(e.g. vinylsilane) and a catalyst during extruding and then performing a post-extrusion curing. The chemical process of vulcanization is a type of cross-linking that changes rubber to the hard, durable material associated with car and bike tires. This process is often called sulfur curing; the term vulcanization comes from Vulcan, the Roman god of fire. This is, however, a slower process. 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 catalysing the addition of sulfur chains to the rubber molecules. 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. Physical cross-links Where chemical cross-links are covalent bonds, physical cross-links are formed by weak interactions. For example, sodium alginate gels upon exposure to calcium ion, which allows it to form ionic bonds that bridge between alginate chains. Polyvinyl alcohol gels upon the addition of borax through hydrogen bonding between boric acid and the polymer's alcohol groups. Other examples of materials which form physically cross-linked gels include gelatin, collagen, agarose, and agar agar. 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 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. Note: A rubber which cannot be