Preparation of fumed silica and its application in silicone rubber

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1. Preparation method of fumed silica: Fumed silica is obtained by the high-temperature hydrolysis of halosilanes (such as silicon tetrachloride, silicon tetrafluoride, methyltrichlorosilane, etc.) in a hydrogen-oxygen flame to produce silica particles, which are then rapidly cooled. The particles undergo further processing steps such as aggregation, separation, and acid removal to yield the final product. In the 1960s and 1970s, vapor-phase silica was primarily produced using silicon tetrachloride as a raw material; the production process was easy to control, but the production costs were high. With the development of the silicone monomer industry, the handling of its by-products such as methyltrichlorosilane has become a bottleneck restricting its progress. Generally, they are used in the manufacture of silicone resins and waterproof coatings, but in limited quantities. Therefore, it is urgent to find new ways out. By the 1980s, processes for producing fumed silica using organic silicon monomer by-products or mixtures of such by-products and silicon tetrachloride had been developed; these processes were cost-effective and offered good economic benefits. The principle of producing fumed silica is as follows: SiCl4 + 2H2 + O2 → combustion → SiO2 + H2O + 4HCl; CH3SiCl3 + 2H2 + 3O2 → combustion → SiO2 + 3HCl + CO2 + 2H2O. The emergence of new processes for producing fumed silica has changed the development pattern of this industry, bringing about a closer relationship between it and the silicone monomer industry. It solves the problem of handling by-products from the silicone monomer industry; the by-product generated during fumed silica production (hydrochloric acid) can be reused in the synthesis of silicone monomers. Meanwhile, the fumed silica produced is mostly used in the post-processing of silicone products, thus enabling the recycling of resources. Therefore, most manufacturers of fumed silica choose to locate their plants near large silicone monomer companies, working closely together to promote mutual development. Figure 1 shows a schematic diagram of the resource recycling in the fumed silica industry and the silicone industry. It illustrates the closely related silicone companies and fumed silica companies, which establish their factories in close proximity to one another, fostering mutual development and achieving excellent socioeconomic benefits. 2. Applications of fumed silica in silicone rubber 2.1 Applications of fumed silica in high-temperature vulcanization (HTV) silicone rubber. The use of fumed silica can be seen in silicone materials as well as in other fields; within silicone materials, its usage accounts for nearly 60% of the total amount of fumed silica used. Silicone rubber is the material in which fumed silica is used the most, with addition levels reaching over 50%. Vapor-phase silica plays a primary role in reinforcing HvT silicone rubber. Due to the high flexibility of the silicone rubber molecular chains and the weak intermolecular forces, unreinforced silicone rubber has a very low strength (not exceeding 0.4 MPa), rendering it unsuitable for practical use; it must be reinforced before it can be utilized. Silicone rubber reinforced with vapor-phase silica can see its strength increased by 40 times. 2.1.1 Effect of vapor-phase silica on the mechanical properties of HTv silicone rubber: The reinforcing effect of vapor-phase silica on HTv silicone rubber is influenced by its particle size, specific surface area, and structural characteristics. Generally, the smaller the particle size, the greater the specific surface area, and the higher the structural complexity, the better the reinforcing effect, resulting in higher strength and hardness in the vulcanized rubber. Furthermore, the amount of vapor-phase silica used and its dispersion in the rubber matrix have a significant impact on the properties of the vulcanized rubber. Figure 2 shows the effect of the amount of vapor-phase silica used on the tensile strength of the vulcanized rubber. As can be seen from the graph, as the amount of vapor-phase silica used increases, the strength of the vulcanized rubber increases; generally, a usage level of 35 to 50 parts yields the peak strength. There are also many theories and models regarding the reinforcement mechanism of silica in silicone rubber. The widely accepted explanation is that the free radical groups on the surface of fumed silica form physical or chemical bonds with the molecules of silicone rubber, resulting in an adsorption layer of silicone rubber molecules on the silica surface. This creates a three-dimensional network structure in which fumed silica and silicone rubber molecules are combined together, thereby effectively restricting the deformation of the silicone rubber molecular chains and exerting a reinforcing effect. The variation in the tear strength of vulcanized rubber is similar to that of tensile strength; it increases as the reinforcing effect of vapor-phase silica improves. It rises with increasing amounts of vapor-phase silica, and then decreases slightly after reaching a peak value. 2.1.2 Effect of vapor-phase silica on the processing properties of HTV silicone rubber. The effect of vapor-phase silica on the processing properties of HTV silicone rubber is generally expressed in terms of the degree of structuring (△Crepe). △Crepe is calculated using the plasticity (p28) of the compounded material after being stored at room temperature for 28 days, and the plasticity measured immediately after compounding (P0). ) the difference (see Figure 3), and the plasticity of the rubber compound is related to the amount of vapor-phase silica used, as well as its surface properties and structure. The formation of a structured structure is due to hydrogen bonds forming between the surface silanol groups of vapor-phase silica and the oxygen atoms in silicone rubber, as well as the adsorption of silicone rubber molecular chains on the silica surface. As a result, over time the fluidity of the compound decreases and it becomes harder, which affects its processability. Therefore, during the processing stage, it is necessary to add structured control agents or use vapor-phase silica with a treated surface. The addition of structured control agents and the surface treatment of vapor-phase silica both involve reactions between these agents or treatment chemicals and the silicon hydroxyl groups on the silica surface, thereby reducing the number of surface hydroxyl groups. This leads to a decrease in the number of hydrogen bonds formed with silicone rubber, shortens the mixing time of the compound, increases its plasticity, and helps to reduce the structural effects while improving processability and storage stability. 