NSF’s in-house compostability testing - an incubator with samples hooked up to automatic gas analyzer.

Today’s consumer is more conscientious than ever of their ecological footprint. One way companies are capitalizing on this trend is by offering plastics labeled as “earth friendly,” “biodegradable” and “green plastic,” just to name a few. These compostable plastics are now widely available in the marketplace, present either in the form of packaging materials such as film, foams, and grocery bags; or in the form of finished products such as cups, plates, forks and water bottles.

Some states have even passed legislation that requires all plastic store shopping bags to be eco-friendly. However, many of these labels were placed on plastics that are, in fact, not biodegradable. This type of misuse of eco-friendly terminology has been a large setback for earth-friendly plastic’s reputation. To replace the legitimacy of eco-type plastic, new standards and test methods have been developed to ensure the plastic is indeed compostable.

In the plastics industry, “compostable” is defined as biodegradable under composting conditions, which consist of specified humidity and temperature, presence of microorganism and timeframe. In order to be labeled as compostable, they need to meet the requirement of ASTM D6400, Standard Specification for Compostable Plastics

This test method uses three separate criteria to define compostability:

1.      Biodegradation:  Biodegradation is determined by measuring the amount of carbon dioxide produced from the degrading plastic for up to six months.  In order to pass the test, a minimum of 60% conversion from organic carbon to carbon dioxide is required at 136.4ºF. To perform this test, the sample plastic is cut into approximately 2x2cm squares. Once cut, the plastic is placed in a sealed flask with active compost (AC) and compared to three controlled groups. The three controlled flasks are filled with cellulose and AC, polyethylene and AC, and, finally, just AC to provide positive, negative, and blank data to compare against the amount of carbon dioxide evolution from the sample plastic.

2.      Disintegration:  The disintegration test involves the same test set-up as for biodegradation except gas measurement is not required.  The product passes the test if less than 10% of the initial dry weight material remains after passing through a 2-mm sieve.  

3.      Eco-toxicity:  This is determined by measuring plant growth and germination from soils mixed with different concentrations of compost and product.  Plant growth and the germination rate should not be significantly different from control soils.  The plastic material must not introduce unacceptable levels of heavy metals or other toxic substances into the environment, upon sample decomposition.

In addition to ASTM D6400, manufacturers of plastic-coated products can choose to test against ASTM D6868, Standard Specification for Biodegradable Plastics used as Coatings on Paper and Other Compostable Substrates. Products intended with an end life cycle in an aquatic environment need to meet ASTM D7081, Standard Specification for Non-Floating Biodegradable Plastics in the Marine Environment.

NSF is committed to ensure that products labeled as biodegradable either in natural soil or water environment conditions meet their claim. For more information on NSF’s services, visit www.nsf.org/info/compostability.