The effect of mercapto-modified EVA on rheological and dynamic mechanical properties of NBR/EVA blends

The Effect of Mercapto-Modified EVA on Rheological andDynamic Mechanical Properties of NBR/EVA Blends

PAULO JANSEN,1 2 BLUMA G. SOARES1

1 Instituto de Macromoleculas, Universidade Federal do Rio de Janeiro, Centro de Tecnologia, Bl. J, Ilha do Fundao,21945-970, Rio de Janeiro, RJ, Brazil

2 Departamento de Tecnologia Quımica, Universidade Federal Rural do Rio de Janeiro, 23851-970, Seropedica-RJ, Brazil

Received 29 May 2001; accepted 23 August 20001Published online 20 March 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/app.10497

ABSTRACT: The effect of mercapto-modified ethylene vinyl acetate copolymer(EVALSH) on the rheological and dynamic mechanical properties of acrylonitrilebutadiene rubber (NBR) and ethylene vinyl acetate copolymer (EVA) blends wasevaluated at different blend compositions. The addition of 5 phr of EVALSH in theblends resulted in an increase of the melt viscosity and a substantial decrease of theextrudate swell ratio. These results can be attributed to the interactions occurringbetween the double bond of the NBR phase and the mercapto groups along the EVALSHbackbone. The power–law index also presents a slight increase in the presence ofEVALSH, indicating a decrease in the pseudoplastic nature of the compatibilizedblends. The reactive compatibilization of NBR/EVA blends with EVALSH was alsoconfirmed by the decrease of damping values and an increase of glass transitiontemperature, in dynamic mechanical analysis. © 2002 Wiley Periodicals, Inc. J Appl PolymSci 84: 2335–2344, 2002

Key words: NBR/EVA blends; compatibilization; rheological properties; dynamicmechanical properties

INTRODUCTION

Elastomer–thermoplastic blends have becometechnologically important because they combinethe excellent processing characteristics of ther-moplastic materials at high temperature and thephysical properties of elastomers at service tem-perature. Several researchers have reported use-ful rubber–plastic blends.1–3

Among several elastomers, nitrile rubber(NBR) is a very important rubber component inthese systems because of its excellent oil resis-tance, abrasion resistance, and mechanical prop-erties. Several thermoplastics were blended withNBR, such as poly(vinyl chloride) (PVC),4–5 poly-ethylene,6–8 polypropylene (PP),9–11 nylon,12 andpoly(ethylene-co-vinyl acetate) (EVA).13–15 NBR/PVC blends are miscible depending on the blendcomposition and are normally employed on thedevelopment of several flame-resistant materi-als.4 The other NBR-based TPE blends are incom-patible and present gross phase separation andpoor interfacial adhesion, giving rise to materialswith poor mechanical performance, in almost allcases.

Correspondence to: B. G. Soares ([email protected]).Contract grant sponsor: CNPq; contract grant sponsor:

PADCT-CNPq; contract grant number: 620132/98-1; contractgrant sponsors: FAPERJ; CEPG-UFRJ; CAPES.Journal of Applied Polymer Science, Vol. 84, 2335–2344 (2002)© 2002 Wiley Periodicals, Inc.

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An improvement of the physicomechanicalproperties can be normally achieved with the ad-dition of appropriated interfacial agents, whichact at the interface, thus, promoting an effectiveanchorage between the phases. Block and graftcopolymers were extensively employed for thecompatibilization.16 However, the reactive com-patibilization by the use of functional polymers orcoupling agents is technologically more interest-ing because it is often less expensive. In thesecases, the block or graft copolymer is formed insitu during blend processing. For example, Coranand Patel have successfully compatibilizedNBR/PP blends by using PP modified with maleicanhydride.11 The compatibilization was accom-plished by the in situ formation of block copoly-mer resulted from the reaction between the aminoend groups of the NBR component and the anhy-dride groups of the PP component. Thomas et al.also reported a series of studies concerning thecompatibilization of NBR/polyolefin blends by us-ing the corresponding polyolefin modified withphenolic or maleic anhydride groups.7–8,10

The compatibilization of NBR/EVA blends wasalso investigated in our laboratory by using mer-capto-modified EVA (EVALSH) copolymer.17 Thisfunctionalized copolymer was also employed inother EVA-based blends such as natural rubber(NR)/EVA18–20 and styrene-butadiene rubber(SBR)/EVA blends.21 In both systems, it is be-lieved that an in situ reaction between the mer-capto groups of the functionalized EVA copolymerand the double bond of the unsaturated rubbertakes place during the melt processing. In fact, asubstantial amount of insoluble material was ob-tained in nonvulcanized NR/EVA blends18 andSBR/EVA blends21 containing 5 phr of EVALSH.In addition, an improvement of the ultimate ten-sile strength was observed, indicating a reactivecompatibilization. Concerning NBR/EVA blends,the addition of a low amount of EVALSH alsoresulted in an improvement of the ultimate ten-sile strength, mainly in blend composition con-taining a high amount of NBR.17

