The use of fumed silica can be divided into organic silicon materials and other fields, in which the amount of organic silicon materials accounts for nearly 60% of the total amount of fumed silica. Silicon rubber is the material that uses more fumed silica in organic silicon materials, and its addition amount can be up to more than 50%. Vapor phase silicon dioxide mainly plays a reinforcing role in HVT silicone rubber. Because the molecular chain of silicone rubber is very flexible and the interaction between chains is weak, the strength of unreinforced silicone rubber is very low (no more than 0.4 MPa) and has no use value. It can only be applied after reinforcement. However, the strength of silicone rubber reinforced by vapor phase silicon dioxide can be increased by 40 times.


Effect of fumed silica on the mechanical properties of HTV silicone rubber The reinforcement of fumed silica on HTV silicone rubber is affected by its particle size, comparative area and structure. Generally, the smaller the particle size, the higher the specific surface area, and the higher the structure, the better the reinforcement effect, and the higher the strength and hardness of the vulcanizate. In addition, the amount of fumed silica and its dispersion in the matrix also have a great influence on the properties of the vulcanizate. Figure 1 shows the influence of the amount of fumed silica on the tensile strength of the vulcanizate. It can be seen from the figure that the strength of the vulcanizate increases with the increase of the amount of fumed silica. Generally, the amount reaches the peak at 20-40 phr. There are also many reinforcement mechanisms and models of fumed silica for silicone rubber. The accepted explanation is that the free hydroxyl groups on the surface of fumed silica form physical or chemical combination with silicone rubber molecules, forming a silicon rubber molecular adsorption layer on the surface of silica, forming a three-dimensional network structure of fumed silica and silicone rubber molecules, thus effectively limiting the deformation of silicone rubber molecular chain and its reinforcement effect. The change of tear strength of vulcanizate is similar to that of tensile strength, which increases with the increase of the reinforcement of fumed silica, first increases with the increase of the amount of fumed silica, and then decreases slightly after reaching the peak.


The influence of fumed silica on the processability of HTV silicone rubber The influence of fumed silica on the processability of HTV silicone rubber is generally expressed by the degree of structure (&8710Crepe).&8710Crepe is equal to the difference between the plasticity value (p28) of the rubber mixture after 28 days at room temperature and the plasticity value (P0) measured immediately after mixing (see Figure 3). The plasticity value of the rubber compound is related to the amount of fumed silica, surface properties and structure. The reason for the formation of structure is that the surface hydroxyl of fumed silica forms hydrogen bond with the oxygen atom in the silicone rubber and the silicon rubber molecular chain is adsorbed on the surface of silica, which leads to the decline of the fluidity of the rubber with the extension of time, and the rubber becomes hard, affecting the processing performance. Therefore, in the process of use, it is necessary to add a structure control agent or select the gas phase silicon dioxide with surface treatment. The addition of structure control agent and the surface treatment of gas phase silicon dioxide react with the silicon hydroxyl group on the silicon dioxide surface through the structure control machine or surface treatment agent, so as to reduce the number of surface hydroxyl groups, reduce the number of hydrogen bonds formed with silicone rubber, shorten the mixing time of the rubber mixture, and increase the plasticity, It can reduce the structural effect and improve the processing performance and storage stability.


Application of fumed silica in room temperature vulcanized (RTV) silicone rubber; From the vulcanization mechanism, it can be divided into two systems: condensation type and addition type [14]. All forms of RTV silicone rubber need to be reinforced with fillers to be useful. At present, fumed silica is a widely used and effective RTV silicone rubber reinforcing filler [12.15]. Since RTV silicone rubber is generally used as sealing materials for pouring, caulking, coating, etc., in order to maintain the viscosity and fluidity before vulcanization, the amount of fumed silica is generally much less than that of high-temperature vulcanized silicone rubber, and it is often used together with other reinforcing and semi reinforcing fillers to facilitate construction operations [4.15].


Effect of fumed silica content on tensile strength and hardness of RTV silicone rubber Vapour silica is a very effective reinforcing filler for RTV silicone rubber, which can significantly improve its strength. On the one hand, this is due to the small size effect and large specific surface area of fumed silica particles; On the other hand, because there are many silicon hydroxyl groups on its surface, the particles can form a network structure through hydrogen bonding and van der Waals force. At the same time, silicon dioxide particles also have a strong interaction with polysiloxane molecules, which improves the interface adhesion. The smaller the particle size of fumed silica is, the larger the specific surface area is, the larger the contact surface between particles and rubber is, and there are many junction points. The better the reinforcement performance of RTV silicone rubber is, and the higher the tensile strength, tear strength, performance, and hardness of vulcanizate are. But at the same time, dispersion becomes very difficult, elasticity decreases, and processing performance deteriorates. Therefore, fumed silica with relatively low specific surface area (less than 200m2/g) is generally selected as the filler of RTV silicone rubber [16]. The unreinforced silicone rubber is brittle after vulcanization, and the hardness of silicone rubber increases with the addition of fumed silica.


The influence of the amount of fumed silica added on the rheological properties of RTV silicone rubber The fumed silica aggregate has a three-dimensional branching structure, which can form an interactive network in the dispersion system. With this feature, the fumed silica, as a thickener and thixotropic agent in the sealant field, can increase viscosity, ensure the free flow of rubber, and prevent caking, sagging, collapse, etc. The thickening and thixotropic mechanism of fumed silica is mainly realized through the interaction of hydrogen bonds of surface silicon hydroxyl groups. When it is dispersed in polysiloxane, hydrogen bonds are generated between different particles through its surface silicon hydroxyl groups to form a silica network, which limits the system's fluidity, increases the viscosity, and plays a thickening role; Under the action of shear force, the silica network is damaged, resulting in the decrease of system viscosity and thixotropic effect, which is conducive to construction. Once the shear force disappears, the hydrogen bond is formed again, the silica network is restored, and the viscosity of the RTV silicone rubber compound system is gradually recovered, effectively preventing the sagging phenomenon of the rubber compound during vulcanization [17]. The anti sagging property of the system is closely related to the yield value after shearing and the network reduction rate of the material in use. In practical application, the higher the yield value, the better the anti sagging performance of the compound. The ideal rubber compound should have high yield value, high shear dilution index and reduction rate.


Effect of dispersion of fumed silica on the properties of RTV silicone rubber When fumed silica is added to RTV silicone rubber, attention must be paid to its dispersion in the polymer. After the dispersion process stops, the dispersed fumed silica will form a complete network in the system, with high viscosity and excellent thixotropic properties. When the rubber material is subjected to the shear force, the viscosity will drop significantly, showing a certain fluidity. After the shear force is removed, the viscosity will quickly recover; If the dispersion is insufficient or excessive, only part of the fumed silica network will be formed, resulting in lower viscosity and poor thixotropy. In transparent systems, the higher the light transmittance, the better the dispersion of carbon black. Under the same dispersion conditions, products containing fumed silica with large specific surface area generally have better transparency.


The specific surface area of commonly used fumed silica is 150-400 square meters. The larger the specific surface area is, the higher the reinforcement coefficient is, and the better the transparency is. However, the thickening will be more severe. A better modifier is needed to reduce the viscosity, promote the dispersion of fumed silica, and do not affect its reinforcement effect. Coupling agents such as chlorosilane, siloxane, and silazane can be selected for modification. To sum up, fumed silica is an indispensable reinforcing material for silicone rubber.