Spalling accelerates the methanolic pyrolysis of poly(ethylene terephthalate) in unstirred microwave reactors

Poly(ethylene terephthalate) (PET) is an important commodity polymer that has the potential to be 100% recycled to monomer, allowing for repolymerisation or upcycling. An attractive route is the methanolysis to dimethyl terephthalate (DMT), but it is difficult due to the lower boiling point of methanol. In this work, the PET can be essentially completely reacted to high purity DMT (in one case 99.5% for the crude product) in an unstirred microwave reactor. Unstirred reactors offer a challenge to determining the temperature, but a novel solution utilizing Regnault’s Law was applied to estimate the temperature from the pressure for comparison purposes. During methanolysis in an unstirred microwave reactor, the reaction proceeded nine times faster than in a conventional reactor at the same nominal temperature. This acceleration is apparently due to spalling of the PET, increasing the active surface. Stirring the reactor mitigates this effect by homogenizing the heat distribution, preventing spalling. This is a genuine, although niche, microwave effect, resulting in a far more rapid reaction than a conventional reactor due to increasing the active surface, and hence lowers energy use.