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Homogeneous Styrene Butadiene/AcrylonitrileButadiene Rubber BlendsS. H. Botros a , A. F. Moustafa a & S. A. Ibrahim aa National Research Center, Polymers Department , Dokki, Cairo, EgyptPublished online: 15 Feb 2007.

To cite this article: S. H. Botros , A. F. Moustafa & S. A. Ibrahim (2006) Homogeneous Styrene Butadiene/AcrylonitrileButadiene Rubber Blends, Polymer-Plastics Technology and Engineering, 45:4, 503-512, DOI: 10.1080/03602550600553705

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Homogeneous Styrene Butadiene/Acrylonitrile ButadieneRubber Blends

S. H. Botros, A. F. Moustafa, and S. A. IbrahimNational Research Center, Polymers Department, Dokki, Cairo, Egypt

The graft copolymerization of acrylonitrile (AN) onto butadienerubber (BR) was carried out in toluene at 80�C, using dibenzoyl-peroxide (BPO) as initiator. The synthesized poly acrylonitrile-grafted-butadiene rubber (AN-g-BR) was characterized by N%elemental analysis and Fourier-transform infrared (FT-IR) spec-troscopy. Styrene butadiene rubber/acrylonitrile butadiene rubber(SBR/NBR) blends were prepared with different blend ratios inpresence and absence of AN-g-BR, where the homogeneity of suchblends were examined with intrinsic viscosity (g) measurements, dif-ferential scanning calorimetry (DSC), and scanning electronmicroscopy (SEM). The scanning electron micrographs illustratedisappearance of the macro-scale phase separation of SBR/NBRrubber blend as a result of the incorporation of AN-g-BR into thatblend. Viscosity measurements confirm homogeneity of that blend.Differential Scanning Calorimetry traces exhibit shifts in glass tran-sition temperatures (Tg’s) of SBR and NBR in their blend, indicat-ing some degree of homogeneity. Physico-mechanical properties ofthe rubber blend vulcanizates with different blend ratios, in presenceand absence of AN-g-BR, were investigated before and after accel-erated thermal aging. The SBR/NBR (25/75) homogeneous blendpossessed the best physico-mechanical properties after thermalaging, together with the best swelling behavior in motor oil. Thephysico-mechanical properties of SBR/NBR (25/75) filled blend withdifferent types of inorganic fillers during thermal aging were studied.

Keywords Styrene butadiene rubber; Nitrile rubber; Rubberblend; Graft-copolymerization; AN-g-BR; Homo-geneity

INTRODUCTION

Blends of immiscible polymers obtained by simple mix-ing ultimately show generally poorer properties than theirindividual constituents. This fact is the result of a strongseparation tendency of immiscible components, leading to acoarse phase structure and low interfacial adhesion. On theother hand, immiscibility or limited miscibility of polymersenables formation of various supermolecular structures.

Some of these heterogeneous structures, if stabilized,can impart excellent properties to the final material, hardly

attainable by any other way. It is possible to obtain such astabilized phase structure by bond formation at thepolymer=polymer interface. This procedure, usually calledcompatibilization, generally leads to a finer phase structureand enhanced interfacial adhesion[1].

The use of graft or block copolymers as compatibilizersfor immiscible polymer blends has become an increasinglypopular subject of study in recent years, because it is one ofthe simplest and most efficient means for development ofnew high-performance polymer materials. Usually, suitablychosen graft or block copolymer, whose segments may bechemically identical with those in the respective phases ormiscible with one of the phases, can act as an interfacialagent to reduce interfacial tension and to improve inter-facial adhesion of the immiscible components. However,compatibilizers located only at the interfacial region mayhave different compatibilizing effects from those connect-ing two immiscible components by different chains[2]. Theselection of a copolymer as a compatibilizer for a polymerblend is important to ensure better compatibility. The com-patibilizer must fulfill certain requirements: a) ensure finedispersion during mixing, b) be preferentially located atthe interface between phases, c) provide a stabilizationeffect against gross separation during processing, and d)improve adhesion between blend phases.

