Polyethylene or polythene (abbreviated PE;

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

name polyethene or poly(methylene)) is the most commonly produced

plastic Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...

. It is a

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

, primarily used for

packaging Packaging is the science, art and technology of enclosing or protecting products for distribution, storage, sale, and use. Packaging also refers to the process of designing, evaluating, and producing packages. Packaging can be described as a co ...

(

plastic bag A plastic bag, poly bag, or pouch is a type of container 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, c ...

plastic film Plastic film is a thin continuous polymeric material. Thicker plastic material is often called a "sheet". These thin plastic membranes are used to separate areas or volumes, to hold items, to act as barriers, or as printable surfaces. Plas ...

geomembranes A geomembrane is very low permeability synthetic membrane liner or barrier used with any geotechnical engineering related material so as to control fluid (liquid or gas) migration in a human-made project, structure, or system. Geomembranes are m ...

and containers including

bottle A bottle is a narrow-necked container made of an impermeable material (such as glass, plastic or aluminium) in various shapes and sizes that stores and transports liquids. Its mouth, at the bottling line, can be sealed with an internal stopp ...

s, etc.). , over 100 million

tonne The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton ( United State ...

s of polyethylene

resin In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on natu ...

s are being produced annually, accounting for 34% of the total plastics market. Many kinds of polyethylene are known, with most having the

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

(C₂H₄)_''n''. PE is usually a mixture of similar

s of

ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene i ...

, with various values of ''n''. It can be ''low-density'' or ''high-density'':

low-density polyethylene Low-density polyethylene (LDPE) is a thermoplastic made from the monomer ethylene. It was the first grade of polyethylene, produced in 1933 by Imperial Chemical Industries (ICI) using a high pressure process via free radical polymerization. Its ...

is extruded using high pressure () and high temperature (), while

high-density polyethylene High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a thermoplastic polymer produced from the monomer ethylene. It is sometimes called "alkathene" or "polythene" when used for HDPE pipes. With a high strength-to-density ratio, ...

is extruded using low pressure () and low temperature (). Polyethylene is usually

thermoplastic A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains associate ...

, but it can be modified to become

thermosetting In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and ...

instead, for example, in

cross-linked polyethylene Cross-linked polyethylene, commonly abbreviated PEX, XPE or XLPE, is a form of polyethylene with cross-links. It is used predominantly in building services pipework systems, hydronic radiant heating and cooling systems, domestic water piping, ins ...

History

Polyethylene was first synthesized by the German chemist

Hans von Pechmann Hans von Pechmann (1 April 1850 – 19 April 1902) was a German chemist, renowned for his discovery of diazomethane in 1894. Pechmann condensation and Pechmann pyrazole synthesis. He also first prepared 1,2-diketones (e.g., diacetyl), acetonedicarb ...

, who prepared it by accident in 1898 while investigating

diazomethane Diazomethane is the chemical compound CH2N2, discovered by German chemist Hans von Pechmann in 1894. It is the simplest diazo compound. In the pure form at room temperature, it is an extremely sensitive explosive yellow gas; thus, it is almost u ...

. When his colleagues

Eugen Bamberger Eugen Bamberger (19 July 1857 – 10 December 1932) 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 univer ...

and Friedrich Tschirner characterized the white, waxy substance that he had created, they recognized that it contained long −CH₂− chains and termed it ''polymethylene''. First polythene pillbox

The first industrially practical polyethylene synthesis (diazomethane is a notoriously unstable substance that is generally avoided in industrial application) was again accidentally discovered in 1933 by Eric Fawcett and Reginald Gibson at the

Imperial Chemical Industries Imperial Chemical Industries (ICI) was a British chemical company. It was, for much of its history, the largest manufacturer in Britain. It was formed by the merger of four leading British chemical companies in 1926. Its headquarters were at M ...

(ICI) works in

Northwich {{Infobox UK place , static_image_name = Northwich - Town Bridge.jpg , static_image_caption = Town Bridge, the River Weaver and the spire of Holy Trinity Church , official_name = Northwich , country ...

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...

. Upon applying extremely high pressure (several hundred

atmospheres The standard atmosphere (symbol: atm) is a unit of pressure defined as Pa. It is sometimes used as a ''reference pressure'' or ''standard pressure''. It is approximately equal to Earth's average atmospheric pressure at sea level. History The s ...

) to a mixture of ethylene and

benzaldehyde Benzaldehyde (C6H5CHO) 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. It is a colorless liquid with a characteristic almond-like odor. ...

they again produced a white, waxy material. Because the reaction had been initiated by trace

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

contamination in their apparatus, the experiment was difficult to reproduce at first. It was not until 1935 that another ICI chemist,

Michael Perrin Sir Michael Willcox Perrin, CBE, FRSC (13 September 1905 – 18 August 1988) was a scientist who created the first practical polythene, directed the first British atomic bomb programme, and participated in the Allied intelligence of the Nazi a ...

, developed this accident into a reproducible high-pressure synthesis for polyethylene that became the basis for industrial low-density polyethylene (

LDPE Low-density polyethylene (LDPE) is a thermoplastic made from the monomer ethylene. It was the first grade of polyethylene, produced in 1933 by Imperial Chemical Industries (ICI) using a high pressure process via free radical polymerization. Its ...

