Stress-Strain Behavior and Microstructural Evolution of Hot-Pressed Paper Webs Revealed by X-Ray Tomography

The mechanical performance of fiber-based materials depends not onlyon strength but also on elongation, which is particularly critical inconverting and end-use applications. Wood-fiber-based materials, such asthermomechanical pulp (TMP) paper webs, are typically brittle and exhibitlow breaking strain. However, both dry and wet strengths can besignificantly improved through hot-pressing, especially in the presence oflignin. This study examines the influence of hot-pressing time on thecomplete stress–strain behavior of calendered TMP webs. Dry samplesshowed only minor, systematic changes in mechanical properties withincreasing pressing time at 200 °C. In contrast, wet samples exhibited apronounced increase in breaking strain for pressing times exceeding 1 s,accompanied by increased wet specific strength and tensile energyabsorption. Wet stiffness also increased beyond what could be explainedby densification alone, indicating enhanced inter-fiber bonding. Toelucidate these effects, X-ray microtomography combined with imageanalysis was used to characterize microstructural features, includingporosity, pore size, surface roughness, sheet thickness, and fiber walldensity as functions of pressing time. The results demonstrate thatextended hot-pressing promotes microstructural consolidation andbonding mechanisms that improve mechanical performance under bothdry and wet conditions. The mechanical performance of fiber-based materials depends not only on strength but also on elongation, which is particularly critical in converting and end-use applications. Wood-fiber-based materials, such as thermomechanical pulp (TMP) paper webs, are typically brittle and exhibit low breaking strain. However, both dry and wet strengths can be significantly improved through hot-pressing, especially in the presence of lignin. This study examines the influence of hot-pressing time on the complete stress–strain behavior of calendered TMP webs. Dry samples showed only minor, systematic changes in mechanical properties with increasing pressing time at 200 °C. In contrast, wet samples exhibited a pronounced increase in breaking strain for pressing times exceeding 1 s, accompanied by increased wet specific strength and tensile energy absorption. Wet stiffness also increased beyond what could be explained by densification alone, indicating enhanced inter-fiber bonding. To elucidate these effects, X-ray microtomography combined with image analysis was used to characterize microstructural features, including porosity, pore size, surface roughness, sheet thickness, and fiber wall density as functions of pressing time. The results demonstrate that extended hot-pressing promotes microstructural consolidation and bonding mechanisms that improve mechanical performance under both dry and wet conditions.