Solid‐state polymerization of poly(ethylene terephthalate). III. Thermal stabilities in terms of the vinyl ester end group and acetaldehyde

Poly(ethylene terephthalate) (PET), precursors, and solid‐stated samples were evaluated in terms of changing vinyl ester (VE) concentrations. The results obtained through the application of reaction kinetics gave VE contents ranging from 0.7 to 5.5 mmol/kg of PET. As the initial intrinsic viscosity (IV) of the precursor increased, the VE content also increased, representing the different thermal histories of the samples during melt‐phase polymerization. The VE contents decreased as the solid‐state polymerization (SSP) time increased from 0 to 12 h and as the temperature of SSP increased up to 220°C. A series of acetaldehyde (AA) generation experiments were conducted from 270 to 300°C with samples solid‐stated from three precursors with different initial IVs. The rate of AA generation decreased as the final IV of the solid‐stated PET increased, and this showed that the SSP process improved the thermal stability of PET. The AA generation rates of samples that had similar final IVs but were solid‐stated from different IV precursors were also compared. When the heating temperature was low, the amount and rate of AA generation were higher for samples with higher initial precursor IVs. This tendency, however, became less clear as the generation temperatures increased, probably because interference from the dissociation reactions (occurring between the polymer chains to produce VE) increased with increasing temperature.