Paper wet strength at the press felt seam

Abstract Paper machine press felts often contain seams that facilitate their manufacturing and replacement. However, in industrial practice, breaks in the paper sheet commonly initiate at these seam locations. This study examines how press felt seams affect the local wet strength of high grammage papers. A custom experimental setup was developed to dewater paper using either tensioned seamed felts or two uniform felts separated by a small gap (“gap seam”). The pressed paper was then subjected to tensile testing. The pressure distribution near real press felt seams were mapped with pressure-sensitive film. From this, an “effective gap” was defined as the region with reduced pressure. This effective gap increases linearly with felt tension. The wet strength of paper produced using a seamed felt with a given effective gap matched that of paper pressed with a gap seam of the same size. The tests were conducted under industrially relevant conditions for a cardstock-grade paper, including press felt tension, pressure, furnish, and moisture content. Results showed that paper wet strength near the seam decreases exponentially with increasing seam size. The characteristic length scale of this decay closely matches the average fiber length in the furnish.