Accelerated vulcanization, however, can perform the same operation listed above at the same temperature with an elemental sulfur concentration of 0.5 phr and decrease the reaction time to as low as 1 to 3 minutes . The main mechanism for accelerated vulcanization is listed below:

Figure 6: Mechanism for Accelerated Vulcanization

Where MBT (2-Mercaptobenzothiazole) is the accelerator for the reaction. MBT acts as an initiator by removing the –R-NH2 group from the material allowing it to react with itself quicker. A list of common accelerators is given in the figure below. The most important of which is MBT, which replaced the toxic aniline in 1925. MBT reacts as shown above with the sulfur, thus allowing it to react even faster with the rubber than the elemental sulfur .

Figure 7: Different Types of Accelerators for Vulcanization

The accelerators given above can be used in a variety of roles to either increase the rate of linkage formation, or the extent of formation (di-, tri-, poly-linkages, etc.).

Brass Wire Adhesion

The second area of importance is the brass-coating placed on steel belts for adhesion to the melted rubber. Since carbon steel has a poor affinity for vulcanized rubber, the overall strength of the tire is reduced. Therefore, brass (CuZn) is deposited on the surface of the steel belts so that a stronger bond between the steel and rubber can be formed. As the rubber flows around the steel belts in the mold, a thin copper sulfide (CuS) layer is formed on the surface of the steel belts. Since the layer is porous, the rubber begins to move into the layer. When the vulcanization process is started, the rubber forms cross-links not only with itself, but with the CuS also, resulting in very strong attractions. The entanglements between the rubber and CuS layer help form a powerful bond between the rubber and steel.

The formation of these domains creates a considerable adhesive force between the 2 materials, and is necessary for the long-term durability and strength of the material. A problem with this process is that the presence of either zinc/iron sulfides (ZnS/FeS) will inhibit the process. Neither of these materials exhibits the porosity that CuS does, therefore the entanglements that form the strong bonds do not occur, resulting in weak surface adhesion. Also, the presence of Zn2+ will corrode the rubber because it will form either ZnO or ZnOH will accumulate in the CuS layer and will displace the disulfide linkages. This can be prevented by doping the brass with small amounts of trivalent cobalt.

E. Disposal and Recycling of Used Tires

Today, a variety of recycling techniques encourage the use of tires at the end of their lifetime. They can be used for energy production as a fuel, especially in cement works. Or the materials can be re-used, for example by transforming the tire into a powder which is used for flooring materials, for making rubber objects or in the manufacture of new tires.