View source for Polyethylene

{{Short description|Most common thermoplastic polymer}}
{{About|the plastic|the Radiohead song|Polyethylene (Parts 1 & 2)}}
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{{Use dmy dates|date=September 2021}}
{{Chembox
| Name = 
| ImageFile = Polyethylene_repeat_unit.svg
| ImageFile_Ref = {{chemboximage|correct|??}}
| ImageSize = 100
| ImageName = Skeletal formula of a ris monomer
| ImageClass = skin-invert-image
| ImageFile1 = Polyethylene-3D-vdW.png
| ImageFile1_Ref = {{chemboximage|correct|??}}
| ImageName1 = Spacefill model of polyethylene
| ImageClass1 = bg-transparent
| ImageFile2 = Polyethylene balls1.jpg
| ImageFile2_Ref = {{chemboximage|correct|??}}
| ImageName2 = Sample of granulated polyethylene
| IUPACName = Polyethene or poly(methylene)<ref>{{cite book |title=Compendium of Polymer Terminology and Nomenclature – IUPAC Recommendations 2008 |url=https://www.iupac.org/cms/wp-content/uploads/2016/01/Compendium-of-Polymer-Terminology-and-Nomenclature-IUPAC-Recommendations-2008.pdf |access-date=2018-08-28 }}</ref>
| OtherNames = Polyethylene<br />Polythene
| SystematicName = 
| Section1 = {{Chembox Identifiers
| Abbreviations = PE
| CASNo = 9002-88-4
| CASNo_Ref = {{cascite|correct|CAS}}
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 3W404QE89S
| KEGG = C19503
| KEGG_Ref = {{keggcite|correct|kegg}}
| MeSHName = Polyethylene
| ChemSpiderID = none
| PubChem = 6325
}}
| Section2 = {{Chembox Properties
| Formula = (C<sub>2</sub>H<sub>4</sub>)<sub>''n''</sub>
| MolarMass =
| Appearance =
| Odor =
| Density = 0.88–0.96&nbsp;g/cm<sup>3</sup><ref name="kamalbatra111">{{cite book |last=Batra |first=Kamal |title=Role of Additives in Linear Low Density Polyethylene (LLDPE) Films |url=http://www.slideshare.net/kamalbatra111/polyethylene-pe |access-date=16 September 2014 |year=2014 |page=9}}</ref>
| MeltingPtC = 115–135
| MeltingPt_ref = <ref name="kamalbatra111" /> 
| ThermalConductivity =
| BoilingPtC =
| Solubility = Not soluble
| Solvent1 =
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| VaporPressure =
| pKa =
| RefractIndex =
| MagSus = −9.67×10<sup>−6</sup> ([[High-density polyethylene|HDPE]], SI, 22&nbsp;°C)<ref>{{cite journal |last1=Wapler |first1=M. C. |last2=Leupold |first2=J. |last3=Dragonu |first3=I. |last4=von Elverfeldt |first4=D. |last5=Zaitsev |first5=M. |last6=Wallrabe |first6=U. |title=Magnetic properties of materials for MR engineering, micro-MR and beyond |journal=JMR |date=2014 |volume=242 |pages=233–242 |doi=10.1016/j.jmr.2014.02.005 |pmid=24705364 |arxiv=1403.4760 |bibcode=2014JMagR.242..233W |s2cid=11545416}}</ref>
| LogP = 1.02620<ref name="chemsrc">{{Cite web |url=https://www.chemsrc.com/en/cas/9002-88-4_1088896.html |title=poly(ethylene) |website=ChemSrc}}</ref>
 }}
| Section3 = 
| Section4 = {{Chembox Thermochemistry
| Thermochemistry_ref =
| HeatCapacity = 
| Entropy = 
| DeltaHform = −28 to −29 kJ/mol<ref name=NBS>{{cite journal |last1=Paul L. Splitstone and Walter H. Johnson |title=The Enthalpies of Combustion and Formation of Linear Polyethylene |journal=Journal of Research of the National Bureau of Standards |date=May 20, 1974 |url=https://nvlpubs.nist.gov/nistpubs/jres/78A/jresv78An5p611_A1b.pdf}}</ref>
| DeltaGfree = 
| DeltaHcombust =
| DeltaHfus =
| DeltaHvap =
| DeltaHsublim =
| HHV = 650–651 kJ/mol, 46 MJ/kg<ref name=NBS/>
| LHV =
 }}
| Section5 = 
| Section6 = 
}}
[[File:Polyethylene-repeat-2D.png|thumb|140px|The repeating unit within polyethylene in the most stable<ref>{{cite journal |url=https://www.emerald.com/insight/content/doi/10.1108/S2043-052320230000019004/full/html |title=Challenges of Reducing Polythene and Plastic in Sri Lanka: A Case Study of Attanagalla Secretariat Division |first1=G. P. T. S. |last1=Hemakumara |first2=T. G. Shamal |last2=Madhusankha |journal=Socially Responsible Plastic |series=Developments in Corporate Governance and Responsibility |volume=19 |date=2023 |pages=59–73 |doi=10.1108/S2043-052320230000019004|isbn=978-1-80455-987-1 |url-access=subscription }}</ref> [[staggered conformation]]]]
{{Fibre sidebar}}
'''Polyethylene''' or '''polythene''' (abbreviated '''PE'''; [[IUPAC]] name '''polyethene''' or '''poly(methylene)''') is the most commonly produced [[plastic]].<ref name="Ullmann"/> It is a [[polymer]], primarily used for [[packaging]] ([[plastic bag]]s, [[plastic film]]s, [[geomembranes]] and containers including [[bottle]]s, [[cup]]s, [[jar]]s, [[Presentation folder|folders]], etc.). {{As of|2017}}, over 100 million [[tonne]]s of polyethylene [[resin]]s are being produced annually, accounting for 34% of the total plastics market.<ref>{{Cite journal |last1=Geyer |first1=Roland |last2=Jambeck |first2=Jenna R. |author-link2=Jenna Jambeck |last3=Law |first3=Kara Lavender |date=2017-07-01 |title=Production, use, and fate of all plastics ever made |journal=Science Advances |volume=3 |issue=7 |article-number=e1700782 |doi=10.1126/sciadv.1700782 |pmid=28776036 |pmc=5517107 |bibcode=2017SciA....3E0782G}}</ref><ref>{{Cite web |url=https://www.plasticseurope.org/application/files/5715/1717/4180/Plastics_the_facts_2017_FINAL_for_website_one_page.pdf |title=Plastics: The Facts |website=Plastics Europe |access-date=2018-08-29 |archive-date=4 February 2018 |archive-url=https://web.archive.org/web/20180204170326/https://www.plasticseurope.org/application/files/5715/1717/4180/Plastics_the_facts_2017_FINAL_for_website_one_page.pdf }}</ref>

