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Stand-up paddling has become a popular sport. However, conventional surfboards are made of petroleum-based materials such as epoxy resin and polyurethane.

Researchers at the Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institut, WKI, want to replace plastic boards with sustainable sports equipment: They are developing a stand-up paddle board that is made from one hundred percent renewable raw materials. The ecological lightweight material can be used in many ways, such as in the construction of buildings, cars and ships.

Stand-up paddling (SUP) is a sport that is close to nature, but the plastic boards are anything but environmentally friendly. As a rule, petroleum-based materials such as epoxy resin, polyester resin, polyurethane and expanded or extruded polystyrene are used in combination with fiberglass and carbon fiber fabrics to produce the sports equipment. In many parts of the world, these plastics are not recycled, let alone disposed of correctly. Large quantities of plastic end up in the sea and collect in huge ocean eddies. For Christoph Pöhler, a scientist at Fraunhofer WKI and an avid stand-up paddler, this prompted him to think about a sustainable alternative. In the ecoSUP project, he is driving the development of a stand-up paddle board that is made from 100 percent renewable raw materials and which is also particularly strong and durable. The project is funded by the German Federal Ministry of Education and Research (BMBF). The Fraunhofer Center for International Management and Knowledge Economy IMW is accompanying the research work, with TU Braunschweig acting as project partner.

Recovering balsa wood from rotor blades
“In standard boards, a polystyrene core, which we know as styrofoam, is reinforced with fiberglass and sealed with an epoxy resin. We, instead, use bio-based lightweight material,” says the civil engineer. Pöhler and his colleagues use recycled balsa wood for the core. This has a very low density, i.e. it is light yet mechanically stressable. Balsa wood grows mainly in Papua New Guinea and Ecuador, where it has been used in large quantities in wind turbines for many years – up to six cubic meters of the material can be found in a rotor blade. Many of the systems are currently being disconnected from the grid. In 2020 alone, 6000 were dismantled. A large proportion of this is burnt. It would make more sense to recover the material from the rotor blade and recycle it in accordance with the circular economy. “This was exactly our thinking. The valuable wood is too good to burn,” says Pöhler.

Since the entire sandwich material used in conventional boards is to be completely replaced, the shell of the ecological board is also made from one hundred percent bio-based polymer. It is reinforced with flax fibers grown in Europe, which are characterized by very good mechanical properties. To pull the shell over the balsa wood core, Pöhler and his team use the hand lay-up and vacuum infusion processes. Feasibility studies are still underway to determine the optimal method. The first demonstrator of the ecological board should be available by the end of 2022. “In the interests of environmental protection and resource conservation, we want to use natural fibers and bio-based polymers wherever it is technically possible. In many places, GFRP is used even though a bio-based counterpart could do the same,” Pöhler sums up.

Patented technology for the production of wood foam
But how is it possible to recover the balsa wood from the rotor blade — after all, it is firmly bonded to the glass-fiber reinforced plastic (GFRP) of the outer shell? First, the wood is separated from the composite material in an impact mill. The density differences can be used to split the mixed-material structures into their individual components using a wind sifter. The balsa wood fibers, which are available as chips and fragments, are then finely ground. “We need this very fine starting material to produce wood foam. Fraunhofer WKI has a patented technology for this,” explains the researcher. In this process, the wood particles are suspended to form a kind of cake batter and processed into a light yet firm wood foam that holds together thanks to the wood’s own binding forces. The addition of adhesive is not required. The density and strength of the foam can be adjusted. “This is important because the density should not be too high. Otherwise, the stand-up paddle board would be too heavy to transport.”

Initially, the researchers are focusing on stand-up paddle boards. However, the hybrid material is also suitable for all other boards, such as skateboards. The future range of applications is broad: For example, it could be used as a facade element in the thermal insulation of buildings. The technology can also be used in the construction of vehicles, ships and trains.

As a result of the new combination possibilities for bio-based hybrid-fiber materials achieved at the Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institut WKI, the industrial application possibilities for renewable raw materials, for example in the automotive industry or for everyday objects such as helmets or skis, can be expanded.

