U.S. patent application number 09/832227 was filed with the patent office on 2002-04-18 for polymer composition for pipes.
Invention is credited to Aarila, Jari, Backman, Mats, Heino, Eeva-Leena, Johansson, Solveig, Laurell, Jussi, Lehtinen, Arja, Lindroos, Jarmo, Ora, Marja, Ribarits, Elisabeth, Sihvonen, Teija.
Application Number | 20020045711 09/832227 |
Document ID | / |
Family ID | 20412942 |
Filed Date | 2002-04-18 |
United States Patent
Application |
20020045711 |
Kind Code |
A1 |
Backman, Mats ; et
al. |
April 18, 2002 |
Polymer composition for pipes
Abstract
A multimodal polymer composition for pipes is disclosed as well
as a pipes made thereof. The polymer is a multimodal polyethylene
with a density of 0.930-0.965 g/cm.sup.3, and a viscosity at a
shear stress of 747 Pa (.sub..eta.747Pa) of at least 650 Pa.s, said
multimodal polyethylene comprising a low molecular weight (LMW)
ethylene homopolymer fraction and a high molecular weight (HMW)
ethylene copolymer fraction, said HMW fraction having a weight
ratio of the LMW fraction to the HMW fraction of (35-55):(65-45).
Preferably, the multimodal polyethylene has a viscosity at a shear
stress of 2.7 kPa (.sub..eta.2.7 kPa ) of 260-450 kPa.s; and a
shear thinning index (SHI) defined as the ratio of the viscosities
at shear stresses of 2.7 and 210 kPa, respectively, of SHI.sub.2
7/210=50-150, and a storage modulus (G') at a loss modulus (G") of
5 kPa, of G'.sub.5 kPa.gtoreq.3000 Pa. The pipe is made of the
multimodal polymer composition and withstands a stress of 8.0 MPa
gauge during 50 years at 20.degree. C. (MRS8.0). Preferably, the
pipe has a rapid crack propagation (RCP) S4-value, determined
according to ISO 13477:1997(E), of -5.degree. C. or lower and a
slow crack propagation resistance, determined according to ISO
13479:1997, of at least 500 hrs at 4.6 MPa/80.degree. C. The
polymer composition affords good non-sagging properties to pipe
made thereof.
Inventors: |
Backman, Mats; (Goteborg,
SE) ; Heino, Eeva-Leena; (Helsingfors, FI) ;
Johansson, Solveig; (Stenungsund, SE) ; Laurell,
Jussi; (Borga, FI) ; Lehtinen, Arja;
(Helsingfors, FI) ; Lindroos, Jarmo; (Tolkkinen,
FI) ; Ora, Marja; (Vantaa, FI) ; Ribarits,
Elisabeth; (Spekerod, SE) ; Sihvonen, Teija;
(Borga, FI) ; Aarila, Jari; (Borga, FI) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Family ID: |
20412942 |
Appl. No.: |
09/832227 |
Filed: |
April 10, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09832227 |
Apr 10, 2001 |
|
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PCT/SE99/01679 |
Sep 24, 1999 |
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Current U.S.
Class: |
525/240 |
Current CPC
Class: |
C08L 23/0815 20130101;
C08F 10/02 20130101; C08F 110/02 20130101; Y10S 138/07 20130101;
C08F 210/16 20130101; C08L 23/06 20130101; C08F 10/02 20130101;
C08F 2/001 20130101; C08L 23/06 20130101; C08L 2666/04 20130101;
C08L 23/0815 20130101; C08L 2666/04 20130101; C08F 110/02 20130101;
C08F 2500/02 20130101; C08F 2500/12 20130101; C08F 2500/07
20130101; C08F 2500/17 20130101; C08F 2500/13 20130101; C08F 210/16
20130101; C08F 210/14 20130101; C08F 2500/01 20130101; C08F 2500/12
20130101; C08F 2500/07 20130101; C08F 2500/17 20130101; C08F
2500/13 20130101; C08F 210/16 20130101; C08F 210/08 20130101; C08F
2500/01 20130101; C08F 2500/12 20130101; C08F 2500/07 20130101;
C08F 2500/17 20130101; C08F 2500/13 20130101 |
Class at
Publication: |
525/240 |
International
Class: |
C08L 023/04 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 14, 1998 |
SE |
9803501-7 |
Claims
1. A multimodal polymer composition forpipes, characterised in that
it is a multimodal polyethylene with a density of 0.930-0.965
g/cm.sup.3, and a viscosity at a shear stress of 747 Pa
(.theta..sub.747Pa) of at least 650 kPa.s, said multimodal
polyethylene comprising a low molecular weight (LMW) ethylene
homopolymer fraction and a high molecular weight (HMW) ethylene
copolymer fraction, said HMW fraction having a weight ratio of the
LMW fraction to the HMW fraction of (35-55):(65-45).
2. A multimodal polymer composition as claimed in claim 1, wherein
the multimodal polyethylene has a viscosity at a shear stress of
2.7 kPa (.theta..sub.2.7 kPa) of 260-450 kPa.s; and a shear
thinning index (SHI) defined as the ratio of the viscosities at
shear stresses of 2.7 and 210 kPa, respectively, of
SHI.sub.2.7/210=50-150, and a storage modulus (G') at a loss
modulus (G") of 5 kPa, of G'.sub.5 kPa.gtoreq.3000 Pa.
3. A multimodal polymer composition as claimed in claim 1 or 2,
wherein the multimodal polymer is a bimodal polyethylene produced
by (co) polymerisation in at least two steps.
4. A multimodal polymer composition as claimed in any one of claims
1-3, wherein the ethylene copolymer of the HMW fraction is a
copolymer of ethylene and a comonomer selected from the group
consisting of 1-butene, 1-hexene, 4-methyl-1-pentene, and
1-octene.
5. A multimodal polymer composition as claimed in any one of claims
1-4, wherein the amount of comonomer is 0.1-2.0 mol % of the
multimodal polymer.
6. A multimodal polymer composition according to any of claims 1-5,
having a weight ratio of the LMW fraction to the HMW fraction of
(43-51):(57-49).
7. A multimodal polymer composition as claimed in any one of claims
1-6, wherein the multimodal polymer has an MFR.sub.5 of 0.1-1.0
g/10 min.
8. A multimodal polymer composition as claimed in any one of claims
1-7, wherein the polymer is obtained by slurry polymerisation in a
loop reactor of a LMW ethylene homopolymer fraction, followed by
gas-phase polymerisation of a HMW ethylene copolymer fraction.
9. A multimodal polymer composition as claimed in claim 8, wherein
the slurry polymerisation is preceded by a prepolymerisation
step.
10. A multimodal polymer composition as claimed in claim 9, wherein
the polymer is obtained by prepolymerisation in a loop reactor,
followed by slurry polymerisation in a loop reactor of a LMW
ethylene homopolymer fraction, and gas-phase polymerisation of a
HMW ethylene copolymer fraction.
11. A multimodal polymer composition as claimed in any one of
claims 8-10, wherein polymerisation procatalyst and cocatalyst are
added to the first polymerisation reactor only.
12. A multimodal polymer composition as claimed in claim 11,
wherein the polymerisation catalyst is a Ziegler-Natta type
catalyst.
13. A pipe characterised in that it is a pressure pipe comprising
the multimodal polymer composition according to any one of the
preceding claims, which pipe withstands a pressure of 8.0 MPa gauge
during 50 years at 20.degree. C. (MRS8.0).
14. A pipe as claimed in claim 13, wherein the pipe is a pressure
pipe withstanding a pressure of 10 MPa gauge during 50 years at
20.degree. C. (MRS10.0).
15. A pipe as claimed in claim 13 or 14, wherein the pipe has a
rapid crack propagation (RCP) S4-value, determined according to ISO
13477:1997(E), of -5.degree. C. or lower.
16. A pipe as claimed in claim 15, wherein the pipe has a rapid
crack propagation (RCP) S4-value, determined according to ISO
13477:1997(E), of -7.degree. C. or lower.
