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      Coupling agent is an important and widely used treatment agent. It is mainly used as an assistant for polymer composites. Coupling agent is a kind of material with two different functional groups. The most important characteristic of its molecular structure is that there are two groups with different chemical properties in the molecule. One is the group of inorganic substances, which is easy to react with the surface of inorganic substances. The other is the group of organic substances, which can react with synthetic resins or other polymers chemically or form hydrogen bonds. Therefore, coupling agents are called "molecular bridges" to improve the interfacial interaction between inorganic and organic materials, thus greatly improving the properties of composites, such as physical properties, electrical properties, thermal properties, optical properties and so on. Coupling agent used in rubber industry can improve the wear resistance and aging resistance of tires, rubber sheets, rubber hoses, rubber shoes and other products, and reduce the amount of NR, thereby reducing the cost. The role of coupling agent in composites lies in that it can react not only with some groups on the surface of reinforcing materials, but also with matrix resin, forming an interfacial layer between reinforcing materials and resin matrix, which can transfer stress, thus enhancing the bonding strength between reinforcing materials and resin, improving the properties of composites, and preventing other media from infiltrating into the interface. It is beneficial to aging resistance, stress resistance and electrical insulation performance of the products.

      According to the chemical structure and composition of coupling agents, they can be divided into four categories: organic chromium complexes, silanes, titanates and aluminates.

      Chromium complex coupling agent was developed in the early 1950s. The metal chromium complex formed by unsaturated organic acids and trivalent chromium ions has mature synthesis and application technology, and its cost is low, but the variety is relatively single.

silane coupling agent
      The general formula of silane coupling agent is RSiX3, in which R represents amino group, mercapto group, vinyl group, epoxy group, cyano group and methacryloxy group. These groups and different matrix resins have strong reaction ability. X represents alkoxy group (such as methoxy group, ethoxy group, etc.) which can hydrolyze. Silane coupling agents in China have KH-550, KH-560, KH-570, KH-792 and other models.

Titanate coupling agent
    According to their unique molecular structure, titanate coupling agents include four basic types:
    (1) Monoalkoxy coupling agents are suitable for many kinds of resin matrix composites, especially for filling systems without free water, chemical bonding water or physical water.
    (2) Monoalkoxypyrophosphate coupling agent is suitable for resin-based composite materials, especially for fillers with high moisture content.
    (3) Chelating coupling agents are suitable for many kinds of resin-based composite systems. Because of their excellent hydrolytic stability, these coupling agents are especially suitable for water-containing polymer systems.
    (4) Coordination type. This kind of coupling agent has good coupling effect in many kinds of resin-based or rubber-based composite systems. It overcomes the shortcomings of common titanate coupling agent used in resin-based composite systems.

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