Ethylene Polymers, HDPE.pdf

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ETHYLENE POLYMERS, CHLOROSULFONATED

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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

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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.

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

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ETHYLENE POLYMERS, HDPE

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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

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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

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ETHYLENE POLYMERS, HDPE

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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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