Space-filling model of a polyethylene chain

The repeating unit of polyethylene, showing its stereochemistry

A simpler way of representing the repeating unit. Stereochemistry, a subdiscipline of Chemistry, involves the study of the relative spatial arrangement of Atoms within Molecules An important branch In Polymer chemistry, a structural unit is a building block of a Polymer chain and related to the Repeat unit. Note, however, that the C−H bond angles are not 90° as this diagram would indicate, but are approximately 110°, since each carbon atom is tetrahedral (sp³). Molecular geometry or molecular structure is the three- Dimensional arrangement of the Atoms that constitute a Molecule. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 A tetrahedron (plural tetrahedra) is a Polyhedron composed of four triangular faces three of which meet at each vertex. -->In Chemistry

Polyethylene or polythene (IUPAC name poly(ethene)) is a thermoplastic commodity heavily used in consumer products (notably the plastic shopping bag). The International Union of Pure and Applied Chemistry ( IUPAC) (aɪjuːpæk or ay-yoo-pec) is an international Non-governmental organization A thermoplastic is a Plastic that Melts to a liquid when heated and freezes to a Brittle, very Glassy state when cooled sufficiently Plastic shopping bags, or carrier bags or plastic grocery bags are a common type of Shopping bag in several countries Over 60 million tons of the material are produced worldwide every year.

Description

Polyethylene is a polymer consisting of long chains of the monomer ethylene (IUPAC name ethene). A polymer is a large Molecule ( Macromolecule) composed of repeating Structural units typically connected by Covalent Chemical bonds A monomer (from Greek mono "one" and meros "part" is a small Molecule that may become chemically bonded to other Structure This Hydrocarbon has four Hydrogen Atoms bound to a pair of Carbon atoms that are connected by a Double bond. The International Union of Pure and Applied Chemistry ( IUPAC) (aɪjuːpæk or ay-yoo-pec) is an international Non-governmental organization Structure This Hydrocarbon has four Hydrogen Atoms bound to a pair of Carbon atoms that are connected by a Double bond. The recommended scientific name polyethene is systematically derived from the scientific name of the monomer. ^[1][2] In certain circumstances it is useful to use a structure–based nomenclature. In such cases IUPAC recommends poly(methylene). The International Union of Pure and Applied Chemistry ( IUPAC) (aɪjuːpæk or ay-yoo-pec) is an international Non-governmental organization ^[2] The difference is due to the opening up of the monomer's double bond upon polymerisation.

In the polymer industry the name is sometimes shortened to PE in a manner similar to that by which other polymers like polypropylene and polystyrene are shortened to PP and PS respectively. Polypropylene or polypropene ( PP) is a Thermoplastic Polymer, made by the Chemical industry and used in a wide variety of applications Polystyrene ˌpɒliˈstaɪriːn ( IUPAC Polyphenylethene is an aromatic Polymer made from the aromatic Monomer Styrene In the United Kingdom the polymer is commonly called polythene, although this is not recognised scientifically. The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK or Britain,is a Sovereign state located

The ethene molecule (known almost universally by its common name ethylene) C₂H₄ is CH₂=CH₂, Two CH₂ groups connected by a double bond, thus:

         

Polyethylene is created through polymerization of ethene. Structure This Hydrocarbon has four Hydrogen Atoms bound to a pair of Carbon atoms that are connected by a Double bond. Methylene is the chemical species R2C named after Methane, in which two of the carbon atom's valence electrons form no bonds In Polymer chemistry, polymerization is a process of reacting Monomer Molecules together in a Chemical reaction to form three-dimensional networks It can be produced through radical polymerization, anionic addition polymerization, ion coordination polymerization or cationic addition polymerization. Radical polymerization is a type of Polymerization in which the Reactive center of a polymer chain consists of a radical. An anionic addition polymerization of Vinyl Monomers is an Addition polymerization initiated by a strong base and Anion, such as an Coordination polymerization is a form of Addition polymerization in which Monomer adds to a growing Macromolecule through an Organometallic active This is because ethene does not have any substituent groups that influence the stability of the propagation head of the polymer. Each of these methods results in a different type of polyethylene.

