Agents that alter interfacial interactions are called surfactants. Surfactants have two different chemical groups, one that is compatible with the liquid to be modified and one that has lower surface tension. For example, the surface tension of epoxy resins can be lowered by adding a surfactant that has an alcohol group (compatible with epoxy) on one end and a fluorine-containing group on the other. The alcohol groups will bond to the epoxy, bringing a "tail" of incompatible fluorochemicals to the surface. The epoxy coating behaves as if it were a low surface tension fluorochemical. Adding a small amount of surfactant will allow the epoxy coating to wet difficult low-energy surfaces, even oil-contaminated plastics.
Surfactants are effective in reducing the surface tension of inks, coatings and paints. Usually, 1% or less is sufficient. When dewetting occurs due to the inherently low surface energy of the substrate, the use of surfactants, also known as wetting agents, is recommended. These materials are not a substitute for good housekeeping and proper part preparation. Contamination can cause subsequent bond failures.
Fluorochemicals, silicones and hydrocarbons are common classes of surfactants. Fluorochemicals have the lowest surface tension of all materials and are effective wetting agents. Silicone is less effective and less costly. However, some types of silicones may become airborne, resulting in substrate contamination.
While it may be desirable to lower the surface tension of a coating, the opposite is true for the substrate. It is this agent that helps decorate the material and renders the substrate useless. Silicone contamination can create the notorious dehumidification defect known as "fisheye".
Coatings, paints and inks, once modified with surfactants, often change, even after curing. Their low surface energy makes them difficult to wet, for example, if topcoats are required. There are several ways to overcome this problem. Good practice is to use the least amount of the weakest surfactant to do the job. Start with the hydrocarbon class.
Also make sure the substrate is clean.
Another possibility is to use reactive surfactants. Reagents with functional groups that can react with coatings or adhesives become less reactive after curing. Once a surfactant has fulfilled its role as a wetting agent, it is no longer needed. Another method is to add a surfactant to the second material to be applied. Often the same surfactant will work, especially at slightly higher loads.
