HO-PDMS-OH is produced via a four-step chain: carbothermic reduction of SiO₂ to Si metal, Rochow-process chlorination to dimethyldichlorosilane (M2), hydrolysis/cyclization to D4, and ring-opening polymerization (ROP) of D4 with water as chain-end controller. The water-to-D4 molar ratio at ROP sets OH content and viscosity grade.
Technical Specifications
| Chain Stage | Key Intermediate | Main Cost Driver | Concentration Risk |
|---|---|---|---|
| Reduction | Si metal (98.5% Si) | Electricity (12–15 kWh/kg Si) | Yunnan/Sichuan China |
| Chlorination | Me₂SiCl₂ (M2) | MeCl price, Cu catalyst | China, Germany, USA |
| Hydrolysis/Cyclization | D4 octamethylcyclotetrasiloxane | M2 yield, water purity | China >65% D4 capacity |
| Ring-Opening Polymerization | HO-PDMS-OH (CAS 70131-67-8) | Catalyst, temperature control | Distributed globally |
Upstream Raw Material Map: Silicon Metal to HO-PDMS-OH
HO-PDMS-OH pricing is structurally tied to silicon metal — China controls over 70% of global output from Yunnan/Sichuan smelters. Post-2021 power rationing showed how quickly D4 monomer supply contracts under grid priority shifts. EU carbon border adjustments and anti-dumping tariffs on Chinese organosilicone intermediates add second-order cost pressure. Buyers should model at least ±15% feedstock volatility into raw material cost structures.
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Industrial Applications
RTV-1 and RTV-2 silicone sealants consume 60–65% of global HO-PDMS-OH output, anchored by construction sealing. EV battery pack assembly is a fast-growing segment — thermal interface pads and potting compounds use 5,000–20,000 cs grades for low stress transmission and –50 to +150°C cycling stability. Electronics and LED encapsulants use mid-range 200–1,000 cs grades.
Grade Selection: Viscosity, OH%, and Crosslink Density
Low-viscosity grades (50–200 cs, OH ~4–8%) yield fast-cure, higher-modulus coatings. Mid-range grades (500–2,000 cs, OH ~1.5–2.5%) balance cure speed with flexibility for general-purpose RTV-1 sealants. High-viscosity grades (5,000–20,000 cs, OH ~0.4–0.8%) produce low-modulus, high-elongation elastomers for structural glazing and movement joints. Tin (DBTL) catalyst systems suit most grades; titanium alkoxide catalysts are preferred for food-contact and halogen-free electronics applications.
Frequently Asked Questions
+Q: What does viscosity grade control in a cured RTV silicone product?
A: Viscosity directly sets molecular weight and crosslink density. Lower viscosity (50–500 cs) gives stiffer, faster-cure systems; higher viscosity (5,000–20,000 cs) produces softer, low-modulus elastomers with elongation up to 600%, preferred for movement joints and EV battery potting.
+Q: Can HO-PDMS-OH be cured with both tin and titanium catalysts?
A: Yes. DBTL is the conventional choice for construction sealants. Titanium alkoxide catalysts are specified where halogen-free or food-contact compliance is required — medical device potting, LED encapsulation, and EU food-grade applications.
+Q: What is the CAS number for hydroxyl-terminated PDMS and does it cover all viscosity grades?
A: CAS 70131-67-8 applies across the entire 50–20,000 cs range. The single CAS reflects the same –[Si(CH₃)₂–O]ₙ– backbone with HO– terminal groups; chain length varies by grade. REACH, GB/T, and FDA 21 CFR reference this single CAS regardless of viscosity.
+Q: What OH% should I specify for a 1K moisture-cure RTV-1 window sealant?
A: For standard RTV-1 targeting 100% elongation and Shore A 20–30, the 500–1,000 cs grade (OH ~1.5–2.5 wt%) is industry standard. OH above 3% cures too rapidly and produces brittle film; below 1% cures slowly and may fail in low-humidity environments below 40% RH.
+Q: What internal SEMITECH resources support HO-PDMS-OH sourcing decisions?
A: SEMITECH's silicone oil cluster includes methyl hydrogen silicone fluid, dimethyl silicone fluid, and HO-PDMS-OH — covering 1K moisture-cure, 2K condensation-cure, and addition-cure RTV formulation routes. TDS, lot-specific OH% certificates, and Mn by GPC are available on request.
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