Tests reveal how thermoplastic composites lose strength at elevated temperatures

Researchers from Skoltech and their colleagues have investigated the flexural properties of pultruded glass fiber composites made from three thermoplastic matrices: polyphenylene sulfide (PPS), polyamide-6 (PA6), and polypropylene (PP). The study, published in the journal Next Materials, reveals that a composite's performance under heat is primarily determined by the polymer matrix, not the reinforcement.

When heated from room temperature to 280 °C, PPS-based composites retained strength up to 260 °C and over half of their initial stiffness at 220 °C. PA6-based materials were strongest at room temperature but showed gradual degradation with increasing heat. PP-based composites began to degrade above 120 °C and lost load-bearing capacity by 160 °C.

Thermal analysis confirmed that the materials' behavior under heating is dictated by the properties of the polymer matrix. PPS exhibited the highest glass transition temperature (109 °C) and the highest decomposition temperature (374 °C), contributing to its superior performance. PA6 occupied an intermediate position, while PP degraded most rapidly due to its low melting point (151 °C).

The findings will assist engineers in selecting appropriate materials for high-temperature applications, such as aviation, automotive, and energy industries. The study also emphasizes that even with the same reinforcing fiber, a composite's behavior under heat depends on the polymer matrix.

Sukhwant Pal, the study's first author, notes that no single analytical model accurately describes the temperature degradation of these materials. Different models are required for each matrix type. For gradual degradation (PPS and PA6), the Mahieux and Bosze models are most effective, while simpler models suffice for polypropylene, where properties drop sharply.