2026-04-29
Silicone Grade Sourcing
Looking for PDMS-treated fumed silica for silicone compounds? Get pricing and spec sheets.
Fumed Silica in Silicone Rubber: Reinforcement Mechanism and Grade Selection Without reinforcing filler, unfilled silicone rubber has tensile strength below 1 MPa — barely usable. Fumed silica transforms it into a material with 5–12 MPa tensile strength and excellent tear resistance. This guide explains the reinforcement mechanism, the role of surface treatment, and how to select the right grade for HTV and RTV silicone systems.
Silicone polymer (polydimethylsiloxane, PDMS) is inherently weak because its Si–O backbone is very flexible and the…
Silicone polymer (polydimethylsiloxane, PDMS) is inherently weak because its Si–O backbone is very flexible and the polymer chains have low intermolecular forces. Fumed silica reinforces it through two complementary mechanisms:
At high loadings (20–50 phr), silica particles form a percolating network within the rubber matrix. This network bears load at small strains, dramatically increasing modulus and hardness. At large strains (stretching), the network breaks, showing the characteristic nonlinear “Payne effect” — modulus decreases with strain amplitude.
PDMS chains adsorb strongly onto fumed silica surface silanol groups, forming a “bound rubber” layer that does not desorb under normal processing conditions. This immobilized polymer layer acts as a cross-link equivalent — it transmits stress between the silica network and the free polymer chains, increasing tear strength and elongation at break.
Why fumed silica specifically? Precipitated silica and calcium carbonate can also reinforce silicone, but fumed silica — with its high surface area (90–400 m²/g), non-porous structure, and precisely controlled aggregate morphology — provides the highest tensile strength per gram of filler. It is the preferred reinforcing filler for all high-performance silicone applications.
Unfilled Silicone With 30 phr Fumed Silica With 50 phr Fumed Silica
Elongation at Break (%)100–200300–500200–400 Shore A Hardness10–2040–5555–70 Tear Strength (kN/m)\<515–2520–35 Compression Set (%)HighModerateLow–Moderate
HTV (High Temperature Vulcanization) Used in extruded/molded silicone products (tubing, gaskets, seals). Processed at…
Used in extruded/molded silicone products (tubing, gaskets, seals). Processed at 150–200 °C under pressure. Fumed silica loading: 25–50 phr. Requires high-surface-area grades (150–200 m²/g) for maximum reinforcement. Pre-treated (PDMS-treated or in-situ HMDS) grades preferred to prevent crepe hardening.
Used in sealants, encapsulants, potting compounds. Cures at ambient temperature (condensation or addition cure). Fumed silica loading: 5–20 phr (thixotropy control) or 20–40 phr (reinforced elastomer). Lower BET grades (90–150 m²/g) preferred — easier dispersion without high-shear compounding equipment.
Crepe Hardening in HTV: Untreated (hydrophilic) fumed silica causes “crepe hardening” — the compound stiffens and becomes unprocessable over time as the silanol groups form hydrogen bonds with the PDMS chains. This is why HTV compounds always use pre-treated or in-situ treated fumed silica. RTV systems are less susceptible due to lower viscosity and shorter shelf life before cure.
The key to preventing crepe hardening and maintaining processability is controlling the silanol-PDMS interaction. Three…
The key to preventing crepe hardening and maintaining processability is controlling the silanol-PDMS interaction. Three approaches are used:
Grades like Aerosil R202 and Wacker HDK H20 are pre-treated with PDMS at the factory. The PDMS coating on the silica surface is compatible with the silicone matrix — it reduces the number of free silanols available to form rigid bonds with the polymer. This gives excellent shelf stability of the compound and predictable processing. Highest cost option; most convenient.
Hexamethyldisilazane (HMDS) is added directly to the Sigma mixer or two-roll mill during compounding along with untreated fumed silica. At compounding temperature (150–160 °C), HMDS reacts with silanols, converting them to trimethylsilyl groups in real time. Requires addition of small amount of water to catalyze the reaction. Lower cost than pre-treated grades; widely used in HTV production.
Adding a silanol-terminated PDMS fluid (typically 0.5–2 phr) alongside untreated fumed silica competes for silanol sites on the silica surface, partially blocking crepe hardening. This is a lower-cost approach but provides less consistent protection than HMDS or pre-treatment. Used in less demanding RTV applications.
