Vulcanization

Vulcanization is the process of cross-linking elastomer molecules to make the bulk material harder, less soluble and more durable. It is also called curing. It is the heart of the art and science of rubber compounding.

Related Topics:
Cross-link - Elastomer - Rubber

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Vulcanization is a chemical process in which individual polymer molecules are linked to other polymer molecules by atomic bridges. The end result is that the springy rubber molecules become locked together to a greater or lesser extent. This makes the bulk material harder, much more durable and also more resistant to chemical attack. It also transforms the surface of the material from a sticky feel to a smooth, soft surface which does not adhere to metal or plastic substrates.

Related Topics:
Chemical process - Polymer - Chemical attack

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Vulcanization is an irreversible process, like baking a cake, and must be contrasted strongly with thermoplastic processes (the melt-freeze cycle) which characterize the behavior of the vast majority of modern polymers. This irreversible cure reaction defines cured rubber compounds as thermoset materials, which do not melt on heating, and places them outside the class of thermoplastic materials (like polyethylene and polypropylene). This is a fundamental difference between rubbers and plastics, and sets the conditions for their applications in the real world, their costs, and the economics of their supply and demand.

Related Topics:
Thermoplastic - Thermoset - Polyethylene - Polypropylene - Supply and demand

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Usually, the actual chemical cross-linking is done with sulfur, but there are other technologies, including peroxide-based systems. The combined cure package in a typical rubber compound comprises the cure agent itself, (sulfur or organic peroxide), together with accelerators and retarding agents.

Related Topics:
Cross-linking - Sulfur - Peroxide - Rubber - Accelerators

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Sulfur is an unusual material. In its normal form as it is coming out of the refinery the molecule is a ring structure with 8 sulfur atoms. Carbon and silicon can also form such chains. The curing process makes use of this phenomenon. Along the rubber molecule, there are a number of sites which are attractive to sulfur atoms. These are called cure sites. During vulcanization the 8-membered ring of sulfur breaks down in smaller parts with varying numbers of sulfur atoms. These parts are quite reactive. At each cure site on the rubber molecule, one or more sulfur atoms can attach itself, and from there, a sulfur chain can grow, until it eventually reaches a cure site on another rubber molecule. These sulfur bridges are typically between 2 and 10 atoms long. Contrast this with typical polymer molecules in which the carbon backbone is many thousands of atomic units in length. The number of sulfur atoms in a sulfur crosslink has a strong influence on the physical properties of the final rubber article. Short sulfur crosslinks, so with one or two sulfur atoms in the crosslink give the rubber a very good heat resistance. Crosslinks with higher number of sulfur atoms, up to 6 or 7 give the rubber very good dynamic properties, but with lesser heat resistance. Dynamic properties are important for flexing movements of the rubber article, like for instance the movement of a side-wall of a running tire. Without good flexing properties these movements will rapidly lead to formation of cracks and ultimately to failure of the rubber article.

Related Topics:
Carbon - Silicon - Phenomenon - Molecule - Polymer - Carbon backbone

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