Multi-stage tensile straining during drying of SC paper

Abstract

During multi-cylinder drying on a paper machine, the paper web is under stress in the machine direction, whereas the cross direction is more or less free. The web shrinks due to drying and contracts due to web draws. The machine speed, dryness and tension level determine the speed difference between drive groups, which in turn determines the level of MD straining. Straining and stresses during drying have a significant influence on the elastic properties of paper. The objective of this study was to investigate the effects of multi-stage straining during 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 and the press cylinders. Never-dried SC paper samples were then dried in a C-Impact laboratory tensile tester. The tensile force, strain, surface temperature and dryness of the samples were measured during drying.

The design of the trials was full factorial (multivariate), which enabled the use of statistical methods in the data analysis. Two of the five straining tests were performed on a pilot paper machine during sample preparation, and three of the five during drying on a laboratory tester. The draws were combined in order to form two separate 34 full factorial designs. It was concluded that the use of a fractional factorial design instead of a full factorial design would lead to equally statistically good results, but would also be biased towards the most powerful factor term.

The measured tension of the paper samples during drying was affected by the straining, dryness and tension relaxation of the paper. The straining and drying history of the paper also influenced the tensile properties. Increased straining generally led, almost linearly, to decreased strain at break of the dried paper. The tensile stiffness of the dried paper and the drying tension at the end of drying (final drying tension) were increased considerably by straining. Although a certain level of strain 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 dryness of straining. The C-Impact tensile tester was found to realistically simulate the paper drying process at paper machine conditions.