Tire Rejuvenation:

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Tire Rejuvenation:
- B2 R( f. u; z8 pEfficient Ways to Reuse and Devulcanize Industrial Rubber
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% O9 N. `( o# E+ x5 y9 rThis paper explores the various methods of industrial rubber recycling specifically focusing on the devulcanization of tires. With increased rubber production due to rapid industrialization around the world rubber recycling methods become ever more critical. Current recycling methods are primarily physically based due to the unique structure of the tires. This structure results from a process called vulcanization, the cross-linking of polyisoprene with sulfur at a relatively high temperature. This cross-linking gives tires their unique properties making them ideal for high stress applications. The devulcanization method that this paper is primarily concerned with is ultrasonic devulcanization; the most promising devulcanization method to date. The unique problems that arise in the devulcanization process are also addressed in this paper. Nevertheless it is crucial we explore new methods of rubber recycling that is both economically and ecologically viable for the benefit of all mankind.

2 d6 C% m# w2 Q        There are more than 671,000,000 vehicles on our planet today (Elert).  In addition to emitting numerous pollutants into the atmosphere, these vehicles present another immediate problem to the environment: their tires.  All 671,000,000 vehicles go through an average of three sets of tires in their lifetime, equating to 8,052,000,000 tires in total (Elert).  Modern rubber production reached 21.1 million metric tons in 2006; a number expected to climb in the forthcoming years (Freedonia Group Inc).   With the advent of industrial development in India and China the need to find an effective and efficient way to dispose of these excess tires grows ever more critical.  Current recycling methods include both physical and chemical processes, but both are costly and time-consuming to complete.  This paper will explore the tire recycling processes in use today, as well as revolutionary technologies now being developed to effectively meet the world’s insatiable crave for rubber.
, u( R$ O$ v% X& P( m6 ^        The history of the tire began when Charles Goodyear discovered vulcanization in 1939; a method which cross-links the polymer polyisoprene, at high temperatures, with sulfur (Vulcanization).   
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" J( c  R5 R8 f6 WIt is this process that gives tires and other industrial rubbers their unique chemical and physical characteristics, making them ideal for high stress applications.  This process which served as the catalyst to the industrial revolution carries a steep price: it is extremely difficult to break the sulfur cross-links in the rubber. As a result, the recycling process for used tires is made that much harder.  Unlike other polymers which can be easily separated into their original forms, vulcanized rubber is extremely resistant to the breaking of its sulfur cross-links.  Consequently, current rubber recycling processes are physical recycling methods rather than chemical methods.  As the world rubber consumption exponentially increases to meet world demand, the need to develop more efficient physical and chemical recycling methods becomes increasingly important in the face of limited natural resources.  
        The advent of industrial rubber production raised the question of what to do with the scrap rubber.  For many years, tires were stockpiled in dumps or abandoned across the United States (Department of Environmental Protection).  This easy way to dispose of scrap tires presented even more problems than it avoided.  Many of the tires were not properly stored. As a result, the improperly stored tires containing substantial quantities of oil, ignited and produced horrible fires. This problem arose primarily in 1960 with the advent of synthetic rubber production (Tire Recycling).  These fires emitted numerous pollutants into the atmosphere including carbon monoxide and sulfur dioxide gas, which proved detrimental to the environment (Tire Fire).  A more efficient and effective method was needed to recycle scrap tires, due to new innovations in tire production.  It was not until the 1990’s that a truly efficient means of scrap tire recycling began to take hold in the United States ("Recycling Tires").  This process converted the scrap tires into crumb rubber, a material composed of crumbs of used tires.  The process is a simple one that involving relatively low production costs and carries numerous benefits.  The process begins by grinding up the tires into large chunks, about two inches across.  These chunks are fed through a series of grinders, which reduce the tires into crumbs measuring 3/8 of an inch in diameter ("Recycling Tires").  As a result of grinding up the scrap tires, the steel belts, which maintain the tire’s integrity, are weakened and can be separated from the mixture with a large magnet ("Recycling Tires").  The final product is a mixture rubber crumbs which can be reused in playground mats, construction products, and rubber modified asphalt.  In the year 2001, 996 million pounds of crumb rubber were processed in the United States and Canada alone ("Recycling Tires").  Crumb rubber recycling is the primary means of recycling used today, but the crumb has limited applications. It can only be used in products that do not require unvulcanized, or virgin. rubber.  The technology to extract the original rubber from tires exists currently but its effectiveness is being questioned by many of today’s leading scientists.
4 Z" a; U7 y6 c( k. r        Devulcanization is a process in which the sulfur cross-links of industrial grade rubber are broken rendering the original rubber compound as a byproduct.  This process, seemingly simple in theory, has proven illusive to scientists.  Numerous problems arise with the devulcanization process; the primary of which is the methods that are used to break the sulfur cross-links in the rubber compounds.  The purpose of the devulcanization process is to restore the virgin rubber to its original state. Devulcanized reclaimed rubber can be used in nearly all applications that require virgin rubber; a stark contrast to the physical recycling methods of vulcanized rubber that has limited recyclable applications.  The current methods used to devulcanize rubber are either extremely expensive or compromise the integrity of the final rubber product.  Nevertheless the quest to devulcanize rubber effectively remains one of the primary goals of the tire recycling industry.
