Biodegradable polyethylene

It is universally recognised that polyethylene (PE) is bio-inert. Indeed, conventional LDPE is used as a standard for bio-inertness in biodegradability/compostability test methods. It is not biodegradable because it is hydrophobic and because it consists of molecules having a high molar mass. Nevertheless it is a marvellous source of energy consisting entirely of the elements carbon and hydrogen which can be oxidised completely.

Collectively the various kinds of PE are the most commonly used of all the thermoplastics because of a relatively low cost and a number of other favourable characteristics. PE in thin-film form, for example, is strong, tough and flexible over a wide range of temperatures, and has high wet strength. These useful properties have led to the development of many familiar consumer products. It is reasonable to suppose that biodegradable PE would be even more useful.

Since PE is not biodegradable, it must be changed into something else that is biodegradable in order that microorganisms will be able, ultimately, to convert the carbon and the hydrogen to carbon dioxide and water, respectively. Hydrocarbon molecules, even relatively large ones, that do undergo such microbial conversion are hydrophilic and have molar mass values that are an order of magnitude below the average values found in commercial PE's. The hydrolysis products of linear polyesters, for example, fit this description having acid and alcohol end groups which derive from the ester linkages in the original macromolecules.

There is a remarkable 'fit' between the molecular properties which are required for microbial susceptibility and the nature of the normal oxidation products of conventional PE's (Carlsson and Wiles, 1986). Figure 1 is a schematic summary of the oxidative degradation of PE, using as an example the chemistry that is initiated at a branch point, i.e., a tertiary carbon - hydrogen bond. An analogous reaction sequence occurs as a result of oxidation initiated at secondary carbon - hydrogen sites along the molecules.

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