Vulcanization
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Vulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives". These additives modify the polymer by forming crosslinks (bridges) between individual polymer chains.[1] The vulcanized material is less sticky and has superior mechanical properties. A vast array of products are made with vulcanized rubber including tires, shoe soles, hoses, and hockey pucks. The process is named after Vulcan, Roman god of fire. Hard vulcanized rubber is known as ebonite or vulcanite and is used to make hard articles such as bowling balls and clarinet mouth pieces.
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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 the material to change shape. Crosslinking introduced by vulcanization prevents 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 article will revert 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 main polymers subjected to vulcanization are polyisoprene (natural rubber) and styrene-butadiene rubber (SBR), which are used for most passenger tires. The "cure package" is adjusted specifically for the substrate and the application. The reactive sites - "cure sites" - are allylic hydrogen atoms. These C-H bonds are adjacent to carbon-carbon double bonds. During vulcanization, some of these C-H bonds are replaced by chains of sulfur atoms that link with a cure site of another polymer chain. These bridges contain between one and eight atoms. The number of sulfur atoms in the crosslink strongly influences the physical properties of the final rubber article. Short crosslinks give the rubber better heat resistance. Crosslinks with higher number of sulfur atoms 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:
- Sulfur systems
- Peroxides
- Urethane crosslinkers
- Metallic oxides
Vulcanization with sulfur
By far the most common vulcanizing methods depend on sulfur. Sulfur, by itself, is a slow vulcanizing agent and will not vulcanize synthetic polyolefins. Even with natural rubber, 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 additives, comprising the "cure package," are necessary.
The combined cure package in a typical rubber compound consist of sulfur together with an assortment of 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. The accelerators and activators are catalysts. An additional level of control is achieved by retarding agents that inhibit vulcanization until some optimal time or temperature. Antidegradants are used to prevent degradation of the vulcanized product by heat, oxygen, and ozone.[2]
Vulcanization of silicones
"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.
History of vulcanization of rubber
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 [3] In the western world, rubber remained a curiosity although it was used in the production of waterproofed products such as Mackintosh rainwear.
Goodyear's contribution
Charles Goodyear (1800–1860) invented vulcanization of rubber when he was experimenting, and heated a mixture of rubber and sulfur. The Goodyear story is one of either pure luck or careful research, but both are debatable. 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 his discovery was not readily accepted.
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 Shehan, Massachusetts and carried out a series of systematic experiments to optimize the curing of 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 by Hancock and his colleagues led to a more repeatable and stable process.
In 1905 George Oenslager discovered that a derivative of aniline called thiocarbanilide accelerated the action of sulfur to rubber, leading to shorter cure times and reducing energy consumption. This breakthrough, although less famous, 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 faster, improved the reliability of the vulcanization process and, although not obvious at the time, enabled vulcanization to be applied to synthetic polymers. 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.
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.
References
- ↑ “Science and technology of rubber. 2005. pp. 768. ISBN 0-12-464786-3.
- ↑ Hans-Wilhelm Engels, Herrmann-Josef Weidenhaupt, Manfred Pieroth, Werner Hofmann, Karl-Hans Menting, Thomas Mergenhagen, Ralf Schmoll, Stefan Uhrlandt “Rubber, 4. Chemicals and Additives” in Ullmann's Encyclopedia of Industrial Chemistry 2004, Wiley-VCH, Weinheim. doi:10.1002/14356007.a23_365.pub2
- ↑ D Hosler, SL Burkett and MJ Tarkanian (1999). "Prehistoric Polymers: Rubber Processing in Ancient Mesoamerica". Science 284 (5422): 1988–1991. doi:10.1126/science.284.5422.1988. PMID 10373117.