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Polyolefin resins

Contributed by: Bruce Bersted, Steven A. Cohen

Publication year: 2014

Polymers derived from hydrocarbon molecules that possess one or more alkenyl (or olefinic) groups.

The term polyolefin typically is applied to polymers derived from ethylene, propylene, and other alpha-olefins, and from isobutylene, cyclic olefins, and butadiene and other diolefins. Polymers produced from other olefinic monomers, such as styrene, vinyl chloride, and tetrafluoroethylene, generally are considered separately. See also:

ALKENE .

Polyethylene

The term polyethylene generally refers to any homopolymer or copolymer in which ethylene is the major component monomer. Polyethylene is a semicrystalline polymer of low to moderate strength and high toughness. ◦ ◦ The melting point varies from approximately 105 to 125 C (220 to 257 F), depending on the degree of crystallinity. The density of polyethylene reflects the degree of crystallinity, and is generally used in classifying 3 commercial grades. Low-density polyethylene, having densities ranging from 0.905 to 0.936 g ∕ cm, , can be subdivided into high-pressure low-density polyethylene (LDPE), and low-pressure low-density polyethylene copolymers, referred to as linear low-density polyethylene (LLDPE). This last term denotes a copolymer of polyethylene that is produced by the low-pressure polymerization process, as opposed to low-density polyethylene (or conventional polyethylene), which is produced by an older high-pressure polymerization process. In a typical linear low-density polyethylene, the percentage of comonomer would be on the order of 2–3 3 mol %, typically butene. High-density polyethylene (HDPE) covers the density range from 0.941 to 0.967 g ∕ cm, .

See also: COPOLYMER .

The various types of polyethylene are characterized by differences in structure ( Fig. 1 ). High-density polyethylene generally consists of a polymethylene (CH, 2 ), n chain with no, or very few, side chains to disrupt crystallization, while linear low-density polyethylene contains side chains whose length depends on the comonomer used. The spacing and uniformity of the incorporation of the comonomer in linear low-density polyethylene depends on both the process and the type of comonomer, but the incorporation of comonomer is generally nonuniform. To some extent, the uniformity tends to decrease with the size of the comonomer.

Polyethylene with structures different from those produced with previous-generation catalysts are being produced with metallocene catalysts. Metallocene materials can cover the density range of both the high-density polyethylene and the linear low-density polyethylene, with the important distinction that the structures are very uniform in both molecular weight and comonomer incorporation. See also: METALLOCENES . AccessScience from McGraw-Hill Education Page 2 of 10 www.accessscience.com

WIDTH:BFig. 1 Polyethylene structures. ( a ) Conventional low-density polyethylene. ( b ) Linear low-density polyethylene and high-density polyethylene with alpha-olefin comonomer. ( c ) Linear high-density polyethylene with no comonomer [a polymethylene chain]. ( d ) Metallocene low-density polyethylene.

Polymerization

The monomer for polyethylene is produced by cracking aliphatic hydrocarbons separated in the refining process. 2 Low-density polyethylene is formed by the polymerization of ethylene at high pressures (15,000–50,000 lb ∕ in., ◦ ◦ or 100–350 megapascals) and at temperatures of 150–300 C (300–570 F) in the presence of a small amount of organic peroxide or oxygen. The density is controlled by regulating the temperature, where lower temperatures lead to higher densities. Linear low-density polyethylene is produced commercially by the copolymerization of ◦ ◦ ethylene with alpha-olefins ranging from C, 3 to C, 8 at temperatures up to 250 C (480 F) at low pressure 2 (300–1100 lb ∕ in., or 2–7.6 MPa) in the presence of a chromium catalyst or titanium-based Ziegler-Natta catalyst.

See also: CATALYSIS ; SINGLE-SITE CATALYSTS (POLYMER SCIENCE) .

High-density polyethylene is formed at temperatures similar to those for linear low-density polyethylene and at 2 pressures of generally less than 1000 lb ∕ in., (7 MPa) by using either chromium or Ziegler catalysts. The molecular-weight distribution of high-density polyethylene is largely controlled by the type of catalyst, while the molecular weight is most often controlled with hydrogen level.

