ASTM G21

Fungus Resistance

ASTM G21; MIL-STD 810; IPC-SM-840; IPC-TM-650

Many products that operate in warm, humid environments must be tested to withstand fungal attacks, since this problem can cause myriad of operational challenges. To varying degrees, live fungi will use wood, paper, leather, hydrocarbons, PVC, polyurethanes, certain plastics and paints, along with other materials, as fuel for growth.

The Fungus Resistance test is used to determine the resistance of key materials to fungi, and to determine if these materials are adversely affected by fungi under conditions favorable for their development… namely, high humidity, warm atmosphere, and presence the of inorganic salts.

Types of Fungal Attacks

Fungus can disrupt systems and cause damage in several ways, including:

  • Breaching protective covers to mar the appearance and degrade the optical capabilities of your product
  • Shedding metabolic waste products that cause corrosion or etching
  • Prompting electrical systems to develop undesirable conducting paths
  • Blocking delicate moving parts
  • Introducing internal moisture, which can affect system performance

Fungus Testing

Test methods are used to determine the effect of fungi on the properties of synthetic polymeric materials in the form of molded and fabricated articles, tubes, rods, sheets, and film materials. The test can reveal if the specimen being analyzed supplies sufficient nutrition for fungal growth. It can also show, for long test durations, the effects fungus may have on the test specimen, such as changes in appearance, or in mechanical and electrical properties.

NTS engineers are extremely careful handling samples for fungus testing, as the cleanliness of the specimens is important prior to testing – especially testing being conducted on a cleaning or decontamination process. Contamination can have a negative effect on the test results since it could easily provide nutrients to the fungus for growth. In order to minimize contamination, good practices include not removing the test specimens from the packaging until it is time for inoculation. If handling is necessary gloves, should be worn while handling test specimens. If the samples are mishandled or suspected of contamination prior to testing, they should be cleaned, Testing cannot be conducted on specimens until 72 hours have passed.

Spores are grown for approximately 2 – 3 weeks, followed by inoculation of the spore suspension (the combined spores that are selected for testing) and placed in a test chamber for 28 days (or as specified). The fungus results are usually determined and recorded as a growth rating:

  • “0” is no growth
  • “1” is <10% observed growth
  • “2” is 10 to 30% or 100% cobwebby growth (white growth) observed growth
  • “3” is 30 to 60% observed growth
  • “4” is greater than 60% observed growth

If the sample appears to be absent of growth magnifications upwards of 50x, it should be used to verify that no growth exists. There could be fungal spores on the samples that are not readily visible to the eye due to the size of the spores, localized areas of growth, or because the color of the spores are similar to those of the test specimen.

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