Effects of Processing Parameters on the Mechanical, Thermo-Mechanical and Creep-Recovery Properties of Unidirectional Carbon Fiber Reinforced Thermoplastic Polypropylene Composites

Processing parameters play a key role in the mechanical, thermo-mechanical and creep-recovery properties of unidirectional carbon fiber reinforced thermoplastic polypropylene (CF/PP) composites because of high matrix viscosity, which governs their impregnation and interfacial bonding. This study systematically investigates the effects of molding temperature (190~210 °C), pressure (1~3 MPa), and holding time (5~15 min) on its short beam strength (SBS), storage modulus, loss modulus, tan δ, creep strain, strain recovery, and crystallinity using a Taguchi experimental design. The results presented that processing parameters have a huge effect on CF/PP composites’ SBS, and through the experimental design, the SBS could be improved by 68.1% (21.3~35.8 MPa). Holding time is the most influential parameter for SBS and damping performance, while temperature and pressure interact strongly, highlighting the importance of parameter synergy. There was a strong negative correlation between the crystallinity degree and the SBS of CF/PP composites, and a higher crystallinity degree means a sharper and higher melting peak. Creep-recovery tests reveal near-complete recovery (87~102%) at 30 °C, which decreases to 71~79% at 80 °C due to increased matrix mobility. Finally, it was confirmed that the relatively low SBS of CF/PP composites comes from the void and incomplete matrix impregnation of fibers. The above results advance the design of high-performance, sustainable thermoplastic composites for civil and structural engineering applications.