Due to the wide range of properties of plastics (e.g. low density), more and more conventional materials are substituted by polymer materials. Complex requirement profiles on technical parts increase the demand for joining processes that enable the reliable joining of otherwise incompatible thermoplastics. In this case, material bonded connections are approaching their limits. In the following study two incompatible thermoplastic polymers were welded by using polymer blends that are compatible to both components. Industrially relevant thermoplastics polyethylene (PE) and polyamide 12 (PA12) were chosen to demonstrate the potential of an innovative joining technology.

In order to achieve short cycle times, gas temperatures of 500 °C or higher are used for welding plastics by means of serial hot gas welding. These temperatures are often distinctly higher than the decomposition temperature of the plastic to be joined. The thermal stresses can lead to chemical degradation and physical aging. This can negatively affect the lifetime of the weld and lead to premature failure. In this study, the fusion layers of polypropylene and polyamide are analyzed in terms of their thermal and thermal oxidative degradation using viscometry and FTIR spectroscopy. The results show that a detection of chain degradation and degradation products in the individual melt layer depths is possible. Furthermore, correlations between the depth of degradation and the weld strength can be identified. This characterization shows the first results of a holistic approach to the influence of thermal degradation in the weld seam as a function of the process parameters and their effects on weld seam properties. With the right joining strategy, it is possible to achieve high weld strength without the use of nitrogen and the associated material degradation. This could result in a significant cost reduction for the industry.