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Vulcanization, or curing, of rubber, 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.
Reason for vulcanizing
Uncured natural rubber will begin to deteriorate within a few days, gradually breaking down into a wet crumbly mess. The process of perishing partly consists of proteins being broken down much as milk proteins do, and also of the large rubber molecules breaking up as they oxidize in the air due to oxygen molecules attacking the double bonds.
Rubber which has been inadequately vulcanized also may perish, but more slowly. The process of perishing is encouraged by long exposure to sunlight.
Vulcanization is generally considered to be an irreversible process (see below), and must be contrasted strongly with thermoplastic processes (the melt-freeze cycle) which characterize the behavior of most 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 organic peroxide), together with accelerators and retarding agents.
Along the rubber molecule, there are a number of sites which are attractive to sulfur atoms. These are called cure sites. During vulcanization the 8-membered ring of sulfur breaks down in smaller parts with varying numbers of sulfur atoms. These parts are quite reactive. At each cure site on the rubber molecule, one or more sulfur atoms 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. 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, so with 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 6 or 7 give the rubber very good dynamic properties, but with lesser heat resistance. Dynamic properties are important for flexing movements of the rubber article, like for instance the movement of a side-wall of a running tire. Without good flexing properties these movements will rapidly lead to formation of cracks and ultimately to failure of the rubber article.
Overview and history
The history of vulcanized rubber 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 vulcanized rubber as early as 1600 BC !
Furthermore, the Aztecs and Mayans created a non-vulcanized rubber by extracting natural rubber latex from the Hevea brasiliensis trees in the local forests. They formed the latex into balls, and played the Mesoamerican ballgame with the resulting bouncy balls. The losers were sometimes ritually executed.
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.
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 that 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.
The rubber industry has been researching the devulcanization of rubber for many years. The difficulty in recycling rubber has been with devulcanizing the rubber without compromising its properties. The process of devulcanization is the treatment of rubber granulate with heat and/or softening agents to return its elastic qualities to enable the rubber to be reused. Several experimental processes have met with varying degrees of success at the laboratory level, but are less successful at commercial production levels. Also, different processes result in different levels of devulcanization, for example the use of a very fine granulate and a process which results in surface devulcanization will yield a product with some of the desired qualities of rubber. Typically, alternative devulcanization processes have resulted in low levels of devulcanization, have failed to produce consistent quality or have been prohibitively expensive.
The rubber recycling process commences with the collection and shredding of discarded tires, which reduces the rubber to a granular material and all steel and other fibers are removed. After a secondary grinding, the resulting rubber powder is then ready for product remanufacture. However, the volume of applications that can utilize this inert material is limited to products that do not require vulcanization.
Similar to vulcanization, in the rubber recycling process, devulcanization begins as a process by which the sulfur molecules are delinked from the rubber molecules, thereby facilitating the formation of new cross-linking structures. Two general rubber recycling processes have been developed: modified oil process and water-oil process. With each of these processes, oil and a reclaiming agent are added to the reclaimed rubber powder, treated at high temperatures and under high pressure for a long period (5-12 hours) in special equipment, and requiring strong mechanical post- processing efforts. The reclaimed rubber from these processes have changed properties and is generally not suitable for use in many products, including tire compounds. Typically, these alternative devulcanization processes have resulted in low levels of devulcanization, have failed to produce consistent quality or have been prohibitively expensive.
In the mid-90s researchers at the Guangzhou Research Institute for the Utilization of Reusable Resources in China patented a method for the reclamation and devulcanizing of recycled rubber. Their technology called the AMR Process is claimed to produce a new polymer with consistent properties that approach the properties of natural and synthetic rubber, with potential a significant price advantage.
The AMR Process exploits the molecular characteristics of vulcanized rubber powder in conjunction with the use of an activator, a modifier and an accelerator reacting homogeneously with particle of rubber. The chemical reaction that occurs in the mixing process facilitates the delinking of the sulfur molecules thus enabling a re-enactment of the characteristics of natural or synthetic rubber. A mixture of chemical additives are added to the recycled rubber powder in a mixer for approximately five minutes, after which the powder passes through a cooling process and is then ready for packaging. Proponents also state that the process does not release any toxins, by-products or contaminants. Reactivated rubber may then be compounded and processed in accordance with specific requirements.
Currently Rebound Rubber Corp., which holds the North American license for the AMR Process, has built a plant facility and research/quality control lab in Dayton, Ohio. The plant performs production runs on a demonstration basis or at small commercial levels. The recycled rubber from the Ohio plant is currently being tested by an independent lab to establish its physical and chemical properties.
Regardless of whether the AMR Process becomes successful or not, the market for new, raw rubber or equivalent is enormous, with North America alone using over 10 billion pounds every year. The auto industry consumes approximately 79% of new rubber and 57% of synthetic rubber. To date, recycled rubber in significant quantities has not been used as a replacement for new or synthetic rubber, largely because required properties could not be achieved. Used tires are the most visible of waste rubber products; it is estimated that North America alone generates approximately 300 million waste tires annually, with over half of them being added to the already huge stockpiles. It is estimated that less than 10% of waste rubber is reused in any kind of new product. Furthermore, the United States, the European Union, Eastern Europe, Latin America, Japan and the Middle East collectively produce about one billion tires annually with estimated accumulations of three billion in Europe and six billion in North America.