Polyethylene
Overview
 
Polyethylene or polythene (IUPAC name polyethene or poly(methylene)) is the most widely used plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

, with an annual production of approximately 80 million metric tons. Its primary use is within packaging (plastic bag
Plastic bag
A plastic bag, polybag, or pouch is a type of packaging made of thin, flexible, plastic film, nonwoven fabric, or plastic textile. Plastic bags are used for containing and transporting goods such as foods, produce, powders, ice, magazines, comic books, chemicals and waste.Most plastic bags are...

, plastic films, geomembranes
Geomembranes
Geomembranes are a kind of geosynthetic material made up of an impermeable membranes. Their use includes lining canals, ponds and also waste containment.-Composition:Geomembranes are made of various materials...

, etc.).
Polyethylene is a thermoplastic
Thermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...

 polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 consisting of long chains produced by combining the ingredient monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...

 ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...

 (IUPAC name ethene), the name comes from the ingredient and not the actual chemical resulting.
Encyclopedia
Polyethylene or polythene (IUPAC name polyethene or poly(methylene)) is the most widely used plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

, with an annual production of approximately 80 million metric tons. Its primary use is within packaging (plastic bag
Plastic bag
A plastic bag, polybag, or pouch is a type of packaging made of thin, flexible, plastic film, nonwoven fabric, or plastic textile. Plastic bags are used for containing and transporting goods such as foods, produce, powders, ice, magazines, comic books, chemicals and waste.Most plastic bags are...

, plastic films, geomembranes
Geomembranes
Geomembranes are a kind of geosynthetic material made up of an impermeable membranes. Their use includes lining canals, ponds and also waste containment.-Composition:Geomembranes are made of various materials...

, etc.).

Description

Polyethylene is a thermoplastic
Thermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...

 polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 consisting of long chains produced by combining the ingredient monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...

 ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...

 (IUPAC name ethene), the name comes from the ingredient and not the actual chemical resulting. The ethylene actually converts to ethane as it takes its place in a polymer and straight sections of the polymer are the same structure as the simple chain hydrocarbons, e.g., propane, decane and other straight single-bonded carbon chains. As with any polymer, the structure of the resulting substance defies molecular description due to cross branching of the chains. The scientific name polyethene is systematically derived from the scientific name of the monomer. In certain circumstances it is useful to use a structure-based nomenclature; in such cases IUPAC recommends poly(methylene) (poly(methanediyl) is a non-preferred alternative). The difference in names between the two systems is due to the opening up of the monomer's double bond upon polymerization.

The name is abbreviated to PE in a manner similar to that by which other polymers like polypropylene
Polypropylene
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...

 and polystyrene
Polystyrene
Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...

 are shortened to PP and PS respectively. In the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 the polymer is commonly called polythene, although this is not recognized scientifically.

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 contains the chemical elements carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

.

Polyethylene is created through polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...

 of ethene. It can be produced through radical polymerization
Radical polymerization
Free radical polymerization is a method of polymerization by which a polymer forms by the successive addition of free radical building blocks. Free radicals can be formed via a number of different mechanisms usually involving separate initiator molecules...

, anionic addition polymerization
Anionic addition polymerization
Anionic addition polymerization is a form of chain-growth polymerization or addition polymerization that involves the polymerization of vinyl monomers with strong electronegative groups. This polymerization is carried out through a carbanion active species. Like all addition polymerizations, it...

, ion coordination polymerization
Coordination polymerization
Coordination polymerization is a form of addition polymerization in which monomer adds to a growing macromolecule through an organometallic active center...

 or cationic addition polymerization. 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
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 and branching
Branching (chemistry)
In polymer chemistry, branching occurs by the replacement of a substituent, e.g., a hydrogen atom, on a monomer subunit, by another covalently bonded chain of that polymer; or, in the case of a graft copolymer, by a chain of another type...

. The mechanical properties of PE depend significantly on variables such as the extent and type of branching, the crystal structure and the molecular weight. With regard to sold volumes, the most important polyethylene grades are HDPE, LLDPE and LDPE.
  • Ultra high molecular weight polyethylene
    Ultra high molecular weight polyethylene
    Ultra-high-molecular-weight polyethylene , also known as high-modulus polyethylene or high-performance polyethylene , is a subset of the thermoplastic polyethylene. It has extremely long chains, with molecular weight numbering in the millions, usually between 2 and 6 million...

