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Home > Timeline > 1913 - 1997 > Growth Of The Synthethics

It was obvious to all Victorian scientists that man could improve on nature and as early as 1860 Williams thermally decomposed rubber and identified “spirit”, “oil” and “tar” - the “spirit”, or volatile substance, he named isoprene and correctly gave its elemental composition as C5H8. In 1879 Bouchardat re-combined isoprene to a rubbery material and Tilden wrote in 1884 of its possible industrial significance (if it could be synthesised cheaply enough). One of life’s perpetual problems!

In 1900 Kondakoff polymerised 2,3-dimethylbutadiene to obtain “methyl rubber” and this became the first commercial rubber when it was produced by Bayer in 1909. Interestingly Tilden had carried out this reaction some twenty years earlier but possibly by accident as he never recorded it and his material was only identified recently.

In 1912 a few car tyres were made of this elastomer and one set went to Kaiser Wilhelm II. One, at least, of these is still in existence and was displayed at an exhibition of the history of rubber which toured Europe during 1995-6.

However, it was still not a commercially viable material so Germany’s obtained most of its natural rubber from the US. This obviously ended when the States entered the Great War in 1916 and production of “methyl rubber” was re-commenced with some 2½ thousand tons being manufactured by the war’s end.

Russia was also active during this period, polymerising many monomers including 1,4-butadiene in 1910 to give butadiene rubber (BR), but neither the Russian nor the American synthetic rubber industries was under the same pressures as Germany and, with the price of natural rubber low again, there was little incentive for anything other than academic research.

The situation changed drastically in 1922 when the Stephenson Reduction Plan was introduced. This was designed to cut production from the British-controlled plantations and so force up the price of natural rubber (NR). Over the next three years there was a ten fold price rise - followed by an equally rapid and catastrophic fall as producers outside the control of Britain flooded the market. It was this political intervention in the free market which triggered the next phase in the development of the synthetics.

One of the first of the new materials was far removed from the work of the preceding years in that it was prepared, by accident in the early 20's (although not patented until 1932). This was an ethylene polysulphide - the first of the ‘Thiokols’ which are still in use as sealants today. Working independently in Switzerland, Baer produced a similar material in 1926 on which IG Farbenindustrie based its Perdurens whilst in the States the thiokol rubbers were referred to as GR-P rubber.

IG (which now included Bayer) had resumed its research in 1925 and soon came on stream with polybutadiene rubber which it called Buna, as well as two copolymers synthesised by mixing two different monomers together before the polymerization stage - Buna S (styrene butadiene rubber, SBR or GR-S in America) and Buna-N (butadiene acrylonitrile rubber, NBR or GR-A). These had reached laboratory production by 1930 but were not industrially developed when reaction to the Stephenson Reduction plan made the price of NR head for the floor again.

However, in 1933, when Hitler came to power, work restarted with a vengeance, only now with solution polymerisation, rather than gas phase polymerisation being the preferred route forward.

Given the political situation in the 30’s it is perhaps surprising that IG and the Standard Oil Co. of New Jersey formed a joint study group - although they did maintain carefully identified areas of privacy. At that time IG was making acetylene, its primary feedstock for elastomer synthesis, from calcium carbide. This was a private area but in about 1930 it changed to natural gas as the primary feedstock - the use of which was a joint area so Standard Oil became entitled to an interest. This led to Standard holding the US patents to all the Buna rubbers, a crucial factor in the development of the American synthetic rubber industry as the Second World War developed and the output of NR fell essentially to zero.

A further valuable material to come out of the IG/Standard agreement was butyl rubber (IIR). Its precursor was synthesised by IG as polyisobutylene (IM) and had no olefinic groups so it could not be vulcanized. Standard added a few % of butadiene (or isoprene) to give a low level of residual unsaturation and thus a vulcanizable elastomer; butyl rubber.