By increasing the proportion of flax fiber in hybrid-fiber materials to up to 50 percent, the scientists have demonstrated that it is possible to significantly increase the biogenic proportion in composite materials. The special aspect of the tested methods: The fabrics can be individually composed with the help of a weaving machine. In this way, process steps in industrial production, in which materials first have to be merged together, can be omitted. This will achieve reductions in energy and CO2 throughout the entire production process.

Successfully woven: Different hybrid fabrics
In view of the increased demands being placed upon environmental and climate protection, science and industry are seeking sustainable alternatives to conventional materials in all branches of production. As a material, natural fibers offer a sustainable solution. Due to their low density and simultaneous high stability, natural fibers can be used to produce highly resilient light-weight-construction materials which are easy to recycle. In the “ProBio” project, scientists from the Fraunhofer WKI have therefore addressed the question as to how the proportion of natural fibers in bio-based hybrid-fiber materials can be increased as significantly as possible. A double-rapier weaving machine with Jacquard attachment was thereby utilized in order to produce the bio-based hybrid-fiber materials.

The researchers thereby focused specifically on bio-based hybrid-fiber composites (Bio-HFC). Bio-HFC consist of a combination of cellulose-based fibers, such as flax fibers, and synthetic high-performance fibers, such as carbon or glass fibers, for reinforcement. Bio-HFC can be utilized in, for example, vehicle construction. As an innovation in the “ProBio” project, the researchers interwove differing fiber-material combinations, reinforcing fibers and matrix fibers with the aid of the double-rapier weaving machine. This procedure differs from the process in which finished fabrics are layered on top of one another.

“We have combined the advantageous properties of the fiber materials within a composite material in such a way that we have been able to compensate for weak points in individual components, thereby achieving new properties in some cases. In addition, we have succeeded in increasing the proportion of bio-based fibers to up to 50 percent flax fibers, which we have combined with 50 percent reinforcing fibers,” says project team member Jana Winkelmann, describing the procedure. The bio-hybrid textiles, each consisting of 50 percent by weight carbon and flax fabric, are introduced into a bio-based plastic matrix. The composite material possesses a flexural strength which is more than twice as high as that of the corresponding composite material made from flax-reinforced epoxy resin. This mechanical performance capability can significantly expand the application range of renewable raw materials for technical applications.

With the weaving machine, the scientists have successfully combined innovative light-weight-construction composite materials with complex application-specific fabric structures and integrated functions. Reinforcing fibers, such as carbon and natural fibers, as well as multilayer fabrics and three-dimensional structures, can be woven together in a single work step. This offers advantages for industrial production, as production steps in which materials first have to be merged together can be omitted. “We have succeeded, for example, in utilizing conductive yarns or wires as sensors or conductor paths directly in the weaving process, thereby producing fabrics with integrated functions. The introduction of synthetic fibers as weft threads enables the production of bio-hybrid composites with isotropic mechanical properties,” explains Ms. Winkelmann.

Weaving technology makes it possible to create new products with a high proportion of bio-based components on a pilot scale. The project results provide an insight into the diverse combination possibilities of natural and reinforcing fibers and demonstrate opportunities for utilization not only in vehicle construction but also for everyday objects such as helmets or skis. The results will be presented within the framework of the 4th International Conference on Natural Fibers, ICNF, July 2019 in Porto, Portugal. The “ProBio” project, which ran from 1st July 2014 to 30th June 2019, was funded by the Lower Saxony Ministry of Science and Culture (MWK).

Background
Sustainability through the utilization of renewable raw materials has formed the focus at the Fraunhofer WKI for more than 70 years. The institute, with locations in Braunschweig, Hanover and Wolfsburg, specializes in process engineering, natural-fiber composites, wood and emission protection, quality assurance of wood products, material and product testing, recycling procedures and the utilization of organic building materials and wood in construction. Virtually all the procedures and materials resulting from the research activities are applied industrially.