Fiber-reinforced thermosetting resins, usually glass and carbon, give rise to light and very resistant materials that are highly attractive in strategic sectors such as aeronautics (5.3% CAGR 2018-2023), wind energy (4.7% CAGR 2018-2023), automotive (3.8% CAGR 2018-2023) or construction (4.0% CAGR 2018-2023) due to its high performance. Demand in these sectors will continue to drive growth in the coming years. However, they still have technical limitations, especially at their end of life due to their inherent complexity and the difficulty of applying an eco-design, generating plastic waste.
Plastic waste is one of the main threats to our ecosystems. Currently the world production of plastics exceeds 350 million tons, approximately 85% corresponding to the production of thermoplastics and 15% to the production of thermosets. That is why, in recent years, various initiatives, efforts and investments have been launched to increase the recycling rate or promote materials with more controlled biodegradation in the thermoplastics sector. Thermosetting composites, unlike thermoplastics, cannot be melted, making it difficult to reuse them. In thermosetting materials, the chemical bonds that are generated produce an irreversible three-dimensional network through the application of an energy source, usually heat, in a process called curing. The material hardens permanently making it difficult to recycle.
For this reason, there are far fewer strategies and technologies available to manage the end of life of thermosetting plastics and composites, making them a challenge for waste management. Today, these materials are either stored in landfills (24.9%), including airplane, wind blade, and railway graveyards; or they end up being valued for energy (42.6%), where the resin is incinerated and only the reinforcement is recovered, although it is usually damaged and cannot be used for the same purpose.
The AITIIP Technological Center is working on a circular approach that allows the valorization of composite materials and their components at all levels. From selective dismantling for reuse and remanufacturing in different sectors, promoting industrial symbiosis, to new sustainable chemical and biological recycling technologies, where the aim is to be able to value both resin and fiber efficiently and without losing their original value. In this way, AITIIP intends to contribute its grain of sand to solve a problem that, despite the youth of said composite materials, is already a reality. In the coming years, it will be necessary to give a sustainable end of life to 12,000 aircraft, 3,800 wind blades in the US and 8,000 in the US, 11% of composite plastic waste generated by the construction sector or the more than 6 million boats abandoned recreation… among other applications.
The VIBES and BIZENTE projects, coordinated by AITIIP, come to develop and guarantee a circular ecosystem for these materials, contributing to the objectives of the European green pact to achieve a climate-neutral Europe in 2050. Both projects are part of the European program of public financing -Private biobased industrial companies (BBI).
The VIBES project presents an innovative solution based on the development of a new green technology focused on the controlled separation and recovery of the components through the development of functional chemical structures that have adherence properties in normal working conditions of the material, but that against certain stimuli (temperature, UV or electrical pulse) are reversible and facilitate the chemical separation between the fiber and the resin. These chemical structures are incorporated either in the resin or at the interface between the fiber and the resin; and are based on combinations of dynamic covalent networks (vitrimers, Diels-Alder reactions and molecular interactions based on supramolecular architecture). Once the stimulus is induced and said dissociation occurs internally, chemical recycling is carried out in a reactor using sustainable and biobased solvents under mild conditions of temperature and pressure. The solvent breaks down the resin into monomers or primary units that can be recovered to re-produce resins or other chemical compounds. For its part, the fiber is recovered and valued in new applications and products.
The BIZENTE project presents a biological alternative to chemical recycling, a completely new approach based on the development of enzymes that selectively depolymerize the resin without attacking the fiber. Currently, there are no known enzymes that naturally attack these materials. However, technologies such as directed molecular evolution (nobel prize in chemistry in 2018) allow the design of enzymes with improved characteristics and new enzymatic functions not required in natural environments. Therefore, it is possible to design enzymes with improved catalytic capabilities, greater specificity and stability before certain temperatures or organic solvents. Within the BIZENTE project, we are working with low molecular weight model compounds in the laboratory that contain all the structural characteristics of the target resins, so that enzymes can be determined and trained to attack the bonds of said model compounds and study the products that are obtained. Once the enzymes with the greatest degradation potential of the selected resins are obtained (the project works with epoxy, vinyl ester and polyester resin), they are tested in a bioreactor with real composite materials. In this way, the real products obtained are studied and different strategies are proposed for the recovery of both the monomers or chemical blocks and the fibers.
A particularity of the BIZENTE process is the need for the enzyme to be able to find in the composite residue the model compounds or structures that it is capable of identifying and separating. For this, the process requires a stage of pre-treatments, either mechanical or chemical, that allow the matrix structure to be broken and favor enzymatic attack. That is why, during the project, both grinding and micronizing are tested as well as the incorporation of dynamic covalent networks (similar to those developed in VIBES) in the structure of the resin, to evaluate the effect it has on the reactor and see if the process it is more efficient and enzymatic digestion is favored. Both projects are designed to respond to the needs of conventional thermosetting composites (derived from oil), although there is a second phase in both projects in which prototypes developed with substitute biobased resins will be tested of the current ones, as well as with biobased natural fibers such as flax or carbon fiber obtained from lignin. In this way, it is hoped to develop versatile technologies that provide a solution to both the current problem and the new sustainable composites that are to come.
The BIZENTE and VIBES projects have received funding from the Bio Based Industries Joint Undertaking (JU) within the framework of the European Union’s Horizon 2020 research and innovation program under grant agreements No. 886567 and No. 101023190, respectively. The JU receives support from the Horizon 2020 research and innovation program and the Based Industries Consortium.