Many kinds of polyethylene are known, with most having the [[chemical formula]] (C<sub>2</sub>H<sub>4</sub>)<sub>''n''</sub>. PE is usually a mixture of similar [[polymer]]s of [[ethylene]], with various values of ''n''. It can be ''[[Low-density polyethylene|low-density]]'' or ''[[High-density polyethylene|high-density]]'' and many variations thereof.  Its properties can be modified further by crosslinking or copolymerization.  All forms are nontoxic as well as chemically resilient, contributing to polyethylene's popularity as a multi-use plastic. However, polyethylene's chemical resilience also makes it a long-lived and decomposition-resistant pollutant when disposed of improperly.<ref>{{cite journal |last1=Yao |first1=Zhuang |last2=Jeong Seong |first2=Hyeon |last3=Jang |first3=Yu-Sin |title=Environmental toxicity and decomposition of polyethylene |journal=  Ecotoxicology and Environmental Safety|date=2022 |volume=242 |pages=1, 3 |doi=10.1016/j.ecoenv.2022.113933|pmid=35930840 |doi-access=free |bibcode=2022EcoES.24213933Y }}</ref> Being a [[hydrocarbon]], polyethylene is colorless to opaque (without impurities or colorants) and combustible.<ref>{{cite web |last1=Sepe |first1=Michael |title=Understanding the 'Science' of Color |url=https://www.ptonline.com/articles/understanding-the-science-of-color |website=Plastics Technology |date=8 April 2024 |access-date=April 25, 2024}}</ref> 
{{Toclimit|3}}

==History==
Polyethylene was first synthesized by the German chemist [[Hans von Pechmann]], who prepared it by accident in 1898 while investigating [[diazomethane]].<ref name=vPechmann1898>{{cite journal |author=von Pechmann, H. |year=1898 |url=https://gallica.bnf.fr/ark:/12148/bpt6k90751n/f312.image.langEN |title=Ueber Diazomethan und Nitrosoacylamine |journal=Berichte der Deutschen Chemischen Gesellschaft zu Berlin |volume=31 |pages=2640–2646}}</ref>{{efn|{{lang|de|Erwähnt sei noch, dass aus einer ätherischen Diazomethanlösung sich beim Stehen manchmal minimale Quantitäten eines weissen, flockigen, aus Chloroform krystallisirenden Körpers abscheiden; ...}} [''It should be mentioned that from an ether solution of diazomethane, upon standing, sometimes small quantities of a white, flakey substance precipitate, which can be crystallized with chloroform; ...''].<ref name=vPechmann1898/>{{rp|2643}} }}<ref name=BambergerTschirner1900>{{cite journal |first1=Eug. |last1=Bamberger |author1-link=Eugen Bamberger |first2=Fred. |last2=Tschirner |year=1900 |url=https://gallica.bnf.fr/ark:/12148/bpt6k907551/f960.image.langEN |title=Ueber die Einwirkung von Diazomethan auf β-Arylhydroxylamine |trans-title=On the effect of diazomethane on β-arylhydroxylamine |journal=Berichte der Deutschen Chemischen Gesellschaft zu Berlin |volume=33 |pages=955–959 |doi=10.1002/cber.190003301166 }}</ref>{{efn|{{lang|de|Die Abscheidung weisser Flocken aus Diazomethanlösungen erwähnt auch v. Pechmann (diese Berichte '''31''', 2643);<ref name=vPechmann1898/> er hat sie aber wegen Substanzmangel nicht untersucht. Ich hatte übrigens Hrn.&nbsp;v.&nbsp;Pechmann schon einige Zeit vor Erscheinen seiner Publication mitgetheilt, dass aus Diazomethan ein fester, weisser Körper entstehe, der sich bei der Analyse als (CH<sub>2</sub>)<sub>x</sub> erwiesen habe, worauf mir Hr.&nbsp;v.&nbsp;Pechmann schrieb, dass er den weissen Körper ebensfalls beobachtet, aber nicht untersucht habe. Zuerst erwähnt ist derselbe in der Dissertation meines Schülers. (Hindermann, Zürich (1897), S.&nbsp;120)}}<ref name=BambergerTschirner1900/>{{rp|footnote&nbsp;3 on page&nbsp;956}} [''Von Pechmann (these ''Reports'', '''31''', 2643)<ref name=vPechmann1898/> also mentioned the precipitation of white flakes from diazomethane solutions; however, due to a scarcity of the material, he didn't investigate it. Incidentally, some time before the appearance of his publication, I had communicated to Mr.&nbsp;von&nbsp;Pechmann that a solid, white substance arose from diazomethane, which on analysis proved to be (CH<sub>2</sub>)<sub>x</sub>, whereupon Mr.&nbsp;von&nbsp;Pechmann wrote me that he had likewise observed the white substance, but not investigated it. It is first mentioned in the dissertation of my student. (Hindermann, Zürich (1897), p.&nbsp;120)''].}} When his colleagues [[Eugen Bamberger]] and Friedrich Tschirner characterized the white, waxy substance that he had created, they recognized that it contained long −CH<sub>2</sub>− chains and termed it ''polymethylene''.<ref>{{cite journal |first1=Eugen |last1=Bamberger |author1-link=Eugen Bamberger |first2=Friedrich |last2=Tschirner |year=1900 |url=https://gallica.bnf.fr/ark:/12148/bpt6k907551/f960.image.langEN |title=Ueber die Einwirkung von Diazomethan auf β-Arylhydroxylamine |trans-title=On the effect of diazomethane on β-arylhydroxylamine |journal=Berichte der Deutschen Chemischen Gesellschaft zu Berlin |volume=33 |pages=955–959 |doi=10.1002/cber.190003301166 |quote=[page 956]: {{lang|de|Eine theilweise – übrigens immer nur minimale – Umwandlung des Diazomethans in Stickstoff und Polymethylen vollzieht sich auch bei ganz andersartigen Reactionen; ...}} [''A partial – incidentally, always only minimal – conversion of diazomethane into nitrogen and polymethylene takes place also during quite different reactions; ...''] }}</ref>
[[File:First polythene pillbox.JPG|thumb|right|A [[pill box (container)|pill box]] presented to a technician at ICI in 1936 made from the first pound of polyethylene]]

