U.S. patent application number 12/085539 was filed with the patent office on 2009-04-16 for polyethylene composition suitable for the preparation of films and process for preparing the same.
This patent application is currently assigned to BASELL POLYOLEFINE GMBH. Invention is credited to Jorg Auffermann, Paulus De Lange, Manfred Hecker, Rainer Karer, Jennifer Kipke, Shahram Mihan, Harald Schmitz.
Application Number | 20090099315 12/085539 |
Document ID | / |
Family ID | 37680749 |
Filed Date | 2009-04-16 |
United States Patent
Application |
20090099315 |
Kind Code |
A1 |
Kipke; Jennifer ; et
al. |
April 16, 2009 |
Polyethylene Composition Suitable for the Preparation of Films and
Process for Preparing the Same
Abstract
A polyethylene composition, in particular suitable for the
preparation of films, and a process for preparing the same are
described. The polyethylene composition of the invention comprises
from 50 to 89% by weight of a first polyethylene component
comprising at least one multimodal polyethylene including a
plurality of ethylene polymer fractions having distinct molecular
weights and comonomer contents, at least one of said plurality of
ethylene polymer fractions being prepared by the use of a single
site catalyst, and from 50 to 11% by weight of a second
polyethylene component comprising a low or medium density
polyethylene.
Inventors: |
Kipke; Jennifer; (Frankfurt,
DE) ; Mihan; Shahram; (Bad Soden, DE) ; Karer;
Rainer; (Kaiserslautern, DE) ; Auffermann; Jorg;
(Freinsheim, DE) ; Hecker; Manfred; (Neustadt
Wied, DE) ; De Lange; Paulus; (Wesseling, DE)
; Schmitz; Harald; (Weinheim, DE) |
Correspondence
Address: |
LyondellBasell Industries
3801 WEST CHESTER PIKE
NEWTOWN SQUARE
PA
19073
US
|
Assignee: |
BASELL POLYOLEFINE GMBH
WESSELING
DE
|
Family ID: |
37680749 |
Appl. No.: |
12/085539 |
Filed: |
November 14, 2006 |
PCT Filed: |
November 14, 2006 |
PCT NO: |
PCT/EP2006/068444 |
371 Date: |
November 10, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60749791 |
Dec 13, 2005 |
|
|
|
Current U.S.
Class: |
525/240 |
Current CPC
Class: |
C08L 23/0815 20130101;
C08L 2205/02 20130101; C08L 23/04 20130101; C08L 2666/06 20130101;
C08L 2666/02 20130101; C08L 23/04 20130101; C08L 2205/025 20130101;
C08L 2205/035 20130101; C08L 2308/00 20130101; C08L 23/06 20130101;
C08L 2314/06 20130101; C08L 23/0815 20130101 |
Class at
Publication: |
525/240 |
International
Class: |
C08L 23/06 20060101
C08L023/06 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 28, 2005 |
EP |
05025875.5 |
Claims
1-10. (canceled)
11. A polyethylene composition comprising: (a) from 50 to 89% by
weight of a first polyethylene component comprising a multimodal
polyethylene including a plurality of ethylene polymer fractions
having distinct molecular weights and co-monomer contents, at least
one of said plurality of ethylene polymer fractions being prepared
by the use of a single site catalyst; and (b) from 50 to 11% by
weight of a second polyethylene component comprising a low density
polyethylene or a medium density polyethylene.
12. The polyethylene composition of claim 11 having a density of
0.915 to 0.955 g/cm.sup.3.
13. The polyethylene composition of claim 11, wherein said first
polyethylene component has a density of from 0.920 to 0.960
g/cm.sup.3.
14. The polyethylene composition of claim 11, wherein said second
polyethylene component has a density of from 0.910 to 0.940
g/cm.sup.3.
15. The polyethylene composition of claim 11, wherein the first
polyethylene component comprises a bimodal polyethylene including a
low molecular weight ethylene homopolymer and a high molecular
weight ethylene copolymer.
16. The polyethylene composition of claim 15 having a density of
0.915 to 0.955 g/cm.sup.3.
17. The polyethylene composition of claim 15, wherein said first
polyethylene component has a density of from 0.920 to 0.960
g/cm.sup.3.
18. The polyethylene composition of claim 15, wherein said second
component has a density of from 0.910 to 0.940 g/cm.sup.3.
19. The polyethylene composition of claim 15, wherein said high
molecular weight ethylene copolymer comprises 1 to 10% by weight of
a comonomer selected from the group consisting of propene,
1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene,
1-octene and 1-decene, and mixtures thereof.
20. A process for producing a polyethylene composition, said
process comprising: (a) subjecting ethylene, optionally with
comonomer(s), to a plurality of polymerization stages, wherein at
least one of the plurality of polymerization stages is carried out
in the presence of a single site catalyst to prepare a multimodal
first polyethylene component; (b) preparing a second polyethylene
component comprising a low density polyethylene or a medium density
polyethylene; and (c) combining said second polyethylene component
and said multimodal first polyethylene component to obtain a
polyethylene composition comprising from 50 to 89% by weight of the
first polyethylene component and from 50 to 11% by weight of the
second polyethylene component.
21. The process of claim 20, wherein the single site catalyst
comprise a metallocene.
22. The process of claim 20, wherein the polyethylene composition
has a density of 0.915 to 0.955 g/cm.sup.3.
23. The process of claim 20, wherein said first polyethylene
component has a density of from 0.920 to 0.960 g/cm.sup.3.
24. The process of claim 20, wherein said second component has a
density of from 0.910 to 0.940 g/cm.sup.3.
25. The process of claim 20, wherein the first polyethylene
component comprises a bimodal polyethylene including a low
molecular weight ethylene homopolymer and a high molecular weight
ethylene copolymer.
26. A film comprising a polyethylene composition of claim 11.
27. The film of claim 26, wherein the polyethylene composition has
a density of 0.915 to 0.955 g/cm.sup.3.
28. The film of claim 26, wherein said first polyethylene component
has a density of from 0.920 to 0.960 g/cm.sup.3.
29. The film of claim 26, wherein said second component has a
density of from 0.910 to 0.940 g/cm.sup.3.
30. The film of claim 26, wherein the first polyethylene component
comprises a bimodal polyethylene including a low molecular weight
ethylene homopolymer and a high molecular weight ethylene
copolymer.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a novel polyethylene
composition, to a process for the preparation thereof, as well as
to a film comprising such a polyethylene composition.
[0002] In the field of preparation of polyethylene films,
particularly in the field of medium density (MDPE) and high density
(HDPE) films, there is a long-felt need of providing films having,
at the same time, a number of mechanical and physical properties,
and in particular adequate mechanical strength, processability and
transparency, which are normally conflicting with each other.
[0003] In the present description and in the following claims, the
expression "medium density film" is used to indicate a film having
a density ranging from above 0.930 to 0.940 g/cm.sup.3, while the
expression "high density film" is used to indicate a film having a
density above 0.940 g/cm.sup.3.
[0004] In polyethylene film applications, a possible way to
evaluate the above-mentioned properties may be made through the
following parameters which, in the present description and in the
following claims, are defined and determined as specified
hereinbelow.
[0005] The mechanical strength of a polyethylene film may be
effectively evaluated, for example, by means of the dart drop
impact, which gives a measure of the puncture resistance of a film
under shock loading. In the present description and in the
following claims, the dart drop will be referred to as determined
by ASTM D 1709, Method A.
[0006] The processability of the composition on which the
polyethylene film is based may be determined in terms of MFR
according to standard ISO 1133, condition G, corresponding to a
measurement performed at a temperature of 190.degree. C. and under
a weight of 21.6 kg.
[0007] The transparency of a polyethylene film may be expressed in
terms of haze, gloss and/or of clarity. In the present description
and in the following claims, the haze will be referred to as
determined by ASTM D 1003-00 on a BYK Gardener Haze Guard Plus
Device on at least 5 pieces of 10.times.10 cm film, while the gloss
will be referred to as determined by ISO 2813 and the clarity will
be referred to as determined by ASTM D 1746-03 on a BYK Gardener
Haze Guard Plus Device, calibrated with calibration cell 77.5, on
at least 5 pieces of film 10.times.10 cm.
