Microplastic (MP) is defined as plastic particles smaller than 5mm. We differentiate between primary MP, which is made this size and enters the marine environment more or less by chance, and secondary MP, which comes from larger plastic pieces. Examples for primary microplastics are micro particles in cosmetics or pellets, which are the primary material for plastic products. Textile fibres or tyre friction are also counted as secondary MP.
The following mechanisms are responsible for plastics getting brittle and disintegrating in the sea: long periods of sunlight can cause a photochemical decomposition of plastics, whereby the ultraviolet rays (UV) in sunlight cause oxidation of the polymer matrix, which then leads to a cleavage of the bonding. A process like this can lead to additives in the plastics being washed out, which had been added to increase shelf life and resistance to corrosion. Whereas cold water slows down the disintegration of larger pieces of plastic, plastic rubbish on beaches is exposed to abrasion by sand and rocks and the structures of such plastic pieces can be broken down by photo degradation through UV-sunlight. This way plastics get more and more brittle and break into smaller and smaller bits which are transported by waves and wind into the sea.
Once the structures of larger pieces of plastic are broken down and they get smaller and smaller they can also be ingested by smaller and smaller organisms. Apart from this the breakdown helps additives like plasticisers and BPA Bisphenol A enter the environment, which had been introduced to make the plastics more flexible and elastic.
Microplastic is transported by the currents in the oceans and can be found in so-called water columns as well as floating in or on water, or in the sediment of the coasts down to the deep-sea floor. Microplastic as such can be found in most environments worldwide.1)
1) Microplastics as contaminants in the marine environment: A review
Matthew Cole, Pennie Lindeque, Claudia Halsband, Tamara S. Galloway
Marine Pollution Bulletin, ISSN 0025-326X, 10.1016/j.marpolbul.2011.09.025.