S-EPDM

A route to compatibility involving ionomers has been described recently by Eisenberg and coworkers [250-252]. The use of ionic interactions between different polymer chains to produce new materials has gained tremendous importance. Choudhury et al. [60] reported compatibilization of NR-polyolefin blends with the use of ionomers (S-EPDM). Blending with thermoplastics and elastomers could enhance the properties of MPR. The compatibility of copolyester TPE, TPU, flexible PVC, with MPR in aU proportions, enables one to blend any combination of these plastics with MPR to cost performance balance. Myrick has reported on the effect of blending MPR with various combinations and proportions of these plastics and provided a general guideline for property enhancement [253]. 

From X-ray and electron microscopy data (21). it was found that the ZnSt in S-EPEW phase separates into crystalline domains of less than 500 nm. It was also inferred from the data that the interactions between the sulfonate groups and the ZnSt occur on preferred crystal planes. In the context of stress relaxation results, it was proposed that the long cell axis of ZnSt lies at 90 to the stress axis, and that relaxation of the stress occurs by "interaction hopping . Sane stress relaxation studies of ZnSt plasticized S-EPDM were also reported by Granick (35). 

A thermopolastic elastomer based on sulfonated-EPDM, S-EPDM, was developed in the 1970 s by Exxon and more recently by Uniroyal. Unlike the synthesis of the carboxylate ionomers described above, S-EPDM is prepared by a post-polymerization sulfonatlon reaction(28). Compared to the metal neutralized S-EPDM, the sulfonic acid derivative is not highly associated. The free acid materials possess low strengths and are less thermally stable. The metal salts of S-EPDM have properties comparable to crosslInked elastomers, but they do exhibit viscous flow at elevated temperatures. In the absence of a polar cosolvent, such as methanol, hydrocarbon solutions of the metal salts of S-EPDM are solid gels at polymer concentrations above several percent(31). With the addition of 1 to 5% alcohol the polymer solution becomes fluid with solution viscosities of the order of 10 to 100 poise. 

Duvdevani(40) have been directed at modification of ionomer properties by employing polar additives to specifically interact or plasticize the ionic interactions. This plasticization process is necessary to achieve the processability of thermoplastic elastomers based on S-EPDM. Crystalline polar plasticizers such as zinc stearate can markedly affect ionic associations in S-EPDM. For example, low levels of metal stearate can enhance the melt flow of S-EPDM at elevated temperatures and yet improve the tensile properties of this ionomer at ambient temperatures. Above its crystalline melting point, ca. 120°C, zinc stearate is effective at solvating the ionic groups, thus lowering the melt viscosity of the ionomer. At ambient temperatures the crystalline additive acts as a reinforcing filler. 

In order to enable melt processing of ion containing polymers, such as S-EPDM, it is necessary to introduce a mechanism that weakens the ionic interactions. This can be achieved by the addition of a polar ingredient that would plasticize" ionic domains at elevated temperatures only. A variety of such ionic-plasticizers were described by Makowski and Lundberg (10). A particularly attractive combination was found to be zinc stearate with a zinc salt of S-EPDM. It was shown that for such a combination melt... 

Figure 4. Differential scanning thermograms of Zn-S-EPDM with various loadings of zinc stearate.

Figure 7. Engineering stress-strain at ambient conditions of Zn-S-EPDM samples with various loading levels of zinc stearate (see text for sample size).

Figure 10. Normalized modulus of Zn-S-EPDM filled with zinc stearate compared to other filled elastomers and segmented polyuretane.

Figure 18. A proposed morphological representation of zinc stearate crystals interacting with Zn-S-EPDM molecules under relaxed and deformed states. Interactions occur between sulfonated sites on the polymeric backbone and polar sites on the surface of the crystals.

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