U.S. patent application number 13/508903 was filed with the patent office on 2012-09-20 for process for the polymerisation of olefins.
This patent application is currently assigned to INEOS COMMERCIAL SERVICES UK LIMITED. Invention is credited to Stephan Detournay, Christophe Moineau.
Application Number | 20120238714 13/508903 |
Document ID | / |
Family ID | 43558263 |
Filed Date | 2012-09-20 |
United States Patent
Application |
20120238714 |
Kind Code |
A1 |
Detournay; Stephan ; et
al. |
September 20, 2012 |
PROCESS FOR THE POLYMERISATION OF OLEFINS
Abstract
The present invention relates to a process for polymerisation of
olefins, in particular gas phase polymerisation of olefins, with
the aid of a supported chromium oxide based catalyst.
Inventors: |
Detournay; Stephan;
(Stombeek-Bever, BE) ; Moineau; Christophe;
(Nivelles, BE) |
Assignee: |
INEOS COMMERCIAL SERVICES UK
LIMITED
HAMPSHIRE
GB
|
Family ID: |
43558263 |
Appl. No.: |
13/508903 |
Filed: |
November 1, 2010 |
PCT Filed: |
November 1, 2010 |
PCT NO: |
PCT/EP2010/066557 |
371 Date: |
May 14, 2012 |
Current U.S.
Class: |
526/106 |
Current CPC
Class: |
C08F 10/00 20130101;
C08F 10/00 20130101; C08F 10/00 20130101; C08F 210/16 20130101;
C08F 2500/07 20130101; C08F 2500/12 20130101; C08F 4/025 20130101;
C08F 210/16 20130101; C08F 2500/12 20130101; C08F 10/00 20130101;
C08F 210/14 20130101; C08F 4/69 20130101; C08F 4/24 20130101 |
Class at
Publication: |
526/106 |
International
Class: |
C08F 2/34 20060101
C08F002/34; C08F 10/02 20060101 C08F010/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 10, 2009 |
EP |
09175489.5 |
Nov 10, 2009 |
EP |
09175490.3 |
Claims
1-14. (canceled)
15. Process for the polymerisation, preferably the gas phase
polymerisation, of at least one alpha olefin containing from 2 to
12 carbon atoms in the presence of a supported chromium oxide based
catalyst characterised in that the supported chromium oxide based
catalyst is a titanium modified supported chromium oxide based
catalyst; the chromium oxide based catalyst is supported on a
refractory oxide; and the titanium modified supported chromium
oxide based catalyst has been subjected to a 2 steps thermal
treatment wherein the first step consists in bringing the catalyst
under an inert atmosphere to a temperature comprised between 600
and 900.degree. C. and then keeping the catalyst during a holding
time of less than or equal to 8 hours under an inert atmosphere at
a temperature comprised between 600 and 900.degree. C., and the
second step consists in the treatment of the catalyst coming from
step 1 under an oxidizing atmosphere, preferably air, at a
temperature comprised between 400 and 700.degree. C. during a
holding time comprised between 2 and 10 hours, and wherein the
maximum temperature of step 2 is always lower than the maximum
temperature of step 1 and wherein the difference between the
maximum temperature of step 1 and the maximum temperature of step 2
is comprised between 50 and 250.degree. C.
16. Process according to claim 15 wherein the refractory oxide
support for the chromium oxide based catalyst used in the process
of the present invention is spherical and/or spheroidal and is
preferably chosen amongst silica, alumina, aluminophosphate, metal
oxides such as oxides of titanium, zirconium, boron, zinc,
magnesium, and the like, or combinations thereof.
17. Process according to claim 16 wherein the spherical and/or
spheroidal refractory oxide support consists of silica.
18. Process according to claim 16 wherein the spherical and/or
spheroidal refractory oxide support has a pore volume higher than 1
mL/g.
19. Process according to claim 16 wherein the spherical and/or
spheroidal refractory oxide support has a specific surface area
ranging from 100 to 800 m.sup.2/g.
20. Process according to claim 15 wherein the chromium oxide based
catalyst contains 0.3 to 2% by weight of chromium, preferably 0.3
to 1% by weight of chromium, more preferably between 0.3 and 0.7%
by weight of chromium, and most preferably between 0.4 and 0.6% by
weight of chromium.
