Vulcanization (or Vulcanisation) refers to a specificcuring process of rubber involving highheat and the addition of sulfur or otherequivalent curatives. It is a chemicalprocess in which polymer molecules arelinked to other polymer molecules by atomic bridges composed ofsulfur atoms or carbon to carbon bonds. The end result is that thespringy rubber molecules become cross-linked to agreater or lesser extent. This makes the bulk material harder, muchmore durable and also more resistant to chemical attack. It alsomakes the surface of the material smoother and prevents it fromsticking to metal or plastic chemical catalysts.
This heavily cross-linked polymer has strong covalent bonds,with strong forces between the chains, and is therefore aninsoluble and infusible, thermosettingpolymer.
The process is named after Vulcan, Roman god of fire.
A vast array of products are made with vulcanized rubberincluding ice hockey pucks, tires, shoe soles, hoses and manymore.
[edit] Reason forvulcanizing
Uncured natural rubber issticky and can easily deform when warm, and is brittle when cold.In this state it cannot be used to make articles with a good levelof elasticity.The reason for inelastic deformation of unvulcanized rubber can befound in the chemical nature: rubber is made of long polymerchains. These polymer chains can move independently relative toeach other, and this will result in a change of shape. By theprocess of vulcanization crosslinks are formed between the polymerchains, so the chains cannot move independently anymore. As aresult, when stress is applied the vulcanized rubber will deform,but upon release of the stress, the rubber article will go back toits original shape.
[edit] Description
Vulcanisation is generally considered to be an irreversibleprocess (seebelow), similar to other thermosets and must be contrastedstrongly with thermoplasticprocesses (the melt-freeze process) which characterize the behaviorof most modern polymers. Thisirreversible cure reaction defines cured rubber compounds asthermoset materials,which do not melt on heating, and places them outside the class ofthermoplasticmaterials (like polyethylene andpolypropylene).This is a fundamental difference between rubbers andthermoplastics, and sets the conditions for their applications inthe real world, their costs, and the economics of their supply and demand.
Usually, the actual chemical cross-linking isdone with sulfur, but there areother technologies, including peroxide-basedsystems. The combined cure package in a typical rubber compound comprisesthe 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 anumber of sites which are attractive to sulfur atoms. These arecalled cure sites, and are generally sites with an unsaturatedcarbon-carbon bond, like in polyisoprene, the basic material ofnatural rubber,and in styrene-butadienerubber (SBR), the basic material for passenger tires. The active sitesare allylic hydrogen atoms; that means they are hydrogen atomsconnected to the first saturated carbon atom connected to thecarbon-carbon double bond. During vulcanization the eight-memberedring of sulfur breaks down in smaller parts with one to eightsulfur atoms. These small sulfur chains are quite reactive. At eachcure site on the rubber molecule, such short sulfur chain canattach itself, and eventually reacts with a cure site of anotherrubber molecule, and so forming a bond between two chains. This isnamed a cross-link. These sulfur bridges are typically between twoand eight atoms long. The number of sulfur atoms in a sulfurcrosslink has a strong influence on the physical properties of thefinal rubber article. Short sulfur crosslinks, with just one or twosulfur atoms in the crosslink, give the rubber a very good heatresistance. Crosslinks with higher number of sulfur atoms, up tosix or seven, give the rubber very good dynamic properties but withlesser heat resistance. Dynamic properties are important forflexing movements of the rubber article, e.g., the movement of aside-wall of a running tire. Without good flexing properties thesemovements will rapidly lead to formation of cracks and, ultimately,to failure of the rubber article.
[edit] Vulcanizationmethods
There are various vulcanization methods. The economically mostimportant method i.e the vulcanization of tires uses increasedpressure and temperature. A typical vulcanization temperature for apassenger tire is 10 minutes at 170 degrees C. This type ofvulcanization is an example of the general vulcanization methodnamed compression molding. The rubber article is intended to adoptthe shape of the mold. Other methods for instance those used tomake door profiles for cars use hot air vulcanization or microwaveheated vulcanization (both continuous processes).