[0008] In the field of films, the above-mentioned mechanical and
optical parameters should range in ranges meeting the requirements
set by the packaging industry in the production, for example, of
hygiene films and laminating films for food packaging, where
transparency should be as high as possible.
PRIOR ART
[0009] Several polyethylene films are known whose properties
essentially depend, in addition to on the nature of the composition
on which the films are based, also on the way in which the film is
prepared and, in particular, on the kind of process used to prepare
the same. Among the different steps used to carry out the process,
a key role is played by the catalyst system selected in the
(co)polymerization step(s) which are carried out to obtain the
polyethylene starting from ethylene and, optionally, one comonomer
or more comonomers.
[0010] Accordingly, in the present description and in the following
claims, the term "polymer" is used to indicate both a homopolymer,
i.e. a polymer comprising repeating monomeric units derived from
equal species of monomers, and a copolymer, i.e. a polymer
comprising repeating monomeric units derived from at least two
different species of monomers, in which case reference will be made
to a binary copolymer, to a terpolymer, etc. depending on the
number of different species of monomers used.
[0011] Among the prior art medium density polyethylene films, films
prepared by means of chromium catalysts are known. Although
substantially suitable for the purpose, the polyethylene films
based on chromium catalysts suffer from an insufficient mechanical
strength and a very poor transparency. By way of illustrative
example, the known polyethylene films prepared by means of a
chromium catalyst have a dart drop impact ranging from 150 to 200
g, a MFR (190/21.6) ranging from 10 to 15 g/10 min, a haze ranging
from 70 to 80%, and a clarity ranging from 8 to 15%, such values
being essentially a function of the film thickness.
[0012] Such values of mechanical strength and transparency are
considered unacceptable, particularly in food packaging
applications. In the attempt of improving the transparency, low
density polyethylene (LDPE) prepared by high-pressure
polymerization, which is known for being transparent, has been
added to the medium density polyethylene prepared by means of
chromium catalysts. In the present description and in the following
claims, the term LDPE is used to indicate a polyethylene having a
density from 0.910 to 0.930 g/cm.sup.3.
[0013] For example, an LDPE film having a density of 0.930
g/cm.sup.3 and a MFR (190/2.16) of 1 g/10 min may have a clarity of
above 99% at a thickness of 50 .mu.m.
[0014] Although the compositions made of MDPE and LDPE show an
increased transparency, for example in terms of a certain increase
of clarity up from an initial value of about 13% (MDPE alone) to a
final value 56% (MDPE added with LDPE) at a thickness of 50 .mu.m,
a first disadvantage of these compositions is that such increase of
transparency is still insufficient for film applications in food
industry. A second disadvantage is that such a relative increase of
transparency is obtained at the expenses of the mechanical
strength. In particular, for example, MDPE films having a dart drop
impact of 180 g, when added with LDPE, may have a dart drop impact
in the range of 130-165 g depending on the amount of LDPE added to
MDPE. Such worsening of the mechanical properties of the mixture is
deemed to depend on the intrinsic poor mechanical dart drop impact
of the LDPE.
[0015] Thus, no significant improvement in transparency has been
attained by adding a LDPE to a MDPE prepared by means of a chromium
catalyst and the relative improvement of the transparency
inevitably results in an unacceptable worsening of the mechanical
properties of the film.
[0016] It is also known to use a blend of a metallocene-catalyzed
medium density polyethylene (mMDPE) with low density polyethylene
(LDPE) and/or a linear low density polyethylene (LLDPE), to produce
blown films, as for example described by patent U.S. Pat. No.
6,114,456. Compositions of such kind have sufficient processability
and are used to make blown films which have to some extent the good
optical properties of LDPE and the good mechanical properties of
mMDPE. However, such compositions have the main disadvantage in
that the dart drop impact sensibly decreases as the density
increases.
[0017] Patent application WO 01/62847 discloses a bimodal extrusion
composition of polyethylene which is prepared by (co)polymerizing
ethylene in a multistage polymerization sequence of successive
polymerization stages in the presence of a single site catalyst.
According to WO 01/62847, the bimodal composition of polyethylene
can be extruded with addition of small amounts, namely 10 wt-% or
less, of high pressure LPDE by blending or by coextrusion. The
addition of LDPE to such a bimodal composition, however, does not
allow to obtain a film product having adequate optical
properties.
[0018] A polyethylene film made from a composition of a
high-density polyethylene (HDPE) and a low-density polyethylene
(LDPE) prepared by high-pressure polymerization process is also
known and has been hitherto used as a packaging material utilizing
its transparency. However, the mechanical strength of the
polyethylene film is yet insufficient. Therefore, there have been
attempts to improve the impact resistance thereof. In order to
improve the impact resistance, U.S. Pat. No. 6,426,384 for example
teaches to prepare a polyethylene film for packaging starting from
a polyethylene resin composition comprising a linear low-density
polyethylene prepared using a metallocene-based catalyst and a
high-density polyethylene prepared using a Ziegler type catalyst.
However, the increase of the impact resistance is still
insufficient.
[0019] EP-A1-1 470 185 describes a blend from about 20% by weight
to about 80% by weight of a high-molecular weight, medium density
polyethylene having a multimodal molecular weight distribution and
about 20% by weight to about 80% by weight of a linear low density
polyethylene. The medium density polyethylene is prepared by using
Ziegler catalysts. The blend may optionally contain a third
polymer, such as for example low density polyethylene, in an amount
preferably less than 50% by weight of the total blend. However, the
dart drop impact and the tear strength of the films prepared
starting from such blend are inadequate.
SUMMARY OF THE INVENTION
[0020] In view of the above, the Applicant has perceived the need
of providing a polyethylene composition, as well as a process for
the preparation thereof and a film comprising such a polyethylene
composition which, in sharp contrast to the prior art, although
having a density which may range in the medium-high density range,
has a high dart drop impact and a high transparency, while
maintaining a good degree of processability so as to permit to use
low working temperatures.
[0021] In other words, the technical problem underlying the present
invention is that of providing a polyethylene composition having a
suitable processability, while simultaneously achieving an improved
balance between both mechanical and optical properties, in
particular in terms of impact resistance and clarity. Such problem,
as discussed above, is particularly felt in the medium-high density
range film applications.
[0022] According to a first aspect of the present invention, the
above-mentioned technical problem is solved by a polyethylene
composition comprising:
[0023] a) from 50 to 89% by weight of a first polyethylene
component comprising at least one multimodal ethylene polymer
including a plurality of ethylene polymer fractions having distinct
molecular weights and comonomer contents, at least one of said
plurality of ethylene polymer fractions being prepared by the use
of a single site catalyst; and
[0024] b) from 50 to 11% by weight of a second polyethylene
component comprising a low or medium density polyethylene.
[0025] In the present description and in the following claims, the
expression "single site catalyst" is used to indicate any
transition metal coordination compound comprising at least one
ligand, such as for example a compound selected in the group of
cyclopentadienyl derivatives, phenoxyimin derivatives, as well as
neutral or charged bidentate or tridentate nitrogen ligands with 2
or 3 coordinating nitrogen atoms.
[0026] In the present description and in the following claims, the
expression "low or medium density polyethylene" is used to indicate
any polyethylene having a density in the range 0.910 to 0.940
g/cm.sup.3.
[0027] For the purpose of the present description and of the claims
which follow, except where otherwise indicated, all numbers
expressing amounts, quantities, percentages, and so forth, are to
be understood as being modified in all instances by the term
"about". Also, all ranges include any combination of the maximum
and minimum points disclosed and include any intermediate ranges
therein, which may or may not be specifically enumerated
herein.
[0028] Thanks to the fact that the first polyethylene component
includes a plurality of ethylene polymer fractions having distinct
molecular weights, i.e. thanks to the fact that the first
polyethylene composition is multimodal, the composition of the
invention, on the one side, may have a broad molecular
distribution, which advantageously permits to improve the
processing of the composition. Furthermore, thanks to the fact that
the multimodal first polyethylene component of the invention
includes a plurality of ethylene polymer fractions having distinct
comonomer contents, the composition of the invention, on the other
side, may be tailored in such a way to preferentially include
relatively greater amounts of comonomer within the relatively
higher molecular weight fractions, and relatively smaller amounts
of comonomer within the relatively lower molecular weight
fractions, which advantageously permits to improve the mechanical
properties of the composition, and in particular the puncture
resistance as well as the tensile and tear strength of the film
products prepared therefrom.
