Silane coupling agent is one of the earliest studied and applied coupling agents. Because of its unique properties and the constant emergence of new products, its application fields are gradually expanding, and it has become an important branch of silicone industry.
Silane coupling agent is a kind of silicone product which has developed rapidly in recent years. There are many kinds of silane coupling agent with novel structure. There are more than 100 kinds of silane coupling agent with known structure.
1. Development of silane coupling agent
Around 1945, UC and DOWCORNING of US developed and published a series of silane coupling agents with typical structure.
In 1955, UC company put forward the silane coupling agent containing amino group for the first time. A series of modified amino silane coupling agents have appeared since 1959. The silane coupling agents with peroxysilane in the early 1960s and the silane coupling agents with diazo and azide structure in the late 1960s greatly enriched the varieties of silane coupling agents.
In recent decades, with the development of glass fiber reinforced plastics, the research and development of various coupling agents have been promoted. The synthesis and application of modified amino silane coupling agent, peroxy silane coupling agent and azido silane coupling agent are the main achievements in this period.
2. Structure of silane coupling agent
The general formula of silane coupling agent is RNSIX(4-N), where R is a non hydrolytic organic functional group that can be combined with high molecular polymer. According to the different properties of polymer, R should have strong affinity or reactivity with polymer molecules, such as methyl, vinyl, amino, epoxy, sulfhydryl, acryloyloxy propyl, etc. X is a hydrolyzable group, which can decompose with water solution, water in air or water adsorbed on inorganic surface. It has good reactivity with inorganic surface.
Typical X groups are alkoxy, aryloxy, acyl, chloro and so on. Methoxyl and ethoxyl are the most commonly used by-products in the coupling reaction. Chlorosilane forms corrosive by-product hydrogen chloride in coupling reaction, so it should be used appropriately.
Silane coupling agent has these two kinds of chemical groups in the molecule. Therefore, it can not only react with hydroxyl groups in inorganic compounds, but also interact with long molecular chains in organic compounds. The mechanism of action can be divided into three steps:
(1) The X group is hydrolyzed to hydroxyl group;
(2) The hydroxyl group forms hydrogen bond or dehydrates to ether bond with the hydroxyl group on the inorganic surface;
(3) R-groups are combined with organic compounds.