Recycling of Rubbers
Microwave Method
It was proposed to devulcanize waste rubber.
This process applies the heat very quickly and uniformly on the waste rubber. The method employs the application of a controlled amount of microwave energy to devulcanize a sulfur vulcanized elastomers containing polar groups or components, to a state in which it could be compounded and revulcanized to useful products, such as hoses, requiring significant physical properties. On the basis of the relative bond energies ofcarbon-carbon, carbon-sulfur, and sulfur-sulfur bonds, it was presumed that the scission of the sulfur-sulfur and sulfur-carbon crosslinks actually occurred. However, the material to be used in the microwave process must be polar enough to accept energy at a rate sufficient to generate the heat necessary for devulcanization. This method is a batch process and requires expensive equipment.
Thermogravimetry was also employed to study the changes occurring in rubber vulcanizates during devulcanization carried out by microwave treatment . The degree of degradation of the polymer chains in response to microwave treatment was obtained, establishing the conditions of devulcanization in order to obtain the best properties of rubber devulcanizates for reuse in rubber processing.
Microwave technology has been used in inorganic chemistry since the late 1970s. However, its use in organic chemistry started with the first reports to accelerate organic chemical reactions by the groups in 1986. The development of microwave irradiation for organic chemistry was slow in the initial phase due to lack of controllability and reproducibility and a lack of a clear understanding of basic microwave dielectric heating and operational risk . Since the late 1990s, the use of microwave irradiation in organic synthesis has become increasingly popular in pharmaceutical and academic fields, and thousands of articles have been published in few years. By taking advantage of microwave irradiation as an efficient source of energy, compound libraries for lead generation and optimization can be assembled in a fraction of the time required by classical thermal metods. An alternative method for performing microwave-assisted organic reactions, termed “Enhanced Microwave Synthesis,” has also been used, where external cooling is applied to the reaction vessel while simultaneously administering microwave irradiation. This technique keeps the temperature of the reaction low enabling more energy input directly to the reaction mixtures.
Hetrocyclic compounds are extremely important and are ranked high among pharmaceuticals, natural and synthetic materials. The remarkable ability of hetrocyclic nuclei to serve as biometrics and active pharmacophores has largely contributed to their unique value as numerous drugs.
This chapter will deal with the application of microwave irradiation for the synthesis of a variety of heterocycles . For a better understanding, the chapter is organized on the basis of the type of heterocycle, in order of increasing complexity, starting from five-membered heterocycles.
4.2 History of the microwave oven , it was developed for radar during World War II, was put to peaceful uses immediately after the end of the war. Dr Percy Spencer is credited with inventing the microwave oven while employed by Raytheon Co. of Woburn, MA. In the manufacture of radar systems in those days, open air operation of magnetrons at ‘exhaust stations’ (sometimes referred to as aging racks) was common practice. At the exhaust station, a worker would mount a tube in a fixture and connect a vacuum line between the magnetron tube and a high vacuum pump. After a predetermined level of vacuum had been achieved, the tube would be baked out at high temperature for at least one hour to further degas the tube. Finally, the cathode would be slowly energized and voltage applied to the anode until full power was achieved. Dr Spencer observed that a candy bar in his shirt pocket had melted when he was in the vicinity of these magnetrons.
In 1952, Raytheon licensed its oven technology to Tappan. Three years later, Tappan introduced the first home microwave oven. It was a built-in wall unit, about the size of a dishwasher. The magnetron was air cooled, so a plumbed water connection was not needed. However, the cost of $1200–1300 was still an impediment to sales.
By the mid-1950s other manufacturers had also taken an interest in the microwave oven. Manufacturers such as General Electric (GE), Bruder Corporation (later to become a part of Litton Industries) and Philips Company of the Netherlands were soon selling commercial microwave ovens. The competition between companies, directed by feedback from users, pushed the engineers to design ovens with more power and more features but in a smaller size.
The early 1960s saw demonstrations of the first conveyorized microwave oven for foodservice by Philips Company. A working model was exhibited at numerous venues across Europe in 1961–62. The oven was capable of continuously heating 150–200 pre-plated meals per hour from ‒25 °C to 80 °C. Although a technical success, efforts to market the equipment in Europe and the United States were not successful.
In 1961, Sharp Corporation developed Japan’s first microwave oven, the Model R-20. By 1962, the first Japanese-built microwave oven, the Sharp Model R-10 was on the market. Panasonic was also selling commercial microwave ovens in Japan.
Throughout the 1960s manufacturers tweaked and fine-tuned the technology in order to develop the one microwave oven with the broadest appeal. To address the lack of browning, resistance heating elements were added to the top of the oven. Microwave ovens were sometimes combined with conventional ovens in a ‘kitchen center’ approach. GE produced one model that operated at 915 MHz to address the problem of limited microwave penetration at 2450 MHz. Behind the scenes, magnetron manufacturers were feverishly designing smaller, more efficient and more reliable magnetrons.
