Rubber Compounding - Chemistry and Applications Part 3 pptx

Weight-average molecular weight Mw Summation of the number of polymerchains N with a given molecular weight m times the square of the molecularweight of each polymer chain divided by the

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General-purpose elastomers played a critical role in the history of the last half

of the 20th century In 1942 the Rubber Reserve program developed both thebasic technology and manufacturing capability to make emulsion styrenebutadiene rubber (SBR) just a few years after World War II had interruptednatural rubber supplies Historians have noted that the scientiﬁc contribution

to that eﬀort is comparable to the nuclear research program at Los Alamosthat occurred at the same time (1) After the petroleum shortages of the 1970s,fuel economy became a primary driving force in the automotive industry, andthe tire industry was challenged to develop new products that would improvegas mileage New elastomers based on solution SBR technology proved to bepart of the answer

Today the tire industry is challenged to meet new environmentalstandards while maintaining or improving the vehicle handling, ride, anddurability that has already been achieved To meet this challenge, the rubbertechnologist must have a thorough understanding of how general-purposeelastomers (i.e., polybutadiene, styrene/butadiene, and styrene/butadiene/isoprene) affect compound processability, tire rolling resistance, tire traction,tire treadwear, and overall cost of tire components Use of these elastomersoutside of the tire industry requires the same type of understanding offundamental polymer characteristics and how they affect the final applica-tion This review will describe the basic structure–property relationshipsbetween general-purpose elastomers and end-use properties, with a focus on

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the tire industry The processes used to make the general-purpose elastomerswill be described with an emphasis on how the polymerization variables(mechanism, catalyst, process) aﬀect the macrostructure and microstructure

of the polymer It is polymer microstructure and macrostructure thatdetermine whether a polymer is suitable for a particular application, notthe type of process or catalyst used to produce the polymer

Some important terms used in this chapter are deﬁned in Table 1

GENERAL-PURPOSE ELASTOMERS USED

IN TIRE APPLICATIONS

Prediction of tire properties based on laboratory properties has met withvarious degrees of success, depending on which property was being predicted.There is a good correlation between the rolling resistance of tires and the treadcompound tangent delta at 60jC and 40 Hz (2) There is a reasonable

distribu-Number-average molecular weight (Mn) Summation of the number of polymerchains (N) with a given molecular weight (m) times the molecular weight of eachchain divided by the total number of polymer chains: SmiNi/SNi

Weight-average molecular weight (Mw) Summation of the number of polymerchains (N) with a given molecular weight (m) times the square of the molecularweight of each polymer chain divided by the total number of polymer chains timesthe molecular weight of each chain: Sm2iNi/SmiNi

Molecular weight distribution Mw/Mn

Glass transition temperature (Tg) Temperature at which local molecular motion in

a polymer chain virtually ceases General-purpose elastomers behave like a glassbelow this temperature

Weight-average Tg Average Tgof a compound:

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correlation between tire traction and tangent delta of the tread compound at0jC and 40 Hz (2) Tire wear is more difficult to predict, with one researcherobserving, ‘‘Despite more than 50 years of effort to devise laboratory abradersthat give a good prediction of the wear resistance in real-world situations, noabrasion device currently exists that does an acceptable job’’ (3) Typically,DIN abrasion or some type of blade abrader is used as a general indicator,however Rubber processability has been defined in a number of ways (4) but

is usually determined by what type of equipment will be used to process therubber Mooney stress relaxation time to 80% decay (MSR t-80) is a rapid,eﬀective processability test that works well with both emulsion (5) andsolution SBR (6) Other more sophisticated instruments such as the rubberprocessability analyzer (RPA) or capillary rheometer are now becoming morepopular

The most important elastomer variable in determining overall tire ance is the glass transition temperature, Tg Aggarwal et al (2) showed thatthe tangent delta at 60jC of ﬁlled rubber vulcanizates made from ‘‘conven-tional rubbers’’ correlated with tire rolling resistance and then determinedthat the tangent delta values were approximately a linear function of thecompound’s Tgvalue This was true whether the polymers were made by asolution process or an emulsion process They did not compare solution andemulsion polymers at the same glass transition temperature

perform-Oberster et al (7) showed that traction and wear properties were notdependent on the way the polymer was manufactured but were functions ofthe overall glass transition temperature of the compound, as shown inFigures