The compatibilizer may form an interphase between theimmiscible blend components, so that imposed stresses canbe transferred between the phases via the covalent bondsalong the copolymer backbone. However, understandingof blend morphology is important because the propertiesof polymer blends are strongly dependent upon it[3,4]. Ithas also been observed that the procedure used to preparecompatibilized blends has a significant effect on their mor-phology and ultimate mechanical properties[5,6]. The degreeof compatibility of rubber blend plays an essential role inthe applicability of the final product in many industrialfields[7]. Blending of styrene butadiene rubber=acrylonitrilebutadiene rubber (SBR=NBR) leads to an incompatibleblend. The degree of compatibility of that blend has beenstudied using ultrasonic methods[8]. Graft copolymers have

Address correspondence to Dr. S. H. Botros, NationalResearch Center, Polymers Department, Dokki-12622, Cairo,Egypt. E-mail: sh50 botros@hotmail.com

Polymer-Plastics Technology and Engineering, 45: 503–512, 2006

Copyright # Taylor & Francis Group, LLC

ISSN: 0360-2559 print/1525-6111 online

DOI: 10.1080/03602550600553705

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also been used as compatibilizers in immiscible rubberblends. Investigators have produced graft copolymers bythe free radical copolymerization technique[9].

Researchers have focused on the compatibilization ofSBR=NBR blends with dichlorocarbene-modified styrene-co-butadiene rubber (DCSBR) as a function of concen-tration of compatibilizer and composition of the blend.To prepare DCSBR, alkaline hydrolysis of chloroformwas performed using cetyltrimethylammonium bromideas a phase-transfer agent. Fourier-transform IR studies,differential scanning calorimetry, and dynamic mechanicalanalysis revealed molecular-level miscibility in the blendsin the presence of the compatibilizer. The formation ofinterfacial bonding was assessed by analysis of swellingbehavior, cure characteristics, stress-strain data, andmechanical properties. These studies showed that the com-patibilizing action of DCSBR became more prominent asthe proportion of NBR in the blend increased. The resist-ance of the vulcanizate toward thermal and oil agingimproved with compatibilization. The change in techno-logical properties was correlated with crosslink density ofthe blends assessed from swelling and stress-straindata[10,11]. The effect of 5 phr polyester on the mechanicaland physical properties of SBR=NBR rubber blend wasstudied. The ultrasonic velocity and attenuation wereinvestigated for both types of rubber with and withoutpolyester resin. It was found that the apparent activationenergy of the main relaxation process was temperaturedependent and increased with the addition of polyesterresin[7]. Unsaturated polyester (UPE) resin was added tothe SBR=NBR blend as a compatibilizer. The degree ofcompatibility was enhanced by the addition of 10 phrUPE to the blend12. Mechanical properties and AC impe-dance analysis on dual-phase polymer electrolytes (DPE)prepared from NBR=SBR mixed lattices have beenreported[13,14]. In the present work, grafting of butadienerubber (BR) with acrylonitrile (AN) using a chemicalmethod was carried out. The application of AN-g-BRproduced as a compatibilizer for SBR=NBR blends arediscussed. Physico-mechanical properties of the homo-geneous SBR=NBR blend with different blend ratios wereevaluated before and after accelerated thermal aging. Theswelling resistance of SBR=NBR blend vulcanizates withdifferent blend ratios in both toluene and motor oil wasassessed in the presence and absence of AN-g-BR. Theeffects of different types of inorganic fillers on the physico-mechanical properties of SBR=NBR (25=75) vulcanizatesbefore and after accelerated thermal aging were investigated.

EXPERIMENTAL

Materials

Acrylonitrile monomer, a product of Merck, Darmstadt,Germany; was vacuum distilled twice before use.

Dibenzoyl-peroxide (BPO), a product of Acros, New Jersey,U.S.A, was recrystallizated from chloroform=methanol(50=50) v=v twice. Toluene, chloroform, and methanol(analytical grade) products of El Nasr Chemical Company,Cairo, Egypt, were used as received. Dimethylsulfoxide(DMSO), tetrahydrofuran (THF), acetone, and anhydroussodium sulfate, products of Riedel de Haen, Seelze,Germany, were of analytical grade and used as received.Butadiene rubber (BR) of 35 Mooney viscosity [ML

(1þ 4) at 100�C] and NBR (Krynac 3450) of 34% acrylo-nitrile content and 50 Mooney viscosity [ML (1þ 4) at100�C] are products of Bayer Company, Leverkusen,Germany. Styrene butadiene rubber (SBR) (1502) of 52Mooney viscosity [ML (1þ 4) at 100�C] is a product ofExxon, Machelen, Belgium.

