vulcanization of blends – crosslink distribution and … of blends – crosslink distribution and...

ELASTOMERE UND KUNSTSTOFFEELASTOMERS AND PLASTICS

Vulcanization of Blends –Crosslink Distribution andits Effect on Properties1

Vulcanization � Blends � Crosslink dis-tribution measurement

This review is focussed on the crosslinkdistributions between rubber phases,which arise when blends of rubbers arevulcanized, how these distributionsmay be evaluated and controlled, andhow they impact upon the propertiesof the blends. The blends are dividedinto three categories – rubbers differ-ing primarily in polarity, rubbers dif-fering primarily in degree of unsa-turation, and rubbers differing little ineither polarity or degree of unsatura-tion. Control of crosslink distribution isimportant if the best is to be obtainedfrom vulcanized blends. When cross-links are very unevenly distributed in ablend, improvement of the distribu-tion leads to improved physical prop-erties.

A. V. Chapman and A. J. Tinker,

Brickendonbury, Hertford

(Großbritannien)

1 Based on a presentation at the DIK Workshop

„Vulcanisation Prozess-Technik-Wirkung“;

Hannover, 27 March, 2003.

Vernetzung von Verschnitten-Verteilung der Netzknoten undderen Einfluss auf die Ei-genschaften.

Vulkanisation � Verschnitte � Mes-sung der Verteilung von Netzknoten

Der Ûbersichtsartikel bezieht sichauf die Verteilung der Vernet-zungsdichte in diskreten Polymer-phasen, und auf die experimentelleErmittelung und Steuerung sowieauf den Einfluss dieser Verteilungauf die Eigenschaften von Kau-tschukverschnitten. Die Verschnittewerden aufgrund ihrer in drei Ka-tegorien eingeteilt: Verschnitte mitunterschiedlicher Polaritat, Ver-schnitte mit unterschiedlicher Pola-ritat und Ungesattigtheitsgrad so-wie Verschnitte mit geringen Un-terschieden dieser beiden Eigen-schaften. Wird die stark unter-schiedliche Vernetzungsdichtedurch geeignete Maßnahmen ver-mindert so fuhrt dieses allgemein zueiner Verbesserung der physikali-schen Eigenschaften.

The majority of rubber is used in the formof blends, an industrial fact of life, which issufficient in itself to show the importanceof vulcanization of blends. The aim ofblending is to combine the desirable fea-tures of each component, but often theproperties obtained are worse than antici-pated from those of the component rub-bers and, generally, the properties of vulca-nized blends cannot be linearly interpo-lated from those of the individual rubbervulcanizates. The following factors arisespecifically for blends and determine theirproperties:* The rubbers [1, 2] and their ratio [2–6]* The phase morphology [2–4, 7–13]* The distribution of filler between the

rubbers [1, 3, 8, 10, 11, 14–23] or atthe interface [24–25]

* The distribution of plasticizer betweenthe rubbers [26, 27]

* The interface: interpenetration of poly-mer chain segments, adhesion andcrosslinking [2, 14, 24, 28–42]

* The distribution of crosslinks betweenthe rubbers [1, 2, 43–46]

This review is focussed mainly on the last ofthese factors, but interfacial crosslinking isalso considered. For more general reviewsof rubber blends the reader is referred toCorish [1], Hess et al. [3], Manjaraj [14]and Roland [7], while McDonnel et al.[47] have reviewed blends used in tyresand Tinker [43] has previously reviewedcrosslink distribution in blends.Vulcanization is most commonly achievedby using a sulphur based cure system, andthe complexities of this are well documen-ted, if not completely understood yet. Thiscomplexity increases when rubber blendsare vulcanized. The specific additional fac-tors at work in vulcanization of blends areconsidered here, rather than the details ofvulcanization chemistry.

Whilst most of the issues and conse-quences have been understood for manyyears, it is only recently that techniqueshave been developed to allow accurate, in-formed study of the vulcanization ofblends. This has led to better control ofthe two key factors which are peculiar tothe vulcanization of blends – distributionof crosslinks between the rubber phasesand interfacial crosslinking between therubbers – and hence to improved physicalproperties and performance. Emphasis willbe placed on the former here, although itwill be shown that the two are not unre-lated.For crosslink distribution to be considered amatter for concern, it is implicit that theremust be the potential for a difference invulcanization rate prevailing in the rubberscomprising the blend. At its simplest, thismeans that there is a difference in concen-tration of the reactants, or a difference inreactivity of the sites for crosslink forma-tion or both.A difference in the concentration of thechemicals responsible for crosslink forma-tion can arise through preferential solubi-lity (partition) of the added curatives [48–51] and/or the vulcanization intermedi-ates. Differences in solubility parametersof the rubbers will cause both, and parti-tion will be further enhanced when thereis the possibility of specific interactions,such as hydrogen bonding or dipolar inter-actions in one of the rubbers. Whilst equi-librium concentrations of curatives in eachrubber might be expected to be achievedduring mixing of the curatives in a rubberblend, equilibrium is unlikely to beachieved for vulcanization intermediates.Indeed, phase size will dictate whether mi-gration of intermediates during vulcaniza-tion will play a significant role in dictatingthe eventual crosslink distribution. When

KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 10/2003 533

phase sizes are small and there is a concen-tration gradient relative to the concentra-tions dictated by the partition coefficientfor a particular vulcanization intermediate,the extent to which migration will occurwill be dictated by the balance betweendiffusion rate [44, 50–54] and rate of re-action with the host elastomer. Nonethe-less, some migration might be expected gi-ven that the time for diffusion of species ofthe size of vulcanization intermediates isabout 1 second for 1 lm [44, 52]. How-ever, when phase sizes are large, any mi-gration will only affect the outcome ofcrosslinking in a minor proportion of theblend close to the interface.The other reactant in crosslinking is thecrosslink site, or more commonly sites,on the elastomer. For sulphur vulcaniza-tion, these are associated with the unsa-turation in the elastomer. Some blendshave an obvious difference in concentra-tion of double bonds within the rubberphases, such as blends of the highly-unsa-turated natural rubber (NR) with ethylene-propylene-diene rubber (EPDM). However,significant differences in molar concentra-tion of double bonds also exist for blendsof what are normally considered to beequivalently highly unsaturated elasto-mers, such as blends of NR and cis-1,4-polybutadiene (BR). The concentration ofallylic sites is more relevant, since theseare the sites for crosslinking in sulphur vul-canization, and the reactivity of these sitesis also crucial.As the curatives or vulcanization inter-mediates become depleted in the faster re-acting rubber phase, diffusion from theslower reacting phase will occur. This willlimit the crosslinking in the latter phaseand increase it in the former, eitherthroughout when phase sizes are small,or close to the interface when they arelarge.The potential difficulties which can ariseduring vulcanization of rubber blends,due to the action of the factors discussedabove, have been appreciated for sometime. Gardiner estimated distribution coef-ficients for a number of curatives and com-binations of elastomers [48]. Although thework was undertaken in the context of plyadhesion, the relevance to the vulcaniza-tion of blends was noted and this topic re-ceived further attention later [44]. Aknowledge of the difficulties associatedwith vulcanizing blends of EPDM withdiene rubbers, such as NR and NBR, alsohas a similar long history. This led to inves-tigation of modifications to EPDM to en-

hance vulcanization [55–61] or of modifi-cations of the cure system – using curativesmore soluble in EPDM [50, 51, 62, 63] orinsoluble, immobile curatives [64, 65].Early studies of crosslink distribution andthe efficacy of attempts to manipulatethis relied on simple techniques such as ex-tractability of the rubbers from a vulcani-zate [44] or changes in physical properties.Over the years, numerous techniques havebeen proposed for estimating crosslinkdensities in vulcanized blends, most ofwhich have proved to lack foundation,or generality or sensitivity. The advent ofat least one independently verified techni-que has allowed both the accurate evalua-tion of crosslink distribution and informedstudies of control of this important para-meter, which has opened new prospectsfor improving the performance of vulca-nized blends. The techniques proposedfor the investigation of crosslink densitiesin vulcanized blends will be first reviewedbriefly before examining observations forblends typifying three categories – rubbersdiffering primarily in polarity, rubbers dif-fering primarily in degree of unsaturation,and rubbers differing little in either polarityor degree of unsaturation. The impact ofcrosslink distribution on properties willbe demonstrated.

Estimating crosslink densitiesin vulcanized blends

The procedures which have been claimedto give an indication of crosslinking in vul-canized blends are considered briefly be-low in chronological order; not all haveproved to be valid in any respect. Amore thorough review of these procedureswas made in 1995 [43].

Sol-gel analysis

For many years this simple procedure hasbeen used to gain an insight into crosslinkdistribution [44, 66], particularly in ex-treme systems, for which it can provide auseful indication of cure systems givingreasonable crosslinking in each phase.However, it is limited in utility and cannotprovide estimates of crosslink density.

Dynamic mechanical thermalanalysis (DMTA)

Huson et al. proposed that DMTA could beused to provide an indication of the relativedegree of vulcanization in each phase of arubber blend [67, 68]. The technique reliedon vulcanization in a custom mould

clamped in a Dupont 981 DMA. Theheight of the damping maximum asso-ciated with the glass transition of eachelastomer was observed to decrease as vul-canization progressed. The relative changein damping peaks due to NR and BR, or IRand BR, was interpreted in terms of cross-linking of the elastomers concerned. How-ever, workers in the same group subse-quently concluded that the decrease indamping was associated with loss of pres-sure in the mould due to shrinkage of theelastomers during vulcanization [69].

Differential scanningcalorimetry (DSC)

Three different procedures involving DSChave been proposed:Cure characteristics: DSC cure curves forblends were compared with those of theindividual elastomers in which the blendcuratives were distributed in a wide varietyof proportions [70]. Combined with cross-link density measurements by equilibriumswelling this was used to estimate crosslinkdensities in the individual phases of theblends. However, there is no direct mea-surement of individual crosslink densitiesof the rubber phases and the techniqueis time-consuming even for simple curesystems. The procedure relies on estimatesof “overall” crosslink density from equili-brium swelling of the blend and, presum-ably, use of a common polymer-solvent in-teraction parameter for the elastomersconcerned (NR/BR and IR/BR in this in-stance).Freezing point depression: The freezingpoint of a solvent is depressed when itswells a rubber vulcanizate to a degree de-pendent on the volume fraction of rubberpresent in the swollen gel [71], and hencecan be correlated with crosslink density viathe Flory-Rehner relationship [72]. It hasbeen proposed that this can provide thebasis for estimating crosslink densities inblends because the nucleation of crystal-lites is biased towards the higher swellingphase [69, 73]. There are number of draw-backs to this approach [43], including theneed to decide which phase is the morehighly swollen and a lack of sensitivity –and hence accuracy – at low to moderatecrosslink densities. Later work [74] sug-gested that a linear relationship betweenfreezing point depression and volume frac-tion of rubber in a swollen gel does not ne-cessarily hold. Recently [75, 76], IR/SBRblends have been studied using freezingpoint depression, together with swelling