The first industrially practical polyethylene synthesis (diazomethane is a notoriously unstable substance that is generally avoided in industrial syntheses) was again accidentally discovered in 1933 by Eric Fawcett and Reginald Gibson at the [[Imperial Chemical Industries]] (ICI) works in [[Northwich]], [[England]].<ref>{{cite news |url=http://archive.thisischeshire.co.uk/2006/8/23/275808.html |archive-url=https://web.archive.org/web/20100121071050/http://archive.thisischeshire.co.uk/2006/8/23/275808.html |archive-date=21 January 2010 |title=Winnington history in the making |date=23 August 2006 |work=This is Cheshire |access-date=20 February 2014 }}</ref> Upon applying extremely high pressure (several hundred [[atmosphere (unit)|atmospheres]]) to a mixture of ethylene and [[benzaldehyde]] they again produced a white, waxy material. Because the reaction had been initiated by trace [[oxygen]] contamination in their apparatus, the experiment was difficult to reproduce at first. It was not until 1935 that another ICI chemist, [[Michael Perrin]], developed this accident into a reproducible high-pressure synthesis for polyethylene that became the basis for industrial low-density polyethylene ([[Low-density polyethylene|LDPE]]) 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&nbsp;II, secrecy imposed, and the new process was used to produce insulation for UHF and SHF [[coaxial cable]]s of [[radar]] sets. During World War&nbsp;II, further research was done on the ICI process and in 1944, [[DuPont]] at Sabine River, Texas, and [[Union Carbide|Union Carbide Corporation]] at South Charleston, West Virginia, began large-scale commercial production under license from ICI.<ref>{{cite news |date=July 1949 |title=Poly – the all-star plastic |work=Popular Mechanics |via=Google Books |url=https://books.google.com/books?id=GtkDAAAAMBAJ&pg=PA126 |pages=125–129 |volume=91 |issue=1 |publisher=Hearst Magazines |access-date=20 February 2014}}</ref><ref>{{cite book |title=A History of Union Carbide Corporation |page=69 |url=https://www.carbidecamps.net/UCC%20Articles/HistoryOfUCC/History%20of%20UCC%20-%20complete%20-%20single%20file.pdf}}</ref>

The landmark breakthrough in the commercial production of polyethylene began with the development of [[catalyst]]s that promoted the [[polymerization]] at mild temperatures and pressures. The first of these was a catalyst based on [[chromium trioxide]] discovered in 1951 by [[Robert Banks (chemist)|Robert Banks]] and [[J. Paul Hogan]] at [[Phillips Petroleum]].<ref>{{cite book |title=Handbook of Transition Metal Polymerization Catalysts |last1=Hoff |first1=Ray |last2=Mathers |first2=Robert T. |editor1-last=Hoff |editor1-first=Ray |editor2-last=Mathers |editor2-first=Robert T. |year=2010 |publisher=John Wiley & Sons |isbn=978-0-470-13798-7 |doi=10.1002/9780470504437.ch10 |chapter=Chapter 10. Review of Phillips Chromium Catalyst for Ethylene Polymerization}}</ref> In 1953 the German chemist [[Karl Ziegler]] developed a catalytic system based on [[titanium]] [[halide]]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–Natta catalyst|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]]. Catalytic systems based on soluble catalysts, the [[metallocene]]s, were reported in 1976 by [[Walter Kaminsky]] and [[Hansjörg Sinn]]. The Ziegler- and metallocene-based catalysts families have proven to be very flexible at copolymerizing ethylene with other [[olefin]]s and have become the basis for the wide range of polyethylene [[resin]]s available today, including [[#Very-low-density polyethylene (VLDPE)|very-low-density polyethylene]] and [[linear low-density polyethylene]]. Such resins, in the form of [[Ultra-high-molecular-weight polyethylene#UHMWfiber|UHMWPE fibers]], have (as of 2005) begun to replace [[aramid]]s in many high-strength applications.

==Properties==
The properties of polyethylene depend strongly on type.  The molecular weight, crosslinking, and presence of comonomers all strongly affect its properties.  It is for this structure-property relation that intense effort has been invested into diverse kinds of PE.<ref name=Ullmann/><ref name="Kaiser">{{cite book|last1= Kaiser|first1= Wolfgang|title= Kunststoffchemie für Ingenieure von der Synthese bis zur Anwendung|date=2011|publisher= Hanser|location=München|isbn= 978-3-446-43047-1|edition=3.}}</ref> LDPE is softer and more transparent than HDPE. For medium- and high-density polyethylene the melting point is typically in the range {{convert|120|to|130|C|F}}. The melting point for average commercial low-density polyethylene is typically {{convert|105|to|115|C|F}}. These temperatures vary strongly with the type of polyethylene, but the theoretical upper limit of melting of polyethylene is reported to be {{convert|144|to|146|C|F}}. Combustion typically occurs above {{convert|349|C|F}}.

