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ASTM G21
Standard Method for Determining Resistance of Synthetic Polymeric Materials to Fungi
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Quick understanding of the test
ASTM G21 - Fungal Resistance Test
This test evaluates a material’s susceptibility to fungal growth to ensure its durability in microbial-prone environments.
Applicable to a wide range of materials, including plastics, rubber, paints, and more.
- Aspergillus brasiliensis
- Talaromyces pinophilus
- Chaetomium globosum
- Trichoderma virens
- Aureobasidium pullulans
- Test specimens and controls are placed on nutrient-salt agar and are inoculated with spore suspension.
- Samples are incubated at specified temperatures for up to 28 days.
- Specimens are rated on a 0–4 scale based on fungal coverage.
- Helps manufacturers to improve material formulations for better antifungal efficacy.
- Assists in meeting industry and safety standards for antifungal properties.
Turnaround Time
It typically takes 4-5 weeks.
The results are evaluated on a 0–4 scale based on fungal growth coverage.
Passing criteria
The passing criterion for ASTM G21 is achieving a rating of 0 or 1, indicating no or minimal fungal growth.
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Overview of ASTM G21 fungal resistance test
ASTM G21 is an international standard that describes the test method for determining the resistance of plastics and polymers to fungal growth. In general, synthetic polymers are resistant to the growth of fungi as they do not provide the nutrients required for their growth. However, the addition of certain substances such as plasticizers, lubricants, stabilizers, cellulose and colorants during the processing can promote fungal growth. Therefore, products that claim to be antifungal must be thoroughly tested to ensure their efficacy.
Why test for fungal resistance?
The growth of fungi on plastics enhances the rate at which they deteriorate, affecting the strength and performance of the material. This is a major concern for plastic products used in areas exposed to moisture and organic load that can stimulate the growth of fungi. Antifungal testing helps identify susceptible materials and allows manufacturers to improve their formulations, ensuring long-lasting products.
ASTM G21 test requirements
At Microbe Investigations Switzerland, the test is performed using the following bacterial strains: Staphylococcus aureus (ATCC 6538), Escherichia coli (ATCC 8739), Pseudomonas aeruginosa (ATCC 15442). Additional strains can be added on customer request.
ISO 21149 test method for aerobic mesophilic bacteria detection
Test specimens
The test requires specific specimens to be tested for fungal resistance:
- Standard specimens
Typically, specimens with dimensions of 50mm×50 mm are selected.
- Rod or tube specimens
Specimens with a minimum size of 76 mm are used.
- Film-forming materials
Films with dimensions of 50mm×25 mm (2 inches×1 inches) are selected.
- Differing surfaces
If a specimen has different surfaces, three specimens are tested each with different surface orientations (face up and face down).
ASTM G21 test methodology
Preparation and inoculation
- The test specimens are placed on the solidified surface of nutrient-salts agar and inoculated with a spore suspension. This setup mimics real-world conditions where fungi encounter the materials in a nutrient-rich environment.
- Untreated control samples are also inoculated and incubated alongside the test specimens to provide a baseline for comparison.
Incubation
- The inoculated samples are then sealed and incubated at a temperature range of 28°C to 30°C, and under conditions of 90% relative humidity.
- The standard incubation period is 28 days but the test may be terminated earlier if the test specimens show considerable mycelial growth, usually indicated by a growth rating of two or more.
Evaluation and rating
After the incubation period, the samples are rated on a scale of 0-4 as mentioned below:
| Rating Scale | Description |
| 0 | No growth on the specimen |
| 1 | Traces of growth on the specimen (less than 10 %) |
| 2 | Light growth (10 to 30 %) |
| 3 | Medium growth (30 to 60 %) |
| 4 | Specimens completely covered with growth (60 %) |
This rating scale provides a clear and quantifiable measure of the antifungal efficacy of the tested materials.
