Modified Silane Polymer Adhesive: Effect of Aluminum Substrate Treatment Methods and the Adhesive Thickness on Adhesive Durability and Joint Strength

Hypothesis: 

Adhesives and sealants based on modified silane polymer (MS polymer) are increasingly gaining market share in adhesive and sealant market place due to their desirable performance in various applications.

In order to find the most optimal adhesive processing parameters, different stages of aluminum surface treatment in three stages were investigated, including the surface degreasing, sanding and priming. In this manner, the performance of four different types of degreasing agents and two kinds of silane-based primers were investigated individually. The optimum bond line thickness was also determined through adhesive joint shear strength. The effect of different degreasing agents and silane-based primers were evaluated through lap-shear strength. The effect of adhesive layer thickness was studied through lap-shear strength and cleavage strength tests.

Optimum bond line thickness was determined at 0.5 mm based on the results of the adhesive shear strength, whereas the results of the fracture toughness indicated 1.25 mm as the optimum adhesive layer thickness. The lower adhesive strength in thicker bond lines may well be due to the possible formation of air traps and lower constrained effect of the substrate. Therefore, the least requirements for aluminum surface treatment was surface degreasing followed by sanding which results in a transition from adhesive fracture mode to cohesive fracture. The use of two kinds of silane-based primers containing the amine group, triethoxy silylpropylamine (APS) and the other with epoxy group, glycidoxypropyl trimethoxysilane (GPS) was more efficient in improving joint durability even after 2500 h cyclic aging with the more pronounced results for the APS.