Hornification: Implications from Paper Manufacturing to Recycling

Swedish Karlstat University informs about new insights into hornification regarding paper production.

When paper dries and is subsequently rewetted, its properties change permanently. This phenomenon is known as hornification. According to the university, new research shows that the process is more complex than previously assumed, and that temperature, humidity, and fiber type all play decisive roles.

Hornification means that fibers in paper products lose some of their ability to absorb water. This has major implications for everything from paper manufacturing to recycling, where controlling the material’s strength and durability is crucial. “Fundamentally, hornification is more about removing water than adding heat, and this means that we can actually control the material’s properties and avoid unnecessary strength losses,” Björn Sjöstrand, Associate Professor of Chemical Engineering and project leader for the research project, was quoted.

Björn Sjöstrands, Associate Professor of Chemical Engineering (Photo: Karlstat University)

As reported, one of the most surprising findings is that hornification does not increase steadily with temperature. Instead, there is a distinct “dip zone” around drying at 40–60 °C, where structural changes in the fibers are minimal and the material is at its strongest. “This pattern has now been confirmed in several different pulp types, lending weight to earlier isolated observations reported in previous studies.” The study would also clarify something that has previously been uncertain: it is primarily the removal of water, not the heat itself, which drives hornification. “When the effect of high temperature was separated from the drying process, the results showed that heat alone has almost no influence on the fiber structure. This provides a clearer picture of what actually happens within the material.”

The results further show that hardwood pulp is affected more than softwood pulp. The reason is that hardwood fibers have a more complex structure and swell more in water before drying, which leads to greater collapse and stronger hornification during drying compared with softwood fibers.

Furthermore, the researchers have identified a linear relationship between the loss of material strength and the degree of hornification. “This means that microscopic structural changes can now be directly linked to paper strength – something that previously lacked a clear quantitative basis. Taken together, the study strengthens the theory that hornification is primarily governed by hydrogen bonding between fibers, while also showing how the interaction between temperature, moisture loss, and fiber type determines how the process develops.” By gaining deeper insight into what happens at the fiber level, the industry can optimize processes and reduce quality losses during reuse, the researchers at Karlstat University are convinced.

Read the full study: diva-portal.org/smash/get/diva2:2033815/FULLTEXT01.pdf

(Published in GLOBAL RECYCLING Magazine 2/2026, Page 30)