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Glossary of Silicone Terms

The Glossary of Silicone Terms is an excerpt from The Silicone Elastomer Handbook

Written By: David Brassard

Edited By: Mr. David Crossan, Sr. Editor

Abhesion – lack of adhesion, easy release from substrates.

Abrasion – surface wear from friction.

Acetoxy cure system – a tin catalyzed moisture cure system that liberates an acetic acid/ vinegar odor by-product upon curing.

Adhesion – the bonding to a substrate by an adhesive or other agent.  May be chemical, mechanical, or a combination of both.

Adhesion promoter – a silane or zirconate compound which enables a self-priming bonding capability when utilized in adhesives.

Adhesive – elastomers, resins or PSA’s that are designed to display self-bonding properties to substrates without primers

Addition cure system – a reaction between a two PDMS polymers, where no by-products are generated.  Typically referred to as Platinum cure systems, where vinyl terminated PDMS and a methyl hydrogen copolymer PDMS crosslink with a platinum catalyst.

Si-H + CH2 = CHSi ————> SiCH2CH2Si

Can also be a peroxide initiated, free radical cure between two PDMS methyl groups or a methyl and a vinyl group, or two methyl groups resulting in crosslinking of a silicone.

Agglomerates - are large clusters of aggregates and primary particles.  In fumed silica’s, they are attracted by hydrogen bonding of the surface silanols.  They can be broken apart through shearing.

Aggregates – are clusters of primary particles.  In fumed silica’s, they are attracted by hydrogen bonding of the surface silanols.  They can be broken apart through shearing.

Alkoxysilane – a compound containing a central silicon atom and an Si-OR functionality, where R represents a aliphatic organic group.  Upon hydrolysis, generates a alcohol molecule.

Application time/life – the usability time of the product after catalyzation is measured here.  Often identified as how long until a product will not flow, enable tooling or application into the assembly.  Also known as work life, pot life.

Application rate – used to measure the flow of RTV sealants from a semco tube. Material is extruded from a semco tube at a stated pressure, usually 60 or 90 PSI through a standard orifice. Reported in grams per minute.

Antifoam – a liquid product that when added to a composition retards or inhibits the development of a foam in the composition. 

Arc Resistance – denotes the resistance of a material to a to an arc produced by an electrical current between two electrodes.

ASTM – American Society of Testing Materials, they define standard test methods for industry.

Base – a silicone compound that is only partially compounded with fillers.  Often un-colorized and un-catalyzed, fabricators of finished rubber parts use bases in formulating their own special compounds.

BET surface area – is a method developed by Brunaer, Emmet and Taller in 1931, which is a means of displaying the surface area of powders by measuring the amount of gas absorption under varying pressure conditions.  The larger the BET gets, the smaller the primary particle and the larger the surface area of the powder.  The BET value relays the square meters of surface area per gram of powder.

Bladder cure - a surface only cure condition in a deep section cured product 

Bleeding – the surface release of a plasticizer or incompatible liquid component from a cured elastomer.   Can also identify a oil separation from an uncured product.

Blow-out resistance – a green strength – resistance to be displaced while in an uncured state found in some sealants, useful in assembly applications, especially automotive engine assembly.  

Brittle point – denotes the lowest temperature than an elastomer can be subjected without breaking or shattering upon sudden impact or stress.

Bulk modulus – is the elastic/energy absorption capability property of an elastomer when compressed.  Silicone rubber will deform approximately only one half of what an organic rubber will when exposed to the same stresses. This is due to the siloxane bond spacing and the high free volume in a crosslinked matrix.

By-products – the evolved reaction materials of a condensation cure silicone with moisture. These typically generate the odor found when silicones cure.

Catalyst – is a reagent that when added to a compound, initiates a chemical reaction, but is not consumed by the reaction.

Caulk – is a compound that is used to fill crevices to enable a moisture or weather-proof seal.

Centipoise = a unit of measure for liquid materials.  One hundredth of a poise.  (centistokes x specific gravity)

Centistokes = like centipoises, it is also a method of defining the viscosity of liquids.  (centipoises / specific gravity)

Chain extension – a process where a high molecular weight linear polymer is fabricated from many lower molecular weight linear components.