Classification

Polyethylene is classified into several different categories based mostly on its density and branching. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Polymer chemistry, branching occurs by the replacement of a Substituent, e The mechanical properties of PE depend significantly on variables such as the extent and type of branching, the crystal structure and the molecular weight. Polymer chemistry, branching occurs by the replacement of a Substituent, e The molecular mass (abbreviated m of a substance, more commonly referred to as molecular weight and abbreviated as MW, is the Mass of one

• Ultra high molecular weight polyethylene (UHMWPE)
• Ultra low molecular weight polyethylene (ULMWPE - PE-WAX)
• High molecular weight polyethylene (HMWPE)
• High density polyethylene (HDPE)
• High density cross-linked polyethylene (HDXLPE)
• Cross-linked polyethylene (PEX)
• Medium density polyethylene (MDPE)
• Low density polyethylene (LDPE)
• Linear low density polyethylene (LLDPE)
• Very low density polyethylene (VLDPE)

UHMWPE is polyethylene with a molecular weight numbering in the millions, usually between 3. Ultra high molecular weight polyethylene ( UHMWPE) also known as high-modulus polyethylene ( HMPE) or high-performance polyethylene ( HPPE High-Density Polyethylene ( HDPE) or PolyEthylene High-Density ( PEHD) is a Polyethylene Thermoplastic made from Petroleum Medium Density Polyethylene, or MDPE is a type of Polyethylene defined by a density range of 0 Properties LDPE is defined by a density range of 0910 - 0940 g/cm³ Linear Low-density polyethylene ( LLDPE) is a substantially linear polymer ( Polyethylene) with significant numbers of short branches commonly made by 1 and 5. 67 million. The high molecular weight results in less efficient packing of the chains into the crystal structure as evidenced by densities of less than high density polyethylene (for example, 0. In Mineralogy and Crystallography, a crystal structure is a unique arrangement of Atoms in a Crystal. 930–0. 935 g/cm³). The high molecular weight results in a very tough material. The molecular mass (abbreviated m of a substance, more commonly referred to as molecular weight and abbreviated as MW, is the Mass of one Toughness, in Materials science and Metallurgy, is the resistance to Fracture of a material when stressed. UHMWPE can be made through any catalyst technology, although Ziegler catalysts are most common. Because of its outstanding toughness and its cut, wear and excellent chemical resistance, UHMWPE is used in a wide diversity of applications. These include can and bottle handling machine parts, moving parts on weaving machines, bearings, gears, artificial joints, edge protection on ice rinks and butchers' chopping boards. A bottle is a container with a neck that is narrower than the body and a "mouth It competes with Aramid in bulletproof vests, under the tradenames Spectra and Dyneema, and is commonly used for the construction of articular portions of implants used for hip and knee replacements. Aramid fibers are a class of heat-resistant and strong Synthetic fibers They are used in aerospace and military applications for ballistic rated body armor A ballistic vest is an item of protective clothing that absorbs the impact from gun-fired Projectiles and shrapnel fragments from explosion An implant is a Medical device made to replace and act as a missing biological structure (as compared with a transplant, which indicates transplanted Biomedical Hip replacement, also hip Arthroplasty, is a surgical procedure in which the Hip joint is replaced by a prosthetic implant. Knee replacement, or knee Arthroplasty, is a common surgical procedure most often performed to relieve the pain and disability from degenerative arthritis most commonly

HDPE is defined by a density of greater or equal to 0. 941 g/cm³. HDPE has a low degree of branching and thus stronger intermolecular forces and tensile strength. HDPE can be produced by chromium/silica catalysts, Ziegler-Natta catalysts or metallocene catalysts. A Ziegler-Natta catalyst is a Reagent or a mixture of reagents used in the production of Polymers of 1-alkenes (α-olefins A metallocene is a compound with the general formula (C5R52M consisting of two cyclopentadienyl anions (Cp bound to a metal center in the oxidation The lack of branching is ensured by an appropriate choice of catalyst (for example, chromium catalysts or Ziegler-Natta catalysts) and reaction conditions. Chromium (ˈkroʊmiəm is a Chemical element which has the symbol Cr and Atomic number 24 HDPE is used in products and packaging such as milk jugs, detergent bottles, margarine tubs, garbage containers and water pipes.

PEX is a medium- to high-density polyethylene containing cross-link bonds introduced into the polymer structure, changing the thermoplast into an elastomer. Cross-links are bonds that link one Polymer chain to another They can be Covalent bonds or Ionic bonds "Polymer chains" can refer An elastomer is a Polymer with the property of Elasticity. The term which is derived from elastic polymer, is often used interchangeably with the term The high-temperature properties of the polymer are improved, its flow is reduced and its chemical resistance is enhanced. PEX is used in some potable-water plumbing systems because tubes made of the material can be expanded to fit over a metal nipple and it will slowly return to its original shape, forming a permanent, water-tight, connection.