Compliance and industry standards for ASTM G21 testing
The ASTM standard is used by most industries globally to assess the resistance of materials to fungal growth. Compliance is usually required for materials used in construction, aerospace, marine, and electronics due to a high probability of these materials being exposed to conditions that favor fungal growth. Ensuring compliance with this standard ensures that materials meet the necessary requirements for durability and safety.
Importance of ASTM G21 fungal resistance test
Ensuring material longevity and durability
The test is highly useful in assessing the durability of any synthetic polymer as well as plastic material. Fungal growth on such products can greatly reduce their structural durability and visual appeal thus affecting the product’s integrity.
Enhancing safety and hygiene
Several industrial sectors including healthcare and food packaging, can be negatively affected by fungal presence. Fungal growths on medical devices, packaging material or any surface that comes into contact with foods lead to contamination and spread of infections. This test makes sure that the materials used for these vital applications will not support the growth of fungi hence increasing safety and sanitation. This is particularly important in sterile environments where even minimal fungal contamination can have severe consequences.
Supporting innovation and development
This standard is a good tool for research and development in material science. Understanding how different additives and formulations affect fungal resistance can help researchers develop new materials with unique, improved properties. This test provides key information necessary for the formulation of new plastics and polymers that have higher resistance to fungal degradation while at the same time being durable, biodegradable, and sustainable.
Versatility in application
This test is applicable to a wide range of materials and products. From coatings and films right through to tubes, rods and fabricated articles, the test can be applied to various types of synthetic polymers and plastics.
Enhancing consumer confidence
Quality assurance of products plays a significant role in determining the confidence levels of consumers. Hence it is crucial to understand that certification for fungal resistance testing helps build consumer confidence regarding the quality, safety, and durability of the product.
Supporting sustainable practices
The test contributes to sustainability by encouraging the use of strong and fungus-resistant products that help to avoid their rapid deterioration by fungi thus reducing regular replacements. This serves to reduce waste and conserve the available resources to the maximum possible extent.
Real-world applicability
The test method effectively simulates real-world conditions making it a reliable predictor of how materials will perform during actual use. This is particularly important for industries where the material can be exposed to environments that favor fungus growth such as the marine sector, construction, and healthcare sectors.
Strengths of ASTM G21 test method
The test is highly valued as it is simple to perform and is reproducible. It gives a clear and objective measurement of fungal resistance using standard procedures and organisms. Furthermore, the test accommodates a very wide range of materials, thereby making it applicable in a myriad of industries.
Conclusion: The role of ASTM G21 testing in modern industries
In modern industries where materials are exposed to humid environments, it has become important to ensure the fungal resistance of products. The ASTM standard provides a standardized method to evaluate how well materials resist fungal growth, thus assuring manufacturers of their product performance. The test also supports innovation, regulatory compliance, and sustainability efforts that are critical for businesses that aim to meet the demands of today’s markets.
At Microbe Investigations (MIS), we test the efficacy of antifungal plastic finishes using the ASTM G21 test and other standard test methods like AATCC-30. With a team of experienced microbiologists and advanced technological capabilities, MIS provides a comprehensive suite of quality antiviral, antibacterial and antifungal testing services worldwide.
Contact our experts here to learn about the pricing, turnaround times and other related queries.
Frequently Asked Questions
DR. Martinoz Scholtz
MIS Labs
1. What types of materials are tested under ASTM G21?
Plastics, rubber, paints, coatings, textiles, and other synthetic materials are some commonly tested materials.
2. What are the grading criteria in ASTM G21?
The standard uses a rating scale of 0 to 4. 0 indicates no growth (highly resistant), and 4 indicates extensive growth (very low resistance).
3. Can ASTM G21 be used to test coatings?
Yes, this standard is applicable to coatings, including those used on various substrates to enhance fungal resistance.
4. What are the limitations of this standard?
The standard assesses fungal growth by visual method and does not evaluate bacterial contamination or other impacts by microorganisms.
5. What is the significance of a '4' rating in this test?
A ‘4’ rating means high fungal growth in the material; that is, it possesses low resistance to fungi.
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