Buna-N (Nitrile) Gaskets and Oil

Jun 06, 2016

When working with a gearbox, a pump, or part of a fuel system, it’s essential to select a gasket material with resistance to hydrocarbons. Mineral and vegetable oils, along with diesel, gasoline and similar fuels, attack many materials, causing them to swell or break down. The notable exception is nitrile rubber. Nitrile rubber gaskets, often referred to as Buna-N or NBR gaskets, offer excellent resistance to oil.

Nitrile Rubber Chemistry

Nitrile rubber is made from a blend of acrylonitrile (chemical formula C3H3N) and butadiene (C4H6.) Each component is mixed with water as an emulsion before being combined in a polymerization reaction. This makes the carbon (C) and hydrogen (H) atoms form into long chains that tangle round another to form a rubbery compound.

Increasing the proportion of acrylonitrile makes the rubber stronger and reduces its gas permeability. The trade-off is that it results in a rubber that’s stiffer at low temperatures.

Rubber and Swelling

Most rubber absorbs oil and swells up in size. This is a problem in gaskets used in gearboxes and engines because it can lead to leaks. However, acrylonitrile reduces this tendency. So the greater the proportion of acrylonitrile used in the nitrile rubber formulation the less it will swell.

Swelling is covered in the ASTM D2000 specification. Most nitrile gasket materials are classed as ‘BF,’ ‘BG’ or ‘BK.’ The ‘B’ indicates an upper temperature limit of 100°C and the second letter shows how much the rubber will swell. ‘F’ is swell of 60% by volume, (under specified conditions,) while ‘G’ is 40% and ‘K’ 10%.

When and When Not to use Nitrile Gaskets

Good applications are those where peak temperatures are moderate and there’s exposure to oils. However, nitrile rubber does not hold up well to ozone and oxygen, so should not be used in places where these are present. Water purification equipment is one such location.

The Application Dictates the Gasket Material

If there’s any question about the properties of various gasket materials, consult the specialists at Hennig Gasket. They can explain the material choices available for your application.

Buna N, Nitrile Gaskets
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How Flanges Influence Gasket Material Selection

May 23, 2016

If flange and enclosure door surfaces were perfectly smooth and perfectly aligned, gaskets wouldn’t be needed. In the real world though, uneven gaps are always present and must be sealed to prevent leaks or contamination. Sealing options range from inexpensive red rubber and buna N materials to advanced silicone rubber gaskets, and include materials as diverse as graphite, PTFE and paper.

When replacing gaskets it’s common to use the same material that’s just been removed. If joints never change, that approach is often adequate. But by considering the nature and design of the sealing surfaces or flanges, it may be possible to select a longer-lasting material.

Impact of flange material

Some flanges can’t take high clamping forces, especially as they age. Plastics tend to become brittle and some metals lose ductility as they age, particularly if put through repeated temperature cycles. This means a soft, easily compressed gasket material is needed.

Impact of flange geometry

Bolt patterns or the position of clamps and latches can distort the mating surfaces, leading to uneven gaps. For example, an enclosure door with a single central latch can leave large gaps at the corners when closed. Also, a flange that’s been assembled and dissembled repeatedly for many years will start to distort, creating uneven gaps.

Flange alignment can change over time. After years of service it’s possible that piping will have moved, with the result that flange faces are no longer parallel. Again, the result is an uneven gap. Another problem is surface imperfections resulting from careless gasket removal.

These problems demand thicker gasket material that provides more compression. But thicker material needs higher loads to compress down in the joint, and those loads can lead to more distortion in the flanges.

Things change

Flanges and mating surfaces change over time and products that performed well, perhaps red rubber or buna N gaskets, may no longer be up to the job. When replacing gaskets, consider the condition of the sealing surfaces or flanges. A different material may last longer in the joint.

 

FAQs, Flange Gaskets
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ASTM Testing for Creep Relaxation

May 09, 2016

Open an electrical enclosure and you may see that the neoprene gasket material has taken on the imprint of the door or cover. In technical terms, it’s taken a compression set. In many gasket applications compression set can lead to sealing problems, due to a phenomenon known as creep relaxation.

Introduction to creep

Apply a load to an elastic material and it compresses. This happens because unlike in a metal, the atoms are linked in a way that lets them move. In gasket materials this is good because it lets the gasket deform to take up the irregularities between the two surfaces being sealed. However, there is a downside to this compressibility.

Rubber and rubber-like materials, as used in neoprene gaskets for example, have the ability to spring back. Release the load and the material returns to its original shape, more or less. Some materials do this better than others. The issue is that the material takes on a permanent deformation, or worse still, continues to deform. This behavior is called “creep” or more accurately, “viscoelastic creep.” It’s related to both the strength of the material and the time and temperature to which it’s subjected.

