The modification of the properties of rubber by chemical treatment, originally and still mainly (except for certain synthetic rubbers) with sulphur. Other chemicals may speed the vulcanization process or serve as extenders. The technique, which originated with Charles Goodyear in 1839, improves tensile strength, elasticity, and abrasion resistance.
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 cross-linked to a greater or lesser extent.
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 are, 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.
Description
Vulcanization is generally considered to be an irreversible process (see below), similar to other thermosets and must be contrasted strongly with thermoplastic processes (the melt-freeze process) 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). The combined cure package in a typical rubber compound comprises the cure agent itself, (sulfur or peroxide), together with accelerators and retarding agents.
Along the rubber molecule, there are a number of sites which are attractive to sulfur atoms. 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. 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.g., the movement of a side-wall of a running tire.
Overview and history
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.
In the mid-19th century rubber was a novelty material, but it did not find much application in the industrial world. 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.
Goodyear's contribution
Most textbooks have it that Charles Goodyear (1800–1860) was first to use sulfur to vulcanize rubber.
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. He describes the scene in a rubber factory where his brother worked:
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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.
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.
Later developments
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.
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.
Further experiments in the processing and compounding of rubber were carried out, mostly in the UK by Hancock and his colleagues.
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. In the subsequent century, various chemists have developed other accelerators, and so-called ultra-accelerators, that make the reaction very fast, and are used to make most modern rubber goods.
Devulcanization
The rubber industry has been researching the devulcanization of rubber for many years. The main difficulty in recycling rubber has been devulcanizing the rubber without compromising its desirable properties. The process of devulcanization involves treating rubber in granular form with heat and/or softening agents in order to restore its elastic qualities, in order to enable the rubber to be reused. Also, different processes result in different levels of devulcanization: for example, the use of a very fine granulate and a process that produces surface devulcanization will yield a product with some of the desired qualities of unrecycled rubber.
The rubber recycling process begins with the collection and shredding of discarded tires. After a secondary grinding, the resulting rubber powder is ready for product remanufacture.
In the rubber recycling process, devulcanization begins with the delinking of the sulfur molecules from the rubber molecules, thereby facilitating the formation of new cross-linkages. Two main rubber recycling processes have been developed: the modified oil process and the water-oil process. With each of these processes, oil and a reclaiming agent are added to the reclaimed rubber powder, which is subjected to high temperature and pressure for a long period (5-12 hours) in special equipment and also requires extensive mechanical post-processing. The reclaimed rubber from these processes has altered properties and is unsuitable for use in many products, including tires.
In the mid-1990s, 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, known as the AMR Process, is claimed to produce a new polymer with consistent properties that are close to those of natural and synthetic rubber, and at a significantly lower potential cost.
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, thereby enabling the characteristics of either natural or synthetic rubber to be recreated. A mixture of chemical additives is 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. The reactivated rubber may then be compounded and processed to meet specific requirements.
Currently Rebound Rubber Corp., which holds the North American license for the AMR Process, has built a rubber reprocessing plant and research/quality control lab in Dayton, Ohio. The recycled rubber from the Ohio plant is currently being tested by an independent lab to establish its physical and chemical properties.
Whether or not the AMR Process succeeds, the market for new raw rubber or equivalent remains enormous, with North America alone using over 10 billion pounds (circa 4.5 million tons) every year. The auto industry consumes approximately 79% of new rubber and 57% of synthetic rubber. To date, recycled rubber has not been used as a replacement for new or synthetic rubber in significant quantities, largely because the desired properties have not been achieved. Used tires are the most visible of the waste products made from rubber; It is estimated that less than 10% of waste rubber is reused in any kind of new product.
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