Paint Microbiology

film cross section

Introduction

The three main classes of microorganisms that can colonise paints are

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 two main classes:-

These biocides can be further sub-divided into those which have anti-fungal, anti-algal and anti-bacterial properties.

Bio-degradation of Liquid Coatings

The requirement of algae for sunlight means they 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 cause a number of problems including:-

Bio-degradation 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 Paint Films

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 paint 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 colonisation 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 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 Paint Films

In order to grow, algae need sunlight and a surface which can provide nutrients and 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 appears as an unsightly greenish slime. The actual damage that the organism does to paintwork 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.

Bacterial Growth on Paint Films

Bacterial growth on a paint film is not visible to the naked eye and it does not cause any significant damage to the film. Consequently, as a general rule, when paint manufacturers add anti-bacterial 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. 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 colonisation. Hygienic coatings can provide protection under these conditions.

Services

PRA provides a service specialising in all aspects of microbiology relating to the coatings industry, and is able to carry out a wide range of standard and in-house test methodology. These include:-

Test Methods

These test methods can be sub-divided into those applicable to liquid coatings and those for dry films. In addition to paints we can also test adhesives, boards, electrical and electronic equipment etc.

Microbial Resistance Testing of Liquid Paints

Test Standard
Bacterial Resistance In-house method
Bacterial Resistance ASTM D 2574
Fungal Resistance In-house method

Microbial Resistance Testing of Paint Films

Test Standard
Fungal Resistance BS 3900-G6
Fungal Resistance VdL-RL 06
Algal Resistance BBA MOAT No 33
Algal Resistance VdL-RL 07
Anti-bacterial Activity of Hygienic Coatings JIS Z 2801

Microbial Resistance Testing of Miscellaneous Materials

Test Standard
Fungal Resistance: Building Materials BS 1982 Part 3
Fungal Resistance: Ceramic Tile Adhesives BS 5980
Fungal Resistance: Wallpaper Adhesives BS 3046
Fungal Resistance: Electrical and Electronic Equipment BS 2011-2.1J, IEC 68-2-10
Fungal Resistance: Synthetic Polymers ASTM G 21
Fungal Resistance: General MIL-STD-810E

Further Information

Please contact Peter Collins for further details.

PRA also run the Hygienic Coatings Special Interest Group, and are currently planning a research project into Testing And Evaluation For Surface Hygiene Applications - contact Richard Holman for further information.