Thermoplastic elastomer based on ionomer.

Ionomers are ion containing polymers, composed of a hydrocarbon backbone, containing pendant acid groups which are neutralized at least partially and the concentration of salt groups in ionomers is, in general, less than 10 mole % (ref. 1). Ionomers are emerging as important industrial polymers (ref 2). The ionic groups aggregate in the bulk and form physical crosslinks which cause profound improvement in the physical properties of the ionomers (refs. 3-7).

Ionomers in general show unusually high melt viscosities due to strong intermolecular associations and the relatively high stability of the ionic clusters, thus making their melt processing difficult (ref 8). As is done in the case of conventional polymers, plasticizers may be incorporated in ionomers for lowering their melt viscosity and improving their processability. The two phase morphology and the large difference in the polarity of the hydrocarbon and ionic phases offers two possibilities for the plasticization of ionomers, namely the plasticization of the hydrocarbon phase, and the plasticization of the ionic clusters.

Bazuin and Eisenberg (ref.9) have studied the effects of polar (glycerol) and nonpolar (ethyl benzene) plasticizers on the dynamic mechanical properties of styrene/methacrylic acid ionomers. Recently Hara et al (ref 10) have reported the role of dimethyl formamide (DMF) as a dual plasticizer for sulfonated polystyrene ionomers. DMF is known as a polar solvent that disrupts ionic interactions (ref 11). Navaratil and Eisenberg (ref 12) have reported the use of dimethyl sulfoxide (DMSO) as an ionic plasticizer for sodium-neutralized poly(styrene-co-methacrylic acid).

The first part of this article reports the results of studies on paraffinic oil and DMSO plasticization of the zinc salt of sulfonated ethylene-propylene-diene terpolymer rubber containing 75 weight % ethylene.

While studying the effect of fillers on the properties of this material, we observed improvement in physical properties on incorporation of HAF carbon black. The second part of this article reports the results of studies on the effect of HAF carbon black on physical properties and processability of zinc sulfonated EPDM.

Experimental

The polymers used in this study are:

* Thermoplastic ethylene-propylene-diene terpolymer (EPDM) containing 75% ethylene, 20% propylene and 5% 5-ethylidene-2-norbornene, and

* zinc salt of the sulfonated ethylene-propylene-diene terpolymer formed by the sulfonation of the pendent unsaturation in the thermoplastic EPDM followed by neutralization of the resultant EPDM sulfonic acid using the procedure described by Makowski et al (ref 13).

The level of sulfonation in the zinc sulfonated EPDM was 30 meq/100 g polymer. The number average molecular weight of EPDM was 52,000 and the weight average molecular weight was 151,000. The paraffinic oil used was rubber grade and had a density of 0.83 g/cc. The dimethyl sulfoxide (DMSO) had a density of 1.10 g/cc. The carbon black used was high abrasion furnace type (HAF); N-330 grade; surface area [80m.sup.2]/g; ph 7.6.

Rubber compounds were mixed in a Brabender Plasticorder (model PLE-330) using cam type rotors for six minutes at a rotor speed of 80 rpm and at a temperature of 150[degrees]C. The neat polymers were also masticated under the same conditions. The test specimens were prepared by molding in an electrically heated hydraulic press for five minutes at 160[degrees]C.

Dynamic mechanical analysis were performed in a viscoelastometer at a frequency of 3.5 Hz and a strain amplitude of 0.0025 cm. The measurements were carried out over a temperature range of -150[degrees]C to +200[degrees]C at a heating rate of 1[degrees]C/minute.

The hardness was determined as per ASTM D2240 (1986) and expressed in Shore A units. The stress-strain properties were determined at 25[degrees], 50[degrees] and 70[degrees]C according to ASTM D412 (1987) using dumbbell specimens in a universal testing machine (UTM), fitted with a temperature controlled cabinet using a cross-head speed of 500 mm/minute. The tension set at 200% elongation was determined as per ASTM D412 (1987). The tear resistance was determined as per ASTM D624 (1986) using unnicked 90[degrees] angle test pieces (die C) at 25[degrees]C at a cross head speed of 500 mm/min. in the UTM. The abrasion resistance was determined in an abrasion tester (BS 903: Part A9 1957 method C) and expressed as abrasion loss, which is the volume in [cm.sup.3] abraded from a test specimen per hour. The hysteresis loss was determined according to ASTM D412 (1980) by stretching dumbell specimens to the desired strain level.

