Polymer Testing 22 (2003) 93–100www.elsevier.com/locate/polytest

Material Characterisation

Compatibility study of natural rubber and ethylene–propylene diene rubber blends

S.H. El-Sabbagha,∗

a Department of Polymers and Pigments, National Research Centre, Dokki, Cairo, Egypt.

Received 19 October 2001; accepted 2 May 2002

Abstract

Recently, blending of polymers has become an increasingly important area of research activity. Compatibility ofnatural rubber (NR) and ethylene–propylene diene monomer (EPDM) is poor and can be enhanced by the addition ofcompatibilizers.In the present study, the polymeric blends were modified either by high-energy radiation (γ-rays) inorder to create crosslinks (chemical bonds) between the rubber chains at the domain boundaries, or by the addition ofa compatibilizer EPDM-g-MAH prepared by radiation-induced graft copolymerization of EPDM with maleic anhydride,or by adding polymers such as polybutadiene rubber (BR), chlorinated rubber, chlorosulfonated polyethylene and poly-vinylchloride (PVC).The compatibility of NR/EPDM blends with and without different compatibilizers was evaluatedby viscosity measurements, differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Theresults obtained reveal that the addition of a small percentage of compatibilizer decreases the domain size of thedispersed phase, and also the compatibility and properties of the blend were greatly enhanced. 2002 Published byElsevier Science Ltd.

Keywords: Compatibility; Blend; Natural rubber; Ethylene; Propylene diene rubber; Graft; Maleic anhydride; Viscosity

1. Introduction

Blending of two or more types of rubber is a usefultechnique for the preparation of materials with propertiesabsent in the component rubbers [1–4].

Natural rubber has been studied and reported on exten-sively because of its superior performance in tire appli-cations. The incorporation of a suitable amount of EPDMinto a diene rubber results in a significant improvementin heat and ozone resistance [5–7]. The polymer blendsgenerally exhibit poor mechanical properties due toincompatibility and phase separation [8–11].

Several trials were carried out to minimize phase sep-aration and increase interfacial adhesion. These includethe addition of a compatiblizing agent such as a third

∗ Tel.: +20-2-335-5146; fax:+20-2-337-0931.E-mail address: salwaFelsabbagh@hotmail.com (S.H. El-

Sabbagh).

0142-9418/02/$ - see front matter 2002 Published by Elsevier Science Ltd.PII: S0142 -9418(02 )00056-9

polymer—a graft or block copolymer which improvesthe interaction between the constituent polymers [12–16].

The aim of the present work is to improve the com-patibility of NR/EPDM blend by introducing a thirdpolymer, such as Polybutadiene (BR), Styrene–butadienerubber (SBR), chlorinated rubber, Chlorosulfonated PEand Polyvinylchloride ( PVC), or exposure toγ-radiationto create crosslinks between the rubber chains, or to pro-duce maleic anhydride-grafted EPDM.

2. Materials and techniques

2.1. Materials

� Natural rubber (NR), ribbed smoked sheets RSS-1,specific gravity 0.913± 0.005, Mooney viscosity ML(1 + 4) at 100°C 60–90, supplied by Transport andEngineering Company, Alexandria, Egypt.

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� Ethylene propylene diene rubber (EPDM), Vistalon6505, produced by ESSO Chemi Germany. Diene(ethylidene norbornene) content 9%; ethylene content55%; Mooney viscosity ML (1 + 8) at 127 °C 48–52;density 0.86.

2.1.1. Compatibilizers� Polybutadiene rubber (BR) of 1,4 cis form (97%),

specific gravity 0.915 ± 0.005, Mooney viscosity ML

(1 + 4) at 100 °C 35 ± 3.� Polyvinyl chloride (PVC); a suspension polymer, K

value 68.� Chlorinated rubber: C10H11Cl17 yellowish powder,

specific gravity 1.63–1.66.� Chlorosulfonated polyethylene: white chips, specific

gravity 1.12, sulfur content 1.4% and chlorine con-tent 26.

� Maleic acid anhydride; m.p. 52.5 °C; b.p. 202 °C,specific gravity 1.48.

