The magical effect of fumed silica in silicone rubber

Fumed silica is one of the most important high-tech ultra-fine inorganic new materials. Due to its small particle size, it has a large specific surface area, strong surface adsorption, large surface energy, high chemical purity, good dispersion performance, thermal resistance, It has specific properties in terms of electrical resistance, etc. Its superior stability, reinforcement, thickening and thixotropic properties are unique in many disciplines and fields and have an irreplaceable role. Today, Xiaobian will work with you to learn how to apply fumed silica in silicone rubber.

Application of fumed silica in high temperature vulcanization (HTV) silicone rubber
The use of fumed silica can be divided into silicone materials and other fields, in which the amount of organosilicon materials accounts for nearly 60% of the total amount of fumed silica, and silicone rubber is the most used fumed silica in silicone materials. It can be added in an amount of up to 50% or more. Fumed silica plays a major role in the reinforcement of HVT silicone rubber. Since the silicone rubber molecular chain is very compliant and the interchain interaction force is weak, the unreinforced silicone rubber has very low strength (not exceeding 0.4 MPa). The value of use must be reinforced before it can be applied, and the silicone rubber reinforced by fumed silica can be increased in strength by up to 40 times.

Effect of fumed silica on mechanical properties of HTV silicone rubber
The reinforcing of HTV silicone rubber by fumed silica is affected by its particle size, comparative area and structure. Generally, the smaller the particle size, the higher the specific surface area, the higher the structure, the better the reinforcing effect, and the vulcanized rubber. The higher the strength and hardness. In addition, the amount of fumed silica and its dispersion in the matrix have a great influence on the properties of the vulcanizate. Figure 1 is a graph showing the effect of the amount of fumed silica on the tensile strength of the vulcanizate. As can be seen from the figure, as the amount of fumed silica increases, the strength of the vulcanizate increases, generally reaching a peak at 35-50 parts. There are also many reinforcing mechanisms and models for fumed silica on silicone rubber. The more accepted explanation is that the free hydroxyl groups on the surface of fumed silica form physical or chemical bonds with silicone rubber molecules, forming silicone rubber on the surface of silica. The molecular adsorption layer constitutes a three-dimensional network structure in which fumed silica and silicone rubber molecules are integrated, thereby effectively limiting the deformation of the silicone rubber molecular chain and reinforcing it. The change of the tear strength of the vulcanizate is similar to the tensile strength, which increases with the increase of the fusibility of fumed silica, and increases first with the increase of the amount of fumed silica, and decreases slightly after reaching the peak.

Effect of fumed silica on the processing properties of HTV silicone rubber
The effect of fumed silica on the processing properties of HTV silicone rubber is generally expressed by the degree of structuring (∆Crepe). The ∆Crepe is equal to the plasticity value (p28) of the mixture after 28 days of storage at room temperature and immediately after mixing. The difference in plasticity value (P0), the plasticity value of the rubber compound is related to the amount of fumed silica, surface properties and structure. The reason for the structuring is that the surface hydroxyl group of fumed silica forms a hydrogen bond with the oxygen atom in the silicone rubber and the silica gel molecular chain is adsorbed on the surface of the silica, resulting in a decrease in fluidity of the rubber compound over time. The material becomes hard and affects the processing performance. Therefore, it is necessary to add a structure control agent or a surface treated fumed silica during the use process, the addition of a structure control agent, and the surface treatment of fumed silica by a structure control machine or a surface treatment agent and silica. The surface of the silanol reacts, thereby reducing the amount of surface hydroxyl groups, resulting in a decrease in the number of hydrogen bonds with the silicone rubber, shortening the mixing time of the rubber compound, increasing the plasticity, reducing the structuring effect, and improving the processing property and storage stability. the goal of.

Application of fumed silica in room temperature vulcanization (RTV) silicone rubber
Room temperature vulcanization (RTV) silicone rubber is divided into two categories: single component (RTV-1) and two component (RTV-2). From the vulcanization mechanism, it is divided into two systems: condensation and addition. . Various forms of room temperature vulcanized silicone rubber need to be reinforced with fillers. At present, fumed silica is the most effective and most effective RTV silicone rubber reinforcing filler. Since RTV silicone rubber is generally used as a sealing material for casting, 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 often with other supplements. Strong and semi-reinforcing fillers are used together to facilitate construction operations.Effect of fumed silica addition on tensile strength and hardness of RTV silicone rubber
Fumed silica is a very effective reinforcing filler for RTV silicone rubber and can significantly increase its strength. This aspect is due to the small size effect of the fumed silica particles and the large specific surface area; on the other hand, because the surface contains many silicols, the particles can form a network structure through the interaction of hydrogen bonds and van der Waals forces, while silica The particles also interact strongly with the silicone molecules, improving interfacial adhesion. The smaller the particle size of fumed silica, the larger the specific surface area, the larger the contact surface between the particles and the rubber compound, and the more bonding points, the better the reinforcing performance for RTV silicone rubber, the tensile strength, tear strength and resistance of the vulcanized rubber. The abrasiveness and hardness are also high, but at the same time, the dispersion becomes very difficult, the elasticity is lowered, and the processability is deteriorated. Therefore, the RTV silicone rubber generally uses fumed silica having a relatively low specific surface area (200 m 2 /g or less) as a filler. The unreinforced silicone rubber is brittle after vulcanization. After the addition of fumed silica, the hardness of the silicone rubber increases as the amount of silica added increases.

Effect of fumed silica addition on rheological properties of RTV silicone rubber
The fumed silica aggregate has a three-dimensional branching structure, which can form an interaction network in the dispersion system. With this property, fumed silica can increase viscosity in the field of sealant as a thickener and a thixotropic agent. To ensure the free flow of the rubber compound, to prevent agglomeration, sag, collapse and the like. The thickening and thixotropic mechanism of fumed silica is mainly achieved by the interaction of hydrogen bonds on the surface silanol. When dispersed in the polysiloxane, hydrogen bonds are generated between different particles through the surface of the silicon hydroxyl group. Forming a silica network to limit the fluidity of the system, increase the viscosity, and thicken the role; when subjected to shearing force, the silica network is damaged, resulting in a decrease in viscosity of the system. Variable effect is conducive to construction. Once the shearing force disappears, the hydrogen bond is reformed, the silica network is restored, and the viscosity of the RTV silicone rubber compound system gradually rises, effectively preventing the sag of the rubber compound during the vulcanization process. The anti-sagging characteristics of the system are closely related to the yield value and network reduction rate after shearing of the material during use. In practical applications, the higher the yield value, the better the anti-sag performance of the rubber compound. The ideal compound should have a high yield value, a high shear dilution index, and a fast reduction rate.

Effect of fumed silica dispersibility on properties of RTV silicone rubber
When adding fumed silica to RTV silicone rubber, attention must be paid to its degree of dispersion in the polymer. After the dispersion process is stopped, the fumed silica that achieves the best dispersion condition will form a complete network in the system, with high viscosity and excellent thixotropic properties. When the rubber is subjected to shearing force, the viscosity is greatly reduced. After a certain fluidity, the viscosity will recover quickly after the shearing force is removed; if the dispersion is insufficient or excessively dispersed, only a part of the vapor phase white carbon black network will be formed, resulting in lower viscosity and poor thixotropic properties. In a transparent system, the higher the transmittance, the better the dispersion of silica. Under the same dispersion conditions, fumed silica products containing larger specific surface areas generally have better transparency.