When paper dries and is subsequently rewetted, its properties change permanently. This phenomenon is known as hornification. New research now shows that the process is more complex than previously assumed, and that temperature, humidity, and fibre type all play decisive roles.
Hornification means that fibres 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,” says Björn Sjöstrand, Associate Professor of Chemical Engineering and project leader for the research project.
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 fibres 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 also clarifies something that has previously been uncertain: it is primarily the removal of water, not the heat itself, that 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 fibre 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 fibres 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 fibres.
Another important piece of the puzzle is that 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 fibres, while also showing how the interaction between temperature, moisture loss, and fibre type determines how the process develops.
What could this research lead to?
By gaining deeper insight into what happens at the fibre level, the industry can optimise processes and reduce quality losses during reuse.
Read the full study
https://www.diva-portal.org/smash/get/diva2:2033815/FULLTEXT01.pdf
Facts
The work is funded by the Åforsk Foundation and is part of the project Hornification and Fibre Properties, which runs until the end of 2027. The study is based on the master’s thesis “The influence of temperature and drying conditions on hornification and paper properties: Further investigation of the hornification phenomenon”, published in 2025 by Sandra Mellroth within the project.