The processability studies were carried out by a processability tester (PT) at the shear rates of 36, 90, 181 and 289 [sec.sup.-1] and a temperature of 170[degrees]C. The capillary length (29.97 mm) to diameter (1.50 mm) ratio was 20 with an entrance angle of 45[degrees] and 60[degrees] (compound). The preheat time for each sample was five minutes.

The reprocessability of the polymer was tested by extruding the sample in the PT at 179[degrees]C, at a shear rate of 18 [sec.sup.-1], and the extrudate as re-extruded under similar conditions and the process was repeated up to three cycles. Before each extrusion the sample was preheated in the barrel of the PT at 160[degrees]C for 10 minutes. Apparent viscosity and apparent shear stress after each cycle were noted. The extrudate sample after each cycle was allowed to rest 24 hours before the tensile strength was measured.

Results and discussions

Effect of plasticizers

Variation of dynamic mechanical properties with temperature gives an idea about the different transition in the polymers. Figure 1a shows the plots of loss tangent (tan [delta]) versus temperature of EPDM and zinc sulfonated EPDM. The glass-rubber transition (Tg) occurs at around -26[degrees]C for both the polymers. Apart from Tg, zinc sulfonated EPDM shows two other transitions, one at +119[degrees]C, which is believed to be due to the melting of crystalline one of the polyethylene block, and another broad transition in the temperature range of +27 to 80[degrees]C, which is ascribed to the ionic aggregates. It has been reported earlier that in the case of crystalline ionomers, transition due to ionic aggregates occurs at a lower temperature than the crystallite melting temperature (refs. 14 and 15). In the case of rubbery ionomers, transition due to ionic groups has been found to occur in the same temperature range (ref. 16), as observed in the present study. In the case of EPDM, expectedly the broad transition (27[degrees] to 80[degrees]C) due to ionic aggregates was not observed and also the high temperature transition due to crystalline zone could not be detected due to softening of the sample above 100[degrees]. In the case of zinc sulfonated EPDM, the presence of ionic aggregates makes the matrix rigid enough or crystallite melting zone to be detected (refs. 14 and 15).

Figure 1b shows the effects of plasticization on the dynamic mechanical properties of zinc sulfonated EPDM. Paraffinic oil is known to be a plasticizer of EPDM (ref 17). Paraffinic oil causes a reduction in the glass rubber transition (Tg) due to the plasticization of the backbone chain and the transition due to ionic aggregates is only marginally affected. However, DMSO causes no reduction in Tg, but the transition due to ionic aggregates is not detectable due to the disruption of the ionic domains. Results of dynamic mechanical analysis are summarized in table 1. The melting of the crystallites occurs at a lower temperature in presence of plasticizer, and the effect is more pronounced in the case of DMSO than oil.

[TABULAR DATA OMITTED]

These observations can be explained on the basis of the shell core model for the distribution of salt groups in ionomers (refs. 18-20). The shell core model postulates that in the dry state a cluster of ~0.1 mm in radius is shielded from the surrounding matrix ions not incorporated into the clusters by a shell of hydrocarbon chains. The surrounding matrix ions which cannot approach the cluster more closely than the outside of the hydrocarbon shell will be attracted into the cluster by electrostatic forces. The matrix ions associate to form multiplets which act as ionic crosslinks and affect the properties of the matrix.

