On Improving the Mechanical Properties of Polypropylene Composites: Airflow Processing and Application of Waste Carbon Fiber

To investigate the effect of steam kinetic milling on morphology retention of waste carbon fibers and the reinforcing performance of their composites, steam, ambient air, and high‐temperature air were used as grinding media at classifier frequencies of 30, 45, and 60 Hz to prepare carbon fiber powders with different aspect ratios, denoted as SKM‐CF. Four representative powders were incorporated into polypropylene, PP, at 10, 20, and 30 wt% to fabricate SKM‐CF/PP composites. Powder morphology, tensile properties, flexural properties, impact properties, and fracture morphology were systematically analyzed. The results showed that steam grinding reduced fiber breakage and better retained fiber morphology, with the highest aspect ratio obtained at 45 Hz. Compared with neat PP, the composite containing 10 wt% S45 powder exhibited superior overall performance, with tensile strength, flexural strength, and notched impact strength increased by 42.65%, 30.16%, and 16.65%, respectively. In addition, the tensile modulus and flexural modulus increased overall with increasing SKM‐CF content, indicating enhanced stiffness and flexural rigidity. This study demonstrates that steam kinetic milling effectively retains the aspect ratio of waste carbon fibers and verifies their reinforcing effect in PP, providing an experimental route for converting low‐value recycled carbon fiber powders into reinforcements for thermoplastic composites.