1and2 In actual tire tests, results are more complicated The weight-average

Tgof the tread compound is still a major variable, but it is not as dominant as

in laboratory tests A comprehensive study of tire wear under a variety ofenvironmental and road conditions showed that tire wear improves linearly asthe ratio of BR to SBR is increased in BR–SBR tread compounds (lowerweight-average Tg) The wear behavior was more complex in BR–NR blendswith low carbon black levels and was shown to be a function of ambient testtemperature (3)

Nordsiek (8) expanded the concept of using the glass transition perature to using the entire damping curve to predict tire performance Hedivided the damping curve into regions that inﬂuenced various tire properties(Fig 3) The damping curves for an emulsion SBR, a high-vinyl polybutadi-ene, and a medium-vinyl SBR at the same Tgwere compared and shown to be

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diﬀerent at temperatures of 20–100jC This led to the proposal of an ‘‘integralrubber’’ that would have a compilation of damping curves from a number ofpolymers and would incorporate damping behavior that would lead to the

‘‘ideal’’ elastomer for tread compounds It was implied that this elastomerconsisted of segmented blocks of diﬀerent elastomers with diﬀerent glasstransition temperatures An ‘‘integral rubber’’ was prepared and compared to

Figure 1 Eﬀect of Tgon traction of (x) solution polymers and (n) emulsion mers (From Ref 7.)

poly-Figure 2 Eﬀect of Tgon wear of (x) solution polymers and (n) emulsion polymers.(From Ref 7.)

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natural rubber and SBR 1500 controls in a laboratory compounding study.The ‘‘integral rubber’’ had a hot rebound within one point of the naturalrubber control and was three points higher than the SBR 1500 control.Abrasion resistance was better than that of the natural rubber control butslightly worse than that of the SBR 1500 The 0jC rebound was lower thanthat of either control.

C Molecular Weight and Molecular Weight Distribution

The molecular weight aspect of polymer macrostructure affects the rollingresistance (via hysteresis) and processability of the tread compound As themolecular weight is increased, the total number of free chain ends in a rubbersample is reduced, and energy loss of the cured compound is reduced Thisleads to improved rolling resistance, but at the expense of processability.Caution should be used in extrapolating lab data on high molecular weightrubbers to factory-mixed stocks, because filler dispersion is not as efficientwith large-scale equipment Thus, low hysteresis in lab compounds may nottranslate into low hysteresis in commercial tire compounds There is anoptimum balance between molecular weight and processability that is definedFigure 3 Damping curve of ESBR 1500 tread compound (From Ref 8.)

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by the type of mixing equipment used Increasing the molecular weightdistribution at equivalent molecular weight by branching produces more freechain ends and more hysteresis but at moderate levels can improve otherproperties Saito (9) showed that in silicon-branched solution SBR the eﬀect

on hysteresis could be minimized and ultimate tensile strength could beimproved because of better carbon black dispersion In emulsion polymers,the branching is uncontrolled and the polymers have poorer hysteresis thanthe corresponding solution polymer (10) From a practical standpoint, somebranching in tire polymers is necessary to prevent cold ﬂow and ensure thatthe elastomer bales will retain their dimensions on storage

Polymer scientists have worked hard to take advantage of the ship between free chain ends and hysteresis In one case, an attempt was made

relation-to eliminate chain ends completely by preparing cyclic polymers Hall (11)polymerized butadiene with a cyclic initiator and claimed to have made amixture of linear and cyclic polybutadiene Cyclic structure was inferred from

a comparison of the viscous modulus of the cyclic polymer to that of a linearcontrol All of the cyclic polymers had a lower viscous modulus than thecontrols No compounding data were reported, however

A more popular method of reducing the effective number of free chainends is to functionalize the end of the polymer chain with a polar group.Functional end groups can enhance the probability of cross-linking near thechain end and interact directly with the filler, thus reducing end effects.Ideally, difunctional low molecular weight polymers would be mixed withfiller and then chemically react with the filler during vulcanization to give anetwork with no free chain ends This ideal can be approached, depending onhow effectively the polymer chains are functionalized and the strength of theinteraction of the functional group with the filler This will be discussedfurther in the section on anionic polymerization and anionic polymers(Section IV)

Day and Futamura (12) compared diﬀerent 35% styrene solution SBRs atequivalent molecular weights and found that hysteresis is a linear function ofthe block styrene content The eﬀect of the polystyrene block length onhysteresis is shown inFigure 4