MDPE is defined by a density range of 0. 926–0. 940 g/cm³. MDPE can be produced by chromium/silica catalysts, Ziegler-Natta catalysts or metallocene catalysts. MDPE has good shock and drop resistance properties. It also is less notch sensitive than HDPE, stress cracking resistance is better than HDPE. MDPE is typically used in gas pipes and fittings, sacks, shrink film, packaging film, carrier bags and screw closures.

LLDPE is defined by a density range of 0. 915–0. 925 g/cm³. LLDPE is a substantially linear polymer with significant numbers of short branches, commonly made by copolymerization of ethylene with short-chain alpha-olefins (for example, 1-butene, 1-hexene and 1-octene). A heteropolymer or copolymer is a Polymer derived from two (or more Monomeric species as opposed to a Homopolymer where only one monomer is Alpha-olefins (or α-olefins) are a family of Organic compounds which are Olefins or Alkenes with a Chemical formula C LLDPE has higher tensile strength than LDPE, it exhibits higher impact and puncture resistance than LDPE. Lower thickness (gauge) films can be blown, compared with LDPE, with better environmental stress cracking resistance but is not as easy to process. LLDPE is used in packaging, particularly film for bags and sheets. Lower thickness may be used compared to LDPE. Cable covering, toys, lids, buckets, containers and pipe. While other applications are available, LLDPE is used predominantly in film applications due to its toughness, flexibility and relative transparency.

LDPE is defined by a density range of 0. 910–0. 940 g/cm³. LDPE has a high degree of short and long chain branching, which means that the chains do not pack into the crystal structure as well. In Mineralogy and Crystallography, a crystal structure is a unique arrangement of Atoms in a Crystal. It has, therefore, less strong intermolecular forces as the instantaneous-dipole induced-dipole attraction is less. In Physics, Chemistry, and Biology, intermolecular forces are forces that act between stable Molecules or between functional groups of This results in a lower tensile strength and increased ductility. Tensile strength \sigma_{UTS} or S_U is the Stress at which a material breaks or permanently deforms Ductility is a mechanical property used to describe the extent to which materials can be deformed plastically or "stretched" into "wires" without LDPE is created by free radical polymerization. Radical polymerization is a type of Polymerization in which the Reactive center of a polymer chain consists of a radical. The high degree of branching with long chains gives molten LDPE unique and desirable flow properties. LDPE is used for both rigid containers and plastic film applications such as plastic bags and film wrap.

VLDPE is defined by a density range of 0. 880–0. 915 g/cm³. VLDPE is a substantially linear polymer with high levels of short-chain branches, commonly made by copolymerization of ethylene with short-chain alpha-olefins (for example, 1-butene, 1-hexene and 1-octene). VLDPE is most commonly produced using metallocene catalysts due to the greater co-monomer incorporation exhibited by these catalysts. VLDPEs are used for hose and tubing, ice and frozen food bags, food packaging and stretch wrap as well as impact modifiers when blended with other polymers.

Recently much research activity has focused on the nature and distribution of long chain branches in polyethylene. In HDPE a relatively small number of these branches, perhaps 1 in 100 or 1,000 branches per backbone carbon, can significantly affect the rheological properties of the polymer. Rheology is the study of the flow of matter mainly liquids but also soft solids or solids under conditions in which they flow rather than deform elastically

Ethylene copolymers

In addition to copolymerization with alpha-olefins, ethylene can also be copolymerized with a wide range of other monomers and ionic composition that creates ionized free radicals. A heteropolymer or copolymer is a Polymer derived from two (or more Monomeric species as opposed to a Homopolymer where only one monomer is Common examples include vinyl acetate (the resulting product is ethylene-vinyl acetate copolymer, or EVA, widely used in athletic-shoe sole foams) and a variety of acrylates (applications include packaging and sporting goods). Vinyl acetate is an Organic compound with the formula CH3COOCH=CH2 Ethylene vinyl acetate (CAS# 24937-78-8 also known as EVA) is the copolymer of Ethylene and Vinyl acetate. The acrylate ion ( C[[Hydrogen H]]2=CHC OO sup>&minus is the Ion of Acrylic acid.

