In general, surface coatings are multi-phase fluids, usually one or more liquid phase, which may or may not be water based, in which particulate matter is suspended. The effectiveness of the coating depends on many things, including the properties of both phases and their interaction, the nature of the substrate, the method of application of the coating to the substrate, and diverse processing conditions including temperature, mixing, time, etc.
The performance of a coating fluid has traditionally been assessed using either full-scale process equipment, leading to down time and production loss, or (often expensive) purpose built pilot plant. The terms usually referred to when coating performance is assessed are diverse, for example, tack, surface tension, opacity, brushability, leveling, sagging, flop, length and so on. It is widely accepted that rheological parameters including high- and low-shear viscosity, thixotropy, visco-elasticity and yield point are reliable pointers for coating performance. Their influence, and the influence of environmental and process conditions upon them is a critical factor for processability and product quality.
Shear viscosity and yield stress influence processing through power consumption during pumping, mixing etc. Viscosity is usually shear dependent and is important for flowability during application (moderate shear for brushing, high shear for spraying) and even cover after application. Yield stress dominates the mechanical properties governing sag. Thixotropy is important for management of post application spread (in inks for example). Visco-elastic effects are prominent during application and define the length of a coating fluid, low elasticity can lead to early breakup of strands, high elasticity results in reduced flow-rates after the gap in rolls, or higher than expected power consumption whilst pumping or mixing. These properties are manipulated to obtain optimal performance for particular surface coating applications.
Processing conditions such as ambient or process temperature and roller/mixer/pump speed severely effect the rheological properties of fluids. It is of inestimable importance to quantify their effect in order to fully control processing and application issues, and to predict the effects of environmental changes.
Sensitive, high precision versatile equipment is available for repeatable and reliable measurement of all flow properties in the laboratory. These rheometers measure shear viscosity, yield stress, thixotropic hysteresis loops and visco-elastic properties of surface coatings. Recently, extensional rheometers have become commercially available, for the first time, which are ideal for length measurement. These units have been shown to be of value for both QC and product development.
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