Painting Q&A: Inconsistent Viscosity Measurements
How can we get more consistent viscosity measurement results?
Q. I am servicing a customer site and we have been having a lot of trouble with inconsistent viscosity measurements on our waterborne basecoat. We use an efflux cup (Fisher No. 2) and get a lot of variability even on the same batch of paint. We are doing our best to control the temperature. How can we get more consistent results?
A. This is a common question with many of today’s high performance waterborne coatings. It sounds like you have good control of the temperature variable. However, these waterborne formulations typically have non-Newtonian behavior meaning the measured viscosity is dependent on the shear rate and the shear history (time dependency). This viscosity behavior arises from the various types of rheology control packages used in waterborne formulations to provide the application workability and good final appearance.
This fluid behavior is named after Sir Issac Newton (1642-1727) who first described viscosity as the ratio of the shear stress to the shear rate. Newtonian fluids, like water for example, have a proportional relationship of shear stress and shear rate, meaning the measured viscosity is constant over the measured shear range. So while water is a good approximate example of a Newtonian fluid, the formulated waterborne coatings display a high amount of non-Newtonian behavior. For the waterborne non-Newtonian coatings, the important terms you should keep in mind are pseudoplastic and thixotropic. Pseudoplastic, simply, means that the viscosity of the material decreases (at constant temperature) with increasing shear, thus the common term shear thinning is often used. The opposite of pseudoplastic is dilatant which means that the viscosity increases with increasing shear. This would not be a desirable viscosity behavior for coatings circulated through high shear pumps. The term thixotropic is used for a time-dependent shear thinning behavior at constant temperature. These two terms are often used interchangeably in the industry, though this is technically not a correct usage. The most common misuse is the term “thixo” , or thixotropic, meaning an isothermal change in viscosity at constant shear. Many of today’s high performance waterborne formulations display these types of non-Newtonian behavior.
For more consistent measurements, take the sample from a consistent point in the process and then measure within a consistent time. For example, measuring a sample that has been in a sample container overnight might give you a different measurement than a sample taken directly from your production circulation system and measured immediately. This is critical when dealing with the thixotropic behavior. Controlling the sample procedure and shear-history of the sample can help measurement consistency. The sampling procedure needs to be carefully connected in a consistent protocol to the actual measurement process for consistent results. Some chemists suggest adding a preconditioning shear step to the testing protocol to improve the consistency of the measurements
There are limitations of the efflux cup, especially with non-Newtonian fluids and especially with the thixotropic behavior. These limitations arise from the basic geometry of the cup reservoir and orifice outlet flow, so they cannot be overcome. Another option for consistent viscosity measurements is to consider the use of a viscometer for these non-Newtonian waterborne materials. A rotational viscometer based on cone and plate geometry would be a good choice in this case. This instrument provides a flat plate for a small amount of sample and a very shallow cone to make contact with the sample. The torque required to turn the cone at a constant speed (shear rate) is a function of the viscosity of the material. There are many instruments available now which fit nicely in the production QA lab and give quick and easy measurements controlling the shear. In some cases, a measurement is taken at both a low and a high shear rate. For example the shear rates of 1000 sec-1 and 1 sec-1 are often used. This is a great way to characterize the amount of shear thinning behavior of your waterborne material. Sometimes a ratio of these two numbers is taken with the higher low shear value in the numerator and the lower high shear value in the denominator. This gives an index of the amount of shear thinning behavior. Sometimes this is called a Thixo index, but really it is a measure of the amount of non-Newtonian behavior of the sample. A preconditioning shear time can also be incorporated to help. Another advantage is that these viscometers will automatically give you temperature control.
Overall, a viscometer can be really helpful to ensure the consistency of the waterborne non-Newtonian material, especially when the material has strong thixotropic behavior.
Originally published in the January 2016 issue.
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