Silicone Gasket Material – The Ends and Outs

What a difference an “e” makes! Silicon is the material of electronics. It’s hard and brittle and makes lousy gaskets. Silicone on the other hand is soft and elastic, which makes it a good choice in many gasket applications. Here’s what makes this polysiloxane material so useful.

Basic Chemistry

Silicon and silicone are closely related. Silicon is a naturally-occurring element while silicone is a polymer that combines silicon atoms with those of oxygen and the H3C hydrocarbon compound. The result is a soft, plastic-like material that springs back after being compressed.

More Silicone Properties

In addition to compressibility, other useful features of silicone include:

  • Poor adhesion, so it doesn’t mark surfaces
  • Low toxicity, making it useful for food and medical applications
  • Resists degradation by ultraviolet light (sunlight)
  • Low electrical conductivity
  • Repels water
  • Ozone resistant
  • Retains its flexibility over a temperature range of -94 to +392°F

These properties make silicone gaskets a good choice in a range of food, medical and electrical applications. It won’t taint foods and it handles a wider temperature range than many other gasket materials.

Silicone Weaknesses

In some regards, silicone performs less well as a gasket material than the alternatives. Some others have higher strength and better compression set recovery for instance. It’s also attacked by hydrocarbons like most oils and fuels, and resistance to acids and alkalines is poor. In short, unless you need the special properties of silicone there may be better alternatives.

Silicone Forms

Silicone is available in both solid sheet and as a foamed or cellular material. Silicone foam may be either closed or open cell.

Silicone comes in many colors, (which may not be food grade, so check before ordering.) There are also many variants tailored for specific application needs. Some will go to lower temperatures than that given above, while others have been engineered for higher strength or even electrical conductivity.

If you’re considering using silicone gaskets we respectfully suggest speaking with one of our material specialists. There are many instances where silicone is an excellent choice, but sometimes other materials may perform better.

Don’t Overspecify Your Gasket Requirements

When choosing a gasket always consider the application’s TEMP – that’s the temperature, environment, media and pressure. This will lead you to the best material for the job, providing you can predict what those values will be. In reality, all but the media can vary. In response, engineers sometimes select gasket material to handle the worst possible combination of conditions. This is not a good idea, and here’s why.

Compromising Performance, at a Price

Consider an application where a simple nitrile rubber gasket will handle the normal operating conditions. Then throw in the remote possibility of exceptionally low ambient temperatures or hotter-than-normal media. This could lead you to look at silicone or PTFE gasket materials.

These will handle extremes better than nitrile rubber, but both are considerably more expensive. And there’s another point to consider: will they work as well as the nitrile over the normal working range? If nitrile best satisfies the typical needs of the application, that’s probably the material to go with.

Consider Risks and Probabilities

There are of course exceptions. If the likelihood of failure is related to the chance of extreme deviations from normal conditions, how much deviation do you design for? Four-sigma? Six? More? It depends on how much risk you’re willing to accept, and that is driven by the cost of failure.

If you’re sealing steam in an accessible location the consequences of a gasket failure are probably not too severe. But if the application is sealing-in sulfuric acid in a high volume processing plant, the costs of both downtime and failure could be very high.

Make an Engineering Decision

Here’s the bottom line: gasket failure always has a cost. You can probably reduce the risk of failure and extend the period between gasket replacement by specifying more sophisticated gasket material – Viton/FKM rather than neoprene for example. But, this increases the upfront costs. So estimate risks and costs – a Failure Mode Effect Analysis (FMEA) might help – and make an informed decision about the right gasket material for your application.

Where to Buy Gasket Material

Gasketing is complicated. Every application has a unique combination of temperature, environment, media, and pressure to consider and there’s almost never a ‘one-material-suits-all’ solution. As a result, even a small HVAC, food processing or chemical application might need gasket materials ranging from nitrile rubber to EPDM, silicone, fiber and even PTFE.

Many companies will sell you some of these. The danger is that when they don’t carry a full range they’ll propose a material that’s not ideal for your application. Here’s some advice for deciding where to buy your gasket material.

Consider Material Range

Stocking nitrile rubber and neoprene foam does not constitute a full range. You should look for a vendor that carries all these types of gasket material:

  • Fiber
  • Felt
  • Cork and cork/rubber
  • Fiberglass
  • Compressed Non-Asbestos
  • Various types of rubber
  • Closed and open-cell foam
  • Graphite
  • PTFE
  • Specialized brand name materials like Viton and Santoprene

Look for Rapid Turnaround Capabilities …

It’s rare to have the luxury of buying gasket material just to put into storage. Usually, there’s an immediate need, and you can’t wait for delivery. Keeping a full range of gasket material in stock ties up money and space though, so some businesses don’t want to do it. Always look for a material supplier who maintains a comprehensive inventory.

