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Home > Chemistry, Technology & Conservation > Vulcanization

The vulcanization of polyolefinic rubbers can be defined as the process by which the reaction between the polyolefin and sulphur results in greatly increased elastic properties of the polyolefin and the maintenance of these properties over a comparatively wide temperature range.

The term ‘vulcanization’ was proposed by Mr Brockedon to Thomas Hancock who took out the first patent for the process in 1843, although it is now universally accepted that the action of sulphur, lead oxide and heat on natural rubber to ‘cure’ it of its propensity to turn brittle when cold and sticky when hot was discovered by Charles Goodyear in 1839. Indeed, in the US the terms ‘cure’ and ‘curing’ tended to be used instead of ‘vulcanize’ and ‘vulcanizing’ and this has resulted in a number of problems and anachronisms which will be addressed later.

It was rapidly established that, whilst the basic ingredients of a vulcanizing system were sulphur, a base oxide such as lead oxide or zinc oxide and heat, the last ranging today from about 120oC to 200oC, other chemicals could be added to the mix to speed up the process at any given temperature and even today new chemicals (called generically ‘accelerators’) are being brought into commercial use. Apart from the obvious reason of getting a slice of an existing market the reasons tend to be threefold:

  1. To modify the vulcanization process in such a way as to make it more cost-effective.
  2. To modify the vulcanization chemistry and thereby obtain a product with improved performance characteristics.
  3. For environmental or ‘health & safety’ reasons.

Because a rubber product can contain materials such as fillers and oils which are not part of the vulcanization process, it is the ratio of sulphur and accelerators to the rubber which is important in defining the type of vulcanising system so quantities are expressed not as %’s but as ‘parts by weight per hundred parts of rubber’ (pphr) or (phr).

However, before considering actual quantities it is worth considering what happens during the vulcanization process. For close to a century after the discoveries of Goodyear and Hancock the argument raged as to what was the interaction between the rubber and sulphur. As early as 1898 Ostromislenski (Ostromuisslenski) had proposed a combined chemical/physical theory of vulcanization whilst in 1902 Weber proposed a purely chemical one and in 1910 Ostwald opted for some sort of physical mixture or ‘alloy’ formation. Although the evidence for a chemical inter-reaction became overwhelming to most scientists there was still a bitter argument between the two sides at an international conference on vulcanization as late as 1939.

The chemical structure of the natural rubber vulcanizate was finally resolved during the 1960’s and 70’s at the Malaysian Rubber Producers’ Research Association by studies of the reaction between sulphur and various accelerators with low molecular weight olefins which could be taken as models of the rubber olefinic unit. This work led to the conclusion that the polymer chains of an unvulcanized rubber were joined together (crosslinked) by sulphur bridges to give a three dimensional network and it was this which provided the increased elasticity of the vulcanized product.

The detailed picture was extremely complex, showing that the sulphur could initially attach itself to one of several carbons within the monomer unit and could then either remain there or undergo a chemical shift to another carbon atom. The length of the sulphur bridge was also variable, being 1, 2, 3 or even more sulphur atoms whilst the final complication was that the sulphur bridge might not have been formed between two adjacent polymer molecules but it could have looped back to the same polymer chain from whence it started.

An understanding of these factors enabled some quantitative observations obtained almost a century ago to be understood.

In the early days of vulcanization, before the introduction of modern accelerators but when the advantages of adding a little organic base were appreciated, trial and error had shown that around 2.5 pphr sulphur gave a good useful vulcanized rubber product and this came to be known as a conventional vulcanizate. This level of sulphur is used today, with around 0.5 pphr accelerator and what the model olefin studies, together with calculations of the crosslink density of the vulcanizate, showed is that it is not an efficient system. The polysulphidic crosslinks and other non-crosslinking sulphur chemistry means that it needs about 12 sulphur atoms to produce one crosslink. If the levels of the ingredients are reversed to 0.5 pphr sulphur and 2.5 pphr then we have an efficient vulcanizate (EV) with only about 3 sulphurs required for each crosslink. Not surprisingly it is possible to design semi-EV systems between these two extremes. Zinc oxide is today the preferred inorganic base, usually added at around 5 pphr, although for applications where transparency is desired (baby feeding teats, medical tubing etc.) it can be as low as 1 pphr.

Just because the terms conventional and efficient are used these should not be equated with ‘old’ and ‘new’ (better). The different systems give different properties to the vulcanizates at the same level of crosslinks and the system chosen will depend on the anticipated use of the product.

Many vulcanizing systems for rubbers using chemicals other than sulphur have been investigated over the last 150 years but very few exist commercially today and here we tend to move from the word ‘vulcanization’ to ‘cure’. The use of sulphur chloride (1846 - Parkes Process) is called the sulphur chloride cure process and we also have peroxide cured rubbers. Cold vulcanization processes, such as the ubiquitous cycle repair patch, are called ‘cold cure’ processes. To add a further layer of complication, the chemicals which may be added to a rubber mix to effect vulcanization, including the accelerators, are collectively called curatives and the remains of these chemicals after vulcanization, cure residues.

It should also be remembered that early stories of the preparation of ‘dry’ rubber from latex in South America involved the smoking over open fires of layers of rubber built up on paddles whilst smoking sheets of coagulated and semi-dried natural rubber is still practised today in eastern plantations and cooperatives. This process was familiar to the British as it mirrored that used to smoke or ‘cure’ fish. Inevitably therefore the smoking process for rubber also became known as curing and for some time the two quite different meanings were used with much confusion. Eventually it became accepted that ‘curing’ only referred to the reaction with sulphur and the other process was called ‘smoking’.

Vulcanization of Latex

In the early 1920’s it was discovered that natural rubber latex could be vulcanized over a period of several hours by the addition of the usual ingredients – sulphur, zinc oxide and an accelerator – to the warm latex. Visually the vulcanized latex appeared indistinguishable from the untreated material but when it was coagulated and/or dried it behaved as if vulcanized. The procedure of treating the latex in this way is called pre-vulcanization and the further short heating period after coagulation, post-vulcanization. The chemistry of this process was finally understood in the 1990’s, again following work at the Malaysian Rubber Producers’ Research Association in the UK. It was shown using transmission electron microscopy that there was initial crosslinking between the different elastomer chains within each particle of latex and that, on drying to a film, loose ends of the polymer, projecting from the particle, acted like ‘Velcro’ to hold the particles together. Slowly at room temperature, or more rapidly at elevated ones, the vulcanization chemistry continued with S-S bonds in the polysulphidic crosslinks breaking and reforming. Some of the reformed bonds were inevitably between the entangled polymer ends of different particles and the whole product then became chemically ‘fused’ together.

Two particular advantages of vulcanized latex over vulcanized ‘dry’ rubber are:

  1. that the latex tends to be very much stronger and elastic as the elastomer chains have not been degraded by the mechanical work needed to incorporate curatives into the dry material.
  2. since even post-vulcanization takes place at a relatively low temperature (generally below 120oC) it is possible to incorporate colouring chemicals which would not survive the high temperatures used for cost-effective conventional vulcanization (typically 160oC to 200oC).

Moving from sulphur and polyolefinic rubbers to other systems is to move completely from vulcanize to cure. Perhaps the three most common materials in a domestic environment being the cold curing silicone or acrylate sealants, the peroxide cured car filler and the two part epoxy resins.