Phase behavior and reaction of polyethylene terephthalate-water systems at pressures up to 173 MPa and temperatures up to 490°C

The phase behavior and reaction rate of polyethylene terephthalate (PET) in water were studied with a hydrothermal diamond anvil cell (DAC), visual microscopy and Raman spectroscopy. Experimental runs were made to observe PET under subcritical water, supercritical water and pyrolysis conditions, and also terephthalic acid (TPA) under supercritical water conditions. Over the range of PET concentrations studied (12 to 59wt%), most systems became homogeneous. For the case where samples were heated slowly, the homogenization temperature was around the PET melting point (241°C). For PET samples that were rapidly heated and underwent a solid-liquid transition upon heating, the homogeneous temperature was between 297°C and 318°C. For samples that were rapidly heated and underwent crystallization during heating, the homogenization temperature was between 360°C and 396°C. The homogenization temperature for TPA+water was found to be around 356°C. PET particles that were well dispersed into the aqueous phase and rapidly heated to supercritical conditions were found to undergo surface crystallization and subsequent melting of the crystalline oligomers. In general, the crystals formed during both heating and cooling were oligomeric forms of TPA, as apparent from the high dissolution temperatures and as confirmed by Raman spectroscopy after cooling.