In the United States, the U.S. Environmental Protection Agency (EPA) regulates antimicrobial products as pesticides, and the U.S. Food and Drug Administration (FDA) regulates antimicrobial products as drugs/antiseptics.

As mentioned in my previous article,¹ if a product label claims to kill, control, repel, mitigate or reduce a pest, it is a pesticide regulated by the U.S. EPA.² When manufacturers make this kind of claim on the label, they must also include:

• application instructions that are effective at killing or controlling the pest, and
• first aid instructions, in case of accidental exposure.

In Europe, the classification of biocides in the Biocidal Products Regulation (BPR, Regulation (EU) 528/2012)³ is broken down into 22 product types (i.e. application categories), with several comprising multiple subgroups. There are four main groups, with most coatings applications falling under Main Group 2:

Main Group 2: Preservatives

• Product-type 6: In-can preservatives
• Product-type 7: Film preservatives
• Product-type 8: Wood preservatives
• Product-type 9: Fibre, leather, rubber and polymerised materials preservatives
• Product-type 10: Masonry preservatives
• Product-type 11: Preservatives for liquid-cooling and processing systems
• Product-type 12: Slimicides
• Product-type 13: Metalworking-fluid preservatives

How to avoid and treat microbial contamination

The prevention of microbial contamination starts with good housekeeping and the safe storage of raw materials. Most any material will support invasive contamination and even contribute to its growth. This is one of the reasons why powders are kept dry and liquids are kept in closed containers and both are handled with clean equipment for transfer.

Many facilities use waste wash water in production to minimize impact on release into the environment. This water is typically separated by color and interior vs exterior waste water, since the latter may contain zinc oxide or other reactive pigments incompatible with an interior formulation.

The water is often allowed to settle to decrease suspended solids, and then treated with a biocide to prevent contamination. This biocide may differ from what will be used in a paint that will act as an in-can preservative. It can also serve to protect the coating from accumulation of microbes which can result in human infection, film degradation and mildew. In an extreme case of contamination, the holding tank will be emptied and steam power-washed, perhaps with bleach.

As an in-can preservative, antimicrobials provide protection from microbes that could otherwise utilize various components of coatings as food. This could result in a change in viscosity, discoloration, malodor, pH shift and other degradation of properties.

Antimicrobial coating formulation considerations

Regulatory

Recent legislation has eliminated the use of certain antimicrobial materials based on more stringent parameters for health and safety. Therefore, the first action a formulator must undertake when formulating with an antimicrobial (A-M) is to ensure that the materials pass current regulatory scrutiny in the region where the coating will be manufactured and sold.

Environment

The second consideration is the type of microbes that will be found in the environment of the applied coating. Some questions to consider – will the environment be:

• A common interior wall?
• An interior room which will see high levels of humidity such as bathroom?
• An exterior coating that will be applied on a north-facing side and therefore subjected to less drying from the sun, and little-to-no mold-inhibiting UV?

pH

The third consideration is pH. All biocides operate in an optimal pH range, with more efficacy with some microbes and little to none with others. Some recent blends of products and sold-as-new materials, have addressed both of the previous issues without requiring re-registration of products as “new.”

Compatibility

The next consideration is compatibility. The Technical Data Sheet or/and a technical representative from the antimicrobial company can help the formulator avoid the use of an A-M that is not compatible with other ingredients.

Cost

And the very last contemplation is cost. What is the cost efficacy of the material compared to other choices that you have? In the case of an exterior coating, an organic antimicrobial at a lower loading complemented with zinc oxide (ZnO) may provide the protection required vs perhaps two times the A-M/ZnO combination, but at a much lower price.

For most exterior applications, mildew and mold are removed with sodium hypochlorite (bleach), Trisodium phosphate (TSP), an oxalic acid-based compound, etc. The Forest Products Laboratory has many monographs covering the topic of wood preparation and mildew removal. Of course, wood is just one substrate where microbes grow.

A recent concern, leaching is the migration of the A-M to the surface, and should be monitored. It can be removed by contact, washing or even high humidity to some extent. Most organic antimicrobials function by solubility and mobility in the film. There is more interest in A-M moieties that are covalently bonded to a monomeric backbone, or have the ability to be covalently bonded to mitigate or eliminate leaching.

Be sure to follow this ever-evolving technology pool to evaluate newly approved materials as replacements for existing products could become regulated out of use.

References:

1. A Primer on Microbials
2. Title 40, Part 152 - Pesticide Registration and Classification Procedures, Protection of the Environment; Code of Federal Regulations.
3. Understanding Biocidal Products Regulation (BPR, Regulation (EU) 528/2012)
4. Antimicrobial Resistance Learning Site: Antimicrobials: An Introduction
5. National Pesticide Information Center: Antimicrobials Fact Sheet