Rheometers—TA Instruments (New Castle, DE) Advanced Rheometric Expansion System (ARES) controlled-strain and AR550 controlled-stress rotational rheometers were used to conduct post-cure dynamic deformation experiments. The full-scale torque was 200 mN■ m and 50 mN ■ m for the ARES and AR550, respectively. In dynamic testing mode, the frequency range capability of the ARES and AR550 was 10-5 to 500 rad■ s-1 (10-5 8 to 80 Hz) and
10-3 2 to 250 rad■ s-1 (10-4 to 40 Hz), respectively. The ARES was equipped with a forced (air or N2) convection oven while the AR550 used a controlled convection/radiant-heating environmental chamber.
Plate Geometries—Rotational rheometers are typically accessorized with two different plate geometries shown schematically in Fig. 2. Plate geometry fixtures with a radius R of 4 mm were used to impose a shear deformation, represented in Fig. 2 by the angular displacement ®, on the test specimens based on the instrument design specifications, the modulus of rigidity of the cured sealants, and the testing parameters. The parallel-plate fixture was the primary geometry used. It had the advantage of allowing for a gap h between the plates, corresponding to the specimen thickness, to be specified, which can be useful if a certain aspect ratio of the test specimen is desired. In this work,
the thickness of the sealant test specimens was approximately 1.5 mm. An autotension capability allowed the gap to be adjusted automatically during the cure cycle to compensate for forces generated as a result of volume shrinkage from leaving reaction by-products. The ARES was equipped with plates constructed from stainless steel whereas a stainless steel upper plate and aluminum bottom plate was utilized with the AR550. Both rheometers had the option of using disposable plate fixtures constructed from different materials.
A cone-and-plate geometry was used in controlled-strain experiments with the ARES rheometer to validate the results obtained with the parallel plates. The cone angle 3 was 0.0999 radians and the cone tip was truncated to a gap of 0.044 mm. This truncation gap needed to be maintained regardless of temperature, which, unlike the parallel plate geometry, did not permit the thickness of the test specimen to be varied.