Misconceptions in the application of the Arrhenius equation to plastics - environmental degradation of plastics unlikely below their glass transition temperature

The term ‘’plastic’’ is derived from the Greek word ''plastikos'', meaning fit for moulding, highlighting the main feature that makes plastics so useful, i.e., their malleability during manufacture, allowing plastics to be formed into a variety of shapes such as bottles, bags and straws1. However, this malleability is temperature-dependent requiring the constituent polymers to be in the molten state and thus to flow over one another; upon cooling to room temperature the plastic becomes solid. Thus, plastics have three main temperature/property regimes: the glassy state, the rubbery state (above the glass transition temperature, Tg) and the malleable state (above the moulding temperature, Tm). It has recently been suggested that the Arrhenius equation can be used to extrapolate the rate of degradation of Polyethylene Terephthalate (PET) determined in test tubes at highly elevated pressure (>3MPa/26.9 atmospheres) and temperature (190-215 °C) to predict the rate of degradation under environmental conditions. It was suggested that oceans, with their high metal salt contents, could act as catalysts resulting in potential degradation hot-spots in limited pockets of the worlds’ oceans that reach 35 °C. We contest this on multiple grounds as described below, the most important being that the Arrhenius equation as implemented in this study applies only to the plastic in its molten state (>Tm), and that there will be separate rates and activation energies in the intermediate rubbery state (>Tg but >Tm) and for the plastic at temperatures