Free radical polymerization of ethylene initiated by
organic peroxides with high productivity
The present invention relates to a process for manufacturing
polyethylene or an ethylene copolymer by 5 highpressure
polymerization in the presence of a particular
peroxide polymerization initiator in a particular
temperature range.
10 Low-density polyethylenes and ethylene copolymers are
generally manufactured in an autoclave reactor or
tubular reactor under very high pressure, by continuous
introduction of ethylene, of one or more optional
comonomers and of one or more organic peroxide initia15
tors generally diluted in an organic solvent. The
pressure inside the reactor is generally between 500
and 5000 bar. The temperature during the initiation of
the reaction is generally between 80 and 250°C. The
maximum reaction temperature is generally between 120
20 and 350°C.
The degree of conversion into polymer generally
obtained with this type of process is about from 15% to
25%. In the same way, the productivity of such a
25 process, expressed in grams of polyethylene produced
per gram of peroxide initiator used, is generally
between 1000 and 3000 g/g, and more particularly less
than 2500 g/g.
30 The search for gain in productivity and therefore cost
is a constant preoccupation of polyethylene producers.
There is a need for a polyethylene manufacturing
process which has a high productivity while at the same
time retaining an advantageous degree of conversion
35 into polymer.
A process for polymerizing ethylene in the presence of
a 2,2-bis-(tertiary butyl peroxy)butane is known from
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document US 2 650 913, but this initiator results in a
low productivity (cf. example 1 of that document and
test 3 below).
Also known is document FR 2 946 653 which 5 discloses
2,2-di(t-amylperoxy)propane, but the latter is
absolutely not used as a initiator.
Finally, documents US 2008/0226891, EP 0273090 and
10 EP 0259537 are known, which disclose the use of
2,2-di(t-amyl peroxy)butane, but the latter is used for
the manufacture of polymers that are very distinct from
ethylene polymers or ethylene copolymers.
15 The applicant has discovered, surprisingly, that the
use, at a temperature ranging from 150 to 200°C, of a
peroxide initiator of formula
20 wherein R1 and R8 independently represent a C2-C6 alkyl
group, R2, R3, R6 and R7 independently represent a C1-C5
alkyl group, and R4 and R5 independently represent a
C1-C6 alkyl group, makes it possible to solve these
problems, and to obtain a productivity of greater than
25 3000 g/g, while at the same time retaining a degree of
conversion into polymer of between 13% and 25%.
The subject of the invention is therefore a process for
manufacturing polyethylene or an ethylene copolymer,
30 comprising a step of free-radical polymerization or
copolymerization of ethylene at a pressure ranging from
500 to 3000 bar.
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According to the invention, the polymerization or
copolymerization is initiated at an initiation temperature
ranging from 150 to 200°C, in the presence of a
peroxide polymerization initiator chosen from the
peroxide compounds 5 s of formula
wherein
10 R1 and R8 independently represent a C2-C6 alkyl group,
R2, R3, R6 and R7 independently represent a C1-C5 alkyl
group, and
R4 and R5 independently represent a C1-C6 alkyl group.
15 According to a first embodiment, R2, R3, R4, R6 and R7
each represent a methyl group.
According to a second embodiment, R1 and R8 independently
represent a C2-C4 alkyl group.
20
According to a third embodiment, R5 represents a C2-C4
alkyl group.
These three embodiments can be combined in pairs or all
25 three together.
Thus, according to one particular embodiment, R2, R3,
R4, R6 and R7 can each represent a methyl group and R1
and R8 can independently represent a C2-C4 alkyl group.
30
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According to another particular embodiment, R2, R3, R4,
R6 and R7 can each represent a methyl group and R5 can
represent a C2-C4 alkyl group.
According to another particular embodiment, R1 and 5 R8
can independently represent a C2-C4 alkyl group and R5
can represent a C2-C4 alkyl group.
According to another particular embodiment, R2, R3, R4,
10 R6 and R7 can each represent a methyl group, R1 and R8
can independently represent a C2-C4 alkyl group, and R5
can represent a C2-C4 alkyl group.
