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       • • • • • • • • Glossary   |
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Elastomer Industry Glossary By R. J. DelVecchio, Technical Consulting Services (919) 567-2288, DelTCS@aol.com © Corry Rubber Corporation and R. J. DelVecchio ASTM - acronym for American Society for Testing and Materials; the organization that codifies and publishes testing procedures for many materials (rubber is covered in Section 9 of their annual books on test methods) ASTM D2000 - a system for writing specifications to describe rubber compounds using various physical and chemical characteristics of the material; however, such descriptions are not unique, two compounds can both meet the specification requirements yet still differ significantly from each other in important ways abrasion resistance - the capacity of a material to withstand various forms of surface wearing or cutting by exterior forces and suffer minimum loss of mass absorption - the transport of a material, usually a fluid, through the surface of the elastomer to within its molecular matrix (see swelling) accelerated test - subjecting the elastomer to controlled conditions related to, but more intensive than, those known to have significant effect on the rubber over time, in order to evaluate the material’s resistance to that type of condition in a shortened time frame accelerator - various chemicals that take part in the crosslinking reaction for an elastomeric polymer, which have more effect in increasing the rate of reaction than on the final level of crosslinks (some of these are called ultra-accelerators) Acrylonitrile Rubber - a copolymer of butadiene and acrylonitrile with a primary characteristic of oil resistance; also known as Buna-N, Nitrile Rubber, and NBR Activator - chemicals which in some way catalyze crosslinking reactions; for sulfur cures zinc oxide is the primary activator, usually in combination with stearic acid Adhesion - the joining of two dissimilar surfaces together in such a way that it takes considerable force to separate them, sometimes so much force that one or the other of the joined materials fails cohesively rather than at their interface Adhesive - usually a mixture of chemicals in some solvent which will wet out on a solid surface and dry to form a film which promotes physical and chemical bonding of rubber to the underlying surface; often but not exclusively during vulcanization (see primer) Adsorption - migration of a material onto the surface of a solid so that part or all of the surface is covered with a layer of the material that is intimately joined to it and will resist removalaging oven - an air oven, usually laboratory scale, that is used to expose rubber test specimens to elevated temperatures in an accelerated test of the compound’s resistance to normal aging age resistance - the capacity of a compound to undergo very minimal changes in properties in a normal environment with the passage of many months or even years air trap/check - typically shallow surface marks or depressions on molded parts due to air in the mold being trapped between the mold wall and the rubber flowing to fill the cavity; also called cold checking aniline point - a method of evaluating the aggressiveness of petroleum oils, with lower aniline point indicating a more aromatic nature of the oil, which is associated with greater effects on general purpose elastomers antioxidant - a chemical which prevents or retards the natural attack of atmospheric oxygen on rubber antiozonant - a chemical which prevents or retards the attack of ozone on the surface of many elastomers (some chemicals will act to a certain extent to block both oxygen and ozone, although usually the activity is far more effective against one or the other) Arrhenius Method - a technique for developing a predictive model of material lifetime by exposing samples to a series of temperatures and noting when failure (by some criterion) occurs; a mathematical model can then be derived for interpolation or limited extrapolation of life at other temperatures Autoclave - a chamber that can be sealed and heated, usually by the circulation of live steam under pressure, in order to transfer heat effectively to anything placed in it Backrinding - distortion of a molded part, located at the mold parting line, usually as folds, wrinkles, or tears Banbury - the name of the original brand of internal mixing machine for rubber, now sometimes used generically Band - the continuous layer of rubber that forms on a mill roll during mixing Bank - the accumulated mass of rubber that stays above the point at which the mill rolls approach each other most closely; the bank should be in constant movement, rolling and losing some of its material into the nip while other material comes into it from the band (also called the rolling bank) Bloom - migration to the surface of a rubber article of some ingredients or chemical debris from vulcanization; sometimes desired/intended, as with wax protective layers, sometimes unintended as with sulfur bloom (if the ingredient migrating out is a liquid, the term “bleed” may be used, if it is hard and adheres to the