Zinc Pyrithione: The most potent biocide

Excerpt: The paint industry has long been exposed to the problem of microbial attack on painted surfaces. Microbial growth can lead to both aesthetic and physical degradation of the coating or painted surface.

*ABSTRACT*

**The paint industry has long been exposed to the problem of microbial attack on painted surfaces. Microbial growth can lead to both aesthetic and physical degradation of the coating or painted surface. In addition to the obvious defacing effects of mold, mildew and algae growth, physical deterioration by their secretion can lead to physical degradation. This includes an increase in porosity of the surface coating or a loss of adhesion to the substrate. Moisture penetration can lead to fungal decay of the underlying surfaces. Biodegradation is not limited to the surface coating or dry paint films; it can also occur during production and storage of the paint.

Keywords: Zinc Pyrithione, antifungal/anti algal water based paints, Low VOC (Volatile Organic Compound), wall exterior protection coating.

Introduction

A RANGE of fungicides and algaecides are available that can be incorporated into masonry paints and plaster to protect against microbial attack. However, the new labeling regulation from the EU (European Union) restricts the use of fungicides and algaecides. Unlike most of the conventional additives that function as either fungicides or algaecides, zinc pyrithione displays potent antimicrobial activity towards fungi and algae and to a lesser extent, bacteria. Long term exposure tests have shown that zinc pyrithione is a competent alternative to conventional fungicide/algaecide blends that would not require any labeling in the finished paint. It is well - known that most exterior coatings and many interior coatings require additives to prevent the growth of microorganisms on the surface. Formulation ingredients of emulsion paints are good nutrients, especially the low molecular weight hydrophilic additives such as thickeners and surfactants. The extent of colonization by different organisms varies according to the substrate (wood or plaster or walls) and the prevailing conditions (temperature, substrate pH, humidity and light intensity), but under the favorable circumstances fungal and algal growth can cause massive defacement of exterior surfaces. Fortunately, there are a number of chemical solutions to the problem and a range of fungicides and algaecides have been identified that can be incorporated into paints and plaster to provide a measure of protection from microbial attack. The range of suitable products is not extensive, however, and is unlikely to grow given the additional financial constraints on new active development imposed by the Biocidal Products Directive (BPD).

The main active ingredients used to control microbiological growth on exterior surfaces and listed under Product Type 7 of the BPD are:

  • 3-iodo-2-propynyl butylcarbamate (IPBC)

  • Benzimidazole carbamic acid

methyl ester (carbendazim or

BCM)

  • Zinc pyrithione (ZPT)

  • 2-n-octyl-4-isothiazolin-3-one (OIT)

  • 4,5-dichloro-2-n-octyl-4-isothiazolin -3-one (DCOIT)

  • 2-methylthio-4-tert.- butylamino-6-cyclopropylamino-s-triazine (Irgarol)

  • 2-methylthio-4-tert.- butylamino-6-ethyllamino-s-triazine (Terbutryn) n-3,4-dichlorophenyl-n,n-dimethyl urea (Diuron)

Zinc pyrithione displays dual functionality

All the above mentioned active ingredients are either fungicides or algaecides with the exception of zinc pyrithione and DCOIT. Zinc pyrithione displays dual fungicidal/ algaecidal activity and is not a skin sensitizer like DCOIT. DCOIT (and Isothiazolinones in general) must carry a sensitizer label if present above 500 ppm in paint. The antifungal/antialgal selectivity of most active ingredients has meant that it has been common practice in Europe to mix two or more active ingredients to deliver the appropriate spectrum of activity at the coating surface. Biocide combinations, or blends, of fungicides and algaecides have therefore become the norm in Europe.

A good example of a blend for exterior surface protection is the combination of carbendazim, OIT and diuron that has been used to good effect for many years. Carbendazim is an agrochemical fungicide with a selective mode of action (it prevents aggregation of tubulin, a component of the fungal cytoskeleton and severely inhibits DNA synthesis [2]). It is a potent fungicide with negligible algaecidal activity but it does have gaps in its spectrum and fails to control Alternaria alternata species in particular. For this reason it is often combined with OIT, which is highly active against Alternaria and plugs the gap in the spectrum of carbendazim. Diuron, an agrochemical herbicide, is then added as the third component to deliver the algaecidal effect. Used in combination, the three active ingredients deliver very good microbial control at the coating surface. The current list of suitable active ingredients has come under more pressure as products such as carbendazim have come under scrutiny by the European Chemicals Board. Under the 29th ATP (Adaptation to Technical Progress), it was announced that carbendazim has been reclassified and products containing more than 1000 ppm (0.1%) carbendazim attracts "skull and crossbones" and "dead fish plus dead tree" labels together with the phrases "Toxic and Dangerous for the Environment", "Mutagen Category 2" and "Reprotox Category 2" [3] .

