Fumed silica (also known as fumed silicon dioxide) is a nano-scale amorphous white powder generated through high-temperature hydrolysis reactions of silicon halides (such as silicon tetrachloride, methyltrichlorosilane, etc.) in an oxyhydrogen flame, followed by processes like agglomeration and deacidification. Its primary particle size is extremely small (about 7~40 nanometers), with a large specific surface area (usually between 50~400 m²/g), and its surface is rich in highly active silanol groups (Si-OH). This special microstructure makes it extremely easy to form a three-dimensional network structure through hydrogen bonds in liquid systems, thereby endowing materials with excellent thickening, thixotropic, and anti-settling properties.
In the field of silicone rubber, fumed silica is an indispensable core reinforcing filler, mainly playing the following key roles:
1. Significantly Enhancing Mechanical Properties
Unreinforced pure silicone rubber has extremely low tensile strength. After adding fumed silica, a stable "filler-rubber" network can be formed with silicone rubber molecular chains through physical adsorption and chemical bonding. This can significantly improve the tensile strength, tear strength, and wear resistance of silicone rubber. The tensile strength of some high-end formulations can be increased to 3-10 MPa or even higher, giving the material extremely high toughness.
2. Optimizing Heat Aging Resistance and Medium Resistance
Fumed silica itself has excellent high-temperature resistance and chemical stability. When filled into silicone rubber, it can effectively inhibit the fracture and degradation of rubber molecular chains under high-temperature conditions, delaying the aging process. At the same time, it can significantly improve silicone rubber's tolerance to media such as water, oil, acid, and alkali, reducing material degradation and extending service life.
3. Improving Processing/Molding and Rheological Properties
Fumed silica exhibits significant thickening and thixotropic effects. During the processing of rubber compounds, it can effectively prevent sagging, collapse, or sedimentation during molding; when subjected to shear forces (such as stirring or extrusion), its internal network is destroyed, causing viscosity to drop, and it can quickly recover once the external force disappears. This makes mixing and shaping of compounded materials much easier.
4. Enhancing Electrical Insulation Performance
As an excellent insulator, fumed silica can increase the volume resistivity and breakdown voltage of organic silicone rubber, endowing products with superior dielectric properties. This makes it highly suitable for the encapsulation and protection of insulating components in various electronic and electrical equipment.
It should be specifically pointed out that because the active hydroxyl groups on the surface of fumed silica easily cause structuralization during the storage of silicone rubber (manifesting as hardening of the compound and increased viscosity), in actual production, it is usually necessary to use "structure control agents" such as hydroxyl silicone oil and silazane to passivate the surface active groups, thereby maintaining good processing performance of the compound.
