The effects of fungal growth on building products, paper and textiles have been a major concern of manufacturers for over 60 years. In recent years, the marketplace has been inundated with fungal resistant coatings and products designed to inhibit fungal growth. Several related industry organizations have developed testing standards for antifungal performance testing.
Fungi grow in the form of multicellular filaments called hyphae. A connected network of these tubular branching hyphae is referred to as a colony or a mycelium. They secrete hydrolytic enzymes, mainly from the hyphal tips. These enzymes degrade complex biopolymers such as starch, cellulose and lignin into simpler substances which can be absorbed by the hyphae. In this way, fungi play a major role in causing decomposition of organic material, enabling the recycling of nutrients throughout ecosystems. Fungi may also degrade man-made polymer additives such as plasticizers, cellulosics, lubricants, stabilizers and colorants contained in organic coatings and plastics and other building products.
Fungi are grouped into four Phyla:
1: The Ascomycota Phylum contains 25,000 species, including Aspergillus, Penicillium, Trichoderma, Aureobasidium and, Chaetomium. These fungi decompose cellulose (wood, paper and paperboard), textiles, paint coatings, plastics, insulation and leather, and are, therefore, employed in most of the ASTM Standard Test Methods.
These fungi which produce “fuzzy” mycelial colonies on organic matter are frequently referred to as “Molds.”
The 1,500 species of yeasts belong to this phylum also. They are microscopic fungi growing as single cells and do little damage to building products as their main source of carbon is obtained from hexose sugars such as glucose and fructose, or disaccharides such as sucrose and maltose.
2: The Basidiomycota Phylum contains 25,000 species of saprophytic mushrooms which obtain nutrients from decaying organic matter.
3: The Zygomycota Phylum contains 600 species, including bread mold.
4: The Imperfect Fungi Phylum contains 25,000 species which do not fit into the other Phyla and are grouped together for convenience. Many of these fungi are parasitic plant pathogens.
The following standards have been published for testing of materials for resistance to fungi:
1: ASTM G 21 – Determining Resistance of Synthetic Polymeric Materials to Fungi.
2: ASTM C 1338 – Fungi Resistance of Insulation Materials and Facings.
3: ASTM D 2020 – Standard Test Methods for Mildew (Fungus) Resistance of Paper and Paperboard.
4: ASTM D 5590 – Standard Test Method for Determining the Resistance of Paint Films and Related Coatings to Fungal Defacement by Accelerated Four-Week Agar Plate Assay.
5: ASTM D 3273 – Standard Test Method for Resistance to Growth of Mold on the Surface of Interior Coatings in an Environmental Chamber.
6: ASTM E 154 – Resistance to Deterioration from Organisms and Substances in Contacting Soil.
7: ASTM D 4300 – Standard Test Methods for Ability of Adhesives to Support or Resist the Growth of Fungi.
8: ASTM D 4578 – Standard Method for Mold Growth Resistance of Blue Stock (Leather).
9: MIL-STD 810G - Method 508.6. Determines whether fungus will grow on a device exposed to warm, moist air in the presence of fungus spores?
10: TAPPI T 487 – Fungus Resistance of Paper and Paperboard.
11: AATCC 30 – Antifungal Activity, Assessment on Textile Materials: Mildew and Rot Resistance of Textile Materials.
12: ISO 16869 – Assessment of the Effectiveness of Fungistatic Compounds in Plastics Formulations.
13: ASTM D 6329 - Standard Guide for Developing Methodology for Evaluating the Ability of Indoor Materials to Support Microbial Growth Using Static Environmental Chambers
The ASTM G-21 and TAPPI T 487 procedures require a composite fungal spore suspension to be sprayed upon the sample materials with an atomizer while the samples are horizontal in a nutrient growth agar deficient in a carbohydrate source. Therefore, no organic carbon nutrients are available. Photographic images are taken each week of the four-week exposure. The fungal spores may grow on the sample only if they are capable of extracting organic carbon from the sample.
The ASTM D 2020, ASTM D 4300, ASDM D 4578 and ASTM 5590 procedures call for the specimen on a PDA (Potato Dextrose Agar) petri dish to be sprayed with a composite of fungal spores. The specimens in the environmental chamber are thus surrounded with heavy fungal growth in each petri dish for 28 days to test for antifungal activity and leaching of the anti-fungal components into the PDA agar.
The ASTM D-3273 standard is for fungal growth in soil, contained in the humidified environmental chamber. Air circulation within the environmental chamber circulates the fungal spores in the chamber to the vertically suspended samples four inches over the soil. Weekly photographic images of each sample are emailed to customers so they can follow the progress of the testing.
The ASTM E-154 standard requires sample submersion in soil containing fungal spores and bacteria. This three-month exposure inside the environmental chamber exposes building materials to bacterial and mold attack. Each month, the weight of the samples is checked to determine which samples are losing weight due to degradation by soil bacteria and fungi.
The MIL-STD 810 involves spraying the test items with a composite of fungal spores and supporting the test items in the environmental chamber for 28 days.
The test Standards call for specific fungi to be used for resistance testing. The ATCC (American Type Culture Collection) identification numbers are provided in each test method:
Microbac Laboratories, Inc. is prepared to analyze your antifungal performance standards and assist you in developing products with superior fungal resistance. The Venice (FL) Division of Microbac Laboratories maintains pure fungal cultures and performs several of the accelerated ASTM, MIL-STD, TAPPI, AATCC and ISO Standard Tests for fungal growth in humidity and temperature-controlled environmental chambers. Antifungal performance validations are valuable to customers who develop, sell, and use products which may be subject to attack by fungi.