Material of the month – rubber

Materials World magazine
1 Sep 2016

From the playthings of the ancient Mesoamericans to Wellington boots, car tyres to hot water bottles, natural and synthetic rubber has a rich and storied history, as Anna Ploszajski finds out.  

Sir Isaac Newton’s Principia is the cornerstone of physics. Nestled between the laws of motion, classical mechanics and the famous law of universal gravity was the hypothesis that, because of the outward thrust caused by a spinning sphere, the Earth would be slightly flattened at the poles and bulge at the equator. To test this postulation, the French Académie des Sciences in Paris sent mathematicians to the Arctic and the equator to measure the length of one-degree latitude – if Newton was right then the distance would be longer at the equator than near the pole. 

French mathematician Charles Marie de La Condamine drew the long straw and, in 1736, found himself in Quito, Ecuador. It was here that he became the first European to encounter rubber, and he brought some samples back with him when he eventually returned to Paris in 1745. On his travels, he had met French botanist François Fresneau, and their collaboration resulted in Fresneau’s 1751 paper, which described the properties of this strange, exotic substance. 

Samples made their way around to a few curious Europeans, including Joseph Priestley, who famously discovered oxygen, but who also observed in 1770 that the material was very good for rubbing pencil marks off paper, giving the substance the name ‘rubber’. For the next century, limited quantities of rubber made its way across the Atlantic from Brazil, but the trade was closely protected. This changed in 1876, when British explorer Henry Wickham brought 70,000 rubber tree seeds back from Brazil to Kew Gardens in London. 2,800 seedlings from this batch were sent to warmer climes within the Empire – India, Ceylon, Indonesia, Singapore and British Malaya – for commercial cultivation. The plantations in Thailand, Indonesia and Malaysia remain the largest producers of natural rubber today. 

From tree trunks to tyres

The chemical make-up of latex, or natural rubber, is mostly cis-1,4-polyisoprene, with a molecular weight ranging from 72,000 to 4,500,000 daltons. In the form that it emerges from the tree, it usually contains up to 5% of other materials such as proteins, fatty acids, resins and salts. To get it out of the tree, incisions are made into the bark and the sticky, milky substance that slowly leaches out is collected in cups, in the process of tapping. The latex tubes within the bark ascend the trunk in an anticlockwise spiral pattern, so tapping cuts usually ascend to the left in order to cross more tubes and increase yields. The trees are tapped every two or three days, the optimal time being morning, when the internal pressure of the tree is highest. 

CC BY-SA 3.0When the Europeans arrived in Central and South America, the people there had already been tapping rubber trees for more than 3,000 years. The ancient Mesoamericans used the material to make the solid rubber balls with which they played games. Although the exact rules are not known, archaeological evidence suggests that small teams kept the ball in play by hitting it off their hips. These balls have also been discovered as part of burials or ritual offerings, indicating the significance placed on rubber by these cultures. The game was adopted by the Maya, for whom it was even a substitute for warfare – they played it to settle territory disputes or hereditary issues, and even to predict the future. They built grand stone courts, which can still be visited today, and added variations such as hoops to get the ball through. Sacrifice of the losers was routine, a tradition that was continued by the Aztecs. 

Making the balls involved mixing the latex with the juice of the morning glory vine, which stabilised it and made it bouncier. This represents a precursor to the modern process of vulcanisation, developed by American self-taught chemist Charles Goodyear after many long years of experimentation, and patented in 1844. The process involves the cross-linking of polymer rubber chains by disulphide bonds, which limits the degrees of freedom of the chains so that they tighten more quickly, making the material harder, more durable and less extensible. 

Race to the patent

The English entrepreneur Thomas Hancock had filed a patent for vulcanisation eight weeks prior to Goodyear, although it was contested on the grounds that Hancock had copied Goodyear after seeing some of his vulcanised samples. After several chemists testified that this would not have been possible, Hancock’s patent was deemed sound, and his efforts to develop waterproof fabrics led to a collaboration with Charles Mackintosh, and ultimately to the Mackintosh coat. Fittingly, the Hancock Medal is awarded every other year by IOM3 for outstanding service to the rubber industry.

A few years later, Goodyear came across the American industrialist Hiram Hutchinson, to whom he licenced the patent for the manufacture of footwear, having decided to focus his own attention on tyres. Hutchinson moved to France and established La Compagnie du Caoctchouc Souple (The Flexible Rubber Company) and the brand À L’Aigle with the purpose of making rubber boots, inspired by the quintessentially English Wellington boot prototype made from leather, originally commissioned and popularised, of course, by the Duke of Wellington. 

Around the same time, two Americans, Henry Norris and Spencer Parmelee, landed on Scottish soil and founded the North British Rubber Company (now Hunter Boot Ltd). The business boomed, and they not only made waterproof rubber boots, but tyres, golf balls, hot water bottles, rubber flooring and conveyor belts. The boot side of the business really took wind during the First World War. British soldiers were suffering terribly in the muddy, flooded trenches of Europe, and the War Office commissioned North British Rubber to produce suitable footwear. The factory production went into overdrive and is thought to have produced more than one million boots to meet the army’s requirements. 

