Polyethylene terephthalate (PET) is the third most common synthetic polymer and accounts for about 18% of world polymer production. It is an aromatic/aliphatic polyester which possesses very practical thermal properties that are not found in the all aliphatic commodity thermoplastics polyethylene or polypropylene: a glass transition temperature (Tg) near 67°C and a melting temperature (Tm) of 265°C.   But like those two, it is derived from fossil fuels: the key aromatic component of PET, terephthalic acid, is derived from petroleum, while ethylene glycol is derived from petroleum or natural gas.