A
ccording to one particularly preferred embodiment, the
15 peroxide polymerization initiator is 2,2-di(tertamylperoxy)
butane.
The peroxide polymerization initiator is generally
present in a weight amount of between 20 and 1000 ppm
20 relative to the weight amount of ethylene.
The polymerization initiator is generally diluted in a
solvent or a mixture of solvents. The solvent(s) may be
chosen from C1-C20 alkanes.
25
As previously explained, the polymerization or copolymerization
of the ethylene is carried out at an
initiation temperature ranging from 150 to 200°C,
preferably from 160 to 190°C.
30
The polymerization or copolymerization can be carried
out in the presence also of an additional peroxide
initiator.
35 This additional peroxide initiator can be chosen from
tert-butyl peroxyneodecanoate, tert-butyl peroxypivalate,
tert-amyl peroxypivalate, bis(3,5,5-trimethylhexanoyl)
peroxide, dodecanoyl peroxide, tert-amyl
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peroxy-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexanoate,
tert-butyl peroxy-3,5,5-trimethylhexanoate,
tert-amyl peroxy-3,5,5-trimethylhexanoate, tert-butyl
peroxybenzoate, tert-butyl peroxyacetate and di-tertamyl
5 amyl peroxide.
The polymerization or copolymerization can be carried
out in the presence of at least one additive, preferably
chosen from antioxidants; UV protection agents;
10 processing agents, having the function of improving the
final appearance when it is used, such as fatty amides,
stearic acid and its salts, ethylenebis(stearamide) or
fluoro polymers; antifogging agents; antiblocking
agents, such as silica or talc; fillers, such as
15 calcium carbonate, and nanofillers, for instance clays;
coupling agents, such as silanes; crosslinking agents,
such as peroxides; antistatic agents; nucleating
agents; pigments; dyes; plasticizers; fluidizers and
flame-retardant additives, such as aluminum hydroxide
20 or magnesium hydroxide.
These additives are generally used in contents of
between 10 ppm and 10 000 ppm by weight relative to the
weight of the final polyethylene or ethylene copolymer.
25 The plasticizers, the fluidizers or the flame-retardant
additives can reach amounts well above 10 000 ppm.
The polymerization or copolymerization is carried out
at a pressure ranging from 500 to 3000 bar, preferably
30 from 1200 to 3000 bar and better still from 1200 to
2600 bar.
H
igh-pressure polymerization is generally carried out
in an autoclave reactor or tubular reactor. The
35 reaction temperature is generally between 150°C and
320°C.
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When a tubular reactor is used, the mixture of the
ethylene and of the optional comonomer(s) is preferably
introduced at the top of the tubular reactor. The
initiator or the mixture of initiators is injected
using a high-pressure pump at the top of the 5 reactor,
after the site of introduction of the mixture of the
ethylene and of the optional comonomer(s).
The mixture of the ethylene and of the optional comono10
mer(s) can be injected at at least one other point of
the reactor; this injection is itself followed by a
further injection of initiator or of mixture of initiators,
and the term multipoint injection technique is
then used. When the multipoint injection technique is
15 used, the mixture is preferentially injected in such a
way that the weight ratio of the mixture injected at
the reactor inlet to the total mixture injected is
between 10% and 90%.
20 Other tubular high-pressure polymerization or copolymerization
processes which can be used are, for
example, those described in US 2006/0149004 A1 or in
US 2007/0032614 A1.
25 It is also possible to use an autoclave reactor for
carrying out the free-radical high-pressure polymerization.
An autoclave reactor generally consists of a cylin30
drical reactor in which a stirrer is placed. The
reactor can be separated into several zones connected
to one another in series. Advantageously, the residence
time in the reactor is between 30 and 120 seconds.
