Microbes are all around but since we can’t see them, we aren’t always conscious of them. These unseen entities play a big role in human lives, from causing disease to enabling beer brewing. While microbes can be beneficial to humans in some instances, their interactions with coatings are usually negative. Microbes such as bacteria and fungi can grow in wet paint in the container or on the dried coating. Awareness of the possible negative effects of microbes on coatings doesn’t solve the problem so we need to find ways to prevent or correct contamination.
Strictly speaking, a biocide is a compound that can kill or neutralize a living organism such as a bacteria, plant, or animal. A more specific term for the biocides used in coating formulations would be antimicrobial agents since we are only concerned with controlling contamination by microbes. We will explore the reasons to use biocides in coatings, and discuss families of biocides based on chemical structure, biocide regulations, and the future of biocides in paints.
Why Use Biocides in Paints?
Two main goals for using biocides in paints and coatings are in-can preservation and dry film protection. Microbial contamination by bacteria or fungi in the container may cause unpleasant odors, loss of viscosity, and even container failure due to gas generation. Opening a can of “spoiled” paint contaminated with microorganisms is a very negative customer experience. It’s very hard to restore confidence in your product afterward. Contamination of the dried film by fungi and algae leads to surface erosion and dirt pickup. Extreme cases of fungi or algae growth can cause film failure and damage to the substrate. An emerging application of biocides in coatings is to prevent contamination of a surface by pathogenic microbes such as viruses and bacteria.
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What are Biocides? How can we Classify Biocides?
We can classify biocides several ways, chemical structure, target organisms, common applications, or even how they are regulated. However you choose to classify biocides, most applications require consideration of several performance factors when choosing a biocide. When selecting an in-can preservative a survey or plant hygiene audit to identify specific contaminants and where in the manufacturing process contamination might have been introduced is very helpful to ensure the right choice. In the case of determining the proper dry film preservative samples can be taken from exterior exposure panels or end user feedback such as customer complaints for identification of microbial contaminants.
Chemical structure determines performance and regulatory benchmarks for a biocide so let’s start by grouping them according to chemical structure.
- Isothiazolinone derivatives are some of the most widely used biocides. This group of effective in-can preservatives includes: BIT (1,2-Benzisothiazolin-3-one ), MIT ( Methylisothiazolinone), OIT (Octylisothiazolinone), DCOIT (Dichlorooctylisothiolinone),and CMIT (Chloromethylisothiazolinone). Often blends of these biocides are used to achieve quick kill and long term in-can paint preservation. Effective May 2019, paints containing OIT cannot be manufactured or sold in Canada. BIT is especially useful for long-term in-can preservation.
- Organic Zinc Complexes are versatile since they can serve as both in-can and dry film preservatives. The most common Zinc Complex biocide used in coatings is Zinc Pyrithione, extremely versatile it can be used as a bactericide, fungicide and algaecide. One precaution involves paints containing metal driers as these can form inactive chelates, this can also occur when process water with high amounts of iron is used in waterborne paint manufacturing.
- Inorganic Zinc compounds such as Zinc Oxide are often used as cost effective dry film preservatives. The partially soluble Zinc Oxide slowly releases ions which prevent the growth of algae and fungi. When using Zinc Oxide as a biocide the stability of the paint must be evaluated. Improper stabilization of the Zinc Oxide or a reaction between other coating components and Zinc Oxide can cause viscosity increase, settling, pH change or even gelation.
- Formaldehyde releasing agents are low-cost bactericides but have multiple regulatory and voluntary use restrictions. Considered a toxic air contaminant, formaldehyde contributes to the poor indoor air quality issue often referred to as “sick building syndrome”. Formaldehyde is heavily regulated as an air toxic in many regions. Besides use as a biocide other formaldehyde generating chemicals like adhesives and wood product binders can release formaldehyde.
- Metals/Metal ions can be used as broad spectrum antimicrobial agents, some such as Mercury are not in use anymore because of toxicity and regulatory restrictions. Other metals such as Silver and Copper are finding use to control pathogens including viruses and bacteria on surfaces. In this application the purpose of the biocide is to kill or impede the growth of microorganisms to prevent the spread of disease. Because of these public health claims, the process to certify products designed to control pathogens on surfaces varies greatly by region and regulating agency.
| Chemical Family | Organisms Controlled | Best Use | Regulatory Notes |
| Isothiazolinone Derivatives | Bacteria, Fungi | In-can Preservation | Regulated as pesticides in the US, OIT banned in Canada, EU regulating certain compounds in class |
| Organic Zinc Complexes | Bacteria, Fungi, Algae | In-can and Dry Film Preservation | Regulated as pesticides, limited new regulations pending |
| Inorganic Zinc Compounds | Fungi, Algae | Dry Film Preservation | Zinc Oxide has many uses in paints (anti-corrosion), stability issues, lightly regulated |
| Formaldehyde Generators | Bacteria | In-can Preservation | Formaldehyde toxicity, strictly regulated, use banned in many regions |
| Metal Ions | Bacteria, Viruses | Control Surface Pathogens | Complex regulatory process in US to authorize products |
Regulation of Biocides: Current Assessment and Future Overview
The purpose of biocides is to interrupt the growth of microorganisms by destroying their cells or disrupting growth and reproduction. This implies that biocides contain a degree of toxicity. For this reason, biocides are highly regulated in most cases. In the US, primary biocide regulations consider the paint a treated article. This means that the purpose of the biocide is to protect the can of paint or dried film from damage by microorganisms. As long as no other claims such as protecting the coated substrate from damage are made, the less rigorous treated article regulatory classification applies.
Even when used within the scope of treated articles some specific chemical compounds are prohibited or can only be used at limited levels. The Isothiazolinone derivatives family is a good case in point of this specific target regulation. All of these compounds have regulatory guidelines mandating maximum use levels for specific uses such as in-can preservation of waterborne paints. In the US these limits are enforced by the Environmental Protection Agency (EPA). In the EU even stricter limits or outright bans of selected Isothiazolinone biocides are either in effect or proposed by the European Chemicals Agency (ECHA).
Future Overview
By most predictions, future trends for biocides in coatings will center on means to meet ever tightening regulations. Advances which allow reductions in dosage of and/or elimination of the need for regulated biocides will be implemented to accomplish this goal. Let’s explore some of the routes we can use to reduce the use of these regulated biocides.
- Reducing the need for biocides by advances in raw materials. There are several methods which show promise in this approach. Raw material suppliers are developing and bringing to market formula ingredients which don’t require preservation thus reducing biocide amounts in the final product. For example, several suppliers are marketing additives such as rheology modifiers and colorants packaged as dry powders. Since these materials don’t contain water, they do not easily sustain microbial growth.
- Non-regulated compounds which increase the efficacy of biocides can reduce the amount of biocides required for effective suppression of microbial activity. Developing synergistic compounds to increase biocide effectiveness is an ongoing research goal for many coatings material suppliers.
- Another non-regulated class of compounds are those which have antimicrobial activity on their own but do not cause harm to higher animals and plants or the general environment. The development timeline for these agents is longer than that of synergistic compounds. Work with chitin-based materials derived from shellfish has shown promise in replacing biocides with non-hazardous antimicrobial agents not requiring regulation.
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