Understanding the relationship between the structure of an organofunctional methoxysilane bilayer and its properties as adhesion primer for Zn-epoxidized rubber system

The structure of a new organofunctional methoxysilane bilayer system, which was found to improve the adhesion of epoxidized natural rubber (ENR) to zinc, is analyzed by High Resolution Electron Energy Loss and Infrared Reflection Absorption Spectroscopy. This bilayer is formed when zinc plate is successively dip-coated in water–alcohol solutions of γ-mercaptopropyltrimethoxysilane and aminopropyltrimethoxysilane. The optimal adhesion, obtained when both silane layers are cured at 110°C, is due to the specific structure of this bilayer, which depends dramatically on the cure temperature of the first layer. From HREELS results under the conditions of an impact mechanism, it is shown that the surface density of NH2 groups (which react with the epoxy groups of rubber and provide the adhesion of the whole system) is much higher when the γ-MPS layer is cured at 110°C instead of 70°C or room temperature. This high NH2 group surface density is also confirmed by wettability measurements, which indicate a marked increase in the basic component of the surface energy at 110°C. An IRRAS study of the γ-MPS layer deposited on zinc shows that up to 70°C hydrolysis of the SiOCH3 groups is poor, and becomes effective only after the zinc/γ-MPS layer is dipped in the basic APS solution. This makes the penetration of APS molecules across the γ-MPS layer easier and, consequently, amino groups can interact with zinc. Therefore, they are unavailable for a reaction with epoxy groups. In contrast, when the γ-MPS layer is cured at 110°C or higher, hydrolysis of its SiOCH3 groups begins and, simultaneously, lateral cross-linking occurs between SiOH groups. Under these conditions, penetration of the γ-MPS layer by APS molecules is hindered and lateral mixed cross-linking occurs at the interface γ-MPS/APS via the SiOH groups. This entails a greater density of pendant amino groups directed towards the surface, which enhances the practical adhesion of rubber to zinc.