Instruments

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.

Discussion