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Glossary of Rubber and Sealing Terms


abrasion: the surface loss of a material due to frictional forces.

acid resistance: the ability to resist the action of identified acids within specified limits of concentration and temperature.

age resistance: the ability of a material to resist deterioration caused by aging.

aging: the irreversible change of material properties during exposure to a deteriorating environment, for a specified time interval.

ambient temperature:  the environment temperature surrounding the object under consideration.

atmospheric cracking: cracks produced in surface of rubber articles by exposure to atmospheric conditions, usually due to sunlight and/or ozone.

axial seal: a term usually applied to an O-Ring where the squeeze is applied on the top and bottom surfaces. Another term for face seal.


back-up ring: a device made of rubber, PTFE, leather, or another substance that will add strength or support when installed next to a seal. Most commonly used as an anti-extrusion device.

batch: the product of one mixing operation.

bleeding: the exuding of a liquid compounding material from the surface of vulcanized or unvulcanized rubber.

Buna N: a general term for the copolymers of butadiene and acrylonitrile.

butt joint: joining two ends of material whereby the junction is perpendicular to the ID of an O-Ring.

Butyl:  a synthetic rubber of the polybutene type exhibiting very low permeability to gasses.


checking: the short, shallow cracks on the surface of a rubber product, usually resulting from damaging action by environmental elements.

coated fabric: a flexible product composed of a textile fabric and an adherent polymeric material applied to one or both surfaces.

compound: a term applied to a mixture of polymers and other ingredients to produce a usable rubber material.

compound, standard: a control or reference compound prepared according to a prescribed formula and mixing procedure.

conductive rubber: a rubber capable of conducting (generally static) electricity.

copolymer: a polymer formed from two different monomers.

crack: a fissure originating in the surface of a rubber product as a result of natural weathering.

crack, flex: a fissure originating in the surface of a rubber vulcanizate, resulting from cyclic deformation (usually bending).

crack, ozone: fissures originating in the surface of a rubber vulcanizate, caused by exposure to an ozone-containing environment; these fissures are perpendicular to the direction of strain, and usually occur in rubbers having main chain unsaturation.

crazing: formation of a random pattern of shallow cracks on a rubber surface usually due to aging by light. Unlike ozone cracking, crazing does not depend on the presence of a tensile strain in the rubber.

cross section:  an O-Ring as viewed if cut at right angles to the axis showing internal structure.

cure date: the date a rubber product was molded. Normally expressed, for example, as 1Q04, meaning the first quarter of the year 2004.

cylinder: chamber in which piston, plunger, ram, rod or shaft is driven by or against the pressure medium.


durometer: an instrument for measuring the indentation hardness of rubber.

dusting: the application of a powder to a rubber surface, generally to prevent adhesion to another surface.

dynamic properties: mechanical properties exhibited under repeated cyclic deformation.

dynamic seal: a seal which is subjected to reciprocating, rotational, or oscillating motion.


elasticity: the rapid recovery of a material to its approximate initial shape and dimensions after substantial deformation by force and subsequent release of that force.

elastomer: an elastic polymer.

elongation: extension produced by a tensile stress.

elongation, percent: the extension of a uniform section of a specimen expressed as percent of the original length.

elongation, ultimate: the elongation at the time of rupture.

explosive decompression: the rupture of a rubber article caused by the rapid reversal of pressure, causing dissolved gasses in the rubber to escape quickly to the surface of the vulcanizate.

extender: a material (usually organic) used to augment the polymer in a compound.

extrusion: the continuous shaping of a material during passage through a die.

extrusion (seal): distortion, under pressure, of a portion of a seal into the clearance between mating metal parts.


face seal: a seal between two flat surfaces. In an O-Ring, this means it seals on the top and bottom, not the ID and OD. Also referred to as an axial seal.

fatigue breakdown: the deterioration of an elastomeric product during repeated deformation.

fatigue life: the number of deformations required to produce a specified state of fatigue breakdown in a test specimen or product that is deformed under a prescribed set of conditions.

filler: a solid compounding material, usually in finely divided form, which may be added in relatively large proportions to a polymer for technical or economic reasons. The most commonly used filler is carbon black. Most fillers also function as reinforcing agents.

flex life: the number of cycles required to produce a specified state of failure in a specimen that is flexed in a prescribed method.

formula: a list of the materials and their amounts used in the preparation of a compound. Also called a recipe.

friction: resistance to motion due to the contact of surfaces.

frosting: the formation of a matte, whitish appearance on a rubber surface exposed to air, resulting from the action of ozone. Often confused with bloom.


gland: a seal assembly which includes an O-Ring, the groove which holds the O-Ring, and contacting surfaces.

groove: the machined recess into which a seal is fitted.


hardness: a physical property of a rubber vulcanizate, characterized by resistance to indentation.

hardness, Shore A: durometer reading in degrees of hardness of the rubber.  (Shore A hardness of 35 is soft; 90 is hard.)

homogeneous: general-a term of uniform composition throughout.

