April 2016

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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