Figure 2 shows the log-log plots of apparent viscosity versus shear stress and shear rate. The reduction in viscosity with increasing shear stress is due to the pseudoplastic nature of the compounds. At all shear stresses and shear rates the compounds show lower viscosity than the unplasticized zinc sulfonated EPDM and the viscosity values follow the order, unplasticized > oil > DMSO. In conformity with the results of DMA studies, disruption of ionic aggregates of the zinc sulfonated EPDM by DMSO causes sharp fall in the viscosity. It is apparent that the plasticization of ionic domains by 10 phr of DMSO is more efficient than the plasticization of the backbone hydrocarbon by 10 phr of paraffinic oil.

The results of hysteresis studies are shown in figure 3. The area under the hysteresis curve is lowest for EPDM and highest for zinc sulfonated EDPM. Although the plasticized compounds show lower hysteresis than the neat zinc sulfonated EPDM, the compound containing DMSO shows lower hysteresis than the compound containing paraffinic oil. The high value of hysteresis shown by zinc sulfonated EPDM compared to EPDM is due to the additional energy dissipation mechanisms in the former (ref. 21), because the ionic aggregates may be considered to behave as ultrafine particles of a reinforcing filler in the host polymer in addition to acting as multifunctional crosslink (ref. 22). The reduction in hysteresis shown by the compounds containing paraffinic oil and DMSO may be attributed to the plasticization of the hydrocarbon backbone, and the ionic aggregates, respectively.

Figure 4 shows the tensile stress-strain behavior of EPDM, Zinc sulfonated EPDM and the plasticized compounds at 25[degrees], 50[degrees] and 70[degrees]C. Modulus and tensile strength values are higher in the case of zinc sulfonated EPDM as compared to EPDM at all three temperatures. Zinc sulfonated EPDM showed less reduction in tensile strength at higher temperatures compared to EPDM. This is attributed to the presence of physical crosslinks in th zinc sulfonated EPDM. The physical crosslinks or hard domains in the zinc sulfonated EPDM arises out of the ionic aggregates and the crystalline region. The presence of the ionic aggregates makes the matrix rigid, thereby enabling the crystalline regions to exert their influence which is not observable in the case of EPDM. The morphological structure in zinc sulfonated EPDM is believed to be similar to that of conventional thermoplastic elastomers, that is, a combination of hard domains and soft segments as depicted in figure 5. Both the plasticizers cause reduction in physical properties at the three test temperatures, which is due to plasticization of main chain in the case of paraffinic oil and disruption of ionic structure in the case of DMSO. As compared to the paraffinic oil, DSMO causes greater fall in physical properties at higher temperatures.

Effect of HAF carbon black

The physical properties of the neat polymers and HAF carbon black filled zinc sulfonated EPDM compounds are summarized in table 2. Zinc sulfonated EPDM showed higher hardness than thermoplastic EPDM. Hardness is a measure of modulus of elasticity at low strain (ref. 23). The increase in modulus of elasticity by ionic aggregates in zinc sulfonated EPDM may be the reason for its higher hardness. As expected, the hardness of zinc sulfonated EPDM was found to increase as the carbon black loading increased.

[TABULAR DATA OMITTED]

The stress-strain behavior of EPDM, zinc sulfonated EPDM and the compounds of zinc sulfonated EPDM containing 10, 20 and 35 phr of carbon black filler at 25[degrees], 50[degrees] and 70[degrees]C is shown in figure 6. Incorporation of filler causes gradual increase in modulus and decrease in elongation at break, of zinc sulfonated EPDM, but the tensile strength remains almost constant. In presence of carbon black, zinc sulfonated EPDM showed greater retention of the properties at elevated testing temperatures. Incorporation of carbon black filler seems to strengthen the physical crosslinks arising out of the ionic aggregates present in zinc sulfonated EPDM, as elaborated later in this article, thereby increasing the high temperature strength properties. For the same reasons tension set values (table 2) gradually decreased with increase in filler loading. Incorporation of carbon black in EPDM increases the room temperature stress-strain properties due to weak rubber-filler interaction, but at higher test temperatures the properties drop down sharply. This is in sharp contrast to the observation made with carbon black filled zinc sulfonated EPDM.