) production beginning in 1939. Because polyethylene was found to have very low-loss properties at very high frequency radio waves, 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, or coax (pronounced ) is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ( insulating material); many coaxial cables also have a p ...

s of

radar Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...

sets. During World War II, further research was done on the ICI process and in 1944, Du Pont at Sabine River, Texas, and Bakelite Corporation at Charleston, West Virginia, began large-scale commercial production under license from ICI. The landmark breakthrough in the commercial production of polyethylene began with the development of

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

s that promoted the

polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer, monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are ...

at mild temperatures and pressures. The first of these was a catalyst based on

chromium trioxide Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an 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-purple ...

discovered in 1951 by Robert Banks and J. Paul Hogan at

Phillips Petroleum Phillips Petroleum Company was an American oil company incorporated in 1917 that expanded into petroleum refining, marketing and transportation, natural gas gathering and the chemicals sectors. It was Phillips Petroleum that first found oil in the ...

. In 1953 the German chemist

Karl Ziegler Karl Waldemar Ziegler (26 November 1898 – 12 August 1973) 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 ...

developed a catalytic system based on

titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...

halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fluor ...

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 heavily used industrially. By the end of the 1950s both the Phillips- and Ziegler-type catalysts were being used for high-density polyethylene (HDPE) production. In the 1970s, the Ziegler system was improved by the incorporation of

magnesium chloride Magnesium chloride is the family of inorganic compounds with the formula , where x can range from 0 to 12. These salts are colorless or white solids that are highly soluble in water. These compounds and their solutions, both of which occur in natu ...

. Catalytic systems based on soluble catalysts, the

metallocene A metallocene is a compound typically consisting of two cyclopentadienyl anions (, abbreviated Cp) bound to a metallic element, metal center (M) in the oxidation state II, with the resulting general formula Closely related to the metallocenes are ...

s, were reported in 1976 by

Walter Kaminsky Walter Kaminsky (born 7 May 1941) 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 (MAO) mixtures as catalysts for olefin p ...

and Hansjörg Sinn. The Ziegler- and metallocene-based catalysts families have proven to be very flexible at copolymerizing ethylene with other

olefin In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...

s and have become the basis for the wide range of polyethylene

s available today, including very-low-density polyethylene and

linear low-density polyethylene Linear low-density polyethylene (LLDPE) is a substantially linear polymer (polyethylene), with significant numbers of short branches, commonly made by copolymerization of ethylene with longer-chain olefins. Linear low-density polyethylene differs s ...

. Such resins, in the form of UHMWPE fibers, have (as of 2005) begun to replace

aramid Aramid fibers, short for aromatic polyamide, 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 marine cordage, ma ...

s in many high-strength applications.

Properties

The properties of polyethylene can be divided into mechanical, chemical, electrical, optical, and thermal properties.

Mechanical

Polyethylene is of low strength,

hardness In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...

and rigidity, but has a high

ductility Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

and

impact strength In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing.creep under persistent force, which can be reduced by addition of short fibers. It feels waxy when touched.

Thermal

The commercial applicability of polyethylene is limited by its low melting point compared to other thermoplastics. For common commercial grades of medium- and high-density polyethylene the melting point is typically in the range . The melting point for average commercial low-density polyethylene is typically . These temperatures vary strongly with the type of polyethylene, but the theoretical upper limit of melting of polyethylene is reported to be . Combustion typically occurs above .

Chemical

Polyethylene consists of nonpolar, saturated, high-molecular-weight

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...

s. Therefore, its chemical behavior is similar to

paraffin Paraffin may refer to: Substances * Paraffin wax, a white or colorless soft solid that is used as a lubricant and for other applications * Liquid paraffin (drug), a very highly refined mineral oil used in cosmetics and for medical purposes * Alkane ...

. The individual macromolecules are not

covalently A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...

linked. Because of their symmetric molecular structure, they tend to crystallize; overall polyethylene is partially crystalline. Higher

crystallinity Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, Transparency and translucen ...

increases

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

and mechanical and chemical stability. The standard method to test plastic density is ISO 1183 part 2 (gradient columns), alternatively ISO 1183 part 1
MVS2PRO density analyzer
. Most

MDPE Medium-density polyethylene (MDPE) 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 can be produced by chromium/silica catalysts, Ziegler-Natta catalysts or met ...

, and

HDPE High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a thermoplastic polymer produced from the monomer ethylene. It is sometimes called "alkathene" or "polythene" when used for HDPE pipes. With a high strength-to-density ratio, ...

grades have excellent chemical resistance, meaning that they are not attacked by strong acids or strong bases and are resistant to gentle oxidants and reducing agents. Crystalline samples do not dissolve at room temperature. Polyethylene (other than cross-linked polyethylene) usually can be dissolved at elevated temperatures in

aromatic hydrocarbon Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member of aromatic compounds is benzene. The word "aromatic" originates from the past grouping ...

s such as

toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) at ...

xylene In organic chemistry, xylene or xylol (; IUPAC name: dimethylbenzene) are any of three organic compounds with the formula . They are derived from the substitution of two hydrogen atoms with methyl groups in a benzene ring; which hydrogens are sub ...

