Investigating the Molecular Behaviors of Titanium Catalyst and Silane Cross-Linker at the Buried Silicone Interface

The curing and adhesion properties of many silicone adhesives are mediated by organometallic catalysts (e.g., titanates, zirconates, or aluminates) and silane cross-linker and coupling agents in the formulation. However, how these catalysts and cross-linkers behave at the interface to enhance adhesion is not well established. In this study, sum frequency generation (SFG) vibrational spectroscopy was utilized to investigate the behaviors of titanium catalysts and silane cross-linkers at buried interfaces. When a titanium catalyst (titanium diisopropoxy-bis ethylacetoacetate (TDIDE)) was mixed with methyltrimethoxysilane (MTMS), a new SFG signal centered at ∼2815 cm^-1 emerges, especially in the formulations where there were substantially more MTMS than titanium catalysts. At the same time, methoxy signals (∼2845 cm^-1) were found to increase in intensity as more MTMS was present in the sample. In contrast, mixtures that had more titanium catalysts than MTMS did not show the ∼2815 cm^-1 peak or the strong methoxy signals. It is concluded that the ∼2815 cm^-1 peak is attributed to the C-H stretching mode of a Ti-OCH₃ moiety, an intermediate species formed by the reaction between MTMS and titanium catalyst that results in ligand exchange. SFG experiments were also conducted with TDIDE-MTMS mixtures incorporated into PDMS. Similar interfacial behaviors of TDIDE and MTMS could be observed at the interface. To the best of our knowledge, this is the first time that the mechanism has been elucidated as to the molecular interactions that occur between titanium catalyst and silane in silicone adhesive formulation at buried interface.