Rubber Compounding - Chemistry and Applications Part 6 pot

sequen-In a 40 Shore D copolyester COPE elastomer based upon polybutyleneterephthalate PBT hard blocks and polytetramethylene oxide/terephthal-ate PTMO-T soft blocks, the hard sequence l

Thermoplastic Elastomers:

Fundamentals and Applications

Tonson Abraham and Colleen McMahan

Advanced Elastomer Systems, L.P., Akron, Ohio, U.S.A

I INTRODUCTION

In the fifteenth century, Christopher Columbus witnessed South Americansplaying a game centered around a bounceable ‘‘solid’’ mass that wasproduced from the exudate of a tree they called ‘‘weeping wood’’ (1) Thismaterial was first scientifically described by C.-M de la Condamine andFrancßois Fressneau of France following an expedition to South America in

1736 (2) The English chemist Joseph Priestley gave the name ‘‘rubber’’ to thematerial obtained by processing the sap from Hevea brasiliensis, a tallhardwood tree (angiosperm) originating in Brazil, when he found that itcould be used to rub out pencil marks (2) A rubber is a ‘‘solid’’ material thatcan readily be deformed at room temperature and that upon release of thedeforming force will rapidly revert to its original dimensions

Rubber products were plagued by the tendency to soften in the summerand turn sticky when exposed to solvents This problem associated withnatural rubber was overcome by Charles Goodyear in the 1840s by subjectingthe rubber to a vulcanization (after Vulcanus, the Roman god of fire) process.Natural rubber was vulcanized by heating it with sulfur and ‘‘white lead’’(lead monoxide) (2) In May 1920 the German chemist Hermann Staudingerpublished a paper that demonstrated that natural rubber was composed of achain of isoprene units, that is, a polymer (from the Greek poly, many, andmer,part) of isoprene (3) In vulcanization the rubber macromolecules arechemically bonded to one another (‘‘cross-linked’’ in a thermosetting process)

to form a three-dimensional network composing a giant molecule of infinite

molecular weight At present the word ‘‘rubber’’ is associated with molecules that exhibit glass transition below room temperature and have

macro-‘‘long-chain,’’ ‘‘organic,’’ carbon-based backbones or ‘‘inorganic’’ bones typified by polysiloxanes and polyphosphazenes

back-‘‘Elastomer’’ is always used in reference to a cross-linked rubber that iselastic (Greek elastikos, beaten out, extensible) An elastomer is highlyextensible and reverts rapidly to its original shape after release of thedeforming force Entropic forces best describe rubber elasticity (4) However,

it should be noted that under relatively much smaller deformation, plasticmaterials and even metals can exhibit elasticity due to enthalpic factors (4).Gases and liquids also exhibit elastic properties due to reversible volumechanges as a result of pressure and/or heat (4) Nevertheless, the term

‘‘elastomer’’ is always used in reference to rubber elasticity

A plastic material is one that can be molded (Greek plastikos), and athermoplastic can be molded by the application of heat A rubber compound(a blend of rubber, process oil, filler, cross-linking chemicals, etc.) is thermo-plastic and is ‘‘set’’ after several minutes in a hot mold, with loss ofthermoplasticity A thermoplastic material can be molded in a matter ofseconds, and the molded part can be reprocessed The viscous character of thethermoplastic melt readily allows control of the appearance of the surface offinished goods In comparison, the effect of ‘‘melt elasticity’’ of a rubbercompound on end product surface appearance is not as readily controlled.The origin of the first thermoplastic material can be traced to ChristianSchonbein, a Swiss scientist who broke a beaker containing a mixture of nitricand sulfuric acid and used his wife’s cotton apron to clean up the spillage!Unfortunately for his wife, but fortunately for science, he left the washedapron near a fireplace to dry The cotton apron soon combusted withoutleaving any residue! Schonbein realized that the cotton of the apron wasconverted to ‘‘gun cotton,’’ a nitro derivative of the naturally occurring poly-mer cellulose (1) This learning may have been instrumental in the preparation

of the first plastic by the English chemist and inventor Alexander Parkes in

1862 First called Parkesine, it was later renamed Xylonite This substancewas nitrocellulose softened by vegetable oils and a little camphor During thistime, elephant tusks, which were used to make ivory billiard balls, amongother things, became scarce In 1869, motivated by the need to find a suitablesubstitute for ivory, John W Hyatt in the United States recognized the vitalplasticizing effect of camphor on nitrocellulose and developed a product thatcould be molded by heat He named this product obtained from cellulose

‘‘Celluloid’’ (Greek oid, resembling) Though primarily regarded as a tute for ivory and tortoiseshell, Celluloid, despite its flammability, foundsubstantial early use in carriage and automobile windshields and motionpicture film (3)

substi-A Definition of Thermoplastic Elastomer

A thermoplastic elastomer (TPE) is generally considered a bimicrophasicmaterial that exhibits rubber elasticity over a specified service temperaturerange but at elevated temperature can be processed as a thermoplastic(because of the thermoreversible physical cross-links present in the material)

It offers the processing advantages of a highly viscous melt behavior and ashort product cycle time in manufacturing due to rapid melt hardening oncooling

B Classification of Commercially Available Thermoplastic

Elastomers

The TPE products of commerce listed in Table 1 are classified inTable 2onthe basis of their polymer microstructure Representative examples areincluded for each polymer class Segmented block copolymers, triblockcopolymers, and thermoplastic vulcanizates represent a significant portion

of the TPE family

The fundamental aspects of structure–property relationships in moplastic polyurethanes (TPUs), styrenic block copolymers (SBCs) [withemphasis on styrene/ethylene-1-butene/styrene (SEBS) copolymers andSEBS compounds], and thermoplastic vulcanizates (TPVs) produced frompolypropylene and ethylene/propylene/diene monomer (EPDM) rubber wereselected for review in this chapter, as representative of the most commerciallysignificant and the closest in performance to thermoset elastomers

ther-Table 1 Thermoplastic Elastomer Products of Commerce

Product

First commercialized(year, company)Plasticized poly(vinyl chloride) 1935, B F Goodrich

Thermoplastic polyolefin elastomers 1972, UniroyalStyrenic block copolymers (hydrogenated) 1972, Shell

Thermoplastic vulcanizates (PP/EPDM) 1981, Monsanto

Chlorinated polyolefin/ethylene interpolymer rubber 1985, DuPont

Thermoplastic vulcanizates possess sufficient elastic recovery to lenge thermoset rubber in many applications, and insights into TPE elasticrecovery and processability are presented based upon the latest developments

chal-in the field The poor elastic recovery of TPEs at elevated temperature is a keydeficiency that has prevented these materials from completely replacing theirthermoset counterparts

Thermoplastic elastomers owe their existence as products of commerce

to the fabrication economics and environmental advantage they offer overthermoset rubber TPEs, of course, are designed to flow under the action ofheat; hence their upper service temperature is limited in comparison tothermoset rubber Thus a major hurdle to overcome in the replacement ofthermoset rubber with TPEs is the improvement in elastic recovery, partic-ularly at elevated temperature, especially compression set, because in manyapplications elastomers are subjected to compression The scope of thischapter includes those TPEs that in our opinion come reasonably close inproperties to thermoset elastomers, as listed in Table 1 Not included, forexample, are plastomers that are ethylene/a-olefin copolymers generallyproduced using metallocene catalysts (5).* These materials can be rubberlikeonly at room temperature They are thermoplastic owing to the thermorever-sible cross-links provided by crystallization of the ethylene sequences in thepolymer but are deficient in elastomeric character above room temperature orwhen under excessive strain Thermoplastic elastomers based on melt-blendedpolyolefins, ethylene/vinyl acetate copolymers, and ethylene/styrene co-polymers are also omitted from the list (6,7) Although thermoplastic olefins(TPOs) represent a commercially important class of materials, they areincluded primarily as comparative points to their more elastomerically per-forming counterparts, TPVs

Plasticized poly(vinyl chloride) (PVC) is used as a flexible plastic andnot an elastomer but is included in Table 1 because it was the first commer-

Table 2 Thermoplastic Elastomer Classification

Segmented block

copolymers

Triblockcopolymers

Thermoplasticvulcanizates

Polymerblends

*Note that Ziegler–Natta-based plastomers are also commercially available For example, some

of Dow’s Flexomer products are based on ethylene/1-butene copolymers.

cially produced thermoplastic elastomer PVC, produced by free radicalpolymerization, contains crystallizable syndiotactic segments, the crystalli-zation of which is enhanced on mobilization of the polymer chain in thepresence of a plasticizer (8) However, imperfections in the crystalline phaselimit the upper service temperature of PVC.

