The hypothesis is that it should be possible to modify papermaking conditions in line with the softening properties of high yield pulp fibres and achieve similar strength properties to conventional chemical pulp based paper. We therefore investigated the rheological and physical properties of high yield pulp based papers during hot-pressing. Our results confirm that increased temperature combined with sufficient pressure enables permanent densification by softening of lignin, producing very high tensile strength. This treatment also significantly improved the wet tensile strength in comparison to bleached kraft pulp without using wet strength agents. The high yield pulps used here were spruce based thermomechanical pulp, chemi-thermomechanical pulp, and high temperature chemi-thermomechanical pulp, and birch-aspen based neutral sulphite semi chemical pulp, with spruce-pine based bleached kraft pulp as reference. Rapid Köhten sheets of and 50 % dryness were hot-pressed in a cylinder-press at 20–200 °C, 7 MPa, and 1 m/min. The mechanical properties showed great improvements in these high yield pulp papers, with tensile index increased to 75 kNm/kg and compression strength index to 45 kNm/kg; levels close to and better than bleached kraft. Wet strength increased to 16 Nm/g compared to 5 Nm/g for bleached kraft.
Funding statement: Knowledge Foundation (KKS) with FORIC (Forest as Resource Industrial Research College, a part of Treesearch), the Kempe Foundation, IPCO AB, MoRe Research Örnsköldsvik AB, and Mid Sweden University.
We gratefully acknowledge financial support from the Knowledge Foundation (KKS) with FORIC (Forest as Resource Industrial Research College, a part of Treesearch), the Kempe Foundation, IPCO AB, MoRe Research Örnsköldsvik AB, and Mid Sweden University. We would also like to thank the laboratory personnel at MoRe for their excellent cooperation.
Conflict of interest: The authors declare no conflicts of interest.
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