2.2. Techniques

2.2.1. Blend preparationThe graft polymer (EPDM-g-MAH) was prepared

using EPDM rubber as a backbone polymeric chain andmaleic anhydride. γ-radiation was utilized as a poly-merization initiator. Blending of the components wasachieved by mastication on a two-roll mill for 5 min,then each blend was mixed in a Brabander Plasticorderat a rotor speed of 70 rpm for 5 min. The mixing tem-perature was 150 °C. The amount of the compatibilizer,BR, chlorinated rubber, chlorosulfonated polyethyleneand PVC was 10 parts in each case. The compatibilizerswere added subsequently to the blend composition andmixing was continued for another 5 min.

2.3. Measurements

� The viscosity measurements were carried out using amodified Ostwald dilution viscometer [17].

� Infrared spectroscopy (IR) and differential scanningcalorimetery (DSC) were carried out at the MicroAnalytical Center, Cairo University. The IR spectrawere recorded on a Bruker Vector 22 (made inGermany) spectrophotometer using KBr. The DSCused a DSC-50 from Shimadzu.

� Scanning Electron Microscopy (SEM) was carried outusing a electron microscope model JSM-T20 fromJEOL, Japan.

� Irradiation of the rubber samples was carried out inthe Middle East Regional Radioisotope Center for theArab Countries. A gamma chamber, type 4000, wasused to give a dose rate of about 96 rad/sec. The tem-perature during irradiation was about 40 °C.

3. Results and discussion

The viscosity measurements were used as principalmeans for examining the compatibility of the NR/EPDMrubber blends [18]. It was interesting to study the rheol-ogical behavior of rubber blends in solution in order toestablish the general shape of the viscosity curves for theblends under investigation. The degree of compatibilityof natural rubber with ethylene propylene diene mono-mer was determined according to the linearity of thecurves. The specific viscosity (ηsp) for different concen-trations (C) was measured and ηsp/C plotted against (C);straight lines were obtained. Their intersections with theηsp/C axis represent the intrinsic viscosity [η]. Thechange of intrinsic viscosity [η] with NR/EPDM blendcomposition is shown in Fig. 1(a). The straight line rep-resents the additive values of [η] at different compo-sitions, while the experimental data is the S-shape curve.The observed non-linear behaviour, i.e., deviation fromthe additivity line, indicates the incompatibility of thisblend.Also, when the heat of mixing [19] over the entirerange of compositions and weight percent in NR/EPDMwas calculated, it was found to lie in the range28.7 × 10�3 and 72.7 × 10�3 J/mole and these valuesare above the limiting value for compatibility. This result

Fig. 1. a) The variation of intrinsic viscocity [η] with blendratio of NR/EPDM. b) Heat of mixing as a function of compo-sition in blend of NR and EPDM.

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implies that macromolecules in this mixture are in a dis-ordered state and therefore this blend system is con-sidered as incompatible (Fig. 1(b)).

The heterogeneity of the NR/EPDM blend system isemphasized by the difference in the solubility parameterof the two rubbers. This is calculated theoretically fromthe heat of mixing using the Schneier equation [19]. Thesolubility parameter of NR is known [20] (d �8.35(cal /cc)1/2). However, the solubility parameter of

EPDM is not known so it was calculated according to

Krause et al. [13a,b,21], i.e., d � �di�i, where �1 is

the volume fraction of each component from Small’s

equation [13], δ � ρ�Fi /M, where ρ is the density of

the polymer at the temperature of interest, M is the mol-ecular weight of the repeat unit in the polymer chain and� Fi is the sum of the molar attraction constants of all thechemical groups in the polymer repeat unit. δ (EPDM) =7.74 (cal/cc)1/2. Paul and Barlow [10] showed that a closematch of solubility parameters (δ1–δ2 = 0.1) will resultin complete miscibility of two polymers as long as theirmolecular weight exceeds 30,000; a wide gap of solu-bility parameter (= 0.61) between NR and EPDM pre-dicts incompatibility.

In order to overcome the problem of phase separationbetween NR/EPDM blends, trials were made using dif-ferent compatibilizers; i.e., the compatibility ofNR/EPDM blends could be improved either by theaddition of a third component (compatibilizer) such aspolybutadiene (BR), chlorosulfonated polyethylene, polyvinyl chloride (PVC) [15,16], or EPDM-g-MAH graftcopolymer or by creating some crosslinks between thecomponents of the blend by γ-irradiation.

3.1. Compatibilization via g-irradiation

NR/EPDM blends with different ratios were irradiatedat different doses of γ-rays (2, 4, 6 and 8 k gray).The

Fig. 2. The variation of intrinsic viscocity [η] with blend ratioof NR/EPDM after irradiation for 2 Kgray.

