Elastomers ethylene-propylene rubber

Polypropylene polymers are typically modified with ethylene to obtain desirable properties for specific applications. Specifically, ethylene—propylene mbbers are introduced as a discrete phase in heterophasic copolymers to improve toughness and low temperature impact resistance (see Elastomers, ETHYLENE-PROPYLENE rubber). This is done by sequential polymerisation of homopolymer polypropylene and ethylene—propylene mbber in a multistage reactor process or by the extmsion compounding of ethylene—propylene mbber with a homopolymer. Addition of high density polyethylene, by polymerisation or compounding, is sometimes used to reduce stress whitening. In all cases, a superior balance of properties is obtained when the sise of the discrete mbber phase is approximately one micrometer. Examples of these polymers and their properties are shown in Table 2. Mineral fillers, such as talc or calcium carbonate, can be added to polypropylene to increase stiffness and high temperature properties, as shown in Table 3. 

Kennedy JP, Tornqvist EGM (1969) In Natta G, Dall Asta G (eds) Polymer chemistry of Synthetic Elastomers, Ethylene — Propylene Rubbers, Interscience Ihiblishers, New York... 

Vulcanization or cross-linking chains by reacting the double bond in natural rubber with sulfur and other crosslinking agents is used to limit stress induced flow in natural rubber. Various means were tried to crosslink the synthetic elastomer ethylene propylene rubber (EPR). The technique commonly used is based on experience with natural rubber. A small amount of a diene [a monomer with two double bonds] is incorporated into the EPR chain to furnish sites for vulcanization reactions. 

Polymers Resins I Butyl Rubber, Epichlorohydrin Elastomers, Ethylene Propylene Rubber, Hypalon (TM) Production, Neoprene Production, Nitrile Butadiene Rubber, Polybutadiene Rubber, Polysulfide Rubber, Styrene-Butadiene Rubber Latex 07/31/97... 

Numerous nylon blends prepared by compatibilization or reactive blending are commercially successful. The modifiers fiequenfly utilized in commercial nylon blends include polyolefin, thermoplastic polyolefin, thermoplastic polyunethane, ionomer, elastomer, ethylene-propylene rubber, nitrile mbber, polyftetrafluoroethylene), poly (phenylene ether), poly(ether amide), silicone, glass fiber, and carbon fiber. The nonpolar modifiers such as polyolefin, maleic anhydride or a polar vinyl monomer such as acrylic acid, methaciylic acid and fimiaric acid is fiequently incorporated to introduce reactive sites in nylon. 

Copolymers of polypropylene with other monomers are also available, the most common monomer being ethylene. Copolymers usually contain between 1 and 7 wt % of ethylene randomly placed in the polypropylene backbone. This disrupts the ability of the polymer chain to crystallize, giving more flexible products. This also improves the impact resistance of the polymer, decreases the melting point, and increases flexibility. The degree of flexibility increases with ethylene content, eventually turning the polymer into an elastomer (ethylene propylene rubber). The copolymers also exhibit increased clarity and are used in blow molding, injection molding, and extrusion. 

Polyolefins. In these thermoplastic elastomers the hard component is a crystalline polyolefin, such as polyethylene or polypropylene, and the soft portion is composed of ethylene-propylene rubber. Attractive forces between the rubber and resin phases serve as labile cross-links. Some contain a chemically cross-linked rubber phase that imparts a higher degree of elasticity. 

Ethylene—Propylene Rubber. Ethylene and propjiene copolymerize to produce a wide range of elastomeric and thermoplastic products. Often a third monomer such dicyclopentadiene, hexadiene, or ethylene norbomene is incorporated at 2—12% into the polymer backbone and leads to the designation ethylene—propylene—diene monomer (EPDM) mbber (see Elastomers, synthetic-ethylene-propylene-diene rubber). The third monomer introduces sites of unsaturation that allow vulcanization by conventional sulfur cures. At high levels of third monomer it is possible to achieve cure rates that are equivalent to conventional mbbers such as SBR and PBD. Ethylene—propylene mbber (EPR) requires peroxide vulcanization. 

Elastomers. Ethylene—propylene terpolymer (diene monomer) elastomers (EPDM) use a variety of third monomers during polymerization (see Elastomers, ethyiene-propylene-diene rubber). Ethyhdenenorbomene (ENB) is the most important of these monomers and requires dicyclopentadiene as a precursor. ENB is synthesized in a two step preparation, ie, a Diels-Alder reaction of CPD (via cracking of DCPD) with butadiene to yield 5-vinylbicyclo[2.2.1]-hept-2-ene [3048-64-4] (7) where the external double bond is then isomerized catalyticaHy toward the ring yielding 5-ethyhdenebicyclo[2.2.1]-hept-2-ene [16219-75-3] (ENB) (8) (60). 

In the early stages of development of polypropylene rubbers, particularly butyl rubber, were used to reduce the brittleness of polypropylene. Their use declined for some years with the development of the polypropylene copolymers but interest was greatly renewed in the 1970s. This interest has been centred largely around the ethylene-propylene rubbers which are reasonably compatible in all proportions with polypropylene. At first the main interest was with blends in which the rubber content exceeded 50% of the blend and such materials have been designated as thermoplastic polyolefin elastomers (discussed in Section 11.9.1). There is also increasing interest in compounds with less than 50% rubber, often referred to as elastomer-modified thermoplastics. It is of interest to note... 

A manufacturer considering using a thermoplastic elastomer would probably first consider one of the thermoplastic polyolefin rubbers or TPOs, since these tend to have the lowest raw polymer price. These are mainly based on blends of polypropylene and an ethylene-propylene rubber (either EPM or EPDM) although some of the polypropylene may be replaeed by polyethylene. A wide range of blends are possible which may also contain some filler, oil and flame retardant in addition to the polymers. The blends are usually subject to dynamic vulcanisation as described in Section 11.9.1. 

