Depolymerization of poly(butylene terephthalate) in sub- and supercritical ethanol in a fused silica capillary reactor or autoclave reactor

Depolymerization of poly(butylene terephthalate) (PBT) in sub- and supercritical ethanol was performed both in a fused silica capillary reactor (FSCR) or a batch autoclave reactor. The phase behavior of PBT in ethanol during heating and cooling was studied in the FSCR under a microscope and captured by a digital camera. The results showed that PBT could dissolve in supercritical ethanol at 330 °C and form a homogeneous solution. Under autogenous pressure in the batch autoclave reactor, the effects of the ethanol/PBT mass ratio (8 g/2 g to 24 g/2 g), reaction temperature (200-280 °C) and reaction time (5-60 min) on depolymerization of PBT in sub- and supercritical ethanol were investigated. The main liquid products of depolymerization were identified and quantified as diethyl terephthalate (DET) and 1,4-butanediol (1,4-BD) by liquid chromatography mass spectrometry, gas chromatography mass spectrometry, and gas chromatography, respectively. PBT could be completely depolymerized at 240 °C in 60 min with an optimal ethanol/PBT mass ratio of 20 g/2 g (10:1). The highest yields of DET and 1,4-BD were 97.7% and 89.4%, respectively. Reaction kinetics analysis showed that the PBT depolymerization in sub-critical ethanol reaction was first order and activation energy was 38.8 kJ mol−1. A reaction pathway was proposed based on the experimental results.