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Basics Polyethylene

Basics Polyethylene History and outlook Rigid toy made of polyethylene Polyethylene is a plastic material that has been known for more than 100 years. It is found in millions of applications from simple film, through containers, to toys or technical components such as plastic fuel tanks for cars. Polyethylene was discovered by the chemist Hans von Pechmann in 1898. In 1933 polyethylene was successfully produced, at a pressure of 1400 bar and a temperature of 170 °C, at the ICI laboratories. For a large scale industrial process these conditions were, however, difficult to produce and were highly energy intensive. In 1953 polymer chemistry saw a major breakthrough. The chemists Karl Ziegler and Giulio Natta succeeded in synthesising polyethylene from ethene (also called ethylene) at normal pressure using catalysts. The establishment of this process led to the introduction of large scale polyethylene synthesis and the use of polyethylene as a mass market material. In 1963 they were jointly awarded the Nobel Prize for Chemistry in recognition of this achievement. Polyethylene has been used industrially in huge quantities since 1953, principally for gas and water pipelines, cable insulation and as a packaging material, such as shrink packaging film. Polyethylene and polypropylene opened up the age of plastics. Polyethylene today is the most widely used plastic material in the world, with about a 30% market share. Karl Ziegler was born in 1898 and studied chemistry in Marburg. He graduated in 1923. Major stages in his academic career were at the Universities of Frankfurt, Heidelberg, Halle and Chicago. From 1943 he was head of the Kaiser Wilhelm Institute for Coal Research (today the Max Planck Institute in Mülheim) where he devoted his energies to research into the combination of organic compounds with metals. From 1948 to 1969 Ziegler taught, as an honorary professor, at the RWTH technical college in Aachen. (Photo: dpa) The basis of the polyethylene polymer chain is ethene (ethylene), which is a highly flammable gas. The synthesis of ethene was originally carried out by the dehydrogenation of pure alcohol (ethanol). Today‘s technically relevant processes are the cracking of natural gas and higher hydrocarbons. These technologies are based on fossil resources whose availability is limited and which are subject to major price fluctuations. According to estimates there is enough crude petroleum to last for another 40 years at current demand rates. This shows a clear need for the development of polyethylene based on renewable resources. With the introduction of the Kyoto Protocol on February 6th 2005 the industrial nations committed themselves to a reduction of greenhouse gases and the avoidance of carbon dioxide emissions. The protocol also envisages the scavenging and conversion of carbon dioxide by green vegetation. Ethanol (pure alcohol) is seen, in the search for alternative sources for the synthesis of ethene, as a possibility based on regenerative bio-mass. The production of „bio-ethanol“ from renewable resources is achieved by the enzymatic conversion of starch and cellulose. For years bio-ethanol has been used as a biogenous fuel for cars. It therefore seems logical that to use bioethanol as the basis for synthesising polyethylene by the polymerisation of bio-ethene. 26 bioplastics MAGAZINE [01/08] Vol. 3

Basics and Bio-Polyethylene by Dr Thomas Isenburg The current annual production level of bio-ethanol is some 35 to 40 million tonnes. The basis for the synthesis is sugar cane, maize starch, wheat starch and sugar beet. By catalytic extraction of water bio-ethene can be obtained from bio-ethanol. At the moment the majority of the ethanol so produced is used as motor fuel. It is however theoretically possible to produce 20 % of the world demand for ethylene using the process described above. During the 1980s the French chemicals company Rhodia set up and operated a plant for the production of ethene from ethanol in Sao Paulo, Brazil. After the withdrawal of the government bio-ethanol subsidy, and the low petroleum prices that the world was enjoying at that time, the plant was closed down. During this period there was a good deal of work done on the development of a catalyst; work which could be used today as the basis of further research. In Brazil ethanol is currently sold at about 330 to 350 US Dollars per tonne. This leads to ethene production costs in the order of 700 Dollars per tonne. The price of ethene obtained from fossil resources fluctuates enormously. In 2003, when crude oil was 28 Dollars a barrel, the price of ethene was between 500 and 600 Dollars per tonne. By 2005 (with crude oil at 54 Dollars a barrel) the price of ethene had rapidly grown to over 900 Dollars per tonne. Today, with crude oil at 90 Dollars a barrel, the price of ethene is over 1100 Dollars per tonne. Brazil, as one of the world‘s major sugar producers, has a considerable interest in producing bio-ethylene via the synthesis of sugar-based bio-ethanol. The first plants are in the planning stage but none is so far in operation. The Brazilian ethanol price is something of a special case which is related not so much to the particularly attractive conditions for purchasing cane sugar, but more to general production cost levels in that country. In Europe and the USA the production costs for bio-ethanol are about double those in Brazil. This effectively means that bio-ethanol will only be competitive when crude oil reaches 120 Dollars a barrel. Ethanol can be transported by sea. Ethylene is highly reactive (a flammable, explosive gas) and can only be transported via a pipeline. Companies in Brazil can therefore use their competitive advantage mainly at the polymer level, and for products made from the polymer. Because Europe is a leading chemical industry location, with a high level of exports of downstream products, it is nevertheless not unreasonable to consider producing bio-ethylene in Europe despite the generally higher costs. If the carbon dioxide problem is also included in the equation ethylene from renewable resources offers an added bonus. Giulio Natta was born in 1903 in Imperia, Italy, and from 1933 to 1935 was professor of chemistry at the University of Pavia. From 1936 to 1938 he was director of the Institute for Industrial Chemistry at the Turin Polytechnic and from 1938 was director of the Institute for Industrial Chemistry at the Milan Polytechnic. (Photo: dpa) Packaging applications made of polyethylene bioplastics MAGAZINE [01/08] Vol. 3 27

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