Vulcanization or
vulcanisation is a
chemical processIn a "scientific" sense, a chemical process is a method or means of somehow changing one or more chemicals or chemical compounds. Such a chemical process can occur by itself or be caused by somebody. Such a chemical process commonly involves a chemical reaction of some sort...
for converting
rubberNatural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...
or related
polymerA polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s into more durable materials via the addition of
sulfurSulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
or other equivalent "curatives." These additives modify the polymer by forming crosslinks (bridges) between individual polymer chains. 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
VulcanVulcan , aka Mulciber, is the god of beneficial and hindering fire, including the fire of volcanoes in ancient Roman religion and Roman Neopaganism. Vulcan is usually depicted with a thunderbolt. He is known as Sethlans in Etruscan mythology...
, Roman god of
fireFire is the rapid oxidation of a material in the chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition....
. Hard vulcanized rubber is sometimes sold under the brand names
eboniteEbonite is a brand name for very hard rubber first obtained by Charles Goodyear by vulcanizing rubber for prolonged periods. It is about 30% to 40% sulfur. Its name comes from its intended use as an artificial substitute for ebony wood...
or vulcanite, and is used to make hard articles such as
bowling ballA bowling ball is a spherical ball made from plastic, reactive resin, urethane or a combination of these materials which is used in the sport of bowling. Ten-pin bowling balls generally have a set of three holes drilled in them, one each for the ring and middle finger, and one for the thumb;...
s and
saxophoneThe saxophone is a conical-bore transposing musical instrument that is a member of the woodwind family. Saxophones are usually made of brass and played with a single-reed mouthpiece similar to that of the clarinet. The saxophone was invented by the Belgian instrument maker Adolphe Sax in 1846...
mouth pieces.
Natural vs vulcanized rubber
Uncured natural rubber is sticky, deforms easily when warm, and is brittle when cold. In this state, it is a poor material when a high level of
elasticityIn physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....
is required. The reason for inelastic deformation of un-vulcanized 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 lets the material change shape. Crosslinking introduced by vulcanization prevents the polymer chains from moving independently. As a result, when stress is applied the vulcanized rubber deforms, but upon release of the stress, the article reverts to its original shape.
Process
Vulcanization is generally irreversible, similar to other thermosets and in contrast to
thermoplasticThermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...
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
peroxideOrganic peroxides are organic compounds containing the peroxide functional group . If the R' is hydrogen, the compound is called an organic hydroperoxide. Peresters have general structure RCOOR. The O-O bond easily breaks and forms free radicals of the form RO·...
-based systems.
The main polymers subjected to vulcanization are polyisoprene (natural rubber) and
styrene-butadieneStyrene-Butadiene or Styrene-Butadiene-Rubber is a synthetic rubber copolymer consisting of styrene and butadiene. It has good abrasion resistance and good aging stability when protected by additives, and is widely used in car tires, where it may be blended with natural rubber...
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
allylAn allyl group is a substituent with the structural formula H2C=CH-CH2R, where R is the connection to the rest of the molecule. It is made up of a methylene , attached to a vinyl group . The name is derived from the Latin word for garlic, Allium sativum. Theodor Wertheim isolated an allyl...
ic 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 rapidly form cracks and, ultimately, make the rubber article fail.
Vulcanization methods
A variety of methods exist for vulcanization. The economically most important method (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).
Six types of curing systems are in common use. They are:
- Sulfur systems
- Peroxides
- Urethane crosslinkers
- Metallic oxides
- Duece
- Acetoxysilane
Vulcanization with sulfur
By far the most common vulcanizing methods depend on sulfur. Sulfur, by itself, is a slow vulcanizing agent and does 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 oxideZinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber , lubricants, paints, ointments, adhesives, sealants,...
and
stearic acidStearic acid is the saturated fatty acid with an 18 carbon chain and has the IUPAC name octadecanoic acid. It is a waxy solid, and its chemical formula is CH316CO2H. Its name comes from the Greek word στέαρ "stéatos", which means tallow. The salts and esters of stearic acid are called stearates...
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.
