PHOTOOXIDATION AND BIODEGRADATION OF COMMERCIAL PHOTODEGRADABLE POLYETHYLENES

Representative samples of commercial photodegradable polyethylenes have been examined with respect to rate and extent of oxidation as measured by carbonyl (carboxylic acid and ester) formation, molar mass reduction and ability to support microbial growth when used as the only source of carbon. An ethylene-carbon monoxide (E/CO) copolymer was found to photodegrade most rapidly but to biodegrade most slowly. An antioxidant iron dithiocarbamate photodegradable polyethylene (Scott-Gilead) and a starch filled iron catalysed polyethylene were shown to produce more carboxylic acids during photooxidation than did the E/CO polymer, resulting in more rapid microbial growth. After removing the microorganisms, the surface of the oxidised polyethylene was found to be eroded with substantial reduction in sample thickness, while the molar mass of the polymer remained unchanged. It is shown that the microbial exo enzymes are able to recognise relatively high molar mass carboxylic acids and remove them from the surface of the polymer under conditions where water is not able to remove them by leaching. From this it is concluded that the oxidation products of oxidised polyethylene are unlikely to present a threat to the environment and that by conversion to biomass they contribute to the fertility of the soil. Abiotic iron catalysed photo- or thermooxidation is the rate-limiting step in the bioassimilation process. It is concluded that abiotic oxidation must precede the onset of biotic degradation, which is shown to occur readily at M(w) as high as 40 000. Bioassimilation involves further oxidation catalysed by transition metal ions and probably by exo enzymes from the microorganisms.