Page - 4 - in Freshwater Microplastics - Emerging Environmental Contaminants?

polyhydroxyalkanoates (PHA), polylactides (PLA), aliphatic polyesters, andpoly- saccharides [35]. PLAison thevergeof entering intobulkproduction,whilePHA production is betweenpilot plant and commercial stage [36, 37]. 2.2 DescribingPlasticMaterials Plastics are processablematerials based on polymers [38], and tomake them into materials fit for purpose, they are generally processed with a range of chemical additives (Table 2). These compounds are used in order to adjust the materials properties and make them suitable for their intended purpose. Therefore, within polymerclassificationsplasticmaterialscanstilldiffer instructureandperformance depending on the type and quantity of additives they are compoundedwith.More recently, technological advanceshaveseen thedevelopmentofnewapplicationsof elementsbasedonnanoscales that arenowproducingplastic nanocomposites.The plastics industry isexpected tobeamajorfield fornanotechnology innovation. It is estimatedthatby2020, theshareofnanocompositesamongplastics in theUSAwill be7%[39].Thesenanocomposites includematerials thatare reinforcedwithnano- fillers (nano-clay and nano-silica) forweight reduction, carbon nanotubes (CNTs) for improved mechanical strength, and nano-silver utilised as an antimicrobial agent in plastic foodpackagingmaterials. 2.3 Microplastics asContaminants ofEmergingConcern The term ‘microplastics’ commonly refers to plastic particles whose longest dia- meter is<5mmandisthedefinitionusedbymostauthors.Ithasbeensuggestedthat the termmicroplasticsbe redefinedas items<1mmto includeonlyparticles in the Table 2 Aselective list of additive compoundsused tomakeplastics fit for purpose Additive compounds Function Plasticisers Renders thematerial pliable Flame retardants Reducesflammability Cross-linking additives Links together polymer chains Antioxidants andother stabilisers Increases thedurability of plastics by slowingdown the rate atwhichoxygen, heat, and light degrade thematerial Sensitisers (e.g. pro-oxidant transi- tionmetal complexes) Used to give accelerated degradationproperties Surfactants Used tomodify surface properties to allowemulsionof normally incompatible substances Inorganicfillers Used to reinforce thematerial to improve impact resistance Pigments For colour 4 S.Lambert andM.Wagner