The term “sustainability” was defined in the late 1980s by the UN World Commission on Environment and Development as “meeting the needs of the present without compromising the ability of future generations to meet their own needs.” For decades, the concept has been observed, discussed and considered, but there has been little in the way of enforcing it. Arguably, it has only been within the last few years that sustainability has truly been deemed an urgent concern on a more global scale, and as a result, has become a priority for nearly every country. This movement was led by the legally binding Paris Agreement signed in 2015 and has grown ever since, aided by regulations, corporate goals and customer demands.

The composites industry is making considerable efforts to cut emissions, reduce waste, use natural resources and improve energy management from material development to product fabrication throughout the product’s life cycle and its end of life. This pursuit of complete circularity is a complicated and challenging path, and new and ongoing developments are constantly altering what practices are considered “sustainable.” Key approaches in composites include the development of biomaterials, recycling and energy savings via equipment, sensors and digitization.

In this first article, to be part of an ongoing series, CW summarizes some of the biomaterial developments that companies have shared within the last year. Subsequent articles will address recycling and energy-saving initiatives.

Natural fibers and beyond

Materials are the most obvious choice when pursuing sustainability in composites and are being developed in several ways:

• Increased use of natural fibers;
• Incorporation of thermoset or thermoplastic resins that include bio-derived content (e.g., resins and fillers sourced from biomass versus petroleum) and increasing the percentage of what is bio-derived;
• And a combination of these two approaches.

The use of natural fibers — also known as natural fiber-reinforced polymers (NFRP) or natural fiber composites (NFC) — has become a popular choice for companies wanting to provide their customers with sustainable application options. These fibers come from a range of sources, such as plants (hemp, jute, sisal, flax, kenax and bamboo) and even animals (silk and wool), and are characterized by their renewability, less energy-intensive extraction, biodegradability and reduced carbon impact, while still maintaining high mechanical properties like more traditional synthetic options such as carbon or glass fiber.

NFRPs have increasingly graced a variety of end markets, but perhaps none so consistently as automotive, where flax fiber reinforcement in particular decks out everything from aesthetic interior components to safety-critical structures.

Bcomp Ltd.’s (Fribourg, Switzerland) ampliTex technical fabrics and powerRibs reinforcement grid have become synonymous with this movement as the company continues its push to accelerate flax fiber adoption in automotive and beyond. Its materials are already being seen on an industrial scale — visible interior components in the new electric Volvo EX30 and the Polestar’s interior and exterior flax fiber composite panels; bus A/C covers; and a motorsports adaptation like the exterior bodywork components on the BMW M4 GT4. In April 2024, Bcomp closed a $40 million funding round in support of the material’s entry into new Asian and North American markets.

Flax fiber producer Groupe Depestele (Normandy, France) is another name in the realm of automotive flax fiber applications. Together with Greenboats (Breman, Germany), the company showcased the Greenlander Sherpa expedition vehicle at JEC World 2024, featuring skins, panels and complete structures using dry impregnated natural fiber tapes.

In addition, Volkswagen (Wolfsburg, Germany) has entered into a cooperation with textiles manufacturer startup Revoltech GmbH (Darmstadt, Germany) to develop and test a substitute for imitation leather — materials incorporating industrial hemp fibers and a fully bio-based adhesive — for use as surface materials in Volkswagen vehicles starting in 2028.

Along these same lines, the German Institutes of Textile and Fiber Research Denkendorf (DITF) and RBX Créations (Jonzac, France) won an award for the development of a hemp-based pulp that is being further processing into filament-spun cellulose fibers. The raw hemp is grown and obtained locally, which is then processed into a fine-fibered pulp in a process patented by RBX Créations and dissolved in an ionic liquid. This then serves as the raw material for a wet spinning process developed at the DITF and patented under the name HighPerCell. The solution is spun into cellulose fibers in a precipitation bath — using a solvent that can be completely recovered and reused — for development of textile fibers, yarns and fabrics.

