What is Viscosity?
Viscosity tells us how thick or thin a liquid is. Viscosity is the property of a fluid that causes it not to flow easily because of the resistance of its molecules. When we use coatings, we need to understand viscosity, because it is key to accurate application, and a finished coated product that achieves target design.
Viscosity of Cork manufactured aqueous coatings is measured at 70 degrees F utilizing a Brookfield viscometer which yields an accurate, reproducible measurement in centipoise (cps). Other simpler, less expensive devices, called orifice viscometer cups, numbered according to hole size, are also used. Commonly used Zahn type cups should only be used as a comparative tool to evaluate or adjust similar coatings, but should not be considered a viscometer.
So What is the Problem with Viscosity?
First of all, it MUST be understood that the viscosity of aqueous coatings is dependent upon temperature. Simply, the higher a coatings temperature, the less resistance there is to flow, and the lower the viscosity becomes. Conversely, the lower a coatings temperature the more resistance to flow and viscosity is higher.
Depending on formulation, the Zahn #3 viscosity of an aqueous coating could vary across a wide range of as much as 60 seconds, when measured at 50 degrees F, and when measured at 100 degrees F.
In measuring viscosity, you MUST measure viscosity at a stabilized 70 F. temperature to expect to come close to the viscosity specified by the coating manufacturer.
In application, running a colder than 70 degree coating will mean that the coating will be thicker and higher in viscosity, producing a situation where more coating will be deposited by most coaters. This can produce the possibility of slower drying, slinging, shrink cracking and/or a lower gloss textured orange peel finish. On the other hand, running a warmer than 70 degree coating will mean that the coating will be thinner or lower in viscosity. Most coaters will apply less coating, which will have the effect of reducing such final properties as gloss and rub resistance. Also, if the viscosity of a cold coating is lowered by the addition of water or a 50% water/50% isopropyl alcohol solution, it must be remembered that the coating solids will be lowered.
Field Testing using Zahn Cups
The field testing of coatings by Zahn type cup is very imprecise with results that are not reproducible under the best conditions. Not only are test results not reproducible when using the same exact cup, operator and sample, but cup to cup variations exist from the same cup manufacturer and major differences are apparent among different cup manufacturers. However, the use of these cup devices is popular because they are cheap and seemingly easy to use. If there is to be any hope of obtaining a usable viscosity comparison by the use of viscosity cups, then certain prescribed recommended procedures MUST be followed!
Measuring Viscosity With A Zahn Cup Correctly
- Use a dry, clean, room temperature (approx. 70 degree F), dent free cup only, to compare the flow of coatings.
- Take a clean coating sample (no debris or foam), in a clean container large enough to fully sub-merge a viscosity cup.
- Stir the coating sample, in order to completely equalize its temperature, then measure the coatings temperature.
- Adjust the coating sample temperature, up or down to 70 degrees F +/- 1 degree, placing the sample container in a hot or cold water bath as necessary.
- To measure, submerge the viscosity cup with the upper rim below the surface of the liquid.
- Simultaneously with a vertical withdrawal of the cup, (when the cups top rim breaks the liquid surface), start a stop-watch.
- The draining cup is held by a finger or a lab. Stand about 6″ above the sample container.
- Stop the stop-watch at the first break (interruption), in the liquid draining from the cup orifice.
- The elapsed time that’s been recorded on the stop-watch gives a measurement of flow or viscosity, expressed in the number of seconds to empty the volume of the cup, per the cup’s orifice.
Example: 20 seconds, #3 Zahn
When recording the drain time in seconds, if the count is less than 20 seconds, repeat the test using the next lower cup size. If the count is greater than 50 seconds, repeat using the next higher cup size. For accuracy, repeat test 3 times and average results.
Questions about Viscosity and Coatings? Contact the Experts at Cork Industries Here.