Crosslinks vulcanization

Examples are the introduction of carboxyl or hydroxy groups to increase hy-drophilicity, crosslinking (vulcanization) of polydienes with sulfur, or conversions that proceed only at the terminal groups under retention of the molecular backbone (chain-analogous conversion). 

These highly amorphous elastomers have relatively low Tt values (—73 C) and tend to crystallize when stretched. The cold flow of these thermoplastic polymers is reduced when they are crosslinked (vulcanized) with a small amount (2%) of sulfur. Since these polymers of isoprene have a solubility parameter of 8.0 H, they are resistant to polar solvents but are soluble in many aliphatic and aromatic hydrocarbon solvents. The cross-linked derivatives swell but do not dissolve in these solvents. 

Consider the case of polymer chains with an arbitrary distribution of chain lengths and with a number of potentially reactive sites equal to the degree of polymerization (every monomeric unit of the polymer chain constitutes one potentially reactive site). The random crosslinking (vulcanization) of these linear primary chains may be considered a stepwise homopolymerization of Afl species, where the functionality of every species is directly proportional to its molar mass,... 

Latex balloons are still made in essentially the same way. A metal form shaped like the uninflated balloon is first coated with a chemical that will cause the latex to coagulate on it. The form is immersed upside-down into a vat of latex also containing a crosslinking (vulcanizing) agent, and then dried in an oven to cure the rubber. After the balloon is removed from the mold, it is ready to be imprinted or packaged. A modern, automated factory can turn out a million or so latex balloons a day. 

In contrast to thermoplastic materials, crosslinked (vulcanized) elastomer products cannot be remelted. This puts narrow limits on material recycling or reuse. The case is different with the recycling and reuse of tires, the main elastomeric product. 

One of the disadvantages of vulcanized soft natural rubber is its lack of resistance to deterioration and to reactants which attack the residual double bonds present. Thomas and Sparks tailor-made a diene copolymer (butyl rubber) which contained enough unsaturation for crosslinking but was more resistant to deterioration than hevea mbber and the crosslinked (vulcanized) elastomer contained few double bonds. 

In order to get some idea about the validity of the above general formulation and to determine whether any result can be substantiated by experimental data, crosslinked vulcanized rubber has... 

Useful rubber articles are made by vulcanization. Unvulcanized rubbers are generally not very strong, do not maintain their shape after a large deformation, and can be very sticky (about the same consistency as chewing gum). In order to produce a rubber having the proper elastomeric physical properties, the polymer chains must be very flexible and chemically crosslinked. Vulcanization can be defined as a chemical crosslinking process that generates a rubber material with useful properties. 

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