Two structural adhesives were rheologically characterized, with the purpose to define a testing protocol that would be useful for differentiation of structural adhesives before thy are tested on the production line. In production, the adhesives are dispensed through small orifices and then are expected to set quickly to avoid creeping and flowing past the adhesive channels, a condition called squish. The adhesives were filled epoxies, labeled "A" and "B". By static measurements (viscosity, modulus in "at rest" conditions), "A" is a better material; it has almost twice the modulus and viscosity of "B". These observations are further kept throughout the temperature range until full cure is obtained (where they are equal). "A" starts curing quicker (approx85C vs. approx92C), but full cure is not achieved for both until approx 108deg C. However, under dynamic conditions (measurements immediately after the adhesive undergoes a large shear deformation, as occurs during dispensing), a different profile is observed. "B" regains its structure almost five times faster than "A" (30 vs. 145 seconds). This feature allows "B" to regain its elasticity and therefore stop flowing or creeping quicker than "A". Experience on the line correlates with the dynamic experiments, with "B" a better performer for squish. Since the adhesives are never under static conditions, typical characterization under these conditions does not make sense. The relaxation dynamic test is a good indicator of flow behavior under deformation and is a better predictor of squish under our fast production process.
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