Solvent effects on high temperature polyimides and their bonded joints

Environmental stress crazing/cracking (ESCR/C) of adhesives under organic s olvent 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 degra de considerably after both prolonged and momentary exposure to solvents und er a state of stress. In this study, the solvent resistance of three struct ural polyimide adhesives was studied, with respect to organic solvents that may come in contact with the adhesives during their service life. Initiall y, dog-bone samples of the resin systems were prepared, according to ASTM S tandard D638-91a, and these were soaked in the solvents to obtain equilibri um solvent mass uptake curves. The solvents used in the study included acet one, methyl ethyl ketone (MEK) and ethylene glycol, among others. The equil ibrated samples were then tested in a miniature tensile testing machine, to obtain stress-strain characteristics. Secondly, samples equilibrated in th e solvents were held in a vacuum environment at 150 degrees C (which is bel ow the glass transition temperature of the adhesives) to desorb the solvent , and these were then tested to obtain residual properties. To study the du rability of bonded joints under solvent exposure, Ti-6Al-4V/adhesive bonds were prepared and wedge tests were performed on them for periods up to seve ral 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) measurem ents help quantify the durability of the bonded joints on exposure to the v arious solvents, and further help in ranking the adhesives in terms of solv ent resistance.