Relationship between mechanical properties and flame retardancy in halogen-free sandwich structures based on PET foams and glass fiber-reinforced epoxide resins

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Sandwich structures, consisting of a cellular polymer core and thin compact skin layers, are gaining increasing relevance as structural materials since they combine a low density with a relatively high strength. Sandwich materials based on polyethylene terephthalate (PET) foams were introduced only a few years ago. These structures are innovative materials with outstanding shear and compressive strength. However, they have the disadvantage of being flammable compared to other foam materials. This undesired flammability must be overcome by adding flame-retardant additives to PET. Concerning flame-retardant PET-based sandwich materials, the relationship between the microscopic morphology of both the foam core and the skin layer phases on the structure mechanical properties and fire resistance has not been systematically investigated. The aim of this project is to investigate and clarify the mentioned structure-property relationship and set the foundations on the knowledge on materials science for further improvement of these materials. The selected materials composing the sandwich materials are foam-extruded PET as the core and a high-performance glass-fiber reinforced epoxy resin as skin layer. These sandwich structures will be obtained by Vacuum Assisted Resin Infusion (VARI). The flame retardancy of PET foams will be improved through a novel additivating concept. This concept consists on introducing in PET phosphorous- and sulfur-based compounds having synergetic flame retardant properties. The epoxy resin will be also additivated with phosphorous-based compounds to improve its flame retardant properties. These compounds will not affect the resin processability for VARI infusion nor its ultimate mechanical properties. To understand the influence of both phases on the overall properties of the sandwich material, the core and skin layers will be studied separately. As such, all relevant material and process parameters for PET foam extrusion can be investigated. The effect of the foam extrusion parameters on the foam microstructure will be herein elucidated. Afterwards, the influence of the flame-retardant additives on the foam morphology will be investigated. Then, the relationship between the foam morphology on the mechanical properties and fire resistance will be established. Finally concerning the sandwich structures, the surface interactions between the core and the skin layer will be investigated and then related to the mechanical properties and fire resistance of the sandwich. Especial attention will be paid to the fire resistance of halogen-free flame-retardant sandwich samples, with the fire-retardant mechanisms of such structures being elucidated. For this joint research project, the scientific competences of UBT (material science) and TUD (flame retardants) are optimal for a research collaboration.