Techniques

Synthesis of AN-g-BR

Grafting of AN onto BR was carried out in a 2-L, nitrogen-flushed, three-neck, round-bottom flask. A BR 15 g=Ltoluene solution was introduced into the reaction vesseland heated up to 80�C. Dibenzoyl peroxide 2.7 mmoland AN 603 mmol were added step-wise to the reactionmedium over 60 minutes. The copolymerization reactionmixture was stirred for 5 h at a speed of 600 rpm. Thecopolymerization product was precipitated in methanolover night, decanted and washed several times with meth-anol. The AN-g-BR was purified to get rid of poly acry-lonitrile homopolymer by being soaked in DMSO for 24 hand then filtered. The precipitate was then rinsed withwater as well as methanol several times and finally driedin vacuum oven at 40�C for 7 days.

Characterization of AN-g-BR

The synthesized AN-g-BR was characterized by means of:

1. FT-IR spectrophotometer, Nicolet, Nexus 821, Madison,WI, U.S.A.

2. Waters GPC instrument, Waters Millipore Corporation,Milfords, MA, U.S.A., in order to determine numberaverage molecular weights Mn and polydispersitiesMw=Mn. The GPC instrument was equipped with a ser-ies of styragel columns (100, 500, 103, 104, 105 A). Thesecolumns were calibrated with narrow molecular weightdistribution polystyrene linear standards. The GPCinstrument was connected to a Waters 410 differentialrefractometer (DRI) detector.

3. Elemental analyzer, Perkin-Elmer, Elementar, Hanau,Germany; to determine N% in prepared AN-g-BR.

Compatibility Investigation

The morphology of compatibilized and uncompatibilizedblends was studied using a scanning electron microscope,

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Model JXA-840A, JEOL, Technics Co. Ltd., Tokyo, Japan,at magnification M ¼ 500�. The surface of the polymerwas mounted on a standard specimen stub. A thin coating10 A of gold was deposited onto the polymer surface andattached to the stub prior to examination in the micro-scope to enhance the conductivity. Tg’s of rubber blendswere determined utilizing DSC calorimeter, Shimadzu,DSC-50, Foster City, CA, U.S.A. The DSC was operatedat a heating rate of 10�C min�1 within the temperaturerange from�100� to 100�C. Rubber blend specimens wereheated up to 100�C, then cooled to �100�C twice toremove their thermal history. The intrinsic viscosities (g)of the blend solutions were determined with a modifiedOstwald viscometer at 25� 1�C. Various blend ratios ofuncured SBR=NBR blend with and without AN-g-BRas a compatibilizer were dissolved in toluene; to obtain0.8, 0.6, 0.4, 0.2, and 0.1 g=100 mL solutions required forg measurements.

Mixing, Vulcanization, and Testing of Rubber Blends

The rubber blends were mixed with curatives and othercompounding ingredients on an open two-roll mixing millof 170 mm diameter and 300 mm working distance at24 rpm speed of slow roll and gear ratio of 1:1.25 at70�C. The AN-g-BR was first mixed with SBR, thenNBR was added onto the mill, followed by the other com-pounding ingredients. The rheometric characteristics[15]

were assessed with a Monsanto Oscillating Disc RheometerR-100 (Akron, OH, U.S.A.) at 152� 1�C. The blends werethen cured for their respective optimum cure time, in ahydraulic press at the same temperature and pressure of4 MPa on the mold. The physico-mechanical propertieswere determined with a Zwick-1425 tensile tester[16]

(Munich, Germany) at 25� 1�C and crosshead speed of500 mm min�1. Accelerated thermal aging of the rubbervulcanizates was carried out in an air-circulated oven at90�C. Swelling tests of the rubber blend vulcanizates intoluene were carried out at 25� 1�C for 48 h[17]. However,the swelling test in motor oil was conducted at 100� � 1�Cfor 8 days, in a thermostated electric oven. The physico-mechanical properties and the swelling data were measuredin five replicates.