History

Polyethylene was first synthesized by the German chemist Hans von Pechmann who prepared it by accident in 1898 while heating diazomethane. Germany, officially the Federal Republic of Germany ( ˈbʊndəsʁepuˌbliːk ˈdɔʏtʃlant is a Country in Central Europe. Hans von Pechmann (1850 - 1902 was a German Chemist, renowned for his discovery of Diazomethane, Pechmann condensation and Pechmann pyrazole synthesis Diazomethane is the chemical compound CH2N2 In the pure form at room temperature it is a Yellow Gas, but it is almost universally used When his colleagues Eugen Bamberger and Friedrich Tschirner characterized the white, waxy, substance that he had created they recognized that it contained long -CH₂- chains and termed it polymethylene. Eugen Bamberger was a German chemist and discoverer of the Bamberger rearrangement.

The first industrially practical polyethylene synthesis was discovered (again by accident) in 1933 by Eric Fawcett and Reginald Gibson at the ICI works in Northwich, England. Imperial Chemical Industries ( ICI) is a British chemical subsidiary of a Dutch conglomerate and one of the largest chemical producers Northwich is a town in Cheshire, England. It lies in the heart of the Cheshire Plain, at the confluence of the rivers England is a Country which is part of the United Kingdom. Its inhabitants account for more than 83% of the total UK population whilst its mainland ^[3] Upon applying extremely high pressure (several hundred atmospheres) to a mixture of ethylene and benzaldehyde they again produced a white, waxy, material. Benzaldehyde (C6H5CHO is a chemical compound consisting of a Benzene ring with an Aldehyde substituent Because the reaction had been initiated by trace oxygen contamination in their apparatus the experiment was, at first, difficult to reproduce. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the It was not until 1935 that another ICI chemist, Michael Perrin, developed this accident into a reproducible high-pressure synthesis for polyethylene that became the basis for industrial LDPE production beginning in 1939. Sir Michael Willcox Perrin (1905 – 1988 was a scientist who created the first practical Polythene, directed the first British atomic bomb programme and managed the Allied

Subsequent landmarks in polyethylene synthesis have revolved around the development of several types of catalyst that promote ethylene polymerization at more mild temperatures and pressures. Catalysis is the process in which the rate of a Chemical reaction is increased by means of a Chemical substance known as a catalyst The first of these was a chromium trioxide-based catalyst discovered in 1951 by Robert Banks and J. Paul Hogan at Phillips Petroleum. The acid Anhydride of Chromic acid, CrO3 is chromium trioxide or chromium(VI oxide; industrially this compound is sometimes sold as " Robert L Banks ( November 24, 1921 – January 3. 1989 was an American Chemist. John Paul Hogan (born August 7, 1919) is an American research chemist Phillips Petroleum Company was founded in 1917 by LE Phillips and Frank Phillips, of Bartlesville, Oklahoma. In 1953 the German chemist Karl Ziegler developed a catalytic system based on titanium halides and organoaluminium compounds that worked at even milder conditions than the Phillips catalyst. Karl Waldemar Ziegler ( November 26, 1898 &ndash August 12 1973) was a German Chemist who won the Nobel Prize Titanium (taɪˈteɪniəm is a Chemical element with the symbol Ti and Atomic number 22 A halide is a Binary compound, of which one part is a Halogen Atom and the other part is an element or radical that is less The Phillips catalyst is less expensive and easier to work with, however, and both methods are used in industrial practice.

By the end of the 1950s both the Phillips- and Ziegler-type catalysts were being used for HDPE production. A Ziegler-Natta catalyst is a Reagent or a mixture of reagents used in the production of Polymers of 1-alkenes (α-olefins Phillips initially had difficulties producing a HDPE product of uniform quality and filled warehouses with off-specification plastic. However, financial ruin was unexpectedly averted in 1957 when the hula hoop, a toy consisting of a circular polyethylene tube, became a fad among youth in the United States. A hula hoop is a Toy Hoop, usually made of Plastic, that is twirled around the waist limbs or neck The United States of America —commonly referred to as the

