|MadSci Network: Chemistry|
Making plastics directly from carbon can actually turn out to be a very expensive and laborious process. This is because carbon by itself is very stable. One has to go to high temperatures and/or pressure to make it reactive. Once it reacts however a lot of useful compounds can be made from it including plastics (but at a high cost). For example acetylene can be produced from carbon as follows. Coke (which is nothing but carbon) is heated with limestone in an electric furnace to form calcium carbide. CaO + 3C = CaC2 + CO This calcium carbide that is formed reacts with water to form calcium hydroxide and acetylene. There are other ways of producing acetylene from carbon. One such important process is using an electric arc furnace. Now we have synthesized acetylene. How would you convert acetylene to a plastic? You can polymerize it to polyacetylene which can be called a plastic. However even this procedure is difficult. You have to use compounds called initiators to initiate the polymerization process. Some examples of these initiators are triethyl aluminium (Al(C2H5)3) and Ti(O-I- C4H9)4. (see reference 1). Polyacetylene when doped (mixed) with iodine is used as a polymeric semiconductor. As you can see it a complicated and expensive process to convert carbon to a plastic. In addition you have to use additional reagent/chemicals to produce the plastic. The most important step in the above process is the first step wherein you produce acetylene from carbon. Once you produce acetylene you can make a host other compounds from it such as aldehydes, ethylene etc. These compounds can be polymerized to give other plastics such as polyethylene. For example acetylene can be hydrogenated to produce ethylene in the presence of a catalyst such as Platinum, Palladium or Nickel. Ethylene can be polymerized by heating it to above 160-180C and at high pressures of 120-300 megapascals. You can find details of this process in references 2 and 3. This particular plastic is called low density polyethylene. You can also make high density polyethylene and the process is also given in reference 3. During World War I, the synthesis of vinyl acetylene by Julius Nieuwland led to the development in 1932 of the synthetic rubber, neoprene, by DuPont. Its annual output reached 120,000 tons by 1960. In Germany after World War I, butadiene made from acetylene was the basis of a rubber substitute that made the country self-sufficient in rubber. One can thus come a long way with starting from carbon to acetylene to plastics. Before I end one last example. Acetylene can be made to react with water to produce vinyl alcohol which when polymerized yields polyvinyl alcohol. This plastic is one of the substances used to dye the Chicago river green during St. Patrick's. A green dye is dispersed in PVA and released into the river. The plastic itself is harmless and dissolves in the water. Reference: (1) Ito H, Shirakawa H., Ikeda S., J. Polym. Sci. Poly. Chem. Ed., V12, 11, 1974 (2) Doak K. W., Low density polyethylene (High Pressure), pp386-429 in "Encyclopedia of Polymer Science and Engineering", Vol6, 2nd ed., H. F. Mark, N.M. Bikales, C.G. Overberger, G. Menges, Eds., Wiley-Interscience New York, 1986. (3) G.Odian, Principles of Polymerization, pp 303-306, John Wiley and Sons , NY, 1991
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