Chain stopper – an M unit utilized in polymerization reactions, which limits chain growth by having only one hydrolyzable or reactive functional group on the molecule.

Chlorosilane – a compound containing at least one Si-Cl bond.  Silanes typically being tetrafunctional, have multiple chlorines and/or organic components.   These are the building blocks of silicones.

CIPG – cure in place gasket.  Applying a wet silicone to a part & allowed to dry.  Then the part is compressed onto the other substrate compressing the gasket.  Compression forces maintain the seal.  Adhesion to only the first substrate is required to hold the gasket in-place during assembly.

Coefficient of thermal expansion or CTE– the material’s fractional change in dimensions typically measured in length for a given unit change in temperature.

Cohesion – the strong attraction of a substance to itself enabling a high resistance to splitting or tearing.  Often accomplished through the chemical bonds from crosslinking and reinforcement.

Condensation cure system – a composition, which is capable of a reaction between silanol terminated polydimethylsiloxane and multifunctional crosslinker with a catalyst.  This reaction creates a water molecule.

Si-OH + AcOSi ———- > Si-O-Si + AcOH

Compatible – materials that can be mixed or blended without any chemical reaction or separation taking place. 

Compound – this has very different meanings:

  1. a substance containing two or more elements united in definite proportions by weight.
  2. to mix or incorporate finely divided solids into a polymeric material.

Compression set – the reduction in thickness of a material caused by the loss of a spring-back capability after being in a compression condition for a period of time.   This spring-back capability id key as it creates the sealing forces of a rubber in a compression gasket application.

Copolymer – is a compound that contains two different chemical structures within the backbone as the repeating units.

Creep – is a cold flow like deformation of a cured elastomer under a load

Crepe Hardening – the thickening of an uncured silicone upon ageing caused by hydrogen bonding.  Can be offset by softening on a mill or shearing in a mixer.

Crosslinker – a silane or methyl hydrogen compound, which when utilized properly in compositions, will enable the formation of chemical bonds.

Crosslink density – the concentration of chemical bonds within a cured rubber or elastomer.  Measured by durometer and modulus.

Crosslinking – a series of reactions that occur as an uncured silicone material cures to form a cured material. 

Cryogenic – having a very low temperature, in the range of –200oF and lower.

Crystalline – is a solid with a very regular, lattice molecular architecture.  

Cure moderator – cure moderators are added to formulations to function as a timer to delay cure.   Very low molecular weight components will preferentially crosslink prior to the higher molecular weight components.  Slowly increasing in molecular weight until the viscosity meets the base polymer, then the product would rapidly cure.  Through formulation, they enable a very accurate 2 to 30 minute open time prior to a rapid cure.

Cure speed/time – The time required to convert the silicone elastomer to cured rubber. Example: 24 hours for 1/8” diameter bead at 77° F and 50% relative humidity or 15 minutes @ 150oC.

Curing– a crosslinking reaction which enables silicone liquids, pastes or rubber compounds to harden to a three dimensional matrix.

Curing agent – a material which when added to a second material, brings about a chemical reaction, usually causing the hardening of the entire mass.  

Cureometer – a test device, which measures polymer properties as it cures/crosslinks.

Cure time – the time lapsed between the addition of a catalyst and the complete hardening of a material.

Cyclomethicone – a low molecular weight cyclic PDMS usually D-3, 4, or 5 and sometimes blends. This is a term utilized by the healthcare industry. 

Cyclics – a term used to depict the volatile oligomeric components often found in silicone materials as a result of incomplete copolymerization.  These can be removed through post baking parts or de-volatization of polymers.  Cyclics are compounds that are in closed rings as opposed to linear, aliphatic chains.

D-4 – is industry shorthand to typically depict octamethylcyclotetrasiloxane, a volatile low molecular weight cyclic oligomer.   Vinyl D-4 has a vinyl functional group substituted on each silicon atom for a methyl, resulting in a methyl and vinyl group being pendant on each silicon atom.

Deep section cure – a silicone product that is formulated to enable cure to any depth or dimension.  Most typically found in a 2-part product where all components required for cure are incorporated.  Unlike a 1-part moisture cure, which requires atmospheric moisture permeation and cures slowly from the outside in.