[0029] Furthermore, thanks to the presence of a second polyethylene
component comprising a polyethylene having a density ranging in the
low and medium density range, the composition of the invention has,
in addition to the above-mentioned suitable processability and
mechanical properties, also improved optical properties, in
particular in terms of clarity and gloss.
[0030] Surprisingly, such improvement of the optical properties
does not substantially affect the mechanical and processability
properties of the composition of the invention. So, the present
invention advantageously allows to obtain a balance between optical
and mechanical properties, which are normally conflicting with each
other.
[0031] If the second polyethylene component is present in an amount
lower than 11%, the transparency of the polyethylene composition is
inadequate, while if the second polyethylene component is present
in an amount higher than 50%, an unacceptable worsening of the
mechanical properties is observed.
[0032] Preferably, the polyethylene composition comprises from 55
to 85% by weight of said first polyethylene component and from 45
to 15% by weight of said second polyethylene component. More
preferably, the polyethylene composition comprises from 60 to 85%
by weight of said first polyethylene component and from 40 to 15%
by weight of said second polyethylene component. Still more
preferably, the polyethylene composition comprises from 60 to 80%
by weight of said first polyethylene component and from 40 to 20%
by weight of said second polyethylene component.
[0033] Within such preferred composition ranges, it is
advantageously possible to prepare films having a further improved
combination of optical and mechanical properties, while being at
the same time easily processable.
[0034] In order to obtain films having a particularly advantageous
combination of mechanical and optical properties, a preferred
embodiment of the composition of the invention provides a
polyethylene composition comprising from 70 to 80% by weight of
said first polyethylene component and from 30 to 20% by weight of
said second polyethylene component.
[0035] The first polyethylene component has preferably a density of
from 0.920 to 0.970 g/cm.sup.3, more preferably of from 0.920 to
0.960 g/cm.sup.3, still more preferably of from 0.930 to 0.950
g/cm.sup.3 and, in particular, of from 0.932 to 0.945
g/cm.sup.3.
[0036] The above-mentioned advantageous effects of the invention in
terms of improved processing, mechanical resistance and optical
properties are particularly pronounced when the density of the
multimodal first polyethylene component ranges in the medium-high
density range, e.g. in the range from 0.932 to 0.945
g/cm.sup.3.
[0037] Preferably, the polyethylene composition has a density of
from 0.915 to 0.965 g/cm.sup.3, more preferably from 0.915 to 0.960
g/cm.sup.3, still more preferably from 0.915 to 0.955 g/cm.sup.3,
particularly preferably from 0.915 to 0.945 g/cm.sup.3. According
to further preferred embodiments of the invention, the polyethylene
composition has preferably a density of from 0.920 to 0.955
g/cm.sup.3, more preferably from 0.930 to 0.950 g/cm.sup.3 and,
still more preferably, from 0.935 to 0.940 g/cm.sup.3. A further
improvement of the optical properties without a substantial
affection of the mechanical properties and an increase in the
stiffness is advantageously achieved when the density falls in
these preferred ranges. In other words, an improved balance between
optical and mechanical properties is advantageously obtained.
[0038] Preferably, at least one fraction of the above-mentioned
plurality of ethylene polymer fractions of the first polyethylene
component comprises an ethylene copolymer containing a comonomer
including at least one 1-olefin.
[0039] Preferably, the at least one 1-olefin has formula
R.sup.1CH.dbd.CH.sub.2, wherein R.sup.1 is hydrogen or an alkyl
radical with 1 to 12 carbon atoms and, more preferably, wherein
R.sup.1 is an alkyl radical with 1 to 10 carbon atoms.
[0040] In the above-mentioned ethylene copolymer, in addition to
ethylene it is possible to use any 1-olefin having from 3 to 12,
preferably to 3 to 10, carbon atoms, e.g.
propene,1-butene,1-pentene, 1-hexene, 4-methyl-1-pentene,
1-heptene, 1-octene and 1-decene. More particularly, the ethylene
copolymer preferably comprises 1-olefins having from 4 to 8 carbon
atoms, e.g. 1-butene, 1-pentene, 1-hexene, 4-methylpentene or
1-octene, in copolymerized form as comonomer unit. Particular
preference is given to 1-olefins selected from the group consisting
of 1-butene, 1-hexene and 1-octene.
[0041] The above-mentioned comonomers can be present either
individually or in a mixture with one another.
[0042] According to a preferred embodiment of the polyethylene
composition of the invention, the first polyethylene component
comprises a multimodal polyethylene in which the lower molecular
weight ethylene polymers are preferably homopolymers or,
alternatively, copolymers containing less than 1% by weight of
comonomer(s), more preferably less than 0.5%, while the higher
molecular weight ethylene polymers are preferably copolymers
containing a predetermined amount of comonomer(s) which is
preferably greater than 1% by weight. Preferably, such
predetermined amount of comonomer(s) of the copolymers either
increases as a function of the molecular weight of the higher
molecular weight ethylene polymers or remains equal, the amount of
comonomer(s) of the highest molecular weight ethylene polymer being
of 2-10% by weight based on the copolymer.
[0043] Preferably, the first polyethylene component comprises a
bimodal polyethylene including a relatively low molecular weight
ethylene polymer and a relatively high molecular weight ethylene
polymer. Preferably, the bimodal polyethylene has a density
comprised in the range from 0.932 to 0.945 g/cm.sup.3, more
preferably from 0.930-0.940 g/cm.sup.3.
[0044] Preferably, the relatively low molecular weight component
and the relatively high molecular weight component of the bimodal
first polyethylene composition have an intrinsic viscosity in
decalin at 135.degree. of from 0.6 to 1.2 dl/g and, respectively,
of from 2.5 to 5 dl/g as determined according to EN ISO
1628-3:2003.
[0045] In this way, the balance between optical and mechanical
properties of the polyethylene composition of the invention is
further improved.
[0046] More preferably, the composition of the invention comprises,
as a first polyethylene component, a bimodal polyethylene component
including a relatively low molecular weight component having a MFR
(190/21.6) of from above 5 to 100 g/10 min and a relatively high
molecular weight component having a MFR (190/21.6) of from 5 to 15
g/10 min, and in any case lower than the MFR (190/21.6) of the
relatively low molecular weight component.
[0047] According to a preferred embodiment of the polyethylene
composition of the invention, the first polyethylene component
comprises a bimodal polyethylene, in which said relatively low
molecular weight ethylene polymer is preferably a homopolymer or,
alternatively, a copolymer containing less than 1% by weight of
comonomer, more preferably less than 0.5%, while said relatively
high molecular weight ethylene polymer is preferably a copolymer
containing a predetermined amount of comonomer preferably higher
than 1%, for example comprised between 1% and 10% by weight,
preferably from 2 to 8%, more preferably from 2.5 to 5% and, still
more preferably, from 3 to 4% by weight.
[0048] In this way, and in particular thanks to the absence of
comonomer or, at the most, thanks to a limited content of comonomer
in the relatively low molecular weight fraction of the first
polyethylene, content which, as said above, is preferably not
higher than 1% and more preferably not higher than 0.5%, the
composition of the invention is particularly easily processable,
which advantageously allows to use lower working temperatures, for
example in the range of 180-250.degree. C. Preferably, the
relatively high molecular weight ethylene copolymer comprises from
1% to 10% by weight, preferably from 2 to 8%, more preferably from
2.5 to 5% and, still more preferably, from 3 to 4% by weight of a
comonomer, which preferably includes at least one of the comonomers
described above, in particular a comonomer selected from the group
of propene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene,
1-heptene, 1-octene and 1-decene.
[0049] According to a preferred embodiment of the invention, at
least the ethylene polymer fraction of the multimodal (e.g.
bimodal) polyethylene of the first polyethylene component having
the lowest molecular weight is prepared by means of the
above-mentioned single site catalyst.
[0050] Preferably, the above-mentioned single site catalyst used to
prepare the at least one ethylene polymer fraction of the
multimodal polyethylene of the first polyethylene component is a
metallocene.
