Silicone O-rings: food-safe and flexible

What are silicone O-rings?

Silicone O-rings are made of silicone rubber with the material code VMQ. They are suited above all to use in contact with sensitive products, for example in medical and food technology. The decisive factors are the excellent heat resistance, the high suitability for low temperatures and the favorable physiological behavior.

VMQ retains its sealing-relevant properties across a very wide temperature band and changes them only slightly even in severe cold. The separately listed fluorosilicone rubber FVMQ behaves similarly to VMQ, but is additionally more resistant to mineral-oil products and thus closes the most important gap in silicone's resistance profile.

The most important advantages of silicone O-rings

• Superior low-temperature resistance: silicone remains flexible down to −60 °C and thus stands out from the common O-ring materials.
• Exceptional hot-air resistance: can be used continuously in air up to +200 °C; special grades likewise reach +200 °C.
• Good physiological properties: silicone hardly interacts with other media, is odorless and tasteless and barely affects products.
• Resistant to vegetable and animal oils and fats: an important argument for use in the food industry.
• Insensitive to weathering: resistant to weather and ozone.

From the lower service limit at −60 °C to the maximum temperature of +200 °C in hot air, the sealing-relevant properties vary only slightly. Silicone shows its strength above all in the low-temperature range. Because the material hardly reacts with other media, unwanted influence on the product by the seal can largely be ruled out. This is one of the decisive arguments for silicone O-rings in production plants in medical and food technology.

What is the temperature resistance of silicone O-rings?

Silicone O-rings have a temperature resistance from −60 °C to +200 °C. Even in standard grades they thus cover a very wide temperature range. Their suitability for low temperatures stands out in particular.

With similar heat suitability, silicone O-rings retain their sealing properties in even more extreme cold than EPDM O-rings. The decisive factor is the material's good low-temperature flexibility. That the sealing-relevant properties depend only slightly on temperature is shown, for example, by the compression set. It remains low even at high temperatures, so that silicone O-rings largely return to their original shape after a compressive load and close the sealing gap.

However, this applies to silicone in air. In contact with steam, the compression set rises significantly. This permanent deformation reduces the sealing effect and limits the service range. It is therefore important: the temperature resistance from −60 °C to +200 °C applies to the medium air. When water is involved, silicone O-rings are suitable continuously up to 100 °C.

What is the media resistance of silicone O-rings?

Silicone O-rings are resistant to a range of technical media, but they also have clear limits.

Resistant to:

• Weather and ozone
• Aliphatic engine and gear oils
• Glycol-based brake fluids
• Flame-resistant hydraulic fluids
• Oils and fats of vegetable and animal origin

Not resistant to:

1. Steam above 120 °C
2. Strong acids and alkalis
3. Fuels and mineral-oil-based products
4. Silicone oils and silicone greases (caution during installation)

Silicone owes its widespread use in the beverage and food industry to its resistance to oils and fats of vegetable and animal origin. The material shows weaknesses in contact with steam above 120 °C. This acts as a limiting factor in steam sterilization in food-processing plants.

Because silicone O-rings have only limited durability in contact with silicone oils and silicone greases, installation requires particular care. Common silicone-based lubricants can damage the O-rings. In general, exposure to the wrong media causes silicone to lose hardness and tensile strength. Unlike many other elastomers, however, the material does not swell in the process. You can check specific media in the media resistance tool.

What are the mechanical properties of silicone O-rings?

The mechanical properties of silicone are weak compared with other sealing materials. This is the material's most important limitation.

• High abrasion: mechanical stress from movement in the sealing system quickly leads to material loss on the O-ring. The abrasion can contaminate the medium and impair the sealing effect.
• Low tensile strength: the tensile strength is at a low level.
• Low elongation at break: the elongation is also low, which must be taken into account when stretching during installation.

In addition, there is the low extrusion resistance. When high pressure acts on silicone O-rings, they are easily forced into the sealing gap on the low-pressure side, and peeling can occur. The unfavorable abrasion behavior also rules out the dynamic use of silicone O-rings. Overall, these properties require particular care in the design of the installation spaces and during installation.

What should be considered for design and installation?

Because silicone O-rings show weaknesses under high pressure loads, the groove design must take this into account.

1. Carry out the groove design in accordance with DIN 3771 Part 5.
2. Reduce the sealing gap by 50 % compared with other elastomers to lower the risk of gap extrusion.
3. Ensure a minimum compression of the cross-section of 6 %.
4. Protect silicone O-rings against excessive pressure with a back-up ring.
5. Choose a manufacturing process with particular focus on tight tolerances.

A few additional points apply to installation:

1. Provide lead-in chamfers between 15 and 20 degrees.
2. Deburr the bores.
3. Plan for sharp edges with a minimum radius of 0.1 to 0.3 mm.
4. Use a suitable mineral-oil-based installation aid, not a silicone-based one.
5. Avoid twisting the O-ring during installation.

Because of the low elongation at break, excessive stretching during installation must be avoided at all costs. You can conveniently determine groove dimensions and tolerances with the O-ring groove calculation and the O-ring tolerances.

Typical fields of application

Are silicone O-rings food-safe?

Yes, silicone O-rings are available in food-safe grades. They are available in many FDA-compliant grades and thus meet the requirements for minimal product influence. The American FDA (Food and Drug Administration) sets requirements for the extraction of material constituents from the seal. You can read more about this in the article on FDA approval.

In addition to the FDA, further industry-specific regulations can play a role, such as those of the BfR (German Federal Institute for Risk Assessment), the 3-A Sanitary Standards or USP. We are happy to clarify directly with you which approval your application requires.

Are there alternatives to silicone?

The choice of material depends on the medium, temperature and mechanical load. If mineral oils or fuels are the main concern, FVMQ (fluorosilicone) is the obvious alternative, as it behaves similarly to silicone but is more resistant to oil products. Read more in the article FVMQ O-rings.

If the main concern is cold and weathering resistance without oil contact, EPDM is an economical option. If universal media resistance matters and high elasticity is not required, FEP-encapsulated O-rings are an alternative, though considerably more rigid. Tell us your requirements and we will recommend the right material.

Frequently asked questions