Page - 8 - in Freshwater Microplastics - Emerging Environmental Contaminants?

The prediction of plastic fragmentation rates is not a simple process. Kinetic fragmentationmodelshavebeen investigated in themathematics andphysics liter- atures, and thekinetics of polymer degradationhas been researched extensively in the polymer science literature. Thesemodels describe the distribution of fragment sizes that result from breakup events. These processes can be expressed by rate equations that assumeeachparticle is exposed to an average environment,mass is the unit used to characterise a particle, and the size distribution is taken to be spatially uniform [69, 70]. These processes can be described linearly (i.e. particle breakup is driven only by a homogeneous external agent) or nonlinearly (i.e. additional influences also play a role), and particle shape can be accounted for by averagingoverall possible particle shape [69]. Themodels used to describe these degradation process are often frequently complicated, but as a general rule focusonchain scission in thepolymerbackbone through(a) randomchainscission (all bonds break with equal probability) characterised by oxidative reactions; (b) scission at the chain midpoint dominated by mechanical degradation; (c) chain-end scission, a monomer-yielding depolymerisation reaction found in thermal and photodecomposition processes; and (d) in terms of inhomogeneity (different bonds have different breaking probability and dispersed throughout the system) [71–73].Theestimationofdegradationhalf-liveshasalsobeenconsidered for strongly hydrolysable polymers through the use of exponential decay eqs. [65, 74, 75]. However, the applicability of modelling the exponential decay of more chemically resistant plastics requires greater investigation [74]. Important variables that will influenceMP degradation and fragmentation are environmental exposure conditions, polymer properties such as density and crys- tallinity (Table3),and the typeandquantityofchemicaladditives.Molecularchar- acteristics that generally counteract degradationare the complexityof thepolymer Table 3 Polymer type, density, and crystallinity Polymer type Density (g cm 3) Crystallinity Natural rubber 0.92 Low Polyethylene–lowdensity 0.91–0.93 45–60% Polyethylene–highdensity 0.94–0.97 70–95% Polypropylene 0.85–0.94 50–80% Polystyrene 0.96–1.05 Low Polyamide (PA6andPA66) 1.12–1.14 35–45% Polycarbonate 1.20 Low Cellulose acetate 1.28 High Polyvinyl chloride 1.38 High Polylactic acid 1.21–1.43 37% Polyethylene terephthalate 1.34–1.39 Described as high in [76] and as 30–40%in [77] Polyoxymethylene 1.41 70–80% Informationoncrystallinitywas taken from [76, 77] 8 S.Lambert andM.Wagner