Published: Jan 2012
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THIS ARTICLE PRESENTS A BRIEF REVIEW OF principles and methods of rheological analysis for the coatings industry. However, the information and methodology discussed applies equally well to the characterization of coatings, adhesives, inks, and sealants. These products share a common task, in that they are applied to a substrate—and must function—as a thin film. The process of application and film formation obviously requires not only a large total deformation, but also a high degree of control of flow, in order to achieve success. Flow cannot be controlled unless it is properly measured. Typical industrial viscosity test methods have several faults: (1) the methods do not closely relate to actual industrial processes, (2) the test devices are unsuitable for non-Newtonian fluids, (3) the result is usually a single point, and (4) in general, such measurements do not correlate well to performance. The objective for the applied rheologist, therefore, is to develop methods of rheological characterization that 1. yield accurate data for complex fluids and 2. can be specifically related to the critical processes that paints must undergo. To meet the latter goal requires characterization methods that cover a wide range of stresses and time scales. Though all other properties be acceptable, a coating will usually not meet with success if the rheology is not. Experienced formulators say that more than half the cost of new product development is consumed in “getting the rheology right.” Furthermore, “minor” changes in a raw material or process can create significant and unexpected variability in product rheology, and such problems naturally require urgent solution. For these reasons and others, rheological analysis is a vital and cost-effective tool for the coatings industry. The performance of coatings is governed by viscosity variations over a span of several orders of magnitude in shear rate or shear stress (at least four to as many as eight), if processes ranging from coating application, to leveling and sagging, to pigment settling are considered. To characterize fluid rheology over such a broad range of stress and deformation requires a “research-quality” rheometer. In order to take advantage of the capabilities built into the sophisticated rheological instrumentation now available, more than a passing knowledge of the subject of rheology is required. This chapter is therefore tutorial in style, aiming to provide the novice an entry point to the discipline. Certain concepts are discussed at some length because of their importance or complexity. One of these is viscoelasticity. It is worth the time because (a) a number of commercial instruments are capable of viscoelastic characterization of materials and (b) it is established that viscoelasticity exerts control over coating processes when present.
Eley, Richard R.
Senior Scientist, Strongsville Research Center, Strongsville, OH