Is sealing with silicone rubber gaskets right for your application? Silicones have many desirable properties but can cost more than other polymers. That’s why some engineers and product designers avoid them. At the same time, newer materials such as thermoplastic elastomers are interesting but even more expensive.
If you’re wondering whether silicone rubber is the right choice for sealing, it’s time to take a closer look at these synthetic elastomers. First, we’ll examine the advantages of sealing with silicone and compare some silicone gasket materials. We’ll then categorize silicone rubber in different ways and examine some applications.
To discuss your application for sealing with silicone, contact Elasto Proxy.
Advantages of Sealing with Silicone
Silicones resist moisture, chemicals, heat, cold, ozone, and ultraviolet (UV) radiation. They’re also stable, flexible, long-lasting, and aesthetically-pleasing. The advantages of silicone rubber include:
- Good thermal stability and low chemical reactivity
- Excellent resistance to ozone, sunlight, and oxygen
- Consistent properties at high and low temperatures
- Repels water, resists moisture, and forms watertight seals
- Good electrical insulation and excellent gas permeability
- Come in a range of durometers and custom colors
- Available in special grades and with filler materials
Sealing with Silicone: Solid, Sponge, and Foam Rubber
There are three basic types of silicone gasket materials: solid, sponge, and foam.
- Solid silicone gaskets have a tightly-packed molecular structure. They aren’t always “soft”, but they can have a lower durometer (hardness) than sponge silicone. Lower-durometer solid profiles provide strong environmental sealing with low closure force. Higher durometers require greater closure force.
- Sponge silicone gaskets have a cellular structure that offers cushioning and padding. There are two basic types of sponge silicones: open-cell and closed-cell. Open-cell silicones contain interconnected pockets that allow water, air, and chemicals to pass unless the gasket is compressed. Closed-cell silicones have pockets that are filled with nitrogen to prevent these substances from passing at low pressures.
- Foam silicone gaskets are available in soft, medium, firm, and extra-firm grades for different amounts of compressibility. Foam silicones offer improved flame resistance, too. There are three varieties of silicone foams: open-cell, closed-cell, and foamed materials with both open cells and closed cells.
It’s also important to understand that silicones can be divided into various types, classes, and industrial classifications. Note that “solid” materials also include silicone rubber with a cellular or foamed structure. These silicone profiles, sheets, and rolls support water jet cutting, splicing, taping, and other processes used in gasket fabrication.
Special Grades and Filler Materials for Silicone Gaskets
Silicone gasket materials are available in grades that meet FDA requirements, UL 94 standards, and military and aerospace requirements. They can also contain filler materials to add strength, or to provide shielding against electromagnetic interference (EMI).
- FDA silicone gaskets meet U.S. Food and Drug Administration (FDA) requirements for food contact applications and medical devices.
- UL 94 silicone gaskets meet testing requirements from Underwriters Laboratories (UL) for fire safety and flame resistance.
- EMI silicone gaskets that contain metal or metal-coated particles can meet MIL-DTL-8352 shielding requirements from the U.S. Department of Defense (DoD).
- Low flex-fatigue resistance gaskets can also meet military and aerospace requirements. They resist cracking during repeated take-offs and landings.
- Fluorosilicone gaskets contain fluorine additions for added chemical and fuel resistance. They can also contain metal or metal-coated particles for EMI shielding.
Applications for Sealing with Silicone Rubber Gaskets
If you’re still wondering if sealing with silicone is the right choice, consider some uses for this versatile polymer. The examples you’ll read about aren’t the only uses for silicone, but they’re representative.
Mobile equipment manufacturers need sealing and insulation materials that can withstand demanding conditions such as engine heat, extreme outdoor temperatures, wind, water, and mud. Examples of silicone products for mobile equipment include shaft seals, spark plug caps, radiator heating hoses, O-rings, and roller gaskets.
Mobile equipment manufacturers also need floor mats, door and window seals, and thermal and acoustic insulation. Silicone rubber isn’t used for all these applications, but engine bay insulation can have a silicone-coated fiberglass facing and a layer of open cell silicone foam. This thermal-acoustic insulation is fire-resistant and withstands continuous temperatures up to 500° F.
Defense, Aerospace and Aviation
Defense contractors need rubber for the hatch seals on military vehicles and for other demanding extreme environments. Sometimes, military-grade silicones are required. For example, MIL-DTL-83528 defines the requirements for elastomeric shielding gaskets that combine environmental sealing with protection against electromagnetic interference (EMI).
The aerospace industry also needs silicone gaskets with specialized properties. For example, silicones that meet the requirements of the A-A-59588A specification provide strong flex-fatigue resistance – a measure of an elastomer’s ability to withstand repeated flexing or bending. Standard silicones can withstand a range of temperatures, but not all silicones resist higher levels of fatigue.
Food equipment manufacturers need rubber that can withstand high heat from commercial ovens and cold temperatures from freezers and refrigerators. Silicones handle these extreme temperatures and can resist the frequent cleaning that’s required with food and beverage equipment. From baking mats to oven seals, silicone rubber also resists greases and oils.
Depending upon the application, manufacturers of stainless steel and food equipment may need to use FDA approved silicones materials. FDA silicones are non-toxic, non-marking, and non-allergenic. They’re also tasteless, odorless, and resistant to the natural growth of bacteria. Not all food-grade silicones are FDA approved, however.
Enclosures and Building and Construction
Enclosure sealing with flurosilicons is used with tableting equipment in the pharmaceutical industry. Silicones are also used in the enclosures for electronic and electrical equipment. In buildings, silicone rubber can be used in window seals and door seals. With other types of built structures, silicones are used in expansion joints because these elastomers allow for thermal expansion without distortion.
With their strong dielectric properties, silicones are used with cables and cable terminations, corona-resistant insulation tubing, keyboards, and contact mats. Particle-filled silicones that provide EMI shielding are used in conductive seals because metal or metal-coated particles also provide electrical conductivity. UL 94 silicones that resist the spread of fire are used in rubber gaskets for electrical enclosures.
Medical and Healthcare
Silicones for medical and healthcare applications are used in tubing for medical devices and equipment, bellows for artificial respirators, and EMI gaskets. As with food and beverage equipment, FDA approved rubber may be required. Yet not all silicones for healthcare environments require FDA approvals. Examples include the silicone door seals in walk-in bathtubs for hospital patients.
Silicones are also used with the handles for patient lifts. A metal tube can support a patient’s weight, but stainless steel is cold, hard, and sometimes slippery. Slip-on silicone foam handles support easier gripping for greater patient safety and comfort. To learn more about sealing with silicone, contact Elasto Proxy.