, or in chlorinated solvents such as

trichloroethane Trichloroethane (CHCl) may refer to either of two isomeric chemical compounds: * 1,1,1-Trichloroethane (methyl chloroform, CClCH) * 1,1,2-Trichloroethane 1,1,2-Trichloroethane, or 1,1,2-TCA, is an organochloride solvent with the molecular formula ...

trichlorobenzene Trichlorobenzene (TCB) may refer to any of three isomeric chlorinated derivatives of benzene with the molecular formula C6H3Cl3. Trichlorobenzenes are man-made chemical compounds that occur in three different forms. Even though the forms have the ...

.Whiteley, Kenneth S.; Heggs, T. Geoffrey; Koch, Hartmut; Mawer, Ralph L. and Immel, Wolfgang (2005) "Polyolefins" in ''Ullmann's Encyclopedia of Industrial Chemistry''. Wiley-VCH, Weinheim. . Polyethylene absorbs almost no

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

; the gas and water vapour permeability (only polar gases) is lower than for most plastics.

Oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

and

flavorings A flavoring (or flavouring), also known as flavor (or flavour) or flavorant, is a food additive used to improve the taste or smell of food. It changes the perceptual impression of food as determined primarily by the chemoreceptors of the gustat ...

, on the other hand, can pass it easily. PE can become brittle when exposed to sunlight,

carbon black Carbon black (subtypes are acetylene black, channel black, furnace black, lamp black and thermal black) is a material produced by the incomplete combustion of coal and coal tar, vegetable matter, or petroleum products, including fuel oil, fluid ...

is usually used as a UV stabilizer. Polyethylene burns slowly with a blue flame having a yellow tip and gives off an odour of paraffin (similar to

candle A candle is an ignitable wick embedded in wax, or another flammable solid substance such as tallow, that provides light, and in some cases, a fragrance. A candle can also provide heat or a method of keeping time. A person who makes candles i ...

flame). The material continues burning on removal of the flame source and produces a drip. Polyethylene cannot be imprinted or bonded with adhesives without pretreatment. High-strength joints are readily achieved with

plastic welding Plastic welding is welding for semi-finished plastic materials, and is described in ISO 472 as a process of uniting softened surfaces of materials, generally with the aid of heat (except solvent welding). Welding of thermoplastics is accomplishe ...

Electrical

Polyethylene is a good

electrical insulator An electrical insulator is a material in which electric current does not flow freely. The atoms of the insulator have tightly bound electrons which cannot readily move. Other materials—semiconductors and conductors—conduct electric current ...

. It offers good

electrical treeing In electrical engineering, treeing is an electrical pre-breakdown phenomenon in solid insulation. It is a damaging process due to partial discharges and progresses through the stressed dielectric insulation, in a path resembling the branches of ...

resistance; however, it becomes easily electrostatically charged (which can be reduced by additions of

graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...

antistatic agent An antistatic agent is a compound used for treatment of materials or their surfaces in order to reduce or eliminate buildup of static electricity. Static charge may be generated by the triboelectric effect or by a non-contact process using a high ...

s). When pure, the dielectric constant is in the range 2.2 to 2.4 depending on the density and the loss tangent is very low, making it a good dielectric for a

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

Optical

Depending on thermal history and film thickness, PE can vary between almost clear (

transparent Transparency, transparence or transparent most often refer to: * Transparency (optics), the physical property of allowing the transmission of light through a material They may also refer to: Literal uses * Transparency (photography), a still, ...

), milky-opaque (

translucent In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions a ...

) and

opaque Opacity or opaque may refer to: * Impediments to (especially, visible) light: ** Opacities, absorption coefficients ** Opacity (optics), property or degree of blocking the transmission of light * Metaphors derived from literal optics: ** In lingu ...

. LDPE has the greatest, LLDPE slightly less, and HDPE the least transparency. Transparency is reduced by

crystallite A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel longulites. Stru ...

s if they are larger than the wavelength of visible light.

Manufacturing process

Monomer

The ingredient or

monomer In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification Mo ...

(

name ethene), a

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

eous

with the formula C₂H₄, which can be viewed as a pair of

methylene group In organic chemistry, a methylene group is any part of a molecule that consists of two hydrogen atoms bound to a carbon atom, which is connected to the remainder of the molecule by two single bonds. The group may be represented as , where the '< ...

s (−−) connected to each other. Typical specifications for PE purity are <5 ppm for water, oxygen, and other

alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...

s contents. Acceptable contaminants include N₂, ethane (common precursor to ethylene), and methane. Ethylene is usually produced from petrochemical sources, but also is generated by dehydration of ethanol.

Polymerization

Polymerization of ethylene to polyethylene is described by the following

chemical equation A chemical equation is the symbolic representation of a chemical reaction in the form of symbols and chemical formulas. The reactant entities are given on the left-hand side and the product entities on the right-hand side with a plus sign between t ...