… And Gasket Cutting Services

Material is one thing, but you still have to cut gaskets from it. A stockist with waterjet cutting capabilities can produce installation-ready gaskets in just a few hours, sometimes faster. That saves you time and results in a more accurate gasket with better edge quality.

Location is another factor to consider. Today’s rapid delivery services mean material can ship almost anywhere in the country in 24 to 48 hours, but there are still advantages to finding a local vendor. Closer is faster, and likely cheaper too.

Range and Speed

When searching for gasket material, similar isn’t good enough. It’s essential to buy the right material for each application and get it without a long wait. That’s why you should look for a supplier with a broad inventory and the ability to deliver quickly.

Cutting with Water: How the Waterjet Machine Works

Waterjet cutting produces small or large gaskets in just minutes. There’s no tooling, it’s fast and edge quality is extremely good. Waterjet eliminates the wait for tooling that goes with steel rule die cutting and it avoids the heat and fumes of laser cutting. That’s why we prefer it for many different gasket materials. Here’s how it works and why we’re such fans.

High Intensity

A jet of water from a garden hose can sting the skin. In a waterjet cutting machine that jet is far thinner and traveling at supersonic speeds. The water is powered by an intensifier pump that produces 60,000 psi or more. This forces the water through a 0.006” diameter hole in a piece of sapphire.

With such high pressure and velocity, the jet of water slices through material like a knife. For cutting harder materials, adding an abrasive powder to the water increases cutting speed.

Machine Operation

The cutting head is mounted on a gantry. This spans the machine bed, which is where the material being cut is placed. Here at Hennig Gasket we can handle sheets up to 8′ x 6′. The gantry provides one axis of motion and the sheet moves under the gantry for the second. By interpolating motion between the two it’s possible to cut incredibly intricate shapes.

Waterjet machines are programmed directly from CAD files and there’s no special tooling needed. The programming package helps maximize material utilization by nesting gaskets efficiently. This, plus a very small kerf (the cut width,) helps minimize waste.

Types of Gasket Material

Cutting speed and maximum material thickness depend on hardness. Speeds of up to 500”/minute are possible, but this drops as thickness increases. Sheet EPDM, neoprene, cork and silicone gasket material can be cut quickly and accurately, and maximum thickness can be up to 6”.

Fast Turnaround

Using a waterjet to cut gaskets from sheet material is fast and cost-effective. Perhaps the biggest benefit though is the speed with which we can satisfy an order. Curious how fast that is? Put us to the test.

Gaskets for Vacuum Chambers

Vacuum chambers are used in many industries. Their largest application area in the physical vapor and directed vapor deposition, (PVD and DVD,) process industries. Here they are used for applying both decorative finishes and hard protective coatings. Vacuum eliminates contaminants that would cause oxidation or reduce purity. The cutting tool, semiconductor and nuclear industries are all big users. They’re also used in scientific research even for growing engineered diamonds.

A Difficult Sealing Environment

The gaskets used for sealing these chambers are critical pieces of the equipment. They fit around access ports where they have to withstand high clamping forces as well as extremely low vacuum. That means they need strength and good compression set resistance. Another requirement is a wide temperature range and there’s also a fourth, more specialized challenge.

Outgassing

Emptying a vacuum chamber of air, (pumping it down,) takes time because molecules cling to the interior surfaces. These surfaces must be given time to give up these molecules in a process called “outgassing”.

Materials give up their attached air molecules at different rates, which makes outgassing behavior an important consideration when selecting gasket material for vacuum chambers. Slower outgassing means longer pump-down times, which in turn reduces chamber throughput.

Suitable Materials for Vacuum Chamber Gaskets

The most popular choice is Viton®. Technically a polymer from the fluorocarbon family, (Viton® is the DuPont trade name,) this has a wide temperature range, (-20 to +400°F) and good compression set resistance. Most importantly though, it provides shorter outgassing times than the alternatives.

These alternative materials are silicon, Butyl, Buna-N and EPDM. Silicon gasket material outgasses more slowly than Viton® but has a wider temperature range and good ozone resistance. In ultra-low vacuum applications, meaning pressures below 7.5×10-10 Torr, elastomeric gaskets are replaced by copper.

Finding the Right Material for Vacuum

Every vacuum chamber has access ports, and access ports need gaskets. An important consideration for the gasket material is outgassing behavior as this affects pump-down time. If outgassing is a concern in your gasket application, the specialists at Hennig Gasket will be happy to offer advice.

What UL Ratings Mean for Gasket Materials

Industrial enclosures often have to meet NEMA and IP standards for ingress protection. These aren’t the only standards designers should consider though. There’s also UL50 and UL50E. Established by Underwriters Laboratories, these were developed to ensure that electrical enclosures would be safe. Safety depends at least in part on the gasket, so there are UL standards for gasket materials too.

