Tensile strength of two-part epoxy paste adhesives: Influence of mixing technique and micro-void formation

Two-part epoxy paste adhesives are frequently used to bond metals and composite materials in many structural applications. After mixing two reactive parts (by weight or volume ratio), adhesive paste is applied to the substrate surfaces and cured at elevated temperatures. Air-entrapment during mixing and/or application process often produces micro-voids in the adhesive bondlines and influences the strength of the bonded joints. In this work, void formation was investigated using two adhesive mixing techniques: (a) dual-cartridge and static-mixer with a dispenser and (b) hand-mix. Flat adhesive sheets were cured by mixing a two-part epoxy adhesive, and bulk specimens with notches were cut using CNC-machining. Using X-ray microtomography scans, the micro-voids were detected and material porosity was evaluated. Furthermore, tensile tests were performed on the specimens and two-dimensional digital image correlation (2D DIC) was employed to analyse the surface strain concentrations near the notches. The fracture surfaces were examined using optical and scanning electron microscopy. The results indicated that mixing technique influences the formation of micro-voids and thus the tensile strength of two-part epoxy paste adhesives.