The Influence of Molecular Adhesion on Paper Strength

Abstract: This thesis deals with the influence of molecular adhesion on paper strength. By combining the use of high-resolution techniques and silica/cellulose surfaces, with various fibre–fibre and sheet testing techniques, new information regarding the molecular mechanisms responsible for paper strength has been obtained.Large parts of this research were devoted to the polyelectrolyte multilayer (PEM) technique, i.e. a charged surface is consecutively treated with oppositely charged polyelectrolytes. Application of PEMs incorporating polyallylamine hydrochloride (PAH) and polyacrylic acid (PAA) onto dried, fully bleached softwood fibres, prior to sheet preparation, increased tensile strength. No linear relationship was detected between the amounts of PAH and PAA adsorbed onto the fibres and the developed tensile strength, which suggests that the adsorbed amount is not the only important factor determining the tensile strength. Closer examination of PEM formation on silica indicated that both exponential PEM film growth and the occurrence of a PEM film in which the polyelectrolytes are highly mobile, favour the strength-enhancing properties of sheets containing PEM-treated fibres. This indicates that a water-rich, soft PEM film allows the polyelectrolytes to diffuse into each other, creating a stronger fibre–fibre joint during consolidation, pressing, and drying of the paper. In addition, when PAH capped the PEM film, the paper strength was higher than when PAA capped the film; this could be related to the structure of the adsorbed layer. Further analysis of the sheets revealed that the increase in tensile strength can also be linked to an increase in the degree of contact within a fibre–fibre joint, the number of efficient joints, and the formation of covalent bonds. The relative bonded area (RBA) in the sheets, as determined using light-scattering measurements, indicated no significant change until a certain tensile strength was obtained. The RBA, as determined using nitrogen adsorption via BET analysis, did show significant changes over the whole investigated tensile strength range. From this it can be concluded that light scattering cannot give any direct information regarding molecular interactions within a sheet. Furthermore, it was shown that PEMs involving cationic and anionic starch display an almost linear relationship with out-of-plane strength properties regarding the amount of starch in the sheets, whereas the tensile strength was more dependent on the physical properties of the starch, as was the case with PAH and PAA.Cationic dextran (DEX) and hydrophobically modified cationic dextran (HDEX) were used to test the importance of having compatible surface layers in order to obtain strong adhesive joints. DEX and HDEX phase separated in solution, however, this incompatibility of HDEX:DEX mixtures was not reflected in wet or dry joint strength. For both wet and dry measurements, adhesion between DEX and HDEX coated surfaces was intermediate to the adhesion of DEX:DEX and HDEX:HDEX surfaces.In addition, various types of cellulose surfaces, different regarding their crystallinity, were investigated. Depending on the preparation techniques and solution conditions used, i.e. pH and salt concentration, steric, electrostatic, and van der Waals interactions were obtained between the surfaces in aqueous solutions. The adhesion forces between polydimethylsiloxane and cellulose surfaces, measured under ambient conditions, were influenced by the degree of crystallinity. This suggests that amorphous cellulose offers more possibilities for surface groups to arrange themselves to participate in molecular interactions in the joint. Higher relative humidity could increase this adhesion force further, water probably acting as a plasticizer during joint formation.