A new method showing the impact of pulp refining on fiber-fiber interactions in wet webs

There is still an on-going discussion about current theories regarding fiber-fiber bonds at dryness below 60%. The goal of this paper is to show the impact of refining on the initial wet web strength (IWWS) and to reveal the role of macro fibrils in wet webs for strength development. The study was carried out with laboratory hand sheets, made of commercial bleached softwood pulp, which was refined in a laboratory valley beater. In addition to measuring the IWWS, the paper specimens were prepared by cryo fixation and subsequently freeze dried. This sample preparation enables the imaging of fiber alterations by means of high-resolution Field Emission-Scanning Electron Microscopy (FE-SEM). As expected, the effect of the dryness level as well as the degree of refining are positive and quite big on IWWS. The images reveal the behavior of macro fibrils in the fiber-water phase. These interactions between the fibrous material prompts the electrostatic forces as well as the frictional connection and entanglements. Acting in a self-assembly manner, strong cohesive joints are formed within the fiber structure. The IWWS values, in conjunction with the images, reveal the processes involved in the approach of fibers and macro fibrils even when acquired at high moisture levels.