17. A pipe as claimed in any one of claims 13-16, wherein the pipe
has a slow crack propagation resistance, determined according to
ISO 13479:1997, of at least 500 hrs at 4.6 MPa/80.degree. C.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a multimodal polymer
composition for pipes and a pipe prepared thereof.
BACKGROUND OF THE INVENTION
[0002] Nowadays, pipes of polymer material are frequently used for
various purposes, such as fluid transport, i.e. transport of liquid
or gas, e.g. water or natural gas, during which the fluid can be
pressurised. Moreover, the transported fluid may have varying
temperatures, usually within the temperature range from about
0.degree. C. to about 50.degree. C. Such pressure pipes are
preferably made of polyolefin plastic, usually unimodal ethylene
plastic such as medium density polyethylene (MDPE; density:
0.930-0.942 g/cm.sup.3) and high density polyethylene (HDPE;
density: 0.942-0.965 g/cm.sup.3). By the expression "pressure pipe"
herein is meant a pipe which, when used, is subjected to a positive
pressure, i.e. the pressure inside the pipe is higher than the
pressure outside the pipe.
[0003] Polymer pipes are generally manufactured by extrusion, or,
to a smaller extent, by injection moulding. A conventional plant
for extrusion of polymer pipes comprises an extruder, a die-head, a
calibrating device, cooling equipment, a pulling device, and a
device for cutting or for coiling-up the pipe.
[0004] The manufacture of PE materials to be used in pressure pipes
is discussed in an article by Scheirs et al [Scheirs, Bohm Boot and
Leevers: PE100 Resins for Pipe Applications, TRIP Vol. 4, No 12
(1996) pp. 408-415]. The authors discuss the production technology
and properties of PE100 pipe materials. They point out the
importance of proper comonomer distribution and molecular weight
distribution to optimize the slow crack growth and rapid crack
propagation.
[0005] The European patent application EP 739937 A2 discloses a
pipe having improved properties. The pipe is made of a bimodal PE
resin, and it has a specified stress cracking resistance, impact
strength and stiffness. The publication discloses that preferably
the material should have an MFR.sub.5 not higher than 0.35 g/10
min.
[0006] The properties of conventional polymer pipes are sufficient
for many purposes, although enhanced properties may be desired, for
instance in applications requiring high pressure resistance, i.e.
pipes that are subjected to an internal fluid pressure for a long
and/or short period of time. As examples of properties which it is
desirable to improve may be mentioned the processability, the
impact strength, the modulus of elasticity, the rapid crack
propagation resistance, the slow crack growth resistance, and the
design stress rating of the pipe.
[0007] A problem when manufacturing large diameter pipes,
particularly from multimodal, such as bimodal, polymer material, is
that it is difficult to maintain uniform dimensions all over the
pipe. This is due to gravity flow of the polymer melt, causing it
to flow from the upper part of the pipe to the lower part (often
called "sagging"). Thus, the wall thickness at the upper part of
the pipe becomes smaller than at the lower part of the pipe. The
sagging problem is particularly pronounced for thick-walled large
diameter pipes.
[0008] The above sagging problem has been discussed in the German
patent application DE 19604196 A1. It discloses a process to
manufacture a large-bore, thick walled pipe of polyethylene. The
pipe is extruded through a ring formed die and cooled on both inner
and outer surfaces. This double sided cooling is said to eliminate
the deformation of the pipe due to gravity-induced flow of the melt
emerging from the die.
[0009] The sagging problem has also been discussed in an article by
D. N. Githuku and A. J. Giacomin, "Elimination of Sag in Plastic
Pipe Extrusion", Intern. Polymer Processing VII (1992) 2, 140-143.
The conventional way to reduce sag is by manually adjusting the die
eccentricity which typically requires three or four tries at
start-up to get an acceptable thickness profile. The article
proposes a new way to reduce sag, namely by rotating the pipe
during cooling.
[0010] A mathematical mode of cooling and solidification, coupled
with gravity induced flow during the cooling of extruded plastic
pipes is set up and solved by the finite element method in an
article by J. F. T. Pittman, G. P. Whitman, S. Beech, and D. Gwynn,
"Cooling and Wall Thickness Uniformity in Plastic Pipe
Manufacture", Intern. Polymer Processing IX (1994) 2, 130-140. Melt
rheology and determination of melt flow properties at the very low
stress levels that are relevant at sag is also discussed.
SUMMARY OF THE INVENTION
[0011] It has now been discovered that the above sagging problem
can be overcome by preparing the pipe from a specific, well defined
type of multimodal polyethylene. More particularly, the multimodal
polyethylene should have a medium to high density, a high viscosity
at low shear stress, a carefully selected ratio between its low
molecular weight fraction and high molecular weight fraction, and
include a comonomer in its high molecular weight fraction only.
Preferably, the multimodal polyethylene should have a specific
molecular weight and a well defined molecular weight
distribution.
[0012] Thus, the present invention provides a multimodal polymer
composition for pipes, characterised in that it is a multimodal
polyethylene with a density of 0.930-0.965 g/cm.sup.3, and a
viscosity at a constant shear stress of 747 Pa (.eta..sub.747Pa) of
at least 650 kPa.s, said multimodal polyethylene comprising a low
molecular weight (LMW) ethylene homopolymer fraction and a high
molecular weight (HMW) ethylene copolymer fraction, said HMW
fraction having a weight ratio of the LMW fraction to the HMW
fraction of (35-55):(65-45).
[0013] It is much preferred that the multimodal polyethylene has a
viscosity at a shear stress of 2.7 kPa (.eta..sub.2.7 kPa) of
260-450 kPa.s; and a shear thinning index (SHI), defined as the
ratio of the viscosities at shear stresses of 2.7 kPa and 210 kPa,
respectively, of SHI.sub.2.7/210=50-150, and a storage modulus (G')
at a loss modulus (G") of 5 kPa, of G'.sub.5 kPa .gtoreq.3000 Pa.
Preferably densities in the range 0.937-0.942 g/cm.sup.3 are used
for smaller diameter MD pressure pipes while higher densities of
0.943-0.955 g/cm.sup.3 are used for larger diameter HD pressure
pipes.
[0014] The present invention also provides a pipe comprising said
multimodal polymer composition, which pipe withstands a hoop stress
of 8.0 MPa gauge during 50 years at 20.degree. C. (MRS8.0).
[0015] Preferably, the pipe has a rapid crack propagation (RCP)
S4-value, determined according to ISO 13477:1997(E), of -5.degree.
C. or lower and a slow crack propagation resistance, determined
according to ISO 13479:1997, of at least 500 hrs at 4.6
MPa/80.degree. C.
[0016] Other distinguishing features and advantages of the
invention will appear from the following specification and the
appended claims.
DETAILED DESCRIPTION OF THE INVENTION
[0017] As stated above, the pressure pipe composition of the
present invention is made from a specific multimodal polyethylene.
This is in contrast to prior art polyethylene pipes which usually
are made of unimodal polyethylene or bimodal polyethylene which
does not have the specific molecular weight distribution and
composition of the multimodal polyethylene of the present
invention.
[0018] The "modality" of a polymer refers to the form of its
molecular weight distribution curve, i.e. the appearance of the
graph of the polymer weight fraction as function of its molecular
weight. If the polymer is produced in a sequential step process,
utilizing reactors coupled in series and using different conditions
in each reactor, the different fractions produced in the different
reactors will each have their own molecular weight distribution.
When the molecular weight distribution curves from these fractions
are superimposed into the molecular weight distribution curve for
the total resulting polymer product, that curve will show two or
more maxima or at least be distinctly broadened in comparison with
the curves for the individual fractions. Such a polymer product,
produced in two or more serial steps, is called bimodal or
multimodal depending on the number of steps. In the following all
polymers thus produced in two or more sequential steps are called
"multimodal". It is to be noted here that also the chemical
compositions of the different fractions may be different. Thus one
or more fractions may consist of an ethylene copolymer, while one
or more others may consist of ethylene homopolymer.
