By the time that Christopher Columbus discovered America, the
Indians of the tropical Americas had developed the technology
of rubber product manufacture to a fine art - within their scientific
limitations. Most of their products were manufactured by successive
dippings of a former with fresh latex and each layer was dried
(and sterilized) by smoking over a wood or nut fire before the
next was applied. The pre-Columban 'curing' referred to in some
literature uses the word in the same sense as the curing of meat
by smoking and this should not be confused with Goodyear's
and Hancock’s
vulcanization or 'curing' some centuries later. The tropical climate
is relatively consistent in temperature and the manufacturing
protocol which had been developed over many centuries gave products
which were adequate for the demands placed upon them in that environment.
With the arrival of the western entrepreneur in the middle of
the second millennium there were many attempts to ship latex to
Europe so that the manufacturing industry could produce value-added
goods on its home base but these generally failed as the latex
coagulated during the Atlantic crossing. This led to the dissolution
of dried rubber in a suitable solvent and the use of that in much
the same way as latex. Unfortunately, the very act of preparing
a film in this way, coupled with the broader climatic variations
experienced in Europe and the lack of the sterilizing and smoking
step gave the lie to a series of claims to have overcome the problems
associated with rubber - stickiness/brittleness, putrefaction
and degradation. The products simply failed the test of the real
world.
The discovery of vulcanization in the middle of the 19th century
was believed to be the ultimate holy grail - Hancock, in his classic
book on rubber technology, published in 1857, repeated his observation
of about 1826:
"The injurious effects of
the sun's rays upon thin films of rubber we discovered and provided
against before much damage accrued"
It was obviously realised that sunlight induced degradation of
the rubber but in spite of Hancock's assertion it was certainly
not 'provided against'. Indeed, Hancock did not realise that light
catalysed degradation was oxidation and it was left to Spiller,
in 1865, to show that degraded rubber had actually been oxidized.
40 Or so years later, it was discovered that amines and amine-based
materials offered considerable protection against oxidative degradation
but it took another 30 years for specifically manufactured amine-based
antioxidants and antiozonants to be brought to the market. These
materials, however, had one considerable handicap; although they
could be prepared very pure and pale straw in colour, they themselves
soon oxidized to various shades of blue, purple and black. Whilst
this might not be too important in a black vulcanizate such as
a car tyre, it was certainly of little use in the many light-coloured
products, such as rubber thread or strip coming to the market
at that time and used in the clothing industry.
Today para-phenylenediamines (PPD's) are regularly used and function
as both antiozonants and antioxidants. As antiozonants they operate
by reacting more readily than the rubber double bonds with any
ozone present at the surface of the article. In so doing they
build up a protective film which, as it thickens by migration
and further reaction of the migrated antiozonant, eventually provides
an impermeable barrier to the gas. Any damage to the skin, such
as by cracking, is repaired by further migration. As well as discolouring
the product, since part of the mechanism by which these antidegradents
operate is by migrating to the product surface, they could then
further migrate into any material in contact with the protected
rubber and then oxidize, producing a dark stain. There are many
different PPD's and one of the main reasons for a manufacturer
to select a particular one is its solubility and rate of migration
in the polymer system being protected, obviously crucial factors
in long term protection. However, since PPD's oxidize to blue/purple/black
materials and readily stain, their use must obviously be selective.
It took a further generation (the late 1940's/early 1950's) for
the 'non-staining' phenolic antioxidants to be produced and brought
to the market. Everything is relative and it was soon realised
that, whilst these antioxidants were certainly 'non-staining'
compared to many of the amine-based materials, it was not true
to say that they had no effect on their colour, or that of materials
in contact with them, where pronounced yellowing was sometimes
observed. Phenolic antidegradents are not considered antiozonants,
only antioxidants. When oxygen attacks a rubber molecule various
chain reactions occur which can result in polymer chain breakage
and/or the insertion of further crosslinks. The phenols offer
an alternative path in the chain reaction sequence which stops
it progressing. It is essential to realise that they do not stop
the initiation step so their effect is, at best, to slow down
the oxidative breakdown, perhaps by up to five times. They do
not form a protective 'skin' if oxidized on the rubber surface
indeed, because of their mode of operation, they need to be intimately
dispersed or dissolved in the rubber to function. However, conventional
diffusion theory predicts that if two materials are in contact
with one another and one contains a substance not present in the
second, then that substance will migrate from one to the other
and it is often the case that light coloured rubber vulcanizates
are in contact with fabrics.
Whilst it must be realised that the discolouration of ‘white’
vulcanizates may be related to poor quality or inappropriate grades
of white filler or brightener (although this hardly ever results
in the staining of contact fabrics) and also that a common source
of organic discolouration or staining is the reaction between
dithiocarbamate cure residues and trace amounts of copper or to
a lesser extent iron there are certainly cases where some yellowing
has been traced to phenolic antioxidants. This was initially put
down to impurities in the chemicals but it soon became apparent
that it was oxidative degradation of the antioxidant itself which
was producing a derivative which was coloured yellow. Obviously
staining could be due to either migration of that yellow derivative
or migration of the unoxidized antioxidant followed by its oxidation.
The yellowing effect was stronger in urban or industrial environments
than rural ones suggesting that the extent of oxidation is increased
by the oxides of nitrogen which are nowadays major atmospheric
pollutants, being produced as by-products of the high temperature
combustion of fuels by motor vehicles, industrial boilers etc.
These oxides of nitrogen (collectively known as NOX's) have the
potential to cause colouration by the introduction of two chromophores
(light absorbing groups) into the phenolic antioxidant molecule,
namely the nitro group by electrophilic substitution and the carbonyl
group via the oxidation of phenols to the corresponding quinone
(in this case the conjugation of the molecule also contributes
to the colour). The more chromophoric groups a compound contains,
the deeper its colour will be, whilst the presence of auxochromes
such as the OH group will also deepen, though not cause, any colouration.
In order to try and resolve the detailed chemistry of the yellowing,
twenty-one antioxidants were investigated and ten were found to
discolour in the presence of NOX fumes. Using separative techniques
followed by mass spectroscopy it was shown that four distinct
reactions are responsible for their discolouration :
- para-oxidation
- ortho-nitration
- para-nitration
- oxidation of the phenol group
Comparison of the structures of those antioxidants which discoloured
with those which did not, showed that oxidation or nitration of
the para- position only occurred when there was no meta- substitution
and here it is important to remember that the yellow derivatives
are not only the nitrated phenols which obviously require the
NOX’s but also the quinones which do not - they are simple
oxidation products. Of the antioxidants that did not discolour,
all were either para-substituted or, if they had no para- group,
they were meta-substituted, with the substituent groups hindering
the introduction of the relatively large nitro group into the
para- or 4-position or preventing the oxidation to the quinone.
It should be noted that none of these relates to the way in which
a phenolic antioxidant is supposed to operate and each molecule
which is oxidized by itself is one molecule which is lost to the
protective system. The yellowing reactions are therefore undesirable
competing reactions which should be prevented for good commercial
and longevity reasons as well as the problem of discolouration.