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OH-Terminated PDMS / 107 Silicone Rubber Base (α,ω-Dihydroxy-PDMS, CAS 70131-67-8): The Condensation-Cure Base Polymer for RTV-1 Sealants and RTV-2 Mould Rubber
SEMITECH OH-terminated PDMS — "107 silicone rubber base" — is α,ω-dihydroxy-polydimethylsiloxane, the workhorse base polymer for tin-catalysed RTV-1 sealants and RTV-2 condensation-cure mould rubber. Multiple viscosity grades from 700 cP to 80,000 cP. 200 kg HDPE drum / 1 t IBC.
Contents
| 70131-67-8 | 700–80,000 cP | "107" base |
|---|---|---|
| CAS number | Viscosity range | Industry name |
Chemistry & Specifications
α,ω-dihydroxy-PDMS chain; clear colourless to slightly cloudy liquid; OH content scaled inversely with chain length.
OH-terminated PDMS (CAS 70131-67-8) has structure HO-Si(CH₃)₂-O-[Si(CH₃)₂-O]ₙ-Si(CH₃)₂-OH, with reactive silanol groups on both chain ends. Grades span 700–80,000 cP, MW 4,000–100,000 g/mol, OH content 0.10% (low-MW) to 0.04% (high-MW).
SEMITECH stocks five tiers: 700 cP (OH ~0.10%), 5,000 cP (~0.07%), 20,000 cP (~0.05%), 50,000 cP (~0.045%), 80,000 cP (~0.04%). All at ≥98% active polymer, water ≤500 ppm, APHA ≤30.
Tin-Catalysed Condensation Cure: OH + Alkoxy/Oxime/Acetoxy Crosslinking
OH groups condense with alkoxy/oxime/acetoxy silane crosslinker under tin catalysis, releasing alcohol/oxime/acetic acid byproduct.
- 700 cP grade — RTV-1 sealant base polymer for cartridge-packaged construction sealants
- 5,000 cP grade — RTV-2 cast moulding rubber base, soft to medium hardness
- 20,000 cP grade — RTV-2 mould rubber base, medium hardness; specialty RTV-1 high-modulus
- 50,000–80,000 cP — high-modulus RTV-2 mould rubber, dental and prosthetic silicone
For RTV-1 alkoxy-cure sealants: OH-PDMS 50–60%, MTMS crosslinker 3–5%, hydrophobic fumed silica 6–12%, plasticiser 5–15%, DBTDL catalyst 0.05–0.15%. For RTV-2 mould rubber: OH-PDMS 70–85% with TEOS/ES-40 crosslinker and DBTDL gives 30–60 min pot-life and 12–24 hr cure to Shore A 25–60.
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Applications: RTV-1 Sealants, RTV-2 Mould Rubber, Construction, Dental & Prosthetic
RTV-1 sealants (largest), RTV-2 cast mould rubber, dental impression silicone, prosthetic-orthotic silicone.
RTV-1 condensation-cure sealants are the largest application — construction glazing, sanitary, and industrial bonding. The 700 cP grade is the standard sealant base for nearly all global RTV-1 brands. RTV-2 cast mould rubber for prototyping, decorative casting, and dental impressions uses 5,000–20,000 cP.
Dental impression silicone uses 50,000–80,000 cP grades for putty and light-body. Prosthetic silicone uses 5,000–20,000 cP with T-9 stannous octoate catalyst.
Procurement, Storage and Quality Control
CoA per shipment with OH content verification; 200 kg drum / 1 t IBC; 24-month shelf life; non-flammable; moisture-stable.
MOQ 200 kg per grade. Lead time 1–2 weeks to Asia for stocked grades, 4–6 weeks to Europe/North America. Non-flammable, no nitrogen blanket required. Shelf life 24 months sealed. REACH-registered; FDA 21 CFR 175.300 listed.
OH-Terminated PDMS Specification Sheet
SEMITECH stocked grades (700 / 5,000 / 20,000 / 50,000 / 80,000 cP); CoA per batch with OH-content verification.
| Grade | Viscosity (cP) | OH % (target) | MW (typical) | Primary Application |
|---|---|---|---|---|
| 107-700 | 700 ± 70 | 0.10 ± 0.01% | 5,000 g/mol | RTV-1 sealant base for cartridge-packaged sealants |
| 107-5k | 5,000 ± 500 | 0.07 ± 0.01% | 20,000 g/mol | RTV-2 cast mould rubber soft-medium hardness |
| 107-20k | 20,000 ± 2,000 | 0.05 ± 0.005% | 50,000 g/mol | RTV-2 mould rubber medium hardness; specialty RTV-1 |
| 107-50k | 50,000 ± 5,000 | 0.045 ± 0.005% | 80,000 g/mol | High-modulus RTV-2; dental impression putty |
| 107-80k | 80,000 ± 8,000 | 0.04 ± 0.005% | 100,000 g/mol | High-hardness RTV-2; prosthetic-orthotic |
FAQ
+What is the difference between "107 silicone rubber base" and OH-terminated PDMS?
Same chemistry — "107" is the legacy Chinese industry name from a 1960s Wacker product code for α,ω-dihydroxy-PDMS. SEMITECH uses both names interchangeably in CoA and SDS.
+Which OH-PDMS viscosity grade should I order for RTV-1 alkoxy-cure construction sealant?
700 cP (107-700) is the dominant grade. Lower viscosity enables cartridge dispensing, proper fumed silica thixotropy development, and plasticiser compatibility. The 5,000 cP grade is used only in specialty high-modulus RTV-1 formulations.
+Can I use the same OH-PDMS for both alkoxy-cure and acetoxy-cure RTV-1 sealants?
Yes — the same 700 cP base works for all three cure chemistries (alkoxy, oxime, acetoxy). The differentiation is in the crosslinker and catalyst choice, not the base polymer.