Performing composite materials: thermoplastic matrix for more circular components, from monomer to in situ polimeryzation

To move towards eco-sustainable and circular materials, one of the most effective solutions is to create thermoplastic composites. The strong commitment of world organizations in the field of safeguarding the planet has directed the research of these materials towards production processes with a lower environmental impact and strong propensity to recycle the polymeric part. The will to produce a composite with a thermoplastic matrix lies in its intrinsic value nature, which is the òpposite of thermosetting one, that is the possibility of recycling the material: the thermosetting polymer, such as epoxy resin, certainly guarantees the best mechanical characteristics, but once that the material has finished its function, it cannot be reworked and reused for other pùrposes. Up to now, indeed, incineration and landfilling are the main approaches for disposing of composite wastes. These routes, however, are not viable tools in view of the strong expected growth in waste production because they completely discard the related environmental impact, the waste accumulation of composites and they especially imply the loss of all the high-added value. On the other hand, the thermoplastic polymer can undergo a softening process which allows to obtain again a melt capable of being subjected to a new type of processing with a new purpose. What is being studied in recent times is the search for the application and implementation of impregnation processes used for thermosetting matrix composites towards thermoplastic matrix ones. The attempted infusion methods involved the use of an already polymerized thermoplastic matrix which was brought back to the molten state by applying heat, subsequently proceeding with the impregnation of the fiber fabrics. The problem encountered, however, lies in the difficulty of the actual impregnation due to the high viscosities of the thermoplastic polymers in the molten state. Under its chemical properties, Nylon 6 is the polymer that best satisfies this specific trade-off above all thanks to its precursor, Ɛ-caprolactam, a molecule that melts at about 70 °C which allows to obtain a liquid phase characterized by similar water viscosity. With this low viscosity it is possible to obtain a potentially optimal impregnation of the fibers, with subsequent reaction activated by the increase in temperature. The solution, therefore, lies in in-situ polymerization precisely because it no longer allows starting from a polymer, in which macromolecules are already formed, which results in high viscosity and process temperatures from 170 °C up to 200 °C, but from monomers which therefore allow a process not too far from those already known for thermosetting. However, it will be necessary to move towards an optimization of the process parameters and, consequently, of the impregnation phase, guaranteeing a greater homogeneity of dispersion of the reactive mixture and a better adhesion between fiber and matrix, aiming at the study of sizing compatible with thermoplastics matrices. These resolutions will lead to the real goal, which is the production of performing composite materials with a thermoplastic matrix, with increasing volumetric quantities of reinforcement, up to values equal to 40%, allowing a true comparison, in terms of both the quantity of filling and mechanical properties, with composite materials with a thermosetting matrix. It is a fact that, when taking into account factors such as climate change, global warming, environmental sustainability and circular economy, the landfill or incineration of composites wastes must be avoided. The future research studies must be focused on the following points: the production process and the chemistry of the matrices to obtain a performing thermoplastic matrix composite, with mechanical–functional characteristics suitable for completely replacing thermosetting agents; the use, recovery, and disposal of thermosetting and thermoplastic composites which re-enter the circulation to obtain manufactured articles; evaluation of the potential to close the life cycle loop of composites and reducing energy consumption and recycling cost. Thanks to the intelligent reaction and research of different groups around the world, the future looks bright for new possibilities.