Vulcanization of polychloroprene
The vulcanization of
neopreneNeoprene or polychloroprene is a family of synthetic rubbers that are produced by polymerization of chloroprene. Neoprene in general has good chemical stability, and maintains flexibility over a wide temperature range...
or polychloroprene rubber (CR rubber) is carried out using metal oxides (specifically MgO and ZnO, sometimes PbO) rather than sulphur compounds which are presently used with many natural and synthetic rubbers. In addition, because of various processing factors (principally scorch, this being the premature cross-linking of rubbers due to the influence of heat), the choice of
accelerator- In computing :*Download accelerator, or download manager, software dedicated to downloading*Hardware acceleration, the use of dedicated hardware to perform functions faster than a CPU...
is governed by different rules to other diene rubbers. Most conventionally used accelerators are problematic when CR rubbers are cured and the most important
accelerantAccelerants play a major role in chemistry. Most chemical reactions can be hastened with an accelerant. Accelerants alter a chemical bond, speed up a chemical process, or bring organisms back to homeostasis. Accelerants are not necessarily catalysts as they may be consumed by the process...
has been found to be ethylene thiourea (ETU) which, although being an excellent and proven accelerator for polychloroprene, has been classified as reprotoxic. The European rubber industry has started a research project SafeRubber to develop a safer alternative to the use of ETU.
Vulcanization of silicones
"Room-temperature vulcanizing" (RTV)
siliconeSilicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....
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 due to the action of atmospheric humidity, a catalyst and acetoxysilane. Acetoxysilane, when exposed to humid conditions will form
acetic acidAcetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
. 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
prehistoricPrehistory is the span of time before recorded history. Prehistory can refer to the period of human existence before the availability of those written records with which recorded history begins. More broadly, it refers to all the time preceding human existence and the invention of writing...
times. The name "
OlmecThe Olmec were the first major Pre-Columbian civilization in Mexico. They lived in the tropical lowlands of south-central Mexico, in the modern-day states of Veracruz and Tabasco....
" means "rubber people" in the
AztecThe Aztec people were certain ethnic groups of central Mexico, particularly those groups who spoke the Nahuatl language and who dominated large parts of Mesoamerica in the 14th, 15th and 16th centuries, a period referred to as the late post-classic period in Mesoamerican chronology.Aztec is the...
language. Ancient
MesoamericaMesoamerica is a region and culture area in the Americas, extending approximately from central Mexico to Belize, Guatemala, El Salvador, Honduras, Nicaragua, and Costa Rica, within which a number of pre-Columbian societies flourished before the Spanish colonization of the Americas in the 15th and...
ns, spanning from ancient Olmecs to Aztecs, extracted
latexLatex is the stable dispersion of polymer microparticles in an aqueous medium. Latexes may be natural or synthetic.Latex as found in nature is a milky fluid found in 10% of all flowering plants . It is a complex emulsion consisting of proteins, alkaloids, starches, sugars, oils, tannins, resins,...
from
Castilla elasticaCastilla elastica, the Panama Rubber Tree, is a tree native to the tropical areas of Mexico, Central America, and northern South America. It was the principal source of latex among the Mesoamerican peoples in pre-Columbian times...
, a type of rubber tree in the area. The juice of a local vine,
Ipomoea albaIpomoea alba, sometimes called the moonflower or moon vine, is a species of night-blooming morning-glory, native to tropical and subtropical regions of the New World, from northern Argentina north to Mexico and Florida...
, was then mixed with this latex to create processed rubber as early as 1600 BC. In the western world, rubber remained a curiosity, although it was used to produce waterproofed products such as
MackintoshThe Mackintosh or Macintosh is a form of waterproof raincoat, first sold in 1824, made out of rubberised fabric...
rainwear.
Modern Developments
Thomas HancockThomas Hancock , elder brother of inventor Walter Hancock, was an English inventor who founded the British rubber industry...
(1786–1865), a
scientistA scientist in a broad sense is one engaging in a systematic activity to acquire knowledge. In a more restricted sense, a scientist is an individual who uses the scientific method. The person may be an expert in one or more areas of science. This article focuses on the more restricted use of the word...
and
engineerAn engineer is a professional practitioner of engineering, concerned with applying scientific knowledge, mathematics and ingenuity to develop solutions for technical problems. Engineers design materials, structures, machines and systems while considering the limitations imposed by practicality,...
,
invented vulcanization of rubber when he heated a mixture of rubber and sulfur. He patented the process in the UK on November 21, 1843, eight weeks before Goodyear applied for his own UK
patentA patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
.
Charles GoodyearCharles Goodyear was an American inventor who developed a process to vulcanize rubber in 1839 -- a method that he perfected while living and working in Springfield, Massachusetts in 1844, and for which he received patent number 3633 from the United States Patent Office on June 15, 1844Although...
(1800–1860) patented his process in June 15 1844, but claimed that he had discovered vulcanization earlier, in 1839. He wrote the story of the discovery in 1853 in his autobiographical book
Gum-Elastica.
Here is Goodyear's account of the
inventionAn invention is a novel composition, device, or process. An invention may be derived from a pre-existing model or idea, or it could be independently conceived, in which case it may be a radical breakthrough. In addition, there is cultural invention, which is an innovative set of useful social...