II THERMOPLASTIC ELASTOMERS: APPLICATIONS

OVERVIEW

Thermoplastic elastomers are found in thousands of applications, rangingfrom commodity TPOs used in automotive bumper and facia applications,through plastomers used as impact modifiers for plastics, and TPVs and SBCs

in sealing applications, to TPUs and copolyesters in numerous engineeringapplications TPEs replace EPDM rubber in many sealing applications, butylrubber where permeation resistance is required, and nitrile rubber for oil andfuel resistance

World demand for thermoplastic elastomers will grow at over 6% peryear through 2006, according to a recent study (9) The 1.6 million metric tonTPE industry will remain concentrated in the United States, Western Europe,and Japan, although underdeveloped markets such as Asia grow at a fasterrate

The most important driver for TPE growth through thermoset rubberreplacement is cost savings This is normally achieved through a combination

of material selection, part redesign, and fabrication economics Recyclabilityand weight reduction provide additional drivers in some markets Colora-bility is another important TPE attribute that increases design flexibility.Further, use of TPEs allows introduction of designs, processes, and value-added features not possible at any cost with thermoset rubber

Almost all commercial TPEs have one feature in common: they aremicrophase separated systems in which one phase is hard at room tempera-ture while another phase is soft and elastomeric The harder phase gives TPEtheir strength and, when softened, their processability The soft phase givesTPEs their elasticity Each phase has its own glass transition temperature, Tg,

or crystal melting point, Tm, and these in turn determine the temperatures atwhich the TPEs exhibit their transition properties Thus, the TPE servicetemperature on the lower end is bounded by the Tgof the elastomeric phase,whereas the upper service temperature depends on the Tmof the hard phase.Note that the practical service range also depends on the softening point,stress applied, and article design (10)

The ability of TPEs to repeatedly become fluid on heating and solidify

on cooling gives manufacturers the ability to produce rubberlike articles using

the fast processing equipment designed for the plastics industry Scrap canusually be reground and recycled Output of parts is generally increased andlabor requirements reduced compared to parts manufactured from thermosetrubber Thermoplastic elastomers can be fabricated by conventional thermo-plastic methods including injection molding, blow molding, and extrusion.Injection molding processes range from single- to multiple-cavity,including up to 48 or more cavities per mold, hot runner mold technologyfor runnerless part production, insert molding with other materials, andcoinjection molding of two materials sequentially or simultaneously Toolssuch as MoldflowR (11) allow fast development of tooling and processconditions for many TPEs Another significant advantage is that injectionmolding of TPEs allows dimensional tolerances not achievable in thermosetrubber This allows snap fits and ‘‘living hinges’’ to be designed into the parts.Flexible, nonblooming, flashless parts are easily produced on largely auto-mated molding equipment A compatible thermoplastic can give excellentbond strength with two-shot injection molding For noncompatible materials,

a physical lock or interference fit is used over a rigid substrate of metal, plastic,

or even glass (12)

Blow molding is practiced by injection blow molding, extrusion blowmolding, or press blow molding processes Complex designs can be easilymanufactured by three-dimensional sequential blow molding with multiplematerials Fabrication process equipment is available today that can blowmold three-dimensional parts from combinations of thermoplastic and ther-moplastic elastomer materials in up to seven layers by precise material de-livery, robotic parison manipulation, and perfectly timed mold positioning,all computer-controlled in a largely automated process (13)

Extrusion of thermoplastic elastomers includes single-extrusion, extrusion, and triple-extrusion processes Multiprofile dies for extrusionsfrom a single line provide important improvements in efficiency for simpleextrusions Hard–soft combinations with other polymers, including polyole-fins, polystyrene, and other TPEs, are commonly practiced Recent develop-ments include coextrusion of thermoset EPDM with TPVs (14,15) Specialextrusion processes have been developed to produce foamed profiles usingwater as the blowing agent (16,17) and create low-friction surfaces with acoextruded slipcoat, offering low-cost environmentally friendly alternativesfor specific applications Robotic extrusion of TPVs, through a systemcomposed of a moving die, flexible heated hose, and 3D robot, has been used

co-to apply seals directly co-to auco-tomotive parts (18,19) Secondary processes such

as heat welding, thermoforming, coating, printing, and painting add cant value at moderate cost in many applications

signifi-Thermoplastic elastomers can offer the design engineer greater designflexibility as well as part size and weight reduction In the case of thermoset

rubber replacement, the part is usually redesigned to leverage the physicalproperties and processing characteristics of the TPE The use of TPEsfrequently allows designers to reduce the amount of material per part and,combined with the lower specific gravity of TPEs in comparison with thermo-sets, significantly reduce the overall part weight compared with thermosetrubber (20) An important advantage in redesign is the opportunity for partsconsolidation through combinations of thermoplastic elastomer and otherthermoplastic components.

Thermoplastic elastomer grades have been developed that bond to awide range of engineering thermoplastics, including polypropylene, polyeth-ylene, polystyrene, polyamides, polyesters, acrylonitrile/butadiene/styrene(ABS) rubber modified plastic, cured EPDM rubber, polycarbonates, andcopolyesters The bond is typically formed through an autoadhesion (diffu-sion) mechanism during thermoplastic processing (21,22) In many cases,bond strengths at levels comparable to material strength can be achieved

A Thermoplastic Elastomers in Automotive Applications

The automotive industry has always been a major end-use market for TPEsand accounts for about 60% of the total demand in North America Tiresaccount for most of the thermoset elastomeric content in a vehicle The rest isspread over 600 or more elastomer applications from simple grommets tocomplex constant-velocity joint boots and radial lip seals Automotiveelastomeric parts serve in a wide range of operating environments They alsoprovide numerous functions such as air, vacuum, and fluid seals; mechanicalshock absorption; flexible couplings; and soft-touch interior components Aswith any elastomer, TPEs have their limitations They do not have thecombination of abrasion resistance, flexural strength, deformation resistance,and high-temperature use that thermoset elastomers display; therefore, thesematerials have found no significant use in pneumatic tires

Key automotive trends have provided a demand for increasing use ofTPEs The most important is the drive for cost reduction in every possiblecomponent of the vehicle Even though TPEs are more expensive as a rawmaterial than thermoset elastomers, the cost of the TPE finished part isusually significantly lower than that of a functionally comparable thermosetrubber part through redesign including lighter weight, shorter cycle time,lower energy usage, lower scrap, and recyclability

Another significant automotive trend is the increased level of ment regulations, which has forced the world’s automotive manufacturers toput major emphasis on improving safety and increasing fuel efficiency,recyclability, and the use of environmentally friendly materials As Germanyled the world in reduction of nitrosamine-containing cure package compo-

govern-nents for thermoset rubber, the European Union leads with respect tolegislation requiring higher recyclable content and lower overall vehicleemissions (23) Recyclability has provided a consistent driver in the Japanesemarket Thermoplastics and thermoplastic elastomers are key to reaching thetarget (24) Vehicle manufacturers have taken a lead as well, including targetsfor increase in recyclable content and elimination of PVC use in certain autointerior skin applications The relatively low price of PVC compounds,however, makes replacement by olefinic systems difficult from a cost view-point (25).

In addition, the automotive industry is trying to respond effectively to

an increased level of technical performance requirements Higher mance engines, operating at higher temperatures with lowered emissions,coupled with improved aerodynamics due to decreased frontal and grille area,contribute to increasing under-the-hood temperatures Longer lived automo-biles also require elastomers with improved ultraviolet resistance Soft-touch,color-matched interior parts, featuring low odor and low fogging, add toesthetics and consumer-recognized value

perfor-Engine compartment timing belt covers with a flexible segment ofrubber and a rigid segment of polypropylene have successfully employedTPE Fuel line covers from specially formulated flame-retardant grades, rack-and-pinion boots taking advantage of the outstanding flex fatigue resistance

of TPVs, and clean air ducts featuring innovative convolute designs incombination with polypropylene are just a few examples of automotiveapplications that leverage the unique properties of TPVs Thermoplasticelastomers, especially thermoplastic vulcanizates, are moving quickly intoautomotive weatherseal applications; this market provides significant growthpotential for TPEs in the future TPEs are injection molded for glassencapsulation and cutline seals They are extruded for belt line and glassrun channel seals Extruded seals can be coated with specially formulated lowfriction TPEs and joined at the corners with specialty molding TPVs toreplace flocked thermoset EPDM seals with 100% recyclable parts

B Thermoplastic Elastomers in Industrial Applications

Thermoplastic vulcanizates are found in hundreds of industrial applications

In most cases the drivers for TPE use are the same as in other industries, i.e.,thermoset elastomer performance with the advantages of thermoplasticeconomics The building and construction industry takes advantage of TPEperformance to provide critical sealing in places such as architectural glazingseals, bridge deck seals, pipe seals, and roofing Industrial hose applicationsform a growing segment of TPV applications, including fire hose, washdownhoses, and specialty grades for handling potable water and food Excellent

TPV resistance to detergents, acids, and bases, combined with superior flexlife and weatherability compared to thermoset rubber, drive application inthousands of small sealing parts such as gaskets and bushings in appliancesand mechanical devices worldwide Specialty TPEs featuring low flameretardancy, good abrasion resistance, dielectric strength, and wet electricalperformance are used in electrical applications, especially wire and cablecoverings, insulators, and flexible connectors (26) Conductive thermoplasticelastomers incorporating carbon or metal powders are used for staticdissipative and conductive properties or in electromagnetic interference/radiofrequency interference (EMI/RFI) shielding (27).

Multilayer coated sheets are used in roofing, and their use is expanding

to innovative applications such as pillow tank liners

C Thermoplastic Elastomers in Consumer Applications

Thermoplastic vulcanizates are found in a variety of consumer products, mostrecognizably those incorporating grips for soft but secure handling of powertools, housewares, and toothbrushes Good sealing properties and goodchemical resistance make them well suited for kitchen appliances (28).Because many TPEs have consistent frictional characteristics over a range

of temperatures and in wet and dry conditions, they are well suited for use inthis growing market The ability to adhere to a variety of substrates by two-shot or overmolding allows processing ease with excellent adhesion Trans-parent and translucent products are readily available

