What are the Surface Modification Methods for Silica?
Chemical modification is currently the primary direction for silica modification research and industrial application due to its advantages such as high stability, easy controllability, and excellent product performance. The main methods include: surface grafting modification, coupling agent modification, ionic liquid modification, macromolecular interface modification, and combined/modified blending.
(1) Surface Grafting Modification
The principle of surface grafting modification involves chemically grafting macromolecular polymers onto the silica surface that have the same properties as the matrix polymer (such as rubber). On one hand, this enhances the interaction force between the particles and the matrix and alters the polarity of the particle surface. On the other hand, it also improves the dispersibility of the silica itself.
Surface grafting methods can be divided into "Grafting from" and "Grafting to" based on the technique used. "Grafting from" allows for precise control over the molecular weight and density of the grafted chains, but suffers from high steric hindrance in later stages, resulting in a lower grafted molecular weight. "Grafting to" can graft higher molecular weight chains and has relatively lower steric hindrance, but the reaction conditions are more demanding.
(2) Coupling Agent Modification
Coupling agent modification works by utilizing functional groups on the coupling agent to chemically react with the hydroxyl groups on the silica surface. This alters the structure and distribution of surface groups, thereby improving compatibility with the matrix and its own dispersibility. Coupling agent modification offers good modification results and high reaction controllability, making it one of the most widely used modification methods today. Commonly used coupling agents include silane coupling agents, siloxane coupling agents, and silazane coupling agents. In practical applications, the main process for coupling agent modification typically involves four steps: mixing reaction, cooling separation, repeated washing, and precipitation.
(3) Ionic Liquid Modification
Ionic liquids, also known as room-temperature ionic liquids, are molten salts composed of organic cations and organic or inorganic anions that remain in a liquid state below 100°C. Ionic liquid modification uses ionic liquid modifiers to replace traditional organic phase modifiers for silica. Compared to traditional organic modifiers, ionic liquids offer advantages such as being liquid at room temperature, high electrical conductivity, high stability, and good solubility. Additionally, they are non-volatile, less prone to causing pollution, and better aligned with the requirements of green production. The process for ionic liquid modification is relatively simple. A certain amount of anhydrous ethanol is added to a mixed system of silica and ionic liquid, and the mixture is then placed in a constant temperature water bath for reaction. After sufficient reaction, the product is dried to obtain modified silica.
(4) Macromolecular Interface Modification
The modifier used in macromolecular interface modification is a macromolecular polymer containing polar groups. During the modification reaction with silica particles, the main chain of the macromolecular interface modifier can introduce a significant number of polar epoxy groups while maintaining its basic main chain structure. This improves the compatibility between the silica particles and the matrix, achieving an effective interfacial modification result.
(5) Combined Modification (Blending with Carbon Black)
Both carbon black and silica are excellent reinforcing agents in the rubber industry. Carbon black is one of the most commonly used reinforcing agents. Its unique structure can enhance the tensile and tear strength of rubber materials and improve properties such as abrasion resistance and low-temperature resistance. Silica, as a reinforcing agent, can significantly improve the rolling resistance and wet skid resistance of rubber products, but its effect when used alone is not as good as carbon black.
Numerous studies have shown that using a combination of carbon black and silica as a reinforcing agent can leverage the advantages of both to improve the overall performance of rubber products.
