U.S. patent application number 11/259053 was filed with the patent office on 2006-02-16 for novel polyethylene films.
This patent application is currently assigned to BP Chemicals Limited. Invention is credited to Frederic Alarcon, Christopher James Frye, David George Gilbert, Brian Leslie Turtle.
Application Number | 20060036039 11/259053 |
Document ID | / |
Family ID | 26244404 |
Filed Date | 2006-02-16 |
United States Patent
Application |
20060036039 |
Kind Code |
A1 |
Alarcon; Frederic ; et
al. |
February 16, 2006 |
Novel polyethylene films
Abstract
Novel stretch and blown films are prepared based on copolymers
of ethylene and alpha-olefins having (a) a density in the range
0.900 to 0.940 (b) an apparent Mw/Mn of 2-3.4 (c) I.sub.21/I.sub.2
from 16 to 24 (d) activation energy of flow from 28 to 45 kJ/mol
(e) a ratio Ea(HMW)/Ea(LMW) >1.1, and (f) a ratio
g'(HMW)/g'(LMW) from 0.85 to 0.95. The films exhibit an excellent
combination of strength and processability and are particularly
suitable for use as either stretch films or blown films for use as
heavy duty sacks. The preferred films show a dart impact of
>1100 g and MD elongations of >500%.
Inventors: |
Alarcon; Frederic; (Allee
des Argelas, FR) ; Frye; Christopher James;
(Dunblane, GB) ; Gilbert; David George;
(Teddington, GB) ; Turtle; Brian Leslie; (Oxshott,
GB) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Assignee: |
BP Chemicals Limited
|
Family ID: |
26244404 |
Appl. No.: |
11/259053 |
Filed: |
October 27, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10296830 |
Jun 23, 2003 |
|
|
|
PCT/GB01/02266 |
May 22, 2001 |
|
|
|
11259053 |
Oct 27, 2005 |
|
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Current U.S.
Class: |
525/191 |
Current CPC
Class: |
C08L 23/22 20130101;
C08J 2323/08 20130101; C08L 23/0815 20130101; C08L 2205/02
20130101; C08J 5/18 20130101; C08L 2666/04 20130101; C08L 23/0815
20130101 |
Class at
Publication: |
525/191 |
International
Class: |
C08F 8/00 20060101
C08F008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 1, 2000 |
GB |
0013344.7 |
Jun 1, 2000 |
GB |
0013343.9 |
Claims
1-6. (canceled)
7. A blown film having (a) dart impact of >450 g, (b) MD tear
strength >190 g/25 .mu.m, and (c) MD elongation >450% said
film comprising a copolymer of ethylene and an alpha-olefin having
from 3 to 10 carbon atoms, said copolymer having (a) a density
>0.920, (b) an apparent Mw/Mn of 2-3.4, (C) I.sub.21/I.sub.2
from 16 to 24, (d) activation energy of flow from 28 to 45 kJ/mol,
(e) a ratio Ea(HMW)/Ea(LMW) >1.1, and (h) a ratio
g'(HMW)/g'(LMW) from 0.85 to 0.95.
8. A blown film according to claim 7 having a dart impact >600
g.
9. A blown film according to claim 8 having a dart impact >1100
g.
10. A blown film according to 7 having a MD elongation of >500
g.
Description
[0001] The present invention relates to copolymers of ethylene and
alpha-olefins in particular to low density copolymers and also to
novel films produced from said copolymer having improved properties
in particular improved stretch and creep characteristics.
[0002] In recent years there have been many advances in the
production of polyolefin copolymers due to the introduction of
metallocene catalysts. Metallocene catalysts offer the advantage of
generally higher activity than traditional Ziegler catalysts and
are usually described as catalysts which are single-site in nature.
Because of their single-site nature the polyolefin copolymers
produced by metallocene catalysts often are quite uniform in their
molecular structure. For example, in comparison to traditional
Ziegler produced materials, they have relatively narrow molecular
weight distributions (MWD) and narrow Short Chain Branching
Distribution (SCBD).
[0003] Although certain properties of metallocene products are
enhanced by narrow MWD, difficulties are often encountered in the
processing of these materials into useful articles and films
relative to Ziegler produced materials. In addition, the uniform
nature of the SCBD of metallocene produced materials does not
readily permit certain structures to be obtained.
