Over eighty percent of architectural paints are waterborne.1 The presence of water makes these paints very susceptible to microbial attack both in the wet state and as dry film. The addition of an in-can preservative will protect these coatings in the wet state during storage and transport. But after a coating has been applied and dried, it becomes susceptible to colonization by fungi and/or algae. The addition of a dry-film preservative (fungicide and/or algaecide) ensures long-term performance of the coating.

Currently, there is no single biocide that possesses all the properties of an ideal biocide. However, blends of bio-actives may be used to optimize performance and approach the efficacy of the "ideal" biocide.

Prevention and Protection

Biocides are important to prevent the microbial deterioration of many coatings. The two main applications of biocides are: (a) to prevent spoilage of the wet-state product during storage and transportation (in-can protection); and (b) to ensure long-term performance of the coating (dry-film protection).2

Microbial growth in the wet state is usually manifested by a loss of product functionality and may include gas formation, offensive odors and changes in pH, viscosity and color.3 Microbial contaminants can be introduced with water (process water, wash water), with raw materials (latex, fillers, pigments, etc.) and by poor plant hygiene. Bacteria are the most common spoilage organisms, but fungi and yeasts are sometimes responsible for product deterioration. Spoilage of the waterborne products, which may go unnoticed until the product reaches the consumer, can result in significant economic loss. Good plant hygiene and manufacturing practices, when combined with the use of an optimized biocide, will minimize the risk of microbial spoilage of the coating.4,5

Upon drying, both water- and solventborne coatings are susceptible to colonization by fungi and/or algae. Microorganism growth on dry-film not only affects the coating appearance (discoloration), but may also compromise its performance (biodeterioration). Fungi can penetrate coatings, resulting in cracking, blistering and loss of adhesion, leading to decay or corrosion of the underlying substrate. Algae colonies, which seem to grow more rapidly on porous substrates such as stucco, cement and bricks, can occlude water. The freezing and thawing of this entrapped water may induce cracking or increase the permeation properties of the coating, leading to failure. The presence of water may also encourage colonization by other microorganisms, which in turn may cause biodeterioration.6

The type of microorganism that can colonize the coating will depend on several factors, including moisture content of the surface, presence of nutrients, and substrate and coating composition.3,7 For example, surface moisture content is affected by climatic conditions (amount of rainfall, dew, humidity, temperature and time of the year) as well as by local conditions (surfaces sheltered from winds and shaded areas will contain higher moisture content). Nutrient sources include constituents of the coating itself (such as polymers, thickeners, etc.), partially biodegraded substances produced by other microorganisms, or simply material deposited on the coating from the atmosphere, such as dirt. The composition of the substrate may affect the pH of the surface, making it suitable for microbe colonization. For example, fungi favor more acidic conditions, such as those provided by wood. Some species of wood are more susceptible to colonization by fungi than others (e.g., pine is more susceptible than cedar). Algae, on the other hand, favor alkaline conditions, such as those provided by masonry. Lastly, the coating composition and properties (polymer type, water repellency, porosity, hardness, chalking and roughness) determine the type of microbial community that will colonize the coating.

Blending Biocides

There is no universal biocide active ingredient that is compatible with all formulations and meets the requirements of the coatings manufacturer. Among the various actives used for in-can preservation, there are fast-acting biocides, e.g., O-formals, N-formals and 5-chloro-N-methylisothiazolin-3-one (CIT); and slow-acting biocides, e.g., benzisothiazoline (BIT) and chloroallyl-3,5,7-azoniaadamantane chloride (adamantane). There are biocides with deficiencies in their activity spectra, such as BIT and N-methylisothiazolone (MIT), which have limited antifungal activity.8 Some biocides (e.g., isothiazolones) are known to be skin irritants and sensitizers. Others may release formaldehyde. These include O-formals, N-formals, triazines and adamantanes.

Bioactives suitable for dry-film preservation differ in water solubility. Two of the more water-soluble are 2-n-octyl-4-isothiazolin-3-one (OIT) and 3-iodo-2-propylbutyl carbamate (IPBC). Some, such as the fungicide carbendazim, have deficiencies in antifungal spectra. Carbendazim does not inhibit the common fungal isolates, Alternaria sp.

The use of a single active ingredient may be sufficient to protect a coating against in-can spoilage or dry-film defacement; however, in many cases it may be advantageous to use a blend of actives. For example, the combination of certain active ingredients can result in synergy (less amounts of each active are necessary to bring about the same inhibitory effect as the use of either active alone).

