Industrial silicone falls into twelve formulation families. All share the VMQ base, but filler, cure system and additives change, and with them the service temperature, the tear strength and the certifications each one can hold. The family is chosen by the most restrictive requirement of the application: mandatory standards first, real service temperature next, and cost only at the end.
| Family | Chosen when | Temperature |
|---|---|---|
| General purpose | No special requirement beyond sealing | −60/+200 °C |
| High tear | The part flexes or stretches repeatedly | −60/+200 °C |
| Food grade | There is food contact (EC 1935/2004, FDA) | −60/+200 °C |
| Medical grade | Patient or fluid contact (USP VI, ISO 10993) | −60/+200 °C |
| Rail | Rolling stock under EN 45545-2 | −60/+200 °C |
| High temperature | Continuous service above 200 °C | −60/+300 °C |
| Low temperature (PVMQ) | Cryogenics, LNG, extreme cold | −110/+200 °C |
| Fluorosilicone (FVMQ) | Fuels, mineral oils, solvents | −60/+170 °C |
| Steam resistant | Autoclave and sterilisation cycles | −60/+200 °C |
| Low compression set | Static sealing that is never re-tightened | −60/+200 °C |
| Electrically conductive | EMI shielding or electrostatic discharge | −50/+210 °C |
| Cellular (sponge) | Available closing force is very low | −60/+200 °C |
Where the family is already settled, these are the pages: general purpose, high tear, food grade, medical grade, rail EN 45545-2, high temperature, low compression set, electrically conductive and cellular. The full catalogue is under silicone products.
The order in which the decision is made
These five steps run in this order because each one rules out whole families. Reversing them leads to over-specification: starting from hardness ends in medical grade silicone on a cabinet gasket.
- 1. Real service temperature. Above +200 °C continuous → high temperature. Below −60 °C → PVMQ. Between the two, carry on.
- 2. Mandatory standards. Food contact → food grade, stating FDA, BfR or EC 1935/2004 by market. Medical device → medical grade, stating USP VI, ISO 10993 or implantable. Rolling stock → EN 45545-2, stating the hazard level.
- 3. Chemical environment. Hydrocarbons, fuels or solvents → fluorosilicone. Repeated autoclave cycles → steam resistant.
- 4. Mechanical demand. Critical tear → high tear. Static sealing over years → low compression set. Low closing force → cellular. Electrical continuity → electrically conductive.
- 5. Availability. Confirm the formulation exists in the hardness required, suits the process and keeps its certifications in the specified colour.
Cure system: peroxide or platinum
The cure system governs which certifications the part can hold and what it costs. Peroxide decomposes under heat and crosslinks the chains through free radicals: it covers 10 to 90 Shore A, suits extrusion, compression and injection, and is the cost-optimised route. It leaves volatile by-products, so applications sensitive to odour or taste may call for post-curing.
Platinum cures by addition and leaves no by-products: odourless and tasteless, higher tear strength, better clarity, and mandatory for demanding medical and high-purity food applications. In exchange it costs more, it is sensitive to contaminants — sulphur, amines, tin, certain metals — and it demands tighter process control.
The twelve families and their figures
The table holds the parameters that settle a specification. Hardness is tested to ISO 7619-1 and tear strength to ISO 34-1, with an industrial tolerance of ±5 Shore A per batch.