" {6 Z$ o6 F9 T" P8 A& BOne would think that such an important process as rubber devulcanization would be an established industrial process in today's modern society, but the concept of rubber devulcanization is relatively new.  Although no economically viable method has been developed to devulcanize rubber, there is one method that is particularly interesting to scientists around the world: ultrasound devulcanization technologies.  Ultrasound devulcanization processes, unlike other devulcanization methods, promise to deliver the best results with minimal material or production costs.  The idea of ultrasonic devulcanization was discussed first by Okuda and Hatano in 1987 (CalRecovery).  Their idea was to expose crumb rubber to ultrasonic waves of 50 kHz, under standard conditions, for approximately 20 minutes (CalRecovery).  During the time of exposure the C-S and S-S bonds decomposed while the C-C bonds still maintained their integrity.  Okuda and Hanto found the resulting product closely resembled virgin rubber with only minor structural alterations, a breakthrough compared to other, more primitive, devulcanization methods.  Further research yielded new breakthroughs in the quest for a practical ultrasonic devulcanization technology.  Scientists found that exposing the crumb rubber to higher ultrasonic radiation combined with higher pressures and temperatures quickly cleaved the sulfur cross-links holding the rubber together (CalRecovery).  This new process greatly reduced the devulcanization time and did not require the use of harmful catalysts or chemicals.  The devulcanized rubber produced from this new method exhibits softer properties than the original virgin rubber from which it was synthesized.  This allows for the rubber to be easily formed in molds and utilized in various applications where virgin rubber is needed.  These tests are very promising to scientists and environmentalists alike but their use in the real world has limitations especially in the area of tire recycling.
% d1 W: T# A. C: M# Y; ~Currently many factors act as inhibitors to the ultrasonic devulcanization methods application in commercial processes.  One of these factors is distributing ultrasonic energy evenly across the surface of the rubber.  The rubber in tires is essentially composed of three bonds; C-C (347 kJ/mol), C-S (301 kJ/mole), and S-S (271.7 kJ/mole) (Kleps).  In order for devulcanization to occur nearly all of the C-S and S-S bonds need to be broken while, ideally, all of the C-C bonds remain.  The problem is a 46 kJ difference between the energies of the C-C bonds and the C-S bonds.  This leaves an extremely small energy window before the integrity of the C-C bonds is compromised and the polymer is depolymerized.  Further tests on rubber devulcanized ultrasonically show exactly this trend.  The C-C bonds in the rubber are partially degraded and the resulting rubber produced after the devulcanization process loses many of it previous properties (Kleps).  Because the rubber does not possess its original properties, its real world applications are limited and the devulcanization process becomes impractical.  Another problem scientists face when trying to devulcanize road tires is the actual composition of the tire.  The tires on today’s vehicles are a compilation of modern rubber technology.  Almost every part of the tire is composed of different types of rubber, and tire manufacturers tires, use different blends depending on their tires applications.  The inconsistency in tire composition complicates the devulcanization process to the point where it no longer becomes feasible to extract the virgin rubber while still maintaining its integrity.  Ultrasonic technology and tire composition currently inhibit commercial devulcanization of automobile tires.  Further research in needed, be the speed with which the research is pursued depends significantly on the world’s demand for recycled rubber.  Given the planets limited natural resources, the quest to find an efficient way to devulcanize industrial rubber will continue.
Devulcanization is a recycling process in its infancy it would be possible for an up-and-coming scientists may develop innovative devulcanization methods using natural energy sources.  Preliminary studies of devulcanization have revealed a common trend among the methods used to cleave the C-S and S-S bonds; they employ relatively weak radiation sources (from either ultrasound or microwaves).  Because relatively weak radiation sources are needed to devulcanize it might be possible to use a natural energy source, such as ultraviolet radiation.  To further optimize the devulcanization process the reaction vessel could be evacuated and filled with pressurized hydrogen gas.  Theoretically once the C-S and S-S bonds are broken the sulfur atoms will react with the hydrogen gas producing sulfur dioxide.  The resulting polymer, if its properties are desirable, can be reused or resold for various applications.  Alternatively, the resulting polymer could be burned as an energy source if all the sulfur is completely removed from the vulcanized rubber.  The advantage to this process, if it works, is the energy source is completely natural and will never run out.  Natural radiation sources, although not used currently in devulcanization methods, could be the new revolutionary technology used to recycle vulcanized materials.  
/ ]) Z4 L- _# f7 V: d6 v; HClearly, the world’s industrial rubber production is increasing at an exponential rate.  As newly industrialized nations such as China and India begin to demand more vehicles, the amount of vulcanized rubber produced will reach new highs.  The recycling methods we use today only compensate for a fraction of the total annual output of rubber.  As more rubber floods the market and foreign oil sources become more scarce, the need to devulcanized industrial rubber becomes ever more crucial.  The devulcanization methods being developed today may work in small scale lab experiments but their practical application is very limited.  Daunting obstacles stand in the way of industrial rubber recycling but scientists and citizens alike must continue to pursue the goal of completely efficient rubber devulcanization.