Many ballpoint pens now feature a soft grip made from a TPE.Cosmetic containers, food containers, and water bottles incorporate TPEsfor soft-grip feel, color, and design innovation The demand for thermoplasticrubber soft grips is also growing in sports applications, such as tennis racket

or golf club grips Other sports and leisure applications include toys, skiequipment, and sports balls (e.g., soccer ball inner bladder) made from butylrubber–based TPVs Consumer products emphasize good esthetic design aswell as functionality, and the ability of TPEs to be decorated is a realadvantage Techniques such as permanent laser marking and the application

of hot stamping foils, heat transfer labels, or screen or tampo printing havebeen used for marking various products, including multicolored flexiblelabels Logos can be integrally designed into products by using overmolding

of hard–soft combinations Effects linked to other materials such as mineralscan be obtained through the use of innovative pigments; marble and graniteare the most commonly imitated materials (29) Newer application areas forTPEs in consumer products include personal electronics and a growing range

of household and garden tools

III SEGMENTED BLOCK COPOLYMER TPEs

The segmented block copolymer TPEs included inTable 1– contain ces of ‘‘hard’’ and ‘‘soft’’ blocks within the same polymer chain Solubilitydifferences between the polymer segments and association and/or crystalliza-tion of the hard blocks produce phase separation in the molten elastomer as itcools The hard blocks form the thermoreversible cross-links and reinforce-ment (increasing stiffness) of the elastomeric soft phase The rate of crystal-lization or association of the hard blocks will impact product fabrication time.Polymer microstructure and morphology is depicted inFigure 1 These TPEsare produced by condensation or addition step growth polymerization andhave low molecular weight segments Although this is desirable, segment mo-lecular weight and molecular weight distribution cannot be readily controlled

sequen-In a 40 Shore D copolyester (COPE) elastomer based upon poly(butyleneterephthalate) (PBT) hard blocks and poly(tetramethylene oxide/terephthal-ate) (PTMO-T) soft blocks, the hard sequence length varies from 1 to 10 (30).PBTmolecular weight of sequence length 10 is 2200, whereas high molecularweight PBTthat is commercially available could easily have an Mnof 50,000!Thus, a sufficient number of hard blocks have to associate to produce a highenough melting crystal phase to provide a reasonably high elastomer upperservice temperature This necessitates increasing the hard-phase content of theTPE, which results in a hard elastomer (‘‘filler’’ effect) Note that for a givenhard-phase content, the lower the number of hard domains (more hardsegments per domain), the greater the entropic penalty imposed on theelastomeric phase and the less favored the phase-separated morphology.Increased hard phase content also causes more hard segments to berejected into the amorphous elastomeric phase, thus raising the rubber glasstransition temperature (Tg) and therefore also the TPE lower service temper-ature In the case of an increase in the number of hard domains, the soft-phase

Tg is also elevated owing to the increased ‘‘cross-link density.’’ Theseconsiderations allow the commercial viability of only hard COPEs This is

a major deficiency in this class of TPEs as the softest product available has ahardness of 35 Shore D Also based on the above discussion, the more or lesscontinuous hard phase in commercially available COPEs where fibrillarcrystalline lamellae (due to short hard segments) are connected at the growthfaces by short tie molecules can readily be rationalized The amorphous phase

is also continuous (31)

It is difficult to produce useful soft elastomeric products from segmentedblock copolymers except in the case of thermoplastic polyurethanes (TPUs).The strong association of hard blocks even at low hard block content allowsthe preparation of soft elastomeric TPUs TPUs with hardness as low as 70Shore A are available commercially

Noveon: EstaneR58137

EMS-Chemie:

GrilonRELX2112

DuPont: HytrelR

5 (100jC ASTM D 395A,constant load)

sulfur-curedthermoset

IV THERMOPLASTIC POLYURETHANES

Thermoplastic polyurethane (TPU) was the first thermoplastic product thatcould truly be considered an elastomer (32) The bulk of commercially avail-able TPUs are produced from hard segments based on 4,4V-diphenylmethanediisocyanate (MDI) and 1,4-butanediol (BDO, a ‘‘chain extender’’), witheither poly(tetramethylene oxide) (PTMO) glycol, or poly(1,4-tetramethyleneadipate) (PTMA) glycol or poly (q-caprolactone) (PCL) glycol as the soft

Figure 1 Polymer microstructure and morphology of segmented block copolymers(TPU, COPE, COPA) A, crystalline domain; B, junction area of crystalline lamella;

C, polymer hard segment that has not crystallized; D, polymer soft segment

a two-stage process In the former, the diisocyanate, chain-extender diol, andsoft segment diol are mixed and heated to yield the final product, whereas inthe latter the soft-segment diol is first ‘‘end-capped’’ by using an excess ofdiisocyanate and the chain-extending short-chain diol is subsequently added

to form the hard segments and to attach them to the soft segments in analternating manner to yield a TPU of high molecular weight by addition step-growth polymerization A representation of a TPU molecule is presented inFigure 2 A TPU’s Mw can be as high as about 200,000, with Mn about100,000, although the individual hard and soft segments are of much lowermolecular weight For example, poly(tetramethylene oxide) glycol of Mn1000

or 2000 is used commercially for TPU production, thereby fixing the softblock length The longer the soft segment, the lower its hydroxyl end groupconcentration, which would allow preferential step growth of the hardsegments by reaction of the short-chain diol with the diisocyanate Hence,the longer the soft segment, the longer the hard segment Because the number

of soft segments will equal the number of hard segments, for a large number ofalternating segments,

Weight % SS

Mnss

Mnhsor

weight % SSFigure 2 Polyether-based TPU

where soft segments and hard segments are abbreviated SS and HS, tively For given soft segment molecular weight, the number-average molec-ular weight of the hard segment is directly proportional to the hard segmentcontent and inversely proportional to the soft segment content (33) Mwsscan

respec-be obtained by measurements on the polyol, but obtaining the hard-segmentweight-average molecular weight is difficult Bonart developed a theoreticalmethod to calculate Mwhs (34) The average number of hard segments for aTPU (MDI/BDO hard segments; polyoxypropylene end-capped with poly-oxyethylene soft segments) with a 50 wt% hard phase has been calculated to

be six (35) Peebles mathematically modeled the soft and hard segment lengthdistribution in TPUs (36,37)

The infrared studies of Cooper demonstrated that the urethane NUH ishydrogen-bonded to the oxygen atoms of the urethane moiety as well as to theoxygen atoms of the polyether or polyester soft segments (38) This hydrogenbonding and soft segment polarity can retard and lower the ultimate degree ofphase separation in TPUs Poor phase separation is reflected in the increase in

Tgof the mostly amorphous soft phase due to the presence of dissolved hardsegments The hard microphase is formed by association of the relativelyshort hard segments and by their crystallization into fibrillar microcrystals.The poorer phase separation in polyester TPUs compared with polyetherTPUs is presumably due to the greater polarity of and stronger hydrogenbonding (with the NUH of the hard segments) in the soft phase of the formercompared with the latter (39) A 1:2:1 (molar polyester:MDI:BDO) TPU(polyester polyol Mn= 1000) exhibited a single phase, but the correspondingpolyether-based TPU system was phase-separated (40) The degree of phasemixing is also dependent upon soft segment content For a polyether-basedTPU, complete phase mixing was observed at 80 wt% soft segment content(41,42) Phase mixing is also dependent upon segment molecular weight, asdemonstrated in the case of TPUs containing low molecular weight poly-caprolactone soft segments (43)

Phase separation in TPUs is driven by the solubility parameter ence between the polymer segments and by association and/or crystallization

differ-of the hard segments and is limited by the geometry differ-of the molecule and thehydrogen bonding and polarity effects discussed In addition, the kinetics ofTPU phase separation will also be influenced by the mobility (Tg) of thepolymer segments

A TPU Morphology and Microstructure

The mechanical behavior (Young’s modulus, elastic recovery, elongation,flexural modulus, heat sag, thermomechanical penetration probe behavior) ofTPUs suggests a transition from discrete to continuous hard microdomain

morphology at hard segment content above about 45 wt% (33,41–46) Thesmall-angle X-ray studies of Abouzahr and Wilkes (42) and Cooper andcoworkers (43) and the small-angle X-ray and neutron scattering analysis ofLeung and Koberstein (41) suggested an interlocking hard domain morphol-ogy at high hard segment content Depending upon processing conditions andhard phase type and content, crystalline TPU systems may exhibit a fringedmicellar texture of thickness equal to the hard segment length or clear-cutconnectivity of the crystalline hard phase The hard domain diameter in aTPU produced from a 1:6:5 polycaprolactone (Mn= 2000)/MDI/BDO moleratio was estimated to be 400 A˚ by transmission electron microscopy (TEM)(46) (hence ‘‘hard microdomain’’), although for the typical TPU materialsmentioned in this review this number is expected to be about 100 A˚.

Using small-angle X-ray scattering (SAXS), Leung and Koberstein (41)studied the hard segment microdomain thickness (which corresponds to thelength of the hard segments) in TPUs in which the hard segment contentvaried from 30 to 80 wt% The SAXS measurement provided an overallcharacterization of the microdomain morphology averaged over crystallineand noncrystalline structures The hard microdomain thickness varied from

2 nm (corresponding to a hard segment length containing two MDI residues)

to 5.4 nm (hard segment length with four MDI residues) for the 60 wt% hardsegment content TPU, after which the thickness did not increase further withincreased TPU hard segment content Because the hard segment lengthincreases with increased TPU hard segment content, chain folding via theflexible BDO segments to accommodate longer hard sequences within thecrystal is thought to occur Other possible explanations for this phenomenonhave been discounted The extended chain crystal structure, irrespective ofTPU hard segment length, that has been demonstrated to occur by wide-angleX-ray diffraction (WAXD) may well be characteristic of the TPU samplesstudied that were treated (annealed, etc.) to maximize crystallinity so as to beamenable to analysis by the WAXD method (41)

Spherulitic structure for high hard segment content (>40 wt%) TPUshave been observed in samples crystallized in the laboratory (33,46,47) In onecase, because of the large spherulite diameter (several micrometers) and theabsence of a hard phase Tg, the spherulites may have contained occluded softphase (33) Hard phase Tgis rarely discernible even in high hard phase contentTPUs A hard phase Tgwas observed in a melt-quenched TPU with 80 wt%hard segment content (33) Owing to the tendency of the relatively short TPUhard segments to associate or crystallize or to be miscible in the TPU softphase, amorphous hard segments may exist only as tie molecules connectingmicrofibrillar crystalline segments Low TPU amorphous hard phase contentwould preclude Tgdetection Moreover, hard phase Tgobservation would beobscured by other transitions (discussed later) Spherulitic soft segment

structure in a high PTMO soft segment content TPU has been observed (33).Generally, TPU parts that are fabricated by commercial processing equip-ment exhibit crystallinity but no spherulitic structure (48).