Dehydrogenation – a chemical reaction where hydrogen is released from a compound.

Density – mass per unit volume, as compared to water.  Typically reported in grams/cubic centimeter.

Dielectric strength – the point when a material loses its insulation capability and enables an electrical current to pass through.  Typically measured in volts at a given thickness.

Dilatant – a rheological capability to display increasing hardness upon exposure to shear. 

Dimethicone – another healthcare industry term for Trimethyl terminated PDMS fluid, It is also called simethicone. Found in many consumer products, skin and anti-flatulence preparations just to name a few. 

Dimethylsiloxane – see PDMS 

Dipodal – a silane having twice the level of the typical trimethoxy functionality.  This silane molecule will have two functional pendant hydrolysable trialkoxy groups.  One is found at each ends of the molecule.  

Dispersions- elastomeric or resinous product dissolved in a solvent carrier.  Dispersions are applied to substrates and allowed for the solvent to evaporate prior to use. A dispersion allows a very thin film to be applied.

Durometer – resistance to indentation. The measure of the crosslink density of a rubber sample via ASTM D-2240.

Endcapping – The functionalization of a polymer terminus.

Elastomeric – is a rubber-like substance displaying a stretching capability.  Some define as the ability to stretch to at least twice its length under light stress and snaps back to its original length upon the release of the stress.

Elongation – (elongation at break) the ability of the test piece to be stretched without breaking. Given in % of original dimension @ maximum elongation. Maximum elongation is determined by attaining the point of breakage. Typically measured via ASTM D-412.

Emulsion – is very small droplets of oil uniformly suspended in water, typically accomplished through the aid of chemical emulsifiers and high speed milling or high pressure homogenization.

EMI/RFI – electromagnetic interference or radio frequency interference are applications that require shielding of cables and apparatus.  Electrically conductive silicones are used for shielding.

Encapsulation – the complete surrounding of an electronic component or complex of components with a coating of an insulative material. The purpose is to protect the component from adverse environmental conditions.

Extrusion rate – a measurement of the flow capabilities of a material by dispensing through a narrow orifice under pressure and measuring the grams per minute.   Typically tested at 60 or 90 PSI and through a 1/8 orifice.

Fast cure – an order of magnitude faster than typical – Most R.T. cures are slow

Filler – a finely divided powder formulated into a composition to either enhance physical properties or reduce the overall cost of the composition.

FIP – form in place gasketing, RTV’s are applied wet, conforming perfectly to the gap upon mating excess sealants pushed-out.  The wet sealant then cures forming a gasket.  The gasket seals the two flanges by adhering to each exposed part.

Firm time - an old GE term, the time to obtain 40 Shore A min. in an 2-part tin catalyzed system

Flash point – the temperature that an materials vapors will ignite when it is exposed to an ignition source.

Flowable – a material that is in a liquid state at room temperature and will fill all of the voids in the container into which it is poured.

Fluid – typically defined as a material that will take the shape of any vessel containing it.  However, in silicone technology it also refers to a non-functional silicone PDMS polymer of low viscosity whose terminal groups will be trimethyl.  A fluid is the opposite of a resin, which is functional and can be crosslinked.   Repeating groups on the polymer backbone can be of a wide variety. 

Fluorosilicone – a silicone PDMS material with varying levels of trifluoropropyl functionality along the backbone chain.  They can be either functional or non-functional.

Foam – a silicone rubber containing many gas pockets formed during cure, creating a cellular structure.  These voids can be caused from the release of hydrogen gas or a blowing agent, often thermally activated.

Freshening – the milling or working of an uncured silicone to break hydrogen bonds and enable better processability.

Fumed silica – a very fine silicon dioxide, typically formed through the pyrolysis/flame hydrolysis of silanes. 

Functionalization – the complexing or chemical reaction of a pendant group with a reactive species, often silanes, which results in a change in activity or potential.

Gel – a soft, compliant, lightly crosslinked silicone elastomer.  Some display a semi solid-form.

Gel time – The period of time from initial mixing of flowable reactants in which the material becomes too viscous to flow or be tooled caused by crosslinking to a pre-cure elastomeric state.  