: : (gas) → − (solid) Δ/ = Ethylene is a stable molecule that polymerizes only upon contact with catalysts. The conversion is highly

exothermic In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...

Coordination polymerization Coordination polymerisation is a form of polymerization that is catalyzed by transition metal salts and complexes. Types of coordination polymerization of alkenes Heterogeneous Ziegler–Natta polymerization Coordination polymerization started in t ...

is the most pervasive technology, which means that metal chlorides or metal oxides are used. The most common catalysts consist of

titanium(III) chloride Titanium(III) chloride is the inorganic compound with the formula TiCl3. At least four distinct species have this formula; additionally hydrated derivatives are known. TiCl3 is one of the most common halides of titanium and is an important catalys ...

, the so-called

Ziegler–Natta catalyst A Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, is a catalyst used in the synthesis of polymers of 1-alkenes (alpha-olefins). Two broad classes of Ziegler–Natta catalysts are employed, distinguished by their solubility: * He ...

s. Another common catalyst is the

Phillips catalyst The Phillips catalyst, or the Phillips supported chromium catalyst, is the catalyst used to produce approximately half of the world's polyethylene. A heterogeneous catalyst, it consists of a chromium oxide supported on silica gel. Polyethylene, ...

, prepared by depositing

chromium(VI) oxide Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an 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-purple s ...

on silica. Polyethylene can be produced through

radical polymerization In polymer chemistry, free-radical polymerization (FRP) is a method of polymerization by which a polymer forms by the successive addition of free-radical building blocks (repeat units). Free radicals can be formed by a number of different mechanis ...

, but this route has only limited utility and typically requires high-pressure apparatus.

Joining

Commonly used methods for joining polyethylene parts together include:Plastics Design Library (1997). ''Handbook of Plastics Joining: A Practical Guide''. Norwich, New York: Plastics Design Library. p. 326. . *

Welding Welding is a fabrication (metal), fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing Fusion welding, fusion. Welding is distinct from lower ...

Hot gas welding Hot-gas welding is a manual plastic welding process for joining thermoplastic materials. A hot-gas torch is used to direct hot air to both the joint surface and weld rod, heating the materials to their softening temperature. Application of pressure ...

** Infrared welding **

Laser welding Laser beam welding (LBW) is a welding technique used to join pieces of metal or thermoplastics through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequen ...

Ultrasonic welding Ultrasonic welding is an industrial process whereby high-frequency ultrasonic acoustic vibrations are locally applied to work pieces being held together under pressure to create a solid-state weld. It is commonly used for plastics and metals, and ...

Heat seal A heat sealer is a machine used to seal products, packaging, and other thermoplastic materials using heat. This can be with uniform thermoplastic monolayers or with materials having several layers, at least one being thermoplastic. Heat sealin ...

ing **

Heat fusion Heat fusion (sometimes called heat welding, butt welding or simply fusion) is a welding process used to join two different pieces of a thermoplastic. This process involves heating both pieces simultaneously and pressing them together. The two piec ...

Fastening A fastener (US English) or fastening (UK English) is a hardware device that mechanically joins or affixes two or more objects together. In general, fasteners are used to create non-permanent joints; that is, joints that can be removed or disman ...

Adhesives Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation. The use of adhesives offers certain advant ...

Pressure-sensitive adhesive Pressure-sensitive adhesive (PSA, self-adhesive, self-stick adhesive) is a type of nonreactive adhesive which forms a bond when pressure is applied to bond the adhesive with a surface. No solvent, water, or heat is needed to activate the adhesive ...

(PSAs) *** Dispersion of solvent-type PSAs **

Polyurethane Polyurethane (; often abbreviated PUR and PU) refers to a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethan ...

contact adhesives ** Two-part polyurethane **

Epoxy adhesive Epoxy is the family of basic components or Curing (chemistry), cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional ...

s ** Hot-melt adhesives **

Solvent bonding Solvent bonding is one of several methods of adhesive bonding for joining plastic joining, plastics. Application of a solvent to a thermoplastic material softens the polymer, and with applied pressure this results in polymer chain interdiffusion a ...

—

Adhesive Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation. The use of adhesives offers certain advant ...

s and

solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

s are rarely used as

solvent bonding Solvent bonding is one of several methods of adhesive bonding for joining plastic joining, plastics. Application of a solvent to a thermoplastic material softens the polymer, and with applied pressure this results in polymer chain interdiffusion a ...

because polyethylene is

nonpolar In chemistry, polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end. Polar molecules must contain one or more polar ...

and has a high resistance to solvents.

s (PSA) are feasible if the surface chemistry or charge is modified with

plasma activation Plasma activation (or plasma functionalization) is a method of surface modification employing plasma processing, which improves surface adhesion properties of many materials including metals, glass, ceramics, a broad range of polymers and textiles ...

flame treatment Flame treatment is the application of a gas flame to the surface of a material to improve adhesion. Polyolefins, especially polyethylene and polypropylene bond poorly, because they consist of long non-polar molecules. Without special treatment, ad ...

, or

corona treatment Corona treatment (sometimes referred to as air plasma) 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 ...

