Ionic elastomer blends of zinc salts of maleated EPDM rubber and carboxylated nitrile rubber

Ionic Elastomer Blends of Zinc Salts of Maleated EPDMRubber and Carboxylated Nitrile Rubber

SANTANU DUTTA, P. P. DE, S. K. DE

Rubber Technology Centre, Indian Institute of Technology, Kharagpur 721302, India

Received 13 February 1996; accepted 24 November 1997

ABSTRACT: Blends of zinc salts of maleic anhydride-grafted EPDM rubber and carboxyl-ated nitrile rubber behave as ionic elastomers. Measurement of physical propertiessuggest that the blend is compatible. It is proposed that the compatibility arises pre-sumably due to formation of interfacial ionic aggregates. Dynamic mechanical andinfrared spectroscopic studies reveal that the ionic aggregates can be solvated by ammo-nia. q 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 153–160, 1998

Key words: ionomeric polyblend; Zn-salt of maleated EPDM rubber; Zn-salt of car-boxylated nitrile rubber; ionic elastomer; compatible blends.

INTRODUCTION ment of physical properties, dynamic mechanicalproperties, and infrared spectroscopic studies.

Both carboxylated nitrile rubber (known as XNBR)and maleated EPDM rubber (known as m-EPDM) EXPERIMENTALare known to form ionic elastomers on neutraliza-tion with a suitable base like zinc oxide (ZnO).1–5

m-EPDM was supplied by Uniroyal Chemical Co.,Both the ionic elastomers can be processed like (Naugatuck, CT). Its weight average molecularthermoplastics in the presence of zinc stearate.4,6

weight (Mw ) was 3.991 105, and its number aver-The present article aims at the development of age molecular weight (Mn ) was 1.16 1 105. The

an ozone-resistant and oil-resistant ionic thermo- maleic acid/anhydride content was 1%. Mooneyplastic elastomer based on the blend of XNBR and viscosity, ML1/4 at 1007C, was 60.m-EPDM. One important consideration in mak- XNBR was supplied by Goodyear Rubber anding such a blend of two chemically dissimilar poly- Tire Co. (Akron, OH). Its carboxyl content was 1mers is the lack of their compatibility. Very often, mol %. Mooney viscosity, ML1/4 at 1007C, was 48.such systems are grossly incompatible, and in- ZnO was laboratory-grade reagent and was ob-terfacial agents are required to obtain compatible tained from BDH Chemicals (Calcutta, India).blends.7,8

Composition of the blends is shown in TableAlthough blends of ionomers with other poly- I. Blends were prepared in a two-roll mixing

mers have been studied,9–12 studies on the rub- mill at room temperature at a nip gap of 2 mm.bery ionomers have received little attention. The First, XNBR was premasticated for about 1 minpresent article reports the results of studies on so that it can form a smooth band. Next, ZnOthe binary blend of zinc oxide neutralized was mixed separately with m-EPDM for aboutm-EPDM and XNBR. Studies include measure- 2 min. Finally, the ZnO–m-EPDM rubber mas-

terbatch was blended thoroughly with XNBRfor about 3 min.

Correspondence to: S. K. De.The blends were molded for 1 h at 1707C. Mold-

Journal of Applied Polymer Science, Vol. 69, 153–160 (1998)q 1998 John Wiley & Sons, Inc. CCC 0021-8995/98/010153-08 ing was done in an electrically heated press at a

153

8E39 5221/ 8e39$$5221 04-08-98 19:43:19 polaa W: Poly Applied

154 DUTTA, DE, AND DE

Table I Physical Propertiesa

Formulation,b XNBR/m-EPDM (in parts by weight)

Property 100/0 80/20 50/50 20/80 0/100

100% Modulus (MPa) 2.11 1.93 1.52 1.28 1.12(0.76) (1.10) (1.03)

300% Modulus (MPa) 4.25 4.14 3.08 2.15 1.84(0.81) (1.30) (1.43)

Tensile strength (MPa) 17.84 14.12 11.20 8.32 4.40(2.02) (2.87) (3.82)

Elongation at break (%) 944 850 863 1010 850(900) (650) (600)

Tear strength (N/cm) 441 419 373 349 332Tension set at 100% extension (%) 17 22 22 13 13

a Values in parentheses are the results of measurements of ammonia-treated samples.b Each formulation contains 10 parts of ZnO.

