Wet Web Rheology on a Paper Machine

The runnability of a wet web is the sum of many factors, ranging from furnish variables to papermaking running parameters (speed, draws, distance from wet pressing etc). The relative importance of these factors was studied using several different experimental methods. The dynamic stress-strain relationship was determined in situ by measuring it on a wet web winder installed on a pilot paper machine. It was then compared with values obtained by testing the wet rolls on a separate running device. The comparison suggests that tensile strength is a more fundamental characteristic of the stress-strain curve than the dynamic stiffness affected by creep. Tensile strength is dominated by moisture content in a transition region where free water enters the fiber network. Its sensitivity to moisture content weakens as the paper becomes very wet. The location of the transition region depends on the fiber saturation point. This leads to complex changes in ranking when different pulps are compared at different moisture contents. The fines content of the furnish has a significant impact on wet web strength, whereas the fiber stiffness affects the measured dynamic stiffness but not tensile strength.