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A

ABRASION - progressive wearing away of a surface in service by mechanical action such as scraping, rubbing, or erosion.

ABRASION RESISTANCE - resistance of a rubber compound to wearing away when in dynamic contact with an abrasive surface.

ABSORPTION - physical mechanism by which one substance attracts and takes up another substance (liquid, gas, or vapor) into its interior.

ACCELERATED LIFE TEST - any set of test conditions designed to reproduce in a short time the deterioration obtained under normal service conditions.

ACCELERATED SERVICE TEST - bench or service test in which a particular service condition, such as speed, temperature, or continuity of operation, is exaggerated so as to obtain a more rapid result.

ACCELERATOR - chemical which speeds the vulcanization of an elastomer, so that it takes place in a shorter time or at a lower temperature. Picking up where an activator leaves off, an accelerator is often used in conjunction with a catalyst, hardener, or curing agent.

ACID RESISTANT - able to withstand the degrading effects of acids.

ACTIVATOR - chemical which initiates the vulcanization of an elastomer.

ACTUAL SIZE - exact size of an O-ring or seal in decimal dimensions (inches or millimeters), including tolerances.

ADDITIVE - material added to an elastomeric compound to alter its properties, e.g. a reinforcing agent to improve strength or a plasticizer to aid flexibility and processibility.

ADHERE - (a) to cling or stick together; or (b) to cause two surfaces to stick together.

ADHESION - tendency of rubber or other material to stick to a contact surface; may result from chemical or physical interlocking.

ADDHESIVE - substance used to hold materials together.

ADSORPTION - physical mechanism by which one substance attracts another substance (either solid, liquid, gas, or vapor) to its surface.

AERATION - air (or gas) bubbles built up within a liquid.

AFTER CURE - uncontrolled continuation of vulcanization after the desired cure has been effected and the heat source removed; not the same as post cure.

AGING - change in rubber characteristics over time brought about by environmental factors such as heat and light.

AIR CHECKS / TRAPS - surface marks or depressions on a molded rubber product resulting from air getting trapped between the material being cured and the mold surface.

AIR CURING - vulcanization of rubber in air as opposed to steam or press vulcanization.

ALCOHOLS - organic compounds containing the hydroxyl (-OH) group; used as starting points in the production of synthetic resins, synthetic rubbers, and plasticizers.

ALIPHATIC HYDROCARBONS - organic compounds recognizable by their straight chains of carbon atoms. Three subgroups comprise aliphatic hydrocarbons: paraffins (alkanes), olefins (alkenes), and acetylenes (alkynes).

AMBIENT TEMPERATURE - temperature of the environment surrounding a component; not necessarily the same as atmospheric temperature.

AMINE - chemical used as a curing agent for fluoroelastomers; also a film-forming inhibitor used to prevent corrosion in oil-field tubular goods.

AMORPHOUS - non-crystalline in structure; may be used in reference to polymers whose molecular chains are irregular and that therefore do not fit closely together.

ANILINE POINT - lowest temperature at which equal volumes of aniline and a petroleum fluid will completely dissolve in one another. The aniline point of oil is a measure of the aromatic content or the amount of unsaturated hydrocarbons present. The lower the aniline point, the higher the level of unsaturants, and the higher the potential for swelling certain rubber compounds.

ANTI-DEGRADANT - chemical added to an elastomeric compound to shield against the degrading effects of environmental elements like oxygen or ozone.

ANTI-EXTRUSION RING (DEVICE) - relatively hard, high modulus ring (or similar device) placed in the gland between the O-ring and the groove side walls, to prevent extrusion of the seal into the clearance gap; also known as a back-up ring.

ANTIOXIDANT - chemical added to a rubber compound to resist oxidation.

ANTIOZONANT - chemical added to a rubber compound to resist ozone (O3) degradation.

AROMATIC HYDROCARBONS - organic compounds recognizable by their rings of carbon atoms. Benzene, for example, is a six carbon ring with three double bonds. Other aromatic hydrocarbons include toluene and xylene (see Figure 148).

AS 568A - Aerospace Standard Uniform Dash Numbering System; specifies O-ring sizes based on their inside diameter (I.D.) and cross-section (W); supersedes and cancels AS 568 and ARP 568.

ASSEMBLED STRETCH - amount of stretch as measured once a seal is seated in the groove.

ATMOSPHERIC CRACKING - cracking and degradation of the physical properties of a rubber product exposed to atmospheric conditions; also known as weathering.

ATOM - smallest unit of an element that (a) still retains all the properties of that element; and (b) is capable of entering into a chemical reaction.

ATOMIC NUMBER - the number of protons within the nucleus of an atom. For example, carbon has six protons and its atomic number is 6. Elements are listed in order of their increasing atomic numbers in the Periodic Table.

ATOMIC WEIGHT - sum of the masses of the protons and neutrons within the nucleus of a given atom. Because their weight is negligible, electrons are not included in this total; also known as atomic mass.

AXIAL SEAL - an O-ring that seals on a plane perpendicular to its axis instead of on its outside diameter (O.D.) or inside diameter (I.D.); also known as a face seal.

AXIAL SQUEEZE - compression on an O-ring's top and bottom surfaces, as with face (flange) type designs.

B

BACK-UP RING - relatively hard, high modulus ring placed in the gland between the O-ring and the groove side walls, to prevent extrusion of the seal into the clearance gap; also known as an anti-extrusion ring or device.

BACKRIND - ragged indentation at the parting line of a finished rubber product resulting from molding stresses.

BANBURY MIXER - specific type of internal mixer in which rubber compounds are blended.

BI-DIRECTIONAL SEAL - seal which provides fluid sealing on both sides (see Figure 149).

BLEEDING - migration of plasticizers, waxes, or other compound ingredients to the surface of a molded rubber product; also known as blooming.

BLEMISH - mark or deformity on the surface of a molded product.

BLISTER - raised area on the surface of a molded product caused by the pressure of internal gases.

BLOOM - creamy or dusty deposit appearing on the surface of a molded rubber product; caused by the migration of certain compound ingredients to the rubber’s surface after molding and storage.

BLOOMING - migration of plasticizers, waxes, or other compound ingredients to the surface of a molded rubber product; also known as bleeding.