534 KGK Kautschuk Gummi Kunststoffe 56. Jahrgang, Nr. 10/2003

and sol-gel measurements. Although theSBR phase crosslinks more slowly andthus would be the more swollen phase,it was proposed that the observed freezingpoint depression actually reflects the highcrosslinking in the surrounding IR phase,which restricts the swelling of the dis-persed SBR phase.Dependence of Tg on crosslinking: For anygiven cure system, the glass transition tem-perature (Tg) of a vulcanizate cured to op-timum is dependent on curative level used[77–80], and hence on crosslink density.Provided the Tgs of the elastomers in theblend may be resolved, the values ob-served may be used as an indication ofcrosslink density. However, Tg is not depen-dent solely on crosslink density, the natureof the cure system plays a role – includingthe efficiency (sulfur:accelerator ratio) of asulfur cure system. At a given crosslinkdensity, Tgs of NR vulcanizates rank inthe order: Conventional sulfur > semi-EV> EV � peroxide. Thus, changes in the ef-ficiency of the sulphur cure system operat-ing in a particular rubber phase due to dif-ferential partition of sulphur and accelera-tor or accelerator derived species will causeerrors in converting Tg to crosslink density.Furthermore, the dependence of Tg oncrosslink density is low, typically about1 8C/20 mol/m3 for NR, and even very care-ful DSC measurements provide rather un-certain estimates of crosslink density.

Stress-strain modelling

It has been suggested that the concept ofmechanical modelling commonly used instructural investigations of elastomer-ther-moplastic and thermoplastic blends maybe extended to the large strain deforma-tion behaviour of elastomer blends [81], in-

cluding black-filled blends [82]. The dis-persed phase must have a lower modulusthan the continuous phase. Crosslink den-sities have not been evaluated in this work.

Swollen-state NMR spectroscopy

This technique [43, 45, 83–85] is based onthe observation that signals in NMR spec-tra of polymers are broader than those ofsimple molecules, and that signal width in-creases as the polymer is crosslinked as aconsequence of a progressive reductionin chain mobility. Spectra of vulcanizatescannot normally be obtained using a li-quids NMR spectrometer, but swellingthe vulcanizate allows spectra to be ob-tained with sufficient resolution to identifysignals from protons in different environ-ments. The effect of crosslink density onsignal width is readily evident simply bycomparison of spectra (Fig. 1), but maybe quantified through estimation of signalwidth.This technique is the only one providing es-timates of crosslink densities in rubberblends that has been used by several inde-pendent groups and applied to a wide vari-ety of blends. Details of the technique havebeen reviewed [43], and only the essentialswill be presented here.The technique is not absolute; it relies oncorrelations of peak width in 1H or 13CNMR spectra with estimates of crosslinkdensity by established procedures suchas equilibrium volume swelling [72] orthe Mooney-Rivlin C1 constant derivedfrom stress-strain measurements [86]. Hav-ing established correlations between peakwidth and crosslink density for the elasto-mers used in a blend, spectra of vulcanizedblends may be analysed by a simple decon-volution procedure to obtain estimates of

peak width for signals from each elastomerand thence crosslink densities. An impres-sion of the reproducibility which may be at-tained can be gained from Fig. 2, whichdepicts the dependence of the measureof peak width, H%, on crosslink densityfor NR vulcanizates cured with a wide vari-ety of sulphur cure systems.Although the presence of carbon blackcauses additional peak broadening, it ispossible to apply the swollen-state NMRspectroscopy technique to black-filledblends [84, 87, 88]. Peak widths can be cor-related either with curative levels or withphysical crosslink densities obtained usinga modified stress-strain measurement[89]. Since peak width is affected by bothblack loading and particle size, althoughapparently not structure [84], it is necessaryto have calibration curves correlating peakwidth and crosslink density for each combi-nation of elastomer, grade and loading ofcarbon black of interest. It is also necessaryto prepare the blends by crossblendingwell-mixed black masterbatches to ensurethat each phase of the blend contains therequisite level of carbon black.

Network visualization microscopy

Vulcanizate is swollen to equilibrium withstyrene, which is then polymerized. The re-sulting composite, which can be termed asemi-inter-penetrating network (IPN), issectioned, stained with osmium tetroxideto provide contrast, and viewed by trans-mission electron microscopy [90–92]. Aconstrained phase separation occurs dur-ing polymerization of the styrene, resultingin a mesh structure comprising bundles ofnetwork chains (Fig. 3 and 4). The meanmesh size is linearly correlated with mole-cular weight between crosslinks [91, 92]

Fig. 1. 300 MHz 1H NMR spectra of swollen, gum NR vulcanizates withlow (37.5 mol/m3) and high (100 mol/m3) crosslink densities

Fig. 2. Dependence of olefinic peak width, H%, on physical crosslinkdensity for gum NR vulcanizates (300 MHz NMR)


(Fig. 5). Although the technique has beenapplied to blends (Fig. 4), and reasonableaccord observed with estimates made byswollen-state NMR spectroscopy [91, 92],accuracy is not good at normal technolo-gical crosslink densities and the procedureis time-consuming.The technique also has potential to provideinsight into crosslinking at the interface invulcanized blends. The phase separationprocess will exploit weaknesses in a vulca-nizate, such as around zinc oxide particles,and regions of pure polystyrene will form –as evident in Fig. 3. If there is inadequatecrosslinking between the elastomers inthe interfacial region, similar accumula-tions of polystyrene will occur as shownin Fig. 4a.

