Four Major Surface Modification Technologies and Research Progress of Silica

1. Coupling Agent Modification

In the rubber industry, to improve the compatibility of fumed silica with natural rubber (NR), silane coupling agents are used for surface treatment of the silica.

Using coupling agents such as Si69 or A151 to modify fumed silica and reinforce NR, DCP-cured NR/fumed silica composites have been successfully prepared. The proportion of coupling agent significantly influences the curing rate, curing time, maximum torque, tensile strength, tear strength, and hardness of the silica composite.

2. Surfactant Modification

Adding surfactants to nano-silica can cause the silica to form a network structure, increasing its specific surface area and improving its oil absorption performance. The addition of certain active agents can also impart aerogel characteristics to the silica.

A novel cold-storage composite phase change material (PCM) was prepared by adsorbing dodecane into hydrophobic fumed silica without any chemical reaction. Based on results from pore size analysis, scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FTIR), it was concluded that the relatively suitable pore size and hydrophobic groups of the hydrophobic fumed silica are key to ensuring good dodecane penetration, solving leakage problems. Furthermore, this composite PCM is malleable and can be compressed into any shape, offering broad application prospects. Simultaneously, the composite PCM exhibits low thermal conductivity, which was found to decrease with an increasing mass percentage of hydrophobic fumed silica.

3. Surface Grafting Polymer Modification

Through combined modification processes, surface polymer grafting modification of nano-silica and effective dispersion of nano-silica aggregates can be achieved. Targeted modification can be realized by altering reaction conditions, the type of grafted monomer, and their ratios.

Wu Chenggao treated the surface of nano-SiO₂ particles with the silane coupling agent KH550 to obtain aminated nano-SiO₂. Subsequently, through the ring-opening reaction between glycidyl 2-bromoisobutyrate and the amino groups, both the ring-opening polymerization (ROP) initiator (-OH) and the atom transfer radical polymerization (ATRP) initiator (-Br) were simultaneously bonded to the surface of the nano-SiO₂ particles (SNPs-f-OH/Br). Using SNPs-f-OH/Br as the initiator, ROP and ATRP were carried out to graft mixed polymer brushes of polycaprolactone (PCL) and polystyrene (PS) onto the nano-SiO₂ particle surface. The resulting nanocomposite particles were characterized and tested using FTIR, TEM, thermogravimetric analysis (TGA), and gel permeation chromatography (GPC), confirming the successful grafting of mixed polymer brushes onto the nano-SiO₂ particle surface.

4. Alcohol Modification

Alcohol modification involves a reaction between an alcohol and the hydroxyl groups on the silica surface, with the elimination of water molecules. The hydroxyl groups on the nano-silica surface are replaced by alkoxy groups. This reaction typically requires specific temperature and pressure conditions. The effectiveness of this method is closely related to the number of carbon atoms in the alcohol and the reaction conditions. Generally, a more significant surface modification of nano-silica is observed when the alcohol contains more than 8 carbon atoms.

Zhao Peng et al. successfully prepared silica samples with good hydrophobicity using octadecanol as the modifier. Through single-factor experiments, the optimal conditions were determined to be: octadecanol dosage 28%, additive sodium dodecylbenzene sulfonate dosage 4%, modification temperature 85°C, and aging twice for 30 minutes and 60 minutes.

Currently, the technology for ordinary industrial-grade silica in China is relatively mature, and production capacity is sufficient. However, high-end fumed silica is still in the early stages of development.