|Each compound is made as per a specific recipe. The raw materials have to be stored under certain temperature conditions protected from sunlight in order to avoid oxidation processes. Natural rubber may need to be decristallized before it is used. The mixing is similar to making a cake. All ingredients are weighed with high accuracy according to a formula and then mixed together. The number of raw materials is quite high (eventually a dozen). In the mixer all the ingredients are well distributed and dispersed. Still being unvulcanized resp. plastic, a maximum storage has to be kept. The raw rubber batches are then usually processed into cover sheets either in a calendar for textile conveyor belts or in an extruder for steel cord conveyor belts.After confectioning the rubber sheets with the tensile member, the entire matrix is cured in a press, initializing the vulcanization.
The 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. 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 surface.
This irreversible cure reaction defines cured rubber compounds as thermo set materials, which do not melt on heating, and places them outside the class of thermoplastic materials (like polyethylene, polypropylene and PVC). This is a fundamental difference between rubbers and plastics.
Usually, the actual chemical cross-linking is done with sulphur, but there are other technologies, including peroxide-based systems. The combined cure package in a typical rubber compound comprises the cure agent itself (sulphur or peroxide), together with accelerators and retarding agents.
Sulphur is an unusual material. Given the right circumstances, it will form chains composed of strings of its own atoms. The curing process makes use of this phenomenon. Along the rubber molecule, there are a number of sites which are attractive to sulphur atoms. These are called cure sites. At each cure site on the rubber molecule, a sulphur atom can attach itself, and from there, a sulphur chain can grow, until it eventually reaches a cure site on another rubber molecule. These sulphur bridges are typically between 2 and 10 atoms long.