Table 1 shows an example of a synergistic blend for in-can protection. The blend contains a formaldehyde donor (FD) type biocide (N-methyl-2-hydroxymethylenoxypropyl-2'hydroxypropylamine) and 1,2-Benziosthiazolin-3-one (BIT), as the second ingredient. The table also shows minimally inhibitory concentration (MIC) data for each active ingredient acting alone or acting in combination for selected bacterial and fungal species. Synergy is indicated by a synergy index (SI) of less than one.9 Because MIC data provide only an indication of activity, it is desirable to confirm the results in a real paint matrix.

An in-can preservation test was conducted following the test method described in Standards on Materials and Environmental Microbiology by the American Society for Testing and Materials (ASTM D 2574-97).10 As shown in Table 2, a use level of 0.2% by wt. of either the FD- or the BIT-based biocides was required to protect the tested paint. However, only 0.1% by wt. of the blend was required to offer the same degree of protection. Thus, blends of actives may allow manufacturers to protect a product at reduced levels, providing not only a potential cost benefit but also a product that is more environmentally friendly. In addition, blends of actives may have a better compatibility profile than when each of the active ingredients is used alone. Furthermore, the ability to protect a product with lower use levels of certain actives may circumvent labeling issues (

A second example describes a synergistic blend for dry-film protection. As shown in Table 3, synergy is observed when combining the fungicides tetrachloroisophthalonitrile (CTL) and IPBC. An additional benefit of this blend is provided by the differences in water solubility properties of the active ingredients. That is, the water solubility of IPBC is about 190 mg/L11 and that of CTL is about 0.6 mg/L.12

Figure 1 shows the result of test-fence exposure studies over a period of 24 months, during which cedar panels were treated with polyvinyl acrylic (PVA) paint containing (a) 1% by wt IPBC (40% active), (b) 1% by wt. CTL (40.4% active), (c) 1% by wt. of CTL+ IPBC (30% total active) or (d) no-treatment (control). After 12 months of exposure, no defacement was observed in any of the treated panels. After 18 months, the paint containing only IPBC showed 30% defacement. The paint containing only CTL showed 20% defacement, however the paint containing the combination of CTL and IPBC was defaced only by 10%. The control paint (untreated) was defaced 50% after 24 months' exposure.

Because all treatments were added at the same levels (1%), and the total amount of actives was lower in the blend (30% vs. 40%), the exposure tests clearly show the benefits of this synergistic blend. By combining the actives CTL and IPBC with different solubility properties, the synergistic blend provides a dual action: fast acting/long lasting protection.

Yet another type of blend is one in which active ingredients with different target microorganisms are combined, providing a broader spectrum of antimicrobial activity. These blends include (a) combinations of fungicides plus algaecide [e.g., CTL plus S-triazine, IPBC plus N-(3,4-dichlorophenyl)-N,N-dimethylurea (diuron), carbendazim plus diuron, CTL plus diuron, etc.], (b) combinations of fungicides with certain deficiencies in their antimicrobial spectrum (e.g., carbendazim plus OIT or carbendazim plus IPBC), or (c) combinations of an in-can bactericide plus a fungicide (triazines plus carbendazim, etc.). While many of these combinations are regarded as "silver bullet" biocides (formulas can be simplified, inventory can be reduced), they also have some limitations. For example, the fixed ratio of active ingredients does not allow product optimization.

To Blend or Not

Not all blends are made equal. Some blends seem to be more effective than others. For example, in comparing the performance of blends containing combinations of fungicides and algaecides, some blends seem to outperform others (Figure 2). Performance may be related to the actual active ingredients blended, to the matrix where the blend is used, or - in the case of dry film protection - to the climatic conditions where exposures take place.

As with the use of any single active ingredient, compatibility issues should be ruled out before using a blend. A major disadvantage of any synergistic blend is that, if one of the active ingredients is deactivated, the level of the second active ingredient alone may not be sufficient to protect the product.

In Summary

The ideal biocide - one that is compatible with all formulations and all processing conditions, active against all microorganisms, environmentally friendly, and cost effective - has yet to be discovered. While the use of single active ingredients may be sufficient to prevent in-can microbial spoilage or dry film defacement, blends of actives may be used to optimize biocide performance. Depending on the blend, the advantages of using blends of actives may include lower use levels, fast-acting/longer term protection, broader antimicrobial spectrum, improved compatibility profile, increased cost effectiveness and environmentally friendly end products.


The author wishes to thank Dr. William Woods, Larry Liu and Daniel Brown, International Specialty Products, for their help and contributions to this article.


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