Influence of the addition of glycerol-derived polymers on the properties of post-consumer recycled PET

In the present work, the influence of glycerol prepolymers as potential chain extenders in the reactive mixture with post-consumer recycled PET (PET-PCR) was studied, namely: poly(glycerol citrate) (PGC), poly(glycerol succinate) (PGSu) and poly(glycerol citrate-co-succinate) (PGCSu). Mixtures of recycled PET with 2 wt% of glycerol prepolymers or Joncryl ADR®4638, a commercial chain extender, were processed in an internal mixer at 265 °C for 6 min and 60 rpm. The compositions were characterized by viscosimetry, carboxylic end groups (CEG) analysis using the Pohl method, crosslinking degree as the percentage of insoluble gel, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The addition of glycerol prepolymers increased the fluidity in the post-consumer PET resin, consequently, decreasing its viscosity by 33% for PETPCR/PGC, 24% for PET-PCR/PGSu, 44% for PET-PCR/PGCSu and 17% PET-PCR/Joncryl; the molecular weight in the order of 42% for PET-PCR/PGC, 31% for PET-PCR/PGSu, 55% for PETPCR/PGCSu and 23% for PET-PCR/Joncryl and increasing the number of carboxylic end groups. Although they did not show extensive chain crosslinking, the samples were fragile and brittle, thus making the tensile test unfeasible. Thermogravimetry showed that the polyesters were thermally stable up to approximately 350 °C and the sample that presented the lowest thermal stability was the PETPCR/PGCSu corroborating to the highest reduction in the molecular weight recorded. The DSC results showed an almost constant melting and glass transition temperature among the samples, a decrease of 8 °C in the crystallization temperature and an increase in the degree of crystallinity relative to neat PET-PCR, which corroborates to the intrinsic viscosity results. These results indicated that at the concentration and processing conditions used, the potential additives to chain extenders had an opposite effect to the desired one, that is, they reduced the molecular weight of recycled PET and the same effect occurred for the commercial chain extender. However, the additive that showed the greatest potential for use as chain extender was PGSu, which presented results similar to the commercial extender Joncryl ADR®4638, composed of multifunctional epoxy groups.