RESULTS AND DISCUSSION

Inhomogeneous SBR/NBR Blends

The SBR=NBR rubber blends with different blend ratioswere prepared. The formulations and the rheologicalproperties of SBR, NBR, and their blends are listed inTable 1. The cure times (tc90) of the SBR=NBR blendswere shorter and the cure rate indices were greater thanthose of the individual rubbers. The rubber mixes were thenvulcanized at their cure times. The physico-mechanicalproperties of SBR, NBR, and their blends with different

blend ratios were measured and plotted vs. SBR=NBRblend ratio in Fig. 1. Tensile strength and elongation atbreak of SBR=NBR show an irregular relationship withthe blend ratios. This points to the incompatibility of thatblend[18]. The rubber vulcanizates under investigation werethen subjected to accelerated thermal aging. The physico-mechanical properties of SBR, NBR, and their blends weredetermined after thermal aging for different periods up to7 days. Figures 2 and 3 illustrate that NBR and the

TABLE 1Formulations (weight parts, phr) and rheological and

physico-mechanical properties of SBR=NBR rubber blendswith different blend ratios

Designation S1 S2 S3 S4 S5

SBR 100 75 50 25 0NBR 0 25 50 75 100Zinc oxide 5 5 5 5 5Stearic acid 2 2 2 2 2Carbon black (HAF) 40 40 40 40 40Processing oil 5 5 5 5 5CBS� 1 1 1 1 1Sulfur 2 2 2 2 2Rheological properties

Minimum torque, Nm 8 9 7 8 7Maximum torque, Nm 68 60 64 67 70Cure time (tc90), min 25 18.5 18.5 20 23Scorch time (ts2), min 4.5 5 5 4.25 3.8Cure rate index

(CRI), min�11.65 2.0 1.8 1.7 1.6

�N-Cyclohexyl-2-benzothiazole sulfenamide.

FIG. 1. Elongation at break and tensile strength of SBR=NBR vulcani-

zates vs. blend ratio.

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NBR-rich blend possess more thermally stable tensilestrength and elongation at break, respectively, than SBR-and SBR-rich blend. This can be attributed to thethermally stable acrylonitrile content in NBR. The 100%moduli (Fig. 4) of all vulcanizates under investigationincrease with aging time without stability.

Grafting of Acrylonitrile onto BR

Figure 5 shows the conversion-time curve of graft copo-lymerization of AN onto BR as a function of N% contentin AN-g-BR. The induction period of the graft copolymer-

ization is quite low, �30 minutes. The conversion to AN-g-BR increases with time up to 5 h; thereafter it remainsconstant. The probability of achieving high molecularweight poly acrylonitrile (PAN) segment is certainly verylow, due to the insolubility of the growing PAN macro rad-ical segment in toluene[19].

Gel Permeation Chromatography

Figure 6 illustrates the GPC trace of AN-g-BR. It showsa marked decrease in Mn and Mw=Mn when compared with

FIG. 2. Tensile strength of SBR=NBR vulcanizates with various blend

ratios vs. aging time at 90�C.

FIG. 3. Elongation at break of SBR=NBR vulcanizates with various

blend ratios vs. aging time at 90�C.

FIG. 4. 100% modulus of SBR=NBR vulcanizates with various blend

ratios vs. aging time at 90�C.

FIG. 5. Conversion-time curve of graft copolymerization of acryloni-

trile onto butadiene rubber.

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that of the mother BR; Mn decreases from 3.66� 105, to2.54� 105, and Mw=Mn decreases from 1.13 to 1.00 forBR and AN-g-BR, respectively. This drop in Mn andMw=Mn is attributed to the difference in the molecularshapes of linear BR and branched AN-g-BR; in effect thisdrop assures the relatively considerable amount of graftingobtained.

Elucidation of AN-g-BR Structure

Figure 7 shows the FT-IR spectra of BR and AN-g-BR.Spectrum (a) shows the CH2 and C=C groups character-istic of BR at 1450 and 1665 cm�1, respectively. Spectrum(b) shows the �C�N group characteristic of PAN at2240 cm�1. The peak at 1715 cm�1 is attributed to the car-bonyl group of the BPO initiator. However, the peak at1770 cm�1 is attributed to the C=N group, which resultsfrom some degree of self cyclization of PAN[19].

