Cross-links are covalent bonds linking one polymer chain to another, are the characteristic property of thermosetting plastics materials. They 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.