surface, the term “frosting” applies) bond - a connection between two different materials that keeps them in intimate contact until made to separate by force (see adhesion); bonds can be physical or chemical or both bonding agent - see adhesive bond stress - the force that acts to break apart the interfacial layer between joined materials, which is transmitted to the bond area from whatever external force is applied to deform or disassemble the bonded article brush coat - a layer of liquid chemical, such as an adhesive, that has been applied to a surface by use of a brush brittleness - the tendency for a compound to fracture rather than bend when rapidly deformed at some low temperature; brittleness tests give one indication of when a compound may no longer be expected to function well as an elastomeric material bumping - the practice of opening a mold slightly after it has closed and filled with compound; done to allow air to escape and facilitate better mold fill by the compound Butyl Rubber - an elastomer based on isobutylene, known for resistance to gas permeation and high damping Calendaring - a process in which compound is forced over a series of rolls mounted on the same frame (a calender) to form a continuous sheet of controlled thickness carbon black - very fine powders of graphitic carbon particles produced by incomplete combustion of oil or gas, which act as reinforcing filler for many elastomers; available in many grades which differ in particle size and complexity catalyst - normally a chemical which affects the rate of a chemical reaction without itself being used up in the reaction; for vulcanization by peroxides, the peroxide is often referred to as “catalyst” even though it does break down completely during the reaction chalking - formation of a powdery surface residue through disintegration of the uppermost layer of the compound due to weathering or other form of attack checking - a pattern of short, shallow cracks on the rubber surface due to environmental attack; see ozone checking chlorination - the treatment of rubber articles by some form of chlorine, either from bleach or water solution of the gas, to change the surface and make it sterile, harder, tougher, or more chemically resistant Chloroprene Rubber - commonly known as Neoprene, polychloroprene rubber is a very useful general purpose rubber with some resistance to oils, and offering high strength, good aging, and flex life closed cell sponge - foam (cellular) rubber in which the individual gas pockets are all independent of each other, fully encapsulated by the polymer matrix; these do not absorb fluids readily, and are resilient when subjected to deformation coagent - a reactive chemical that acts in conjunction with a nonsulfur cure system, typically peroxide, to increase efficiency of the crosslinking reaction coefficient of expansion - a measure of the degree to which a compound grows in size as its temperature is increased; elastomers have higher coefficients of expansion than do metals, which leads to differences between the room temperature dimensions of a mold and the room temperature dimensions of a rubber part made from that mold (see shrinkage) coefficient of friction - (COF) the ratio of tangential force to normal load during a sliding process; dry rubbers have high coefficients of friction unless specially formulated, but any lubrication of rubber by water, soapy water, or light oil will drop the apparent COF very quickly cold flow - slow deformation of polymer or uncured rubber due to its liquid-like state; sometimes due to simple gravity, but also through imposed force compound - usually a reference to the uncured but completed mix of the entire formula, ready for forming and vulcanization (sometimes called stock) compression molding - a process in which a quantity of the compound is placed in the mold cavity, where it is compressed by the action of the mold closing and forced to flow to fill the entire cavity compression set - the amount (in percent of the original strain) that a sample of rubber does not recover after being forced to deform in compression, usually by a fixed strain of 25%, or sometimes by a fixed constant load; considered an indicator of resistance to stress relaxation, but used more as a QC measurement copolymer - a polymer made up of two different monomers, e.g., Styrene-Butadiene Rubber creep - the slow continuous increase in strain of a rubber part under constant loading; often observed to be linear with the logarithm of time critical strain - also threshold strain, which is the level of stretching below which ozone attack on the compound does not take place; dependent on the particular compound, temperature, concentration of ozone and UV crosslink - a chemical joining of two separate polymer chains, usually by means of a small molecular bridge between them; different kinds of chemistry are used to crosslink different polymers, such as sulfur chemistry for many, peroxides for some others, metal oxides for a few, and other specialized systems for various specialty polymers crystallization - the process in which some kinds of polymer chains go from random independent coils to alignment in a regular pattern; this