Handbook of Materials for Product Design Part 8 ppsx

Articles made from natural rubber possess high strength and sion resistance and are very resilient with low heat buildup in dy-namic applications.. 6.6 Characterizing Heat and Oil Resist

Natural and Synthetic Rubbers 6.3

6.2 Properties of Polymers

Both rubbers and plastics are in the family of polymers, a term from

change as an increasing number (n) of repeating units (CH2-CH2) arejoined to form a high-molecular-weight (MW) polymer

As the number of units joined together and the molecular weight crease, the melting and boiling points increase, and the products gofrom gases to liquids to waxes Only at sufficiently high molecularweight is the polymer capable of high strength to make useful load-bearing parts

in-High-molecular-weight polyethylene (PE) is used to make milk jugs.The regularity of its structure allows adjacent chain segments to align

in perfect order to form crystals, which are the source of opacity orcloudiness in articles made from PE To become rubbery and recoverfrom large deformation, the amount of crystallinity must be con-trolled

One approach to controlling crystallinity is to add a differentlyshaped co-monomer such as propylene Ethylene propylene co- andter-polymers (EPM and EPDM, respectively) are rubber polymersused in weatherstrip door seals and the white sidewalls of tires Theoptional third monomer in EPDM is a diene that allows crosslinking

by sulfur cure systems EPM copolymers must be crosslinked withperoxides (see Fig 6.2)

Many rubbers are based on diene monomers in which only one of thetwo double bonds polymerizes The double bond remaining in the poly-

TABLE 6.1 Properties of Hydrocarbons -(CH 2 -Ch 2 ) n

Melting point, °C

Boiling point, °C

6.4 Chapter 6, Part 1

mer prevents free rotation of the polymer chain and minimizes thepossibility of crystallization There are two possible isomers, cis andtrans, depending on whether the polymer continues on the same side(cis) or the opposite side (trans) of the double bond Normal emulsionpolymerization gives a mixture of cis and trans structures, and nocrystallization occurs in these rubbers

If the cis or trans content is very high (>90%), then some tion can occur on stretching, which provides high strength in gum (un-filled) compounds Two strain-crystallizing rubbers are shown in Fig.6.3

crystalliza-The glass transition is the temperature at which a polymer becomesstiff and brittle As such, it determines the low temperature servicelimit of rubbers The effect on glass transition (Tg) as the polymercomposition changes from pure polybutadiene (BR, a rubber) to purepolystyrene (a plastic) is shown in Table 6.2 Copolymers of 23% sty-rene and 77% butadiene (SBR) are used in tires

Crosslinking or joining adjacent polymer chains is necessary to vent flow Adhesives and chewing gum are applications of un-crosslinked rubber Charles Goodyear used molten sulfur to cure

pre-Figure 6.2 Structure of polyethylene plastic and ethylene propylene rubber.

Figure 6.3 Structure of natural rubber and

Natural and Synthetic Rubbers 6.5

rubber of its tendency to soften and flow Elemental sulfur is still themost widely used means to crosslink or vulcanize rubber Other curesystems have been developed over the years to improve certain prop-erties or to crosslink fully saturated polymers that cannot becrosslinked with sulfur

6.3 General-Purpose Rubbers

General-purpose rubbers are low-cost hydrocarbon polymers that finduse in tires as well as other large-volume applications The 1994 worldconsumption of general purpose rubbers is shown in Table 6.3

Natural rubber (NR) was the only available rubber for many years

It is produced primarily in the Far East (Malaysia, Indonesia, andThailand), either as a concentrated liquid latex or coagulated, dried,and baled Latex is used to make thin-walled articles such as glovesand balloons Rubber bales are usually mixed with fillers for tires andmechanical goods But NR can also be used unfilled to make translu-cent articles such as rubber bands and baby bottle nipples

Articles made from natural rubber possess high strength and sion resistance and are very resilient with low heat buildup in dy-namic applications Their heat resistance is limited, and the rubberparts are susceptible to attack by oxygen, ozone, and sunlight

abra-Polyisoprene (IR) is the synthetic equivalent of natural rubber andpossesses many of the same characteristics and limitations IR is free

of the nonrubber components contained in NR, including tree proteinsthat cause allergic reactions in some individuals IR is also more con-

TABLE 6.3 Consumption of General-Purpose Rubbers 1

Consumption, thousands of metric tons

SBR is offered as a latex or in baled form The baled rubber can bepure, clear polymer as well as having carbon black and/or processingoil incorporated These low-cost polymers are extensively used in tiresand general mechanical goods The use of a reinforcing filler is neces-sary to develop good tensile and tear strength It may be blended with

NR, IR, or other polymers for cost or performance purposes

Polybutadiene (BR) is a polymer of 1,3-butadiene, which can havevarying amounts of cis, trans, and vinyl 1,2 structures incorporated inthe polymer The pendant vinyl structure can also be incorporated indifferent ways, leading to an array of polymers with varying physicalproperties and processing characteristics

Polybutadiene is mainly used in polymer blends, with the majorconsumption in tires High-cis polybutadiene is used in tire compo-nents because of its high resilience, abrasion resistance, and good flexfatigue Polybutadiene with high vinyl content is used in tire treadsfor low rolling resistance and good fuel economy Non-tire applicationsinclude high-impact polystyrene and solid-core golf ball centers

Butyl rubber is a copolymer of isobutylene with a few percent of acure site monomer The cure site is typically isoprene (IIR), which may

be halogenated to produce bromobutyl (BIIR) and chlorobutyl (CIIR)rubbers Halobutyl rubbers have faster cure rates and so may beblended and co-cured with high-diene polymers such as NR, SBR, and

BR Polyisobutylene with a brominated para-methylstyrene cure sitemonomer (Exxpro® BIMS) has recently been introduced

The polyisobutylene polymers have improved heat resistance pared to the foregoing high-diene rubbers with double bonds in the re-peating structural unit The polymers have low air permeability,leading to their use in inner tubes (butyl) and tire liners (halobutyl).Polyisobutylene rubbers also are very energy absorbent, which pro-vides ideal characteristics for articles in dynamic service

com-Ethylene propylene rubbers may be either a fully saturated mer (EPM) or a terpolymer containing <10% of a diene (EPDM),typically ethylidene norbornene, to enable vulcanization with sulfurcuring systems

copoly-EP rubbers are the largest-volume rubber used in non-tire tions They combine the heat resistance of a fully saturated polymer

applica-Natural and Synthetic Rubbers 6.7

backbone with the ability to use high levels of low-cost fillers and ticizers Examples of EP uses are hose, automotive weatherstrip, sin-gle-ply roofing membranes, and high-temperature-service wire andcable insulations

plas-6.4 Specialty Rubbers

Specialty rubbers have chlorine, fluorine, nitrogen, oxygen, or sulfurincorporated into the repeating structure These polar atoms provideresistance to swelling in hydrocarbon fluids such as gasoline and mo-tor oil The 1994 world consumption of specialty rubbers is shown inTable 6.4

Polychloroprene (CR) was the first oil-resistant rubber It may belikened to natural rubber in which the pendant methyl group is re-placed with a polar chlorine atom Like NR, CR has high strength inunfilled (gum) compounds Copolymerization with sulfur leads to highresistance to flex fatigue, whereas using a thiuram polymerizationmodifier improves heat resistance Some grades use 2,3-dichlorobuta-diene as a co-monomer to obtain resistance to crystallization andhardening at low temperature

Polychloroprene is used in adhesives, v-belts, molded goods, andjackets for electrical wire and cables Latex grades are available fordipped goods manufacture or foaming into mattress applications

