1 heat treatment method
The heat treatment can reduce the amount of adsorbed water on the surface of the nano-silica, because the high temperature heating will promote the dehydration reaction of the linked silanol groups associated with hydrogen bonding to form a stable bond, resulting in a decrease in the amount of adsorbed water. Although this method is economical and simple, the combination effect of nano-silica and organics cannot be improved by heat treatment. Therefore, the actual heat treatment process is usually performed by adding a zinc-containing compound at a temperature of 200 to 400 ° C, or by first treating the nano-silica with a silane and a transition metal ion, and then performing a heat treatment.

2 chemical modification methods
The chemical methods of nano-silica surface modification can be divided into inorganic modification and organic modification. The modification of inorganic materials usually uses titanium dioxide to coat nano-silica, while the modification of organic materials is the main method for surface modification of nano-silica, which will be mainly introduced below.
1) Coupling agent modification method
Among the commonly used coupling agent modification methods of nano-silica, silane coupling agents are most widely used, which can condense silicon-oxygen bonds with hydroxyl groups on the surface of nano-silica.

When the coupling agent is used to modify the surface of the nano-silica, the coupling agent needs to be hydrolyzed before reacting with the nano-silica. The hydrolysate will self-condensate, which will hinder the reaction between the hydrolysate and the hydroxyl groups on the surface of the silica, reduce the coupling efficiency to a certain extent, and make the surface modification of the nano-silica incomplete.

2) Alcohol ester modification method
The alcohol ester method uses fatty alcohols to react with the hydroxyl groups on the surface of nano-silica under high temperature and high pressure conditions to achieve the purpose of changing the surface wettability of silica.

Compared with the silane coupling agent method, the alcohol ester method has the advantages that the modifier fatty alcohol is cheaper, easier to synthesize, and easier to control the structure. However, the modification effect is affected by the length of the alkyl chain of the alcohol, and it needs to be performed at high temperature and pressure, which requires higher reaction conditions.

3) Polymer graft modification
Grafting the polymer to the surface of nano-silica in a specific way can effectively improve the hydrophobicity of the particles and improve their interfacial affinity in the nanocomposite. The long-chain structure of the graft polymer can create chain entanglement with the matrix polymer, making this modification more uniform and close. At the same time, different grafting monomers and grafting conditions can be selected according to needs to make the modification more Diversity and controllability.

Polymer grafted modified nano-silica can be divided into "Grafting to" and "Grafting from" according to the different grafting methods.

The "Grafting to" method generally refers to the covalent attachment of a terminally functionalized polymer to a nano-silica surface.

The "Grafting from" rule uses a large number of hydroxyl groups on the surface of the nano-silica to first introduce active sites that can initiate polymerization, such as cations, anions, or free radicals, onto the surface of the nano-silica, and then trigger the polymerization of surrounding monomers on the surface of the particles. To make the polymer grow on the surface of nano-silica.

The "Grafting from" method introduces polymers in situ by grafting prepolymer segments, and steric hindrance does not limit the graft growth of smaller monomer molecules at the active initiation site, and has higher grafting efficiency. . However, in the process of compounding with the material, the long-chain polymers connected to the surface of the nano-silica may entangle and cause the adjacent silicas to re-aggregate again, which is not conducive to further dispersion in the polymer matrix.

4) In-situ modification method
Generally, chemical modification can effectively reduce the agglomeration of nano-silica, but there is also a problem that the nano-silica agglomerates before modification. Therefore, it may be considered to complete the modification in the preparation process of the nano-silica to obtain surface-functionalized silica particles.

Huang Fen et al. Used the sol-gel method to generate silicone-modified nano-silica particles in situ, combined with epoxy polyester to prepare TH1178-2 corona-resistant solvent-free insulating paint, and directly doped nano-dioxide Compared with silicon modification, the in-situ generated modified nano-silica particles have more uniform dispersion without obvious agglomeration, and the corona-resistant paint, electrical properties, and mechanical properties of the prepared corona-resistant paint are more uniform. excellent.