Silicone properties
General
Section titled “General”Silicone rubber (polydimethylsiloxane, VMQ) is a synthetic elastomer that combines a wide thermal stability (−50 to 200 °C continuously, peaks of 300 °C), excellent chemical resistance (especially to acids, alkalis and water) and notable dielectric properties. It is marketed in RTV (room temperature vulcanization), HTV (hot vulcanization) and LSR (liquid silicone rubber) forms and, once cured, maintains its flexibility over a temperature range unusually wide for an elastomer.
Chemical composition
Section titled “Chemical composition”Silicone rubber consists of a main chain of silicon–oxygen (Si–O) bonds, with two organic groups (usually methyl, –CH₃) attached to each silicon atom. Its polysiloxane structure may include fillers such as fumed silica to adjust the viscosity in the uncured state and the final mechanical properties. Curing is carried out by addition (platinum), condensation (acetoxy, alkoxy or oxime) or peroxide systems, producing a three-dimensional elastomeric network without by-products in the case of addition curing.
Mechanical properties in uncured state
Section titled “Mechanical properties in uncured state”Before curing, silicone is a viscoelastic fluid whose rheological properties depend on molecular weight and filler content. Typical dynamic viscosities range from 10,000 to 1,000,000 mPa·s (100–10,000 P) for RTV formulations; LSR versions present lower values, around 50,000 mPa·s, allowing injection molding. The density in the liquid state is similar to that of the cured product (1.1–1.3 g/cm³).
Mechanical properties in cured state
Section titled “Mechanical properties in cured state”Once crosslinked, silicone rubber offers the following representative mechanical values (data integrated from AZoM and supplemented with usual hardness ranges):
| Property | Minimum value (SI) | Maximum value (SI) | Unit (SI) | Minimum value (Imperial) | Maximum value (Imperial) | Unit (Imperial) |
|---|---|---|---|---|---|---|
| Tensile strength | 2,4 | 5,5 | MPa | 348 | 798 | psi |
| Young modulus | 0,001 | 0,05 | GPa | 0,145 | 7,25 | 10³ psi |
| Elongation at break | 80 | 530 | % | 80 | 530 | % |
| Shear modulus | 0,0003 | 0,02 | GPa | 0,0435 | 2,90 | 10³ psi |
| Compressive strength | 10 | 30 | MPa | 1,45 | 4,35 | ksi |
| Yield strength | 2,4 | 5,5 | MPa | 348 | 798 | psi |
| Fatigue limit | 2,28 | 5,23 | MPa | 331 | 759 | psi |
| Fracture toughness | 0,03 | 0,7 | MPa·m½ | 0,027 | 0,637 | ksi·in½ |
| Hardness (Shore A) | 10 | 60 | – | 10 | 60 | – |
| Poisson’s coefficient | 0,47 | 0,49 | – | 0,47 | 0,49 | – |
| Modulus of rupture | 2,4 | 5,5 | MPa | 348 | 798 | psi |
| Loss coefficient | 0,4 | 1,6 | – | 0,4 | 1,6 | – |
Note: The mechanical values correspond to general-purpose filled silicones; formulations with high specific surface silica reinforcement can achieve tensile strengths up to 10 MPa and elongations above 700 %.
Physical properties
Section titled “Physical properties”| Property | Minimum value (SI) | Maximum value (SI) | Unit (SI) | Minimum value (Imperial) | Maximum value (Imperial) | Unit (Imperial) |
|---|---|---|---|---|---|---|
| Density | 1,1 | 2,3 | Mg/m³ (g/cm³) | 68,7 | 143,6 | lb/ft³ |
| Bulk modulus | 1,5 | 2 | GPa | 0,218 | 0,290 | 10⁶ psi |
| Average atomic volume | 0,0035 | 0,0055 | m³/kmol | 213,6 | 335,6 | in³/kmol |
| Energy content | 200 | 300 | MJ/kg | 21 668 | 32 502 | kcal/lb |
Thermal properties
Section titled “Thermal properties”| Property | Minimum value (SI) | Maximum value (SI) | Unit (SI) | Minimum value (Imperial) | Maximum value (Imperial) | Unit (Imperial) |
|---|---|---|---|---|---|---|
| Glass transition temperature (Tg) | 150 | 200 | K | −189,7 | −99,7 | °F |
| Minimum service temperature | 200 | 225 | K | −99,7 | −54,7 | °F |
| Maximum service temperature (continuous) | 500 | 560 | K | 440,3 | 548,3 | °F |
| Specific heat | 1 050 | 1 300 | J/(kg·K) | 0,813 | 1,006 | BTU/(lb·°F) |
| Thermal conductivity | 0,2 | 2,55 | W/(m·K) | 0,374 | 4,77 | BTU·ft/(h·ft²·°F) |
| Coefficient of thermal expansion | 250 | 300 | 10⁻⁶ /K | 450 | 540 | 10⁻⁶ /°F |
Note: The temperature values correspond to the AZoM database; in industrial practice, general-purpose silicone rubber works continuously between −50 °C and +200 °C, with peaks of 300 °C in intermittent service. There is no defined melting point; above 300 °C the material degrades without melting.
