Applied rheology and architectural coating performance

Paint rheology is understood to play a vital role in both product performance and customer acceptance. Consequently, the ability to formulate paints having the necessary flow properties is essential for paint technologists. Experienced formulators have said that as much as half the cost of new product development can be consumed in getting the rheology right. In fact, the quality-control viscosity measurement devices in everyday use in the development laboratory are of little help in this endeavor. Among other shortcomings, most such instruments apply shear stresses which are far from those involved in important coating flow processes. The rheological properties required for a successful coating must be defined with due regard to the prevailing conditions of stress involved in application and film formation. This requires that measurements should be taken over a wide range of shear stress and timescales. The task for the applied rheologist is to bridge rheology and technology, but it is often unclear how to connect rheological data with the real-world performance of paints, due to the complexity of coating flows. This review in part discusses the use of controlled-stress rheometry to characterize coatings, and presents ways of applying the results effectively to the analysis of paint flow. The methodology is fundamental but not unduly time-consuming, since the objective is to provide sound yet timely guidance to formulators. Thirteen commercial semigloss latex paints were analyzed rheologically to develop correlations to paint performance. Using the method of shear stress mapping, key regions of the non-equilibrium flow curve are identified for the control of paint flow processes. With this approach, simple but strong correlations were obtained of paint flow metrics to viscosity chosen at the relevant stresses. The fact of high correlation means one can expect that an appropriate viscosity adjustment will correspondingly improve performance. It is argued that shear stress, not shear rate, is the correct independent variable both for experimentation and for the graphical presentation and analysis of viscosity data. The yield stress parameter, particle flocculation, and sedimentation are also discussed, and an oscillatory shear method of direct measurement of yield stress is described.