Analysis of the Effect of Natural Aging and Burning on the Degree of Crystallization in the Heat‐Affected Zone of Polypropylene Used in Medical Applications

ABSTRACT Many medical products made from polypropylene rely on long storage periods before actual use. Natural aging can lead to cumulative changes in the crystalline structure of the material, which may affect its thermal response when exposed to localized heat sources or accidental combustion during service. This article investigates the combined effects of natural aging, long‐term storage and localized heat exposure on the crystallization behavior and structural stability of polypropylene (PP). Samples of PP manufactured in 2023 and 2025 were examined using UL‐94 HB, DSC, XRD analysis. The results of the UL‐94 HB test showed that the aged PP (2023) has a lower apparent combustion rate compared to the new PP, but at the same time, it exhibits a noticeable expansion in the heat‐affected zone (HAZ), indicating an increase in the polymer's sensitivity to heat transfer despite its reduced flammability. The DSC results showed that natural aging and storage do not lead to a noticeable change in the melting point, confirming the stability of the dominant crystalline phase; however, they clearly affect the crystallization kinetics and final crystallinity, as a slight increase in latent heat of fusion and crystallinity was observed, corresponding to a decrease in the cooling crystallinity, which indicates a slowdown in the crystallization process during cooling. The XRD results also confirmed that all samples retained the α crystalline phase of polypropylene, with differences in peak intensity and width reflecting changes in crystalline regularity and crystal size resulting from aging and thermal history, rather than a change in crystalline phase. In the HAZ, thermal exposure acted as a localized heat treatment, leading to partial decomposition of the crystalline structure followed by recrystallization during cooling. This response was more efficient in the newer samples due to the integrity of the polymer chains. The results confirm that the HAZ is a more sensitive indicator than the original material for tracking the effect of natural aging and storage history on the crystalline structure of PP.