Homogeneity of SBR/NBR Blend

For the homogeneity investigation, SBR=NBR blendswith different blend ratios were prepared on an opentwo-roll mill, in the presence and absence of AN-g-BR(10 phr). Figure 8 illustrates g of the uncured SBR=NBRblends vs. the blend ratios. The blends (S1-S5) in theabsence of AN-g-BR show S-shape relation, whereas theblends (S6-S10) in the presence of AN-g-BR show a

straight line; this indicates an improvement in homogeneityof the SBR=NBR blend containing AN-g-BR due to inter-facial interactions.

FIG. 6. GPC traces of (a) butadiene rubber and (b) acrylonitrile grafted

butadiene rubber.

FIG. 7. FT-IR of (a) BR and (b) AN-g-BR.

FIG. 8. Intrinsic viscosity vs. NBR content in SBR=NBR blend, in pres-

ence and absence of AN-g-BR compatibilizer.

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The SBR=NBR (50=50) blend with and without AN-g-BR (10 phr) was selected for the microscopy investigation.Figure 9 shows scanning electron micrographs of theuncured blends with 500� magnification. It is clear thatthe micrograph (Fig. 9a) illustrates two different phases forthe individual rubbers, indicating phase separation andincompatibility of the SBR=NBR blend. However, themicrograph (Fig. 9b) shows change in the morphology,where no macro-scale phase separation takes place, indi-cating enhancement of the homogeneity of the SBR=NBRblend. Improvement in the homogeneity can be a result ofco-continuous phases, where both SBR and NBR formcontinuous phase after incorporation of AN-g-BR. Thisis confirmed by the DSC study that is used to detect quali-tatively the homogeneity of the SBR=NBR blend. The glasstransition (Tg) is kept from the second scan. Figure 10a, billustrates DSC traces of the SBR=NBR blend with andwithout AN-g-BR. Tg’s of SBR and NBR in the blendappear at �69.9�C and �40.6�C, respectively, with Tg dif-ference of 29.3�C, which shows the presence of microlevelinhomogeneity. However, Tg’s of SBR and NBR in the

blend with AN-g-BR appear at �64.8�C and�41�C,respectively, with Tg difference of 23.8�C. This data illus-trate that Tg’s of SBR and NBR became closer to eachother upon incorporation of AN-g-BR. This can be attrib-uted to the increase of adhesion between phases as a resultof dipole interactions assumed between the cyanide groupsof acrylonitrile repeat units present in NBR and cyanidegroup of acrylonitrile present in AN-g-BR, which leadsto some degree of homogeneity for that blend.

Homogeneous SBR/NBR Blends

The formulations and rheological properties of SBR,NBR, and their blend mixes containing AN-g-BR are listedin Table 2. The optimum cure times (tc90) of the compati-bilized blends are slightly longer than those of the individ-ual rubbers. During vulcanization, interface crosslinkingoccurs via dipole interactions between the cyanide polargroups in NBR and AN-g-BR. This leads to an increasein the cure time. Comparing cure times (tc90) of the compa-tibilized and the uncompatibilized blends with differentblend ratios (Tables 1, 2), one can notice that the presenceof the compatibilizer resulted in a decrease in cure time ofthe corresponding blend. This can be attributed to thereduction in the interfacial area.

The rubber mixes were then vulcanized for their curetimes (tc90). Physico-mechanical properties of the rubbervulcanizates were measured and plotted vs. SBR=NBR blend

FIG. 9. SEM micrographs of SBR=NBR (50=50) blends. (a) uncompa-

tibilized and (b) compatibilized with AN-g-BR. M ¼ 500�.

FIG. 10. DSC traces of SBR=NBR blend (a) uncompatibilized and (b)

compatibilized with AN-g-BR.

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ratio, as shown in Fig. 11. The disappearance of the irregu-lar pattern in tensile strength and elongation at break canbe clearly observed. It is also obvious that tensile strengthand elongation at break values of the rubber vulcanizateslie on straight lines, indicating some sort of blend hom-ogeneity.

