Towards Sustainable Recycling of Epoxy-Based Polymers: Approaches and Challenges of Epoxy Biodegradation

Composites are especially important for the development and implementation of sustainable technologies such as wind power, energy-efficient aircrafts, and electric cars. Despite their advantages, their non-biodegradability raises challenges for the recycling of polymer and composites in particular.

Figure 1. Production volume (status 2021) and end-of-life fate of plastics (status 2017) [7,11]; LD/HD-PE—Low-density/High-density polyethylene, PP—Polypropylene, PVC—Polyvinyl chloride, PET—Polyethylene terephthalate, PUR—Polyurethane, PS—Polystyrene.

Epoxy polymers are widely used in various industries, e.g., as coatings, adhesives, and for lightweight construction due to their unique properties such as high strength, chemical resistance, and adhesion to various surfaces. Therefore, one of the most prominent applications is their use as matrix material in fiber-reinforced composites, which are heavily employed in the aerospace sector. However, the disposal of epoxy polymers and composites thereof has become a significant concern due to their recalcitrant nature and the adverse environmental effects caused by traditional recycling methods.

In this context, the overall production of plastic waste is projected to double within the next 20 years, with only 18 percent currently being recycled, leading to the deposition of around 12,000 Mt of plastic waste in landfills and the environment by 2050 (see figure 1). Even though epoxy polymers only accounted for a rather small share of the global plastic production volume of around 7.1 percent in 2021, the demand for fiber-reinforced polymers is steadily increasing. This is closely linked to emerging sustainable technologies such as off and on-shore wind power plants or electric vehicles that rely on lightweight construction materials for increased efficiency. To cover the resource demand of the composite industry and to reduce the environmental impact of accumulated epoxy waste, there is an urgent need for the development of sustainable recycling methods.

Conventional recycling methods for epoxy polymers, such as solvolysis, pyrolysis, or nitric acid treatment, involve harsh chemicals and the application of high temperatures and pressures. Not only does this lead to the emission of large quantities of CO2 and other pollutants, but it also generally results in a reduction of the material properties. Particularly with regard to composites, the fibers often suffer from the extreme reaction conditions, resulting in a reduction in their quality.

As a result, plastic-degrading biocatalysts have attracted growing attention due to their advantageous properties such as milder process conditions, high substrate specificity, and overall reduced environmental impact. Enzymatic PET (polyethylene terephthalate) depolymerization is one example of the significant progress that has been made within the last decade in the field of plastic degradation. However, the development of biodegradation approaches for recalcitrant plastics such as epoxy polymers has been less comprehensive and is still limited to a few studies undertaking this challenge. In this regard, the exploitation of biodiversity resources, especially in combination with metagenome mining and environmental screenings, represents a promising approach towards the development of novel biocatalysts for plastic degradation. This approach has been successful in identifying enzymes that can degrade plastics such as PET and can potentially be used to develop biocatalysts for the degradation and recycling of epoxy polymers.

There are a variety of existing approaches taken towards the development of biochemical degradation methods for epoxy and epoxy-based fiber-reinforced composites. As the development of bio-based recycling technologies also requires comprehensive analysis of the underlying processes, an overview of available analytical approaches is given.

The recycling of epoxy resins with bio-based methods is an important area of research, as it offers a sustainable and environmentally friendly alternative to conventional methods. While challenges exist in terms of recalcitrant epoxy degradation and the need for robust biocatalysts, advancements in metagenome mining, directed evolution, and utilization of oxidative enzymes provide avenues for overcoming these hurdles. Additionally, research on bio-degradable epoxy and bio-reinforced materials contributes to a holistic approach towards a more sustainable epoxy industry. Continued interdisciplinary efforts in exploring and optimizing bio-based epoxy recycling methods will pave the way for a greener future and a circular economy.

This work was performed by Leon Klose, Neele Meyer-Heydecke, Sasipa Wongwattanarat and Jennifer Chow for the University of Hamburg and the Office of Naval Research Global. For more information, download the Technical Support Package (free white paper) below. TSP-12231

This Brief includes a Technical Support Package (TSP).
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Towards Sustainable Recycling of Epoxy-Based Polymers: Approaches and Challenges of Epoxy Biodegradation

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