Environmental\nEffects on the Interfacial Chemical\nReactions at RTV Silicone–Silica Interfaces

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