Durability of sheet molding compound/metal adhesive bonds

Durability of a variety of sheet molding compounds (SMC)/ adhesive /metal systems involving steel, aluminum, phase α-SMC, Budd SMC, and polyurea adherends and polyurethane and epoxy adhesives has been investigated. Three specimen geometries with different modes of stress were employed: lap shear, wedge, and butt torsion. Specimens were exposed to an environmental cycle or to fixed conditions of temperature and humidity with or without load.

For phase α-SMC/urethane/ELPO steel adhesive bonds, all visually determined adhesive failure occurs at the SMC/adhesive interface, and it is in reality a mixture of interfacial failure and near surface SMC debonding. The near surface SMC material and the thin layer of primer on the SMC surface are the weak links in the bond. An improvement in durability of phase α-SMC/urethane/ELPO steel adhesive bonds requires an increase in the strength of the outer layer of the SMC material.

The metal surface pretreatment proved to be very important for the durability of phase α-SMC /urethane/aluminum adhesive bonds. The adhesive bonds prepared with solvent cleaned aluminum primed with epoxy or phenolic primer displayed poor durability. This is attributed to the contaminants on the aluminum surface after solvent cleaning which hinder the interactions of aluminum surface species with primers. The use of alkaline cleaned aluminum primed with diisocyanate primer improved durability only in dry air conditions. This can be explained by the high susceptibility of the NCO functionality to water, which can rapidly weaken the primer/aluminum bonds. Priming alkaline cleaned aluminum adherends with phenolic or epoxy primers improved dramatically the adhesive bond durability under all testing conditions. Enhanced durability is attributed to chemical and hydrogen bonds established between components of phenolic and epoxy primers and aluminum surface species. Reflection absorption FTIR studies of the interactions between identical or similar compounds that exist in the primers, e.g. diglycidyl ether of bisphenol A, phenol, and methylene diphenyl diisocyanate, and alkaline cleaned aluminum surface species indicated chemical processes.

Exposure of bonded samples at 60°C for up to seven days did not reduce the residual strength. Some loaded samples failed during durability tests, indicating that loading is a significant factor in durability. Butt torsion samples proved to be much more sensitive to stress and temperature than the lap shear and wedge samples, showing the important role of the mode of stress in durability. Humidity had a detrimental effect on durability for all systems investigated. For the system Budd SMC/adhesive/ELPO steel an epoxy adhesive exhibited better durability than a polyurethane adhesive. Enhanced durability is explained by higher cohesive strength, a lower moisture absorption rate and higher resistance to hydrolysis of the epoxy adhesive. An increase in bondline thickness proved to be beneficial for adhesive durability, a fact attributed to a more uniform stress distribution in thicker bonds.