The Microstar Lab

Fungal and Bacterial Tests - ISO 846 Methods

Plastics – Evaluation of the action of microorganisms

Fungal Testing - ASTM G21 - Plastic Materials Testing

This method has 4 methods for determining the deterioration of plastics due to the action of fungi, bacteria, and soil microorganisms by visual examination, changes in mass, and/or changes in other physical properties. 

Method A: Fungal Growth Test

Samples are exposed to a mixed suspension of fungus spores in the presence of ≥95% relative humidity. The fungi can only grow at the expense of the test sample. If the sample contains no nutritive component, the fungi cannot develop and there will be no fungal growth or deterioration.

Method B: Determination of Fungistatic Effects

Samples are exposed to a mixed suspension of fungus spores in the presence of a complete nutrient medium (with carbon source). Even if the plastic does not contain any nutritive elements, the fungi can grow over the sample and their metabolic products can attack the material by metabolizing the nutrient-agar medium. Any inhibition of growth either on the plastic or in the nutrient-agar medium (zone of inhibition) shows fungistatic activity of the plastic or the presence of a fungicidal treatment.

Method C: Resistance to Bacteria

Samples are inoculated and incubated on an incomplete medium without a carbon source. If there is no growth in the agar surrounding the specimen, then the specimen does not contain any nutritive components.

Method D: Resistance to Microbially Active Soil (soil-burial test)

Samples are completely buried in natural soil with a known water-holding capacity and a specified moisture content. The soil-burial test has been included in this document because many plastics are used in permanent contact with soil and exposed to high humidity.

MicroStar Lab does not perform breakage testing and will only report on visual assessment of fungal growth. Samples can be sent to an outside lab for strength loss determination. MicroStar can also return items to the customer for testing or work with an outside lab.