Woven fabrics made from hybrid yarns facilitate manufacturing of natural fiber-reinforced organic sheets

Locally sourced bast fibers, like hemp from Germany or flax from France and Belgium, are an excellent basis for manufacturing of natural fiber-reinforced polymer composites. Short transportation routes influence the greenhouse gas emissions only marginally compared to plant fibers sourced in Southeast Asia. For this reason, the Durobast project (funding reference: 2220NR090C) pursues development of a process chain fully based in Germany, beginning with the selection and treatment of fibers, followed by yarn and fabric manufacturing to demonstrator production for various market segments including sports, leisure, and automotive. Both industrial and scientific partners are involved in the collaborative development.

For more information, please visit www.durobast.de.

Manufacturing of natural fiber-reinforced organic sheets, which means fully impregnated and consolidated flat semi-finished products, must be realized satisfying conflicting priorities. Especially the so-called micro-impregnation, in which air is displaced and the space in between individual fibers is filled with molten polymer, requires sufficient process temperatures to reduce the melt viscosity. At the same time, the temperature must not exceed a specific threshold to avoid thermal degradation of bio-based fiber components and thus reduce mechanical performance of the composite. The alternating preparation of woven fabrics alongside polymer films or powder within a film stack streamlines fiber impregnation solely in the thickness direction while simultaneously displacing air from the outermost yarn inward. Due to the high melt viscosity of the thermoplastic polymer, this process requires sufficient process time to prevent pores. Hybridyarn with a defined polymer fraction can present an efficient alternative. Polymer fibers allow yarn stabilization at lower titer and therefore a finer mesh within the woven fabric. The more homogeneous mixture of polymer and reinforcemend fibers facilitates short flow distance of the polymer from inside the yarn outwards which prevents entrapment of air and formation of pores. A schematic representation of the impregnation process in filmstacks and hybridyarns is depicted in Figure 1. For continuous processes, there is also the advantage that only the fabric layers made from hybrid yarns need to be fed into the double belt or continuous compression molding equipment. With film stacks, on the other hand, fabric and polymer films must be fed from the creel into the press zone according to the defined layer structure. Figure 2 shows a woven fabric made from hybridyarns and an organic sheet after impregnation and consolidation in a hot press process.

Results from the project Durobast focusing on manufacturing of woven fabric reinforced organic sheets with a recycled PLA matrix have recently been published in a peer-reviewed open-access paper: https://www.mdpi.com/2073-4360/15/22/4357

A comparison between properties of hemp- and flax reinforced organic sheets made from virgin and recycled PLA has been presented and discussed at a workshop within the framework of the Greener Manufacturing Show (Cologne, Germany) in November 2023. The presentation slides are accessible in German at: https://durobast.de/publikationen/

Further project results have been presented on March 20-21, 2024 at the 3rd Online Conference Bioplastics organized by the Institute for Circular Economy of Bio: Polymers at Hof University. The project Durobast was also represented at the Hannover Messe trade fair from April 22-26^, 2024 at the FNR joint booth in Hall 2, booth A35.

The project Durobast is funded by the Federal Ministry of Food and Agriculture based on a decision by the German Bundestag (funding reference: 2220NR090C).