21. Process according to claim 15 wherein the chromium oxide based
catalyst contains between 0.5 and 5% by weight of titanium,
preferably between 1.5 and 4%, most preferably between 2 and 4%,
and most preferably between 2 and 3% by weight of titanium.
22. Process according to claim 15 wherein the holding time during
the first thermal treatment is less than or equal to 6 hours,
particularly less than or equal to 4 hours; and/or said holding
time is at least 30 minutes, particularly at least 1 hour, more
particularly at least 1.5 hours.
23. Process according to claim 15 wherein the temperature described
in the first thermal treatment step is at least 650.degree. C.,
preferably at least 700.degree. C., for example at least
730.degree. C.; and/or said temperature is less than or equal to
850.degree. C., for example less than or equal to 800.degree.
C.
24. Process according to claim 15 wherein the second step has a
holding time of at least 3 hours, particularly at least 4 hours;
and/or said holding time is less than or equal to 9 hours,
particularly less than or equal to 8 hours, for example less than
or equal to 7 hours.
25. Process according to claim 15 wherein the range of temperatures
described in the second step is comprised between 500 and
700.degree. C., preferably comprised between 600 and 700.degree.
C.
26. Process according to claim 15 wherein the maximum temperature
of step 2 is always lower than the maximum temperature of step 1
and wherein the difference between the maximum temperature of step
1 and the maximum temperature of step 2 is comprised between 50 and
200.degree. C.; preferably, such difference in temperatures is of
at least 80.degree. C., preferably at least 100.degree. C.
27. Process according to claim 15 wherein step 1 is performed under
nitrogen.
28. Process according to claim 15 wherein the total duration of any
thermal treatment (non oxidising and oxidising steps) above
300.degree. C. of our titanium modified supported chromium oxide
based catalyst is always less than 24 hours, preferably less than
22 hours, more preferably less 20 hours and even more preferably
less than 18 hours.
Description
[0001] The present invention relates to a process for
polymerisation of olefins, in particular gas phase polymerisation
of olefins, with the aid of a supported chromium oxide based
catalyst.
[0002] In particular, the present invention relates to a process
for polymerisation of olefins, in particular gas phase
polymerisation of olefins, with the aid of a supported chromium
oxide based catalyst which has been subjected to a two-steps
activation method.
[0003] The "two step activation" of supported chromium oxide
catalysts, by which the catalyst is subject to a single activation
in two steps, is well-known and commercially operated. In general,
the two-step activation comprises heating the catalyst to a first
temperature in a non-oxidising atmosphere (e.g. an inert or
reducing gas) and subsequently lowering the temperature and
replacing the non-oxidising gas with an oxidising one, usually air.
The process is also known as "reduction and reoxidation activation"
or "R&R activation", and is further described, for example, in
McDaniel "A Review of the Phillips Supported Chromium Catalyst and
Its Commercial Use for Ethylene Polymerization" in Advances in
Catalysis, Vol. 53, Chapter 3, Section 12.4 "Commercial
Practice".
[0004] Such activations are also described, for example, in U.S.
Pat. No. 4,147,849, which discloses activation of a chromium
containing catalyst under a non-oxidative atmosphere at an
activation temperature of 600-2000.degree. F. followed by
subjecting the activated catalyst to an oxidative treatment at a
lower, although still elevated, temperature.
[0005] A similar process is described in WO 2008/074467, albeit
that at the highest temperature during the activation steps the
catalyst is preferably exposed to an oxidising atmosphere. In
particular, WO2008074467 discloses a process for producing chromium
catalysts, comprising the steps of: a) applying one or more
chromium compound(s) to a finely divided inorganic support to form
a catalyst precursor, b) thermally treating the catalyst precursor,
the step of thermally treating the catalyst precursor being carried
out for at least part of the time in an oxidizing atmosphere and in
such a manner that a maximum temperature of from 350.degree. C. to
1050.degree. C. is not exceeded, wherein the duration of the
thermal treatment step at a temperature of above 300.degree. C. is
at least 1500 minutes. Whilst, as exemplified by the prior art in
the field, it is common general knowledge to produce polymers in
gas phase in the presence of supported chromium oxide based
catalyst, the man skilled in the art is still trying to obtain
polymers with the right combination of product properties,
particularly properties appropriate for pipe purposes, while still
having a high productivity and avoiding production problems, e.g.