| Application | System | Recommended Grade | BET (m²/g) | Loading (phr) |
|---|---|---|---|---|
| High-strength HTV (seals, gaskets) | HTV peroxide cure | Aerosil R202 / HDK H20 | 100 ± 20 | 30–50 |
| HTV compound (in-situ HMDS) | HTV peroxide cure | Aerosil 200 + HMDS | 200 ± 25 | 25–45 |
| LSR (Liquid Silicone Rubber) | Addition cure, injection molded | Aerosil R202 / R972 | 100–150 | 20–40 |
| RTV-1 sealant (acetoxy/neutral) | Condensation cure | Aerosil R202 / HDK H20 | 100 ± 20 | 5–15 |
| RTV-2 sealant/encapsulant | Addition cure (Pt) | Aerosil R972 / R202 | 110–150 | 8–20 |
| Silicone potting compound | Addition cure, low viscosity | Aerosil R972 | 110 ± 20 | 3–8 |
| Silicone foam | HTV or RTV | Aerosil R202 | 100 ± 20 | 15–30 |
Why lower BET (90–150 m²/g) for silicone vs. higher BET for coatings? In silicone systems, the goal is reinforcement through polymer-filler interaction and aggregate networking, not solution-phase hydrogen bonding. Lower BET grades have larger aggregates with higher structure — more branched chains that entangle with polymer. Higher BET grades (300–380 m²/g) are harder to compound and can reduce elongation at break by creating excessive cross-link density.
HTV Compounding Equipment HTV silicone compounds are mixed in a sigma (Z-blade) mixer or on a two-roll mill. A typical…
HTV silicone compounds are mixed in a sigma (Z-blade) mixer or on a two-roll mill. A typical sigma mixer compounding cycle:
Charge PDMS gum polymer into mixer
Add 50% of fumed silica; mix at 80–100 °C for 15 min
Add HMDS (if in-situ treatment); add water catalyst (0.5–1 phr); continue mixing 15 min at 150–160 °C
Add remaining 50% fumed silica; mix additional 30–45 min
Add crosslinker, catalyst, and other additives on the two-roll mill at room temperature
Post-heat treatment (150–200 °C, 2–4 h) to remove residual HMDS byproducts (ammonia)
RTV systems are mixed at ambient or mild temperatures using planetary mixers, dual asymmetric centrifuge (SpeedMixer), or high-speed dissolvers. Key points:
Add fumed silica to Part A (base polymer) only — never to the catalyst/crosslinker side
Use pre-treated grades (R202) to avoid stability issues in one-pack systems
Mix under vacuum to eliminate air entrainment before packaging
For addition-cure (Pt catalyst) systems: even trace amounts of amine or sulfur from HMDS treatment can poison the Pt catalyst — always use R202 (PDMS-treated) rather than HMDS-treated grades in addition-cure formulations
Platinum Catalyst Poisoning: HMDS-treated fumed silica can leave residual nitrogen compounds that inhibit platinum addition-cure catalysts. Always use PDMS-treated grades (R202, HDK H20) in addition-cure RTV systems. If HMDS grades are necessary, post-heat at 200 °C for 4+ hours before use to drive off residual amines.
We supply HDK H20 and equivalent PDMS-treated grades for silicone rubber compounding. Contact us for pricing, samples, and technical data sheets.
Yes, but only for HTV compounding with in-situ HMDS treatment. You add Aerosil 200 together with HMDS to the sigma mixer at elevated temperature, which converts the surface to hydrophobic in real time. Using Aerosil 200 without treatment will cause severe crepe hardening — the compound becomes a stiff, unprocessable brick within days or even hours at ambient temperature. For RTV systems, always use pre-treated grades.
Practical maximum is 50–60 phr for HTV systems and 25–35 phr for RTV. Above these levels, the compound becomes too viscous to process. Tensile strength also plateaus or decreases at very high loadings because excess silica creates too many cross-links, reducing chain mobility needed for energy dissipation. Typical optimal loading for maximum tensile strength is 35–45 phr for HTV.
This is a moisture diffusion limitation for condensation-cure RTV-1 systems. Moisture from the air must diffuse through the cured skin to reach uncured material — fumed silica does not cause this, but a high fumed silica loading can reduce moisture permeability and slow deep cure. For deep-section applications, use addition-cure RTV-2 systems (mixed before pouring) rather than moisture-cure RTV-1.
High-quality fumed silica at proper dispersion can produce transparent to translucent silicone rubber because the refractive index of fumed silica (1.46) is close to that of PDMS (1.40–1.43). Agglomerates scatter light and cause haze. For optical-grade transparent silicone (LED encapsulants, optical bonding), use well-dispersed R202 or R972 at moderate loadings (15–25 phr) with optimized mixing. Certify with haze measurements before production scale-up.
Both are PDMS-treated fumed silica grades with similar BET (100 ± 20 m²/g) and intended for silicone rubber reinforcement. HDK H20 (Wacker) and Aerosil R202 (Evonik) are functionally equivalent and interchangeable in most applications. Minor differences exist in carbon content and PDMS chain length — request data sheets and run a comparison compound to verify before switching suppliers in a validated product.
Get Samples & TDS
Free samples for qualified buyers · reply within 24h. Tell us how you plan to use Fumed Silica for Silicone Rubber: Reinforcement & Grades.