It is understood that fumed silica (fumed white carbon black) is white granular and inactive in nature. It does not interact with halogens and halogenated substances other than fluorine and hydrogen fluoride. Through the action of hydrogen, sulfuric acid, nitric acid, and perchloric acid (except for hot concentrated phosphoric acid), the fumed silica particles are fully and evenly dispersed into the resin material, which can fully achieve the purpose of improving the properties of the resin-based material.
The particle size of fumed silica is very small, so it has a large specific surface area, strong surface adsorption, high surface energy, and is very easy to agglomerate. Fumed silica has high chemical purity and good dispersion properties. It also has specific properties in terms of thermal resistance and electrical resistance. With its superior stability, reinforcement, thickening and thixotropy, it is unique in many disciplines and fields. Characteristics play an irreplaceable role.
Application and role of fumed silica in adhesives:
Thickening, thixotropy and reinforcement are necessary for fumed silica in the use of adhesives. An indispensable feature.
The viscosity of the adhesive during production is less than the standing viscosity after production. When external force is applied and stirred sufficiently, the viscosity will return to the production viscosity. This entire process is called thixotropy. In adhesives, fumed silica can be used as a thixotropic agent and reinforcing agent to control rheology, prevent settlement and reinforce at the same time. During adhesive construction, fumed silica also improves the adhesion and assists in improving the surface. Bonding properties.
After fumed silica is evenly dispersed in the adhesive system, hydrogen bonds are formed between a large number of particles through the silicon hydroxyl groups (Si-OH) on the surface, which can form a silica aggregate network and make the system The fluidity is restricted and the viscosity increases, thereby acting as a thickener. Under the action of shear force, hydrogen bonds and silica network are destroyed, causing the viscosity of the system to decrease, resulting in thixotropy, which facilitates construction. Once the external force is removed, hydrogen bonds are re-formed with the silica network. In this way, the adhesive can be effectively prevented from sagging during construction and use.
A very important application of fumed silica in adhesives is that it serves as an active reinforcing filler to improve the mechanical properties of the system, such as tensile strength, tear strength and elongation at break. This reinforcing effect is mainly attributed to the interaction between silica particles and between the particles and the cross-linked network. Activity means that the groups it contains can interact with the matrix material or directly react and cross-link. As a filler, since the primary particle diameter of fumed silica is only 7-40nm, the aggregate particle diameter is generally 200-500nm. , it is easier to penetrate into the gap between the polymer chains of the base material and the adhesive film, thereby improving the strength, toughness and ductility of the material, increasing the bonding area and increasing the riveting strength of the material.
PS; Adding an appropriate amount of fumed silica to epoxy adhesive can improve the tensile strength and shear strength of the product. Gas silicone has high surface activity and certain compatibility with resin. It can form a high bonding force with epoxy resin and has high interface strength, which improves the material strength. However, if too much is added, the gaseous silicon cannot be dispersed evenly, and large agglomerations are easily formed, thereby reducing the strength.
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