Monday, October 29, 2012

What "Rubber Vulcanisation" Is All About and How It All Happened?

Some of the first natural rubber products such as rubber balls, jars and boots were discovered by the Europeans when they travelled 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 crosslinked 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.

The possibility of vulcanizing the rubber molecules within the dispersed rubber particles in the Natural Rubber (NR) latex was first investigated by Philip Schidrowitz in the period 1914 - 1918. His idea was to use prevulcanised NR latex for making latex foam products in order to eliminate the vulcanization stage for economical reason.

Most of the subsequent studies by Schidrowitz and other researchers found that prevulcanised latex was generally not suitable for foam products due to the inherently poor gel strength.

While the chemistry of dry rubber vulcanization is complex, the mechanism of vulcanisation of rubber particles in latex is even more so and remains a wonder even today, almost 90 years after the advent of the discovery of prevulcanisation of latex. Unlike dry rubber, where the curatives could be intimately mixed and dispersed within the rubber matrix itself, similar curatives remain dispersed and suspended separately in the latex along with the rubber particles at least immediately after compounding and mixing. At best, the curatives could only collide with the rubber particles.

Many researchers had put forward their different schools of thoughts over the past decades. While some of these were contradictory with one another, others raised more questions.

It seems likely that the first important step in the vulcanization of latex is the formation of sulphur-accelerator species in the serum which then becomes soluble in the presence of the hydrophilic non-rubbers. It is possible that the resultant active sulphurating agent attains some degree of surface active property. Hence a logical mode of transfer would be its absorption onto the rubber particle surface from the serum or the aqueous phase.

Once on the rubber particle surface, the sulphurating agent would lose some of its hydrophilic moieties resulting in an increase in the hydrophobicity enabling itself to migrate into the hydrophobic interior of the rubber particle to trigger the initial formation of polysulphidic cross-links followed by cross-link shortening with recycling of the sulphur into additional cross-linking, modification of polyisoprene chains etc.

Prevulcanised latex is therefore compounded latex in which the molecules of the rubber particles are chemically cross-linked (i.e. vulcanised). However there is no change in rubber particle size, shape and particle size distribution, the latex still retaining its original fluidity and colloidal property.