[0051] So, for example, in the preferred embodiment according to
which the first polyethylene component comprises a bimodal
polyethylene including two ethylene polymer fractions having
distinct molecular weights and comonomer contents, namely a
relatively high molecular weight ethylene polymer fraction
preferably including copolymers containing a predetermined amount
of comonomer preferably greater than 1% by weight, and a relatively
low molecular weight ethylene polymer fraction preferably including
homopolymers or copolymers containing less than 1% by weight of
comonomer, more preferably less than 0.5%, the relatively high
molecular weight ethylene polymer fraction is preferably obtained
by means of the above-mentioned single site catalyst, for example
by means of a metallocene.
[0052] According to a preferred embodiment of the invention, a
mixed type catalyst may be used, i.e. a catalyst comprising
particles each containing a plurality of different kind of active
species, in which at least one active specie is a single site
catalyst.
[0053] Thanks to the fact that in the case of a mixed type catalyst
containing at least two active species at least two different
polymerization catalysts are provided within the same catalyst
system, on the one side the first polyethylene compound is
multimodal and, on the other side, it is advantageously possible to
prepare the first polyethylene component by means of a
polymerization process carried out in a single reactor.
[0054] When the mixed type catalyst contains only two active
species, for example, a bimodal first polyethylene component of the
composition may be advantageously obtained, which permits, on the
one side, to prepare a broad molecular weight distribution
composition and, on the other side, to polymerize both the
relatively low molecular weight component and the relatively high
molecular weight component in a parallel way, i.e. substantially in
a simultaneous manner, in one single reactor.
[0055] By way of illustrative example, the mixed catalyst may
contain at least one metallocene (by way of illustrative and not
limiting example, a hafnocene or a zirconocene) component and one
iron component. In particular, the mixed catalyst may contain one
metallocene (e.g. hafnocene or zirconocene) component and one iron
component.
[0056] However, any other combination of active species which are
able to polymerize ethylene in such a manner as to obtain a
relatively high molecular weight component containing preferably at
least 1% of comonomer and, respectively, a relatively low molecular
weight component containing an amount of comonomer preferably lower
than 1%, is acceptable for the purpose of the invention.
[0057] Preferably, in the preferred embodiment in which the
catalyst contains one metallocene, for example a hafnocene or a
zirconocene, component and one iron component, the iron component
has preferably a tridentate ligand bearing at least two aryl
radicals, each bearing a halogen or alkyl substituent in the
ortho-position(s) as described by formula (B) disclosed in
W02005/103095 in the name of the Applicant, which is hereby
incorporated by reference.
[0058] The mixed catalyst may for example comprise, as active
species, at least one first component and at least one second
component, as well as at least one activating compound so as to
advantageously improve the polymerization activity of the first and
second component. The activation of the at least one first
component and of the at least second component of the catalyst may
be effected using the same activating compound or different
activating compounds. The molar ratio of the first component to the
activating compound, as well as the molar ratio of the second
component to the activating compound, may range in a first and,
respectively, in a second predetermined range which, with reference
to the illustrative example of the catalyst comprising one
metallocene component and one iron component, is preferably as
follows. The molar ratio of the metallocene component to the
activating compound may range from 1:0.1 to 1:10000, preferably
from 1:1 to 1:2000. The molar ratio of the iron component to the
activating compound is also usually in the range from 1:0.1 to
1:10000, preferably from 1:1 to 1:2000.
[0059] Suitable activating compounds which are able to react with
one of the components of the mixed catalyst, for example with the
hafnocene component or the iron component, to convert the same into
a catalytically active or more active compound are, for example,
compounds such as an aluminoxane, a strong uncharged Lewis acid, an
ionic compound having a Lewis-acid cation or an ionic compound
containing a Bronsted acid as cation.
[0060] The catalyst may further comprise at least one support. The
preferred catalyst composition according to the invention comprises
one support or a plurality of supports, which may be organic or
inorganic. The first component and/or the second component and the
optional activating compound of the catalyst, in particular, may be
supported, for example on different supports or together on a
common support.
[0061] Preferably a finely divided organic or inorganic solid
support, such as for example silica, hydrotalcite, magnesium
chloride, talc, montmorillonite, mica, or an inorganic oxide or a
finely divided polymer powder (e.g. polyolefin or a polymer having
polar functional groups) is used. The catalyst system may further
comprise a metal compound, preferably a metal of group 1, 2 or 13
of the Periodic Table and preferably different from the
above-mentioned activating component, which is used as constituent
of the catalyst for the polymerization or copolymerization of
olefins, for example to prepare a catalyst solid comprising the
support and/or be added during or shortly before the
polymerization.
[0062] It is also possible for the catalyst system firstly to be
prepolymerized with a .alpha.-olefin, preferably with a linear
C.sub.2-C.sub.10-1-alkene and in particular ethylene or propylene.
The resulting prepolymerized catalyst solid may then be submitted
to the actual polymerization step.
[0063] Furthermore, a small amount of an olefin, preferably an
.alpha.-olefin, for example vinylcyclohexane, styrene or
phenyldimethylvinylsilane can be added as additive during or after
the preparation of the catalyst. Other additives, such as for
example wax or oil, can be also added during or after the
preparation of the catalyst.
[0064] Preferably, the first polyethylene component of the
polyethylene composition has a molar mass distribution width
M.sub.w/M.sub.n of from 5 to 30. Preferably, the first polyethylene
component has a weight average molar mass M.sub.w of from 50 000
g/mol to 500 000 g/mol. Preferably, the first polyethylene
component has a z-average molecular weight M.sub.z of less than 1
Mio. g/mol.
[0065] Preferably, the first polyethylene component of the
polyethylene composition has a molar mass distribution width
M.sub.w/M.sub.n in the range from 6 to 20 and, more preferably,
from 7 to 15.
[0066] Preferably, the weight average molar mass M.sub.w of the
first polyethylene component of the polyethylene composition is in
the range from 100 000 g/mol to 300 000 g/mol and, more preferably,
from 120 000 g/mol to 250 000 g/mol.
[0067] The z-average molar mass M.sub.z of the first polyethylene
component of the polyethylene composition is preferably in the
range of from 250 000 g/mol to 700 000 g/mol and, more preferably,
from 300 000 g/mol to 500 000 g/mol. The definition of z-average
molar mass M.sub.z is used herewith in accordance with the
definition given in High Polymers Vol. XX, Raff und Doak,
Interscience Publishers, John Wiley & Sons, 1965, page 443.
[0068] According to a particularly preferred embodiment of the
present invention, the first polyethylene component has the
following preferred features: [0069] a molar mass distribution
width M.sub.w/M.sub.n of from 5 to 30; [0070] a weight average
molar mass M.sub.w of from 50000 g/mol to 500 000 g/mol; and [0071]
a z-average molecular weight M.sub.z of less than 1 Mio. g/mol.
[0072] Such a preferred combination of features advantageously
permits to provide a polyethylene composition in which the first
polyethylene component has improved and balanced processability and
mechanical properties, which in turn advantageously permits to add
sensibly great amounts of the second polyethylene component, for
example in the range of 35-50% by weight, with advantageous
increase of the transparency without substantially altering the
processing and mechanical properties.
[0073] The first polyethylene component of the polyethylene
composition has a MFR (190/21.6) which is preferably in the range
of from 5 to 100 g/10 min, more preferably in the range of from 7
to 60 g/10 min and, still more preferably, of from 9 to 50 g/10
min.
[0074] In the present description and in the following claims, the
MFR (190/21.6) is the melt flow rate measured in accordance with
ISO 1133, condition G, namely at 190.degree. C. and under a load of
21.6 kg.
[0075] The first polyethylene component preferably comprises a
fraction having a molar mass of below 1 Mio. g/mol as determined by
Gel Permeation Chromatography (GPC) in the standard determination
of the molecular weight distribution according to standard DIN
55672 with 1,2,4-trichlorobenzene at 140.degree. C. More
preferably, said fraction amounts to at least 95.5% by weight of
the first polyethylene component.
