The influence of particle shape of coating pigments on their packing ability and on the flow properties of coating colours

Sammanfattning: 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(>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.Keywords: Aspect ratio, capillary viscometry, coatingcolour, filtration, particle packing, pigment, pore structure,rheology, shape factor, slit die viscometry, spheroid.