Testing Microbial Resistance of Paint Films

These test methods are designed to determine the fungal or algal resistance of paints, varnishes and lacquers after application. They are applicable to coatings intended for use on wood, metal, plaster and building boards.

Fungal Resistance Testing: BS 3900-G6

Scope and Field of Application

This test method is designed to determine the fungal resistance of paints, varnishes and lacquers after application. It is applicable to coatings intended for use on wood, metal, plaster and building boards.

There are no pass/fail criteria defined in the standard. This is a matter of agreement between the parties concerned.

The actual standard is a copyright-protected document and we are not able to provide you with a copy. If required however, you can easily obtain one from the British Standards Institution.

Summary of Method

The standard describes the preparation of the chosen substrate and the application of the coating. For exterior coatings the prepared panels can be conditioned prior to testing, such as by leaching in running water or using artificial weathering. The test method described here is based on BS 3900-G6. The main differences are that we use a different control paint than the one specified, re-inoculation after 7 days and a shorter incubation period.

A control paint is selected which is susceptible to mould growth, its purpose being to check that the inoculum is viable and that the incubation conditions support mould growth.

A mixed spore suspension is prepared from ten fungal species (additional optional species may be included) and used to spray inoculate the coated panels. At this stage the severity of the test can be increased, if required, by applying a soiling medium to the inoculated test panels. The panels are then placed in fungal test cabinets (ie. humidity chambers operating on a time cycle to give 2 hours in every 12 hours at 4 °C above ambient).

After 7 days incubation, the painted panels are re-inoculated as above and incubation continued. Growth of fungi on the panels is monitored by microscopic examination after 28 and 42 days, with a final growth assessment being made 56 days after inoculation.

For assessment the standard defines a six-point scale ranging from 0 (no growth) to 5 (more than 70% of the coating covered by fungal growth). A control paint rating of 5 at the 56 day point indicates that fungal growth conditions in the test cabinet are optimal.

Sample Requirements

We require a 500 ml sample.

Fungal Resistance Testing: VdL–RL 06

Scope and Field of Application

This test method is designed to determine the resistance of coating materials to fungal growth. It can be used to test interior or exterior paints, stucco and plaster. Filter paper is normally used as the support substrate. Alternative substrates include wood, calcium silicate blocks and plastics.

In March 2005, VdL–RL 06 was in draft form awaiting approval. It is a copyright-protected document and we are not able to provide you with a copy. If you require a copy you should contact Verband der Lackindustrie e.V. (VdL), (Association of the Varnish Industry), Frankfurt/Main.

Summary of Method

The coating material is applied aseptically to a sterilized filter paper disc and conditioned for at least five days at specified temperature and relative humidity conditions.

Exterior coatings and interior coatings for wet rooms are leached in water and then dried for at least 3 days at specified temperature and relative humidity conditions. The method allows for the use of accelerated weathering in place of leaching.

The filter paper discs are placed on agar in Petri dishes and inoculated with a mixed inoculum prepared from two fungal species.

The test pieces are then incubated for 3 weeks at 28°C and rated every seven days using a 5-point scale ranging from zero to 100% fungal growth.

The method states that an effective fungicidal coating should show no growth in the absence of leaching and no more than 10 % growth after leaching.

Sample Requirements

We require a 100 ml sample.

Algal Resistance Testing: MOAT No 33

Scope and Field of Application

Method of Assessment and Test (MOAT) No. 33 "The Assessment of Masonry Coatings" is published by the British Board of Agrément (BBA) and includes a number of tests for masonry coatings.

The test to determine resistance to algal growth was developed by PRA. Originally it required the use of cylindrical test specimens and a PRA algal test cabinet. This has been simplified and the procedure now utilizes flat test specimens without the need for a specialized test cabinet.

There are no pass/ fail criteria defined in the standard. This is a matter of agreement between the parties concerned.

MOAT No. 33 is a copyright-protected document and we cannot provide you with a copy. If required however, you can easily obtain one from the BBA.

Summary of Method

The test paint is applied to a panel cut from a calcium silicate building board. A PRA control paint known to be susceptible to algal growth is similarly applied. After drying, the test and control panels are leached in running water for 150 hours.

A mixed inoculum of six algal species is applied to the painted surface of the panels which are then incubated in small humidity chambers over water saturated vermiculite and under 'daylight' fluorescent lamps.

All test pieces are re-inoculated after 7 days incubation and assessed after 28 days and 42 days, with a final growth assessment being made 56 days after inoculation. At each assessment the panels are rated using a 5-point scale ranging from no growth to growth covering more than 70% of the surface. A rating of 5 for the control paint demonstrates that the inoculum was viable and that the exposure conditions favoured growth.

Sample Requirements

We require a 500 ml sample.

Algal Resistance Testing: VdL–RL 07

Scope and Field of Application

This test method is designed to determine the resistance of coating materials to algal growth. It can be used to test exterior coatings, interior coatings for wet rooms, stucco and plaster. Filter paper is normally used as the support substrate. Alternative substrates include wood, calcium silicate blocks and plastics.

In March 2005, VdL–RL 07 was in draft form awaiting approval. It is a copyright-protected document and we are not able to provide you with a copy. If you require a copy you should contact Verband der Lackindustrie e.V. (VdL), (Association of the Varnish Industry), Frankfurt/Main.

Summary of Method

The coating material is applied to a sterilized filter paper disc and conditioned for at least five days at specified temperature and relative humidity conditions.

Exterior coatings and interior coatings for wet rooms are leached in water and then dried for at least 3 days at specified temperature and relative humidity conditions. The method allows for the use of accelerated weathering in place of leaching.

The filter paper discs are placed on agar in Petri dishes and inoculated with a mixed inoculum prepared from two algal species.

The test pieces are then incubated for 3 weeks at 23°C under daylight fluorescent lighting (16 hours illumination/8 hours darkness and rated every seven days using a 5-point scale ranging from zero to more than 75% algal growth.

The method states that an effective algicidal coating should develop no growth or only very slight growth.

Sample Requirements

We require a 100 ml sample.

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