occurs either when the temperature drops to a range particular to the polymer, or when the polymer is strained past a certain level (Natural Rubber and Neoprene are among the best known crystallizing polymers) cure, curing - common terms for vulcanization curative - chemicals which react to create crosslinks between polymer chains, along with or independently of a primary crosslinker like sulfur damping - denotes a low rate of energy return from a forcibly deformed mass of elastomer, associated with energy absorbing or "dead" rubbers; the opposite of resilient die - a metal element with a shaped aperture in its center through which uncured rubber is extruded; alternately, a convoluted metal shape in outline with sharp edges, used to cut shapes from flat pieces of cured rubber die swell - increase in dimension when extruded rubber exits the die, caused by the stress the molecules were subjected to in the extruder and while passing through the die relieving itself; this must be taken into account when the die is designed if the shape of the final extrudate is to be tightly controlled dispersion - the degree to which ingredients in a mixture have been broken down into very small domains, the better to interact with each other; like grinding different kinds of rock into sand, after which they can mix more intimately with each other distribution - the degree to which a mixture of different ingredients has been made homogeneous, like mixing black sand and white sand to a perfectly even gray mixture; good dispersion & distribution produce more consistent rubber with better properties durometer - a device which measures the resistance of a rubber surface to penetration by a small specially shaped indentor; these devices come in several different scales of hardness, the most common of which is the Shore A scale, with Shore D being for harder compounds and Shore O for softer (the IRHD scale is similar to Shore A) dynamic properties - characteristics of rubber behavior when subjected to comparatively rapid cycles of deformation (E*, complex modulus; E’, elastic modulus; E”, loss or viscous modulus; Tangent Delta or Loss Factor, a measure of damping) EPDM - general purpose rubber made by copolymerizing ethylene and propylene and a third component (which allows sulfur curing), excellent for resistance to oxygen, ozone, and aging in general; lack of the third component makes it EPR, which must be vulcanized by peroxides Ebonite - an elastomeric polymer (usually Natural Rubber) vulcanized with 30+ phr of sulfur to produce a very hard material that is no longer rubbery but more like a plastic; also known as hard rubber efficient vulcanization - (EV) the use of sulfur based curatives with no or minimal elemental sulfur in a cure system; use of minimal to low sulfur levels is called a semi-EV cure electrically conductive rubber - elastomers are natural insulators with very high resistivity ratings; by loading with special carbon blacks resistivity can be reduced to the point where static electrical charges will bleed off through the rubber, which is semi-conductive at that point; truly conductive rubber is only obtained by heavy loading with highly conductive particles (like ground silver), which has serious effects on the physical properties of the compound extraction - the migration of some ingredients out of the rubber due to attraction into solvents or oils extruder - a machine for forcing compound to flow through a forming die so as to produce a cross-sectional shape continuously; typically using a rotating screw in a heated barrel to soften and drive the compound forward, although ram extruders also exist filler - finely divided nonreactive solid ingredient for compounds; these range from having substantial effects in improving some rubber properties (a reinforcing filler) to acting primarily to dilute the polymer and reduce cost (then termed an inert filler) finite element analysis - (FEA) a methodology for modeling parts using very small sections of the part geometry in a computer simulation; FEA can be used to design parts, understand how they react to deforming forces, or to simulate the flow of compound in a mold in order to optimize a molding process flame resistance - most elastomers, being based on organic chemicals, will burn, but they can be compounded to resist ignition for some period of time, and only burn slowly when finally ignited flash - excess compound that has flowed out of a mold cavity and cured; often in thin sheet form, extending from the mold parting line flex life - long term flexing of rubber will eventually cause it to soften slightly and begin to deteriorate, with cracks forming and spreading and ultimately causing failure; a variety of flex life tests are used to evaluate elastomers, but they are not very accurate and are subject to wide scatter flow marks/lines/cracks - surface imperfections in a rubber part created by the imperfect melding together of the hot compound during forming; some types of these defects are called knit marks fluoroelastomers - specialty polymers (Viton༮, Fluorel༮, etc) with high percentages of fluorine in them; extremely resistant to oils and heat, but not good at low