TABLE 6.4 Consumption of Specialty Rubbers 1

Consumption, thousands

6.8 Chapter 6, Part 1

Nitrile rubber (NBR) is a copolymer of butadiene with 20 to 40%acrylonitrile, typically 33% Oil resistance increases in proportion tothe amount of acrylonitrile in the copolymer; low-temperature resis-tance improves in proportion to the amount of butadiene Nitrile rub-ber containing carboxyl functionality has exceptionally good toughnessand abrasion resistance Built-in antioxidants can improve heat resis-tance, and hydrogenation of the double bonds can maximize high-tem-perature performance

Nitrile rubber is used in the tube and cover of fuel hose, curb pumphose, hydraulic hose, and oil-resistant molded parts Hydrogenated ni-trile rubber (HNBR) is used in automotive power transmission belts

Polyurethane has exceptional toughness and abrasion resistance.There are two main types, produced by the reaction of an isocyanatewith a diol, either an ether (EU) or an ester (AU) Ether-based poly-urethanes have higher resilience and somewhat better low-tempera-ture and water resistance

Solid tire applications are a mainstay of polyurethane uses, ing fork lift tires, caster wheels, and skate wheels Polyurethanes arealso used to cover rubber rolls and line pumps and pipes in abrasiveservice

includ-Polyacrylates (ACM) and acrylic elastomers (EAM) have carboxyl ter groups in the repeating structural unit A small percentage of acure site monomer is also incorporated during polymerization The po-lar ester group provides oil resistance with the usual sacrifice in low-temperature resistance

es-These polymers are widely used for high-temperature oil seals such

as transmission lip seals and shaft seals They are energy absorbentfor dynamic applications and are used in wire and cable

Chlorinated polyethylene (CPE) has a fully saturated polymerbackbone for improved heat resistance as compared to the first twooil-resistant polymer families discussed For crosslinking flexibility,chlorosulfonated grades (Hypalon® CSM and an analog containingbranching, Acsium® ACSM) are available

CPE, CSM, and ACSM are used for improved heat resistance inhose and belt applications Colorable compounds can be provided thatare resistant to outdoor exposure

Silicone rubber (MQ) has a repeating polymer backbone of ing silicon and oxygen atoms Each silicon atom has two methylgroups attached For improved low-temperature properties, some me-thyl groups are replaced with phenyl groups (PMQ) For crosslinkingwith peroxides, a vinyl silicone monomer is incorporated (VMQ) Sili-cone rubber has the broadest temperature range of any rubber It is asgood as polybutadiene on the low-temperature side and is superior tomost all hydrocarbon based rubbers on the high-temperature side

alternat-Natural and Synthetic Rubbers 6.9

The uses of silicone include high-temperature seals and gaskets,electrical insulation for spark plug and appliance wires, and aerospace(both aircraft and spacecraft) These take advantage of the broad ser-vice temperature range

Fluorocarbon rubber (FKM) replaces the oxidizable gen bond with a thermally stable carbon-fluorine bond The polar fluo-rine atom provides exceptionally good resistance to oils and solventsthat would attack most all other rubbers

carbon-hydro-Many fluorocarbon applications involve parts that are small butprovide a critical function And they are used in applications where noother material will work, such as flue duct expansion joints The mod-ern automobile uses fluoroelastomer-lined hose in fuel-injected en-gines

Other rubbers include epichlorohydrin (CO), which is usually a polymer with ethylene oxide (ECO) or a terpolymer containing a sul-fur or peroxide crosslinking site (GECO); polysulfide copolymers withethylene dichloride (T); polynorbornene (PNR); tetrafluouroethylene-propylene copolymers (Aflas®); and fluorosilicone (FVMQ)

co-6.5 Thermoplastic Elastomers

Thermoplastic elastomers have two phases that are intimately mixed One phase is a rubbery phase that provides elastic recoveryfrom deformation The other phase is a hard phase that softens andflows at elevated temperature Above the melting point of the hardphase, the polymers will flow and can be shaped Below the meltingpoint of the hard phase, the material behaves like a conventionalrubber

inter-Unlike conventional rubbers, the hard phase can be melted manytimes, and the scrap can be recycled The melting of the hard phaselimits high-temperature service and detracts from compression set.The 1994 world consumption of thermoplastic elastomers is shown inTable 6.5

Styrene block copolymers have a polystyrene hard phase at each end

of the polymer with a midblock of butadiene (SBS), isoprene (SIS), orhydrogenated butadiene (SEBS) They are used in footwear and adhe-sives

Thermoplastic polyolefins (TEO or TPO) have a polyolefin hardphase, typically polypropylene, physically mixed with a rubbery phasesuch as EPDM The rubber phase has little or no crosslinking TEOsare used in automotive exterior panels and in lower-temperature wireand cable applications

Thermoplastic vulcanizates (TPV) also have a polyolefin hardphase with a crosslinked elastomer phase The crosslinking provides

6.10 Chapter 6, Part 1

improved resistance to compression set and creep The improvedtemperature resistance permits use in under-the-hood automotiveapplications

Thermoplastic polyurethanes combine the toughness and abrasionresistance of urethanes with the ability to be recycled

Thermoplastic polyesters have a terephthalate ester hard phase andsoft phase, the difference being the length of the alkylene diol joiningterephthalate groups The polymers are very stiff relative to conven-tional rubbers, which allows less material to be used to realize weightand cost savings Applications that take advantage of the polymer’shigh strength and flexibility include fuel tanks, gear wheels, and skiboots

6.6 Characterizing Heat and Oil Resistance

The heat and oil resistance of natural and synthetic rubbers may becharacterized for automotive applications by a specification systemthat has been jointly developed by the American Society for Testingand Materials (ASTM) and the Society of Automotive Engineers(SAE) ASTM Test Method D2000, or the corresponding SAE MethodJ200, characterizes the heat and oil resistance by the retention ofproperties after exposure to a standard time and temperature Thecomposition of the oil is well characterized and supplied by ASTM Inaddition to property retention minimums, the volume change upon oilimmersion is a key requirement

The relative heat and oil resistance for rubbers is shown in Fig 6.4according to the ASTM/SAE scheme Both the heat resistance and oilresistance of the polymers shown are not absolute, immutable prop-erties

During exposure to high temperature, the properties of the rubbervulcanizate will continue to change with time And, within a particu-

TABLE 6.5 Consumption of Thermoplastic Elastomers 1

Consumption, thousands

of metric tons Styrene block copolymers SBS, SIS, SEBS 294

Thermoplastic polyolefins TEO, TPO, TPV 192

Natural and Synthetic Rubbers 6.11

lar rubber type, the amount of change may vary to some extent pending on the rubber formulation—particularly the heat resistance

de-of the cure system and the use de-of antidegradants The typical rangeand variation with time for three rubbers of different recipes is shown

in Figure 6.5

The composition of polymer and the immersion fluid affect the ume swell and change in properties of a rubber compound This is illus-trated in Figs 6.6 and 6.7 for compounds based on nitrile-butadienerubber (NBR) and fluoroelastomer (FKM), respectively

vol-Figure 6.4 Heat and oil resistance per ASTM D200/SAE J200

scheme.

Hours to 100% elongation for three rubbers.

6.12 Chapter 6, Part 1

6.6.1 Initial Physical Properties

The heat and oil resistance encountered in the application helps thedesign engineer to select the type of polymer most likely to perform inthe intended application In addition, the initial physical propertiesplay a significant role in determining the suitability for use TheASTM D2000/SAE J200 system characterizes the basic initial physicalproperties across the range of properties shown in Table 6.6 Thedurometer A hardness is measured by an indentor of specific radius

Figure 6.6 Effect of acrylonitrile content on volume change of NBR.

Figure 6.7 Effect of fluorine content on volume change of FKM.

