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Silane Coupling Agents: Chemistry and Applications

Release time:

2016-02-19 08:55

Silane Coupling Agents: Chemistry and Applications
Silane Coupling Agents: Chemistry and Applications

I. Chemistry of Silane Coupling Agents

1. The structure of silane

Y-R- Si Men X3-n

Y (organic group)

(1) Epoxide; (2) Methacrylate; (3) Mercaptan (SH); (4) Amine; (5) Alkyl; (6) Isocyanate; (7) Vinyl.

Note: X represents a hydrolyzable group that can react with inorganic materials containing hydroxyl groups. The larger the X group, the slower the hydrolysis; furthermore, the Me groups on the main chain further retard the hydrolysis reaction.

X (hydrolyzable group)

Cl, Me-O-, Et-O-, i-Pr-O-, MeO-CH2CH2-O-

Note: Y represents an organic functional group that can undergo chemical reactions with polymers or form hydrogen bonds, thereby establishing a strong bond with the polymer.

2. Commonly used silanes

3. Coupling and Crosslinking Effects of Silanes

II. Chemical bonds formed between organic (polymer) molecules

Each grafted silane group (-Si[OR]3) contains three crosslinkable sites;

Crosslinking requires moisture and releases alcohol.

Crosslinking enhances chemical resistance and UV resistance.

Suitable for: Cross-linked polyethylene cables and pipes; water resistance, chemical resistance, and scrub resistance of water-based coatings.

1. The coupling effect of silanes

(1) Formation of chemical bonds at the organic-inorganic interface;

(2) Prevent moisture from penetrating along the interface;

(3) Prevent interface failure;

(4) Transfer the strength from the filler to the resin;

(5) Improve the dispersibility of the filler or reduce the viscosity of the material.

2. Hydrolysis reaction of silane

All silanes are sensitive to moisture.

Typically, X = methoxy, ethoxy, isopropoxy, and the like.

X=Cl undergoes rapid hydrolysis, but it is rarely used because the hydrolysis product (HCl) is corrosive.

III. Factors Affecting the Hydrolysis and Polycondensation Reactions of Silanes

1. The concentration of silane in water;

2. The pH value of the silane aqueous solution;

3. Catalyst;

4. Water solubility of silanes;

5. Steric hindrance and polarity factors.

IV. Hydrolysis and Polycondensation Reaction Rates of A-187 and Its Triol

1. Principle of the hydrolysis reaction of silanes

(1) Special acids and common bases can catalyze hydrolysis reactions;

(2) The hydrolysis reaction is slowest at pH 7.

(3) At pH values below 4 or above 7, the formation of oligomers can be promoted.

(4) Electron-withdrawing groups can accelerate hydrolysis reactions, such as amino silanes;

(5) Silanes that are immiscible with water and have a slow release in water-based systems;

(6) Large groups on the silicon chain will slow down the hydrolysis reaction;

(7) Alcohol solvents slow down the hydrolysis reaction;

(8) Silane triol is unstable and readily undergoes further polycondensation.

2. Catalyst for silane hydrolysis

Any acid or base can catalyze a hydrolysis reaction:

(1) Natural acids, such as hydrochloric acid, sulfuric acid, and phosphoric acid;

(2) Hydroxides and alkoxides, such as caustic soda;

(3) Tin(IV) and tin(II) complexes, such as dibutyltin dilaurate;

(4) Titanates, such as tetraisopropoxytitanate;

(5) Zirconium salts, such as ethoxyzirconium;

(6) Hydrolysis products of phosphate and phosphite complexes;

(7) Amines, such as tributylamine and imidazoline.

3. Main chemical reactions of silane organic groups

(1) Alkenylsilane coupling agent

CH2＝CH－SiX3

CH2＝CH－(CH2)n－SiX3

a. Polymerization and Copolymerization Reactions

b. Addition reaction

(2) Amino-containing silane coupling agents

H2N-C3H6Si(OR)3

a. Addition reaction

b. Reaction with carboxylic acid derivatives

(3) Epoxy-containing silane coupling agents

a. Addition reaction with amino groups

b. Reaction between epoxy groups and carboxyl groups

c. Addition reaction with hydroxyl groups catalyzed by strong bases or bimetallic catalysts

(4) Silane coupling agents containing thiol groups

a. Addition reaction with organic compounds containing unsaturated groups

V. Silane Applications

1. Functions of silane coupling agents

(1) It can serve as an elastic bridging agent between matrices, improving the adhesion between two materials with different chemical properties and thereby enhancing the overall performance of the product, including mechanical properties, electrical insulation, aging resistance, and water repellency.