Fig. 3. The variation of intrinsic viscocity [η] with blend ratioof NR/EPDM after irradiation for 4 Kgray.

Fig. 4. The variation of intrinsic viscocity [η] with blend ratioof NR/EPDM after irradiation for 6 Kgray.

Fig. 5. The variation of intrinsic viscocity [η] with blend ratioof NR/EPDM after irradiation for 8 Kgray.

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compatibility of the blends was evaluated by viscositymeasurements. Figs. 2–5 represent the relation between[η] and the blend ratios. One can see that radiation doses6 and 8 k gray are the most suitable to create reasonablecrosslinking necessary for mixing homogeneity of NRand EPDM, judging by the good straight lines between[η] and the blend ratios, which demonstrate the compati-bility of this blend system.

3.2. Compatibilizing by using EPDM-g-MAHcopolymer

The EPDM was grafted by different concentrations ofMAH (maleic anhydride) (0.05, 0.07, 0.1 and 0.2) underthe action of γ-irradiation. The IR spectra of EPDMgrafted by MAH are shown in Figs. 6a and b).It wasfound that, in the case of EPDM-g-MAH, four strongabsorption bands at 1740, 1638.5, 1155.6, and 722 cm–

Fig. 6. (a) The infrared spectrum. a) EPDM, b) EPDM grafted by MAH with cnc. 0.05% at 2 Kgray, c) EPDM grafted by MAHwith cnc. 0.07% at 2 Kgray, d) EPDM grafted by MAH with cnc. 0.1% at 2 Kgray. (b) The infrared spectrum. a) EPDM, e) EPDMgrafted by MAH with cnc. 0.05% at 4 Kgray, f) EPDM grafted by MAH with cnc. 0.07% at 4 Kgray.

1 are observed due to stretching absorption ofC=O,C=C,–C–Oand CH bending frequency of MAH,which confirms that grafting by MAH occurred on theEPDM chains. The optimum grafting yield was found tobe 14%, as shown in Table 1, at 4 k gray and at MAHconcentration of 0.07%.The concentration of the compa-bitilizer was selected as 10%, which was found to bevery suitable. Fig. 7 shows the linear relationship of [η]vs. blend composition of NR/EPDM compatibilized by10% of EPDM-g-MAH graft coploymer. This confirmsthe complete homogeneity of these two rubbers witheach other. In other words, a grafted polymer can act asan effective compatibilizer for NR/EPDM blends.

3.3. Effect of some other compatibilizers

Some other compatibilizers such a chlorinated rubber,BR, chlorosulfonated polyethylene and PVC with an

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Table 1Graft yield (G.Y.%) of EPDM grafted by (MAH)

Radiation Dose k. Concentration of G.Y. %gray MAH %

2 0.05 5.42 0.07 7.262 0.1 6.12 0.2 Gel4 0.05 84 0.07 144 0.1 Gel4 0.2 Gel6 0.05 Gel6 0.07 Gel6 0.1 Gel6 0.2 Gel

Fig. 7. The variation of intrinsic viscocity [η] with blend ratioof NR/EPDM containing 10 phr MAH-g-EPDM.

Fig. 8. The variation of intrinsic viscocity [η] with blend ratioof NR/EPDM containing 10 phr chlorinated rubber.

Fig. 9. The variation of intrinsic viscocity [η] with blend ratioof NR/EPDM containing 10 phr BR.

Fig. 10. The variation of intrinsic viscocity [η] with blendratio of NR/EPDM containing 10 phr chlorosulfonated PE.

Fig. 11. The variation of intrinsic viscocity [η] with blendratio of NR/EPDM containing 10 phr PVC.

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optimum concentration of 10 phr were investigated bythe viscometric technique; the results obtained are illus-trated in Figs. 8–11. The linearity obtained in these fig-ures confirms the great improvement of homogeneity ofNR/EPDM blend in the presence of these compatibiliz-ers.

