Stress–strain curve of single pulp fibres and paper

Jarmo Kouko, Marina Jajcinovic, Wolfgang Fischer, Annika Ketola, Ulrich Hirn, Elias Retulainen

    Research output: Chapter in Book/Report/Conference proceedingConference abstract in proceedingsScientific


    Fibre properties have a central role in the mechanical properties of paper. Fibre strength determines the strength potential of paper. Also the elongation potential of fibre network is strongly related to the behaviour of single fibres. When aiming at improved strength or elongation a better knowledge of the properties of single fibre and the effect of mechanical treatment is needed. The literature results typically suggest that the stress-strain curve of straight single fibre is practically linear or slightly concave downwards and the typical elongation is 2-5%. Refining has been shown to be an efficient way to improve the strength of paper, mainly due to improved bonding. High consistency refining has been shown to increase elongation of paper. This has been attributed to the deformations (curl, dislocations and micro-compressions) introduced by the treatment. But is that due to the improved fibre elongation of single fibres? The effect of mechanical treatment on the stress-strain behaviour of single fibres of bleached softwood kraft from a Nordic mill was investigated and compared also to the stress-strain curve of paper. The results showed that the individual fibres were strong and showed considerably higher elongation than expected. They also had a very wide variation in those properties. The measured strength and elongation ranged from 200 MPa to 1100 MPa, and from 8% to 32%, respectively. The refining did not improve the measured strength, nor did the HC refining increase the elongation of the single fibres. The effects of the refining seemed to correlate with the number of dislocations (measured using HCl cleavage test) in fibres. With increasing number of dislocations both the average breaking stress and strain decreased. High consistency refining introduced dislocations, but low consistency refining seemed to be able heal some of them. The shape of the stress strain-curves was rarely a linear, but often the slope increased with straining which behaviour could be related to the straightening of disordered regions in the fibre wall. Test also showed that fibres show a considerable plastic elongation and stress relaxation. Comparison of the single fibre tests with the paper test results, suggest that the main effect of refining on paper strength is not due to increased strength nor elongation of fibred but due to the increased bonding of the fibres, which is mainly responsible for the increased strength. Increased paper elongation caused by HC refining is not due to improved fibre elongation but due to reduced axial stiffness of the wet fibres that results in increased drying shrinkage of the paper.
    Original languageEnglish
    Title of host publicationProceedings of Paper and Biorefinery Conference 16-17 May 2018
    Subtitle of host publicationAbstracts
    PublisherAPV Graz - Akademischer Papieringenieurs Verein
    Number of pages2
    Publication statusPublished - 16 May 2018
    MoE publication typeNot Eligible
    EventPaper and Biorefinery Conference - Graz, Austria
    Duration: 16 May 201817 May 2018


    ConferencePaper and Biorefinery Conference


    • tensile strength
    • strain
    • fiber
    • paper


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