Experimental and Analytical Study of Thermal Degradation and Creep Behaviour of Polyethylene Pipe Materials in Nigeria’s Hot Tropical Environment

Polyethylene pressure pipes are extensively used in water distribution and gas transmission infrastructure across Nigeria, yet their long-term mechanical performance under the combined effects of sustained internal pressure, elevated service temperatures, and UV exposure in the hot tropical environment has not been systematically characterised under local conditions. This study presents an experimental and analytical investigation of thermal degradation and creep behaviour of medium-density and high-density polyethylene pipe grades commonly used in Nigerian infrastructure projects. Pipe specimens were subjected to accelerated thermal ageing at temperatures of 60, 70, and 80 degrees Celsius for periods of up to 2,000 hours under atmospheric conditions. Tensile strength, elongation at break, and melt flow index were measured at regular intervals to track degradation progression. Creep compliance tests were conducted on pipe ring specimens under sustained hoop stress levels of 4, 6, and 8 MPa using custom-built dead-weight loading rigs at 23 and 50 degrees Celsius. Time-temperature superposition was applied to construct master creep compliance curves extending predicted behaviour to 50-year service lifetimes. Fourier transform infrared spectroscopy was used to characterise chemical degradation at the polymer chain level, identifying carbonyl index growth as the primary marker of oxidative chain scission. Results showed that high-density polyethylene retained 85 percent of initial tensile strength after 2,000 hours at 60 degrees Celsius but only 61 percent at 80 degrees Celsius, indicating significant sensitivity to temperature above 70 degrees Celsius. Keywords: polyethylene pipe, thermal degradation, creep behaviour, tropical environment, long-term performance.