CHAPTER 11. Natural Rubber/Engineering Thermoplastic Elastomer Blends

Abstract

Blending of two or more polymers has now emerged as a major tool to obtain new polymeric materials with desirable and novel properties that cannot be achieved from any of the individual components. Blending also provides an unusual combination of mechanical, thermal, chemical and morphological properties. Polymer blends are being used extensively in numerous applications. They play an important role in the fabrication of various products for applications ranging from footwear to space vehicles. A blend of elastic, rubbery polymers with a rigid plastics form thermoplastic elastomers (TPEs). These materials have the processing characteristics of a thermoplastic and the functional performance of a conventional rubber. They possess a wide range of physical properties of conventional elastomers at room and service temperatures in combination with the excellent processing characteristics of thermoplastic materials at high temperature. Such materials can be reprocessed, display good processability and have economic advantages. There is a wide variety of commercially available rubbers and plastics that can be considered for the preparation of TPEs. Different types of rubbers like NR, ENR, SBR, NBR, EPDM, XSBR, XNBR, etc. and different types of thermoplastics like PP, PVC, PS, PMMA, polyamides, nylons, LDPE, HDPE, etc. are used for blending to prepare TPEs. In rubber-modified thermoplastics, rubber improves the impact strength and ductility of the plastic and the plastic increases the stiffness of the rubber. A thermoplastic material known as thermoplastic natural rubber (TPNR) is produced on blending thermoplastic with NR. A variety of TPNRs have been developed by blending NR with polystyrene, polyamide 6, ethylene-vinyl acetate (EVA) copolymer and poly(methyl methacrylate), etc. The research into the preparation and evaluation of TPNR was initiated in the early 1960s at the Malaysian Rubber Producers’ Research Association (MRPRA) in London. TPNRs, with their unique properties, find use in various fields like packaging industries, manufacturing of household materials, automotive industries, biomedical applications, space technology etc. Natural rubber is characterized by good elastic properties, good resilience and damping behaviour but poor chemical resistance and processability. On the other hand, thermoplastics exhibit superior processing characteristics. However, it is extremely brittle. TPNRs from are expected to exhibit good processability, impact strength, good flexibility and rubbery nature. The properties the blend depend upon the way in which the blends are prepared, the nature of the homopolymers and additives used, and the amount of constituents. This chapter deals with the recent developments in the area of natural rubber/engineering TPE blends with emphasis on the different processing parameters, characterization techniques, structure–property relationship and their applications.