On the role of water and humidity in the dry strength of paper and fiber-fiber bonds

Paper strength is known to increase as water is removed, typically attributed to fiber stiffness. However, whether inter-fiber contact/bonding linked to capillary bridges (CapB) and Laplace pressure (LP) also contributes to this strength gain during drying remains unclear. Theoretically, paper strength should peak before CapB collapses during drying. This study aims to clarify the roles of fiber stiffness and CapB in paper strength. To that end, the mechanical properties of untreated, unsized paper made from bleached hardwood pulp with 17% PCC filler were tested across different temperatures and relative humidity (RH), extending to extreme dryness (0% RH, 120 °C). At 27 °C, strength, stiffness, and E-modulus rose as RH decreased to 10%, then apparently plateaued between 10% and 0% RH. Notably, even at 0% RH and 120 °C, the paper still absorbed some water. To explain the mechanisms behind these results, we evaluated several possible scenarios. Based on this analysis, a new hypothesis is proposed: increased paper strength during drying results from a combination of Laplace pressure and adhesion forces, driven by inter-fiber molecular contact/bonding area, together with fiber stiffness. However, the experiments could not confirm or rule out a contribution from capillary bridges to paper dry strength.