[0019] By properly selecting the different polymer fractions and
the proportions thereof in the multimodal polyethylene a pipe with
good non-sagging properties together with good processability, good
slow crack growth resistance, rapid crack propagation resistance,
and a high design stress rating is obtainable.
[0020] The pressure pipe composition of the present invention is a
multimodal polyethylene, preferably a bimodal polyethylene. The
multimodal polyethylene comprises a low molecular weight (LMW)
ethylene homopolymer fraction and a high molecular weight (HMW)
ethylene copolymer fraction. Depending on whether the multimodal
polyethylene is bimodal or has a higher modality the LMW and HMW
fractions may comprise only one fraction each or include
subfractions, i.e. the LMW may comprise two or more LMW
sub-fractions and similarly the HMW fraction may comprise two or
more HMW sub-fractions. It is a characterising feature of the
present invention that the LMW fraction is an ethylene homopolymer
and that the HMW fraction is an ethylene copolymer, i.e. it is only
the HMW fraction that includes a comonomer. As a matter of
definition, the expression "ethylene homopolymer" used herein
relates to an ethylene polymer that consists substantially, i.e. to
at least 97% by weight, preferably at least 99% by weight, more
preferably at least 99.5% by weight, and most preferably at least
99.8% by weight of ethylene and thus is an HD ethylene polymer
which preferably only includes ethylene monomer units. Preferably,
the lower limit of the molecular weight range of the HMW fraction
is 3500, more preferably 4000, which means that almost all ethylene
copolymer molecules in the multimodal poly-ethylene pipe
composition of the invention have a molecular weight of at least
3500, preferably at least 4000. This preferred lower limit of the
HMW fraction gives a pressure pipe with enhanced strength.
[0021] In the present invention it is further important that the
proportions of the LMW and HMW fractions (also known as the "split"
between the fractions) are selected properly. More particularly,
the weight ratio of the LMW fraction to the HMW fraction should lie
in the range (35-55):(65-45), preferably (43-51):(57-49), most
preferably (44-50):(56-50). It is important that the split lies
within these ranges, because if the proportion of the HMW fraction
becomes too great it results in too low strength values and if it
is too low it results in an unacceptable formation of gels.
[0022] In order to achieve the non-sagging characteristics of the
multimodal polyethylene of the present invention it is important
that the polymer after being extruded into a pipe and before being
cooled does not flow by gravity from the upper part of the pipe
down to the lower part of the pipe and thus creates a non-uniform
distribution of polymer over the cross-section of the pipe.
[0023] The tendency of a polymer to display gravity flow may be
determined by means of a conventional melt index apparatus, such as
a Gottfert melt index apparatus. Generally, a polymer sample is
introduced into the bore (9.550 mm diameter; ISO 1133) of the melt
index apparatus, the temperature is set at 230.degree. C., the
bottom die is removed, and the polymer loaded with a weight
corresponding to the force of gravity that would have acted upon
the polymer if it had constituted the upper part of a pipe. One has
found that the piston (which weighs 120 g) of the melt index
apparatus corresponds to the gravity force on the polymer at the
upper part of a 2.2 m diameter pipe, and it is therefore suitable
to use the piston without any extra weight as the gravity force
acting upon the polymer sample. During the test the polymer flow is
determined at intervals for 75 min and the average gravity flow is
then determined in mm/10 min. At the present invention the gravity
flow of the polymer should be less than 0.1 mm/10 min. A more
detailed description of the steps of the gravity flow determination
method is given below.
[0024] 1. Set the temperature to 230.degree. C. and let it
stabilise.
[0025] 2. Weight the piston to an accuracy of 0.1 g.
[0026] 3. When the temperature is stable insert 6-8 g of the
material to be measured.
[0027] 4. Let the material heat soak for 10 min.
[0028] 5. After 10 min open the bottom holder for the die and press
out the die by pressing the melt pool from above.
[0029] 6. Take away the die and apply the piston. Press down the
piston until the lower marking scratch on the piston is 29 mm above
the filling hole.
[0030] 7. Let the melt pool relax for 10 min as some materials have
a more pronounced melt elasticity than others and the induced
elasticity from the pressing down of the melt pool may influence
the result.
[0031] 8. Start the measurement by measuring the height of the
lower marking scratch above the filling hole with a sliding caliper
to an accuracy of 0.1 mm. Start the stop watch.
[0032] 9. Make a measurement of the height above the filing hole
each 20 min and make a final measurement after 75 min.
[0033] 10. Make notes and present the results of the height each 20
min. Calculate the travelling distance each 20 min in mm as well as
the travelling speed expressed as mm/10 min. Finally calculate the
average travelling distance and velocity after 75 min (travel.
dist/75) and make a report.
[0034] 11. Clean the equipment in the normal manner.
[0035] Although the above method is a simple and easy way of
determining the sagging property of a pipe material, the accuracy
for very low gravity flow materials is somewhat uncertain. In order
to remedy this disadvantage it is preferred to use another method
which correlates well with the above described gravity flow method,
but gives greater accuracy for materials with very low gravity
flow. This preferred method, which is used in connection with the
present invention relates to the rheology of the polymer and is
based on determination of the viscosity of the polymer at a very
low, constant shear stress. A shear stress of 747 Pa has been
selected for this method. The viscosity of the polymer at this
shear stress is determined at a temperature of 190.degree. C. and
has been found to be inversely proportional to the gravity flow of
the polymer, i.e. the greater the viscosity the lower the gravity
flow. At the present invention the viscosity at 747 Pa and
190.degree. C. should be at least 650 kPa.s. A more detailed
description of the steps of the method for determination of the
viscosity of the polymer at 747 Pa and 190.degree. C. is given
below.
[0036] The determination is made by using a rheometer, preferably a
Bohlin CS Melt Rheometer. Rheometers and their function have been
described in "Encyclopedia of Polymer Science and Engineering", 2nd
Ed., Vol. 14, pp. 492-509. The measurements are performed under a
constant stress between two 25 mm diameter plates (constant
rotation direction). The gap between the plates is 1.8 mm. An 1.8
mm thick polymer sample is inserted between the plates.
[0037] The sample is temperature conditioned during 2 min before
the measurement is started. The measurement is performed at
190.degree. C. After temperature conditioning the measurement
starts by applying the predetermined stress. The stress is
maintained during 1800 s to let the system approach steady state
conditions. After this time the measurement starts and the
viscosity is calculated.
[0038] The measurement principle is to apply a certain torque to
the plate axis via a precision motor. This torque is then
translated into a shear stress in the sample. This shear stress is
kept constant. The rotational speed produced by the shear stress is
recorded and used for the calculation of the viscosity of the
sample.
[0039] Rheology measurements according to ASTM D 4440-95a may also
be used to characterise other important properties of the polymer,
such as the molecular weight and molecular weight distribution
(MWD).
[0040] The use of rheology is advantageous in those cases where the
high molecular weight end of the molecular weight distribution is
important. Typically, size exclusion chromatography (gel permeation
chromatography), which often is used to measure the molecular
weight distribution, is not sensitive enough in this molecular
weight range.
[0041] The storage modulus (G') and the loss modulus (G") together
with the absolute value of the complex viscosity (.eta.*) as a
function of the frequency (.omega.) or the absolute value of the
complex modulus (G*) are obtained by rheology measurements.
.eta.*={square root}{square root over
((G.sup.'2+G.sup."2))}/.omega.
G*={square root}{square root over ((G.sup.'2+G.sup."2))}
[0042] According to Cox-Merz rule the complex viscosity function,
.eta.*(.omega.) is the same as the conventional viscosity function
(viscosity as a function of shear rate), if frequency is taken in
rad/s. If this empiric equation is valid, the absolute value of the
complex modulus corresponds to the shear stress in conventional
(that is steady state) viscosity measurements. This means that the
function .eta.*(G*) is the same as the viscosity as a function of
shear stress.