Sakakibara et al (13) made block polymers of polybutadiene and SBRwith anionic polymerization and compared them to an SBR with the sameoverall microstructure They found that the block polymers had broader glasstransition temperatures that resulted in better wet skid resistance and lowerrolling resistance than the corresponding random SBRs They also found thatblocky styrene in the SBR block was detrimental to overall performance

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III EMULSION POLYMERIZATION AND EMULSION

POLYMERS

The copolymerization of styrene and butadiene is accomplished by dispersingthe monomers in water in the presence of a surfactant, an initiator, and achain transfer agent The process oﬀers limited control over polymer micro-structure, and the polymers are branched Emulsion SBR, however, hasplayed and continues to play an important role in tire compounds

The best way to consider the overall emulsion process is to examine theoriginal recipe used to produce GR-S rubber at the beginning of World War II(14) (Table 2)

It is important that the polymerization be done in the absence of oxygen.Oxygen is removed from the water by bubbling nitrogen through it prior tothe polymerization, and the polymerization is conducted under a nitrogenatmosphere When the ingredients are mixed, the monomers are partitionedbetween the water, micelles, and monomer droplets The water solubility ofstyrene and butadiene is very low, so there is little of either in the water phase.Micelles are aggregates of surfactant (fatty acid soap) with the polar carbox-ylic group on the outside oriented toward the polar water and the nonpolarhydrocarbon tail oriented toward the inside of the micelle The nonpolarstyrene and butadiene are ‘‘soluble’’ inside the nonpolar environment of themicelle Still, only a small portion of the monomer is located in micelles There

Figure 4 Eﬀect of block styrene on hysteresis in SBR (From Ref 12.)

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are approximately 1017–1018micelles per milliliter of emulsion (15) Most ofthe monomer is contained in monomer droplets, which are in lower concen-tration (1010–1011monomer droplets per milliliter emulsion) and much largerthan the micelles (15) When the mixture is heated to 50jC, the potassiumpersulfate decomposes into radicals in the aqueous phase Because the surfacearea of the micelles is much greater than that of monomer droplets, theradicals are more likely to inoculate the micelles to begin the polymerization.

A representation of this is shown inFigure 5

As the polymerization proceeds, monomer migrates from the monomerdroplets to the micelles until the monomer droplets are gone Chain transfer

to the mercaptan controls polymer molecular weight Conversion is stopped

at approximately 70% by addition of a radical trap such as the salt of adithiocarbamate or hydroquinone The latex is stabilized, then coagulated togive crumb rubber

A major improvement in this process was the development of the redoxinitiation system shortly after World War II (16) (Table 3) With this recipe,the polymerization could be conducted at 5jC by changing the initiatorsystem from potassium persulfate to cumene hydroperoxide The iron(II) saltlowers the activation energy for the decomposition of the cumene hydroper-oxide and is oxidized to iron(III) during the process The dextrose is present toreduce the iron(III) back to iron(II) so more peroxide can be decomposed.The importance of the lower polymerization temperature is shown in

Figure 6 As the polymerization temperature is decreased, the ultimate tensilestrength of cured rubber increases dramatically (17) This is because there isless low molecular weight material and less branching at the lower polymer-ization temperature (18)

There is little control over butadiene polymer microstructure in theemulsion process It remains fairly constant at 12–18% cis, 72–65% trans, and16–17% vinyl as the polymerization temperature is increased from 5jC to

Table 2 GR-S Recipe for Emulsion SBRa

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50jC Butadiene microstructure does not vary signiﬁcantly as the styrenecontent is changed (19) The glass transition temperature of emulsion SBR iscontrolled by the amount of styrene in the polymer.

It is easy to incorporate a functional monomer into an emulsion polymer aslong as there is some water solubility Emulsion butadiene or styrene

Figure 5 Species present during emulsion polymerization (From Ref 15 printed by permission.)

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Table 3 ‘‘Custom’’ Recipe for Emulsion SBR

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butadiene rubbers containing acrylate, amine, cyano, and hydroxyl groupshave been made Although some recent work has been done in exploring theinteraction of functional emulsion rubbers with ﬁllers, more work could bedone Emulsion SBR containing 3–5% acrylonitrile displays better abrasionresistance than the corresponding unfunctionalized rubber in carbon blackcompounds (20) Emulsion SBRs containing one to four parts of copolym-erized amines were compounded into silica-containing stocks and showedgood processability, improved tensile strength, lower hysteresis, and betterabrasion resistance than a corresponding emulsion SBR control (21).