Natural and Synthetic Rubbers 6.13

The durometer measurement generally correlates to the load-bearingcapability of the rubber article A tolerance of ±5 durometer A pointsfrom the specified target is typically allowed

Accuracy declines toward either end of the 0 to 100 durometer Ascale Because rubber includes soft sponge and extremely hard, plas-tic-like urethane, other hardness measurements are typically usedfor these materials The OO durometer has a blunter indentor forsponge; the more pointed D durometer is used for hard rubber Someapplications use hardness measurements specific to their industry,such as the Pusey and Jones (P&J) scale in rubber-covered rolls forpaper mills

Minimum values for the ultimate tensile strength and the tion at break are two other basic properties in the ASTM D2000/SAEJ200 system These two properties are useful to control the uniformity

elonga-of a particular compound but generally do not correlate with end-useperformance At a particular hardness, not all levels of tensile andelongation may be obtainable

To measure tensile and elongation, the test specimen is secured inthe jaws of a tensile test machine and stretched at a specified rate un-

TABLE 6.6 Basic Initial Physical Properties of Rubbers

Polymer

Hardness durometer A

Min tensile strength, MPa

6.14 Chapter 6, Part 1

til it breaks The final force required for the break is recorded, alongwith the amount of stretch that was achieved at the break point Theforces in effect at various degrees of elongation of the specimen usu-ally are also recorded These forces are used to calculate the stressesper unit area at those elongations, which are reported as tensile mod-uli These are typically written as the M-100 (or L-100), which is thestress at 100% strain, the M-200, M-300, etc

It is very important to understand that none of these numbers is aclassic modulus, that is, a basic ratio of stress to strain that appliesacross a wide range of strains With the possible exception of a narrowregion of moderately low strain, the stress-strain plot for elastomers isalways nonlinear, and these are secant moduli drawn to various points

of the particular curve that applies at that temperature and rate ofstrain In Figure 6.8, a typical stress-strain plot is displayed, with thelines drawn on it that illustrate what the secant and tangent moduliare (The tangent modulus, which estimates the force required to de-form the rubber in the strain region of interest, is more meaningful formany engineering applications but is not commonly used.)

For a few materials, such as steel, a comparatively pure moduluscan be measured that is not a function of strain, temperature, or rate

of strain It is a single point of information, so to speak, that appliesvery broadly In contrast, the response of rubber (and many other poly-meric materials) to a deforming force is not a point; it is a three di-mensional surface, the axes of which are temperature, amount ofstrain (deformation), and rate of strain

What can be said simply about responses of rubber to different ditions is that the material will become stiffer as the temperaturedrops or as the rate of strain is increased There is also the well knownMullins effect, which says that, for many rubber compounds, the force

con-Stress/strain plot for a rubber specimen.

Natural and Synthetic Rubbers 6.15

necessary to deform them the very first time will be significantly morethan will be required on subsequent deformations Also, for moderatedeformations (10–50%), rubber undergoes what engineers refer to as

strain softening; this means that, as the rubber is forced to deform, ittakes less force per unit of deformation to achieve a high strain than alower one As an example, the dynamic modulus of a compound inshear at a level of ±10% might be 150 psi, but under the greater strain

of ±20%, that modulus will be less than 300 psi It will still take moreforce to make the rubber deform to the greater strain, but not twice asmuch force

These special characteristics of rubber add up to the important cept The single-cycle stress-strain curve to rupture of a rubber speci-men at room temperature cannot generate data with the kind ofmeaning that exists in tensile testing other materials—definitely notthe kind of meaning that exists in tensile testing of metals

con-6.6.2 Specifying Heat and Oil Resistance

The ASTM D2000/SAE J200 system specifies a maximum change inproperties after heat aging The allowable change is measured after 70hours at the maximum service temperature (see Table 6.9 for the testtemperature) The basic change in properties that is permitted is iden-tical for all rubbers For particularly severe applications, the allow-able change in properties after heat aging may be tightened byincorporating suffix requirements The basic and most stringent suffixproperty changes are summarized in Table 6.7

For oil-resistant rubbers, the ASTM D2000/SAE J200 system fies a maximum change in volume after immersion an oil of standardcomposition, called ASTM No 3 oil (Since the supply of No 3 oil hasbeen depleted, testing is currently being performed with its replace-ment, 903 oil.) The allowable change is generally is measured after 70hours immersion at the normal maximum service temperature Above150ºC, the oil tends to degrade, and immersion tests on higher temper-ature rubbers are not run above this limit The basic property is amaximum volume swell that depends on the specific rubber Incorpo-rating suffix requirements that reduce the allowable change on vol-ume swell and limit the change in physical properties may helpperformance in severe oil immersion applications The basic and moststringent suffix property changes for immersion in No 3 Oil are sum-marized in Table 6.8

speci-6.7 Other Properties

The heat and oil resistance encountered in the application may not fine all the requirements necessary for successful use Depending on

de-6.16 Chapter 6, Part 1

the particular application, certain additional properties may be

re-quired

Power transmission belts often require good tear strength and

re-sistance to crack growth during flexing O-rings and seals generally

specify a maximum compression set, or the allowable unrecovered

de-formation after aging while compressed between two steel plates (see

Table 6.9) Burst strength is an important property in hose As with

many applications, both part design and polymer selection are

impor-tant for best performance

Frequently, the attainment of one property involves a trade-off or a

sacrifice in other areas An example is the balance that must be struck

in tire treads among abrasion resistance, fuel economy, and wet

trac-tion

Some of the comparative properties of rubbers are shown in Table

6.10 The ratings are only a general guideline, because the specific

TABLE 6.7 Basic and Suffix Heat Resistance of Rubbers

Hardness ∆ , pts Tensile ∆ , % Elongation ∆ , % max.

Polymer Basic Suffix Basic Suffix Basic Suffix

IIR, CIIR, BIIR ±15 +10 max ±30 –25 max –50 –25

Natural and Synthetic Rubbers 6.17

compounding, processing, and part design can affect actual

perfor-mance

For example, electrical properties are highly dependent on the type

of filler used The addition of nonconductive mineral fillers is

em-ployed in wire and cable insulations for high resistivity and good

di-electric strength Conversely, the use of high-structure carbon blacks

achieves antistatic or electrical conductive properties to drain static

charges from walk-off mats and mouse pads Electrical conductivity

versus carbon black concentration in EPDM is shown in Figure 6.9 for

four high-structure blacks

Another example of the general nature of the ratings shown in Table

6.10 is the resistance to hydrocarbons and oils Crankcase lubricants

are based on paraffinic hydrocarbon base oils The fully formulated

lu-bricant may contain about 20% functional additives that improve

spe-cific performance properties: lower friction, increased viscosity index,

dispersancy and/or detergency provided, etc These additives can

at-tack certain rubbers that one might expect to be impervious to the

base oil itself In any contemplated use, the rubber part should be

ex-perimentally tested under actual use conditions before adoption in

production

TABLE 6.8 Basic and Suffix No 3 Oil Resistance of Rubbers

Vol ∆ , % max.

Hardness ∆ , pts (suffix)

Tensile ∆ ,

% max (suffix)

Elong ∆ ,

% max (suffix) Polymer Basic Suffix

6.18 Chapter 6, Part 1

6.7.1 Rubber in Motion

When rubber is deformed and then allowed to recover, not all the

en-ergy input is recovered That is, rubbers are not purely elastic but

ex-hibit a significant viscous component that can be used for energy

management purposes The ratio of elastic to viscous response depends

on several factors: temperature, frequency, strain amplitude (both

static and dynamic), the particular polymer, and how it is compounded

To characterize rubber’s dynamic response, it is helpful to examine

both purely elastic and purely viscous responses as shown in Figure

6.10 In the deformation of a purely elastic Hookean spring, there is a

linear response of stress to strain The ratio of stress to strain does not

change, no matter how rapidly the strain is applied Metal springs

typically exhibit Hookean elasticity

TABLE 6.9 Basic and Suffix Compression Set of Rubbers

Compression set, % max.