B Thermal Characteristics of TPUs

Although the structure of TPUs changes constantly during differentialscanning calorimetry (DSC), DSC coupled with SAXS has proven to be apowerful tool in uncovering TPU microstructure and thermal behavior, as inthe masterful research work of Koberstein and coworkers (35,41,49,50), whostudied polyether TPUs with MDI/BDO hard segments Molten TPUs from ahomogeneous melt state were rapidly quenched to and held at variousannealing temperatures for specific time periods Generally, three distinctendotherms were observed by DSC of the annealed samples The firstendotherm (TI) is dependent upon the annealing temperature, annealingtime, and TPU hard segment content This endotherm is observed at 20–40jC above the annealing temperature, which was varied from 30jC to 170jC,depending upon TPU hard segment content Higher hard segment contentTPUs gave higher TIvalues The exact origin of TIis still unknown, but it islinked to a short-range order dissociation endotherm in the hard microphaseand not in the interphase, because this transition is also observed in pure hardsegment materials as suggested by Cooper and coworkers (51,52) For a softTPU with a discrete hard phase and a total hard phase content of 30 wt%, the

Tgof the soft phase kept increasing with increased annealing temperature up

to 170jC Annealing above 170jC did not change the soft phase Tg, indicatingthat the microdomain structure is completely disordered above this temper-ature (35) The Tg increase of the soft phase was related to increasedsolubilization of hard segments into the soft phase Increasing annealingtemperature caused the solubilization of hard segments of high molecularweight into the soft phase that already contained lower molecular weight hardsegments It has also been suggested that ‘‘cross-linking’’ by soft segment–hard segment hydrogen bonding is another factor that contributes to in-creased soft phase Tg in addition to the physical presence of TPU hardsegments in the soft phase (53) By studying the change in TPU heat capacity

at its glass transition temperature, it was concluded that below an annealingtemperature of 80jC hard segment solubilization into the soft phase occursand above 80jC, which is near the hard segment Tg, soft segments that aretrapped in the hard microphase also enter the bulk soft phase in addition tofurther hard segment dissolution into the soft phase

The TIIendotherm is also dependent upon annealing temperature, andfor the soft TPU under discussion the TIImaximum is 175jC This transitionwas identified by Koberstein as the microphase separation transition (MST),

where the partially ordered ‘‘noncrystalline’’ segments in the hard domain are mixed into the soft TPU phase The TPU with 30 wt% hardsegment content did not exhibit a microcrystalline melting TIIIendotherm,which is observed for higher hard segment content TPUs The identification

micro-of TII as the MSTwas further confirmed by simultaneous DSC/SAXSmeasurements in a TPU with 50 wt% hard segment content (49) The TPUinterdomain spacing increased dramatically beginning at TII T his T PUexhibited a higher TIIIendotherm corresponding to the melting of a micro-crystalline hard phase within the ‘‘noncrystalline’’ ordered hard domain.For the TPU with 50 wt% hard segment content, the TI endothermmerged with the TII endotherm when annealing took place at 155jC.Annealing above 155jC raised the TIIendotherm and decreased its intensitywhereas the intensity of the TIII microcrystalline peak melting endothermincreased TIIIwas the only DSC peak endotherm observed at 210jC whenannealing was conducted at 175jC At annealing temperatures of 175–190jC,the TIII endotherm diminished in magnitude and the TII endotherm reap-peared These findings are consistent with an expected decrease in crystallinity

at low undercoolings where crystallization is controlled by nucleation Abovethe MST, TPU crystallization occurs from a homogeneous mixed melt phase(‘‘solution’’ crystallization) Crystallization occurs within the hard micro-domains (‘‘bulk’’ crystallization) below the MST For harder TPUs (70 wt%hard segment content), melting endotherms corresponding to different crystalstructures have been observed, depending upon annealing conditions.The thermogravimetric analysis (TGA) trace of the TPUs of the Hu andKoberstein study (50) demonstrates initial weight loss around 300jC, which iswell above the annealing temperatures used to probe the TPU microstructure

A small change in annealing temperature (from 190jC to 195jC) exhibited adramatic increase in TPU Mnand Mwvalues [gel permeation chromatogra-phy (GPC) measurements] The increased MW is presumably the result of

‘‘trans urethanation’’ reactions that result from cleavage of the urethane bond

in a polymer segment back to the isocyanate and alcohol, and subsequentallophanate formation by addition of the newly formed isocyanate to theurethane NUH bond of another polymer chain, thus creating a branchedstructure Crystallization of the branched TPU molecules appears to be hind-ered in comparison with their linear counterparts Reduction in the heat offusion is observed for TPU samples where molecular weight was increased byannealing at high temperature, due to ‘‘trans-urethanation’’ reactions Itshould also be reiterated here that the sequence length of the hard segmentsthat are incorporated into the soft phase increases with increased annealingtemperature For more on trans-urethanation reactions and TPU thermaldegradation mechanisms, the reader is referred to the work of Macosko andcoworkers (54)

According to Koberstein, all three TPU endotherms TI, TII, and TIIIareaccompanied by the mixing of hard and soft microphases The Kobersteinschematic model for the morphological changes that occur during the DSCscans of TPUs is presented inFigure 3 It should be noted that Koberstein’swork is grounded on the pioneering TPU research work of Wilkes andCooper and coworkers, who had previously recognized the time- andtemperature-dependent morphological and mechanical properties of TPUs(51,55–61) The increased mutual solubility of TPU hard and soft phases withincreasing temperature was recognized, as was the influence of hydrogenbonding and soft phase Tgon phase mixing and demixing over a broad temp-erature range Both phase mixing and demixing have been observed on TPUmechanical deformation, depending upon sample thermal history, includingchanges in phase continuity (59) TPU morphology is complex, and a smallchange in the polymer segment type can result in diverse melting behavior.For example, TPUs produced from MDI/BDO hard segments and poly(hexa-methylene oxide) soft segments exhibited five melting endotherms that wereattributed to hard segment sequences containing one to five MDI-derivedunits (62) There is continued interest in elucidating the origin of multiplemelting endotherms in TPUs (63).

It is now readily understood how TPU morphology is dependent uponprocessing conditions and what thermally induced phase transitions canoccur that would be detrimental to product elastic recovery at elevatedtemperature

Based upon the information presented so far, it would appear that TPUsthat are designed for improved phase separation (decreased hard and softphase compatibility) should provide improved elastic recovery However,TPU mechanical properties are adversely affected when the desired micro-structure is difficult to achieve due to incompatibility of the TPU buildingblocks under the polymerization conditions, including incompatibility of thereactants with the polymer produced This is the case for TPUs (for improvedhydrolysis resistance) produced with polybutadiene diol or hydrogenatedpolybutadiene diol (for improved heat and hydrolysis resistance) soft seg-ments and MDI-based hard segments (64–68) Molecular heterogeneity inchemical composition and average hard segment length is expected to be thekey factor contributing to the poor mechanical properties of these hydrocar-bon soft segment TPUs compared with conventional TPUs, based on, forexample, MDI/BDO/PTMO (69–71) Hydrocarbon diols are being promotedfor nonelastomeric polyurethane applications, as in the preparation ofcastable polyurethanes for moisture-resistant adhesives, coatings, and elec-trical potting compounds (72)

Thermoplastic polyurethanes produced with 2,6-toluenediisocyanate(2,6-TDI) hard segments with BDO as chain extender and PTMO as the soft

Figure 3 Schematic model for the morphological changes that occur during DCscans of polyurethane elastomer (a) below the microphase mixing transition temp-erature, (b) between the microphase mixing temperature and the melting temperature,and (c) above the melting temperature The microcrystalline hard-segment domainsare indicated (From Ref 49.)

phase undergo cleaner phase separation than the corresponding 2,4-TDIbased TPUs (53) The use of 2,6-TDI as the hard phase isocyanate mayprovide TPUs with excellent elastic recovery, but difficulty in 2,6-TDI/2,4-TDI isomer separation makes this approach commercially unfeasible More-over, the volatility of TDI over MDI makes the latter isocyanate preferablebecause of toxicity considerations However, TDI, the first isocyanatedeveloped for the thermoset polyurethane industry, is still used in NorthAmerica in the manufacture of thermoset polyurethane foam (73–75) TPUsproduced with aromatic diol chain extenders such as hydroquinone bis(2-hydroxyethyl) ether in, for example, the conventional MDI/PTMO systemare emerging as elastomers with improved elastic recovery (76).

Aliphatic and aromatic diamines can be used as chain extenders to formTPU ureas with high melting point hard segments, but these materials meltwith some decomposition and well above the processing temperature of TPUs(32) and hence are not commercially feasible as thermoplastic elastomers withimproved elastic recovery

However, owing to improved elastic recovery after high strain and ahigher use temperature due to the urea hard segments, solution-processedaromatic polyurethaneureas are preferable to conventional melt-processedaromatic polyurethanes in fiber applications (clothing, upholstery, andcarpet) Spandex is the generic trade name given by the Federal TradeCommission to synthetic elastomeric fibers that contain at least 85% seg-mented polyurethane In comparison with natural rubber threads, Spandexfibers are readily dyeable, lightweight materials with excellent abrasionresistance, tensile strength, and tear strength They have better resistance tooxidation, sunlight, and dry cleaning fluids than natural rubber threads andare also tolerant to bleach containing a low chlorine level Although curednatural rubber fibers have the advantage of low hysteresis and stretchcrystallinity, they are being replaced by Spandex, which can also be curedduring the fiber-forming process (77)

C Aliphatic TPUs

Aliphatic TPUs are used in light-stable (nonyellowing) applications and canhave mechanical properties comparable to those of aromatic TPUs (78).These materials are synthesized from hydrogenated MDI diisocyanate/BDO

or hexamethylenediamine diisocyanate/BDO hard segments and polyestersoft segments (Polyether soft phase would reduce TPU UV resistance.)Conventional MDI-based aromatic TPUs yellow on exposure to UV lightowing to the formation of quinone imides The quinone imides are UVabsorbers that dissipate UV energy as heat and hence retard further TPUdegradation On UV exposure, the aliphatic TPUs undergo a greater reduc-

tion in mechanical properties than their aromatic counterparts but withoutcolor change or loss of transparency Hence, UV-stabilized aliphatic TPUsare used in outdoor applications where the abrasion resistance of TPUs isnecessary For example, some outdoor signs enclosed in transparent acrylicare laminated with aliphatic TPUs Aircraft canopies are fabricated withhigh-impact-resistant layered structures produced from polycarbonate and a

‘‘flexibilizing’’ aliphatic TPU ‘‘glue.’’

