Interphases developed by interfacial reactions in polypropylene-aluminum joints unveiled by local thermomechanical analysis

Abstract

Blending of maleic anhydride-grafted PP (PPgMA) into isotactic PP (iPP) has been discovered to improve its bonding strength with aluminum (Al) significantly. The optimal bonding performance is achieved with a 20 wt% of PPgMA, whereas pure PPgMA fails to adhere to Al. The results suggested that the chemical reaction between MA and the Al surface alone does not directly account for the bonding properties. To investigate the interfacial phenomena between iPP/PPgMA blends and the Al substrate, we employed local thermomechanical analysis using NanoTA. Remarkable variations in the thermomechanical response were obtained near the Al/PP interface compared to the bulk region and even the surface. The interphase, located approximately 10 µm from the Al/PP interface towards the bulk of PP, was identified through these analyses. Combining the findings from NanoTA, Differential Scanning Calorimetry (DSC), and scanning electron transmission electron microscopy (STEM) revealed that the chemical interaction between PPgMA and the Al surface triggers the formation of a wide interphase through the phase separation of iPP and PPgMA. Importantly, the interfacial chemical reaction does not directly enhance the bonding strength; rather, it generates the interphase, which plays a critical role in bonding strength.