Propylene-ethylene polyallomer

The physical properties of polyallomers are generally intermediate between those of the homopolymers prepared from the same monomers, but frequently represent a better balance of properties than blends of the homopolymers. This is illustrated by comparing the properties of a propylene-ethylene polyallomer containing 2.5% ethylene with polypropylene, high-density polyethylene, and a blend of 5% high-density polyethylene and 95% polypropylene. 

Figure 6.28. Infrared curves of (1) linear polyethylene, (2) polypropylene, (3) propylene-ethylene polyallomer. (Hagenmeyer and Edwards, 1966.)...

Comparison of Properties of Propylene-Ethylene Polyallomers and Polypropylene"... 

PALL Polyallomer - a block copolymer of propylene, ethylene... 

One unfortunate characteristic property of polypropylene is the dominating transition point which occurs at about 0°C with the result that the polymer becomes brittle as this temperature is approached. Even at room temperature the impact strength of some grades leaves something to be desired. Products of improved strength and lower brittle points may be obtained by block copolymerisation of propylene with small amounts (4-15%) of ethylene. Such materials are widely used (known variously as polyallomers or just as propylene copolymers) and are often preferred to the homopolymer in injection moulding and bottle blowing applications. 

Copolymers of propylene and a small amount of ethylene (3%) are called polyallomers. These copolymers are not as flexible as EPDM, but they are more ductile than //-PP. 

Many attempts have been made to synthesise ethylene/propylene block copolymers, referred to as polyallomers, with isospecific Ziegler-Natta catalysts. However, true block copolymers can hardly be synthesised. This is due to the short life of the growing polymer chains [68,241]. Therefore, only in a few cases, when the copolymers are synthesised by adding two comonomers sequentially and under very specific conditions in order to reduce chain transfer reactions, does unambiguous evidence for true block copolymer formation with isospecific catalysts exist [457]. 

Isotactic poly(4-methyl-l -pentene) Ethylene/isotactic propylene block (compact) copolymers (polyallomers)... 

Ziegler-Natta catalysts have been used to prepare a number of block copolymers, though in most cases these are mixed with significant amounts of homopolymers. The Ziegler-Natta method is clearly inferior to anionic polymerization for preparing block copolymers of controlled compositions. Nevertheless, block copolymers of ethylene and propylene (Polyallomers), which are high-impact plastics exhibiting crystallinity characteristics of both isotactic polypropylene and linear polyethylene, have been made by this process as a commercial product. 

A proprietary polymerization process, developed in the mid 1960s by staff researchers of Eastman Chemical Products, produces copolymers of 1-olefins that give a degree of crystallinity normally obtained only with homopolymers. The term polyallomer was coined to identify the polymers manufactured by this process and to distinguish them from conventional copolymers. The polyallomer materials available today are based on block copolymers of propylene and ethylene. 

Amongst copolymers of PE, there are polyallomers (ethylene-propylene copolymers), copolymers with cyclo-olefins and with vinylic monomers [with vinyl acetate ethylene-vinyl acetate, (EVA), with methacrylic acid, MA, and with vinyl alcohol ethylene - vinyl alcohol, (EVOH)], and chlorinated PE (CPE). CPE, although it exhibits exceptional UV and chemical resistance, gives rise to a high amount of hydrogen chloride gas evolution if combusted. 

Polyallomer Crystalline thermoplastic block copolymers of ethylene, propylene, and other olefins. Has good impact strength, flex life, and low density. 

The polyallomers constitute the class of block copolymers where both components are capable of crystallizing independently (Coover et al, 1966 Hagenmeyer and Edwards, 1966, 1970 Eastman Chemical Products, n.d.). The most important member of this family contains crystalline, stereoregular polypropylene as the major component and polyethylene as the minor component. As expected for a block copolymer, these products differ greatly in behavior from mechanical blends of polyethylene and polypropylene, and also from their random copolymers, poly(propylene-co-ethylene). When crosslinked with a diene monomer, the latter copolymers are known as EPDM rubbers (Lee et a/., 1966 Rodriguez, 1970, Chapter 13), while the former blends are of apparently little interest. In Figure 6.28 and 6.29 the... 

EP Poly(ethylene-b-propylene) Polyallomer Eastman Chemical Products... 

It is the ethylene-propylene block copolymers which are traditionally used in place of the homopolymers. Such block, copolymer materials may be known as polyallomers or, as propylene copolymers and referred to as either PP or, PP-K or, as PP-C or, as PP-B. The K simply means that it is a copolymer K is used instead of C as, in some circles, C stands for chlorination. PP-B stands for block copolymer. PP may therefore refer to either a homopolymer or, to a copolymer (PP-H and/or PP-K and/or PP-B). Copolymers have a lower heat distortion temperature (HDT), less clarity, less gloss and, less rigidity but greater impact strength. As the proportion of ethylene increases the material becomes softer and tougher. 

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