Consolidation and structure of paper coating and fibre systems

Sammanfattning: Paper coatings or coating colours are high solidssuspensions that are applied to paper to improve chieflyprintability and optical properties. A paper coatingsuspension, including pigments, binders and thickeners, is acomplex system which must be understood if the end-useproperties of paper are to be improved. Some of the importantissues that need to be addressed are the mechanisms ofmigration of latices in a drying film, the influence ofthickener on the overall consolidation and the behaviour ofpaper coatings under compression.U nderstanding theconsolidation of paper coatings is essential as the aggregatestructures formed at this stage largely determine theperformance of the paper coating.Similarly, the forming ofpaper itself from pulp fibres requires a detailed understandingof the consolidation and dewatering process.In this thesis several approaches have been used in order tostudy the consolidation of paper coatings before and afterapplication to substrates. The techniques utilised arerheometry, centrifugation with NMR-imaging, scanning electronmicroscopy (SEM) and atomic force microscopy (AFM).Furthermore, investigations of the compressional rheology offibre systems has indicated that stepped pressure filtrationcan be a useful tool in the study of fibre dewatering.The primary findings presented in this thesis can besummarised as follows. Studies of the compressive yield stressand rheological properties of ground calcium carbonate (GCC)suspensions indicate that the addition of a commonly usedthickener can cause flocculation, most likely of a depletiontype, which leads to more compact structures during dewatering.When applied to substrates, migration and film formation ofbinder latices in the presence of calcium carbonate pigmentsand CMC thickeners occurs concurrently and collectively. Thisis shown to influence the consolidation of the top layer of thecoating to a great extent. It was found that CMC retards themigration of latices and that ethyl(hydroxyethyl)cellulose(EHEC) adsorbs to latex and creates a composite material uponlocal film formation. The composite retains an internalhoneycomb structure in the film, that is revealed by mapping ofelastic variations using AFM phase imaging.For fibre systems, the novel method of stepped pressurefiltration has been applied to study the compressional rheologyof pulp suspensions, and the effect of added surfactantsinvestigated. The technique is shown to be a promising tool forthe study of structure-dewatering relationships in fibresuspensions.