The role of thixotropy and yield behavior in causing and preventing problems.

I discussed flow and defects in an earlier column (April 2006), but recent questions have prompted me to go into more detail regarding two aspects of the rheology of coatings: thixotropy and yield behavior. Both exist because of the formation of three-dimensional structures in paint. Thixotropy means "to change by touch" and is the name given to time-dependent behavior of a fluid. Thixotropic paints show a decrease in viscosity when sheared at a given rate. When the shearing stops, the viscosity begins to increase, rapidly with some types of coatings, slowly with others. The pattern on application is for the paint viscosity to drop to a very low level with shear by a brush, spray gun, or roll coater, then increase in viscosity with time. If the recovery is too rapid (which can be due to fast evaporating solvents as well as the building of structure), surface irregularities such as brush-marks, roller patterns, and spray droplets will not have an opportunity to flow out. The result will be poor appearance. If the recovery is too slow, then sagging and other defects may occur. Careful formulation with thickeners or thixotropes (preferably more than one), attention to solvent evaporation rates and control of pigment flocculation should allow sufficient flow and adequate recovery.

A material that shows yield (plastic) behavior will act like a solid until a critical stress (yield stress) is applied, then it will flow. Many architectural paints are solid-like in the can, and only become fluid on shaking or stirring. There is controversy as to whether such materials truly yield, or only show apparent yield due to high viscosity at very low shear rates. This is a case where the true situation is less important than the practical consequences. If a paint or pigment paste behaves much like a solid with a high yield stress, it may jam pumps, plug lines, cause inaccurate metering and dispensing, etc. A paint with a high yield stress may not level well. I have seen all of these things happen. However, yield behavior is not all bad. A low yield stress is good for the prevention of settling in paints and other dispersions. Yield behavior after application is useful in the prevention of sag and surface defects and in holding aluminum and mica flakes in place so that they are oriented properly for good light reflection. I recall one case where lack of a yield stress in a new automotive sealant formulation caused pump fouling. When the crew took a break and stopped applying the material, the sealant would ooze around the piston of the pump and prevent restarting. It turned out that the product had structure and a moderate yield stress that prevented flow through the pump, but was not high enough to cause other problems. A new product had to be reformulated to include some structure rather than just having a mass of material to provide the needed viscosity.

Unfortunately, accurate measurements of yield stresses and degrees of thixotropy are difficult and shear histories and other variables impact results. However, a step-shear method for thixotropic recovery is a useful test. The specimen is sheared at a high shear rate to simulate application, and then the shear rate is suddenly reduced to a very low level. The resultant recovery curve can be analyzed to provide a time constant and identify the highest viscosity reached. Yield stresses can be measured with a controlled stress rheometer. The stress is gradually increased and the resultant shear rate measured. Before flow, the shear rate will be zero and the viscosity infinite, but once the yield stress is reached, flow begins and there is a shear rate and a finite viscosity. Often, the results of practical non-viscometer tests or measurements (spray-outs, sag tests, drop tests, pouring on an inclined plane) are used to compare rates of viscosity build and how different formulas or batches are set up. A test that we used on the old and new formulas of the sealant previously described was to pull them up into large syringes. The material was then extruded for a few seconds followed by a stopping of the pressure on the plunger. The sealant with a yield stress stopped flowing immediately, but the one without just kept flowing and flowing even though the viscosity was high.

An excellent reference for yield, thixotropy and methods to measure them is Eley, R.R., "Rheology and Viscometry," in Paint and Coating Testing Manual, Gardner-Sward Handbook, 14th ed., Koleske, J. (Ed.), ASTM, Philadelphia, 1995, pp. 333-368.

"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences, and/or photos of coatings defects to Cliff Schoff, c/o "Coatings Clinic," CoatingsTech, 492 Norristoum Rd., Blue Bell, PA 19422; or email [e-mail].