Classification

Polyethylene is classified by its

and branching. Its mechanical properties depend significantly on variables such as the extent and type of branching, the crystal structure, and the

molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

. There are several types of polyethylene: *

Ultra-high-molecular-weight polyethylene Ultra-high-molecular-weight polyethylene (UHMWPE, UHMW) is a subset of the thermoplastic polyethylene. Also known as high-modulus polyethylene, (HMPE), it has extremely long chains, with a molecular mass usually between 3.5 and 7.5 million amu. T ...

(UHMWPE) *Ultra-low-molecular-weight polyethylene (ULMWPE or PE-WAX) *High-molecular-weight polyethylene (HMWPE) *

High-density polyethylene High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a thermoplastic polymer produced from the monomer ethylene. It is sometimes called "alkathene" or "polythene" when used for HDPE pipes. With a high strength-to-density ratio, ...

(HDPE) *High-density cross-linked polyethylene (HDXLPE) *

Cross-linked polyethylene Cross-linked polyethylene, commonly abbreviated PEX, XPE or XLPE, is a form of polyethylene with cross-links. It is used predominantly in building services pipework systems, hydronic radiant heating and cooling systems, domestic water piping, ins ...

(PEX or XLPE) *

Medium-density polyethylene Medium-density polyethylene (MDPE) 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 can be produced by chromium/silica catalysts, Ziegler-Natta catalysts or meta ...

(MDPE) *

Linear low-density polyethylene Linear low-density polyethylene (LLDPE) is a substantially linear polymer (polyethylene), with significant numbers of short branches, commonly made by copolymerization of ethylene with longer-chain olefins. Linear low-density polyethylene differs s ...

(LLDPE) *

Low-density polyethylene Low-density polyethylene (LDPE) is a thermoplastic made from the monomer ethylene. It was the first grade of polyethylene, produced in 1933 by Imperial Chemical Industries (ICI) using a high pressure process via free radical polymerization. Its ...

(LDPE) *Very-low-density polyethylene (VLDPE) *

Chlorinated polyethylene Chlorinated polyethylene (PE-C or CPE) is an inexpensive variation of polyethylene, where chlorine is substituted for some of the hydrogen atoms. CPE has a chlorine content from 34 to 44%. Due to its soft, rubbery texture, CPE is added to polyvin ...

(CPE) With regard to sold volumes, the most important polyethylene grades are HDPE, LLDPE, and LDPE.

Ultra-high-molecular-weight (UHMWPE)

Stainless steel and ultra high molecular weight polythene hip replacement (9672239334)

UHMWPE is polyethylene with a molecular weight numbering in the millions, usually between 3.5 and 7.5 million amu. The high molecular weight makes it a very tough material, but results in less efficient packing of the chains into the

crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystal, crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pat ...

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

-handling machine parts, moving parts on weaving machines, bearings, gears, artificial joints, edge protection on ice rinks, steel cable replacements on ships, and butchers' chopping boards. It is commonly used for the construction of articular portions of implants used for

hip In vertebrate anatomy, hip (or "coxa"Latin ''coxa'' was used by Celsus in the sense "hip", but by Pliny the Elder in the sense "hip bone" (Diab, p 77) in medical terminology) refers to either an anatomical region or a joint. The hip region is ...

and

knee replacement Knee replacement, also known as knee arthroplasty, is a surgical procedure to replace the weight-bearing surfaces of the knee joint to relieve pain and disability, most commonly offered when joint pain is not diminished by conservative sources a ...

s. As

fiber Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...

, it competes with

in bulletproof vests.

High-density (HDPE)

HDPE is defined by a density of greater or equal to 0.941 g/cm³. HDPE has a low degree of branching. The mostly linear molecules pack together well, so intermolecular forces are stronger than in highly branched polymers. HDPE can be produced by chromium/silica catalysts,

s or

catalysts; by choosing catalysts and reaction conditions, the small amount of branching that does occur can be controlled. These catalysts prefer the formation of free radicals at the ends of the growing polyethylene molecules. They cause new ethylene monomers to add to the ends of the molecules, rather than along the middle, causing the growth of a linear chain. HDPE has high tensile strength. It is used in products and packaging such as milk jugs, detergent bottles, butter tubs, garbage containers, and HDPE pipe, 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.

Cross-linked (PEX or XLPE)

PEX is a medium- to high-density polyethylene containing cross-link bonds introduced into the polymer structure, changing the thermoplastic into a thermoset. 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.

Medium-density (MDPE)

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.

Linear low-density (LLDPE)

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). LLDPE has higher tensile strength than LDPE, and 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 they are not as easy to process. LLDPE is used in packaging, particularly film for bags and sheets. Lower thickness may be used compared to LDPE. It is used for cable coverings, 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 2013, the world LLDPE market reached a volume of US$40 billion.

Low-density (LDPE)

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

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 and increased

. LDPE is created by Radical polymerization, free-radical polymerization. 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 2013, the global LDPE market had a volume of almost US$33 billion. The radical polymerization process used to make LDPE does not include a catalyst that "supervises" the radical sites on the growing PE chains. (In HDPE synthesis, the radical sites are at the ends of the PE chains, because the catalyst stabilizes their formation at the ends.) Secondary free radical, radicals (in the middle of a chain) are more stable than primary radicals (at the end of the chain), and tertiary radicals (at a branch point) are more stable yet. Each time an ethylene monomer is added, it creates a primary radical, but often these will rearrange to form more stable secondary or tertiary radicals. Addition of ethylene monomers to the secondary or tertiary sites creates branching.