As illustrated by the data inTable 3, the compression set of TPUs ismuch poorer than that of thermoset rubber Under compression at elevatedtemperature, irreversible deformation in TPUs occurs by continued phaseseparation and/or reorganization of the hard and soft segments over thatestablished after part manufacture

Hydrogen bonding in the hard phase and in the interphase (the regionwhere the polymer composition changes from 100% hard segment to 100%soft segment) between the hard and soft domains provides a ready mechanismfor chain slip because hydrogen bonds can reorganize readily by the partialformation of ‘‘new’’ hydrogen bonds as the ‘‘old’’ hydrogen bonds arepartially broken Increasing the amount of the hard phase (to provide moresecure thermoreversible cross-links at the TPE upper service temperature)increases compression set because the now higher modulus material issubjected to much higher stress under compression compared to thecorresponding softer material (under constant deflection) Increased hardphase volume fraction in TPUs also restricts polymer motion in the soft phase(increased elastomer cross-link density), and there is an increased presence ofhard segments in the soft phase These factors cause an increase in the softphase Tg that raises the product’s lower use temperature The hard TPUproduct, of course, would have an advantage in constant load applications.Thermoplastic polyurethanes may also contain thermoreversible allo-phanate branch points resulting from the reaction of the urethane NUH bondwith excess diisocyanate It is not feasible to design allophanate bonds into

a TPU, but these fortuitously present cross-links may contribute to improvedTPU elastic recovery Nevertheless, elastic recovery in the various types ofTPUs does not approach that of thermoset rubber In some cases the elasticrecovery of a soft product can be worse than that of a harder product because

of product design For example, it may be necessary to produce a soft TPUwith a low rate of crystallization to achieve desirable processing character-istics in film applications This may be accomplished by the use of a lowmolecular weight soft segment in which the TPU crystallization rate islowered owing to increased phase mixing Continued phase separation inthe finished product is one factor that would raise set

Amorphous materials exhibit a gradual decrease in viscosity withincreasing temperature beyond Tg, compared with crystalline materials, in

which viscosity drops sharply on melting due to the Tmbeing much greaterthan the Tg In crystalline hard phase TPUs the viscosity drop on crystal phasemelting may not be as precipitous as expected because of association amongthe hard phase molecules that are still present just after melting because ofincompatibility with the soft phase Even so, this viscosity drop in a crystallinehard phase TPU may cause it to lack desirable processing characteristics, andTPUs with a high amorphous hard segment content may be designed for animproved processing window and for transparency The excellent impactproperties, processability, and transparency of Dow’s IsoplastTMare credited

to the amorphous hard segment that makes up most of this TPU engineeringplastic In the finished product, elastic recovery is controlled by both rawmaterial properties and part design

V ELASTOMERIC COPOLYESTERS AND COPOLYAMIDES

Elastomeric copolyesters (COPEs) (31) and elastomeric copolyamides(COPAs) (79) are similar in structure to TPUs and suffer similar drawbacks

in rubber performance The hydrogen bonding present in TPUs and COPAs isabsent from COPEs Commercially available COPEs are based upon crys-talline polybutyleneterephthalate (PBT) hard segments and poly(tetramethy-lene oxide) (PTMO) soft segments PBT monofilaments exhibit only a 1%permanent set after 11% extension at room temperature, owing to a reversiblea- to h-crystal transition (80,81) This reversible crystal transition, whichwould be beneficial in the elastic recovery of COPEs, has been observed inPBT/PTMO COPEs with a high enough level of the PBT hard phase that theamorphous phase is hard enough (due to the presence of PBThard segments

in the amorphous phase) to bear the level of tensile stress necessary to causethe reversible deformation behavior in the hard phase (82) Although it isgenerally thought that segmented block copolymers have a homogeneousamorphous phase consisting of hard and soft blocks, experimental evidenceindicates that a biphasic amorphous phase consisting of a PTMO phase and amixed PBT/PTMO phase can exist in certain COPEs (83,84) The lack ofhydrogen bonding in COPEs and the reversible crystal transformationpossible in the PBThard phase are responsible for the modest improvement

in elastic recovery of these materials over TPUs and COPAs However, atelevated temperature, the motion of the soft segments cannot be adequatelyrestrained by the crystalline polymer chains, thus causing reorganization inthe hard phase that leads to irreversible deformation COPEs cannot matchthe elastic recovery of thermoset rubber (Table 3)

In addition to the disadvantage of poor elastic recovery at elevatedtemperature that is characteristic of most TPEs, the segmented block copoly-

mers suffer the additional disadvantage of the lack of commercially availablesoft products due to inadequate physical properties as already discussed Inthe case of TPUs, the association of the hard segments is strong enough toconfer excellent physical properties to soft products (70 Shore A), butdifficulty in pelletization of the soft product during manufacture and pelletagglomeration on storage have to be overcome.

Addition of plasticizer to hard segment block copolymers is not a viableoption for the production of soft products, because the plasticizer wouldlower the melting point of the polar hard phase in addition to softening thepolar elastomeric phase, which, in any case, cannot hold a high level of addedplasticizer Moreover, continued phase separation after processing can causethe exudation of plasticizer from the molded product Commercially availablesegmented block copolymer TPEs are plasticizer-free

Elastic recovery is an important property for elastomer performance.Because of the price and performance requirements in diverse applications,the hydrocarbon oil-resistant segmented block copolymers discussed aresuccessful products of commerce

The most important end use of the polyurethane-elastomer, elastomer, and polyester-elastomer block copolymers has been in thermosetrubber replacement Their crystalline hard segments make them insoluble inmost liquids Products feature exceptional toughness and resilience, creep andflex fatigue resistance, impact resistance, and low-temperature flexibility Allthree types are generally used uncompounded, and the final parts can bemetallized or painted Thus, they are often used as replacements for oil-resistant rubbers such as neoprene because they have better tensile and tearstrength at temperatures up to about 100jC Automotive applications includeflexible couplings, seal rings, gears, timing and drive belts, tire chains, andbrake hose Special elastomeric paints have been developed that match theappearance of automotive sheet metal; such parts have been used in car bodies(31,32,79) Flexible membranes, tubing, hose, and wire and cable jackets areincluded in the long list of applications

polyamide-VI STYRENIC BLOCK COPOLYMERS

The advent of hydrogenated styrene/butadiene/styrene (SBS), i.e., styrene/ethylene-1-butene/styrene (SEBS), triblock copolymer compounds repre-sented an advance in the elastic performance of thermoplastic elastomers atelevated temperature SEBS is almost always compounded; one can achieveprocessable soft compositions (0–30 Shore A) that are not possible in the case

of segmented block copolymers The key features of SEBS will be describedbefore we discuss SEBS compounds Phase separation in these triblock

copolymers is more complete and occurs more readily than in the segmentedblock copolymers This is reflected in the Tgof the rubber phase, which isnearly unaffected by the polymer styrene content The Tgof the styrene phasedepends upon its molecular weight More phase mixing with the rubber can beexpected with decreasing styrene molecular weight when the material isheated to the Tg of styrene (85) Both the polystyrene end block contentand polystyrene molecular weight in SEBS is designed to be lower than that ofthe rubber midblock For example, Kraton G1651(SEBS) of Kraton Poly-mers has a plastic block of molecular weight 29,000 (33 wt%) and a rubberblock of molecular weight 116,000 (68 wt%) (86) The rubber block isdesigned to have a 40 wt% butene content to limit crystallinity due to thepolyethylene segments (low crystallinity would increase the rubber’s oil-holding capacity) and lower Tg (low Tg for improved low-temperatureperformance) (87) Simplistically, SEBS has a ‘‘spaghetti and meatball’’morphology, in which the styrenic microdomains (200–300 A˚) are dispersed

in a continuous rubber matrix (88) The polystyrene microdomain size reflectsthe entropic penalty that would be imposed on the rubber in the case of largerplastic domains The higher molecular weight and narrower molecular weightdistribution of SEBS than those of the segmented block copolymers arefactors that favor improved phase separation in the former system in spite ofthe smaller solubility parameter difference between the phases in SEBS versusthe segmented block copolymers (89,90) Molecular architecture also favorsbetter phase separation in SEBS than in the segmented block copolymers Thepolystyrene phase will flow above its Tg(f95jC), and these microdomainsform the thermoreversible cross-links in the SEBS thermoplastic elastomer.The styrenic cross-links, however, do not contribute much to the ‘‘cross-link’’density of the rubber phase that is dominated by the trapped entanglementswithin it (91) This can readily be inferred by a comparison of the modulus(initial slope of the stress–strain curve and also the plateau modulus) of SEBSwith other styrenic block copolymers such as styrene/butadiene/styrene (SBS)and styrene/isoprene/styrene (SIS) The modulus in these systems is directlyrelated to the molecular weight between entanglements in the rubber phase(88) The modulus of SEBS (lowest molecular weight between entanglementsand highest entanglement density) is greater than that of SBS, which in turnhas a higher modulus than SIS (highest molecular weight between entangle-ments and lowest entanglement density)

Thus the function of the styrenic domains is to prevent disentanglement

of the rubber segments when these styrenic block copolymers (SBCs) aresubjected to load For example, Kraton G1651 has a 33.3 wt% PS content and

a rubber molecular weight of 116,000 (MngMw) Neglecting the interphase,the total PS phase volume in 100 g of SEBS would be 31.71 cm3(PS density =1.05 g/cm3) Assuming spherical 200 A˚ diameter PS domains, the volume per

domain is 4.19  10
18cm3, which translates to 7.57  1018domains in theSEBS sample The number of PEB macromolecules is 35.29  1019 (68/116,000 = 5.89  10
4gmol = 5.86  10
4 6.023  1023

macromolecules).Assuming a molecular weight between entanglements for PEB of 1800, thenumber of entanglements per chain is 64 (116,000/1800) If entanglementsoccur only by the crossing of two different rubber chains, the total number ofentanglements in the rubber is 1129  1019(35.29/2  1019 64), which results

in 1490 entanglements in the rubber phase per PS domain A representation ofSEBS polymer microstructure and morphology is presented inFigure 4 Notethat in SBS and SIS the rubber block has a high 1,4-copolymerized dienecontent that maximizes phase separation (due to maximized incompatibilitybetween the plastic and rubber phases) for improved elastic properties but isalso detrimental to product processability On the other hand, SEBS isproduced by the hydrogenation of high- ‘‘vinyl’’ (low 1,4-copolymerizeddiene) SBS for reasons already discussed Hydrogenation of commerciallyavailable SBS would yield a crystalline plastic instead of an elastomericpolymer midblock

