CA-2K: Sustainable alternatives for hard-soft compounds

Composite of a biobased hard component and a biobased soft component for the 2K injection moulding process

Wheel guards and handles of a bicycle pump

SKS metaplast Scheffer-Klute GmbH was responsible for testing the developed material as wheel guards and handles for bicycle pumps.

Bio-based toothbrush handles

Sunstar Interbros GmbH produced toothbrush handles from the developed material by 2K injection moulding.

Project goals

The development of a bio-based plastic composite consisting of a hard component and a soft component for 2K injection moulding is the aim of a joint research project of Fraunhofer UMSICHT, the Institute for Material Technology of the University of Kassel and FKuR Kunststoff GmbH. Other industrial partners along the entire value chain from raw material manufacturers to plastics processors support the project as associated partners.

Project benefits

The use of hard-soft-plastic composites is versatile and everyday life would be unthinkable without them. In the consumer goods sector, they are used, for example, for sports equipment or toys, in the hygiene products sector for e.g. toothbrushes, and in the office products sector for writing utensils and the like. They are also frequently found in technical applications such as vehicle construction, medical technology or the electrical industry. In addition to the desired technical properties of the materials used, product development is increasingly focusing on factors such as sustainability and environmental compatibility. Innovative bioplastics composites can make an important contribution in this context. This is where the consortium under the leadership of Fraunhofer UMSICHT comes in.

Cellulose acetate (CA), polylactic acid (PLA) blends and polyhydroxyalkanoates (PHA) were selected as possible hard components for the composite. The soft components used were biobased thermoplastic polyurethanes (TPU) and elastomers (TPE). For an applicable bio-based hard-soft composite, both plastic components must be well compatible with each other so that they adhere firmly to each other. The boundary layer between the hard component and the soft component is of decisive importance. The main challenge here is the different polarity and surface tension of the two plastic materials. However, other parameters such as processing conditions or additives used also influence adhesion.

Innovative modification of the hard component

The bio-based hard components and Bio-TPE as a soft component are not compatible and do not have sufficient mutual adhesion without suitable modification. To reduce the differences in polarity and surface tension, the research project produced compounds with different compatibilizers to improve phase compatibility. This also made it possible to specifically improve the adhesive effect of the 2-component composites.

The most important factor influencing the properties of the adhesive bond is the interfacial tension between the two materials. The surface energies of the individual materials were therefore determined using Drop Shape Analysis (DSA) at the University of Kassel and used to calculate the interfacial tension. In addition, the process settings for the production of hard-soft composites in 2-component injection moulding were also optimized there.

Significant improvement in adhesion

By means of peel tests according to VDI 2019, testing the adhesion of thermoplastic elastomers (TPE) to substrates, the bond strength between soft components and both incompatible and compatible hard components was determined at the University of Kassel. The measurements showed that the compatible components lead to a significantly better bond strength. The peel force could be increased to a remarkable 174 N by modifying 0 N (corresponds to no adhesion, the components already separate when removed from the injection mold). 

Project result

The production of selected compounds was successfully transferred to industrial production scale at FKuR Kunststoff GmbH and made available to other industrial partners for sampling tests. The company SKS metaplast Scheffer-Klute GmbH manufactured wheel guards and air pump handles. These were each produced by means of a transfer technique in 2K injection moulding, whereby the hard component is first injected in a single cavity. On a second injection moulding machine, the hard component is inserted into another mould and overmoulded with the soft component. At WEGU GmbH Leichtbausysteme, air ducts for the engine compartment of passenger cars were sampled. For this purpose, an injection moulding machine with two injection units was used, so that both components could be produced simultaneously in a split mould. The trials at the companies Ritter-Pen GmbH (hard-soft handles for writing instruments) and Sunstar Interbros GmbH (toothbrush handles) were also successful.

Project partners

  • Institut für Werkstofftechnik der Universität Kassel
  • FKuR Kunststoff GmbH 
  • Other associated industrial partners 


Duration: December 2016 to May 2020
Funding code: 22000915, 22011816, 22011916