A substantial percentage of the rubber used in tire compounds is oil-extendedemulsion SBR, which is prepared by adding an emulsion of oil to SBR latexprior to coagulation Oil extension allows higher molecular weight elastomers

to be used without processing problems, and incorporating the oil into thelatex is much easier than putting it in the compound at the mixer The oils used

in compounding rubber are classified as paraffinic, naphthenic, and aromaticdepending on the aromatic content of the oil The different types of oils affectrubber compounds differently, and they cannot be directly substituted foreach other without compounding changes The more paraffinic the oil is, thelower its Tg, which will lead to different compound properties than a higher Tg

naphthenic or aromatic oil Direct comparison of SBR 1712 (37.5 phraromatic oil) with SBR 1778 (37.5 phr of naphthenic oil) in a sulfur-vulcanized stock showed that the 1778 stock had a six point higher roomtemperature rebound and a higher 300% modulus but poorer wet traction(22) Schneider et al suggested using a higher surface area black and addingsmall amounts of a higher TgSBR to match the 1712 performance Since thelate 1980s the aromatic oil used in SBR 1712 has come under ﬁre forcontaining polycyclic aromatics that may be a factor in causing cancer.Compounders must be ready to make the necessary changes to eliminatethe high aromatic oil if necessary

Carbon black and carbon black–oil masterbatches of emulsion SBR havebeen used commercially for a long time They are prepared by blending adispersion of carbon black and oil with latex followed by coagulation.Masterbatching oﬀers the advantages of improved black dispersion and

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shorter mix times A major problem with masterbatching is that it limitscompound flexibility to compounds that contain the type of black that is inthe masterbatch There can also be unexpected effects on the vulcanizationrate (23) Surprisingly, there is no commercial counterpart in an emulsionSBR silica masterbatch, although there have been a number of patents on thesubject (24–27) In most of these patents, a dispersion of silica and some ma-terial to reduce the filler–filler interaction is blended with the latex prior tocoagulation The problems encountered with carbon black masterbatch arealso expected in silica masterbatches.

The International Institute of Synthetic Rubber Producers (IISRP) classifiescommercial emulsion polymers as shown in Table 4 Specifics (soap type,Mooney viscosity, coagulation, and supplier) for different grades of polymersare provided in the detailed section of the IISRP Synthetic Rubber Manual(28)

A schematic representation of a commercial continuous emulsion SBRprocess is shown inFigures 7and8 Most of the ingredients are mixed andcooled, then combined with a solution of initiator immediately before theyenter the ﬁrst reactor The number of reactors is chosen to control theresidence time to reach 60–65% conversion in 10–12 hr The polymerization

is shortstopped, and the latex is pumped to a blowdown tank and ﬂash tanks

to remove most of the residual butadiene A dispersion of an antioxidant isadded to protect the polymer through the subsequent processing steps and

Table 4 Numbering System for Commercial Emulsion

Polymers

less parts of oil per 100 parts SBR

than 14 parts of oil per 100 parts SBR

Source: Ref 28.

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Figure 7 Emulsion polymer process—polymerization (Courtesy of G Rogerson, Goodyear Tire & Rubber Co.,Akron, OH.)

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Figure 8 Emulsion polymer process—ﬁnishing (Courtesy of G Rogerson, Goodyear Tire & Rubber Co., Akron, OH.)

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storage prior to use The latex is then steam stripped to remove the rest of thebutadiene and all of the styrene Crumb rubber is produced by coagulation in

a solution of acidic sodium chloride After washing, the crumb is dried andbaled (19)

Anionic polymerization oﬀers the rubber technologist the maximum tility in preparing new elastomers The procedure involves reaction of alithium alkyl with a diene or combination of styrene and diene(s) in ahydrocarbon solvent The polymerization typically produces a polymer with

versa-a nversa-arrow moleculversa-ar weight distribution becversa-ause eversa-ach initiversa-ator moleculeproduces one polymer chain, and initiation is fast relative to propagation.Polymer microstructure is strongly influenced by a judicious choice of polarmodifier The resulting polymer can be further treated with electrophiles toprepare functional polymers The polymerization process is straightforward,although care must be given to purification of all reagents, and the polymer-ization must be run in an inert atmosphere A laboratory reactor setup forpreparative quantities of polymer has been described in the literature (29)