BOND - (a) to unite two materials; or (b) the mechanical, chemical, or adhesive force which binds an elastomer to another object. Mechanical bonds use interlocking design characteristics to ensure continued physical contact. Chemical bonds are based on internal cross-linking. Adhesive bonds rely on cements or other external adhesives.

BREAK-OUT FRICTION - static frictional force which must be overcome to initiate movement; also known as static friction or stiction (see Figure 161).

BRITTLENESS - tendency to crack upon deformation.

BRITTLENESS POINT - lowest temperature at which a rubber sample will not fracture or crack when struck once.

BUNA N - copolymer of butadiene and acrylonitrile; also known as NBR or nitrile rubber.

BUNA S - copolymer of butadiene and styrene; also known as SBR or styrene butadiene rubber.

BUTT JOINT - joining two seal ends such that the junction is perpendicular to the mold parting line.

BUTYL - copolymer of isobutylene and isoprene.

C

C (° C) - degrees Centigrade (Celsius).

CATALYST - chemical that causes or accelerates the cure of a rubber compound, but that does not usually become a chemical component of the end product.

CAVITY - hollow space within the mold in which uncured rubber is shaped and vulcanized; also known as mold cavity.

CHAIN EXTENDER - chemical combined with a polyurethane pre-polymer; acts much like a cross-linking or vulcanizing agent used to cure rubber.

CHAIN SCISSION - breaking of molecular bonds within the backbone of a polymer due to chemical or thermal attack that divides the polymer chains into smaller segments, with a resulting loss in physical properties; also known simply as scission.

CHAMFER - beveled edge in a component to facilitate assembly of a seal onto a rod or shaft, or into a cylinder or housing; also known as a lead-in chamfer (see Figure 155).

CHECKING - cracking or crazing of an elastomer’s surface due to the action of sunlight; also known as sun checking.

CHLORINATED HYDROCARBONS - organic compounds having chlorine and hydrogen atoms in their chemical structure. Examples include trichloroethylene, methylene chloride, and methyl chloroform.

CHLORINATION - surface treatment using chlorine gas that reduces break-out and running friction in molded rubber seals.

CLEARANCE GAP - the gap between two mating surfaces.

CLEAVAGE - breaking of any chemical bond; most commonly refers to the breaking of cross-link bonds between polymer chains or sidegroups that are pendent to the polymer backbone.

COEFFICIENT OF THERMAL EXPANSION - may be linear or volumetric: (a) the coefficient of linear thermal expansion is the change in length per unit of length for a one degree rise in temperature; and (b) the coefficient of volumetric thermal expansion is the change in volume divided by the product of the original volume and the change in temperature. The coefficient of volumetric thermal expansion is three times the coefficient of linear thermal expansion for a solid material.

COLD FLEXIBILITY - ability of an elastomeric product to resist cracking or breaking when flexed or bent at low temperatures; also known as low temperature flexibility.

COLD FLOW - increasing deformation of a rubber material under a constant compressive load; also known as creep.

COLD RESISTANT - able to function in low temperature applications.

COMMERCIALLY SMOOTH - surface smoothness that is acceptable for use.

COMPATIBILITY - a seal material's resistance to having its chemical (and by extension, its physical) properties degraded (either temporarily or permanently) as a result of contact with a liquid or gas.

COMPOSITE SEAL - seal composed of two (or more) separate materials, such as rubber and metal, generally bonded together.

COMPOUND - (a) molecules made up of differing atoms; and (b) a mixture of polymers and other ingredients to produce an elastomeric material.

COMPRESSION MODULUS - ratio of compression stress (force in psi) to resulting compression strain (noted as a percentage of the original specimen thickness).

COMPRESSION MOLDING - thermoset molding technique (see Figure 150) in which the uncured rubber compound is put in a heated, open mold cavity and the mold is closed under pressure (often in a hydraulic press). The material flows to completely fill the cavity. Pressure is maintained until curing is complete.

COMPRESSION SEAL - seal effected by compressing a rubbery material between mating surfaces.

COMPRESSION SET - (a) the amount, expressed as a percentage of deflection, by which a rubber specimen does not return to its original thickness following release of a compressive load; and (b) the end result of a progressive stress relaxation. In terms of the life of a seal, stress relaxation is like dying, whereas compression set is like death.

CONDUCTIVE RUBBER - rubber material that is capable of conducting electricity, usually static electricity. To be classified as conductive, an elastomer must have a direct current resistivity of less than 105 ohm/cm.

COPOLYMER - polymer composed of two different monomers, chemically combined. For example, Buna N is a copolymer of butadiene and acrylonitrile.

CORROSION - progressive wearing away of a surface in service by chemical action.

CORROSIVE - material property that promotes corrosion of a mating sealing surface.

COVALENT BOND - bond between atoms consisting of a pair of shared electrons.

CRACKING - sharp breaks or fissures in a rubber surface caused by excessive strain and/or exposure to detrimental environmental conditions, such as ozone, weather, or ultraviolet (UV) light; also known as crazing.

CREEP - increasing deformation of a rubber material under a constant compressive load; also known as cold flow.

CRITICAL TEMPERATURE (Tc) - (a) regarding gases, the temperature above which a gas cannot be liquefied, regardless of the amount of pressure applied to it; and (b) regarding rubber compounds, the temperature above which a rubber can no longer strain crystallize.

CROSS-SECTION - (a) view of a seal, cut at right angles to the mold parting line, exposing the seal’s internal structure; and (b) one-half the difference between the outside diameter (O.D.) and inside diameter (I.D.) of a seal; also known as width (W).

CRYOGENIC - pertaining to very low temperatures. Some molded articles are deflashed in cryogenic chambers.

CRYSTALLINE - containing crystals; may be used in reference to polymers whose molecular chains are very regular and that therefore fit closely together into a rigid pattern.

CURE - heat-induced process whereby the long chains of the rubber molecules become cross-linked by a vulcanizing agent to form three-dimensional elastic structures. This reaction transforms soft, weak, non-cross-linked materials into strong elastic products; also known as vulcanization.

CURE CURVE - graphic representation plotted by a batch testing device (such as an oscillating disk rheometer) showing a rubber sample's state of cure for a given time and temperature.

CURE DATE - the quarter and year indicating the molding date of a rubber part. For example, “1Q00” denotes a cure date in the first quarter (January, February, or March) of 2000.