The foregoing discussion is based upon the ‘‘spaghetti and meatball’’SEBS morphology described earlier In the case of lower molecular weightSEBS, a higher modulus has been observed compared to those of thecorresponding higher molecular weight counterparts This has been attribut-

ed to the presence of a larger interphase in the former case due to greater phasemixing (92) If the TPE hard block content is high enough to form acontinuous phase, a higher modulus can be expected

Upon increasing PS content, the discrete plastic phase morphology inSEBS can change to a cocontinuous rubber and plastic phase, and further to adiscrete rubber phase in a plastic matrix Also, the shape of the plastic phasecan change from spheroidal to cylindrical to plate-like with increasing SEBS

PS content These regular shapes can be achieved only under carefullycontrolled annealing or shearing conditions

Compared with a corresponding low molecular weight polymer, highmolecular weight SEBS exhibits superior mechanical properties and can be

Figure 4 High rubber content SEBS triblock copolymer microstructure and phology

mor-used to produce lower cost end products owing to its ability to absorb largeamounts of paraffinic oil However, high molecular weight SEBS is notprocessable, because this polymer alone does not flow well under polyolefinplastic processing conditions (93) This is due to phase incompatibility thatnecessitates high temperature and high shear (for increased phase-mixingkinetics) conditions to transform biphasic SEBS to a molten single-phasesystem That is, SEBS has a high order–disorder transition temperature(TODT) that is related to the segmental molecular weight and composition

of this triblock copolymer For example, the TODT of Kraton G1650 (PSblock 29 wt%, MW = 13,500; PEB block 71 wt%, MW = 66,400), which isconsidered a medium molecular weight product, is estimated to be 350jC (94).Moreover, this transformation would not occur instantaneously at thistemperature; it is expected to be retarded due to the highly entangled nature

of the rubber phase One way of determining the TODTis to experimentallymeasure the temperature at which there is a precipitous drop in polymerelastic modulus when measured as a function of temperature at a fixedfrequency, although this approach may not yield the true TODT(95), becausesome order may still exist in the polymer melt at this temperature For anexcellent discussion of SEBS TODTthe reader is referred to the work of Chunand Han (94), Kim et al (95), Baetzold and Koberstein (96), and thereferences cited in these publications In spite of the saturated backbone inhigh molecular weight SEBS, polymer degradation occurs before the TODTisreached, and hence it is difficult to measure this temperature experimentally(94) Lower molecular weight SEBS polymers could be readily processedunder normal polyolefin plastic processing conditions (200–250jC) butcannot provide the necessary price–performance balance to become a product

of commerce as an elastomer A SEBS polymer with a PS end block MW of

3400 (31.8 wt%) and a PEB midblock MW of 14,600 (68.2 wt%) exhibits a

TODTof 142jC (96)

A SBCs as Compounded Materials

In elastomer applications, SEBS is never used alone; it is always compounded

to improve product processability and performance and to lower productcost Polypropylene (PP), paraffinic oil, and fillers make up the bulk of a SEBSelastomer compound In elastomer applications, high molecular weight SEBS

is extended with from 200 to over 400 phr of paraffinic oil In certain oil-gelapplications, the concentration of SEBS is as low as 5 wt% (97) The oilcontributes to compound processability and lowers cost without sacrificingthe elastomer upper service temperature Paraffinic oil is chosen to selectivelyswell the continuous rubber phase, leaving the discrete polystyrene domainsunplasticized, thereby maintaining the integrity of these virtual cross-links at

the elastomer upper service temperature The molecular weight of the PS endblocks is high enough to prevent significant plasticization by paraffinic oil and

to provide sufficient incompatibility with the rubber phase for balancing TPEelastic properties (better with increased phase incompatibility) with process-ability (better with increased phase compatibility) Because SEBS is produced

by the selective hydrogenation in solution of the high vinyl butadiene rubbermidblocks in SBS (98), the narrow molecular weight distribution of the plasticand rubber blocks in SBS (99) (synthesized by anionic polymerization) ismaintained in SEBS Thus, a truly uniform rubber network structure swollen

in paraffinic oil can be expected for SEBS due to the reorganization possible(at elevated temperature) in the polystyrene domains

The presence of a uniformly entangled rubber network and the lowinterphase volume (due to polystyrene and rubber phase incompatibility—theinterphase would hold less oil than the rubber phase) expected in highmolecular weight SEBS would explain the large oil-holding capacity of thismaterial The rubber polymer chains can be viewed as being surrounded by a

‘‘tube’’ of oil, where the oil molecules are generally restricted to move withinthe tube but can cross over between tubes The absorption of oil by SEBS isdriven by the configurational entropy gain by the oil, which overcomes theconformational entropic losses on stretching of the rubber segments Theremay be some lowering of system internal energy due to the adoption of low-energy conformations by the rubber segments There also may be a limitedenthalpic attraction between the oil and rubber The rubber and the oil arenonpolar; therefore, no preferred orientation around the rubber molecule isexpected for the oil in order to maintain the expected enthalpic attraction,thus minimizing the loss in entropy of the oil There is a slight increase in the

Tgof SEBS rubber (40 wt% 1-butene) when it is plasticized by paraffinic oil(100)

The viscosity of SEBS drops when it is plasticized by oil, but there is noincreased phase mixing in the ‘‘melt.’’ The apparent viscosity is reduced owing

to the reduction in frictional forces between the rubber phase (when swollen inoil) and the wall of the capillary rheometer This friction is not affected much

by shear rate or temperature, so the apparent viscosity varies inversely withshear rate and is almost independent of temperature (92,93) The flow ofSEBS is best described by plug flow resulting from wall slip

The presence of both polypropylene (PP) and paraffinic oil is requiredfor a dramatic improvement in the processability of a SEBS compound.Molten PP forms the viscous medium that allows ready transport of thebiphasic SEBS during processing The oil in the SEBS partitions between theSEBS and PP phases (100) (molten PP is miscible with paraffinic oil), thusreducing the viscosity of the molten PP and increasing its volume, whichtranslates into improved SEBS compound melt processability On cooling,

the molten PP crystallizes, and the oil rejected from the crystalline phasepartitions between the SEBS and amorphous PP phases On cooling, theSEBS compound ‘‘hardens’’ rapidly due to the crystallization of the PP phase,thereby allowing rapid cycle time in end product manufacture.

PP level The rubber and PP molecules are then entangled, and, on cooling,the trapped entanglements allow good adhesion between the phases and PP isnucleated across the phase boundary so that cocontinuity is maintainedbetween the phases (86,100,101) It is conceivable that the entanglement with

a rubber molecule of an amorphous PP tie chain is anchored if the tie molecule

is trapped within the same or different PP lamellae as it emerges from therubber phase Even at high elongations the cocontinuous blends show astress–strain behavior similar to that of rubber, with no sign of the typicalnecking phenomenon normally associated with PP at large deformation Itseems reasonable to propose that PP is present as thin coiled sheets andligaments that simply uncoil during deformation, so that the PP phase itself isnot subject to much stress and most of the deformation occurs in the SEBS.From the foregoing discussion it can also be understood how SEBScompounds with a hardness of about 0 Shore A can readily be produced.SEBS can absorb large quantities of paraffinic oil to yield a soft rubber, and

limited amount of the PP, which forms a cocontinuous hard phase alongsidethe cocontinuous SEBS rubber phase During processing of the SEBScompound melt it is simply slipping along the processing conduits on a thinfilm of a molten PP solution in oil High filler and oil loading allows theproduction of low cost SEBS compounds.

Because the oil-holding capacity of SEBS has been discussed, it is worthmentioning the oil-holding characteristics of commercially available SBS inconnection with the wrist rest application, an example of which is a pad thatspans the length of a computer keyboard support The highest molecularweight linear triblock SBS (Kraton D1101) is medium in molecular weightcompared to the highest molecular weight SEBS that is commerciallyavailable (Kraton G1651) (102) Kraton D can probably hold only 100–150phr of paraffinic oil without oil bleed However, the oil gel of the wrist restpresumably contains low molecular weight SBS extended with perhaps 200phr or more paraffinic oil The SBS then increases oil viscosity, and oil bleedfrom the gel is prevented by encapsulation of the oil in an oil- and abrasion-resistant polyurethane cover The oil-extended low molecular weight SBS can

be readily processed (poured into a mold in the wrist rest application) at about150jC because of its lower (than SEBS) TODT Moreover, the slight miscibility

of the low molecular weight PS end blocks with paraffinic oil would furtherreduce hard and soft phase incompatibility, thereby improving gel process-ability The high damping characteristics (103) of this gel (perhaps due to thelarge interphase volume created by the low molecular weight of the polymerand the mixing of small quantities of paraffinic oil into the styrene micro-domains) may not be important in the wrist rest application The lower cost ofSBS compared to SEBS and the high oil loading (which also lowers cost)allowable without bleed due to the oil-resistant polyurethane cover make SBScompetitive in this low-end application where the product UV or thermo-oxidative stability requirement is minimal (104,105) Moreover, intellectualproperty concerning SEBS gels and the large number of SBS manufacturerscompared to SEBS manufacturers also allow the entry of SBS oil gels into thismarket

C SEBS Compound Upper Service Temperature

Improvement

Even though the Tgof polystyrene is about 95jC, under stress the polystyrenesegments will flow at a temperature lower than its Tg SBS loses most of itsstrength at 60–70jC (106,107) In this case, the additional barrier to flow due

to phase incompatibility between the rubber and the plastic is not sufficient toallow the polystyrene microdomains to be good enough anchors to preventdisentanglement of the rubber chains and thus prevent viscous flow Viscousflow occurs in low molecular weight SEBS (Kraton G1652, Table 4) at

65jC (107), in spite of the increased incompatibility between the rubber andplastic phases compared with SBS (elevated temperature stress–strain data).

