Application of Silane Coupling Agents in the Field of Conversion Coatings and Their Hydrolysis Significance

Silane coupling agent is an organic silicon compound that contains two different chemical functional groups in its molecular structure. It can be represented by the formula YnSiX4-n, where Y is an unhydrolyzable functional group, such as an alkenyl group (ethylene group) and hydrocarbon groups with end groups such as Cl, NH2, SH, epoxy, and (methyl) acryloxy; X is a hydrolyzable functional group that can generate Si-OH, including Cl, OMe, OEt, and so on.

X and Y are two types of active groups with different reaction characteristics. X groups are more likely to bond well with inorganic substances, minerals, metals, and other materials, while Y groups are more likely to bond well with organic materials such as resins and rubbers. Because silane coupling agents have properties that simultaneously affinity for organic and inorganic substances, they can be used as intermediates to connect materials with significantly different chemical structural types and affinities. For example, the silane coupling agent KH570 can increase the bond strength between organic coatings and inorganic substrates such as metals.

Application of silane coupling agent in conversion film field

Exposed metal substrates are easily corroded under natural conditions. Applying a layer of coating to the metal substrate surface is the most common and convenient method for alleviating metal substrate corrosion. However, due to the different chemical properties of the metal substrate and the coating, the bond strength between them is very limited, and the coating is easy to peel off. This causes the metal substrate to be re-exposed to a corrosive environment.

Industrial conversion film processes such as phosphating are commonly used to increase the bond strength between the metal substrate and the coating. Conversion film technologies are simple and have stable performance. However, products (such as phosphating and passivation) widely used on the market usually contain heavy metals, volatile acids, phosphorus, and other pollutants, and their use is becoming increasingly restricted. Silane coupling agents, as environmentally friendly organic materials that enhance the bond strength between coatings and metal substrates, have led to the development of environmentally friendly conversion film products that use silane coupling agents to enhance the bond strength between coatings and metal substrates.

The significance of hydrolysis in silane coupling agents

Conversion film products that only use silane coupling agents have thin and poorly corrosion-resistant conversion films. Although they can enhance the bond strength between coatings and metal substrates, metal substrates are prone to rusting before applying coatings, and this conversion film layer is difficult to meet the rust prevention requirements between steps. Therefore, other film-forming materials such as fluoro-zirconic acid, borate, and molybdate need to be added for compounding to enhance the comprehensive performance of the conversion film. Since these materials are mostly water-soluble inorganic substances, they cannot be directly mixed with silane coupling agents. Water needs to be used as a carrier for silane coupling agents and inorganic substances to coexist. This requires the oil-soluble silane coupling agent to be added to water for hydrolysis. Without hydrolysis, the silane coupling agent will float in the aqueous solution like small oil droplets, unable to evenly disperse in the aqueous solution, which will significantly reduce the effectiveness of the silane coupling agent.