The procurement question
You're formulating an RTV-1 silicone sealant or RTV-2 silicone mould. The base polymer is hydroxy-terminated polydimethylsiloxane (HO-PDMS), also called silanol-terminated PDMS. Different vendors offer different OH contents and viscosity grades. Which one for your product?
The OH content + viscosity combination directly controls:
- Sealant rheology (gun-cartridge squeeze, vertical-bead slump)
- Cure rate (how fast skin formation happens)
- Final elastomer hardness, elongation, and tear strength
- Shelf life (more OH = faster cure = shorter shelf life if catalyst is present)
The three OH content × viscosity combinations
East Materials offers three commercial viscosity ranges with engineered OH content for each:
| Grade | Viscosity (cSt) | OH content (%) | Typical chain length | Best for |
|---|---|---|---|---|
| EHT-750 | 750 ± 100 | 4.0-6.0 | Short (Mn ~10,000) | Low-viscosity RTV-2 moulds |
| EHT-20000 | 20,000 ± 2,000 | 0.05-0.15 | Medium (Mn ~70,000) | Standard RTV-1 sealants |
| EHT-80000 | 80,000 ± 5,000 | 0.04-0.08 | Long (Mn ~150,000) | HCR-grade, mill-processable |
The pattern: as viscosity increases, OH content decreases. This is because the OH groups are terminal — they exist only at the polymer chain ends. Longer chains = fewer ends per gram = lower OH content.
Why OH content matters
In RTV-1 acetoxy / alkoxy silicone sealant, the cure reaction is:
HO–PDMS–OH + (CH₃COO)₃–Si–CH₃ → PDMS–O–Si(OOCCH₃)₂–CH₃ + CH₃COOH
The OH groups are the reactive endpoints. Each OH reacts with one functional group on the crosslinker, building a 3D silicone network. The OH content determines:
- Crosslink density — more OH per gram = denser network = harder, less elongation
- Cure rate — more OH per gram = more reactive sites = faster skin formation
- Shelf life — more OH = more catalyst hydrolysis pathway = shorter cartridge life
For a typical 100-percent-silicone sealant with 20,000 cSt EHT-20000 (0.10% OH content), the cured rubber has:
- Tensile strength: 1-2 MPa
- Elongation: 400-600%
- Hardness: 30-40 Shore A
- Skin time: 10-30 minutes
- Full cure: 24 hours at 23°C/50% RH
Grade selection by application
EHT-750 (low viscosity, high OH)
Use for: room-temperature vulcanizing RTV-2 silicone moulds for resin / polyurethane casting. The low viscosity allows easy mould-fill of complex geometries; the high OH content gives fast cure (working pot life 30 min, cured in 4-8 hours at 23°C).
Formulation: EHT-750 at 70-80% in Part A. Tin or platinum catalyst at 0.1-0.5%. For tin-cure (acetoxy or alkoxy): tin octoate or DBTDL. For platinum-cure: Karstedt's catalyst with vinyl-functional crosslinker.
Avoid for: cartridge-applied sealants. The low viscosity will not hold a bead shape on vertical surfaces.
EHT-20000 (medium viscosity, low OH)
Use for: standard RTV-1 cartridge-applied silicone sealant. The viscosity is engineered for cartridge gun squeeze — gun-friendly without sag.
Formulation: EHT-20000 at 50-60% of total. Hydrophobic fumed silica (HJSIL R272 or R110) at 8-12 phr. Crosslinker (methyltriacetoxysilane MTOS for acetoxy cure; vinyltrimethoxysilane VTMS for alkoxy cure) at 4-7%. Plasticiser (non-reactive PDMS 350 cSt) at 5-15%. Catalyst (DBTDL for acetoxy; titanate for alkoxy) at 0.1-0.5%.
Result: Industry-standard RTV-1 sealant with 12-month shelf life in moisture-tight cartridge, gun-friendly extrusion, vertical-bead application, 30 min skin time, 24-hour cure depth 2 mm.
EHT-80000 (high viscosity, lowest OH)
Use for: high-consistency silicone rubber (HCR) compound base. Mill-processable — meaning it can be compounded on a two-roll mill with fillers + crosslinker, then extruded or moulded. The very low OH content gives long shelf life and slow ambient cure (good for mill processing windows).
Formulation: EHT-80000 at 70-80%. Fumed silica (hydrophobic R-grade) at 20-35 phr. Peroxide crosslinker (dicumyl peroxide DCP) for thermal cure at 0.4-1.0%.
Result: HCR rubber compound with full mill-mixing capability, 10+ minute open time, fast vulcanization at 170-180°C for 5-10 minutes.
Substituting from Dow Corning Silres or Wacker E series
| Existing supplier grade | EHT equivalent | OH content match |
|---|---|---|
| Dow Corning Silres 7607 (RTV-2 base) | EHT-750 | 4-6% OH match |
| Wacker E series (RTV-1 base, 20,000 cSt) | EHT-20000 | 0.10% OH match |
| Genesee Polymers GP-1 (high-viscosity RTV) | EHT-80000 | 0.05% OH match |
Direct 1:1 substitution in the formulation. Run one validation batch to confirm cure rate and shelf-life are within your specification tolerance. Common formulation adjustments after substitution: catalyst dose may need ±10% tuning to match cure speed.
Compatible silica filler — quick reference
The HO-PDMS + filler combination is critical. The filler must be:
- Hydrophobic (DMDS, HMDS, or PDMS-treated) — water on the filler hydrolyses the catalyst
- Compatible with the crosslinker chemistry — for acetoxy cure, any hydrophobic fumed silica works; for platinum-cure, prefer PDMS-treated R620
See HJSIL® Fumed Silica Hub for the full grade lineup and selection guide.
Procurement notes
MOQ: 200 L (one drum) per grade. Lead time 4-6 weeks FOB Shanghai. Each batch ships with CoA covering kinematic viscosity, OH content (titration), water content (Karl Fischer), and trace metals (Pb, As, Cd, Hg) for pharma-grade variants.
FAQ
Can I substitute EHT-750 with EHT-20000 in my RTV-2 mould formulation?
No, viscosity is too different. EHT-20000 at 20,000 cSt will not fill complex mould geometries; you'll get bubbles and surface defects. Use EHT-750 for RTV-2 moulds.
What's the shelf life of HO-PDMS itself (before formulation)?
18 months at 25°C in original sealed drum. Storage above 30°C accelerates dimer formation that reduces OH availability. The actual sealant shelf life (after formulation) depends on the catalyst + crosslinker + moisture exclusion — typically 12 months in moisture-tight cartridge.
How do I measure OH content?
ASTM D5901 acetic anhydride method, or proton NMR. For incoming-material QC, the acetic anhydride titration method gives ±5% accuracy and is the standard procurement spec method. Each East Materials batch ships with the titration result on CoA.
Why does my sealant get harder if stored too long?
The OH groups slowly self-condense in storage, forming Si-O-Si dimers that reduce the available crosslink sites. Over 12+ months, this can shift the final cured hardness by 3-5 Shore A points. To minimize, store cartridges in cool dry conditions (15-20°C ideal) and use FIFO inventory rotation.
Related
- Silicone Oils Hub — full silicone fluid families
- Fumed Silica in RTV-2 and LSR Silicone Rubber — silica filler companion
- HJSIL® Fumed Silica Hub — hydrophobic grade selection
- chemzip.com Silicone Curing Catalysts — DBTDL, tin octoate, titanate catalysts (external)
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