Editing Vulcanization (section)

==Vulcanization with sulfur==
[[File:The Employment of Women in Britain, 1914-1918 Q28267.jpg|thumb|Two factory workers placing rubber tubing into a vulcanizer (1918)]]
{{Main|Sulfur vulcanization}}

The most common vulcanizing methods depend on sulfur. Sulfur, by itself, is a slow vulcanizing agent and does not vulcanize synthetic [[polyolefin]]s. Accelerated vulcanization is carried out using various compounds that modify the kinetics of crosslinking;<ref>Hans-Wilhelm Engels, Herrmann-Josef Weidenhaupt, Manfred Pieroth, Werner Hofmann, Karl-Hans Menting, Thomas Mergenhagen, Ralf Schmoll, Stefan Uhrlandt “Rubber, 4. Chemicals and Additives” in ''Ullmann's Encyclopedia of Industrial Chemistry'', 2004, Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a23_365.pub2}}</ref> this mixture is often referred to as a cure package. The main polymers subjected to [[sulfur vulcanization]] are [[polyisoprene]] ([[natural rubber]]) and [[styrene-butadiene]] rubber (SBR), which are used for most street-vehicle tires. The cure package is adjusted specifically for the substrate and the application. The reactive sites—cure sites—are [[allyl]]ic hydrogen atoms. These C-H bonds are adjacent to [[Alkene|carbon-carbon double bonds]] (>C=C<). During vulcanization, some of these C-H bonds are replaced by [[polysulfide|chains of sulfur]] atoms that link with a cure site of another polymer chain. These bridges contain between one and several atoms. The number of sulfur atoms in the crosslink strongly influences the physical properties of the final rubber article. Short crosslinks give the rubber better heat resistance. Crosslinks with higher number of sulfur atoms give the rubber good dynamic properties but less heat resistance. Dynamic properties are important for flexing movements of the rubber article, e.g., the movement of a side-wall of a running tire. Without good flexing properties these movements rapidly form cracks, and ultimately will make the rubber article fail.