Very-low-density (VLDPE)

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 one in 100 or 1,000 branches per backbone carbon, can significantly affect the rheology, rheological properties of the polymer.

Copolymers

In addition to copolymerization with alpha-olefins, ethylene can be copolymerized with a wide range of other monomers and ionic composition that creates ionized free radicals. 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 of Acrylic resin, acrylic copolymer include packaging and sporting goods, and superplasticizer, used in cement production.

Types of polyethylenes

The particular material properties of "polyethylene" depend on its molecular structure. Molecular weight and crystallinity are the most significant factors; crystallinity in turn depends on molecular weight and degree of branching. The less the polymer chains are branched, and the lower the molecular weight, the higher the crystallinity of polyethylene. Crystallinity ranges from 35% (PE-LD/PE-LLD) to 80% (PE-HD). Polyethylene has a density of 1.0 g/cm³ in crystalline regions and 0.86 g/cm³ in amorphous regions. An almost linear relationship exists between density and crystallinity. The degree of branching of the different types of polyethylene can be schematically represented as follows: The figure shows polyethylene backbones, short-chain branches and side-chain branches. The polymer chains are represented linearly.

Chain branches

The properties of polyethylene are highly dependent on type and number of chain branches. The chain branches in turn depend on the process used: either the high-pressure process (only PE-LD) or the low-pressure process (all other PE grades). Low-density polyethylene is produced by the high-pressure process by radical polymerization, thereby numerous short chain branches as well as long chain branches are formed. Short chain branches are formed by Intramolecular reaction, intramolecular chain transfer reactions, they are always butyl group, butyl or ethyl group, ethyl chain branches because the reaction proceeds after the following mechanism: : PE-LD mechanism en

Environmental issues

Polyethylene is produced from ethylene, and although ethylene can be produced from renewable resources, it is mainly obtained from petroleum or natural gas. Moreover, the widespread usage of polyethylene poses difficulties for waste management if it is not recycled. Polyethylene, like other synthetic plastics, is not readily biodegradable, and thus accumulates in landfills. Recycling is made easier if marked with a recycling codes, recycling code or resin identification code. This can read "PE" or "02" ("plastic number 2") for PE-HD and "04" ("plastic number 4") for PE-LD. In Japan, getting rid of plastics in an environmentally friendly way was the major problem discussed until the Fukushima Daiichi nuclear disaster, Fukushima nuclear disaster in 2011 became a larger issue. It was listed as a US$90 billion market for solutions. Since 2008, Japan has rapidly increased the recycling of plastics, but still has a large amount of plastic wrapping which goes to waste. In 2010, a Japanese researcher, Akinori Ito, released the prototype of a machine which creates oil from polyethylene using a small, self-contained vapor distillation process.

Biodegradability

Polyethylene, like other synthetic plastics, is not readily biodegradable, and thus accumulates in landfills. However, there are a number of species of bacteria and animals that are able to degrade polyethylene. In May 2008, Daniel Burd, a 16-year-old Canadian, won the Canada-Wide Science Fair in Ottawa after discovering that ''Pseudomonas fluorescens'', with the help of ''Sphingomonas'', can degrade over 40% of the weight of plastic bags within six weeks. He later guessed that it would be gone after six more weeks. The thermophilic bacterium ''Brevibacillus borstelensis'' (strain 707) was isolated from a soil sample and found to use low-density polyethylene as a sole carbon source when incubated together at 50 °C. Biodegradation increased with time exposed to ultraviolet radiation. ''Acinetobacter'' sp. 351 can degrade lower molecular-weight PE oligomers. When PE is subjected to thermo- and photo-oxidization, products including alkanes, alkenes, ketones, aldehydes, alcohols, carboxylic acid, keto-acids, dicarboxylic acids, lactones, and esters are released. In 2014, a Chinese researcher named Jun Yang discovered that Indian mealmoth larvae could metabolize polyethylene from observing that plastic bags at his home had small holes in them. Deducing that the hungry larvae must have digested the plastic somehow, he and his team analyzed their gut bacteria and found a few that could use plastic as their only carbon source. Not only could the bacteria from the guts of the ''Plodia interpunctella'' moth larvae metabolize polyethylene, they degraded it significantly, dropping its tensile strength by 50%, its mass by 10% and the molecular weights of its polymeric chains by 13%. In 2017, researchers reported that the caterpillar of ''Galleria mellonella'' eats Plastic pollution, plastic garbage such as polyethylene.

