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OQD is a Physical Chemist, Materials Scientist, Rheology Expert, Coatings Scientist and Colloidal Chemistry Consultant, Polymer Science And Technology Specialist with world-class expertise in polymer rheology, polymer melt and solution processing, coatings formulations design, testing and problem solving.
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Coatings science and technology involve an interdisciplinary approach. For example, polymer science, physics, colloidal chemistry, surface chemistry, organic chemistry, engineering, and other disciplines are all required to some degree to develop new coatings and to make incremental improvements to existing technologies. We will focus now on coatings rheology, a branch of physics, and its relationship to coatings science.
Rheology is defined as the science of the flow and deformation of matter. Several important material functions are defined by rheologists. The coatings formulator typically deals with the viscosity as measured by a viscometer. However, most coatings formulations have viscoelastic properties. These viscoelastic properties are generally inaccessible to the formulator but can lead to unexpected results for a given formulation viscosity including rod-climbing behavior, stringing from spray nozzles, shear thinning of viscosity, spatter from roller brush applications, poor atomization from spray applications, variability in roll-coating applications, differences in applied film thickness, and undesirable dried film properties such as brush marks.
The most commonly known and misinterpreted of these viscoelastic features is the one called shear thinning of viscosity. Most coatings formulations are colloidal systems and most colloidal systems have a steady state shear viscosity that varies with shear rate. Shear rate can be defined as the velocity of the moving fluid being applied to a substrate divided by the gap or distance between the substrate and the applicator. Or, for a spray application, the shear rate to a first approximation varies as the flow rate divided by the cube of the hole radius. The shear rate has units of reciprocal seconds, sec-1.
The structure of a colloidal system, e.g., a paint formula, leads to a transient network of elastic junctions that can be characterized by an elastic modulus proportional to the number density of segments between network junctions and a characteristic time given by the ratio of the limiting zero shear rate viscosity divided by the modulus. As the system is sheared at a given shear rate, the non-linear viscoelastic response results in a change in the number of network junctions--usually a decrease in junctions. A decrease in network junctions results in a decrease in the steady state shear viscosity and the fluid satisfies the definition of shear thinning.
In designing new formulas or improving existing formulas, the objective is to match the shear viscosity of the formula to the expected shear rate of the application environment. Types of applications problems include pigment settling and shelf stability, sag and leveling, brushing, roller application, spray application, and transport via piping systems. This act of matching of viscosities of a formulation to a specific application shear rate can easily be done with current rheometer technology; however, low budget low cost users may only have access to viscometers. Viscometers can be used for the same purpose with more difficulty if the test shear rate(s) are thoroughly understood.
For example, here is a list of desired room temperature viscosities for water based paints based on our research of the literature and our experience:
1. Minimize pigment settling and coagulation. Shear rate < 0.01 sec-1. Viscosity > 500 poise.
2. Maximize leveling and minimize brush marks. Shear rate 0.01 to 0.1 sec-1. Viscosity < 100 poise.
3. Minimize sagging and appearance problems. Shear rate 0.01 to 0.1 sec-1. Viscosity > 100 poise.
4. Optimize brush loading (from paint can to paint brush). Shear rate 50 to 100 sec-1. Viscosity of 10 to 12 poise.
5. Optimize brush drag on paint application. Shear rate 1000 to 10000 sec-1. Viscosity less than 3 poise.
6. Optimize film build and film thickness. Shear rate 2000 to 10000 sec-1. Viscosity 1 to 3 poise.
7. Optimize droplet formation for spray applications. 5000 to 10000 sec-1. Viscosity less than 0.6 poise.
Different application techniques and film property objectives will require a different set of guidelines. For example, a large volume spray paint application for automobiles will require a special set of properties to minimize pinholes, craters, and other appearance defects.
Some of the limits shown above are in conflict. Therefore, it is common in coatings formulations to make trade-offs between various objectives. For example, a paint formula with excellent sag control would probably exhibit poor leveling and appearance problems from brush marks. To make matters worse, rheology control is critical at every level in formulations from pigment dispersion quality, to letdown of the pigment dispersion with the binder system, to selection of the rheology modifiers and their order of addition, to the time behavior of the coatings rheology after the final additions have been made. And, selection of a rheology control package must take in to account the effect of the additive on the colloidal stability of the dispersion. Some thickeners may result in a water sensitive coatings or appearance problems due to volume exclusion flocculation of the pigment.
In summary, formulation design can require knowledge of many areas of science and engineering. We have given some guidelines for rheology control of formulations in this short article. Our experience in designing formulations that meet the many challenges of formulation design has been very rewarding.
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