When creep is a problem

In a bolted joint the compressed gasket creates the torque in the securing bolts. But as the gasket material creeps and the gasket thins, the bolts are able to relax. That reduces the torque and the joint begins to loosen.

Polyurethane, silicone and nitrile gaskets tend to have lower creep than some other materials, as quantified by testing to ASTM 38.

ASTM testing

The principle is to measure the thickness of a sample of gasket material, subject it to load, temperature and time, then release the load. The recovered thickness is measured and the difference used to calculate a percentage reduction.

Taking creep relaxation numbers into account when choosing gasket material.

As with testing to ASTM 36, the absolute test values are less important than the ability to make comparisons between gasket materials. It’s a parameter of particular importance when lasting bolt tightness is essential.

Neoprene Gaskets
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Interpreting ASTM F36 Compressibility Data

Apr 25, 2016

Gaskets seal gaps of varying size by compressing under load. Best practice is usually to keep that load as low as possible, which is why softer gasket materials are preferred. For products like silicone or PTFE gaskets durometer numbers give a good indication of material hardness, (following the ASTM D2240 standard,) but they don’t show how that material will perform in a joint. That means turning to the ASTM F36 test data.

Compressibility and Recovery

When selecting gasket material it’s important to understand its compression and recovery behavior. This is because joints tend to move, whether due to varying temperatures, (media and environmental,) or loads. A material that compresses easily but has no recovery may not do a good job of sealing a joint that experiences a lot of cycling.

ASTM F36 provides a standardized method of testing and measuring compressibility and recovery. The test has two parts. First, the material is put under a load of 5,000 psi for 60 seconds and the reduction in thickness measured. Then the load is taken off and the material given another 60 seconds to spring back before the thickness is measured again. Both compressibility and recovery are expressed as percentages.

Caveats

The conditions F36 testing is done under don’t necessarily reflect the actual usage conditions as temperatures, pressures and loads will almost certainly be different. Neither do they take time into account, which in reality is a significant factor when dealing with viscoelastic materials, (where properties change over time.) What the numbers do provide is a basis for comparing between different gasket materials.

Typical F36 Numbers

Compressibility and recovery values vary greatly between different materials. For example, expanded PTFE has a compressibility of around 68% but recovery of just 12%, while the same numbers for a neoprene gasket could be 7 to 17% compressibility and 50% recovery. This would suggest the neoprene material would perform better in an application where flange faces are in good condition but gasket loads cycle. Of course, other factors such as temperatures, media and pressures must also be considered.

FAQs
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ASTM and Gaskets

Apr 11, 2016

Specifications for rubber or elastomeric gasket materials often reference an ASTM classification. For example, silicone gasket sheet material might be shown as “ISO/ASTM Designation FE” while material for a nitrile gasket could be BF. These references come from ASTM D2000, one of many standards addressing gasket design, gasket material and gasket classification. Buyers don’t have to know these standards, but understanding what they address helps when selecting material.

ASTM and their gasket standards

ASTM International develops voluntary consensus standards. These help manufacturers and buyers alike by standardizing aspects of design, testing and manufacture.

For gasket materials the first two standards to be aware of are F104 and D2000. F104 is a system for classifying non-metallic gasket materials. The idea is to simplify material and gasket selection by translating application needs into a six digit code. F104 covers asbestos, cork, cellulose, PTFE, graphite and other non-asbestos materials. Rubber and rubber-like materials are excluded from this system and come under D2000 instead.

Material properties like compressibility and tensile strength are covered under a range of other standards. For example, D2240 addresses testing of rubber hardness, (durometer,) while F36 describes compressibility and recovery and F37 covers sealability test methods.

Interpreting ASTM classifications

The D2000 standard does the same for vulcanized rubber as F104 does for non-metallic gasket materials, namely, it sets out a standard way of describing every type of material. A complete D2000 specification covers maximum temperature, swelling performance, hardness and tensile strength, plus optional characteristics such as fuel and water resistance.

Maximum temperature is defined as the temperature at which material performs degrades to a set level. This is indicated by letter where “A” means a maximum of 70°C and K is 300°C. Swelling performance is also shown by letter with B the highest.

These two letters are used to describe many rubber-like materials. A “FE” designation for silicone gasket material shows that it’s performance degrades only slightly at 200°C but under defined conditions it will swell by 60%. Likewise, a nitrile gasket designated “BF” has the same swell behavior but is only good to 100°C.

