Heterophasic copolymers

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. 

Random insertion of ethylene as comonomer and, in some cases, butene as termonomer, enhances clarity and depresses the polymer melting point and stiffness. Propylene—butene copolymers are also available (47). Consequendy, these polymers are used in apphcations where clarity is essential and as a sealant layer in polypropylene films. The impact resistance of these polymers is sligbdy superior to propylene homopolymers, especially at refrigeration temperatures, but still vastiy inferior to that of heterophasic copolymers. Properties of these polymers are shown in Table 4. 

Polypropylene polymers are typically modified with ethylene to obtain desirable properties for specific applications. Specifically, ethylene-propylene rubbers are introduced as a discrete phase in heterophasic copolymers to improve toughness and low temperature impact resistance. 

Products The process can produce a broad range of propylene-based polymers, including mono- and bimodal (medium/wide/very wide molecular weight distribution) homopolymer PP, high stiffness homopolymers, random copolymers and terpolymers, high-clarity random copolymers as well as two compositions (homopolymer/random copolymer, twin-random or random/heterophasic copolymer). Conventional hetero-phasic impact copolymers (with improved stiffness/impact balance) can be produced with the second additional gas phase reactor, with ethyl-ene/propylene rubber content up to 40%. 

Application The Borstar polypropylene (PP) process is a versatile technology. Through the choice of reactor combinations, homopolymers, random copolymers, heterophasic copolymers, and very high-rubber content heterophasic copolymers can be produced. 

Description Polypropylene with a melt flowrate ranging from 0.1 to 1,200 can be produced with the Borstar PP process. Currently, Ziegler Natta catalysts are used, but there is a potential to use single-site catalysts latter. When producing homopolymers and random copolymers, the process consists of a loop reactor and a gas-phase reactor in series. One or two gas-phase reactors are combined with this arrangement when heterophasic copolymers are produced. Propylene, catalyst, cocatalyst, donor, hydrogen, and comonomer... 

In the case of heterophasic copolymers, the polymer from the gas-phase reactor is transferred into another, smaller gas-phase reactor where the rubbery copolymer is made. After this step, hydrocarbon residuals are removed, and the powder is transferred to the extrusion section. Polymerization conditions in each reactor can be independently controlled, enabling production of both standard uni-modal and broad molecular weight multimodal grades. The production rate ratios between the reactors can be adjusted to meet the targeted product properties. 

Ethylene propylene copolymers and their blends exhibit diverse degradation behavior under the influence of light, heat and radiation. In spite of many papers in this area, little, if any, mechanistic data on degradation and stabilization of this important class of materials is available in the literature. The present paper reviews the published literature in this area organised under five distinct class of materials, namely, thermoplastic, elastomeric, and heterophasic copolymers, thermoplastic elastomer and blends. Of this, elastomeric ethylene-propylene copolymers appears to have been most exhaustively studied. Very few studies have reported on thermoplastic copolymers, both random as well as heterophasic as well as thermoplastic elastomers and blends. Specific mechanisms of degradation and stabilization of each of these classes of materials are discussed. 

In general, there is a paucity of information on the relationship between polymer structure and degradation kinetics. This becomes especially critical in multiphase polymers like heterophasic copolymers, thermoplastic elastomers and blends. This review should stimulate research in this important area, which could ultimately lead to polymers with better photo-thermal and radiation resistance as well as more effective stabilizers. 

The crystalline copolymers of propylene with ethylene are thermoplastic materials. They can be further classified as random and block or heterophase copolymers. 

Although it is reasonable to expect differences in degradation behaviour between random and heterophase copolymers, literature seldom defines clearly the nature of copolymers studied. Well characterized heterophase E-P copolymers, however, are of very recent origin. 

Polymerization prepolymerization polymerization in the MZCR (homopolymer grades, medium-wide-very wide MWD, random copolymers and terpolymers, two composition polymers, homopolymer/random copolymer - twin random copolymers - random/heterophasic copolymers) (Figure 17.3). 

For heterophasic copolymers, polymer from the gas-phase reactor is transferred to another, smaller gas-phase reactor where the rubbery copolymer is made. After this processing step, hydrocarbon residuals are removed, and the powder is transferred for extrusion. 

Heterophasic polyropylene copolymers are used in large automotive parts that must withstand high temperatures without distortion. Heterophasic copolymers are formed when a rubber phase, usually ehtylenepropylene rubber, is polymerized with the homopolymer phase during manufacture. The addition of rubber increases the impact resistance of the material, while the homopolymer provides... 

From the three basic categories of polypropylene, namely, homopolymers, heterophasic copolymers, and random copolymers (with ethylene), there are specialty resins with enhanced capabilities for specific applications. Producers of large blow-molded or thermoformed parts can thus utilize grades with high melt strength to fabricate heat-resistant under-the-hood automotive parts. 

Figure 3. Cross-section of polypropylene-EFR heterophasic copolymer. (X 11.800)...

The Spheripol process enable the production a wide range of homopoly mer, random and heterophasic copolymers with a very broad product capability without additional compounding operations... 

Heterophasic copolymers, produced with a combination of loop and gas phase reactors, give an extremely uniform distribution of the dispersed phase within the homopolymer granule. Products with outstanding low temperature behavior, high-impact strength, and good stiffness can be obtained in a wide range of melt viscosities. 

Figure 5. Interpenetration polymer lattice network structure of "soft" polypropylene heterophasic copolymers. 

ICP = blends of isotactic propylene homopolymer with ethylene-propylene rubber. These materials are commonly called "impact copolymers," "heterophasic copolymers," or, incorrectly, "block copolymers." These are typically prepared during the polymerization process using a series of reactors. L = low rubber (less than about 15% rubber by weight typically witli an ethylene content of less than about 10%). H = high rubber content blends (greater than about 15% rubber by weight typically with an ethylene content of at least 7%). 

Impact copolymers (heterophasic copolymers), also known as block copolymers, are made in a two reactor system where the homopolymer matrix is made in the first reactor and then transferred to the second reactor where ethylene and propylene are polymerized to create ethylene propylene rubber (EPR) in the form of microscopic nodules dispersed in the homopolymer matrix phase. These nodules impart impact resistance both at ambient and cold temperatures to the compound. This type has intermediate stiffness and tensile strength and is quite cloudy. In general, the more ethylene monomer added, the greater the impact resistance with correspondingly lower stiffness and tensile strength. 

Heterophasic copolymer resins (so called because their morphology typically shows two or more phases) have lower stiffness and improved toughness at low temperature, down to -40°C (depending on the dispersed phase type and amount). These resins often demonstrate more complex thermal behavior (e.g., two or more melting points and reduced stiffness at elevated temperature). You can find examples of typical grades of polypropylene resins in Table 1.7. Chapter 2 describes propylene structure-property relationships that suit a variety of end-use applications. 

Figure 2 Phase morphologies of heterophasic copolymers with different matrices, (a) homopolymer (b) random copolymer.

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