[0076] The first polyethylene component has preferably a
Eta(vis)/Eta(GPC) lower than 0.95, Eta(vis) being the intrinsic
viscosity as determined according to ISO 1628-1 and -3 and Eta(GPC)
being the viscosity as determined by GPC according to DIN 55672,
with 1,2,4-Trichlorobenzene, at 140.degree. C.
[0077] According to a preferred embodiment of the composition of
the invention, the second polyethylene component has a density of
from 0.910 to 0.940 g/cm.sup.3, preferably of from 0.910 to 0.933
g/cm.sup.3, more preferably of from 0.915 to 0.933 g/cm.sup.3 and,
still more preferably, of from 0.925 to 0.930 g/cm.sup.3.
[0078] Preferably, the second polyethylene component has a density
lower than the density of the first polyethylene component.
[0079] The second polyethylene component of the composition of the
invention has preferably a MFR (190/2.16) of from 0.2 to 50 g/10
min, more preferably from 0.3 to 10 g/10 min, and, still more
preferably, from 0.3 to 5 g/10 min.
[0080] According to a second aspect thereof, the present invention
relates to a process for producing a polyethylene composition,
comprising the steps of:
[0081] a) preparing a multimodal first polyethylene component
by:
[0082] a1) providing at least one single site catalyst;
[0083] a2) subjecting ethylene, optionally with at least one
comonomer, in the presence of said at least one single site
catalyst, to a plurality of polymerization stages intended to
obtain a respective plurality of ethylene polymer fractions;
[0084] a3) distinguishing said plurality of ethylene polymer
fractions with respect to each other on the basis of molecular
weights and comonomer contents;
[0085] b) preparing a second polyethylene component comprising a
low or medium density polyethylene;
[0086] c) adding said second polyethylene component to said
multimodal first polyethylene component so prepared so as to obtain
a composition comprising from 50 to 89% by weight of the first
polyethylene component and from 50 to 11% by weight of the second
polyethylene component.
[0087] Thanks to the fact that the first polyethylene component is
of the multimodal type and that the second polyethylene component
comprises a low density polyethylene or a medium density
polyethylene, it is advantageously possible to obtain a
polyethylene composition which is easily processable and has
improved optical properties. The addition--in the above-mentioned
predetermined amount--of a second polyethylene component including
a LDPE or a MDPE to a multimodal first polyethylene component
defined as above, advantageously allows to prepare a polyethylene
composition having simultaneously an improvement of the optical
properties, in particular in terms of haze, clarity and gloss,
without substantially compromising the mechanical properties, in
particular in terms of dart drop impact, as well as the
processability of the composition. An improved balance among
conflicting properties is therefore achieved, and this improvement
is particularly pronounced when the density of the multimodal first
polyethylene component ranges in the medium-high density range.
[0088] The step of providing at least one single site catalyst is
preferably carried out in such a manner to obtain a catalyst
according to any one of the preferred embodiments described above
with reference to the composition of the invention. So, for
example, if the catalyst is a mixed type catalyst, it is
advantageously possible to prepare the multimodal first
polyethylene component by means of a polymerization process carried
out in a single reactor.
[0089] Said step of preparing the multimodal first polyethylene
component is carried out in such a manner as to obtain a first
polyethylene component having a density of from 0.920 to 0.955
g/cm.sup.3, more preferably from 0.930 to 0.950 g/cm.sup.3 and,
still more preferably, from 0.932 to 0.945 g/cm.sup.3.
[0090] Ethylene with at least one comonomer, and optionally
preferably with hydrogen as preferred molar mass regulator, is
subjected, in the presence of said at least one single site
catalyst, to a plurality of polymerization stages, preferably to a
two polymerization stages so as to conveniently obtain a relatively
low molecular weight component and a relatively high molecular
weight component.
[0091] Preferably, the process is carried out so as to obtain a
relatively low molecular weight component and a relatively high
molecular weight component having an intrinsic viscosity in decalin
at 135.degree. C. of from 0.6 to 1.2 dl/g and, respectively, of
from 2.5 to 5 dl/g as determined according to EN ISO
1628-3:2003.
[0092] In this way, the processability of the composition is
further improved.
[0093] Preferably, the process is carried out so as to obtain a
bimodal first polyethylene component comprising a relatively low
molecular weight component having a MFR (190/21.6) of from above 5
to 100 g/10 min and a relatively high molecular weight component
having a MFR (190/21.6) of from 5 to 15 g/10 min, and in any case
lower than the MFR (190/21.6) of the relatively low molecular
weight component
[0094] According to a preferred embodiment of the process of the
invention, the ethylene may be copolymerized with at least one
1-olefin, such as for example one or more of the 1-olefins
described above with reference to the preferred embodiments of the
composition of the invention. So, for example, the ethylene is
preferably subjected to copolymerization with at least one 1-olefin
having formula R.sup.1CH.dbd.CH.sub.2, wherein R.sup.1 is hydrogen
or an alkyl radical with 1 to 12 carbon atoms and, more preferably,
with 1 to 10 carbon atoms. As a comonomer, any 1-olefin having from
3 to 12 carbon atoms, e.g. propene, 1-butene, 1-pentene, 1-hexene,
4-methyl1-pentene, 1-heptene, 1-octene and 1-decene may be used.
The comonomer preferably comprises 1-olefins having from 4 to 8
carbon atoms, e.g. 1-butene, 1-pentene, 1-hexene, 4methylpentene or
1-octene, in copolymerized form as comonomer unit. Particular
preference is given to 1-olefins selected from the group consisting
of 1-butene, 1-hexene and 1-octene.
[0095] The above-mentioned comonomers can be present either
individually or in a mixture with one another.
[0096] Preferably, the temperature at which ethylene is
(co)polymerized is carried out is of from 20 to 200.degree. C.
Preferably, the pressure at which ethylene is (co)polymerized is
carried out is from 0.05 to 1 MPa.
[0097] The step of preparing the multimodal first polyethylene is
preferably carried out in such a way as to obtain a first
polyethylene component having: [0098] a molar mass distribution
width M.sub.w/M.sub.n of from 5 to 30; [0099] a weight average
molar mass M.sub.w of from 50000 g/mol to 500 000 g/mol; and [0100]
a z-average molecular weight M.sub.z of less than 1 Mio. g/mol.
[0101] Preferably, the step of distinguishing the plurality of
ethylene polymer fractions with respect to each other on the basis
of molecular weights is carried out by using at least two active
catalytic species.
[0102] More preferably, such at least two active catalytic species,
of which at least one is of the single site type, are incorporated
in the same catalyst particle. In such a preferred embodiment, a
corresponding plurality of polymerization stages is advantageously
carried out in a substantially simultaneous manner in a parallel
mode and the result of such plurality of substantially simultaneous
polymerization stages is a multimodal polyethylene composition.
Thanks to these preferred features, it is advantageously possible
to prepare the first polyethylene component by means of a single
step polymerization process in a single reactor, thus
advantageously reducing both the plant costs and the energy
consumption with respect to the processes carried out in a
plurality of reactors.
[0103] Alternatively, the above-mentioned at least two active
catalytic species are incorporated in different catalyst particles.
Also in this case, by providing a mixture of at least two
particulate catalysts, a corresponding plurality of polymerization
stages is advantageously carried out in a substantially
simultaneous manner in a parallel mode and the result of the
different substantially simultaneous polymerization stages is a
multimodal polyethylene composition.
[0104] The step of distinguishing the plurality of ethylene polymer
fractions with respect to each other on the basis of molecular
weights may be also carried out by polymerizing ethylene in a
respective plurality of reactors arranged in series with each
other. In this case, a corresponding plurality of polymerization
stages is advantageously carried out in a serial mode, and the
result of the different subsequent polymerization stages is a
multimodal polyethylene composition. Thanks to these preferred
steps, it is advantageously possible to prepare the first
polyethylene component by means of a multistage polymerization
process in which the polymerization stages are subsequent to each
other.
[0105] Independently of the number of reactors used, with each of
these three alternative methods, good mixing of the polyethylene is
advantageously achieved and the control of the molecular weight
fractions of the various polymers and of the molecular weight
distributions is conveniently simple.
