Composites are growing increasingly popular as construction materials. Nowadays, not only wind turbines but also airplanes, high speed trains and automobiles are manufactured using composites. Their advantageous properties like light weight and lack of corrosion make them attractive materials, and lightweight constructs allow commercial transportation to run more ecologically efficient. However, composites are composed of plastics, usually epoxy resins, which contributes to the fire risks in case of fire. To increase their safe usage, flame retardants are often implemented additively, yet in some cases, this affects mechanical properties. Moreover, the materials are rarely recyclable.
Conversely, so-called vitrimers offer an interesting alternative to thermoset matrices in composites. Due to their reversible covalent bonds, vitrimers share some material properties with both thermosets and thermoplastic polymers. Depending on their composition, vitrimers may be poured like epoxy resins or pressed into form like thermoplasts at high pressure and temperature. At elevated temperatures, vitrimers liquify and upon cooling, they form tough materials similar to thermosets like epoxy resins. This way, prepregs, i.e. pre-coated fiber components for composites, may be easily prepared. Additionally, due to their reversible melting behavior, vitrimers are easily recyclable compared to conventional thermosets. Thus, vitrimers are exceedingly interesting compounds for recyclable composites.
This paper discusses phosphorus containing vitrimers and investigates their role as intrinsically flame-retarded potential alternatives to common thermosets in composites. In a collaborative effort between BAM, the MPIP in Mainz and the Fraunhofer IFAM in Bremen, the publication elucidates not only the synthesis and chemical characterization, but also the fire behavior and mechanical properties of a phophonate-based vitrimer glass fiber composite. In a direct comparison to a non-phosphorus vitrimer, the phosphorus-containing material displayed a 27% reduction of the fire load, and the mechanical properties shared some similarities to epoxy resin composites. Moreover, the principle recycling capabilities of the material was demonstrated. These results highlight that vitrimers are a potential multifunctional alternative to common polymer matrices for composites.
Intrinsic flame retardant phosphonate-based vitrimers as a recyclable alternative for commodity polymers in composite materials
Jens C. Markwart, Alexander Battig, Tobias Urbaniak, Katharina Haag, Katharina Koschek, Bernhard Schartel und Frederik R. Wurm.
published in Polymer Chemistry, Vol. 11, 2020, pages 4933-4941.
BAM, Division Technical Properties of Polymeric Materials