The Essence of Aqueous Coatings
The practice of applying a coating successfully is a result of understanding and being attentive to a few fundamentals. These are pressure, viscosity, and pile temperature. Success requires that these key variables be under control.
Pressure in Aqueous Coatings
PRESSURE: “KISS” is the key to a successful trouble-free coating application. Using the least amount of pressure possible, the coating should be transferred and rolled on from blanket to substrate with the least amount of squeeze.
Each coater manufacturer will supply specifications for properly setting standard nip widths between rollers. A multitude of coating application problems can result from improperly set nip pressures. These may be observed as coating blurring, blotching, thickening at both edges, and transfer and adherence of ink to the blanket cylinder. To avoid these issues, be sure to set nip width stripes to the coater manufacturer’s specifications.
Depending on the coater manufacturer and the number of rollers, recommended nip width settings may include pan roller to application roller, pan roller to intermediate roller to application form roller, application form roller to blanket, anilox roller to blanket or plate, and back cylinder (impression) to blanket or plate cylinder.
Application and Anilox Settings
The application form or anilox should be set to 3-5 mm wide nip width, then backed away until the transferred image breaks up, finally adjusting just enough to bring the image back to a “KISS” impression of from 1-2mm.
Pack the blanket cylinder to the correct blanket height following the coater manufacturers’ specifications. A blanket height of .000 or +.002 to or over bearer is common. Cut the packing square for the image area only leaving the color bars uncoated. The reason for striping a blanket or plate is to verify that it is set to square or parallel.
Set the impression cylinder pressure so that the coating is transferred, then back the cylinder away until the coating breaks up showing voids. Then increase the pressure until voids just disappear. You are adjusting to bring the coating back to a “KISS” impression.
The Viscosity of Aqueous Coatings
VISCOSITY: Viscosity is a measurement of a liquid’s ability to flow. High viscosity liquids are thicker and flow with difficulty, while thin low viscosity liquids flow easily. Viscosity cups which are metal cups with a precisely measured hole in their bottom are used to determine the viscosity of a liquid. This is done by measuring the time in seconds that it takes for a cup to empty a liquid fill. Different size cups allow the measurement of differing ranges of viscosity. Typically, #2 and #3 cups accommodate the range of coatings used in the printing industry.
Zahn Cups Measure the Viscosity of a liquid
It is important to note that cups from different manufacturers will measure differently. The gold standard in viscosity cups is the GARDCO EZ Zahn (ASTM) and the S80/Zahn Signature Dip Cups manufactured by the Paul N. Gardner Co. Inc. The GARDCO EZ Dip Cups with a machined orifice hole are considered to be the superior dip cup. Cups that are scratched, dented, or dirty will not produce accurate results. Unfortunately, dip cups are not able to produce completely accurate reproducible results. Nevertheless, viscosity cups remain a practical choice for measuring viscosity.
It is essential to remember that the viscosity of a coating is dependent upon temperature. In order to achieve a valid comparison always measure viscosity at the temperature noted on the coating supplier’s technical datasheet. Further, any aqueous coating will increase in viscosity over time as it evaporates volatiles as it is circulated through the coater and pumping system. Also, this circulation typically raises the temperature of a coating.
Monitor the viscosity of a coating before and at least every 2-3 hours during a run, aiming to keep viscosity at recommended levels.
PROCEDURE FOR MEASURING VISCOSITY:
1./ Select the proper viscosity range dip cup & inspect it. It must be damage-free and clean.
2./ Collect a clean coating sample (no debris or foam) in a clean container deep enough to fully submerge the dip cup.
3./ Stir & adjust the test coatings temperature to 70° F. +/- 1°.
4./ Submerge the dip cup at an angle into foam-free coating for 30 seconds bringing the cup to the fluids’ temperature.
5./ Lift the cup vertically using the finger ring, starting the stop-watch as the cup’s top rim first breaks the surface of the coating.
6./ Hold the draining cup about 6 inches above the sample container.
7./ Stop the timer when the fluid stream breaks, recording time, Zahn cup # used, & temperature.
8./ Clean & flush the cup promptly, using a suitable solvent, and a nylon fish line to clean the orifice, then dry thoroughly.
DELIVERY PILE TEMPERATURE:
Very efficient drying systems have been evolved by the sheetfed offset printing press and dryer manufacturers. Extended delivery, allowing added drying time, combined with hot, warm, and ambient air knives, and I-R emitting dryer systems are proven to be able to evaporate great quantities of water quickly. High volume air movement is essential to drying 60%-70% water containing aqueous coatings. Three types of air movement are generally present in dryer systems. These include air knives to facilitate striping moisture-laden air from a coated substrates’ surface, air extraction to remove moisture-laden air from the delivery path and beneath the press, and cool/ ambient air applied in delivery to remove heat.
When printed/coated sheets are accumulated and piled in delivery at press end, inks and coating need to be acceptably set. Drying can then be expected to be completed in the pile load. Too high a pile temperature build must be avoided as excessive heat can soften a dry coating resulting in blocking. A stem thermometer can be useful to measure temperature within the pile. Always measure temperature on the darkest color image area for the most accuracy.
The following pile temperatures are recommended for paper/paperboard substrates.
Single side sheet coating: Pile temperature at 100° F. +/- 5°.
Two side coating-Work & turn: First pass pile temperature at 100° F. +/- 5°. Second pass pile temperature at 95° F. +/- 5°.
Okay, that’s a lot to think about. If you have any specific questions regarding this material and Aqueous Coatings, please reach out to the experts at Cork Industries.