CURING TEMPERATURE - temperature at which a rubber product is vulcanized.

CYCLE TIME - the time that elapses between a given point in one molding cycle and the same point in the next cycle (for example, loading of raw stock, through molding and unloading of finished parts, then back to reloading again). Generally speaking, the longer the cycle time, the more the process costs and the more expensive the finished part will be.

CYLINDER - chamber in which a piston, ram, rod, or shaft operates.

D

DAMPER - device capable of minimizing motion or dissipating energy, such as a shock absorber. Because an elastomer has a viscous phase, it can be thought of as a damper, i.e. the elastomer resists motion (deformation), making it an effective seal material.

DASH NUMBER - three-digit number preceded by a dash as specified by SAE Aerospace Standard 568A to indicate the O-ring size based on its inside diameter (I.D.) and cross-section (W); also known as size number.

DEFLASH - process of removing excess material (flash) from the parting line of a molded rubber product.

DEFLECTION - change in the shape of a seal as a result of compression; also known as deformation.

DEGASSING - the intentional, controlled evaporation of volatile substances out of a rubber material.

DEGRADATION - breakdown in chemical structure and/or loss of physical properties after exposure to harmful agents (such as heat, sunlight, oxygen, ozone, or weather).

DIAMETRAL CLEARANCE GAP - the difference in diameters between two mating surfaces to be sealed.

DIENE RUBBER - rubber containing a double bond in the main chain; such double bonds are vulnerable to attack (such as by oxygen, ozone, and UV light).

DIFFERENTIAL PRESSURE - difference in the amount of force being exerted on the high-pressure side of a seal (the side facing system pressure) relative to the low-pressure side (the side facing away from system pressure). Differential pressure is responsible for forcing a seal toward the low pressure side of a gland (see Figure 151).

DIISOCYANATE - hard segment in the polyurethane backbone; imparts toughness and heat resistance.

DOUBLE-ACTING SEAL - dynamic reciprocating seal capable of sealing in both directions of movement.

DOUBLE BOND - covalent bond consisting of two pairs of shared electrons. A double bond occurring between two carbon atoms (such as is found in the butadiene segment of nitrile rubber) is inherently more chemically reactive and is a site for both cross-linking and chemical attack (see Figure 152).

DRY RUNNING - absence of liquid or lubrication in a dynamic sealing application.

DUROMETER - (a) an instrument that measures the hardness of rubber by its resistance to surface penetration of an indenterpoint; and (b) the numerical scale indicating the hardness of rubber. See also “Shore A Durometer” and “Shore D Durometer.”

DYNAMIC - describes an application in which the mating surfaces to be sealed are in relative motion to each other.

DYNAMIC FRICTION - friction resulting from relative motion between two contacting surfaces.

DYNAMIC SEAL - seal functioning in an environment in which there is relative motion (e.g. reciprocating, rotary, or oscillating) between the mating surfaces being sealed.

E

ELASTICITY - an elastomer’s inherent ability to readily regain its original size and shape after being released from a deforming load.

ELASTOMER - any natural or synthetic material meeting the following requirements: (a) it must not break when stretched 100%; and (b) after being held at 100% stretch for five minutes then released, it must return to within 10% of its original length within five minutes.

ELASTOMERIC COMPOUND - combination of a base polymer and additives.

ELECTRON - small, negatively-charged particle orbiting the nucleus of an atom; for electrically-neutral atoms, the number of electrons equals the number of positively-charged protons within the nucleus.

ELEMENT - term referring to a single type of atom making up a substance.

ELONGATION - percentage increase in original length (strain) of a specimen produced by a tensile force (stress) applied to the specimen. “Ultimate elongation” is the elongation at the moment the specimen breaks.

ENCAPSULATION - enclosure or jacket surrounding another material; for example, a Teflon® encapsulation over an O-ring core molded from a different material.

ENDOTHERMIC - absorbing heat.

EVAPORATION - direct conversion of a fluid from liquid to vapor.

EXOTHERMIC - giving off heat, as during a chemical reaction.

EXPLOSIVE DECOMPRESSION - phenomenon occurring in rubber seals after exposure to high-pressure gas. This gas permeates into the elastomer through flaw sites present in all molded rubber products. During an equilibrium shift (lowered pressure), the gas then expands within the seal, causing internal ruptures in high shear modulus (hard) materials and surface blisters in low shear modulus (soft) materials. Explosive decompression can be likened to “getting the bends.”

EXTEND - add fillers or other low-cost materials to an elastomeric mixture in an effort to reduce costs and to increase the amount of compound that is available for use, i.e. “extend” its usage.

EXTENDER - relatively inexpensive and inert material added to an elastomeric compound to reinforce or modify properties (e.g. physical, mechanical, electrical, thermal), impart certain processing properties, or reduce costs; also known as a filler.

EXTRACTION - removal from a material, as when fuel or other system fluids chemically remove a compound’s plasticizer, leading to seal shrinkage.

EXTRUSION - pressure-induced distortion or extension of part of a seal into the clearance gap between mating seal surfaces.

F

F ( ° F) - degrees Fahrenheit.

FACE - front surface of a seal; in an O-ring, the two surfaces that are perpendicular to its axis.

FACE SEAL - an O-ring that seals on a plane perpendicular to its axis instead of on its outside diameter (O.D.) or inside diameter (I.D.); also known as an axial seal.

FATIGUE RESISTANCE - capable of withstanding fatigue caused by repeated bending, extension, or compression; also known as flex resistance.

FILLER - relatively inexpensive and inert material added to an elastomer to reinforce or modify properties (e.g. physical, mechanical, electrical, thermal), impart certain processing properties, or reduce cost; also known as an extender.

FLASH - excess rubber remaining on the parting line of a molded rubber product (see Figure 153).

FLAWS - surface imperfections that occur infrequently (i.e. not in a pattern), as with an isolated scratch or crack in the metal of a gland.

FLEX CRACKING - surface cracks caused by repeated flexural cycling.

FLEX RESISTANCE - capable of withstanding fatigue caused by repeated bending, extension, or compression; also known as fatigue resistance.

FLOW LINES - imperfections in a molded rubber product caused by imperfect flow of the material during molding; also known as flow cracks or flow marks.

FLUID - a liquid or a gas.

