Morphology of Elastomeric Alloys

Abstract

Abstract The field of thermoplastic elastomers has shown an explosive growth with the successful commercialization of elastomeric alloys (EAs) in 1981, based on the original work of Coran, Das, and Patel on dynamic vulcanization and the discovery of preferred cure system by Abdou-Sabet and Fath. These discoveries have led to the development of commercial products having true elastomeric properties while maintaining excellent thermoplastic processing. The success of EAs in the marketplace has led to the introduction of new products by Monsanto and others at a rate of 60 products per year in the last half of the eighties. Elastomeric alloys have been characterized as compositions containing rubber particulate domains approximately 1–2 µm in diameter in a matrix of thermoplastic resin. Such dispersed phase morphology has not been widely accepted, especially when it came to explaining the true elastomeric properties of the soft elastomeric products, i.e. 64 and 55 Shore A hardness products. Interaction among the rubber particles leading to a network of vulcanized elastomer phase that gave the appearance of two continuous networks has been proposed. In this paper, the morphology of EPDM/polypropylene elastomeric alloys is examined with some detail, and evidence leading to dispersed phase morphology is provided. There are several variables to such an investigation which can be grouped under the following headings: 1. Molecular weight of EPDM and polypropylene (PP). 2. Ratio of EPDM to PP. 3. Crosslinked or uncrosslinked blend. 4. Degree of crosslinking. 5. Type of crosslinks. 6. Typical and commercial products. It is not the subject of this paper to review the morphology of different binary polymer blends, which have been extensively covered in the literature. It can be concluded that a variety of morphologies can be obtained, however, depending on the mixing conditions, polymer ratios, relative surface energies of the polymer pair, and viscosities and molecular weights of the two polymers. In this study, the mixing conditions were kept similar as much as possible to eliminate the possibility of morphological changes as a function of the applied mixing intensity as influenced by shear rate, mixing time, and temperature.