(2) It can be used as a surface modifier for materials, imparting properties such as water resistance, antistatic performance, mold resistance, odor resistance, anti-coagulation, and physiological inertness.

(3) It serves as a crosslinking curing agent for non-crosslinked polymer systems, enabling them to cure at room temperature and atmospheric pressure.

2. Application of Silanes in Coatings

(1) Enhance adhesion

a. Enhance adhesion between organic polymers and inorganic oxides.

b. Enhancing adhesion on difficult-to-bond organic substrates

(2) Crosslinking agent (improves water resistance)

a. Reactive polymer

b. Exhibits characteristics of moisture-curing reaction

3. Application of silanes in rubber

Silane coupling agents are primarily used in white carbon black, where they react with both the white carbon black and the rubber. Their main functions are:

(1) Enhance the adhesion between white carbon black and rubber;

(2) Enhance the wear resistance of the product;

(3) Reduce the rolling resistance of the product.

Mechanism of action of silicone-silane-rubber coupling agents:

4. Application of Silane in Adhesives

Performance advantages: Excellent adhesion, enhanced tensile strength, and improved elasticity.

Mechanism of action: Y represents organic functional groups such as vinyl, epoxy, amino, and others, which typically react with the organic functional groups in the adhesive matrix resin. X denotes groups like chloro, methoxy, and ethoxy—these groups readily hydrolyze to form silanols, which then react with oxides or hydroxyl groups on the surfaces of inorganic materials (such as glass, silica, metals, etc.), forming stable silicon-oxygen bonds. Thus, by using a silane coupling agent—a “molecular bridge”—two completely different materials can be bonded together.

5. Enhancing the performance of polymer filled with fillers

(1) Improve the dispersibility of fillers in polymers;

(2) Enhance mechanical strength (tear strength, tensile strength, elongation at break, impact strength);

(3) Reduce the heat generated during extrusion and processing;

(4) Prevent premature thermal decomposition;

(5) Enhance wet electrical performance and wet mechanical performance.

The organic functional groups of silanes, when matched with the resin, significantly enhance performance.

6. Water resistance of silanes

Performance advantages:

(1) Remarkable hydrophobic properties;

(2) Significantly reduces the water absorption rate of building materials such as gypsum and cement;

(3) Good breathability;

(4) Enhance the resistance of building materials to erosion by alkalis, salts, and acids.

Applications: Tiles, concrete, stone.

Silane: YC-1004, YC-1005

7. Silane-olefin crosslinking reaction

Thermoplastic polyethylene forms a three-dimensional molecular structure through chemical bonds between molecules, thereby preventing the material from melting again.

Function:

(1) Increase the long-term use temperature; (2) Enhance creep resistance; (3) Improve solvent resistance; (4) Boost resistance to stress cracking.

Application:

(1) Wires and Cables: Insulation for low-voltage and medium-voltage power cables; (2) Pipes: Water pipes and underfloor heating pipes; (3) Heat-Shrink Products: Sheets for power cables; (4) Films: For applications requiring resistance to creep and shrinkage.

8. Application of silanes in glass fibers

Performance Advantages – Enhancing the Mechanical Properties of Fiberglass Reinforced Plastic

9. Reactivity of organosilanes toward inorganic materials

VI. Classification and Model Numbers of Common Silane Coupling Agents

1. Amino: A-1100, A-1102, A-1110, A-1170

2. Diamino: A-1120, A-2120

3. Triamine: A-1130

4. Isocyanate: A-1310

5. Epoxy: A-186, A-187

6. Methacryloyloxy: A-174

7. Thiol group: A-1891

8. Polysulfide: A-1289, A-1589

9. Urea-based: A-1160, A-1524

10. Vinyl: A-171, A-151, A-172, A-2171, RC-1

11. Silicone Grease: A-137, A-162, A-1230, A-1630

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