Photo-biodegradable polyethylene

Figure 3.2 Mechanical property change of photo-biodegradable polyethylene carrier bags during outdoor exposure in England (Reproduced with permission from D. Gilead and G. Scott, in Developments in Polymer Stabilisation - 5, ed. G. Scott, Applied Science Publications, 1982, p. 103)...

Photo-biodegradable polyethylene using the above system was developed commercially for use in agriculture by D. Gilead of Plastopil Hazorea in Israel in collaboration with the author and is now widely used as Plastor in mulching films in Europe and Plastigone in the USA. It is also used in polypropylene baler twines as Cleanfields by AMBRACO in the USA, and in controlled release fertilisers as Nutri-cote by Chisso-Asahi Fertilizer Company in Japan. The biodegradation mechanism will be discussed below. 

Figure 5.4 Photo-biodegradable polyethylene mulch (a) after planting, (b) after cropping, (c) after ploughing...

S-G photo-biodegradable polyethylene is now being used in a different way to reduce the pollution of water courses by fertilisers. By encapsulating the fertiliser in porous photo-biodegradable capsules fertiliser release times can be achieved from 40 days to one year. Nitrogenous fertiliser based on this principle are manufactured by Chisso-Asahi Fertilizer Company of Japan and scientific studies by Kawai of Okayama University have shown (personal communication) that the empty polymer capsules biodegrade rapidly in soil. 

Figure 5.8 Bioerosion of photo-biodegradable polyethylene Plastor S-G) films after exposure to oxidising environments. SE micrographs (x 5000) obtained after microbial incubation (6 months) in the absence of any other source of carbon...

Mid-IR spectroscopy, alongside gravimetric and molecular weight determinations, has also been used to analyse the biodegradation by a thermophilic bacterium (isolated from soil) of an LDPE film [44], The mid-IR studies were undertaken using the ATR sampling technique on control samples, samples that had been UV irradiated, and samples that had been UV irradiated then incubated with bacteria. The study showed that the particular bacterial strain was capable of utilising standard and photo-oxidised polyethylene as the sole carbon source. 

Four main types of polymer are currently accepted as being environmentally degradable. They are the photolytic polymers, peroxidisable polymers, photo-biodegradable polymers and hydro-biodegradable polymers. Commercial products may be composite materials in which hydrolysable and peroxidisable polymers are combined (e.g. starch-polyethylene composites containing prooxidants). The application, advantages and limitations of each group will be briefly discussed. 

Polyethylene oxide) (PEO) is a semicrystalline water-soluble polymer [64, 65], with a crystallinity that is very sensitive to the thermal history of the sample, making this property interesting as an indicator of degradation. Because it is biodegradable and biocompatible, PEO is a good candidate for environmental and medical applications [66-68]. The mechanisms of thermo- and photo-oxidation of PEO have already been investigated [69, 70] on the basis of IR identification of the oxidation products and are summarized in Scheme 10.1. 

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