Glass transition temperature, Tg – the temperature where a brittle solid will soften to a liquid, gel, or if an elastomer, start to display elastomeric behavior. 

Grease – lubricating products made by thickening fluids with suitable fillers.

Green strength – a resistance to be displaced while in an uncured state found in some sealants, useful in assembly applications.

Gum – a very viscous polymer sometimes filled with silicas used as a base for rubber compounding.  Viscosity is often > 1,000,000 cps.

Hardness – Shore A – a scale of measurement used to define the relative hardness of materials.

HAV – hot air vulcanization, typically refers to either the curing technique or the actual ovens used to cure elastomers.  The ovens can be vertical or horizontal.

HCE – stands for high consistency elastomer, or heat cure elastomer.  An older term, which is less utilized today. 

HCR – heat cured rubber or high consistency (silicone) rubber.  Crosslinkable with heat, HCR is typically molded with very high pressures.  Often highly filled, the very thixotropic silicone rubber appears to be a solid in an uncured state.  Also called a HCE, heat cured elastomer.

HDPE – stands for high density polyethylene.

Headspace – the unfilled portion of a container that is occupied by a gas.

Heat cured – 1 or 2 part product that typically crosslinks into a hard, cured product upon exposure to heat

Heavy-bodied – an uncured polymer exhibiting a very high green strength or blow-out resistance in a assembly test condition.

Heat cure – a reaction where external heat is applied to either accelerate or initiate a chemical crosslinking to result in a desired end product.

Heat sink – the ability to rapidly transmit heat from a generating component.

Hidden silanols – silanols that are hidden inside of the particle not deactivated in a treatment process. 

HMDZ – is an industry shorthand term for hexamethyldisilazane, a very hygroscopic water scavenger and treatment agent.

Humidity – the amount of water vapor present in the air at a given time and place.

Hydrolysis – the reaction of a reactive species like a silane with water generating a by-product. 

Hydrolyzable – a reactive species like a silane that will rapidly react with a water or a silanol functionality.

Hydrosilation – a chemical reaction in which a vinyl functional polymer will crosslink with a hydrofunctional polymer.

Hydrolytic stability – the degree of relative resistance to attack of a material by water or water vapor.

Hydrophillic / Hydrophillicity – is a high affinity or attraction to water.

Hydrophobic / Hydrophobicity – is a repulsion of a material to water.

Hygroscopic – is the capacity in which a material will absorb or retain water.

Inhibitor / Inhibition – a volatile reagent intentionally added to a batch, used to delay curing and control open time or work time.  Typically evaporated during heat curing. 

Instant cure – a rapid room temperature or heat accelerated room temperature cure, usually within 1 minute.

Insulate – to prevent the passage of an electrical current by imposing a barrier substance in the path of the current.

Joint movement – is the movement of two rigid structures within an assembly.  It can be caused from differential thermal expansion rates, vibration, shock or other means.  Also defined as the ability of a sealant to conform to these movements through high flexibility and maintain a bond.

Killed polymerization – the cessation of a polymerization due to deactivation of the catalyst and/or introduction of a chain stopper.

Lamination – is fully enclosing an adhesive between two sheets, disallowing exposure of the surface, preventing off gas and surface cure. Condensation cured silicones require exposure to air for full cure and cannot be used in a lamination application.

LDPE – low density polyethylene, can allow moisture to permeate in the gas phase.

Ligand – is a pendant organic component of a molecule.

LMW – low molecular weight

LSR – liquid silicone rubber.  Actually it’s a misnomer, the products are mostly of a thick paste consistency.  It’s a pumpable, moldable and heat curable silicone composition.  Also called LIM, liquid injection molding (grade).

Masterbatch – various additives or pigments are pre-dispersed into PDMS liquids to allow ease of addition to batches in production.

Mechanical adhesion – the interlocking of a uncured elastomer with the surface irregularities, which upon curing results in a secure bond.

MeH – industry shorthand term for a methyl hydrogen copolymer.  The crosslinker utilized in addition cured platinum catalyzed reactions.  The hydrogen functionality can be pendent interchain, on the chain terminus, or both.

Methyl – a central carbon surrounded by three hydrogens, a typical non-functional pendant group on a PDMS molecule, typically written as CH3.

