Polyethylene biodegradation products

Recently, the possibility of replacing petroleum-derived synthetic polymers with natural, abundant and low-cost biodegradable products has gained much interest in both academic and industrial fields. " For instance, the production of plastics in Europe reached 57 million tons in 2012, mostly divided between polyethylene, polypropylene, poly(vinyl chloride), polystyrene and poly(ethylene terephthalate) production. These fossil-based plastics were consumed and discarded into the environment, generating 10.4 million tons of plastic waste, most of which ended up in landfills (Figure 1). 

During the 1970s there was a popular but rather naive belief in industry that the incorporation of biodegradable materials such as starch into synthetic polymers such as polyethylene would transform these relatively intractable materials into environmentally biodegradable products. This was subsequently shown not to be the case but it... 

Table 6. Biodegradable products formed from polyethylene in abiotic and biotic environments [20].

BPEO means that in practice the same disposable product may end up in any one of the alternative options discussed above. Consequently the material used should ideally be accommodated in any of the procedures used. Thus for example, if a biodegradable product is to be mechanically recycled, it should be capable of being reprocessed at the same temperature as the rest of the polymeric waste. This has proved to be difficult in the case of many bio-based materials. Degradable polyethylene can be recycled normally at polyolefin processing temperatures [10] whereas most hydro-biodegradable polymers depolymerise or scorch at these temperatures and cannot be recycled with commercial synthetic polymers in standard reprocessing equipment. 

Perhaps owing to some misunderstandings created by laboratory work that was published more than 30 years ago, there was a commonly held conviction that only hydrocarbon molecules below a molar mass of 500 would support microbial growth. The results of that work were vaUd, but they have no relevance to the biosusceptibility of polyethylene oxidation products in various environments. The actual molar mass values at which biodegradation of these molecules occurs is very much higher, probably in the tens of thousands. 

These monomers are biodegradeable and used for the production of bioplastics. PHAs produced from the process are usually composed of 100-30,000 monomers and exist in a short chain. Naturally, the properties of PHAs are similar to thermoplastics that are obtained from petrochemical industry such as polypropylene (PP) and polyethylene (PE) as shown in Table 1 (Evan and Sikdar, 1990). 

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