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Silicon dioxide, commonly known as white carbon black, is also called hydrated silicic acid or light silicon dioxide. It has high electrical insulation, is porous, has a large internal surface area, is hygroscopic and non-toxic. So, what are the influences on the surface properties of silicon dioxide? Let's take a look together.
1. Moisture
The moisture content of silica includes: free water and bound water. The free water on the surface of silicon dioxide is easily removed when heated. Too little free water indicates that there are not many light groups on the surface of silica, which reduces the activity of silica. During the mixing process, it promotes the aggregation of silica and makes it difficult to distribute within the rubber compound. The lack of surface water not only affects the vulcanization speed but also reduces the reinforcing effect accordingly. The free water content is slightly high. The operation performance of the mixed rubber is good, the powder-eating speed is fast, the dust flying is less, the roller wrapping performance is also good, and the vulcanization speed is slightly fast.
2. The influence of impurities
During the preparation of silicon dioxide, impurities such as Cu, Fe and Mn are prone to be introduced. Although these trace impurities have little impact on the physical properties of the rubber compound, in order to make silicone rubber products have excellent oxidation resistance and aging resistance, the content of Cu, Fe and Mn is still required to be reduced to a certain limit.
3. PH value
The acidity or alkalinity of silica has a significant impact on the vulcanization of rubber compounds. Acidic silica delays the vulcanization of rubber compounds, while alkalinity promotes vulcanization. However, excessive alkalinity is not conducive to reinforcement. Silica with a slightly acidic PH is beneficial to the tensile strength and wear resistance of rubber compounds and products. Therefore, the PH value of silica used in silicone rubber is generally 6 to 8, close to neutral.
4. Surface modification treatment
The surface modification treatment of silica refers to the addition of coupling agents in silica to achieve the purpose of its hydrophobicity. After the addition of the coupling agent, the CH3O group (methoxy group) at one end of the coupling agent is prone to react with the silanol group on the surface layer of silica, while the other end reacts with the gelatin-like molecules, that is, the coupling agent plays a "bridging" role between silica and silicone rubber. There are many ways to modify the surface of silica. Currently, the commonly used one is to treat it with silane coupling agents (such as Si-69).
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