Silicone Oil Specification FAQ

Specification and Technical Questions

Q: What viscosity should I specify — kinematic or dynamic?

Silicone oil is universally specified in kinematic viscosity in centistokes (cSt) at 25 °C. Dynamic viscosity (mPa·s or cP) is rarely used. Since PDMS density varies little across grades (0.760–0.978 g/cm³), kinematic and dynamic viscosity values are numerically close for most grades, but cSt is the industry standard. Most supplier certificates of analysis report viscosity in cSt at 25 °C using ASTM D445 or equivalent method.

Q: What is the difference between technical grade, industrial grade, and food grade?

• Technical/industrial grade: Standard purity PDMS with no specific regulatory compliance. Cyclic siloxane (D4/D5/D6) content may be 0.5–2%. No COA documentation for food or pharmaceutical compliance. Lowest cost.
• Food grade: Meets FDA 21 CFR 172.878 or 178.3570 requirements. Requires cyclic siloxane content below specified limits, minimum purity testing, and documented compliance statement. 20–40% price premium over industrial.
• Pharmaceutical grade (USP Dimethicone): Complies with USP Dimethicone monograph. Requires viscometry test (USP <1121>), limit tests for volatile components, and Certificate of Conformance to USP specification. More stringent than food grade.

Q: What does "viscosity 1000 cSt ± 5%" mean in practice?

A specification of 1000 cSt ± 5% means acceptable range is 950–1050 cSt. For general lubrication, ±10% is typically acceptable. For precision damping applications, ±2% or even ±1% may be required. The tighter the tolerance, the more expensive the product — narrow-specification grades require more precise process control and batch rejection of out-of-specification material.

Q: How do I specify silicone oil for a heat transfer application?

Heat transfer silicone oil specifications should include:

1. Kinematic viscosity at operating temperature (not room temperature)
2. Flash point minimum (recommend >250 °C for safety margin in heated baths)
3. Pour point (if low-temperature startup is required)
4. Thermal stability test results (request TGA data showing decomposition onset above your operating temperature)
5. Oxidative stability (PDMS inherently good, but confirm no additives that could oxidize)

For continuous use above 200 °C, phenyl-modified PDMS or specifically formulated heat transfer grades are preferred over standard dimethyl PDMS.

Q: What is the shelf life of silicone oil and how should it be stored?

Unopened, properly stored dimethyl PDMS has a shelf life of 5+ years. PDMS is chemically inert and does not hydrolyze or polymerize under normal storage conditions. Storage recommendations:

• Keep away from strong acids, bases, and oxidizers (can catalyze chain scission or polymerization)
• Store below 40 °C (no degradation concern, but prevents any softening of drum seals)
• Keep container closed (PDMS is hygroscopic at low viscosities — absorbed water affects electrical properties for insulation applications)
• UV exposure is not a concern for standard PDMS (no UV-sensitive groups)

Functional grades (amino silicone, methyl hydrogen silicone) have shorter shelf lives — typically 12–24 months — due to the reactivity of their functional groups. Follow supplier-specific shelf life recommendations for these grades.

Q: My supplier COA shows viscosity at 20 °C, not 25 °C. How do I convert?

PDMS viscosity decreases approximately 2–3% per °C increase in temperature in the medium viscosity range. A product measuring 1000 cSt at 20 °C will measure approximately 875–940 cSt at 25 °C. For precise conversion, use the supplier's viscosity-temperature data. For specification compliance purposes, confirm with the supplier which temperature the specification refers to — most international standards use 25 °C as reference, but some national standards (especially older German DIN standards) use 20 °C.

Q: What certifications should I require for a food-contact application?

Minimum documentation for FDA-regulated food contact:

1. Certificate of Analysis showing viscosity, specific gravity, refractive index, flash point
2. FDA 21 CFR 172.878 compliance letter (signed by supplier's regulatory affairs contact)
3. Cyclic siloxane (D4/D5/D6) content test result
4. Allergen-free statement (if required by your facility's allergen program)

For EU food contact, additionally require EFSA authorization confirmation (EC No 10/2011 for plastic food contact, or EFSA GRAS opinions for direct food use). Note that EU regulations differ from FDA — confirm with your EU regulatory affairs contact which regulations apply to your application.

Q: What is cyclic siloxane content and why does it matter?

During PDMS manufacturing, some cyclic oligomers (D4 = octamethylcyclotetrasiloxane, D5 = decamethylcyclopentasiloxane, D6 = dodecamethylcyclohexasiloxane) remain in the product. In industrial grades, D4+D5+D6 content may be 0.5–2%. In food and cosmetic grades, these are purified out because:

• EU REACH restriction: D4 and D5 are restricted in rinse-off cosmetic products at >0.1% (EC 2020/1215). Products must be certified compliant.
• Bioaccumulation concern: D5 has been identified as persistent, bioaccumulative, and toxic (PBT) in aquatic environments under EU assessment.
• FDA purity: 21 CFR 172.878 specifies purity and composition requirements that effectively limit cyclic content.

For cosmetics and food contact: request D4+D5+D6 content on the COA.

Q: Can I blend two viscosity grades to get a custom viscosity?

Yes. PDMS viscosity blending follows an approximately log-linear relationship:

log(blend viscosity) ≈ w₁ × log(V₁) + w₂ × log(V₂)

where w₁ and w₂ are weight fractions and V₁, V₂ are viscosities. For example, to achieve approximately 500 cSt by blending 100 cSt and 1000 cSt:

From the equation: weight fraction of 100 cSt ≈ 0.5, weight fraction of 1000 cSt ≈ 0.5 gives approximately 316 cSt. Adjust ratios iteratively to reach target. In practice, request blend ratio tables from your silicone oil supplier — they typically have empirical blending data more accurate than the theoretical calculation.

Q: How do I verify silicone oil identity — that I received what I ordered?

Standard identity verification methods:

1. Viscosity measurement (ASTM D445): Fastest, cheapest, confirms grade
2. FTIR spectroscopy: Confirms Si-O-Si fingerprint spectrum, identifies type (dimethyl vs. phenyl vs. amino by characteristic peaks)
3. Specific gravity (ASTM D1298): Simple, confirms grade range
4. Refractive index (ASTM D1218): Quick, sensitive to silicone type (phenyl grades have higher RI ~1.47 vs. dimethyl ~1.40)

For high-value pharmaceutical applications, full USP identity testing per the Dimethicone monograph is required, including IR spectrum comparison against reference standard.