Metallocene, or more generally single-site, catalysts have a transition metal sandwiched between organic ring compounds. They allow molecular weight and comonomer distributions to be closely controlled, such that each polymer molecule closely resembles the others. Additionally, these catalyst systems permit the incorporation of comonomers once thought incompatible with previous catalyst generations. See also: POLYMERIZATION ; SINGLE-SITE

CATALYSTS (POLYMER SCIENCE) .

Properties

The physical properties of polyethylene, regardless of process, are primarily dependent on the degree of crystallinity, which is related to the density and the molecular weight. Uniformity in both molecular weight and comonomer can also affect the observed properties. The stiffness, yield strength, and thermal and mechanical AccessScience from McGraw-Hill Education Page 3 of 10 www.accessscience.com

properties increase with the crystallinity, which can be systematically reduced by the addition of comonomer.

Failure properties, such as toughness and ultimate tensile strength, increase with molecular weight, while to a lesser extent failure properties tend to decrease with broadening of the molecular-weight distribution for a given peak molecular weight. Polyethylene generally shows excellent toughness at low temperatures, because of the ◦ ◦ low glass transition temperature (generally believed to be between − 80 and − 120 C or − 112 and − 184 F).

Environmental stress cracking, in the presence of detergents, is reduced for polyethylene copolymers as the size of the alpha-olefin comonomer is increased and as the density is decreased. Clarity is generally increased with decreasing crystallinity, although the morphology of the crystalline phase can also affect clarity.

The melt flow properties are affected by molecular weight and long-chain branching. Choice of fabrication technique to produce items of commerce from polyethylene generally determines the melt flow properties needed and therefore the type of polyethylene used. Among commodity polymers, polyethylene is unique with respect to the ease of formation of long-chain branching, the presence of which can dramatically affect melt fabrication options. Generally, the ease of flow for both extrusion and injection molding increases with molecular-weight distribution and long-chain branching, but decreases with molecular weight. Consequently, low-molecular-weight resins are favored for injection-molding applications. Processes requiring melt strength and strain hardening of the molten polymer, such as blown film and foam, favor high molecular weight, the presence of long-chain branching, and to a lesser extent broadening of the molecular-weight distribution. For a given molecular weight, long-chain branching can either enhance or diminish the melt viscosity at low shear rates (melt strength), with the high levels in most low-density polyethylenes yielding lower viscosity at a given molecular weight. See also: PLASTICS PROCESSING .

Use and fabrication

Polyethylene is used in greater volume worldwide than any other plastic, because it is relatively inexpensive, extremely versatile, and adaptable to a large array of fabrication techniques. Its advantages are chemical inertness

(resistance to solvents, acids, and alkalis) and good dielectric and barrier properties. Major uses include packaging films, plastic containers, molded articles, foam, pipe, cable sheathing, and coatings.

Because of its broad melting range, low-density polyethylene can be used in heat-seal operations. Film applications include food packaging bags, shrink wrap, and garment bags. Blow-molded containers are used in milk and other household containers, while injection-molded items include housewares, pails, and toys. Because of its low degree of crystallinity, low-density polyethylene is limited for applications requiring high stiffness, temperature resistance, tensile strength, and good barrier properties.

Linear low-density polyethylene is a versatile material that finds wide application in plastic films. Films produced by extrusion, blown, and cast film processes are widely used as garbage bags, stretch cling films, and other items that require the properties of flexibility, tear and puncture resistance, and toughness. The toughness of linear low-density polyethylene is generally superior to low-density polyethylene and increases with the size of the AccessScience from McGraw-Hill Education Page 4 of 10 www.accessscience.com

comonomer. The very low density formulations of linear low-density polyethylene are also used for heat-seal applications, and are stronger than those using low-density polyethylene. While the relatively narrow molecular-weight distribution makes extrusion-coating applications somewhat difficult, specific resin grades are commercially available for this purpose. The molded items made from linear low-density polyethylene are similar to those made from low-density polyethylene, such as trash cans, food containers, and closures. Sheathing and

flexible pipe are applications that take advantage of the flexibility and low-temperature toughness of linear low-density polyethylene.

