Oxo-biodegradable Plastics in Landfills

Many plastic film products end up in landfills after use as a result of municipal waste collection. This is probably the fate of most carrier bags and food wraps although recycling programs of post-consumer plastics are fortunately available in a number of communities. It is of interest then to compare the behavior of conventional carrier bags with those containing TDPA® pro-degradant additives. It is mandated in a number of countries, e.g., Australia, USA, that sanitary landfills are to be operated under conditions as dry as possible, presumably so as to minimize the amount of leachate formed. Since dry conditions do not favour microbial activity, it is assumed by many that biodegradation is not a factor in the conversion of organic waste to gaseous products in landfills. Nevertheless it is a fact that the temperatures in landfills are well above ambient and this is due to microbial activity. Detailed discussion of this situation is found in refs 3 & 4. It is clearly an advantage to have discarded carrier bags and the like degrade in landfills, and repeated testing has proven that polyethylene film products containing TDPA® degrade relatively rapidly in landfills. For example, films made from LLDPE/LDPE blends were shown to undergo significant oxidation with resulting embrittlement as a result of being buried 2 meters below the surface of landfills in China and in Canada for several winter months – only if those films contained TDPA® (ref. 5). Further, the effects of landfill burial (a UK site) for 10 months on LDPE film with and without TDPA® pro-degradant are even more spectacular (ref. 1, 2 & 6). There were no changes in the buried or unburied control or in the unburied TDPA–PE films. The M_w of the recovered TDPA-PE films had decreased from a starting value of 115,000 to 4,250. At that lower value the oxidized plastic fragments are readily biodegradable.

There is significant microbial activity in landfills owing to the high carbon content of the material in municipal solid waste (MSW), and the ubiquity of microorganisms and moisture. You cannot expect to operate landfills as ‘dry tombs’ when you are adding food waste and lots of cellulosic materials at the active face. At the surface and for several meters below there will be enough oxygen and water that aerobic biodegradation of the organic matter will occur. A vast array of fungal species and aerobic bacteria will convert carbon to carbon dioxide. A limit to this activity will be the impervious plastic bags, sheets and films that prevent the free movement of gasses and liquids through the mass of organic waste that is contained in or ‘protected’ by this ordinary plastic. After a time, (months, years) the MSW that is well below the surface or active face will no longer have an adequate supply of oxygen and water to support the aerobes. Then whatever anaerobic bacteria are present will convert (much more slowly) the carbon in the remaining organic material largely to methane, which is 24.5 times more potent as a greenhouse gas than is carbon dioxide. It follows that there are advantages, both environmental and commercial to encouraging rapid aerobic biodegradation while there is enough oxygen and water in the upper levels of the MSW in the landfill. A simple and inexpensive way to do this is to use TDPA® - based oxo-biodegradable polyolefins in packaging since most packaging is deposited in landfills.

Under normal conditions, degradation of EPI’s TDPA® - based polyethylene film will begin approximately thirty days after disposal in a landfill (ref. 3). This time to the onset of degradation could be as short as two weeks under ideal conditions or it could be as long as several months under cold, wet conditions. Fragmentation of the films, bags and other containers will follow after abiotic oxidation, aided by the inevitable mechanical stresses (e.g., from compacting equipment) in the landfill environment. This will allow the free circulation of air and water through the upper levels of the waste mass for some time: months at least, perhaps a year or more and the readily biodegradable organic materials in the MSW will be bioassimilated by aerobic microorganisms. This relatively rapid conversion will reduce the volume of waste and this will prolong the useful life of the landfill. Obviously, with so much more material bioassimilated during the aerobic phase, there will be much less to undergo slower anaerobic degradation to produce more damaging methane. Furthermore the site, after filling and capping, will be available that much sooner for other purposes such as recreational fields.

References

1. E. Chiellini, A. Corti and G. Swift, “Biodegradation of thermally oxidized, fragmented low-density polyethylenes”, Polymer Degradation and Stability, 81, pp. 341-351, 2003.
2. E. Chiellini, A. Corti, S. D. Antone and R. Baciu, “Oxo-biodegradable carbon backbone polymers – oxidative degradation of polyethylene under accelerated test conditions,” Polymer Degradation and Stability, vol. 91, pp. 2739 - 2747, 2006.
3. G. Swift and D. M. Wiles, “Biodegradable and degradable polymers and plastics in landfill sites,” in Encyclopedia of Polymer Science and Technology, J. I. Kroschwitz (ed.) Hoboken, John Wiley & Sons, 2004.
4. D. M. Wiles in Biodegradable Polymers for Industrial Applications, Ray Smith (ed.) Cambridge, Woodland Publishing (CRC Press) chapter 16, pp. 437-450, 2005.
5. G. Scott and D. M. Wiles, “Degradable hydrocarbon polymers in waste and litter control,” in Degradable Polymers: Principles and Applications, 2nd ed. G. Scott (ed.) Dordrecht, Kluwer Academic Publishers, pp. 454-457, 2002.
6. N. C. Billingham, M. Bonora and D. De Corte, “Environmentally degradable plastics based on oxo-biodegradation of conventional polyolefins,” in Proceedings of the 7th World Conference on Biodegradable Polymers and Plastics, Pisa, June 4-8, 2002.