Polymer Test temp., °C Basic Suffix

Natural and Synthetic Rubbers 6.19

TABLE 6.10 Comparative Properties of Rubbers (from Ref 2)

SBR AA

EPR EPDM DA

CR BC

IIR AA

BIIR CIIR BA

NBR BK

CSM CE Density, mg/m 3 0.93 0.94 0.86 1.23 0.92 0.92 1.00 1.10

Hardness, Shore A 20–90 40–90 40–90 20–95 40–75 40–75 20–95 45–95

Typical tensile strength

Pure gum, MPa 21 7 3 21 10 10 7 14

Pure gum, psi 3000 1000 400 3000 1500 1500 1000 2100

A = hexane, isooctane, etc., B = acetone, methyl-ethyl ketone, etc., C = chloroform, etc., D = toluene,

TABLE 6.10 Comparative Properties of Rubbers (Continued)

CM BC

ACM EH

AU EU BG

T AK

MQ GE

FKM HK

FVMQ FK Density, mg/m 3 1.27–1.36 1.16–1.32 1.09 1.02 1.20 1.1–1.6 1.85 1.47 Hardness, Shore A 40–90 40–95 40–90 60–95 20–80 10–85 60–95 40–70

Typical tensile strength

Pure gum, MPa –– 10 3 42 1 1 14 –– Pure gum, psi –– 1500 400 6000 200 200 2000 –– Reinforced, MPa 14 14 12 42 9 8 14 10 Reinforced, psi 2000 2000 1800 6000 1300 1100 2000 1500

Viscous behavior is typified by a shock absorber, i.e., a cylinder filledwith a fluid through which a piston is moved For pure viscous behav-ior, the fluid must be Newtonian That is, the fluid will exhibit a linearresponse to strain rate and show no dependency on displacement The viscoelastic stress/strain response of a typical rubber is shown

in Figure 6.11 Initially, stress increases in response to strain Thestress will then almost plateau at a level that depends on strain rate

At higher deformation, the finite extensibility of the polymer chains isreached, and the curve bends upward toward the break point This lat-ter part of the curve can not be readily modeled theoretically, and rub-bers are generally not used at strains of this magnitude—at least forlong periods of time

Figure 6.9 Effect of carbon blacks on electrical conductivity in EPDM.3

Figure 6.10 Elastic and viscous stress/strain responses.

The specific contributions of the elastic and the viscous componentscan be separated by repeatedly cycling the rubber through a deforma-tion range as indicated in Figure 6.12 The observed stress responsewill lead the strain deformation by a certain amount, the phase angle

δ With this angle and the force measured at maximum deformation,the respective elastic and viscous components can be calculated Theelastic component is in phase with the applied deformation; the vis-cous component is 90º out of phase Often, the ratio of the viscous toelastic components (loss factor or tangent δ) is computed, because it isrelated to the amount of kinetic energy that is converted to heat en-ergy

When the deformation of rubber occurs to a constant energy, thehysteresis (heat generated) is equal to tangent δ When the deforma-tion is to constant strain, the hysteresis is determined by the product

Figure 6.11 Viscoelastic stress/strain response of

rub-ber.

Determining the viscous and elastic components.

of the elastic response (E´ if in tension or G´ if in shear) times tangent

δ Finally, for a constant dynamic stress or load input, hysteresis is

equal to the viscous response (E´´ or G´´) divided by the square of the complex (observed) response (E* or G*) Because the elastic response

normally is much larger than the viscous response, the constant loadhysteresis is approximated by the ratio of tangent δ to E´ (or G´).

As the temperature is lowered, rubbers become stiff and like, where the tangent δ goes through a maximum The maximum intangent δ occurs slightly before the rubber transitions to a rigid,glassy state

leather-Increasing the test frequency has the same effect as lowering thetemperature For ideal viscoelasticity, a tenfold increase in frequency

is approximately equal to a 10°C decrease in temperature Some gum(unfilled) compounds exhibit nearly ideal viscoelasticity in which time(frequency) and temperature can be superimposed on a single mastercurve of dynamic behavior

However, the incorporation of fillers complicates the situation, andideal viscoelasticity is not observed in these compounds, which arerepresentative of most rubber articles The addition of fillers signifi-cantly increases the low strain dynamic modulus However, at higher(1 to 10%) dynamic strain amplitude, the filler network breaks down,resulting in a rapid decrease in dynamic modulus and a maximum intangent δ At still higher strain amplitudes, the filled compound ap-proximates the dynamic response of an unfilled gum compound.The geometric design of the part also determines the dynamic re-sponse A shape factor is calculated as the ratio of the loaded area of

the part (A) to the area that is free to deform (L) The shape factor can

be used to estimate various moduli (shear, compression, etc.), springrates, and damping coefficients for simple shapes However, complexshapes are less readily modeled In this case, finite element analysis isapplied for static deformation estimations, and dynamic simulationsmay be possible in the future

For a given rubber part, the spring rate (K) and the damping cient (C) characterize the dynamic response For simple geometries, K

coeffi-is equal to E´ times A divided by L, where A and L are the loaded area

and the area free to deform The damping coefficient is the viscous

re-sponse, calculated from E´´ times A and divided by the product of L

times ω, the test frequency The “C to K ratio” then becomes the E´´ vided by E´ times ω (or tangent δ/ω)

di-In many applications, the rubber part supports a vibrating machine

As the speed of the machine changes, it affects the frequency of the brations that the rubber article partly absorbs and partly transmits.Transmissibility is the ratio of the transmitted force to the applied

vi-force (T = F t /F a)

The transmitted force is greater than the applied force in the low

frequency attenuation region The natural or resonant frequency (f n)

is determined by the spring rate of the rubber part (K´) and the mass

of the system (M) by the equation

At resonance, transmissibility goes through a maximum that is portional to the reciprocal of the damping coefficient or tangent δ (seeFigure 6.13) Normally, it is desirable to design the system so that res-onance is experienced only occasionally, such as during startup

pro-In a plot of transmissibility versus frequency (Fig 6.14), the

high-frequency region is called the isolation region, and transmissibility is

less than 1.0 The spring rate increases with frequency, and theamount of increase is generally greater with higher damping rubbercompounds In a log-log plot, the rolloff rate is therefore greater forlower damping compounds, and they transmit less vibration in the iso-lation region where the machines are typically designed to operate.Over time, the flexing of a rubber part can cause fatigue, as evi-denced by the development and growth of cracks The fatigue life isstrongly dependent on the dynamic strain (or stress, if deformed toconstant load) In the extreme, a total failure can occur in one cycle.Alternatively, the part may last indefinitely at very low dynamic de-formations

Generally, higher tearing energy gives longer flex fatigue life, butthe relationship is not always linear Higher temperatures mayshorten flex life due to lower tear strength of the rubber at elevatedtemperature However, as modulus also declines with an increase intemperature, if the deformation is to constant strain, less energy is in-put per cycle, and longer flex life may ensue.

Strain-crystallizing rubbers (such as NR and CR) exhibit longer flexlife if the minimum strain does not go to zero In this case, it is be-lieved that crystallites form at the tip of the growing crack, wheremaximum strain is encountered The crystals blunt the crack tip andforce the tear to travel around the crystals

Both the vulcanization system and the antidegradant package canaffect flex life In general, flex life improves in going from peroxide tosulfur-donor to elemental sulfur cures The use of antioxidants andantiozonants can improve fatigue life

A final complication occurs in the measurement of flex fatigue life.Laboratory tests can have poor correlation with actual application re-sults Reproducibility can also be a problem, since crack initiation isthought to occur at microscopic inhomogenities in the rubber sample,which depend on how well each test specimen is prepared To achievemore reproducible results, the laboratory specimen is often cut ornicked to measure crack growth rather than crack initiation

When rubber moves in relation to a contacting surface, wear of therubber can occur Again, laboratory tests generally do not correlatewell with application results, because the loss of rubber from the sur-face depends on the service conditions, which typically vary with time,temperature, frequency, strain, etc

Figure 6.14 Transmissibility in the high-frequency isolation region.

