Hydro-biodegradation

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. 

Fig. 14.9 Energetic profiles in oxo- and hydro-biodegradables primary steps...

The second group of biodegradable pol3rmers is formed by hydro-biodegradable materials,... 

Biomax is, according to the producer, a standard PET with addition of special monomers to allow degradation to take place. Comparable to PLA, the degradation mechanism is described as an initial attack of water to the special monomers which are sensitive to hydrolysis. Oligomers formed by this first abiotic degradation step can be transported into microbial cells and there be metabolised (Biomax is claimed to be (hydro/ biodegradable). 

Figure 1. Biodegradation routes for 0x0- and hydro-biodegradable polymers .

Manufacturers and users of oxo-biodegradable polyolefins view with concern the development of standards for degradable polymers which demand a high level of mineralization as the primary criterion. This protocol was originally developed for hydro-biodegradable polymers, which will primarily end up in sewage. For these polymers and in this application, such test methods are entirely acceptable but they are totally inappropriate for compost, litter and agricultural applications. 

It will be seen in Section 4 that the rate of the abiotic and hence the biodegradation process can be readily controlled by antioxidants whereas no comparable control process has yet been developed for hydro-biodegradable polymers. A second conclusion was that starch plays no part in the biodegradation of a polyethylene matrix until the latter has been extensively peroxidised in the presence of transition metal ions. Similar conclusions have been reached by Wool [56] who showed that in the absence of PE degradation in starch-PE blends, biodegradation is controlled by the rate of migration... 

Carbon is retained in the soil during oxo-biodegradation in a form accessible to growing plants, rather than by being eliminated to the environment as carbon dioxide as is the case with hydro-biodegradable polymers (e.g. pure cellulose and starch). The time-scale for complete oxo-bioassimilation of the synthetic polyolefins is very similar to that for their natural analogues such as c/5(polyisoprene) and related plant exudates and lignocellulose, the structural material of plants.. 

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. 

A primary target for degradable plastics is in waste and litter control and most manufacturers of such materials have made claim to the environmental acceptability of their products as replacements for the commodity packaging polymers. A primary criterion of acceptability is cost and few hydro-biodegradable polymers can at the moment approach the hydrocarbon polymers in this respect. Consequently, in the following Sections the emphasis will be on synthetic commodity polymers with enhanced biodegradability in the natural environment. By way of clarification. 

Hydro-biodegradation. Most organic polymers are much more hydrophobic than those found in nature, and living organisms have not evolved enzymes which can effectively penetrate and cleave them. Polymers which resist hydrolysis are also resistant to biodegradation, since enzymes operate in aqueous media. Thus synthetic polymers are typically much more resistant to biological attack than are the natural polymers, such as proteins and polysaccharides. Biodegradability of polymers has been well reviewed (see eg. Refs. 166-169). 

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