Hypalon: commercial name for chlorosulphonated derivative of polyethylene.


impact resistance: resistance to fracture under shock force.

impact strength: a measure of the toughness of the material, as the energy required break a specimen with a single blow.

inhibitor: a material used to suppress a chemical reaction.



logy: sluggish, low snap or recovery of a material


masterbatch: a homogeneous mixture of rubber and one or more materials in known proportions for use as a raw material in the preparation of the final compounds. Masterbatches are used to facilitate processing or enhance the properties of the final product, or both.

mastication: the breakdown or softening of raw rubber by the combined action of mechanical work (shear), and atmospheric oxygen, sometimes accelerated by the use of a peptizer, and frequently at elevated temperatures.

memory: the tendency of a rubber material to return to its original shape after deformation.

mixer: a machine that incorporates and disperses compounding ingredients into rubber to form a mix or a compound through the action of mechanical work (shear).

mixer, internal: a machine with a closed chamber in which specially shaped rotors masticate the rubber or incorporate and disperse compounding materials, or both, into the rubber.

monomer: a low molecular weight substance consisting of molecules capable of reacting with like or unlike molecules to form a polymer.


nominal length: the desired length from which tolerances are set.


oil resistant: ability of a vulcanized rubber to resist the swelling and deteriorating effects of various type oils.

O-Ring: an elastomeric seal of homogeneous composition molded in one piece to the configuration of a torus with circular cross-section (doughnut).  The O-Ring is used as a dynamic or static seal usually installed in a machined groove.

oxidation: the reaction between oxygen and a rubber vulcanizate, usually detected by a change in the appearance or feel of the surface, or by a detrimental change in physical properties.

oxygen bomb: a pressure-resistant apparatus, used in an aging test, in which rubber is deteriorated in hot, compressed oxygen.

ozone resistant: withstands the deteriorating effects of ozone (generally cracking).


packing (mechanical): a deformable material used to prevent or control the passage of matter between surfaces that move in relation to each other.

permeability: the permeation rate divided by the pressure gradient of the gas or vapor.

pigment: an insoluble compounding material used to impart color to rubber.

plasticity: a characteristic of unvulcanized rubber indicated by the degree of retention of deformation after removal of the deforming force.

polymer: a substance consisting of molecules characterized by the repetition of one or more types of monomeric units.


rebound test: the method of determining the resilient properties of vulcanized rubber by measuring the rebound of a steel ball or pendulum from a defined height onto a rubber sample.

recipe: a formula, mixing procedure, and any other instructions needed for the preparation of a rubber compound.

reclaimed rubber: vulcanized rubber that has been thermally, mechanically, and/or chemically plasticized for use as a rubber diluent, extender, or processing aid.

resilience: capable of returning to original size and shape after deformation.

recovery: the degree to which a rubber product returns to its normal dimensions after being distorted.

RMS: root mean square. A measure of surface roughness, calculated as the square root of the sum of the squares of micro-inch deviation from true flat.

rubber: a material that is capable of recovering from large deformations quickly and forcibly, and can be, or already is, modified to a state in which it is essentially insoluble.

rubber, bound: the portion of the rubber in a mix that is so closely associated with the filler as to be unextractable by the usual rubber solvents.

rubber, cellular, closed cell: a cellular material in which practically all the individual cells are non-connecting.

rubber, expanded: cellular rubber having closed cells made from a solid rubber compound.

rubber hardness degree, international: a measure of hardness, the magnitude of which is derived from the depth of penetration of a specified indenter into a specimen. Commonly referred to as IRHD.

rubber, Nitrile (Buna N): Copolymer of butadiene and acrylonitrile, known commercially as Butaprene, Chemigum, Hycar OR, Perbuna and Paracril.

rubber, oil-extended: a grade of raw rubber containing a relatively high proportion of processing oil.

