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Traditional carbon-fiber-reinforced polymer (CFRP) composite materials are neither recyclable nor repairable due to the thermoset polymers' permanently crosslinked network. Strong and durable like thermoset plastics, yet moldable and recyclable like their thermoplastic counterparts, vitrimers provide exciting opportunities to recycle thermosets and CFR composites.
From aerospace and automotive industries to construction and sporting goods manufacturing, CFRP composite applications are constantly expanding because of the material's outstanding mechanical properties, reduced weight, and better long-term durability.
The rapidly growing market for CFRP composites raises serious concerns about the availability of cost-efficient waste management and disposal methods for these materials. Because of the thermoset polymer matrix's inherent heterogeneity and the reinforcement fibers, the CFRP composites exhibit poor recyclability.
Reducing the Environmental Impact of Carbon-Fiber-Reinforced Composites
At present, such materials are usually down-cycled as fuel for power generation with a little recovery of valuable materials such as reinforcement carbon fibers.
The ongoing shift from the take-make-dispose approach towards more sustainable economic models that would allow keeping raw materials in use for as long as possible requires developing innovative recyclable CFRP composites.
Composite materials are made of at least two different constituent components, thus benefitting from combining the properties of each of the individual components.
These materials are generally composed of reinforcing material, usually carbon or glass fiber, and a matrix phase, which acts as a glue to bind the whole structure together. Components that can enhance the composite properties, such as fire resistance or electrical conductivity, can also be added.
In conventional CFRP composites, the binder (matrix) components are usually thermoset polymers, such as epoxy resins. These materials are preferred due to their superior thermal and chemical resistance. Upon curing, the thermosetting polymers form an irreversibly crosslinked network resulting in a rigid and robust structure that contributes to the increased strength and resistance of the CFPR composites.
However, recycling such material is extremely difficult and poses a significant challenge. An additional difficulty arises from the need to strip away and separate the polymer from the embedded carbon fibers.
Vitrimer Epoxy Resins Offer New Possibilities for Innovative Composite Materials
A research team led by Professor Jinwen Zhang at the School of Mechanical and Materials Engineering, Washington State University (WSU) in the USA, has developed a recyclable carbon-fiber-reinforced composite material that could directly substitute conventional non-recyclable CFRPs into the existing manufacturing processes.
The WSU researchers used an entirely new matrix polymer material called epoxy vitrimer, which can be recycled or reused more easily.
The term vitrimer refers to the vitreous-like thermal behavior of the material. At high temperatures, the material's viscosity decreases and enables stress relaxation and malleable mechanical properties. Upon cooling, the material behaves like an elastic thermoset (or elastomer).
This allows the vitrimer thermoset plastics to be reprocessed using conventional thermoplastic processing techniques, e.g., injection and compression molding, and extrusion. Importantly, unlike thermoplastics, vitrimers maintain stable intermolecular connectivity via densely crosslinked networks even at elevated temperatures.
Vitrimers consist of a covalent organic network that can rearrange its topology via reversible exchange reactions that preserve the total number of network bonds (or the crosslinking density) and their molecular architecture.
Self-Healing High-Strength Composite Materials
The vitrimer epoxy resin synthesized by Professor Zhang's team does not require an external catalyst for the curing reaction. Instead, it relies on an internally catalytic response triggered by the precursors' abundant hydroxyl groups that act as catalytic species to promote the crosslinking reaction and the subsequent exchange reactions.
Unlike the conventional epoxy materials' permanent crosslinked networks, these dynamic exchange reactions between the vitrimer's polymer chains enable internal stress release associated with applied external strain while still retaining the crosslinked network.
As a result, at elevated temperatures (above 150 °C), the material exhibits fast stress relaxation and excellent self-healing properties. To demonstrate the new material's outstanding mechanical properties, the researchers prepared CFRP composite material with three layers of carbon fiber fabric and vitrimer epoxy as a binding matrix.
The new CFRP composite exhibited a tensile strength of 356 MPa, comparable to that of conventional CFRP materials. When heated above the glass transition temperature of the vitrimer (> 200 °C), the composite material demonstrated a remarkable shape-changing ability, unlike the thermoset CFRP composites.
Recyclable Vitrimer CFRP Composites for Future Applications
More importantly, the composites' vitrimer matrix was efficiently hydrolyzed and degraded in pure water at temperatures above 160 °C (in a pressurized vessel) without the need for a catalyst. The convenient catalyst-free hydrothermal degradation occurs due to internal tertiary amines, making the crosslinked network easily hydrolyzable at elevated temperatures.
The hydrolysis reaction products are insoluble in water, permitting an easy and complete recovery and recycling of the vitrimer precursor and the reinforcing carbon fibers. The recycled carbon fibers exhibited similar tensile strength to the virgin ones because of the mild degradation conditions.
According to Professor Zhang, the new vitrimer-based composite would require only minor changes to the established manufacturing processes to accommodate the epoxy vitrimer instead of traditional epoxy resins. This can provide a sustainable solution to non-recyclable composite material waste by providing readily recyclable CFRP composite with robust mechanical performance for future industrial applications.
References and Further Reading
T. Liu, et al. (2021) Carbon Fiber Reinforced Epoxy Vitrimer: Robust Mechanical Performance and Facile Hydrothermal Decomposition in Pure Water. Macromol. Rapid Commun. 42, 2000458. Available at: https://doi.org/10.1002/marc.202000458
S. Kumar and S. Krishnan (2020) Recycling of carbon fiber with epoxy composites by chemical recycling for future perspective: a review. Chem. Pap. 74, 3785–3807. Available at: https://doi.org/10.1007/s11696-020-01198-y
D. A. Kissounko, et al. (2018) New material: vitrimers promise to impact composites. Reinforced Plastics 62, 162-166. Available at: https://doi.org/10.1016/j.repl.2017.06.084
T. Hilding (2021) Researchers develop recyclable composites. [Online] www.news.wsu.edu Available at: https://news.wsu.edu/2021/01/21/researchers-develop-recyclable-composites (Accessed on 12 February 2021)
SpecialChem (2021) New Recyclable Carbon Fiber-reinforced Composite with Self-healing Properties. [Online] www.omnexus.specialchem.com Available at: https://omnexus.specialchem.com/news/industry-news/new-recyclable-cfrp-composite-000223785 (Accessed on 12 February 2021)