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Ethylene Polymers, Materiały studia, materiały polimerowe
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ETHYLENE POLYMERS, CHLOROSULFONATED
Vol. 2
ETHYLENE POLYMERS, HDPE
Introduction
Polyethylene (PE) is the most widely used plastic throughout the world, and high
density PE (HDPE) is the most widely used type of PE. HDPE has generally been
taken to mean the product of ethylene polymerization having density greater than
about 0.935 (or 0.94). It includes ethylene homopolymers and also copolymers
of ethylene and alpha-olefins such as 1-butene, 1-hexene, 1-octene, or 4-methyl-
1-pentene. Other types of PE include low density PE (LDPE), made through a
free-radical process, and linear low density PE (LLDPE).
An analysis of commercial usage of the world’s major plastic types is shown
in Table 1. In terms of amount consumed, PE dominates the other types. Table 2
lists the U.S. usage of the various types of PE. In comparison to the other types of
PE, HDPE is by far the most versatile. HDPE consumption in the United States
accounted for about 6.95 billion kilograms per year in 1999, or almost half of total
U.S. production of all PE types. Figure 1 shows how the demand for HDPE in the
United States has developed historically since its invention in 1951.
History of PE
Early Work.
The history of HDPE (and polyolefins in general) actually be-
gan in the 1890s with the synthesis of “polymethylene” from the decomposition of
diazomethane. Between 1897 and 1938, numerous reports of such polymers ap-
peared in the literature (1–6). Catalysts such as unglazed china, amorphous boron,
and boric acid esters were used for the decomposition. The empirical formula of
such products was found to be CH
2
. Later reproductions of work (3,6) established
Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.
Vol. 2
ETHYLENE POLYMERS, HDPE
383
Table 1. Worldwide Usage of the Most Common Plastics Types
Polymer type
Share of world usage, %
PE (LDPE, EVA, LLDPE, and HDPE) 40.3
Polypropylene 21.9
PVC 21.1
Polystyrene 11.6
ABS/SAN 5.1
Total 100
a
Data from Digest of Polymer Developments, Series I, Number 95, STR
Publishing, Enfield, Conn., May, 2000.
Table 2. Polyolefins Usage in the United States in 1999
Consumption in
U.S. polyolefin
U.S. Polyolefin
Polymers
United States, 10
6
t consumption, % consumption, %
Total PE
14.7
67.8
100
LDPE (including EVA)
3.6
16.6
24.5
LLDPE
4.1
19.1
28.2
HDPE
7.0
32.1
47.4
Total Polypropylene 7.0 32.3
Total Polyolefins 21.7 100
a
Data from Digest of Polymer Developments, Series I, Number 95, STR Publishing, Enfield, Conn.,
May, 2000.
8
7
6
5
4
3
2
1
0
1950
1960
1970
1980
1990
2000
Year
Fig. 1.
HDPE consumption in the United States.
that the polymethylene thus obtained was a high molecular weight linear polymer
having a melting point of 134–137
◦
C and a density of 0.964–0.970 g/cm
3
(7). Thus,
it can be stated that although no commercial use was initially made of it, HDPE
was discovered long before the well-known LDPE was introduced (8,9).
In another early approach Pichler (10) and Pichler and Buffleb (11) described
during 1938–1940 the preparation of high molecular weight (
M
n
=
23,000) poly-
mers from the hydrogenation of carbon monoxide over ruthenium and cobalt
384
ETHYLENE POLYMERS, HDPE
Vol. 2
catalysts. The reported melting point of 132–134
◦
C and density of 0.980 g/cm
3
indicate again that linear HDPE was obtained.
Free-Radical Process.
The first commercial PE was developed by Im-
perial Chemical Industries from 1932 to 1938 using a free-radical process (12).