[edit] Overview andhistory
Although vulcanization is a 19th century invention, the historyof rubber cured by other means goes back to prehistorictimes. The name "Olmec" means "rubberpeople" in the Aztec language. AncientMesoamericans,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 in1770, when Edward Nairne was selling cubes of natural rubber fromhis shop at 20 Cornhill,London. The cubes, meant to be erasers, sold for theastonishingly high price of 3 shillings per half-inchcube.
In the early 19th century rubber was a novelty material, but itdid not find much application in the industrial world. It was usedfirst as erasers, and then as medical devices for connecting tubesand for inhaling medicinal gases. With thediscovery 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 volumesand the material did not last long. The reason for this lack ofserious applications was the fact that the material was notdurable, was sticky and often rotted and smelled bad because itremained in its uncured state.
[edit] Goodyear'scontribution
Most textbooks point out that Charles Goodyear (1800–1860) invented vulcanisation 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 insisted that it was the latter, though many contemporaneous accounts indicate the former.
Goodyear claimed that he discovered vulcanization in 1839, butdid not patent the invention until June 15, 1844, and did not writethe story of the discovery until 1853 in his autobiographical bookGum-Elastica. Meanwhile, ThomasHancock (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 fromGum-Elastica. Although the book is an autobiography,Goodyear chose to write it in the thirdperson, so that 'the inventor' and 'he' referred to in the textare in fact the author. He describes the scene in a rubber factory where hisbrother worked:
... The inventor made some experiments to ascertain the effectof heat on the same compound that had decomposed in the mail-bagsand other articles. He was surprised to find that the specimen,being carelessly brought into contact with a hot stove, charredlike leather.
Goodyear goes on to describe how he attempted to call theattention of his brother and other workers in the plant who werefamiliar with the behavior of dissolved rubber, but they dismissedhis appeal as unworthy of their attention, believing it to be oneof the many appeals he made to them on account of some strangeexperiment. Goodyear claims he tried to tell them thatdissolved rubber usually melted when heated excessively, but theystill ignored him.
He directly inferred that if the process of charring could bestopped at the right point, it might divest the gum of its nativeadhesiveness throughout, which would make it better than the nativegum. Upon further trial with heat, he was further convinced of thecorrectness of this inference, by finding that the India rubbercould not be melted in boiling sulfur at any heat ever so great,but always charred. He made another trial of heating a similarfabric before an open fire. The same effect, that of charring thegum, followed; but there were further and very satisfactoryindications of success in producing the desired result, as upon theedge of the charred portion appeared a line or border, that was notcharred, but perfectly cured.
Goodyear then goes on to describe how he moved to Woburn,Massachusetts and carried out a series of systematicexperiments to discover the right conditions for curing rubber.
... On ascertaining to a certainty that he had found the objectof his search and much more, and that the new substance was proofagainst cold and the solvent of the native gum, he felt himselfamply repaid for the past, and quite indifferent to the trials ofthe future.
Goodyear never made any money out of his invention. He pawnedall his family's possessions in an effort to raise money, but onJuly 1, 1860, he died with debts of over $200,000.
[edit] Laterdevelopments
Whatever the true history, the discovery of the rubber-sulfurreaction revolutionized the use and applications of rubber, andchanged the face of the industrial world.
Up to that time, the only way to seal a small gap between movingmachine parts, such as between a piston and its cylinder in a steamengine, was to use leather soaked in oil.This was acceptable up to moderate pressures, but above a certainpoint, machine designers had to compromise between the extrafriction generated bypacking the leather more tightly and greater leakage of precioussteam.
Vulcanized rubber offered the ideal solution. Withvulcanized rubber, engineers had a material which could be shapedand formed to precise shapes and dimensions, and which would acceptmoderate to large deformations under load and recover quickly toits original dimensions once the load was removed. These, combinedwith good durability and lack of stickiness, are the criticalrequirements for an effective sealing material.
