What is "Vulcanization"?

Manufacturer:

I have been in the rubber industry for many years. Over the years many had asked me what "vulcanisation" is all about and how it all happened? How do you tackle this question?

John Woon:

Some of the first natural rubber products such as rubber balls, jars and boots were discovered by the Europeans when they traveled to The Amazon of Brazil in the 1730's. These were brought back to Europe but to their surprise, these products were too brittle during the winters and too soft and sticky in the summers.

It was later discovered that this undesirable phenomenon was due the fact that the rubber molecules had not been vulcanized i.e. they had not been cross-linked. The molecules could easily flow and slide pass each other at elevated temperatures making the rubber soft while in the cold they tend to crystallize resulting in an increase in stiffness and brittleness.

Charles Goodyear in 1839 discovered by accident (although he insisted that it was his sheer hard work) that the rubber molecules could be cross-linked with sulphur when exposed to heat. The term "vulcanisation" was coined from the words "Vulcan", the God of fire and "volcano" to signify that both heat and sulphur which is of volcanic origin were involved in the reaction.

Today it is also commonly used even for non-sulphur curing system such as radiation vulcanized natural rubber latex (RVNRL) and peroxide vulcanized natural rubber latex (PVNRL).

A rubber becomes a thermoset after vulcanization.Unlike a thermoplastic, it is no longer sensitive to extreme temperatures.

Since the advent of this discovery, vulcanization has been a very essential part of all processes concerned with the manufacturing of rubber products based on dry rubber. Hence the understanding of the chemistry of vulcanisation was biased towards dry rubber technology.

The solid rubber must first be masticated to soften it in order to remove its "nerviness" to allow for the addition and homogeneous mixing of the curatives, namely sulphur, zinc oxide, stearic acid and accelerators. Poor mixing would lead to uneven curing. Any poorly dispersed sulphur would inevitably lead to localized over-curing or "reversion" indicating poor heat ageing resistance.