Base silicone polymers (polysiloxanes) exhibit very low mechanical strength on their own. To achieve industrial utility in High Consistency Rubber (HCR) and Liquid Silicone Rubber (LSR), reinforcing fillers are absolutely critical. However, integrating untreated silica introduces profound rheological challenges due to the hydrogen bonding between silica silanol groups and the siloxane backbone.
Figure 1: Ensuring superior optical clarity: Our fumed silica provides excellent reinforcement while maintaining the transparency of liquid silicone rubber formulations.
SEMITECH's surface-modified hydrophobic fumed silica resolves these compounding issues at the molecular level. By precisely capping the reactive silanol groups, we deliver unparalleled mechanical reinforcement without compromising processing flow.
The Ultimate Anti-Structuring Agent
Un-treated silica causes the polymer network to stiffen over time, a phenomenon known as crepe hardening. Our fully treated silica acts as a permanent anti-structuring agent for silicone elastomers, ensuring long shelf-life and excellent workability for compounded HCR stocks without needing extra hydroxyl silicone oil.
High Transparency for LSR
Medical and optical-grade liquid silicones demand zero haze. Our specialized HMDS treated silica for high transparency LSR utilizes hexamethyldisilazane to replace surface hydroxyls with non-polar trimethylsilyl groups. This perfectly matches the refractive index of the LSR matrix, eliminating yellowing and opacity.
Tear Strength Reinforcement
Whether molding keypads, seals, or medical tubing, mechanical durability is paramount. Our high surface area products serve as an elite tear strength reinforcement fumed silica, vastly improving tensile strength, elongation at break, and dynamic fatigue resistance.
PDMS Treatment for High Loading
For highly filled extrusion profiles, PDMS treated fumed silica is the ideal choice. The polydimethylsiloxane polymer chains on the silica surface provide superior steric hindrance, allowing for massive filler loading while keeping the uncured rubber highly plastic and easy to extrude.
Recommended Hydrophobic Series
We highly recommend SemiSIL R620 (HMDS) for high-clarity LSR, and SemiSIL R202 (PDMS) for maximum anti-structuring in heavy-duty HCR.
1. How to prevent crepe hardening in HCR silicone rubber?
To solve how to prevent crepe hardening in HCR silicone rubber, you must eliminate the interaction between the silica's surface hydroxyl groups and the siloxane polymer. Instead of adding temporary process aids (like short-chain silanol fluids) during milling, using a fully pre-treated HMDS treated fumed silica permanently shields these groups, ensuring the compounded rubber remains soft and processable even after months of storage.
2. Why is HMDS treated silica necessary for LSR?
Liquid Silicone Rubber (LSR) requires low initial viscosity for injection molding and high optical clarity for the final part. An HMDS treated silica for high transparency LSR provides a highly hydrophobic, non-reactive surface. This prevents premature thickening in the liquid state and ensures a crystal-clear, non-yellowing cure.
3. What makes a good tear strength reinforcement fumed silica?
A premium tear strength reinforcement fumed silica requires a high specific surface area (e.g., BET >200 m²/g) to maximize contact points with the polymer matrix, combined with the correct organic surface treatment to ensure uniform dispersion without agglomeration, which acts as stress-concentration failure points.
4. What is the difference between HMDS and PDMS treated fumed silica?
An HMDS treated fumed silica caps the surface with small trimethylsilyl groups, offering excellent transparency and maintaining some reinforcing stiffness. A PDMS treated fumed silica coats the particles with longer silicone polymer chains, providing the ultimate hydrophobicity and the strongest anti-structuring agent for silicone elastomers, ideal for highly loaded, soft extrusion HCR.
5. Do I still need to use hydroxyl silicone oil (process aids)?
If you formulate entirely with our highly treated hydrophobic fumed silica, you can significantly reduce or completely eliminate the need for in-situ process aids (hydroxyl silicone oil). This simplifies your compounding process and prevents the volatile outgassing issues associated with low-molecular-weight silanols during high-heat post-curing.
Whether you are formulating ultra-clear LSR or highly loaded HCR extrusion profiles, our surface-treated silica provides the ultimate rheological stability. Request a technical sample now.