Most [[Low-density polyethylene|LDPE]], [[Medium-density polyethylene|MDPE]], and [[High-density polyethylene|HDPE]] 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]]s such as [[toluene]] or [[xylene]], or in chlorinated solvents such as [[trichloroethane]] or [[trichlorobenzene]].<ref name="Ullmann">{{Cite encyclopedia |title=Polyolefins |encyclopedia=Ullmann's Encyclopedia of Industrial Chemistry |date=2000 |doi=10.1002/14356007.a21_487 |isbn=3-527-30673-0 |last2=Heggs |first2=T. Geoffrey |last3=Koch |first3=Hartmut |last4=Mawer |first4=Ralph L. |last5=Immel |first5=Wolfgang |last1=Whiteley |first1=Kenneth S.}}</ref>

Polyethylene absorbs almost no [[water]].  The permeability for water vapor and polar gases of is lower than for most plastics.  On the other hand, non-polar gases such as [[Oxygen]], [[carbon dioxide]], and [[flavorings]] can pass it easily.

Polyethylene burns slowly with a blue flame having a yellow tip and gives off an odour of paraffin (similar to [[candle]] flame). The material continues burning on removal of the flame source and produces a drip.<ref>{{cite web |url=http://www.boedeker.com/burntest.htm |title=How to Identify Plastic Materials Using The Burn Test |work=Boedeker Plastics |access-date=8 May 2012}}</ref>

Polyethylene cannot be imprinted or bonded with adhesives without pretreatment. High-strength joints are readily achieved with [[plastic welding]].

===Electrical===
Polyethylene is a good [[insulator (electricity)|electrical insulator]]. It offers good [[electrical treeing]] resistance; however, it becomes easily [[triboelectric effect|electrostatically charged]] (which can be reduced by additions of [[graphite]], [[carbon black]] or [[antistatic agent]]s).
When pure, the [[dielectric constant]] is in the range 2.2 to 2.4 depending on the density<ref>{{cite web |title=Electrical Properties of Plastic Materials |url=https://www.professionalplastics.com/professionalplastics/ElectricalPropertiesofPlastics.pdf |website=professionalplastics.com |publisher=Professional Plastics |access-date=11 October 2022 |archive-url=https://web.archive.org/web/20220920173045/https://www.professionalplastics.com/professionalplastics/ElectricalPropertiesofPlastics.pdf |archive-date=20 September 2022 |language=en-US |url-status=live}}</ref> and the loss tangent is very low, making it a good dielectric for building [[capacitor]]s. For the same reason it is commonly used as the insulation material for high-frequency coaxial and twisted pair cables.

===Optical ===
Depending on thermal history and film thickness, PE can vary between almost clear ([[transparency and translucency|transparent]]), milky-opaque ([[translucent]]) and [[Opacity (optics)|opaque]]. LDPE has the greatest, LLDPE slightly less, and HDPE the least transparency. Transparency is reduced by [[crystallite]]s if they are larger than the wavelength of visible light.<ref>Chung, C. I. (2010) ''Extrusion of Polymers: Theory and Practice''. 2nd ed.. Hanser: Munich.</ref>

==Manufacturing process==

===Monomer===
{{multiple image
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The ingredient or [[monomer]] is [[ethylene]] ([[IUPAC]] name ethene), a [[gas]]eous [[hydrocarbon]] with the formula C<sub>2</sub>H<sub>4</sub>, which can be viewed as a pair of [[methylene group]]s (−{{chem|CH|2}}−) connected to each other. Typical specifications for PE purity are <5&nbsp;ppm for water, oxygen, and other [[alkene]]s contents. Acceptable contaminants include N<sub>2</sub>, ethane (common precursor to ethylene), and methane. Ethylene is usually produced from [[Cracking (chemistry)|petrochemical sources]], but is also generated by [[Ethanol#Dehydration|dehydration of ethanol]].<ref name="Ullmann"/>

===Polymerization===
[[File:CNX Chem 20 01 monomer.png|thumb|500x500px]]
Polymerization of ethylene to polyethylene is described by the following [[chemical equation]]:

: {{var|n}} {{chem|CH|2|{{=}}CH|2}} (gas) → {{chem|[−CH|2|−CH|2}}−{{chem|]|{{var|n}}}} (solid){{pad|2em}}[[Standard enthalpy of reaction|Δ{{var|H}}]]/{{var|n}}&nbsp;=&nbsp;{{convert|-25.71|+/-|0.59|kcal/mol|kJ/mol|abbr=on}}<ref>Victor Ostrovskii et al. [https://www.researchgate.net/publication/255729774_Ethylene_Polymerization_Heat Ethylene Polymerization Heat] (abstract) in Doklady Chemistry 184(1):103–104. January 1969.</ref>

Ethylene is a stable molecule that polymerizes only upon contact with catalysts. The conversion is highly [[exothermic]]. [[Coordination polymerization]] is the most pervasive technology, which means that metal chlorides or metal oxides are used. The most common catalysts consist of [[titanium(III) chloride]], the so-called [[Ziegler–Natta catalyst]]s. Another common catalyst is the [[Phillips catalyst]], prepared by depositing [[chromium trioxide|chromium(VI) oxide]] on silica.<ref name="Ullmann"/> Polyethylene can be produced through [[radical polymerization]], but this route has only limited utility and typically requires high-pressure apparatus.

==Joining==
{{See also|Solvent bonding}}
Commonly used methods for joining polyethylene parts together include:<ref name="pdl">Plastics Design Library (1997). ''Handbook of Plastics Joining: A Practical Guide''. Norwich, New York: Plastics Design Library. p. 326. {{ISBN|1-884207-17-0}}.</ref>

* [[Plastic welding|Welding]]
** [[Hot gas welding]]
** [[Plastic welding#Non-contact/IR welding|Infrared welding]]
** [[Laser welding]]
** [[Ultrasonic welding]]
** [[Heat seal]]ing
** [[Heat fusion]]
* [[Fasteners|Fastening]]
* [[Adhesives]]<ref name="pdl"/> 
** [[Pressure-sensitive adhesive]] (PSAs) 
*** Dispersion of solvent-type PSAs
** [[polyurethane adhesive|Polyurethane]] contact adhesives
** Two-part polyurethane
** [[Epoxy adhesive]]s
** [[Hot-melt adhesive]]s
** [[Solvent bonding]] – [[Adhesive]]s and [[solvent]]s are rarely used as [[solvent bonding]] because polyethylene is [[nonpolar]] and has a high resistance to solvents. 
[[Pressure-sensitive adhesive]]s (PSA) are feasible if the surface chemistry or charge is modified with [[plasma activation]], [[flame treatment]], or [[corona treatment]].

