Fumed Silica in RTV-2 and LSR Silicone Rubber: Formulation Guide

The procurement question

You're formulating an addition-cure silicone rubber — LSR (liquid silicone rubber, for injection moulding), RTV-2 (room-temperature vulcanizing, two-component), or HCR (high-consistency rubber, mill-processed). Fumed silica is the reinforcing filler. Which grade — and how much — without compromising flow, cure, or final mechanical properties?

Three points the marketing copy from your current supplier probably doesn't explain:

1. Hydrophobic grade is mandatory (catalyst inhibition issue)
2. PDMS-treated grade dominates LSR despite higher cost
3. The "more silica = better mechanical" assumption fails above 20-25 phr

Why hydrophobic, not hydrophilic

Addition-cure silicone (LSR, RTV-2, HCR addition-cure) uses platinum catalyst (Karstedt's complex or Speier's catalyst) for hydrosilylation between vinyl-terminated PDMS and Si-H functional crosslinker. Hydrophilic fumed silica has a problem here: silanol groups on the surface adsorb water, and water deactivates the platinum catalyst by displacing alkene ligands. Result: incomplete cure, sticky surface defects, batch-to-batch unpredictability.

Hydrophobic fumed silica solves this by capping the silanol groups via DMDS, HMDS, or PDMS surface treatment. The resulting silica is non-adsorptive for water and cosmologically inert to the platinum catalyst.

Mandatory: always use HJSIL R272, R274, R110, or R620 (hydrophobic) in addition-cure silicone. Hydrophilic HJSIL 150/200/300/380 will work in condensation-cure RTV-1 (tin or titanate catalysed) but will sabotage Pt-catalysed cure.

R620 vs R272/R274 for LSR — why PDMS-treated wins

Three reasons R620 (PDMS-treated) is the choice for LSR despite being 20-25% more expensive than R272/R274 (DMDS-treated):

1. Lower compound viscosity at same reinforcement

PDMS surface chemistry is identical to the V-PDMS base polymer. R620 disperses into the silicone matrix without phase boundary — the silica particles "blend in" rather than form distinct phases. DMDS-treated silica has a small phase boundary that resists wetting by the base polymer.

At 20 phr loading in 1000 cSt V-PDMS:

• R620: compound viscosity 250,000-400,000 cps at 0.1 s⁻¹
• R274: compound viscosity 500,000-700,000 cps at 0.1 s⁻¹

For LSR injection moulding, lower viscosity = lower clamp force, faster cycle, less wear on machine. The 20-25% material cost premium pays for itself in the first 6 months of production.

2. Better post-cure mechanical properties at same dose

The PDMS surface enables stronger polymer-filler hydrogen bonding (the polymer chain interacts with PDMS-surface-anchored silanol residues rather than the more reactive trimethylsilyl groups). Result: at 20 phr R620, typical 60 Shore A LSR shows:

• Tensile strength: 10-11 MPa (vs 8-9 MPa with R274)
• Elongation: 450-550% (vs 350-450%)
• Tear strength (Die B): 30-35 N/mm (vs 22-28 N/mm)

3. Compatible with addition cure without inhibitor reshuffle

DMDS-treated grades carry trace residual Si-Cl groups that interact with Pt catalyst, requiring careful balance with the hexamethyldisiloxane inhibitor in Part B. R620 has no Si-Cl residue and gives more predictable working-pot life.

The formulation arithmetic

Standard LSR (50 Shore A, injection-mouldable) at 100 phr V-PDMS base:

Part A (V-PDMS + silica + Pt) and Part B (V-PDMS + silica + PMHS + inhibitor) mix 1:1 at injection. Cure at 110-180°C in 30 seconds to 5 minutes (faster at higher T or with less inhibitor). Final hardness adjustable 30-80 Shore A by tweaking vinyl content (more vinyl) and crosslinker ratio (more PMHS).

The "more silica = better mechanical" trap

Below 20 phr, increasing silica monotonically improves tensile, tear, hardness. Above ~25 phr, performance starts to degrade:

• Tensile strength decreases (filler-filler network bears stress instead of polymer)
• Elongation collapses (less polymer chain mobility)
• Working compound becomes too stiff for LSR injection moulding

Optimal LSR window: 18-22 phr R620. Push to 25-28 phr only for high-stiffness applications (durometer 70+) where ultimate elongation is not critical.

For HCR (high-consistency rubber, mill-mixable), the range extends to 30-40 phr because the milling process can break up the filler-filler network and re-establish polymer wetting.

What about RTV-2 (room-temperature addition cure)?

Same principles, lower hardness targets, more attention to working-pot life:

• Base polymer: V-PDMS 100-1000 cSt (lower viscosity for mould-fill)
• Silica: HJSIL R620 at 15-20 phr (lower loading for lower hardness)
• Cure: room temperature, 30 min to 24 hours (depends on Pt concentration and inhibitor)
• Final hardness: 25-50 Shore A (lower than LSR)

The major design constraint: you want long working-pot life (15-60 minutes at 23°C) AND fast cure once the part is in the mould. The trick is using slow-reacting Pt complex (e.g., Pt-CL Karstedt with hexamethyldisiloxane inhibitor) tuned to your specific application.

Procurement notes

MOQ 1 pallet (1.2 t) for HJSIL R620 commercial supply. Lead time 4-6 weeks peak season. For LSR injection moulding manufacturers running 50-200 t/yr, contract supply with 6-month forecasts gets price 8-12% below spot and rolling stock at Shanghai port.

FAQ

Can I use HJSIL R274 instead of R620 to save cost?

In LSR with high-end finish requirements: no. R274 will give higher viscosity (slower cycle, higher clamp force) and slightly inferior post-cure mechanical properties. In HCR (mill-processed) and lower-end RTV-2: yes — R274 saves ~20% material cost with manageable trade-offs.

What's the working-pot life of a typical LSR Part A + Part B mix?

At 23°C: 15-45 minutes depending on inhibitor concentration. At 60°C: 1-3 minutes (you need to inject quickly). At 110°C: <10 seconds (cure is essentially instant — works for fast injection moulding cycles).

Are there platinum-catalyst-free addition-cure systems?

Currently no commercially viable alternative for industrial addition-cure silicone. Platinum is at ~10-50 ppm in finished compound — small but expensive at $50,000/kg+ for Pt. Recent research on Ir, Rh, and Pd alternatives shows promise but not yet at commercial scale.

Related

• HJSIL® Fumed Silica Hub — full 8-grade lineup
• Silicone Oils Hub — V-PDMS, PMHS, and other silicone fluids
• How to Choose Vinyl Silicone Fluid for Silicone Rubber — companion to base-polymer selection
• HJSIL R274 Selection Guide — neighbouring DMDS grade detail