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‐125 ‐Low‐density PE LDPE Chain structure with long branches 0.89–0.93 85–125 ‐High density PE HDPE Chain structure with minimal branches 0.94–0.98 130–140 Polypropylene PP Schematic illustration of the structure of Polypropylene. 0.83–0.92 ‐20 to ‐5 Polystyrene PS Schematic illustration of the structure of Polystyrene. 1.04–1.1 0.05 (Foam)b 90–100 Poly(vinyl chloride) PVC Schematic illustration of the structure of Poly(vinyl chloride). 1.16–1.58 87 Poly(ethylene terephthalate) PET Schematic illustration of the structure of Poly(ethylene terephthalate). 1.37–1.45 67–80 Polyurethane PU Schematic illustration of the structure of Polyurethane. 1.2 0.05–0.96 (Foam)b ‐20 Polyamide (Nylon 66) PA Schematic illustration of the structure of Polyamide. 1.31 50–60 Cellulose acetate CA Schematic illustration of the structure of Cellulose acetate.D.S ~ 2.45c 1.29–1.31 ~187

      a Glass transition temperature

      b Density of foamed plastics depends on the fraction of air or other gas in the plastic

      c D.S, the degree of substitution for CA used in cigarette filters (a the predominant constituent of marine beach debris), is about 2.45 but CA is available different values of D.S from 0–3.0 for various applications.

      

      1.6.2 Thermal Transitions

Schematic illustration of the scanning Electron Micrograph of an ultra-high molecular weight HDPE fiber heated at 110 °C for one hour.

      Source: Left: Courtesy of Miao et al., (2018).

      With semi‐crystalline plastics, however, the thermal behavior is somewhat different. The amorphous fraction of the plastic behaves the same as described above and soften above its Tg (°C), but this temperature has no impact at all on the crystalline domains. But, when the plastic is heated to even higher temperatures well above the Tg, the melting temperature of the crystallites, Tm (°C) is reached, when the material changes into a fully amorphous polymer. Further heating above the Tm (°C) (and therefore >Tg [°C]), converts the polymer into a viscous liquid. Neither the Tm (°C) nor Tg (°C) are inherent properties of polymers but can be changed by its thermal and stress history.

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