Synthesis of a Novel Aromatic–Aliphatic Copolyester from Poly(ethylene terephthalate) Using Reactive Extrusion

Reactive extrusion (REX) was used to synthesize an aromatic–aliphatic copolyester by copolymerizing poly(ethylene terephthalate) (PET) and polycaprolactone (PCL) as a novel PET recycling strategy. Critical design parameters were evaluated, including reaction kinetics, screw configuration, and processing conditions, to optimize the REX process in a co-rotating twin-screw extruder. By integrating kneading blocks with varying staggering angles, an optimized screw design was proposed to mitigate significant challenges such as short residence times, mixing inefficiencies, and barrel blockage caused by drastic changes in melt viscosity. Although the feed rate is more influential than screw speed in determining the residence time, controlling screw speed is crucial for optimizing the degree of fill and shear rate to promote transesterification. In addition, a minimum of 4 min of residence time is required. To reach equilibrium in an equimolar PET/PCL blend with 1.5 pph Ti(OBu)₄, kneading blocks covering over 50% of the screw length, it was essential to achieve a degree of fill of 0.5, ensuring sufficient mixing efficiency in low-viscosity melts against gravitational effects. Copolymers synthesized from virgin PET and municipal PET waste sources exhibit comparable thermomechanical properties and backbone structures, demonstrating that hard-to-recycle PET can be upcycled by catalytic transesterification. This study provides valuable insights and advances the design of REX systems, especially in the context of depolymerization-driven chemical recycling that involves significant viscosity changes.