Evaluation of the structural degradation of polypropylene in the heat-affected zone induced by natural ageing using TGA/DTG and FTIR analysis.

An investigation of the structural changes occurring in the heat-affected zone (HAZ) of polypropylene used to manufacture orthopaedic limb supports after exposure to burning is the primary objective of the current study, which also examines how this zone is affected by the natural aging of the polymer. TGA and DTG analyses further confirmed that natural aging significantly reduces the thermal stability of polypropylene, as evidenced by a clear shift of the onset decomposition temperature toward lower values and an increase in the maximum degradation rate in aged samples. These effects were more pronounced within the heat-affected zone (HAZ) after burning, indicating that prior aging accelerates thermal decomposition and intensifies heat-induced damage. Also, it is evident from FTIR spectra that the 2023 sample exhibited high levels of oxidative degradation due to surface aging during storage, as opposed to the 2025 sample, which demonstrated low levels of oxidative degradation due to normal storage conditions. FTIR spectra indicated that the hydroxyl and carbonyl groups had increased in intensity and the hydrocarbon bonds had decreased relative to them, indicating that polymer chains had been scissiled and polar groups had been formed. According to the analyses performed on the horizontal burning samples (UL-94 HB), the effects of fire were more noticeable in older samples, since initial aging increased the decomposition process within the heat-affected zone, which resulted in a double appearance of new oxidized groups and a greater loss of crystallographic structure regularity when compared with samples manufactured more recently. Furthermore, structural changes within the burned area were noted, including a loss of crystallinity and an increase in amorphous phases, as reflected by the decline in crystalline structure peaks. As a result of these findings, it is evident that the effects of heat on polymers are not independent of their history, but are intensified when there has been previous degradation brought on by storage or prolonged exposure to adverse environmental conditions. Also, the results of optical inspection and surface roughness analysis showed that sample 2023 recorded higher values for surface roughness, indicating local morphological changes due to thermal exposure.