Throughout this period all the work in the US was privately funded and when the Second World War started production was minimal. It only rose from 2,000 tons in1939 to 10,000 tons in 1941. However, the first government-owned plant came on stream in mid 1942 and by 1945 the year’s production exceeded 830,000 tons. Over this same early period Germany’s production went from 22,000 to a peak of about 100,000 tons in the middle years of the war and then, not surprisingly, fell to zero in 1945.

There is one other major elastomer which made its appearance between the wars and this was polychloroprene (CR).

Its origin can be traced to the academic work of Father Julius Nieuwland in 1922 into the dimerization of acetylene to form vinyl acetylene. When Du Pont heard of this, it realised that the addition of hydrogen chloride across the acetylenic bond would produce 2-chloro-1,3-butadiene, a substance structurally similar to isoprene except that the side chain methyl group had been replaced by a chlorine atom. This chemical, which they called chloroprene was polymerised by Carothers and his team in 1930 and in commercial production the next year as polychloroprene although this is still more often called by Du Pont’s trade name which was initially Duprene and later became Neoprene. It was the first really viable commercial synthetic rubber.

The American contribution to synthetic rubber production during the war had been a vast amount of fundamental research together with production technology and plant but, when the war finished in 1945, the cycle of cheap natural rubber returned, leading yet again to reduced commercial interest in the synthetics.

It was left to Ziegler and Natta to re-activate the cycle in the early 50’s when they developed catalysts which enabled high cis 1,4-polybutadiene to be synthesised. The third phase of production techniques had arrived. (Gas phase reactions, emulsion phase and now catalysed stereo-regular emulsion phase.

In the early 1960’s Du Pont echoed the pre-war work of IG/Standard when, instead of co-polymerizing just ethylene and propylene to make ethylene prolylene rubber (EPM) with no crosslinking sites, they included a small amount of ethylene norbornene which provided, after co-polymerization, olefinic crosslinking sites and gave us EPDM. It should be noted that this is still an “M” rubber (see nomenclature of synthetic elastomers) as the main polymer chain is saturated and the olefinic double bond forms part of the pendant norbornene group. Butyl rubber is classified as an “R” type because the residual double bond from butadiene or isoprene is in the polymer chain itself.

All of the elastomers discussed so far have been either homopolymers, (that is one monomer polymerised), or random co-polymers of two (or three) monomers but, when some structure is fed into this randomness, we get quite different properties.

This is the principle behind some of today’s thermoplastic elastomers. In these materials there are soft ‘rubbery’ regions to provide extensibility coupled with ‘glassy’ regions which serve as physical network junctions at their operating temperatures but melt, making the material mouldable (or re-mouldable) when they are heated. These materials have been around for some 25 years and, in a world where recycling is king, they are taking an ever-increasing share of the elastomer market with. recent figures suggesting about 20% of the non-tyre market.

In this brief summary of the synthetics, many materials have been omitted but one class must be mentioned since it is unique in containing no carbon. This is the silicone rubbers, introduced as early as in 1944 and today ubiquitous, being found in almost every environment from the most hi-tech to every DIY fanatic’s toolbox.

Although many people think that today’s world is ruled by synthetics and NR is dead, its output has continued to grow throughout the last century although certainly the synthetics have expanded more rapidly as demand for general purpose elastomeric materials, which can replace the natural material in many applications, has grown more rapidly the latter can be produced and speciality elastomers, which are used for a range of dedicated applications for which natural and general purpose rubbers are not suited, have been developed. In round figures, there are about 6 million tons of NR and 9 million tons of synthetics (SR) produced per year in the late 1990’s.

There is a final thought: most of the major synthetic elastomers are today made from that finite material – oil. Natural rubber comes from a renewable resource and chemical treatments of NR are being investigated and used to enable modified NR to operate in areas once considered the sole preserves of the synthetics. The biosynthesis of natural rubber also consumes the greenhouse gas carbon dioxide and it produces timber as a by-product.

The story of the synthetics vs the natural elastomer is by no means over.