Materials

Table 1 summarizes the four moisture curing materials used for this work: a pair of one-part RTV and a pair of two-part structural silicone sealants. The listed movement capability that ranged from 12.5 % for the two-part sealants to 50 % was determined following ASTM C719. Each test specimen was allowed to cure in situ, to optimize material-substrate contact with the rheometer plate fixtures, at ambient conditions (23 ° C and 40 % RH) for one week under static conditions. For future considerations and in the event that proposed standard testing parameters are established [11], rheometers can also be used to cure specimens under dynamic conditions while simultaneously measuring its change in rheological properties over time.

TABLE 1—Description of moisture curing silicone sealants.

Sealant

Designation

Type

Si Cure Chemistry

Rated Movement Capability, %

Shear Modulus, MPa

A

One-part

Acetoxy

25

0.4

B

One-part

Alkoxy

50

0.7

C

Two-part

Alkoxy

12.5

1.5

D

Two-part

Alkoxy

12.5

1.4

Materials
Подпись: h
Подпись: R

image225

FIG. 2—Schematic diagrams for parallel-plate (left) and cone-and-plate (right) geom­etries with a plate radius R.

Using dimensions defined by the rheometer fixtures described in the next section, full cure of the 8-mm diameter test specimens under ambient condi­tions (22°C, 40 % RH) was predicted to be 73±25h and 81±10h for the one-part RTV sealants A and B, respectively. This was based on preliminary in-house experiments using (1) rheometry to monitor the modulus build up to an asymptotic (full cure) value using small-amplitude deformation (1 % strain), and (2) cure-in-depth measurements of one-part RTV sealants. After the seven – day cure period, the environmental test chamber for the rheometer was en­gaged to compartmentalize the test setup and the cured sealant was allowed to equilibrate at 25 ° C for 24 hours. A preliminary stress-strain test was con­ducted to approximate the shear modulus of the elastomer (Table 1).