[0043] In the present method the viscosity at a low shear stress or
.eta.* at a low G* (which serves as an approximation of the so
called zero viscosity) is used as a measure of average molecular
weight.
[0044] According to the invention, .eta..sub.2.7 kPa (viscosity at
2.7 kPa shear stress) should be between 260-450 kPa.s.
[0045] On the other hand, shear thinning, that is the decrease of
viscosity with G*, gets more pronounced the broader the molecular
weight distribution is. This property can be approximated by
defining a so called shear thinning index, SHI, as a ratio of the
viscosity at two different shear stresses. In the present invention
the shear stresses (or G*) 2.7 kPa and 210 kPa are used for
calculating the SHI.sub.2..sub.7/210 as a measure of the broadness
of the molecular weight distribution.
SHI.sub.2.7/210=.eta.*.sub.2.7/.eta.*.sub.210
[0046] where
[0047] .eta.*.sub.2.7 is the complex viscosity at G*=2.7 kPa
and
[0048] .eta.*.sub.210 is the complex viscosity at G*=210 kPa.
[0049] According to the invention, SHI.sub.2.7/210 should be
between 50-150.
[0050] The storage modulus, G', may also be used as a measure of
the molecular weight distribution. As mentioned above, the storage
modulus function, G' (.omega.) and the loss modulus function G"
(.omega.), are obtained as primary functions from dynamic
measurements. The value of the storage modulus at a specific value
of loss modulus increases with broadness of the molecular weight
distribution. However this quantity is highly dependent on the
shape of the molecular weight distribution of the polymer.
Especially it is a measure of the high molecular weight end of the
molecular weight distribution. According to the invention, the
material should have a G'.sub.5 kPa (G'at G"=5 kPa).gtoreq.3000
Pa.
[0051] Rheological measurements were made using the dynamic
rheometers Bohlin CS Melt Rheometer like Rheometrics RDA II. The
measurements were performed at 190.degree. C. under nitrogen
atmosphere using plate & plate test fixture with diameter of 25
mm. The strain amplitude was chosen so that a linear working range
was obtained. From the measurements storage modulus (G') and loss
modulus (G") together with absolute value of complex viscosity
(.eta.*) as a function of frequency (.omega.) or the absolute value
of complex modulus (G*) were obtained.
[0052] It has been found that when the polymer has been prepared to
have the above-mentioned characteristics, the resulting material
has low tendency for sagging. It also has a good extrudability and
good mechanical properties. All the rheological mesurements (except
the determination of G', which was made using a Rheometrics RDA II
Dynamic Rheometer) have been made using a Bohlin CS Melt Rheometer
and were carried out at 190.degree. C. under nitrogen
atmosphere.
[0053] The melt flow rate (MFR), which is equivalent to the term
"melt index" previously used, is another important property of the
multimodal polyethylene for pipes according to the invention. The
MFR is determined according to ISO 1133 and is indicated in g/10
min. The MFR is an indication of the flowability, and hence the
processability, of the polymer. The higher the melt flow rate, the
lower the viscosity of the polymer. The MFR is determined at
different loadings such as 2.1 kg (MFR.sub.2.1; ISO 1133, condition
D) or 5 kg (MFR.sub.5; ISO 1133, condition T). At the present
invention the multimodal polyethylene should have an MFR.sub.5 of
0.1-1.0 g/10 min, preferably 0.15-0.8 g/10 min.
[0054] Another characterising feature of the present invention is
the density of the multimodal polyethylene. For reasons of strength
the density lies in the medium to-high density range, more
particularly in the range 0.930-0.965 g/cm.sup.3. Preferably, lower
densities of 0.937-0.942 g/cm.sup.3 are used for smaller diameter
MD pressure pipes, while higher densities of 0.943-0.955 g/cm.sup.3
are used for larger diameter HD pressure pipes. The pressure pipes
of medium density multimodal polyethylene are somewhat more
flexible than pressure pipes of high density multimodal
polyethylene and may therefore more easily be coiled into a roll.
On the other hand it is possible to obtain pressure pipes of a
higher design stress rating with high density multimodal
polyethylene than with medium density multimodal polyethylene.
[0055] It should be noted that the multimodal polymer composition
of the present invention is characterised, not by any single one of
the above defined features, but by the combination of all the
features defined in claim 1. By this unique combination of features
it is possible to obtain pressure pipes of superior performance,
particularly with regard to sagging, processability, rapid crack
propagation (RCP) resistance, design stress rating, impact
strength, and slow crack propagation resistance.
[0056] The processability of a pipe (or rather the polymer thereof)
may be determined in terms of the number of screw revolutions per
minute (rpm) of an extruder for a predetermined output of pipe in
kg/h, but also the surface appearance of the pipe is then
important.
[0057] The rapid crack propagation (RCP) resistance of a pipe may
be determined according to a method called the S4 test (Small Scale
Steady State), which has been developed at Imperial College,
London, and which is described in ISO 13477:1997(E). According to
the RCP-S4 test a pipe is tested, which has an axial length not
below 7 pipe diameters. The outer diameter of the pipe is about 110
mm or greater and its wall thickness about 10 mm or greater. When
determining the RCP properties of a pipe in connection with the
present invention, the outer diameter and the wall thickness have
been selected to be 110 mm and 10 mm, respectively. While the
exterior of the pipe is at ambient pressure (atmospheric pressure),
the pipe is pressurised internally, and the internal pressure in
the pipe is kept constant at a pressure of 0.5 MPa positive
pressure. The pipe and the equipment surrounding it are
thermostatted to a predetermined temperature. A number of discs
have been mounted on a shaft inside the pipe to prevent
decompression during the tests. A knife projectile is shot, with
well-defined forms, towards the pipe close to its one end in the
so-called initiating zone in order to start a rapidly running axial
crack. The initiating zone is provided with an abutment for
avoiding unnecessary deformation of the pipe. The test equipment is
adjusted in such a manner that crack initiation takes place in the
material involved, and a number of tests are effected at varying
temperatures. The axial crack length in the measuring zone, having
a total length of 4.5 diameters, is measured for each test and is
plotted against the set test temperature. If the crack length
exceeds 4 diameters, the crack is assessed to propagate. If the
pipe passes the test at a given temperature, the temperature is
lowered successively until a temperature is reached, at which the
pipe no longer passes the test, but the crack propagation exceeds 4
times the pipe diameter. The critical temperature (T.sub.crit) i.e.
the ductile brittle transition temperature as measured according to
ISO 13477:1997(E) is the lowest temperature at which the pipe
passes the test. The lower the critical temperature the better,
since it results in an extension of the applicability of the pipe.
It is desirable for the critical temperature to be around
-5.degree. C. or lower. A pressure pipe made of the multimodal
polymer composition according to the present invention preferably
has an RCP-S4 value of -1.degree. C. (minimum requirement for an MD
PE80 pipe) or lower, more preferably -40.degree. C. (minimum
requirement for an HD PE80 pipe) or lower, and most preferably
-7.degree. C. (minimum requirement for an HD PE100 pipe) or
lower.
[0058] The design stress rating is the circumferential stress a
pipe is designed to withstand for 50 years without failure and is
determined for different temperatures in terms of the Minimum
Required Strength (MRS) according to ISO/TR 9080. Thus, MRS8.0
means that the pipe is a pipe withstanding a hoop stress of 8.0 MPa
gauge for 50 years at 20.degree. C., and similarly MRS10.0 means
that the pipe withstands a hoop stress of 10 MPa gauge for 50 years
at 20.degree. C. A pressure pipe made of the multimodal polymer
composition according to the present invention preferably has a
design stress rating of at least MRS8.0, and most preferably
MRS10.0.
