Structure control of silica reinforced silicone rubber compound
Silica, especially fumed silica, is a good reinforcing agent for silicone rubber, but there is a problem that hardens the rubber mixture, which is generally called "structural effect". Its structure increases with the prolongation of rubber storage time, even to the extent that it can not be recycled or scrapped. There are two explanations for this, one is that the end group of silicone rubber condenses with the hydroxyl group on the surface of silica, and the other is that the silicone oxygen chain of silicone rubber forms hydrogen bond with the hydroxyl group on the surface of silica. There are two ways to prevent the structure. One is to add some substances that can react with the hydroxyl groups on the surface of silica, such as hydroxyl silicone oil, diphenyl silicon glycol, silazane, etc. When diphenylsilanediol is used, it should be treated at 160 ~ 200 ℃ for 0.5 ~ 1H after mixing. In this way, the structure of silica filled silicone rubber can be prevented. Another way is to modify the surface of silica in advance, remove part of the surface hydroxyl, and eliminate the structure fundamentally.
When fumed silica is dispersed in nonpolar liquid, the silanol groups on different particle surfaces are bridged by hydrogen bond interaction, and the thickening effect is produced by forming three-dimensional structure. When the structure system is subjected to external mechanical force, such as stirring or shaking, the structure is damaged. The type and duration of the external mechanical force determine the degree of damage. As a result, the thickened system becomes liquid again. If kept in a static state, the fumed silica particles will be connected again to restore the original viscosity. This phenomenon is called pseudoplasticity.
It takes a small amount of energy for fumed silica thickening system to start flowing, in other words, there is a so-called "yield point". The thickening effect of hydrophilic fumed silica on polar or semi polar liquid is not significant. In these systems, especially in liquid mixtures or solutions, surface modified fumed silica displays.