Effect of rPET Content and Preform Heating/Cooling Conditions in the Stretch Blow Molding Process on Microcavitation and Solid-State Post-Condensation of vPET-rPET Blend: Part I—Research Methodology and Results

Polyethylene terephthalate (PET) is widely used in bottle production due to its cost-effectiveness and low environmental impact. The first part of this article describes the research and statistical analysis methodology of the influence of the virgin PET (vPET) and recycled PET (rPET) content in the vPET-rPET blend, as well as the preform heating/cooling conditions in the stretch blow molding (SBM) process on the microscopic bottle properties. Microscopic properties such as crystallinity, density, viscosity, relaxation degree of the amorphous phase, and microcavitation in PET were examined. This study reveals that microcavity and solid-state post-condensation effects occur during PET deformation in the SBM process. The increase in free volume, indicating microcavitation, was confirmed by measuring positron annihilation lifetime spectroscopy (PALS). PALS and density of the amorphous phase studies prove a reduction in the dimensions of the free volumes, with a simultaneous significant increase in their number and ellipsoidization. It can be associated with crystallite rotation in a temperature-dependent non-crystalline matrix. The occurrence of solid-state post-condensation effects was confirmed by measuring the intrinsic viscosity. The conclusions resulting from the analysis of the microstructure affecting the mechanical strength of the material were validated by pressure resistance tests of the bottles.