     (UHMWPE)
  • Ultra low molecular weight polyethylene (ULMWPE or PE-WAX)
  • High molecular weight polyethylene (HMWPE)
  • High density polyethylene
    High density polyethylene
    High-density polyethylene or polyethylene high-density is a polyethylene thermoplastic made from petroleum. It takes 1.75 kilograms of petroleum to make one kilogram of HDPE...

     (HDPE)
  • High density cross-linked polyethylene (HDXLPE)
  • Cross-linked polyethylene (PEX or XLPE)
  • Medium density polyethylene
    Medium density polyethylene
    Medium-density polyethylene is a type of polyethylene defined by a density range of 0.926–0.940 g/cm3. It is less dense than HDPE, which is more common....

     (MDPE)
  • Linear low density polyethylene
    Linear low density polyethylene
    Linear low-density polyethylene is a substantially linear polymer , with significant numbers of short branches, commonly made by copolymerization of ethylene with longer-chain olefins. Linear low-density polyethylene differs structurally from conventional low-density polyethylene because of the...

     (LLDPE)
  • Low density polyethylene
    Low density polyethylene
    Low-density polyethylene is a thermoplastic made from petroleum. It was the first grade of polyethylene, produced in 1933 by Imperial Chemical Industries using a high pressure process via free radical polymerization. Its manufacture employs the same method today. LDPE is commonly recycled and has...

     (LDPE)
  • Very low density polyethylene (VLDPE)


UHMWPE is polyethylene with a molecular weight numbering in the millions, usually between 3.1 and 5.67 million. The high molecular weight makes it a very tough
Toughness
In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing; Material toughness is defined as the amount of energy per volume that a material can absorb before rupturing...

 material, but results in less efficient packing of the chains into the crystal structure
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...

 as evidenced by densities of less than high density polyethylene (for example, 0.930–0.935 g/cm3). 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 diverse range of applications. These include can and bottle
Bottle
A bottle is a rigid container with a neck that is narrower than the body and a "mouth". By contrast, a jar has a relatively large mouth or opening. Bottles are often made of glass, clay, plastic, aluminum or other impervious materials, and typically used to store liquids such as water, milk, soft...

 handling machine parts, moving parts on weaving machines, bearings, gears, artificial joints, edge protection on ice rinks and butchers' chopping boards. It competes with aramid
Aramid
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 fabric and ballistic composites, in bicycle tires, and as an asbestos substitute. The name is a portmanteau of "aromatic polyamide"...

 in bulletproof vests, under the tradenames Spectra and Dyneema, and is commonly used for the construction of articular portions of implants
Implant (medicine)
An implant is a medical device manufactured to replace a missing biological structure, support a damaged biological structure, or enhance an existing biological structure. Medical implants are man-made devices, in contrast to a transplant, which is a transplanted biomedical tissue...

 used for hip
Hip replacement
Hip replacement is a surgical procedure in which the hip joint is replaced by a prosthetic implant. Hip replacement surgery can be performed as a total replacement or a hemi replacement. Such joint replacement orthopaedic surgery generally is conducted to relieve arthritis pain or fix severe...

 and knee replacement
Knee replacement
Knee replacement, or knee arthroplasty, is a surgical procedure to replace the weight-bearing surfaces of the knee joint to relieve the pain and disability of osteoarthritis. It may be performed for other knee diseases such as rheumatoid arthritis and psoriatic arthritis...

s.

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 catalyst
Ziegler-Natta catalyst
A Ziegler–Natta catalyst is a catalyst used in the synthesis of polymers of 1-alkenes . Three types of Ziegler–Natta catalysts are currently employed:* Solid and supported catalysts based on titanium compounds...

s or metallocene
Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...

 catalysts. The lack of branching is ensured by an appropriate choice of catalyst (for example, chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

 catalysts or Ziegler-Natta catalysts) and reaction conditions. HDPE is used in products and packaging such as milk jugs, detergent bottles, margarine tubs, garbage containers and water pipes. One third of all toys are manufactured from HDPE. In 2007 the global HDPE consumption reached a volume of more than 30 million tons.

PEX is a medium- to high-density polyethylene containing cross-link
Cross-link
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...

 bonds introduced into the polymer structure, changing the thermoplast into an elastomer
Elastomer
An elastomer is a polymer with the property of viscoelasticity , generally having notably low Young's modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred...