Preferentially, the length/diameter ratio of the
35 reactor is between 3 and 25. The ethylene alone and the
optional comonomer(s) are injected into the first zone
of the reactor at a temperature of between 50 and
120°C. An initiator is also injected into this first
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reaction zone when the reaction zone reaches a
temperature of between 150 and 200°C. During the
reaction, the temperature can be between 150 and 320°C,
since the reaction is exothermic. If the reactor is a
multizone reactor, the stream of ethylene and 5 of
optional comonomers which have not reacted and also the
polymer formed then pass through the subsequent
reaction zones. In each reaction zone, ethylene,
optional comonomers and initiators can be injected, at
10 an initiation temperature of between 150 and 200°C. The
temperature of the zones after initiation is between
150 and 320°C. The pressure of the reactor ranges from
500 to 3000 bar, preferentially from 1200 to 3000 bar
and better still from 1200 to 2600 bar.
15
The invention is illustrated by the examples which
follow.
Example 1
20
Polymerization of ethylene is carried out with a
conventional initiator, tert-butyl peroxy-3,5,5-trimethylhexanoate
(sold by the company Arkema under the
reference Luperox 270).
25
In a 435 ml high-pressure stirred reactor of autoclave
type, ethylene is injected until a pressure of 1800 bar
is reached, i.e. approximately 207 g. The reactor wall
temperature is then fixed at 177°C by means of heater
30 rods placed in the walls of the reactor.
The temperature of the reaction medium in the reactor
is measured by means of two thermocouples.
35 The tert-butyl peroxy-3,5,5-trimethylhexanoate
(14.8 mg) is diluted in heptane (1.3 g) and propanaldehyde
(0.89 g) upstream of the reactor and at low
temperature (25°C), so as not to initiate the reaction
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before the entry into the reactor. This mixture is then
injected into the reactor using a high-pressure pump.
The polymerization is initiated as soon as the peroxide
is injected at an initial temperature of 177°C
5 (initiation temperature).
The reaction is allowed to take place until the final
temperature returns to the same value level as the
initial temperature (i.e. a test time of approximately
10 25 to 50 minutes).
At the outlet of the reactor, the ethylene/polyethylene
mixture is directly decompressed to 3 bar and the polymer
is separated from the unreacted ethylene by passing
15 it through a separation vessel.
The amount of polymer recovered after polymerization is
determined by weighing, which makes it possible to
express the degree of conversion into polymer, the
20 specific peroxide consumption and the productivity of
the process.
The productivity is expressed in grams of polyethylene
produced per gram of peroxide initiator used.
25
The conversion is defined by the amount of polyethylene
produced relative to the amount of ethylene introduced.
The results are the following:
30
Degree of conversion into polyethylene: 10.2%
Amount of low-density polyethylene produced: 22.4 g
Productivity: 1510 g/g.
35 It is therefore concluded therefrom that the conventional
peroxide initiators result in a low productivity.
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Example 2
Polymerization of ethylene is carried out according to
the invention with the initiator 2,2-di(tert-amylperoxy)
butane (sold by the company Arkema 5 under the
reference Luperox 520M50).
The procedure described in example 1 is reproduced,
except for the fact that the 14.8 mg of tert-butyl
10 peroxy-3,5,5-trimethylhexanoate are replaced with
10.2 mg of 2,2-di(tert-amylperoxy)butane used according
to the invention.
The results are the following:
15
Degree of conversion into polyethylene: 13.9%
Amount of low-density polyethylene produced: 30.7 g
Productivity: 3010 g/g.
20 The use of the 2,2-di(tert-amylperoxy)butane initiator
during a polymerization at a temperature of between 150
and 200°C therefore makes it possible to obtain a high
conversion and a high productivity.
25 Example 3
Polymerization of ethylene is carried out according to
the invention with the initiator 2,2-di(tert-amylperoxy)
butane.
30
The procedure described in example 2 is reproduced,
except for the fact that the initial polymerization
temperature (initiation temperature) is changed. Said
temperature is fixed at 192°C instead of 177°C. The
35 initiation is carried out according to the invention as
in example 2 with 10.2 mg of 2,2-di(tert-amylperoxy)-
butane.
