Abstract

Reprocessable, repairable, and recyclable thermoset composites have the potential to improve the circularity of composites and ensure their sustainability. Disulfide-based epoxy matrices are under development to meet the specifications of key sectors (transport, wind energy, etc.). The manufacturing of reinforced composites by different techniques (infusion, prepregs, etc.) has been validated (TRL 4-5). The incorporation of new functionalities, such as fire resistance, and the validation of recycling are now on the table. Conventional chemical recycling separates the reinforcement from the composite for reuse, leaving aside the polymer matrix. The new functionalities incorporated require recovery and validation of the matrix. Dynamic resins allow milder conditions to be applied during recycling, and the challenge remains to find new applications that reuse these recycled products. On the other hand, the rational use of resources demands for new recycling routes that do not require separation of the components, apply few steps, be economically viable, and maintain the value of the reinforcement and the matrix. Vitrimers allow the thermoforming of composite parts through the application of heat and pressure. This process opens the way from reusing final parts with new shapes to generating short new fibre composites from mechanical pre-crushing. This communication will present the latest developments and objectives of several European projects funded under the HE framework (SURPASS, GA no. 101057901; MC4, GA no. 101057394) in which work on the maturity of the technologies is in progress.

Abstract

Vitrimers are polymeric materials that behave as thermosets at room temperature but, when heated, they exhibit a plastic flow like thermoplastics, enabling their reprocessability. This reprocessability is particularly appealing for sustainable material design, as it allows damaged components to be repaired or recycled, reducing waste and extending their lifespan. When heated above the glass transition temperature, vitrimer networks undergo dynamic covalent bond exchange, allowing damaged material to recover its original mechanical properties while maintaining its shape. The objective of this study is to present a new constitutive model to computationally predict the mechanical behaviour, damage and healing evolution in materials with self-healing properties. The model is informed by experimental data derived from tailor-made aromatic disulfide-containing epoxy vitrimer samples. The model is formulated at the material point level following thermodynamic principles within a continuum mechanics framework. The damage evolution function depends on thermodynamic forces while the healing function, modelled as reverse damage, is based on the kinetic results of the chemical transformation process of the material. Simulations were conducted to observe temperature and stress distributions in a sample subjected to a cyclic sequence of damage-inducing loading, unloading, healing-inducing heating, and a new cycle of reloading and unloading. The results replicate the force-driven mechanical behaviour and temperature-driven self-healing properties of the vitrimer. Validation against both experimental data and literature demonstrates the model’s capability to capture the self-healing material behaviour. 

Abstract

In the railway sector, represented by TALGO, there is growing concern about the recyclability and reusability of composite parts at the end of their useful life. To address this concern and comply with Spanish and European directives and legislation, an innovation project (divided into "PART 1" and "PART 2") has been carried out in which AITEX, CIDETEC, iTREE, and TALGO have collaborated. The main results of the first part are as follows:

1. Recycling process of the composite material of the first prototype of the Lightweight Running Gear Frame: This process focuses on recovering the carbon fiber contained in the material by combining several chemical-physical steps.
2. Development of a 3R (recyclable) epoxy resin formulation: This resin meets the thermal, mechanical, and FST (Fire, Smoke, and Toxicity) requirements of the railway sector and is suitable for use with prepreg technology.

The results indicate that the recycling process is suitable for producing post-consumer parts and that the formulated resin meets the required standards. The overall conclusions of the project will be revealed in PART 2 of this study.