fines and/or temperature upsets. Therefore, it would be desirable
to obtain a resin product with desirable properties, e.g.,
sufficiently high Environmental Stress Crack Resistance (ESCR),
high Creep behaviour and high resistance against rapid crack
propagation with the right melt index (MI), while also achieving
high catalyst activity, in particular using a gas phase process,
more particularly a fluidized bed gas phase process.
[0006] It is therefore an objective of the present invention to
provide a process for preparing ethylene (co-) polymers, preferably
in gas phase, having high Environmental Stress Crack Resistance
(ESCR) and high Creep behaviour in the presence of a supported
chromium oxide based catalyst having good activity.
[0007] The subject of the invention is therefore a process for the
polymerisation, preferably the gas phase polymerisation, of at
least one alpha olefin containing from 2 to 12 carbon atoms in the
presence of a supported chromium oxide based catalyst characterised
in that [0008] the supported chromium oxide based catalyst is a
titanium modified supported chromium oxide based catalyst; [0009]
the chromium oxide based catalyst is supported on a refractory
oxide; and [0010] the titanium modified supported chromium oxide
based catalyst has been subjected to a 2 steps thermal treatment
wherein [0011] the first step consists in bringing the catalyst
under an inert atmosphere to a temperature comprised between 600
and 900.degree. C. and then keeping the catalyst during a holding
time of less than or equal to 8 hours under an inert atmosphere at
a temperature comprised between 600 and 900.degree. C., and [0012]
the second step consists in the treatment of the catalyst coming
from step 1 under an oxidizing atmosphere, preferably air, at a
temperature comprised between 400 and 700.degree. C. during a
holding time comprised between 2 and 10 hours, and [0013] wherein
the maximum temperature of step 2 is always lower than the maximum
temperature of step 1 and wherein the difference between the
maximum temperature of step 1 and the maximum temperature of step 2
is comprised between 50 and 250.degree. C.
[0014] According to a preferred embodiment of the present
invention, the raw catalyst or the activated catalyst itself is not
contacted with an alkylboron compound before or during (co-)
polymerization. Thus, preferably, the present invention does not
contemplate the activation of our catalyst by a cocatalyst such as
with an alkylboron.
[0015] The refractory oxide support for the chromium oxide based
catalyst used in the process of the present invention is preferably
a spherical and/or spheroidal refractory oxide. It can preferably
be silica, alumina, aluminophosphate, metal oxides such as oxides
of titanium, zirconium, boron, zinc, magnesium, and the like, or
combinations thereof; more preferably, it essentially consists of
silica or more preferably silica doped with titanium, aluminium or
boron.
[0016] The spherical and/or spheroidal refractory oxide support may
suitably be prepared by spray drying of washed and aged hydrogel
particles or spray setting of a hydrosol. Such processes are well
known in the art and typically result in spherical and/or
spheroidal particles. The particle size may be adjusted by
selection of conditions. In this invention the median (volume)
particle diameter of the spherical and/or spheroidal particles
measured after drying is from 10 to 250 .mu.m, preferably from 20
to 200 .mu.m and most preferably from 20 to 150 .mu.m. The
International Standard ISO 13320:2009 ("Particle size
analysis--Laser diffraction methods") can be used for measuring
said median particle size characteristic. Particle diameters are
indeed typically measured using light scattering techniques. For
example, Malvern Instruments' laser diffraction systems can
advantageously be used, e.g. a Malvern Mastersizer S or a Malvern
Mastersizer 2000; such instrument together with its operating
manual meets or even exceeds the requirements set-out within the
ISO 13320 Standard. The resulting spherical and/or spheroidal
particles may be further classified e.g. by sieving to tailor the
median particle diameter and reduce the amounts of fine and/or
coarse particles.
