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ASTM G21
Standard Method for Determining Resistance of Synthetic Polymeric Materials to Fungi
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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
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).
Test organisms
Aspergillus brasiliensis, Aureobasidium pullulans, Chaetomium globosum, Trichoderma virens, Penicillium funiculosum are the test organisms. These fungi are chosen due to their prevalence in various environments and their ability to grow on a wide range of materials including materials containing plasticizers, cellulose, lubricants, stabilizers, and colorants as food sources.
ASTM G21 test methodology
Preparation and inoculation
- The test specimens are placed on the solidified surface of nutrient salt 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.
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.
FAQs
ASTM G21 is a standard test method that is used to qualitatively assess the resistance of synthetic polymer materials to fungal growth.
A wide range of materials can be tested using this standard. These include but are not limited to molded and fabricated articles, tubes, rods, coatings, pipes, marine plastics, sheets and film materials. This wide range of applicability makes it an important test for various industries.
It takes approximately 4-6 weeks to complete this test. This period allows for the thorough incubation and growth of the test specimens thus ensuring accurate and reliable results.
At MIS, Aspergillus niger (ATCC-16888) is tested for ASTM G21. However, additional strains like Penicillium funiculosum, Chaetomium globosum, Gliocladium virens, Aureobasidium pullulans can be included based on the customer’s specific testing requirements.
To prepare the fungal spore suspension, streak the fungal culture on the agar medium and allow it to sporulate. Harvest the spores by flooding the Petri dish with sterile water or saline and filter the suspension through sterile gauze or cheesecloth to remove the mycelial fragments. Dilute the spore concentration by adding sterile water or saline followed by enumeration.
This test includes temperature and humidity parameters which usually range between 28-30°C and the humidity of around 90-100%. Samples are incubated at 28 days and test materials are exposed to a spore suspension of the selected fungi.
This test is used to measure the ability of synthetic polymeric materials to resist fungal growth using a standard fungal culture which is known to decompose plastics. It sets specific conditions of temperature, humidity, and incubation time, and visually evaluates the degree of fungal growth on a scale, which defines material resistivity.
This test identifies fungal susceptibility to improve formulations for better material durability and longevity and ensures quality assurance through standardized methods. This shall enable manufacturers to enhance their product quality and consistently ensure improved quality of their products.
Product samples for testing can be prepared by cutting the samples into the required size—usually 50×50 mm, cleaning them, labeling them with appropriate identification marks, and then placing them inside a sterile container so as not to contaminate them. Detailed documentation of material specifications and handling instructions is to be provided, together with test requirements.
Plastics, rubber, paints, coatings, textiles, and other synthetic materials are some commonly tested materials.
The standard uses a rating scale of 0 to 4. 0 indicates no growth (highly resistant), and 4 indicates extensive growth (very low resistance).
Yes, this standard is applicable to coatings, including those used on various substrates to enhance fungal resistance.
The standard assesses fungal growth by visual method and does not evaluate bacterial contamination or other impacts by microorganisms.
A ‘4’ rating means high fungal growth in the material; that is, it possesses low resistance to fungi.
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