==Classification==
Polyethylene is classified by its [[density]] and [[Branching (polymer chemistry)|branching]]. Its mechanical properties depend significantly on variables such as the extent and type of branching, the crystal structure, and the [[molecular weight]]. There are several types of polyethylene:

*[[Ultra-high-molecular-weight polyethylene]] (UHMWPE)
*Ultra-low-molecular-weight polyethylene (ULMWPE or PE-WAX)
*High-molecular-weight polyethylene (HMWPE)
*[[High-density polyethylene]] (HDPE)
*High-density cross-linked polyethylene (HDXLPE)
*[[Cross-linked polyethylene]] (PEX or XLPE)
*[[Medium-density polyethylene]] (MDPE)
*[[Linear low-density polyethylene]] (LLDPE)
*[[Low-density polyethylene]] (LDPE)
*Very-low-density polyethylene (VLDPE)
*[[Chlorinated polyethylene]] (CPE)

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

===Ultra-high-molecular-weight (UHMWPE)===
{{main|Ultra-high-molecular-weight polyethylene}}
[[File:Stainless steel and ultra high molecular weight polythene hip replacement (9672239334).jpg|thumb|Stainless steel and ultra-high-molecular-weight polyethylene hip replacement]]
UHMWPE is polyethylene with a molecular weight numbering in the millions, usually between 3.5 and 7.5 million [[Atomic mass unit|amu]].<ref>{{cite book|author=Kurtz, Steven M. |title=UHMWPE Biomaterials Handbook. Ultra-High Molecular Weight Polyethylene in Total Joint Replacement and Medical Devices|edition= 3rd| page=3|isbn=978-0-323-35435-6 |year=2015|publisher=Elsevier|doi=10.1016/C2013-0-16083-7}}</ref> The high molecular weight makes it a very [[toughness|tough]] material, but results in less efficient packing of the chains into the [[crystal structure]] as evidenced by densities of less than high-density polyethylene (for example, 0.930–0.935&nbsp;g/cm<sup>3</sup>). 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]]-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 [[implant (medicine)|implants]] used for [[hip replacement|hip]] and [[knee replacement]]s. As [[Ultra-high-molecular-weight polyethylene#UHMWfiber|fiber]], it competes with [[aramid]] in [[bulletproof vests]].

===High-density (HDPE)===
{{main|High-density polyethylene}}
[[File:Site Photo of HDPE Pipe.jpg|thumb|HDPE pipe on site during installation in outback Western Australia. The white outer layer, Acu-Therm, is co-extruded to provide a reduction of thermal heating.]]
HDPE is defined by a density of greater or equal to 0.941&nbsp;g/cm<sup>3</sup>. 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, [[Ziegler–Natta catalyst]]s or [[metallocene]] 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]].

===Cross-linked (PEX or XLPE)===
{{Main|Cross-linked polyethylene}}
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)===
{{Main|Medium-density polyethylene}}
MDPE is defined by a density range of 0.926–0.940&nbsp;g/cm<sup>3</sup>. 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)===
{{Main|Linear low-density polyethylene}}
LLDPE is defined by a density range of 0.915–0.925&nbsp;g/cm<sup>3</sup>. LLDPE is a substantially linear polymer with significant numbers of short branches, commonly made by [[copolymerization]] of ethylene with short-chain [[alpha-olefin]]s (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.
===Low-density (LDPE)===
{{Main|Low-density polyethylene}}
LDPE is defined by a density range of 0.910–0.940&nbsp;g/cm<sup>3</sup>. LDPE has a high degree of short- and long-chain branching, which means that the chains do not pack into the [[crystal structure]] as well. 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 [[ductility]]. 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.

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&nbsp;g/cm<sup>3</sup>. 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.

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 [[acrylate]]s. 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&nbsp;g/cm<sup>3</sup> in crystalline regions and 0.86&nbsp;g/cm<sup>3</sup> in amorphous regions. An almost linear relationship exists between density and crystallinity.<ref name="Kaiser" />

The degree of branching of the different types of polyethylene can be schematically represented as follows:<ref name="Kaiser" />
{| class="wikitable skin-invert-image"
|-
| PE-HD || [[File:PE-HD schematic.svg|Schematic representation of PE-HD (high-density polyethylene)]]
|-
| PE-LLD ||
[[File:PE-LLD schematic.svg|Schematic representation of PE-LLD (linear low-density polyethylene)]]
|-
| PE-LD || [[File:PE-LD schematic.svg|Schematic representation of PE-LD (low-density polyethylene)]]
|}
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:
:[[File:PE-LD mechanism en.svg|class=skin-invert-image|Mechanism for the emergence of side chains during synthesis of polyethylene (PE-LD).]]

==Environmental issues==
[[File:Polyethylene.jpg|thumb|140px|A recyclable bag manufactured from polyethylene, [[resin identification code]]]]
[[File:Say no to polythene. Sign. Nako, H.P. India.jpg|thumb|''Say no to polythene.'' Sign. [[Nako, Himachal Pradesh]], India.]]
{{See also|Plastic pollution}}
The widespread usage of polyethylene poses potential difficulties for [[waste management]] because it is not readily biodegradable.  Since 2008, Japan has increased plastic recycling, but still has a large amount of plastic wrapping which goes to waste. Plastic recycling in Japan is a potential US$90 [[billion]] market.<ref name="prideaux2007">{{cite news |title=Plastic incineration rise draws ire |first=Eric |last=Prideaux |url=http://www.japantimes.co.jp/text/nn20071103f2.html |newspaper=The Japan Times|date=3 November 2007 |access-date=8 May 2012 |archive-url=https://web.archive.org/web/20121122082604/http://www.japantimes.co.jp/text/nn20071103f2.html |archive-date=22 November 2012 }}</ref>