, taken from
Gum-Elastica. Although the book is an
autobiographyAn autobiography is a book about the life of a person, written by that person.-Origin of the term:...
, Goodyear chose to write it in the
third personGrammatical person, in linguistics, is deictic reference to a participant in an event; such as the speaker, the addressee, or others. Grammatical person typically defines a language's set of personal pronouns...
, so that "the inventor" and "he" referred to in the text are the author. He describes the scene in a rubber
factoryA factory or manufacturing plant is an industrial building where laborers manufacture goods or supervise machines processing one product into another. Most modern factories have large warehouses or warehouse-like facilities that contain heavy equipment used for assembly line production...
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, 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. 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, MassachusettsWoburn is a city in Middlesex County, Massachusetts, USA. The population was 38,120 at the 2010 census. Woburn is located north of Boston, Massachusetts, and just south of the intersection of I-93 and I-95.- History :...
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
The discovery of the rubber-sulfur reaction revolutionized the use and applications of rubber, and changed the face of the
industrialIndustry refers to the production of an economic good or service within an economy.-Industrial sectors:There are four key industrial economic sectors: the primary sector, largely raw material extraction industries such as mining and farming; the secondary sector, involving refining, construction,...
world.
Formerly, the only way to seal a small gap between moving machine parts was to use
leatherLeather is a durable and flexible material created via the tanning of putrescible animal rawhide and skin, primarily cattlehide. It can be produced through different manufacturing processes, ranging from cottage industry to heavy industry.-Forms:...
soaked in oil. This was acceptable at moderate pressures, but above a certain point, machine designers had to compromise between the extra
frictionFriction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
generated by packing the leather more tightly and greater leakage of steam.
Vulcanized rubber offered the ideal solution. It could be formed to precise shapes and dimensions. It accepts moderate to large deformations under load and recovers quickly to its original dimensions once the load is removed. These, combined with good durability and lack of stickiness, are critical for an effective sealing material. Further experiments in the processing and compounding of rubber by Hancock and his colleagues led to a more reliable process.
In 1905
George OenslagerGeorge Oenslager was a chemist who discovered that a derivative of aniline accelerated the vulcanization of rubber with sulfur.Oenslager received his PhD under Theodore William Richards....
discovered that a derivative of
anilineAniline, phenylamine or aminobenzene is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Being a precursor to many industrial chemicals, its main use is in the manufacture of precursors to polyurethane...
called
thiocarbanilideThiocarbanilide is an organic chemical compound with the formula 2CS. This white solid is a derivative of thiourea. It is prepared by the reaction of aniline and carbon disulfide.-Uses:...
accelerated the action of sulfur to rubber, leading to shorter cure times and
reducing energy consumptionEnergy conservation refers to efforts made to reduce energy consumption. Energy conservation can be achieved through increased efficient energy use, in conjunction with decreased energy consumption and/or reduced consumption from conventional energy sources...
. This breakthrough is almost as fundamental to the development of the rubber industry as Goodyear's sulfur cure. Accelerators made the cure process faster, improved the reliability of the process and 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, chemists have developed other accelerators and ultra-accelerators, and are used to make most modern rubber goods.
Recycling and devulcanization
The market for new raw rubber or equivalent remains enormous, with
North AmericaNorth America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
alone using over 10 billion
poundThe pound or pound-mass is a unit of mass used in the Imperial, United States customary and other systems of measurement...
s (circa 4.5 million
tonThe ton is a unit of measure. It has a long history and has acquired a number of meanings and uses over the years. It is used principally as a unit of weight, and as a unit of volume. It can also be used as a measure of energy, for truck classification, or as a colloquial term.It is derived from...
s) 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
tireA tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...
s 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 UnionThe European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...
,
Eastern EuropeEastern Europe is the eastern part of Europe. The term has widely disparate geopolitical, geographical, cultural and socioeconomic readings, which makes it highly context-dependent and even volatile, and there are "almost as many definitions of Eastern Europe as there are scholars of the region"...
,
Latin AmericaLatin America is a region of the Americas where Romance languages – particularly Spanish and Portuguese, and variably French – are primarily spoken. Latin America has an area of approximately 21,069,500 km² , almost 3.9% of the Earth's surface or 14.1% of its land surface area...
,
JapanJapan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
and the
Middle EastThe Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far 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 shredding. After the
steelSteel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
and reinforcing
fiberFiber is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.They are very important in the biology of both plants and animals, for holding tissues together....
s are removed, and a secondary grinding, the resulting rubber powder is ready for product remanufacture. Until now this inert material could only be used in applications that do not require vulcanization. In the rubber recycling process, devulcanization begins with delinking of the sulfur molecules from the rubber molecules, 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,
oilAn oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....
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.