The rubber vulcanizates containing AN-g-BR were thensubjected to accelerated thermal aging. Physico-mechanical

properties of the vulcanizates were measured and plottedvs. the aging periods. Figure 12 illustrates that the tensilestrength of SBR is the lowest and that of NBR is the high-est upon aging, due to thermal stability of AN content inNBR. However, the SBR=NBR (25=75) blend possessesmedium tensile strength values together with better thermalstability.

Figure 13 indicates that NBR possesses the longest elon-gation at break. However, the SBR=NBR (25=75) blend

TABLE 2Formulations (weight parts, phr) and rheological and

physico-mechanical properties of SBR=NBR rubber blendswith different ratios compatibilized with AN-g-BR

Designation S6 S7 S8 S9 S10

SBR 100 75 50 25 0NBR 0 25 50 75 100AN-g-BR 10 10 10 10 10Zinc oxide 5 5 5 5 5Stearic acid 2 2 2 2 2Carbon black (HAF) 40 40 40 40 40Processing oil 5 5 5 5 5CBS� 1 1 1 1 1Sulfur 2 2 2 2 2Rheological properties

Minimum torque, Nm 8 9 8 9 7.5Maximum torque, Nm 70 62 66 68 71Cure time (tc90), min 14 17.5 18 18 17Scorch time (ts2), min 3.5 3.75 3 3.25 2.5Cure rate (CRI), min�1 1.6 1.9 1.7 1.7 1.6

Physico-mechanical propertiesTensile strength, MPa 13 13.5 16.5 15.5 18.5100% Modulus, MPa 1.4 1.8 1.8 1.9 1.92Elongation at break, % 780 715 770 825 890�N-Cyclohexyl-2-benzothiazole sulfenamide.

FIG. 11. Elongation at break and tensile strength of SBR=NBR vulca-

nizates compatibilized with AN-g-BR vs. blend ratio.

FIG. 12. Tensile strength of SBR=NBR vulcanizates with various blend

ratios compatibilized with AN-g-BR vs. aging time at 90�C.

FIG. 13. Elongation at break of SBR=NBR vulcanizates with various

blend ratios compatibilized with AN-g-BR vs. aging time at 90�C.

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shows medium and suitable values of elongation at breakwith better thermal stability. On the other hand,SBR=NBR (50=50) and (25=75) vulcanizates (Fig. 14) showhigher 100% moduli than the individual rubbers; this isbecause of the co-curing of SBR and NBR. All mechanicalproperties indicate that the most desirable blend isSBR=NBR (25=75); this can be attributed to its highNBR content.

The investigated vulcanizates were subjected to swellingtests in toluene and in motor oil. The weight swell data are

shown in (Fig. 15). It is observed that the weight swell per-cent decreases with increasing NBR content in the blend.This can be attributed to the polarity of AN existing inNBR. Low-weight swell values of the compatibilized vulca-nizates confirm that the compatibilizer brings aboutadhesion between the phases. This is due to the dipole-dipole interactions among the blend constituents[20]. Asseen from Fig. 15, the weight swell is below the additiveaverage in the presence of the compatibilizer. If interfacialbonds are formed during vulcanization, the lightly swollenphase will restrict swelling of the highly swollen phasebelow the additive average[21]. This confirms that inter-facial bonds are formed in blends of SBR and NBR inthe presence of AN-g-BR.

The SBR=NBR (25=75) blend was selected for furtherstudy utilizing different inorganic fillers, namely silica, talc,and kaolin. Formulations and rheological and physico-mechanical properties of the blend vulcanizates in the pres-ence and absence of AN-g-BR compatibilizer are tabulatedin Table 3. It is obvious from the table that the silica-filledblends, in the presence and absence of the compatibilizer,possess the highest values of maximum and minimumtorques, since silica is a reinforcing filler. Also, those silicafilled blends show the longest cure times and the lowest

FIG. 14. 100% modulus of SBR=NBR vulcanizates with various blend

ratios compatibilized with AN-g-BR vs. aging time at 90�C.

FIG. 15. Weight swell in toluene for 2 days at 25�C and in motor oil for

8 days at 100�C vs. NBR content in SBR=NBR blend with and without

AN-g-BR as compatibilizer.