Ethylene was polymerized at high pressure (142 MPa or 1400 atm) and at about
180
◦
C. It was discovered by accident that oxygen impurity could serve as the ini-
tiator. An ICI British patent filed in 1936 (13) disclosed pressures of ca 50–300
MPa (500–3000 atm), temperatures of 100–300
◦
C, the necessity of removing heat
to control temperature, and the necessity of controlling the oxygen content of the
ethylene used.
The PE produced at this time had a melting point of 115
◦
C and a density of
0.91–0.92 g/cm
3
. In 1940, Fox and Martin (14) found by infrared analysis that there
were more methyl groups in the polymer than could be accounted for by the end
groups of a linear chain. Thus the importance of chain branching was recognized,
and subsequent studies of branching led to a better understanding of its effect
on mechanical properties and polymer morphology. This material is called low
density PE (LDPE) and is still in high commercial demand today. Further work
on the free-radical process extended the pressure. Larcher and Pease (15) disclosed
PE with densities of 0.95–0.97 g/cm
3
, melting points above 127
◦
C, and branching
of less than one side chain per 200 carbon atoms.
Transition-Metal Catalysts.
Today’s “low pressure” catalytic processes,
from which HDPE now comes, were discovered in the early 1950s. The term “low
pressure” refers to operating pressures of generally 1.4–6.9 MPa (200–1000 psig),
in contrast to the ICI free-radical process. Patent applications were filed by Stan-
dard Oil of Indiana (16), in 1951, by Phillips Petroleum (17) in early 1953, and by
Ziegler and co-workers (18) in late 1953.
The Standard Oil patent (16) describes a supported reduced molybdenum
oxide or cobalt molybdate on alumina, with the ethylene preferably contacting
the catalyst in an aromatic solvent to affect the polymerization. Operating tem-
peratures of 100–270
◦
C were disclosed and the molecular weight could be varied
from very high to low such as those of greases.
At about the same time (1951) it was discovered that supported chromium
oxide catalysts would also polymerize ethylene at low pressures to produce high
molecular weight polymers (17). Reaction temperatures were in the range of
60–190
◦
C. Polymer characteristics, particularly, molecular weight and molecu-
lar weight distribution, could be varied by reactor temperature, pressure, and
activation temperature of the “Phillips catalyst.”
Shortly thereafter, yet another transition-metal catalyst (Ziegler catalyst)
capable of polymerizing ethylene at low pressure was discovered in Germany (18).
This approach used a transitional metal halide, or other complex, activated by an
aluminum alkyl cocatalyst. Transition-metal compounds of Groups IVa through
VIa (Ti preferred) were claimed.
Although the Standard Oil discovery came first, commercialization was slow
(19,20). Three plants were eventually built between 1961 and 1971, but the pro-
cess had poor economics and was soon scrapped. In contrast, the Phillips and
Ziegler discoveries were both commercialized rapidly and still exist today in more
advanced forms. At Phillips, the first plants were brought on stream in 1955 and
1956. However, Phillips management concluded that no one manufacturer could
Vol. 2
ETHYLENE POLYMERS, HDPE
385
develop the full market potential of the Phillips HDPE and therefore decided to
license the process. By 1956, nine companies in seven countries became licensees
(21–24). Ziegler also began to license his patent, following the discovery that good
activity could be obtained by use of titanium halides in combination with alu-
minum alkyls. However, the patent included only catalyst knowledge, and each
licensee had to develop a process. The first Ziegler plant was brought on stream
in late 1956 by Hoechst. A second one was built in 1957 by Hercules. By 1960 U.S.
production of HDPE via the Phillips process had reached over 91,000 t annually,
while 32,000 t came from the Ziegler process.
Today HDPE is still made almost entirely through chromium or Ziegler sys-
tems. The two systems produce different types of polymer, which is useful for
different applications. The Phillips catalyst generally produces broader molecu-
lar weight distributions than that of typical Ziegler catalysts.
Catalysts Used for HDPE Production
Chromium Catalysts.