pressure of 10 MPa. After molding, the mixes RESULTS AND DISCUSSIONwere cooled to room temperature by circulation ofcold water through the platens. Physical Properties

Physical properties were determined in a ZwickThe representative stress–strain plots for differ-UTM 1445 at a crosshead speed of 500 mm min01 .ent mixes are shown in Figure 1. Results of mea-Tensile stress–strain and tension set tests weresurement of the physical properties are summa-done using dumbbell specimens cut according to

ASTM D412 specification. Tension set was mea-sured at 100% extension. Tear strength was deter-mined with 907 nick-cut crescent tear samples cutaccording to ASTM-624 die C specification.

Dynamic mechanical studies were made undertension mode, using a viscoelastometer, namelyRheovibron (DDV-III-EP) of M/s (Orientec Corp.,Tokyo, Japan). Sample size was 3.5 cm 1 6.5 mm1 2 mm. Testing was conducted at a frequency of3.5 Hz over a temperature range of 01007C to/2007C at a heating rate of 17C min01 .

Infrared spectra were taken in a Perkin-Elmerdispersive infrared spectrophotometer, model 843,attached with a computer for data processing.The spectrophotometer was equipped with acoated deuterated triglycine sulfate pyroelec-tric type detector, and protected by a cesiumiodide window and interchangeable dispersiongratings. Spectra were taken at a resolution of3.2 cm01 with an in-built noise level of 0.3%transmittance. The inherent frequency accu-racy was {4 cm01 in the range of 4000 to 2000cm01 and {2 cm01 in 2000 to 200 cm01 . A set of16 scans were signal-averaged.

Ammonia treatments were done by keeping Figure 1 Representative stress–strain plots forthe samples in a desiccator over ammonia vapor m-EPDM/XNBR/ZnO systems. —- — , 100/0/10; - - -,

50/50/10; and , 0/100/10.for 48 h.


IONIC ELASTOMER BLENDS OF ZINC SALTS 155

Results of dynamic mechanical properties aresummarized in Table II. The results show thatthe ZnO-neutralized XNBR shows two peaks inthe tan d curve. The most intense peak occursaround 0.77C. This low temperature tan d peakis accompanied by a pronounced fall of storagemodulus from the glassy state to the rubberystate. This transition is believed to be due to mainchain glass-rubber transition, and the correspond-ing temperature is abbreviated as Tg2 . The secondpeak occurs at a higher temperature around 607C,which is in the same temperature range reportedearlier in the case of XNBR–ZnO system.6 In ionicpolymers, the high temperature peak is reportedto be due to relaxation of immobile chains presentin the ion-rich domains, which constitute the sec-ond phase. The ionic forces of interaction leads toformation of ionic aggregates called multipletsand clusters that greatly restrict the mobility ofthe adjacent chains and becomes responsible forthe formation of the biphasic structure.13–15 Thehigh temperature relaxation is labeled as Ti .

The m-EPDM–ZnO ionic elastomer shows themain glass-rubber transition around 041.87C(known as Tg1) and a weak high temperaturetransition around /407C. That the weak transi-tion is not due to melting of any crystallites issubstantiated by the fact that m-EPDM contains50% of ethylene units and the material is essen-

Figure 2 Variation of physical properties with blendratio in m-EPDM/XNBR/ZnO systems. (a) Tensilestrength and 300% modulus. (b) Hysteresis loss andtear strength. Solid lines indicate the calculated aver-age values. Experimental results are shown by thepoints on the lines.

rized in Table I. From Figure 1 and Table I, it isevident that the XNBR–ZnO system shows maxi-mum tensile strength, modulus, and tear resis-tance, whereas the m-EPDM–ZnO system showsthe lowest values. For a compatible blend of twopolymers, the physical properties should lie in be-tween the two component polymers and follow theadditivity rule.8 From Figure 2, it is evident that Figure 3 Representative plots of storage modulusthe physical properties follow the additivity line. and tan d against temperature. (a) — , 100/0 m-EPDM/

ZnO; -----, 100/0 m-EPDM/ZnO after ammonia treat-ment; – r– , 100/0 XNBR/ZnO; –

I– , 100/0 XNBR/Dynamic Mechanical Properties

ZnO after ammonia treatment. (b) — , 50/50/10 m-In Figure 3 is shown typical plots of storage modu- EPDM/XNBR/ZnO system; -----, 50/50/10 m-EPDM/

XNBR/ZnO system after ammonia treatment.lus and loss tangent (tan d ) against temperature.



