Application Center for Wood Fiber Research HOFZET®

Research focus

Hybrid (bio-) composite materials

In light of the strongly-increasing demands concerning the environmental friendliness of products, focus is being increasingly placed upon lightweight solutions – particularly in aircraft and automotive manufacture. Fiber-reinforced plastics enable the production of lightweight, stable components and also utilize the specific advantages of the applied fibers. At HOFZET, we process not only natural fibers such as flax, hemp and jute but also technical fibers such as glass, carbon and aramid as well as polyamide and viscose fibers. In addition to the thermosetting plastics, thermoplastics also serve as matrix systems.

© Photo Fraunhofer WKI | Sebastian Kalka

Schematic representation of organic sheet production and two alternatives for further processing

© Photo Fraunhofer WKI | Sebastian Kalka

Assorted hybrid thermoplastic composite materials comprised of carbon and flax fibers as well as biogenic polyamide (organic sheet)

© Photo Fraunhofer WKI | Kathrin Morawietz

Partially-impregnated organic sheet: the cushioning function is realized through backfilling and the omission of polymer wetting of the textile components.

The main focus at the HOFZET currently lies on composites with a thermoplastic matrix, which are also referred to as organic sheets. Compared to conventional thermosetting composite materials or prepregs, these offer the advantages of reversible deformation and virtually unlimited storage. This results in new fields of design freedom which enlarge the component spectrum of fiber-reinforced composites. As an example, the shaping can be carried out simply and quickly through thermoforming in a die or in the clamping unit of an injection molding machine. For the new and further development of these composites, we have modern plant technology at our disposal. Similar plants are already being used in some areas of industrial large-scale production.

The targeted combination of different substances to form hybrid materials is also an important research area. Within the framework of organic sheet production, we examine widely-differing laminated structures made from, for example, carbon with natural fibers, honeycomb cores, glass fibers and assorted fabrics. The advantageous properties of natural fibers, such as low weight, excellent insulation effect and good CO2 balance, can hereby be combined with the excellent mechanical properties of, for example, carbon fibers or aramid fibers. A further advantage of the substitution of high-priced carbon fibers or aramid fibers through natural fibers is the cost-saving.

For fiber-reinforced plastics, thermoset matrix systems still represent the state of the art, which is why we also carry out research in this field at the HOFZET. The surface modification of natural fibers as well as the vacuum infusion of widely-differing textile semi-finished products are the main issues here, in order to further improve the mechanical and physical properties of the materials.

As a research partner at the Fraunhofer Project Centre Wolfsburg, we are involved in the development of future technologies for the automotive industry. At present, the focus lies upon the hybridization of metals, foams and fiber-reinforced plastics. Here we examine in particular the aspects of fiber-reinforced thermoplastics as well as material recycling. We are also investigating the potential of biogenic technical textiles as well as biogenic polymers.

Our goal is to maximize the proportion of bio-based materials in components for the large-scale production of passenger cars and commercial vehicles. Innovative procedures, for example through the direct combination of the organic sheet technology with injection molding, enable us to produce complex parts with integrated injection geometries, such as reinforcing ribs, in one production step. Through the partial impregnation of textile semi-finished products with matrix, we are opening up new fields of application and functional integration.

© Photo Photo Fraunhofer WKI | Carsten Aßhoff, CAD raw data from Schmidt & Heinzmann

3D model of a fiber-spraying head with exiting cut fibers and resin/curing agent mixture

Tailored Composites

With the help of a robot-controlled fiber-spraying facility, we develop and implement innovative hybrid (bio) fiber-reinforced composite materials (FRC) and corresponding process chains in cooperation with project partners from industry and research.

With fiber spraying, continuous filaments and yarns are cut to a specific length by a cutting unit whilst simultaneously being wetted in an air stream with a mixture of resin and curing agent from the spray head and sprayed onto a shaping tool.

The procedure enables the automated production of components from (bio/hybrid) fiber composite materials as well as a reinforcement of technical textiles or finished components. In addition to conventional high-performance fibers such as carbon, glass and aramid, cellulose-based fibers such as viscose, flax and hemp can also be cut and sprayed on using the fiber-spraying system. The technology simultaneously offers a wide selection of materials as regards the polymer matrices which are to be processed.

Further information can be found in our fact sheet Tailored Composites.