Cross-link is a bond formed between polymer chains, either between different chains or between different parts of the same chain. Cross-links can be covalent bonds or ionic bonds. Polyethylene consists of many random molecular
chains with no particular orientation and no chemical bonds existing between chains. When heat is applied to such a material, the chains are free to slip and flow under relatively small outside force. Such a material is called a thermoplastic. If we are able to introduce cross-linking bonds between adjacent molecular chains, this adds form stability at higher temperatures. There will still be some loss of strength at elevated temperatures, but the cross-linked molecular chains are much more resistant to flow when stress is applied. The chemical process of vulcanization is a type of cross-linking and it changes the property of rubber to the hard, durable material we associate with car and bike tires. This process is often called sulfur curing, and the term vulcanization comes from 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 disulfide. Both of these contain a sulfur atom in the molecule that initiates the reaction of the sulfur chains with the rubber. Accelerators increase the rate of cure by catalyzing the addition of sulfur chains to the rubber molecules.
The resulting modification of mechanical properties depends strongly on the cross-link density. Low cross-link densities decrease the viscosities of polymer melts. Intermediate cross-link densities transform gummy polymers into materials that have elastomeric properties and potentially high strengths. Very high cross-link densities can cause materials to become very rigid or glassy, such as phenol-formaldehyde materials.
Cross-links are the characteristic property of thermosetting plastic materials. In most cases, cross-linking is irreversible, and the resulting thermosetting material will degrade or burn if heated, without melting. Especially in the case of commercially used plastics, once a substance is cross-linked, the product is very hard or impossible to recycle. 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. When polymer chains are linked together by cross-links, they lose some of their ability to move as individual polymer chains. For example, a liquid polymer (where the chains are freely flowing) can be turned into a "solid" or "gel" by cross-linking the chains together.
Chemical covalent cross-links are stable mechanically and thermally, so once formed are difficult to break. Therefore, cross-linked products like car tires cannot be recycled easily. A class of polymers known as thermoplastic elastomers rely on physical cross-links in their microstructure to achieve stability, and are widely used in non-tire applications, such as snowmobile tracks, and catheters for medical use. They offer a much wider range of properties than conventional cross-linked elastomers because the domains which act as cross-links are reversible, so can be reformed by heat. The stabilising domains may be non-crystalline (as in styrene-butadiene block copolymers) or crystalline as in thermoplastic copolyesters.
