At NEDC, fungus resistant gaskets are one of the more specialized product lines we cut, but they come up more often than you might think. Military equipment deployed in tropical environments, marine electronics housings on vessels that spend months in humid conditions, outdoor telecommunications enclosures in the Southeast, all of these applications share a common problem: fungal growth on gasket materials. In this post, I will walk through why fungus resistance matters in military and marine environments, how ASTM G21 testing works, and which materials we recommend. For our full product overview, see our fungus resistant gaskets pillar page.
Why fungus resistance matters in military applications
In military applications, fungus resistance is driven by MIL-STD-810 Method 508, the standard test method for evaluating fungus resistance of military equipment. The concern is real: military equipment gets deployed in tropical and subtropical environments where humidity is consistently high and temperatures stay warm enough for fungal growth year-round.
Fungal growth on a gasket causes three specific problems in military equipment. First, the fungus can physically degrade the gasket surface, creating leaks in what was supposed to be a sealed enclosure. Second, in optical systems and sensor housings, fungal growth can cloud lenses and affect the performance of the equipment. Third, in electronic enclosures, fungal growth can trap moisture against circuitry and, in some cases, create conductive pathways that cause short circuits. For military grade gasket requirements, fungus resistance is often specified alongside other MIL-STD requirements. A mil-spec gasket that passes MIL-STD-810 Method 508 gives designers confidence that the material will not support fungal growth over the life of the equipment. For EMI shielding applications requiring fungus resistance, MIL-DTL-83528F gaskets may also be specified.
Marine environments and fungal growth on gaskets
Marine environments are particularly harsh for gasket materials because they combine high humidity, salt spray, prolonged moisture exposure, and temperature cycling. These conditions create an ideal environment for fungal growth, especially on elastomers that contain plasticizers or organic additives that fungi can use as a food source.
In marine applications, the gasket is often the only barrier between the interior of an enclosure and the corrosive marine environment. If the gasket material supports fungal growth, the fungus can degrade the seal over time, allowing moisture and salt to penetrate the enclosure. Once that happens, corrosion of internal components is not far behind. For marine fungus resistant gaskets, EPDM gaskets are a common choice because EPDM has excellent water, UV, and ozone resistance and certain compounds pass ASTM G21. Neoprene gaskets are another option, though their fungus resistance depends on the specific compound formulation.
ASTM G21 testing for fungus resistant gaskets
ASTM G21 is the standard test method for determining the resistance of synthetic polymeric materials to fungi. The test works like this: gasket material samples are exposed to five specific fungal strains (Aspergillus niger, Penicillium pinophilum, Chaetomium globosum, Gliocladium virens, and Aureobasidium pullulans) for 28 days under conditions designed to maximize fungal growth, which means high humidity and a temperature around 85F.
After the 28-day exposure, the samples are rated on a scale of 0 to 4:
- 0: No growth (the material is fully fungus resistant)
- 1: Trace growth (less than 10% coverage)
- 2: Light growth (10-30% coverage)
- 3: Medium growth (30-60% coverage)
- 4: Heavy growth (60-100% coverage)
A rating of 0 or 1 is generally considered passing for fungus resistant gasket applications. The key point is that the material itself must not provide a food source for the fungi. Synthetic polymers like silicone, EPDM, and certain urethane formulations are naturally resistant because they do not contain the organic compounds that fungi metabolize.
Materials for fungus resistant gaskets
The most common materials we cut for fungus resistant gaskets are:
- EPDM (solid and foam): Excellent water, UV, and ozone resistance. Certain EPDM compounds pass ASTM G21 with a rating of 0. Widely used in outdoor and marine applications.
- Neoprene: Good all-around elastomer with moderate fungus resistance depending on the compound. Often specified in military applications where MIL-STD-810 Method 508 compliance is required.
- PORON 4701 series urethane foam: PORON urethane foam products have been tested to ASTM G21 and certain grades pass with a rating of 0 or 1. PORON provides both fungus resistance and excellent compression set resistance, making it ideal for military and marine enclosure gaskets.
- Silicone: Naturally fungus resistant because silicone polymers do not provide a food source for fungi. Silicone also offers a wide temperature range and UV resistance.
For applications requiring both soft compression and fungus resistance, soft gaskets in PORON or silicone foam are good options. For applications requiring both low temperature performance and fungus resistance, see our low temperature gaskets page.
Specifying fungus resistance in your gasket requirements
When specifying a fungus resistant gasket, there are two common ways the requirement gets called out:
Z suffix in ASTM D2000: In the ASTM D2000 specification system for rubber materials, the Z suffix indicates a special requirement. When fungus resistance is needed, the Z suffix is used with a reference to ASTM G21 and the required rating (typically 0 or 1).
Z suffix in ASTM D1056: Similarly, in ASTM D1056 (the cellular elastomer specification), the Z suffix calls out fungus resistance for sponge and foam materials. This ensures that the foam or sponge grade you specify has been tested and meets the fungus resistance requirement.
The key is to specify the exact standard, the test method, and the required rating. A vague callout like “fungus resistant” without a test standard leaves room for interpretation, while “ASTM G21, Rating 0” is unambiguous.
More information on fungus resistant gaskets
If you need fungus resistant gaskets for a military or marine application, or if you need help specifying the right material and test standard, contact us at sales@nedc.com. We cut fungus resistant gaskets from EPDM, Neoprene, PORON urethane foam, and silicone, and we can provide ASTM G21 test data and material recommendations for your specific environment.
FAQ
What is ASTM G21 testing for fungus resistant gaskets?
ASTM G21 is the standard test method for determining the resistance of synthetic polymeric materials to fungi. Samples are exposed to five fungal strains for 28 days at high humidity and warm temperature. After exposure, samples are rated 0 (no growth) to 4 (heavy growth). A rating of 0 or 1 is considered passing for fungus resistant gasket applications.
What is MIL-STD-810 Method 508?
MIL-STD-810 Method 508 is the military standard test method for fungus resistance. It evaluates whether materials and equipment can withstand fungal growth in tropical and subtropical deployment environments. The test exposes samples to fungal spores under controlled humidity and temperature for 28 days, then evaluates growth and physical degradation. Passing this test is a requirement for many military equipment specifications.
What gasket materials pass ASTM G21?
Common gasket materials that pass ASTM G21 testing include EPDM (solid and foam), Neoprene, certain silicone formulations, and select PORON urethane foam products. The specific performance depends on the compound formulation, so it is important to verify that the exact material grade has been tested. EPDM and PORON are the most commonly specified materials we cut for fungus resistant gasket applications.
How is fungus resistance called out in gasket specifications?
Fungus resistance is typically called out using the Z suffix in ASTM D2000 (for solid rubber) or ASTM D1056 (for cellular elastomers). The Z suffix indicates a special requirement, and it is paired with a reference to ASTM G21 and the required rating (typically 0 or 1). For example, a specification might read “ASTM D2000 M2BC510A14B14E014Z1 – ASTM G21 Rating 0.” This ensures the material has been tested and meets the fungus resistance requirement.