April 2003 Q&A

Optimizing Biocidal Additives

How may biocidal additives be more efficiently utilized in waterborne coating applications?

A common practice to provide applied coatings with properties to resist contamination by microorganisms is to include biocidal additives as part of the formulation. Biocides must be mobile so that they can migrate to the coating-microorganism interface and travel across the microorganism’s membrane to destroy the microbes. The application of biocidal additives in waterborne formulations presents the opportunity for these agents to undergo significant aqueous extraction, leading to the need for higher loading of the additive in the formulation. Since most biocides are fundamentally toxic, increased levels of these additives in applied coatings potentially pose a risk to ecosystems in the environment of the coating.

A logical and demonstrated strategy is to deliberately limit the release of such additives to sufficiently inhibit the aqueous extraction of the biocide and retain their antimicrobial properties. In addition, the controlled delivery facilitates a dynamic equilibrium to maintain a minimum inhibitory concentration at the coating interface over an extended time.

The methods employed to provide controlled release of these species include encapsulating the additive in a permeable coating film, hosting the additive molecules within a macromolecular framework and bonding the additive molecule to another (generally larger) molecule.

Mechanistically, these bound additives provide the desired function through reduced water-solubility characteristics, complex guest-host-water equilibria and the physical mobility limitations of active molecules.

General examples of controlled release include methods applicable to coating systems, such as clathrate and inclusion complexes of inorganic and organic foundations, water-permeable polymers and ligating polymers that exhibit a strong affinity for a specific biocidal agent.

Waxes in Powder Coatings

What role do waxes serve in powder coatings?

In recent years, powder coatings have become a technically sound alternative to traditional, organic solventborne and waterborne coatings. As with traditional coatings, powder coatings require primarily a mixture of synthetic resins, pigments and additives offering economical and ecological advantages compared to other coating systems. These raw materials are mixed, extruded and ground to coating powders. When applied, these coatings are expected to provide thin, adhering films with performance properties that match or exceed the performance of traditional coatings.

As in traditional coatings, waxes play an important part as an additive in powder coatings. Production, storage and processing, as well as the properties of the applied powder coating, can be influenced positively by using different waxes. Examples of the application of waxes in powder coatings include improving film texture, reducing gloss and enhancing film slip and mar properties. Waxes and blends of waxes, including polyethylene and polyamide waxes, have been employed.

Silicone Additives for UV-Cure Coatings

What specific consideration is made for the application of silicone additives for UV-curable coatings?

Additives are necessary to optimize the performance of low-VOC, radiation-curable coatings. The basic criteria valid for the selection of surface-active additives for conventional or waterborne coatings are applicable for radiation-cure coatings as well. However, specific requirements for radiation-curing systems often prohibit the use of nonreactive migratory additives. To overcome migration problems and the deterioration often seen with typical nonreactive silicone additives in radiation-cure coatings, reactive additives are necessary for such systems to provide improvement in flow and leveling, substrate wetting, air release, slip properties, scratch resistance and adhesion promotion.

Typical paint-film defects, such as craters and orange peel, are avoided, and the negative effects of floating are reduced. In addition, these additives promote smooth surfaces, thus improving gloss and rendering the coating more resistant to scratching, blocking and soiling.