A Initiation

Conventional organolithium species are highly associated in hydrocarbonmedia, and the resulting aggregates are not very reactive in polymerization(30) The aggregates are in equilibrium with less associated organolithiumspecies, which actually initiate most if not all of the polymerization (Fig 9).Conducting the polymerization in more polar solvents such as diethylether or tetrahydrofuran (THF) increases the concentration of less associatedspecies and increases the reaction rate Typically, however, small amounts ofpolar compounds are added to the polymerization in nonpolar media toachieve the same effect These materials complex with the lithium to break upthe agglomerates In ‘‘modified’’ polymerizations (polymerizations where asmall amount of a polar compound is added), most alkyllithium compoundsare suitable initiators, but for an unmodified polymerization secondary or

Figure 9 Aggregation of organolithium species

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tertiary lithium compounds are required to rapidly initiate the tion This is because primary organolithium compounds such as n-butyl-lithium are more associated than the secondary organolithium compoundsand thus are less reactive (31,32).

polymeriza-Functional organolithium reagents are used to make functional mers (33) This technique is generally better than functionalizing a livingpolymer by reaction with an electrophile, because there are fewer sidereactions with initiation The reactivity of the lithium portion of the initiatorrequires that the functional group be protected in most cases, but the availablefunctionality is surprisingly diverse The key issues with functional initiatorsare storage stability and solubility in solvents suitable for polymerization.Lithiated acetals (34) and lithiated trialkylsilyl ethers (35) are used to formhydroxyl-terminated polymers after deprotection Amine-terminated poly-mers have proven to be more useful for the preparation of tire elastomers Thesynthetic routes diagrammed in Figure 10 can prepare these initiators.The reaction of imine 1 with n-butyllithium produced initiator 2 SBRwas prepared with this initiator, but the number-average molecular weightwas much higher than predicted, which indicates that the alkyllithiumreaction with the imine produced less than 100% of 2 or that the initiator isnot completely eﬃcient for initiation The compounded SBR did exhibitimproved hysteresis compared to a butyllithium-initiated control (36,37) Thereaction of secondary amines with butyllithium seems like an easy way toprepare n-lithium amides, but most of them are insoluble in nonpolar media

poly-Figure 10 Synthesis of lithium amide initiators (From Refs 36–38.)

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Cheng (38) prepared a series of simple secondary lithium amides, but in allcases they were insoluble in hexane The heterogeneous initiators were used topolymerize dienes, but the polymerizations did not go to completion and theresulting polymers most likely had a very broad molecular weight distribu-tion Lawson et al (39a,39b) showed that preparation of lithium amides in thepresence of two equivalents of THF gave soluble initiators that could be used

to make a medium vinyl SBR at high conversion The resulting polymer wascoupled with tin tetrachloride and showed a 40% reduction in hysteresis asmeasured by tan y at 50jC compared to a butyllithium-initiated controlpolymer A partial list of the amide initiators studied and their solubilities isgiven inTable 5

Interestingly, although almost all of the amide initiators eﬀectivelyinitiated polymerization, not all of the resulting polymers showed reducedhysteresis on compounding

N-Lithiohexamethyleneimine 3 and clo[3.2.1]octane 4 were studied further They were both shown to be stable for

N-lithio-1,3,3-trimethyl-6-azabicy-‘‘several days.’’ Initiator 4 produced polymers with a broader molecularweight distribution than initiator 3 (40) One diﬃculty in working with theseinitiators is that the amine group is lost during polymerization by themechanism shown inFigure 11 This reaction becomes more signiﬁcant inthe presence of excess initiator and at temperatures above 80jC

Initiators 5 and 6 (Fig 12) can eliminate head group loss because theadditional carbon atom between the nitrogen and lithium prevents elimina-tion (41)

The diﬃculty with the lack of solubility of simple lithium amides can beovercome by in situ formation of the initiator Immediately after charging areactor with solvent, monomer, randomizer (THF or potassium amylate),and butyllithium, a secondary amine is added to the mixture The amide ismade in situ, and high molecular weight polymers are formed that have lowerhysteresis than the corresponding polymers made with butyllithium Approx-imately 85–90% of the chains have amine head groups when this procedure isused (42)