Micron – 1 x 10-6 meter 

Modulus – see Youngs and Bulk Modulus – the amount of force or energy that can be absorbed by a rubber specimen.

Monomer – an elementary molecule capable of being polymerized into a polymer.

MQ resin – a silicone macromolecule if functionalized, it will have a multitude of crosslink sites.  When used properly in silicone elastomers, provides regions of high crosslink density.

Mudcracking – is the surface cracking of a sealant or coating during cure.  Caused from poor formulation, weak surface, or high joint movement during cure.

Nanometer – one millionth of a meter

Neutral cure – a condensation cure system that liberates no corrosive (to metals) by-products upon curing. Alkoxy/Alcohol cure systems are neutral cure.

Oil resistance the ability of an elastomer to survive and display minimal changes in a long-term oil immersion condition.   

One-part – is a ready-to-use silicone material that does not require mixing with a catalyst or other additive to form a durable rubber or adhesive.

Open time – typically defined as the time that a compound takes to double in viscosity after catalyzation.  Some define it as how long a product will flow, enable tooling or application into the assembly. 

Outgassing – the release of gaseous components from a product often accompanying a curing product by-product release or evaporation of low molecular weight components.

Oxime cure system – a tin catalyzed moisture cure system, which liberates an alcohol methylethyl ketoxime byproduct upon curing. This byproduct smells like latex paint.

Parts – (per hundred) is a rubber industry term depicting formulation components utilization in a formula in a amount relative to the resin (base polymer) depicted as 100 parts

PDMS – polydimethyl siloxanes (see siloxane)

Peel strength– an adhesion test measuring both the adhesion to the substrate, and the cohesive forces found within a cured rubber.  It is measured in pounds per linear inch, PLI, and percent cohesive and adhesive failure.  The test is characterized in ASTM D-794. 

Peelable – displaying no adhesion for easy removal

Peroxide – a thermally decomposing agent added to silicone products to result in a free radical addition cure.  These free radicals create reactive species from non-reactive groups.  These reactive groups then form crosslinks during the curing of the compound.  Both vinyl specific and general-purpose peroxides are used.  Following this crosslinking route, by-products are generated which must be removed, via post-baking.

Peroxide cure – an addition reaction in which free radicals create reactive sites and form crosslinks.

Phenyl – a pendant fully conjugated benzene ring functionality on a PDMS molecule, typically written as C6H5.

Plasticizers – is typically non-functional diluents enabling a lower viscosity, or a more desired plasticity.  In methyl silicones typically trimethyl terminated PDMS.

Platinum cure – an addition reaction between a hydride functional PDMS with a vinyl functional PDMS, which is catalyzed by a platinum salt, typically chloroplatinic acid.

Poisons/poisoning – an industry term that identifies a list of materials (non-toxic) that can complex and or deactivate a platinum catalyst.  ie. sulfur, amines, etc.

Polydispersity – a characteristic of polymer where it is composed of multiple polymer molecular weight/chain lengths.  It is contrary to a unimodal polymer.

Polymerization – a series of chemical reactions with oligomers, which result in long chains of repeated units being formed.

Porosity – the presence of void or air pockets in a cured composition.

Post baking / post cure – the exposure of a crosslinked material component after the initial heated cure to elevated temperatures in an effort to improve the mechanical properties or evaporate undesirable volatile components.   

Pot life – the amount of time available between the moment that a catalyst is stirred into a base material and the approximate moment that it becomes too thick to apply in the recommended manner.

Potting material – an electrically insulative, moisture resistant material, supplied in a liquid or putty-like form and used as a protective coating on sensitive areas of electrical and electronic equipment.

Pourable – is a self-leveling composition that can be easily dispensed from a package as a liquid.

Precipitated silica – is wet process silica formed through the reaction of sodium silicate and sulfuric acid.  Typically can contain up to 7% water.

Primer – a organo-functional silane in a multi-solvent solution.  Upon application to a substrate, a silanol rich surface remains, rendering the surface highly prone to adhesion reactions.

