Silica, also known as hydrated silica, is a crucial reinforcing agent in the rubber industry. Currently, most of the products on the market are manufactured using the precipitation method. The primary particles of silica have a particle size of 30-50nm and are approximately spherical. These primary particles contact each other and form aggregates through van der Waals forces. These aggregates then interconnect to form agglomerates. The abundance of hydrogen bonds on the surface of the primary particles is the main reason they tend to cluster together.
1. Why is surface modification of silica necessary?
Due to this unique physical and chemical structure, standard silica particles tend to exhibit several issues when forming aggregates, which negatively impact their industrial application: a strong tendency to adsorb water molecules on their surface, a propensity for secondary aggregation between molecules, and an excessively large specific surface area. These issues specifically manifest as:
(1) Hydrophilic Surface
The surface of silica aggregates contains a large number of polar hydroxyl groups. These groups strongly adsorb external water molecules, causing the surface to become coated with water. During the processing of rubber products, these adsorbed water molecules can vaporize during vulcanization, creating bubbles and voids in the rubber compound and thus compromising the performance of the final product. Furthermore, rubber product surfaces are generally non-polar, while the silica surface is highly polar due to the hydroxyl groups. This difference in polarity leads to poor compatibility between the two, which diminishes the reinforcing effect of the silica.
(2) Poor Dispersibility
Silica has a branched, chain-like aggregate structure. The common silica produced by the precipitation method is prone to secondary aggregation during formation, and this structure is difficult to break down. Compared to silica particles made by the fumed (pyrogenic) process, precipitated silica particles often cross-link into a network during aggregation. This significantly reduces their ability to disperse evenly. When used as an industrial reinforcing agent, this poor dispersibility hinders the integration of the silica with the matrix material it is meant to reinforce, thereby lowering its overall effectiveness.
(3) Excessively Large Specific Surface Area
Silica particles are very small, which gives them a large specific surface area. While a large surface area is generally beneficial for rubber reinforcement, an excessively large area can increase the cohesive force between silica particles. This makes them difficult to disperse uniformly within the rubber matrix. Additionally, during the processing of rubber products, this can lead to problems such as adsorbing a large amount of curatives (accelerators), delaying the vulcanization time, generating excessive heat, and even causing the rubber compound to scorch.
Therefore, to enhance its performance in industrial applications, silica typically requires modification before it can be used effectively.