The mechanism by which wear or loss of rubber occurs depends onservice conditions Sliding abrasion, such as observed with a tiretread, is caused by hard surface projections cutting the rubber Im-pingement abrasion, such as encountered in a sandblasting hose, isdue to high-speed particles impacting the rubber Less frequently en-countered is adhesive wear in which rubber particles are transferred

to another surface because of high adhesion to the surface

Sliding abrasion can be improved by adding reinforcing carbonblacks of small particle size to the rubber compound Generally, there

is an optimal level of carbon black that depends on the specificblack(s), polymer(s), and operating conditions On the other hand, theresistance to impingement abrasion is often best in unfilled gum com-pounds

6.7.2 Friction

Dry rubber surfaces are generally accepted as having high coefficients

of friction, but measurement of COF can be done in many ways (ASTMD1894 is one method), which will generate very different numbers.For instance, static COF, the force needed to start movement across arubber surface, can be quite high, with levels ranging easily as high as

1 and often appreciably greater, such as 2–4 Dynamic COF, the forcerequired to maintain movement, is always less and can range from 0.2

up toward 1 It should be noted that actual movement across a rubbersurface is almost always in the mode of a stick-slip process, and usu-ally the COF is calculated from average force recorded

Furthermore, many factors affect the frictional force measured,which include the rubber hardness, the load and speed used in thetest, and the particular material and surface morphology of the sur-face against which the rubber is pressed Compounding differences ofpolymer type and especially additives (which can bloom to the rubbersurface and act to lubricate or tackify) have major effects Lubrication

of rubber surfaces by light oils or soapy water can render them veryslippery, with the COF dropping to levels below 0.1 and the disappear-ance of the stick-slip phenomenon With all these variables in play, thedetermination of frictional properties of any rubber compound should

be carefully considered in light of the actual application and its ronment

envi-6.7.3 Permeability

The ability to retain air is a key property of the modern tire and otherarticles Relative to other materials of construction, however, rubber isrelatively permeable to the migration of small molecules

Permeability is mainly determined by polymer, and specifically its

Tg In general, the higher is the temperature (above Tg), the greater isthe permeability to gases However, use of plasticizer and the selection

of filler also modify permeability Plasticizers tend to depress Tg andtherefore increase permeability Fillers, particularly platy ones such

as talc and clay, can decrease permeability by creating a longer, moretortuous diffusion path The permeability of selected rubbers to vari-ous penetrant molecules is shown in Table 6.11

6.7.4 Flame Resistance

Being composed of oxidizable carbon and hydrogen, most polymerswill burn Incorporating antimony oxide (Sb2O3) and a halogen (typi-cally at a 1:3 ratio) is a cost-effective method to achieve flame retar-dance Polymers such as CR and CPE, as well as halogenatedplasticizers, can supply the halogen source However, this technique of

Sb2O3 and halogen generates smoke (which can obscure exit signs)and acid that corrodes electrical equipment (and can be toxic)

Other methods for low-smoke (halogen-free) rubber compounds clude materials that form a glassy char on the surface, such as zinc bo-rate or phosphate plasticizers, and/or the use of magnesium salts,such as carbonates or hydroxides, that release carbon dioxide and wa-ter, respectively The use of polymers with a high oxygen content, aswell as inorganic fillers such as clay to dilute the oxidizable compo-

in-TABLE 6.11 Permeability, (10 –9 )(m 2 )/(sec)(Pa), of Some Rubbers to Various Gases (from Ref 4)

nents, is also beneficial Due to the large quantity of additives quired, the nonhalogen systems usually have inferior properties.Alumina trihydrate (ATH) is incorporated in rug backing com-

re-pounds to release water on heating and help the material pass a pill

test, when a hot metal disk is placed on the carpet, which should notignite The decomposition temperature of ATH is too low for many ap-plications, and it detracts from physical properties

Reinforcing fillers are typically carbon blacks, because they providethe best properties Carbon blacks are characterized by two parame-

ters: particle size and structure, the way in which the particles are

fused together Particle size is typically measured by surface area, ther nitrogen absorption or iodine number, with larger surface areacorresponding to smaller particle size Structure is measured by dibu-tyl phthalate (DBP) absorption The more adsorption of the relativelylarge DBP molecule into the chain-like structure of the aggregates ittakes to wet the black, the greater is the black’s structure

ei-Smaller particle, larger surface area blacks provide higher hardnessand tensile strength, with poorer resilience and more difficulty in ob-taining a uniform dispersion in the rubber Higher structure blacks in-crease hardness and modulus, decrease ultimate elongation, dispersemore rapidly in the rubber, and give higher dimensional stability or

“green” strength to the uncured compound

There are also non-black reinforcing fillers, such as precipitated ica and hard clay, that can approximate the properties of carbon black.These or the extending fillers can be used to give bright white or color-able compounds for ready identification Treating some non-black fill-ers with silanes, molecules that increase polymer-to-filler bonding, canbring the performance of non-black fillers even closer to that of carbonblack or, for some properties, even surpass carbon blacks

sil-Plasticizers represent a second large volume category of

compound-ing compound-ingredients These are typically either hydrocarbon oils or esterplasticizers

Oils are used in hydrocarbon polymers to provide lower hardness,better low-temperature performance, and improved processing Oilsfor rubber are available in three general types (aromatic, naphthenic,and paraffinic), although each type contains a mixture of the threestructures Each type is available in various viscosities, with thehigher-viscosity oils giving low volatility and good permanence in thefinished vulcanizate Lower-viscosity oils are more volatile and affectprocessing more than end-use properties.

The ester plasticizers are more polar and provide similar properties

in the oil-resistant specialty rubbers A wide variety of esters areavailable to serve a range of cost and performance goals In some criti-cal seal applications, plasticizers should be avoided, because they arenot permanently bound into the polymer and can either be extracted

by the fluid being contained or volatilized (lost) during high ture exposure

tempera-Vulcanizing agents provide a critical function, that of crosslinkingadjacent polymer chains together, which prevents flow and permits therecovery from deformation The vulcanizing system can be the simpleaddition of one additive such as a peroxide or metal oxide, or it can be acomplex mixture of several ingredients, the components of which maydiffuse into different phases of a polymer blend before reacting Liter-ally hundreds of materials are available for curing rubbers

Vulcanizing agents may be broadly grouped into five categories: oxides, elemental sulfur, sulfur donors, metal oxides, and multifunc-tional additives—typically amines, phenols, or thiophenols Inaddition to the crosslinking agent itself, various accelerators, activa-tors, and retarders are used in the vulcanizing system

per-Peroxides generate the most thermally stable carbon-to-carboncrosslink bonds Peroxide-cured vulcanizates exhibit outstanding heatresistance and compression set They are deficient in some failureproperties, particularly flex fatigue, and can cause problems in pro-cessing, such as tacky surfaces if exposed to air during cure

Sulfur donor cure systems have good thermal stability, approachingthat of peroxides Table 6.12 compares sulfur and peroxide cure sys-tems in nitrile rubber (NBR) The high-temperature performance ofthese systems can be improved by adding a good antioxidant package,

as indicated in compounds B and D

The sulfur donor cure depends on the ability of certain molecules todonate an atom of sulfur for crosslinking purposes This monosulfidecrosslink has better heat resistance that the polysulfide crosslinksformed when elemental sulfur (an eight-membered sulfur ring) isused

Elemental sulfur is the most widely used cure system for purpose rubbers It provides good failure properties and high flex fa-

general-tigue resistance, and it is very economical Its deficiencies are in poorhigh-temperature aging and compression set See Table 6.13 for acomparison of a normal sulfur and a sulfurless efficient vulcanizing(EV) cure system in an SBR/IR blend.