It is possible to rapidly convert polyethylene to hydrogen and [[graphene]] by heating. The energy needed is much less than for producing hydrogen by electrolysis.<ref>{{cite journal |last1=Alex Wilkins |title=Waste plastic can be recycled into hydrogen fuel and graphene |journal=New Scientist |date=Sep 29, 2023 |url=https://www.newscientist.com/article/2394144-waste-plastic-can-be-recycled-into-hydrogen-fuel-and-graphene/#}}</ref><ref>{{cite journal|display-authors=etal |last1=Kevin Wyss |title=Synthesis of Clean Hydrogen Gas from Waste Plastic at Zero Net Cost |journal=Advanced Materials |date=Sep 11, 2023 |volume=35 |issue=48 |article-number=e2306763 |doi=10.1002/adma.202306763|pmid=37694496 |bibcode=2023AdM....3506763W }}</ref>

===Biodegradability===
Several experiments have been conducted aimed at discovering an [[enzyme]] or organisms that will degrade polyethylene. Several plastics - such as polyesters, polycarbonates, and polyamides - degrade either by hydrolysis or air oxidation.  In some cases the degradation is increased by bacteria or various enzyme cocktails.  The situation is very different with polymers where the backbone consists solely of C-C bonds.  These polymers include polyethylene, but also polypropylene, polystyrene and acrylates.  At best, these polymers degrade very slowly, but degradation experiments are difficult because yields and rates are very slow.<ref name= Enzyme>{{cite journal |doi=10.1021/acs.chemrev.2c00644 |title=Enzymes' Power for Plastics Degradation |date=2023 |last1=Tournier |first1=Vincent |last2=Duquesne |first2=Sophie |last3=Guillamot |first3=Frédérique |last4=Cramail |first4=Henri |last5=Taton |first5=Daniel |last6=Marty |first6=Alain |last7=André |first7=Isabelle |journal=Chemical Reviews |volume=123 |issue=9 |pages=5612–5701 |pmid=36916764 |url=https://hal.science/hal-04150645/file/Andre_Chemical_Reviews_HAL.pdf }}</ref> Further confusing the situation, even preliminary successes are greeted with enthusiasm by the popular press.<ref>{{cite magazine |url=https://www.spiegel.de/wissenschaft/natur/forscherin-entdeckt-zufaellig-plastik-fressende-raupe-a-1144619.html | title=Forscherin entdeckt zufällig Plastik-fressende Raupe| magazine=[[Der Spiegel]]| language=de |date=24 April 2017 |access-date=24 April 2017}}</ref><ref>{{cite news|work=BBC News|author=Briggs, Helen |title=Plastic-eating caterpillar could munch waste, scientists say|url=https://www.bbc.co.uk/news/science-environment-39694553|access-date=24 April 2017}}</ref><ref>{{cite web |url=http://woohooreport.com/2009/09/wci-student-isolates-microbe-that-lunches-on-plastic-bags/ |publisher=The Record.com |title=CanadaWorld – WCI student isolates microbe that lunches on plastic bags |access-date=2014-02-20 |author-first1=Karen|author-last1=Kawawada|archive-url=https://web.archive.org/web/20110718081923/http://woohooreport.com/2009/09/wci-student-isolates-microbe-that-lunches-on-plastic-bags/ |archive-date=18 July 2011}}</ref> Some technical challenges in this area include the failure to identify enzymes responsible for the proposed degradation.  Another issue is that organisms are incapable of importing hydrocarbons of molecular weight greater than 500.<ref name= Enzyme/>

===Bacteria and insect case studies===


The [[Indian mealmoth]] larvae are claimed to metabolize polyethylene based on observing that plastic bags at a researcher's 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%.<ref>{{cite web|url=http://www.fondriest.com/news/discovery-plastic-eating-bacteria-may-speed-waste-reduction.htm |date=27 January 2015 |title=Discovery of plastic-eating bacteria may speed waste reduction |first=Lori |last=Balster |publisher=fondriest.com}}</ref><ref>{{cite journal|doi=10.1021/es504038a |pmid=25384056 |title=Evidence of Polyethylene Biodegradation by Bacterial Strains from the Guts of Plastic-Eating Waxworms |journal=Environmental Science & Technology |volume=48 |issue=23 |pages=13776–84 |year=2014 |last1=Yang |first1=Jun |last2=Yang |first2=Yu |last3=Wu |first3=Wei-Min |last4=Zhao |first4=Jiao |last5=Jiang |first5=Lei |bibcode=2014EnST...4813776Y }}</ref>

The caterpillar of ''[[Galleria mellonella]]'' is claimed to consume polyethylene. The caterpillar is able to digest polyethylene due to a combination of its [[gut microbiota]]<ref>{{Cite journal |last1=Cassone |first1=Bryan J. |last2=Grove |first2=Harald C. |last3=Elebute |first3=Oluwadara |last4=Villanueva |first4=Sachi M. P. |last5=LeMoine |first5=Christophe M. R. |date=2020-03-11 |title=Role of the intestinal microbiome in low-density polyethylene degradation by caterpillar larvae of the greater wax moth, Galleria mellonella |journal=Proceedings of the Royal Society B: Biological Sciences |language=en |volume=287 |issue=1922 |article-number=20200112 |doi=10.1098/rspb.2020.0112 |issn=0962-8452 |pmc=7126078 |pmid=32126962}}</ref> and its saliva containing [[Enzyme|enzymes]] that oxidise and depolymerise the plastic.<ref>{{Cite journal |last1=Sanluis-Verdes |first1=A. |last2=Colomer-Vidal |first2=P. |last3=Rodriguez-Ventura |first3=F. |last4=Bello-Villarino |first4=M. |last5=Spinola-Amilibia |first5=M. |last6=Ruiz-Lopez |first6=E. |last7=Illanes-Vicioso |first7=R. |last8=Castroviejo |first8=P. |last9=Aiese Cigliano |first9=R. |last10=Montoya |first10=M. |last11=Falabella |first11=P. |last12=Pesquera |first12=C. |last13=Gonzalez-Legarreta |first13=L. |last14=Arias-Palomo |first14=E. |last15=Solà |first15=M. |date=2022-10-04 |title=Wax worm saliva and the enzymes therein are the key to polyethylene degradation by Galleria mellonella |journal=Nature Communications |language=en |volume=13 |issue=1 |page=5568 |doi=10.1038/s41467-022-33127-w |pmid=36195604 |pmc=9532405 |bibcode=2022NatCo..13.5568S |issn=2041-1723}}</ref>