Sales of commercial microwave ovens climbed in a slow but steady rate until 1966, when Litton Industries introduced the Model 500, a compact countertop microwave oven with 1 kW of power, operating on standard 110 V and priced under $1000. In the same year Sharp Corporation introduced the Model R-600, the first Japanese-built microwave oven with a turntable. By 1970, total sales of microwave ovens in the United States had reached approximately 40 000 units at prices from $300 to $400 .
Development of combination ovens began in the late 1960s. Hot air convection combined with microwave achieved the quality available from a conventional oven with the speed of a microwave oven. However, it was not until the mid-1970s that this type of oven was available for sale. They gained some popularity in foodservice and in the galleys of passenger trains. The first marketing efforts for this type of oven began in England with the Hirst Corporation Articair oven (Andrews, 1989). Litton soon followed in the United States with their Jet-Wave oven.
The 1970s were the hey-day of the consumer microwave oven. By 1971, prices of Japanese-manufactured ovens were as low as $200. By 1978 about 10–12% of US households had microwave ovens. Sales grew steadily through the decade, hitting the 6 million mark in 1983. Today, annual sales are about 13 million ovens. This equates to a market penetration of over 90% (Anonymous, 2007).
Manufacturers introduced many new features in an effort to attract the consumer. The first humidity sensor was invented by P.O. Risman of Sweden and was initially used in Husqvarna ovens. Amana brought microwave ovens into the digital age with the introduction of the Touchmatic™ Radarange. The Touchmatic™ replaced the rotary knobs with a digital touchpad. The consumer could, for the first time, program defrosting, cooking, and reheating with the help of the built-in microprocessor. It was an instant hit, and soon every manufacturer was marketing a touchpad/microprocessor equipped oven.
By the mid-1970s, microwave ovens were becoming a common fixture in American kitchens, with sales of microwave ovens exceeding sales of gas ranges for the first time. Food manufacturers began to take notice of the increased sales and began marketing food specifically for this technology. Also, third party manufacturers were marketing accessories for the microwave oven, such as a pressure cooker, popcorn popper ,wind-up turntable , the microwave griddle , and microwave steamer.
The microwave griddle transforms microwave energy into thermal energy which then browns the food in contact with the griddle. Microwave griddles can be made from steel rods and lossy dielectrics, such as ceramic. While technically not a griddle, ferrite-loaded rubber and plastic have been used as a thermal heating surface on microwave cookware. All of these thermal heating devices act as an additional load to the microwave oven, thus robbing the food of microwave energy and, in most cases, prolonging the cook time.
It was believed in the 1970s that the microwave oven would eventually become the primary cooking appliance in the home. Virtually every manufacturer produced well-made, hardback cookbooks which shipped with their ovens. Food companies and independent writers also produced cookbooks. Eventually oven prices fell to the point where (in the mid-1980s) it was no longer economical to include a book with each oven.
Prices of microwave ovens continued to fall through the 1980s–1990s. Nevertheless, the list of available features continued to grow, leading to those commonly found today. Sensor assisted cooking and specialized, preprogrammed buttons (such as Popcorn and Defrost) are now available on many models. Sensor cooking uses built-in humidity sensors to measure moisture released from the food while cooking. Special cooking algorithms automatically adjust the microwave power and cook time to achieve optimum results.
Over the past decade,microwave technologies has been applied in such varied fields as drying, food processing, organic synthesis, ceramic processing,composite joining, decomposition processing, and waste treatment .The reason was that microwave processing has attracted potential as an alternative to thermal heating because of the inherent advantages of microwave heating which is selective, direct, rapid, internal, and controllable . The heating effect associated with microwaves in chemical reaction is mainly due to dielectric polarization such as dipolar and interfacial polarization, although superheating can also be important at rapid heating .Polar solvents such as water, methanol, DMF, ethyl acetate, acetone, chloroform, acetic acid and dichloromethane are all heated when irradiated with microwaves. Non-polar solvents such as hexane,toulene, diethyl ether, and CCl4 do not couple and therefore do not heat with microwave irradiation. Phthalocyanines have continuously been the subject of research due to their wide application fields, such as in organic pigment, chemical sensor, electro-chromic display devices,photovoltaic cell , xerography,optical disks, catalysis, and nonlinear optics. These versatile features have stimulated attempts on the synthesis of various metal phthalocyanines or new phthalocyanine derivatives with objective of developing new materials which may show improved or more functional characteristics.
The introduction of microwave presents an excellent new option for the synthesis of VOPc from vanadium oxide, dicyanobenzene, and ethylene glycol. In the present study, the effectiveness of synthesizing crude VOPc from vanadium oxide and dicyanobenzene under the two synthetic methods was investigated by comparing reaction temperatures. Also, the preparation of fine crystal VOPc was investigated from the crude VOPc synthesized at optimum condition through the acid-treatment and recrystallization step.
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