Hunter Boot provided similar supplies of boots during the Second World War, as well as rubber ground sheets, life belts, gas masks, and even thigh-high boots for soldiers serving in the flood plains of the Netherlands. By the end of the war, the Wellington boot was firmly rooted in British society, and the traditional green welly we all know and love today was introduced by Hunter in 1955.

Professor Michael Faraday was renowned for his engaging public lectures at the Royal Institution in London, where he also carried out pioneering research in his laboratory downstairs. During his experimentation with hydrogen gas, in 1824, he noted in the Quarterly Journal of Science that ‘The caoutchouc [a Native Amazonian word for rubber] is exceedingly elastic’, such that it could be expanded by hydrogen ‘to form balloons with considerable ascending power’. It didn’t take long for toy balloons to catch on, but have you noticed what happens to the balloons when the party is over? 

In their uninflated state, the polymer chains in the rubber are wrinkled and disorganised, but upon inflating, the material stretches and the polymer chains are pulled into alignment. These parallel chains are susceptible to crystallisation over time. You may have noticed a rubber balloon shrivels over the course of several days – when you touch it to pick it up, there is a rather horrible feeling as the balloon rapidly shrinks beneath your fingers. This is because your hand is warm enough to melt the crystals that form when the balloon is inflated. 

The Dunlop story

Little Johnnie Dunlop’s tricycle gave him a headache when he rode it around the rough streets of Belfast. His dad, a vet and dab hand at making things out of rubber, set about redesigning the wheels. He used an inflated tube of sheet rubber, wrapped around the wheels, providing much better cushioning, and Johnnie’s headaches went away. His father, John Boyd Dunlop, had succeeded in making the first pneumatic tire, and he quickly patented the invention for use in cycles and light vehicles in 1888. Unfortunately, unbeknownst to Dunlop, fellow Scottsman Robert William Thomson had already filed a patent in 1847 for that very thing, but had never brought the idea into fruition. Nevertheless, the patent office ruled Dunlop’s patent invalid.

This could have spelled the end for Dunlop and his tyres but, with the benefit of hindsight, we know it wasn’t. In 1889, Irish cyclist Willie Hume was demolishing his opponents thanks to Dunlop’s tires. Among the losers were the two sons of Dublin-born financier and cycle enthusiast, Harvey Du Cros. Rather than wallow in his sons’ failures, Du Cros saw an opportunity, and went into business with Dunlop, despite the legal difficulties surrounding the patents. Fortunately, the pair were able to acquire some rights and other patents that gave them protection over their business. Just seven years later, the business was sold for £3 million, and Dunlop remains a household name to this day.

The booming bicycle tyre business attracted the attention of the motor companies, but it became apparent that the production of natural rubber in the tropics couldn’t keep up with the demand from the rapidly industrialising nations, so chemists began to seek synthetic alternatives. The first to succeed was Fritz Hofmann’s team at Beyer Laboratories, who successfully polymerised methyl isoprene in 1909. This was followed the next year by Russian scientist Sergei Vasiljevich Lebdev who synthesised rubber from butadiene. 

Synthetic revolution

As the price of rubber grew, more and more businesses were seeing the commercial sense of producing rubber synthetically. A man of the church seems an unlikely candidate to be the father of wetsuits, but it was Father Julius Nieuwland, also Professor of Chemistry at the University of Notre Dame, whose work on acetylene resulted in the discovery of neoprene (see Materials World, April 2016). DuPont purchased the patent from the university, and Wallace Carothers, also known for developing nylon and polyester (see Materials World, March 2014 and November 2014 respectively), collaborated with Nieuwland under the direction of Elmer K Bolton to develop the material and bring it to
market in 1931.

By the time of the Second World War, it wasn’t just boots that demanded rubber for the war effort – rubber components became integral in a plethora of war machines. Russia, Germany and the USA all made synthetic rubber breakthroughs, and many rubber factories on allied soil were targeted by the Nazi forces. The Germans themselves developed a series of synthetic rubbers in 1935 known as Buna rubbers.

The Monowitz concentration camp, a subcamp of Auschwitz, was set up to provide slave labour to the ‘Buna Werke’ (Buna Works) factory of German chemical company IG Farben. The name Buna derives from the butadiene synthetic rubber the factory produced, and sodium (Na), which was the catalyst. Around 12,000 mainly Jewish prisoners were held at Monowitz between October 1942 and January 1945, including the Italian Chemist and writer Primo Levi, whose poignant survival account, If This Is a Man, includes details of gruelling toil for the Buna Werke. 

The most common synthetic rubbers today are styrene-butadiene rubbers, but others include isoprene, chloroprene, isobutylene and of course neoprene. Synthetic rubbers have the advantage that their physical, mechanical and chemical properties can be more easily tailored than their natural counterpart, as well as the obvious relief from reliance on natural resources with limited geography. However, synthetic rubbers are dependent on something else – crude oil – since they derive from petroleum by-products. Nearly 27 million tonnes of rubber were produced globally in 2015, 54% of which was synthetic.