[0059] The impact strength is determined as Charpy Impact Strength
according to ISO 179. A pressure pipe made of the multimodal
polymer composition according to the present invention preferably
has an impact resistance at 0.degree. C. of at least 10 kJ/m.sup.2,
more preferably at least 14 kJ/m.sup.2, and most preferably at
least 15 kJ/m.sup.2.
[0060] The slow crack propagation resistance is determined
according to ISO 13479:1997 in terms of the number of hours the
pipe withstands a certain pressure at a certain temperature before
failure. A pressure pipe made of the multimodal polymer composition
according to the present invention preferably has a slow crack
propagation resistance of at least 1000 hrs at 4.0 MPa/80.degree.
C., and more preferably at least 500 hrs at 4.6 MPa/80.degree.
C.
[0061] The modulus of elasticity is determined according to ISO
527-2 (with test specimen 1B). A pressure pipe made of the
multimodal polymer composition according to the present invention
preferably has a modulus of elasticity of at least 800 MPa, more
preferably at least 950 MPa, and most preferably at least 1100
MPa.
[0062] A pressure pipe made of the multimodal polymer composition
of the present invention is prepared in a conventional manner,
preferably by extrusion in an extruder. This is a technique well
known to the skilled person an no further particulars should
therefore be necessary here concerning this aspect.
[0063] It is previously known to produce multimodal, in particular
bimodal, olefin polymers, such as multimodal polyethylene, in two
or more reactors connected in series. As instance of this prior
art, mention may be made of EP 517 868, which is hereby
incorporated by way of reference as regards the production of
multimodal polymers.
[0064] According to the present invention, the main polymerisation
stages are preferably carried out as a combination of slurry
polymerisation/gas-phase polymerisation. The slurry polymerisation
is preferably performed in a so-called loop reactor. The use of
slurry polymerisation in a stirred-tank reactor is not preferred in
the present invention, since such a method is not sufficiently
flexible for the production of the inventive composition and
involves solubility problems. In order to produce the inventive
composition of improved properties, a flexible method is required.
For this reason, it is preferred that the composition is produced
in two main polymerisation stages in a combination of loop
reactor/gas-phase reactor. Optionally and advantageously, the main
polymerisation stages may be preceded by a prepolymerisation, in
which case up to 20% by weight, preferably 1-10% by weight, more
preferably 1-5% by weight, of the total amount of polymers is
produced. The prepolymer is preferably an ethylene homopolymer
(HDPE). At the prepolymerisation all of the catalyst is preferably
charged into a loop reactor and the prepolymerisation is performed
as a slurry polymerisation. Such a prepolymerisation leads to less
fine particles being produced in the following reactors and to a
more homogeneous product being obtained in the end. Generally, this
technique results in a multimodal polymer mixture through
polymerisation with the aid of a Ziegler-Natta or metallocene
catalyst in several successive polymerisation reactors. Chromium
catalysts are not preferred in connection with the present
invention. In the production of, say, a bimodal polyethylene, which
according to the invention is the preferred polymer, a first
ethylene polymer is produced in a first reactor under certain
conditions with respect to hydrogen-gas concentration, temperature,
pressure, and so forth. After the polymerisation in the first
reactor, the polymer including the catalyst is separated from the
reaction mixture and transferred to a second reactor, where further
polymerisation takes place under other conditions. Usually, a first
polymer of high melt flow rate (low molecular weight, LMW) and with
no addition of comonomer is produced in the first reactor, whereas
a second polymer of low melt flow rate (high molecular weight, HMW)
and with addition of comonomer is produced in the second reactor.
As comonomer of the HMW fraction various alpha-olefins with 4-8
carbon atoms may be used, but the comonomer is preferably selected
from the group consisting of 1-butene, 1-hexene,
4-methyl-1-pentene, and 1-octene. The amount of comonomer is
preferably such that it comprises 0.1-2.0 mol %, more preferably
0.1-1.0 mol % of the multimodal polyethylene. The resulting end
product consists of an intimate mixture of the polymers from the
two reactors, the different molecular-weight-distribution curves of
these polymers together forming a molecular-weight-distribution
curve having a broad maximum or two maxima, i.e. the end product is
a bimodal polymer mixture. Since multimodal, and especially
bimodal, ethylene polymers, and the production thereof belong to
the prior art, no detailed description is called for here, but
reference is had to the above mentioned EP 517 868.
[0065] As hinted above, it is preferred that the multimodal
polyethylene composition according to the invention is a bimodal
polymer mixture. It is also preferred that this bimodal polymer
mixture has been produced by polymerisation as above under
different polymerisation conditions in two or more polymerisation
reactors connected in series. Owing to the flexibility with respect
to reaction conditions thus obtained, it is most preferred that the
polymerisation is carried out in a loop reactor/a gas-phase
reactor. Preferably, the polymerisation conditions in the preferred
two-stage method are so chosen that a comparatively low-molecular
polymer having no content of comonomer is produced in one stage,
preferably the first stage, owing to a high content of
chain-transfer agent (hydrogen gas), whereas a high-molecular
polymer having a content of comonomer is produced in another stage,
preferably the second stage. The order of these stages may,
however, be reversed.
[0066] In the preferred embodiment of the polymerisation in a loop
reactor followed by a gas-phase reactor, the polymerisation
temperature in the loop reactor preferably is 92-98.degree. C.,
more preferably about 95.degree. C., and the temperature in the
gas-phase reactor preferably is 75-90.degree. C., more preferably
82-87.degree. C.
[0067] A chain-transfer agent, preferably hydrogen, is added as
required to the reactors, and preferably 200-800 moles of
H.sub.2/kmoles of ethylene are added to the reactor, when the LMW
fraction is produced in this reactor, and 0-50 moles of
H.sub.2/kmoles of ethylene are added to the gas phase reactor when
this reactor is producing the HMW fraction.
[0068] As indicated earlier, the catalyst for polymerising the
multimodal polyethylene of the invention preferably is a
Ziegler-Natta type catalyst. Particularly preferred are catalysts
with a high overall activity as well as a good activity balance
over a wide range of hydrogen partial pressures. Furthermore, the
molecular weight of the polymer produced by the catalyst is of
great importance. As an example of a preferred catalyst may be
mentioned the catalyst disclosed in FI 980788 and its corresponding
PCT application PCT/FI99/00286. It has surprisingly been found that
when using this catalyst in a multistage process, it is possible to
obtain the polymer having the characterstics described above. This
catalyst also has the advantage that the catalyst (procatalyst and
cocatalyst) only needs to and, indeed, only should be added in the
first polymerisation reactor. The preferred catalyst according to
FI 980788 and its corresponding PCT application FI99/00286 will be
described in more detail below.
[0069] FI 980788 and its corresponding PCT application
PCT/FI99/00286 discloses a process for the production of a high
activity procatalyst, characterized by the steps of reacting a
support comprising a magnesium halide compound having the formula
(1):
MgX.sub.n(OR).sub.2-n (1)
[0070] wherein each same or different R is a C.sub.1-C.sub.20 alkyl
or a C.sub.7-C.sub.26 aralkyl, each same or different X is a
halogen, and n is an integer 1 or 2, an alkyl metal halide compound
having the formula: a) according to PCT application
PCT/FI99/00286
R.sub.nM.sub.mX.sub.(3m-n) (2a)
[0071] wherein Me is B or Al, R being the same or different is a
C.sub.1-C.sub.10 alkyl, X being the same or different is a halogen,
n is an integer of 1 to 5 and m is an integer of 1 or 2, or,
preferably, b) according to FI 980788
(R.sup.1.sub.n.sup.1MeX.sup.1.sub.3-n.sup.1)m.sup.1 (2b)
[0072] wherein Me is B or Al, each same or different R.sup.1 is a
C.sub.1-C.sub.10 alkyl, each same or different X.sup.1 is a
halogen, n.sup.1 is an integer 1 or 2, and m.sup.1 is an integer 1
or 2, a magnesium composition containing magnesium bonded to a
hydrocarbyl and magnesium bonded to a hydrocarbyl oxide, said
magnesium composition having the empirical formula (3):
R.sup.2.sub.n.sup.2(OR.sup.3).sub.2-n.sup.2Mg (3)
[0073] wherein each same or different R.sup.2 is a C.sub.1-C.sub.20
alkyl, each same or different R.sup.3 is a C.sub.1-C.sub.20 alkyl
or a C.sub.1-C.sub.20 alkyl having a hetero element, and n.sup.2 is
between 0.01 and 1.99, and a titanium halide compound having the
formula (4):
(OR.sup.4).sub.n.sup.3TiX.sup.2.sub.4-n.sup.3 (4)
[0074] wherein each same or different R.sup.4 is a C.sub.1-C.sub.20
alkyl, each same or different X.sup.2 is a halogen, n.sup.3 is 0 or
an integer 1-3, and Ti is quadrivalent titanium.