=== Climate change ===
When exposed to ambient solar radiation the plastic produces trace amounts of two [[greenhouse gas]]es, [[methane]] and [[ethylene]]. The plastic type which releases gases at the highest rate is [[low-density polyethylene]] (LDPE). Due to its low density it breaks down more easily over time, leading to higher surface areas.  When incubated in air, LDPE emits gases at rates ~2 times and ~76 times higher in comparison to incubation in water for methane and ethylene, respectively. However, based on the rates measured in the study methane production by plastics is presently an insignificant component of the global methane budget.<ref>{{cite journal | doi = 10.1371/journal.pone.0200574 | pmid = 30067755 | pmc = 6070199 | title = Production of methane and ethylene from plastic in the environment | journal = PLOS ONE | volume = 13 | issue = 8 | article-number = e0200574 | year = 2018 | last1 = Royer | first1 = Sarah-Jeanne | last2 = Ferrón | first2 = Sara | last3 = Wilson | first3 = Samuel T. | last4 = Karl | first4 = David M. | bibcode = 2018PLoSO..1300574R | doi-access = free }}</ref>

== Chemically modified polyethylene ==
Polyethylene may either be modified in the polymerization by [[Chemical polarity|polar]] or non-polar [[comonomer]]s 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 [[crystallinity]] and [[density]] 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.<ref name="Kaiser" />{{rp|238}}<ref name="Vasile">{{cite book|last1=Pascu|first1=Cornelia Vasile: Mihaela|title=Practical guide to polyethylene|date=2005|publisher=Rapra Technology Ltd.|location=Shawbury|isbn=978-1-85957-493-5|edition=[Online-Ausg.].}}</ref>{{rp|19}}

====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.<ref name="Kaiser" />{{rp|238}}

==== Cyclic olefin copolymers (COC) ====
[[Cyclic olefin copolymer]]s are prepared by copolymerization of ethene and [[cycloolefin]]s (usually [[norbornene]]) produced by using metallocene catalysts. The resulting polymers are amorphous polymers and particularly transparent and heat resistant.<ref name="Kaiser" />{{rp|239}}<ref name="Vasile" />{{rp|27}}

=== 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 [[ester]]s 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.<ref name="Domininghaus">{{cite book|last1=Elsner |first1=Peter|last2=Eyerer |first2=Peter|last3=Hirth|first3=Thomas|title=Domininghaus - Kunststoffe|date=2012 |publisher=Springer-Verlag|location=Berlin Heidelberg |isbn=978-3-642-16173-5|edition=8. |page=224}}</ref>{{rp|239}}

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.<ref name="Domininghaus" />{{rp|239}}

==== 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.<ref>{{cite book|author1=Elsner, Peter |author2=Eyerer, Peter |author3=Hirth, Thomas |title=Kunststoffe Eigenschaften und Anwendungen |date=2012|publisher=Springer Berlin Heidelberg |location=Berlin, Heidelberg|isbn=978-3-642-16173-5 |edition=8.}}</ref> However, they are more sensitive to heat and oxidation than ethylene homopolymers. Ethylene/acrylic acid copolymers are used as [[adhesion promoter]]s.<ref name="Kaiser" />

If salts of an unsaturated carboxylic acid are present in the polymer, thermo-reversible ion networks are formed, they are called [[ionomer]]s. Ionomers are highly transparent thermoplastics which are characterized by high adhesion to metals, high abrasion resistance and high water absorption.<ref name="Kaiser" />

=== 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/cm<sup>3</sup>).<ref name="Domininghaus" />{{rp|235}} 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):<ref name="Kaiser" />{{rp|224}} 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 elastomer]]s).<ref name="Domininghaus" />{{rp|235}}

Ethylene-ethyl acrylate copolymers behave similarly to ethylene-vinyl acetate copolymers.<ref name="Kaiser" />{{rp|240}}

=== Crosslinking ===
{{Main|Cross-linked polyethylene}}
A basic distinction is made between peroxide crosslinking (PE-Xa), silane crosslinking (PE-Xb), electron beam crosslinking (PE-Xc) and azo crosslinking (PE-Xd).<ref name="Saechtling">{{cite book |last1=Baur |first1=Erwin |last2=Osswald |first2=Tim A. |author-link2=Tim Osswald |title=Saechtling Kunststoff Taschenbuch |date=October 2013 |isbn=978-3-446-43729-6  |page=443 |publisher=Hanser, Carl }} [https://www.kunststoffe.de/themen/basics/standardthermoplaste/polyethylen-pe/artikel/vernetztes-polyethylen-pe-x-820007.html Vorschau auf kunststoffe.de]</ref>

[[File:Crosslinking PE scheme en.svg|class=skin-invert-image|frameless|upright=2.5|Shown are the peroxide, the silane and irradiation crosslinking]]

<small>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).</small>

*'''Peroxide crosslinking (PE-Xa)''': The crosslinking of polyethylene using [[peroxide]]s (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%<ref name="Ullmanns">{{Cite book |last1=Whiteley |first1=Kenneth S.|chapter=Polyethylene |title=Ullmann's Encyclopedia of Industrial Chemistry |doi=10.1002/14356007.a21_487.pub2|year=2011|isbn=978-3-527-30673-2}}</ref> peroxide is at first mixed at low temperatures in an extruder and then crosslinked at high temperatures (between 200 and 250&nbsp;°C).<ref name="Saechtling" /> 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.<ref name="Koltzenburg">{{cite book |last1=Koltzenburg|first1=Sebastian |last2=Maskos|first2=Michael|last3=Nuyken |first3=Oskar|title=Polymere: Synthese, Eigenschaften und Anwendungen |date=2014|publisher=Springer Spektrum |isbn=978-3-642-34773-3|page=406|edition=1}}</ref> The resulting polymer network is uniform, of low tension and high flexibility, whereby it is softer and tougher than (the irradiated) PE-Xc.<ref name="Saechtling" />
*'''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] [[Catalyst]]s such as [[dibutyltin dilaurate]] may accelerate the reaction.<ref name="Ullmanns" />
*'''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&nbsp;mm. Thereby the interior or specific areas can be excluded from the crosslinking.<ref name="Saechtling" /> However, due to high capital and operating costs radiation crosslinking plays only a minor role compared with the peroxide crosslinking.<ref name="Domininghaus" /> 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.<ref name="Ullmanns" />
*'''Azo crosslinking (PE-Xd)''': In the so-called ''Lubonyl process'' polyethylene is crosslinked preadded [[azo compound]]s after extrusion in a hot salt bath.<ref name="Domininghaus" /><ref name="Saechtling" />