High-density polyethylene is used in food packaging applications, because it gives very good moisture barrier properties. However, like all polyethylenes, it is not used in applications requiring an oxygen barrier. While high-density polyethylene has good chemical resistance, it is prone to environmental stress cracking in the presence of detergents and other surfactants. Longer-chain comonomers and higher-molecular-weight resins improve the environmental stress-crack resistance of high-density polyethylene. This is an important consideration in applications where polyethylene contacts chemicals, such as dishwashing detergent and laundry bleach. Because of the higher crystallinity, high-density polyethylene is used in applications where higher stiffness is required, such as injection-molded beverage cases, thin-walled blow-molded milk bottles, grocery bags, and thermoformed sheet for pickup truck bedliners. Large blow-molded applications, which take advantage of the higher rigidity high-density polyethylene, include large industrial containers. Fibers, having very high tensile strengths and tensile moduli, have been produced by a process known as gel spinning from very high molecular weight high-density polyethylene. In gel spinning, very high molecular weight polyethylene that has been swollen with solvent is spun into an extremely high tenacity fiber. This fiber approaches the strength of spider silk, and is used in medical and other applications where a very high strength fiber is desirable, for example, in fishing line. See also: SPIDER SILK .

Ethylene copolymers

Copolymers of polyethylene are generally limited in comonomer content to preserve some load-bearing properties in the form of enough stiffness to retain their shape. Higher levels of comonomer form rubbery polymers. Nearly all of these rubbery ethylene copolymers are used in compounded mixtures, which include the impact modification of rigid polymers such as polyethylene and polypropylene. These materials generally range in comonomer content from 25 to 60% by weight, with propylene being the most widely used comonomer to form ethylene-propylene rubber. In addition to propylene, small amounts of a diene are sometimes included, forming a terpolymer, that is, a copolymer in which three monomers are used. The distinguishing attribute of this terpolymer is that it can be vulcanized by means of peroxides. Compounded products containing ethylene-propylene rubber and terpolymer have many automotive uses such as in bumpers, dashboard panels, steering wheels, and assorted interior trim.

Another major use of ethylene copolymers is to produce poly(ethylene-co-vinyl acetate). The properties of this polymer are governed by the percentages of ethylene and vinyl acetate. Poly(ethylene-co-vinyl acetate) is AccessScience from McGraw-Hill Education Page 5 of 10 www.accessscience.com

polymerized by using a high-pressure process similar to that used in producing low-density polyethylene. As with all polyethylenes, increasing the amount of vinyl acetate comonomer decreases crystallinity, reduces stiffness, increases clarity, and reduces the melting point. Chemical resistance is similar to low-density polyethylene, while the clarity, low-temperature flexibility, and impact strength are superior. These materials are more permeable to oxygen, carbon dioxide, and water vapor than low-density polyethylene. Applications include specialty film for heat sealing, adhesives, flexible hose and tubing, footwear components, bumper components, and gaskets.

Foamed and cross-linked poly(ethylene-co-vinyl acetate) is used in energy-absorbing applications.

An interesting category of ethylene copolymers is ionomers. These copolymers are produced from the copolymerization of ethylene with a comonomer containing a carboxylic group (COOH). Most often, the comonomer is methyl acrylic acid. When this copolymer is reacted with a salt of sodium or zinc, a polymer with very high tensile strength is obtained having outstanding toughness and abrasion resistance. Ionomers exhibit good adhesion to metal, glass, and many fibers. They are melt-processable, with the ionic bonds acting like cross-links at lower temperatures, yet allowing flow at higher temperatures. Because of toughness, ionomers are widely used in the covers for golf balls.

Bruce Bersted

Polypropylene

Commercial polypropylene homopolymers are high-molecular-weight, semicrystalline solids having melting ◦ ◦ 3 points around 160–165 C (320–329 F), low density (0.90–0.91 g ∕ cm, ), excellent stiffness and tensile strength, and moderate impact strength (toughness). Incorporating small amounts of ethylene or higher alpha-olefins in the isotactic polypropylene structure produces random copolymer resins with lower melting points and densities, reduced stiffness and tensile strengths, but higher impact strengths and clarity than homopolymer polypropylene resins. Intimate blending of homopolymer polypropylene with ethylene-propylene rubber produces impact copolymers that possess both high strength and excellent toughness.