According to the Alliance for European Flax-Linen & Hemp (Paris, France), 2024’s flax harvest reflected positive extraction yields, noting in July that “estimates predict average straw yields of 6-7 tons per hectare for spring flax, a significant milestone that has not been seen since 2019.” A successful harvest, of course, suggests plenty of opportunities to meet industry demand.

Exploration and use of other natural fibers, while often still in the maturation phase, still show potential for what applications they might attract. A novel material targeting aesthetic purposes (for example, automotive, marine and furniture industries) uses Ohoskin’s (Silicia, Italy) leather alternative made from orange and cactus byproducts with ReCarbon’s (Busto Arisizio, Italy) recycled carbon fiber.

Mycelium composites, typically based on mushroom roots, are being studied in academic settings as construction material options, like the University of Bristol (U.K.) where scientists are fine-tuning the properties and production of mycelium composites, or okom wrks labs (Chicago, Ill., U.S.) whose materials “typically incorporate an agricultural residue like hemp hurd as a dispersion phase and fungal mycelium as the matrix phase.” The goal is structural mycelium used in applications such as structural insulation panels (SIPs) for construction.

Sulapac (Helskinki, Finland) was founded in 2016 in an effort to safeguard against continual plastics waste. Its materials portfolio includes wood composites — wood fibers from industrial side streams mixed with biodegradable biopolymers — as well as other bio-based materials which can be recycled and include recycled content. They can be mass produced, and are industrially compostable and sustainably sourced, among other advantages.

In addition, global automotive supplier Toyoda Gosei Co. Ltd. (Kiyosu, Japan) announced the development of a fiber reinforcement formed from magnesium hydroxide (fibriform) derived from seawater, to be targeted for interior and exterior automotive parts; surfaces made from this composite are said to be more scratch-proof and require less reinforcement material during manufacture.

Going bio-based via thermoset resins

Bio-based (also known as bio-derived) resins and additives have become a rapidly growing trend. Rather than simply using natural fibers, companies taking this approach focus either on improving the quality of the resin matrices holding synthetic fibers in place or improving the process of producing those same fibers.

Thermoset resins in particular have been the composite industry’s main target, such as epoxies and vinyl esters. While they have strong mechanical properties and high glass transition temperatures, they are covalently crosslinked, making recycling later in a product’s life cycle difficult and expensive, or nearly impossible. Moreover, these types of resins are often developed through petroleum feedstocks, a nonrenewable and thus unsustainable source.

Announcement or launch of bio-based thermoset resins — specifically, resins that are partially bio-derived — has become increasingly common. Exel Composites (Vantaa, Finland), for instance, is aiming to phase out use of hydrocarbon-derived resins on a commercial scale with the purchase of Ineos’ (Dublin, Ohio, U.S.) Envirez bio-based resins. The system’s chemical composition “features 23% bio-based glycol, compared to traditional crude oil-sourced hydrocarbon resins,” representing a 21% drop in associated manufacturing emissions. The National Renewable Energy Laboratory (NREL, Golden, Colo., U.S.) published findings concerning the biomass-derived “PolyEster Covalently Adaptable Network” or PECAN resin developed for recyclable wind turbine blades. Partners found that it performs well with composites, outperforms some resins and enables chemical recycling at end of life (EOL).

Additional developments include Super Resin Inc.’s (Tokyo, Japan) plant-based epoxy resin (a blend of glycol lignin derived from Japanese Cedar and epoxy), Syensqo’s (Brussels, Belgium) MTM 49-3 resin that contains 30% bio-sourced monomers to meet automotive sustainability goals and Holland Composites’ (Lelystad, Netherlands) Duplicor bio-based, fire-resistant composites, which have made appearances in various composite roof structures and façades, including Netherlands-based Van Gendt Hallen, The Pulse and most recently, the Dutch bank ABN AMRO. “We’re using a plant-based resin with a mix of possible fibers depending on the application,” explains Eric van Uden, managing director of Solico Engineering, working with Holland Composites. “And we no longer use PIR [polyisocyanurate] or EPS [expanded polystyrene] foams in sandwich construction, but just recycled PET foam.” The latter, he concedes, is not bio-based “but it does help to remove plastics from the environment.” Duplicor is now being sold as prepreg and panels worldwide.

