Preparation of Low-Density Microcellular Foams from Recycled PET Modified by Solid State Polymerization and Chain Extension

Recycled polyethylene-terephthalate (RPET) bottle regrinds were physically foamed after two types of industrially feasible molecular weight increasing processes. Intrinsic viscosity (IV) of initial waste (0.71 dL/g) increased both after reactive extrusion carried out using a multifunctional epoxy-based chain extender (0.74 dL/g) and after solid state polycondensation (SSP) (0.78 dL/g), while capillary rheometry revealed higher degree of branching in the chain extended PET material. Despite the relatively low IV values (below 0.80 dL/g), physical foaming, a mild and cost-efficient way, was successful in both cases, uniform microcellular foam structures with void fractions ranging between 75 and 83% were achieved. During the experiments morphology change in the materials was tracked by differential scanning calorimetry (DSC) besides recording IV values. The IV drop during foaming was between 0.03 and 0.10 depending on the pre-processing technology. Structure of foams produced from the two different modified RPET materials was compared with each other based on scanning electron microscopic imaging of cryogenic fracture surfaces. The average cell diameters were measured to be 213 and 360 µm in the case of chain extended and SSP-modified materials, respectively.