FLUOROCARBON - carbon backbone, organic compound having fluorine atoms in its chemical structure. Presence of the fluorine provides increased chemical and high temperature resistance.

FRICTION - motion resistance resulting from contact between mating surfaces, usually accompanied by liberation of heat energy.

FRICTION (BREAK-OUT) - static frictional force which must be overcome to initiate movement; also known as static friction or stiction (see Figure 161).

FRICTION (RUNNING) - dynamic frictional force which must be overcome to maintain movement.

FUEL (AROMATIC) - fuel containing aromatic (ringed) hydrocarbons (such as benzene, toluene, and xylene). Aromatic fuels cause high swell of rubber.

FUEL (NON-AROMATIC) - fuel containing aliphatic (straight chain) hydrocarbons (such as octane). Non-aromatic fuels cause less rubber swell than aromatic fuels.

G

GASKET - static seal effected when a deformable material is sandwiched and compressed between two mating surfaces.

GATE MARK - raised spot or small depression seen on an injection or transfer molded product; caused when the finished molded part is removed from the injection nozzle (gate or sprue) through which the material is injected into the mold cavity; also known as a sprue mark.

GLAND - machined cavity into which an O-ring or other seal is fitted; includes the groove and the mating surface to be sealed.

GLASS TRANSISTION (Tg) - temperature at which a viscous polymer loses all ability to flow or store energy, becoming hard and brittle (like glass).

GOUGH-JOULE EFFECT - tendency of a stretched rubber specimen to retract when heated.

GROOVE - machined recess within a gland into which an O-ring or other seal is fitted.

H

HARDNESS - measure of rubber’s relative resistance to an indenter point on a testing device. Shore A durometers gauge soft to medium-hard rubber. Shore D durometers are more accurate on samples harder than 90 Shore A.

HEAT AGING - loss of physical properties as a result of exposure to heat.

HEAT BUILD-UP - temperature rise in a molded rubber product due to hysteresis during repeated deformations.

HEAT RESISTANCE - rubber compound’s capacity to undergo exposure to some specified level of elevated temperature and retain a high level of its original properties.

HERMETIC SEAL - an airtight seal.

HETEROPOLYMER - polymer composed of differing monomers.

HOMOGENOUS - used to describe a rubber material of uniform composition, with no fabric or metal reinforcement.

HOMOPOLYMER - polymer composed of identical monomers.

HYDROCARBONS - organic compounds with both hydrogen and carbon in their chemistry. Many organic compounds are hydrocarbons. Aliphatic hydrocarbons, such as butane, have a straight-chain structure. Aromatic hydrocarbons, such as benzene, are ringed structures.

HYDROGENATION - addition of hydrogen atoms to an organic compound to reduce the number of carbon-to-carbon double bonds that would otherwise be weak links in the polymer chain. For example, the hydrogenation of nitrile produces a great compound (HNBR) with both high strength and superior oxidation resistance.

HYDROGEN BOND - an electrostatic attraction between a hydrogen atom in one molecule and a small electronegative atom (like fluorine, oxygen, or nitrogen) in an adjoining molecule. Though not nearly as strong as covalent bonds, hydrogen bonds are present in such numbers in hydrocarbon polymers that they are an important source of polymer strength.

HYDROLYSIS - chemical decomposition as a result of contact with water.

HYGROSCOPIC - capable of absorbing moisture, especially from the air.

HYSTERESIS - percent of energy lost per cycle of deformation, or 100% minus the resilience percentage. Hysteresis is the result of internal friction and is evident by the conversion of mechanical energy into heat.

I

I.D. - inside diameter of a seal or component.

IDENTIFICATION - colored stripes or dots on seals to differentiate among rubber compounds.

IMMERSION - putting an article into a fluid so that it is totally covered.

IMMISCIBLE - not capable of being mixed. With elastomers, “immiscible” is generally analogous to “insoluble” and refers to a substance (such as a seal) that cannot be dissolved in a fluid (such as the fluid being sealed). In order to have long seal life, it is important to maximize immiscibility.

IMPACT - forceful contact between two bodies, at least one of which is in motion.

INERT - inactive or non-reactive; often used to describe a material (like Teflon®) that is impervious to many chemicals.

INHIBITOR - chemical added to an elastomeric compound to ensure vulcanization does not proceed too quickly.

INJECTION MOLDING - process in which preheated rubber is injected under pressure from the heating chamber through a series of runners and sprues and into a closed, heated mold cavity, then vulcanized. Injection molding is ideal for high-volume production of molded rubber parts (see Figure 154).

INORGANIC - containing chemical structures not based on the carbon atom.

INSOLUBLE - not susceptible to being dissolved in a fluid.

INSTALLATION STRETCH - amount of stretch that a seal undergoes as it is being placed in the groove.

ION - atom with an electrical charge (either positive or negative) due to unequal numbers of protons and electrons. An ion with more protons than electrons will have a positive charge, whereas an ion with more electrons than protons will have a negative charge.

IONIC BOND - strong electrical attraction between oppositely charged atoms (ions).

ISO - International Organization for Standardization, a non-governmental organization whose primary aim is to develop guidelines on what constitutes an effective quality management system.

ISOTOPE - one of two or more distinct forms of a given element. Isotopes have the same atomic number (due to identical numbers of protons) but different atomic masses (due to unlike numbers of neutrons).

K

K ( ° K) - degrees Kelvin. 0° K (also known as Absolute Zero) is equal to -273° C.

L

LAY - direction of the primary roughness pattern on a gland surface.

LEACHING - removal of soluble components, as when system fluids remove a compound’s plasticizer, leading to seal shrinkage.

LEAD-IN (CHAMFER) - beveled edge in a component to facilitate assembly of a seal onto a rod or shaft, or into a cylinder or housing (see Figure 155).

LEAK RATE - rate at which a fluid (liquid or gas) passes a seal or barrier.

LIFE TEST - laboratory test used to determine the length of a product’s life in a defined set of service conditions.

LOAD - actual pressure at a sealing face; normally the sum of the interference load and the fluid pressure at work on the seal.

LOGY - term used to describe a material with poor visco-elastic properties.

LOW TEMPERATURE FLEXIBILITY - ability of an elastomeric product to resist cracking or breaking when flexed or bent at low temperatures.

