The toughness and the durability under a high humidity condition of the interfaces in dissimilar adhesive joints of carbon-fiber-reinforced thermoplastic with a polyamide-6 matrix and Al alloy were evaluated by two test methods, in which a tensile opening load was applied to the specimens to cleave the interfaces apart in two different ways. In the double cantilever beam (DCB) test, the specimens were continuously pulled apart at a constant velocity, while in the wedge test, the specimens are pulled apart at a constant displacement. The crack growth along the interface in the DCB test was dynamically monitored with the assistance of mechanoluminescence for the accurate detection of the phenomena at the crack tip. The wedge test was employed for the evaluation of the durability of the interfaces under high humidity conditions. It was found that the adhesive joints were failed by various failure modes depending on the surface pretreatment and environmental conditions. Throughout the work, discussion was made concerned with the interfacial structures and the adhesion mechanism of dissimilar adhesive joints.

In the direct melt bonding of isotactic polypropylene (iPP) to aluminum (Al), the blending of a small amount of maleic anhydride-grafted PP (PPgMA) with iPP was found to induce a dramatic improvement of the strength of adhesion. The effect of blending PPgMA was, however, limited, maximizing at ∼20 wt % PPgMA. Incorporation of larger amounts of PPgMA reduced the strength of adhesion. We studied the mechanism of adhesion between Al and iPP by incorporating chemical functionality to the polymer side. The fracture surfaces produced by peeling off the interfaces were investigated by replicating the surface topographic features on a platinum thin film and analyzing them by scanning transmission electron microscopy (STEM) as well as by reconstructing three-dimensional (3D) surface structures with STEM tomography. The replica-STEM technique enabled us to visualize PP surface crystalline lamellar structures and their deformation upon the failure in 3D. We found that polymer/metal interfaces produced surface features in the failure that were similar to those associated with failure of entanglementbased polymer/polymer adhesion via chain pullout. A fractography study by replica-STEM suggested that the formation of a lowmolecular-weight layer with low crystallinity at the interfacial region was responsible for the improvement of adhesion. The adhesion strength depended on the toughness of the "soft layer" and did not depend on the chemical bonding between PPgMA and Al. The interfacial chemical reaction between MA and the Al surface yielded PP with a grafted carboxylic acid (−COOH) group, which may have been excluded from the PP crystalline lamellae. We concluded that chemical bonding was not the primary reason for the improvement of adhesion, but it was necessary to induce the segregation of PPgMA in the interfacial region and the formation of the soft layer.

No abstract

No abstract