Recycling of Thermoset Materials and Thermoset-Based Composites: Challenge and Opportunity

Abstract

Thermoset materials and their composites are characterized by a long life cycle with their main applications in aircrafts, wind turbines and constructions as insulating materials. Considering the importance of recovery and valorization of these materials at their end-of-life, avoiding landfilling, the interest concerning their recycling grows continuously. The thermoset materials and their composites, to be successfully recovered and valorized, must degrade their three-dimensional structures and recover the mono-oligomers and/or fillers. The thermoset materials could successfully degrade through thermal treatment at different temperatures (for example, above 1000 °C for incineration, ca. 500 °C for oxidation/combustion of organic constituents, etc.), chemical degradation by catalyst, irradiation with or without the presence of water, alcohol, etc., and mechanical recycling, obtaining fine particles that are useful as filler and/or reinforcement additives. Among these recycling methods, this mini-review focuses on the formulation and recovery method of innovative thermoset with in-build recyclability, i.e., materials having chemical links that could be degraded on-demand or containing dynamic covalent bonds to have re-processable and/or recyclable thermoset. This issue could be considered the future perspective in developing novel thermoset materials. The aim of this review is to get an overview of the state of the art in thermoset recycling and of the most commonly used thermoset composites, recovering valuable reinforcing fibers. Additionally, in this work, we also report not only known recycling routes for thermoset and thermoset-based composites, but also new and novel formulating strategies for producing thermosets with built-in recyclability, i.e., containing chemical-triggered on-demand links. This mini-review is also a valuable guide for educational purposes for students and specialized technicians in polymer production and recycling.

Recyclable, Repairable, and Fire-Resistant High-Performance Carbon Fiber Biobased Epoxy

“…In mechanical recycling, the composites are finely ground, and the obtained powder is reused as a filler material in the development of new composites for low-value applications. Fiber reclamation consists in recovering of carbon fibers from composites by applying extreme thermal (pyrolysis at 450–700 °C in an inert atmosphere, oxidation in fluidized bed with air at 450–550 °C) or chemical (in catalytic solutions, benzyl alcohol, HCl, or supercritical fluids) processes. , In this work, the CFRC samples were successfully thermally repaired, and also chemically recycled using a simple protocol, at relatively low temperatures and using a more eco-friendly process compared with the already used ones. For example, Xu et al recycled CF from CF/epoxy composites using a two-step method, where initially the composites were pretreated in acetic acid, and then the samples were immersed in a hydrogen peroxide/dimethyl­formamide solution in a hermetic reactor at 80–150 °C for 5–120 min.…”

Nitrile rubber/peroxide treatment transformed recycled polyethylene into mechanically improved materials

“…Recycling such plastics is essential to reduce environmental pollution and global warming related to the dependency on petrochemical feedstock. Compared to thermosets or hazardous thermoplastics such as polyvinyl chloride, polyethylenes are relatively easy to recycle using thermomechanical processing 2–4 . In recent years, overall plastic recycling rates are approximately 9% in the United States and 15% in Europe 5,6 .…”

“…Compared to thermosets or hazardous thermoplastics such as polyvinyl chloride, polyethylenes are relatively easy to recycle using thermomechanical processing. [2][3][4] In recent years, overall plastic recycling rates are approximately 9% in the United States and 15% in Europe. 5,6 In 2021, global plastic production was approximately 391 Megatons, of which polyethylene was around 27%, and a small fraction was recycled.…”

Integrating a Chemicals Perspective into the Global Plastic Treaty

“…90% of the plastics) or thermosets (e.g., epoxy, polyurethane; ca. 10% of the plastics) . Further, various chemicals are added in plastic production, as processing aids to enable or ease the production and processing of the plastics (e.g., lubricants) or as additives to maintain, enhance, and impart specific properties (e.g., plasticizers, flame retardants, antioxidants).…”

“…10% of the plastics). 14 Further, various chemicals are added in plastic production, as processing aids to enable or ease the production and processing of the plastics (e.g., lubricants) or as additives to maintain, enhance, and impart specific properties (e.g., plasticizers, flame retardants, antioxidants). A recent study has identified more than 10,000 chemical substances that may have been used as monomers, processing aids, and additives.…”