===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.<ref name="Domininghaus" /> When chlorinated polyethylene is added to other polyolefins, it reduces the flammability.<ref name="Kaiser" />{{rp|245}}

Chlorosulfonated PE (CSM) is used as starting material for ozone-resistant [[synthetic rubber]].<ref>[http://www.chemgapedia.de/vsengine/vlu/vsc/de/ch/9/mac/andere/kautschuk/kautschuk.vlu/Page/vsc/de/ch/9/mac/andere/kautschuk/chlorsulfon.vscml.html Chlorsulfoniertes Polyethylen (CSM)].  ChemgaPedia.de</ref>

===Bio-based polyethylene===
{{Main|Bioplastics|Renewable 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 {{convert|200000|ST|kg}}, and will produce high-density and low-density polyethylene from [[bioethanol]] derived from sugarcane.<ref>{{cite press release |url=http://www.yourindustrynews.com/blog/?p=2390 |title=Braskem & Toyota Tsusho start joint marketing activities for green polyethylene from sugar cane |publisher=yourindustrynews.com |date=26 September 2008 |access-date=20 February 2014 |archive-url=https://web.archive.org/web/20130521135257/http://www.yourindustrynews.com/blog/?p=2390 |archive-date=21 May 2013 }}</ref>

==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.<ref name=guide/><ref name="Kahovec">{{cite journal|doi=10.1351/pac200274101921|title=Nomenclature of regular single-strand organic polymers (IUPAC Recommendations 2002)|year=2002|last1=Kahovec|first1=J.|last2=Fox|first2=R. B.|last3=Hatada|first3=K.|journal=Pure and Applied Chemistry|volume=74|page=1921|issue=10|doi-access=free}}</ref> The alkene monomer converts to a long, sometimes ''very'' long, alkane in the polymerization process.<ref name="Kahovec" /> 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).<ref name=guide>{{Cite book|url= http://www.acdlabs.com/iupac/nomenclature/93/r93_683.htm |title=A Guide to IUPAC Nomenclature of Organic Compounds (Recommendations 1993) IUPAC, Commission on Nomenclature of Organic Chemistry |year=1993 |publisher=Blackwell Scientific Publications |isbn=978-0-632-03702-5 |access-date=20 February 2014}}</ref> The difference in names between the two systems is due to the ''opening up'' of the monomer's double bond upon polymerization.<ref>{{cite web|url= https://old.iupac.org/reports/provisional/abstract04/favre_310305.html |title=IUPAC Provisional Recommendations on the ''Nomenclature of Organic Chemistry'' |date=27 October 2004 |publisher=International Union of Pure and Applied Chemistry |access-date=20 February 2014}}</ref> 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==
{{notelist|1}}

==References==
{{Reflist|30em}}

==Bibliography==
*{{cite book|last1 = Piringer|first1 = Otto G.|last2 = Baner|first2 = Albert Lawrence|title = Plastic Packaging: Interactions with Food and Pharmaceuticals|publisher = Wiley-VCH|year = 2008|edition = 2nd|url = https://books.google.com/books?id=UbOFgsNEHbAC|isbn = 978-3-527-31455-3|access-date = 2014-02-20}}{{Dead link|date=August 2024 |bot=InternetArchiveBot |fix-attempted=yes }}
*{{cite book|last = Plastics Design Library|title = Handbook of Plastics Joining: A Practical Guide|publisher = William Andrew|year = 1997|edition = illustrated|url = https://books.google.com/books?id=B-F5pXuj_M8C|isbn = 978-1-884207-17-4|access-date = 2014-02-20}}

==External links==
{{Wiktionary}}
{{Commons category|Polyethylene}}
*[https://www.independent.co.uk/news/science/polythenes-story-the-accidental-birth-of-plastic-bags-800602.html Polythene's story: The accidental birth of plastic bags]
*[https://www.ides.com/generics/PE.htm Polythene Technical Properties & Applications]
*[https://pqasb.pqarchiver.com/therecord/access/1482573421.html?FMT=ABS&FMTS=ABS:FT&date=May+22%2C+2008&author=Karen+Kawawada&pub=Waterloo+Region+Record&edition=&startpage=A.1&desc=WCI+student+isolates+microbe+that+lunches+on+plastic+bags Article describing the discovery of ''Sphingomonas'' as a biodegrader of plastic bags] {{Webarchive|url=https://web.archive.org/web/20121105134302/https://pqasb.pqarchiver.com/therecord/access/1482573421.html?FMT=ABS&FMTS=ABS:FT&date=May+22%2C+2008&author=Karen+Kawawada&pub=Waterloo+Region+Record&edition=&startpage=A.1&desc=WCI+student+isolates+microbe+that+lunches+on+plastic+bags |date=5 November 2012 }} Kawawada, Karen, ''Waterloo Region Record'' (22 May 2008).
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{{Plastics}}
{{Fibers}}
{{Packaging}}
{{Authority control}}

[[Category:Polyolefins]]
[[Category:Dielectrics]]
[[Category:Ethylene]]
[[Category:Thermoplastics]]
[[Category:Packaging materials]]
[[Category:Transparent materials]]
[[Category:Bioplastics]]
[[Category:Commodity chemicals]]
[[Category:German inventions]]
[[Category:1898 in science]]
[[Category:1898 in Germany]]