M

MACROMOLECULE - large chainlike molecule, formed during a process called “polymerization,” in which small molecules (monomers) form chemical bonds between one another; also known as a polymer.

MATING SURFACES - points where different parts of an assembly meet.

MAXIMUM CURE - point at which a rubber sample is cured as much as possible without being over-cured.

MAXIMUM TEMPERATURE - highest temperature a rubber compound can withstand prior to undergoing a physical or chemical change.

MEMORY - an elastomer’s ability to regain its original size and shape following deformation.

MICROPORES - very tiny pores on the surfaces of a gland. The presence of micropores, even on finely-machined metal surfaces, contributes to break-out friction. However, these pores also help hold lubricants, so their total elimination is not advantageous.

MINIMUM TEMPERATURE - lowest temperature a rubber compound can withstand prior to losing rubbery properties.

MISCIBLE - capable of being mixed. In the case of elastomers, “miscible” is generally analogous to “soluble” and refers to a substance (such as an elastomeric seal) that can be dissolved in a fluid (such as the fluid being sealed). In order to have long seal life, it is important to minimize miscibility.

MISMATCH - asymmetrical seal cross section caused by dimensional or mating differences in mold sections.

MODULUS - the force in psi (stress) required to produce a certain elongation (strain), usually 100%; a good indication of toughness and resistance to extrusion; also known as tensile modulus or tensile stress.

MODULUS OF ELASTICITY - ratio of the stress (force in psi) to the strain (percent increase in original length) as measured on a rubber specimen; also known as Young’s modulus (E); not the same as tensile modulus.

MOISTURE RESISTANCE - able to resist absorbing moisture from the air or during water immersion.

MOLD - (a) to shape or process a material into a usable form; and (b) metal tools, usually steel or aluminum, machined and assembled so as to create openable cavities for the purpose of shaping and vulcanizing rubber.

MOLD CAVITY - hollow space within the mold in which uncured rubber is shaped and vulcanized; also known simply as a cavity.

MOLD FINISH - surface finish of the mold; determines the surface finish of any product taken from that mold.

MOLD LUBRICANT - coating used in the mold cavity to prevent a molded rubber product from sticking to the cavity during removal; also known as mold release.

MOLD MARKS - imperfections in a molded rubber product replicating surface defects on the mold itself.

MOLD REGISTER - accuracy of alignment of mold plates and cavities. An improperly aligned mold is said to be off-register and will produce mismatched parts.

MOLD RELEASE - coating used in the mold cavity to prevent a molded rubber product from sticking to the cavity during removal; also known as mold lubricant.

MOLD SHRINKAGE - dimensional loss in a molded rubber product that occurs during cooling after it has been removed from the mold.

MOLD STORAGE - holding area in which removable mold plates are stored when not in use (see Figure 156).

MOLECULAR WEIGHT - sum of the atomic masses of the elements forming a molecule.

MOLECULE - an electrically neutral aggregate of chemically bonded atoms.

MONOMER - small molecule capable of reacting with other molecules to form large chainlike molecules (macromolecules) called polymers.

MOONEY VISCOMETER - shearing disk device used to gauge the viscosity of a rubber sample under heat and pressure. Named for developer Melvin Mooney, this was once the standard tool for determining processing characteristics but has now largely been replaced by the rheometer.

MULTIPLE CAVITY MOLD - mold in which more than one article can be made at a time.

N

NEUTRON - non-charged particle within the nucleus of an atom; hydrogen is the only atom which contains no neutrons.

NIBBLING - progressive mode of seal failure that occurs when excessive pressure forces a portion of an O-ring or other rubber seal into a clearance gap. Expansion and contraction of the gap (breathing) caused by pressure cycling traps extruded portions of the seal in the gap, resulting in bite-like portions (nibbles) being removed from the seal (see Figure 157).

NITRILE (BUNA-N) - copolymer of butadiene and acrylonitrile widely used in O-rings and other seals.

NOMINAL SIZE - approximate size of an O-ring or seal in fractional dimensions (inches); typically given solely for reference purposes; also known as nominal dimension.

NON-FILL - defect in a finished molded part caused by the rubber failing to completely fill the mold cavity.

O

OCCLUSION - (a) mechanical process by which vapors, gases, liquids, or solids are entrapped within the folds of a given substance during working or solidification; and (b) the materials entrapped by this process.

O.D. - outside diameter of a seal or component.

OFF-REGISTER - mismatched O-ring cross-section caused by misalignment of mold cavities.

OIL RESISTANT - ability of vulcanized rubber to resist swelling and deterioration due to oil exposure.

OIL SWELL - increase in volume of a rubber product as a result of oil absorption.

OPTIMUM CURE - vulcanization state yielding the most desirable properties.

ORGANIC - containing chemical structures based on the carbon atom.

O-RING - solid elastomer ring seal of circular cross-section; technically, a torus.

OSCILLATING SEAL - rotary seal with limited, reversing travel; as in an on/off valve.

OUTGASSING - phenomenon occurring in vacuums where the volatile materials in a rubber compound are vaporized and released into the environment.

OVER CURE - longer than optimum vulcanization causing some properties to be degraded. Over-cure can be of two types. In the first type, the material continues to harden, the modulus rises, and both tensile strength and elongation fall. In the second type, the rubber begins to break down. The material softens, and the modulus, tensile strength, and elongation all decrease.

OXIDATION - reaction of oxygen with a rubber compound, usually resulting in surface cracking and/or changes in the physical properties of the material.

OZONE (O3) - unstable form of oxygen (usually generated by electricity) that can cause surface cracking in some elastomers.

OZONE RESISTANCE - ability of a rubber material to withstand exposure to ozone without cracking or otherwise deteriorating.

P

PACKING - generic name for a compression-type dynamic seal housed within a gland.

PARTING LINE - mark on a molded rubber article showing where separate parts of the mold cavity met.

PERIPHERAL SQUEEZE - compression applied to the O.D. of a seal when installed in a bore that is smaller than the O.D. of the seal.

PERMANENT SET - amount of deformation in a rubber part after a distorting load has been removed.

PERMEABILITY - measure of the ease with which a liquid or gas can pass through a rubber material (see Figure 158).

PIGMENT - substance included in a material mixture to colorize it in a specific way.