Metal oxides can be used to crosslink certain polymers, notablysome grades of polychloroprene, chlorosulfonated polyethylene, poly-sulfides, and carboxylated polymers The properties provided by

TABLE 6.12 Comparison of Peroxide and Sulfur Donor Cures in NBR (from Ref 5)

metal oxides are generally quite good However, heat resistance is ten limited.

of-Multifunctional additives are mainly used to crosslink specialty resistant rubbers Generally, the crosslinking additive is specific toone or two polymers In addition, an accelerator is often used to speed

oil-up the rate of crosslinking In many cases, an oven post-cure after atypical press cure is required to optimize compression set and otherperformance properties with these cures

Antidegradants are used to prolong the useful life of a rubber cle They may protect against oxidative attack (a bulk effect) or ozoneattack (a surface effect) Mixtures of several antioxidants can be used;one example is shown in Table 6.12 Similarly, more than one antiozo-nant is frequently found in rubber recipes The addition of antiozo-nants and, to a lesser extent, antioxidants may involve the use ofdiscoloring additives

arti-Many other additives are used in typical rubber compounds to vide improved mold flow, release from processing equipment, bonding

pro-to metal inserts, color, tack, resistance pro-to antimicrobial growth, orother desired properties

References

1 James V Fusco, “The World of Elastomers, An Industry Overview,” Paper No G presented at the Rubber Division Meeting of the A.C.S., May 1996.

2 R.F Ohm, ed., The Vanderbilt Rubber Handbook, Vol 13, pp 429–430, 1990.

3 Anon., Ensaco Carbon Blacks in Rubber Compounds, Erachem Europe, s.a cal literature.

techni-4 G J van Amerongen, Diffusion in Elastomers, Rubber Chemistry and Technology,

Vol 37, pp 1065–1152, 1964.

5 Anon., Vanderbilt News, Vol 39, no 1, p.15, 1983.

6 R.F Ohm, ed., The Vanderbilt Rubber Handbook, Vol 13, p 459, 1990.

Suggested Readings

Alan N Gent, ed., Engineering with Rubber, A.C.S Rubber Division, 1992 A practical

introduction and theoretical reference.

P B Lindley, Engineering design with natural rubber, NR Technical Bulletin, The laysian Rubber Producers’ Research Association, 1984.

Ma-Maurice Morton, ed., Rubber Technology, Van Nostrand Reinhold, 3rd ed., 1987 A basic

introduction to rubber technology.

Harry Long, ed., Basic Compounding and Processing of Rubber, A.C.S Rubber Division,

1985.

Frederick R Eirich, ed., Science and Technology of Rubber, Academic Press, 2nd ed.,

1994 An advanced text.

Robert F Ohm, ed., The Vanderbilt Rubber Handbook, 13th ed., 1990, also available on

CD-ROM, 14th ed., 1998 An introduction for neophytes and a reference for gists.

technolo-Robert F Ohm, “Introduction to Rubber Technology,” presented at the Conference on Engineering Design with Elastomers, organized by the New York Rubber Group, Aug 1990.

TABLE 6.13 Comparison of Sulfur and EV Cure Systems in SBR/IR Blend (from Ref 6)

Black filled Mineral filled

Robert F Ohm, “Compounding Rubber for Dynamic Applications,” presented at the ucational Symposium on Designing, Compounding and Testing for Dynamic Proper- ties, organized by the Akron and Northeast Ohio Rubber Groups, April 1995 Robert F Ohm, “Review of Antioxidants,” presented to the Northeast Regional Rubber

Ed-& Plastics Exposition, Sept 1994.

Robert F Ohm, “Review of Antiozonants,” Rubber World, Vol 208, no 8, pp 18–22, Aug.

1993, based on Paper No D at the Rubber Division Meeting of the A.C.S., May, 1993.

R P Brown, Physical Testing of Rubbers, Applied Science, 1979.

TABLE 6.14 Some Common Trade Names for Elastomers and Compound Ingredients

AGERITE SUPERFLEX 3 Acetone-diphenylamine reaction product ADPA AGERITE SUPERLITE 3 Polybutylated Bisphenol A mixture —

AMAX4 N-oxydiethylenebenzothiazole-2-sulfenamide OBTS

EXXPRO8 Polyisobutylene/brominated p-methylstyrene BIMS

VANFRE 4 AP-2 SPECIAL Processing aid —

1 ACSIUM, HYPALON, and Hytrel are registered trademarks of DuPont Dow Elastomers, L.L.C., Wilmington, DE.

2 Aflas is a registered trademark of Dyneon, Minneapolis, MN.

3 AGERITE, STALITE, SUPERFLEX and SUPERLITE are registered trademarks of The B.F Goodrich Company, Akron, OH.

4 ALTAX, AMAX, DIXIE CLAY, CADMATE, MORFAX, RODFORM, TUADS, VANAX, VANFRE, VANOX, VANPLAST, VANWAX and VAROX are registered trademarks of the R T Vanderbilt Company, Inc., Norwalk, CT.

5 CORAX is a registered trademark of Degussa-Hüls Corp., Ridgefield Park, NJ.

6 Cumar is a registered trademark of Neville Chemical Co., Pittsburgh, PA.

7 ENSACO is a registered trademark of Erachem Europe s.a., Brussels, Belgium.

8 EXXPRO is a registered trademark of Exxon Corp., Houston, TX

9 HiSil is a registered trademark of PPG Industries, Inc., Pittsburgh, PA.

10 Hycar is a registered trademark of Zeon Chemicals, Inc., Louisville, KY.

11 NATSYN is a registered trademark of The Goodyear Tire and Rubber Company, Akron, OH.

12 Paraplex is a registered trademark of C.P Hall Co., Chicago, IL.

Note: The R T Vanderbilt Company, Inc sells the capitalized products.

TABLE 6.14 Some Common Trade Names for Elastomers and Compound Ingredients

Part 2 Elastomeric Materials and

6.10 Thermoplastic Elastomers (TPEs)

Worldwide consumption of TPE for the year 2000 is estimated to beabout 2.5 billion pounds, primarily due to new polymer and processingtechnologies, with an annual average growth rate of about 6% be-tween 1996 and 2000 About 40% of this total is consumed in NorthAmerica.4

TPE grades are often characterized by their hardness, resistance

to abrasion, cutting, scratching, local strain, and wear A tional measure of hardness is Shore A and Shore D, shown in Fig.6.15 Shore A is a softer, and Shore D is a harder TPE, with rangesfrom as soft as Shore A 28 to as hard as Shore D 82 Durometer hard-ness (ASTM D 2240) is an industry standard test method for rubbery

conven-* The chapter author, editors, publisher, and companies referred to are not responsible for the use or accuracy of information in this chapter, such as property data, processing parameters, and applications.

6.36 Chapter 6, Part 2

materials, covering two types of durometers, A and D The durometer

is the hardness measuring apparatus, and the term durometer ness is often used with Shore hardness values There are other hard-ness test methods such as Rockwell hardness for plastics andelectrical insulating materials (ASTM D785 and ISO 2039), and Bar-col hardness (ASTM D2583) for rigid plastics While hardness is of-ten a quantifying distinction between grades, it does not indicatecomparisons between physical/mechanical, chemical, and electricalproperties

hard-Drying times depend on moisture absorption of a given resin TPEproducers suggest typical drying times and processing parameters.Actual processing temperature and pressure settings are determined

by resin melt temperatures and rheological properties, mold cavity sign, and equipment design such as screw configuration

de-Performance property tables provided by suppliers usually refer tocompounded grades containing property enhancers (additives) such asstabilizers, modifiers, and flame retardants Sometimes, the suppliers’property tables refer to a polymer rather than a formulated com-pound

6.10.1 Styrenics

Styrene block copolymers are the most widely used TPEs, accountingfor close to 45% of total TPE consumption worldwide at the close of thetwentieth century.1 They are characterized by their molecular archi-tecture, which has a “hard” thermoplastic segment (block) and a “soft”elastomeric segment (block) (see Fig 6.16) Styrenic TPEs are usuallystyrene butadiene styrene (SBS), styrene ethylene/butylene styrene(SEBS), and styrene isoprene styrene (SIS) Styrenic TPEs usuallyhave about 30 to 40% (wt) bound styrene; certain grades have a higherbound styrene content The polystyrene endblocks create a network ofreversible physical cross links that allow thermoplasticity for melt

Figure 6.15 TPEs bridge the hardness ranges of rubbers and plastics (Source:

Ref 10, p 5.2.)

