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ISO 846: 1997 – Evaluation of the action of microorganisms on plastics
ISO 846 specifies test conditions and requirements for the measurement of the resistance of plastics to microbial attack, specifically fungi, bacteria, and other soil microorganisms. Plastics generally are highly durable. However, they are not completely immune to avoid microbial degradation. Therefore, evaluating their resistance to microbial attack is crucial to ensure their durability.
Microbial colonisation on plastic surfaces is influenced by environmental factors such as temperature, humidity and composition of air. Generally, deterioration of plastics by microorganisms occurs in two ways:
- Direct action
In this process, plastics act as a nutrient source and stimulate the growth of microorganisms. Microbial growth over an extended time leads to physical destruction and degradation of plastic material.
- Indirect action
This includes metabolic by-products of microorganisms interacting chemically with plastic surfaces to bring about their breakdown.
These microbial attacks can remarkably alter the physical properties and functionality of plastics, leading to deterioration in service life and impairment in their performance. ISO 846 describes a test method to assess this breakdown by exposing non-porous plastic materials to selected microorganisms under conditions specified in guidelines and subsequent measurement of changes in mass, visual appearance and other physical properties.
It’s important to note that this standard does not test the biodegradability of plastics. The standard specifically tests the resistivity of plastics to microbial attack. Such information is useful for manufacturers in improving the durability and reliability of plastic products exposed to microbial environments.
Importance of ISO 846 Test
The ISO 846 test is essential for assessing the long-term performance of plastics in environments susceptible to microbial growth. By closely reconstructing practical usage scenarios, the test provides critical insights into the microbial resistance of plastic materials. This is important in industries where the degradation of plastics may lead to impaired safety, functionality and product service life. Through visual assessments and measurements of physical properties, the test ensures that plastic products meet the required standards for durability and performance in microbial-rich environments
Industries where ISO 846 test is relevant
ISO 846 tests can be applied to different industrial products that are used in high humidity and soil-rich environments. However, its applicability may expand to other industries too. Some of the relevant industries worth mentioning include:
- Construction industry
These industries rely on this test to assess the performance of plastics in building materials, insulation and structural components, where microbial growth may cause serious damage over time.
- Medical industry
Plastics used in medical devices, surgical instruments and packaging materials are often exposed to sensitive environments. Testing ensures that these materials do not support microbial growth and reduce contamination risks to the patients.
- Agriculture industry
Due to the constant contact with soil and high moisture levels, plastics in agricultural machinery, irrigation systems and wrappings are most susceptible to microbial colonisation. Testing plastics using this standard confirms that these materials can withstand such conditions, ensuring durability and reliability in harsh environmental settings.
- Consumer goods
From household containers to everyday plastic items, consumer products should be resistant to microbial growth to maintain hygiene and extend the life of a product. The standard helps manufacturers ensure that their products remain safe for consumers and functional in homes and other environments where microbial exposure is common.
- Electrical and electronics industry
Plastic parts used in electrical devices and appliances are usually exposed to heat and moisture conditions that are most favourable for microbial growth. Testing ensures that these plastics will not lose their insulating properties and structural integrity, preventing potential failures due to microbial deterioration.
Test organisms
At MIS, the ISO 846 test is generally performed using the following strains:
- Bacteria: Pseudomonas aeruginosa (ATCC 15442)
- Fungi: Aspergillus niger van Tieghem (ATCC 6275), Penicillium funicu/osum Thom (CMI 114933), Paecilomyces variotii Bainier (ATCC 18502), Gliocladium virens Miller et al. (ATCC 9645), and Chaetomium globosum Kunze: Fries (ATCC 6205).
ISO 846 test methods
- Method A: Fungal-growth test
In this method, plastic test specimens treated with antimicrobial treatments are exposed to a mixed suspension of fungal spores with insufficient nutritive medium (no carbon source) under conditions of ≥95% relative humidity. The fungi grow only if the plastic acts as a nutrient source. Suppose the material does not contain nutritive components; the fungal mycelium does not develop, preventing plastic degradation. The test is applicable for comparing the inherent resistance of plastics to attack by fungi in the absence of other organic materials.
- Method B: Determination of fungistatic effects
Method B involves exposing treated plastic specimens to fungal spores on a complete nutrient medium, providing a carbon source for the fungi. Even when the plastic does not contain nutritive elements, fungi can grow on the surface because of the nutrient medium, possibly attacking the material through their metabolic products. The test determines whether the plastic possesses any fungistatic or fungicidal properties by comparing growth between biocide-treated and nontreated specimens.
- Method C: Resistance to bacteria
This test assesses the plastic materials’ resistance to bacteria in an incomplete medium without a carbon source. If no bacterial growth occurs in the agar around the specimen, the plastic lacks nutritive components.
