Interactions of polymers with fibrillar structure of cellulose fibres :a new approach to bonding and strength in paper

The interactions between paper strength enhancing polymers and cellulose fibrils were studied at molecular and microscopic levels with cellulose model surfaces and with fibril and fibre suspensions. Paper sheet experiments were performed to evaluate the influence of different polymers at macroscopic level on the development of bonding and strength in paper. The main objectives of the work were: 1) to further the understanding on the development of tensile properties of paper from a wet sheet to a dry paper and on the mechanisms of action of different strength additives 2) to resolve the specific interactions of certain polymers with cellulose and 3) to relate the molecular and microscopic level phenomena to the development of bonding and strength in paper. Adsorption of polymers was highly dependent on the interactions between cellulose and the polymers as well as on the adsorption conditions. The dispersing or aggregating effects of polymers on cellulose fibrils were observed at molecular and microscopic levels in model systems and on the surfaces of cellulose fibres. The adsorption of polymers also affected hydration and viscoelastic properties of the fibril/polymer layer. Polymer adsorption, when carefully considered, can provide an easy control over stabilization, compatibilization, and water affinity of fibrillar cellulosic materials. The development of tensile properties of paper upon drying was characteristic for each polymer and adsorption condition. The increased dispersion and plasticization of cellulose fibrils on fibre surfaces by carboxymethyl cellulose and xyloglucan influenced the development of fibre bonding and paper strength during drying. In addition, the development of drying tension showed differences between polymers, thus it could be possible to utilize additive-specific drying conditions to attain the desired end properties of a paper product. The ability of chitosan to act as a wet web strength additive in paper was related to the pH dependent adsorption behaviour of the polymer. Chitosan was found to adsorb on cellulose in the absence of electrostatic attraction, demonstrating the specific interaction between the polymers. The wet web strength improvement was partly attributed to increased wet adhesion between chitosan coated cellulose surfaces at high pH but covalent bonding was likely to impart the wet web strength as well