Application of silica in silicone rubber
Silica is one of the most important high-tech ultra-fine inorganic materials, Because of its small particle size, it has large specific surface area, strong surface adsorption, large surface energy, high chemical purity, good dispersion performance, thermal resistance, electrical resistance and other special properties. With its superior stability, reinforcement, thickening and thixotropy, it has unique characteristics in many disciplines and fields and plays an irreplaceable role.
The use of silicon dioxide can be divided into silicone materials and other fields, in which the amount of silicone materials accounts for nearly 60% of the total amount of fumed silica. Silicone rubber is the most used material in silicone materials, and its addition amount can reach more than 50%. Fumed silica 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 silicone rubber without reinforcement is very low (no more than 0.4MPa) and has no use value. It can only be applied after reinforcement. However, the strength of silicone rubber reinforced by fumed silica can be increased by 40 times.
Effect of fumed silica on mechanical properties of HTV silicone rubber
The reinforcement effect 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 structure, the better the reinforcement effect and the higher the strength and hardness of 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 effect of the amount of fumed silica on the tensile strength of the vulcanizate. It can be seen from the figure that with the increase of the amount of fumed silica, the strength of vulcanizate increases, generally reaching the peak at 35-50 phr. There are also many reinforcement mechanisms and models of fumed silica for silicone rubber. The more accepted explanation is that the free hydroxyl groups on the surface of fumed silica form physical or chemical combination with silicone rubber molecules, forming an adsorption layer of silicone rubber molecules on the surface of silica, forming a three-dimensional network structure of fumed silica and silicone rubber molecules, thus effectively limiting the silicone rubber The deformation of the molecular chain leads to reinforcement. The change of tear strength of vulcanizate is similar to that of tensile strength, which increases with the reinforcement of fumed silica. With the increase of the amount of fumed silica, the tear strength first increases, reaches the peak and then decreases slightly.
Effect of fumed silica on processability of HTV silicone rubber
The effect of fumed silica on the processability of HTV silicone rubber is generally expressed as the degree of structure (△ crepe), which is equal to the difference between the plasticity value (P28) of the compound stored at room temperature for 28 days and the plasticity value (P0) measured immediately after mixing. The plasticity value of the compound is related to the amount of fumed silica, surface properties and structure. The structural reason is that the hydroxyl groups on the surface of fumed silica and the oxygen atoms in the silicone rubber form hydrogen bonds, and the silica surface adsorbs the molecular chain of silicone rubber, which leads to the decrease of the fluidity of the compound with the extension of time and the hardening of the compound, which affects the processing performance. Therefore, it is necessary to add structure control agent or select fumed silica after surface treatment in the process of use. The addition of structure control agent and the surface treatment of fumed silica are all through the reaction of structure control machine or surface treatment agent with silicon hydroxyl on the surface of silica, so as to reduce the number of surface hydroxyl, reduce the number of hydrogen bonds formed with silicone rubber, and make the mixture more stable The mixing time is shortened and the plasticity is increased, which can reduce the structural effect and improve the processability and storage stability.
Application of fumed silica in RTV silicone rubber
Room temperature vulcanized (RTV) silicone rubber can be divided into single component (RTV-1) and two-component (RTV-2) in terms of product morphology, and can be divided into condensation type and addition type in terms of vulcanization mechanism. At present, fumed silica is the most widely used and effective reinforcing filler for RTV silicone rubber. Because RTV silicone rubber is generally used as pouring, caulking, coating and other sealing materials, 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 operation.
Effect of fumed silica content on tensile strength and hardness of RTV silicone rubber
Silica is a very effective reinforcing filler for RTV silicone rubber, which can significantly improve its strength. On the one hand, it is due to the small size effect and large specific surface area of fumed silica particles; on the other hand, it is because there are many silicon hydroxyl groups on the surface of fumed silica particles, which can form a network structure through hydrogen bonding and van der Waals force. At the same time, silica particles also have strong interaction with polysiloxane molecules, which improves the interface adhesion. The smaller the silica particle size, the larger the specific surface area, the larger the contact surface between the particles and the compound, and the more the bonding points, the better the reinforcement performance of RTV silicone rubber, and the higher the tensile strength, tear strength, wear resistance and hardness of vulcanizate. However, at the same time, dispersion becomes very difficult, the elasticity decreases, and the processability deteriorates. Therefore, RTV silicone rubber generally has a relatively low specific surface area (less than 200m2 / g) ）Fumed silica was used as filler [16]. The silicone rubber without reinforcement is brittle after vulcanization. The hardness of silicone rubber increases with the increase of the amount of silica.
Effect of silica content on rheological properties of RTV silicone rubber
Silica aggregate has a three-dimensional branch structure, which can form an interaction network in the dispersion system. Using this characteristic, silica in the field of sealant, as a thickener and thixotropic agent, can increase the viscosity, ensure the free flow of the compound, and prevent caking, sagging and collapse. The thickening and thixotropy mechanism of silicon dioxide is mainly realized by the interaction of hydrogen bonds of silicon hydroxyl groups on the surface. When it is dispersed in polysiloxane, hydrogen bonds are generated between different particles through the silicon hydroxyl groups on the surface, forming a silicon dioxide network, which limits the fluidity of the system, increases the viscosity, and plays a thickening role. When it is subjected to shear force, dioxygen increases The destruction of the siliconized network leads to the decrease of the viscosity of the system and thixotropic effect, which is beneficial to the construction. Once the shear force disappears, the hydrogen bond re forms, the silica network re recovers, and the viscosity of RTV silicone rubber compound system gradually rises, which effectively prevents the sagging phenomenon of compound during vulcanization [17]. The anti sagging property of the system is closely related to the yield value and network reduction rate of the material after shearing. In practical application, the higher the yield value, the better the anti sagging performance of the compound. The ideal compound should have high yield value, high shear dilution index and fast reduction rate.
Effect of silica dispersion on properties of RTV silicone rubber
When silica is added into RTV silicone rubber, the dispersion of silica in polymer must be paid attention to. After the dispersion process stops, the fumed silica with the best dispersion condition will form a complete network in the system, which has high viscosity and excellent thixotropy characteristics. When the compound is subjected to shear force, the viscosity will decrease greatly, showing a certain fluidity. After the shear force is removed, the viscosity will recover rapidly; if the dispersion is not enough or excessive, only part of the fumed silica will be formed Phase white carbon black network, resulting in lower viscosity and poor thixotropy. In the transparent system, the higher the transmittance, the better the dispersion of carbon black. Under the same dispersion conditions, the products with large specific surface area usually have better transparency.
In conclusion, silica is an indispensable reinforcing material for silicone rubber. Due to its unique properties, it has been widely used in other fields, such as coatings, ink, medicine, cosmetics and chemical mechanical polishing (CMP), and has a bright future.