Why Adding Sulphur to Rubber is Important for the Tire Industry?

Learn everything about the process of adding sulphur to rubber in the tire industry and the reasons behind why it is done.

Introduction:

Sulfur, a member of group 16 in the periodic table is one of the most versatile and significant elements used in different industrial sectors, including automotive, medical, and agriculture sectors. In the automotive industry, the process of adding sulphur to rubber during their production is critical for the entire industry.

Vulcanization of Tires:

The natural rubber used in the tire industry has low tensile strength, is squishy, and is quite sticky. This is due to the long hydrocarbon chains with a high molecular weight due to which rubber can’t keep its form. To be usable for tires, this squishy rubber needs to be stronger and more durable so that it can take the wear and tear when used in a tire frame. For this, its long chains need to be firmly bonded together to boost its intermolecular strength.

The process that converts rubber into a stable and stronger state usable for automotive by using sulfur is called vulcanization. First proposed and implemented by Goodyear back in 1839, this process is still an industrial standard and has been followed for almost two hundred years. During this process, Sulfur's high elastic state during compounding, (occurs in the range of 40°C to 70°C) encourages particle elongation and, in turn, breaks these feeble and thin needles into fragments. It also increases the surface area of the particles and facilitates intense diffusion of sulfur into the rubber matrix during melting.

At the end of the process of adding sulphur to rubber, the resultant product has improved physical properties with enhanced viscosity, better hardness, tensile strength, and abrasion resistance. Except for tensile strength, every physical change induced in this rubber is correlated with the rubber network's degree of cross-linking or the number of crosslinks. The entire process is quite slow and can take even days to complete which is why accelerators are added to reduce the entire vulcanization time. Below are the most important steps involved in the accelerated sulfur vulcanization process:

1. Induction Period:

The curatives including sulfur being the main raw material along with other vulcanizing agents are allowed to react with rubber compounds to start cross-linking. The induction period makes sure that all entities are mixed thoroughly to avoid premature curing (a.k.a scorching).

2. Vulcanization of the Rubber

Once proper mixing is completed, sulfur bridges or cross bridges (covalent bonds) are allowed to be formed between the polymer chains of rubber. This process is carried out with special equipment with a stable under heat and pressure. This reaction is made faster by using accelerators that combine with sulfur to produce monomeric polysulfides which further interact with rubber chains to form polymeric polysulfides. Finally, the rubber polysulfides react, either directly or through an intermediate, to develop desrired crosslinks in the rubber matrix.

3. Reversion

This step is not desired for the automotive industry as it includes the breaking of the sulfur cross-link developed in the last step if the rubber is overheated after the optimal curing time needed to achieve an optimum cure. This results in the degradation of the rubber being used in the vulcanization process and results in the failure of the curing process. Controlled temperature and proper pressure with monitored apparatus are required to avoid such a situation. Advancements in current industrial equipment are designed to avoid reversion from happening.

In the current industrial standards, the fundamental basics of the vulcanization process are still the same but certain improvements have been introduced which have made the entire process of tire manufacturing much more efficient. For example, improved accelerators, activators, and retarders are used to enhance the curing efficiency and optimize the end product. Moreover, automation and computer-controlled systems have shrunken the production times and also reduce potential damages from reversion.

Wrapping Up:

The process of adding sulphur to rubber to increase its strength is called vulcanization. During this process, sulfur is used to create cross-links between rubber chains and increase the rubber's strength and flexibility is mineral sulfur. To achieve this crosslinking, rubber is heated for a few hours with sulfur.