Fungi is not able to break down the main resin component in synthetic polymers to use as a source of carbon (sugar) for growth, rather they use other additives in the synthetic polymers such as plasticizers, cellulose, lubricants, stabilizers and colorants as a short-term source of energy that could eventually enable the breakdown of resin. As fungi grows by breaking down the easily digested ingredients, acids are produced as by-products that break down the resin into a more sustaining source of carbon (sugar) for stable growth.
ASTM G21-15 measures the resistance of synthetic polymeric materials to the growth of fungi. The suitable test samples are placed on a specific medium for fungi, Sabouraud dextrose agar, which is previously poured and solidified in sterile petri-dishes forming an agar layer. A concentrated spore suspension containing the fungi is evenly pipetted onto the surface of the samples and covered. The plates are then incubated at 28 to 30°C at >85% relative humidity for 21 days to give desired conditions for possible growth of fungi.
The sample plates are examined after 14 days and 21 days to assess the results according to the macroscopic visual rating system of fungal growth as follows:
|Observed Growth on samples||Rating|
|Traces of growth (less than 10 %)||1|
|Light growth (10 to 30%)||2|
|Medium growth (30 to 60%)||3|
|Heavy growth (60% to complete coverage)||4|
When there is no macroscopic growth after 21 days of incubation, the samples’ surface is further examined under a light microscope for microscopic fungal growth.
In an environmental setting, the synthetic polymer would be exposed to different climatic changes and fungal species that may affect it with different modes of attack. ASTM G21 – 15 is run in a laboratory setting providing ideal growth conditions for fungi and allowing control over conditions which may otherwise interfere and mask the performance of the materials. This enables the samples’ resistance to fungal attack to be measured effectively. Therefore, ASTM G21- 15 offers a realistic alternative to assess whether a product has significant fungal growth resistance.