Isopotential swelling

Elastomers to be used in a blend are firstcured separately and a solvent whichswells both to the same extent is identified.The swelling behaviour of blends in thissolvent is then investigated over the entireblend composition range [93]. In a plot ofdegree of swelling versus blend composi-tion, departures from linearity indicatethat cure interaction between the phaseshas occurred. Among other difficulties,this approach does not provide quantita-tive data.Differential swelling of blend vulcanizates,using solvents which swell one elastomermuch more than the other, has alsobeen used to determine crosslink densities[73]. However, as discussed in Section 3,these measurements are affected by inter-facial crosslinking [29, 35, 78, 94–95]. In-deed, when crosslink densities are known,

differential swelling provides a measure ofthe interfacial crosslinking [78, 95].

Solid-state NMR spectroscopy

High resolution solid-state NMR spectro-scopy has been applied to provide informa-tion on crosslinking in single rubber vulca-nizates, but has yet to provide crosslinkdensity information for vulcanized blends.A combination of spin-spin relaxation andhigh resolution NMR may provide a meansof doing so; this is analogous to the mea-surements performed in swollen-stateNMR spectroscopy. High resolution solid-state NMR spectroscopy has been used re-cently in an endeavour to study crosslink-ing between the rubbers of a NR/BR blend[32]. A new approach, comparing spectraof a blend and a sample comprisingstacked thin sheets of NR and BR vulcani-zates, was employed. Although a signalnot present in spectra of the stacked sam-ple was seen in spectra of the blend, it wasnot possible to conclude with certainty

that this represented crosslinks betweenNR and BR.

NMR imaging

NMR imaging can map local variations incrosslink density [96,97], but the spatial re-solution is insufficient for the phase sizesnormally expected in blends.

Atomic force microscopy (AFM)

AFM is sensitive to crosslink density [98,99] and measurements can be made ona very fine scale – at a spacing of170 nm [98]. Differences in crosslink den-sity between the phases of a blend can beimaged and measured semi-quantitatively[99].

Blends of rubbers differingmainly in polarity

The most extensively studied blends fallinginto this category are blends of NR with ni-trile rubber [poly(acrylonitrile-co-buta-

Fig. 3. TEM micrograph of a sulphur/TMTMcured low crosslink density NR gum vulcani-zate prepared for “network visualization”

Fig. 4. “Network visualization” TEM micrographs of NR/NBR41 blends showing poor (4a), good(4b) and excellent (4c) interfacial crosslinking


diene), NBR], and there have been numer-ous reports of crosslink distribution forblends covering a range of acrylonitrilecontents in the NBR from 18 % to 41 %[78, 83,91, 92, 95, 100–102]. WhilstNBR may appear to have a substantiallylower level of unsaturation relative toNR, due to being a copolymer, in practicethe higher density of NBR and lower mo-lecular weight of the butadiene repeatunit lead to a molar concentration of un-saturation of about 11 � 103 mol/m3 forhigh acrylonitrile NBR in comparison withabout 13 � 103 mol/m3 for NR. The pri-mary influence on crosslink distribution istherefore the difference in polarity of thetwo elastomers and its effect on distribu-tion of curatives and vulcanization inter-mediates.Much of the data available have been re-viewed previously [43]; only the followingwill be considered here: the effect ofchoice of accelerator on crosslink distribu-tion, the effect of acrylonitrile content oncrosslink distribution given by a particularcure system, the effect of phase size oncrosslink distribution, and the effect ofcrosslink distribution on properties and in-terfacial crosslinking.Sulphur will always distribute in favour ofthe NBR phase due to its high solubilityparameter (29.8 MPa1/2). The solubilityparameter of NR is 16.7 MPa1/2, whilstthose for NBR lie between 17.8 MPa1/2

and 21.3 MPa1/2. Control of crosslink dis-tribution will therefore depend largely onhow the accelerator(s), and vulcanizationintermediates, partition between the rub-bers.An extreme example is provided by NBRswith acrylonitrile contents of 18 % and41 % (NBR18 and NBR41 respectively)cured with cure systems containing related

accelerators differing greatly in polarity –TMTD and N,N’-dioctadecyl-N,N’-diisopro-pylthiuram disulphide (ODIP) [101]. Cross-link densities as determined by swollen-state NMR spectroscopy are presented inFig. 6. It should be noted that the twothiuram accelerators were used at equimo-lar levels.The impact of both choice of acceleratorand acrylonitrile content of the NBR is im-mediately apparent. The highly polarTMTD is clearly a poor choice of accelera-tor for NR/NBR blends – the extreme inba-lance of crosslinks in favour of the NBRphase may be attributed to partition ofboth sulphur and TMTD in favour ofNBR. When TMTD is replaced by the lesspolar ODIP, the imbalance in crosslink dis-tribution is reduced in NR/NBR18 blendsthrough a doubling of crosslink densityin the NR phase. This may be attributedto an increase in concentration of accelera-tor in the NR phase. A greater increase inNR crosslink density is seen in NR/NBR41blends, and this is accompanied by a dra-matic decrease in crosslinking of the NBRphase; there is a substantial reduction inoverall crosslink density. This may be ex-plained by the NBR phase containing themajority of the sulphur due to a favourablepartition coefficient, but the NR phase con-taining most of the accelerator. The large

phase sizes in this blend (> 20 lm) pre-clude diffusion of vulcanization intermedi-ates playing a significant role in determin-ing crosslink distribution.This explanation receives support from aconsideration of the type of crosslinks pre-sent in each phase, as determined by acombination of chemical probe treatment(thiol-amines [103, 104]) and swollen-state NMR spectroscopy [101]. The data(Tab. 1) show a decrease in efficiency ofvulcanization in the NBR phase of NR/NBR18 blends and an increase in efficiencyin the NR phase of NR/NBR41 blends whenODIP is substituted for TMTD.Much smaller changes in both acceleratorand acrylonitrile content can have a signif-icant effect on crosslink distribution, as isillustrated in Fig. 7, which depicts datafor blends of NR with NBR41 and NBRwith an acrylonitrile content of 34 %(NBR34) vulcanized with cure systems con-taining the three most common sulphena-mide accelerators at equimolar levels [95].Despite the close similarity of the accelera-tors, the different amine substituents giverise to different crosslink distributions.Likewise, the difference in acrylonitrilecontent, and hence solubility parameter,is sufficient to cause a marked differencein crosslink distribution. For blends withNBR41, TBBS may be considered to be

