Chemical modification method has the characteristics of high stability, easy control and good product performance, which is the main direction of modification research and industrial application of silica, including surface grafting modification, coupling agent modification, ionic liquid modification, macromolecular interface modification and combined modification, etc.
(1) Surface graft modification
The principle of surface grafting modification method is to graft macromolecular polymers with the same properties as matrix polymers (such as rubber) on the surface of silica by chemical grafting method. On the one hand, it can enhance the force between particles and matrix and change the polarity of particle surface. On the other hand, it can also improve the dispersion of silica itself.
Surface grafting method can be divided into "Graftingfrom" (main link grafting method) and "Graftingto" (Graftingto main chain method) according to the different grafting methods. The former can accurately control the molecular weight and density of grafting, but the late steric hindrance is large, and the molecular weight of grafting is small. The latter can graft molecular chains with higher molecular weight and the steric hindrance effect is relatively low but the conditions are harsh.
(2) Modification of coupling agent
The principle of coupling agent modification is to use some functional groups on the coupling agent to react with the hydroxyl group on the surface of silica, so as to change the group structure and distribution on the surface of silica to improve the compatibility with the matrix and its dispersion.
Coupling agent modification is one of the most widely used modification methods with the advantages of good modification effect and high reaction controllability. At present, the widely used coupling agents are silane coupling agent, siloxane coupling agent and silazane coupling agent. In practical application, the modification process of coupling agent can be divided into four steps: mixing reaction, cooling separation, repeated washing and precipitation.
(3) Ionic liquid modification
Ionic liquid, also known as room temperature ionic liquid, is a molten salt composed of organic cations and organic or inorganic anions, which is liquid under 100℃. Ionic liquid modification is to modify silica by using ionic liquid modifier instead of the traditional organic phase modifier. Compared with the traditional organic phase modifier, the ionic liquid has the advantages of being liquid at room temperature, strong electrical conductivity, high stability and good solubility, and it is not volatile and not easy to produce pollution, which is more in line with the requirements of green production.
The ionic liquid modification process is relatively simple. A certain amount of anhydrous ethanol is added into the mixed system of silica and ionic liquid and then put into the constant temperature water bath for reaction. After the reaction is sufficient, the modified silica can be obtained after drying.