Chemnitz: Regional Hemp Bast Used to Reinforce Lightweight Structural Profiles

The cultivation of industrial hemp for the production of ropes and clothing has a long tradition in Saxony. Thanks to its excellent fiber properties, it is also suitable as a renewable raw material for reinforcement in fiber composites. 

The Background

Fiber-reinforced plastics are materials consisting of reinforcing fibers and a polymer matrix that have established themselves as versatile and high-performance materials with high strength and stiffness. The pultrusion process (rod drawing) is a continuous manufacturing process for fiber-reinforced plastics that produces high-quality profiles. Continuous strands of glass, carbon, or natural fibers, known as rovings, are drawn through a thermosetting resin bath into a heated mold and thermally cured. This produces high-quality profiles with a constant cross-section, high strength, and low weight, which are ideal for applications requiring lightness, corrosion resistance, and durability. The petroleum-based components are to be replaced by bio-based raw materials in the future.

The Development Goal

The central challenge in the CannaPul research project is the development and application of bio-based, regionally available materials. The goal is to produce continuous-fiber-reinforced profile structures from hemp bast fibers in combination with a bio-based thermoplastic matrix. A prerequisite for this is, on the one hand, the provision of suitable natural fiber strands from the renewable regional raw material hemp bast. On the other hand, the process requires the application of a thermoplastic biopolymer matrix with low viscosity and correspondingly high flowability, as well as a low melting temperature, to prevent thermal damage or burning of the hemp fibers.

The Collaboration

The STFI is investigating the textile processing of high-strength hemp bast strips into continuous fiber strands and, as a result, is providing bio-based reinforcing fibers for the pultrusion process. The peeled hemp bast strips are separated through a fibrillation process and deposited evenly as a ribbon. The ribbon is stabilized for tensile strength through a coating process and made available as yarn. Fraunhofer IWU addresses the numerous challenges involved in the pultrusion process. This involves matching the melt viscosity and processing temperature of the thermoplastic to the biogenic fiber material.

The Process

In pultrusion, the thermoplastic melt is produced from pellets. The challenge lies in completely impregnating the fibers and ensuring uniform fiber distribution. At the same time, a fiber volume fraction of at least 50% must be achieved to ensure high mechanical properties, particularly a high specific (weight-based) tensile strength. At a pull-off speed of up to 1000 mm/min, a cross-sectional area of 20 mm² must be reliably and completely impregnated with the thermoplastic melt during pultrusion.

Heike Metschies, project manager at STFI, notes: “Initial results are already available in the form of rectangular profiles with hemp reinforcement and a thermoplastic matrix. We were able to present these at AGRA 26 in Leipzig to, among others, Saxony’s State Minister for the Environment and Agriculture, Georg-Ludwig von Breitenbuch, and to a large audience of industry experts at Techtextil in Frankfurt am Main.”

As the CannaPul project progresses, the substitution of the currently petroleum-based thermoplastic matrix with a biogenic thermoplastic matrix will be investigated, the profile cross-section will be enlarged, and further efforts will be made to improve the quality of the hemp fiber roving.

The project is co-financed by tax revenues from the European Union and the Free State of Saxony under the Regional Development Program (ERDF) and the Just Transition Fund (JTF) for the funding period from 2021 to 2027.