Still, other companies have highlighted specific customer applications in which bio-based resin systems are being used or cite transitioning to sustainable resin use in their own product offerings:

• Polyester resin supplier Büfa Composite Systems (Rastede, Germany) now offers styrene-reduced and styrene-free products, bio-based raw materials and products with recycled raw materials.
• Sicomin’s (Châteauneuf les Martigues, France) GreenPoxy 33 bio-resin range has been used for fabrication of eco-friendly twintip kiteboards with flax fiber multiaxial reinforcement plies, a CNC milled wood core and bamboo rail sections.
• Hexcel’s (Stamford, Conn., U.S.) HexPly Nature range of bio-derived prepregs has been transferred to the company’s winter sports application production.
• Kautex Textron’s (Kautex, Bonn, Germany) Green+ products launch, made with 20% biomass materials and/or 25% recycled materials, were demonstrated in the form of Pentatonic Green+, a composite electric vehicle (EV) battery enclosure made from fishing nets discarded in the ocean.

CW has also seen trends in applying biomaterials such as lignin to precursor materials in the production of resins, including bio-based thermoplastics. For example, a consortium led by sustainable technology innovator Sonichem (formerly Bio-Sep, Southampton, U.K.) hopes to establish renewable, cost-effective alternatives to petrochemicals commonly used in the production of plastics, resins and composites within the automotive industry. This includes converting sawdust, the biomass byproduct from forestry operations, into high-quality lignin to serve as the basis for bio-based platform chemicals. Another example is the intensive development work between Synergy Horizon Poland (Dabrowka, Poland) and KraussMaffei (Laatzen, Germany) that has resulted in the development of bio-based PLA using 30% hydrolized lignin during the extrusion processes. KraussMaffei demonstrated the natural material’s incorporation first through its laboratory extruder, a ZE Blue Power 28, and also the company’s small ZE Blue Power 42 production compounder.

Evaluating bio-based precursors

Not only are bio-derived resins being considered, but the processes used for synthetic fibers — primarily carbon fiber — are being re-evaluated. Historically, carbon fiber has been produced from a variety of non-renewable precursors, including polyacrylonitrile (PAN), rayon and pitch. These fibers are chemically treated and carbonized to create the high-strength fibers many manufacturers use today. However, bio-based, carbon-rich precursors such as lignin or bio-based PAN are being increasingly studied and demonstrated, as the ways in which sustainability is achieved in composites manufacturing evolve. DITF is one organization that has been at the forefront of looking to lignin as a source for biocomposites. Its research has included water-spun lignin fibers as a PAN precursor alternative and lignin-based protective coatings for yarns and textile surfaces; its large number of competence and technology centers address everything from molecules and fiber chemistry to end-use products.

Other companies in this field have gone — or will be going — commercial with their bio-based PAN alternatives. For example, a biomass-focused acrylonitrile (CAN) production process developed by Southern Research was licensed by Trillium Renewable Chemicals (Knoxville, Tenn., U.S.) in 2023, and is being commercialized for production of ACN and acetonitrile. The biomass, non-food sugars (or carbohydrates) known as xylose and glucose, are harvested from wood-based biomass. Trillium has since been aiming to construct a market-scale demonstration plant in the U.S.

Through another approach, Airbus (Toulouse, France) flew a bio-based carbon fiber helicopter nose panel (see opening image), which was developed using bio-based acrylontrile precursor produced by the Airbus team. The alternative acrylonitrile is derived from International Sustainability & Carbon Certification (ISCC)-certified non-fossil feedstocks such as wood and food waste, recycled cooking oils and/or algae, as well as renewable sources of ammonia and propylene. Testing showed that the nose panel offered the same performance as conventional carbon fiber-reinforced polymers (CFRP) but with significantly less CO₂.

A new age of adhesives, additives

Additives and adhesives, while perhaps not as widely shared as fibers and biopolymers, still have an important part to play when it comes to a composite part’s sustainability.

