Coatings World - December 2015

previous coat will lower the surface energy. This leads to wetting and overcoatability concerns.

Wetting Agents

One solution to overcoatability is to add a surfactant to the next liquid coating. This would be acting as a wetting agent by lowering the surface tension of the liquid in order to wet out the dried previous coat. Wetting agents and flow and leveling agents are similar chemical answers and differ primarily in the perspective or the problem being solved. This is why those terms are often intermingled.

Foam

At the air/liquid interface, low-surface-tension agents stabilize foam more efficiently than high-surface-tension materials. This is yet another interfacial problem. Again the solution, a defoamer in this case, needs to have a lower surface tension than the foam stabilizer to be effective. With all of these interfaces, is it any wonder the use of additives is so complex?

Slip, COF Reduction or Release Agents

Sometimes one wants to create a coating that is not able to be wetted, for example an anti-grafitti, anti-fouling, or release coating. In that event, migration of these surface-active agents to the interfaces helps. Additive designers would favor silicone or fluoroalkyl surfactants for these applications which can give both flow and leveling, wetting and release.

However, the most potent of these release agents are not the surfactants. A minimally modified or unmodified silicone polymer, fluorocarbon or even hydrocarbon is the most efficient way to accomplish this surface effect. While surfactants are designed to be miscible with the carrier and binders, this diminishes both the amount and the potency of the low-sur-face-energy materials at the interface. An immiscible polymer will migrate very quickly to the interface and cause maximum release properties.

We all learned that oil and water are not miscible, and of course water with silicone oil and water with fluorocarbon oils do not mix either. But some are at first surprised that silicone, fluorocarbon and hydrocarbon oils also do not mix with each other. The additive designer relies on this immiscibility and designs release additives that are chemically closer to pure silicon or fluorocarbon because these are the most potent release agents. The art of this balancing act is to add enough chemical modification to prevent defects and, thereby, send us back into a loop looking for flow and leveling.

Migration

Importantly since these materials are not reacted into the matrix, they are free to migrate through the cured coating to the interfaces. The expression “blooming silicone” is not an Australian condemnation, but rather an accurate description of how these work. They continue to migrate or bloom to the interfaces and further lower coefficient of friction (COF), surface energy and tape release properties.

The UV-cured overprint varnish industry has found a solution to this migration. These formulators commonly use acrylate functional silicones which migrate in the liquid coating to give the slip and release properties desired, but then react into the matrix upon curing to prevent blooming. 6 The same technique can be used with other cure mechanisms if the additives are suitably functionalized to react into the matrix. Care should be taken in slower-cure systems as migration may take days to stop as the cure continues.

Mar and Stain Resistance

Mar and stain resistance are also improved by these low-sur-face-energy materials at the solid/air interface of the cured coating. 7 Mar resistance is a consequence of the slipperiness of the surface; materials that would damage the coating tend to slide across it instead. The coating is not actually harder, but the mar resistance is improved without changing the crosslink density of the film.