[0017] Although handling of the particles may lead to some degree
of breakage, particles are preferably not subjected to any
deliberate comminution processes. Preferably, the spherical and/or
spheroidal particles are prepared by spray setting of a hydrosol,
preferably a silica hydrosol. The resulting spherical and or
spheroidal hydrogel particles are suitably subjected to washing and
aging processes prior to water removal to generate suitable surface
area and pore volume.
[0018] In general, the support has a specific surface area ranging
from 100 to 800 m.sup.2/g, measured according to the BET volumetric
method in British Standard BS 4359/1 (1984). Preferably, the
specific surface area is more than or equal to 250 m2/g or even
more than or equal to 400 m.sup.2/g. The specific surface area is
preferably less than or equal to 600 m2/g.
[0019] Moreover, the support generally has a pore volume higher
than 1 mL/g. It may also be higher than 2 mL/g and even higher than
2.5 mL/g. The term "pore volume" should be understood to mean the
pore volume measured according to the nitrogen penetration method
(BET) with reference to British Standard 13S 4359/1 (1984).
[0020] The chromium oxide based catalyst deposited on a support
used in the process according to the invention usually contains
approximately 0.3 to 2% by weight and even more particularly 0.3 to
1% by weight of chromium, more preferably between 0.3 and 0.7% by
weight of chromium, most preferably between 0.4 and 0.6% by weight
of chromium (these weights being expressed with respect to the
weight of final supported catalyst). Of course, it is possible to
use the catalyst as a blend of 2 different or more different
catalysts, at least one on them being defined as hereinabove. It is
also possible to use the catalyst as a blend of 2 different or more
different catalysts, this blend containing a chromium loading in
average defined as hereinabove.
[0021] Any known chromium containing compounds capable of reacting
with the surface hydroxyl groups of the refractory oxide can be
used in the process of preparation of the catalyst.
[0022] Non-limiting examples of such compounds include chromium
nitrate, chromium trioxide, chromate esters such as chromium
acetate, chromium acetylacetonate and t-butyl chromate, silyl
chromate esters and phosphorous-containing esters, chromium
chloride, chromium sulphate and ammonium chromate.
[0023] The introduction mode of this chromium compound can be a dry
mixing in or outside the activator or by aqueous or non-aqueous
impregnation of the support.
[0024] Any known titanium containing compound capable of reacting
with the surface hydroxyl groups of the refractory oxide can be
used in the process of preparation of the catalyst. These compounds
include those having the structures (R)mTi(OR')n and (RO)mTi(OR')n
where m is 1, 2, 3 or 4; n is 0, 1, 2 or 3 and m+n=4, and where R
and R' are a C1 to C12 alkyl, aryl, cycloalkyl group,
cyclopentadienyl, C2 to C12 alkenyl groups and combination thereof.
These compounds also include those having the structures TiX4
wherein X is chlorine, bromine, fluorine or iodine. The titanium
compound can be solid, liquid or in solution in an hydrocarbon
solvent.
[0025] The introduction mode of the titanium compound can be a dry
mixing in or outside the activator or an impregnation of the
support with a hydrocarbon solution of the titanium compound. The
catalyst used in the process according to the invention usually
contains between 0.5 and 5% by weight of titanium, most preferably
between 1.5 and 4%, most preferably between 2 and 4%, most
preferably between 2 and 3% (these weights being expressed with
respect to the weight of final supported catalyst).
[0026] The preferred method of preparation of the catalyst used in
the present invention is not important as long as it has the
chromium content, the titanium content and the spherical and/or
spheroidal morphology as defined hereinabove. According to the
present invention the preparation of the catalyst is preferably not
carried out by the cogel method. Although the support may contain
small amounts of titanium, for instance as an impurity, the method
of the invention involves the introduction of a titanium compound
on to a support by addition and/or impregnation as defined herein
in addition to any titanium contained within the support
skeleton.
[0027] For the purpose of the present invention and appended
claims, spheroidal shape means shaped like a sphere but not
perfectly round, especially an ellipsoid shape that is generated by
revolving one or more ellipse around one of its axes.
[0028] Thus, for the purpose of the present invention and appended
claims, by "spherical and/or spheroidal refractory oxide", it is
meant that the refractory oxide particles used for the preparation
of the catalyst used in the present invention exhibit a spherical
and/or spheroidal shape morphology. Such spherical and/or
spheroidal morphology of said particles is usually identified by
taking microscopy pictures of said particles; this is currently how
the man skilled in the art can identify the presence of a spherical
and/or spheroidal refractory oxide.