The UL difference

Achieving NEMA or IP certification entails passing some stringent tests. However, these tests only verify dust and water resistance. They don’t explicitly test the gasket material. UL testing does.

Gasket material is tested because environment and use affect how long it lasts. Some gaskets are exposed to UV outdoors. Others, especially if near high voltage equipment, can be attacked by ozone. Some enclosure gaskets will endure periodic recompression when the door is repeatedly opened and closed. Other are in continuous compression, as might be the case around an electronics assembly.

These variables place different demands on the gasket material. Many gaskets might hold up to NEMA and IP testing but the material could degrade slowly and reduce the protection the enclosure provides.

Gasket Material Certification

To address the impact of use and environment on enclosure performance, UL developed tests for gasket material. These are defined in UL157, “Standard for Gaskets and Seals”. This covers both foam and solid elastomeric and composite gasket materials.

Materials passing UL157 can be considered as “UL Recognized Components”. Using components certified this way in an end product simplifies the process of getting UL approval. (Other UL standards may also be applicable.)

Buy UL-Rated Gasket Material

When designing equipment, using UL-rated materials will save time and money in testing. And later, when gaskets are being replaced, it might also be prudent to stick with UL157 materials. These range from neoprene and EPDM foams to silicon and even cork. (But note that adding a pressure sensitive adhesive nullifies any prior UL rating.)

Finding the right UL-rated gasket material for a specific application can be difficult. If unsure what to use, consult a material specialist at Hennig Gasket.

Gasket Material for Natural Gas

Natural gas is widely used for heating and cooking and it’s the energy source for much of our electricity. Perhaps less well known, it’s also essential for fertilizers and plastics.

The systems that store and distribute natural gas need gaskets. Here we’ll discuss the materials most often used, but first, a short primer.

Hydrocarbon Origins and Uses

Like coal and oil, natural gas is formed from plant and animal remains. Decomposition produces methane, (CH4), and when trapped underground we call it natural gas. Natural gas is odorless, so a trace of mercaptan, (CH4S) is added to make it detectable.

Reacting natural gas with steam separates the hydrogen and a second reaction, this time with air, results in NH3, or ammonia. Chemists call this the Haber process and it’s the first step in producing fertilizers.

Like methane, plastics are also composed of carbon and hydrogen atoms. The difference is that the atoms are formed into long chains to create polyethylene, polypropylene and similar materials.

Storage and Distribution

When held above 200 bar (3,000 psi) natural gas is known as compressed natural gas (CNG). This makes it sufficiently dense for use as a vehicle fuel. When pipelines aren’t an option, for easier transportation cooling to below -184°F produces liquefied natural gas (LNG).

The Sealing Challenge

The biggest issues are permeability and flammability. Simply put, the very small methane molecules can find their way through some materials, and they burn readily. Fortunately, as it’s lighter than air, escaping natural gas tends to disperse quickly.

Sealing CNG and LNG presents additional challenges. Gasket materials must retain some flexibility at very low temperatures and should have the strength to resist extrusion through joint faces.

Materials for Natural Gas Gaskets

For most low pressure, ambient temperature applications nitrile and neoprene gasket materials are the first choice. In more challenging applications many engineers opt for more expensive Flexitallic spiral wound gaskets. PTFE and graphite are other options.

Every gasket application has some unique challenges. If you need material for natural gas gaskets, ask a product specialist at Hennig Gasket for advice.

Neoprene Gasket Applications

Neoprene is a synthetic polymer that resembles rubber, but as a derivative of chloroprene, it is far more durable in extreme temperatures, when exposed to oils and several chemicals, and it endures harsh weather conditions. But not all Neoprene gaskets are created equal—there are several variations or “grades” of neoprene. Each grade of neoprene gasket has its ideal application depending on factors such as temperature and pressure tolerances, the environment where it will be used, etc:

  • Commercial Grade Neoprene—15-50% Neoprene mixed with other elastomers; this blend is fine for general commercial gasket applications, but not in situations where high oil resistance is necessary.
  • Medium Grade Neoprene—51%+ Neoprene content; also suitable for many general manufacturing applications where a lower tensile strength will suffice.
  • High Grade Neoprene—100% Neoprene is extremely resilient in the harshest, most demanding environments while maintaining its resiliency, abrasion resistance and tensile strength.
  • FDA Neoprene—A mixture of Neoprene and other FDA-improved materials; gaskets made from this grade are used in the food industry, but also in many government-approved and military applications where oil resistance is key.
  • PSA Ready Neoprene—Available upon request, this Neoprene has one side with a matte finish and is designed to be used with pressure sensitive adhesives.
  • Nylon Cloth Inserted Neoprene—This grade is reinforced with one or two layers of nylon; good for improved creep reduction and stability.
  • Diaphragm Neoprene—Polyester insertions improve performance under high pressure applications requiring high levels of oil and petroleum resistance.
  • Flame Resistant Neoprene—Especially useful in potentially flammable, electric and high-heat applications, it passes the following flammability specifications: UL-94-HF-1, UL-94-HFB, MIL-R-6130C, FMVSS-302.

