What is a silane coupling agent
A silane coupling agent is a silicon-containing compound that can interact with both organic and inorganic materials in the same molecule.
X: Hydrolyzed functional group
Such as: Cl, OCH3, OC2H5, OOCR (acyloxy), NR, etc.
R: organic functional group
Such as: vinyl, epoxy, amino, methacryloxy, mercapto, anhydride, alkyl, halogen, etc.

The principle of action of silane coupling agent
Inorganic surface interaction mechanism model
Silane Monolayer Formation Model on Inorganic Surfaces
(1) The hydrolyzed functional group is hydrolyzed into Si-OH;
(2) The mutual condensation reaction between Si-OH forms oligomers containing -OH;
(3) Oligomers containing -OH form hydrogen bonds with the surface of inorganic substrates;
(4) Dehydration condensation of oligomer -OH and inorganic surface -OH by heating.
Silane Hydrolysis Mechanism
Theoretically the silane coupling agents react sequentially, but these reactions can occur simultaneously after the initial hydrolysis step. At the interface, each silicon of the organosilane is usually bonded to the substrate surface with only one bond, and the remaining two silanol groups exist in condensed or free form. Organic groups are still available for covalent reactions or physical interactions with other substances.

Anhydrous deposition of alkane coupling agents
Reaction time (4-12 hours) at temperature (50 °C-120 °C) Alkoxy groups can be vapor-deposited with hydroxyl-containing substrates for vapor-phase deposition reactions, and the typical silane is a cyclic azasilane.
Mechanism of action on organic material silane:
(1) Improvement of "wetting" property of resin
(2) Improvement of resin mutual solubility
(3) Form a chemical bond with the resin
(4) Form a bond with the hydrogen between the resins

What are the influencing factors of silane coupling agent?
1. Hydrolysis of silane
(1) Leaving group (hydrolysis group)
Si-NR2 > Si-Cl >Si-NH-Si > Si-O(O=C)CH3 > Si-OCH3 > Si-OCH2CH3
(2) pH
The weakly acidic conditions are beneficial to the hydrolysis of silanes, but not conducive to the condensation reaction.
(3) Chemical structure (organic group)
If there are multiple substituents (benzene or tert-butyl), the steric hindrance effect will be increased, which is conducive to the formation of stable silanol bonds.

2. Condensation of silane
(1) pH
Weakly alkaline conditions are beneficial to the condensation reaction, but not conducive to the stability of the hydrolysis.
(2) Chemical structure (organic group)
The fewer substituents in the organic part, the more favorable the dehydration condensation.
(3) temperature
The formation of surface covalent bonds has a certain reversibility. When water is removed, usually by heating to 120°C for 30-90 minutes or vacuum for 2-6 hours, bonds may form, break and recombine to relieve internal stress.
(4) Silane concentration
When silane is added to water and has low solubility, it is beneficial to high degree of polymerization, and the thickness of the polysiloxane layer is also determined by the concentration of the siloxane solution. While monolayer adsorption is often desired, the solutions commonly used yield multilayer adsorption. It has been calculated that deposition from a 0.25% silane solution onto glass can yield between 3 and 8 molecular layers.

3. Aspects of Organic Materials
Different chemical structures, reaction conditions and difficulty are very different.

4. Inorganic materials
(1) Concentration of surface hydroxyl groups
(2) Types of surface hydroxyl groups
(3) Hydrolytic stability of the bond formed
(4) The physical size of the substrate or the characteristics of the substrate
For inorganic materials containing hydroxyl groups, the type and content of hydroxyl groups vary greatly. Under neutral conditions, the content of hydroxyl groups on the surface of inorganic substances that have just been melted is low; the surface of the substrate contains a large amount of adsorbed water, which affects the coupling reaction between the silane coupling agent and the substrate; the o-silanol containing hydrogen bonds and silane coupling Reactive agents are easier to react, while isolated or free hydroxyl groups are less likely to react.

5. Surface tension
Critical surface tension is related to the wettability or release properties of a solid. It can better predict the behavior of solids and a range of liquids. A liquid with a surface tension lower than the critical surface tension of the substrate (γ c ) wets the surface, i.e. exhibits a contact angle of 0 (cosθ e = 1). The critical surface tension is unique to any solid and is determined by plotting the cosine of the contact angles for liquids of different surface tensions and extrapolating to 1.

γsv – γsl = γlv • cosθ Young's modulus equation
γSL = critical surface tension, γLV = liquid surface tension

Hydrophilic behavior is usually observed on surfaces with a critical surface tension greater than 45 dynes/cm. The expected decrease in contact angle is accompanied by stronger adsorption behavior and increased heat release with increasing critical surface tension. Hydrophobic behavior is usually observed on surfaces with a critical surface tension of less than 35 dynes/cm. First, the decrease in critical surface tension is related to lipophilic behavior, i.e. the wetting of surfaces by alkane oils. When the critical surface tension is lowered below 20 dynes/cm, the surface resists wetting by paraffinic oils and is considered oleophobic and hydrophobic.

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