[0029] According to the present invention, the titanium modified
supported chromium oxide based catalyst is then subjected to a 2
steps thermal treatment wherein [0030] the first step consists in
bringing the catalyst under an inert atmosphere to a temperature
comprised between 600 and 900.degree. C. and then keeping the
catalyst under said inert atmosphere and at said temperature
comprised between 600 and 900.degree. C. during a holding time of
less than or equal to 8 hours, preferably less than or equal to 6
hours, for example less than or equal to 4 hours; such holding time
is preferably of at least 30 minutes, more preferably at least 1
hour, for example at least 1.5 hours. Preferably the temperature
described in this first step is at least 650.degree. C., more
preferably at least 700.degree. C., for example at least
730.degree. C.; such temperature is preferably less than or equal
to 850.degree. C., for example less than or equal to 800.degree.
C., and [0031] the second step consists in the treatment of the
catalyst coming from step 1 under an oxidizing atmosphere,
preferably air, at a temperature comprised between 400 and
700.degree. C. during a holding time comprised between 2 and 10
hours. Such holding time lasts preferably at least 3 hours, more
particularly at least 4 hours; such holding time is preferably less
than or equal to 9 hours, more particularly less than or equal to 8
hours, for example less than or equal to 7 hours. More preferably
the range of temperature described in this second step is comprised
between 500 and 700.degree. C. and more preferably comprised
between 600 and 700.degree. C. , and [0032] wherein the maximum
temperature of step 2 is always lower than the maximum temperature
of step 1 and wherein the difference between the maximum
temperature of step 1 and the maximum temperature of step 2 is
comprised between 50 and 250.degree. C., preferably comprised
between 50 and 200.degree. C. For example, such difference in
temperatures is of at least 80.degree. C., preferably at least
100.degree. C. Such difference in temperatures is less than or
equal to 250.degree. C., preferably less than or equal 200.degree.
C. Step 1 according to the present invention is performed under an
inert atmosphere, preferably under nitrogen, more preferably under
dry nitrogen. Any additional thermal treatment steps could also be
performed before our step 1 as claimed hereinabove. However, it is
preferred according to the present invention that the titanium
modified supported chromium oxide based catalyst is not subjected
to any other thermal treatment step at a temperature above
150.degree. C. before our step 1 as claimed in the present
invention.
[0033] It is preferred according to the present invention that step
2 is performed directly after step 1 without any additional
intermediate thermal treatment step.
[0034] After our claimed step 2 treatment, the activated catalyst
is then preferably cooled with the same oxidizing atmosphere from
step 2, preferably air, to a temperature between 400.degree. C. and
300.degree. C. and further cooled from this temperature to room
temperature with inert atmosphere, preferably nitrogen, more
preferably dry nitrogen.
[0035] Also, according to the present invention, it is preferred
that the total duration of any thermal treatment (non oxidising and
oxidising steps) above 300.degree. C. of our titanium modified
supported chromium oxide based catalyst is always less than 24
hours, preferably less than 22 hours, more preferably less 20 hours
and even more preferably less than 18 hours.
[0036] It is obvious for the man skilled in the art that the
thermal treatments according to the present invention can be
performed continuously or in batch in any appropriate reactor. For
continuous mode operations, a configuration with at least two
reactors in series is preferred, one reactor operating under an
inert atmosphere for our claimed step 1 and one reactor operating
under an oxidising atmosphere for our claimed step 2.
[0037] According to an embodiment of the present invention, the 2
thermal treatment steps are performed in batch mode in the same
reactor, preferably in a fluidized bed reactor.
[0038] According to another embodiment of the present invention,
the step of bringing the catalyst under an inert atmosphere to a
temperature comprised between 600 and 900.degree. C. is performed
by introducing the said catalyst into a reactor and gradually
increasing the temperature of the reactor with a thermal speed rate
comprised between 1 and 2.5.degree. C./min until the desired
temperature is reached.
