The significant effect of silane coupling agents in improving the performance of composite materials has long been confirmed, and there are many theories to explain their mechanism of action. The most successful is the chemical bond theory proposed by Arkles, which involves the synthesis of silanol groups or hydrolyzed silanol groups of silane coupling agents with surface hydroxyl groups of inorganic materials, while the organic groups of another carbonyl group form covalent bonds with organic polymer compounds. Therefore, by using silane coupling agents, the interface between two materials with significantly different properties can be coupled together, thereby improving the performance and increasing the bonding strength of the composite material, and obtaining a composite material with excellent performance.
In most applications, silane coupling requires hydrolysis to form silanol based compounds, which then interact with the substrate surface. This reaction is divided into four steps. Firstly, the alkoxy group undergoes hydrolysis, and the resulting silanol group is then dehydrated and polymerized to form oligomeric siloxanes. The silanol groups in oligomers are adsorbed onto the surface of the substrate through hydrogen bonding, and during the heating and curing process, they condense with the hydroxyl groups on the surface of the substrate, forming covalent bonds. It should be pointed out that after hydrolysis occurs, other reactions can proceed simultaneously. It is generally believed that one of the three silicon hydroxyl groups generated by the hydrolysis of silane coupling agents is bonded to the surface of the substrate; The remaining two are either condensed with hydroxyl groups of other silanes or in a free state. In theory, silane coupling agents should form a monolayer, and the monolayer should also be sufficient to endow the substrate with the desired surface properties. In fact, due to the condensation between silane coupling agent molecules, they generally form a three-dimensional network structure thicker than the monolayer on the surface, which does not affect the modification effect. The lower the degree of alkoxyl functional groups in silane coupling agents, the more capable they are of forming monolayers. But the strength and hydrolytic stability of the chemical bonds formed with the surface are lower.
Silane coupling agents can also treat surfaces under anhydrous conditions in the gas phase, but this generally requires a longer time (4-12H) and higher temperatures (50-120 ℃). Only those containing methoxy groups among all alkoxysilane coupling agents can be used in the absence of a catalyst. The most suitable method for vapor deposition is cyclic nitrogen-containing silane.
