Editing Polyethylene (section)

== 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=978-1859574935|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=978-3527306732}}</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 |url-status=dead }}</ref>