Tab. 1. Percentage of each type of sulphidic crosslink in the NR and NBR phases of 50:50 NR:NBRblends cured with 1.5 phr sulphur and either 0.6 phr TMTD or 1.93 phr ODIP (tmax at 150 8C)

Crosslink NBR18 NBR41

TMTD ODIP TMTD ODIP

NBR NR NBR NR NBR NR NBR NR

Poly- 14 100 39 100 24 100 26 22

Di- 41 – 22 – 26 – – 78

Mono- 45 – 39 – 50 – 74 –

Fig. 5. Dependence of mean mesh size in “network visualization” micro-graphs of NR vulcanizates on molecular weight between crosslinks, Mc

Fig. 6. Crosslink distribution in gum vulcanizates of 50:50 NR:NBR blends[101]


ideal in that it gives an even distribution ofcrosslinks, whereas there is a severe inba-lance of crosslinks in favour of the NBR inblends with NBR34.The data in Tab. 2 serve to illustrate boththe susceptibility of crosslink distributionto cure system and the effect of crosslinkdistribution on properties, in the form oftensile strength. Even very low levels ofTMTM, commonly used as a secondary ac-celerator to increase cure rate, or ODIPcause substantial changes in crosslink dis-tribution and tensile strength.The tensile strength which may be attainedin unfilled 50:50 NR/NBR41 blends, whenboth phases are crosslinked well, is re-markable – particularly given the largephase sizes in the blends. This suggestsgood integrity in the blend, and henceadequate crosslinking across the interface.Network visualization microscopy providessome support for this [91, 92, 95]. Whenthere is an imbalance of crosslinks be-tween the phases and tensile strength islow, failure at the interface occurs duringthe preparation of samples for this micro-scopy technique. This is evident as large re-gions of pure polystyrene up to 200 lm indiameter at the interface, the white areasin Fig. 4a. When there is a near even dis-tribution of crosslinks, such failure at theinterface is either very limited, as inFig. 4b, which shows a thin (about30 lm thick) band bridged by network ma-terial, or does not occur at all (Fig. 4c).Access to crosslink density data for vulca-nized blends allows correct interpretationof differential swelling data [78]. Thismethod of investigating the strength ofthe interfacial region was first proposed

by Zapp [29, 94], and involves comparisonof equilibrium swelling of a vulcanizedblend as given by the volume fraction ofrubber in the swollen gel (tr) with the swel-ling expected from a vulcanizate in the ab-sence of the non-swelling rubber phase(tro). A solvent is chosen which swellsthe major one well and the minor onepoorly. If the interface is strong, tro/tr

will be < 1 due to restriction of swellingby the non-swelling rubber phase, but ifthe interface is weak and failure occurs un-der the stresses generated in differentialswelling, tro/tr will be � 1 due to solventcollecting at the interface. There is a corre-lation between tro/tr and tensile strength/blend composition [95] as illustrated inFig. 8. The NR-rich vulcanizates have lowertro/tr and hence better interfacial adhesion

than the NBR-rich vulcanizates. The highvalues of tro/tr (in NBR-rich blends) arealso associated with low overall crosslinkdensities, as this declines with increasingNBR content. Increasing the curative level,and hence the overall crosslink density, re-duces tro/tr (Tab. 3). It would seem thatcrosslinking between the two rubbers isadequate provided that both phases areadequately crosslinked, despite the verylimited mixing at the interface to be ex-pected in two rubbers differing so muchin solubility parameter.Phase size also plays a part in controllingcrosslink distribution in a blend of elasto-mers differing greatly in solubility para-meter. In the absence of compatibilizer,such blends normally have large phasesizes. However, compatibilizers are often

Tab. 2. Effect of crosslink distributionz on tensile strength of gum 50:50 NR:NBR (41 % acry-lonitrile) blends [100]

Accelerator(s)†, phr nphys NR : nphys NBR Tensile strength, MPa

TBBS, 1.17 1.01 26.8

TBBS, 1.17/TMTM, 0.1 0.67 17.8

TBBS, 1.17/ODIP, 0.37 1.75 19.8

z As given by the ratio of physical crosslink densities, nphys, in the two rubbers.† With 1.3 phr sulfur.

Fig. 7. Crosslink distribution in 50:50 NR:NBR blends cured at 150 8C with1.3 phr sulphur and 1.3 phr CBS, or 1.24 phr MBS, or 1.17 phr TBBS

Fig. 8. Dependence of tensile strength of NR/NBR41 gum blend vulcani-zates on swelling restriction ratio, ?ro/?r: * 75:25 NR:NBR, ^ 65:35NR:NBR, ~ 35:65 NR:NBR, & 25:75 NR:NBR

Tab. 3. Effect of curative level on swelling restriction ratio for gum NR/NBR41 blend vulca-nizates

NR:NBR vro/vr

1.3phr sulphur 3phr sulphur

35:65 0.943 0.913

35:65 1.02 0.905

25:75 1.21 0.936


used to reduce phase size and improvephysical properties. Polychloroprene (CR)is effective in NR/NBR blends [102], butin addition to reducing phase size thereis a marked change in crosslink distribution(Tab. 4), which is difficult to attribute to thepresence of the low level of CR used in theblend (5 phr). The smaller phase size hasreduced the imbalance of crosslink densityin favour of NBR, presumably by allowingdiffusion of vulcanization intermediatesbetween the phases to play a greaterrole in the vulcanization process.

