The Vulcanization of Rubber
The Vulcanization of Rubber
"A Short Description
Vulcanization is the process in which rubber is heated at high temperatures and sulfur is added to give it certain qualities such as strength, elasticity and resistance to solvents as well as moderate heat and cold. The process involves the formation of cross-links between the polymer chains of the rubber's molecules. During vulcanization polymer molecules are linked to other polymer molecules by atomic bridges composed of sulfur atoms or carbon to carbon bonds. The end result is that the springy rubber molecules become cross-linked an extent. This makes the bulk material harder, much more durable and more resistant to chemicals. 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 was invented by Charles Goodyear in 1839 who was showing off his latest concoction of gum-and-sulphur at a general store. When he was ridiculed, he waved his fist in the air and a bit of the substance flew off to land on a hot stove. When Goodyear scraped it off he noticed an elastic rim had formed at the edge of the hardened material.
He had just got a glimpse of vulcanization rubber. He reasoned sulphur and heat were the key, but was unsure about the proportions. Finally, Goodyear came upon the answer: the application of steam to sulphurized rubber for several hours under pressure, at a temperature of about 270F (132C), resulted in weatherproofing.
But he sent samples of his weatherproof rubber to British rubber plants before obtaining a foreign patent, and Thomas Hancock, a man who had been trying to make weatherproof rubber for 20 years saw one of the samples. He reinvented the weather proofing process in 1843 and patented it. So when Goodyear applied for a British patent, he found Hancock had beaten him to it.
Goodyear never made any money out of his invention and though he pawned all his family's possessions to raise money, he died on July 1, 1860 with debts of over $200,000.
Vulcanization is named after Vulcan, Roman god of fire. This name was suggested by a friend of Hancock's. The multi-billion dollar Goodyear Tire and Rubber Co., which is the largest rubber company in the world, is named after Charles Goodyear even though there are no family relations.
Vulcanized rubber serves many purposes including the manufacture of products such as ice hockey pucks, tires, shoe soles, ...
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Goodyear never made any money out of his invention and though he pawned all his family's possessions to raise money, he died on July 1, 1860 with debts of over $200,000.
Vulcanization is named after Vulcan, Roman god of fire. This name was suggested by a friend of Hancock's. The multi-billion dollar Goodyear Tire and Rubber Co., which is the largest rubber company in the world, is named after Charles Goodyear even though there are no family relations.
Vulcanized rubber serves many purposes including the manufacture of products such as ice hockey pucks, tires, shoe soles, hoses and many more.
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"Why rubber is vulcanized
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. The reason why unvulcanized rubber has poor elasticity is because rubber is made of long polymer chains. These polymer chains can move independently relative to each other, and this will result in a change of shape. By the process of vulcanization, cross links 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.
"Methods of Vulcanization
The most important method economically, i.e. the vulcanization of tires, uses increased pressure and temperature. A typical vulcanization temperature for a passenger tire is 10 minutes at 170 degrees C. This type of vulcanization is an example of the general vulcanization method named compression molding. The rubber article is intended to adopt the shape of the mould. Other methods, for instance those used to make door profiles for cars use hot air vulcanization or microwave heated vulcanization.
"About Vulcanization
Vulcanization is considered to be irreversible and its products are similar in properties to other thermosets. Vulcanized rubber compounds are grouped with thermoset materials, which do not melt on heating, unlike thermoplastic materials (polyethylene and polypropylene).
Usually, the actual chemical cross-linking is done with sulfur, but other technologies, including 'peroxide-based systems' are used. The process of vulcanization consists typically of the cure agent itself, (sulfur or peroxide), together with accelerators, like zinc oxide and stearic acid and antidegradants. To prevent vulcanization starting too early, retarding agents are added. Antidegradants are used to prevent degradation by heat, oxygen and ozone.
There are areas which attract sulfur atoms along the rubber molecule. These are called 'cure sites', and are generally places 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 (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 into smaller parts with one to eight sulfur atoms. These small sulfur chains are quite reactive. At each cure site on the rubber molecule, a sulfur chain can attach itself, and then reacts with a cure site of another rubber molecule, forming a bond between two chains. These are known as cross-links. The 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 product. Short sulfur cross links, with just one or two sulfur atoms in the crosslink, give the rubber a very good heat resistance. Cross links 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.
"Later developments
Previously, the only way to seal a small gap between moving machine parts 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 steam. Vulcanized rubber was 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.
George Oenslager discovered that a derivative of aniline called thiocarbanilie was able to accelerate the action of sulfur on the rubber, leading to much shorter cure times and reduced energy consumption. 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
Recycling rubber is difficult because it has to be devulcanized without compromising its desirable properties. Devulcanization involves treating rubber in granular form with heat and/or softening agents to restore its elastic qualities, so the rubber can be reused. Several experimental processes have achieved certain degrees of success in the laboratory, but have been less successful when scaled up to commercial production levels.
Recycling rubber begins with the collection and shredding of discarded tires resulting in a granular material. All the steel and reinforcing fibers are removed and 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.
Devulcanization begins with the delinking of the sulfur molecules from the rubber molecules, facilitating the formation of new cross-links. Two main rubber recycling processes have been developed: the modified oil process and the water-oil process. 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 extremely expensive.
-Elisha George
SR 4E
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