Highly resilient and recyclable UD tapes made from stereocomplex PLA

High-performance technical fibers are used in various applications, but they are often made from fossil raw materials and are not biodegradable. This is where the "scPLA-UD-Tape" project of the Fraunhofer Cluster of Excellence „Circular Plastics Economy CCPE” comes in, with the aim of creating intrinsically reinforced materials for lightweight construction that are highly resilient, biodegradable and significantly easier to recycle.

© Fraunhofer IAP
scPLA filament yarns produced at the Fraunhofer IAP.
© Fraunhofer IMWS
UD Tape in the manufactoring process.
© Fraunhofer IMWS
Example of a UD tape.

High-performance technical fibers can be found in seat belts, breathable clothing, particularly stable concrete structures and medical implants. In fiber-reinforced plastics, they ensure highly durable and lightweight components, for example for the automotive industry. Every year, tens of tons of such fibres are produced in Germany, often from plastics such as polyester (PET) and polyamide (PA), which are made from fossil raw materials and are not biodegradable.

In the search for more environmentally friendly alternatives, polylactic acid (PLA) is an attractive candidate. This biopolymer, which is often produced from corn or sugar beet, is becoming increasingly widespread in many fields of application. However, PLA does not yet have a sufficient thermo-mechanical profile for use as a raw material for reinforcing fibers and cannot keep up with the performance of PET and PA.

In previous research activities, the Fraunhofer Institute for Applied Polymer Research IAP has succeeded in advancing the production process of high-quality stereocomplex PLA (scPLA) filament yarns to such an extent that thermally resilient scPLA yarns can be produced whose mechanical property profile is very similar to that of technical PET fibers. To produce scPLA, two types of PLA are combined, resulting in crystal structures that allow the melting temperature to rise by up to 60 Kelvin. In cooperation between the Fraunhofer IAP and the Fraunhofer Institute for Chemical Technology ICT, it was also demonstrated that scPLA filament yarns can act as reinforcing fibers in PLA-based organic sheets and offer decisive advantages for recycling. "Because the matrix and fibers are made from the same material, a single-component composite material is created that no longer requires complex separation steps. Lightweight materials and components can thus be better managed in the cycle and make an additional contribution to sustainability," explains Dr. Evgueni Tarkhanov, Head of Development of scPLA fibers at the Fraunhofer IAP.

"We want to use these improved properties of scPLA fibers in a UD tape as a continuous fiber-reinforced semi-finished product for self-reinforcement. This will enable us to achieve material properties that are suitable for a wide range of applications and significantly increase the maturity of this technology," says Ivonne Jahn, who is leading the project at the Fraunhofer Institute for Microstructure of Materials and Systems IMWS. UD tapes are thin thermoplastic layers with a thickness of 200-300 µm and a very high fiber content. The reinforcing fibers are aligned unidirectionally. As a result, they can be laid on top of each other in a load-oriented and precisely fitting manner during further processing, which takes place in a very efficient melt impregnation process. This offers enormous potential for lightweight construction. As a semi-finished product, the UD tape is intended to prevent fiber undulations (a curvature of the fiber that can lead to reduced strength and stiffness of the material) and increase the fiber content in the material. As a local reinforcement in components, the use of UD tapes is also significantly more material-saving and therefore more resource-efficient than, for example, the use of fabric-based organic sheets as a reinforcement component, which, always available as a large semi-finished product, result in more waste when fitting into the component geometry.

Particular challenges in the project are the sensitive process control during tape production and further processing. The identification of critical influencing factors on the fibre-matrix interaction, the resulting improvement of the fibre-matrix adhesion and the further optimization of the mechanical properties are declared project goals. In their project, which will run until the end of the year, the project partners want to lay the foundations for near-net-shape material deposition of the UD tapes in the subsequent component in order to significantly reduce material waste.

In order to be successful, both the scPLA reinforcing fibres are being adapted and different PLA matrix types with improved flow properties are being tested, while the UD tape production process is also being optimized for this application so that the bio-fibres are damaged as little as possible and the fibre-matrix bonding is as successful as possible. "If we master the material and process development as planned, the findings can also be transferred to other self-reinforced material systems," says Sascha Kilian, project manager at Fraunhofer ICT. "There is already great demand for such materials, from mobility applications to the medical and sports equipment sector and additive manufacturing."