Tab. 4. Effect of compatibilizer, CR, on crosslink distributionz in NR/NBR34 blends [102]

Cure system (phr) nphys NR : nphys NBR

NR:CR:NBR3450:0:50

NR:CR:NBR3450:5:45

S/CBS (1.3/1.3) 0.41 0.59

S/CBS/TETD (1.3/1.3/0.14) 0.39 0.54

S/CBS/TBTD (1.3/1.3/0.2) 0.47 0.80

z As given by the ratio of physical crosslink densities, nphys, in the two rubbers.

Tab. 5. Effect of modification of EDPM with N-chlorothioamide on physical properties of 70:30IR/EPDM blendz [56]

Property Unmodified EPDM Modified EPDM†

Rheometer Dtorque, Nm 6.52 7.73

M300, MPa 12.9 14.3

Tensile strength, MPa 17.7 22.8

Elongation at break, % 400 450

z Blends of Natsyn 200 with Nordel 1470 containing 50 phr FEF black, 4 phr ZnO, 1.5 phr stearic acid, 1phrphenolic antioxidant, 2 phr sulphur, 1 phr MBS.

† Modified with 0.14 mol/kg N-chlorothio-N-methyl-p-toluenesulphonamide.

Tab. 6. Effect of modification of EDPM with maleic anhydride on physical properties of 70:30NR/EPDM blendz [58]

Property Unmodified EPDM Modified EPDM†

M300, MPa 7.7 8.0

Tensile strength, MPa 14.8 23.3

Elongation at break, % 500 602

Fatigue life:

0–100% Strain, kcs 26 46

0–10kg/cm2 Energy, kcs 18 41

z Blends of SMR5 with Epsyn 70A containing 50 phr N326 black, 10 phr oil, 5.5 phr ZnO, 2 phr stearicacid, 2 phr sulphur, 0.5 phr TBBS.

† Modified with 2 % maleic anhydride.

Tab. 7. Effect of modification of EPDM on crosslink density in 70:30 NR:EPDM blends [105, 106]

Modification N-Chlorothioamide Maleic anhydride

None 0.114 mol/kg None 2wt%

EPDM nphys, mol/m3 6.8 25.7 11.0 25.6

NR nphys, mol/m3 95 92 61/56* 52/53*

* Reference [107].

The advances made in the understandingof vulcanization of NR/NBR blends and itseffect on properties have led to the devel-opment and commercial uptake of formu-lations for milking inflations.

Blends of rubbers differing pri-marily in degree of unsaturation

The classic example of this type of blend isNR with EPDM, and the great commercialpotential of this system has resulted in nu-merous attempts [50, 51,55–59, 61] to

overcome the inherent difficulties asso-ciated with vulcanizing two elastomers dif-fering so much in unsaturation. It shouldalso be recognized that there will be a ten-dency for curatives and vulcanization inter-mediates to partition in favour of the NRphase [2, 3]; indeed the use of dithiopho-sphate accelerators, which have high solu-bility in both NR and EPDM, has beenfound to lead to improved blend properties[50, 51, 61].Success in increasing crosslinking in theEPDM phase was generally inferred froman improvement in physical properties,particularly modulus and tensile strengthas illustrated in Tab. 5 and 6, which sum-marize results obtained by Hopper whenmodifying EPDM with N-chlorothioamides[56] and Coran when modifying EPDMwith maleic anhydride [58]. Although thetwo approaches are different, the formeraiming to enforce sulphur vulcanizationin the EPDM by attaching a pendent pre-vulcanization inhibitor and the latter aim-ing to introduce a second, ionomeric net-work in the EPDM, the net result is similar.Subsequently, both of these approacheshave been examined further by swollen-state NMR spectroscopy to allow an assess-ment of the extent to which the EPDM iscrosslinked in a blend with NR [105]. Giventhe marked effect on physical properties,the results are very interesting; the cross-link density attained in the EPDM is modestin both cases (Tab. 7), compared to that inthe NR phase. This encouraged investiga-tion of modification by reactive mixing andled to practical procedures for modificationof EPDM with sulphur donors during nor-mal masterbatch mixing [106, 108–111].Although levels of modification are low,crosslink densities attained by the modifiedEPDM during sulphur vulcanization ofblends with NR are of the order notedabove – and there are commensurate in-creases in modulus and tensile strength(Tab. 8). This technology has been provedin full formulations mixed in a factory in aNo. 11 Farrel Banbury mixer [111]. A num-ber of other properties, such as abrasionresistance and resilience, are improved inblack-filled blends, but other factors aris-ing from the modification and consequentimprovement of EPDM-black interactionwill also play a part in inducing thesechanges.Similar levels of crosslinking may be at-tained in NR/EPDM blends if the EPDMhas very high ENB level [112] and is alsoof very high molecular weight [113–115], as shown in Tab. 9. The effect of


both ENB level and molecular weight isconfirmed by a swollen-state NMR studywhich did not go to the lengths of calibrat-ing peak width against crosslink density[116]. Nonetheless, the trends of increas-

ing peak width for signals arising fromEPDM – and hence crosslink density –with increasing ENB content or molecularweight, as indicated by Mooney viscosity,are evident (Tab. 10).