In addition to mechanical and morphologicalproperties, it is well established that the compati-bilization also affects the flow behavior of polymerblends.10,22–26 Normally, the viscosity of the sys-tem increases with the reactive compatibilizationas a consequence of chemical reactions occurringbetween the components of the blends. For exam-ple, the use of styrene-(ethylene-co-butylene)-sty-rene block copolymer grafted with maleic anhy-

dride (SEBS-MA) in polypropylene/polyamideblends resulted in an increase in the viscosity ofthe blends as a consequence of the chemical reac-tions taking place between amine and anhydridegroups.22 Also, the compatibilization of NBR/PPblends by phenolic-modified PP increased the vis-cosity of the system.10

In this article, we examine the influence of theEVALSH on the flow characteristics and dynamicmechanical properties of nonvulcanized NBR/EVA blends.

EXPERIMENTAL

Materials

Nitrile rubber NBR615 (33 wt % of acrylonitrile;Mooney viscosity � 32) was kindly supplied byNitriflex S.A., Rio de Janeiro, Brazil. EVA copol-ymer [18 wt % vinyl acetate; melt flow index(MFI) � 2.3 g/10 min at 120°C] was kindly sup-plied by Petroquimica Triunfo S.A., Triunfo, Bra-zil. Mercapto-modified EVA copolymer (EVALSH)was obtained in our laboratory by esterifying thehydrolyzed EVA copolymer with mercaptoaceticacid, according to literature.27 The amount ofmercapto groups in the functionalized copolymercorresponded to 62 mmol %, as determined byFTIR and TGA analysis.

Blend Preparation

The blends were prepared in a Berstoff two-rollmill at 110°C and 30 rpm. NBR was first masti-cated for 2 min and then compounded withEVALSH and EVA in this order. The total mixingtime was 6 min in all blends.

Rheological Measurements

Rheological measurements were carried out in anInstron capillary rheometer (model 4204) at dif-ferent plunger speeds. The plunger speed variedfrom 1.0 to 12 mm min�1. The capillary used hasan l/d ratio of 100. The samples were preheatedfor 3 min and extruded through the capillary atpreselected speeds of the crosshead. The mea-surements were done at 120 and 150°C. The trueshear rate was calculated from the apparentshear rate by using the Weissenberg–Rabinowishcorrection, assuming that the blend behaves as apower–law fluid. The power–law index was deter-mined by regression analysis according to

2336 JANSEN AND SOARES

n� �d�log ��

d�log ��

where � is the shear stress and � is the apparentshear rate. The shear viscosity, �, was calculated

from the ratio between shear stress and the trueshear rate, obtained by the Weissenberg–Rabi-nowish correction.

For extrudate swell, the extrudates were care-fully collected as they emerged out from the cap-

Figure 1 Variation of the melt viscosity with the shear rate of NBR/EVA blends as afunction of the blend composition; (�) without EVALSH; (f) with 5 phr of EVALSH.(Experiments performed at 120°C.)

MERCAPTO-MODIFIED EVA ON NBR/EVA BLENDS 2337

illary die, taking care to avoid any deformation.The diameter of the extrudate was measured af-ter 48 h of extrusion. The die swell was calculatedfrom de/dc ratio, where de is the diameter of theextrudate and dc is the diameter of the capillary.

Dynamic Mechanical Analysis

Dynamic mechanical analyses of the compatibi-lized, noncompatibilized, and individual polymerswere performed on a Rheometric Scientific MKIII

Figure 2 Variation of the melt viscosity with the shear rate of NBR/EVA blends as afunction of the blend composition; (�) without EVALSH; (f) with 5 phr of EVALSH.(Experiments performed at 150°C.)


DMTA. Samples of 50 � 5 � 0.5 mm3 were usedfor this study. The experiments were carried outat a frequency of 1 Hz over a temperature range of�50–30°C in a bending mode. The tan � andstorage modulus were measured for each samplein this temperature range.

RESULTS AND DISCUSSION

Rheological Properties

Figures 1 and 2 compare the effect of theEVALSH on melt viscosity of NBR/EVA blends inexperiments performed at 120 and 150°C, respec-tively. The viscosity of all blends decreased withthe increase of the shear rate, indicating pseudo-plastic behavior. The addition of 5 phr ofEVALSH resulted in an increase of the melt vis-cosity, mainly in blends containing higheramounts of NBR. Such increase in viscosity oncompatibilization of immiscible polymer blendswas reported by several researchers.10,24–26 Thisbehavior is better illustrated in Figures 3 and 4,where the viscosity of the blends at different ap-parent shear rates is plotted as a function of theweight percentage of EVA.