3.4. Electron microscope investigations

Electron micrographs represented in Fig. 12 (a) showthe morphology of 50/50 wt % blend of NR/EPDM. Theinspection of this micrograph indicates two phases withirregular domain size and shape. This means that NR/EPDM blends are completely immiscible where largeEPDM domains are dispersed in the NR matrix. Thecompatibility of the NR/EPDM system is improved bythe addition of a compatibilizer as can be seen from Fig.12 (b, c, d, e, f and g.) Also, the treatment resulted innoticeable surface hardening and the physical changes in

Fig. 12. Scanning electron micrograph with magnification1000. a) 50/50 NR/EPDM without compatibilizer, b)NR/BR/EPDM, c) NR/PVC/EPDM, d) NR/chlorinatedrubber/EPDM, e) NR/chlorosulfonated PE/EPDM, f) NR/γ-radiation/EPDM, g) NR/g-MAH/EPDM.

the surface are expected to influence both the defor-mation and adhesion of the two rubbers. In other words,compatibilizers improved both the morphology and thecompatibility of the blend due to the reduction of theinterfacial tension between EPDM and NR rubbers. Itwas seen that the size of the dispersed phase (EPDM)domain decreases with the addition of compatibilizer andno gross phase separation is present in the blend. Gener-ally, the obtained results comply with the theory of Kob-erstein et al. [22], Legge et al. [23] and Meier [24],which states that compatibilizers reduce phase domainsize.

3.5. Differential scanning calorimatry (DSC)

The thermal characteristics of both NR, EPDM andtheir blend (50/50 NR/EPDM) were examined by DSCin the temperature range from –100 to +50 °C, whichallows the identification of the glass transition tempera-tures, Tg. Table 2 contains the Tg values for NR, EPDMand their blend with and without compatibilizers, andFig. 13 illustrates the scan traces. It can clearly be seenthat for a 50/50 wt % blend of NR/EPDM, there are twodistinct glass transitions, the lower glass transition is dueto the EPDM phase and the higher glass transition is dueto the NR phase. The large melting endotherm is attri-buted to the high crystallinity of the EPDM. It is worthnoticing that the specific heat capacity (� Cp) valuedepends on the type of rubber, compatibilizer and con-centration of the blend. From inspection of Table 2 onecan see that the mean Tg value of pure NR is –63 °Cand changes to –64 °C in the blend; the Tg value of pureEPDM is –37 °C and changes to –45 °C in the blend.This may be due to some interaction between NR andEPDM at the boundaries of their phases forming a thirdphase [25].

The DSC thermographs which show the compatibiliz-ing effect of BR, PVC, EPDM-g-MAH and γ- radiationon NR/EPDM are displayed in Fig. 13 (d, e, f and g). Itappears that the Tg of each component in the blendshows temperature shift: NR/BR/EPDM � NR/γ-radiation/EPDM � NR/g-MAH/EPDM �NR/PVC/EPDM Only one glass transition temperature isdetected after adding chlorinated rubber or chlorosulfon-ated PE to NR/EPDM blends, indicating improved com-patibility or dominace of this phase as shown in Fig. 13(h and m). Also, when the compatibilizers are added tothis blend, the glass transition becomes less distinct indi-cating improved compatibility.

4. Conclusion

The incorportion of compatibilizers into NR/EPDMblends greatly enhances their compatibility and greatlyimproves the rheological properties of rubber blends.

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Table 2DSC results obtained for NR, EPDM and 50/50 NR/EPDM without and with compatibilizers

Contribution from NR Contribution from EPDMCode Sample Tg, °C Shift in NR Tg,°C Shift in EPDM Tg (°C)

Tg (°C)

NR - 63 — — —EPDM — — –37 —NR/EPDM (without compatibilizer, control) –64 — –45 —NR/BR/EPDM –58 +6 –44 + 1NR/PVC/EPDM –62 +2 –42.5 +2.5NR/g-MAH/EPDM –60 +4 –40 +5NR/γ-radiation/EPDM –59.6 +4.4 –45.6 –0.6NR/chlorinated rubber / EPDM (one glass transition) –48 +12 –48 –3NR/chlorosulfonated/EPDM (one glass transition) –60 +4 –60 –14

Fig. 13. DSC spectrum of a) NR, b) EPDM, c) 50/50 NR/EPDM, d) NR/BR/EPDM, e) NR/PVC/EPDM, f) NR/g-MAH/EPDM, g)NR/γ-radiation/EPDM, h) NR/chlorinated rubber/EPDM, m) NR/chlorosulfonated PE/EPDM.

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The compatibilizers are able to create a well-dispersedbicontinuous phase that exhibits rheological propertiesvery similar to those obtained for compatible blends hav-ing one glass transition.

Acknowledgements

The author wishes to express her sincere appreciationand thanks to Prof. Dr. A Yehia for his encouragementand support during the progress of this work. Also, mydeepest thanks to Hussein S Hegazy, Department ofPhysical Chemistry for his helpful discussions in theIR interpretation.

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