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 the target design.
The 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 the temperature of a coating, the less resistance there is to flow, and the lower the viscosity becomes. Conversely, the lower the temperature of a coating the more resistance to flow and viscosity is higher.
Depending on the 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 degrees 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 submerge a viscosity cup.
- Stir the coating sample, in order to completely equalize its temperature, then measure the temperature of the coating.
- 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 cup’s top rim breaks the liquid surface), start a stopwatch.
- 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.
- Clean the cup immediately after use including the 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 the test 3 times and average results.
The viscosity of inks and coatings should be tested as close to Newtonian as possible as non-Newtonian, thixotropic liquids will cause inaccuracy in a cup reading. Foam and volatiles that have been evaporated from a sample will also produce erroneous readings.
So, what is it about viscosity? What do you think?
You can see that viscosity is not too difficult to understand, but at the same time, it is not so easy to measure the same way twice. When all is said and done, most questions about viscosity are a matter of temperature, faulty test procedures, or cup differences. Follow test procedures exactly and test to compare samples only.
NIST standard traceable Newtonian oils are available from Cannon Instrument Co. 1-800-533-6232, 1-814-353-8000 for correlating viscosity test equipment. The gold standard is viscosity dip cups are the GARDCO EZ™ Zahn (ASTM) Dip Cups and theS90/Zahn Signature Dip Cups made by Paul N.
Questions about Viscosity and Coatings? Contact the Experts at Cork.