Why and How Viscosity is Measured

Viscosity Zahn Cups

Viscosity is a measure of a fluid’s resistance to flow. It describes the internal friction of a moving fluid due to its molecular makeup. A high viscosity (thick) fluid resists motion because of high internal friction, while a low viscosity (thin) fluid flows readily because of low internal friction.

Viscosity is measured because it is important to the successful application of coatings. Viscosity has a direct relationship to the amount of coating applied by coating equipment. Coatings are formulated to be applied at a particular viscosity in order to optimize final properties.

Cork’s quality control labs measure the viscosity of its batch manufactured coatings at a climate controlled 70°F. A high quality, Brookfield viscometer is used to obtain an accurate, reproducible measurement in centipoise (cps).

Coating users MUST understand that viscosity is key to successful coating application.

First, coating viscosity varies with temperature!

• As a coating’s temperature rises, its viscosity drops and its flow increases.
• As a coating’s temperature drops, its viscosity rises and its flow decreases.
• The viscosity of an aqueous coating can vary as much as 60 seconds, Zahn #3 when measured at 50°F and at 100°F.
• Accurate viscosity comparison demands that a sample coating’s temperature be stabilized to agree with that shown on the coating’s Technical Data Sheet.
Example: 19 seconds #3 EZ Zahn @70°F.

Second, on press running a colder than 70°F coating MEANS:

• The coating will be thicker and have a higher viscosity
• Most coaters will apply more coating
• Slower drying might be experienced
• Slinging might be seen
• Shrink cracking, orange peel and a lower gloss could result
*The viscosities of UV & EB coatings are affected more by temperature than aqueous coatings.

Third, on press running a warmer than 70°F coating MEANS:

• The coating will be thinner and have a lower viscosity
• Most coaters will apply less coating
• Lower gloss & rub resistance are likely
*The above variations on press will be dependent on the working range of the coating.

MEASURING VISCOSITY

The viscosity of Newtonian fluids may be measured by viscometers and efflux cups. Expensive very accurate viscometers are mainly limited to the laboratories of coating manufacturers, while cheaper efflux cups are practical on the site of coating use. The field testing of coatings by efflux cups is very imprecise with the results not often reproducible even under the best of conditions. They should only be used to compare, evaluate or adjust similar coatings.

viscometerVISCOMETERS

• Can very accurately and reproducibly measure the viscous drag of a fluid
• The unit of measurement is centipoise, a universally used metric unit of viscosity

EFFLUX CUPS

• Flow cups based on different design types, Zahn, Shell, Gardco EZ Zahn, Gardco Signature Zahn, and Din
• The unit of measurement is seconds or the time it takes to drain the test cup
• All cups hold a standard amount of fluid
• They feature an accurately sized orifice (hole) in the cup bottom to drain a fluid
• Viscosity ranges are accommodated by cups numbered according to the hole size

SETTING UP TO TEST

• Identify the coating to be tested
• Check the coating’s Technical Data Sheet
• Find the coating’s viscosity range
• Determine the proper test cup size
• Stay within a 15 sec to 60 sec range Example: Gardco EZ Zahn #3
o 15 seconds = 115 cps
o 60 seconds = 600 cps
• Adjust the test coating’s temperature to agree with that shown on the Technical Data Sheet. Example: 19 seconds ± 3 seconds #3 EZ Zahn @ 70°F.

TESTING CORRECTLY WITH AN EFFLUX CUP

  1. Always use a clean, dent free, appropriately numbered cup & an accurate stopwatch.
  2. Prepare a clean, stirred & mixed coating sample (no debris or foam), adjusting to 70° F ±1° as needed by placing sample container in a hot or cold-water bath, or test at ambient temperature.
  3.  To test, hold the cup by its handle and submerge the cup into the coating sample at an angle so the cup fills without trapping any air until the top rim of the cup is below the coating surface. 
  4.  Holding the cup using its handle with one hand, and the stopwatch in the other, proceed by lifting the cup out of the coating with a quick steady motion, starting the stopwatch when the top rim of the cup first breaks the coating surface.
  5. Lift and hold the draining cup to no more than 6 inches above the sample surface, watching the coating drain from the cup.
  6. Observe when the draining stream of coating is first broken (interrupted) and immediately stop the stopwatch.
  7. Record the stopwatch time indicated, sample temperature, & cup number used. Example: 19 seconds, # 3 EZ Zahn @70°F.
  8. Clean the cup immediately after use.

CLEANING THE CUP

Clean by flushing with a suitable solvent to remove all traces of the tested product. Clean the orifice using a length of nylon fishing line. Completely dry the cup using a lint-free cloth. Inspect visually and clean a second time if any residual test material remains.

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About the Author

Elmer W. Griese Jr, having accumulated 35+ years of knowledge working in the coatings and printing ink industries has now authored the Cork Tech Talk News, newsletter since 1992 producing 112 issues. He remains dedicated to educating and illuminating technological progress that offers the potential to advance coating technology and its applications.

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