Production methods summary
The first poly(ethene) to be manufactured was low density (LDPE). Its discovery dates back to 1933. Although it was used in the development of airborne radar during World War II, consumer products only became available in 1948, when it was used for domestic items like washing-up bowls. Other types of poly(ethene) appeared in the 1960s, with the use of new Ziegler-Natta catalysts, and recent years have seen more sophisticated control of properties using another class of catalyst, the metallocenes.
The process used for Low Density Poly(Ethene) (LDPE) initially involved very high pressures, between 1000 and 3000 atmospheres, and temperatures up to 350°C. Plants will now typically use temperatures of 200°C and pressures of 2000 atmospheres. Under these conditions, a small amount of oxygen is sufficient to initiate the reaction, though organic peroxides may also be used.
The resulting polymer is highly branched, so consequently the chains do not pack closely and the density is low.
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Work by Karl Ziegler and Giulio Natta with organometallic materials demonstrated that poly(ethene) could be produced at much lower temperatures and pressures using a combination of a titanium catalysts and an aluminium co-catalyst.
The resulting polymer has very little branching compared with LDPE, which alters the properties of the polymer, including its density. It is therefore known as High Density Poly(Ethene) (HDPE)
Information on the development of Ziegler-Natta catalysts can be found on the catalysis site.
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Ziegler and Natta received the 1963 Nobel Prize in chemistry for their work
When using Ziegler catalysts, the degree of branching can be adjusted by the addition of other alkene monomers, giving control over the properties of the polymer. This enables the production of a polymer consisting of linear chains with short branches, called Linear Low Density Poly(Ethene) (LLDPE). This structure gives the material much better resilience, tear strength and flexibility without the use of plasticisers.
LLDPE is also generally produced under mild conditions, as low as 1 atmosphere pressure and no more than 100 °C. This reduces the energy requirements of the process, and makes the process safer.
Find out more about LLDPE here
For more information about metallocene catalysts, see the Catalysis site