Further experiments in the processing and compounding of rubberwere 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 anilinecalled thiocarbanilidewas able to accelerate the actionof sulfur on the rubber, leading to much shorter cure times andreduced energyconsumption. This work, though much less well-known, is almostas fundamental to the development of the rubber industry as thatof Goodyear in discovering the sulfur cure. Acceleratorsmade the cure process much more reliable and more repeatable. Oneyear after his discovery, Oenslager had found hundreds of potentialapplications for his additive.
Thus, the science of accelerators and retarders was born. Anacceleratorspeeds up the cure reaction, while a retarder delays it. In thesubsequent century, various chemists have developed otheraccelerators, and so-called ultra-accelerators, that make thereaction very fast, and are used to make most modern rubber goods.
[edit] Devulcanization
The rubber industry has been researching the devulcanization ofrubber for many years. The main difficulty in recycling rubber hasbeen devulcanizing the rubber without compromising its desirableproperties. The process of devulcanization involves treating rubberin granular form with heat and/or softening agents in order torestore its elastic qualities, in order to enable the rubber to bereused. Several experimental processes have achieved varyingdegrees of success in the laboratory, but have been less successfulwhen scaled up to commercial production levels. Also, differentprocesses result in different levels of devulcanization: forexample, the use of a very fine granulate and a process thatproduces surface devulcanization will yield a product with some ofthe desired qualities of unrecycled rubber.
The rubber recycling process begins with the collection andshredding of discarded tires. This reduces therubber 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 thedelinking of the sulfur molecules from the rubber molecules,thereby facilitating the formation of new cross-linkages. Two mainrubber recycling processes have been developed: the modified oilprocess and the water-oil process. With each of theseprocesses, oil and a reclaiming agentare added to the reclaimed rubber powder, which is subjected tohigh temperature and pressure for a long period (5-12 hours) inspecial equipment and also requires extensive mechanicalpost-processing. The reclaimed rubber from these processes hasaltered properties and is unsuitable for use in many products,including tires. Typically, these various devulcanization processeshave failed to result in significant devulcanization, have failedto achieve consistent quality, or have been prohibitivelyexpensive.
In the mid-1990s, researchers at the Guangzhou Research Institute for the Utilization of ReusableResources in China patented a methodfor the reclamation and devulcanizing of recycled rubber. Theirtechnology, known as the AMR Process, is claimed to producea new polymer with consistent properties that are close to those ofnatural and synthetic rubber, and at a significantly lowerpotential 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 Americanlicense for the AMR Process, has built a rubber reprocessingplant and research/quality control lab in Columbus, Ohio. The plantperforms production runs on a demonstration basis or at smallcommercial levels. The recycled rubber from the Ohio plant iscurrently being tested by an independent lab to establish itsphysical and chemical properties.
Whether or not the AMR Process succeeds, the market fornew raw rubber or equivalent remains enormous, with North Americaalone using over 10 billionpounds (circa 4.5million tons) every year. The autoindustry consumes approximately 79% of new rubber and 57% ofsynthetic rubber. To date, recycled rubber has not been used as areplacement for new or synthetic rubber in significant quantities,largely because the desired properties have not been achieved. Usedtiresare the most visible of the waste products made from rubber; it isestimated that North America alone generates approximately 300million waste tires annually, with over half being added tostockpiles that are already huge. It is estimated that less than10% 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.
Recently a new method of devulcanization was developed by CoralGROUP, in Dnepropetrovsk,Ukraine. This method ofdevulcanization, includes impregnation of rubber with specialsolvent with additives of catalysts and reagents. In this processrubber is restructured, sulfuric "bridges are torn up, sulfurchemically connects, and rubber becomes plastic, suitable formolding. All that remains is to add 2-4% of sulfur, and new rubber products can be made. The quality of the obtained rubber compound is not worse than obtained from the initial materials, i.e. it is completely possible to make new automobile tires or other rubber products from the devulcanized rubber.
[edit] References
- ^D Hosler, SL Burkett and MJTarkanian (1999). "Prehistoric Polymers: Rubber Processing inAncient Mesoamerica". Science 284: 1988–1991.doi:10.1126/science.284.5422.1988