[0075] By "magnesium composition" above is meant a mixture or a
compound. Note that formula (3) is an empirical formula and
expresses the molar amounts of alkyl R.sup.2 and alkoxy OR.sup.3
relative to the amount of magnesium Mg, which has been defined as
1, and differs from formulas (1), (2a), (2b) and (4), which are
essentially structural formulas and express the molecular structure
of reagents (1), (2a), (2b) and (4).
[0076] Preferably, the process comprises the subsequent steps
of:
[0077] a) providing said support comprising a magnesium halide
compound having the formula (1),
[0078] b) contacting said support comprising a magnesium halide
compound having the formula (1) with said alkyl metal halide
compound having the formula (2a) or (2b), to give a first
product,
[0079] c) contacting said first product with said magnesium
composition containing magnesium bonded to a hydrocarbyl and
magnesium bonded to a hydrocarbyl oxide and having the empirical
formula (3), to give a second product, and
[0080] d) contacting said second product with said titanium halide
compound having the formula (4).
[0081] The support used in the process is preferably in the form of
particles, the size of which is from about 1 .mu.m to about 1000
.mu.m, preferably about 10 .mu.m to about 100 .mu.m. The support
material must have a suitable particle size distribution, a high
porosity and a large specific surface area. A good result is
achieved if the support material has a specific surface area
between 100 and 500 m.sup.2/g support and a pore volume of 1-3 ml/g
support.
[0082] The above catalyst components (2a) to (4) are reacted with a
suitable catalyst support. If the catalyst components (2a) to (4)
are in the form of a solution of low viscosity, good catalyst
morphology and therewith good polymer morphology can be
achieved.
[0083] It is advantageous if in the magnesium halide compound
having the formula (1), R is a C.sub.1-C.sub.20 alkoxy or a
C.sub.7-C.sub.26 aralkoxy. However, it is preferable, if said
compound (1) is a magnesium dihalide, most preferably MgCl.sub.2
for example, the support may comprise solid MgCl.sub.2, either
alone as a powder, or as a powder mixture with other inorganic
powders.
[0084] According to another embodiment, the support comprising a
magnesium halide compound having the formula (1) comprises an
inorganic oxide. Several oxides are suitable, but silicon,
aluminium, titanium, chromium and zirconium oxide or mixtures
thereof are preferred. The most preferred inorganic oxide is
silica, alumina, silica-alumina, magnesia and mixtures thereof,
uttermost preferably silica. The inorganic oxide can also be
chemically pretreated, e.g. by silylation or by treatment with
aluminium alkyls.
[0085] It is good to dry the inorganic oxide before impregnating it
by other catalyst components. A good result is achieved if the
oxide is heat-treated at 100.degree. C. to 900.degree. C. for a
sufficient time, and thereby the surface hydroxyl groups, in the
case of silica, are reduced to below 2 mmol/g SiO.sub.2.
[0086] As was said above, the support may be a mixture of said
magnesium halide compound (1) and another solid powder, which
preferably is an inorganic oxide. According to another aspect, the
support comprises particles having a core comprising said inorganic
oxide and a shell comprising said magnesium halide compound having
the formula (1). Then, the support comprising a magnesium halide
compound having the formula (1) and an inorganic oxide can
conveniently be prepared by treating particles of the inorganic
oxide with a solution of the magnesium halide and removing the
solvent by evaporation.
[0087] When using a support containing both said magnesium halide
compound (1) and another component, the amount of magnesium halide
compound (1) is such that the support contains from 1 to 20% by
weight, preferably from 2 to 6% by weight, of magnesium Mg.
[0088] The process further comprises a step of reacting an alkyl
metal halide compound of the formula:
[0089] a) according to PCT application PCT/FI99/00286
R.sub.nM.sub.mX.sub.(3m-n) (2a)
[0090] wherein Me is B or Al, R being the same or different is a
C.sub.1-C.sub.10 alkyl, X being the same or different is a halogen,
n is an integer of 1 to 5, and m is an integer of 1 or 2, or,
preferably
[0091] b) according to FI 980788
(R.sup.1.sub.n.sup.1MeX.sup.1.sub.3-n.sup.1)m.sup.1 (2b)
[0092] wherein Me is B or Al, each same or different R.sup.1 is a
C.sub.1-C.sub.10 alkyl, each same or different X.sup.1 is a
halogen, n.sup.1 is an integer 1 or 2, and m.sup.1 is an integer 1
or 2. In formulas (2a) and (2b), Me is preferably Al. Each same or
different R or R.sup.1 is preferably a C.sub.1-C.sub.6 alkyl, and,
independently, the preferred same or different halogen X or X.sup.1
is chlorine n or n.sup.1 is preferably 1 and m.sup.1 is preferably
the integer 1 or 2. Most preferably, the alkyl metal halide
compound having the formulas (2a) and (2b) is an alkyl aluminium
dichloride, e.g. ethylaluminium dichloride (EADC).
[0093] The alkyl metal halide compound is preferably deposited on
the support material. An even deposition is achieved if the
viscosity of the agent or its solution is below 10 mPa.s at the
temperature applied. To achieve this low viscosity the alkyl metal
halide agent can be diluted by a non-polar hydrocarbon. The best
deposition is however achieved if the total volume of the deposited
alkyl metal halide solution is not exceeding the pore volume of the
support, or if the excess of diluting hydrocarbon is evaporated
away after the deposition of the alkyl metal halide. A good choice
is to use a 5-25% hydrocarbon solution of ethyl aluminium
dichloride. The chemical addition times and the addition techniques
are preferably adjusted to give an even distribution of the
chemical in the support material.
[0094] In the above mentioned preferred order of reaction steps a)
to d), step b) can advantageously be performed so that undiluted
alkyl metal halide (2a) or (2b) is used to treat the support
comprising a magnesium halide compound having the formula (1).
Alternatively, the support is contacted with a solution of the
alkyl metal halide compound having the formula (2a) or (2b) in an
essentially non-polar solvent, preferably a non-polar hydrocarbon
solvent, most preferably a C.sub.4-C.sub.10 hydrocarbon. The
concentration of the alkyl metal halide compound having the
formulas (2a) or (2b) in said non-polar solvent is usually 1-80% by
weight, preferably 5-40% by weight, most preferably 10-30% by
weight. Advantageously, the support is contacted with a solution of
said alkyl metal halide compound (2a, 2b) in a ratio mol of the
alkyl metal halide compound (2a, 2 b) to grams of the support of
between about 0.01 mmol/g and about 100 mmol/g, preferably about
0.5 mmol/g and about 2.0 mmol/g. The amount of reactants can also
be expressed as molar ratio, whereby it is advantageous, if the
molar ratio of said alkyl metal halide compound (2a, 2b) to said
magnesium halide compound (1) of the support is between about 0.01
mol/mol to about 100, preferably about 0.1 mol/mol to about 10,
most preferably from about 0.2 to about 3.0.
[0095] In step b), the temperature of said contacting is e.g.
5-80.degree. C., preferably 10-50.degree. C., most preferably
20-40.degree. C. The duration of said contacting is 0.1-3 h,
preferably 0.5-1.5 h.
