Introduction to the Microbiology of Coatings
The three main classes of microorganisms that can colonize paints and surface coatings or contaminate surfaces in general are:
- Fungi
- Algae
- Bacteria
All microorganisms need both water and nutrients and some, like algae, also need sunlight.
Microorganisms have the potential to grow in liquid paint before it is applied and also on paint films after application. Paint manufacturers may use biocides in their products in order to prevent these organisms developing. The biocides that are used for this purpose can be divided into three main classes:
- In-can biocides designed to prevent the growth of microorganisms during storage.
- Film biocides designed to protect applied paint films by preventing microbial growth.
- Film biocides designed to impart antimicrobial activity to the coating (to produce a hygienic or antimicrobial coating).
Biocides are also added to other materials, either to preserve it (for example wood), or to provide it with antimicrobial properties (for example certain plastics).
These biocides can be further sub-divided into those which have anti-fungal, anti-algal and anti-bacterial properties, namely fungicides, algicides and bactericides.
All biocides used in the EU are subject to the regulations of the Biocide Products Directive (BPD). Local regulations may apply elsewhere.
Biodeterioration of Liquid Coatings
The requirement of algae for sunlight means these organisms cannot grow in closed containers of liquid paint. Fungi and bacteria both require water and consequently neither can grow in solvent based coatings. This means that in-can biocides do not need to be added to solvent-borne products.
Water-borne coatings however are potentially prone to in-can attack by both bacteria and fungi. If these organisms develop, they can make the product unfit for use by causing a number of problems including:
- viscosity loss
- malodour
- discolouration
- gassing
- frothing
- sedimentation
- pH change
- potential health hazard
Biodeterioration of Applied Coatings
Fungi, algae and bacteria can all grow on applied paint films and solvent and water based coatings are both susceptible. The effect of each class of organism depends on the film environment.
Fungal Growth on Surface Coatings
Fungus (otherwise known as mould or mildew) requires no sunlight and only minimal amounts of water in order to grow. Consequently it will develop on any surface which is periodically moist and which can provide sufficient nutrients. In practice this means that, in the absence of a fungicide, all external coating films are liable to fungal colonization unless they are located in very dry situations.
The relatively dry conditions inside buildings do not favour fungal growth and that which does occur is normally restricted to paint surfaces which are periodically wet. Typically this means window frames and surfaces in kitchens and bathrooms where condensation forms.
Fungal growth on a paint film normally appears as dark spots on the surface and is sometimes confused with dirt. As well as disfiguring the surface, the organism can actually penetrate the film and make it more permeable to water. Growth can also develop under the film and cause loss of adhesion.
Fungal organisms reproduce by producing spores and these can be a significant health hazard to persons with respiratory allergies.
Algal Growth on Surface Coatings
In order to grow, algae need sunlight and a surface which can provide nutrients and which is almost permanently wet. This means that algal growth does not occur on interior paint films and is normally only seen externally on coatings in very wet environments. In situations outside the tropics, a typical example of a favoured environment would be a paint film in a shaded area of a building which was wetted by a leaking gutter.
Algal growth usually appears as an unsightly greenish slime, however orange, red and blue organisms also exist. The actual damage that the organism does to a surface is probably minimal but its presence is a visual indication of very high moisture levels. It is a particularly unwelcome sight on painted wood because the level of moisture present will almost certainly mean that the underlying wood is rotting.
Hygenic Coatings (Antimicrobial Coatings)
Bacterial growth on a surface coating is not visible to the naked eye and does not cause any significant damage to a paint film. Consequently, as a general rule, when paint manufacturers add antibacterial agents, they do so with the sole intention of preventing in-can degradation.
An important exception to this rule is a relatively new class of paints known as hygienic coatings or antimicrobial coatings. These are designed for use in locations such as food preparation areas and hospitals where it is important to limit the growth of bacteria on interior surfaces.
In areas where there is a high level of moisture in the atmosphere, bacteria can multiply rapidly. Even the very act of washing a wall produces a surface that, during the drying period, is susceptible to bacterial colonization. Hygienic coatings can provide protection under these conditions.
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