Verifying the reliability of the steam-jet test on coated thermoplastic olefin substrates by a semi-quantitative peel test

Abstract

The steam-jet test is a standard method to evaluate adhesion of coated lacquer on different substrates in automotive industry. Besides metals, most common for exterior parts is coated injection molded talc filled thermoplastic olefin (TPO). This material often shows high spreading and partially substrate participation in its failure modes. To examine the validity of the steam-jet test on coated TPO, different adhesion qualities are produced by varied flame treatment of the substrate and subjected to the well-known steam-jet test as well as to the peel test. The peel test requires a fabric, embedded inside the coating to measure the peel force, when pulling off the lacquer. Surface characterization is done by XPS and light microscopy. We show that the steam-jet test does not allow even qualitative differentiation between the two best adhering samples. The error is of the order of magnitude of the mean value. By contrast, the peel test showed that the binding force to the substrate is higher than its interior strength and therefore both best adhering samples provide similar peel forces. As a result, the steam-jet test was found to have significant weaknesses in testing coated talc filled thermoplastic olefins.