Environmental Effects on the Interfacial Chemical Reactions at RTV Silicone-Silica Interfaces

Silicone sealants and adhesives are extensively used in construction, automotive, industrial, and electronic applications because they exhibit excellent mechanical properties, strong adhesion, and good weather resistance. Room-temperature vulcanized (RTV) silicones develop good adhesion to many substrates and do not require heat for curing, which leads to flexible use in many applications. Although it is known that various factors such as relative humidity and temperature affect the curing of the RTV silicone adhesives, the interfacial chemistry that occurs during the curing process is still poorly understood but critical for success in adhesive applications. To address this, sum frequency generation (SFG) vibrational spectroscopy was used to probe the molecular details of the buried interface of the RTV silicone adhesive in situ. Time-dependent SFG experiments were conducted on two polydimethylsiloxane (PDMS) matrices, at three humidity levels, and with two kinds of silica surfaces to investigate the behavior of the methoxy groups at the interface and the impact of environmental conditions on the adhesion mechanism. It was found that both the methoxy groups from methyltrimethoxysilane (MTMS) and methoxy-terminated PDMS could segregate to the interface. The diffusion of MTMS and bulk rearrangement of methoxy-terminated PDMS lead to the segregation and ordering of methoxy groups at the interface. After comparing eight samples cured under different environmental conditions, the reactions of the interfacial methoxy groups were found to be facilitated by both the surface water on silica and moisture from the environment. The silylation treatment on the silica slows the reactions of the interfacial methoxy groups, while the high environmental humidity accelerates the consumption of the interfacial methoxy groups. These findings provide insightful information about the adhesion mechanism of RTV silicone adhesives and accelerate new product development.