Page 623 - Handbook of Thermal Analysis of Construction Materials

Section 3.0 - Applications 591

sealants show decomposition, softness, and reversion at high temperatures
and stiffness at lower temperatures after 600 and 1000 hours of exposure to
UVB. Furthermore, PU sealants can show performance similar to commer-
cial silicone sealants under severe test conditions. Finally, he concluded
that rheological additives (used for sag resistance) may play an important
role in the overall performance. Although, the author hints at the usefulness
of DMA as a tool to characterize sealants and to predict their performance,
interpretation of the results is difficult because no data was reported for
unexposed sealant samples.
In another study, Jones, et al., [24] utilized dynamic mechanical
thermal analysis (DMTA) to examine the effects of movement during cure
development on bulk joints sealants. Movement parameters were imposed
upon the joints during cure. These parameters included a combination of
temperature cycling over the relevant amplitudes of ±7.5% and ±12.5% at
temperature ranges of 30° and 60°C, respectively. The joints were sub-
jected to an elevated temperature cycle during compression (35° at -7.5%
and 50°C at -12.5%) and to a low temperature during tension (5°C at + 7.5%
and 50°C at +12.5%). Both one and ten cycles a day were imposed on the
joints. Specimens were also cycled mechanically, but without temperature
cycling. Mechanical testing was performed on these joints. The results
indicated that cyclic movement during cure reduced significantly the
performance of tensile adhesion joints prepared from a one-part system.
However, the effect for two-part systems was minimal. Similar joint
materials and configurations were analyzed using DMTA.
The analysis was carried out in the temperature range of -80° to
40°C at a frequency of 1 Hz, using a shear sandwich arrangement and a
nominal peak-to-peak displacement of 23 µm. Samples from inner and
outer sections of the bead after 1, 3, and 7 days of cure were tested together
with uncured sealant. The curing of the sealants was monitored by observ-
ing the shifting of the primary peak to a higher temperature on the tan δ
versus temperature curves, accompanied by a decrease in the height of the
very broad secondary transition (Fig. 6).
Jones, et al., [24] reported that DMTA results showed that movement
during cure increases the rate of cure for a one-part sealant material, but had
little effect on the actual mechanism of cure as shown in Fig. 7. For the two-
part sealants, it was observed that movements during curing did not affect
the shape and maximum of the tan δ. Therefore, they identified that the
actual mechanism of curing for a two-part sealant is unaffected by move-
ment, and this has no effect on the rate of cure.