It is very important to understand the interaction between plastics and environment in ambient conditions. The plastics degrade because of this interaction and often their surface properties change resulting in the creation of new functional groups. The plastics after this change continue to interact with the environment and biota. It is a dynamic situation with continuous changing parameters. Polyethylene, polypropylene, and polyethylene terephthalate (PET) degrade through the mechanisms of photo-, thermal, and biodegradation. The three polymers degrade with different rates and different pathways. Under normal conditions, photo- and thermal degradation are similar. For polyethylene, photo-degradation results in sharper peaks in the bands which represent ketones, esters, acids, etc. on their infrared spectrum. The same is true for poly propylene but this polymer is more resistant to photo-degradation. The photo-oxidation of PET involves the formation of hydroperoxide species through oxidation of the CH2 groups adjacent to the ester linkages and the hydroperoxides species involving the formation of photoproducts through several pathways. For the three polymers, interaction with microbes and formation of biofilms are different. Generally, biodegradation results in the decrease of carbonyl indices if the sample has already been photo-degraded by exposure to UV. Studies with environmental samples agree with these findings but the degradation of plastics is very subjective to the local environmental conditions that are usually a combination of those simulated in laboratory conditions. For example, some studies suggested that fragmentation of plastic sheet by solar radiation can occur within months to a couple of years on beaches, whereas PET bottles stay intact over 15 years on sea bottoms.
Kalliopi N. Fotopoulou, Hrissi K. Karapanagioti, Chapter, Part of the series The Handbook of Environmental Chemistry, pp 1-22, Date: 13 April 2017