TABLE 3Formulations and rheological and physico-mechanicalproperties of SBR=NBR blend with different inorganic

fillers in presence and absence of AN-g-BR compatibilizer

Ingredients, phr S11 S12 S13 S14 S15 S16

SBR 25 25 25 25 25 25

NBR 75 75 75 75 75 75

AN-g-BR 0 0 0 10 10 10

Zinc oxide 5 5 5 5 5 5

Stearic acid 2 2 2 2 2 2

Silica 40 0 0 40 0 0

Talc 0 40 0 0 40 0

Kaolin 0 0 40 0 0 40

Processing oil 5 5 5 5 5 5

CBS� 1 1 1 1 1 1

Sulfur 2 2 2 2 2 2

Rheological properties

Minimum torque, Nm 11 8 9 13 9 9

Maximum torque, Nm 72 40 40 79 41 42

Cure time (tc90), min 36.5 19 18 29.5 14 17.5

Scorch time (ts2), min 6 8 7.5 4.75 3.25 5

Cure rate index

(CRI), min�1

3.3 9 9.5 4 9.3 8

Physico-mechanical properties

Tensile strength, MPa 6.2 1.7 1.95 10.5 2.5 3.4

100% Modulus, MPa 1.8 1 1.45 1.65 0.9 1.3

Elongation at break, % 1000 900 980 1160 1050 1000

�N-Cyclohexyl-2-benzothiazole sulfenamide.

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cure rate indices, due to the retarding effect of silica-filler.Generally, the mechanical properties of the silica-filled vul-canizates are superior to kaolin and talc ones, as expected,since silica is a reinforcing filler. Physico-mechanicalproperties of the vulcanizates were generally improved inthe presence of the compatibilizer. The physico-mechanicalproperties of SBR=NBR (25=75) vulcanizates with differ-ent inorganic fillers were then measured after acceleratedthermal aging and plotted vs. the aging period. The resultsindicate that silica-filled vulcanizates compatibilized withAN-g-BR possess the highest tensile strength and the long-est elongation at break throughout the aging periods(Figs. 16 and 17). This can be attributed to the reinforcing

effect and thermal stability of silica filler besides the inter-actions between silica and polar CN groups present in bothNBR and AN-g-BR compatibilizer. Also, 100% moduli(Fig. 18) of silica-filled vulcanizates, in the presence andabsence of AN-g-BR, are greater than kaolin- and talc-filled vulcanizates, due to the greater degree of crosslinkingin the former. In general, the compatibilized vulcanizatespossess less desirable 100% moduli than the uncompatibi-lized vulcanizates during aging.

CONCLUSIONS

1. The SEM micrographs, DSC traces, and viscometricstudy confirm change in morphology and improvementof homogeneity for the SBR=NBR blend upon incor-poration of AN-g-BR.

2. Of the entire blend vulcanizates with different blendratios examined, the SBR=NBR (25=75) vulcanizatecontaining AN-g-BR possesses the best thermal stabilitytogether with good physico-mechanical properties.

3. The weight swell of the SBR=NBR vulcanizate intoluene and in motor oil decreases with increasingNBR content in the blend. The NBR and NBR-richblend vulcanizates show the best swelling resistance.

4. The weight swell of SBR=NBR vulcanizates in tolueneand in motor oil is reduced by incorporation of AN-g-BR.

5. Of all vulcanizates filled with inorganic fillers, the silica-filled one possesses higher tensile strength and longerelongation at break.

6. Physico-mechanical properties of the silica-filledSBR=NBR (25=75) vulcanizate containing AN-g-BRshow more thermal aging resistance than the vulcanizatewithout AN-g-BR.

FIG. 17. Elongation at break vs. aging time of SBR=NBR (25=75) with

different inorganic fillers in presence and absence of AN-g-BR at 90�C.

FIG. 16. Tensile strength vs. aging time of SBR=NBR (25=75) with

different inorganic fillers in presence and absence of AN-g-BR at 90�C.FIG. 18. 100% modulus vs. aging time of SBR=NBR (25=75) with dif-

ferent inorganic fillers in presence and absence of AN-g-BR at 90�C.

HOMOGENEOUS RUBBER BLENDS 511

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