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 produceunbleached kraft pulp with high strain at break and tensile energy adsorptionindex. In another project at Fibre Science and Communication Network, GunillaPetterson and Lars Wågberg have developed a strategy for formation ofpolyelectrolyte multilayers on cellulose fibres. This method increases thejoint strength between fibres. The aim of this thesis work was to use the twotechniques mentioned above and to optimize them in order to produce a paperwith both high tensile index and strain at break.In this study high consistency refined pulp from an atmospheric and apressurized refiner was low consistency refined in an Escher-Wyss laboratoryrefiner to a SR-number between 17,5-20,5. Ordinary ISO-sheets and freelydried sheets were manufactured from those pulp samples. The laboratory sheetsmade from pulp samples from the pressurized system had higher strain at breakand tensile energy adsorption index but lower tensile index compared tosheets from the pulp sample refined in a conventional atmospheric highconsistency system. ISO-sheets and freely dried sheets were also made from refined pulp sampleswhose fibres had been coated with one or three layers of polymer.Polyelectrolyte multilayers were built by sequential provision of anionicstarch and cationic CMC. Three different stock preparation strategies wereused for the provision of polymer. Adsorption equilibrium was determinedindirectly by charge titration of non-adsorbed polyelectrolyte in a particlecharge detector. Physical paper properties of the sheets showed that theaddition of only one layer of starch increased strain at break, tensileindex, tensile energy adsorption index and Scott-Bond significantly. By usingthe multilayer technique tensile index could be increased further compared tosheets from fibres with one single starch layer, however, no major positiveeffects on strain at break and tensile energy adsorption index was obtained.When the fibres were treated with multilayers Scott-Bond, which is a measureof the internal bonding strength, increased dramatically compared to thesamples with one layer of starch.