Aqueous Coatings are Complex Blends
Aqueous coatings are complex blends of materials with a typical formula consisting of a mixture of aqueous emulsions, blended with additives to improve properties and performance. These additives are commonly, small quantities of an amine, a plasticizer, waxes, a surfactant, coalescent aids, and anti-foam agents. The balance of each material portion to the total is important and enables a formula’s given target performance to be attained.
Agitation or Aeration can cause Foaming Issues
Aqueous coatings, being 30-40% solids are low-viscosity liquids, and as such are capable of creating foam when agitated or aerated.
Foam present in a coating during its application is capable of producing unwanted defects in a dried coating film. These objectionable visible defects may be seen as clear distinct round spots or voids in the surface of a coating. This kind of defect is caused by a bubble of air that produces a characteristic round hole with a clear center, void of coating. Called “pinholes” these voids are a result of the applied coating failing to wet the substrate surface, due to the presence of a foam bubble.
One key component of an aqueous coating is the amine used to solubilize acrylic resin, which is also the primary determinant of a coating’s pH, or the acidity/alkalinity balance. Loss of the amine through evaporation from an open container must be prevented as this will lower a coatings’ pH. The amine loss will cause a rise in a coating’s viscosity which must be avoided because viscosity is very important to a coating’s running characteristics. Remember, it is always recommended that a container of coating be covered while in use to avoid evaporation issues.
Cork typically recommends that its coatings be used as received. This recommendation is made because it is too easy to upset the chemical balance of a formula when attempting to make field adjustments. Once compromised, both the running and final film properties of a coating can be affected. Considering the subject of “foaming”, IMPROPER pH CAN CONTRIBUTE TO FOAMING! Therefore, take care to maintain the pH of an aqueous coating as formulated and as received.
Blending, or thorough continuous mixing of an aqueous coating during use is important to maintain a coatings integrity. Cork recommends that a container of coating be stirred, rocked, or rolled before use. Recommended room temperature 72° F. storage will assure that the supplied coating viscosity is that which goes to press. A “lightning” mixer, or similar blade type mixer can be used to effectively mix a container of coating before use. An alternative is stirring by hand using a hand paddle.
How to Control Foaming of Aqueous Coatings
- Diaphragm pumps are recommended to pump and circulate coating from the supply container to the coater and back. Gear pumps are not recommended since high shear can break down a coating and cause foam.
- The pump should be run only at a rate that satisfies coater demand and does not recirculate coating excessively. The return line must not be allowed to suck and pump air into the coating supply. The coating level in the supply fountain must never be allowed to drop too low.
- The pump set-up should be properly maintained with pump seals and line connections regularly checked to avoid air leaks.
- A loop formed in the return line will slow coating flow speed, allowing any entrained air picked up in the fountain overflow vortex to escape.
- When using a drum or kit as a supply container direct the return line flow through a perforated pipe extending from the container cover to the bottom of the supply container. Don’t return a stream of splashing, foam-creating coating to the container.
- If your pump has a flow bypass, make sure that the exit line also extends below the lowest level of coating ever expected in the container.
- Watch out for detergent/soap wash-up solutions that will cause foam in a coating if all traces are not thoroughly rinsed away after use.
Beware Of Adding Defoamers to Aqueous Coatings
Sometimes a bad foaming situation may be remedied by the addition of a defoamer solution press side under the advice of a Cork representative. While this situation will rarely occur, follow the advice with care and caution. A little defoamer addition may help, but too much can be harmful potentially disrupting the delicate chemical balance of the coating, affecting performance.
In conclusion, there are as you have now read, a number of factors that contribute to and cause foaming when coatings are used. Some of these you can control by following the recommended coating practices outlined above.
Contact Cork Technical experts if you require an aqueous coating with enhanced performance properties for a demanding application.
Cork’s business is the development and formulation of Aqueous, energy-curing Ultraviolet (UV), and Electron Beam (EB) specialty coatings and adhesives.