Ask the Expert Question-and-Answer Archive

by Ron Joseph

June, 2005

Measuring Paint Coating Viscosity

Q. I have some problems, that has frustrated me ....
1. Some of my customer used nk2 cup to measure the paint viscosity, is that appropriate?
2. What is the difference between measuring non-newtonia and newtonia paint? Is that measurement trustable?
3. Kreb Stormer uses 200 rpm (constant shear rate) to measure viscosity. What about others?
(e.g my HQ in japan used stormer but measured in second and weight loaded. How do I interpret?
4. What is the most used uv curing strength? curing time? uv light power? and how do I measure the uv light power loss?

A. Thank you for your questions. Measuring viscosity with a Kreb's Stormer viscometer is considerably more accurate than measuring with a cup. In the US we use a Zahn cup and I'm not familiar with an nk2 cup, but I assume that they are much the same. Also, the Kreb's Stormer viscometer is well suited to non-Newtonian fluids, such as paints and coatings.

You cannot measure viscosity accurately using a viscosity cup that allows the paint to efflux due to gravity. That might be a reliable method for Newtonian fluids, but most paints are non-Newtonian. Even here, some paints behave more like Newtonian fluids than other paints. Therefore, there is no acceptable general rule regarding the accuracy of the efflux cup method for non-Newtonian fluids. Remember also that temperature is an extremely important parameter when measuring viscosity.

Unfortunately I cannot answer your questions regarding the UV curable coatings.

Best wishes,

Ron Joseph