Elastomeric Materials and Processes 6.37

processing or solvation With cooling or solvent evaporation, the styrene domains reform and harden, and the rubber network is fixed

poly-in position.2

Principal styrenic TPE markets are: molded shoe soles and otherfootwear; extruded film/sheet and wire/cable covering; pressure-sensi-tive adhesives (PSA) and hot-melt adhesives; and viscosity index (VI)improver additives in lube oils, resin modifiers, and asphalt modifiers.They are also popular as grips (bike handles), kitchen utensils, clearmedical products, and personal care products.1,4 Adhesives and seal-ants are the largest single market.1 Styrenic TPEs are useful in adhe-sive compositions in web coatings.1

Styrenic block copolymer (SBC) thermoplastic elastomers were duced by Shell Chemical (KRATON®) and are available from FirestoneSynthetic Rubber and Latex, Division of Bridgestone/Firestone (Ste-reon®*), Dexco Polymers (Vector®†), and EniChem Elastomers (Euro-prene®‡) SBC properties and processes are described for these fourSBCs

pro-KRATON§ TPEs are usually SBS, SEBS, and SIS, as are SEP rene ethylene/propylene) and SEB (styrene ethylene/butylene).2 Thepolymers can be precisely controlled during polymerization to meetproperty requirements for a given application.2

(sty-Two KRATON types are chemically distinguished: KRATON G and ton D, as described below A third type, KRATON Liquid®, poly(ethyl-

(ra-* Stereon is a registered trademark of Firestone Synthetic Rubber and Latex Company, Division of Bridgestone/Firestone.

† Vector is a registered trademark of Dexco, A Dow/Exxon Partnership

‡ Europrene is a registered trademark of EniChem Elastomers

§ K RATON styrenics were developed by Shell Chemical In March 2001, the company formally announced finalization of sale to Ripplewood Holdings For information, contact Kate Hill, Shell International Media Relations, telephone 44(0)2079342914.

Figure 6.16 Structures of three common styrenic

block copolymer TPEs: a and c = 50 to 80; b = 20 to

100 (Source: Ref 10, p 5.12.)

6.38 Chapter 6, Part 2

ene/butylene), is described thereafter KRATON G and D have differentperformance and processing properties KRATON G polymers have sat-urated midblocks with better resistance to oxygen, ozone and ultravio-let (UV) radiation, and higher service temperatures, depending onload, up to 350°F (177°C) for certain grades.2 They can be steam steril-ized for reusable hospital products KRATON D polymers have unsatur-ated midblocks with service temperatures up to 150°F (66°C).2 SBCupper service temperature limits depend on the type and weight per-cent (wt%) thermoplastic and type and wt% elastomer, and the addi-tion of heat stabilizers A number of KRATON G polymers are linearSEBS, while several KRATON D polymers are linear SIS.2 KRATON Gpolymer compounds’ melt process is similar to polypropylene; KRATON

D polymer compounds’ process is comparable to polystyrene (PS).2Styrenic TPEs have strength properties equal to those of vulcanizedrubber, but they do not require vulcanization.2 Properties are deter-mined by polymer type and formulation There is wide latitude in com-pounding to meet a wide variety of application properties.2 According

to application-driven formulations, KRATONs are compounded with ahardness range from Shore A 28 to 95 (Shore A 95 is approximatelyequal to Shore D 40), sp gr from 0.90 to 1.18, tensile strengths from

150 to 5000 lb/in2 (1.03 to 34.4 MPa), and flexibility down to –112°F(80°C) (see Table 6.15).2

KRATONs are resistant to acids, alkalis, and water, but long soaking inhydrocarbon solvents and oils deteriorates the polymers.2

Automotive applications range from window seals and gasketing toenhanced noise/vibration attenuation.1 The polymers are candidatesfor automotive seating, interior padded trim and insulation, hospital

TABLE 6.15 Typical Properties of a K RATON D and K RATON G Polymer for Use

as Formulation Ingredients and as Additive (U.S FDA Compliance)

Property [74°F (23°C)]

K RATON

D D1101 (linear SBS)

K RATON

G G1650 (linear SEBS) Specific gravity, g/cm 3

Hardness, Shore A

Tensile strength, lb/in2 (MPa)

300% modulus, lb/in2 (MPa)

4600 (32)

400 (2.7) 880 10

<1 4000 31/69

0.91 75

5000 (34)

800 (5.5) 550

—

— 8000 29/71

Elastomeric Materials and Processes 6.39

padding, and topper pads.1 SEBS is extruded/blown into 1-mil filmsfor disposable gloves for surgical/hospital/dental, food/pharmaceutical,and household markets.1

KRATONs are used in PSAs, hot-melt adhesives, sealants, applied coatings, flexible oil gels, modifiers in asphalt, thermoplastics,and thermosetting resins.2 When KRATONs are used as an impactmodifier in nylon 66, notched Izod impact strength can be increasedfrom 0.8 ft-lb/in for unmodified nylon 66 to 19 ft-lb/in Flexural modu-lus may decrease from 44,000 lb/in2 (302 MPa) for unmodified nylon

solution-66 to about 27,000 lb/in2 (186 MPa) for impact-modified nylon 66.SBCs are injection molded, extruded, blow molded, and compressionmolded.2

KRATON Liquid polymers are polymeric diols with an aliphatic, mary OH– group on each terminal end of the poly(ethylene/butylene)elastomer They are used in formulations for adhesives, sealants, coat-ings, inks, foams, fibers, surfactants, and polymer modifiers.13

pri-Two large markets for Firestone’s styrenic block copolymer SBS reon TPEs are (1) impact modifiers (enhancers) for flame-retardantpolystyrene and polyolefin resins and (2) PSA and hot-melt adhesives.Moldable SBS block copolymers possess high clarity and gloss, havegood flex cycle stability for “living hinge” applications, and come inFDA-compliant grades for food containers and medical/hospital prod-ucts.1 Typical mechanical properties are 4600-lb/in2 (31.7-MPa) tensilestrength, 6000-lb/in2 (41.4-MPa) flexural strength, and 200,000-lb/in2(1.4-GPa) flexural modulus.1

Ste-Stereon stereospecific butadiene styrene block copolymer is used as

an impact modifier in PS, high-impact polystyrene (HIPS), polyolefinsheet and films, such as blown film grade linear low-density polyethyl-ene (LLDPE), to achieve downgauging and improve tear resistanceand heat sealing.1 Blown LLDPE film modified with 7.5% stereospe-cific styrene block copolymers has a Dart impact strength of 185°F per

50 g, compared with 135°F per 50 g for unmodified LLDPE film Thesecopolymers also improve environmental stress crack resistance(ESCR) (especially to fats and oils for meat/poultry packaging trays),increase melt flow rates, increase gloss, and meet U.S FDA 21 CFR177.1640 (PS and rubber modified PS) with at least 60% PS for foodcontact packaging.1 When used with thermoformable foam PS, flexibil-ity is improved without sacrificing stiffness, allowing deeper draws.1The stereospecific butadiene block copolymer TPEs are easily dis-persed and improve blendability of primary polymer with scrap for re-cycling

Vector SBS, SIS, and SB styrenic block copolymers are produced asdiblock-free and diblock copolymers.29 The company’s process to makelinear SBCs yields virtually no diblock residuals Residual styrene