The Phillips chromium catalyst, which perhaps
accounts for about half of the total HDPE production, is usually made by impreg-
nating a chromium compound onto a porous, high surface area oxide carrier, such
as silica, and then calcining it in dry air at 500–900
◦
C (25). This latter activa-
tion step converts the chromium into a hexavalent surface chromate, or perhaps
dichromate, ester. Because each Cr atom is individually attached to the surface,
the support is not inert but exerts a strong influence on the polymerization be-
havior of the site.
The first step in the development of polymerization activity occurs when
these hexavalent surface species are then reduced by ethylene in the reactor to
a lower valent active precursor, probably Cr(II) (25–28). Because Cr(VI) is tetra-
hedrally coordinated, and the reduced species can be octahedral, the resultant
expansion of the coordination sphere creates a high level of coordinative unsat-
uration which plays a role in the polymerization mechanism. Thus, the reduced
species chemisorbs olefin readily. However, this same trait makes the catalyst very
sensitive to small levels of polar impurities in the feed streams, such as alcohols,
water, amines, etc (29,30). Commercial catalysts usually contain about 1.0 wt%
Cr, but only a small fraction of this, perhaps 10–20% or even less, is actually active
for polymerization (25,27,30).
The second step in the development of polymerization activity is an alkyla-
tion reaction in which the first chain begins growing on the reduced chromium
species. Exactly how this reaction occurs has been unclear. Several possibilities
have been suggested over the years, but no clear answer has yet been established
(25,31–35). Recent studies suggest that initial adsorption of olefin on Cr(II) sites
may cause an oxidation to an alkylated Cr(IV) site as the active polymerization
386
ETHYLENE POLYMERS, HDPE
Vol. 2
Polymerization time
Fig. 2.
Kinetic profiles of Cr-based catalysts.
Oxidized
k
1
Reduced
k
2
Alkylated
*
k
3
Dead
[C
∗
]
=
k
1
k
2
e
−
k
1
t
(
k
2
−
k
1
)
+
e
−
k
2
t
k
2
)
−
e
−
k
3
t
k
1
)(
k
3
−
(
k
1
−
k
2
)(
k
3
−
(
k
1
−
k
3
)(
k
2
−
k
3
)
species (34,35). These two initial steps, reduction followed by alkylation, cause
the catalyst to display an induction time. That is, the onset of polymerization oc-
curs gradually after a delay which can last from a few minutes to over an hour
depending on reaction conditions (25,27).
Alternatively, the reduction step can also be accomplished before the catalyst
contacts ethylene in the reactor, by exposure to carbon monoxide at 350
◦
C (25,27).
In this case the reduced species has definitely been identified as Cr(II) (28,36–40).
This catalyst behaves much like its Cr(VI)/silica parent when introduced into the
reactor, producing similar polymer at similar activity in most cases. However, the
onset of polymerization often occurs more rapidly when the catalyst is prereduced,
since one step is omitted (25,27).
Once the polymerization reaction has developed, a decay in activity can also
sometimes be observed because of a chemical instability of the active species.
Thus, the kinetic profile of the polymerization reaction can be defined as a series
of three consecutive reactions: (
1
) reduction, (
2
) alkylation, and (
3
) decay. Each step
in the series has its own dependency on ethylene concentration, temperature, and
catalyst composition. Figure 2 shows some typical kinetic profiles that are often
obtained and that can be produced by varying the individual rate constants of the
three steps (41).
Metal alkyl cocatalysts, such as alkyl boron, aluminum, zinc, lithium, etc,
can also be added to the reactor as a way of enhancing the activity of the catalyst.
Such agents act by accelerating the reduction step, by alkylating the chromium, or
by scavenging minor amounts of poisons such as water and oxygen. These agents
are sometimes used commercially for specific resin types, although they are not
essential and in most cases are not used (41,42).
In another, less common, variation of the catalyst, lower valent
organochromium compounds can be deposited onto an already calcined support to
produce very active catalysts (43–51). These compounds react with surface hydrox-
yls to become attached to the support, often losing one or more ligands. Examples
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