Rubber Gaskets
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PTFE Makes an Excellent Material for FDA Gaskets

Mar 28, 2016

Buying gaskets for dairy, brewery and food processing equipment is difficult. The choice is limited to FDA gaskets or FDA approved gasket material because it’s essential to avoid tainting or contaminating the product. Cleaning and sterilization routines are a way of life, so gasket material must stand up to high temperatures and caustic cleaning agents. And last, clamping forces are low in equipment like kettles, mixers and sanitary pipe fittings, so the material must be soft.

Polytetrafluoroethylene, (PTFE) sometimes referred to as Teflon®, meets all these requirements. That’s why it’s used for gasketing throughout the food industry. PTFE is listed by the FDA under 21 CFR 177.1550, although this approval really only covers virgin PTFE material. PTFE with markings, adhesive backing or filler can be used in food applications, providing the inks, adhesives and fillers also meet FDA standards. If purchasing gaskets like these for a food industry gasket application, have the material supplier confirm they qualify as FDA gaskets.

Some gasket materials will absorb traces of food preservatives, but PTFE is almost completely non-reactive. It doesn’t pick anything up and neither does it pass anything over so there’s no risk of food being contaminated with traces of elastomers or cleaning agents. The Shore D hardness of PTFE is around 50, so it’s soft and easily compressible. That’s important when pipes and vessels are secured with clamps rather than bolted flanges. PTFE also has the advantage of retaining its properties at temperatures as high as 400°F (204°C). That’s why it survives steam cleaning.

PTFE is not perfect though. It has a tendency to creep, which could lead to reduced clamping loads. This is because, unlike elastomeric gasket material, PTFE doesn’t cross-link, so bolted joints with PTFE gaskets may need occasional re-torquing.

In food processing it’s essential to use FDA gaskets. These will typically be PTFE, although some other materials are FDA approved too. If a gasket is going to contact food products it’s always best to discuss the application with the material supplier and make sure the correct material is chosen.

Food Grade Gaskets, PTFE Gaskets
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Does Your Application Need a Graphite Seal or Gasket?

Mar 14, 2016

High temperatures challenge many gasket materials. Nitrile gaskets will go up to about 95oC, silicone gaskets to 200oC and PTFE to 260oC, but what if you need to go higher? One option is to go with a metal gasket. A better one is to ask about graphite. Graphite seals and gaskets retain their properties at temperatures as high as 450oC, and have some other very useful sealing characteristics.

Properties of graphite

Graphite is a form of carbon where the atoms are arranged in layers or sheets. That lets them slide over one another easily, which translates to a slippery feel when rubbed between finger and thumb. (It’s also what lets a “lead” pencil write – graphite actually rubs off onto the paper.)

This slipperiness or low coefficient of friction is useful in gasketing or sealing. As mating surfaces are brought together graphite allows some slip. That accommodates any rotation or sideways movement as the clamping load goes on without damaging the sealing material. Used as a dynamic or shaft seal the low friction properties of graphite minimize energy losses and heat build-up while maintaining an effective barrier.

Graphite is also a very soft material, (which might seem odd when you consider it’s a cousin of diamond,) but that lets it flow into surface irregularities, which is what provides the sealing function.

Like PTFE, graphite is quite inert. It resists attack from most corrosive chemicals, even at high temperatures, and likewise doesn’t contaminate them.

Graphite seals and gaskets

It’s possible to buy graphite for use as a gasket or seal. It’s also used as a coating for some metal gaskets, (Kammprofile gaskets are an example,) where it provides excellent sealing performance along with temperature and chemical resistance. If you’re looking for gasket material for a high temperature application, ask Hennig Gasket if graphite seals might be right for you.

Graphite Gaskets
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Failure Mechanism at High Temperatures

Mar 07, 2016

Previous blog posts emphasized the importance of assessing the peak temperatures anticipated in a gasketed joint. Left unsaid has been why this matters. It’s probably obvious that polyurethane, nitrile and silicone gaskets all have a temperature at which they melt. More important, all will likely fail under prolonged exposure to temperatures near their melting point, due to a phenomenon called “creep.”.

Viscoelastic materials

Most gasket materials are viscoelastic. The “viscous” part means they have a propensity to flow slowly, like a thick gel and “elastic” refers to their ability to stretch and return to their original dimensions. However, elasticity has its limits. If the material is stretched too far it can’t return to its original size or shape, resulting in permanent “plastic deformation.”

Place a viscoelastic gasket material like polyurethane under load and it becomes thinner while simultaneously spreading outwards. This is “creep.” Releasing the load lets the material recover, but only to the extent that it has not been deformed plastically.

