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Vulcanization is the process of cross-linking elastomer molecules to make the bulk material harder, less soluble and more durable. It is also called curing. It is the heart of the art and science of rubber compounding.
Vulcanization is a chemical process in which individual polymer molecules are linked to other polymer molecules by atomic bridges. The end result is that the springy rubber molecules become locked together to a greater or lesser extent. This makes the bulk material harder, much more durable and also more resistant to chemical attack . It also transforms the surface of the material from a sticky feel to a smooth, soft surface which does not adhere to metal or plastic substrates.
Vulcanization is an irreversible process, like baking a cake, and must be contrasted strongly with thermoplastic processes (the melt-freeze cycle) which characterize the behavior of the vast majority of modern polymers. This irreversible cure reaction defines cured rubber compounds as thermoset materials, which do not melt on heating, and places them outside the class of thermoplastic materials (like polyethylene and polypropylene). This is a fundamental difference between rubbers and plastics, and sets the conditions for their applications in the real world, their costs, and the economics of their supply and demand.
Usually, the actual chemical cross-linking is done with sulfur, but there are other technologies, including peroxide-based systems. The combined cure package in a typical rubber compound comprises the cure agent itself, (sulfur or peroxide), together with accelerators and retarding agents.
Sulfur is an unusual material. Given the right circumstances, it will form chains composed of strings of its own atoms. Carbon and silicon can also form such chains. The curing process makes use of this phenomenon. Along the rubber molecule, there are a number of sites which are attractive to sulfur atoms. These are called cure sites. At each cure site on the rubber molecule, a sulfur atom can attach itself, and from there, a sulfur chain can grow, until it eventually reaches a cure site on another rubber molecule. These sulfur bridges are typically between 2 and 10 atoms long. Contrast this with typical polymer molecules in which the carbon backbone is many thousands of atomic units in length.
Overview and history
The history of rubber goes back to prehistoric times, when the Aztecs and Mayans bled natural rubber latex from the Hevea brasiliensis trees in the local forests, formed the gunk into balls, and played the Mesoamerican ballgame with the resulting bouncy balls. The losers were sometimes ritually executed. Those balls cannot have lasted much longer than the losing players. Uncured natural rubber turns very smelly within a few days as it starts to rot. The rotting process is partly to do with proteins being broken down much as milk proteins do, but also due to the large rubber molecules breaking up as they oxidize in the air. (Chain scission , for the technically-minded)
The first reference to rubber in Europe appears to be in 1770, when Edward Nairne was selling cubes of natural rubber from his shop at 20 Cornhill in London. The cubes, meant to be erasers, sold for the astonishingly high price of 3 shillings per half-inch cube.
From these early days to the mid-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. The reason for this lack of serious applications was the fact that the material was not durable, was sticky and often rotted and smelled bad because it remained in its uncured state.
Most textbooks have it that Charles Goodyear (1800–1860) was first to use sulfur to vulcanize rubber. Depending on whom you read, the Goodyear story is either one of pure luck, or one of careful research. Goodyear insists that it was the latter, though there are many contemporaneous accounts which indicate the former.
Goodyear claimed he discovered sulfur-based vulcanization in 1839, but did not patent the invention until July 5, 1843, and did not write the story of the discovery until 1853 in his autobiographical book, Gum-Elastica. Meanwhile, Thomas Hancock (1786-1865), a scientist and engineer, patented the process in the UK on November 21, 1843, eight weeks before Goodyear applied for his own UK patent.
The Goodyear Tire and Rubber Company adopted the Goodyear name because of its activities in the rubber industry, but it has no other links to Charles Goodyear and his family.
Here is Goodyear's account of the invention, taken from Gum-Elastica. Although the book is an autobiography, Goodyear chose to write it in the third person, so that 'the inventor' and 'he' referred to in the text are in fact, the author. He describes the scene in a rubber factory where his brother worked.
...The inventor made some experiments to ascertain the effect of heat on the same compound that had decomposed in the mail-bags and other articles. He was surprised to find that the specimen, being carelessly brought into contact with a hot stove, charred like leather.
Goodyear goes on to describe how he attempted to call the attention of his brother and other workers in the plant who were familiar with the behavior of dissolved rubber, but they dismissed his appeal as unworthy of their attention, believing it to be one of the many appeals he made to them on account of some strange experiment. Goodyear claims he tried to tell them that dissolved rubber usually melted when heated excessively, but they still ignored him.
He directly inferred that if the process of charring could be stopped at the right point, it might divest the gum of its native adhesiveness throughout, which would make it better than the native gum. Upon further trial with heat, he was further convinced of the correctness of this inference, by finding that the India rubber could not be melted in boiling sulfur at any heat ever so great, but always charred. He made another trial of heating a similar fabric before an open fire. The same effect, that of charring the gum, followed; but there were further and very satisfactory indications of success in producing the desired result, as upon the edge of the charred portion appeared a line or border, that was not charred, but perfectly cured.
Goodyear then goes on to describe how he moved to Woburn, Massachusetts and carried out a series of systematic experiments to discover the right conditions for curing rubber.
...On ascertaining to a certainty that he had found the object of his search and much more, and that the new substance was proof against cold and the solvent of the native gum, he felt himself amply repaid for the past, and quite indifferent to the trials of the future.
Goodyear never made any money out of his invention. He pawned all his family's possessions in an effort to raise money, but on July 1, 1860, he died with debts of over $200,000.
Whatever the true history, the discovery of the rubber-sulfur reaction revolutionized the use and applications of rubber, and changed the face of the industrial world.
Up to that time, the only way to seal a small gap on a rotating machine, or ensure that the fuel used to power that machine(usually steam) in a cylinder applied its force to the piston with minimal leakage, was by using leather soaked in oil. This was acceptable up to moderate pressures, but above a certain point, machine designers had to compromise between the extra friction generated by packing the leather ever more tightly, or face greater leakage of the precious steam.
Vulcanized rubber offered the ideal solution. With vulcanized rubber, engineers had a material which could be shaped and formed to precise shapes and dimensions, and which would accept moderate to large deformations under load and recover quickly to its original dimensions once the load was removed. These, combined with good durability and lack of stickiness, are the critical requirements for an effective sealing material.
Further experiments in the processing and compounding of rubber were carried out, mostly in the UK by Hancock and his colleagues. These led to a more repeatable and stable process.
In 1905, however, George Oenslager , discovered that a derivative of aniline called thiocarbanilide was able to accelerate the action of sulfur on the rubber, leading to much shorter cure times and reduced energy consumption . This work, though much less well-known, is almost as fundamental to the development of the rubber industry as that of Goodyear in discovering the sulfur cure. Accelerators made the cure process much more reliable and more repeatable. One year after his discovery, Oenslager had found hundreds of potential applications for his additive.
Thus, the science of accelerators and retarders was born. An accelerator speeds up the cure reaction, while a retarder delays it. In the subsequent century, various chemists have developed other accelerators, and so-called ultra-accelerators, that make the reaction very fast, and are used in the compounds used to make most modern rubber goods.
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