rubber, raw: natural or synthetic elastomer, usually in bales or packages, that is the starting material for the manufacture of rubber articles.

rubber, silicone: semi-organic elastomer, containing silicone.

rubber, sponge: cellular rubber consisting predominantly of open cells and made from a dry rubber compound.


seal (mechanical): any material or device that prevents or controls the passage of matter across the separable members of a mechanical assembly.

service: operating conditions to be met.

shaft: rotating member within cylinder; not in contact with walls.

shelf life: the period of time after production during which a material or product, that is stored under specified conditions, retains its intended performance capabilities.

shock load: the sudden application of an external force.

shrinkage: the linear contraction, upon cooling, of a molded rubber part.

size, actual: the actual dimensions of an O-Ring, including tolerance limits.

size, nominal: the approximate size of an O-Ring expressed in fractional dimensions.

skin: a relatively dense layer at the surface of a cellular polymeric material.

softener: a compounding material used to produce a mix of reduced viscosity, which facilitates incorporation of rubber additives.

spiral failure: seal failure in reciprocating applications, that results from a twisting action that strains or ruptures the rubber.

splice: the uniting of two parts of a vulcanized rubber product to form a continuous length.

squeeze: the deformation or interference placed on a rubber component to affect a seal.

stabilizer: a substance present in or added to raw rubber to maintain properties at or near their initial values during its production, processing, and storage.

static seal: part designed to seal between parts having no relative motion. Opposite of a dynamic seal.

stiffness, bending: the force required to produce a bent configuration under specified conditions.

strainer: a machine designed to force a rubber or rubber mix through a sieve to remove extraneous material.

stock: unvulcanized, mixed rubber compound of a definite composition.

stress: force per unit of original cross-sectional are required to stretch a specimen to a stated elongation.

sun checking: surface cracks or crazing caused by exposure to direct or indirect sunlight.

swelling: the increase in volume of a specimen immersed in a liquid or exposed to a vapor.


tack, rubber: the property that causes contacting surfaces of unvulcanized rubber to adhere to each other.

tear: mechanical rupture initiated and propagated at a site of high-stress concentration caused by a cut, defect, or localized deformation.

tear strength: the maximum force required to tear a specified test specimen, the force acting substantially parallel to the major axis of the specimen.

temperature range: lowest temperature at which rubber remains flexible and highest temperature at which it will function.

tensile set: the extension remaining after a specimen has been stretched then allowed to retract in a specified manner. Expressed as a percentage of the original length.

tensile strength: force in pounds per square inch (or kilograms per square centimeter) required to cause the rupture of a specimen of a rubber material.

tensile stress: a stress applied to stretch a test specimen.

terpolymer: a polymer consisting of three different monomers chemically combined.

thermal carbon black: a soft carbon black formed by the repeated tensile deformation.

tension set: the extension remaining after a rubber specimen has been stretched and allowed to retract thermal decomposition of natural gas.

thermal degradation: irreversible and undesirable change in the properties of a material due to exposure to heat.

thermoplastic: a polymer that repeatedly can be softened by heating and hardened by cooling through a temperature range characteristic of the polymer, and in the softened state, can be shaped into articles.

thermoplastic elastomer (TPE): a diverse family of rubber-like materials that, unlike conventional vulcanized rubbers, can be processed and recycled like thermoplastic materials. Often referred to as TPR (thermoplastic rubber).

torque: the turning power of a shaft.


U-cups or U-Seals: U-cups are a lip seal, named for the cross-section’s distinctive “U” shape. They are used for both dynamic and static applications. The “U” shape energizes the sealing lips as the application pressure increases. U-cups come in Loaded, Unloaded, Homogenous, Z-Seal and Spring energized styles.


viscoelasticity: a combination of viscous and elastic properties in a material, with the relative contribution of each being dependent on time, temperature, stress, and strain rate.

viscosity: the resistance of a material to flow under stress.

viscosity, Mooney: a measure of the viscosity of a rubber or rubber compound determined in a Mooney shearing disk viscometer.

volume swell: increase in physical size caused by the swelling action of a liquid.


water absorption: the amount of water absorbed by a material under specified test conditions.

weathering: the surface deterioration of a rubber article during outdoor exposure.

width: radial dimension. For packing rings, or sets the term “packing space” is preferred.

wiper ring: a ring employed to remove excess fluid, mud, etc., from a reciprocating member before it reaches the packing.

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