Paper strength correlates with hornification for kraft pulps dried at different temperatures

Abstract This study examines the impact of hornification on the strength of paper made from chemical pulps. Two key aspects are examined: how varying degrees of hornification influence the mechanical properties of paper sheets, and how temperature affects hornification in papermaking pulps when water remains within the fibre structure. Experiments were conducted on hardwood and softwood kraft pulps that were dried and heated under controlled conditions. The results revealed that hornification reduces the strength of paper made from three different types of pulp. A local minimum of hornification and local maxima of tensile strength occurred around 40–60 °C. A strong linear correlation was observed between decreased tensile strength, decreased water retention, and the hornification ratio. Temperature treatments applied without water removal did not affect the water retention value (WRV) or the strength of the paper. This confirms that water removal is essential for hornification and strength loss to occur. These findings refine our understanding of hornification, suggesting that careful process control during drying can exploit the positive effects of moderate drying while minimizing hornification, and thus, the risk of excessive fibre closure. Such control strategies could improve strength optimization in industrial pulp and paper manufacturing.