In a win for chemistry, inventors on the Department of Energy’s Oak Ridge National Laboratory have designed a closed-loop path for synthesizing an exceptionally tough carbon-fiber-reinforced polymer, or CFRP, and later recovering all of its starting materials.
A light-weight, strong and difficult composite material, CFRP is helpful for reducing weight and increasing fuel efficiency of automobiles, airplanes and spacecraft. Nevertheless, conventional CFRPs are difficult to recycle. Most have been single-use materials, so their carbon footprint is important. In contrast, ORNL’s closed-loop technology, which is published in Cell Reports Physical Science, accelerates addressing that grand challenge.
“We incorporated dynamic crosslinking right into a commodity polymer to functionalize it. Then, we added a crosslinker to make it like thermoset materials,” said ORNL chemist and inventor Md Anisur Rahman. “Dynamic crosslinking allows us to interrupt chemical bonds and reprocess or recycle the carbon fiber composite materials.”
A standard thermoset material is permanently crosslinked. Once synthesized, cured, molded and set right into a shape, it can’t be reprocessed. ORNL’s system, then again, adds dynamic chemical groups to the polymer matrix and its embedded carbon fibers. The polymer matrix and carbon fibers can undergo multiple reprocessing cycles without lack of mechanical properties, comparable to strength and toughness.
Rahman led the study with ORNL chemist Tomonori Saito, who was honored by Battelle in 2023 as ORNL Inventor of the 12 months. Rahman and ORNL postdoctoral fellow Menisha Karunarathna Koralalage conducted a lot of the experiments. The trio has applied for a patent for the innovation.
“We invented a troublesome and recyclable carbon fiber composite,” said Saito. “The fiber and the polymer have a really strong interfacial adhesion as a result of the presence of dynamic bonds.” The interface locks materials together through covalent interactions and unlocks them on demand using heat or chemistry. Saito added, “The functionalized fiber has dynamic exchangeable crosslinking with this polymer. The composite structure is de facto tough due to interface characteristics. That makes a really, very strong material.”
Conventional polymers like thermoset epoxies are typically used to permanently bond materials comparable to metal, carbon, concrete, glass, ceramic and plastic to form multicomponent materials comparable to composites. Nevertheless, within the ORNL material, the polymer, carbon fibers and crosslinker, once thermoset, could be reincarnated back into those starting materials. The fabric’s components could be released for recycling when a special alcohol called a pinacol replaces the crosslinker’s covalent bonds.
Closed-loop recycling at laboratory scale leads to no lack of starting materials. “Once we recycle the composites, we get better 100% of the starting materials — the crosslinker, the polymer, the fiber,” Rahman said.
“That is the importance of our work,” Saito said. “Other composite recycling technologies are inclined to lose the component starting materials throughout the recycling process.”
Other benefits of the reversibly crosslinked CFRPs are quick thermosetting, self-adhesive behavior and repair of microcracks within the composite matrix.
In the long run, closed-loop recycling of CFRPs may transform low-carbon manufacturing as circular lightweight materials grow to be incorporated into clean-energy technologies.
The researchers drew inspiration from nature, which employs dynamic interfaces to create robust materials. Nacre, the iridescent mother-of-pearl contained in the shells of marine mussels and other mollusks, is exceptionally tough: it could possibly deform without breaking. Furthermore, marine mussels strongly adhere to surfaces but dissipate energy to release when obligatory. The researchers aimed to optimize interfacial chemistry between the carbon fibers and the polymer matrix to spice up interfacial adhesion and enhance CFRP toughness. “Our composite’s strength is sort of two times higher than a traditional epoxy composite,” Rahman said. “Other mechanical properties are also superb.”
The tensile strength, or the stress a fabric can bear when it’s pulled, was the very best ever reported amongst similar fiber-reinforced composite materials. It was 731 megapascals — stronger than stainless-steel and stronger than a traditional epoxy-based CFRP composite for automobiles.
Within the ORNL material, the dynamic covalent bonding between the fiber interface and the polymer had 43% greater interfacial adhesion in comparison with polymers without dynamic bonds.
The dynamic covalent bonds enable closed-loop recycling. In a traditional matrix material, the carbon fibers are difficult to separate from the polymer. ORNL’s chemical method, which clips fibers on the functional sites, makes it possible to separate fibers from the polymer for reuse.
Karunarathna Koralalage, Rahman and Saito modified a commodity polymer, called S-Bpin, with assistance from Natasha Ghezawi, a graduate student on the Bredesen Center for Interdisciplinary Research and Graduate Education of the University of Tennessee, Knoxville. They created upcycled styrene ethylene butylene styrene copolymer, which contains boronic ester groups that covalently bond with a crosslinker and fibers to generate the tough CFRP.
Because CFRP is a fancy material, its detailed characterization required diverse expertise and instrumentation. ORNL’s Chris Bowland tested tensile properties. With Raman mapping, ORNL’s Guang Yang showed the distribution of chemical and structural species. Catalin Gainaru and Sungjin Kim, each of ORNL, captured rheological data, and Alexei Sokolov, a UT-ORNL Governor’s Chair, elucidated it. Scanning electron microscopy by Bingrui Li, of ORNL and UT, revealed that carbon fiber maintained its quality after recycling. Vivek Chawla and Dayakar Penumadu, each of UT, analyzed interlaminar shear strength. With X-ray photoelectron spectroscopy, ORNL’s Harry Meyer III confirmed what molecules attached to fiber surfaces. ORNL’s Amit Naskar, a renowned expert in carbon fiber, reviewed the paper.
The scientists found that the degree of dynamic crosslinking is vital. “We found 5% crosslinking works higher than 50%,” Rahman said. “If we increase the crosslinker amount, it starts making the polymer brittle. That is because our crosslinker has three hand-like bulky structures, capable of make more connections and reduce the polymer’s flexibility.”
Next, the research team would really like to conduct similar studies with glass-fiber composites, which maintain high performance while lowering the price and carbon footprint of applications in aerospace, automotive, marine, sporting, construction and engineering. In addition they hope to cut back costs of the brand new technology to optimize business prospects for a future licensee.
“This step will open more applications, especially for wind turbines, electric vehicles, aerospace materials and even sporting goods,” Rahman said.
The Vehicle Technologies Office in DOE’s Office of Energy Efficiency and Renewable Energy sponsored the research. DOE’s Office of Electricity sponsored Raman mapping.