Tin-containing initiators are also important compounds used to preparehigh-performance tire rubbers Addition of lithium metal to tributyltinchloride in an ether solvent produces a solution of the desired initiator that

is ﬁltered to remove lithium chloride (43) (Fig 13) The initiator is stable atroom temperature and can be stored for approximately 8 weeks before a loss

in activity is observed Polymer with a lower vinyl content and narrowermolecular weight distribution is obtained if the initiator is made in dimethylether rather than THF This is illustrated inTable 6for the polymerization ofbutadiene Carbon black compounds based on these polymers have lowerhysteresis than corresponding unfunctionalized controls

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Table 5 Solubility and Eﬀectiveness of Lithium Amide Initiators

Source: Ref 39.

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B Propagation

Propagation takes place at typical reaction temperatures (20–75jC) in inertsolvents such as hexane or benzene without chain transfer or termination Athigh temperature, however, the growing polymer chain can eliminate lithiumhydride, which stops the polymerization and broadens the molecular weightdistribution The mechanism is shown inFigure 14

Elimination of lithium hydride is a ﬁrst-order process that yields apolymer terminated with a diene Addition of living polymer doubles themolecular weight of the chain and provides an active site that can react withadditional butadiene to form a branched polymer (44)

The ratio of monomer to initiator has a major inﬂuence on the cis/transratio in the homopolymerization of both butadiene and isoprene in unmod-iﬁed polymerizations, as shown inTable 7 (45,46) The higher the ratio of

Figure 12 Functional initiators to avoid head group loss

Figure 11 Head group loss in functional polymers

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monomer to initiator, the higher the cis/trans ratio produced with bothbutadiene and isoprene.

Two kinetic factors aﬀect the diene microstructure The ﬁrst involvesthe relative rates of propagation versus isomerization of the initially formedallyl anion Monomer is inserted initially to form the allyl anion in the antiform If propagation is rapid, the microstructure of the penultimate unit will

be cis If, however, the allyllithium has suﬃcient time to isomerize to thethermodynamically more stable syn form, then the penultimate unit will betrans Thus, at a high monomer/initiator ratio that favors rapid propagation,the microstructure is primarily cis As the monomer is depleted and themonomer/initiator ratio decreases, more trans microstructure will be formed(Fig 15) (47,48) The second factor is the relative rate of addition of monomer

to the syn or anti isomer Butadiene will add approximately twice as fast to theanti form as to the syn form With isoprene the factor is eight times as fast (49)

In addition to increasing the rate of polymerization, polar solvents orpolymerization modiﬁers also aﬀect the vinyl content and sequence distribu-tion in polybutadiene, as shown inTable 8(50,51)

Large amounts of weak complexing agents such as diethyl ether ortriethylamine must be used to significantly affect the microstructure, butstrongly chelating modifiers such as tetramethylethylenediamine (TMEDA)

or 1,2-dipiperidinoethane increase the vinyl content dramatically at lowlevels The effect of polymerization temperature and its interaction withmodifier is also illustrated by the data Vinyl content is increased as thetemperature is reduced for all polymerizations, but the effect is morepronounced at low modifier/lithium ratios

In the copolymerization of styrene and butadiene, the sequence bution is strongly aﬀected by the addition of polar modiﬁers or salts InFigure 13 Synthesis of tributyltin lithium

distri-Table 6 Polymerization of Butadiene with Tributyltin Lithium

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hydrocarbon solvents without polar materials, most of the butadiene willpolymerize ﬁrst, followed by the styrene This process is used to prepare

‘‘tapered’’ block polymers where there is a butadiene block, a mixed ene–styrene block, and a styrene block (52)

butadi-Addition of polar compounds will randomize the styrene and increasethe rate of polymerization Choice of modifier is critical to get the properdegree of randomization and control the vinyl content Modifiers such aspotassium tert-butyl alkoxide (t-BuOK) are used to randomize the styrenewithout significantly increasing the vinyl content At a ratio of t-BuOK/n-BuLi of 0.1, there is only a small increase in vinyl content (Fig 16), but this issufficient to randomize styrene in an SBR (53)

For higher vinyl SBR, a more powerful randomizer such as TMEDA isused that produces high vinyl polymers at relatively low modiﬁer/lithiumratios (54) Very high vinyl SBR and polybutadiene can be prepared with amodiﬁer consisting of a mixture of TMEDA and an alkali metal salt of analcohol (55)

Table 7 Eﬀect of Monomer/Initiator Ratio on Microstructure

Monomer/

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Figure 15 Microstructure formation during lithium polymerization (Adaptedfrom Refs 47 and 48.)

