Ethylene-propylene rubbers blends with other elastomers

Blending within the family of polyolefins has, however, been more common (Plochocki 1978). Although they are usually immiscible with each other, there exists some degree of mutual compatibility between them. The similarity of their hydrocarbon backbones and the closeness of their solubility parameters, although not adequate for miscibility, account for a relatively low degree of interfacial tension. For example, the solubility parameters of polyethylene, polyisobutylene, ethylene-propylene rubber, and polypropylene are estimated to be 16.0,16.4,16.5, and 17.0 cm respectively, all very close to each other (VanKrevelen 1990). Similarly, the interfacial tension coefficients between PE or PP and EP-elastomers are quite small (typically ca. 0.1 MN/m) (Shih 1990 Wu 1989). Hence, polyolefin blends have been made since the early days of polyolefin commercialization via a simple melt mixing, without a compatibUizer. 

Polyolefin thermoplastic elastomers are generally blends of polypropylene with up to 65% ethylene-propylene rubber and it is supposed that short propylene blocks in the latter co-crystallize with segments of the polypropylene chains to give microcrystalline regions which act as cross-links. A recent development in this field has been the use of highly cross-linked ethylene-propylene rubbers (and other rubbers) in the blends to give so-called thermoplastic vulcanizates (TPVs). In these blends the rubber is present as finely dispersed particles in a polypropylene matrix. Compared to the simple blends, these materials have generally enhanced properties. 

This class of TPEs consists of blends of the thermoplastic PP with an unvulcanized ethylene propylene rubber/elastomer. May contain smaller amounts of other materials, for example, HDPE, plasticizers and carbon black. 

Elastomers, plastics, fabrics, wood and metals can be joined with themselves and with each other using nitrile rubber/epoxy resin blends cured with amines and/or acidic agents. Ethylene-propylene vulcanizates can also be joined using blends of carboxylated nitrile rubber, epoxy resin and a reactive metal filler (copper, nickel, cobalt). However, one of the largest areas of use of nitrile rubber modified epoxy systems is in the printed circuit board area [12]. 

On the other hand, it is not always necessary that an interfacial agent be present. Polypropylene is available in impact-modified grades which are made by simply blending polypropylene with suitable olefin-based elastomers. Most often the elastomer is a suitably chosen ethylene-propylene-based rubber. Evidently, the required adhesion develops naturally in these systems without the need for an interfacial agent. However, proper control of phase morphology during mixing is essential. 

The EP thermoplastic elastomers are distinguished from the crossUnked analogues, which are not thermoplastics since reforming is impossible. A very important thermoplastic elastomer is comprised of a blend of an EP copolymer with an ethylene-propylene-diene (EPDM) terpolymer. This latter material is, of course, a crosslinkable thermoset however, these materials can be processed as thermoplastics if the crosslinkable component is present at low enough concentration to be present as an isolated phase. Melt-processing causes the formation of chemical bonds within the isolated rubber phase, a process called dynamic vulcanization. A commercial example of this type of material is Santoprene [4] manufactured by Advanced Elastomer Systems. Other blends of noncrosslinkable TPEs with crosslinkable materials are used commercially. These materials are classified as elastomer blends and are the subject of Chapter 12. 

As a consequence, before 1953, the only possible blends were those of LDPE with other polymers than PO or with elastomers (e.g., chlorosulfonated polyethylene rubber, CSR chlorinated butyl mbber, CBR ethylene/propylene/diene copolymers, EPR, EPDM thermoplastic olefinic elastomer TPE, TPO). However, in addition to the original autoclave polymerization, already in 1938, a tubular reactor was introduced and its product had different properties than that from the autoclave. Also varying the reaction condition affected the degree of short- and long-chain branching in LDPE thus, blending different LDPEs offered a way for optimizing the resin to specific applications. 

Thermoplastic polyolefin (TPO) n. Any of a group of elastomers produced by either of two processes. In one, polypropylene is melt-blended with from 15 to 85% of ter-polymer elastomer, ethylenepropylene rubber, or styrene-butadiene rubber. In the other, propylene is co-polymerized with ethylene-propylene elastomer in a series of reactions. The smaller elastomeric domains obtained in the latter process are claimed to provide improved properties over the blended materials. 

Development of TPEs with various types of elastomers and polyolefins has been extensively reported by many researchers. Ethylene-propylene-diene monomer (EPDM) or its modified form is used as the elastomer in most polyolefin TPEs. Natural rubber (NR) and thermoplastic blends have become an area of interest only recently. These materials are known as thermoplastic natural rubber (TPNR). The development of TPNR was principally based on the criteria set by EPDM blends with thermoplastics. Two types are known, one belonging to the TPO class and the other belonging to the TPV class. 

The blends of EPDM terpolymers and isotactic PP with curing agents, such as peroxide, phenol resins, and sulfur, are termed as thermoplastic vulcanized elastomer (TPV) since the rubber domains are vulcanized. Polyolefin copolymers, such as random copolymer of propylene with ethylene, copolymers of other olefins, elastomeric PP, and elastomeric PE, are developed with recent advances of... 

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