Research on precipitated silica surface modification technology mainly focuses on improving the dispersion problems caused by its hydrophilicity. By introducing organic groups onto the surface through chemical and physical methods, its compatibility, dispersibility, and functional properties with organic matrices are significantly improved, thus expanding its application value in fields such as rubber, plastics, and coatings.
I. Principles and Objectives of Surface Modification
1. Modification Principles
Surface hydroxyl group conversion: Precipitated silica surface contains a large number of silanol groups (Si-OH). Through chemical reactions, these are converted into organosilicon bonds (Si-R), reducing surface polarity.
Improved interfacial compatibility: After modification, a stable interfacial bond is formed between the modified precipitated silica and the organic matrix, reducing phase separation.
Enhanced dispersion stability: The modified layer can effectively prevent the aggregation of precipitated silica particles, improving dispersion stability in organic media.
2. Modification Objectives
Improved dispersibility: Solving the problem of easy aggregation of precipitated silica in organic matrices and improving dispersion uniformity.
Enhanced interfacial bonding: Improving the interfacial bonding force between precipitated silica and the organic matrix, enhancing the performance of composite materials.
Imparting new functions: Introducing specific functional groups through surface modification to expand application fields.
Reducing production costs: Improving the effective utilization rate of precipitated silica in composite materials and reducing the amount used.

II. Main Surface Modification Technologies
1. Coupling Agent Modification Technology
Silane coupling agent modification: The most widely used modification method, where the functional groups on the silane coupling agent react with the hydroxyl groups on the precipitated silica surface, forming stable chemical bonds.
Common coupling agents: Si69, A151, KH550, KH560, KH570, etc.
Modification mechanism: One end of the coupling agent reacts with the hydroxyl groups on the precipitated silica surface, and the other end forms a chemical bond or physical entanglement with the organic matrix.
Process flow: Mixing reaction → cooling and separation → repeated washing → precipitation and drying.
Sulfur-containing and sulfur-free silane coupling agents:
Sulfur-containing type (e.g., Si69, Si75): Interacts with rubber molecular chains at high temperatures through sulfur and vulcanization accelerators. Sulfur-free type (e.g., KH550, KH560): One end is an alkoxy group, and the other end is a double bond group, epoxy group, or amino group, etc.
2. Surface Grafting Modification Technology
Grafting principle: Grafting macromolecular polymers with properties similar to the matrix polymer onto the surface of fumed silica through chemical grafting.
Technical characteristics:
Suitable for grafting polymers with smaller molecular weights; high molecular weight grafting requires stringent conditions.
It can enhance the interaction force between particles and the matrix, and change the polarity of the particle surface.
Application example: Wu Chenggao et al. treated the surface of nano-SiO₂ with silane coupling agent KH550 to obtain aminated nano-SiO₂, and then successfully grafted polycaprolactone and polystyrene mixed polymer brushes through the ring-opening reaction of glycidyl methacrylate with amino groups.
3. Ionic Liquid Modification Technology
Modification principle: Using ionic liquid modifiers instead of traditional organic phase modifiers. Ionic liquids are composed of organic cations and organic or inorganic anions, and are liquid below 100°C.
Technical advantages:
Liquid at room temperature, strong conductivity, high stability, and good solubility.
Non-volatile and less polluting, meeting the requirements of green production.
Process flow: Add anhydrous ethanol to the fumed silica and ionic liquid mixed system → react in a constant temperature water bath → drying.
Limitations: The modification effect is worse than that of coupling agent modification.
4. Macromolecular Interface Modification Technology
Modification principle: Using macromolecular polymers containing polar groups as modifiers. During the modification reaction, the main chain of the macromolecular interface modifier maintains its basic main chain structure while introducing polar epoxy groups.
Technical characteristics:
It can improve the compatibility between fumed silica particles and the matrix, achieving a better interface modification effect.
It can synergistically reinforce the matrix with coupling agents, but the reinforcing effect is lower when used alone.
5. Combined Modification Technology
Modification principle: Modifying fumed silica in combination with other materials, combining their respective advantages to improve overall performance. Typical Applications:
Combined use of carbon black and silica: Carbon black enhances the tensile and tear strength of rubber, while silica improves rolling resistance and wet skid resistance.  Combining the two leverages their advantages to improve the overall performance of rubber products.
Modifier ratio: The modification effect is closely related to the modifier ratio; the ratio needs to be optimized to obtain the best performance.
6. Alcohol Modification Technology
Modification principle: Alcohols react with the hydroxyl groups on the surface of silica, removing water molecules and replacing the hydroxyl groups with alkoxy groups.
Technical characteristics:
The modification effect is significantly related to the number of carbon atoms in the alcohol and the reaction conditions.
Generally, the modification effect is more pronounced when the number of carbon atoms is greater than 8.
Application example: Zhao Peng et al. prepared a hydrophobic silica sample using octadecanol as a modifier. The optimal conditions were 28% octadecanol, 4% sodium dodecylbenzenesulfonate additive, modification temperature of 85℃, and aging for 30 and 60 minutes.
7. Surfactant Modification Technology
Modification principle: Adding surfactants to silica forms a network structure, increasing the specific surface area and oil absorption performance.
Technical characteristics:
It can give silica aerogel characteristics.
The appropriate pore size and hydrophobic groups of hydrophobic fumed silica are key to ensuring good penetration of dodecane.
Application example: Dodecane is adsorbed onto hydrophobic fumed silica to prepare a new type of cold energy storage composite phase change material, which is plastic and can be compressed into any shape.

