Today’s coatings are applied to a wide variety of substrates for both decorative and protective purposes. While the ability of the coating to deliver acceptable properties depends primarily on the performance of the dry film, the rheology of the coating plays a critical role in the actual process of applying the coating to the substrate. This process not only impacts the ease and cost of the application, but it may also affect the final decorative and protective properties of the dry paint film. Thus, it is critical that the coating have the correct rheology, and hence, selecting the right rheology modifier package is critical to formulating such coatings.

The rheological needs of most coatings can be grouped according to the shear rate of the particular phenomenon. Shear rate is a measure of the severity of a coating’s flow during the process of interest, with that severity being a ratio of the speed of the flow to the thickness of the film. Thus, the shear rate of the processes range from very low for pigment settling (very low velocity and very thick film) to very high for brush drag (thin film under the brush and rapid movement). Shear rate is expressed in units of reciprocal seconds (sec-1), with values ranging from as low as 0.001 sec-1 for pigment settling to 10,000 sec-1 or greater for brushing and spraying. By characterizing the coating’s rheology as a function of shear rate, a few simple measurements can provide good direction to the design of the optimum rheology package. Because an instrumental measurement is typically much quicker, easier and more objective than a real-world test, pushing this testing to later in the development cycle will save time and money.