Elastomeric Gasket

From www.gasketwiki.com

Elastomeric Gaskets, also referred to as rubber gaskets, are made of one type of solid rubber which is either cut from a sheet/roll or moulded/extruded to a specification and a hardness.  The reasons for the wide-spread use of rubber gaskets are their relatively low price, the ease of cutting into final gaskets and their wide range of service and application capabilities.

These gaskets are suited for low pressure, low heat environments and will create a seal with a low bolt load.  Rubber gaskets typically can only withstand a maximum application pressure of 150 PSI.  Pressures over 150 PSI will likely cause the gasket to fail.  This is ideal for flanges that use low grade low quality bolts and/or non-asme flanges and joints.  Common forms of rubber gaskets come as flat sheet, extruded and spliced, lathe cut from sleeves and molded to specific shapes (such as Topoge gaskets).  O-rings fall into this class of gasket and are particularly suited to tongue and groove applications.

The use of these gaskets are limited to the operational working conditions that the type of rubber exhibits.  And since rubber gaskets will not compress, only deform, excess bolt load will only extrude the gasket out of position into the inner diameter of the flange causing a disturbance of the process fluid and cause the gasket to lose thickness and allow a loss of seal.

Cellular elastomers such as foam/sponge are made by mechanically or chemically expanding uncured rubber compounds.  Closed cell foam is made with separate cells of trapped gas within its structure.  This makes the foam highly compressible and impermeable to fluids where as solid rubber is incompressible.  They are ideal for use as weather seal.

Elastomeric gasket can also have a reinforcing fabric material to provide higher strength.  They can contain one or more plies of the material which is made of several fibers, typically nylon, cotton, polyester, as well as the less frequently used asbestos and glass.

Contents 1 Common Rubber Gasket Materials 2 Hardness 3 Tensile Strength 4 Elongation / Stretch 5 Compression Set 6 Resilience 7 Specific Gravity 8 FDA Compliance / Food Grade

Common Rubber Gasket Materials

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Natural Rubber -  Excellent physical properties including abrasion and low temperature resistance.  Poor resistance to petroleum-based fluids.

Urethane -  Good aging and excellent abrasion, tear, and solvent resistance.  Poor high temperature properties.

Neoprene -  Good weathering resistance.  Flame retarding.  Moderate resistance to petroleum-based fluids.  Good physical properties.

Nitrile -  Excellent resistance to petroleum-based fluids.  Good physical properties.

Butyl -  Very good weathering resistance.  Excellent dielectric properties.  Low permeability to air.  Good physical properties.  Poor resistance to petroleum-based fluids.

EPDM -  Excellent ozone, chemical, and aging resistance.  Poor resistance to petroleum-based fluids.

Styrene Butadiene (Buna-S, SBR) -  Excellent abrasion resistance and low temperature properties. High resilience.

Silicone -  Excellent high and low temperature properties.  Fair physical properties.

Hypalon -  Excellent ozone, weathering, and acid resistance.  Good abrasion and heat resistance.  Fair resistance to petroleum-based fluids.

Viton -  Excellent oil and air resistance both low and high temperatures.  Very good chemical resistance.

Hardness

Refers to the ease of which rubber can be deflected.  There are two standard hardness scales, Shore A and Shore D.  Shore A is determined with a blunt indentor and is used for soft and medium hard elastomers with a reading from 20 through 100.  Shore D is determined with a needle point which is graduated so that it registers a reading of 100 on smooth glass.  A reading of Shore D at 50 is approximately a reading of Shore A at 100.

Tensile Strength

A property that indicates the overall character of the rubber, as it is a measure of resistance to rupture.  This property is often expressed in PSI or a cross section.  However, this is not typically a critical factor in gasketing.

Elongation / Stretch

A measure of rubber's ability to stretch without breaking.  This term is typically expressed as the maximum elongation of the material and is expressed as a percentage.  Modulus will often refer to the measure of resistance to stretch, where a high modulus means high tension to stretch, and a low modulus means a low tension to stretch.

Compression Set

Compression set, referred to as memory, is a measure of the lack of recovery by rubber after deformation for a given time.  A low compression set is more desirable by a rubber gasket as it will have the tendency to recovery to its original form more readily.  This feature is particularity important for applications that require frequent loading and unloading, ie. door seals.

Resilience

The ratio of energy given up by the elastomer upon releasing it from a definite deformation to the energy required to produce the deformation.

Specific Gravity

The ratio of weight of a given substance, ie. rubber, to the weight of an equal volume of water.  Most rubbers have a gravity greater than 1.00, which means that they are heavier than water (sinks in water).  The specific gravity range for elastomers is around 0.95 to 2.00.  This plays an important part when considering the sale of rubber by the pound.

FDA Compliance / Food Grade

Only a certain few elastomers are considered FDA compliant.  Food grade rubber is always white in colour; coloured material contains carbon black which is not acceptable in food applications.  Typical food grade rubbers are White Nitrile and White Silicone.

¹ "Sheet Rubber Handbook - Gasket and Packing Materials", Rubber Manufactureers Association(RMA) & The Rubber Association of Canada

"Industrial Gasketing Course", Garlock Sealing Technologies