Factors influencing mechanical long-term stability of condensation curing silicone elastomers

This work represents a tutorial overview of necessary considerations, which need to be made when working with condensation curing silicone elastomers. Due to the ability to cure at room temperature, the condensation curing silicone elastomers are commonly used as protective coatings and sealants. Nevertheless, if they are not designed with special precautions, their long-term stability can be negatively influenced. Hereby, we first demonstrate the mechanical long-term stability of different network structures consisting of hydroxyl-terminated polydimethylsiloxanes (HO-PDMS-OH) and methyltrimethoxysilane (MTMS). Even though, the instantaneous mechanical properties of the prepared elastomers are promising, long-term stability tests revealed severe post-curing effects making the overall network properties dynamic. We propose two phenomena explaining the network instability: 1) condensation of unreacted and/or dangling polymer chains, and 2) reaction between the cross-linker molecules. Since the extent of the post-curing reactions is being highly dependent on the stoichiometric ratio between the functional groups of the cross-linker and the polymer (r=n(OCH3)/n(OH)), the factors influencing the originally calculated r are investigated. Consequently, it is shown how an unfortunate choice of the cross-linker type/purity and the catalyst concentration can significantly influence the original stoichiometric ratio between the functional groups and therefore also the curing reaction and mechanical long-term stability.