Presented by Peter Leitner, Senior Vice President
Pigments present a different challenge for mixing engineers, as the two principle requirements are dispersion and particle size reduction. The ability to reduce the pigment to a very small 1 to 5 micron particle size or less is critical for success.
Pigments are indispensable to the cosmetics industry for creating makeup, lipstick, sunscreen, and soaps. They are also used in the paint and pharmaceuticals industries, and photographic media and limited applicability in food processing, like candy coatings or shells. Pigments can be inorganic (metallic oxides, metal powder suspensions, carbon black), or organic (animal melanin or vegetable carotene, lakes and toners). Most of our experience is with titanium dioxide, lakes and some iron oxides.
Particle size reduction occurs with a high shear mixer running in a batch operation, or in a continuous operation using in-line high shear mixers, shearpumps, or shearmills. Combining batch mixing with recirculation through an in-line device offers the best setup. Pre-mixing helps ensure that all surfaces are wetted out and agglomerates are broken down. A good pre-mix can drastically reduce the burden put on shearpumps or mills downstream to achieve the final particle size. It is possible to perform both the pre-mixing and final milling stages in one operation. The refractory properties of the materials and the hardness of the particles determine the equipment needed, and whether it can be done in one step.
When pigments are used for making an emulsion of mostly soft to semi-hard particles, either the DynaShear can be used as an inline high shear continuous mixer or the Rotosolver can be used in a batch mode. Both machines deliver particle sizes for this application in the 2-4 micron range. Most pigments can be completely processed with our Boston Shearmill set up in a recirculation mode where multiple passes over 10-30 minutes will achieve the desired result (down to 1 micron or less). Many hard particle pigments, however, can be premixed and "milled" to particles in the 4-10 micron range in a batch reactor alone using our Rotosolver or Rotostat set at a high tip speed, eliminating the need for further downstream refinement. As with many applications, we recommend a factory test when dealing with pigments. Running both benchtop and 10-100 gallon pilot tests ensure success of the full-scale larger batches or continuous runs.
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