Effects of heating, drying and straining on the relaxation and tensile properties of wet paper: Dissertation

Producers, converters and printers of paper try to maximize the production and minimize the waste. The runnability of paper web plays an important role when the efficiency of processes is improved. This investigation was here focused on evaluating in-plane viscoelastic strength properties of paper when miming the conditions typical of the paper machine press and dryer section. In order to study the effects of refining, straining, heating and drying on wet paper strength properties, new test methods were developed. The advantage of these methods was to produce more realistic laboratory data for evaluating the runnability of wet paper web in the paper machine and that of dry paper web in the converting and printing processes. Measurement of the residual tension in a tension relaxation test is essential for describing the runnability of a pulp furnish in the paper machine. In addition to this, several parameters are needed to describe the runnability of wet and dry paper web. The tension-strain behavior of wet paper and web handling in the paper machine determines to large extent the runnability potential of dry paper. Wet web draws during drying influence the tension-strain properties of dry web that govern the dry web runnability. In wood containing printing paper grades (NEWS, LWC, SC) the runnability potential of mechanical pulps (TMP, PGW) is very good compared with that of chemical reinforcement pulps. The effect of addition of bleached softwood reinforcement pulp on the tension-strain properties of mechanical-pulp-based furnish is different for dry and wet paper. Generally, in mechanical-pulp-based furnishes, bleached softwood reinforcement pulp is not able to improve the tensile stiffness of wet paper. However, the reinforcement ability of bleached softwood kraft pulp in wet paper depends on the strength properties of the mechanical pulp there. Heating has a strong negative influence on the strength and relaxation properties of wet paper. Decrease of its tensile stiffness can be explained by softening of polymers in the cell wall of fibers. The observed temperature-accelerated relaxation in wet paper seems to be analogous to the moisture-accelerated creep phenomenon in dry paper. Elevated temperatures used in the paper machine have a strong influence on the wet web tensile behavior, e.g. in the transfer from the press to the dryer section, as the tensile stiffness is decreased and increasing tension relaxation of the wet web. This work raises some fundamental aspects in the complex subject of the paper web runnability, and aim to provide tools for the controlling the wet web tension-strain behavior, and ensuring properties of the end product.