PSA- pressure sensitive adhesive, typically supplied in a dispersion.  Upon evaporation of the carrier, a permanently tacky, gel-like adhesive results that sticks to substrates through a light pressure application, like applying a label.  The common label and sticker backings are PSA’s.

Quenching – is the secondary addition of a crosslinker, often with an adhesion promoter targeted at providing an enhanced shelf-stability potential.

Reinforcement – is adding to the strength of an elastomer system.  This is typically accomplished by adding reinforcing fillers and resins.

Rheology – the study of the deformation and flow of materials, in terms of stress, strain and time.

Resin – a polymeric material often of a high molecular weight and flowable, utilized as a base raw material in elastomeric compositions.  It is often a crystalline material in a solvent carrier.

Reversion – a de-polymerization of a cured silicone rubber into its original polymeric components. 

R.T. – room temperature cure

R.T.V. – room temperature vulcanization (curing without heat)

RTV-1 – a 1-part RTV system.  No mixing, ready to use.

RTV-2 – a 2-part RTV system.  The mixing of the two components is required prior to use.

Sag resistance – a test that measures the resistance to flow of a one-inch plug of uncured sealant after being placed on a vertical plane.

 Scavenged /scavenger – the chemical reactions of silanes or MeH with silica silanols, water or other functionalities rendering them less reactive. 

Scorch – a cure speed that exceeds the design of the targeted utilization.  Curing too fast at a given temperature to fully fill mold cavities.

Self-leveling – a high flow, syrup-like condition, unlike thixotropic

Shawningan black – a carbon black manufactured from a natural gas stream that is free from sulfur contamination.  Typically used in platinum catalyzed addition cure systems.

Shear – the introduction of energy into matter via applied displacement force and or restriction of flow.

Shelf-life – the period of time that a packaged product is useable.

Silane – A highly reactive molecule having a Silicon atom as the central unit.  Outside of their main utilization to produce monomers or silica, in silicones they are typically functionalized and used as crosslinkers and adhesion promoters.

Silbond ™ 40 – a 40% hydrolyzed tetraethoxy silane, typically used as a primer, hydrophobizing agent and adhesion promoter.

Silica – silicon dioxide in a finely divided powder form.  Can be generated from quartz, precipitated from sodium silicate or created via flame hydrolysis of certain silanes.  

Silicate – a salt of silicic acid

Silicone plasticizers – trimethyl terminated siloxane fluid having no functionality that cannot crosslink without the use of peroxides.

Silicone rubber – a crosslinked silicone polymer.  It maintains its elasticity and electrical properties over a wide range of temperatures and is widely found in sealants, gaskets, insulation, tapes, etc.

Silanol – a Si-OH functionality.  It can be found in a silica, glass and silicone polymers.

Siloxane fluids – polymers having a regular alternating silicon and oxygen central atoms, typically having pendant dimethyl functionality.  These polymers can be functionalized interchain and at chain terminus for a wide range of applications.

Silsequioxane – a ladder polymer, which can provide reinforcement in compositions.

Simethicone – trimethyl terminated siloxane fluid, also called dimethicone by non-silicone industry formulators.  Found in skin and anti-flatulence preparations. 

Skin-over – the formation of an integral skin over the surface of a quantity of sealant.

Softening – reducing the viscosity of a compound by shearing which breaks hydrogen bonds.  Freshening is required of HCR on 2-roll mills prior to processing.

Solubility – the ability of a substance to dissolve in a solvent.

Solvent – a material that has the ability to dissolve of other materials.

Specific gravity – is the ratio of the weight of any volume of a material to the weight of an equal volume of water.

Specific heat  - the quantity of heat required to raise the temperature of one gram of a substance to one degree centigrade at 15oC.

Starved – a condition where a insufficient level of crosslinker or adhesion promoters exist in the uncured silicone composition.  This results in very high product viscosities and or very slow to incomplete curing capabilities.

Stoichiometry – is the chemical balance of reactants in a reaction.  Ideally one mole of A will fully react with one mole of B.

Substrate – a materials surface in which an adhesive or other agent is in contact with and acted upon.

Surface inhibition of cure – contamination of a platinum cured product by “poisons” found on a substrate, or oxygen inhibition of a peroxide cured system.

Surface tension – is an effect of the forces of attraction existing between molecules of a substance.  These forces are only exerted on the surface layers.