temperatures and not inexpensive fluxing - the mixing together of ingredients under conditions of high temperature and shear in order to achieve the best possible dispersion foaming agent - chemicals which break down to liberate gases during vulcanization, in order to form cells in the viscous uncured rubber matrix which stabilize as curing is completed to produce a rubber foam (formerly referred to as blowing agents) fractioning - the use of a calender to force rubber to flow over and into fabric to make a composite product gate - a final restriction to rubber flow right at the entrance to a mold cavity glass transition point - the lowered temperature at which a polymer’s chains lose mobility, so it goes from being rubbery to hard or leathery, subject to breaking on impact; often written as Tg grain - the properties of rubber can be anisotropic, different along one axis of a specimen than along a perpendicular axis; this is due to the polymer chains aligning to some extent during processing, and the direction of that alignment is called the grain of the rubber green strength - the resistance to deformation of the uncured compound guillotine - a machine with a power driven vertical blade used to cut up bales of polymer or compound into conveniently sized pieces gum compound - a formulation using a minimum of ingredients other than polymer heat buildup - steady increase in temperature of a rubber part due to conversion of mechanical energy used to flex the rubber into heat through hysteresis heat history - the overall amount of heat to which some compound has been exposed during processing and storage; sufficient heat history causes scorch hydrolysis - a destructive reaction with water that some polymers (e.g. urethanes, acrylates) can undergo hysteresis - the transformation of mechanical energy into heat energy that takes place to greater or lesser extent when rubber is flexed; resilient compounds have low hysteresis, damped compounds have high hysteresis inhibitor - a chemical that acts to prevent or slow an undesirable chemical reaction; inhibitors are used in rubber to prevent premature onset of vulcanization (see scorching), also known as retarders injection molding - a process in which compound is heated in a separate device mounted on the mold, then forced to rapidly flow from there through passages (sprues and runners) into the mold cavity insert - any solid component, usually metal, which is placed in a mold in order to have rubber vulcanized in contact with it, so as to become an integral part of the final product internal mixer - various types of machines comprised of a closeable chamber containing two convoluted rotors, which mix rubber compounds efficiently; these have replaced rubber mills for most volume production latex - a water dispersion of a polymer; Natural Rubber is obtained from a rubber tree in the form of a latex, but synthetic latices have been produced for many years mandrel - a metal component of a mold which has rubber formed and vulcanized all around it, without bonding, so it can be removed from the final part to leave an aperture or passage; also the core of a die around which rubber is extruded to form a center hole marching modulus - the tendency of some compounds to reach high crosslink concentration, yet continue to have crosslink creation or rearrangement that causes a slight stiffening of the material even after it has achieved a stable level of cure masterbatch - any mixture of some but not all of the components of a formulation; this is done to allow hotter mixing (in which addition of curatives would precipitate some premature vulcanization in the mixer) or for some other processing convenience modulus - in engineering, modulus is the ratio of stress to strain but in rubber technology modulus is the force per unit area required to achieve a particular elongation, such as the M-100 (100% modulus), M-200, etc; these tensile moduli are indicators of the rubber’s resistance to deformation, but are used most often as QC measurements mold - a tool, usually of metal, with one or more shaped cavities used to form compound into the desired product geometry and transfer heat sufficient to accomplish its vulcanization (also referred to as tooling) mold fouling - deposits of ingredients and chemical debris from the compound onto the mold surface; various compounds contribute very differently to fouling, which is sometimes a problem in production practice mold release - materials applied to a mold surface to form a thin layer that allows easy removal of the part after curing monomer - a small molecule capable of linking up to other small molecules (of its own type or other types) to form a polymer chain Mooney Scorch - a measure of a compound’s sensitivity to heat history; a rating of comparatively short time implies the compound is likely to begin vulcanizing if exposed to even mild heat Mooney Viscosity - a measure of the viscosity of a polymer or compound, used frequently for QC purposes but also an indicator of how the material may resist flowing during mixing or processing Mullins Effect - the phenomenon whereby the first few deformations of a piece of rubber require more force than