Zinc sulfonated EPDM showed higher tear strength as compared to the control thermoplastic EPDM. Tear resistance of elastomers is a measure of crack propagation. It is known that tear strength is enhanced by factors which tend to dissipate energy (ref. 21). The ionic domains of zinc sulfonated EPDM may be acting as tear deviators or arrestors. Incorporation of carbon black caused further deviation in tear path and thereby causing further improvement in the strength. Results of hysteresis studies are shown in figure 7. Areas under the hysteresis loop run parallel with the tear strength. The hysteresis of zinc sulfonated EPDM was found to increase with the incorporation of the carbon black and it gradually increased as the loading of the filler increased. The increase in hysteresis on addition of filler is due to additional energy dissipation mechanisms, such as motion of filler particles, chain slippage or breakage, and dewetting at high strains (ref. 21). Additional energy dissipation in zinc sulfonated EPDM and its carbon black filled compounds may be the reason for their higher tear strength. Abrasion resistance also follows the same pattern as the tear strength.

Results of dynamic mechanical analyses are summarized in table 3. Zinc sulfonated EPDM showed higher storage modulus than the control EPDM. Incorporation of carbon black increased the storage modulus of zinc sulfonated EPDM.

[TABULAR DATA OMITTED]

Figure 8 shows the variation of loss tangent (tan [delta]) with temperature for EPDM, zinc sulfonated EPDM, and the compounds of zinc sulfonated EPDM containing HAF carbon black. The glass-rubber transition (Tg) occurs around -26[degrees]C in all cases. The tan [delta.sub.max] (that is, the [delta] value at Tg ) was less in the case of zinc sulfonated EPDM as compared to EPDM. This is because of the stiffening imparted by the ionic domains in zinc sulfonated EPDM (ref. 24). Incorporation of HAF carbon black causes lowering of tan [delta.sub.max] of the zinc sulfonated EPDM due to strong rubber-filler interaction involving the backbone chains. Incorporation of filler increased the tan [delta] value at the high temperature ionic transition (Ti). The tan [delta] value at Ti increased as the loading of carbon black increased.

From the above results it can be inferred that the rubber-filler interaction in the case of the black filled zinc sulfonated EPDM is of two types:

* The interaction between the filler particles and the nonpolar polymer backbone of zinc sulfonated EPDM, which is similar to the interaction involving diene rubbers and reinforcing fillers, as manifested in the lowering of tan [delta] at Tg, and

* The interaction between the ionic groups of the polymer and the polar groups (-OH,>C=0 etc.) present on the surface of the filler particles, which is manifested in an increase in tan [delta] at Ti.

While the rubber-filler interaction involving the nonpolar polymer backbone is of weak Van der Waals type, the same due to ionic aggregates can be of much stronger type, as proposed in figure 9. This may be the reason for higher retention of stress-strain properties at elevated test temperatures in the case of black filled compounds, as compared to the neat zinc sulfonated EPDM.

Figure 10 shows the variation of apparent viscosity, and tensile strength of the extrudate versus the number of cycles in extrusion through PT. It was observed that the viscosity of the mix and tensile strength of the extrudate remain almost constant by repeated preheatings and extrusions. This shows that zinc sulfonated EPDM behaves as a thermoplastic elastomer and can be reprocessed be mechanical recycling without deterioration in properties.

Conclusions

Zinc sulfonated EPDM containing 75% ethylene behaves as an ionic thermoplastic elastomer. Dynamic mechanical analyses of zinc sulfonated EPDM show two transitions in addition to Tg, which are ascribed to the ionic aggregates and the melting of the crystallites. Paraffinic oil acts as the backbone plasticizer and DMSO acts as the ionic domain plasticizer. Studies using the processability tester show that both paraffinic oil and DMSO reduce the melt viscosity of the zinc sulfonated EPDM and the reduction is higher in the case of DMSO. Zinc sulfonated EPDM shows higher hysteresis compared to EPDM. Although oil and DMSO cause reduction in the hysteresis, modulus and tensile strength of zinc sulfonated EPDM, the reduction is more pronounced in the case of DMSO, due to the plasticization of the ionic domains.

