Why is sulphur used for the vulcanization of rubber?
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Sulphur is the first chemical discovered by Charles Goodyear to vulcanize Natural rubber compounds.
Till late 1950, only sulphur was used to vulcanize rubber compounds.
Its not true that only sulphur can be used for vulcanisation
Actually, sulphur can only vulcanise unsaturated rubbers (polymer chain containing double bonds) like Natural rubber, Styrene Butadiene rubber, Nitrile Butadiene rubber, etc.
While certain organic peroxide, Phenolic resin in general can vulcanise all the rubbers ( saturated and unsaturated).
There are also other specific curatives(vulcanising agents) like ZnO/MgO which is used to vulcanise Chloroprene rubber, Epichlorohydrin rubber.
Most of the rubber compounds in day to day activities contains Natural rubber, Styrene Butadiene rubber, Butyl rubber. So sulphur is very common and mostly known to every body.
Dynamic properties are very good for sulphur vulcanised compounds when compared to other curatives like peroxide or resin cure. This is the reason why sulphur vulcanisation is used in all dynamic products like tyres, transmission belts, etc.
Though there are other vulcanising agents (curatives), sulphur dominates the market mainly because of its reliability, flexibility and controllability of vulcanisation apart from its excellent dynamic properties.
Sulphur alone cannot be used for vulcanisation. It requires accelerators and activators. Accelerators are the organic chemical compound which can accelerate or slow down the vulcanisation process. Generally, addition of accelerators accelerates the vulcanisation. Activators (ZnO/Stearic acid) is also very important for Vulcanisation process.
If you ask me why only sulphur and not phosphorus or other inorganic compounds can be used for vulcanisation, then this is one question really nobody knows the answer. Though there are lot of speculation or probable mechanism, but still true reason is not clearly understood.
In case, you want to know more about what/why/how's of vulcanisation. Feel free read below.
Let's get to basics first. Let me be as simple as i can.
Let's me introduce you this first -
Rubber is made up of 1000s of polymer chains entangled together.
Just to visualise - its like a noodles in a bowl.
Now going to your answer:
What is Vulcanisation?
Vulcanisation is nothing but the process of applying heat to link the polymer chains.
Some times vulcanisation process is also done together with product shape formation, with the help of Compression moulding, Injection moulding, etc.
It can loosely be called curing or pressing.
Why vulcanisation is important.?
Rubber is a amorphous material which means that there is very low interaction between the polymer chains (low intermolecular attraction). So in general, even if you give very little force or even a very slight change in temperature, these polymer chains start moving. That means the material flows or deforms (permanently) as there is no restriction by the nearby molecules of polymer chains (low intermolecular attraction). This deformation or flow ability is also not reversible. This means you don't have a stable material.(This was also the reason why rubbers flowed at summer and become hard at winter.)
So to stop the mobility of polymer chains, these polymer chains are fixed to each other /linked to each other. So this process of fixing or linking together the polymer chains is called Vulcanisation/crosslinking.
So inshort , if you need a stable useful material, you need to vulcanize or in simple words, you need to create a cross-link between the polymer chains.
Now Why Sulphur is used??
To create a crosslink, a chemical/element should hold the two or more polymer chains.
Charles Good year has accidentally discovered sulphur was able to crosslink the polymer chains. So actually till now, nobody really understand why sulphur only can do this while other materials like phosphorus or other inorganic materials were not able to do it.
The prime difference between using Sulphur and other materials is that
Sulphur has very good dynamic properties when compared to other curatives.That's reason why sulphur is s used for dynamic products like tyres. The disadvantage of sulphur is that it cannot withstand high temperature. That's where the other curatives like peroxide, phenolic resin comes in to play.
Now, the question why sulphur is good for dynamic properties?.
Well, it has to done with the type of sulphur used. Generally, Elemental polymeric sulphur is used (S-S6-S). So sulphur itself has a chain and so on applying the dynamic force, the force can be distributed or flexed by the sulphur chain.
