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==Synthetic polymers== :[[image:DryOilSteps.svg|thumb|260px|left|Chemical reactions associated with crosslinking of [[drying oil]]s, the process that produces [[linoleum]].]] Crosslinking generally involves covalent bonds that join two polymer chains. The term ''[[curing (chemistry)|curing]]'' refers to the crosslinking of [[thermosetting]] resins, such as unsaturated [[polyester]] and [[epoxy]] resin, and the term ''[[vulcanization]]'' is characteristically used for [[rubber]]s.<ref>{{cite book|author1=Hans Zweifel|author2=Ralph D. Maier|author3=Michael Schiller|title=Plastics additives handbook |date=2009 |publisher=Hanser |location=Munich |isbn=978-3-446-40801-2 |page=746 |edition= 6th}}</ref> When polymer chains are crosslinked, the material becomes more rigid. The mechanical properties of a polymer depend strongly on the cross-link density. Low cross-link densities increase the viscosities of [[Crystallization of polymers|polymer melt]]s. Intermediate cross-link densities transform gummy polymers into materials that have [[elastomer]]ic properties and potentially high strengths. Very high cross-link densities can cause materials to become very rigid or glassy, such as [[phenol formaldehyde resin|phenol-formaldehyde]] materials.<ref>{{cite book|url=https://books.google.com/books?id=q034u2kLAagC&pg=PA22|title=Engineering with Rubber: How to Design Rubber Components|first=Alan N.|last=Gent|date=1 April 2018|publisher=Hanser|access-date=1 April 2018|via=Google Books|isbn=9781569902998}}</ref> [[File:MethmethacrylateBPA-glyc.png|thumb|left|352px|Typical [[vinyl ester resin]] derived from [[bisphenol A diglycidyl ether]]. Free-radical polymerization gives a highly crosslinked polymer.<ref name="Ullmann">{{cite encyclopedia|last1=Pham|first1=Ha Q.|last2=Marks|first2=Maurice J.|title=Epoxy Resins|encyclopedia=Ullmann's Encyclopedia of Industrial Chemistry|year=2012|doi=10.1002/14356007.a09_547.pub2|isbn=978-3527306732}}</ref>]] In one implementation, unpolymerized or partially polymerized [[resin]] is treated with a '''crosslinking reagent'''. In [[vulcanization]], sulfur is the cross-linking agent. Its introduction changes [[rubber]] to a more rigid, durable material associated with car and bike [[tire]]s. This process is often called sulfur curing. In most cases, cross-linking is irreversible, and the resulting thermosetting material will degrade or burn if heated, without melting. Chemical covalent cross-links are stable mechanically and thermally. Therefore, cross-linked products like car [[tire]]s cannot be recycled easily. A class of polymers known as [[thermoplastic elastomer]]s rely on physical cross-links in their microstructure to achieve stability, and are widely used in non-tire applications, such as [[snowmobile]] tracks, and [[catheter]]s for medical use. They offer a much wider range of properties than conventional cross-linked elastomers because the domains that act as cross-links are reversible, so can be reformed by heat. The stabilizing domains may be non-crystalline (as in styrene-butadiene block copolymers) or crystalline as in thermoplastic copolyesters. [[File:Si69.svg|thumb|The compound [[bis(triethoxysilylpropyl)tetrasulfide]] is a cross-linking agent: the [[siloxy]] groups link to silica and the [[polysulfide]] groups vulcanize with [[polyolefin]]s.]] [[Enamel paint|Alkyd enamels]], the dominant type of commercial oil-based paint, cure by oxidative crosslinking after exposure to air.<ref>{{Citation |last1=Abraham |first1=T.W. |title=Lipid-Based Polymer Building Blocks and Polymers |date=2012 |url=https://linkinghub.elsevier.com/retrieve/pii/B9780444533494002533 |work=Polymer Science: A Comprehensive Reference |pages=15β58 |publisher=Elsevier |language=en |doi=10.1016/b978-0-444-53349-4.00253-3 |isbn=978-0-08-087862-1 |access-date=2022-06-27 |last2=HΓΆfer |first2=R.|url-access=subscription }}</ref> ===Physical cross-links=== In contrast to chemical cross-links, physical cross-links are formed by weaker interactions. For example, sodium [[alginate]] gels upon exposure to calcium ions, which form ionic bonds that bridge between alginate chains.<ref>{{cite journal |doi= 10.1021/acs.biomac.6b00378|pmid= 27177209|title= Structural Characterization of Sodium Alginate and Calcium Alginate|journal= Biomacromolecules|volume= 17|issue= 6|pages= 2160β2167|year= 2016|last1= Hecht|first1= Hadas|last2= Srebnik|first2= Simcha}}</ref> [[Polyvinyl alcohol]] gels upon the addition of [[borax]] through hydrogen bonding between [[boric acid]] and the polymer's alcohol groups.<ref>{{cite web |title=Experiments: PVA polymer slime |url=https://edu.rsc.org/experiments/pva-polymer-slime/756.article |website=Education: Inspiring your teaching and learning |publisher=Royal Society of Chemistry |access-date=2 April 2022 |date=2016 |quote=A solution of polyvinyl alcohol (PVA) can be made into a slime by adding borax solution, which creates cross-links between polymer chains.}}</ref><ref>{{cite journal |doi= 10.1021/ed063p57|title= The gelation of polyvinyl alcohol with borax: A novel class participation experiment involving the preparation and properties of a "slime"|journal= Journal of Chemical Education|volume= 63|issue= 1|pages= 57|year= 1986|last1= Casassa|first1= E.Z|last2= Sarquis|first2= A.M|last3= Van Dyke|first3= C.H|bibcode= 1986JChEd..63...57C}}</ref> Other examples of materials which form physically cross-linked gels include [[gelatin]], [[collagen]], [[agarose]], and [[agar agar]].
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