A recent study published in Engineering presents a significant advancement in improving the toughness of polypropylene (PP), a widely used thermoplastic material. The research, led by Zhiyi Zhang and Qiang Zheng from Taiyuan University of Technology and Zhejiang University, focuses on developing a novel core–shell structured composite to enhance PP’s performance at low temperatures.

PP has many advantages, such as high thermal and chemical resistance, but its low-temperature toughness is a limiting factor in various applications. To address this issue, the researchers introduced an innovative approach. They blended PP with high-density polyethylene (HDPE) and polystyrene–polyethylene–polypropylene–polystyrene (SEPS) to create an unusual SEPS@HDPE core–shell structure, where SEPS serves as the core and HDPE as the shell.

The experimental results are quite remarkable. The addition of HDPE to the PP/SEPS system led to a significant brittle–ductile transition in the PPM/HDPE composites (PPM stands for the PP/SEPS composite with the fixed mass ratio of 70/30). The brittle–ductile transition temperature T_bd could be effectively adjusted, and the impact strength improved substantially. For example, at 10 °C, the impact strength of PPM increased rapidly from 13.6 to 49.3 kJ/m² at −5 °C when HDPE was added. What’s more, the tensile strength was well-maintained, with only a slight decrease in the yield strain and Young’s modulus, which was within an acceptable range considering the overall mechanical performance.

The phase morphology evolution of the composites was carefully investigated. Through scanning electron microscopy and transmission electron microscopy analysis, it was found that HDPE addition refined the size of the dispersed SEPS microparticle phase and promoted a more uniform phase distribution. At low HDPE contents, a stacked structure of the dispersed phase was observed, while at high HDPE contents, a core–shell structure formed. This morphological change was explained by the spreading coefficient theory.

The rheological behavior of the composites also provided important insights. A long-term relaxation unit was found to be related to the percolated network microstructure formed by the dispersed particles. A relatively low concentration of HDPE (3%) promoted the formation of physical networks of SEPS particles, while an excessive HDPE content (over 10%) weakened the physical network.

The toughening mechanism of the PPM/HDPE composites was attributed to the soft-core hard-shell structure. HDPE reduced the size of the dispersed phase, suppressed SEPS coalescence, and formed an intermediate layer that dispersed stress concentrations. This promoted matrix shear yielding and effectively suppressed crack progression.

This research not only deepens our understanding of the toughening mechanism of PP composites but also provides a practical low-temperature toughening strategy that may be applicable to other thermoplastic polymers. It offers new possibilities for improving the performance of polymer materials in various industries, from automotive to packaging.

The paper “Phase Morphology Evolution and Rheological Behavior of Toughened Polypropylene Composite with Controllable Brittle soft-core hard-shell Ductile Transition Temperature Using SEPS@HDPE Core soft-core hard-shell Shell Structure,” authored by Jiahao Shen, Zhiyi Zhang, Wenwen Yu, Jiayi Wang, Weixuan Wang, Yonggang Shangguan, Qiang Zheng. Full text of the open access paper: https://doi.org/10.1016/j.eng.2024.04.027. For more information about the Engineering, follow us on X (https://twitter.com/EngineeringJrnl) & like us on Facebook (https://www.facebook.com/EngineeringJrnl).