Vulcanization

Rubber ball obtained from a vulcanization process

Vulcanization or vulcanisation is a chemical process for curing rubber or related polymers that involve the addition of sulfur or other equivalent "curatives". These additives modify the polymer by crosslinking polymer chains by formation of bridges or crosslinks. The result is a springy material with improved durability that retains its shape. The vulcanized material is smoother and less sticky toward metals or other compounds. A vast array of products are made with vulcanized rubber including ice hockey tires, shoe soles, hoses, pucks, and many more. The process is named after Vulcan, Roman god of fire. Hard vulcanized rubber is known as ebonite or vulcanite and is used to make bowling balls and clarinet mouth pieces.

Natural vs vulcanized rubber

Uncured natural rubber is sticky, easily deforms when warm, and is brittle when cold. In this state it cannot be used to make articles with a good level of elasticity. The reason for inelastic deformation of unvulcanized rubber can be found in its chemical structure: rubber is composed of long polymer chains. These chains can move independently relative to each other, which enables a change of shape. Crosslinking introduced by vulcanization prevent the polymer chains from moving independently. 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.

Process

Vulcanization is generally irreversible, similar to other thermosets and in contrast to thermoplastic processes (the melt-freeze process) that characterize the behavior of most modern polymers. The cross-linking is usually done with sulfur, but other technologies are known, including peroxide-based systems.

The combined cure package in a typical rubber compound comprises the cure agent itself (sulfur or peroxide), together with an assortment of chemical compounds that modify the kinetics of crosslinking and stabilize the final product. These additives include accelerators, activators like zinc oxide and stearic acid and antidegradants. Prevention of vulcanization starting too early is achieved by addition of retarding agents. Antidegradants are used to prevent degradation of the vulcanized product by heat, oxygen, and ozone.

The main polymers subjected to vulcanization are polyisoprene (natural rubber) and styrene-butadiene rubber (SBR), which is used in for most passenger tires. The cure package is adjusted specifically for the substrate. The reactive sites - "cure sites" - are allylic hydrogen atoms: these C-H bonds are adjacent to carbon-carbon double bonds. During vulcanization thsee C-H bonds are replaced by chains of sulfur atoms which links with a cure site of another polymer chain, thereby forming a crosslink between two chains. These polysulfur bridges typically contain between two and eight atoms. The number of sulfur atoms in the crosslink strongly influences the physical properties of the final rubber article. Short sulfur crosslinks, with just one or two sulfur atoms in the crosslink, give the rubber better heat resistance. Crosslinks with higher number of sulfur atoms, up to six or seven, give the rubber 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

A variety of methods exist for vulcanization. The economically most important method (the vulcanization of tires) uses high pressure and temperature. A typical vulcanization temperature for a passenger tire is 10 minutes at 170 °C. This type of vulcanization is called compression molding. The rubber article is intended to adopt the shape of the mold. Other methods, for instance to make door profiles for cars, use hot air vulcanization or microwave heated vulcanization (both continuous processes).

Four types of curing systems are in common use. They are:

1. Sulfur systems
2. Peroxides
3. Urethane crosslinkers
4. Metallic oxides

By far the most common vulcanizing methods are those dependent on sulfur.

Sulfur, by itself, is a slow vulcanizing agent. Large amounts of sulfur, as well as high temperatures and long heating periods are necessary and one obtains an unsatisfactory crosslinking efficiency with unsatisfactory strength and aging properties. Only with vulcanization accelerators can the quality corresponding to today's level of technology be achieved. The multiplicity of vulcanization effects demanded cannot be achieved with one universal substance, a large number of diverse materials is necessary.

Recycling and devulcanization

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 existing stockpiles. It is estimated that less than 10% of waste rubber is reused in any kind of new product. 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.

The rubber recycling process begins with the shredding. After the steel and reinforcing fibers are removed and a secondary grinding, the resulting rubber powder is ready for product remanufacture. 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.

Technologies have been commercialized to address this opportunity. 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 company Green Rubber, based in Malaysia has promoted thier mechano-chemical devulcanisation process to supply devulcanised compound made from tire waste. The Coral Group, in Dnepropetrovsk, Ukraine has commercialized an alternative devulcanization process that involves impregnation of rubber with special solvents, catalysts, and reagents.

Room-temperature vulcanization

Room-temperature vulcanizing (RTV) silicone is constructed of reactive oil base polymers combined with strengthening mineral fillers. There are two types of room-temperature vulcanizing silicone:

RTV-1 (One-component systems)

RTV-1 hardens directly under the action of atmospheric humidity. The curing process begins on the outer surface and progresses through to its core. The product is packed in airtight cartridges and is either in a fluid or paste form. RTV-1 silicone has good adhesion, elasticity and durability characteristics. The Shore A hardness can be varied between 18 and 60. Elongation at break can range from 150% up to 700%. They have excellent aging resistance due to superior resistance to UV radiation and weathering. Industrial RTV-1 products are referred to as CAFs.

RTV-2 (Two-component systems)

RTV-2 elastomer are two-component products that, when mixed, cure at room-temperature to a solid elastomer, a gel, or a flexible foam. RTV-2 remains flexible from -80 °C to +250 °C. Break down occurs at temperatures above 350 °C leaving an inert silica deposit that is non-flammable and non-combustible. They can be used for electrical insulation due to their dielectric properties. Mechanical properties are satisfactory. RTV-2 is used to make flexible moulds, as well as many technical parts for industry and paramedical applications.

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

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. For use as erasers, they sold for the exorbitant 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. Soon after, the rubberized Mackintosh coat became popular.

Nevertheless, most of these applications were in small volumes and the material was short-lived. The reason for this lack of serious applications was that the the material was not durable, was sticky and often rotted and smelled bad due to its uncured state.

Goodyear's contribution

Most textbooks state that Charles Goodyear (1800–1860) invented vulcanization of rubber as used today by the addition of sulfur with high heat. The Goodyear story is one of either pure luck or careful research. Goodyear insisted 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.

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 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 did not profit from his invention.

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 George Oenslager discovered that a derivative of aniline called thiocarbanilide would accelerate the action of sulfur on rubber, leading to 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 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 ultra-accelerators, that make the reaction extremely fast, and are used to make most modern rubber goods.