subsequent deformations, but after repeated deformations the rubber’s response to force approaches equilibrium Natural Rubber - polyisoprene from a plant source, available in numerous grades, known for good flex life, but vulnerable to environmental attack; synthetic polyisoprene is also available, with most of the characteristics of Natural Rubber Nerve - the toughness and elastic resistance to deformation of compound during processing Nitrile Rubber - a copolymer of acrylonitrile and butadiene made in particular for oil resistance (also known as NBR or the older term Buna-N); available in many grades, also sometimes in combination with PVC polymer (polyvinyl chloride) for improved processing and ozone resistance open cell sponge - foam (cellular) rubber in which the structure is like a series of tiny caves that open into each other; this is like true sponges that will readily soak up fluids as all the cells fill up with it oven cure - vulcanization of formed compound through exposure to heat in an oven; often used for extruded or calendered products (see autoclave) ozone cracking - the visible signs of ozone attack on a rubber surface, small parallel cracks at right angles to the direction of strain in the rubber parting line - the point at which separate plates of a mold abut each other; also the visible mark on the molded part from that joint peptizer - reactive chemicals which promote the breakdown of very long polymer chains into shorter, more easily processable chains; most often used with Natural Rubber pigment - formally an ingredient used to color some mixture; in past practice, used as a synonym for filler, especially carbon black plasticity - another way to evaluate viscosity, it is a measure of the tendency for polymer or compound to flow when stressed plasticizer - ingredients which lower the polymer viscosity and final rubber hardness, and often improve the low temperature tolerance of the rubber as well; typically oils and liquid esters, but also some resins and waxy materials platen - the flat plates of a molding press that open and close, in contact with or fastened to the mold; usually equipped with built-in heaters to maintain the vulcanization temperature porosity - the presence of many small holes or voids in part of the mass of a rubber part postcure - a deliberate heat exposure to the vulcanized rubber part, typically in an oven, in order to optimize final properties by further vulcanization reactions or by volatilization of unwanted chemical residues from the rubber perform - usually a shaped piece of compound made to more efficiently fill a compression mold press - abbreviated term for hydraulic presses used for molding processes; most often opening vertically, but sometimes horizontally, and ranging from mechanically simple to quite complex primer - most often the first coat of a two-coat adhesive system for bonding rubber; although sometimes a single coat system will be referred to as the primer processing aid - any type of compounding ingredient which improves the processability of the compound; this may be by improving its flow, the dispersion of other ingredients, surface appearance of the final product, energy consumption during mixing, etc. RHC - an abbreviation for “percent rubber hydrocarbon”, which is the percent of the formulation made up of elastic polymer(s); the quality of a compound depends in some ways on not having too low RHC RTV - acronym for room temperature vulcanization; a few kinds of reactions exist which can crosslink an elastomeric polymer, most typically silicone, at room temperature Reclaim - some kinds of cured rubber can be mechanically and chemically processed into a state suitable for use in mixing new batches of compound; this is called reclaim material, generally used at a moderately low level Reinforcement - see filler Resilience - denotes a high rate of energy return from a forcibly deformed piece of cured rubber (resilient rubber balls bounce very high) reversion - heat related chemical breakdown of some elastomers, notably Natural Rubber, into shorter chains; this results in a softer, sticky material rheometer - a device which exposes a small sample of compound to vulcanization temperature and tracks its changes in viscosity as it cures, to produce a kind of processing profile of the material; available as oscillating disk rheometers (ODR) and the newer moving die rheometers (MDR) rubber mill - two counter-rotating steel rolls mounted closely in a frame, used by a skilled worker to mix rubber compounds; available from lab scale to 100” across the rolls runner - a channel for rubber to flow through in filling a multi-cavity mold, from which other channels (see sprue) branch off towards the mold cavities SBR - (Buna-S) copolymer of styrene and butadiene, the highest volume rubber made, in many grades, used for general purpose but especially in tires Scorch - premature crosslinking in the compound; some formulas are sensitive enough to heat so that exposure to warm weather or a hot factory environment over time causes vulcanization to begin, which raises the viscosity of the compound and can ultimately make the batch of material unfit to process set up - compound which has scorched enough to be unusable is said to be set up shape factor - the ratio of one load bearing surface of a rubber part to the combined area of all unloaded surfaces free to expand sideways when the part is subjected to a compressive force; an important characteristic for engineered parts shelf life - for unvulcanized compound it is the period of exposure to normal ambient temperature which is known to be unlikely to have any scorching occur (usually in months); for rubber products it is the period of time (in years) of storage under normal conditions which is known to not have significant change in the rubber properties shrinkage - when rubber cools from its vulcanization temperature, it decreases slightly in volume; in molded articles there will be a consistent difference between the dimensions of the mold cavity and the corresponding part dimension, typically 2-4% smaller Silicone Rubber - an elastomer in which alternating silicon and oxygen atoms make up the backbone chain of the polymer; lower physical properties than other rubbers but less sensitive to heat and cold than any other elastomer slab dip - mixed rubber is often taken off a mill in sheets or slabs, which can stick together strongly if piled on each other; a slurry of water and chemicals (soaps, powders) is used to coat the slabs to prevent this solubility parameter - a number which relates to the chemical structure of molecules; ingredients with similar solubility parameters will mix readily to form a stable matrix, but dissimilar solubilities indicate poor mixing and instability will be seen in the compound sprue - a primary channel for rubber to flow through in filling a mold; the leftover cured rubber from the channel is also called the sprue sprue mark - the detectable mark on a molded part where the sprue was removed stress relaxation - a steady lessening of internal stress over time in the matrix of rubber subjected to stress or strain; this results in both a reduction in the force the rubber exerts against the external stress and an increase in permanent strain (set) in the rubber even after the external stress is removed staining - some ingredients, notably antidegradants, will not only bloom to the surface of a rubber article, they will migrate to contacting surfaces and at times make a visible stain that may be objectionable swelling - compatible fluids will migrate into the mass of a rubber article, which usually results in a gain in volume of the affected portion of the part; the more compatible the fluid, the greater the volume increase, which can sometimes be well over 100% tack - a slight stickiness of the uncured rubber surface which is sufficient to cause separate pieces of rubber to conform to each other and make a significant physical bond over time tear strength - the capacity of a compound to resist the initiation of a break in its surface or the propagation of such a break; several kinds of tear tests exist, but close correspondence of any of them to field experience has not been established terpolymer - a polymer made up of three different monomers, e.g., EPDM thermoplastic elastomer - (TPE) some families of polymer mixtures or copolymers that will soften and flow when heated, but when cool exhibit dimensional stability and rubber-like properties even though they do not undergo a vulcanization reaction thixotropic - fluids which change viscosity when subjected to shear forces over time, so that the more they are made to flow, the less resistance to flow they have transfer molding - a process in which a quantity of compound is placed in a chamber (the transfer pot) in one part of the mold body, and the mold closure forces a projection (the transfer ram) into the chamber, thereby forcing the compound to flow through passages (sprues) into the mold cavity to fill it undercure - failure of the vulcanization reaction to go to completion or very close to completion, so that the crosslink concentration in the elastomer is not sufficient for full development of final properties viscoelasticity - term used to summarize the complex response of rubber to a deforming force; part of the energy used to accomplish the deformation is stored (elastic response) and part is dissipated (viscous response) viscosity - the resistance to flow of a fluid; since elastomeric polymers are fluids and uncured compounds are also fluids (even though they can feel like a soft solid to the touch), they have intrinsic viscosity levels which are useful to know for processing purposes volatilization - a process of material loss due to some ingredients (typically plasticizers) in compounds slowly evaporating from the rubber surface with heat and time, which can cause the rubber properties to change vulcanization - the creation of chemical links between polymer chains that converts them from a viscous liquid to a stable elastomeric solid; so named after Vulcan, Roman god of fire, since it usually takes heat to drive the reaction Corry Rubber will be glad to answer your questions regarding design and engineering of rubber materials and products. Just Ask Dr. Stretch at www.CorryRubber.com. |
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