[0096] In the process, the magnesium composition containing
magnesium bonded to a hydrocarbyl and magnesium bonded to a
hydrocarbyl oxide having the empirical formula (3), each same or
different R.sup.2 is preferably C.sub.2-C.sub.10 alkyl, most
preferably a C.sub.2-C8 alkyl. Each same or different R.sup.3 is
preferably a C.sub.3-C.sub.20 alkyl, more preferably a branched
C.sub.4-C.sub.10alkyl, most preferably a 2-ethyl-1-hexyl or a
2-propyl-1-pentyl.
[0097] The magnesium composition containing magnesium bonded to a
hydrocarbyl and magnesium bonded to a hydrocarbyl oxide having the
empirical formula (3) can also be expressed by its preparation.
According to one embodiment, it is a contact product of a dialkyl
magnesium having the formula (5):
R.sup.2.sub.2Mg (5)
[0098] wherein each same or different R.sup.2 is defined as above,
and an alcohol. Preferably, the dialkyl magnesium having the
formula (5) is dibutyl magnesium, butyl ethyl magnesium or butyl
octyl magnesium.
[0099] The magnesium composition can also be defined in that the
magnesium composition containing magnesium bonded to a hydrocarbyl
and magnesium bonded to a hydrocarbyl oxide having the empirical
formula (3) is a contact product of a dialkyl magnesium and an
alcohol having the formula (6):
R.sup.3OH (6)
[0100] wherein each same or different R.sup.3 is the same as above.
Preferably, the alcohol having the formula (6) is a 2-alkyl
alkanol, preferably 2-ethyl hexanol or 2-propyl pentanol. It was
found, that such branched alcohols gave better results than linear
alcohols.
[0101] Preferably, the magnesium composition containing magnesium
bonded to a hydrocarbyl and magnesium bonded to a hydrocarbyl oxide
having the empirical formula (3) is a contact product of a dialkyl
magnesium and an alcohol in a molar ratio alcohol to dialkyl
magnesium of 0.01-100 mol/mol, preferably 1.0-5.0 mol/mol, more
preferably 1.7-2.0 mol/mol, most preferably 1.8-1.98 mol/mol. The
dialkyl magnesium and the alcohol are conveniently contacted by
adding the alcohol to a solution of said dialkyl magnesium in an
organic solvent, e.g. a C.sub.4-C.sub.10 hydrocarbon. Then, the
concentration of the solution is preferably between 1 and 50% by
weight, most preferably between 10 and 30% by weight. The
contacting temperature between the dialkyl magnesium and the
alcohol is preferably 10-50.degree. C., preferably from about
20.degree. C. to about 35.degree. C.
[0102] In step c) of the above mentioned preferred order
a).fwdarw.d) of the process, the contacting product of the support
with the alkyl metal halide compound (2) (=said first product) is
contacted with said magnesium composition containing magnesium
bonded to a hydrocarbyl and magnesium bonded to a hydrocarbyl oxide
and having the empirical formula (3).
[0103] Preferably, said first product is contacted with said
magnesium composition (3) in a ratio moles of magnesium/g of the
support between 0.001-1000 mmol/g, preferably 0.01-100 mmol/g, most
preferably 0.1-10 mmol/g (g of the support means, in the case of
said first reaction product, the support which was used as starting
material for the first reaction product).
[0104] A good deposition of said magnesium composition as a
solution is achieved if the volume of the magnesium composition (3)
solution is about two times the pore volume of the support
material. This is achieved if the concentration of the composition
in a hydrocarbon solvent is between 5-60% in respect of the
hydrocarbon used. When depositing the magnesium composition on the
support material its hydrocarbon solution should have a viscosity
that is lower than 10 mPa.s at the temperature applied. The
viscosity of the magnesium complex solution can be adjusted for
example by the choice of the group R.sup.4 in the formula (3), by
the choice of the concentration of the hydrocarbon solution, by the
choice of the ratio between the magnesium alkyl and the alcohol or
by using some viscosity lowering agent. The titanium compound can
be added to the support material with or without a previous drying
of the catalyst to remove the volatile hydrocarbons. Remaining
hydrocarbons can if desired be removed by using slight
underpressure, elevated temperature or nitrogen flash.
[0105] In the process, the transition metal compound is a titanium
halide compound having the formula (4). R.sup.4 is preferably a
C.sub.2-C.sub.8 alkyl, most preferably a C.sub.2-C.sub.6 alkyl.
X.sup.2 is preferably chlorine and, independently, n.sup.3 is
preferably 0. Said titanium halide compound having the formula (4)
is advantageously titanium tetrachloride.
[0106] According to one embodiment, in addition to said titanium
compound having the formula (4), a titanium compound having the
formula (7):
(R.sup.5O)n.sup.4TiX.sup.3.sub.4-n.sup.4 (7)
[0107] wherein each same of different R.sup.5 is a C.sub.1-C.sub.20
alkyl, preferably a C.sub.2-C.sub.8 alkyl, most preferably a
C.sub.2C.sub.6 alkyl, each same or different X.sup.3 is a halogen,
preferably chlorine, n.sup.4 is an integer 1-4, and Ti is
quadrivalent titanium, is reacted. The titanium compound (7) always
has at least one alkoxy group, which helps dissolving the titanium
compound (4) which does not necessarily have alkoxide, into an
organic solvent before the contacting. Naturally, the more alkoxide
groups compound (4) has, the less is the need for compound (7). If
compound (7) is used, the preferable combination is that of
titanium tetrachloride and a titanium tetra C.sub.1-C.sub.6
alkoxide.
[0108] In step d) of the preferred step sequence a).fwdarw.d), said
second product is advantageously contacted with the titanium
compound having the formula (4) in a ratio moles of said titanium
compound/g of the support of 0.01-10 mmol/g, preferably 0.1-2
mmol/g. Preferably, said second reaction product is contacted with
said titanium compound (4) in a ratio moles of said titanium
compound (4)/total moles of the magnesium of 0.05-2 mol/mol,
preferably 0.1-1.2 mol/mol, most preferably 0.2-0.7 mol/mol. The
temperature is usually 10-80.degree. C., preferably 30-60.degree.
C., most preferably from about 40.degree. C. to about 50.degree.
C., and the contacting time is usually 0.5-10 h, preferably 2-8 h,
most preferably from about 3.5 h to about 6.5 h.
[0109] Above, the process for the preparation of a high activity
catalyst component for the production of olefin polymers of
different molecular weight and homogenous consistence, have been
described in detail.
[0110] The catalyst component has high activity both when producing
low melt flow rate ethylene polymer and high melt flow rate
polymer. High molecular weight polymer has high melt viscosity,
i.e. low melt flow rate, and low molecular weight polymer has low
melt viscosity, i.e. high melt flow rate.
[0111] Simultaneously or separately, it preferably produces
ethylene homopolymer and copolymer with low gel content. Most
preferably it produces ethylene homopolymer having a gel number,
expressed as number of gel spots/sq.m in a film prepared from the
material, of 0. This means, that by the standards used, the
catalyst components can be used to produce totally homogenous
(gelless) low and high molecular weight ethylene polymer.
[0112] The alkyl metal halide compound of the formula (2) can, if
used, act completely or partially as a cocatalyst. However, it is
preferable to add a cocatalyst having the formula (9):
R.sup.6.sub.n.sup.5AlX.sup.4.sub.3-n.sup.5 (9)
[0113] wherein R.sup.6 is a C.sub.1-C.sub.20 alkyl, preferably a
C.sub.1-C.sub.10 alkyl, most preferably a C.sub.2-C.sub.6 alkyl
such as ethyl, X is halogen, preferably chlorine, n is 1 to 3, more
preferably 2 or 3, most preferably 3, to the polymerization
mixture. The cocatalyst having the formula (9) can be used
irrespective of whether said alkyl metal halide compound (2) is
used or not.