In SEBS compounds, the presence of PP helps to improve elastic recovery atelevated temperature Nevertheless, because of the permanent plastic defor-mation of the styrenic domains and their reorganization as discussed earlier,the continuous use temperature of compounds containing high molecularweight SEBS is limited to 70jC, with a 100jC use temperature possible inapplications where there is a limited load on the product

Table 4 lists the physical properties of paraffinic oil blends of SEBSproducts KratonR G1651, G1650, and G1652 prepared by mixing in alaboratory Brabender The SEBS materials have approximately the same

PS content and are listed in order of decreasing molecular weight Note thatfor SEBS triblock copolymers, reducing PS molecular weight while keepingthe same weight percent of PS would require a reduction in rubber molecularweight The compression set increase with decreased SEBS molecular weightcan be related to permanent deformation of the PS microdomains and to the

Table 4 SEBS Block Copolymers: Characterization and Properties

PS(wt%)

PS(MwgMn)

PEB(wt%)

PEB(MwgMn)

reorganization of these discrete PS domains by interdomain movement of the

PS chain ends through the continuous rubber phase Permanent plastic formation of the PS phase may contribute only minimally to the compressionset, because the volume of this phase is only 13% for a 30 wt% PS SEBS,assuming that all the added oil is present in the rubber phase and the inter-phase is neglected (PS density 1.05 g/mL; plasticized rubber density 0.86 g/mL) Hence, lowered SEBS molecular weight must facilitate increasedinterphase movement at the molecular level, as discussed, due to the increase

de-in phase compatibility Increased phase compatibility is reflected de-in de-increasedphase mixing for the lower molecular weight SEBS (higher hardness andhigher M100; M100 = modulus at 100% elongation) in comparison with thehigher molecular weight materials (lower hardness and lower M100) Notethat a continuous rubber phase is expected for these SEBS compositions Theincreased Tgexpected for the rubber phase of the low molecular weight SEBSwould contribute to the increased set observed, but it is believed that the bulk

of the set observed is due to interdomain movement of the PS segments Theproperty changes observed when high molecular weight SEBS is processed atdifferent temperatures reflect the difficulty in achieving an equilibriummorphology even after long processing times (compare columns 1 and 2 in

Table 4)

The elastic recovery of SEBS compounds at elevated temperature can beimproved by increasing the hard phase Tgwhile maintaining or exceeding theincompatibility between the rubber and plastic phases over that of SEBS The

Tgof the hard phase can be increased by chemical modification of the PS endblocks in SEBS, by synthesis of triblock copolymers where the PS blocks arereplaced by higher Tg hard blocks, and by compounding with a high Tg

polymer that is miscible with the PS domains of SEBS Alkylation of thepolystyrene phase of SEBS increases the hard phase Tg, but reduces thecompatibility difference between the rubber and hard phases (93)

A recent publication (107) reviewed the methodology to enhance thehigh-temperature properties of SEBS by chemical modification, whichincreases the Tgof the PS glassy phase

Poly(a-methylstyrene) (PaMS) has a Tgof 165jC and a-methylstyrene(a-MS) can be polymerized by anionic, cationic, and free radical polymeriza-tion Triblock copolymers with a polyisoprene midblock and PaMS end blockshave been produced by anionic polymerization, although hurdles have to beovercome due to the low ceiling temperature of aMS (108) An unsuccessfulattempt to synthesize by cationic polymerization an aMS/isobutylene/aMStriblock copolymer has been reported (109) TPEs based on PaMS are notexpected to be of commercial value because of reversion of the polymer to mo-nomer at elevated temperature (110) Hence the use of PaMS hard segments

is unsuitable for improving the high-temperature compression set of SBCs

Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) is the high Tgadditive ofchoice for increasing SEBS upper service temperature Paul and coworkers(111) showed that the solubility of PPO is much greater than that of PS itself inthe PS microdomains of SEBS, owing to the exothermic heat of mixing in thecase of PPO The PPO molecular weight should be less than or equal to that ofthe PS molecular weight of the SEBS microdomain for miscibility, owing tothe limited conformations available to it in this confined geometry (112–114).Also, PPO of greater molecular weight would lose much of its configurationalentropy and gain only a small amount of translational entropy upon mixing(115) The exothermic heat of mixing can partially compensate for theunfavorable entropic effects associated with PPO confinement in the case ofPPO–SEBS mixing.

Baetzold and Korberstein (96) studied solvent-blended low molecularweight PPOs [Mn/Tg(jC): 1000/116, 2000/161, 6000/196] with low molecularweight SEBS PS [(31.8 wt%), MW = 3400, PEB: MW = 14,600] [PEB =poly(ethylene/butene) SEBS rubber midblock] PPO is thought to be homo-geneously distributed among the styrene microdomains but heterogeneouslysolubilized within them PPO is concentrated in the center of the PS micro-domain, with the PPO concentration becoming more diffuse with increasingmolecular weight The SEBS thus modified exhibited two high temperature Tg

values—one due to PS and one due to the PPO/PS core of the PS domains PPO of Mn2000 could be solubilized into the SEBS PS domains toonly about 26 wt% of the total glassy phase Beyond this concentration, PPOformed a separate phase Therefore, there is a limit to which the SEBS hardphase Tgcan be increased by compounding with PPO

micro-An additional disadvantage of this method is the continued presence ofunmixed PS Koberstein and Baetzold could increase the TODTof SEBS from142jC to 180jC by solution blending with PPO It should be mentioned thatPaul and coworkers had previously published similar results (102,111) Pauldemonstrated that PPO with Mnvalues of 15,500–29,400 is miscible in allproportions with high molecular weight SBS (Kraton D1101: 28.8 wt% PS,

MW = 14,500; PBD: MW = 67,500; 16 wt% diblock content) and highmolecular weight SEBS (Kraton G1651) The morphology in these materials

is expected to change from a discrete plastic phase to a continuous one withgreater additions of PPO

Thermoplastic elastomers with improved elevated temperature pression set have been produced by melt blending SEBS, PPO, PP, andparaffinic oil (116) Gel compositions with softening points above 100jC wereachieved by solution blending of PPO and SEBS and subsequent plasticiza-tion of the product isolated with paraffinic oil (117) Replacement of paraffinicoil in this system with very low molecular weight EPR as plasticizer (toprevent evaporative losses) results in soft SEBS gel compositions (hard-

com-Low molecular weight SEBS is readily processable but has a limitedupper service temperature (see earlier discussion) High molecular weightSEBS or SEBS with a high TODThas an increased upper service temperature atthe expense of reduced processability PP that is useful in SEBS compoundingbecause of its partial melt miscibility with oil and PEB represents another limitbeyond which the upper service temperature of SEBS cannot be raised PEB

is not compatible with the commercially available, higher melting (Tm =240jC), isotactic poly(4-methyl-1-pentene) Hence, the latter plastic cannot

be used to improve the upper service temperature of SEBS compounds.The increased Tgof the hard phase modified SEBS would necessitate anincrease in compound manufacturing temperature, time, and mixing intensity

to achieve a near-equilibrium polymer morphology that would be stable at theTPE service temperature Polymer thermo-oxidative and mechanical degra-dation may preclude these aggressive manufacturing conditions

With the additional expense of material and compounding, SEBScompounds (presumably modified with PPO) have matched the 70jC tem-perature elastic recovery of PP/EPDM TPVs (Table 5) (119)

Commercially important SBCs include the SBS and SIS polymers (forexample, Kraton Dk) and SEBS and SEPS (Kraton Gk) HydrogenatedSBCs show improved UV and ozone resistance and better strength at higher

Table 5 Property Comparison of SEBS Compounds vs PP/EPDM TPVs

Manufacturer, trade name

Property

Multibase, Inc., lowcompression setSEBS compound

Advancedelastomer systems,PP/EPDM TPVMultiflex TPE A

5001 E LC

SantopreneR101–55W185

temperatures than the corresponding unsaturated copolymer Increasingstyrene content increases the strength of the materials; conversely, lowerstyrene content increases elongation of SBCs.

Properties of SBCs include high elasticity and tensile strength, lowdensity, low permeability, good optical clarity and surface appearance, andchemical resistance to acids, bases, and aqueous media The materials arenormally custom compounded depending upon the application and can beformulated to yield a wide range of performance characteristics Combina-tions of SBCs with other materials—oils, resins, fillers, processing aids,antioxidants, etc.—yields the desired combination of performance and costdepending upon the application Compounds usually contain SBC for elasticproperties, plasticizers for softening and improved fabrication, and sufficientthermoplastic resin to reach the desired hardness Various fillers, includingcarbon black, clay, and talc, lower cost; color is incorporated through directpigment or pigment masterbatch addition Additives include zinc stearate as aprocess aid and stabilization packages for improved temperature or weath-ering resistance

As already described, SBCs are well known for their oil-holding acity, and relatively large amounts of oil can be incorporated without adetrimental effect on performance Paraffinic oils are preferred for SBCsbased on their elastic phase compatibility; hardness is decreased, but materialstrength is relatively unaffected Aromatic oils should be avoided because theywill soften the styrenic hard domain Compounding with polypropylene iscritical for good injection molding or extrusion fabrication processability forthe SEBS materials because it creates a distinct continuous polypropylenephase

cap-Compounding of SBCs can be achieved with batch or continuousmixers Some ingredients, including color or additive concentrates, may beprecompounded; others are added in separate feeders The final form isusually a finished free-flowing pellet; however, continuous processes formaterial compounding and extrusion have been developed

Compounded SBCs can be fabricated by conventional thermoplasticmethods including injection molding, blow molding, and extrusion Thestyrenic character dominates the melt behavior; materials exhibit strongthixotropic or shear thinning behavior at melt temperatures and higherviscosity at higher styrene block Mw Generally, the melt viscosities are inthe order SEBS>SBS>SIS (due to increasing phase compatibility); onlythe SIS and SBS show a Newtonian region at the low end of the shear raterange