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The results are the following:
Degree of conversion into polyethylene: 14.4%
Amount of low-density polyethylene produced: 31.1 g
Productivity: 3100 5 g/g.
The use of the 2,2-di(tert-amylperoxy)butane initiator
during a polymerization at a temperature of between 150
and 200°C therefore makes it possible to obtain a high
10 conversion and a high productivity.
Example 4
Conventional polymerization of ethylene is carried out
15 with a 2,2-di(tert-amylperoxy)butane initiator, but at
a temperature above the temperature used in the process
according to the invention.
Thus, the procedure described in example 2 is repro20
duced, except for the fact that the initial polymerization
temperature (initiation temperature) is fixed at
215°C, and not 177°C.
The initiation is carried out as in example 2 with
25 10.2 mg of 2,2-di(tert-amylperoxy)butane.
The results are the following:
Degree of conversion into polyethylene: 5.7%
30 Amount of low-density polyethylene produced: 12.1 g
Productivity: 1200 g/g.
Thus, an ethylene polymerization carried out at a
temperature above 200°C results in a low conversion and
35 a low productivity.
Supplementary tests were carried out in order to
compare the process for manufacturing polyethylene or
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an ethylene copolymer according to the invention (tests
1 and 2 below) with a process according to the prior
art (according to document US 2 650 913 in which the
initiator is 2,2-bis(tertiary butyl peroxy). Only in
the case of the initiators of tests 1 and 2 is 5 it
possible to obtain a productivity greater than 3000 g/g
with a degree of conversion into polymer of between 13%
and 25%, whereas, with the abovementioned initiator
targeted in document US 2 650 913, these objectives are
10 not achieved.
Test 1: Polymerization with initiation using
2,2-di(tert-amylperoxy)butane (according to the
invention) or Luperox 520M50:
15
Polymerization of ethylene according to the invention
is carried out with the 2,2-di(tert-amylperoxy)butane
initiator (sold by the company Arkema under the
reference Luperox 520M50).
20
In a 435 ml high-pressure stirred reactor of autoclave
type, the ethylene is injected until a pressure of 1800
bar is reached, i.e. approximately 207 g. The reactor
wall temperature is then fixed at 177°C by means of
25 heater rods placed in the walls of the reactor.
The temperature of the reaction medium in the reactor
is measured by means of two thermocouples.
30 The 2,2-di(tert-amylperoxy)butane (10.2 mg) is diluted
in heptane (1.3 g) and propanaldehyde (0.89 g) upstream
of the reactor and at low temperature (25°C), so as not
to initiate the reaction before the entry into the
reactor. This mixture is then injected into the reactor
35 using a high-pressure pump. The polymerization is
initiated as soon as the peroxide is injected at an
initial temperature of 177°C (initiation temperature).
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The reaction is allowed to take place until the final
temperature returns to the same value level as the
initial temperature (i.e. a test time of approximately
25 to 50 minutes).
5
At the reactor outlet, the ethylene/polyethylene
mixture is directly decompressed to 3 bar and the polymer
is separated from the unreacted ethylene by passing
it through a separation vessel.
10
The amount of polymer recovered after polymerization is
determined by weighing, which makes it possible to
express the degree of conversion into polymer, the
specific peroxide consumption and the productivity of
15 the process.
The productivity is expressed in grams of polyethylene
produced per gram of peroxide initiator used.
20 The conversion is defined by the amount of polyethylene
produced relative to the amount of ethylene introduced.
In the test, the following results were recorded:
25 Degree of conversion into polyethylene: 13.9%
Amount of low-density polyethylene produced: 30.7 g
Productivity: 3010 g/g.
Test 2 - Polymerization with initiation using
2,2-di(tert-amylperoxy)butane (invention) or Luperox 30
520M50:
Polymerization of ethylene is carried out according to
the invention with the initiator 2,2-di(tert-amyl35
peroxy)butane.
The procedure described in the previous test (test 1)
is reproduced, except for the fact that the initial
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polymerization temperature (initiation temperature) is
changed. Said temperature is fixed at 192°C instead of
177°C. The initiation is carried out as in the previous
test with 10.2 mg of 2,2-di(tert-amylperoxy)butane.
