Research on the Causes of Cracking of Pipes Derived from Recirculated Polyethylene

The objectives of the research was to determine the causes of cracks in pipes made of high-density polyethylene (HDPE) regranulate during their storage. A visual assessment of the cracks was carried out and their fatigue nature was found. Due to the insufficient information on the composition and properties of the processed regranulate, tests were carried out on the density distribution and mass flow rate of plastic samples taken from various areas of the pipe wall. Comparative tests were performed using infrared spectrophotometry of a plastic sample taken from a pipe, commercial HDPE and commercial polypropylene (PP) by attenuated total reflection/Fourier-transform infrared spectroscopy (ATR-FTIR), thermogravimetry (TG) and differencial scaning calorimetry (DSC) method. There were differences in density and flow rate depending on the distance from the outer surface of the pipe, reaching several percent. Based on the conducted spectroscopic, thermogravimetric and differential scanning calorimetry studies, none of the expected plastics contamination, mostly PP, has been found in recirculated polyethylene. Futhermore, the expected decrease in the temperature of phase transformations and the beginning of decomposition of polyethylene (PE) after recycling, compared to the original one, were visible. Possible causes of cracks were considered to be differences in the PE structure in the pipe wall and the occurrence of possible porous structures, which when storing pipes in variable load and temperature conditions could cause the propagation of microcracks.