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).