The use of a hybrid accelerated sulphur/peroxide cure has also been advocated[117, 118]. Although some partitioningof the peroxide is to be expected, any per-oxide in the EPDM phase will result incrosslinking of the EPDM. Only low levelsof peroxide will be necessary to inducethe moderate crosslink density known tobe needed for good properties, and0.6 phr dicumyl peroxide has been foundto give improvements in cut growth anddynamic ozone resistance. This approachhas parallels with that of Coran [58], inthat the crosslinks formed in the EPDMmay be expected to be predominantlynot sulphidic in nature. Recent studieshave indicated that satisfactory blendproperties can be achieved if an EPDMwith high ethylene content is used [18,119]; the importance of filler distributionwas also stressed.Blends of NR or SBR with chloro- or bromo-butyl rubber (CIIR or BIIR) have been widelyinvestigated because of their commercialimportance in tyre inner liners [29, 35,44, 81, 120–122]. Crosslink distributionshave not been determined, although theNR phase would crosslink more rapidlythan the CIIR, which can lead to curativediffusion from the CIIR and the formationof a highly crosslinked layer in the NRphase adjacent to the interface [122].Much evidence has been advanced to in-dicate that CIIR, when cured appropriately,will form interfacial crosslinks with highlyunsaturated rubbers [23, 29, 35, 44].

Blends differing little inpolarity or unsaturation

These blends are exemplified by blends ofthe general purpose rubbers – NR, BR andSBR. Of these, blends of NR or IR and BR

Fig. 9. Dependence of NR:BR crosslink density ratio on rheometer curetime for 70:30 NR:BR blends vulcanized at 150 8C with conventional curesystems [106, 128]

Fig. 10. Dependence of NR:BR crosslink density ratio on rheometer curetime for 50:50 NR:BR blends vulcanized at 150 8C with semi-EV cure sys-tems [128]

Tab. 8. Crosslink densities in and tensile properties of 60:40 NR:EPDM blends vulcanized with2 phr sulphur/0.6 phr CBS [108]

EPDMmodifier

nphys, mol/m3 M100, MPa Tensilestrength, MPa

Elongation atbreak, %

NR EPDM

None 65.5 12.5 0.91 16.9 645

BAPDz 85 20 0.95 23.2 705

DTDC† 82 20.5 0.92 21.7 730

DTDM§ 90 19 0.96 21.8 685

z Bis-alkylphenoldisulphide.† Dithiodicaprolactam.§ Dithiodimorpholine.

Tab. 9. Crosslink densities in 60:40 NR:EPDMz blends cured to optimum (t95þ 5) and overcuredat 166 8C (2 phr sulphur, 0.5 phr MBS) [113]

Cure time, min 12 30

NR nphys, mol/m3 61 47

EPDM nphys, mol/m3 25.5 25

z Polysar experimental polymer: 10.5wt% ENB, Mooney viscosity ML(1þ 4) at 150 8C ¼ 70.

Tab. 10. Peak width at half height of NR and EPDM signals in 13C NMR spectra of swollenvulcanizatesz [116]

Material EPDM W1/2, Hz, at chemical shift, ppm

Ethylene,wt%

ENB,wt%

Mooneyviscosity

NR32.5

NR26.5

EPDM37.5

EPDM33

EPDM27.5

NR – – – 20 19 – – –

EPDM 52 8 63 – – 80 91 82

Blend 52 8 63 26 25 26 16 20

Blend 52 8 46 24 23 19 15 15

Blend 52 8 33 27 26 15 12 11

Blend 55 4 63 24 23 20 17 17

Blend – 24 62 23 23 25 23 24

z Cured with 1.5 phr sulphur/0.5 phr MBT/1 phr TMTD; blends are 70:30 NR:EPDM.


have received most attention. At first sight,these elastomers would appear to differ solittle that it might be anticipated that aneven distribution of crosslinks would bethe norm. In practice, there are significantdifferences, and not those which may beinferred from a simple comparison of therheometer cure behaviour of comparablecompounds of the two; this shows theNR to cure much quicker, but the naturallyoccurring cure activators and acceleratorsmight be expected to partition fairly evenlybetween the two rubbers once they areblended, and so NR would lose this advan-tage.A deeper consideration of the rubbers andthe literature points to BR being likely tocrosslink preferentially in a blend withNR. Both sulphur and the common sulphe-namide accelerators will partition slightlyin favour of the BR [48, 124]. Moreover,it has been argued that the unsaturationin BR may be more reactive towards sul-phur vulcanization [125]. The concentra-

tion of double bonds is also greater forBR, about 17 mol/dm3 versus about13 mol/dm3 for NR.The first reports of crosslink density distri-bution for NR/BR blends cured with sul-phur/sulphenamide or sulphur/TMTDwere in accord with this prediction: theBR was the more highly cured phase [69,126]. Subsequently, a study of IR/BR blendsthrough the cure by swollen-state NMRspectroscopy indicated that, whilst theBR phase was the more highly crosslinkedat optimum cure, crosslinks formed prefer-entially in the IR phase in the early stages ofvulcanization [127]. However, a later re-port of the vulcanization of NR/BR blendswith conventional and semi-EV cure sys-tems based on the three most commonsulphenamide accelerators indicated thatthe BR phase begins to cure before theNR phase at 150 8C, and that the lattertends not to catch up (Fig. 9 and 10)[128]. This observation was supported bynetwork visualization microscopy [128]. Si-