- Method D: Resistance to microbially active soil (soil-burial test)
This test method evaluates plastics which are in contact with soil for long-term periods, such as those used in outdoor applications. Test specimens are buried in microbially active soil of known water-holding capacity and moisture content. The aim is to determine the resistance of the plastic material to microbial attack during an extended period of exposure to high humidity and soil microbiota.
In addition to ISO 846 microbial testing for plastics, MIS performs other testing methods such as ISO 22196, JIS Z 2801, ASTM G21 and ISO 21702 to check the antimicrobial efficacy of treated plastic materials.
Benefits of testing
- Regulatory compliance
The standard helps manufacturers meet industry standards and regulatory requirements for plastics to ensure the product meets various global expectations on safety and durability especially in industries such as construction, medical and consumer goods.
- Market advantage
Plastics which successfully demonstrate resistance to microbial attacks gain a competitive advantage in the market. Manufacturers can promote their products as durable and microbial-resistant, attracting customers, especially for industries where long-lasting performance is a necessity.
- Product development
By identifying potential issues in plastic formulations, testing provides valuable data for improving material composition. This aids manufacturers in making their products stronger and more resistant to degradation via microbial activity.
- Consumer trust
Tested products instil confidence in consumers by confirming that the tested products will remain safe, hygienic and functional over time. This is very important for plastics used in medical devices, packaging and food-related applications.
- Environmental impact
The standard indirectly helps reduce waste and environmental impact by guaranteeing product durability and microbial resistance, as products do not break down prematurely and end up in landfills.
- Brand reputation
Customers perceive companies operating according to ISO standards as a reliable and responsible business partner in both B2B and consumer markets, thus enhancing their brand reputation.
Guidelines to follow for conducting ISO 846 Test
Specimen preparation
- Ensure plastic test specimens are clean and of the proper size (typically 50 mm x 50 mm) and shape as specified by the particular test method.
- Prepare test specimens in sufficient numbers to meet the requirements for all test methods being conducted (fungal, bacterial, soil-burial).
Selection of microorganisms
- Choose appropriate test organisms based on the environmental exposure of the plastic (e.g., fungi like Aspergillus niger or bacteria like Pseudomonas aeruginosa).
- Testing shall be performed with microorganisms from recognised culture collections, e.g. ATCC or equivalent in order to ensure standardised testing.
Testing conditions
- Maintain controlled environmental conditions during testing, such as temperature (typically 29°C) and relative humidity (≥95%) for fungal growth tests.
- In the case of the soil burial tests, the soil shall possess suitable water holding capacity and moisture content so as to simulate realistic environmental exposure.
Exposure time
- Test specimens should be exposed to microorganisms for a specified period of time, typically determined by the standard and application requirements (often weeks to months).
Visual and physical assessments
- After the exposure period, conduct a visual examination of the specimens to assess microbial growth or discoloration.
- Change in physical properties like mass loss, surface gloss flexural properties and hardness must be measured to determine the amount of deterioration.
Use of controls
- Always include control specimens, such as stainless steel discs, to validate test results by showing the extent of microbial growth in comparison to the plastic material.
- Use positive and negative controls (biocide-treated and untreated specimens) where applicable to determine fungistatic or antibacterial efficacy.
Test documentation
- Thoroughly observe the type of microbial growth, degree of physical changes and any other variations between treated and untreated test pieces.
- Prepare a standard test report in compliance with the requirements of ISO including test methods, microorganisms used, and results obtained.
At Microbe Investigations Switzerland (MIS), we offer comprehensive antimicrobial testing services for products claiming microbial protection, including textiles, disinfectants and coatings. Our advanced microbiology testing facility enables precise evaluation of antimicrobial efficacy, ensuring compliance with international standards.
For more information on ISO 846 testing or to explore our antimicrobial plastic testing services, contact us today.
FAQs
ISO 846 is an international standard that provides test methods for determining the resistance activity of plastics to microbial attacks.
Non-porous plastic materials such as roofing membranes, storage containers, vinyl siding, medical devices and ceramics can be tested using this standard.
It takes approximately 28 days to complete the test, depending on the test organisms.
At Microbe Investigations, we perform the test using Pseudomonas aeruginosa (ATCC 15442) and Aspergillus niger (ATCC 16888). Other strains can also be added as per the customer’s request.
The test helps in determining the resistance of plastics to microbial degradation when they are exposed to fungi, bacteria and microorganisms present in soil. It also helps maintain product durability and achieve compliance with regulatory standards.
The standard consists of four methods: Method A for testing fungal resistance; Method B for assessing fungistatic effect; Method C for testing bacterial resistance; and Method D, a soil-burial test.
A high resistance rating would suggest that the material possesses great durability to microbial growth, promoting longer product life and ensuring that regulatory requirements are adequately met.
Temperature affects the growth rate of microorganisms and also the rate of degradation of plastics. Therefore, it is necessary to conduct the tests at specified temperatures for consistent and reliable results.
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