Degradation assessment of high-density polyethylene (HDPE) debris after long exposure to marine conditions

The degradation of high-density polyethylene (HDPE) in marine environments was investigated under various weathering conditions. HDPE debris were collected from coastal areas near Korinthos, Greece which had been exposed to marine conditions for durations ranging from a few months to several decades; they were analysed alongside with laboratory-manufactured HDPE specimens subjected to controlled weathering exposure. Four (4) different cases were investigated, including exposure to different conditions, namely to (a) natural atmospheric and (b) sea weathering conditions, (c) accelerated ultraviolet (UV) radiation, and finally (d) submersion to artificial seawater for up to twelve (12) months. The degradation assessment was proposed based on performed tensile mechanical tests, while the chemical/microstructural changes were assessed through Fourier Transform Infrared (FTIR) spectroscopy and Scanning Electron Microscopy (SEM). FTIR spectroscopy indicated the emergence of carbonyl groups, with peaks appearing between 1740 cm^-1 and 1645 cm^-1, which are crucial indicators of photo-oxidative degradation. Key findings revealed that HDPE specimens experienced significant (8 %) ultimate tensile strength (σ_UTS) only after 3 months of atmospheric exposure, while this decrease can reach up to 60 % over the period of 35 years exposure. A strong correlation was observed between the σ_UTS decrease between the (a) natural environment and (b) accelerated UV weathering exposure. It is noticed that 1½ month of accelerated UV exposure corresponded to similar ultimate tensile strength decrease for 6 months of natural atmospheric degradation. A linear correlation is proposed to assess the long-term materials' tensile properties degradation in marine environments.