What Are the Factors that Influence the Effects of Silicone Coupling Agents?

Silane hydrolysis

• Leaving group (hydrolysis group);

• Weakly acidic pH conditions are favorable for hydrolysis of silane, and unfavorable for condensation reactions;

• Chemical structure (organic group), if there are multiple substitution groups (phenyl or tert-butyl), increased steric hindrance is favorable for the formation of a stable silanol bond.

Silane condensation

• pH. Weakly alkaline conditions are favorable for condensation reactions and unfavorable for hydrolysis stability;

• Chemical structure (organic group). The fewer organic substituents, the more favorable for dehydration condensation;

• Temperature. The formation of surface covalent bonds has a certain reversibility. When water is removed, heating to 120℃ and 30-90 minutes or vacuum for 2-6 hours is usually used to form, break, and recombine bonds to relieve internal stress;

• Silane concentration. Adding silane to water and having low solubility is favorable for increasing the degree of polymerization. The thickness of the polysiloxane layer is also determined by the concentration of the siloxane solution. Although single-layer adsorption is usually required, the solution used will typically produce multiple-layer adsorption. It is calculated that depositing from a 0.25% silane solution onto glass can produce 3 to 8 molecular layers.

The impact of organic materials on silane coupling agents

Different chemical structures have a great difference in the difficulty and conditions of the reaction.

The impact of inorganic materials on silane coupling agents

• Concentration of surface hydroxyl groups;

• Type of surface hydroxyl groups;

• Hydrolytic stability of the formed bond;

• Physical size or characteristics of the substrate.

Inorganic materials containing hydroxyl groups have large differences in the type and content of hydroxyl groups. Under neutral conditions, the surface hydroxyl content of freshly fused inorganic materials is low; the large amount of adsorbed water on the substrate surface affects the coupling reaction of silane coupling agents with the substrate; neighboring silanol alcohols with hydrogen bonds are more likely to react with Silane coupling agents, while isolated or free hydroxyls are less responsive.

Surface tension affects the silane coupling agents

The critical surface tension is related to the wetting or release properties of the solid. It can better predict the behavior of solids with a range of liquids. Liquids with surface tensions lower than the critical surface tension of the substrate will wet the surface, showing a contact angle of 0. The critical surface tension is unique to any solid and is determined by plotting the cosine graph of the contact angle of liquids with different surface tensions and extrapolating to 1.

Surfaces with critical surface tensions greater than 45 dynes/centimeter are usually observed to exhibit hydrophilic behavior. As the critical surface tension increases, the expected drop in contact angle is accompanied by increased adsorption behavior and increased release of heat. Surfaces with critical surface tensions less than 35 dynes/centimeter are usually observed to exhibit hydrophobic behavior. Firstly, the decrease in critical surface tension is related to the hydrophobic behavior, that is, the wetting of aliphatic hydrocarbons on the surface. When the critical surface tension drops below 20 dynes/centimeter, the surface resists wetting by aliphatic hydrocarbons and is considered to be oleophobic and hydrophobic.