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Silicone Transformer Oil (STO-50) — 50 cSt PDMS Dielectric Fluid

CAS 63148-62-9

A 50 cSt polydimethylsiloxane dielectric fluid for power transformers and thermal-management systems, offering high fire point and stable non-conductivity.

Part of Silicone Fluids & Oils · Silicone & Polyethylene Softeners · Process Defoamers & Wetting Agents · Silicone Antifoams / Defoamers · Silicone Processing Aids · Silicone Fluids & Additives · Crop Protection Adjuvants · Agricultural Manufacturing Process Aids
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CAS Number
63148-62-9
Material Family
Silicone Fluids & Oils
At a Glance
Material Family
Silicone Fluids & Oils
Primary Role
Lubrication · Defoaming / Foam Control
Functional Roles
Lubrication Defoaming / Foam Control Slip / Release
Physical Properties
Appearance
Crystal-clear liquid
Density (25°C)
0.96 g/cm³
Viscosity (25°C)
50 cSt
Water Content
< 50 ppm (low-moisture)
Specific Heat
1.51 kJ/kg·K
Thermal Conductivity
0.151 W/m·K
Refractive Index (25°C)
1.404
Breakdown Voltage
50 kV
Permittivity (25°C)
2.5
Dissipation Factor (25°C)
0.0001
Volume Resistivity (25°C)
1.0 × 10¹⁴ Ω·cm
Flash Point (open cup)
300°C
Fire Point (open cup)
> 340°C
Pour Point
< -50°C
Neutralization Value
< 0.01 mg KOH/g
Dielectric Dissipation Factor
< 0.001
DC Resistivity
> 100 GΩ·m
Standards
IEC 60836 (K-class) / ASTM D 4652 / IEEE C57.111
Specification Basis
Typical values per RS-STO 50 TDS; lot CoA governs
Documents

Values shown are typical for the current commercial grade. A lot-specific Certificate of Analysis accompanies every shipment; the grade-specific SDS is available on request.

Safety & Handling
Full SDS available on request

A grade-specific Safety Data Sheet (SDS) — with the complete hazard classification, handling precautions, and transport information — is supplied with every shipment and available on request. Confirm all safety and regulatory details against the SDS for your specific grade.

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Chemical Identity
CAS Number
63148-62-9
Synonyms & Trade Names
STO-50 RawSil HT-50 50 STO Silicone Transformer Oil Silicone Heat Transfer Fluid PDMS Dielectric Fluid Polydimethylsiloxane 50 cSt Silicone Dielectric Coolant
Full Description

Silicone Transformer Oil (STO-50), product code RS-STO 50 (formerly supplied as RawSil HT-50), is a 50 cSt polydimethylsiloxane (PDMS) fluid engineered as a low-moisture dielectric and heat-transfer fluid for electrical and thermal-management service. Its wide liquid range, high fire point, and stable dielectric properties make it a less-flammable alternative to mineral oil in transformers and a clean heat-transfer medium where thermal stability and non-conductivity matter.

50 cSt PDMS
Fire point >340°C
Breakdown 50 kV
Less-flammable (NEC 450.23)

Applications

  • ▸Power & distribution transformers — dry-type and liquid-filled; a less-flammable dielectric coolant meeting IEC 60836 and ASTM D 4652-92, with acceptance per IEEE C57.111.
  • ▸Silicone heat-transfer fluid — thermal-management and process heat-transfer service over a wide temperature range, with low volatility and oxidative stability.
  • ▸Electronics & data-center thermal management — dielectric, non-conductive fluid for cooling sensitive equipment.

STO-50 vs. PMX-561 — Specification Comparison

STO-50 vs. PMX-561
50 cSt PDMS dielectric & heat-transfer fluid · specification equivalence

Drop-in alternative

Parameter
STO-50

RawSource
PMX-561

Dow

 Match
Chemical composition PDMS PDMS ✓
Viscosity @ 25°C 50 cSt 50 cSt ✓
Density @ 25°C 0.96 g/cm³ 0.96 g/cm³ ✓
Specific heat 1.51 kJ/kg·K 1.51 kJ/kg·K ✓
Thermal conductivity 0.151 W/m·K 0.151 W/m·K ✓
Breakdown voltage 50 kV 50 kV ✓
Volume resistivity @ 25°C 1.0 × 1014 Ω·cm 1.0 × 1014 Ω·cm ✓
Flash point (open cup) > 285°C > 300°C ≈
Fire point (open cup) >340°C 370°C ✓
Standards (IEC 60836 / ASTM D 4652) Meets Meets ✓
✓
9 of 10 parameters equivalent
— a direct drop-in for PMX-561 in transformer dielectric & heat-transfer service.

✓ equivalent value · ≈ closely comparable. PMX-561 is a trademark of Dow Inc.; RawSource is not affiliated with, endorsed by, or sponsored by Dow Inc. Figures are from publicly available technical data sheets and are subject to change; STO-50 values are typical and not a guaranteed specification. Customers should validate suitability for their specific application.

CAS 63148-62-9 (polydimethylsiloxane) · conforms to IEC 60836, ASTM D 4652 and IEEE C57.111 · qualifies as less-flammable under NEC 450.23. Supplied in North America with reduced lead times — specifications, samples, and certificates of analysis available on request.

Regulatory & registration requirements

  • TSCA (US):
  • REACH (EU): Not determined from public registry
  • EC number: 687-578-3

TSCA Inventory flag XU: a substance exempt from reporting under the Chemical Data Reporting Rule (40 CFR 711) (EPA flag legend).

Source: EPA TSCA Inventory (July 2025 release) · ECHA CHEM — retrieved 2026-07-12

Disclaimer. Information on this page — including properties, identifiers, hazard, transport (DOT/UN) and tariff (HS) classifications, and applications — is provided for general reference and is compiled from authoritative public sources (e.g. PubChem/ECHA, 49 CFR 172.101, the Harmonized Tariff Schedule). Values are typical and are not a guaranteed specification; the Certificate of Analysis (CoA) for the lot purchased governs. Products are sold for industrial and professional use only. Nothing here is a medical, health, or efficacy claim or advice. Always consult the current Safety Data Sheet (SDS) before handling, storage, transport or disposal, and confirm regulatory status, classification and suitability for your application and jurisdiction. Hazard, transport and tariff classifications must be verified for your specific shipment. RawSource makes no warranty, express or implied, and assumes no liability for use of this information. Trademarks. Third-party trademarks and brand names are the property of their respective owners; any reference is nominative — used only to identify a comparable product — and does not imply affiliation with, sponsorship by, or endorsement by the trademark owner.
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