Characterizing interfacial structure of TPO/CPO/TPO adhesive joints by PFM–AFM and SEM

The influence of annealing and EBR component in injection-molded thermoplastic polyolefin (TPO) plaques on adhesion strength of CPO to TPO was investigated by a lap shear test. The TPO was fabricated as a blend of highly crystalline isotactic polypropylene (iPP) and low crystalline poly(ethylene-butene) impact modifier (EBR28). The CPO was a maleated chlorinated polypropylene containing 21.8 wt% Cl. High resolution pulsed force mode–atomic force microscopy (PFM–AFM) combined with the image analysis was used to characterize the interfacial properties of the lap shear joints. Based on PFM–AFM stiffness images, a “transition zone” with a width on the order of 600–1500 nm was observed between CPO and the TPO substrate that may play an important role in affecting the adhesion strength. This zone exhibits enhanced stiffness after annealing at 120 °C. The PFM–AFM images further show that the interface between iPP and CPO without annealing is very sharp and the interface between TPO and CPO without annealing is wider than the interface between iPP and CPO. Annealing (120 °C/20 min) leads to broadening of the interface between TPO and CPO. The thickness of the interface in lap shear joints was obtained from the z-directional line profiles of the stiffness maps. The fracture surface morphology was revealed by scanning electronic microscope (SEM), which showed that the fracture structure varied with both the addition of EBR28 in TPO plaques and the annealing condition. Finally, a correlation of interfacial properties to adhesion was obtained: higher stiffness in the transition zone coupled with a thicker interface resulted in stronger adhesion and cohesive failure within the CPO and TPO. In the case of CPO/iPP, the narrow interface and absence of a clearly defined transition zone correlated with interfacial failure between these components.