Paint products with over 1000 ppm of carbendazim are restricted for sale to professionals only. Diuron is also under review as a possible sensitizer and currently is labeled as "Carcinogen Category 3" and "R48-Danger of serious damage to health by prolonged exposure" if added at any concentration in a paint. New options are required for paint film protection and in the absence of new active ingredients there is a need to focus on existing chemistries that can be adapted to deliver effective protection of exterior coatings. Zinc pyrithione has low solubility in water (TABLE 1) and this property of Zinc Pyrithione helps in claiming anti algal/antifungal protection throughout the lifecycle of paint.

Unique mode of action sets zinc pyrithione apart

Virtually all metal salts of the pyrithione moiety display potent antimicrobial activity towards fungi and algae and to a lesser extent, bacteria. However, to date only the water-soluble sodium salt and the water-insoluble zinc and copper complexes have been developed as antimicrobial agents for broad commercial use. Zinc pyrithione inhibits the proton-linked ATPase in the fungal plasma membrane [4]. The ATPase ejects protons from the cytoplasm of the cell into the external medium, thereby generating an electrochemical gradient that is used to pump essential nutrients into the cell. Zinc pyrithione inhibits the ATPase, leading to the collapse of the electrochemical gradient across the plasma membrane, cessation of nutrient uptake and eventual cell death. It is also further proposed that pyrithione acts as a lipophilic weak acid chelator, becoming an ion carrier across microbial membranes, destroying the cell's ability to transport and store energy, effectively starving the cell to death. This unique mode of action distinguishes zinc pyrithione from other film fungicides in that it is not consumed or degraded in the process of killing the organism. Pyrithione brings toxic ions into the microbial cell.

Zinc pyrithione outperforms the blends that use a triazine as the algaecide or IPBC as the fungicide. Although Carbendazim/diuron/OIT blend performs better overall, there is always a possibility of Alternaria alternate growth, if a suitable co-fungicide is not used. For paint manufacturers, zinc pyrithione is an attractive alternative that would not require any labeling in the finished paint as long as the concentration is under 3.0%. Zinc pyrithione has very good efficacy against both fungi and algae and could be used alone, or in combination with other actives.

Fungal defacement is typically the focus of paint formulation biocide activity that requires strategic protection. The local biocide regulations and environmental public awareness considerations often limit the selection of available active agents or limit the amount of these actives that can be present in a coating. In cases where it is desired to increase the algal resistance of a paint film, the paint film fungicide can be co-formulated with an algaecide.

Zinc pyrithione exhibits superior thermal stability (Upto 240°C) and exceptional pH stability (pH 4 to 10) in comparison to other commonly used fungicides. Chlorothalonil shows tendency associated with fading or chalking while zinc pyrithione does not indicate this characteristics and it can be used for both whites and colours. Zinc pyrithione with added Zinc Oxide exhibits a potential formulation which provides colour stabilisation with typical prevention against discoloration. Zinc oxide is more soluble than Zinc Pyrithione in alkaline pH and prevents transchelation (Common Ion effect).

Conclusion

Recent changes in EU regulations have applied considerable pressure on some key active ingredients that until now have been the mainstays of European fungicide/algaecide formulations. In particular, reclassification of labeling of Carbendazim and Diuron means that tried and tested Carbendazim/Diuron formulations may soon be more limited in paint products. Zinc pyrithione has a proven track record as a commercial antimicrobial agent and possesses all of the key features of a paint film fungicide/algaecide including broad spectrum activity (fungi and algae), low aqueous solubility, ease of formulation and ability to deliver long lasting protection. However, it is yet to be exploited fully in the exterior coatings sector as a major film fungicide/algaecide. Given the small number of active ingredients currently available, zinc pyrithione is worth investigating for film protection either on its own (e.g. as a standalone fungicide/algaecide) or as the major component of a blend. Zinc pyrithione has the added advantage that it can be added to a paint formulation at concentrations of 3.0% by weight of wet paint without attracting any hazard labels on the packaging. Typical zinc pyrithione use levels for decorative paint applications are in the range 0.2- 0.6% active and, therefore, significantly below the labeling threshold. This is important for an active ingredient used in paints that will be sold at consumer outlets.

References

Directive 1999/45/EC of the European Parliament and the Council of May 1999 concerning the approximation of the laws, regulations, and administrative provisions of the Member States relating to the classification, packaging, and labeling of dangerous preparations.

G.P. Clemens & H.B Sisler, Localization of the Site of Action of a Fungitoxic Benomyl Derivative. Pestic. Biochem Biophys., 1971. 1, 32-43.

Directive 2004/73/EC of 29 April- 2004 adapting to technical progress for the 29th time Council Directive 67/548/EEC on the approximation of the laws, regulations and administrative provisions to the classification, packaging, and labeling of dangerous substances.

E.Emolayeva, and D. Sanders, Mechanism of Pyrithione-Induced Membrane Depolarization in Neurospora crassa. Applied and Environmental Microbiology. 61 (Sept. 1995), 3385-3390.

J.A. Sterpka, M.W. Glynn, R.T. Vinopal, R.F. Vieth and R.W. Coughlin, 1996. Pyrithione Collapse of the Delta-pH and Cation Leakage in Escherichia coli. Abstracts of the 96th Annu. Meet. Am. Soc. Microbiol., A-80, p. 147.**

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