Abstract

Thermosets comprise a family of chemically cross-linked networks formed by irreversible chemical reactions, resulting in polymers with outstanding properties such as superior mechanical properties, high thermal stability and chemical resistance. However, their high stability precludes reprocessing or recycling, thus making it difficult to move towards a circular economy and sustainability. During the last years, research has focused on dynamic chemistry, in order to introduce reversible bonds along the polymer chain and synthesize reprocessable and/or recyclable thermosets. In this context, the Diels-Alder reaction is of special interest, as it can give thermoreversible reactions with a relatively low coupling and high decoupling temperature. Within this work, a trifunctional polyol containing the furan-maleimide Diels-Alder adduct has been developed to facilitate the recycling of cross-linked polyurethanes. The maleimide-furan adduct forms covalent bonds at low temperatures, but when heated these bonds break down, splitting the polymer chain into smaller chains and softening the polymer, allowing it to be reprocessed. Then, when the temperature drops, the bonds between the maleimide and the furan are reformed and the polymer regains its mechanical properties. Thereby, a thermoset polyurethane was synthesized reacting the triol containing Diels-Alder adduct with a polymeric isocyanate. The reprocessability and/or recyclability of the synthesized polyurethane was then evaluated by breaking it into small pieces and reprocessing them by pressing at high temperature, in order to obtain new sheets and evaluate their mechanical properties, being similar to the original ones.

Abstract

This chapter describes the different strategies that can be carried out with polymeric matrix composite materials once they have passed through the recycling centres currently available on an industrial scale.

The work describes how, depending on the nature of the matrices, as well as the fibers, the reuse of this type of fiber should be directed towards one type of process or another and how, through diverse technologies, it is possible to develop products with a greater or lower fiber orientation. In the same way, the chapter collects information on how some composite manufacturing processes can affect the fiber length and therefore modify the final properties of the product.

Thus, the different technologies currently available and under development in Spain with focus on giving a new life to carbon and glass fibers, while retaining most of their added value and properties, are described. Namely, reuse in thermoset composites, reuse as reinforcement in thermoplastic composites, fiber re-processing (woven or non-woven fiber structures), recycling of fibres that are still embedded, recycling specific to the nature of the fiber.

Throughout the text, some of the projects led by Spanish entities with the greatest relevance in the development of the different technologies are collected. At the end of the text, these same projects and others are highlighted, remarking the work carried out by Spanish centres, universities and companies in the field of re-using and recycling of carbon and glass fibers.