Good extrusion foaming performance of long-chain branched PET induced by its enhanced crystallization property

Poly(ethylene terephthalate) (PET) was modified by regulating different contents of branching agent epoxy-based multifunctional oligomer and chain extender pyromellitic dianhydride in reactive extrusion process. The modified PET with better long-chain branched (LCB) structure boosted its rheological properties, and its enhancement of melt viscoelasticity resulted in excellent foamability in molten-state foaming process using supercritical CO₂ as blowing agent. More importantly, the branched structures acted as crystal sites to accelerate the crystallization kinetic of LCB PET whether under atmospheric pressure or high-pressure CO₂. The shear and elongation flow inside die further quickly induced the crystallization of LCB PET. The rapidly generated fine crystals could both introduce heterogeneous cell nucleation and suppress CO₂ escape, so the cell morphology of LCB PET in continuous extrusion foaming process exhibited a three-fold increase in cell density and smaller uniform cell size with respect to those of other foam-grade PET with long-chain structure.