01 | 2008

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