Titanium Dioxide in Coatings, Rheology
Rheology is the science of flow and deformation of matter. However, it is more simply put as the study of viscosity or flow under changing, or a series of different, conditions. Attempting to describe rheological properties often means using words like "buttery", "rubbery", "thin" etc. All of these are totally subjective and tell us little about the actual properties of the material except for its superficial appearance. Coatings formulators always have to balance the "ideal" rheology/viscosity for paint application with such properties as sag resistance, sharp edge coverage and film build. Today, these objectives often have to be met within the added constraints of environmental legislation. This limits the use of organic solvents and has led to the introduction of water based, low VOC and high solids coatings for industrial application.
Millennium Inorganic Chemicals produces a range of TiO2 pigments designed to meet the demands of these new technologies which maximize flow and give consistent rheological properties. We have also developed a high level of expertise that can help you to achieve the optimum balance of rheological properties for your formulation.
About Rheology…
The study of rheology or flow needs to consider what happens when a liquid has shear forces impressed upon it, say by stirring, or application. When the stirrer or brush applies these forces, a shearing force is developed causing the liquid paint to move at a speed and in a direction controlled by the force applied. The response of the liquid to that application of force can be taken as its rheological behavior. It can respond in a number of ways.
Definitions
Shear Rate is the velocity gradient or the change in velocity with distance in the system when the force is applied.
Shear Stress is the force per unit area acting on a plane within the liquid.
Viscosity is defined as the shear stress required to produce unit shear rate in a liquid, i.e.
Viscosity = shear stress
shear rate
Basic Rheological Types
Newtonian (Simple) Flow
Newtonian flow is defined as that flow where shear stress is directly proportional to shear rate. The simplest example of a liquid exhibiting Newtonian flow is water. Paints exhibiting Newtonian flow would give significant problems due to the very high level of flow they exhibit. They would tend to allow pigment settling, as well as separation and syneresis in the can. Newtonian flow paints would also flow away from sharp corners and edges and give significant sagging or curtaining problems when applied.
Non-Newtonian Flow
a) Plastic Flow
The yield point associated with plastic flow gives excellent settling, separation and syneresis resistance in the can. However, if the yield point is too high, flow problems can occur on application, lack of flow of application patterns (brush marks, orange peel) is a distinct possibility. If too low, sagging and curtaining become problems.
b) Pseudoplastic Flow
Pseudoplastic flow can be considered to be a combination of Newtonian flow and plastic flow. As such it combines the properties of these two rheological conditions. At low rates of shear, Newtonian flow properties exist, leading to the possibility of settling, separation and syneresis in the can but excellent flow on application. At higher shear rates, including those at application, plastic flow conditions exist developing the wet film properties associated with plastic flow.
c) Dilatant Flow
Slightly dilatant flow is a desirable property in millbases for dispersing TiO2 in High Speed Impeller Mills. The increased shear rate with applied shear stress means that the forces applied to the pigment agglomerates increase their rate of break down, with increased stirring speed. This speeds the dispersion process. If dilatancy becomes too high, stalling of the mill can take place or the blade may simply cut a hole in a localized section of the millbase. Dilatancy in a finished paint is not desirable due to its adverse influence on application properties. The high shear rate under application shear stress means that application becomes difficult with the possibility of the paint "powdering" on application and being unable to re-flow after the force has been removed.
d) Thixotropic Flow
Thixotropy is a special case of pseudoplasticity and occurs frequently in coatings systems. Fully controlled, thixotropy in paints can give excellent control of the various properties of the finished paint. Control of yield point gives good control of in-can properties. Control of the application properties (how far the viscosity breaks down under shear and how quickly it recovers) can give excellent control over wet film defects. Excessive thixotropy at the application stage is to be avoided as it can give flow problems involving a number of major film defects. However, a controlled level of thixotropy is desirable in most wet paints.
Methods of Measurement of Viscosity
Viscosity is measured by the application of a controlled force to the liquid and measuring its rheological behavior. There are many different viscometers/rheometers available. This fact alone emphasizes the difficulty in measuring the rheology of a system, particularly one as complex as a paint or millbase. Some of the more commonly used ones are
All of these viscometers or rheomoters have their place in the paint laboratory, but their weaknesses must be understood in order to gain full value from their use. The more complex viscometers or rheometers (Brookfield and the various Cone and Plates) give information over a wide range of applied shear and can be very useful research tools. However, for routine QC work the simpler equipment is often the best to use.
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