[0106] A further possible alternative in order to distinguish the
plurality of ethylene polymer fractions with respect to each other
on the basis of molecular weights is that of blending a plurality
of polymer fractions each obtained by the use of a respective
catalyst. In this case, by blending such a plurality of polymer
fractions, it is advantageously possible to obtain a multimodal
polyethylene composition in a parallel mode, as a result of the
blending of polymer fractions which have been separately prepared,
either simultaneously or subsequently to each other, by the use of
respective catalyst in respective polymerization stages.
[0107] The above-mentioned addition step is preferably carried out
so as to obtain a composition comprising from 50 to 89% by weight
of said first polyethylene component and from 50 to 11% by weight
of said second polyethylene component, more preferably from 55 to
85% by weight of said first polyethylene component and from 45 to
15% by weight of said second polyethylene component, still more
preferably the polyethylene composition comprises from 60 to 85% by
weight of said first polyethylene component and from 40 to 15% by
weight of said second polyethylene component and, in
particular.
[0108] Within such preferred composition ranges, it is
advantageously possible to prepare films having a further improved
transparency.
[0109] In order to obtain films having a particularly advantageous
combination of mechanical and optical properties, a preferred
embodiment of the process of the invention provides an additional
step which is carried out so as to prepare a polyethylene
composition comprising from 65 to 80% by weight of said first
polyethylene component and from 35 to 20% by weight of said second
polyethylene component and, more preferably, from 70 to 80% by
weight of said first polyethylene component and from 30 to 20% by
weight of said second polyethylene component
[0110] According to a preferred embodiment of the process of the
invention, the above-mentioned step of adding the second
polyethylene component to the first polyethylene component is
carried out by blending.
[0111] In this way, a good mixing of the first polyethylene
component and of the second polyethylene component is
advantageously achieved.
[0112] Alternatively, the step of adding the second polyethylene
component to the first polyethylene component is carried out by
compounding or by coextrusion.
[0113] The polymerization of ethylene in order to prepare the first
polyethylene component can be carried out using all industrially
known polymerization methods at temperatures in the range from
60.degree. C. to 350.degree. C., preferably from 0 to 200.degree.
C. and particularly preferably from 25 to 150.degree. C., and under
pressures of from 0.5 to 4000 bar, preferably from 1 to 100 bar,
and particularly preferably from 3 to 40 bar. The polymerizations
effected to prepare the first polyethylene component can be carried
out in a known manner in bulk, in suspension, in the gas phase or
in a supercritical medium in the conventional reactors used for the
polymerization of olefins. It can be carried out batchwise or, more
preferably, continuously in one stage (for example, as described
above, if a mixed catalyst is used) or in more stages. Solution
processes, suspension processes, stirred gas-phase processes and
gas-phase fluidized-bed processes are all possible. The second
polyethylene component is preferably prepared by conventional
high-pressure polymerization processes in tube reactors or
autoclaves.
[0114] The mean residence times are preferably from 0.5 to 5 hours.
The advantageous pressure and temperature ranges for carrying out
the polymerizations usually depend on the polymerization
method.
[0115] In the case of suspension polymerizations, for example, the
polymerization is usually carried out in a suspension medium,
preferably an inert hydrocarbon such as isobutane or mixtures of
hydrocarbons or else in the monomers themselves. The polymerization
temperatures are generally in the range from -20.degree. C. to
115.degree. C., and the pressure is generally in the range from 1
to 100 bar. The solids content of the suspension is generally in
the range from 10% to 80%. The polymerization can be carried out
either batchwise or continuously, e.g. in stirring autoclaves, in
tube reactors, such as for example in loop reactors. Particular
preference is given to employing the Phillips PF process as
described in US-A U.S. Pat. No. 3,242,150 and US-A U.S. Pat. No.
3,248,179. The gas-phase polymerization is generally carried out in
the range from 30 to 125.degree. C. at pressures of from 1 to 50
bar.
[0116] In the case of high-pressure polymerization processes, which
are conventionally carried out at pressures of from 1000 to 4000
bar, in particular from 2000 to 3500 bar, high polymerization
temperatures are generally also set. Advantageous temperature
ranges for these high-pressure polymerization processes are from
200.degree. C. to 320.degree. C., in particular from 220.degree. C.
to 290.degree. C. In the case of low-pressure polymerization
processes, it is usual to set a temperature which is at least a few
degrees below the softening temperature of the polymer. In
particular, temperatures of from 140.degree. C. to 310.degree. C.
are preferably set in these polymerization processes.
[0117] Among the above-mentioned polymerization processes used to
prepare the first polyethylene component, particular preference is
given to gas-phase polymerization and, more in particular, in
gas-phase fluidized-bed reactors, solution polymerization and
suspension polymerization, such as for example in loop reactors and
stirred tank reactors. The gas-phase polymerization may also be
carried out in the condensed or supercondensed mode, in which part
of the circulating gas is cooled to below the dew point and is
recirculated as a two-phase mixture to the reactor. Furthermore, it
is possible to use a multizone reactor in which at least two
reciprocally linked polymerization zones are provided, so that the
polymer is passed alternately through these at least two zones a
predetermined number of times. The at least two zones may also be
subjected to different polymerization conditions. Such a multizone
reactor is described, for example, in WO 97/04015. The different or
identical polymerization stages, as already explained above, may
also, if desired, be carried out in a serial manner, namely in a
plurality of reactors arranged in series to each other so as to
form a polymerization cascade. A parallel reactor arrangement using
two or more identical or different processes is also possible.
Furthermore, molar mass regulators, such as for example hydrogen,
or conventional additives, such as for example antistatics, may
also be used in the polymerizations. If hydrogen is added and if
the temperature is increased, a lower z-average molar mass is
advantageously achieved.
[0118] The polymerization is preferably carried out in a single
reactor, in particular in a gas-phase reactor. The polyethylene
powder so obtained is advantageously more homogeneous with respect
to the polyethylene obtained as a result of a cascade process,
where a number of polymerization stages are carried out in a serial
manner in a plurality of reactors arranged in series to each other,
so that, unlike the powder obtainable by means of the cascade
process, a possible subsequent extrusion is conveniently not
necessary in order to obtain a homogeneous product.
[0119] The composition of the invention may also be prepared by
blending a first polyethylene component and a second polyethylene
component as defined above, preferable by intimate mixing of
individual components, for example by melt extrusion in an extruder
or kneader (as described, for example, in "Polymer Blends" in
Ullmann's Encyclopedia of Industrial Chemistry, 6.sup.th Edition,
1998, Electronic Release).
[0120] According to a further aspect thereof, the present invention
relates to the use of a polyethylene composition as defined above
for producing a film.
[0121] Furthermore, the present invention relates to a film
comprising a polyethylene composition as defined above, as well as
to a particularly preferred film selected from the group of stretch
films, hygienic films, films for office uses, sealing layers,
automatic packaging films, composite and laminating films.
[0122] Films in which the polyethylene of the invention is present
as a significant component are ones which contain from 50 to 100%
by weight, preferably from 60 to 90% by weight, of the polyethylene
of the invention, based on the total polymer material used for
manufacture. In particular, films including a plurality of layers
in which in which at least one of the layers contains from 50 to
100% by weight of the polyethylene of the invention are also
included.
[0123] In general the films are preferably produced by
plastification of the polyethylene composition of the invention at
a melt temperature in the range of from 190 to 230.degree. C., by
forcing the plasticized polyethylene through an annular die and
cooling. The film may further comprise of from 0 to 30% by weight,
preferably 0.1 to 3 by weight of auxiliaries and/or additives known
per se, e.g. processing stabilizers, stabilizers against the
effects of light and heat, customary additives such as lubricants,
antioxidants, antiblocking agents and antistatics, and also, if
appropriate, dyes.
[0124] The polyethylene composition of the invention may be used to
prepare films with a thickness of from 5 .mu.m to 2.5 mm. The films
can for example be prepared via blown film extrusion with a
thickness of from 5 .mu.m to 250 .mu.m or via flat film extrusion,
such as cast film extrusion with a thickness of from 10 .mu.m to
2.5 mm. During blown film extrusion the polyethylene melt is forced
through an annular die. The bubble which is formed is inflated with
air and hauled off at a higher speed than the die outlet speed. The
bubble is intensively cooled by a current of air so that the
temperature at the frost line is lower than the crystallite melting
point. The bubble is then collapsed, trimmed if necessary and
rolled up using a suitable winding instrument. The polyethylene
composition of the invention may be extruded either according to
two alternative configurations known in the art, namely according
to a "long stalk" configuration or according to a "conventional"
configuration depending on the density of the polyethylene. In the
"long stalk" configuration, which is normally suitable for blowing
high density polyethylene, the bubble of polymer blown into a film
has a well defined and longer neck height with respect to the
"conventional" configuration, which is suitable in blowing low
density polyethylene.