Numerous precompounded grades have been developed for specificpurposes After priming, the parts can be coated with paints that are alsoflexible Applications cover a wide range of products and are found commer-

cially in automotive, medical, footwear, wire and cable, and consumer andindustrial goods SBCs can be formulated into hot melt adhesives for use inlabels and tapes, eliminating the solvents used in conventional polymersolution products (120) Some of these formulations can be covalentlycross-linked by radiation after coating (120) Elastic films and sheets are used

in medical and diaper films

Acoustic barriers for dash panels, wheel wells, firewalls, and floorsprovide a significant part of SEBS compound use in automotive applications

in North America Other automotive uses include seals, gaskets, airbag doorcovers, and soft-touch interior parts

In Europe SBS block copolymers are widely used as asphalt modifiers(121) Used at relatively low concentrations, these materials provide recycla-ble and safe solutions to improve the performance of asphalt by forming athree-dimensional structure within the material Special polymers have beenengineered to provide the appropriate balance of compatibility and flowproperties for road structures and roofing applications (121)

VII THERMOPLASTIC VULCANIZATES

A significant advance in polyolefin-based thermoplastic elastomers resultedfrom the discovery that EPDM rubber, when selectively cross-linked undershear (dynamic vulcanization) during melt blending with a compatible plastic,namely isotactic homopolypropylene, results in a thermoplastic elastomerwith mechanical properties and fabricability far superior to those obtainedfrom a simple blend of the elastic and plastic materials (122–129) Indeed, theperformance, price, and environmental impact of PP/EPDM TPVs haveprovided impetus for replacement of thermoset rubber by these thermoplasticelastomers Penetration of the thermoset rubber market by PP/EPDM TPVshas been made possible by the breadth of the product service temperature(
40jC to 135jC), hardness range (35 Shore A to 50 Shore D), excellentfabricability and fabrication economics, and the ability of product scrap to bereprocessed, among other desirable environmental characteristics PP/EPDMTPVs are products of commerce in thermoset rubber replacements throughfinished part cost savings realized by fabrication, design, and materialeconomics Compared to SEBS compounds, PP/EPDM TPVs exhibit betterelastic recovery at a higher service temperature (100jC vs 70jC) In ‘‘static’’applications PP/EPDM TPVs can provide service at 135jC, versus 100jC forSEBS compounds Very soft TPEs (0–5 Shore A) are based on SEBS; PP/EPDM TPVs with hardness lower than 35 Shore A are not commerciallyavailable

A Definition of Dynamic Vulcanization

Dynamic vulcanization is the process of producing a thermoplastic elastomer

by selective cross-linking of the rubber phase during mixing of a ically compatible or compatibilized rubber and plastic blend of high rubbercontent while minimally affecting the plastic phase Rubber cross-linking isaccomplished only after a well-mixed molten polymer blend is formed, andintensive blend mixing is continued during the curing process The elasto-meric thermoplastic vulcanizate thus formed should ideally consist of a plasticmatrix that is filled with 1–5 Am cross-linked rubber particles

technolog-B Development of Dynamic Vulcanizates:

Historical Perspective

Dynamic vulcanization has its origin in the work of Gessler and Haslett (130)

at Esso, where they demonstrated that carbon black–filled blends of isotactic

PP and chlorobutyl rubber with good tensile strength could be obtained bycuring the rubber (with zinc oxide) after the components were blended on amill at room temperature, by further milling the blend at the curing temper-ature and above the melting point of PP Tensile strength was lower when theblend was ‘‘statically’’ cured Both ‘‘dynamic’’ and ‘‘static’’ cure resulted inthermoplastic compositions The first patent claim, limited to chlorobutylrubber, included blends with up to 50 wt% rubber in PP that could be curedwith any curative that did not break down the PP plastic material Theincorporation of plasticizer oils into the blends is one of the items outlined inthe second claim The goal of this work may have been the impact modifica-tion of PP, because the rubber content of the blend was limited to 50 wt% inthe patent claims Captured in this work were the essential attributes ofdynamic vulcanization as practiced today, including the use of rubberplasticizer oils, except for recognition of the importance of curing the rubberphase only after the formation of an intimate plastic and rubber blend and thevalue of compositions containing a high rubber content

Gessler and Haslett did not continue their research on the dynamicvulcanization of rubber and plastic blends; the work of Fischer representedthe next advance in this technology It was shown that the properties ofpolyolefin blends with EPM or EPDM rubber (thermoplastic olefins; TPOs)could be dramatically improved if the rubber was first either statically ordynamically (on a mill, Banbury, or extruder) cured to a gel content of up to90% before being melt blended with the plastic The gelled rubber was stillprocessable (could be ‘‘banded’’ on a mill) prior to melt blending with theplastic The TPO rubber content could be as high as 90% The improvement

in TPO properties was quantified by an increased ‘‘performance factor’’

(tensile strength (psi)  elongation at break (%) divided by elongation set atbreak) (131,132) Another route to TPOs with improved properties wasachieved by the use of EPM or EPDM rubber that was branched in acontrolled manner during rubber production (133).

Fischer’s work on TPOs culminated in the dynamic vulcanization (in aBanbury) of molten blends of PP/EPM or PP/EPDM with peroxide (134) Tomaintain thermoplastic processability (‘‘banding’’ of the final product on amill or extrudability as a measure of product processability), the rubber curestate had to be limited In the patent, the maximum cure state claimed for therubber is 90% gel Dynamic vulcanization tremendously increases meltviscosity over that of the TPO melt, which increases with increased rubbercure state, thus reducing thermoplastic vulcanizate (TPV) processability TPVphysical properties, however, improve with increased rubber cure state.Increased tensile strength, improved compression and tension set, lower swell

in hydrocarbon oils, and improved flex fatigue and abrasion resistance aremanifested in a TPV in comparison with the corresponding TPO Neverthe-less, because of PP plastic breakdown by the peroxide rubber curativeemployed by Fischer, the TPVs of the illustrative patent examples did notachieve their full property potential To improve TPV melt processability,Fischer indicates the use of very limited quantities (fone part per 100 parts ofrubber) of process aids (e.g., epoxidized soybean oil, polymeric slip aids).Uniroyal’s polyolefin thermoplastic rubber (TPRR, commercialized in 1972)

is based on Fischer’s work on dynamic vulcanization

About the time Fischer was pursuing his studies on dynamic zation, Paul Hartman at Allied Chemical Corporation claimed that butylrubber could be grafted onto polyethylene by using a difunctional resole-typephenolic resin as rubber curative while the rubber and plastic were meltblended on a mill The grafting was thought to occur via the end olefinicfunctionality in PE The grafted material exhibited superior physical proper-ties and had a greater capacity to disperse fillers such as carbon black and talcthan the simple melt blended product Rubber cross-linking was avoided byusing judicious amounts of the low functional phenolic resin curative Graftedproducts of butyl rubber, EPDM, or diene rubbers such as SBR and NBRonto PP, PE, or poly(1-butene) were claimed (135–137) The completesolubility of the products of this invention in hot xylene was taken as proof

vulcani-of grafting vulcani-of the rubber onto plastic and the absence vulcani-of cross-linked rubber

In all probability, the expected grafting reaction was minimal, and the rubbersimply underwent chain extension in the presence of limited amounts ofcurative during dynamic vulcanization

Monsanto entered the field of dynamic vulcanization with a patent byCoran et al (138) that extended the work of Fischer to dynamic vulcanization

of diene rubbers in a polyolefin matrix The inventors demonstrated that

thermoplastic compositions could be obtained even when the rubber wascured to a high cure state as opposed to Fischer’s finding that the gel content

of the rubber obtained in dynamic vulcanization should be less than 100% inorder to maintain product thermoplastic processability In addition, theMonsanto inventors realized that TPV physical properties improve whendynamic vulcanization is continued to achieve a high rubber cure state TPVphysical property improvement was also attributed to the presence of smallrubber particles (less than 50 Am in diameter) dispersed in a plastic matrix.Subsequently, high rubber content TPVs based on butyl rubber were claimed

by Monsanto (139) The patent claims included compositions containing highlevels of plasticizer oils

In a coup de grace (140), Coran et al demonstrated that, contrary to theFischer partial cure requirement, PP/EPDM TPVs with both excellentphysical properties and processability can be obtained when the rubber iscured to a high cure state Sulfur, which does not degrade PP, was the curative

of choice in the patent examples Processable TPVs with a high rubber curestate could also be obtained by peroxide cure in the presence of a bismaleimidecoagent that undoubtedly limited PP breakdown in addition to allowing theachievement of a fully cured rubber phase It was also recognized that thepresence of a high level of paraffinic oil allowed the preparation of soft,processable TPVs with excellent elastic recovery On TPV plastic phasemelting, the oil in the rubber could partially partition into the molten plasticand also form a separate oil phase (nonequilibrium conditions probably existdue to the short TPV processing time) These factors result in a considerableimprovement in TPV processability When the TPV melt is cooled, the free oiland the oil rejected from the crystallizing plastic are reabsorbed into therubber and the amorphous plastic domain At the same time, Gessler andKresge (141) disclosed that PP/EPDM or EPM TPOs that were producedwith high molecular weight rubber had desirable physical properties but werenot processable The TPOs exhibited both good physical properties andprocessability if paraffinic oil was added to the compositions

Monsanto began its effort to commercialize TPVs with PP/EPDM/paraffinic oil compositions that were cured with sulfur TPV morphologyconsisted of a PP matrix that was filled with cross-linked, micrometer-sized(5–15 Am) rubber particles TPV mechanical properties and fabricability weredependent upon rubber particle size, with the size just indicated beingpreferred During TPV processing, however, the rubber particles increased

in size, presumably due to the breakage and re-formation of the polysulfidiccross-links that occur in the melt during particle collision This unstable meltmorphology (‘‘melt stagnation’’ or phase growth of the dispersed rubber)resulted in poor and variable product fabricability and mechanical properties