Climate change

When exposed to ambient solar radiation the plastic produces two greenhouse gases, methane and

. Of particular concern is the plastic type which releases gases at the highest rate: low-density polyethylene (or

). Due to its low density properties it breaks down more easily over time, leading to higher surface areas. The production of these trace gases from virgin LDPE increase with surface area/time, with rates at the end of a 212-day incubation of 5.8 nmol g-1 d-1 of methane, 14.5 nmol g-1 d-1 of ethylene, 3.9 nmol g-1 d-1 of ethane and 9.7 nmol g-1 d-1 of propylene. When incubated in air, LDPE emits gases at rates ~2 times and ~76 times higher in comparison to water for methane and ethylene, respectively.

Chemically modified polyethylene

Polyethylene may either be modified in the polymerization by Chemical polarity, polar or non-polar comonomers or after polymerization through polymer-analogous reactions. Common polymer-analogous reactions are in case of polyethylene Cross-link, crosslinking, Chlorination reaction, chlorination and sulfochlorination.

Non-polar ethylene copolymers

α-olefins

In the low pressure process Alpha-olefin, α-olefins (e.g. 1-Butene, 1-butene or 1-Hexene, 1-hexene) may be added, which are incorporated in the polymer chain during polymerization. These copolymers introduce short side chains, thus

and

are reduced. As explained above, mechanical and thermal properties are changed thereby. In particular, PE-LLD is produced this way.

Metallocene polyethylene (PE-MC)

Metallocene polyethylene (PE-M) is prepared by means of metallocene catalysts, usually including copolymers (z. B. ethene / hexene). Metallocene polyethylene has a relatively narrow molecular weight distribution, exceptionally high toughness, excellent optical properties and a uniform comonomer content. Because of the narrow molecular weight distribution it behaves less pseudoplastic (especially under larger shear rates). Metallocene polyethylene has a low proportion of low molecular weight (extractable) components and a low welding and sealing temperature. Thus, it is particularly suitable for the food industry.

Polyethylene with multimodal molecular weight distribution

Polyethylene with multimodal molecular weight distribution consists of several polymer fractions, which are homogeneously mixed. Such polyethylene types offer extremely high stiffness, toughness, strength, stress crack resistance and an increased crack propagation resistance. They consist of equal proportions higher and lower molecular polymer fractions. The lower molecular weight units crystallize easier and relax faster. The higher molecular weight fractions form linking molecules between crystallites, thereby increasing toughness and stress crack resistance. Polyethylene with multimodal molecular weight distribution can be prepared either in two-stage reactors, by catalysts with two active centers on a carrier or by blending in extruders.

Cyclic olefin copolymers (COC)

Cyclic olefin copolymers are prepared by copolymerization of ethene and cycloolefins (usually norbornene) produced by using metallocene catalysts. The resulting polymers are amorphous polymers and particularly transparent and heat resistant.

Polar ethylene copolymers

The basic compounds used as polar comonomers are vinyl alcohol (Vinyl alcohol, Ethenol, an unsaturated alcohol), acrylic acid (propenoic acid, an unsaturated acid) and esters containing one of the two compounds.

Ethylene copolymers with unsaturated alcohols

Ethylene/vinyl alcohol copolymer (EVOH) is (formally) a copolymer of PE and vinyl alcohol (ethenol), which is prepared by (partial) hydrolysis of ethylene-vinyl acetate copolymer (as vinyl alcohol itself is not stable). However, typically EVOH has a higher comonomer content than the VAC commonly used. EVOH is used in multilayer films for packaging as a barrier layer (barrier plastic). As EVOH is hygroscopic (water-attracting), it absorbs water from the environment, whereby it loses its barrier effect. Therefore, it must be used as a core layer surrounded by other plastics (like LDPE, PP, PA or PET). EVOH is also used as a coating agent against corrosion at street lights, traffic light poles and noise protection walls.

Ethylene/acrylic acid copolymers (EAA)

Copolymer of ethylene and unsaturated carboxylic acids (such as acrylic acid) are characterized by good adhesion to diverse materials, by resistance to stress cracking and high flexibility. However, they are more sensitive to heat and oxidation than ethylene homopolymers. Ethylene/acrylic acid copolymers are used as adhesion promoters. If salts of an unsaturated carboxylic acid are present in the polymer, thermo-reversible ion networks are formed, they are called ionomers. Ionomers are highly transparent thermoplastics which are characterized by high adhesion to metals, high abrasion resistance and high water absorption.

Ethylene copolymers with unsaturated esters

If unsaturated esters are copolymerized with ethylene, either the alcohol moiety may be in the polymer backbone (as it is the case in ethylene-vinyl acetate copolymer) or of the acid moiety (e. g. in ethylene-ethyl acrylate copolymer). Ethylene-vinyl acetate copolymers are prepared similarly to LD-PE by high pressure polymerization. The proportion of comonomer has a decisive influence on the behaviour of the polymer. The density decreases up to a comonomer share of 10% because of the disturbed crystal formation. With higher proportions it approaches to the one of polyvinyl acetate (1.17 g/cm³). Due to decreasing crystallinity ethylene vinyl acetate copolymers are getting softer with increasing comonomer content. The polar side groups change the chemical properties significantly (compared to polyethylene): weather resistance, adhesiveness and weldability rise with comonomer content, while the chemical resistance decreases. Also mechanical properties are changed: stress cracking resistance and toughness in the cold rise, whereas yield stress and heat resistance decrease. With a very high proportion of comonomers (about 50%) rubbery thermoplastics are produced (thermoplastic elastomers). Ethylene-ethyl acrylate copolymers behave similarly to ethylene-vinyl acetate copolymers.

