advantage of the cascaded process with three reactors in series that the polyethylene
composition can be easily modified by changing the mass fractions as well as
the average molecular mass and the copolymer composition in each reactor as
needed. In this way, the structure of the solid product can be modified to achieve
outstanding final properties for all applications [48–50]. The development of
HD-PE pipe grades is an instructive example of how product development has
been successfully carried out over a period of 60 years. The potential of HD-PE
for this application was realized early. In 1959, Richard et al. reported on the use
of HD-PE as a material for pipes of drinking water lines [57]. The main point to be
clarified experimentally was the life-time under permanent pressure or permanent
stress. The target was a life-time of 50 years at 20
C. It was found that this could
be realized with a permanent pressure not higher than 8 bars. The first tests
were started in 1955 and in 2005 these specimens were still under test, showing
that such pipes can be used for over 50 years without failure by stress cracking
[58, 59]. With the cascaded technology, a new pipe grade was accessible with so-far
unknown properties with respect to the life-time under pressure. This new grade
shows a longer life-time of up to 100 years at a higher hoop stress (comparison
stress at 10 bars). This grade is classified as PE 100 [48]. By introducing the
advanced cascade process this pipe grade could be further upgraded to be applied
for pipes with even larger diameters and thick walls because the sagging problem
could be solved. This grade shows a further favorable behavior in the rapid
crack propagation test for its application in gas distribution systems [58]. Former
polyethylene grades failed at a pressures below 2 bars; the new material can
withstand pressures up to 25 bars. Resistance against slow and rapid crack growth
is caused by the high amount of tie molecules interconnecting the crystalline
lamellae. These tie molecules are further well fixed by having short-chain branches
introduced into the long polymer chains by copolymerization with 1-olefins. The
use and application of HD-PE pipe materials is an interesting example of product
and process development to achieve outstanding qualities through unusual
combinations of properties. A summary of these developments over 50 years can
be found elsewhere [59].
Product development is a very complicated process that comprises scientific,
technical, and commercial aspects as described above. The cascaded polymerization process offers unique possibilities because of the high flexibility to design
the polyethylene structure with respect to average molecular mass, the shape of
the molecular mass distribution, and the comonomer distribution. Although there is
a widespread know-how available on how to design polyethylenes for different
applications, product development is as always based on experimental work.
With a well-designed cascaded pilot plant, product development can be successfully done within a short time because this process offers a broad range of
opportunities for achieving the requirements.
76
L.L. Bo ¨hm
composition can be easily modified by changing the mass fractions as well as
the average molecular mass and the copolymer composition in each reactor as
needed. In this way, the structure of the solid product can be modified to achieve
outstanding final properties for all applications [48–50]. The development of
HD-PE pipe grades is an instructive example of how product development has
been successfully carried out over a period of 60 years. The potential of HD-PE
for this application was realized early. In 1959, Richard et al. reported on the use
of HD-PE as a material for pipes of drinking water lines [57]. The main point to be
clarified experimentally was the life-time under permanent pressure or permanent
stress. The target was a life-time of 50 years at 20
C. It was found that this could
be realized with a permanent pressure not higher than 8 bars. The first tests
were started in 1955 and in 2005 these specimens were still under test, showing
that such pipes can be used for over 50 years without failure by stress cracking
[58, 59]. With the cascaded technology, a new pipe grade was accessible with so-far
unknown properties with respect to the life-time under pressure. This new grade
shows a longer life-time of up to 100 years at a higher hoop stress (comparison
stress at 10 bars). This grade is classified as PE 100 [48]. By introducing the
advanced cascade process this pipe grade could be further upgraded to be applied
for pipes with even larger diameters and thick walls because the sagging problem
could be solved. This grade shows a further favorable behavior in the rapid
crack propagation test for its application in gas distribution systems [58]. Former
polyethylene grades failed at a pressures below 2 bars; the new material can
withstand pressures up to 25 bars. Resistance against slow and rapid crack growth
is caused by the high amount of tie molecules interconnecting the crystalline
lamellae. These tie molecules are further well fixed by having short-chain branches
introduced into the long polymer chains by copolymerization with 1-olefins. The
use and application of HD-PE pipe materials is an interesting example of product
and process development to achieve outstanding qualities through unusual
combinations of properties. A summary of these developments over 50 years can
be found elsewhere [59].
Product development is a very complicated process that comprises scientific,
technical, and commercial aspects as described above. The cascaded polymerization process offers unique possibilities because of the high flexibility to design
the polyethylene structure with respect to average molecular mass, the shape of
the molecular mass distribution, and the comonomer distribution. Although there is
a widespread know-how available on how to design polyethylenes for different
applications, product development is as always based on experimental work.
With a well-designed cascaded pilot plant, product development can be successfully done within a short time because this process offers a broad range of
opportunities for achieving the requirements.
76
L.L. Bo ¨hm