Published: Jan 2012
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CHARACTERIZING THE SHAPE, SIZE AND SIZE-distribution of particles that go into liquid paint or coating formulations as well as dry formulated powdered coatings are very important. These materials may vary from being ground mineral fillers to crystalline epoxy cross-linkers and catalysists used in finished powdered paint products. There is a significant effect on the coating's density, mechanical strength, electrical, thermal as well as sound and small molecule transmission properties as a function of the included average particle size and shape. An example of the importance of particle size is the inclusion of crystalline dicyandiamide (dicy) as a cross-linker in fusion bonded epoxy (FBE) powder coatings or the blowing agent in “fire resistive” paints. Dicy is not stable after melting and decomposes to gaseous products if not consumed in a curing reaction with epoxy. The use of the standard commercial particle size dicy in FBE formulations leads to an unusual type of coating failure. In this case, to remedy the problem, a separate step is performed on the standard commercial-sized powdered dicy used in FBE coatings to produce a “micronized” powdered dicy. If “micronized” dicy is not used, it cannot be fully incorporated into the un-cured FBE matrix during the short time in the extrusion process. This leads to very small regions of the crystalline dicy cross-linker remaining after cure. Because of its slight water solubility, tiny osmotic blisters are formed causing the coating failure. This same material, dicy, is used as a “blowing agent” in the formulation of intumesant paints. In this case the dicy particle size incorporated into the paint dictates the foam density produced on heating and decomposition. This impacts the thermal transfer value of the fire resistive paint. Particle-size measurements associated with the paint and coatings industry has broadened in scope considerably over the past few decades. Not only must we evaluate the size, shape, and size distribution of the pigments, fillers, and emulsified resin particles used in the formulation of the coating system, but major efforts are now put forth to address environmental and applicator's health problems that are specific to particle size, nature, and morphology. Environmental and economic concerns have been a substantial driving force in the development of powder coating technology, which requires monitoring finished powder size and size distribution to ensure consistent application parameters.
Mills, George D.
President, George Mills & Associates International, Inc., Foley, AL