Transfer of flame retardants from the polymer material to fiber-reinforced plastic composite: flame retardancy – fire resistance

Due to their strength and versatility, epoxy resins and fiber-reinforced composites are widely used in the aerospace, automotive, and construction industries. However, ensuring fire safety in these materials requires the incorporation of FRs. While FRs are well-studied in epoxy resins, their transfer from resins to fiber-reinforced composites remains a challenging and unexplored field. The main scientific goals are to analyze the transfer of FRs between epoxy resins to fiber-reinforced composites and the transfer between fire behavior and fire stability utilizing different tools such as the quantification of the flame-retardant modes of action in epoxy resins and fiber-reinforced composites and the FRI. The research begins by fabricating the resins and composites, aiming for a fiber content of around 75 wt.% in the composites. The materials' thermal decomposition, flammability, fire behavior, fire stability, and mechanical properties were analyzed using various techniques to understand the modes of action by which the FRs work, such as flame inhibition and dilution, char formation, and protective layer effect. Analytical methods, including TGA and FTIR, were used to evaluate the thermal decomposition. At the same time, fire testing techniques such as UL-94, LOI, cone calorimetry, SEM, XCT, and bench-scale fire resistance tests provided insights into flammability, fire behavior, and fire stability. By integrating data from these experiments, quantifying the FRI and flame-retardant modes of action allows for a detailed comparison between epoxy resins and their composites as well as between fire behavior and fire stability. Before and after fire exposure, mechanical properties were evaluated with fracture toughness, DMA, and flexural strength tests. Additionally, processing properties such as rheology and DSC were examined to determine how adding FRs influences resin curing and processability. The epoxy resin used in this research was DGEBA, cured with DICY and UR400. DGEBA is widely used because of its excellent chemical and electrical resistance and strong mechanical performance. The study explored varying flame-retardant loadings (8 wt.% to 40 wt.%) of different phosphorus-based FRs, including APP, MPP, SiAPP, and AlPi. These FRs were transferred from the resins to composites reinforced with UD, BD, and WR GFs via prepregs. The results show various tendencies regarding flame retardancy and mechanical integrity. The transfer from epoxy resin to composite significantly changes the flame retardancy modes of action. Composites with only 8 wt.% FR showed reduced char formation, whereas increasing the FR content to 24 wt.% enhanced flame retardancy by improving protective layer formation. However, at 40 wt.% FR, a higher amount of flame retardancy content interfered with resin curing, increased void formation, observed via SEM, and weakened the mechanical strength of the composites. One of the main goals of this study is understanding the transfer between flammability and fire behavior and fire behavior to fire stability. While higher flame-retardant content improved delayed ignition and reduced PHRR, their impact on fire stability (Tf and tf) was more complex. At 8 wt.% of FRs, flame retardancy was hindered by the GFs and the modes of action were suppressed; at 30 wt.% of FRs, the balance between flame retardancy and mechanical performance was optimized, whereas, at 40 wt.% of FRs, the composites exhibited structural degradation, reducing the fire stability. Overall, this research achieves its scientific goals by providing a fundamental understanding of the transfer of FRs from epoxy resins to fiber-reinforced composites as well as the transfer between fire behavior and fire stability. These findings offer crucial insights into balancing fire protection and mechanical integrity for composite materials. The results of this study will contribute to the development of more advanced flame-retardant materials for applications in industries such as transportation, construction, and aerospace, where both fire safety and mechanical performance are critical.