Silane Coupling Agents/Adhesion Promoters
Introduction
Silane are silicon compounds derivative and they have Si-H and Si-C bonds, Silane is a bi-functional synthetic silicon compound, which having the ability to enhance the adhesion between two materials characterized with unsymmetrical chemical composition. Silane is generally used as a coupling agent. Silane coupling agents have two functional groups and can react with organic and inorganic surfaces. They can be broadly classified into three categories according to their functional groups, namely sulfur-containing, amino-containing and epoxy-containing groups. In addition, Silane can also be used for surface modification to improve the adhesion ability of the target.
Silane coupling agents
- Sulfur-containing silane coupling agent
Currently, sulfur-containing silane coupling agents are mainly used in the tire industry. The sulfur-containing silane coupling agent combines with rubber through sulfur in the structure and alkoxy groups with silica surface silicon hydroxyl groups to form a strong network structure, which greatly reduces the rolling resistance of tires.
- Silane coupling agents containing amino groups
Silane coupling agents containing amino groups are one of the most widely used reagents in our life and can react with all polymeric organic resins. Ammonia itself is a weak base, and after dehydrogenation with other molecules, the new compound formed also has a certain alkalinity. The mechanism of action of silane coupling agents shows that in an alkaline environment, the hydrogen ions generated by the reaction will be reacted away, driving the balance to shift positively, the condensation reaction will be more complete, and the polymer obtained will have better performance than the acidic one, and the toughness of the material will be enhanced. Y-urea propyl triethoxy silane (A-1160), a new type of urea-based silane coupling agent, is a neutral water-soluble compound with a pH of about 7. It can react with most resins at low temperatures while maintaining certain ultimate working properties.
- Silane coupling agent containing epoxy group
Silane coupling agent containing epoxy group is different from the first two types of coupling agents, it is a new type of reagent developed in recent years, overcoming the defects of traditional reagents reacting with water and hydrolysis, and avoiding the consumption of hydrolysis reaction. Its effect and speed of action are significantly improved compared with traditional silane coupling agents, with stronger stability, which can maintain its good physical properties for up to one year. Because it does not react with water, it is usually kept in dilute solution to prevent oxidation and reaction with air components. Epoxy-type silane coupling agents represented by 3-(2,3-epoxypropoxy)propyltrimethoxysilane (A-187) maintain good physical properties and adhesion under room temperature conditions for more than 12 months.
Silane adhesion promoters
When silane is used as an adhesion promoter, the functional group of the silane adhesion promoter modifies the target and enhances its adhesion capacity. The adhesion strength promoted by silanes varies with different materials. For example, the surface of pristine multi-walled carbon nanotubes has strong van der Waals forces, which makes them easily entangled, hydrophobic, and not easy to react with ions, thus limiting their applications in many fields, and therefore they need to be modified to break the van der Waals forces on the surface for practical applications. 3-mercaptopropyl trimethoxysilane was used as a silane adhesion promoter to modify multi-walled carbon nanotubes to prepare hydrophobic silane-modified multi-walled carbon nanotubes, which enhanced the adsorption performance of Cd2+.
Application
In recent years, the role of silane coupling agents/adhesion promoters has become more and more important, which oriented to highly composite molecular structures, and the combination of different unit functional groups makes the function more specialized. Functionalized coupling agents/adhesion promoters can change the solubility of materials, can make non-impregnated materials into impregnated materials, such as nanoscale silica particles and glass fibers, etc. the relative molecular mass of the composite becomes larger and the length is constantly longer, improving the anaerobic properties of surface materials. In addition, some silane coupling agents prepared by relying on composite metals and organic materials are widely used in environmental protection coatings and other fields, and there has been a substantial increase in the variety of types and demand.
Reference
- Matinlinna, J. P., Lung, C. Y. K., & Tsoi, J. K. H., Silane adhesion mechanism in dental applications and surface treatments: A review. Dental materials, 2018, 34.1: 13-28.