5
The results are the following:
Degree of conversion into polyethylene: 14.4%
Amount of low-density polyethylene produced: 31.1 g
10 Productivity: 3100 g/g.
The use of the 2,2-di(tert-amylperoxy)butane initiator
during a polymerization at a temperature of between 150
and 200°C therefore clearly makes it possible to obtain
15 a high conversion and a high productivity (conversion
> 13% and productivity > 3000 g/g).
Test 3 - Polymerization with initiation using
2,2-di(tert-butylperoxy)butane:
20
Conventional polymerization of ethylene is carried out
according to the invention with the initiator
2,2-di(tert-butylperoxy)butane.
25 Thus, the procedure described in test 1 is reproduced,
except for the fact that the 2,2-di(tert-amylperoxy)-
butane initiator is replaced with 2,2-di(tert-butylperoxy)
butane.
30 The initiation is carried out with 10.2 mg of
2,2-di(tert-butylperoxy)butane.
The results are the following:
35 Degree of conversion into polyethylene: 11.9%
Amount of low-density polyethylene produced: 24.6 g
Productivity: 2410 g/g.
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It is concluded therefrom that 2,2-di(tertbutylperoxy)
butane results in a lower productivity
(< 3000 g/g) with a conversion lower than 13%.
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CLAIMS
1. A process for manufacturing polyethylene or an
ethylene copolymer, comprising a step of freeradical
polymerization or copolymerization 5 olymerization of
ethylene at an initiation temperature ranging from
150 to 200°C, at a pressure ranging from 500 to
3000 bar, in the presence of a peroxide
polymerization initiator chosen from the peroxide
10 compounds of formula
wherein
R1 and R8 independently represent a C2-C6 alkyl
15 group,
R2, R3, R6 and R7 independently represent a C1-C5
alkyl group, and
R4 and R5 independently represent a C1-C6 alkyl
group.
20
2. The process as claimed in claim 1, characterized
in that R2, R3, R4, R6 and R7 each represent a
methyl group.
25 3. The process as claimed in any one of the preceding
claims, characterized in that R1 and R8 independently
represent a C2-C4 alkyl group.
4. The process as claimed in any one of the preceding
30 claims, characterized in that R5 represents a C2-C4
alkyl group.
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5. The process as claimed in any one of the preceding
claims, characterized in that the peroxide polymerization
initiator is 2,2-di(tert-amylperoxy)-
butane.
5
6. The process as claimed in any one of the preceding
claims, characterized in that the polymerization
or copolymerization of the ethylene is carried out
at an initiation temperature ranging from 160 to
10 190°C.
7. The process as claimed in any one of the preceding
claims, characterized in that the polymerization
or copolymerization is carried out in the presence
15 also of an additional peroxide initiator.
8. The process as claimed in any one of the preceding
claims, characterized in that the polymerization
or copolymerization is carried out in the presence
20 of at least one additive chosen from antioxidants;
UV protection agents; processing agents;
antifogging agents; antiblocking agents; fillers;
coupling agents; crosslinking agents; antistatic
agents; nucleating agents; pigments; dyes;
25 plasticizers; fluidizers and flame-retardant
additives.
ABSTRACT
“FREE RADICAL POLYMERISATION OF ETHYLENE INITIATED BY
ORGANIC PEROXIDES WITH HIGH PRODUCTIVITY”
The present invention relates to a method for manufacturing polyethylene or an
ethylene copolymer, including a step of free radical polymerisation or
copolymerisation of the ethylene at an initiation temperature varying from 150°C to
200°C, at a pressure varying from 500 to 3000 bar, in the presence of a peroxidic
polymerisation initiator selected from among the peroxide compounds of formula (I)
in which R1 and R8 are, separately, a C2-C6 alkyl group; R2, R3, R6 and R7 are,
separately, a C1-C5 alkyl group; and R4 and R5 are, separately, a C1-C6 alkyl group.