Paints & Coatings Resource Center

Ask the Expert Question-and-Answer Archive

by Ron Joseph

January, 2005

Temperature and Viscosity

Q. I work at the Iowa Waste Reduction Center in Cedar Falls, Ia. What is the proper temperature to test the viscosity of paint and do some paints require different temps. to be tested correctly?

A. There is no fixed temperature for testing viscosity. If you are conducting an ASTM test and are comparing the viscosity of a coating with a standard under laboratory conditions, then you might use the ASTM value of 25oC +/- 0.2oC. If you are on a production line and need to measure the viscosity at the application temperature, then that is the temperature at which you will test. Therefore, you will select the temperature and also the method (Zahn cup, Stormer Viscometer, Ford 4 cup, etc.), depending on your reason for conducting the test.

All paints perform differently when you measure their viscosity at a range of temperatures. For some paints the viscosity will drop suddenly when the temperature increases, whereas for others the slope of the graph will be much shallower. Don't forget that the type of instrument you use is critical. Depending on the paint formulation (and the paint's rheology) one instrument might be more appropriate that another.

I hope this help, otherwise please get back to me.

Best wishes,
Ron Joseph

Q2. Thank you for your input. The only question that I have is when you look at a product data sheet and they want the viscosity to be 12-20 sec. in a Zahn #3, are they looking for a certain temp. to check the viscosity at that time or like you said to check the viscosity at the temp you would be spraying at?

A2. In all probability they want you to check at the application temperature. I am assuming that they have tested the paint's ability to be atomized and have found that it's viscosity must be 12-20 seconds at the time of application.

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