PIT OR POCK MARK - small surface void in a molded rubber product caused by mechanical erosion (wear) or chemical action.

PLASTICIZER - chemical substance added to a rubber compound to soften the elastomer, provide flexibility at low temperatures, and improve processing; also known as a softener.

POISSON'S RATIO - ratio of the change in width per unit of width to the change in length per unit of length. For most rubber materials, Poisson’s ratio is essentially equal to 0.5.

POLARITY - imbalance in electrical charge (dipole moment) caused by covalent bonds occurring between two dissimilar atoms. The difference in electrical charges of each atom creates a slight negative charge on one atom and a slight positive charge on the other atom. Since hydrocarbon oils are usually non-polar, they are repelled by polymers that have polarity, resulting in increased oil resistance and other properties not found in elastomers containing only carbon and hydrogen atoms.

POLYMER - large chainlike molecules (macromolecules) made up of small repeating units (monomers). When two different monomers are chemically combined, the resulting product is called a copolymer. When three different monomers are involved, the result is a terpolymer.

POLYMERIZE - to chemically unite two or more monomers or polymers to form a molecule with a higher molecular weight.

POLYOL - soft segment in the polyurethane backbone; imparts rubber-like softness and flexibility.

POROSITY - quality or state of having pores or holes in a material.

POST CURE - controlled continuation of vulcanization, usually in an oven, to complete the curing process, drive off residual byproducts, and provide stabilization of parts; not the same as after cure.

POTABLE - (a) drinkable; and (b) a liquid that is safe or suitable for drinking.

PRE-POLYMER - polyurethane polyol and diisocyanate mixture prior to combination with a chain extender.

PROFILOMETER - an instrument used to gauge surface roughness, i.e. to determine the “profile” of a given surface.

PROTON - positively-charged particle within the nucleus of an atom; for electrically-neutral atoms, the number of protons exactly equals the number of negatively-charged electrons orbiting the nucleus. The number of protons in an atom is also said to be that element’s atomic number, e.g. carbon has six protons, so its atomic number is 6.

Q

QUAD RING - solid elastomeric ring seal with a four-lobed cross-section.

QPL - Military Qualified Products List; listing of commercial products shown in pretesting to meet the demands of a specification, particularly a federal specification.

QS 9000 - Quality System developed by the automotive industry to supplement the ISO 9000 standard.

R

RADIAL SEAL - O-ring or seal having compression applied to its outside diameter (O.D.) and inside diameter (I.D.).

RADIAL SQUEEZE - compression on an O-ring's outside diameter (O.D.) and inside diameter (I.D.), as with cap and plug type configurations.

RADIUS - (a) the distance from the center of a circle to the edge, or one-half the diameter; and (b) to round off a sharp corner, as in the “radiusing” of a gland’s top edges to prevent them from nicking or cutting an O-ring during installation.

RECIPROCATING SEAL - dynamic seal used to seal pistons or rods that are in linear motion.

REINFORCING AGENT - material added to an elastomer to improve physical properties such as tensile strength, tensile modulus, and compression modulus.

RELATIVE HUMIDITY - ratio of the amount of water vapor present in the air to the greatest amount that could be present at a given temperature; expressed as a percentage.

RELAXATION - decrease in the force exerted against a mating part by a rubber component that has been under a constant load for a period of time.

REPEATABILITY - consistency of test results taken within a single lab. For example, the similarity (or lack thereof) of multiple durometer readings taken on a single sample with the same tester.

REPRODUCIBILITY - consistency of test results taken among several different labs. For example, the similarity (or lack thereof) of multiple durometer readings taken on a single sample with a series of different testers.

RESILIENCE - a compound's ability to rapidly regain original size and shape following deformation. Also known as rebound.

REVERSION - condition in an elastomer caused by thermal or chemical attack whereby chemical bonds are broken with a resulting loss in physical properties.

RHEOMETER - cure meter which determines and plots a cure curve illustrating the state of cure for a given time and temperature; typically either an Oscillating Disk Rheometer (ODR) or a Moving Die Rheometer (MDR).

ROOT MEAN SQUARE (RMS) - The square root of the sum of the squares of deviation from true flat; a measure of surface roughness (as with glands or shafts) generally noted in microinches.

ROTARY SEAL - seal capable of sealing between a rotating shaft and an outer surface, such as a groove or housing bore.

ROUGHNESS - closely-spaced irregularities on a gland’s surface that are the result of manufacturing and/or cutting (as by tools or abrasive materials, see Figure 159).

ROUGHNESS AVERAGE (Ra) - measure of the roughness of a metal surface; determined by averaging the absolute value of the deviations from a mean line over a set evaluation length.

RUBBER - natural or synthetic elastomeric substance.

RUNNING FRICTION - dynamic frictional force which must be overcome in order to maintain movement. Running friction generally necessitates the use of some form of lubrication.

RUNOUT (SHAFT) - phenomenon which occurs when the shaft’s axis and the axis of rotation are different, causing the shaft to wobble or gyrate; expressed in inches followed by the abbreviation “TIR” (Total Indicator Reading).

S

SATURATED BONDS - single bonds between carbon atoms and other atoms (such as hydrogen) that are less reactive and less prone to chemical attack than carbon-to-carbon double or triple bonds. The carbon atoms in the backbone of an organic polymer are each capable of forming four individual and separate single covalent bonds.

SATURATION - (a) addition of atoms to a compound to occupy the otherwise “open” or unbonded sites on a polymer chain; results in a more stable, less reactive compound. For example, a highly-saturated organic compound has almost every carbon atom already bonded to a hydrogen atom and therefore has a dramatically reduced ability to interact with other compounds and an increased resistance to chemical attack. Saturation using hydrogen atoms is also known as hydrogenation; and (b) state in which most of the carbon atoms in an organic polymer’s backbone have formed four individual and separate covalent bonds, resulting in increased chemical resistance as there are fewer double bonds that are susceptible to chemical attack (see Figure 160).

SCISSION - breaking of molecular bonds within the backbone of a polymer due to chemical or thermal attack that divides the polymer chains into smaller segments, with a resulting loss in physical properties; also known as chain scission.

SCORCHING - premature curing of rubber during storage or processing, usually caused by excessive heat.

SEAL - device that prevents fluid flow.

