ReSet

Context

Plastics are indispensable materials for industrial development, as they are easy to tailor for applications and also have low production costs. Demand by society keeps rising as plastics can also offer unique material properties, as found in light weight structural components or elastomers. However, as a consequence of this huge success, the plastics industry is also facing a crisis. This is because some of the characteristics that make synthetic polymers such useful materials, also make them environmentally persistent pollutants. Indeed, a significant fraction of current bulk end-of-life synthetic polymers, such as thermosets (105 million tons in 2016) and rubbers, is intrinsically non-recyclable and as a result is ending up in landfills, is being burnt, or, in the worst case, is simply discarded into the environment. In other words, there is an urgent need to reimagine crosslinked plastics, and how their life cycles can be made much more sustainable, while safeguarding their versatility, added value and low production cost.

During the last years, there is a strong drive – both in academic and industrial context – to find effective recycling solutions for the strongly growing market of thermosetting resins and elastomers. These polymer materials are considered unrecyclable as they derive their strength from chemical bonds (or crosslinks) that are embedded throughout the bulk of a material, permanently fixing their shape. Any reprocessing or reshaping of such a material, beyond an elastic response, can only be done by first chemically degrading the polymer, resulting in a loss of its properties, or in a costly purification and ‘chemical recycling’ process. Within the consortium of this project proposal, a strong, world-wide recognized expertise is available on an emerging new strategy in the chemical design, implementation and study of so-called covalent adaptable networks (CANs). 

These innovative materials combine the strength of cross-linked materials with the processability and malleability of non-crosslinked thermoplastics by implementing dynamic covalent crosslinks. The concept has been demonstrated very recently on various simple synthetic polymers using robust chemical platforms, developed by the consortium partners. 

The first purpose of this sprint SBO is to implement these novel dynamic crosslinking chemistries in epoxy and polyester based resins, two major industrially applied material matrices for thermosets, for example in composites, adhesives and coatings. The clear aim would thus be to take up such polymers and their applications into a circular economy paradigm and to “recycle the unrecyclable”.

Goals

Since the introduction of CANs more than a decade ago, vitrimers have taken a central place as an ‘ideal’ polymer material, combining thermoset durability with thermoplastic (re)processing properties. Vitrimers were introduced only in 2011, and reviewed for the first time world-wide in 2015 by some of us(see graphical abstract from Chemical Science), and are now being implemented industrially on small scale (e.g. Mallinda).

The initial moonshot idea of this SBO is to go beyond the current processability limits encountered with vitrimers, focusing on chemical design, using drop-in technologies only. 

All vitrimers have a temperature response similar to that of inorganic glasses: at increased temperatures they can flow, while remaining insoluble and fully crosslinked at all times. Theoretically they can be recycled like inorganic glasses. However, the limited thermal stability of the vitrimers has so far prevented the use of continuous processing techniques, like extrusion, as the required temperatures for fast processing are too high. Only very recent studies (in 2019, UGent) have for the first time shown proof-of-concepts on prototypical vitrimer materials, by controlling and accelerating the dynamic crosslink exchange to the point where it will not limit the processability. Our ambition for this ‘moonshot’ project is to combine the tunable chemical platforms we developed for this purpose with common thermoset formulations and thus to provide a novel material design approach for large volume synthetic polymers. After the finalization of the sprint SBO (thus after 1,5 years), these concepts can be applied to other thermosetting polymeric materials, such as polyurethanes. The circular economy aimed for will a) reduce the carbon footprint of these materials, b) reduce the reliance on petrochemical resources and c) reduce overall waste. 

Given the recent emergence of vitrimers, a detailed understanding of how chemical structure and mechanical properties are related is still lacking. To speed up development, the consortium identified the strong need for an in-silico tool, not yet available, that can fully capture the relation between reaction/processing conditions and the dynamic 3D configurations on macromolecular level. A powerful dynamic polymer network design tool, which will incorporate current understanding of the polymerization and crosslink exchange mechanism/kinetics and the distribution of the dynamic bonds, is expected to result in a predictive model for building block selection and network design, up to and including processing and recycling strategies