Of all the different products that are ground in stirred media mills, many show a non-ideal flow behavior. These non-Newtonian products include paints, varnishes and pigment pastes. They often contain agglomerates and wrinkled polymer structures. The polymers, including polyelectrolytes, are often used for the wetting of particles in the suspension and as dispersants. The solids content is often much higher than 40 weight percent; this means that the probability of agglomeration is quite high because of the low amount of liquid available for wetting the particles. These factors have an influence on flow behavior and, in addition, add to the increase in viscosity caused during grinding by the increase of solid surface area. With decreasing particle size, the adhesive forces increase, further supporting re-agglomeration processes. One main reason for a high-solids content is that it is desirable for processes like drying, and as a result, drying is faster with lower energy costs.
A high-shear stress often has to be applied at the beginning to make the material flow, and this is often due to the structures mentioned above. Therefore, such materials require a careful choice of equipment as well as process parameters. In addition to the specific energy input and the bead size, the throughput plays a major role in the achievable fineness, as well as the optimal operation of the mill. Strong shear forces should be maintained after the start-up in order to attain reasonable grinding progress and to keep the viscosity under control. This can be achieved by adjusting the tip speed in the mill, or by using a gear pump along with high stirrer speeds to shear the product before it enters the mill. Short hoses and pipes should be used to reduce the opportunities for the mentioned structures and the viscosity to rebuild. This article addresses the grinding and milling parameters of a thixotropic pigment paste.
July features our most popular article of the year – the Global Top 10 and PCI 25, a ranking of the leading coatings manufacturers. This issue also includes a guide to antimicrobial coatings, an article that evaluates the use of novel acrylated amine oligomers to address curing deficiencies in UV LED-cured systems, as well as an article on self-cleaning elastomeric cool roof coatings.