During the mixing of EVALSH with NBR/EVAblends, the mercapto groups in the EVALSH areexpected to react with the double bond of the NBRphase, as proposed in our previous article.17

These reactions occurring inside the NBR phaseshould contribute to an increase of the viscosity ofthe blend. The effect is not so high, probably be-cause of the low extent of grafting or crosslinkingreactions between the EVALSH and NBR compo-nents as a consequence of the low amount of themercapto groups along the EVASH backbone. Theextension of the crosslink reaction was analyzedby submitting the NBR/EVA blends to a extrac-tion with hot toluene for 48 h. Pure NBR/EVAblends without EVALSH were completely solubi-lized in hot toluene; that is, no insoluble materialwas present in these blends. Concerning blendscontaining 5 phr of EVALSH, a small amount ofinsoluble material was obtained after the extrac-tion time, confirming the reaction between NBRand EVALSH. Such an amount was, however,small (only about 5–8 wt % of the whole sample)because of the low amount of mercapto groups inthe EVALSH, as mentioned before. As reported inour previous studies,17 the addition of EVALSHin nonvulcanized NBR/EVA blends improved the

ultimate tensile strength in blends containinghigher amounts of NBR. From the mechanicaland rheological results, one can confirm the effi-ciency of the reactive compatibilization, mainly inNBR-richer blends.

The effect of EVALSH on the power–law indexof NBR/EVA blends is shown in Figure 5 for ex-

Figure 3 Variation of the melt viscosity with thecomposition of NBR/EVA blends at different shearrates; (f) without EVALSH; (�) with 5 phr ofEVALSH. (Experiments performed at 120°C.)


periments performed at 120°C. The values of n�less than unity for NBR/EVA blends indicatepseudoplastic behavior, as normally observed inseveral polymer systems. It is interesting to ob-serve that the presence of EVALSH resulted in aslight increase in the n� values in almost all

blends, suggesting an increase of the Newtonianbehavior.

The presence of EVALSH in NBR/EVA blendsexerts a strong influence on the melt elasticity ofthe blends. When a molten polymer flows throughthe capillary, the polymer chains undergo orien-tation due to shear. When the melt emerges fromthe die, the oriented macromolecules have a ten-dency to recoil, resulting in a lateral expansion.This phenomenon is known as die swell. The ef-fect of blend composition and compatibilization onthe extrudate swell at different apparent shearrate is illustrated in Figure 6 for experimentsperformed at 120°C. When EVA is the matrix, theextrudate swell ratio decreases with the additionof NBR in the blend until a composition corre-sponding to 40 wt % of NBR. Thomas et al. alsoreported similar behavior for PP/NBR blends,which was attributed to the elasticity of the rub-ber phase.10 The presence of EVALSH does notexert significant influence on the extrudate swellratio in this composition range probably becauseNBR is the dispersed phase and the interactionsbetween EVALSH and this phase do not contrib-ute for the changes in this property.

For blends containing a higher concentration ofthe NBR component, the extrudate swell ratioincreases substantially as the concentration ofthe rubber phase increases. This phenomenon ismore pronounced at higher shear rate because theresidence time of the blend in the capillary de-creases and more elastic energy is stored in the

Figure 5 Effect of blend ratio and compatibilizationon the power–law index (n�); (a) without EVALSH; (b)with 5 phr of EVALSH.

Figure 4 Variation of the melt viscosity with thecomposition of NBR/EVA blends at different shearrates; (f) without EVALSH; (�) with 5 phr ofEVALSH. (Experiments performed at 150°C.)


polymer. Similar behavior was also observed byKoshy et al. in nonvulcanized NR/EVA blends.28

The presence of as low as 5 phr of EVALSH in theblends resulted in a substantial decrease on theextrudate swell ratio. In addition, the effect of theshear rate on the die swell is less pronounced incompatibilized blends. The results of the presentstudy reveal that the interactions between theNBR phase and the EVALSH decrease the relax-ation process of the blend and increase the elasticrecovery of the NBR matrix.

Dynamic Mechanical Properties

Dynamic mechanical properties of polymers arehighly dependent on the material structure.The dependence of tan � of the pure componentsand the noncompatibilized blends on tempera-ture is shown in Figure 7. NBR displays a sharploss tangent peak at around �16°C [Fig. 7(a)],

which corresponds to the glass transition,whereas EVA displays two distinct peaks ataround �23 and 20°C [Fig. 7(g)]. These EVAtransitions are not clear in Figure 7 because thecorresponding damping values are very lowwhen compared to the damping of the rubbercomponent. These results are also presented inTable I. The blends display a single transition,which is mainly related to the NBR phase. Itsdisplacement toward lower temperature indi-cates an increase of the chain mobility of NBRcaused by the presence of the EVA componentby which the chain mobility of NBR is in-creased. The damping decreases as the EVAcontent in the blend increases, as expected, be-cause this property is related to the volumefraction of the NBR in the blend. These resultsagree with those found by Thomas et al. in theirstudies concerning NBR/EVA blends.13,15

Figure 6 Variation of the extrudate swell ratio with the composition of NBR/EVAblends at different shear rates; (f) without EVALSH; (�) with 5 phr of EVALSH.(Experiments performed at 120°C.)


