a number of mechanical printing grades. By using the tension variation statistics data obtained from a pressroom trial, a parametric study has been conducted to examine the relative magnitude of the effects of key paper properties. The predictions have been compared with field study data. Among the conventional paper
properties, tensile strength and elastic stretch consistently predicted the break frequency. The strength uniformity parameter (Weibull exponent) was shown to have the highest impact on the break frequency based on the parametric study.
In this paper, the impacts of defect type, shape and position on the runnability related strength properties are discussed. As an example, a wood-containing coating base paper is studied. A typical break position on coated mechanical pulp-containing magazine paper production is the first coating unit on a blade coater. There the wetted paper web has to be strong enough and as uniform as possible in order to bear all the stresses under and after the blade.
Very few systematic runnability studies in coated paper production have been reported. Only small step changes and no breaks are tolerated in commercial paper production. Consequently, opinions about relevant test methods for predicting paper runnability have been based on indirect studies. To bypass this obstacle, pilot runnability tests have been included in this study. These results are compared to laboratory sheet studies.
Paper does not break because of its low average apparent strength, but because of a defect in the web of sufficient size, befitting shape, position and orientation. The type of defect is decisive on how reinforcement pulp must be treated and how much of it is needed. Defect type is also important in choosing proper measurement methods in order to predict the endurance of the paper web. Pilot coating trials were used to test base papers where two clearly different types of defects were intentionally made. Defect types were hole and plain cut. These were made at constant intervals into the web. During the coating trial of such defected web, the tension over the coating unit at the moment of a break was considered an indication of the actual strength of paper. In the results, one could clearly distinguish between different behaviour patterns with different defect shapes. Differences were noticed, e.g., as a different maximum tension at a break, and as a different behaviour under the coating blade.
Additionally, handsheet studies using reinforcement pulps refined differently were carried out in order to evaluate their impact on tensile strength of notched samples. Holes and cuts were introduced into the test specimen. The effect of the addition of reinforcement pulp was dependent on the type of mechanical pulp used and on the level of refining of reinforcement pulp so that the effects obtained with notched samples were not predictable while testing undamaged sheets only. FEM (Finite Element Method) analyses simulating stress concentrations around defects gave compatible results with those from pilot coatings and improved our understanding why cut-type defects are so harmful.
A fibre network study was included in order to study the effect of different fibre networks on paper strength. The importance of chemical pulp beating and interaction between mechanical and
chemical pulp was emphasized in these experiments.
The results seemed to be compatible with practical experiences in actual paper manufacturing processes, where paper is coated. The measurements based on the work needed to propagate a cut are not satisfactory in all respects. Fracture toughness may overestimate the benefits of chemical pulp addition and underestimate the benefit of chemical pulp beating. However, fracture toughness is clearly more suitable for predicting coating base paper runnability than the conventional (Elmendorf) tear strength measurement. Tear strength development of paper suggests that almost no beating of chemical pulp is needed, which is clearly not in accordance to our results. Instead, apparent tensile strength tests made on paper specimen with slits in it is a relatively well suited method for predicting the runnability of the base sheet in coating.
The measurement of “damage width” from silicone-impregnated specimens reveals the area in which bond failures and other microscopic fractures take place. We demonstrate that damage width is a reasonable measure of the size of the fracture process zone in the sense of fracture mechanics. Firstly, the decay of cohesive stress against crack widening scales with damage width. Secondly, we can calculate the tensile strength of paper from fracture mechanics using damage width as the size of the fracture process zone. Armed with this interpretation, one can use damage width to evaluate, for example, the effective length and strength of fibers in paper.