The ASTM G21 is a qualitative test that employs a high concentration of spores, from five different fungal species, to determine the resistance of synthetic polymeric materials to fungal growth.
Fungi are generally not able to breakdown the main resin component of synthetic polymer to use as a source of carbon (sugar). However, many additives, such as plasticizers, cellulose, lubricants, stabilizers and colorants that are added to impart desirable characterises, are very susceptible to microbial growth.
Once fungi become established by breaking down the easily digested ingredients, the acids they produce as by-products of growth breakdown the resin components into a more useable food source.
A good analogy is how lighter fluid is used to successfully set charcoal on fire. The additives act as a quick burst of energy that unlocks the stored power in the polymer resin.
The ASTM G21 was first released in 1990 as an official method to test the resistance of synthetic polymers to fungal attack.
The current standard (ASTM G21-15) is the sixth version of this test method. Previous versions are listed below:
Examples of polymers tested using the ASTM G21 include PE, PP, PU, PVC, TPU, TPO, PES, ABS, PC, rubber, etc. They may be in the form of films, sheets, molded materials, coatings, foam, tubes, pipes, pucks, tiles, plaque, rods, etc.
Other materials, such as paint, dry wall, cardboard, fibers and adhesives may also be tested using the ASTM G21 method.
Test samples are placed on an agar medium that is carbon-free (no readily available energy source) but contains the basic mineral salts required for fungi to grow. Having a carbon-free medium forces the fungi to grow on the test specimen and not on the surrounding agar surface.
After being placed on the mineral salts medium, the test sample is inoculated with a concentrated spore suspension (inoculum) that contains five different fungal species.
These fungi were chosen by the ASTM based on their affinity for using plasticisers, cellulose, lubricants, stabilizers and colorants as food sources.
Test plates are incubated at 28C at 90% relative humidity for 28 days to allow any possible fungal growth to develop. The test samples are examined every seven days to monitor progress.
The samples are examined under a dissecting microscope at 40x magnification. A rating score from 0 to 4 is given based on the amount of growth that exists. The description of the rating system is as follows:
0 = Specimen remained free of fungal growth.
1 = Traces of growth on specimen (less than 10%).
2 = Light fungal growth on specimen (10 to 30%).
3 = Medium fungal growth on specimen (30 to 60%).
4 = Heavy fungal growth on specimen (60% to complete coverage)
The ASTM G21 does not outline pass/fail criteria. However, the test method may be terminated earlier for specimens that are experiencing a rating of “2” or higher.
The untreated vinyl sample supports heavy amounts of fungal growth (rating of “4”). The vinyl sample, treated with an antimicrobial agent, remains free of fungal growth (rating of “0”) after 28 days of incubation, as per the ASTM G21 test specifications.
The desired test specimen size is a 50 mm/2-inch square, 50 mm/2-inch diameter circle or a piece that is at least 76 mm/3-inches in length. Before submitting samples for the first time, contact your chosen test lab for more information and guidance.
The cost for testing specimens using third party laboratories varies widely from lab to lab.
In North America, there are test facilities that charge as low as $175 USD per sample and go up as high as $530 USD for the first sample ($340 USD for each additional sample).
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