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Home > Chemistry, Technology & Conservation > Protective Agents

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 :

  1. para-oxidation
  2. ortho-nitration
  3. para-nitration
  4. 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.