Magnetic field-structuring as versatile approach to shape the anisotropic mechanical response of composite foams

The use of a high amount of reinforcing phase improves the mechanical performance of a polymeric composite structure. In composite foams this approach is associated to difficulties in developing a regular cellular structure, due to the hindering effect of such high amount of non polymeric phase on the bubble growth process. Additionally, the compressive response of foams produced with conventional forming processes is isotropic, and cannot fully exploit the reinforcing potential of the reinforcement. To overcome this limitation, the reinforcing phase should be aligned along preferential directions but available industrial methods are costly post-processing procedures that can induce damages in the cellular structure. Recently, several research proposals investigated the use of a contactless magnetic field to build reinforcing structures in polymeric foams, but they were mainly focused on the linear elastic region of the compressive response, while the large strain behaviour was not under control. In this paper, a process based on a time-variable magnetic field is exploited to manage in a simple and fully adaptable way the shape of the stress-strain response of the foam (up to at least 60% strain) during the foaming process. The resulting composite foam is characterized by mechanical anisotropy with a homogeneous and regular cellular morphology. This goal is achieved by imparting suitable geometrical features to the field-structured reinforcing aggregates by means of the proper time-profile and intensity of a magnetic field during the production process.