What does the hydrolysis of silane coupling agents mean
Silane coupling agents are a type of organosilicon compounds that contain two different chemical property groups in their molecular structure. Their main function is to combine the interface between organic and inorganic substances, thereby enhancing the performance and stability of materials. When applying silane coupling agents, it is usually necessary to first prepare a solution of the silane coupling agent for hydrolysis Then it reacts with the active hydroxyl groups on the surface of inorganic materials.
The reason for hydrolyzing the silane coupling agent is that the conversion film product formed by using the silane coupling agent alone is thin and has poor corrosion resistance. Although it can enhance the adhesion between the coating and the metal substrate, the metal substrate is prone to rusting before being coated with the coating, and this conversion film is difficult to meet the rust prevention requirements between processes. Therefore, other film-forming substances need to be added for compounding to enhance the performance of the conversion membrane in all aspects. The film-forming substances that can be added include fluorozirconic acid, borates, molybdates, etc. Since most of these substances are water-soluble inorganic substances, they cannot be directly miscible with silane coupling agents. Therefore, water is needed as the carrier for the coexistence of silane coupling agents and inorganic substances. This requires the addition of oil-soluble silane coupling agents to water for hydrolysis treatment.
If hydrolysis treatment is not carried out, the silane coupling agent will float in the aqueous solution like small oil droplets and cannot be evenly dispersed in the aqueous solution, which will greatly reduce the effect of the silane coupling agent.
What are the hydrolysis conditions for silane coupling agents
The silicon-oxygen bond (Si-O) in silane coupling agent molecules is prone to hydrolysis under the action of water or acid. Generally, the hydrolysis of silane coupling agents needs to meet the following conditions:
The presence of water or acid
During the hydrolysis process, the presence of water or acid is required. Water can promote the breaking of silicon bonds, while acid can further facilitate the hydrolysis reaction.
2. Hydrolysis temperature
Generally speaking, under conditions of higher temperature, hydrolysis reactions occur more rapidly, but excessively high temperatures can also lead to some other adverse effects.
3. Hydrolysis time
The longer the duration of the hydrolysis reaction, the higher the degree of hydrolysis. However, a hydrolysis time that is too long is also not advisable. Silane coupling agents cannot be stored for a long time after hydrolysis, so the hydrolysis time must be well controlled.
Iii. Hydrolysis Methods of Silane Coupling agents
Silane coupling agents are difficult to hydrolyze in water without additives, and the hydrolysis cycle is very long. During the hydrolysis of silane coupling agents, there will be a side reaction of polycondensation. The polycondensation products will precipitate at the bottom of the aqueous solution, affecting the application effect of the product. To ensure the normal progress of the hydrolysis reaction, we usually adopt the following methods:
Firstly, both weakly acidic and weakly basic aqueous solutions can promote the hydrolysis of silane coupling agents. Some silane coupling agents that inherently possess acidic groups (KH560) or basic groups (KH550) are relatively prone to hydrolysis because their own Y groups can affect the pH value of the aqueous solution, making the silane coupling agents more susceptible to hydrolysis. Silanes (such as A151) whose own groups have a relatively weak influence on the pH value of aqueous solutions can adjust the pH value of the aqueous solution by adding substances like acetic acid and ammonia water, making the silane coupling agent easier to hydrolyze. For instance, if the pH value is adjusted to weakly acidic by adding acetic acid and then the hydrolysis of the silane coupling agent A151 is carried out, the hydrolysis rate will be significantly increased.
Secondly, during the hydrolysis of silane coupling agents, a certain amount of solvents such as methanol and ethanol that can be mixed with water in any way will be produced. This is determined by the X group in the silane structure. If the solvents produced during the hydrolysis of silane coupling agents are added in advance to the aqueous solution, the silane coupling agents will be more fully dispersed in the aqueous solution, making the hydrolysate more stable. If a small amount of ethanol is added to the aqueous solution in advance and then the hydrolysis of vinyl silane A151 is carried out, the oil-droplet-like silane coupling agent is miscible with the aqueous solution more quickly and is less likely to condense and precipitate.
In addition, when the silane coupling agent is hydrolyzed, it is necessary to stir thoroughly to ensure that the silane coupling agent comes into more full contact with water, reducing the condensation polymerization reaction between the silane coupling agent molecules due to contact. Once the silane coupling agent undergoes condensation polymerization, it is very difficult to hydrolyze again.