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
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:
- To modify the vulcanization process in such a way as to make
it more cost-effective.
- To modify the vulcanization chemistry and thereby obtain a
product with improved performance characteristics.
- 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
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’.
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:
- 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.
- 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.