The effect of the EVALSH on loss tangent (tan�) values of NBR/EVA blends as a function oftemperature is illustrated in Figure 8. In all blendcompositions studied, the addition of 5 phr ofEVALSH resulted in a decrease of damping and asmall displacement of the glass transition tem-perature toward higher values. These results arealso summarized in Table I. The highest effect ofthe EVALSH on damping was observed for blendscontaining a greater amount of NBR. An increaseof glass transition temperature associated with adecrease in damping indicate a decrease of themolecular mobility of the NBR phase, as a conse-

quence of the interactions between the rubberphase and the EVALSH. These results are inagreement with the rheological data.

Concerning the storage modulus E�, the addi-tion of EVALSH did not affect this property atlower temperatures (i.e., below the glass transi-tion temperature). However, at higher tempera-tures [i.e., in the rubbery region (0 and 20°C)], themodulus slightly increases upon the addition of 5phr of EVALSH (see Table I). These behaviorsalso confirm an increase in the phase interactionswith the presence of the functionalized copoly-mer, EVALSH.

CONCLUSION

The addition of mercapto-modified EVA (EVALSH)in uncrosslinked NBR/EVA blends exerts a sub-stantial influence on the rheological and dynamicmechanical properties. From rheological measure-ments, one can conclude that the presence ofEVALSH increases slightly the viscosity of theblends but decreases significantly the extrudateswell ratio, indicating better dimensional stabilityof the extrudate. In addition, the flow behavior in-dex of the blends increases with the compatibiliza-tion, indicating a decrease in the pseudoplastic na-ture.

Concerning the dynamic mechanical data, theaddition of EVALSH also increases the glass tran-sition temperature of the NBR phase and de-

Figure 7 Variation of tan � of noncompatibilizedNBR/EVA blends with temperature; (a) pure NBR; (g)pure EVA; and their blends containing (b) 85%; (c) 80%;(d) 70%; (e) 60%; and (f) 40% NBR.

Table I Dynamic Mechanical Properties of NBR/EVA Blends as a Function of Blend Ratio andCompatibilization

BlendComposition

Log E� (Pa)

Tg (°C)Maximum

tan ��40°C �20°C 0°C 10°C

NBR EVA a b a b a b a b a b a b

0 100 9.09 — 8.68 — 8.22 — 7.78 — �18.7 — 0.20 —20 80 9.17 9.17 8.52 8.63 7.85 7.88 7.63 7.65 �21.8 �18.3 0.22 0.2040 60 9.17 9.19 8.35 8.63 7.67 7.77 7.49 7.56 �23.8 �23.9 0.32 0.3060 40 9.13 9.20 8.36 8.58 7.46 7.66 7.23 7.43 �23.9 �18.9 0.50 0.4070 30 9.15 9.21 7.77 7.85 6.90 7.48 6.83 7.35 �24.4 �23.0 0.9 0.780 20 9.23 9.26 8.14 7.90 7.06 7.21 6.98 7.09 �24.7 �23.5 1.1 1.085 15 8.86 9.12 7.68 7.74 7.21 7.40 7.04 7.23 �21.2 �19.9 1.2 1.1

100 0 9.12 9.10 7.44 7.68 6.30 6.64 6.27 6.54 �16.0 �14.3 1.5 1.4

a Blends without EVALSH.b Blends with 5 phr of EVALSH.


creases the damping values. During mixing, themercapto groups along the EVALSH backboneare able to react with the double bond of theelastomer phase, thus forming interactions be-tween the phases. These interactions are respon-sible for the better performance in ultimate ten-sile strength, reported in a previous article, andalso for the peculiar behaviors found in dynamicmechanical and rheological properties of the com-patibilized blends. It is important to emphasizethat the influence of EVALSH on the melt viscos-

ity is not so pronounced because the extent of theinteraction between the NBR phase and the func-tionalized copolymer is low because of the lowamount of the mercapto group along the EVALSHbackbone.

This work was sponsored by CNPq, PADCT-CNPq(620132/98-1), FAPERJ, CEPG-UFRJ, and CAPES.The authors are grateful to Ana C. F. Moreira for theassistance given for carrying out the DMTA analysis ofthe blends.

Figure 8 The variation of tan � of compatibilized and noncompatibilized blends as afunction of temperature; (a) without EVALSH and (b) with 5 phr of EVALSH.


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The effect of mercapto-modified EVA on rheological and dynamic mechanical properties of NBR/EVA blends