Cross-links are covalent bonds linking one polymer chain to another. They are the characteristic property of thermosetting plastic materials. In biology, cross-linking has applications in forming polyacrylamide gels for gel electrophoresis and in protein studies. Crosslinking inhibits close packing of the polymer chains, preventing the formation of crystalline regions. The restricted molecular mobility of a crosslinked structure limits the extension of the polymer material under loading.

Cross-links are formed by chemical reactions that are initiated by heat and/or pressure, or by the mixing of an unpolymerized or partially polymerized resin with various chemicals; cross-linking can be induced in materials that are normally thermoplastic through exposure to radiation.

In most cases, cross-linking is irreversible, and the resulting thermosetting material will degrade or burn if heated, without melting. In some cases, though, if the cross-link bonds are sufficiently different, chemically, from the bonds forming the polymers, the process can be reversed. Permanent wave solutions, for example, break and re-form naturally occurring cross-links (disulfide bonds) between protein chains in hair.

The chemical process of vulcanisation is a type of cross-linking and it changes the property of rubber to the hard, durable material we associate with car and bike tyres. This process is often called sulphur curing, or vulcanization after Vulcan, the Roman god of fire. However, this is a slow process, taking around 8 hours. A typical car tire is cured for 15 minutes at 150°C. However, the time can be reduced by the addition of accelerators such as 2-benzothiazolethiol or tetramethylthiuram disulphide. Both of these contain a sulphur atom in the molecule that initiates the reaction of the sulphur chains with the rubber. Accelerators increase the rate of cure by catalysing the addition of sulphur chains to the rubber molecules.

Cross-links can be made also by purely physical means. For example, electron beams are used to cross-link the C type of cross-linked polyethylene. Other types of cross-linked polyethylene are made by addition of peroxide during extruding (type A) or by addition of a cross-linking agent (eg. vinylsilane) and a catalyst during extruding and then performing a post-extrusion curing.

See also branching.

Polymer chemistry

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