Creep relaxation

In a gasketed joint the material is compressed, either by the stretch of the flange bolts as they are tightened or by other retaining clamps. When first placed under load it starts to creep, but as the gasket thins the load lessens until the creep stops. This is termed creep relaxation. With good design this happens before the gasket reaches a point where the joint starts to leak.

Higher temperatures

Creep is related to temperature. When a polymer like polyurethane or styrene butadiene rubber gets warmer the molecular chains slide more readily. As a result it takes less force to produce a given movement. As the temperature approaches the melting point of the material, the force needed to produce a given movement falls quickly. To take one example, this means that at temperatures over 200 F (93 C) a nitrile gasket starts shows considerable creep.

Consider the material properties

Always select gasket material with the knowledge of the maximum temperatures expected. The more safety margin can be incorporated the less creep will be experienced, leading to a longer lasting gasket.

Gasket Material
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Why Thinner Gasket Material Usually Works Better

Feb 15, 2016

Gasket materials come in many thicknesses. To give one example, at Hennig Gasket neoprene gasket material is available from 3/32” all the way up to 2” thickness. Customers will sometimes ask what thickness they should buy, but a gasket material supplier really can’t help with that. It depends completely on the application. However, it’s generally agreed that a gasket should be as thin as possible, providing it still seals. There are four reasons. A thinner gasket:

1. Has greater blow-out resistance. Being thinner, the gasket present less area to the internal pressure, so is less prone to deformation and failure.

2. Has a lower leak rate. All gaskets will allow some quantity of fluid to pass through. This is just a natural function of their structure and the make-up of the fluid being constrained. (Anyone who’s ever tried piping helium knows how its small molecules let it escape from almost anywhere!) So the less gasket material that’s exposed to the fluid, the less will leak.

3. Retains fastener torque better. This stems from the creep relaxation characteristics of the gasket material. When there’s less thickness there’s less creep, (think of it happening on a percentage basis,) so more torque is retained.

4. Is less expensive. Material cost relates more to volume or weight than area, and thicker gaskets need more material. Secondly, thickness also influences cutting method and thicker materials could be more expensive to cut to shape. Neoprene gasket material 3/32” thick die cuts readily, but a thickness of 2” may call for a waterjet.

Note though that points 1 and 2 really only apply to situations where the gasket resists pressure, such as in pipelines. In no-pressure situations such as a gasket sealing around an electrical enclosure, the benefit is primarily Point 4 – cost.

All About the Gap

How thick a gasket should be depends entirely on the application. Remember that it’s purpose is to take up an uneven gap between two surfaces. The key is having enough thickness that the gasket compresses and fills the voids, but no more.

FAQs, Gasket Material, Neoprene Gaskets
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HVAC Sealing Material Primer

Feb 13, 2016

HVAC system seals and gaskets maintain efficiency by preventing the loss of heated and cooled air. Whether installing new ductwork, modifying an existing system, or just replacing worn out gaskets, it’s important to choose appropriate material. Many HVAC specialists consider neoprene gaskets the default choice, but it’s possible better performance could be achieved with EPDM or silicone gaskets.

HVAC Gasket Applications

Gaskets have three main roles in HVAC systems:

  • Sealing opening panels, flaps, and doors
  • Reducing transmission of motor or fan vibration
  • Allowing for thermal expansion and contraction

Sealing

Almost every ducting system includes access doors and panels, along with dampers that close off airflow through “legs” of the system. To minimize closing forces, these need a soft material with good compressibility. Combined with appropriate thickness, such gaskets will also take up the dimensional variation and uneven edges inevitable in most systems.

Reducing Vibration Transmission

Fans and motors can cause a vibration in flat ducting that’s audible as a low hum. To avoid complaints from building tenants, incorporate gaskets at appropriate interfaces. The cellular structure absorbs the vibration and prevents it spreading throughout a system.

Expansion and Contraction

Metal ducting experiences significant dimensional changes in response to switching between warmed and cooled air. A gasket with good recovery takes up these changes while still maintaining a leak-tight seal.

Environmental Factors

Outdoor applications challenge HVAC gasket material as UV light degrades some materials, and moisture penetration must be avoided. Low temperatures and ozone might also be a concern in some applications.

HVAC Gasket Materials

Neoprene gaskets and those made from thermoplastic elastomers (TPE’s) generally perform when soft and resistant to compression set. EPDM gaskets work well outdoors as they stand up to sunlight and other weathering effects. Where air or gas temperatures are high silicone gaskets can be a good choice.

Closed cell materials may be preferable because air and moisture cannot pass through, although these are firmer, requiring higher closing forces.

Installation is simplified by using a pressure sensitive adhesive (PSA). This can be laminated on to the gasket material or can be applied in tape form.

Flange Gaskets
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