SODIUM AND ZINC IONOMERS OF SULFONATED MALEATED EPDM RUBBER

Abstract

Sulfonation of maleated EPDM (m-EPDM) rubber followed by its neutralization by sodium hydroxide or zinc acetate leads to the formation of an ionomer consisting of two types of ionic association, namely carboxylate and sulfonate anions with Na⁺ or Zn²⁺ as the counterion. The products were characterized by infrared spectroscopic studies, dynamic mechanical thermal analyses, dielectric thermal analyses, and measurements of stress-strain properties. Results of dynamic mechanical analyses show that besides the rubbery plateau, there occurs two distinct mechanical loss peaks, of which one is associated with the glass-rubber transition and the other is due to the relaxation of the immobile polymer chains associated with the ionic aggregates and often referred to as the ionic relaxation. The upward shift of the glass transition temperature (T_g ) refers to the formation of physical cross-links in the ionomer. Dielectric thermal analyses show that incorporation of the ionic groups causes profound changes in the dielectric constant (ε′) of m-EPDM. Besides the glass-rubber transition in the low-temperature region, additional dipolar relaxation due to the ionic aggregates occur at a higher temperature. Two types of ionic aggregate, namely multiplets and clusters, give rise to two values of the activation energy for the high-temperature dielectric relaxation. Incorporation of ionic groups causes improvement of physical properties of the polymer, as well as the retention of the properties at elevated temperatures (50°C and 75°C). The zinc ionomer shows higher strength at all temperatures than the sodium ionomer due to the greater extent of physical cross-linking arising from the ionic aggregates.

Keywords:

• Ionomer
• Sodium salts of sulfonated maleated EPDM rubber
• Zinc salt of sulfonated maleated EPDM rubber

ACKNOWLEDGMENTS

The authors wish to thank the Uniroyal Chemical Company (Naugatuck, CT) for providing the maleated EPDM rubber sample used in this study and gratefully acknowledge the financial assistance from the Council of Scientific and Industrial Research, Human Resource Development Group (Extra-Mural Research Division), Government of India, New Delhi.