SEAL WIDTH (W) - axial dimension of a seal. In an O-ring, this is the same as the cross-section.

SERVICE - operating conditions, such as temperature, pressure, chemical environment, and surface speeds, under which a seal must perform.

SERVICE TEMPERATURE - range of temperatures to which a rubber compound will be subjected in a given application.

SHAFT - rotating or reciprocating component that operates within a cylinder or housing.

SHEAR - deformation of a material or surface as a result of sliding or rubbing contact with another surface.

SHEAR MODULUS (G) - measure of stiffness or resistance to deformation taken in shear rather than in tension; technically, the ratio of a shearing stress (force in psi) to shearing strain (amount of linear deflection divided by the specimen thickness). In rubber materials, shear modulus is one-third of Young’s modulus (E); not the same as tensile modulus.

SHELF-AGING - degradation of a rubber material’s properties that occurs in storage over time.

SHELF LIFE - length of time a molded rubber compound can be stored without suffering significant loss of physical properties.

SHORE A DUROMETER - instrument used to gauge soft to medium hard rubber based on resistance to a frustum (truncated) cone indenter point; most accurate for materials below 90 Shore A.

SHORE D DUROMETER - instrument used to gauge hard rubber based on resistance to a sharp, 30° angle indenter point; most accurate for materials at or above 90 Shore A.

SHRINKAGE - (a) after vulcanization, dimensional loss in a molded rubber product that occurs after it has been removed from the mold and allowed to cool; and (b) in seal service, a decrease in seal volume due to extraction of soluble components from the rubber compound by environmental fluids.

SILICONE RUBBER - silicon-oxygen backbone elastomer with excellent high temperature and low temperature properties.

SINGLE-ACTION SEAL - dynamic reciprocating seal capable of sealing in only one direction of movement.

SIZE - actual size refers to the actual dimensions of an O-ring or seal, including tolerances. Nominal size refers to the approximate size in fractional dimensions.

SIZE NUMBER - three-digit number preceded by a dash as specified by SAE Aerospace Standard 568A to indicate the O-ring size based on its inside diameter (I.D.) and cross-section (W); also known as dash number.

SKIVING - slicing of a seal’s surface, as by gland edges during installation.

SOLUBLE - susceptible to being dissolved in a fluid.

SOLVENT - any substance, typically a liquid, capable of dissolving other substances.

SOUR CRUDE - petroleum oil contaminated with hydrogen sulfide (H2S).

SOUR GAS - natural gas contaminated with hydrogen sulfide (H2S).

SPECIFIC GRAVITY - ratio of the weight of a given substance to the weight of an equal volume of water at a specified temperature. Specific gravity is often used to identify rubber compounds.

SPIRAL FAILURE - type of O-ring failure occurring when one portion of the ring tends to roll while another portion slides in the gland, causing twisting and seal failure.

SPRUE MARK - raised spot or small depression seen on an injection or transfer molded product; caused when the finished molded part is removed from the injection nozzle (sprue or gate) through which the material is injected into the mold cavity; also known as a gate mark.

SQUEEZE - compression of an O-ring’s cross-section between mating surfaces; noted as both a decimal measurement (in inches and/or millimeters) and as a percentage of the original cross-section (width). Radial compression occurs on the outside diameter (O.D.) and inside diameter (I.D.). Axial compression occurs on the top and bottom surfaces.

STATIC - describes an application in which there is no relative motion between the mating surfaces to be sealed.

STATIC FRICTION - initial frictional force which must be overcome to initiate movement; also known as break-out friction or stiction (see Figure 161).

STATIC SEAL - seal functioning in an environment in which there is no relative motion between the mating surfaces being sealed.

STICK-SLIP - irregular or jerky seal motion caused by varying amounts of static and dynamic friction.

STICTION - initial frictional force which must be overcome to initiate movement; also known as static friction or break-out friction (see Figure 161).

STOICHIOMETRY, PERCENT - level of curative (chain extender) used on a given pre-polymer. Percentages used have varying effects on the physical properties of the finished elastomer.

STRAIN - amount of deflection, expressed as a percentage of original length, due to an applied force (stress).

STRAIN CRYSTALLIZATION - partial crystallization of an elastomer that temporarily results when a stretching force causes the tangled macromolecular chains to untangle and align to form crystals; the chains revert to their normal state of entanglement when the force is removed. Most elastomers do not strain crystallize, but natural rubber, chloroprene (Neoprene®), and hydrogenated nitrile will.

STRESS - an applied force (in psi) resulting in material deflection (strain).

STRESS RELAXATION - steady decline in sealing force when an elastomer is compressed over a period of time. In terms of the life of a seal, stress relaxation is like dying, whereas compression set is like death.

STRETCH - measured as a percentage increase in the inside diameter (I.D.) of an O-ring, stretch results in a reduction and flattening of the seal’s cross-section. There are two types of stretch: installation stretch (as the seal is being placed in the groove) and assembled stretch (once the seal is seated).

SUBLIMATION - direct conversion of a substance from a solid state to a vapor state, and from a vapor back to a solid. The substance does not become liquid during either transition.

SUN CHECKING - cracking or crazing of an elastomer’s surface due to the action of sunlight; also known simply as checking.

SURFACE FINISH - average value of exterior roughness, often expressed in microinches RMS (Root Mean Square) or Ra (roughness average).

SWELL - volumetric increase of an elastomeric material when in contact with a fluid.

T

TEAR RESISTANCE - resistance to the growth of a nick or cut in a rubber specimen when tension is applied.

TEMPERATURE (MAXIMUM) - highest temperature a rubber compound can withstand prior to undergoing a physical or chemical change.

TEMPERATURE (MINIMUM) - lowest temperature a rubber compound can withstand prior to losing rubbery properties.

TEMPERATURE RANGE - minimum and maximum temperature limits within which a rubber material will effectively perform.

TEMPERATURE (SERVICE) - range of temperatures to which a rubber compound will be subjected in a given application.

TENSILE MODULUS - the force in psi (stress) required to produce a certain elongation (strain), usually 100%; a good indication of toughness and resistance to extrusion; also known as modulus or tensile stress.

TENSILE STRENGTH - force in pounds per square inch (psi) required to break a rubber specimen.