Researchers at the University of Pittsburgh’s (Penn, U.S.) Swanson School of Engineering are one group that have taken inspiration from hydrogels, liquid crystal elastomers and mussels’ natural bioadhesives to optimize adhesive systems at the molecular level. “This technology is critical for strengthening underwater infrastructure such as platforms and piping,” notes Qihan Liu, assistant professor of mechanical engineering and materials science and one of the project’s researchers.

In addition, project partners scattered across northwestern Europe are joining forces under the Biobased Debondable Adhesives (BiDebA) project to engineer bio-based adhesives, derived from renewable resources, facilitating composites debonding as well as circularity in transportation markets.

BioPowder (Birkikara, Malta), a specialist in fiber additives and functional composite fillers provides a good example of how additives can also be sustainable. Its Olea FP line of bio-based and biodegradable functional composite fillers are made from upcycled olive stones. They offer features such as high stability, low water and oil absorption, hardness and abrasion resistance and come with a variety of texture effects.

Fiber and resin sustainability focus

Sustainability has also fueled a variety of ongoing projects where consortium goals attempt to address the entirety of a composite material system. One example is EOLIAN, a result of the intense and extensive effort to recycle wind turbine blades. This EU-funded project will develop blades that are both repairable and recyclable via bio-based vitrimer resins and basalt fibers with integrated sensors to enable structural health monitoring (SHM).

The SUSPENS project is tackling the challenge of reducing the environmental footprint of sandwich composite and hollow structures manufacture for the automotive, nautical leisure and aeronautics industries. Its first year was dedicated to material development, specifically resins with more than 95% bio-sourced epoxy and polyester, combined with sustainable reinforcements made with natural fibers and recycled carbon and glass fibers. The remainder of the project will validate materials performance via coupon testing.

Meanwhile, a wall element demonstrator made for the DACCUS-Pre project led by the DITF features a novel composite material made from natural gabbro stone, lignen-based carbon fibers and biochar, an alternative to reinforced concrete, for use in construction materials.

The CUBIC project, a research and innovation initiative begun in September 2023 and led by the Aitiip Technology Center (Zaragoza, Spain) is focusing its own efforts on bio-based intermediate formats — such as filaments, sheets (organosheets and unidirectional tapes), granules and powder — and their combination, in the hopes of overcoming current technical sector limitations. It is considering aspects such as technical design, materials to be used, processability and life cycle, and so far has created different types of bio-based polyamide, carbon fiber and epoxy resin.

Also of note is the BioStruct project, led by Spanish research center Ideko (Elgoibar), where European partners strive to solve the technical problems associated with the use of bio-based composites in industrial structures for sectors such as wind energy or maritime by 2026. While its aim is more to boost the use of natural fibers and biological resins rather than develop options themselves, this study will still contribute to the “understanding of the mechanical properties of bio-based materials to accurately design structural components and enable their use in such applications.”

In addition to the research projects mentioned above, there are some companies already making parts commercially with bio-based resins and fibers in mind. Greenboats GmbH is a key example. Founded in 2013, it has positioned itself not just as a boat manufacturing company, constructing vessels from flax fiber and bio-based resins, but as a technology company specializing in natural fiber composites. The company is continuously adapting its offerings, currently spanning high-volume production of sandwich panels to high-finish, sustainable campers.

Biomaterials are here to stay

Even with progress being made in this field, there are still many challenges for biocomposites to overcome. For example, depending on the material, they still tend to be higher in cost than more traditionally produced/fossil fuel-based composite materials and may not always be readily available in the volumes that the industry needs (e.g., flax fiber availability depends on crop yield within a given year). In addition, not having the necessary data regarding their performance — particularly regarding their large variety of constituents — has become a large barrier to some biomaterials’ consideration and selection for commercial application.

Regardless of the challenges, development and commercialization of more sustainable composite materials within this wide spectrum — natural fibers, bio-based precursors, 100% or bio-based thermoset resins, and bio-based adhesives and additives — shows a lot of promise in moving the industry’s environmental efforts forward.