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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 — known industry-wide as “107 silicone rubber base” — is α,ω-dihydroxy-polydimethylsiloxane with reactive silanol groups on both chain ends, the workhorse base polymer for tin-catalysed RTV-1 alkoxy/oxime/acetoxy sealants and RTV-2 condensation-cure mould rubber. Multiple viscosity grades from 700 cP (sealant base) to 80,000 cP (high-modulus mould rubber base). 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 — α,ω-dihydroxy-polydimethylsiloxane, CAS 70131-67-8, also widely known by the legacy industry name “107 silicone rubber base” (reflecting its origin as a Wacker-developed product code) — is a linear polysiloxane with the structure HO-Si(CH₃)₂-O-[Si(CH₃)₂-O]ₙ-Si(CH₃)₂-OH. The hydroxyl groups (silanol, Si-OH) on each chain end provide the reactive site for tin-catalysed condensation cure with alkoxysilane, oxime, or acetoxy crosslinkers. Standard commercial grades span 700 cP to 80,000 cP at 25°C, corresponding to chain lengths n = 50 to n = 1,500 silicon-oxygen repeat units, MW 4,000 to 100,000 g/mol, and OH content 0.10% (low-MW) to 0.04% (high-MW) — OH content scales inversely with chain length.
SEMITECH stocks OH-PDMS in five viscosity 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 grades are supplied at ≥98% active polymer (residual cyclic siloxane and linear oligomer ≤2%), water content ≤500 ppm, and APHA colour ≤30. Manufacturing pathway: OH-terminated PDMS is produced by water-quenched ring-opening polymerisation of D4 (or DMC mixture) — the polymerisation is initiated with KOH or H₂SO₄ catalyst, then quenched with stoichiometric water to introduce the hydroxyl chain ends and neutralise the catalyst. Different viscosity grades come from different polymer:water ratios in the polymerisation reactor.
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.
OH-PDMS forms the reactive base polymer of condensation-cure silicone systems. The OH groups are reactive at room temperature with alkoxysilane, oxime-silane, or acetoxy-silane crosslinkers under DBTDL or DOTL tin-catalysis — the silanol attacks the silicon of the crosslinker, displacing the alkoxy/oxime/acetoxy leaving group and forming a Si-O-Si crosslink. The cure is moisture-driven for RTV-1 sealants (atmospheric water hydrolyses the crosslinker, generating reactive Si-OH that then condenses with polymer Si-OH) and reactant-driven for RTV-2 (the crosslinker is added at stoichiometric ratio to polymer OH at the time of compound preparation).
- 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 at 50–60% of total formulation is mixed with methyltrimethoxysilane (MTMS) crosslinker at 3–5%, hydrophobic fumed silica reinforcing filler at 6–12%, plasticiser at 5–15%, and DBTDL tin catalyst at 0.05–0.15%. The compound is packed in cartridges sealed against atmospheric moisture; on dispensing, atmospheric water hydrolyses the MTMS to silanol, which condenses with the OH-PDMS chain ends under DBTDL catalysis to form a 3D network, releasing methanol byproduct. For RTV-2 condensation-cure mould rubber, OH-PDMS at 70–85% paired with TEOS or ES-40 crosslinker and DBTDL gives 30–60 minute pot-life and 12–24 hour full cure to Shore A 25–60 elastomer.
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 single application — construction sealants for glazing, sanitary, and industrial bonding (alkoxy-cure neutral, oxime-cure, acetoxy-cure variants), cartridge-packaged at 280–310 mL retail format. The 700 cP OH-PDMS grade is the standard sealant base for nearly all global commercial RTV-1 sealant brands; SEMITECH supplies approximately 5–8% of global demand for the 700 cP grade. RTV-2 cast mould rubber for prototype tooling, decorative casting, candle moulding, and dental impression uses 5,000–20,000 cP OH-PDMS depending on target hardness and pot-life requirements.
Dental impression silicone (condensation-cure, A-silicone class) uses 50,000–80,000 cP OH-PDMS as the high-viscosity putty and lower-viscosity light-body base, giving the rheology profile needed for two-stage dental impression workflow. Prosthetic and orthotic silicone (skin-contact body-forming RTV, scar-treatment sheets) uses 5,000–20,000 cP grades with T-9 stannous octoate catalyst (lower-toxicity than DBTDL) at 0.20–0.50%. Specialty applications: high-build coatings on stone and concrete (dewatering and waterproofing); silicone-modified alkyd resins (paint and ink crosslinkers); silicone elastomer extenders for fluoropolymer modification.
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.