These grades vary in durometer ratings between 40, 50, 60, 70 and 80, can withstand temperatures between -20F to +180°F and come in sheets with several widths and thicknesses that can be cut (waterjet, flash and die cut) to exact specifications from your blueprints, or even reverse engineered.

Please contact Hennig Gasket & Seals if you have any questions about the different grades of neoprene that might best fit your application: 1-800-747-7661. We can custom manufacture any neoprene gasket you need to the most accurate specs possible.

Gasket Material Selection

It’s said that what comes out of the joint is what goes back into the joint, but sometimes we’re asked if there’s a better material to use. That’s because the expense of replacing a gasket often far outweighs the cost of the part. When that’s the case a little extra spent on better gasket material might save a lot.

Unfortunately, there’s no easy answer to the material question. All we can say is, “It depends.” Here’s why, along with some guidance on selecting the best gasket material.

Four Key Criteria

It’s impossible to determine an appropriate material without knowing the temperature, environment, media and pressure the gasket will experience. It’s also important to determine the actual range of every parameter and every operating condition. Cleaning with caustic agents, for example, creates very different gasket challenges than handling a benign fluid like milk.

Temperature

This refers to the temperature of the media. Many elastomers harden when cold, making them less able to resist pressure and reducing their ability to flex as the joint changes size. Neoprene, for example, has a lower limit of -40°F while high-performance fluoroelastomer (FKM) only goes to -10°F.

Environment

Temperature is one factor, sunlight another. A gasket used outdoors in a midwestern winter could see low temperatures while one exposed to the desert sun will get extremely hot. In addition, UV light damages some common gasket materials. NBR, for example, has poor UV resistance while EPDM holds up much better.

Media

Some gasket materials suffer swelling when exposed to oils and other will oxidize rapidly. Brake fluid is incompatible with nitrile rubber and FKM while silicone and EPDM are a poor match for gasoline.

Pressure

The pressure inside a pipe or enclosure can force gasket material to extrude out sideways. Harder materials generally hold up better but require higher clamping forces. In extreme cases, it may be necessary to consider PTFE, spiral-wound or metal gaskets.

Seek Advice

Every gasket application is different and it’s never easy to say which is the best material to use. Start by determining the four criteria listed above, then consult a material specialist.

Oxygen Compatible Materials

Many industries use oxygen. It’s one of the most reactive elements, which makes it useful in processes from steelmaking to paper production as well as healthcare and waste treatment. This reactivity, (a willingness to form chemical bonds with other elements,) also creates sealing problems. Here’s an overview of the challenges posed by oxygen and a discussion of the best gasket materials to use.

The Air We Breathe

Oxygen is one of the most plentiful elements and makes up some 20% of the air we breathe. It’s also locked into the oceans as water. At temperatures above -297°F (183°C) it’s a gas. Oxygen has a strong affinity for electrons, which is what lets it bond readily with many other chemicals. Rust and oxidation are the common results, although fire and explosions are always possible.

Oxygen Production

Oxygen is produced by cryogenic distillation or pressure swing adsorption (PSA). As the name implies, the cryogenic process entails cooling air until it becomes liquid, allowing the oxygen to boil off. This produces a very high purity gas.

PSA involves pushing air through aluminosilicate minerals. These take up the nitrogen, leaving just oxygen, but at a lower purity than the cryogenic process.

Keeping Oxygen Systems Clean

Given that oxygen is highly reactive, it’s essential that any surface it touches is clean. Some industries have special requirements for oxygen-clean surfaces. The standards applicable to your industry will specify the type and degree of cleaning needed.

Materials for Gaskets in Contact with Oxygen

Liquid oxygen can only be sealed by materials capable of withstanding very low temperatures. PTFE gaskets are a good choice as is FFKM (perfluoroelastomers) gasket material.

Good gasket materials for gaseous oxygen include neoprene, EPDM, silicone, butyl and Viton® (a DuPont brand name for FKM.) Rubber materials such as SBR and natural rubber should be avoided because of their tendency to react with oxygen.

Ask an Expert

Always consider temperature, pressure and the environment along with the media being sealed. For this reason, it’s prudent to consult with a materials specialist before ordering gaskets or gasket material.