. 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-olefin
Alpha-olefin
Alpha-olefins are a family of organic compounds which are olefins or alkenes with a chemical formula CxH2x, distinguished by having a double bond at the primary or alpha position...

s (for example, 1-butene
1-Butene
1-Butene is an organic chemical compound, linear alpha-olefin , and one of the isomers of butene. The formula is .-Stability:1-Butene is stable in itself but polymerizes exothermically. It is highly flammable and readily forms explosive mixtures with air...

, 1-hexene
1-Hexene
1-Hexene is an organic compound with the formula CH2CHC4H9. It is an alkene that is classified in industry as higher olefin and an alpha-olefin, the latter term meaning that the double bond is located at the alpha position, endowing the compound with higher reactivity and thus useful chemical...

 and 1-octene
1-Octene
1-Octene is an organic compound with a formula CH2CHC6H13. The alkene is classified as a higher olefin and alpha-olefin, meaning that the double bond is located at the alpha position, endowing this compound with higher reactivity and thus useful chemical properties. 1-Octene is one of the...

). LLDPE has higher tensile strength than LDPE, it exhibits higher impact and puncture resistance
Puncture resistance
Puncture resistance denotes the relative ability of a material to inhibit the progression of a tear once it has been pierced by a cut or a nick. Tests devised to measure puncture resistance are generally application-specific, covering items such as roofing and packaging materials, protective...

 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. Product examples range from agricultural films, saran wrap, and bubble wrap, to multilayer and composite films. In 2009 the world LLDPE market reached a volume of almost US$
United States dollar
The United States dollar , also referred to as the American dollar, is the official currency of the United States of America. It is divided into 100 smaller units called cents or pennies....

24 billion (€17 billion).

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
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...

 as well. It has, therefore, less strong intermolecular forces as the instantaneous-dipole induced-dipole attraction is less. This results in a lower tensile strength
Tensile strength
Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...

 and increased ductility
Ductility
In materials science, ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized...

. LDPE is created by free radical polymerization
Radical polymerization
Free radical polymerization is a method of polymerization by which a polymer forms by the successive addition of free radical building blocks. Free radicals can be formed via a number of different mechanisms usually involving separate initiator molecules...

. 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. In 2009 the global LDPE market had a volume of circa US$22.2 billion (€15.9 billion).

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
Rheology
Rheology is the study of the flow of matter, primarily in the liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force....

 properties of the polymer.

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. Common examples include vinyl acetate
Vinyl acetate
Vinyl acetate is an organic compound with the formula CH3COOCH=CH2. A colorless liquid with a pungent odor, it is the precursor to polyvinyl acetate, an important polymer in industry.-Production:...

 (the resulting product is ethylene-vinyl acetate
Ethylene-vinyl acetate
Ethylene vinyl acetate is the copolymer of ethylene and vinyl acetate. The weight percent vinyl acetate usually varies from 10 to 40%, with the remainder being ethylene....

 copolymer, or EVA, widely used in athletic-shoe sole foams) and a variety of acrylate
Acrylate
The acrylate ion is the ion of acrylic acid.Acrylates are the salts and esters of acrylic acid. They are also known as propenoates ....

s. Applications of acrylic copolymer include packaging and sporting goods, and superplasticizer
Superplasticizer
Superplasticizers, also known as high range water reducers, are chemicals used as admixtures where well-dispersed particle suspension are required. These polymers are used as dispersants to avoid particle aggregation, and to improve the flow characteristics of suspensions such as in concrete...

, used for cement
Cement
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...

 production.

History

Polyethylene was first synthesized by the German
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

 chemist Hans von Pechmann
Hans von Pechmann
Hans von Pechmann was a German chemist, renowned for his discovery of diazomethane in 1894. , Pechmann condensation and Pechmann pyrazole synthesis...

 who prepared it by accident in 1898 while heating diazomethane
Diazomethane
Diazomethane is the chemical compound CH2N2. It is the simplest of diazo compounds. In the pure form at room temperature, it is a extremely sensitive explosive yellow gas, thus it is almost universally used as a solution in diethyl ether...

. When his colleagues Eugen Bamberger
Eugen Bamberger
Eugen Bamberger was a German chemist and discoverer of the Bamberger rearrangement.-Life and achievements:Bamberger started studying medicine in 1875 at the University of Berlin, but changed subjects and university after one year, starting his studies of science at the University of Heidelberg in...

 and Friedrich Tschirner characterized the white, waxy, substance that he had created they recognized that it contained long -CH2- chains and termed it polymethylene.

