Silica Fume in Epoxy Resins
Epoxy resins are thermosetting polymers containing epoxy groups. As they have characteristics such as excellent mechanical properties, processing properties, corrosion resistance, low price, they are widely used as adhesives, structural materials, composite materials, and laminate materials. However, the cross-linked structure of epoxy resin leads to poor toughness and limited application. Therefore, toughening modification of epoxy resin is particularly important.
The toughness can be improved by introducing flexible chain segments to change the molecular structure of epoxy resin. Even though the effect of this method is significant, its cost is relatively high. Organic additives such as rubber, thermoplastic elastomers are also used in toughness improvement, but this will greatly reduce the stiffness and heat resistance of the epoxy resin. Powder filler materials for epoxy resin potting materials include silica fume, aluminum hydroxide, aluminum oxide, and magnesium hydroxide. Among these, silica fume (usually undensified silica fume is used) has good comprehensive performance and is one of the most commonly used filler materials in epoxy resin potting materials.
Effects of silica fume on epoxy resins
Silica fume is a kind of high-activity, ultrafine solid inorganic filler completely different in structure from epoxy resin. Its addition has effects such as reducing the curing shrinkage rate of epoxy resin potting material, improving dimensional stability, reducing internal stress, effectively preventing cracking, and reducing heat release during curing, improving the heat resistance, dielectric properties and thermal conductivity of the material, as well as reducing the cost of use.
During the curing process of the epoxy resin composite system, shrinkage stress and thermal stress will be created. First, the volume expands when the temperature is heated from room temperature to curing temperature. Then crosslinking reaction occurs at the curing temperature. During gelation, curing and cooling, the volume keeps shrinking, and the total shrinkage rate exceeds 5%, so large stress is generated.
For unmodified epoxy resin matrices, cracking is easy to occur and grows fast at high stress concentration points. Whereas when the ones containing rigid particles are impacted by an external force, crazes in the surrounding resin of the rigid particles and shear deformation in the matrices can absorb the impact energy, thereby achieving a toughening effect. The existence of rigid particles hinders the crack propagation of the matrix, and ultimately prevents crazes from developing into destructive cracks. And the finer the particles, the more energy will be adsorbed as more crazes and shear deformation are induced when impacted.
Because the surface of silica fume has high activity, it has a strong adsorption capacity for the active groups in the silane coupling agent molecule, which adhere to the silica fume surface to form an extremely thin and firm coating layer. When silica fume is filled into the epoxy resin, covalent bonds are formed in the reactions, such as hydrolysis, condensation reaction, between the hydrolyzed groups of the layer and silica fume, creating a flexible network structure between silica fume and epoxy resin. When the stress is transferred to the interface between the silica fume filler and the epoxy resin, it is released and eliminated via the network structure.
Suitable coefficients of linear thermal expansion (CLTE) can solve the defects of material deformation and reduce internal stress. Generally, the CLTE of epoxy resin is relatively large, which is (6 to 15) × 10-5/°C. When an appropriate amount of silica fume filler is added, the CLTE of the cured epoxy resin can be reduced. And as the content of silica fume increases, the coefficient becomes lower.
Generally, the thermal conductivity of polymer insulating materials is very low. While the primary component of silica fume is SiO2, which has a high thermal conductivity. So by adding silica fume, the thermal conductivity of the cured epoxy resin can be improved.