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The paradox of advanced composite materials has long frustrated sustainability efforts. Their exceptional mechanical properties and durability, precisely the characteristics that make them ideal for demanding applications like wind turbine blades, boat hulls and aerospace components, also make them very difficult to recycle at the end of their useful life. With an estimated 43 million tonnes of composite wind turbine blades alone requiring disposal by 2050, and landfill bans taking effect across Europe in 2025, the composites industry faces an urgent need for materials that maintain performance while enabling true circularity.

Traditional thermosetting resins like epoxies and polyesters, which represent approximately 60% of all composite production, form irreversible chemical crosslinks during curing. This chemistry delivers optimal mechanical properties but renders the material impossible to melt and reprocess. Thermoplastic composites offer theoretical recyclability, but their high melt temperature (typically 225°C for engineering grades) and high melt viscosity (approximately 10⁸ centipoise, comparable to chewing gum) make fiber impregnation challenging without specialized high-pressure, high-temperature equipment.

MET-OL Ltd. (Loughborough, U.K.), born out of a need to address these ongoing difficulties, struck out to develop a fundamentally different approach to thermoplastic chemistry that reconceptualizes how thermoplastic polymers interact with reinforcing fibers throughout a composite’s entire manufacturing and recycling life cycle. The initial development phase was financed through a grant from InnovateUK under the LV Wind project (project 10029169), which was executed from July 2022 to June 2024. This work was carried out in collaboration with Hive Composites (U.K.), a specialist partner in thermoplastic materials and part of the same group as MET-OL Ltd.

Engineering a circular materials paradigm

The intelligence behind MET-OL lies in its manipulation of polymer chain architecture. “The challenge was developing a material with the processing advantages of thermosetting resins including low viscosity for efficient fiber impregnation, combined with the recyclability of thermoplastics,” explains Gerry Boyce, managing director of MET-OL Ltd. “Our solution was to completely rethink the polymer’s molecular structure at different stages of its life cycle.”

MET-OL’s thermoplastic technology is based on polybutylene terephthalate (PBT), an engineering thermoplastic with a molecular weight of approximately 220 grams/mole (g/mol), melting temperature of 230°C and melt viscosity around 9×10⁸ centipoise. Through a proprietary chemical process using specific catalysts, solvents and temperature profiles, MET-OL transforms this high molecular weight linear polymer into stable, low molecular weight (3 g/mol) cyclic oligomer rings, essentially breaking long “spaghetti” chains into small “spaghetti-Os.”

This molecular restructuring dramatically changes the material’s physical properties, reducing the melting temperature to 140°C and decreasing viscosity to approximately 30 centipoise — a reduction of nearly eight orders of magnitude that creates a water-like liquid. “This ultra-low viscosity is the key to high-quality fiber impregnation without high pressure processing equipment,” notes Boyce.

After impregnation, a second catalyst initiates in situ polymerization, rebuilding the short cyclic oligomers back into high molecular weight linear PBT chains. This polymerization occurs between 170-200°C with no exothermic reaction. This is an important processing advantage over thermosets, particularly for thick sections.

The circular aspect of this material extends to end-of-life (EOL) recycling. Through a solvolysis technique, MET-OL has demonstrated the ability to chemically separate the PBT matrix from reinforcing fibers, recovering both materials in reusable form. The polymer can be reconverted to its oligomer state for remanufacturing, while the fibers can be resized and repurposed.

Molecular catalysis for polymer transformation

The oligomerization catalyst is designed to achieve complete conversion to cyclic oligomers without side reactions that could introduce impurities or structural defects. MET-OL engineered it to achieve the desired transesterification reactions while minimizing competing pathways like chain scission or random transesterification that would reduce product quality.

Following fiber impregnation, a separate polymerization catalyst distinct from the oligomerization catalyst used in the initial PBT breakdown initiates the in situ polymerization process. This catalyst operates through ring-opening polymerization, specifically targeting the ester linkages in the cyclic oligomers. When activated by heat, it opens these rings and creates reactive end groups that sequentially combine with additional oligomer molecules to build molecular weight.

Temperature-dependent activation enables precise process control during manufacturing. At temperatures below 160°C, the polymerization catalyst remains largely inactive, allowing for extended working time during impregnation and forming operations. As temperature increases above 170°C, catalyst activity accelerates, with polymerization rate approximately doubling with each 10°C increase in temperature. This temperature-responsive behavior enables manufacturers to develop precise time-temperature processing windows tailored to various manufacturing techniques and component geometries.

“The absence of exothermic reaction during polymerization results from the thermodynamic balance of the ring-opening process,” explains Boyce. “Unlike thermoset curing reactions, which release significant heat as crosslinking occurs, the ring-opening polymerization of cyclic oligomers involves minimal net energy change, as the energy required to break the cyclic structure is offset by the energy released in forming new linear bonds.”

Manufacturing implementation pathways

MET-OL’s material processing versatility means it can be used in multiple formats and manufacturing techniques familiar to fabricators working with thermoset resins. For vacuum bag molding applications, the process begins by pre-impregnating fiber reinforcements with MET-OL powder. The prepreg is then laid up in a mold and processed through a three-stage temperature cycle: 100°C for 30 minutes to remove moisture, 140°C for 30 minutes to melt the oligomers and achieve fiber impregnation, and finally 200°C for 90 minutes to catalyze polymerization.