Table II Results of Dynamic Mechanical Analysesa

Formulation,XNBR/m-EPDM/ZnO Tg1

b Tan d at Tg2c Tan d at Ti Tan d at

(Parts by Weight) (7C) Tg1 (7C) Tg2 (7C) Ti

100/0/10 — — /0.7 0.586 51.3 0.297(05.3) (0.535) d d

80/20/10 039.7 0.069 /0.7 0.485 50.8 0.23850/50/10 038.6 0.197 02.2 0.398 42.3 0.180

(034.8) (0.216) (010.5) (0.400) d d

20/80/10 036.2 0.604 09.3 0.189 40.4 0.1300/100/10 041.8 1.192 — — 38.0 0.094

(040.0) (1.199) d d

a Values in parentheses are the results of measurements of ammonia-treated samples.b Tg1 refers to the m-EPDM phase.c Tg2 refers to the XNBR phase.d Transition absent.

tially a random copolymer, which is essentially chains adjacent to the ionic domains. Disruptionof the ionic domains results in loss of physicaldevoid of any degree of crystallinity.16

properties, as discussed previously.That ammonia causes destruction of the ionic

Solvation aggregates by solvation is substantiated by theresults of infrared spectroscopic studies. Ammo-Previously, it has been reported that dimethylnia is reported to have a broad infrared absorptionsulfoxide solvates the ionic aggregates and causesband around 3414 cm01 , and another band atdisappearance of the high temperature peak.4,17

3337 cm01 due to N{H stretching vibration.20It has also been reported that glycols, water, and

The infrared spectra of ammonia-treated 50/50amines are good solvating agents for ionic aggre-XNBR–m-EPDM blend is shown in Figure 4 andgates in ionomers.18,19 Solvation occurs due to ei-compared with the spectra of the untreated blend.ther complex formation with the cation or forma-On ammonia treatment, there occurs a broadtion of hydrogen bonds with the anion. In the pres-band with two peaks around 3460 cm01 and 3360ent case, the blend and the component polymerscm01 . Considering the 3414 cm01 band as thewere solvated by keeping the samples in ammo-main band for ammonia, the low frequency shiftnia-saturated atmosphere for 48 h. This resultsof N{H stretching peak (i.e., from 3414 cm01 toin a drastic fall in physical properties like tensile3360 cm01) can be assigned due to interaction ofstrength and modulus (Table I) . Because XNBRammonia with the ionomer by hydrogen bonding,is more polar than m-EPDM, diffusion of ammo-ion-dipole forces, and complexation.21–25 Lu andnia is greater in XNBR-rich blends, and the fallWeiss21 have reported that the spectrum of poly-is more prominent in the blends with high XNBRamide-6 is characterized by a broad band withcontent.absorbance maxima around 3300 cm01 ,21 whichEffect of ammonia treatment on the plots of tanwas attributed to composite absorption of N{Hd and storage modulus vs. temperature is showngroups present in different phases. They havein Figure 3, and the results of the dynamic me-shown that, on complexation with manganese-sul-chanical studies are shown in Table II. It is evi-fonated polystyrene, the band was broadened anddent that ammonia causes minor reduction in thethe peak maximum was shifted to a lower fre-glass-rubber transition temperature and lowersquency region. It was also observed that a higherdown the storage modulus at the rubbery regionfrequency band of free N{H stretching (3447apparently due to some plasticization of the maincm01) was shifted to a lower frequency region.chain. However, ammonia treatment causes dis-Effects were attributed to complexation of Mn/2appearance of the high temperature ionic transi-by amino groups and formation of H{bonds be-tion. It is believed that the ionic aggregates aretween N{H groups with sulfonate anions. Onsolvated by ammonia, thus suppressing the for-

mation of the rigid phase due to immobility of the other hand, a new peak in the blend appeared



been reported in the case of polyamide-zinc sulfo-nated polystyrene blend.22 Similar reasoning maybe ascribed to the occurrence of the higher fre-quency band at 3460 cm01 in the present case.

