The native Olmec people were known to have used boiled sap from the rubber tree hevea brasiliensis combined with a vine sap to make a stretchy and waterproof material they used for proofing clothing items for some years. However this fabric still went stiff in the cold and too soft under heat, and any stitched holes leaked; but whilst trying to solve these problems, they came across samples sent from the US by Charles Goodyear.
Goodyear had also been trying for years to use rubber in a variety of ways in inventions, and with the more extreme weather in Massachusetts, the poor thermal stability of the uncured natural rubber was an even greater problem for garment makers. He had been asked by a local waterproof material manufacturer to find a way of heat stabilising rubber, but despite building on earlier work by Hayward on sun drying rubber treated with a sulphur solution called solarisation, success eluded him.
That was until one day, reputedly by accident, he dropped a sheet of fabric infused with rubber, sulphur and white lead onto a hot stove.
He was astonished to see that rather than melting, the fabric had hardened into a permanent flexible state, and was far less affected by extremes of heat than it had been.
The story goes that after many years of experimenting, he had spilt or accidentally placed a mixture of rubber, sulfur and lead oxide on a hot stove. The rubber was no longer sticky but had been converted to a tough, elastic substance stable to heat and cold.
It also did not dissolve in the solvents that dissolved natural rubber. He had invented the process now known as vulcanization. Vulcanization is a chemical reaction between sulfur and rubber resulting in cross-links being formed between the rubber polymer chains.
Notice here:. You should remember that double bonds provide a major route to the formation of polymers, so it should not be a surprise to find that these double bonds can serve to provide covalent links between the chains. We can form covalent bonds between the polymer molecules, and if we do this the material will become much more rigid because the chains are no longer free to move apart. The more cross-links between chains, the more rigid the rubber until eventually the polymer is so cross-linked that it is no longer rubbery because there is no flexibility of the chains between the cross-links.
The sulfur reacts with the double bonds and forms sulfur bridges as cross-links between the chains, resulting in a huge three-dimensional network as you can see here. The covalent cross-links survive the stretching and help the molecules to spring back once the tension has been relaxed. This type of network molecular structure lies behind the explanation of why rubber is rubbery. Many synthetic and natural fibres can be stretched, as we can see in the making of polymer fibres:.
The main difference between rubber and other fibres is that rubber goes back to its previous shape and size.
We can see that stretching a fibre aligns the polymer chains. In fibres, this alignment allows forces such as those of hydrogen bonding between chains to have an increased effect, and they will be strong enough to hold the fibres in their stretched, aligned position.
Why did this happen? The rubber back then did not undergo the same process as it does today to chemically change the properties that makes it heat-resistant. Charles Goodyear saw an opportunity to improve upon this rubber material. After 5 years of experimenting, short periods of jail time and a very fortunate accident he lost his grip on his altered rubber and landed on a stove where it did not melt — but instead charred , Goodyear had invented the process of vulcanization. The combined cure package in a typical rubber compound comprises the cure agent itself, sulfur or peroxide , together with accelerators, activators like zinc oxide and stearic acid and antidegradants.
Prevention of vulcanization starting too early is done by addition of retarding agents. Antidegradants are used to prevent degradation by heat, oxygen and ozone. Along the rubber molecule , there are a number of sites which are attractive to sulfur atoms. These are called cure sites, and are generally sites with an unsaturated carbon-carbon bond, like in polyisoprene, the basic material of natural rubber,and in styrene-butadiene rubber SBR , the basic material for passenger tires.
The active sites are allylic hydrogen atoms; that means they are hydrogen atoms connected to the first saturated carbon atom connected to the carbon-carbon double bond. During vulcanization the eight-membered ring of sulfur breaks down in smaller parts with one to eight sulfur atoms. These small sulfur chains are quite reactive.
At each cure site on the rubber molecule, such short sulfur chain can attach itself, and eventually reacts with a cure site of another rubber molecule, and so forming a bond between two chains.
This is named a cross-link. These sulfur bridges are typically between two and eight atoms long. The number of sulfur atoms in a sulfur crosslink has a strong influence on the physical properties of the final rubber article. Short sulfur crosslinks, with just one or two sulfur atoms in the crosslink, give the rubber a very good heat resistance. Crosslinks with higher number of sulfur atoms, up to six or seven, give the rubber very good dynamic properties but with lesser heat resistance.
Dynamic properties are important for flexing movements of the rubber article, e. Without good flexing properties these movements will rapidly lead to formation of cracks and, ultimately, to failure of the rubber article. There are various vulcanization methods. The economically most important method i. A typical vulcanization temperature for a passenger tire is 10 minutes at dgrees C. This type of vulcanization is an example of the general vulcanization method named compression moulding.
The rubber article is intended to adopt the shape of the mold. Other methods for instance those used to make door profiles for cars use hot air vulcanization or microwave heated vulcanization both continuous processes. Although vulcanization is a 19th century invention, the history of rubber cured by other means goes back to prehistoric times.
The name "Olmec" means "rubber people" in the Aztec language. Ancient Mesoamericans, spanning from ancient Olmecs to Aztecs, extracted latex from Castilla elastica , a type of rubber tree in the area.
The juice of a local vine, Ipomoea alba , was then mixed with this latex to create an ancient processed rubber as early as BC [1]. The first reference to rubber in Europe appears to be in , when Edward Nairne was selling cubes of natural rubber from his shop at 20 Cornhill, London.
The cubes, meant to be erasers, sold for the astonishingly high price of 3 shillings per half-inch cube. In the early 19th century rubber was a novelty material, but it did not find much application in the industrial world.
It was used first as erasers, and then as medical devices for connecting tubes and for inhaling medicinal gases. With the discovery that rubber was soluble in ether , it found applications in waterproof coatings, notably for shoes and soon after this, the rubberized Mackintosh coat became very popular. Nevertheless, most of these applications were in small volumes and the material did not last long.
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