TENSILE STRESS - the force in psi (stress) required to produce a certain elongation (strain), usually 100%; a good indication of toughness and resistance to extrusion; also known as modulus or tensile modulus.

TENSION SET - increase in the length of an elastomeric specimen following initial stretching and release.

TERPOLYMER - polymer composed of three different monomers chemically combined (see Figure 162).

TETRAPOLYMER - polymer composed of four different monomers chemically combined.

THERMAL EXPANSION - linear or volumetric expansion of a material due to a temperature increase.

THERMOPLASTIC - an ionically-bonded polymeric material capable of being softened and formed when heated and injected into a cool mold. Upon cooling in the mold, a thermoplastic material will harden (freeze) and regain its original properties. A thermoplastic material can be reprocessed many times.

THERMOSET - polymeric material that forms permanent covalent bonds in an irreversible chemical reaction known as cross-linking, curing, or vulcanizing. Although the cured part can later be softened by heat, it cannot be remelted or reprocessed without extensive chemical treatment.

TIR - Total Indicator Reading; a measurement of shaft eccentricity that results when the shaft centerline is different from its axis of rotation.

TOLERANCE - allowable deviation (plus and minus) from a specified dimension.

TOLERANCE BUILD-UP - sum of the tolerances of all of the elements in a sealing system (e.g. I.D., cross-section, gland dimension); also known as tolerance stack-up.

TOOL - alternative name for a mold.

TORQUE - turning or twisting force that produces, or tends to produce, rotation of a shaft.

TORSIONAL STRENGTH - ability of a material to resist twisting and its damaging effects.

TORUS - donut-shaped ring; another name for an O-ring.

TPE - thermoplastic elastomer with rubber-like properties that is processed by injection molding, blow molding, extrusion, etc.

TPU - thermoplastic polyurethane elastomer that is processed by injection molding, blow molding, or extrusion.

TRANSFER CHAMBER - area within a transfer mold in which the elastomeric compound is heated prior to being squeezed down through a sprue, a runner, and a gate leading into a closed mold cavity to be shaped and vulcanized; also known as a pot.

TRANSFER MOLDING - method of molding thermosetting materials (see Figure 163). The elastomeric compound is placed in a transfer chamber (pot) which is part of the mold, heated, then squeezed down through a sprue, a runner, and a gate leading into a closed mold cavity to be shaped and vulcanized. The advantages of transfer molding are that vulcanization is faster, so the process is more efficient, and the part is formed with little or no flash.

TRIM - removal of excess material from a molded rubber product.

TRIM CUT - damage done to a molded rubber product by excessive trimming.

U

ULTIMATE ELONGATION - amount, expressed as a percentage of original length, that a specimen has stretched at the time of breakage.

UNDER-CURE - degree of incomplete vulcanization resulting in undeveloped physical properties and tackiness.

UNI-DIRECTIONAL SEAL - seal which provides fluid sealing on only one side.

UNSATURATED BONDS - double or triple bonds between carbon atoms creating sites that can undergo numerous chemical reactions, including addition of hydrogen atoms (hydrogenation), cross-linking, or chemical deterioration such as oxidation.

V

VACUUM - condition in which the pressure in a chamber is less than atmospheric pressure.

VALENCE - ability of an atom to form one or more energy bonds with neighboring atoms.

VAN DER WAALS FORCES - weak electrostatic attractions between polymer chains that are adjacent but that have not yet been cross-linked. These intermolecular forces are at their peak when a material is cool. Heating the material weakens the forces and “loosens” the chains, thus increasing pliability and making molding possible.

VAPOR - a gas, whose temperature is below its critical temperature (tc), that normally exists as a liquid under atmospheric conditions.

VAPOR PRESSURE - pressure exerted by a heated liquid or solid in a closed container.

VENT - (a) to give off excess air or pressure so as to avoid build-up and possible rupture; and (b) a shallow hole or channel designed into a mold to facilitate the escape of air as it is displaced by incoming materials to be molded.

VESTIGE - remnants of a runner system visible on the surface of a molded article.

VISCO-ELASTIC - describes rubber-like materials having both a viscous phase (like a damper) and an elastic phase (like a spring).

VISCOMETER - shearing disk device used to gauge the viscosity of a rubber sample under heat and pressure. Often referred to as the Mooney Viscometer, this device was once the most common tool for determining processing characteristics but has now largely been replaced by the rheometer.

VISCOSITY - resistance to flow; the thicker the substance (such as a liquid), the more viscous it is, i.e. the less it flows.

VOID - unintended empty space, such as a pit or air pocket.

VOLATILE - readily vaporizable at a relatively low temperature.

VOLUME CHANGE - increase (swell) or decrease (shrinkage) in the volume of a specimen which has been immersed in a fluid, noted as a percentage of original volume.

VOLUME SHRINKAGE - volumetric decrease of an elastomeric material when in contact with a fluid; also known simply as shrinkage.

VOLUME SWELL - volumetric increase of an elastomeric material when in contact with a fluid; also known simply as swell.

VULCANIZATE - cured rubber compound.

VULCANIZATION - heat-induced process whereby the long chains of the rubber molecules become cross-linked by a vulcanizing agent to form three-dimensional elastic structures. This reaction transforms soft, weak, non-cross-linked materials into strong elastic products; also known as cure.

VULCANIZING AGENT - material added to an uncured batch of rubber that causes the polymer chains to crosslink to one another (vulcanize), forming a three-dimensional elastic structure; also known as curing agent.

W

WAVINESS - irregularities on a gland’s surface with considerably longer wavelengths than those referenced as roughness. Waviness may be caused by machinery vibrations or material warping.

WEATHERING - cracking and degradation of the physical properties of a rubber product exposed to atmospheric conditions; also known as atmospheric cracking.

WEEPAGE - seal leakage of less than one drop per minute; not necessarily an indication of seal failure.

WIDTH (W) - another term for the cross-section of an O-ring.

WIPER - flexible ring used to remove dirt, dust, mud, and other contaminants from a rod or a shaft in order to prevent them from entering a hydraulic, pneumatic, or mechanical system; also known as a wiper ring.

Y

YOUNG'S MODULUS (E) - a measure of material stiffness; defined as the ratio of the stress (force in psi) to the strain (percentage increase in original length) as measured on a rubber specimen; also known as modulus of elasticity; not the same as tensile modulus.