The additives chosen represented two categories: oxo-biodegradables and non-oxo-biodegradables. The oxo- family is described as enhancing biodegradation after initial oxidation of the polymer. The non-oxo additives are described as having a different mode of action. (One of the additives was initially chosen as representative of a combination approach, but the company later dropped that description.)
One environment was a simulated composting system, representing a high-moisture, high-oxygen environment, with moderately elevated temperature to speed up the action of the microorganisms. In this system, we measured the amount of carbon dioxide produced. This experiment ran for 140 days, and then a second experiment ran for 60 days.
The second environment was a laboratory anaerobic digestion system, representing a high-moisture, no-oxygen environment. This type of environment has been used to represent what might occur in an “active” landfill environment. (A typical landfill is much too dry to be as biologically active as this type of system.) We measured the total amount of gas, predominantly methane and carbon dioxide, that was produced. The experiment ran for 464 days.
Our third choice of the environment was soil burial. In this system, the plastic materials were carefully buried in one of the Michigan State University campus fields regularly used to grow crops. Samples were removed periodically and evaluated for any changes in appearance, strength and other characteristics. This experiment ran for three years.
Little going on
In all the experiments, we compared samples of the plastic with and without additives. The additives were provided to us by the suppliers in the form of masterbatches, just as they are supplied to customers of the companies. A polyethylene film was produced using a blown film system, and a sheet of PET plastic was produced using extrusion casting, as is typical in the industry.
Additionally, for one of the additives and polyethylene films, we investigated the effect of extensive exposure to UV light prior to aerobic biodegradation. The light exposure results in some oxidation of the plastic molecules, resulting in both incorporation of oxygen atoms into the molecular structure and reduction in molecular weight. The amount of UV exposure was 152 hours at a temperature of 60 degrees Celsius, which is equivalent to about 58 days of outdoor exposure in Miami, Florida, and was sufficient to make the samples extremely brittle.
The results of the experiments can be summed up very simply: we found no evidence that the additives we tested resulted in any significant biodegradation of either the polyethylene film or the PET sheet in the environments we used. There were no significant differences between the samples with and without the additives and no evidence of substantial biodegradation of any of the samples.
Consumers and makers of plastic products are interested in making plastics that can biodegrade into the environment but making unsubstantiated claims risks causing a backlash with consumers.