Styrene Butadiene Rubber (SBR)

Nineteen fifty-nine was the year the United States finished filling in its own map: Alaska joined the union in January and Hawaii in August, the forty-ninth and fiftieth stars added to a flag that had not changed in shape since 1912. It was also, by general agreement among the people who design cars for a living, the single most extravagant year Detroit ever had. The 1959 Cadillac carried tailfins forty-two inches off the ground, the tallest ever fitted to a production car, on a body built for a country that had never owned more automobiles or driven them over more miles of new road. None of that chrome and legroom asked its owner to think about what the tires underneath it were actually made of. By 1959 the answer, in the overwhelming majority of American cars, was a rubber that had been invented as a wartime expedient a generation earlier and had since become simply the ordinary way a tire was built.

Plate I

Plate II

A Rubber Invented Twice, for Two Different Reasons

The chemistry itself was three decades old by 1959. In 1929, the IG Farben chemists Walter Bock and Eduard Tschunkur, working in the same Leverkusen laboratories where colleagues were separately turning plain butadiene into Buna rubber and, a little later, into the oil-resistant Buna N covered elsewhere in this Atlas, found that copolymerising butadiene with styrene in an emulsion produced a rubber Germany called Buna S. It sat as a modest German industrial product until December 1941, when Japan’s advance through Southeast Asia cut the United States off from the Malayan and Dutch East Indies plantations that supplied nearly all of its natural rubber. Within weeks Washington had organised the largest cooperative chemical engineering effort the country had yet attempted, and its principal product was an American copy of Germany’s own decade-old invention, manufactured under the wartime code name GR-S (Government Rubber-Styrene).

What the War Left Behind

The government-built plants that made GR-S during the war were sold off to private industry through the mid-1950s, and the rubber itself simply kept being made, because it was cheaper than the natural material and, with the wartime emergency over, still perfectly suited to an ordinary tire. By the time the 1959 Cadillac left the assembly line, the industry had dropped the wartime code name in favour of a plain description of the chemistry (styrene-butadiene rubber), and it was the default material in most American tires. The country’s new Interstate Highway System, three years into construction that year, was only going to multiply how many of those tires needed making.

The Molecular Marvel

SBR alternates flexible stretches of butadiene, which give the material its elasticity, with stiffer styrene units that raise its hardness and resistance to wear. The proportion between the two is a dial rather than a fixed recipe (commercial grades run from roughly one part styrene to three of butadiene up to closer to two parts in five), and the butadiene segments themselves carry a mix of cis, trans and vinyl orientations whose balance a compounder can also adjust. Between the two levers, a single family of chemistry covers everything from a soft, springy tread compound to a much harder industrial rubber.

Properties that Pack a Punch

Like most rubbers on this Atlas, SBR sits close to water in density and never develops a true melting point, remaining uniformly amorphous through its service range. Raw and unfilled it is not especially strong, but compounded with reinforcing filler it gains real toughness and, notably, better resistance to abrasion than natural rubber manages in a comparable tread; this is the trade that made it the tire industry’s workhorse in the first place. It tolerates a modest amount of water and resists catching fire about as well as a general-purpose hydrocarbon rubber can, without the added chemical resistance a specialised elastomer offers: aromatic solvents dissolve it outright, and acids, alkalis, esters and chlorinated compounds all degrade it to some degree, so it stays out of jobs that put it in contact with fuel or strong chemicals.

Plate III

The Art and Science of SBR Production

Two distinct processes divide the market. Emulsion SBR, the direct descendant of the wartime GR-S recipe, is still made by polymerising the two monomers in a water emulsion, either “hot” at a higher temperature with a broad resulting molecular weight spread, or “cold,” which gives finer control over the polymer’s architecture and better dynamic properties in a finished tire. Solution SBR, which grew in importance in the years after 1959, is made instead in a hydrocarbon solvent using organolithium initiators under carefully excluded moisture, and gives a compounder far more precise control over the polymer’s microstructure, control that has become increasingly valuable as tire technology has chased better wet grip and lower rolling resistance.

Applications: Where the Rubber Meets the Road

Tires remain SBR’s largest market by a wide margin, still accounting for roughly half of all automobile tires produced. Away from the road it turns up in shoe soles, conveyor belting, adhesives and gaskets, and in two markets a driver would never guess: SBR latex is the single largest binder used in coated printing paper, and, more recently, it has found a role as the binder holding electrode material together inside lithium-ion batteries.

Plate IV

Looking Ahead

Because SBR is neither recyclable in any simple sense nor biodegradable, the same volume that made it the twentieth century’s default tire rubber is now the reason devulcanisation and reclaiming processes are under active development. This is an attempt to give a wartime-born material an end-of-life story to match the one it was given at the start.