SEMITECH issues a CoA on every batch with: OH content (titration with LiAlH₄ or FT-IR, target ±10% of grade specification), viscosity at 25°C (Brookfield, target ±10% of grade specification), volatile content (D4/D5 cyclic siloxane residue, GC, target ≤2%), water content (Karl Fischer, target ≤500 ppm), and APHA colour (target ≤30). Standard packing 200 kg HDPE-lined steel drums; 1 t IBC totes for high-volume RTV-1 sealant manufacturers; 25 t ISO-tank for bulk industrial customers. MOQ 200 kg per grade. Lead time 1–2 weeks ex-Zhejiang to Asia ports for stocked grades, 4–6 weeks to Europe and North America after sea freight.
Storage: stable under ambient conditions; non-flammable (flash point >250°C for higher-MW grades, ~150°C for 700 cP grade); no nitrogen blanket required; tolerates ambient humidity (silanol groups are stable in pure water but condense slowly in concentrated polymer; moisture exposure does not significantly affect OH-PDMS over 24-month sealed storage). Recommended: store sealed below 30°C in original packaging. Shelf life 24 months sealed; opened drums consumable within 12 months without significant OH-content drift. Compatibility note for RTV sealant production: OH-PDMS used in alkoxy-cure RTV-1 must be free of trace organotin (DBTDL contamination from previous batch will accelerate cure) and free of trace acid (from incomplete neutralisation in polymerisation will cause hydrolytic chain scission during cartridge storage). SEMITECH CoA includes pH and tin assay for sealant-grade OH-PDMS to verify these specifications. Health and regulatory: REACH-registered for industrial use; FDA 21 CFR 175.300 listed for indirect food-contact in adhesive components; mild skin and eye irritant; standard PPE — nitrile gloves, splash goggles. Vinyl-PDMS contains residual D4/D5 cyclic siloxane at 1–2% — see DMC and D4/D5 SVHC documentation for downstream regulatory implications.
OH-terminated PDMS / “107” silicone rubber base is the workhorse condensation-cure base polymer — α,ω-dihydroxy-PDMS at 700–80,000 cP grades. Reactive with alkoxy/oxime/acetoxy crosslinker under tin catalysis. Foundation of nearly all global RTV-1 construction sealants and RTV-2 cast mould rubber. 24-month shelf life; ambient handling; no nitrogen blanket needed.
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?
They are the same chemistry — “107” is the legacy industry name (originally a Wacker product code from the 1960s; “107” referred to one of the early commercial OH-PDMS grades that became the de facto industry name in China) for α,ω-dihydroxy-polydimethylsiloxane. International suppliers typically refer to it as “silanol-terminated PDMS” or “α,ω-dihydroxy-PDMS”; Chinese suppliers and downstream RTV sealant manufacturers continue to use “107” or “107胶” as the industry-standard name. SEMITECH uses both names interchangeably in CoA and SDS to ensure clarity for customers. Multiple viscosity grades all share the “107” name with a viscosity-grade modifier (107-700, 107-5k, etc.) — the chemistry is the same, only the chain length differs.
+Which OH-PDMS viscosity grade should I order for RTV-1 alkoxy-cure construction sealant?
700 cP (107-700) is the dominant grade for RTV-1 alkoxy-cure sealants. The lower viscosity is essential for: (1) cartridge dispensing — high-viscosity OH-PDMS would require excessive trigger force on caulking-gun dispensing; (2) compatibility with reinforcing fumed silica thixotrope — the silica builds the formulation viscosity from 5,000 cP (base polymer alone) to 50,000–100,000 cP (finished sealant); starting from 700 cP base allows the silica thixotrope to develop full effect; (3) plasticiser compatibility — most RTV-1 sealant formulations include 5–15% silicone fluid plasticiser to extend the base polymer; the 700 cP grade integrates smoothly. For specialty high-modulus or fast-cure RTV-1 formulations (typically 1–3% of total RTV-1 sealant volume), the 5,000 cP grade may be specified to give higher elastic recovery in the cured film.
+Can I use the same OH-PDMS for both alkoxy-cure and acetoxy-cure RTV-1 sealants?
Yes, the same OH-PDMS base works for all three cure chemistries (alkoxy, oxime, acetoxy). The differentiation is in the crosslinker and catalyst, not the base polymer: (1) alkoxy-cure uses MTMS (methyltrimethoxysilane) crosslinker + DBTDL or DOTL tin catalyst — releases methanol byproduct, neutral cure suitable for sensitive substrates (alkoxy-cure on plastic, painted metal, glass); (2) oxime-cure uses MEKO-silane (methyl tris(methylethylketoximino)silane) crosslinker + DBTDL — releases methylethylketoxime byproduct, neutral cure with stronger amine odour during cure; (3) acetoxy-cure uses methyltriacetoxysilane crosslinker + DBTDL — releases acetic acid byproduct, fast-cure suitable for general industrial bonding but not sensitive substrates (acid corrodes copper, etches marble). All three chemistries use 700 cP OH-PDMS as standard base polymer.