The procurement question
You're formulating an addition-cure silicone rubber — liquid silicone rubber (LSR) for injection moulding, room-temperature platinum-cure RTV-2 silicone, or high-consistency rubber (HCR) addition-cure. The base polymer is vinyl-terminated polydimethylsiloxane (V-PDMS). Vendors offer multiple combinations of viscosity (100 cSt to 100,000 cSt) and vinyl content (0.05 to 0.5 mmol/g). Which one for your product?
The answer depends on what you're optimising for: final hardness (vinyl content drives this), compound flow (viscosity drives this), or cycle time (both interact).
The vinyl content × viscosity matrix
| Grade | Viscosity (cSt) | Vinyl content (mmol/g) | Best for |
|---|---|---|---|
| EVT-100 | 100 | 0.40-0.50 | High-vinyl crosslinker base (HCR thin film) |
| EVT-1000 | 1,000 | 0.13-0.18 | Standard LSR (40-60 Shore A) |
| EVT-10000 | 10,000 | 0.05-0.10 | High-viscosity LSR, encapsulant |
| EVT-100000 | 100,000 | 0.04-0.08 | HCR mill-mixable base |
The pattern: as viscosity increases, vinyl content decreases. Vinyl groups are at the chain ends; longer chains = fewer ends per gram.
Why vinyl content matters
In addition-cure silicone rubber, the cure is hydrosilylation:
V-PDMS-V + H-Si(CH₃)-O-PDMS (PMHS) --(Pt cat)--> Crosslinked network
The vinyl groups (–CH=CH₂) react with Si-H groups in the methyl hydrogen silicone fluid (PMHS) crosslinker, catalysed by platinum. The vinyl content per gram determines:
- Crosslink density — more vinyl per gram = more crosslinks = harder rubber, less elongation
- Pt-catalysed cure speed — more vinyl = faster reaction completion at fixed Pt loading
- Stoichiometric ratio with PMHS — Si-H to vinyl ratio typically 1.2:1 to 1.5:1 (slight excess Si-H for full vinyl consumption)
For a typical LSR base polymer at 1000 cSt + 0.15 mmol/g vinyl:
- Final hardness: 40-60 Shore A (controlled by exact vinyl content + PMHS loading)
- Tensile strength: 7-10 MPa
- Elongation: 400-600%
- Tear strength: 25-35 N/mm
Grade selection by application
EVT-1000 (1000 cSt, 0.15 mmol/g) — standard LSR
The 80% of LSR market. Hardness 40-60 Shore A, injection-mouldable, baby-care + medical + automotive interior + general-purpose LSR.
Formulation: EVT-1000 at 90 phr (90% of base). HJSIL R620 (PDMS-treated fumed silica) at 18-22 phr. PMHS crosslinker at 2 phr (Si-H : vinyl ratio 1.3:1). Pt catalyst 5-10 ppm. Hexamethyldisiloxane inhibitor 0.1-0.3% in Part B.
EVT-100 (100 cSt, 0.45 mmol/g) — high-crosslink-density compound
Used as crosslinker booster in formulations where Si-H side groups alone don't provide enough crosslinks. The 100 cSt fluid disperses uniformly into the higher-viscosity base polymer, raising the effective vinyl count without dramatically increasing viscosity.
Typical loading: 5-15% of total vinyl-functional silicone in the formulation. Boost vinyl content from 0.15 mmol/g (EVT-1000) to 0.18-0.20 mmol/g effective. Gives 5-15 Shore A hardness uplift.
EVT-10000 (10,000 cSt) — encapsulant / mould-fill at higher viscosity
Premium LSR for encapsulation (LED, electronic potting) where the higher viscosity prevents flow into unintended cavities. Also used in addition-cure RTV-2 for two-part moulds.
Hardness window: 40-50 Shore A typical (limited by lower vinyl content).
EVT-100000 (100,000 cSt) — HCR addition-cure
Mill-mixable. Used as the base for HCR compounds processed on two-roll mill with Pt catalyst, PMHS, and reinforcing silica. The very long chain length provides natural processability without sacrificing crosslink density.
Hardness window: 50-80 Shore A (with appropriate silica reinforcement at 30-40 phr).
Substituting from Gelest DMS-V, Wacker V, or Dow XIAMETER PMX-200V
| Existing grade | EVT equivalent | Match basis |
|---|---|---|
| Gelest DMS-V31 (vinyl-PDMS, 1000 cSt) | EVT-1000 | Direct vinyl + viscosity match |
| Gelest DMS-V35 (vinyl-PDMS, 5000 cSt) | EVT-10000 (closest) | Slightly different viscosity |
| Wacker V200 (RTV-2 base, 20,000 cSt) | EVT-10000 to EVT-100000 (intermediate) | Verify in trial |
| Dow XIAMETER PMX-200V (LSR base) | EVT-1000 | Match by hardness target |
Independent testing shows EVT-1000 matches Gelest DMS-V31 within 5% on viscosity, vinyl content, and cured-rubber mechanical properties. Direct 1:1 substitution typically requires no formulation adjustment.
Procurement notes
MOQ 200 L (one drum) per grade. Lead time 4-6 weeks FOB Shanghai. Each batch ships with CoA covering kinematic viscosity, vinyl content (titration or NMR), and trace contaminants.
FAQ
Why does the hardness Shore A change so much with vinyl content?
Crosslink density. Each crosslink point restricts chain mobility. More vinyl per gram → more crosslinks → less elongation → higher Shore hardness. A 50% change in vinyl content (0.10 vs 0.15 mmol/g) can shift Shore hardness from 35A to 60A in otherwise identical formulation.
Is vinyl content reproducible from batch to batch?
±5% on the specified value. East Materials runs NMR confirmation on every batch. The actual rubber-property variance from batch-to-batch is typically less than ±2 Shore A points due to compensating factors (Pt catalyst lot, PMHS lot variation).
What's the difference between vinyl-terminated and vinyl-grafted silicone?
Vinyl-terminated: vinyl groups are at the polymer chain ends only. Standard for crosslinker reaction.
Vinyl-grafted: vinyl groups are pendant along the polymer chain (not just terminal). Less common; gives higher crosslink density but slower reaction kinetics. East Materials EVT series is vinyl-terminated; specialty vinyl-grafted available on request.
Related
- Silicone Oils Hub — full silicone fluid families
- Fumed Silica in RTV-2 and LSR Silicone Rubber — silica filler companion
- Hydroxy-Terminated Silicone Oil Selection — condensation-cure alternative
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