Aqueous coatings formulated for the graphic arts industry may be thermoplastic or thermosetting, solution or emulsion polymer blends. Thermoplastic coatings dominate. They are comprised of approximately 60% water, plus property enhancing components including plasticizers, waxes, surfactants, coalescent aids, antifoams and possibly optical brighteners.
Shipping & Storing Aqueous Coatings in the Winter
When an order of aqueous coating is delivered to a user, it is the responsibility of the receiving dept. to inspect the shipment. Any damaged drums/kits must be reported to the delivery truck driver before he leaves. This is especially important in winter when exposure to sub-freezing temperatures could cause a coating to freeze or partially freeze.
Winter is a tricky time to ship high water content aqueous coatings. Cold snaps can come at any time. Much of the United States is vulnerable to the invasion of cold fronts from the north with even many southern states affected. If in doubt, search for “Polar Vortex” and get an education on the cold weather phenomenon that has affected vast parts of the U.S. during the past several winters.
Despite winter’s effects, the likelihood of a frozen drum of aqueous coating reaching a customer is almost nil. Aqueous coatings can be frozen due to their high-water content. Therefore, during the winter season, sub-freezing temperatures must be respected by handlers. Whether an aqueous coating is in transit or in storage, it is subject to freezing if sub-freezing exposure time is long enough.
Manufacturers of aqueous coatings take precautions whenever there is a threat. First, shipping can be scheduled to eliminate coatings sitting on a cold truck over a weekend. Secondly, heated shipping services can be used, holding product at 50° F. Thirdly, a freeze indicator can be adhered to a drum lid, which when blown, shows RED indicating freezing temperature exposure.
If blown, it doesn’t necessarily mean that the contained coating is frozen or bad, but the fact that the temperature indicator is blown must be reported to the delivery truck driver before he leaves. If blown, the customer must inspect the coating immediately looking for an oatmeal appearance indicating ingredient separation.
A digital temperature probe can be used to check the coatings temperature. If there is any doubt that the coating is compromised the container should be thawed, the coating mixed, a sample drawn and sent to the coating supplier for evaluation. DO NOT USE THE COATING until advised otherwise.
It is recommended that aqueous coatings be stored at room temperature, 20-22°C, 68-72°F. to produce a 6-month shelf life. DO NOT ALLOW AQUEOUS COATINGS TO FREEZE.
Temperature Effects Viscosity of Aqueous Coatings
Prior to use, an aqueous coating should be stirred thoroughly using an air or electrically powered mixer, “Lightning mixer”. After mixing, check viscosity using a clean, undamaged, high quality, Zahn EZ™ cup, recording coating temperature and viscosity. Be wary of low-quality, inaccurate, “Zahn™” cup knock-offs. Aqueous coatings are manufactured to a specific viscosity at a specific temperature i.e., 23 sec Z#3 @ 72°F. Temperature is key to a coatings’ viscosity as flow increases with a temperature rise, and decreases resisting flow, as temperature decreases. No water additions to adjust viscosity should be made at this point.
Connection to the pumping system can now be made creating coating circulation to the coater, which will warm the coating in a short time, simulating a running condition. Another viscosity/temperature check should now be made to determine if a water add viscosity adjustment is needed. Be aware that water adds reduce the coatings solids so that with the same coater settings the wet coat weight is the same, but the dry coating film will be thinner and weigh less.
Specific temperature and weather conditions are crucial to shipping, storing, and handling aqueous coatings. Timing of shipment, heating services, and a freeze indicator on the drum lid are precautionary actions that can minimize freezing issues during the shipping process. Temperature is also important to remember when measuring the viscosity of aqueous coatings and can be adjusted as needed as well as water adds to create the ideal coating.
Monitoring Aqueous Coatings for Smooth Operation
Set up the coater to the manufacturer’s recommendations. Set to run at minimum pressure to a “kiss impression”. Complete make-ready before engaging the coater. Water spray any blanket, plate, or roller surfaces that are not rotating or being rewet with coating whenever the press is stopped. Fast-drying aqueous coating will dry on these surfaces just as it dries on a coated sheet. Before re-starting, wash down these surfaces so that they are tack-free.
Monitoring Coating Drums
Monitor a drum in use and replace it with a fresh drum when it is about 3/4 empty. The remaining coating will typically be thicker because of volatiles evaporation during circulation over the coater cylinders. The coating remaining in the near depleted drum may be added to the fresh drum as it empties providing room. Best practice recommends always covering an opened drum to prevent evaporation of coating volatiles. Further, when storing, place a sheet of polyethylene film into the drum in direct contact with the coating before lid closure to prevent loss of volatiles into the drum’s headspace.
Diaphragm pumps are recommended to circulate coating from the supply container to the coater and back. Suction and return lines can be connected to fittings on a drum lid, which connect to PVC pipes plumbed to reach the drum bottom. The suction feed pipe should be capped with a wire mesh ball filter. Pump to satisfy coater demand only, and not excessively creating foam. Cut the bottom of the return line pipe reaching into the drum to a 45° angle. Also, cut the length of this pipe with spaced vertical 4” long x 1⁄4” wide slots to allow coating to flow into the drum without splashing and creating foam.
Aqueous Coatings Safety
Handle aqueous coatings with care to avoid skin and eye contact by wearing protective clothing, gloves, and glasses. Don’t inhale evaporated vapors for an extended time or ingest. Any coating spills are very slippery and need to be mopped up quickly, or absorbed with an inert material. Dried coating films are tough, but they can be removed from surfaces using a 10% ammonia/water mixture, or an oven cleaner.
After a run, thoroughly wash the application blanket, impression cylinder, rollers, anilox, and associated equipment with water. Drain any coating pan or reservoirs pumping coating back into the supply or other drum. Finish with a clean-up mode, flushing, and rinsing all application equipment and hoses, running at least 5 gallons of water through the entire system. Dispose of empty containers and diluted wash water in accordance with local regulations.
Cork’s business is the development and formulation of Aqueous, energy curing Ultraviolet (UV), and Electron Beam (EB) specialty coatings and adhesives. Cork thrives on its ability to formulate novel, useful specialty products that offer the graphic arts industry printer/coater a competitive advantage. Contact us for your next project, and get expert help from the start.