Abstract

In this work the development of a profile manufactured by pultrusion and thereafter curved by thermoforming is presented. The pultrusion profile is based on a new thermostable epoxy resin with dynamic bonds capable of being reprocessed, repaired and recycled thanks to the incorporation of reversible links within its reticulated structure (3R Technology). Being a resin that contains dynamic bonds, the cured composite shows unexpected properties for thermosetting materials.

In a first phase, a new resin/hardener formulation processable by pultrusion with a viscosity, adhesion to the fibre and speed of curing similar to a conventional formulation have been developed and in a second phase, taking into account the properties of the new composite material, the parameters of the thermoforming process (pressure, temperature or speed of thermoforming) have been analysed and optimized.

Thanks to the combination of pultrusion and thermoforming processes, longitudinal 3R composite profiles acquire a new curved geometry defined by the design of a mould. In this way, the thermoforming of the straight profiles will allow manufacturing curved parts from profiles of thermostable composites with high mechanical performance manufactured by pultrusion in medium-high rates, typical of the automotive sector. Additionally, it will be shown that the profiles manufactured using these composites can be recycled, reducing the amount of waste generated and offering these materials a second useful life.

Abstract

Due to the environmental challenges the aeronautical industry is facing, the development of recyclable aeronautical chemistries have recently raised huge interest. Within this context, different alternatives are being considered with the aim of obtaining new resins with current thermoset resin chemical and mechanical performance, while showing reprocessing, repairability and recyclability capabilities, like thermoplastics.

New vitrimer resins, based on dynamic covalent chemistry, become a very attractive alternative to combine the main advantages of aeronautical thermosets (processing conditions / temperature, means, properties…) and thermoplastics (recycling, welding…). Therefore, they can be the solution to overcome the current limitations of thermoset epoxy resin, which is widely used in composite airframe.

This project, framed within the national funding PTA (Plan Tecnológico Aeronáutico), was focused on the preliminary development of aeronautical vitrimer resin for composite airframe. It was proposed as continuation of the EU Horizon 2020 consortium AIRPOXY led by CIDETEC, where a first generation of aeronautical RTM vitrimer was developed and characterised, while a second generation vitrimer development was only started. This chemistry is based on the use of dynamic aromatic disulfides as hardener.

In the first 2021 project phase, following key activities were performed:

• Definition of targets or reference values for aeronautical infusion resin

• Second generation of infusion vitrimer: development completion and characterization

• Flat CFRP laminates with second generation vitrimer: production and thermoforming trials.

Next steps of the project in 2022 and 2023, will be focused on developing, evaluating and demonstrating an aeronautical prepreg based on vitrimer resin with higher Tg in line with current aeronautical resin values.

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.