[0039] According to the invention, the supported chromium catalysts
modified with titanium having undergone the successive heat
treatments in nitrogen and in air, under suitable conditions of
temperature and duration, are used for the polymerization of
ethylene or for its copolymerization with C3 to C8 olefins. By way
of examples of olefins that can be used as comonomers, mention may
be made of propylene, 1-butene, 4-methyl-1-pentene, 1-hexene,
1-octene, 1-decene, 1-dodecene, styrene, and derivatives thereof.
Of course, several olefins may be used simultaneously, particularly
1-butene and/or 1-hexene and/or 1-octene.
[0040] The process according to the invention applies particularly
well to the manufacture of ethylene (co-) polymers but more
particularly ethylene (co-) polymers having, after compounding:
[0041] a density between 936 and 941 Kg/m.sup.3 and a MI5 between
0.5 and 1.2 dg/min and the pipes produced with this polymer meet
the requirements of ISO 4427 and ISO 4437 in hydrostatic pressure
testing at 20.degree. C. and 9.0 MPa with a ductile failure at more
than 100 hours, or
[0042] a density between 942 and 946 Kg/m.sup.3 and a MI5 between
0.4 and 0.6 dg/min and the pipes produced with this polymer meet
the requirements of ISO 4427 and ISO 4437 in hydrostatic pressure
testing at 20.degree. C. and 9.0 MPa with a ductile failure at more
than 100 hours, or
[0043] a density between 943 and 947 Kg/m.sup.3 and a MI5 between
0.1 and 0.4 dg/min and the pipes made from this polymer and tested
in hydrostatic pressure test at 20.degree. C. under 12.0 MPa will
have a time to failure at more than 100 h, the above hydrostatic
pressure testing being made following ISO1167-1 and 2.
[0044] The (co)polymerization is carried out, in a known manner, by
bringing ethylene and, optionally, other olefins into contact with
the catalyst under polymerizing conditions known per se. According
to the present invention, the (co)polymerization of the ethylene is
preferably carried out in the gas phase. Indeed, the Applicants
have unexpectedly found that it was possible to obtain a resin
product with desirable properties, e.g., sufficiently high
Environmental Stress Crack Resistance (ESCR) and high Creep
behaviour, with the right melt index (MI), while also achieving
high catalyst activity, by using a gas phase process, in particular
a fluidized bed gas phase process. Indeed, the ethylene
(co)polymers produced according to the process of the present
invention exhibit an excellent compromise between processability,
slow crack and creep behaviour. They may be used in any
conventional process for converting thermoplastics, such as, for
example, pipe extrusion, blow moulding and blown film. They are
very suitable for pipe extrusion.
[0045] The present invention also provides ethylene (co-) polymers
but more particularly (co-) polymers having, after compounding:
[0046] a density between 936 and 941 Kg/m.sup.3 and a MI5 between
0.5 and 1.2 dg/min and the pipes produced with this polymer meet
the requirements of ISO 4427 and ISO 4437 in hydrostatic pressure
testing at 20.degree. C. and 9.0 MPa with a ductile failure at more
than 100 hours, or
[0047] a density between 942 and 946 Kg/m.sup.3 and a MO between
0.4 and 0.6 dg/min and the pipes produced with this polymer meet
the requirements of ISO 4427 and ISO 4437 in hydrostatic pressure
testing at 20.degree. C. and 9.0 MPa with a ductile failure at more
than 100 hours, or
[0048] a density between 943 and 947 Kg/m.sup.3 and a MI5 between
0.1 and 0.4 dg/min and the pipes made from this polymer and tested
in hydrostatic pressure test at 20.degree. C. under 12.0 MPa will
have a time to failure at more than 100 h,
the above hydrostatic pressure testing being made following
ISO1167-1 and 2, characterised in that the polymer is obtainable by
a polymerisation process, preferably a gas phase polymerisation
process, of at least one alpha olefin containing from 2 to 12
carbon atoms in the presence of a supported chromium oxide based
catalyst wherein [0049] the supported chromium oxide based catalyst
is a titanium modified supported chromium oxide based catalyst;
[0050] the chromium oxide based catalyst is supported on a
spherical and/or spheroidal refractory oxide; and [0051] the
titanium modified supported chromium oxide based catalyst has been
subjected to a 2 steps thermal treatment wherein [0052] the first
step consists in bringing the catalyst under an inert atmosphere to
a temperature comprised between 600 and 900.degree. C. and
maintaining the catalyst during a holding time of less than or
equal to 8 hours under an inert atmosphere at a temperature
comprised between 600 and 900.degree. C., and [0053] the second
step consists in the treatment of the catalyst coming from step 1
under an oxidizing atmosphere, preferably air, at a temperature
comprised between 400 and 700.degree. C. during a time comprised
between 2 and 10 hours, and [0054] wherein the maximum temperature
of step 2 is always lower than the maximum temperature of step 1
and wherein the difference between the maximum temperature of step
1 and the maximum temperature of step 2 is comprised between 50 and
250.degree. C. The example which follows is intended to illustrate
the process of the invention.