The first industrially practical polyethylene synthesis was discovered (again by accident) in 1933 by Eric Fawcett and Reginald Gibson at the ICI
Imperial Chemical Industries
Imperial Chemical Industries was a British chemical company, taken over by AkzoNobel, a Dutch conglomerate, one of the largest chemical producers in the world. In its heyday, ICI was the largest manufacturing company in the British Empire, and commonly regarded as a "bellwether of the British...

 works in Northwich
Northwich
Northwich is a town and civil parish in the unitary authority of Cheshire West and Chester and the ceremonial county of Cheshire, England. It lies in the heart of the Cheshire Plain, at the confluence of the rivers Weaver and Dane...

, England. Upon applying extremely high pressure (several hundred atmospheres) to a mixture of ethylene and benzaldehyde
Benzaldehyde
Benzaldehyde is an organic compound consisting of a benzene ring with a formyl substituent. It is the simplest aromatic aldehyde and one of the most industrially useful. This colorless liquid has a characteristic pleasant almond-like odor...

 they again produced a white, waxy, material. Because the reaction had been initiated by trace oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 contamination in their apparatus, the experiment was, at first, difficult to reproduce. It was not until 1935 that another ICI chemist, Michael Perrin
Michael Perrin
Sir Michael Willcox Perrin was a scientist who created the first practical polythene, directed the first British atomic bomb programme, and managed the Allied intelligence of the Nazi atomic bomb.- Chemistry career :...

, developed this accident into a reproducible high-pressure synthesis for polyethylene that became the basis for industrial LDPE production beginning in 1939. Because polyethylene was found to have very low-loss properties at very high frequencies, commercial distribution in Britain was suspended on the outbreak of World War II, secrecy imposed and the new process was used to produce insulation for UHF and SHF coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...

s of radar sets. During World War II, further research was done in the United States on the ICI process and in 1944 Bakelite Corporation at Sabine, Texas and Du Pont at Charleston, West Virginia, began large scale commercial production under license from ICI.

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. The first of these was a chromium trioxide
Chromium trioxide
Chromium trioxide is the inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name.This compound is a dark-red/orange brown solid, which dissolves in water concomitant with hydrolysis...

-based catalyst discovered in 1951 by Robert Banks
Robert Banks (chemist)
Robert L. Banks was an American chemist. He was born and grew up in Piedmont, Missouri. He attended Southeast Missouri State University, and initiated into Alpha Phi Omega in 1940. He joined the Phillips Petroleum company in 1946 and worked there until he retired in 1985.He was a fellow research...

 and J. Paul Hogan at Phillips Petroleum
Phillips Petroleum
Phillips Petroleum Company was founded in 1917 by L.E. Phillips and Frank Phillips, of Bartlesville, Oklahoma. Their younger brother Waite Phillips was the benefactor of Philmont Scout Ranch....

. In 1953 the German chemist Karl Ziegler
Karl Ziegler
Karl Waldemar Ziegler was a German chemist who won the Nobel Prize in Chemistry in 1963, with Giulio Natta, for work on polymers. The Nobel Committee recognized his "excellent work on organometallic compounds [which]...led to new polymerization reactions and ... paved the way for new and highly...

 developed a catalytic system based on titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

 halide
Halide
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 electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...

s and organoaluminium compounds that worked at even milder conditions than the Phillips catalyst. 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
Ziegler-Natta catalyst
A Ziegler–Natta catalyst is a catalyst used in the synthesis of polymers of 1-alkenes . Three types of Ziegler–Natta catalysts are currently employed:* Solid and supported catalysts based on titanium compounds...

-type catalysts were being used for HDPE production. 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
Hula hoop
A hula hoop is a toy hoop that is twirled around the waist, limbs or neck.Although the exact origins of hula hoops are unknown, children and adults around the world have played with hoops, twirling, rolling and throwing them throughout history...

, a toy consisting of a circular polyethylene tube, became a fad among youth in the United States.