A third type of catalytic system, one based on metallocenes, was discovered in 1976 in Germany by Walter Kaminsky and Hansjörg Sinn. A metallocene is a compound with the general formula (C5R52M consisting of two cyclopentadienyl anions (Cp bound to a metal center in the oxidation Walter Kaminsky is a German Chemist. His research dwells in Olefin Polymerization, and also in Plastic Recycling. The Ziegler and metallocene catalyst families have since proven to be very flexible at copolymerizing ethylene with other olefins and have become the basis for the wide range of polyethylene resins available today, including very low-density polyethylene and linear low-density polyethylene. In Organic chemistry, an alkene, olefin, or olefine is an unsaturated Chemical compound containing at least one Carbon Resin, not to be confused with Rosin, is a Hydrocarbon Secretion of many Plants particularly coniferous trees. Linear Low-density polyethylene ( LLDPE) is a substantially linear polymer ( Polyethylene) with significant numbers of short branches commonly made by Such resins, in the form of fibers like Dyneema, have (as of 2005) begun to replace aramids in many high-strength applications. Ultra high molecular weight polyethylene ( UHMWPE) also known as high-modulus polyethylene ( HMPE) or high-performance polyethylene ( HPPE Aramid fibers are a class of heat-resistant and strong Synthetic fibers They are used in aerospace and military applications for ballistic rated body armor

Until recently the metallocenes were the most active single-site catalysts for ethylene polymerisation known—new catalysts are typically compared to zirconocene dichloride. Much effort is currently being exerted on developing new, single-site (so-called post-metallocene) catalysts that may allow greater tuning of the polymer structure than is possible with metallocenes. A post-metallocene catalyst is a kind of catalyst for Olefin Polymerization. Recently work by Fujita at the Mitsui corporation (amongst others) has demonstrated that certain salicylaldimine complexes of Group 4 metals show substantially higher activity than the metallocenes. is one of the largest corporate conglomerates ( Keiretsu) in Japan and one of the largest publicly traded companies in the world Biological occurances The group 4 elements are not known to be involved in the biological chemistry of any living systems

Physical properties

Depending on the crystallinity and molecular weight, a melting point and glass transition may or may not be observable. In Mineralogy and Crystallography, a crystal structure is a unique arrangement of Atoms in a Crystal. The molecular mass (abbreviated m of a substance, more commonly referred to as molecular weight and abbreviated as MW, is the Mass of one The melting point of a solid is the temperature range at which it changes state from solid to Liquid. The glass transition temperature, T g is the temperature at which an Amorphous solid, such as Glass or a Polymer, becomes brittle The temperature at which these occur varies strongly with the type of polyethylene. For common commercial grades of medium- and high-density polyethylene the melting point is typically in the range 120 to 130 °C ((250 to 265 °F). The melting point for average, commercial, low-density polyethylene is typically 105 to 115 °C (220 to 240 °F).

Most LDPE, MDPE and HDPE grades have excellent chemical resistance and do not dissolve at room temperature because of their crystallinity. Polyethylene (other than cross-linked polyethylene) usually can be dissolved at elevated temperatures in aromatic hydrocarbons such as toluene or xylene, or in chlorinated solvents such as trichloroethane or trichlorobenzene. Toluene, also known as methylbenzene or phenylmethane, is a clear water -insoluble liquid with the typical smell of Paint thinners redolent of o-Xylene|m-Xylene|p-XyleneThe term xylene or xylol refers to a mixture of three Benzene derivatives which is used as a Solvent in the Printing Trichlorobenzene may refer to any of three Isomeric chlorinated derivatives of Benzene with the molecular formula C6H3Cl3

Environmental issues

The wide use of polyethylene makes it an important environmental issue. Though it can be recycled, most of the commercial polyethylene ends up in landfills and in the oceans (notably the Great Pacific Garbage Patch). Plastic recycling is the process of Recovering scrap or waste Plastics and reprocessing the material into useful products sometimes completely different from their Polyethylene is not considered biodegradable, as it takes several centuries until it is efficiently degraded. Biodegradation is the process by which organic substances are broken down by the enzymes produced by living organisms Recently (May 2008) Daniel Burd, a 16 year old Canadian, won the Canada-Wide Science Fair in Ottawa after discovering that Sphingomonas, a type of bacteria, can degrade over 40% of the weight of plastic bags in less than three months. The Canada-Wide Science Fair ( CWSF) is an annual Science fair in Canada held by the Youth Science Foundation of Canada Sphingomonas was defined in 1990 as a group of Gram-negative, rod-shaped Chemoheterotrophic, strictly aerobic bacteria The applicability of this finding is still a matter for the future.

References

External links

• Polythene's story: The accidental birth of plastic bags
• Polythene Technical Properties & Applications
• Article describing the dicovery of Sphingomonas as a biodegrader of plastic bags Kawawada, Karen, The Record (May 22 2008).