6.40 Chapter 6, Part 2

butadiene and styrene isoprene require endblocks at both ends of the

polymer to have a load-bearing segment in the elastomeric network.29

However, diblocks are blended into the copolymer for certain

applica-tions.29 Vector SBCs are injection molded, extruded, and formulated

into pressure-sensitive adhesives for tapes and labels, hot-melt

prod-uct-assembly adhesives, construction adhesives, mastics, sealants,

and asphalt modifiers.29 The asphalts are used to make membranes

for single-ply roofing and waterproofing systems, binders for

pave-ment construction and repair, and sealants for joints and cracks.29

Vector SBCs are used as property enhancers (additives) to improve the

toughness and impact strength at ambient and low temperatures of

engineering thermoplastics, olefinic and styrenic thermoplastics, and

thermosetting resins.29 The copolymers meet applicable U.S FDA

food additive 21 CFR 177.1810 regulations and United States

Phar-macopoeia (USP) (Class VI medical devices) standards for health-care

applications.29

The company’s patented hydrogenation techniques are developed to

improve SBC heat resistance as well as ultraviolet resistance.29

EniChem Europrene SOL T products are styrene butadiene and

sty-rene isopsty-rene linear and radial block copolymers.1 They are

solution-polymerized using anionic type catalysts.33The molecules have

poly-styrene endblocks with central elastomeric polydiene (butadiene or

isoprene) blocks.33 The copolymers are (S-B)nX type where S =

poly-styrene, B = polybutadiene and polyisoprene, and X = a coupling

agent Both configurations have polystyrene (PS) endblocks, with

bound styrene content ranging from 25 to 70% (wt).1 Polystyrene

con-tributes styrene hardness, tensile strength, and modulus;

polybutadi-ene and polyisoprpolybutadi-ene contribute high resilience and flexibility, even at

low temperatures.1 Higher molecular weight (MW) contributes a little

to mechanical properties but decreases melt flow characteristics and

processibility

The polystyrene and polydiene blocks are mutually insoluble, and

this shows with two T gpeaks on a cartesian graph with tan δ (y axis)

versus temperature (x axis): one T g for the polydiene phase and a

sec-ond T g, for the polystyrene phase A synthetic rubber, such as SBR,

shows one T g.33 The two phases of a styrenic TPE are chemically

bound, forming a network with the PS domains dispersed in the

poly-diene phase This structure accounts for mechanical/elastic properties

and thermoplastic processing properties.33 At temperatures up to

about 167°F (80°C), which is below PS T g of 203 to 212°F (95 to

100°C), the PS phase is rigid.33 Consequently, the PS domains behave

as cross-linking sites in the polydiene phase, similar to sulfur links in

vulcanized rubber.33 The rigid PS phase also acts as a reinforcement,

as noted here.33 Crystal PS, HIPS, poly-alpha-methylstyrene, ethylene

vinyl acetate (EVA) copolymers, low-density polyethylene (LDPE), andhigh-density polyethylene (HDPE) can be used as organic reinforce-ments CaCO3, clay, silica, and silicates act as inorganic fillers, withlittle reinforcement, and they can adversely affect melt flow if used inexcessive amounts.33

The type of PS, as well as its percent content, affect properties.Crystal PS, which is the most commonly used, and HIPS increasehardness, stiffness, and tear resistance without reducing melt rheol-ogy.1 High-styrene copolymers, especially Europrene SOL S typesproduced by solution polymerization, significantly improve tensilestrength, hardness, and plasticity, and they enhance adhesive prop-erties.33 High styrene content does not decrease the translucency ofthe compounds.33 Poly-alpha-methylstyrene provides higher hard-ness and modulus, but abrasion resistance decreases.33 EVA im-proves resistance to weather, ozone, aging, and solvents, retainingmelt rheology and finished product elasticity The highest Shorehardness is 90 A, the highest melt flow is 16 g/10 min, and specificgravity is 0.92–0.96.1

Europrene compounds can be extended with plasticizers that arebasically a paraflinic oil containing specified amounts of naphthenicand aromatic fractions.33 Europrenes are produced in both oil-ex-tended and dry forms.1 Oils were specially developed for optimum me-chanical, aging, processing, and color properties.33 Increasing oilcontent significantly increases melt flow properties, but it reduces me-chanical properties Oil extenders must be incompatible with PS so as

to avoid PS swelling, which would decrease mechanical propertieseven more.33

The elastomers are compounded with antioxidants to prevent mal and photo-oxidation, which can be initiated through the unsatur-ated zones in the copolymers.33 Oxidation can take place during meltprocessing and during the life of the fabricated product.33 Phenolic, orphosphitic antioxidants, and dilauryldithiopropionate as a stabilizerduring melt processing, are recommended.33 Conventional UV stabi-lizers are used such as benzophenone and benzotriazine.33 Depending

ther-on the applicatither-on, the elastomer is compounded with flow enhancerssuch as low MW polyethylene (PE), microcrystalline waxes or zincstearate, pigments, and blowing agents.33

Europrene compounds, especially oil-extended grades, are used inshoe soles and other footwear.1 Principal applications are impact mod-ifiers in PS, HDPE, LDPE, polypropylene (PP), other thermoplasticresins and asphalt; extruded hose, tubing, O-rings, gaskets, mats,swimming equipment (eye masks, snorkels, fins, “rubberized” suits)and rafts; and pressure-sensitive adhesives (PSA) and hot melts.1 SIStypes are used in PSA and hot melts; SBS types are used in footwear.1

The copolymer is supplied in crumb form, and mixing is done by ventional industry practices, with an internal mixer or low-speedroom temperature premixing and compounding with either a single-

con-or twin-screw extruder.33 Low-speed premixing/extrusion ing is the process of choice

compound-Europrenes have thermoplastic polymer melt processing propertiesand characteristics of TPEs At melt processing temperatures, they be-

have as thermoplastics, and below the PS T g of 203 to 212°F (95 to

100°C) the copolymers act as cross-linked elastomers, as noted earlier.Injection-molding barrel temperature settings are from 284 to 374°F(140 to 190°C) Extrusion temperature at the head of the extruder ismaintained between 212 and 356°F (100 and 180°C)

6.10.2 Olefenics and TPO Elastomers

Thermoplastic polyolefin (TPO) elastomers are typically composed ofethylene propylene rubber (EPR) or ethylene propylene diene “M”(EPDM) as the elastomeric segment and polypropylene thermoplasticsegment.18 LDPE, HDPE, and LLDPE; copolymers ethylene vinyl ace-tate (EVA), ethylene ethylacrylate (EEA), ethylene, methyl-acrylate(EMA); and polybutene-1 can be used in TPOs.18 Hydrogenation ofpolyisoprene can yield ethylene propylene copolymers, and hydrogena-tion of 1,4- and 1,2-stereoisomers of S-B-S yields ethylene butylene co-polymers.1

TPO elastomers are the second most widely used TPEs on a tonnagebasis, accounting for about 25% of total world consumption at theclose of the twentieth century (according to what TPOs are included asthermoplastic elastomers)

EPR and polypropylene can be polymerized in a single reactor or intwo reactors With two reactors, one polymerizes propylene monomer

to polypropylene, and the second copolymerizes polypropylene withethylene propylene rubber (EPR) or EPDM Reactor grades are (co)po-lymerized in a single reactor Compounding can be done in the singlereactor

Montell’s in-reactor Catalloy®* (“catalytic alloy”) polymerizationprocess alloys propylene with comonomers, such as EPR and EPDM,yielding very soft, very hard, and rigid plastics, impact grades, or elas-tomeric TPOs, depending on the EPR or EPDM percent content The

term olefinic for thermoplastic olefinic elastomers is arguable because

of the generic definition of olefinic TPVs are composed of a continuous

* Catalloy is a registered trademark of Montell North America Inc., wholly owned by the Royal Dutch/Shell Group.