[0114] Although the invention has been described above with
reference to a specified multimodal polyethylene, it should be
understood that this multimodal polyethylene may include various
additives such as fillers, etc. as is known and conventional in the
art. An especially important additive is carbon black which is used
to colour the pipe black. It should be noted that some additives
may have a significant effect on the properties of the polymer.
Thus, the density of the black compound is typically significantly
higher than the density of the reactor product. Further, the pipe
made of the specified multimodal polyethylene may be a single-layer
pipe or form part of a multilayer pipe including further layers of
other pipe materials.
[0115] Having thus described the present invention it will now be
illustrated by way of non-limiting examples of preferred
embodiments in order to further facilitate the understanding of the
invention.
EXAMPLES
Example 1
[0116] (Preparation of the Catalyst)
[0117] Complex Preparation 8.6 g (66.4 mmol) of ethyl-1-hexanol was
added slowly to 27.8 g (33.2 mmol) of a 19.9% by weight solution of
butyl octyl magnesium. The reaction temperature was kept under
35.degree. C. This complex was used in catalyst preparations. The
molar ratio of 2-ethyl-1-hexanol to butyl octyl magnesium was
2:1.
[0118] Catalyst Preparation
[0119] 3.7 g (1.0 mmol/g carrier) of 20% ethyl aluminium dichloride
was added to 5.9 g of Sylopol 5510 silica/-MgCl.sub.2 carrier and
the mixture was stirred for one hour at 30.degree. C. 5.7 g (0.9
mmol/g carrier) of complex prepared according to "Complex
preparation" was added and the mixture was stirred for 4 hours at
35-45.degree. C. 0.6 g (0.55 mmol/g carrier) of TiCl.sub.4 was
added and the mixture was stirred for 4 hours at 35-45.degree. C.
The catalyst was dried at 45-80.degree. C. for 3 hours. The
composition of the catalyst obtained was Al 1.8%, Mg 3.9% and Cl
18.5% by weight.
Example 2
[0120] Inventive Material A (Two-stage Polymerisation with
Prepolymerised Catalyst)
[0121] Into a 50 dm.sup.3 loop reactor was added 7.0 g/h of
catalyst prepared according to Example 1, 2 kg/h of ethylene, 20
kg/h of propane and 1 g/h of hydrogen. The operating temperature
was 80.degree. C. and the operating pressure 65 bar.
[0122] The slurry was taken out of the reactor and led into a 500
dm.sup.3 loop reactor. The reactor was operated at 95.degree. C.
temperature and 61 bar pressure. The rate of polymer production was
35 kg/h and the MFR.sub.2 of the polymer produced was 280 g/10 min.
No comonomer was fed into the loop reactor.
[0123] From the loop reactor the polymer was transferred into a gas
phase reactor where the polymerisation was continued. The reactor
was operated at 85.degree. C. and 20 bar pressure. Ethylene,
hydrogen and 1-butene were fed into the reactor to obtain such
conditions that the rate of polymer production was 41 kg/h, the
MFR.sub.5 of the polymer produced was 0.24 g/10 min and the density
was 0.951 kg/dm.sup.3.
[0124] The polymer was then compounded with 5.7% by weight of a
carbon black master batch. The MFR.sub.5 of the compounded material
was 0.17 g/10 min and the density was 0.960 kg/dm.sup.3.
[0125] The polymerisation data are collected in Table 1.
Example 3
[0126] Inventive Materials B-D (Further Two-stage Polymerisations
with Prepolymerised Catalyst)
[0127] The procedure of Example 2 was repeated with the exception
that 1-hexene was used as comonomer in the gas phase reactor. The
data are collected in Table 1.
1TABLE 1 Material A B .degree. D Loop 35 31 33 32 production kg/h
Loop H.sub.2/C.sub.2 340 620 620 610 Loop MFR2 280 1000 1010 1060
g/10 min GPR 1-butene 1-hexene 1-hexene 1-hexene comonom. GPR 41 38
40 41 production kg/h GPR H.sub.2/C.sub.2 6 9 7 8 GPR 68 41 37 37
comon./C.sub.2 mol/kmol Fin. resin 0.19 0.40 0.27 0.24 MFR.sub.5
Fin. resin 0.951 0.950 0.952 0.953 density Comonomer 0.50 0.37 0.47
0.34 content, mol-% Split 2/44/54 2/44/54 2/45/53 2/43/55
[0128] The properties of the materials A-D after being compounded
with CB master batch are shown i Table 2.
2TABLE 2 Resin with CB compound A B .degree. D Density 0.960 0.962
0.960 0.960 kg/dm.sup.3 MFR.sub.5 0.17 0.29 0.25 0.21 g/10 min
MFR.sub.21 5.0 11 9.3 7.5 g/10 min FRR.sub.21/5 29 38 37 36
.eta..sub.2.7 kPa 425 292 359 397 kPa.s SHI.sub.2.7/210 66 110 113
109 G'.sub.5 kPa Pa 3100 3420 3405 3435 .eta..sub.747 Pa 1069 710
755 1070 kPa.s Gravity 0.068 0.096 0.068 0.068 flow mm/10 min
Notch/ 1217 (D) >1434 1881 (D) 4.6 Mpa h RCP 4S -7 -7 -11
T.sub.crit .degree. C. Impact 19 13 12 15 strength kJ/m.sup.2
[0129] It is evident from Table 1 that the resins A-D according to
the present invention all have very low gravity flows, below 0.1
mm/10 min, and high viscosities at the low shear stress of 747 Pa
(.eta..sub.747 Pa) of more than 650 kPa.s, i.e. the resins have
good non-sagging properties. In addition, pipes made of the resins
A-D have excellent physical properties, such as resistance to slow
crack propagation, a low T.sub.crit for rapid crack propagation and
good impact strength at 0.degree. C.
[0130] The properties of some commercial bimodal PE100 materials
E-G are shown in Table 3.
3 TABLE 3 Material E F G Density 0.959 0.958 0.958 kg/dm.sup.3
MFR.sub.5 g/10 min 0.35 0.46 0.21 MFR.sub.21 g/10 min 8.7 11.3 6.1
FRR.sub.21/5 25 25 30 .eta..sub.2.7 kpa kPa.s 138 89 264
SHI.sub.2.7/210 27.4 36 35 G'.sub.5 kPa Pa 2630 2250 2480
.eta..sub.747 Pa kPa.s 266 162 556 Gravity flow 0.95 1.58 0.66
min/10 min Notch/4.6 Mpa 1100 1069 2766 h RCP 4S -7 -6 -19
T.sub.crit .degree. C. Impact 14.0 12.5 16.6 strength
kJ/m.sup.2
[0131] The materials E-G all show excellent strength properties but
inferior sagging tendencies.
[0132] The properties of some commercial monomodal PE materials H-K
are shown in Table 4.
4 TABLE 4 Material H (PE80) I (PE63) K (PE80) Density 0.953 0.960
0.953 kg/dm.sup.3 MFR.sub.5 g/10 min 0.51 0.53 0.47 MFR.sub.21 g/10
min 17.0 17.0 13.1 FRR.sub.21/5 34.0 32.0 27.6 .eta..sub.2.7 kPa
kPa.s 305 208 263 SHI.sub.2.7/210 kPa 215 175 94 G'.sub.5 kPa Pa
4190 3960 .eta..sub.747 Pa kPa.s 1392 728 581 Gravity flow 0.13
0.17 0.33 mm/10 min Notch/4.6 Mpa 97 8 100-200 h RCP 4S +29 >+20
>0 T.sub.crit .degree. C. Impact 4.6 3.9 Strength kJ/m.sup.2
[0133] As apparent from the comparative materials, it has hitherto
not been found possible to combine the high strength, in terms of
long term hoop stress resistance, low T.sub.crit in the RCP 4S-test
and high impact strength, with good non-sagging property. The
comparative bimodal PE100-materials all show good strength
properties but have pronounced sagging tendencies, while of the
monomodal materials H and I have low sagging tendency, but less
good strength properties.
* * * * *