Coatings World - December 2015

Additives
December 2015 www.coatingsworld.com Coatings World | 39

Mar and abrasion resistance improvement can also be obtained with increased crosslinking from additives such as or-gano-functional tri-alkoxy silanes. These are designed to react into the matrix with multiple crosslink sites, a concept that also works with high-functional reactive polymers. 8

Stain resistance improvement is due to the low surface energy and poor miscibility of these agents at the interface. Although this property is very dependent on the stain, fluoroalkyl additives are often better for this since less stains are miscible with those. 9

It is important how these additives are aligned at the interface, especially for stain resistance. Different molecular architectures lay at the interface differently and can give very different performance. 10

Foam Control Agents

We touched on foam in the above discussion of flow and leveling agents. Surfactants, including flow and leveling agents and emulsifiers, migrate to the air/liquid interface formed upon mechanical agitation of the coating and some of them stabilize the foam. A surface active agent that does this is referred to as a profoamer. Foam is a property not often desired in coatings.

A defoamer is a material that will cause foam to break when added to it, the simplest way of demonstrating this is to add alcohol to a stable foam - the foam breaks. This low-surface-tension, low-viscosity molecule displaces the profoamer molecules at the interface and allows for a quick collapse of the foam. (As an aside, this is why you add tequila before blending a frozen margarita. Alcohol is a defoamer, but not an antifoam.) In coatings, many mineral-oil-based foam control agents provide this defoaming property.

An antifoam is a more complex additive; it prevents the formation of foam during the agitation. Antifoams work by migrating to the interface, spreading across it and spanning the air gap to rupture the interface. 11 Additive designers develop antifoams which nearly all have an immiscible, low-surface-energy oil and an oil-soluble mechanical particle to accomplish this feat.

Silicone-based antifoams are the norm in coatings and can be used down to ppm levels in many cases. An important but not well known fact is that antifoams can get used up. In an application with constant agitation like a laundry machine, the antifoam eventually gets mechanically dispersed to where it is no longer effective.

Degassing or microfoam elimination is yet another problem. The viscosity and flow resistance of modern carriers, i.e., water-borne and high solids solvents, prevent the smallest of the foam bubbles formed from migrating and coalescing. Buoyancy of a gas bubble in a liquid is proportional to size, so the smallest bubbles do not have enough force to bring them into macro-foam. The result, after curing, is pinhole formation and gas pockets in the coating.

The designer’s solution to microfoam is to go back to
surface active materials choosing those which will quickly
migrate to these interfaces and affect the stability and size
of these tiny bubbles. Cloud point defoaming, which uses
surface active materials with low solubilities, or cloud points,
are a common mechanism here. 12 Foam control theory is es-
tablished far beyond what we are able to present in the scope
of this brief review. Readers are encouraged to see references
cited herein.

Summary

Hopefully you have a better understanding of the complexity and power of additives. In my career, the most common philosophical question I have heard is ‘why can’t we design one additive to do it all’? This additive would lower surface tension to eliminate defects, but not so much as to cause intercoat adhesion or prevent overcoat problems (except in anti-graffiti and marine coatings where we want overcoat problems – but still need adhesion). Also it would give mar and stain resistance, high gloss but not too high, and no foam. The answer is - we’re working on it. CW

References

1 Pierce, P.E.; Schoff, C.K. Coating Film Defects, Rev. Ed. 1994, Federation of Societies for Coatings Technology, Blue Bell, PA.

2 Ferri, J.K.; Stebe, K.J. Which surfactants reduce surface tension faster? A scaling argument for diffusion-controlled adsorption. Advances in Colloid and Interface Science 2000, 2, 85(1), 61-97.

3 Zhang,P. et.al. U.S. 7,129,199 B2, 2006.

4 Ruckenstein et.al. J. Phys. Chem. 1975, 79 ( 24), 2622–2626. 5 ASTM D5767.

6 Ruckle, R.E.; Marengo, P. A. The Effect of Silicone Additive Structure on Foaming in Radcure Systems. In Proceedings of the Radtech Symposium, 1994, Radtech NA.

7Ruckle, R.E.; Cheung, T.; Horne, A. Incorporation of Reactive Silicones into Various Coatings Films and the Resulting Properties. Presented at the Western Coatings Symposium, 2013, Los Angeles Chapter of the Society of Coatings Technology (LASCT).

8 Ruckle, R.E.; Cheung, T. A Structure Property Study of Epoxy Resins Reacted with Epoxy Silicones. Proceedings of Sampetech, 2014, Society for the Advancement of Material and Process Engineering.

9Ruckle, R.E.; Cheung, T. Incorporation of Fluoro-Silicones in Coatings Films and the Resulting Properties. In Proceedings of the Waterborne Symposium, 2014, University of Southern Mississippi.

10 Ruckle, R.E.; Cheung, T. Properties of Silicone Modified UV Cured Acrylate and Epoxy Coatings Films. In Proceedings of the Waterborne Symposium, 2013, University of Southern Mississippi.