A third type of catalytic system, one based on metallocene
Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...

s, was discovered in 1976 in Germany by Walter Kaminsky
Walter Kaminsky
Walter Kaminsky is a German chemist. His research dwells in olefin polymerization, and also in plastic recycling. He discovered the high activity of Group 4 metallocene/methylaluminoxane mixtures as catalysts for olefin polymerization in 1980.He was awarded, among other prizes, the 1999 Benjamin...

 and Hansjörg Sinn. 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 resin
Resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...

s available today, including very low density polyethylene and linear low-density polyethylene. Such resins, in the form of fibers like Dyneema, have (as of 2005) begun to replace aramid
Aramid
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 fabric and ballistic composites, in bicycle tires, and as an asbestos substitute. The name is a portmanteau of "aromatic polyamide"...

s in many high-strength applications.

Until recently the metallocenes were the most active single-site catalysts for ethylene polymerisation known—new catalysts are typically compared to zirconocene dichloride
Zirconocene dichloride
Zirconocene dichloride is an organozirconium compound composed of a zirconium central atom, with two cyclopentadienyl and two chloro ligands. It is a colourless diamagnetic solid that is somewhat stable in air.-Preparation:...

. Much effort is currently being exerted on developing new, single-site (so-called post-metallocene
Post-metallocene catalyst
A post-metallocene catalyst is a kind of catalyst for olefin polymerization. "Post-metallocene" refers to the generation of catalysts following Kaminsky catalysts, which are metallocene catalysts discovered in 1980 by Walter Kaminsky, and have been highly publicized in the olefin polymerization...

) catalysts that may allow greater tuning of the polymer structure than is possible with metallocenes. Recently work by Fujita at the Mitsui
Mitsui
is one of the largest corporate conglomerates in Japan and one of the largest publicly traded companies in the world.-History:Founded by Mitsui Takatoshi , who was the fourth son of a shopkeeper in Matsusaka, in what is now today's Mie prefecture...

 corporation (amongst others) has demonstrated that certain salicylaldimine complexes of Group 4
Group 4 element
The Group 4 elements are a group of chemical elements in the periodic table. In the modern IUPAC nomenclature, Group 4 of the periodic table contains titanium , zirconium , hafnium and rutherfordium . This group lies in the d-block of the periodic table...

 metals show substantially higher activity than the metallocenes.

Physical properties

Depending on the crystallinity
Crystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...

 and molecular weight, a melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...

 and glass transition
Glass transition
The liquid-glass transition is the reversible transition in amorphous materials from a hard and relatively brittle state into a molten or rubber-like state. An amorphous solid that exhibits a glass transition is called a glass...

 may or may not be observable. 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 (248 to 266 F). The melting point for average, commercial, low-density polyethylene is typically 105 to 115 °C (221 to 239 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 hydrocarbon
Aromatic hydrocarbon
An aromatic hydrocarbon or arene is a hydrocarbon with alternating double and single bonds between carbon atoms. The term 'aromatic' was assigned before the physical mechanism determining aromaticity was discovered, and was derived from the fact that many of the compounds have a sweet scent...

s such as toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...

 or xylene
Xylene
Xylene encompasses three isomers of dimethylbenzene. The isomers are distinguished by the designations ortho- , meta- , and para- , which specify to which carbon atoms the two methyl groups are attached...

, or in chlorinated solvents such as trichloroethane
Trichloroethane
Trichloroethane can refer to either of two isomeric chemical compounds:* 1,1,1-Trichloroethane * 1,1,2-Trichloroethane...

 or trichlorobenzene
Trichlorobenzene
Trichlorobenzene may refer to any of three isomeric chlorinated derivatives of benzene with the molecular formula C6H3Cl3:* 1,2,3-Trichlorobenzene* 1,2,4-Trichlorobenzene* 1,3,5-Trichlorobenzene...

.

When incinerated, polyethylene burns slowly with a blue flame having a yellow tip and gives off an odour of paraffin. The material continues burning on removal of the flame source and produces a drip.

Application of polyethylene

Polyethylene is a class of thermoplastics practically ubiquitous in consumer products. In its foam form, polyethylene is used in packaging, vibration damping and insulation, as a barrier or buoyancy component, or as material for cushioning. Polyethylene foam is most frequently seen as a packaging material.

Polyethylene foam is buoyant, making it popular for nautical uses. Many types of polyethylene foam are approved for use in the food industry. Found in all types of packaging, polyethylene foam is used to wrap furniture, computer components, electronics, sporting goods, plants, frozen foods, clothing, bowling balls, signs, metal products, and more.

Environmental issues

The political environment has a divided approach towards the use of plastic. As plastics are mainly based on oil or natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

, there is a general trend, as well as political pressure, towards increased use of renewable sources. The REACH Directive regulates the handling of chemicals in Europe. The directive implies higher demands to cleansing and drives the industry towards increased use of recycled materials.