Electrical properties
Section titled “Electrical properties”| Property | Minimum value (SI) | Maximum value (SI) | Unit (SI) | Minimum value (Imperial) | Maximum value (Imperial) | Unit (Imperial) |
|---|---|---|---|---|---|---|
| Dielectric strength | 11 | 28 | kV/mm | 279 | 711 | V/mil |
| Dielectric constant | 2,9 | 4 | – | 2,9 | 4 | – |
| Volume electrical resistivity | 3.16 × 10¹¹ | 1 × 10¹⁴ | Ω·m | 3.16 × 10¹¹ | 1 × 10¹⁴ | Ω·m |
Resistance to external agents
Section titled “Resistance to external agents”The resistance of silicone rubber to different media is classified on a scale from 1 (poor) to 5 (excellent).
| Agent | Resistance (1‑5) |
|---|---|
| Flammability | 4 |
| Freshwater | 5 |
| Organic solvents | 4 |
| Oxidation at 500 °C | 1 |
| Seawater | 5 |
| Strong acids | 5 |
| Strong alkalis | 5 |
| UV radiation | 4 |
| Wear | 4 |
| Weak acids | 5 |
| Weak alkalis | 5 |
It therefore presents outstanding resistance to acids, bases and water, both fresh and salt, and good resistance to organic solvents. Prolonged exposure to temperatures close to 500 °C severely degrades the material.
Applications by industry
Section titled “Applications by industry”- Automotive: radiator gaskets, connector seals, intercooler hoses, sensors.
- Electronics / Electrical: high-voltage insulators, flexible keypads, semiconductor encapsulations, flame-retardant cables.
- Medical: long-term implants, catheters, peristaltic tubes, pump diaphragms, sterilizable seals.
- Food: pastry molds, oven trays, kitchen utensils, gaskets for food machinery.
- Construction: joint sealants (RTV acetoxy or neutral), weatherstripping profiles, glazing putties.
- Aerospace: seals and insulators that withstand extreme thermal cycles, flexible couplings.
Comparison with similar materials
Section titled “Comparison with similar materials”The following table compares silicone (VMQ) with other common technical elastomers.
| Elastomer | Continuous range | Oil resistance | Mechanical resistance | Relative cost |
|---|---|---|---|---|
| VMQ (silicone) | −50 to 200 °C | Moderate (4/5 to solvents) | Low‑medium (2,4‑5,5 MPa) | Medium |
| NBR (nitrile) | −30 to 100 °C | Excellent | High | Low |
| EPDM | −40 to 150 °C | Poor (not oils) | Medium | Low |
| FKM (fluorocarbon) | −20 to 200 °C | Excellent | Medium | High |
| HNBR | −30 to 140 °C | Very good | High | Medium |
Silicone offers the widest operating temperature range and unmatched low-temperature flexibility, at the cost of lower tensile strength and wear resistance.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the working temperature range of silicone rubber?
Section titled “What is the working temperature range of silicone rubber?”Silicone withstands continuously from −50 °C to 200 °C and reaches peaks of 300 °C in intermittent service.
What chemical resistance does it have?
Section titled “What chemical resistance does it have?”It resists strong acids and bases very well (rating 5/5) and organic solvents (4/5), but oxidizes rapidly at 500 °C (rating 1/5).
What is its typical density?
Section titled “What is its typical density?”Density varies between 1.1 and 2.3 g/cm³ (68.7‑143.6 lb/ft³) depending on the amount and type of filler.
How much does it elongate before breaking?
Section titled “How much does it elongate before breaking?”It presents an elongation at break of 80 % to 530 %; softer formulations can exceed 700 %.
Is it a good electrical insulator?
Section titled “Is it a good electrical insulator?”Yes, it has a dielectric strength of 11 to 28 kV/mm and a volume resistivity on the order of 10¹¹‑10¹⁴ Ω·m.
Can it be used in contact with food?
Section titled “Can it be used in contact with food?”Food-grade silicones (conforming to FDA and EU regulations) are approved for cooking, baking and storing food.
References
Section titled “References”- azom.com: https://www.azom.com/properties.aspx?ArticleID=920
- engineeringtoolbox.com: https://www.engineeringtoolbox.com/elastomers-rubbers-operating-temperatures--d_2242.html
- steelnumber.com: https://www.steelnumber.com/en/steel_composition_eu.php?name_id=427