2.2 Application of vapor-phase silica in room-temperature vulcanizing (RTV) silicone rubber. Room-temperature vulcanizing (RTV) silicone rubber can be divided into two main categories based on its product form: single-component (RTV-1) and two-component (RTV-2); from the perspective of the vulcanization mechanism, it can also be classified into condensation-type and addition-type systems. Silicone rubbers that cure at room temperature in different forms all require filler reinforcement to be practically useful. Currently, vapor-phase silica is the most widely used and effective reinforcing filler for RTV silicone rubber. Since RTv silicone rubber can generally be used as a sealing material for casting, caulking, coating, etc., in order to maintain its viscosity and fluidity before vulcanization, the amount of vapor-phase silica added is usually much less than that in high-temperature vulcanized silicone rubber. It is often used together with other reinforcing and semi-reinforcing fillers to facilitate handling during application. 2.2.1 Effect of vapor-phase silica on the mechanical properties of RTv silicone rubber: Vapor-phase silica is a highly effective reinforcing filler for RTv silicone rubber, capable of significantly improving its strength. On the one hand, this is due to the small size effect and high specific surface area of the vapor-phase silica particles; on the other hand, it is because their surfaces contain many silanol groups, which allow the particles to form a network structure through hydrogen bonds and van der Waals forces. Additionally, the silica particles interact strongly with polysiloxane molecules, thereby improving the interface conditions. Figure 4 shows the effect of the amount of fumed silica used on the tensile strength and Shore A hardness of RTV silicone rubber (with a specific surface area of 153 m2/g). Figure 5 illustrates the effect of the specific surface area of fumed silica on the peeling strength of RTV silicone rubber, while Figure 6 shows the effect of the amount of fumed carbon black used on the tear strength of RTV silicone rubber. As can be seen from the graph, as the amount of vapor-phase silica used increases, the tensile strength, hardness, and tear strength of the RTV silicone rubber all increase. At the same amount of silica used, the peeling strength of the RTV silicone rubber increases as the specific surface area increases. This is mainly because as the amount of vapor-phase silica used increases, it forms a relatively complete network throughout the rubber compound, which can effectively restrict the movement of the silicone rubber molecular chains, thereby providing a reinforcing effect. As the specific surface area increases, the particle size of silica decreases, and the interfacial interaction between silica and silicone rubber molecules strengthens, thereby increasing the peeling strength. 2.2.2 Effect of the amount of vapor-phase silica added on the rheological properties of RTv silicone rubber. Vapor-phase silica aggregates possess a three-dimensional branched structure, which allows them to form an interactive network within the dispersion system. Taking advantage of this property, vapor-phase silica can be used as a thickener and thixotropic agent in sealants; it increases viscosity, ensures the free flow of the adhesive, and prevents problems such as caking, flowing, and sagging. Figures 7–9 show the effect of fumed silica on the rheological properties of RTv silicone rubber. As can be seen from the graph, when the specific surface area is less than 200 m2/g, as the specific surface area of fumed silica increases, the extrusion rate of RTv silicone rubber decreases; it reaches a balance at 200 m2/g. Meanwhile, as the amount of fumed silica used increases, the yield strength of RTv silicone rubber rises. The thickening and thixotropic mechanisms of fumed silica are mainly due to the hydrogen bonding interactions of surface silanol groups. When dispersed in polysiloxane, hydrogen bonds are formed between different particles via the silicon hydroxyl groups on their surfaces, creating a silica network that limits the fluidity of the system and increases its viscosity, thereby acting as a thickening agent. When shear force is applied, this silica network is disrupted, resulting in a decrease in the system’s viscosity and a thixotropic effect, which facilitates application. Once the shear force disappears, hydrogen bonds are reformed, the silica network is restored, and the viscosity of the RTv silicone rubber compound gradually increases, effectively preventing the flow of the compound during the vulcanization process. The anti-sagging property of the system is closely related to the yield value of the material under shear during use and the network recovery rate. In practical applications, the higher the yield value, the better the anti-flowing property of the rubber compound. An ideal rubber compound should have a high yield value, a high shear-thinning index, and a rapid reduction rate. 2.2.3 Effect of the dispersibility of fumed silica on the properties of RTv silicone rubber: When adding fumed silica to RTv silicone rubber, attention must be paid to its degree of dispersion within the polymer. Figure 10 shows the effect of the amount of vapor-phase silica on the mixing time. As can be seen from the graph, as the amount of vapor-phase silica and its specific surface area increase, the mixing time lengthens. The degree of dispersion of fumed silica in the system has a significant impact on the properties of RTv silicone rubber. After the dispersion process is completed, the fumed silica in its optimally dispersed state forms a complete network within the system, resulting in high viscosity and excellent thixotropic properties. When the rubber compound is subjected to shear force, its viscosity drops significantly, giving it a certain degree of fluidity. Once the shear force is removed, the viscosity returns to its normal level rapidly. If the dispersion is insufficient or excessive, only partial silica networks are formed, resulting in lower viscosity and poorer thixotropic properties. In transparent rubber systems, the higher the transparency, the better the dispersion of silica. Under the same dispersion conditions, the transparency of the rubber compound increases as the specific surface area increases. 3. Conclusion In summary, vapor-phase silica is an essential reinforcing material for silicone rubber. Initially it was mainly used in the military industry, but nowadays it is widely employed in various other industrial sectors. Thanks to its unique properties, it finds extensive use in industries such as coatings, inks, pharmaceuticals, agriculture, food, paper, electronics, cosmetics, and chemical mechanical polishing (CMP), and its future prospects are very promising. At present, the domestic market share for vapor-phase silica is mostly held by foreign companies; the total production capacity of domestic manufacturers is less than 1,500 tons per year, which is far from sufficient to meet market demand. Therefore, it has become an urgent task to develop and strengthen China’s vapor-phase silica industry.