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.

<p>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.</p><p>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.</p><p>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.</p><p>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.</p>

The influence of particle shape of coating pigments on theirpacking ability and on the flow properties of coating colourshas been investigated. The particle shapes considered werespherical, flaky and acicular (needle-shaped). In the case ofsuspensions containing monodisperse spherical polystyreneparticles, a concentration gradient appeared in the filter cakeforming during filtration under static conditions. Such agradient, monitoredby non-destructive magnetic resonanceimaging (MRI), is not accounted for in the traditionalfiltration theory used in coating technology. Good agreementwas found between a literature model describing filtrationthrough a compressible filter cake and the concentrationgradients measured by MRI. According to this model, the scaledconcentration gradient was the same at all times. For flaky (mainly kaolin) and acicular (aragonite)particles, a rapid method was evaluated to estimate a shapefactor of the pigment particle. Generalised mathematical modelsof oblate and prolate spheroids were applied to reduce thethree geometrical dimensions of the particle to two, the majoraxis and the minor axis. The shape factor, which is mass-based,was derived from a comparison between the results obtained bytwo different size-assessment instruments, viz. the Sedigraphand an instrument using light scattering. This yields a shapefactor distribution as a function of equivalent sphericalparticle size, but the results are uncertain for small particlediameters, below 0.2 µm. Good agreement was obtainedbetween the shape factor and a mass-based aspect ratio obtainedby image analysis, but the rapid method is generally moreaccurate for flaky than for acicular particles. Results obtained by capillary viscometry showed that therewas a relationship between the viscosity at high shear rates(&gt;105s-1) and the shape factor, but that it was notsufficient to use the median value of the shape factor toachieve proper information. A more complete evaluation requiresknowledge of the shape factor distribution, which is also givenin part by the method mentioned above. However, a large medianshape factor was related to a high high-shear viscosity.Non-Newtonian entrance pressure losses were sometimessignificant in capillary viscometry, indicating that it wasinappropriate to measure the shear viscosity with only onecapillary. Such effects were however relatively much morepronounced in slit die viscometry, especially in the case ofacicular particles, where the aspect ratio was a crucialparameter. The influence of the shape factor of kaolinparticles on the non-Newtonian entrance pressure losses over aslit die was surprisingly small. The high-shear viscosity ofcoating suspensions based on different pigments correlated withthe median pore size of the corresponding coating layer ratherthan with the porosity. <b>Keywords</b>: Aspect ratio, capillary viscometry, coatingcolour, filtration, particle packing, pigment, pore structure,rheology, shape factor, slit die viscometry, spheroid.

<p>The influence of particle shape of coating pigments on theirpacking ability and on the flow properties of coating colourshas been investigated. The particle shapes considered werespherical, flaky and acicular (needle-shaped). In the case ofsuspensions containing monodisperse spherical polystyreneparticles, a concentration gradient appeared in the filter cakeforming during filtration under static conditions. Such agradient, monitoredby non-destructive magnetic resonanceimaging (MRI), is not accounted for in the traditionalfiltration theory used in coating technology. Good agreementwas found between a literature model describing filtrationthrough a compressible filter cake and the concentrationgradients measured by MRI. According to this model, the scaledconcentration gradient was the same at all times.</p><p>For flaky (mainly kaolin) and acicular (aragonite)particles, a rapid method was evaluated to estimate a shapefactor of the pigment particle. Generalised mathematical modelsof oblate and prolate spheroids were applied to reduce thethree geometrical dimensions of the particle to two, the majoraxis and the minor axis. The shape factor, which is mass-based,was derived from a comparison between the results obtained bytwo different size-assessment instruments, viz. the Sedigraphand an instrument using light scattering. This yields a shapefactor distribution as a function of equivalent sphericalparticle size, but the results are uncertain for small particlediameters, below 0.2 µm. Good agreement was obtainedbetween the shape factor and a mass-based aspect ratio obtainedby image analysis, but the rapid method is generally moreaccurate for flaky than for acicular particles.</p><p>Results obtained by capillary viscometry showed that therewas a relationship between the viscosity at high shear rates(>10⁵s^-1) and the shape factor, but that it was notsufficient to use the median value of the shape factor toachieve proper information. A more complete evaluation requiresknowledge of the shape factor distribution, which is also givenin part by the method mentioned above. However, a large medianshape factor was related to a high high-shear viscosity.Non-Newtonian entrance pressure losses were sometimessignificant in capillary viscometry, indicating that it wasinappropriate to measure the shear viscosity with only onecapillary. Such effects were however relatively much morepronounced in slit die viscometry, especially in the case ofacicular particles, where the aspect ratio was a crucialparameter. The influence of the shape factor of kaolinparticles on the non-Newtonian entrance pressure losses over aslit die was surprisingly small. The high-shear viscosity ofcoating suspensions based on different pigments correlated withthe median pore size of the corresponding coating layer ratherthan with the porosity.</p><p><b>Keywords</b>: Aspect ratio, capillary viscometry, coatingcolour, filtration, particle packing, pigment, pore structure,rheology, shape factor, slit die viscometry, spheroid.</p>