[0125] The films may be obtained for example in chill roll lines or
thermoforming film lines. Furthermore composite films essentially
based one the polyethylene composition of the invention may be
produced on coating and laminating lines. Especially preferred are
composite films wherein paper, aluminum or fabric substrates are
incorporated into the composite structure. The films may have a
single layer or a plurality of layers, each obtained by
coextrusion.
[0126] The polyethylene composition of the invention is suitable
for producing films in blown film and cast film plants at high
outputs. The films display improved mechanical properties, in
particular, as better described in the following, high tensile
strength and tear strength together with improved optical
properties, in particular transparency and gloss. The composition
of the invention is suitable, in particular, for preparing
packaging films, such as for example heat sealing films, also for
heavy duty sacks and in particular for films intended to be used in
the food industry.
[0127] The films of the invention are especially suitable in
applications requiring high clarity and gloss such as carrier bags
to permit high quality printing, laminating films in foodstuff
applications, since the films of the invention also have a very low
odor and taste level and automatic packaging films, since the film
can be processed on high-speed lines.
[0128] The films of the invention having a thickness in the order
of 50 .mu.m have advantageously a haze, as determined by ASTM D
1003-00 on a BYK Gardener Haze Guard Plus Device on at least 5
pieces of film of size 10.times.10 cm, below 22%. The dart drop
impact of films having a thickness in the order of 50 .mu.m as
determined by ASTM D 1709 Method A is advantageously above 140 g.
The clarity of films having a thickness in the order of 50 .mu.m as
determined by ASTM D 1746-03 on a BYK Gardener Haze Guard Plus
Device, calibrated with calibration cell 77.5, on at least 5 pieces
of 10.times.10 cm films is advantageously at least 86%. The
20.degree. gloss of films having a thickness in the order of 50
.mu.m as determined by ASTM D 2457-03 on a 20.degree. gloss meter
with a vacuum plate for fixing the film, on at least 5 pieces of
film, is advantageously of at least 15.
[0129] The scrap obtained during the production of these films can
be conveniently recycled. If the films are produced by a first
extruder, film trimmings may be compacted or ground and fed to a
second extruder, where they are melted so as to be ready to be fed
back to the main extruder and, in this way, conveniently recycled.
The film trimmings should be reground to grains having a size which
can be fed into the feed section of the first extruder together
with the virgin polyethylene. The films containing such recycled
material do not show any significant deterioration of the
properties compared to films without recycled material.
[0130] The polyethylene composition of the invention may be also
used to prepare articles by means of a number of techniques, such
as for example blow molding, injection molding, roto-molding and
compression molding.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0131] The present invention will be further described by means of
the following preferred embodiments without restricting the scope
of the invention.
Example 1 (Invention)
[0132] a) Preparation of the Individual Components
[0133] 0.90 kg of 2,6-diacetylpyridine (99%), 2, 56 kg of
phosphorus pentoxide (P.sub.2O .sub.5), and a solution of 2.14 kg
of 2,4-dichloro-6-methylaniline (100%) were solubilized in 20 l of
tetrahydrofuran. The mixture was stirred for 15 min and then heated
under reflux for 18 hours at 70.degree. C. After completion of the
reaction, the obtained suspension was cooled to 20.degree. C.,
stirred for 30 min and then filtered and washed with 6 l of
tetrahydrofuran. The filtrate, having a volume of 26 l, was
concentrated under vacuum (250 mm Hg, 55.degree. C.). The volume
was reduced by rotary evaporation up to a final concentrate of 3.5
l. 20 l of methanol were added so as to obtain crystallization. The
resulting suspension (23.5 l) was filtered and washed with 6 l of
methanol, thus resulting in a volume of 27 l. The humid product
(1.38 kg) resulting from the filtration was set under drying
condition in free air for one night. This gave a first fraction of
1.36 kg of 2,6-Bis[1-(2,4,6-trimethylphenylimino)ethyl]pyridine in
51% yield. The filtrate (27 l) was concentrated as described above
up to a final concentrate of 2.5 kg. 4 1 of methanol were added.
The resulting suspension was agitated for 1 hour at room
temperature and washed with 0.4 l of methanol. A second fraction of
50 g was in this way obtained. Thus, a total of 1400 g of
2,6-Bis[1-(2,4,6-trimethylphenylimino)ethyl]pyridine in 53% yield
were obtained. A reaction with iron(II) dichloride was carried out
as described by Qian et al., Organometallics 2003, 22,
4312-4321.
[0134] b) Support Pretreatment
[0135] 140 kg Sylopol 2107, a spray-dried silica gel from Grace,
was calcinated at 600.degree. C. for 6 hours.
[0136] c) Preparation of the Mixed Catalyst System
[0137] A mixture of 509 g (0.84 mol) of
2,6-Bis[1-(2,4-dichloro-6-methylphenylimino)ethyl]pyridine iron(II)
dichloride, prepared according to the above-mentioned procedure
under a), 4131 g (8.4 mol) of bis(n-butylcyclopentadienyl)hafnium
dichloride, commercially available from Crompton, and 195 l of MAO
(4.75 M in toluene, 926 mol) was stirred at 20.degree. C. for 2 h
and after cooling to 0.degree. C. subsequently added while stirring
to 140 kg of the pretreated support material b). The solution was
added with a flow rate lower than 100 kg/h. The obtained product
was stirred for further 30 minutes and heated to 40.degree. C. The
solid was dried under reduced pressure until it was free-flowing.
After sieving, 320 kg of catalyst were obtained (residual solvent:
41%).
[0138] (d) Polymerization
[0139] The polymerization was carried out in a fluidized-bed
reactor having a diameter of 3.7 m in the presence of the mixed
catalyst described above. The reaction temperature was 105.degree.
C., the pressure in the reactor was 25 bar, the reaction gas had
the following composition: 49 vol % ethylene, 5.1 vol % hexane, 0.6
vol % hexene, 45 vol % nitrogen, 1.5 kg/h trihexylaluminum (2 wt %
in hexane). The output was 5.5 t/h.
[0140] The MDPE polyethylene so obtained had a density of 0.939
g/cm.sup.3 and a MFR (190/21.6) of 28 g/10 min. The MDPE,
conveniently added with 700 ppm of a conventional processing
additive, namely Polybatch.RTM. AMF 705 (available from A.
Schulman) was used as a first polyethylene component, whose main
properties are shown in Table 1 below, while Lupolen 3220 F, which
is a LDPE commercially available from Basell Polyolefine GmbH
having a density of 0.930 g/cm.sup.3, and a MFR (190/2.16) of 0.9
g/10 min, was used as a second polyethylene component in an amount
of 11% by weight.
Examples 2-4 (Invention)
[0141] In Examples 2-4 a first and a second polyethylene components
as those described in Example 1 were used, except for the amount of
LDPE, which was set to 20%, 30% and, respectively, 40% by
weight.
TABLE-US-00001 TABLE 1 First PE component of Examples 1-4 Density
[g/cm.sup.3] 0.939 MFR (190/21.6) [g/10 min] 28 Eta(vis)/Eta(GPC)
2.08 M.sub.w [g/mol] 140000 M.sub.w/M.sub.n 14.4 M.sub.z 462000 GPC
% at molar mass 1 Mio 99.3 --HC.dbd.CH.sub.2 [ 1/1000C] 1.51
total-CH.sub.3 [ 1/1000C] 8.0
[0142] Where [0143] density is the polymer density [0144] MFR
(190/21.6) is the melt flow rate according to standard ISO 1133,
condition G [0145] Eta(vis) is the intrinsic viscosity as
determined according to ISO 1628-1 and [0146] Eta(GPC) is the
viscosity as determined by GPC according to DIN 55672, with
1,2,4-Trichlorobenzene, at 140.degree. C. [0147] M.sub.w is the
weight average molar mass; [0148] M.sub.n is the number average
molar mass [0149] M.sub.z is the z-average molar mass [0150] GPC %
at molar mass 1 Mio is the % by weight according to gel permeation
chromatography below a molar mass of 1 Mio g/mol. [0151]
HC.dbd.CH.sub.2 is the amount of vinyl groups [0152] total-CH.sub.3
is the amount of CH3-groups per 1000 C including end groups.
