*In the original article, the authors cite EN13432 although no anaerobic test is specified in that standard. In 2003 Jakubowicz18 reported the biodegradation of two samples of oxo-degradable polyethylene both containing manganese stearate as oxidation initiator, with the concentration in one sample being twice that of the other. The materials were thermally treated at 70째C in air for 4 weeks to produce oxidation products by thermo-oxidative degradation of the polyethylene. The samples were then incubated in soil at 60째C and the biodegradation was followed by measuring the CO2 released. After 200 days of incubation the biodegradation of the sample containing the lower amount of oxidant was 60.3% and that of the larger amount was 65.1%. The author noted that the shape of the curves indicated that biodegradation was still proceeding at this point. From the data given in the paper, the thermal treatment would have reduced the molecular weight of the samples to 5000 or less, at which point the author maintains that the material becomes biodegradable. The author also concluded, from Arrhenius plots of the data, that for an in-use temperature of 25째C the time taken to reach a molecular weight of 10000 (i.e. twice that of the 5000 required for biodegradability) would be between 2.5 and 4.5 years depending on the manganese stearate content.
Figure 6. Biodegradation of thermally aged (70째C, 4 weeks) oxo-degradable polyethylene in soil18 .
6.2.1.1 A.2.1.1: Studies Involving Specific Microbial Challenge Koutny et. al.11 subjected high density polyethylene film (HDPE) and low density polyethylene film (LDPE) both containing antioxidants and pro-oxidants to an abiotic pretreatment consisting of photooxidation and thermo-oxidation corresponding to about 3 years of outdoor weathering. The samples were then inoculated with defined microbial strains particularly with Rhodococcus rhodochrous and Nocardia asteroides, incubated up to 200 days and during the period their metabolic activities were followed by measuring adenosine triphosphate content. Simultaneously the cultures were also monitored by optical microscopy and FTIR spectroscopy. The first initial phase of fast growth caused by the presence of low molecular extractable compounds was followed by a long period of stabilised metabolic activity suggesting that microorganisms continued to gain energy from the substrate but evidently at a much slower rate. Complementary analysis performed at the end of incubation vii