Liquid silicone rubber (LSR) is widely used in precision injection molding and coating processes due to its excellent processing performance and physical properties. However, low-viscosity LSR base rubbers are prone to sagging or incomplete filling when applied to vertical surfaces or filled in complex molds. Hydrophobic fumed silica, as a highly efficient rheology modifier, achieves precise control over the rheological behavior of LSR by constructing a reversible thixotropic network.

The core influence of hydrophobic fumed silica on rheological properties lies in its thixotropy. When silica is dispersed in liquid silicone rubber, nano-sized silica particles attract each other through intermolecular forces, forming a loose, flocculated structure. Under static or low shear rates, this three-dimensional network structure imparts a high low-shear viscosity to the compound, effectively supporting fillers and pigments, preventing sedimentation, and inhibiting sagging on vertical surfaces. When subjected to high shear forces (such as injection or stirring), this weak flocculated structure is rapidly disrupted, causing a sharp drop in the viscosity of the compound, exhibiting excellent flowability and spreadability. Once the external force is removed, the network structure quickly rebuilds, restoring the high viscosity state.

Hydrophobic modification plays a crucial regulatory role in this process. While untreated hydrophilic silica can also thicken, it often results in an excessively high thixotropic index, poor flowability, and difficulty in processing. Surface-treated hydrophobic silica weakens the excessively strong hydrogen bonds between particles, resulting in a more flexible network structure. This not only improves the dispersibility of silica in LSR and prevents the formation of gel lumps, but more importantly, optimizes the slope of the thixotropic curve. It allows the LSR to maintain good anti-sagging properties while possessing a lower yield value, thus enabling smoother filling of small mold cavities during high-speed injection molding.

Furthermore, hydrophobic fumed silica effectively prevents the structuring phenomenon of LSR during storage (i.e., unintended viscosity increase over time). By shielding surface-active hydroxyl groups, it reduces chemical adsorption between the filler and the matrix, ensuring the long-term stability of LSR viscosity. This perfect balance between static anti-sagging and dynamic flowability makes hydrophobic fumed silica an indispensable rheology control agent in high-end LSR formulations.