Silane Coupling Agents

The silane coupling agent (hereinafter referred to as SCA) is an organic silicon compound. It has two functional groups, non-hydrolyzable group and hydrolyzable group, so it has completely different chemical properties at both ends of the molecule. The classic product of silane coupling agent can be expressed by the general formula YSiX₃. In the formula, Y is a non-hydrolyzable group; X is a hydrolyzable group. Hydrolysis groups include alkoxy groups or carboxyl groups, which are directly connected to the Si atom. Non-hydrolyzable groups include alkene groups, epoxy groups, amine groups, mercapto groups, hydroxyl groups, etc, which are connected to several carbon atoms on the silane. The hydrolyzed group can be hydrolyzed into hydroxyl groups, and then combined with inorganic materials. The non-hydrolyzable group can be combined with the corresponding organic resin through chemical reaction or physical action.

Figure 1. Macromolecular silane coupling agent

Applications:

Due to the advantages of special structure, rich variety and various synthetic methods, silane coupling agents are increasingly used in adhesives, rubber industry, composite materials, aerospace, construction materials and other fields.

• Rubber industry: The silane coupling agent is mainly used in the silicone sealant to improve the adhesion of the sealant to the substrate, adjust the vulcanization time, improve the mechanical properties, and improve the dispersion of the filler in the base rubber. Silicone sealants have excellent high and low temperature resistance, are non-corrosive to metals and their coatings, and exhibit excellent bonding properties to most building substrates, so they are widely used for bonding and sealing. The silicone sealant is made of linear polysiloxane as the main raw material, and is made with the aid of crosslinking agents, fillers, coupling agents, plasticizers and other additives. Partial silane hydrolysis reaction has high activity and can quickly absorb the water in the system, so it can also improve the wet bonding performance of the compound. For example, silane coupling agent γ-aminopropyltriethoxysilane (KH-550), γ-methacryloxypropyltrimethoxysilane (KH-570) can be mixed with base rubber and filler to prepare environmental protection Type silicone sealant. The silicone sealant can be stored stably for 12 months and keep the properties of surface drying time, Shore hardness and tensile strength unchanged.

• Waterproof coating: The application of silane coupling agent in waterproof coating is mainly to enhance the binding ability of each raw material component, so as to obtain waterproof coating with excellent performance. The polymer cement waterproof coating has been developed rapidly in recent years because of its simple construction, high strength of the coating film and strong waterproof ability, and is widely used in the construction industry. The waterproof material not only needs to have high strength and excellent waterproof ability, but also must have good flexibility and durability. Because polymer cement waterproof coatings use organic polymer emulsions and inorganic cement as the main raw materials, the combination of the two limits the improvement of the overall performance of polymer cement waterproof coatings. Adding silane coupling agent to polymer cement waterproof coating can effectively enhance the binding ability between organic materials and inorganic materials, and can improve the performance of waterproof coating, so as to obtain higher tensile strength and elongation at break coated film.

• Metal corrosion protection: The silane coupling agent has applications in the field of metal corrosion protection. The silane coupling agent contains a small surface tension and a large wetting ability and a certain viscosity. The siloxane group of the silane can react with the moisture on the metal surface to form silanol. After hydrolysis, the silanol and the hydroxyl group on the metal surface are bonded together in the form of hydrogen bonds. When drying and curing under air or heating conditions, the silanol generated by the silane coupling agent dehydrates with hydroxyl groups on the metal surface, and finally forms a metal-siloxane covalent bond (Me-O-Si bond). At the same time, the dehydration condensation reaction of silanol itself also proceeds in the form of hydrogen bonding. When drying or curing under air or heating conditions, a dense silane protective film (Si-O-Si) is formed on the metal surface. This dense silane protective film has anti-corrosion effect and can protect metals from corrosion.

Production Processes:

The production processes of the silane coupling agent include a hydrosilylation method, a direct synthesis method, a format method, a halogenated silane dehydrohalogenation method, and a halogenated silanol hydrolysis method.

References:

1. Wirunya Keawwattana. (2011). "The Effect of Coupling Agents on Natural Zeolite-Filled NaturalＲubber." Journal of Chemistry and Chemical Engineering 5, 412-422.
2. Ye Xin. (2012), "Some Interesting Phenomena in Silica-fifilled HNBR with the Addition of Silane Coupling Agent." Journal of Applied Polymer Science 124(2), 927–934.
3. Manna Ajak K. (1999), "Bonding between Precipitated Silica and Epoxidized Natural Rubber in the Presence of Silane." Journal of Applied Polymer Science 4(2), 389-398.