Polypropylene was discovered in the 1950s and is now the second most important thermoplastic material, after polyethylene. The attractive combination of low density, excellent mechanical properties over a wide temperature range, low electrical conductivity, and low cost leads to the widespread use of polypropylene homopolymers and random and impact copolymers in products such as automotive parts, appliances, battery cases, carpeting, electrical insulation, fiber and fabrics, film wraps, food packaging and storage containers, clothing, geotextiles, luggage, medical equipment, rope, and tamper-resistant closures.

Production

Propylene, CH, 3 CH CH, 2 , is produced with ethylene in large quantities at low cost from the cracking of oil and other hydrocarbon feedstocks. It can also be made by propane dehydrogenation. See also: PETROLEUM PRODUCTS . AccessScience from McGraw-Hill Education Page 6 of 10 www.accessscience.com

WIDTH:BFig. 2 Idealized structure of polypropylene materials. ( a ) Isotactic; methyl group always on same side of chain. ( b ) Syndiotactic; methyl group position alternating. ( c ) Atactic; methyl groups randomly distributed.

The polymerization of propylene can yield the three basic types of polypropylene materials—isotactic, syndiotactic, and atactic—which differ by the orientation of the methyl groups (CH, 3 ) along the polymer chain ( Fig. 2 ). Idealized isotactic polypropylene has the same methyl group configuration at each tertiary carbon atom along the polymer backbone, while in perfectly syndiotactic polypropylene the methyl group configuration at the tertiary carbon positions alternate along the chain. Atactic polypropylene possesses a random orientation of methyl groups. These variations in polymer structure produce polypropylenes with widely different properties.

The stereoregular structures of the isotactic and syndiotactic polypropylene chains result in high-melting, crystalline solids, whereas atactic polypropylene is an amorphous, frequently sticky material. In isotactic and syndiotactic polypropylenes, occasional errors in the orientation of the methyl groups disrupt the crystallization of these polymers, lowering the melting point and decreasing stiffness. The choice of catalyst and the polymerization conditions determine the type of polypropylene produced (syndiotactic, isotactic, or atactic) and the number of defects contained in the polymer. Nearly all of the polypropylene resins that are commercially produced possess the isotactic chain structure, although a small amount of atactic polypropylene is manufactured for use in adhesives and roofing materials.

Stereospecific, titanium-based catalysts are used to produce most isotactic polypropylenes. Titanium trichloride catalysts, used with alkylaluminum cocatalysts, were the earliest and the simplest catalysts, yielding the desired isotactic polypropylene as well as a significant amount of the atactic form as a by-product. Organic compounds AccessScience from McGraw-Hill Education Page 7 of 10 www.accessscience.com

containing oxygen, sulfur, or nitrogen were later found to act as catalyst modifiers, increasing the stereoregularity of the isotactic polymer chains while decreasing the amount of atactic polypropylene produced. The most advanced titanium-based catalysts contain titanium tetrachloride and an organic diester modifier supported on magnesium chloride. These titanium ∕ magnesium ∕ diester catalysts, used with an additional silane-ether modifier and trialkylaluminum cocatalyst, produce highly isotactic polypropylene in very high yields with virtually no atactic polypropylene by-product.

Metallocene and other single-site catalysts have been developed for the stereospecific polymerization of propylene. These single-site catalysts are complexes of zirconium and hafnium that have very precise molecular structures. The ability to tailor the structures of these catalysts has allowed the structure and properties of the resulting polymers to be precisely engineered. Single-site catalysts are now used commercially to produce isotactic polypropylene resins having very narrow molecular-weight distributions, very high melt flow rates, and elastomeric properties.

Polypropylene is produced in a variety of slurry-phase and gas-phase processes. Slurry-phase polymerizations are done in hydrocarbon diluents, such as hexane or heptane, or in liquid propylene (also called a bulk slurry process). Gas-phase polymerizations are done in stirred-bed and fluid-bed processes. Hydrogen is used as a chain transfer agent to control the molecular weight of the polymer. More than one reactor in series may be required to produce polymer blends such as impact copolymers. The high-activity titanium ∕ magnesium ∕ diester catalysts are used in both slurry and gas-phase processes, and their development has allowed many processes to be substantially simplified. Single-site catalysts also may be used in the simplified slurry and gas-phase processes. In addition, the advent of the single-site catalyst has led to the commercialization of a high-temperature solution process for producing polypropylene.