Other elements go in favor of increased use of plastics. The low weight reduces energy use and cost related to transportation compared to goods made from wood or paper. Development of advanced food packaging also prolongs shelf life of products and reduces the amount of food disposed of by the consumer. In this respect, plastics are considered to have a favorable environmental profile. However, there is no clear consensus whether or not increased use of plastics reduces emissions.

Plastics can also be produced from renewable sources like ethylene made from, for example, sugar canes. Polyethylene is not considered biodegradable, because, except when it is exposed to UV from sunlight, it takes several centuries until it is efficiently degraded. However, in May 2008, Daniel Burd, a 16-year-old Canadian, won the Canada-Wide Science Fair
Canada-Wide Science Fair
The Canada-Wide Science Fair is an annual science fair in Canada coordinated by . Started in 1962, it is the highest level of competition in Canada, and approximately 400 projects and 500 students are judged at the fair...

 in Ottawa after discovering that Pseudomonas fluorescens
Pseudomonas fluorescens
Pseudomonas fluorescens is a common Gram-negative, rod-shaped bacterium. It belongs to the Pseudomonas genus; 16S rRNA analysis has placed P. fluorescens in the P. fluorescens group within the genus, to which it lends its name....

, with the help of Sphingomonas
Sphingomonas
Sphingomonas was defined in 1990 as a group of Gram-negative, rod-shaped, chemoheterotrophic, strictly aerobic bacteria. They possess ubiquinone 10 as their major respiratory quinone, contain glycosphingolipids instead of lipopolysaccharide in their cell envelopes, and typically produce...

, two types of bacteria, can degrade over 40% of the weight of plastic bags in less than three months.

Bio-derived polyethylene

Braskem
Braskem
Braskem is a Brazilian petrochemical company headquartered in São Paulo. The company is the largest petrochemical in the Americas by production capacity and the fifth largest in the world...

 and Toyota Tsusho Corporation started Joint marketing activities for producing polyethylene from sugar cane. Braskem will build a new facility at their existing industrial unit in Triunfo, RS, Brazil with an annual production capacity of 200000 short tons (181,436,948 kg), and will produce high-density polyethylene (HDPE) and low-density polyethylene (LDPE) from bioethanol derived from sugarcane
Sugarcane
Sugarcane refers to any of six to 37 species of tall perennial grasses of the genus Saccharum . Native to the warm temperate to tropical regions of South Asia, they have stout, jointed, fibrous stalks that are rich in sugar, and measure two to six metres tall...

.

Polyethylene can also be made from other feedstocks, including wheat grain and sugar beet
Sugar beet
Sugar beet, a cultivated plant of Beta vulgaris, is a plant whose tuber contains a high concentration of sucrose. It is grown commercially for sugar production. Sugar beets and other B...

.

Joining

Commonly used methods for joining include:
  • Hot gas welding
  • Fastening
  • Infrared welding
  • Laser welding
  • Ultrasonic welding
    Ultrasonic welding
    Ultrasonic welding is an industrial technique whereby high-frequency ultrasonic acoustic vibrations are locally applied to workpieces being held together under pressure to create a solid-state weld. It is commonly used for plastics, and especially for joining dissimilar materials...

  • Heat sealing


Adhesive
Adhesive
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...

s and solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

s are rarely used because polyethylene is nonpolar and has a high resistance to solvents. Pressure sensitive adhesive
Pressure sensitive adhesive
Pressure sensitive adhesive is adhesive which forms a bond when pressure is applied to marry the adhesive with the adherend...

s (PSA) are feasible if the surface is flame treated or corona treated
Corona treatment
Corona treatment is a surface modification technique that uses a low temperature corona discharge plasma to impart changes in the properties of a surface. The corona plasma is generated by the application of high voltage to sharp electrode tips which forms plasma at the ends of the sharp tips...

.
Commonly used adhesives include:
  • Dispersion of solvent-type PSAs
  • Polyurethane contact adhesives
  • Two-part polyurethane or epoxy adhesives
  • Vinyl acetate
    Vinyl acetate
    Vinyl acetate is an organic compound with the formula CH3COOCH=CH2. A colorless liquid with a pungent odor, it is the precursor to polyvinyl acetate, an important polymer in industry.-Production:...

     copolymer hot melt adhesives

External links

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