Fiber-reinforced polymers in railway application – Flame retardant strategies for low-viscous epoxy

Fibre-reinforced plastic composites (FRP) have become increasingly important in rail transport in recent years due to their high lightweight construction potential. In order to ensure their use, however, not only mechanical parameters are decisive; above all, flame-retardant properties play an important role in the transport sector. In order to bring these into line with the liquid impregnation manufacturing processes for FRP in rail transport, new innovative flame retardant strate-gies are necessary.

Due to the processing method of liquid impregnation for ceiling or floor structures in train compartments, the choice of flame retardants is limited to liquid additives that are soluble in the resin or hardener mixture or nanoscale additives. The additives used in this work are a 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) novolac, a hexa-phenoxy-cyclotriphosphazene (short: cyclophosphazene, CP) and nano-SiO2-particles. The discussion part of the paper is divided into three subchapters. First, the suitability of the flame retardant additives for liquid impregnation processes and their synergistic flame retardant effect is examined. Then the systems are transferred into fibre-reinforced plastic composite, whereby different fabric types with carbon fibre reinforcement and with glass fibre reinforcement are investigated. In the last part, the mechanical properties are evaluated in the compression test of the FRP laminates. The residual stability of the burnt fibre-laminates is determined by means of compression- and three-point bending tests on cone calorimeter specimens („post-fire“ mechanics).

The scientific focus of the work is on the evaluation of flame retardant mechanisms and the in-teraction of gas-phase and condensed-phase active additives during combustion. DOPO used as single additive has a strong gas-phase active character. It is shown that char residues of CP are supported by the partial condensed-phase activity of DOPO. The formation of combustion residues that reduce heat release are the result. Nano-SiO2 in combination with DOPO results in solid-phase activity of phosphorus-based DOPO, leading to the formation of phosphorus-silicon oxide glasses. In addition to the flame retardant behavior in the neat resin, the transfer mechanisms of the fire-retardant properties into fiber reinforced composite are evaluated. The combination effects of the flame retardants have to be matched to the fabric type, especially the mode of action of the condensed-phase active flame retardants is strongly limited by the fiber fabric.The matrix-dominant mechanical properties of the fibre laminates were determined in compres-sion tests. It is shown that the interaction of the flame retardants with each other is decisive. The incorporation of cyclophosphazene increases neat resin modulus and consequently the compres-sive strength of the laminates. The spherical nano-SiO2 particles also show an improvement in strength; only in combination with DOPO is a drop-in strength measurable for CP and SiO2 at high contents. This is due to an increased tendency of the cyclophosphazene to phase separation and the agglomeration formation of the nano-particles. The post-fire mechanical properties are mainly improved by glass-forming residues of the nano-SiO2 systems or their combination with DOPO. The reason for this is that the individual layers „stick together“.