Thermoplastic elastomers are rubbery materials which can be fabricated by techniques usually associated with thermoplastic resins. Classical elastomers rely upon the crosslinked network, developed during vulcanization, to provide the retractive forces of rubber type elasticity. Thermoplastic elastomers contain rubber domains and resinous thermoplastic domains. The thermoplasticity results from the melting characteristics of the hard thermoplastic phase, while the rubber properties arise from the rubbery domains. Thermoplastic elastomers are, therefore, almost by definition, heterogeneous in their phase morphology. Such materials can be blends or block polymers. In the case of block polymers, the rubbery phase is not crosslinked chemically. However, hard or resinous phase domains occur as the hard segments of the block polymer which separate from the composition by agglomeration during cooling from the molten state. These domains act both as well-bonded reinforcing filler particles and as crosslinks. This is, of course, because the hard blocks are connected to the soft or rubbery segments by primary chemical bonds. In the case of the blend compositions, the hard and soft domains are separate polymeric species. However, there must be some form of interaction between the domains if useful properties are to be realized. Recently, uncured or partially cured EPDM rubber has been blended with polyolefin resin to make thermoplastic elastomer-like compositions. However, these compositions suffer deficiencies in performance as well as in certain aspects of fabricability. Only poor to fair performance at temperatures above 70°C in air or in oil has been achieved with the uncured to partially cured compositions. More recently, it has been found in our laboratories that fully cured EPDM compositions which are fabricable as thermoplastics can be prepared. Such compositions, referred to here as thermoplastic vulcanizates, have superior strength, high-temperature mechanical properties, hot oil and solvent resistance, better compression set, etc. This report outlines critical parameters associated with these unique materials.

The results of this work suggest a practical route to hot-oil-resistant thermoplastic elastomers based on NBR and a polyolefin resin (such as polypropylene). Although these two types of polymer are normally grossly incompatible with each other, a melt-mixed blend thereof is technologically improved by the presence of a small amount of a compatibilizing block copolymer which contains both polar and nonpolar segments. Ideally, the block copolymer should contain molecular segments of the types of polymers to be compatibilized. The compatibilizing block (graft) copolymer can form in situ during melt-mixing. Dynamic vulcanization (during melt-mixing) of a compatibilized NBR-polypropylene blend produces a thermoplastic elastomer with mechanical properties about as good as those of a corresponding composition of EPDM and polypropylene (two polymers which are nearly mutually compatible in a thermodynamic sense). The compatibilizing NBR-polypropylene graft copolymer might act by reducing (molten-state) interfacial tension at the NBR-polypropylene interface and also by increasing the interfacial adhesion in the “solidified-state” composition during its use. The hot-oil resistance of the compatibilized NBR-polypropylene thermoplastic vulcanizates is excellent. Also, the NBR-polypropylene compositions can be blended with thermoplastic vulcanizates based on EPDM and polypropylene to obtain thermoplastic elastomeric compositions which exhibit both good hot oil resistance and low temperature brittleness characteristics.

Thermoplastic elaslomeric compositions based on crosslinked NBR and nylon can be produced. A variety of nylons of differing melting points and NBR's of differing AN contents can be used in a broad range of proportions. The compositions can be further altered by the incorporation of fillers and plasticizers. The work indicates that a number of types of nylon-NBR based clastomeric materials, fabricable as thermoplastics and exhibiting good strength and excellent hot oil resistance, can be produced in a range of hardnesses.

Based on a few characteristics of the pure rubber and plastic components, rubber-plastic combinations can be selected, with a high probability of success, to give thermoplastic vulcanizates (by dynamic vulcanization) of good mechanical integrity and elastic recovery. The characteristics used in the selection are estimated surface energies, crystallinity of the hard phase (plastic) material and the critical chain length, of the rubber molecules, for entanglement. The best compositions are prepared when the surface energies of the rubber and plastic material are matched, when the entanglement molecular length of the rubber is low (high entanglement density) and when the plastic material is crystalline. Of course, it is required that neither the plastic, nor the rubber decompose in the presence of the other at temperatures required for melt mixing. Also, a curing system is required, appropriate for the rubber under the conditions of melt-mixing.

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.