It has been stated earlier that any group thatforms complex with the cationic part, as well asforms hydrogen bonds with the anionic part,should result in a shift to a lower frequency of theionic peak (due to anionic stretching).21–23 In thepresent case (e.g., for the 50/50 XNBR–m-EPDMblend), the {COO0 asymmetric stretching fre-quency peak that occurs around 1588 cm01 (refs.27–29) before ammonia treatment is shifted to alower frequency region and forms a doublet at1574 and 1556 cm01 after ammonia treatment.For XNBR–ZnO and m-EPDM–ZnO systems also(spectra not shown), similar observation wasmade. The downfield shift of the {COO0 asym-metric stretching frequency band is thus ex-plained by considering its hydrogen bonding withN{H groups resulting in weakening of the ionicassociates. Appearance of the doublet is believedto be due to the changes in the geometrical ar-rangement of the {COO0 groups surroundingthe central metal ion24 as a result of complexationof the metal ion by ammonia and hydrogen bond-ing of the {COO0 groups with N{H groups.Similar low frequency shift was observed in thehydrated Nafion system.25

It is thus apparent that the high temperaturetransition in XNBR–ZnO and m-EPDM—ZnO

Figure 4 Infrared spectra of 50/50/10 m-EPDM/XNBR/ZnO system. (a) Before ammonia treatment.(b) After ammonia treatment.

at 3520 cm01 . This peak was attributed to delocal-ization of a lone pair of electrons as a result of Figure 5 Dependence of Ti and tan d at Ti on blend

composition of m-EPDM/XNBR/ZnO system.complexation.21 Similar observations have also



systems and their blends is due to the rigid phasearising out of the ionic aggregates, which, how-ever, gets complexed and masked by ammonia,suppressing the formation of the biphasic struc-ture in the constituent polymers and their blend.

Compatibilization

It is apparent from Table II and Figure 3 thatthere occurs two thermal events in the blends thatare ascribed to two glass transitions and the corre-sponding temperatures are Tg1 and Tg2 . Theglass-transition temperature of the two compo-nents shows no appreciable change on blending.Hence, we can conclude that the blend is essen-tially immiscible. This is not unexpected in viewof the difference in polarity of the two polymers.7

The high temperature transition correspondingto Ti in the blends shows the existence of singletransition, which occurs inbetween the Ti’s of thetwo polymers (Fig. 5, Table II) . Tan d values atTi indicate that, for XNBR—ZnO system, tan dis much greater than that for the m-EPDM–ZnOsystem. This is believed to be due to a higher ex-tent of cluster formation in the XNBR—ZnO sys-tem, which in turn depends on the higher extentof carboxylate salt formation. It has been reportedthat higher ion content in an ionic polymer facili-tates phase separated ionic cluster formation.25

Dependence of the tan d at Ti on blend composi-tion is shown in Figure 5. The experimental val-ues are very close to the calculated averages. Ac-cordingly, it is proposed that the ionic domainsarising out of the constituent polymers merge to-gether and form a hard phase at the interfaceof the two polymers. It is believed that the ionicaggregates serve as interfacial crosslinking sites

Figure 6 Infrared spectra of different systems (a)100/10 m-EPDM/ZnO, (b) 100/10 XNBR/ZnO, and (c) Figure 7 Schematic representation of interfacial50/50/10 m-EPDM/XNBR/ZnO. ionic aggregates. , XNBR; , m-EPDM.



Table III Results of Recycling by Repeated 2. In the presence of ammonia, the high temper-Moldings of the Blend Composition ature relaxation due to ionic groups in theXNBR/m-EPDM/ZnO (50/50/10) polymer disappears due to solvation of the

ionic species by ammonia.300% Tensile Elongation 3. Results of reprocessability studies reveal that

Cycle Modulus Strength at Break the blend is reprocessable at least up to threeNo. (MPa) (MPa) (%)cycles of molding.

1 3.08 11.20 8632 3.10 11.11 8543 3.23 11.20 850

The authors are grateful to the Council of Scientific and4 3.28 11.06 841Industrial Research (C.S.I.R.) , New Delhi, for provid-ing the financial support in conducting this work.Thanks are due also to Uniroyal Chemical Co. for sup-and compatibilize the two otherwise incompatibleplying the rubbers.

polymers in the blend.In Figure 6 is shown the infrared spectra of

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Ionic elastomer blends of zinc salts of maleated EPDM rubber and carboxylated nitrile rubber

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