Influence of the Catalyst
The influence of the catalyst amount on weathering behavior was studied for a one-component modified silicone sealant; the results are summarized in Table 8. Cracking is seen at catalyst concentrations which are typically used in MS-1 sealants (around 1.5 phr) only after extended accelerated weathering (2000-3000 h). At higher catalyst loadings, however, the weathering performance becomes increasingly poorer.
From this data, it is obvious that the amount of catalyst has a strong influence on the weathering behavior (crack formation) of MS-1 sealants and will reduce the weathering performance after extended periods of exposure. On the other hand, the catalyst is necessary to control the curing properties of the sealant, such as skinning time and curing speed, and the amount cannot be chosen at random. In practice, the catalyst amount will be a compromise between the necessary curing properties and the weathering behavior needed.
Similar results have been reported in the literature,  where it was also pointed out that the nature of the catalyst plays an important role.