This project demonstrates conditions to disassemble certain carbon fiber-reinforced polymer (CFRP) composites using oxidative catalysis. The technical objectives of this project are as follows:
Polymer composites are widely used across the defense services because of their high performance and light weight relative to conventional metal alloys. Most of the polymer matrices involved are thermoset epoxies, which undergo an irreversible cure reaction, converting them from viscous liquids to stiff, glassy solids. The irreversibility of this curing impedes the productive reuse of scrap and recycling of end-of-life fibre-reinforced plastic (FRP) composites and constitutes a growing obstacle to more wide-spread use. Amplifying the problem is the inefficiency of FRP manufacturing methods: generally, 20% to 30% of purchased material (prepreg) becomes production waste, and no standard approaches exist for reusing production waste or recycling end-of-life FRP composite products.
At present, most composite waste is sent to landfills, often with attendant haz-mat disposal fees. The widespread use of composites in defense applications has led to an urgent need for new strategies for the recycling of scrap and end-of-life composites. Moreover, developing catalytic reactions that will gracefully deconstruct cured FRP matrices will require the development of technology to enable catalysts and reagents to move and react within the complex solid-phase composite architecture. The project team have recently invented methods to enable such catalytic depolymerization of common thermoset polymers used in composites manufacturing, and the long-term goal remains to apply these catalytic methods to the cleavage of FRP matrix linkages in a way that enables the recovery of useful small-molecule monomers and preserves the length and order of composite fibers.
Realizing this goal requires solving the problem of selective polymer cleavage using only sustainable reagents, and understanding the transport of catalysts, reagents, and products through the assembled composite material environment. Having shown the first catalytic oxidation capable of realizing this objective, the project team are now poised to develop the discovery into useful applied technology.
The process used selectively exploits oxidative vulnerabilities in amine-linked epoxy-based CFRP matrices to enable the selective depolymerization of these thermoset polymer materials to return useful fine chemicals. Developing catalytic reactions that will gracefully deconstruct cured carbon fiber reinforced polymer (CFRP) matrices has required basic understanding of how catalysts and reagents move and react within the complex composite architecture. In this project the team transformed discoveries to usable technology for catalytic deconstruction of composite materials.
Concurrently, because the polymer matrix is removed, imbedded carbon fibers are released, substantially undamaged. Such recovered carbon fibers are potentially suitable for remanufacturing into new products of value in the Department of Defense (DoD) supply chain or the public market. The recovered fine chemicals resulting from disassembly of the polymer matrix are similarly suitable for remanufacturing.
A sustainable FRP recycling solution will increase overall efficiency of manufacturing, recovering high-value constituent materials and returning them to service, reducing costs and adverse environmental impact for both production waste and end-of-service-life scrap materials. The results and insights gained from these rigorous studies has advanced the knowledge and understanding on chemical recycling of FRPs and provides a pathway towards improved recycling of both fibers and polymers harvested from current-generation composites.
Following on relationships forged at the 2019 SERDP Symposium, the project team has initiated conversations with DoD stakeholders in the CFRP space. The project team has plans to visit Dr. Ben Harvey at the Naval Air Systems Command China Lake to retrieve CFRP samples for which he had recycling needs. Dr. John La Scala at Army Research Laboratory (ARL) (Aberdeen Proving Grounds), has arranged to send representative samples from ARL of composite materials waste to further challenge the chemical recycling process.