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Abstract
Environmental stress crazing/cracking (ESCR/C) of adhesives under organic solvent exposure is a subject of great practical importance to adhesive end-users, especially in dealing with structural applications. In the past, the mechanical properties of several adhesive systems have been shown to degrade considerably after both prolonged and momentary exposure to solvents under a state of stress. In this study, the solvent resistance of three structural polyimide adhesives was studied, with respect to organic solvents that may come in contact with the adhesives during their service life. Initially, dog-bone samples of the resin systems were prepared, according to ASTM Standard D638–91a, and these were soaked in the solvents to obtain equilibrium solvent mass uptake curves. The solvents used in the study included acetone, methyl ethyl ketone (MEK) and ethylene glycol, among others. The equilibrated samples were then tested in a miniature tensile testing machine, to obtain stress-strain characteristics. Secondly, samples equilibrated in the solvents were held in a vacuum environment at 150°C (which is below the glass transition temperature of the adhesives) to desorb the solvent, and these were then tested to obtain residual properties. To study the durability of bonded joints under solvent exposure, Ti-6A1-4V/adhesive bonds were prepared and wedge tests were performed on them for periods up to several days in solvent baths. Based on the measured crack lengths, the strain energy release rate due to solvent-induced environmental stress cracking (G ESC) was computed as a function of crack growth rate. The G ESC measurements help quantify the durability of the bonded joints on exposure to the various solvents, and further help in ranking the adhesives in terms of solvent resistance.
