Larger plastics eventually undergo

Larger plastics eventually undergo some form of degradation
and subsequent fragmentation, which leads to the formation of
small pieces (Shah et al., 2008; Costa et al., 2010; Andrady, 2011).
Degradation is a chemical change that reduces the average molecular
weight of polymers (Andrady, 2011). The most-used polymer
types (i.e., PE and PP) have high molecular weights and are
non-biodegradable (Shah et al., 2008). However, once in the marine
environment, they start to suffer photo-oxidative degradation
by UV solar radiation, followed by thermal and/or chemical
degradation. This renders plastics susceptible to further microbial
action (i.e., biodegradation) (Shah et al., 2008; Andrady, 2011). The
light-induced oxidation is orders of magnitude higher than other
types of degradation (Andrady, 2011). Any significant extent of
degradation inevitably weakens the plastic, and the material
become brittle enough to fall apart into powdery fragments
(Andrady, 2011) when subjected to sea motion.