III. Evaluation Methods for Modification Effects
1. Surface Characteristics Analysis
Contact angle measurement: KH550 coupling agent increased the contact angle to 120°, indicating a significant increase in hydrophobicity.
Infrared spectroscopy: Used to confirm changes in surface functional groups, such as hydroxyl group reduction and the introduction of organic groups.
X-ray photoelectron spectroscopy: Analyzes the surface elemental composition and chemical state.
2. Dispersion Performance Evaluation
Particle size distribution: After modification, the dispersed particle size of silica in the organic matrix can reach 30-50 nm.
Aggregate size: Aggregates smaller than 1 μm can be achieved through a twin-screw extruder. Sedimentation Stability: Evaluation of the sedimentation behavior of modified silica in solvents.
3. Application Performance Evaluation
Mechanical Properties: Tensile strength, tear strength, hardness, etc., of modified silica composite materials.
Thermal Stability: 92% retention of tensile strength after 168 hours of thermal oxidative aging.
Functional Characteristics: Such as conductivity, thermal conductivity, catalytic activity, etc.

IV. Application Fields of Modified Silica
1. Rubber Industry
Tire Manufacturing: Silica as a reinforcing agent can improve the rolling resistance and wet grip of rubber products.
Seals: Modified silica improves the aging resistance and wear resistance of rubber seals.
Green Tires: 30% replacement of carbon black, reducing rolling resistance by 20% and shortening wet braking distance by 9%.
2. Plastic Modification
Improved Mechanical Properties: A 5% addition can increase the flexural strength of polypropylene by 80% and reduce water absorption by 65%.
Functionalized Plastics: After surface modification, the tensile strength of polyimide is increased by 15 times, and the elongation at break is increased by 3 times.
3. Coatings and Inks
Architectural Coatings: 2% nano-silica enables washability to exceed 10,000 cycles.
Water-based Wood Coatings: Surface tension is reduced to 25 mN/m, improving wettability.
Anti-corrosion Coatings: Salt spray resistance is extended by 40%, improving corrosion resistance.
4. Environment and Energy
Water Treatment: Modified silica is used to remove harmful substances and heavy metal ions from wastewater.
Catalyst Carrier: High specific surface area provides more reaction sites, improving catalytic efficiency.
Phase Change Materials: Compounded with dodecane to prepare cold energy storage materials, solving leakage problems.