In this project, SPECIFIC POLYMERS will notably be in charge of the chemical modification of thermoset resin composition to ease the enzymatic degradation of composite materials without affecting their mechanical properties. To do so, we will custom-designed building blocks containing enzymatically degradable moieties or functional groups easing such degradation mechanisms. In order to achieve this goal, our strategy is based on three main aspects. One of these aspects is based on vitrimer-like technologies in order to be able to dissolve the thermoset material at its end-of-life. What is the role of vitrimers and how do they work?

 Difference between thermoplastics and thermosets: Commonly used in industry, plastics can be classified into two major families, with distinct properties. Thermoplastics are the most commonly used and are characterized by the reversibility of their molecular bonds under the effect of heat. By heating them, these plastics go from a solid to a viscous state, which allows their recyclability. Thermosetting plastics, such as epoxy resins, on the other hand, require a cross-linking step that “freezes” the covalent molecular bonds. More resistant to mechanical stresses, but also to chemical attack, thermosets are used as a matrix for composite and high-performance parts whose strength sometimes rivals that of certain steels. One drawback, however, is that the cross-linking process is not reversible, which prevents these materials from being recyclable.

Vitrimers: The third generation of polymeric materials Vitrimers, materials resulting from work on supramolecular chains, carried out since the 1990s, combine the advantages of both families of polymers. Their internal structure is composed of a Covalent Adaptative Network (CAN) which has the ability to reversibly rearrange its network structure through dynamic covalent bond exchange reactions under the effect of external stimuli such as heat, pH and UV light. These materials thus combine the strength of thermosets with the reversibility of thermoplastics. Indeed, at high temperatures, vitrimers behave as viscoelastic liquids and can be reshaped like traditional glass materials. While at low temperatures, the network structure is comparable to that of traditional thermosets with good network integrity.

To achieve this molecular network rearrangement, it is necessary to introduce specific dynamic covalent bonds. Therefore, various dynamic covalent chemistries were reported such as transesterification, imine amine exchange, vinylogous transamination, olefin metathesis, disulfide exchange and hindered urea exchanges. In contrast to some of the traditional exchange reactions, these associative mechanisms don’t require high temperature treatments, petro-based additional monomer, special processing (e.g., sonication), and/or metal catalysts to allow recycling process. With this whole process, within the BIZENTE project, recovering solubility will clearly ease the enzymatic degradation process which cannot be achieved on crosslinked materials.

References: Recyclability purpose – Vitrimers strategy, Solène Guggari, May 2022