Table 8 Eﬀect of Polar Modiﬁers on Polybutadiene Microstructure DuringLithium Polymerization

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C Termination

Termination is easily accomplished by reaction of the living polymer with anelectrophile In early anionic polymerization studies, the electrophile was aproton donor and termination resulted in a hydrocarbon polymer Reactionwith other electrophiles such as carbon dioxide (carboxylic acid), sultones(sulfonates), ethylene oxide (alcohol), or imines (amines) produce functionalpolymers, but unless conditions are carefully controlled the functional poly-mer is contaminated with other materials (56) Virtually every electrophileknown has been tested as a terminating agent for lithium polymerizations Inone patent alone, the following were claimed for terminating a living trans-polybutadiene polymerization—isocyanates, isothiocyanates, isocyanuric ac-

id derivatives, urea compounds, amide compounds, imides, tuted oxazolydinones, pyridyl-substituted ketones, lactams, diesters,xanthogens, dithio acids, phosphoryl chlorides, silanes, alkoxysilanes, andcarbonates (57), Amine- and tin-containing electrophiles provide the greatestinteraction with carbon black Epoxy compounds and alkoxysilanes are mostbeneﬁcial for silica-ﬁlled compounds The early work focused on terminationwith amine-containing functional groups such as EAB [4,4V-bis-(diethylami-no)benzophenone] (58–60) Black compounds made with these polymersshowed higher rebound, lower heat buildup, higher compound MooneyViscosity, and more bound rubber than the corresponding control rubber.Another study by Kawanaka et al (61) suggested that the mechanism of the

N-alkyl-substi-Figure 16 Eﬀect of potassium butoxide/lithium ratio on polybutadiene structure (n) Percent trans; (E) percent vinyl (From Ref 53.)

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rubber–filler interaction was through an iminium salt formed from thereaction product of the amide and living polymer chain end (Fig 17) Theauthors inferred this because rubber functionalization with amides that couldnot easily form iminium salts did not interact well with carbon black.Termination with tin-containing compounds provides more flexibilitythan with amine compounds RxSnCly(where x + y = 4) can be chosen togive different levels of branching and thus assist in macrostructure control.Phillips pioneered the coupling of solution polymers with tin halides to makeradial polymers in the 1960s but the Japanese Synthetic Rubber Company(JSR) was the first to use the nature of the carbon–tin bond for tire com-pounds Tsutsumi et al (62) outlined the synthesis of tin-coupled solution SBR,the mechanism of how it improves hysteresis, structure–property relation-ships to maximize the effect of tin, and pitfalls to avoid in compounding (62).They first demonstrated that coupling solution SBR with tin tetrachlorideprovided a superior polymer compared to other coupling agents (Table 9).The SBR polymerization was terminated with tin tetrachloride suchthat 50% of the chain ends were coupled The only major difference inperformance among the coupling agents was the low hysteresis exhibited bythe tin-coupled polymer Tsutsumi et al compared a series of tin-coupledpolymers with a polymer containing trialkyltin groups along the backbone.Only tin located at the end of the polymer chain (or branch point) waseffective in reducing hysteresis (Fig 18).

Figure 17 Termination of lithium polymerization with a cyclic amide (From Ref.61.)

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In another study the same group showed that putting the tin group on abutadienyl chain end was more eﬀective in reducing compound hysteresisthan putting it on a styryl chain end Finally, they postulated that themechanism of interaction with carbon black is by formation of a bondbetween the polymer chain and the quinone groups on the carbon black.This was based on a model study of the reaction of tributyltin-capped low

Figure 18 Effect of tin content position on dynamic properties of tin-coupled SBR.(x) Polymer modified on backbone (n) Polymer modified at chain end (From Ref.62.)