Alternatively, the material can be heated to 140°C in a vessel and either injected or vacuum infused into dry fiber preforms. The polymerization kinetics can be tailored through catalyst selection and processing temperature, enabling cycle times ranging from minutes to hours depending on application requirements. At 190°C, for example, complete polymerization occurs in approximately 15-20 minutes, while at 170°C the process extends to several hours.

“The processing flexibility allows manufacturers to integrate MET-OL into existing production workflows with minimal or no equipment modification,” explains Boyce. “We’ve successfully demonstrated vacuum bagging, press molding, resin injection and pultrusion processing techniques.”

The absence of exothermic reaction during polymerization provides an additional manufacturing advantage, eliminating the thickness limitations typically associated with thermoset composites. This enables production of thicker sections without thermal management concerns.

For specialty applications, MET-OL can also be compounded with additives such as UV stabilizers, pigments, fillers and functional materials like metal particles for enhanced erosion resistance or syntactic materials for weight reduction.

Performance validation, mechanical characterization

Extensive mechanical testing has demonstrated that MET-OL composites exhibit mechanical properties comparable or superior to traditional thermoset composites. Independent testing normalized to 53% fiber volume fraction using E-glass tri-axial 1,200 gsm reinforcement shows that MET-OL composites achieve interlaminar shear strength (ILSS) of 44.3 ± 4.1 MPa — approximately 40% higher than equivalent epoxy (31.5 ± 1.4 MPa) or polyester (31.6 ± 1.5 MPa) composites tested to BS EN ISO 14130 standards.

Flexural properties tested according to ASTM D 790-03 demonstrated comparable performance across all three material systems, with MET-OL composites achieving 597 ± 25 MPa strength and 14.6 ± 0.6 GPa modulus, compared to epoxy’s 594 ± 50 MPa strength and 13.9 ± 1.9 GPa modulus.

Tensile testing revealed significant advantages in both strength and strain tolerance. MET-OL exhibited tensile strength of 613.7 ± 10.7 MPa with 1.5 ± 0.1% strain to break, compared to polyester’s 436.7 ± 94.7 MPa and 0.3 ± 0.5% strain to break when tested according to ASTM D 3039, suggesting improved toughness and damage resistance.

“The enhanced interlaminar shear strength is particularly significant,” notes Boyce. “It translates to improved impact resistance and through-thickness properties, which are critical for many demanding applications including wind turbine blades and marine structures.”

Environmental durability testing showed better moisture resistance than polyester or epoxy, with MET-OL composites exhibiting only 0.3% weight gain after 8 weeks in deionized water at 60°C, compared to 0.4% for polyester and 1.65% for epoxy when tested to ASTM D5229 standards.

Solvolysis-enabled material recovery

MET-OL has also considered the thermoplastic technology’s recovery capabilities. The company developed a solvolysis process that employs a specialized solvent system coupled with temperature control and catalytic activation to selectively break the chemical bonds in the PBT polymer matrix. The process begins by introducing the composite material into a temperature-controlled vessel containing the proprietary solvent mixture. The solvent interacts with the ester linkages in the PBT polymer chain, facilitating controlled depolymerization. A catalyst accelerates this reaction, converting the linear polymer chains back into cyclic oligomer structures. Operating temperature is precisely controlled to optimize reaction kinetics without degrading the fiber reinforcement or the base polymer chemistry.

The resulting process creates a solution containing the dissolved oligomer and a separate phase consisting of the reinforcing fibers. The fibers are filtered from the solution, washed to remove residual solvent and dried. Analysis of recovered fibers shows minimal degradation to the fibers from the solvolysis process.

The oligomer-containing solution undergoes a recovery process to separate the solvent for reuse and isolate the oligomer material. This recovered oligomer can then be directly reused in manufacturing new components, completing the circular materials pathway.

Transforming multiple composite markets

MET-OL has developed a number of demonstrators using its product to showcase its potential in different markets. Transportation applications already validated include a composite semi-trailer manufactured with an earlier generation of the technology. This application demonstrated a 1.5-tonne weight reduction compared to conventional steel construction, highlighting the lightweighting potential of the material system.

MET-OL is currently working with selected industrial partners to validate the technology in specific applications while scaling up material production. “We’ve solved the chemistry and developed manufacturing processes that work with existing equipment,” explains Boyce. “The next step is demonstrating the technology in commercial applications and building market demand before licensing the technology to chemical companies capable of large-scale production.

“The circular economy potential of MET-OL extends beyond simple material recycling,” he continues. “Since the technology enables the separation and recovery of both polymer and reinforcement fibers, it creates opportunities for innovative business models where material suppliers could take back EOL components, recover the raw materials, and remanufacture them into new products.”

In parallel to licensing the technology to chemical companies capable of large-scale production, MET-OL is pursuing several research pathways to further enhance the technology and expand its application potential. The company is exploring bio-sourced feedstock options to further improve environmental performance. For example, using butanediol derived from plant sources rather than petroleum, creating an even more sustainable material system.