Vulcanization

Vulcanization refers to a specific curing process of rubber involving high heat and the addition of sulfur. It is a chemical process in which polymer molecules are linked to other polymer molecules by atomic bridges composed of sulfur atoms. The end result is that the springy rubber molecules become cross-linked to a greater or lesser extent. This makes the bulk material harder, much more durable and also more resistant to chemical attack. It also makes the surface of the material smoother and prevents it from sticking to metal or plastic chemical catalysts.

This heavily cross-linked polymer has strong covalent bonds, with strong forces between the chains, and is therefore an insoluble and infusible, thermosetting polymer.

The process is named after Vulcan, Roman god of fire.

A vast array of products are made with vulcanized rubber including hockey pucks, tires, shoe soles, hoses and many more.

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Reason for vulcanizing

Uncured natural rubber is sticky and can easily deform when warm, and is brittle when cold. In this state it cannot be used to make articles with a good level of elasticity (where elasticity is defined as the possibility to return to the original shape after a deformation). The reason for unelastic deformation of unvulcanized rubber can be found in the chemical nature: rubber is made of long polymer chains. These polymer chains can move independently towards each other, and this will result in an irreversible change of shape. By the process of vulcanization crosslinks are formed between the polymer chains, so the chains cannot move independently anymore. As a result, when stress is applied the vulcanized rubber will deform, but upon release of the stress, the rubber article will go back to its original shape.

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). This is a fundamental difference between rubbers and thermoplastics, 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, 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.g., 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.

Vulcanization methods

There are various vulcanization methods. The economically most important method i.e the vulcanization of tires uses increased pressure and temperature. A typical vulcanization temperature for a passenger tire is 10 minutes at 170 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).

Overview and history

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 1600 BC ^[1] .

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, 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. 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.

Goodyear's contribution

Most textbooks point out that Charles Goodyear (1800–1860) invented vulcanization of rubber as used today by the addition of sulfur in high heat. Depending on what you read, the Goodyear story is one of either pure luck or careful research. Goodyear insists that it was the latter, though many contemporaneous accounts indicate the former.

Goodyear claimed that he discovered vulcanization in 1839, but did not patent the invention until June 15, 1844, 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.

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.

Up to that time, the only way to seal a small gap between moving machine parts, such as between a piston and its cylinder in a steam engine, was to use 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 more tightly and greater leakage of 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 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. Several experimental processes have achieved varying degrees of success in the laboratory, but have been less successful when scaled up to commercial production levels. 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. This reduces the rubber to a granular material, and all the steel and reinforcing fibers are removed. After a secondary grinding, the resulting rubber powder is ready for product remanufacture. However, the manufacturing applications that can utilize this inert material are restricted to those which do not require its vulcanization.

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. Typically, these various devulcanization processes have failed to result in significant devulcanization, have failed to achieve consistent quality, or have been prohibitively expensive.

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 particles 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 proponents of the process also claim that the process releases no toxins, by-products or contaminants. The reactivated rubber may then be compounded and processed to meet specific requirements.

Currently, Landstar Rubber, which holds the North American license for the AMR Process, has built a rubber reprocessing plant and research/quality control lab in Columbus, 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.

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 North America alone generates approximately 300 million waste tires annually, with over half being added to stockpiles that are already huge. 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.

References

1. ^ D Hosler, SL Burkett and MJ Tarkanian (1999). "Prehistoric Polymers: Rubber Processing in Ancient Mesoamerica". Science 284: 1988 - 1991. PMID 10373117.