[0055] In a fluidized bed reactor, 15 g of the catalyst PQC35105
were subjected to the following thermal treatment: [0056] increase
of temperature up to 750.degree. C. with a thermal speed rate of
1.5.degree. C./min under nitrogen flow; [0057] keeping temperature
at 750.degree. C. and keeping nitrogen flow during 1 hour; [0058]
decrease of temperature up to 650.degree. C. under nitrogen flow;
[0059] at 650 .degree. C. switch from nitrogen flow to air flow;
[0060] keeping temperature at 650.degree. C. and keeping air flow
during 6 hours; [0061] decrease of temperature up to 350.degree. C.
under air flow; [0062] at 350 .degree. C. switch from air flow to
nitrogen flow; [0063] decrease of temperature up to ambient
temperature under nitrogen flow; [0064] the catalyst was recovered
under nitrogen and stored under nitrogen in a glove box before to
be use for ethylene polymerization.
[0065] After a correct clean-up, 500 g of polyethylene pellets were
introduced into a stainless steel reactor of capacity 5 litres
equipped with a stirrer. Then the reactor was closed, heated at
100.degree. C., stirred at 70 rpm and placed under vacuum during 1
hour. Then vacuum was stopped and replaced with nitrogen and the
reactor was placed overnight under nitrogen flow at 100.degree. C.
Then the nitrogen flow was stopped and the reactor, stored under
nitrogen and stirred at 300 rpm, was ready for the
polymerization.
[0066] Next step was the addition of 150 mg of poison scavenger
prepared with the silica 948 from the Grace Company treated under
nitrogen flow at 600.degree. C. during 5 hours and with 1.5 mmole/g
of triethylaluminium.
[0067] Then the reactor was placed at 93.degree. C. and 15 minutes
after the poison scavenger addition, 260 mg of the C35105 catalyst
prepared as described above, was added. Then 3 bars of hydrogen was
introduced in the reactor. Then ethylene and hexene were introduced
to reach 7 bars of ethylene and a ratio hexene/ethylene of 1.2
mol/mol % controlled by mass spectrometry. Ethylene and hexene were
fed during the reaction to maintain the reactor pressure constant
and to maintain the ratio of hexene/ethylene of 1.2 mol/mol %
always controlled by mass spectrometry.
[0068] The duration of the polymerization was 119 min during which
period 654 g of polyethylene. This corresponds to a productivity of
2515 g/g and a catalytic activity of 181 g/g/h/b. The reactor
content was cooled to 25.degree. C. and were then recovered from
the reactor. A sieve with 2 mm diameter mesh was used to separate
the polyethylene pellets from the powder formed during the
reaction. The recovered polymer powder had the following
properties:
[0069] Melt Index MIS=0.6
[0070] Density MVS=938.5 Kg/m.sup.3
[0071] Melt index MI5 is measured using to standard ISO 1133 at a
temperature of 190.degree. C. under load of 5 Kg.
[0072] Density is measured according to the standard ISO 1183-1
(Method A) and the sample plaque was prepared according to the
standard ASTM D4703 (Condition C) where it was cooled under
pressure at a cooling rate of 15.degree. C./min from 190.degree. C.
to 40.degree. C.
[0073] The hydrostatic pressure testing mentioned above is
described in standard ISO 1167-1 and ISO 1167-2.
* * * * *