Crosslinking

A basic distinction is made between peroxide crosslinking (PE-Xa), silane crosslinking (PE-Xb), electron beam crosslinking (PE-Xc) and azo crosslinking (PE-Xd).Vorschau auf kunststoffe.de
/ref> Crosslinking PE scheme en

Shown are the peroxide, the silane and irradiation crosslinking. In each method, a radical is generated in the polyethylene chain (top center), either by radiation (h·ν) or by peroxides (R-O-O-R). Then, two radical chains can either directly crosslink (bottom left) or indirectly by silane compounds (bottom right). *Peroxide crosslinking (PE-Xa): The crosslinking of polyethylene using peroxides (e. g. dicumyl peroxide, dicumyl or di-tert-butyl peroxide) is still of major importance. In the so-called ''Engel process'', a mixture of HDPE and 2% peroxide is at first mixed at low temperatures in an extruder and then crosslinked at high temperatures (between 200 and 250 °C). The peroxide Radical initiator#Major types of initiation reaction, decomposes to peroxide radicals (RO•), which abstract (remove) hydrogen atoms from the polymer chain, leading to radical (chemistry), radicals. When these combine, a crosslinked network is formed. The resulting polymer network is uniform, of low tension and high flexibility, whereby it is softer and tougher than (the irradiated) PE-Xc. *Silane crosslinking (PE-Xb): In the presence of silanes (e.g. trimethoxyvinylsilane) polyethylene can initially be Si-Functionality (chemistry), functionalized by irradiation or by a small amount of a peroxide. Later Si-OH groups can be formed in a Laboratory water bath, water bath by hydrolysis, which condense then and crosslink the PE by the formation of Si-O-Si bridges. [16] Catalysts such as dibutyltin dilaurate may accelerate the reaction. *Irradiation crosslinking (PE-Xc): The crosslinking of polyethylene is also possible by a downstream radiation source (usually an electron accelerator, occasionally an isotopic radiator). PE products are crosslinked below the crystalline melting point by splitting off hydrogen atoms. β-radiation possesses a penetration depth of 10 Millimetre, mm, Gamma ray, ɣ-radiation 100 mm. Thereby the interior or specific areas can be excluded from the crosslinking. However, due to high capital and operating costs radiation crosslinking plays only a minor role compared with the peroxide crosslinking. In contrast to peroxide crosslinking, the process is carried out in the solid-state chemistry, solid state. Thereby, the cross-linking takes place primarily in the amorphous regions, while the crystallinity remains largely intact. *Azo crosslinking (PE-Xd): In the so-called ''Lubonyl process'' polyethylene is crosslinked preadded azo compounds after extrusion in a hot salt bath.

Chlorination and sulfochlorination

Chlorinated Polyethylene (PE-C) is an inexpensive material having a chlorine content from 34 to 44%. It is used in blends with Polyvinyl chloride, PVC because the soft, rubbery chloropolyethylene is embedded in the PVC matrix, thereby increasing the impact resistance. It also increases the weather resistance. Furthermore, it is used for softening PVC foils, without risking the migrate of plasticizers. Chlorinated polyethylene can be crosslinked peroxidically to form an elastomer which is used in cable and rubber industry. When chlorinated polyethylene is added to other polyolefins, it reduces the flammability. Chlorosulfonated PE (CSM) is used as starting material for ozone-resistant synthetic rubber.

Bio-based polyethylene

Braskem and Toyota Tsusho Corporation started joint marketing activities to produce polyethylene from sugarcane. Braskem will build a new facility at their existing industrial unit in Triunfo, Rio Grande do Sul, Triunfo, Rio Grande do Sul, Brazil with an annual production capacity of , and will produce high-density and low-density polyethylene from bioethanol derived from sugarcane. Polyethylene can also be made from other feedstocks, including wheat grain and sugar beet. These developments are using renewable resources rather than fossil fuel, although the issue of plastic source is currently negligible in the wake of plastic pollution, plastic waste and in particular polyethylene waste as shown above.

Nomenclature and general description of the process

The name polyethylene comes from the ingredient and not the resulting chemical compound, which contains no double bonds. The scientific name ''polyethene'' is systematically derived from the scientific name of the monomer. The alkene monomer converts to a long, sometimes ''very'' long, alkane in the polymerization process. In certain circumstances it is useful to use a structure-based nomenclature; in such cases

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 similar manner polypropylene and polystyrene are shortened to PP and PS, respectively. In the United Kingdom and India the polymer is commonly called ''polythene'', from the ICI trademark, trade name, although this is not recognized scientifically.

Footnotes

References

Bibliography

* *

External links

Polythene's story: The accidental birth of plastic bags
Kawawada, Karen, ''Waterloo Region Record'' (22 May 2008). {{Authority control Polyolefins Dielectrics Ethylene Thermoplastics Packaging materials Transparent materials Bioplastics Commodity chemicals German inventions 1898 in science 1898 in Germany