Increased strength with multilayers on extensible kraft
fibres

Abstract: Jessica Sjöberg and Hans Höglund at Fibre Science and Communication Network,
Mid Sweden University, have developed a refining technique to produce
unbleached kraft pulp with high strain at break and tensile energy adsorption
index. In another project at Fibre Science and Communication Network, Gunilla
Petterson and Lars Wågberg have developed a strategy for formation of
polyelectrolyte multilayers on cellulose fibres. This method increases the
joint strength between fibres. The aim of this thesis work was to use the two
techniques mentioned above and to optimize them in order to produce a paper
with both high tensile index and strain at break.

In this study high consistency refined pulp from an atmospheric and a
pressurized refiner was low consistency refined in an Escher-Wyss laboratory
refiner to a SR-number between 17,5-20,5. Ordinary ISO-sheets and freely
dried sheets were manufactured from those pulp samples. The laboratory sheets
made from pulp samples from the pressurized system had higher strain at break
and tensile energy adsorption index but lower tensile index compared to
sheets from the pulp sample refined in a conventional atmospheric high
consistency system.

ISO-sheets and freely dried sheets were also made from refined pulp samples
whose fibres had been coated with one or three layers of polymer.
Polyelectrolyte multilayers were built by sequential provision of anionic
starch and cationic CMC. Three different stock preparation strategies were
used for the provision of polymer. Adsorption equilibrium was determined
indirectly by charge titration of non-adsorbed polyelectrolyte in a particle
charge detector. Physical paper properties of the sheets showed that the
addition of only one layer of starch increased strain at break, tensile
index, tensile energy adsorption index and Scott-Bond significantly. By using
the multilayer technique tensile index could be increased further compared to
sheets from fibres with one single starch layer, however, no major positive
effects on strain at break and tensile energy adsorption index was obtained.
When the fibres were treated with multilayers Scott-Bond, which is a measure
of the internal bonding strength, increased dramatically compared to the
samples with one layer of starch.