| Family | Series | Hardness (Shore A) | Temperature | Tear | Certifications |
|---|---|---|---|---|---|
| General purpose | S02 | 10–90 | −60/+200 °C | 10–23 kN/m | FDA, BfR, EC 1935/2004 |
| High tear (peroxide) | S07 | 60–70 | −60/+230 °C | 30–35 kN/m | FDA, BfR, EC 1935/2004 |
| High tear (platinum) | — | 40–80 | −60/+200 °C | 33–55 kN/m | FDA, EC 1935, USP VI, ISO 10993 |
| Food grade | S02, S15 | 10–90 | −60/+200 °C | 10–30 kN/m | FDA 21 CFR 177.2600, BfR XV, EC 1935/2004 |
| Medical grade | — | 25–80 | −60/+200 °C | 33–47 kN/m | USP Class VI, ISO 10993 |
| Rail | S16 | 30–85 | −60/+200 °C | 17–25 kN/m | EN 45545-2 HL1–HL3 |
| High temperature | S09 | 40–70 | −60/+300 °C | 12–17 kN/m | FDA |
| Low temperature (PVMQ) | S05 | 50 | −110/+200 °C | 28 kN/m | — |
| Fluorosilicone (FVMQ) | S13 | 40–70 | −60/+170 °C | 10–21 kN/m | — |
| Steam resistant | S14, S22 | 40–80 | −60/+200 °C | 9.5–14 kN/m | FDA |
| Low compression set | — | 40–80 | −60/+200 °C | 10–15 kN/m | FDA |
| Electrically conductive | S11 | 50 and 70 | −50/+210 °C | 5–10 kN/m | — |
| Cellular (sponge) | S15, S33 | by density | −60/+200 °C | 15 kN/m | FDA, EC 1935/2004, EN 45545-2 HL1–HL2 |
High temperature reaches +315 °C on transient peaks. Fluorosilicone rises to +220 °C with an additive but gives up range against standard VMQ. The conductive grade comes in two discrete hardnesses, 50 and 70 Shore A, not a continuous range, with volume resistivity ≤4–12 Ω·cm and in black only, because the carbon filler sets the colour.
Other figures that settle a specification: medical grade reaches 7.9–10.8 MPa tensile and 700–1200 % elongation, with grades validated for tissue contact up to 29 days and an optional radiopaque additive. Low compression set runs at 11–18 % over 70 h at 150 °C, against 20–25 % for a conventional formulation. The cellular grade is closed-cell, 0.5 to 0.8 g/cm³, and its tolerances are class E3 to ISO 3302-1 against class E1 for solid.
Rail EN 45545-2: extrusion against moulding
Series 16 is tested to EN ISO 4589-2 for the oxygen index, EN ISO 5659-2 for smoke optical density and EN 17084 for the toxicity index. The two processes do not give the same result: in extrusion the oxygen index is 32.7 % and smoke density Ds max 84.7; in moulding, 35.1 % and Ds max 45. The toxicity index holds at 0.06 for both. Where smoke emission is the critical criterion of the project, moulding almost halves the opacity.
Both processes cover requirement sets R22 and R23 and all three hazard levels. The level required depends on the operation category and the vehicle design: a driverless metro with no staff on board calls for HL3, while a conventional train may stop at HL2. The cellular version, by contrast, covers HL1 and HL2 but does not reach HL3, and that is its real limitation today. Available colours are black RAL 9017, blue black RAL 5004, grey and cream, with restrictions by formulation.
The six mistakes that cost most
- Mistaking raw material certification for finished product certification. The datasheet covers the raw materials in the formulation. For a CE-marked device the manufacturer must also validate the finished part against its intended use; the material is necessary, not sufficient.
- Specifying the maximum temperature on the datasheet. Brief exposure and continuous service are not the same, and service life shortens near the limit.
- Overlooking colour. Many food and medical certifications are held by the base formulation. Adding pigment can void them, so the combination is worth validating before a special colour is specified.
- Asking for «food grade silicone» and nothing more. That is not a specification. FDA applies in the United States, BfR XV in Germany, EC 1935/2004 in Europe and the Arrêté du 25/11/92 in France. Meeting one does not mean meeting the others.
- Over-specifying out of caution. Asking for medical grade or platinum cure on an industrial gasket raises the price without adding function.
- Forgetting the process. Extreme hardnesses carry manufacturing limits, the conductive grade is optimised for moulding and the cellular grade is obtained by extrusion. Availability is worth confirming before the drawing is closed.
When requirements cross
Some applications call for two properties at once. High tear with high temperature is met by Series 7, which holds −60/+230 °C and 30–35 kN/m. Medical grade with high tear is covered by platinum cure, reaching 33–55 kN/m while holding USP VI and ISO 10993. Food grade with cellular exists with FDA, BfR and EC 1935/2004 certification, at the cost of looser tolerances. The combination that does not exist is high temperature with low cost: thermal stabilisers are paid for, and where real service temperature sits below +200 °C, standard silicone is the right answer.
Formulation review for your application
The technical team checks service temperature, applicable standards, chemical environment and manufacturing process, and confirms the series available in the hardness and colour required.
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