[0153] Example 5-8 (Comparative)
[0154] Innovex LL6910AA, which is a conventional LLDPE prepared by
the use of a Ziegler-Natta catalyst commercially available from BP
(density equal to 0.936 g/cm.sup.3, MFR (190/2.16) of 1.0 g/10
min), conveniently added with 700 ppm Polybatch.RTM. AMF 705, was
used as a first polyethylene component, whose properties are shown
in Table 2, while Lupolen 3220 F was used as a second polyethylene
component in an amount of 11%, 20%, 30% and, respectively, 40% by
weight.
TABLE-US-00002 TABLE 2 First PE component of Examples 5-8 Density
[g/cm.sup.3] 0.936 MFR (190/2.16) [g/10 min] 1.0
[0155] Where [0156] MFR (190/2.16) is the melt flow rate according
to standard ISO 1133, condition D.
Examples 9-12 (Comparative)
[0157] Lupolen 3721 C, which is a MDPE prepared by the use of a
chromium catalyst commercially available from Basell (density equal
to 0.937 g/cm.sup.3, MFR (190/21.6) of 12.5 g/10 min), was used as
a first polyethylene component, whose properties are shown in Table
3, while Lupolen 3220 F was used as a second polyethylene
component.
TABLE-US-00003 TABLE 3 First PE component of Examples 9-12 Density
[g/cm.sup.3] 0.937 MFR (190/21.6) [g/10 min] 12.5 Eta(vis)/Eta(GPC)
2.80 M.sub.w [g/mol] 240000 M.sub.w/M.sub.n 12.1 M.sub.z 1650000
GPC % at molar mass 1 Mio 95.8 --HC.dbd.CH.sub.2 [ 1/1000C] 0.72
total-CH.sub.3 [ 1/1000C] 5.4
[0158] Granulation and Film Extrusion
[0159] The polyethylene compositions of Example 1-12 were
homogenized and granulated on a ZSK 30 (Werner Pfleiderer) with
screw combination 8A. The processing temperature was 220.degree.
C., the screw speed 250/min, the output of 20 kg/h.
[0160] Each polyethylene composition of the Examples above was
extruded into films by blown film extrusion on a Weber film
extruder equipped with a collapsing device with wooden flatted
boards.
[0161] The diameter of the ring die was 50 mm, the gap width was
2/50 and the angle along which the cooling air is blown onto the
extruded film was 45.degree. . No filters were used. The 25D
Extruder with a screw diameter of 30 mm and a screw speed of 50
turns per min gave an output of 5.1 kg/h. The blow-up ratio was 1:2
and the haul-off speed 4.9 m/10 min. The height of the frost line
was 160 mm. Films with a thickness in the order of 50 .mu.m were
obtained. The specific thickness of each film, as well as the
processing properties and optical and mechanical properties of the
different films, are summarized in Tables 4 and 5.
TABLE-US-00004 TABLE 4 processing and optical properties of the
films Thickness Gloss Gloss Haze Clarity Example [.mu.m] 20.degree.
60.degree. [%] [%] 1 51 14 63 22 86 2 50 33 83 16 92 3 51 54 99 14
97 4 51 66 104 12 98 5 50 63 97 14 99 6 51 77 106 13 99 7 51 75 105
12 98. 8 52 71 102 11 99 9 51 1.5 16 61 23 10 51 2.2 22 45 33 11 51
3.2 29 34 47 12 50 4.2 35 30 56
TABLE-US-00005 TABLE 5 mechanical properties of the films Dynamic
Tensile Tear propagation Dart Test strength (Elmendorf method) Drop
[Nm/mm] [N/mm.sup.2] [mN] Example [g] W.sub.s W.sub.tot MD TD MD TD
1 276 11.6 13.1 42.2 35.1 2323 6058 2 241 11.1 12.5 41.8 34.5 2323
14848 3 235 9.8 12.1 40.0 32.6 2072 16387 4 190 8.2 10.8 38.2 32.5
1754 15539 5 119 6.2 9.6 44.5 45.6 1605 8602 6 120 4.7 8.6 41.7
41.7 1185 8319 7 120 4.15 8.7 42.3 39.1 1142 10045 8 117 4.2 8.5
40.3 36.8 1079 8884 9 165 3.0 8.1 -- -- 443 16544 10 146 2.8 8.2 --
-- 426 17674 11 144 2.9 8.7 -- -- 266 16450 12 133 3.2 8.8 -- --
370 14723
[0162] The values presented in the description and in the Tables
were determined in the following way.
[0163] NMR samples were placed in tubes under inert gas and, if
appropriate, melted. The solvent signals served as internal
standard in the .sup.1H- and .sup.13C-NMR spectra and their
chemical shift was converted into the values relative to TMS.
[0164] The degree of branching in the individual polymer fractions
was determined by the method of Holtrup (W. Holtrup, Makromol Chem.
178, 2335 (1977)) coupled with .sup.13C-NMR.
[0165] The density [g/cm.sup.3] was determined in accordance with
ISO1183.
[0166] The determination of the values M.sub.n, M.sub.w, M.sub.z
and of the molar mass distribution M.sub.w/M.sub.n derived
therefrom was carried out by means of high-temperature gel
permeation chromatography on a WATERS 150 C using a method based on
DIN 55672 and the following columns connected in series: 3.times.
SHODEX AT 806 MS, 1.times. SHODEX UT 807 and 1.times. SHODEX AT-G
under the following conditions: solvent: 1,2,4-trichlorobenzene
(stabilized with 0.025% by weight of
2,6-di-tert-butyl-4-methylphenol), flow: 1 ml/min, 500 .mu.l
injection volume, temperature: 140.degree. C. The columns were
calibrated with polyethylene standards with molar masses of from
100 bis 10.sup.7 g/mol. The evaluation was carried out by using the
Win-GPC software of Fa. HS-Entwicklungsgesellschaft fur
wissenschaftliche Hard-und Software mbH, Ober-Hilbersheim.
[0167] For the purposes of the present invention, the expression
MFR (190/21.6), known also as "high load melt flow rate", has been
determined at 190.degree. C. under a load of 21.6 kg in accordance
with ISO 1133, condition G.
[0168] For the purposes of the present invention, the expression
MFR (190/2.16) has been determined at 190.degree. C. under a load
of 2.16 kg in accordance with ISO 1133, condition D.
[0169] In order to determine the reflection properties of the
films, gloss measurements were carried out according to ISO 2813 on
a reflectometer at impingement angles of 20.degree. and 60.degree.,
on at least 5 pieces of film with a thickness of 50 .mu.m.
[0170] The haze was determined by ASTM D 1003-00 on a BYK Gardener
Haze Guard Plus Device on at least 5 pieces of film 10.times.10 cm
with a thickness of 50 .mu.m.
[0171] The clarity was determined by ASTM D 1746-03 on a BYK
Gardener Haze Guard Plus Device, calibrated with calibration cell
77.5, on at least 5 pieces of film 10.times.10 cm with a thickness
of 50 .mu.m.
[0172] In order to determine the puncture resistance of films under
shock loading, the dart drop was determined by ASTM D 1709, Method
A on 10 film samples having a thickness of 50 .mu.m.
[0173] In order to determine the strength of the films under
dynamic loading, dynamic tests were carried out according to DIN
53373, so as to obtain the fracture energy W.sub.s up to the first
tear and the total fracture energy W.sub.tot for the
penetration.
[0174] The tensile strength test was performed according to ISO 527
both in machine direction (MD) and at right angle to the machine
direction, known as transverse direction (TD)
[0175] The tear propagation test, otherwise known as Elmendorf
method, was performed according to ISO 6383/2.
* * * * *