See also Tire compounding antidegradants in, 21 785—790 antioxidants in, 21 789 butyl and halobutyl rubber in, 21 766 elastomers used in, 21 759—772 ethylene-propylene rubber in, 21 765-766... 

High Resolution Spectra of Solutions. An example of high resolution solution spectra of an elastomer system which illustrates the sensitivity of nmr to molecular structure is shown in Figure 1. Shown are spectra of ethylene propylene rubbers... 

ABA ABS ABS-PC ABS-PVC ACM ACS AES AMMA AN APET APP ASA BR BS CA CAB CAP CN CP CPE CPET CPP CPVC CR CTA DAM DAP DMT ECTFE EEA EMA EMAA EMAC EMPP EnBA EP EPM ESI EVA(C) EVOH FEP HDI HDPE HIPS HMDI IPI LDPE LLDPE MBS Acrylonitrile-butadiene-acrylate Acrylonitrile-butadiene-styrene copolymer Acrylonitrile-butadiene-styrene-polycarbonate alloy Acrylonitrile-butadiene-styrene-poly(vinyl chloride) alloy Acrylic acid ester rubber Acrylonitrile-chlorinated pe-styrene Acrylonitrile-ethylene-propylene-styrene Acrylonitrile-methyl methacrylate Acrylonitrile Amorphous polyethylene terephthalate Atactic polypropylene Acrylic-styrene-acrylonitrile Butadiene rubber Butadiene styrene rubber Cellulose acetate Cellulose acetate-butyrate Cellulose acetate-propionate Cellulose nitrate Cellulose propionate Chlorinated polyethylene Crystalline polyethylene terephthalate Cast polypropylene Chlorinated polyvinyl chloride Chloroprene rubber Cellulose triacetate Diallyl maleate Diallyl phthalate Terephthalic acid, dimethyl ester Ethylene-chlorotrifluoroethylene copolymer Ethylene-ethyl acrylate Ethylene-methyl acrylate Ethylene methacrylic acid Ethylene-methyl acrylate copolymer Elastomer modified polypropylene Ethylene normal butyl acrylate Epoxy resin, also ethylene-propylene Ethylene-propylene rubber Ethylene-styrene copolymers Polyethylene-vinyl acetate Polyethylene-vinyl alcohol copolymers Fluorinated ethylene-propylene copolymers Hexamethylene diisocyanate High-density polyethylene High-impact polystyrene Diisocyanato dicyclohexylmethane Isophorone diisocyanate Low-density polyethylene Linear low-density polyethylene Methacrylate-butadiene-styrene... 

Ethylene-propylene rubber (EPR or EPDM) is, basically, a copolymer of ethylene and propylene. Because of the random arrangement of the monomers in the chain, crystallization does not occur, and the material behaves as a rubber. Just as with polyisobutylene, vulcanization with sulphur is impossible (the chain is saturated). Also here, a small amount of another monomer is incorporated, which enables the vulcanization and thus the use as a technical elastomer. EPR has a high resistance against ageing and chemical attack, and is, compared with other specialty rubbers, relatively cheap. 

Sequence distribution studies on several types of rubber by 13C-NMR technique have been reported. Some of the more recent reports include silicone rubbers [28-30], SBR [31], acrylonitrile-butadiene rubber (NBR) [32,33], polyurethane [34,35], polyepichlorohydrin [36], ethylene-norbonene [37] and ethylene-propylene rubber [4, 16, 25, 38-44]. The NMR studies on EPDM have been carried out extensively, because it is one of the important parameters, which control the physical properties of the elastomer. For example, ethylene sequence can influence the crystallisation kinetic and melting behaviour of the rubber [38]. 

Natta, G., Valvassori, A. and Sartori, G., Ethylene-Propylene Rubbers , in Polymer Chemistry of Synthetic Elastomers, Wiley-Interscience, John Wiley Sons, New York, 1969, Part 2, pp. 679-702. 

A comparatively new group of materials— thermoplastic elastomers or thermoplastic rubbers —combines the ease of processing of thermoplastics with qualities of traditional vulcanized rubbers, especially elasticity. Because of convenience in processing there is much interest too in blends of plastics with elastomers, which may be modified by the inclusion of filler or glass fibre. As an example, a rubber-like material that can be processed as a thermoplastic can be made by blending and melt-mixing an ethylene-propylene rubber with polypropylene. The use of such blends may be helpful when there are needs to reclaim and re-process material, and in order to obtain products with qualities intermediate between those of the main components of the blends. 

In addition to the polyolefin blends designed for thermoplastic elastomer applications, a great deal of interest also has centered on other kinds of blends of polyolefins as has been reviewed recently (see chapter 21 of Ref. 10 by Plochocki). In a recent paper (84), we showed that blends involving polypropylene-high density polyethylene-low density polyethylene in various proportions and combinations exhibit additivity of tensile strength however, there are serious losses in ductility in some cases such that the blends are less ductile than either pure component. It is interesting to note, however, that these losses in ductility can largely be restored by addition of rather small amounts of an amorphous ethylene-propylene rubber (84). 

Very Low Density (VLDPE) and Ultra Low Density (ULDPE) Polyethylenes. These are made by copolymerization with increasing amounts of comonomers, especially 1 -octene, reducing regularity/crystallinity (density 0.91- 0.86) down toward ethylene/propylene rubber. These are soft and flexible enough to compete with plasticized polyvinyl chloride and thermoplastic elastomers in some applications. 

Thermoplastic Olefin. These thermoplastic elastomers are primarily blends, or block or graft copolymers, of ethylene/propylene rubber with polypropylene. 

---