Applications

Polypropylene homopolymer, random copolymer, and impact copolymer resins are tailored to fit specific polymer applications and fabrication methods and to achieve the desired product performance.

Low-molecular-weight resins, used for melt spun and melt blown fibers and for injection-molding applications, are produced by single-site catalysts or by oxidative degradation of higher-molecular-weight polymers at elevated temperatures. The latter materials, often called controlled rheology resins, have narrower molecular-weight distributions and lower viscoelasticity. The brittleness of polypropylene homopolymer, particularly at ◦ ◦ temperatures below 0 C (32 F), is greatly reduced by blending it with ethylene-propylene rubber. Compounding with mineral fillers and glass fibers improves product stiffness and other properties. Higher-stiffness resins also are produced by increasing the stereoregularity of the polymer or by adding nucleating agents. Polypropylene resins are used in extrusion and blow-molding processes and to make cast, slit, and oriented films. Stabilizers are added to polypropylene to protect it from attack by oxygen, ultraviolet light, and thermal degradation; other additives improve resin clarity, flame retardancy, or radiation resistance. See also: RHEOLOGY ; STABILIZER (CHEMISTRY) . AccessScience from McGraw-Hill Education Page 8 of 10 www.accessscience.com

Other poly(alpha-olefins)

Isotactic polymers of 1-butene and other higher alpha-olefins are produced by using the same stereospecific catalyst systems used for polypropylene. Poly(1-butene) is a tough and flexible resin that crystallizes from the ◦ ◦ melt in a metastable form (melting point 130 C or 266 F) before transforming to another, more stable crystalline ◦ ◦ form (melting point 138 C or 280 F). This polymer has been used in the manufacture of film and pipe. ◦ ◦ Poly(4-methyl-1-pentene) is a transparent polymer with a high melting point (240 C or 464 F) and exceedingly 3 low density (0.83 g ∕ cm, ). This material is clear and can be autoclaved, and is used in manufacture of chemical and medical equipment. Other poly(alpha-olefins) have been synthesized, including poly(1-pentene), poly(3-methyl-1-butene), and poly(vinylcyclohexane), but they have attracted little or no commercial interest.

Polyisobutylene

High-molecular-weight rubbery solids are produced when isobutylene is polymerized at low temperature via carbocationic intermediates using a Lewis acid catalyst such as aluminum trichloride. Copolymerization of isobutylene with around 3% isoprene produces butyl rubber. Chlorination or bromination of butyl rubber is used to produce halogenated butyl rubbers. All three materials can be vulcanized and have low permeability to gases, making them useful as sealants, inner tubes, and tubeless tire liners. Low-molecular-weight polyisobutylenes are ◦ ◦ prepared near 0 C (32 F) and have molecular weights of 200–3000. They are used in formulations for caulking, sealants, and lubricants. See also: RUBBER .

Polybutadiene and polyisoprene

Butadiene and isoprene can be polymerized by anionic, free-radical, and transition-metal catalysts to give a number of polymer structures. The commercially important forms of polybutadiene and polyisoprene are similar to the cis -1,4-poly(diene) structures possessed by natural rubber, as shown in the structure below.

Unlabelled Image

Synthetic cis -1,4-polyisoprene is produced by alkyllithium catalysts, and high cis -1,4-polybutadiene (that is, mostly cis double bonds) is obtained by using a catalyst containing titanium, iodine, and trialkylaluminum. The

1,4-polybutadiene produced with lithium catalysts contains roughly equal amounts of cis and trans structures, but it is suitable for a variety of compounding applications. Elastomeric copolymers of these dienes with styrene and acrylonitrile can also be produced. See also: POLYACRYLONITRILE RESINS ; POLYMER ; POLYSTYRENE RESIN . AccessScience from McGraw-Hill Education Page 9 of 10 www.accessscience.com

Steven A. Cohen

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Additional Readings

R. Cong et al., A new technique for characterizing comonomer distribution in polyolefins: High-temperature thermal gradient interaction chromatography (HT-TGIC), Macromolecules , 44(8):3062–3072, 2011

DOI: http://doi.org/10.1021/ma200304e

J. B. P. Soares and T. F. L. McKenna, Polyolefin Reaction Engineering , Wiley-VCH, Weinheim, Germany, 2012

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