HAF carbon black increases the hardness, modulus, tear strength, hysteresis and abrasion resistance of the ionic thermoplastic elastomer based on zinc sulfonated EPDM of high ethylene content. Although the room temperature tensile strength of the black filled compounds is similar to the neat polymer, incorporation of the filler improves the retention of stress-strain properties of the polymer at elevated test temperatures. Dynamic mechanical analysis results indicate stiffening of the polymer matrix by ionic aggregates in zinc sulfonated EPDM which is further reinforced by incorporation of the black filler. Thermoplastic elastomeric nature of the polymer is evident from the constancy in the apparent viscosity and tensile strength of the extrudates even after three cycles through the processability tester.

References

[1.] W.J. Macknight and T.R. Earnest, Jr., J. Polym. Sci., Macromol. Rev., 16, 41 (1981). [2.] W.J. Macknight and R.D. Lundberg, Rubber Chem. Technol., 57, 652 (1984). [3.] A. Eisenberg and M. Kirng, "Ion-containing polymers: Physical properties and structure," Academic Press, New York, 1977. [4.] L. Holliday, Ed., "Ionic polymers," Applied Science Publishers Ltd., London, 1975. [5.] A. Eisenberg and F. Bailey, Eds., "Coulombic interactions in micromolecular systems," ACS Symposium Series No. 302, American Chemical Society, Washington, D. C, 1986. [6.] C.W. Lantman, W.J. Macknigh, and R.D. Lundberg, Annu. Rev. Mater. Sci., 1989, 19, 295 1989). [7.] M. Pineri and A. Eisenberg, Eds., "Structure and properties of ionomers," NATO ASI Series 198, D. Reidel Publishing Co., Dordrecht, Holland, 1987. [8.] R.A. Weiss, J.J. Fitzgerald and D. Kim, Macromolecules, 24, 1064 (1991). [9.] C.G. Bazuin and A. Eisenberg, J. Polym. Sci., Polym. Phys. Edn., 24, 1137 (1986). [10.] M. Hara, P. Jar and J.A. Sauer, Polymer, 32, 1380 (1991). [11.] R.D. Lundberg and R.R. Phillips, J. Polym. Sci., Polym. Phys. Edn., 20, 1143 (1982). [12.] M. Navaratil and A. Eisenberg, Macromolecules, 7, 84 (1974). [13.] H.S. Makowski, R.D. Lundberg and Jan Bock, U.S. Patent 4,184,988 (assigned to Exxon Research and Engineering Co.), 1980. [14.] Schinichi Yano, Nobuaki Nagao, Masayuki Hattori, Eisaku Hirasawa and Kenji Tadano, Macromolecules, 25, 368 (1992). [15.] Schinichi Yano, Kenji Tadano, Nobuaki Nagao, Shoichi Kutsumizu, Hitoshi Tachino and Eisaku Hirasawa, Macromolecules, 25, 7168 (1992). [16.] U.K. Mondal, D.K. Tripathy and S.K. De, Polymer, 34, 3832 (1993). [17.] F.P. Baldwin and G. Ver Strate, Rubber Chem. Technol, 45, 709 (1972). [18.] W.J. Macknight, W.P. Taggart and R.S. Stein, J Polym. Sci., Polym. Symp., No 45, 113 (1974). [19.] E.J. Roche, R.S. Stein and W.J. Macknight, J. Polym. Sci., Polym. Phys. Ed., 18, 1035 (1980). [20.] M. Fujimura, T. Hashimoto and H. Kawai, Macromolecules, 15,136 (1982). [21.] G. Kraus, "Science and technology of rubber," F.R. Eirich, Ed., Academic Press, New York, 1978. [22.] B. Hird and A. Eisenberg, Macromolecules, 25, 6466 (1992). [23.] Thomas H. Ferrings in "Handbook of fillers and reinforcements for plastics," Harry S. Katz and John V. Milewski (Eds.), Van Nostrand Rienhold Company, New York, 1978. [24.] P.P.A. Smit, Rheol. Acta., 5,277 (1966).