Editing Polyethylene (section)

=== 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" />