milar behaviour has been seen for vulcani-zation at 175 8C [129, 130]. However, at140 8C there is little difference betweenthe two phases, and the NR phase isseen to crosslink first at 130 8C (Fig. 11)[130]. As noted above, there are reasonsto believe that preferential vulcanizationof BR should occur. However, if the prefer-ential vulcanization of NR at lower tem-peratures is accepted, incursion of radicalcrosslinking of BR at higher temperatureswould explain the observed behaviour(Fig. 12).The question remains as to whether chan-ging the crosslink distribution will improvethe properties of NR/BR blends. Fig. 13shows how the crosslink density distribu-tion in a 70:30 black-filled vulcanizatecan be adjusted by modification of oneof the phases prior to crossblending[131]. This altered crosslink distributionled to improved passenger tyre wear per-formance, as shown in Fig. 14. In a very re-cent study of unfilled NR/BR blends [124],

Fig. 11. Physical crosslink density in NR and BR phases of 70:30 NR:BRblend vulcanized with 2 phr sulphur/1 phr CBS at 130 8C

Fig. 12. Dependence of NR and BR crosslink densities in 70:30 NR:BRblends on cure temperature (2 phr sulphur/1 phr CBS, vulcanized to rhe-ometer tmax) [130]

Fig. 13. Adjustment of the crosslink distribution in a 70:30 black-filledNR/BR vulcanizate by “modification” of one of the phases; semi-EV sul-phur/CBS system cured at 150 8C

Fig. 14. The effect of a “modified” crosslink distribution on acceleratedpassenger tyre wear: 70:30 NR:BR tread compounds cured using a semi-EV sulphur/CBS system at 150 8C


crosslink density distributions were not de-termined, but it was found that promotionof crosslinking in the NR phase (by incor-porating the sulphur, zinc oxide and stearicacid in the NR before crossblending) led toreduced tensile strength and elongation atbreak. However, all of the reported tensilestrengths (of both the blends and the indi-vidual rubbers) were much lower than nor-mally expected for these rubbers.IR/BR blends with varying contents of 1,2-structures in the BR have also been studied[132, 133]. It was concluded that crosslinkdensity in the BR phase decreased with in-creasing content of 1,2-structures.IR/SBR blends have recently been investi-gated by Mallon and McGill [75, 76,134], using freezing point depression,swelling and sol-gel measurements. Theyconcluded that more rapid crosslinking inthe IR phase leads to depletion of curativesin the IR and their diffusion from the SBRphase, creating layers of highly crosslinkedIR surrounding the dispersed SBR phase.Reduced tensile strengths in the blendsare attributed to failure occurring in thesehighly crosslinked layers.

Summary

It may be concluded that uneven distribu-tion of crosslinks between the rubbers of ablend is the norm; an even distributionmust be worked for. Differences in polarity,degree of unsaturation and reactivity ofelastomers all conspire to favour crosslink-ing of one elastomer over the other in ablend.Differences in polarity lead to partition ofcuratives, but careful selection of accelera-tors can offset the consequences of thetendency for sulphur to partition in favourof the more polar elastomer. Whilst a simi-lar degree of crosslinking may be obtainedin the rubbers of a blend by this means, it islikely that the efficiency of vulcanizationwill be different in each rubber – and dif-ferent from that expected on the basis ofthe overall sulphur and accelerator levelsused. The large phase sizes which resultfrom a substantial difference in polarityof the rubbers in a blend will preclude dif-fusion of vulcanization intermediates fromplaying a significant role in determiningcrosslink distribution. The successful useof compatibilizers to reduce phase sizemay allow diffusion of vulcanization to be-come a factor, and can change the appro-priateness of a cure system. Obtaining ade-quate crosslinking in both rubbers of a

blend will help in ensuring adequate cross-linking between them at the interface.The consequences of substantial differ-ences in degree of unsaturation are diffi-cult to overcome. Here, diffusion of vulca-nization intermediates probably plays animportant role in defining the eventualcrosslink distribution. Some means ofeither forcing more sulphur vulcanizationto occur in the less reactive rubber or ofproviding a second network there is neces-sary in order to increase crosslinking in thelow unsaturation rubber. However, it hasbeen shown that only modest increasesin crosslinking of the EPDM in NR/EPDMblends is necessary to give substantial im-provements in a range of physical proper-ties.Even blends which are not considered tobe problematic and which appear to differlittle in either polarity or degree of unsa-turation, such as NR/BR blends, havebeen shown to suffer uneven crosslink dis-tributions in sulphur vulcanization. Im-provements in properties have beenachieved by manipulating the crosslink dis-tribution.Although emphasis has been placed hereon evidence gained in studies of gum vul-canizates, there have been a few studies ofblack-filled blends. These have shown thatcarbon black does not significantly affectthe crosslink distribution [87, 88],although there have been no studies pub-lished in which the carbon black was dis-tributed unevenly between the phases.Control of crosslink distribution is impor-tant if the best is to be obtained from vul-canized blends. Application of the princi-ples described here has provided improve-ments in physical properties and allowedsuccessful use of blends which haveproved problematic in the past.

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The authors

Dr. Andrew Chapman is a Principal Scientist andHead of the Advanced Materials and Product De-velopment Section at the Tun Abdul Razak Re-search Centre (TARRC), Brickendonbury, Hertford,SG13 8NL, UK. Dr. Andrew Tinker is the Directorof Research at TARRC.

Corresponding author:A. V. ChapmanTun Abdul Razak Research CentreBrickendonbury, Hertford, SG13 8NL,Großbritannien


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