How Foam is Formed
Foam consists of bubbles that are formed when air or a gas is trapped in a liquid or solid substance. Certain conditions are required for foam to be formed. Mechanical work or agitation is required, as are surfactants that reduce surface tension, so that work increases surface area. Also, foam must be formed faster than it breaks down. In most foams the volume of air or gas is large with a very thin film of liquid, or solid material enveloping the bubbles.
Foam may also be created by dispersion where a large volume of air or gas is injected into a liquid.
Examples of Foamed Products
There are many examples of products that are common and beneficial to us and we would have trouble living without them. For example, we like the idea of a foam head on a pour of beer. Soap foams or suds improve the cleaning action of clothes and dishwashing detergents, body washes, and toothpastes, etc.
Fire retardant foams and leavened bread are other examples of liquid foam. Additionally, there are many examples of foamed solid products that are beneficial, such as insulated drinking cups and food packaging, cleaning sponges, home and other insulation products, flotation devices, as well as structural rigid products, etc.
Defects in Coating
When liquid coatings are considered, the appearance of foam in a coating during its application is not a good thing. Foam has the prospect of leading to detrimental defects in an applied wet coating film and then in the surface of the final dried coating film. Specifically, these defects are seen as distinctly clear round spots or voids in the coating with no coating in the spot on the dried film.
The spot is caused by a bubble of air that has produced a distinctly round hole, with a clear center identifying it as a foam induced defect. Size can vary from 50 to 1200 microns with the smallest being called “pinholes”. These voids or pinholes are the results of a failure of the applied coating to wet the substrate surface due to a bubble. The final coated product may display a lower gloss level and reduced scuff resistance as well as have visual defects.
Many liquid substances create foam readily when agitated or aerated. This can occur during manufacturing or during use in application. For these reasons anti-foaming agents are often introduced to negate these issues, but mechanical means of prevention are more common and often preferable.
During manufacturing, the use of a central agitator in an unbaffled vessel often leads to a vortex forming. A strong vortex can suck air into a liquid being mixed to form bubbles and foam. This can be easily remedied by using a baffled vessel or a tilted off- center agitator.
A container of coating should be stirred thoroughly before being brought to the application unit. Aqueous products can be effectively mixed using a “lightning mixer” or by stirring at length with a hand paddle. UV & EB products can be thoroughly mixed using a broad blade electric or air powered mixer.
A Feed Line Can Introduce Bubble Forming Air into a Liquid in a Number of Ways
• An empty feed line is air filled. This air will be expelled when a liquid is pumped through it.
• Air bubbles can form on the internal wall of a feed line when a liquid is pumped through. These will disappear with time by the action of the flowing liquid.
• Air pockets in an empty pipe or tubing can trap air but these will be gradually removed by the flowing liquid.
• A loop formed in the return line will cause coating flow speed to slow, and allow any entrained air picked up from the fountain overflow vortex to escape.
A common way to produce bubbles and foam in a coating is to direct a circulating feed pipe downward toward the surface of a coating in a supply container. If the speed of the pumped coating is high enough air is entrained and air bubbles (foam) are formed.
Cork recommends that steps be taken to subdue splashing and the generation of foam when a return line (low velocity) circulates a coating from a coater to a supply container. Plumb the line to reach the bottom of the supply container.
Cut the bottom of the return line to a 45° angle. Ideally, slot the pipe its entire length with a vertical slot made of spaced 4” x ¼” wide slots. A feed line should be limited to only one high point where a bleed valve should be located.
Common Causes of Foam
A major cause of foam is leaking pump seals and/or line connections which must be maintained to avoid air leaks.
Another cause of foam is running the coating circulating pump too fast. Only run the pump as fast as is necessary to satisfy coater demand, without running a low fountain level or recirculating excessively. Never allow the return line to suck and pump air into the coating supply.
Considering coating pumps, only low shear pumps are recommended. Diaphragm pumps are recommended for aqueous coatings. Diaphragm, centrifugal or peristatic pumps are recommended for UV/EB energy curing coatings. High shear gear and piston pumps should not be used.
Always keep a container of aqueous coating covered to prevent amine evaporation and a reduction in pH which will cause a viscosity rise and contribute to foaming.
Detergent /soap wash-up solutions can also generate foam in coatings. When used they must be thoroughly rinsed/removed from equipment and associated plumbing.
New in-line pressure controlled aqueous & UV coating systems consisting of a chambered doctor blade coater, and a coating circulator are also eliminating foaming and dry coating defects.
In conclusion, the introduction of air into a wet liquid coating must be avoided before and during the application process to suppress the formation of foam, air bubbles, and pinhole defects in dry coated films.
Our business at Cork 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.