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Molecular cross-links

The features of the final product depend not only on the original composition, but also on the outworking of this critical manufacturing process 


Massimo Clarke

When speaking of rubber compounds we naturally think of the elastomers and fillers used in producing them and the relative quantities. Yet, there are other no less important factors to consider, that is, the nature and quantity of the additives present in the original raw material, besides the curing process. The latter, as widely known, is produced by the formation of cross-links that join individual polymer chains creating what can be considered a 3-D grid. Through such a process the elastomer is transformed in a compound able to deform itself elastically. The above mentioned cross-links prevent the molecular chains from reciprocal sliding (which happens in the absence of a curing process), which would permanently deform the molecular chains giving them a plastic behavior. These molecular cross-links allow the compound to deform and return to its former shape and dimensions once the load is removed, when the chains return to their original positions (at worst, in case some cross-links are broken, there could be a permanent deformation, but of a minor nature). The curing process increases resistance to traction, the toughness and elasticity of the compound, not to mention wear resistance.

The cross-links involve the presence of one or more sulphur atoms (though in most cases we find 3 to 7), each of which is joined to two carbon atoms belonging to two different molecular chains.

For a correct curing process, besides heat, the original raw materials need a specific amount of an agent able to form polymer cross-links. In the tire world, this essential element is sulphur, the true “prince” of curing agents.

This is not sufficient however since this process would require many hours, consequently, additives such as accelerators are employed. Furthermore, one or more activators are used. This combination or vulcanizing system creates a “cross-linking pattern”. The nature and quantity of each of the additives, along with the curing time and temperature, have a great influence on the final characteristics of the compound, since they determine the nature and density of the cross-links.

This is not all, though, since quite often sulphur-containing substances are also used as well as inhibitors and retardants. Oils and plastifiers are likewise very important, although they act quite differently; they don’t influence the cross-link pattern as such, but rather facilitate the mixing process of the raw materials (by no means a simple procedure) and/or contribute to the performance of the finished product. Oils are valuable “softeners” and plastifiers increase the flexibility of the material after the cross-links are formed, especially at low temperatures.

Accelerators, as the word implies, speed-up the curing process making the formation of cross-links easier and faster. Their application, moreover, allows reducing the temperature and the amount of sulphur needed for the process. These are organic elements whose action is further influenced by the nature of the compound and the vulcanizing parameters.

Zinc oxide is an excellent activator that doesn’t just speed-up the cross-link formation, but improves the thermal stability of the links; it is normally used with a small quantity of stearic acid (alternatively, zinc stearate is directly employed). Its use is generally considered essential.

Sulphur-containing elements make possible the formation of molecular cross-links even when present in very small quantities. Therefore, they can be considered as accelerator and vulcanizing agents simultaneously.

The cross-link formation tends to start immediately after the mixing process (actually, even during the mixing process) and this must be avoided as it would negatively affect the next phase, in which the compound is deformed into the desired shape. This deformation must be plastic, or permanent, but if the compound due to the cross-link formation, this would be impossible. The curing process, therefore, must begin at the right time and not before, and then continue at the right pace until the process comes to the proper conclusion.

To guarantee an adequate amount of time to the production process before the molecular cross-links start forming, retardants are used to prevent the links from forming prematurely. During vulcanization, as the molecular chains form cross-links, resistance to traction is increased as well as toughness and the compound’s elasticity. Measuring this last parameter allows to monitor the progress of the cure and whether the cross-linking has reached the desired degree. This is done using instruments called vulcameters that measure the necessary torque to obtain and maintain a set deformation, and since the torque corresponds to the elasticity of the compound, which in turn depends from the amount of molecular cross-links, this allows to produce a rheometric curve that shows how the curing process is proceeding in relation to time. The length of the process must be just right; if it’s too much, the number of cross-links will be more than required. To the contrary, if the it’s too little due to the reduced amount of cross-links produced the finished product will not guarantee the desired performance. 

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