Predicting adhesion between a crystalline polymer and silane-treated glass surfaces in filled composites

Results reported earlier showing a close correlation between measured adhesion strength of bonds between silane-modified glass and various amorphous thermoplastics and a thermodynamic compatibility criterion based on UNIFAC are extended to the case of a crystalline polymer matrix. The criterion used is the magnitude of the (negative) molar Gibbs free energy of mixing, (-Δ Gmix)0.5, for a pseudo-solution consisting of equal molar amounts of the repeat units of the polymer matrix and the organo-functional group of the silane coupling agent, but the measurements leading to the values of the adhesion strength are carried out using a three-point bending test, in which a rectangular polymer specimen filled to a high volume fraction with silane-treated glass beads is subjected to increasing load until the composite breaks or yields. Earlier work showed a good correlation between the adhesion energy computed from single-partcle composite measurements and the value of (-Δ Gmix)0.5 computed using UNIFAC for a variety of organo-silane-modified spheres imbedded in three different amorphous polymeric matrices. The present work extends the database to a crystalline matrix, viz. poly(ethylene terephthalate), and uses a nonoptical adhesion test to obtain flexural yield strengths. The values of the latter are also found to be in good agreement with the predictive thermodynamic criterion.