Table 9 Coupling of Solution SBRa,b

Coupling agent

ML-1+4(100jC)

CompoundedML-1+4(100jC)

Tensilestrength(MPa)

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molecular weight polybutadiene with a series of compounds containingfunctionality found on carbon black Only the quinones reacted to any extent.The ease of cleavage of the tin–carbon bond is the reason this chemistry cantake place, but it also puts some restrictions on how tin-containing polymerscan be isolated and compounded Acid will cleave the bond and should beavoided until late in the compound cycle Mooney viscosity of coupledpolymers will drop if the tin–carbon bond is broken, but if the polymer iscapped with a trialkyltin halide there will be little change in Mooney viscosity.The importance of complete functionalization is illustrated by a study

by Quiteria et al (63) They examined the effect of the polymer end group andthe effect of unfunctionalized polymer (via incomplete coupling) on thedynamic properties of tin-coupled polymer in a simple black formulation.They synthesized a 25% styrene SBR with 32% vinyl via adiabatic polymer-ization and reacted the living polymer with a small amount of monomer(butadiene, styrene, isoprene, or a-methylstyrene) to ensure a specific endgroup Tin tetrachloride was added to couple 40% of the polymer Theresidual polymer chains were terminated with tributyltin chloride The losstangent as a function of temperature for these polymers is shown in Figure 19.The most important feature of the graph is the effect of unfunctionalizedpolymer on hysteresis (run 5) Compound made with polymer from run 5(uncoupled polymer terminated with a proton) had 15–20% higher tangent

Figure 19 Loss tangent versus temperature for diﬀerent tin–carbon bonds Bd,butadiene; St, styrene; Is, isoprene; MSt, a-methylstyrene; H, hydrogen; Sn, tin.(From Ref 63.)

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delta values at 80jC than runs 1–3 (uncoupled polymer terminated withtributyltin chloride).

For silica compounds, different functional groups are required forpolymer–filler interaction Alkoxysilanes such as 3-triethoxysilylpropyl chlo-ride, chlorodimethylsilane, and bis-(3-triethoxysilypropyl) tetrasulfide reactwith a living isoprene–butadiene chain to give a polymer that is claimed tointeract well with silica (64) Gorce and Labauze (65) showed that 3-glycidoxylpropyl-trimethoxysilane reacted primarily at the silicon instead

of at the epoxy group when used to terminate SBR polymerization Thetangent delta value at 60jC was 28% lower than that of the unfunctionalizedcontrol They also suggested a mixing system for reacting the polymer withthe coupling agent that minimized Mooney viscosity rise after steam strippingand storage This is a serious practical problem with alkoxysilane-terminatedpolymers Hydrolysis of the alkoxysilane group led to hydroxysilyl endgroups that condensed to increase the molecular weight and ultimately gelthe polymer Saito et al (66) compared a number of diﬀerent types offunctional groups (Table 10) in a 35% styrene, 38% vinyl SBR to determinewhich ones interacted most strongly with silica and improved compoundperformance In addition to the structures shown in Table 10, they alsostudied SBRs terminated with tin tetrachloride and silicon tetrachloride.Compounds made from the polymers containing the diglycidylamine group,glycidoxypropyltrimethoxysilane, and dimethylimidazolidinone had very lowhysteresis and better abrasion resistance than the control polymer Theviscosity of the glycidoxypropyltrimethoxysilane- and dimethylimidazolidi-none-modiﬁed polymers rose on storage, however, and would not be suitablefor commercial production

An important consideration for continuous anionic polymerization is trolled termination or chain transfer Very high molecular weight polymerwill form in unagitated areas of a reactor and for practical purposes can beconsidered gel The situation is made worse if the reactor is used for couplingreactions where divinylbenzene, silicon tetrachloride, or tin tetrachloride isused In an adiabatic process, the high temperature can lead to gel via the type

con-of branching process shown inFigure 14 To prevent reactor fouling, a smallamount of a material that can act as a chain transfer agent or a slow ‘‘poison’’must be added Typically, 1,2-butadiene is used (67) Adams et al (68) andlater Puskas (69) investigated the mechanism and found that it is complex Asummary is shown inFigure 20 Organolithium species can isomerize the 1,2-butadiene to 1-butyne or react directly to give a lithiated allene that can befurther lithiated The 1-butyne reacts rapidly with organolithium compounds

to give a lithium acetylide The reaction of poly(butadienyl)lithium with the

4871-9_Rodgers_Ch02_R2_052404

Tiêu đề	General-Purpose Elastomers
Tác giả	Howard Colvin Riba-Fairfield
Trường học	Decatur, Illinois, U.S.A.
Chuyên ngành	Rubber Technology
Thể loại	review article
Năm xuất bản	2004
Thành phố	Decatur

Định dạng
Số trang	54
Dung lượng	1,12 MB