Surface wetting – is the full surface contact of a substrate by an adhesive or coating. 

Tack-free time – the time it takes for a sealant to cure to a point where none of the applied material will stick to your finger, a spatula or a plastic weighted film.

Tear strength – the resistance to the propagation of a nick or cut in a test piece when tension is applied. Given in pounds per inch (ppi), Newton per millimeter (N/mm) or (kN/m) kilo Newton per meter.

Tg – see glass transition temperature

Tensile strength (tensile at break) – the amount of force required to break a test piece. Given in pounds per square inch (PSI), mega Pascals (MPa), or Newton per square millimeter (N/mm2).

Thermal conductivity – the speed with which a material is capable of transferring a given amount of heat through itself.

Two-part – is a dual component A & B silicone material product, which must be mixed together to enable curing to form a durable rubber.

Thixotrope – an additive that imparts thixotropy to a composition generally enabling hydrogen bonding to occur.

Thixotropic – is a non-slump or sag property, resulting in no flow without pressure.  Having high static shear strength and low dynamic shear strength, the material exhibits a highly predictable time-dependant loss of viscosity when exposed to shear. Viscosity decreases under steady shearing then slowly rebuilds.  This is caused by Hydrogen bonds breaking and reforming. 

Treated – a chemical coating applied to fillers to hydrophobize and enable enhanced processing capabilities.

Ultraviolet (UV) cure – is the process of curing resins and adhesives with high intensity UV light.  Through the use of photoinitiators and certain functionalized polymers, this is made possible.

Uncured – a silicone material, which has not yet vulcanized or formed a durable rubber.

Unimodal – a polymer having a uniform molecular weight within a very narrow range.

Untreated silica – is silica filler that has no surface treatment and typically has a high surface silanol content.

Vinyl – is an unsaturated, two-carbon pendant functional group containing a double bond and capable of crossliking with an adjacent hydride functional PDMS.

 Viscosity – a relative measurement, it’s the viscous property of a fluid as measured by shear stress divided by shear rate. Usually considered to be the relative thickness or thinness of a fluid as compared to the thickness (thinness) of water.  Any viscosity value can be changed, by modifying the test conditions of shear stress or shear rate.

VOC’s – volatile organic content, any evaporative component or by-product released from a material.  Measured in grams per liter.

Volatile – a liquid that changes rapidly into a vapor when exposed to air, heat, or both.

Volatiles – low molecular weight un-reacted silicone materials from polymerization.  Found in certain silicone polymers, these materials that can become airborne and create appearance and performance problems. 

Volume expansion – a measurement of the increase in bulk or space occupied by a material in relation to an increase in temperature.

Volume swell – the amount of increase in volume of an elastomer after exposure to an aggressive condition.  Typically identified in liquid immersion testing.  Caused by the polymer dissipating stress via coil chains unwinding and expanding in size three dimensionally.

Youngs modulus – the amount of force per square inch to stretch a test piece to a given elongation. Typically measured @ 50, 100 or 200% elongation.  ASTM D-412 test method is typically utilized.

Yield stress yield point or flow limit is identified as the pressure applied to a sample where plastic flow of a thixotropic material starts. 

Vulcanization – the curing/crossliking of a rubber composition. 

Weep – is a permeability of a seal or gasket to a sealed media resulting in a wicking behavior. 

Weight loss – the loss of volatile materials removed from a substance after being subjected to heat, vacuum or both.

Wettability – the ability of a substrate to attract another substance based on the relative surface tensions between the two materials, or the surface finish. 

Wet-out – is to fully impregnate the surface irregularities of a substrate with an adhesive.  This is accomplished either with an appropriate low viscosity or surface energy of an adhesive or with adequate application forces.

Work time/life – the usability time of the product after catalyzation is measured here.  Often identified as how long up until a product will flow, enable tooling or application into the assembly.  Also known as work life, pot life or gel time.

Yield point – a stress that needs to be applied to a material prior to any plastic deformation or flow to occur.  With thixotropic materials, the reduction of thick phase will only be resultant upon a force or stress being applied.  The yield point identifies the minimum level of stress or force that is required to apply to attain flow.