[0004] Recently a number of patents have published directed to the
preparation of films based on low density polyethylenes prepared
using metallocene catalyst compositions.
[0005] WO 94/14855 discloses linear low density polyethylene
(LLDPE) films prepared using a metallocene, alumoxane and a
carrier. The metallocene component is typically a bis
(cyclopentadienyl) zirconium complex exemplified by bis
(n-butylcyclopentadienyl) zirconium dichloride and is used together
with methyl alumoxane supported on silica The LLDPE's are described
in the patent as having a narrow Mw/Mn of 2.5-3.0, a melt flow
ratio (MFR) of 15-25 and low zirconium residues.
[0006] WO 94/26816 also discloses films prepared from ethylene
copolymers having a narrow composition distribution. The copolymers
are also prepared from traditional metallocenes (eg bis (1-methyl,
3-n-butylcyclopentadienyl) zirconium dichloride and methylalumoxane
deposited on silica) and are also characterised in the patent as
having a narrow Mw/Mn values typically in the range 3-4 and in
addition by a value of Mz/Mw of less than 2.0.
[0007] However, it is recognised that the polymers produced from
these types of catalyst system have deficiencies in processability
due to their narrow Mw/Mn. Various approaches have been proposed in
order to overcome this deficiency. An effective method to regain
processability in polymers of narrow Mw/Mn is by the use of certain
catalysts which have the ability to incorporate long chain
branching (LCB) into the polymer molecular structure. Such
catalysts have been well described in the literature, illustrative
examples being given in WO 93/08221 and EP-A-676421.
[0008] Furthermore, WO 97/44371 discloses polymers and films where
long chain branching is present, and the products have a
particularly advantageous placement of the comonomer within the
polymer structure. Polymers are exemplified having both narrow and
broad Mw/Mn, for example from 2.19 up to 6.0, and activation energy
of flow, which is an indicator of LCB, from 7.39 to 19.2 kcal/mol
(31.1 to 80.8kJ/mol). However, there are no examples of polymers of
narrow Mw/Mn, for example less than 3.4, which also have a low or
moderate amount of LCB, as indicated by an activation energy of
flow less than 11.1kcal/mol (46.7kJ/mol).
[0009] We have now found that it is possible to prepare copolymers
of ethylene and alpha-olefins having narrow Mw/Ma and low or
moderate amounts of LCB. These polymers are suitable for many
applications which will be known to those skilled in the art, but
in particular are advantageous for preparing films with an
excellent balance of processing, optical and mechanical
properties.
[0010] In particular the present invention is particularly directed
to stretch films with excellent cling properties and to blown films
suitable for use for heavy duty sacks.
[0011] Our copending application WO 00/68285 describes copolymers
of ethylene and an alpha olefin having 3 to 10 carbon atoms, said
copolymers having [0012] (a) a density in the range 0.900 to 0.940
[0013] (b) an apparent Mw/Mn of 2-3.4 [0014] (c) I.sub.21/I.sub.2
from 16 to 24 [0015] (d) activation energy of flow from 28 to 45
kJ/mol [0016] (e) a ratio Ea(HMW)/Ea(LMW) >1.1, and [0017] (f) a
ratio g'(HMW)/g'(LMW) from 0.85 to 0.95.
[0018] These copolymers may be used to prepare the full range of
products normally manufactured from polyethylene copolymer products
in the density range 0.900 to 0.940 kg/m.sup.3. Examples of
applications for the copolymers include injection moulding,
rotomoulding, extrusion into pipes, sheets, films, fibres,
non-woven fabrics, cable coverings and other uses which will be
known to those skilled in the art. are particularly suitable for
the production of films and sheets prepared using traditional
methods well known in the art. Examples of such methods are film
blowing, film casting and orientation of the partially crystallised
product. The films exhibit good processability, improved optical
and mechanical properties and good heat sealing properties.
[0019] WO 00/68285 described blown films from such copolymers
having haze ranging from 3 to 20, dart impact >100 g and hexane
extractables in the range 0.1-1.5%. Such films also exhibited a MD
tear strength in the range 106-210 g/25 .mu.m.
[0020] The application of polyethylene films in stretch wrapping
has been considerably enhanced by the use of linear low density
polyethylene (LLDPE) type products. When formed into a film for
stretch wrap application, LLDPE products typically combine a high
extensibility with good mechanical properties to provide a wrapping
or collation function to be achieved in an economic and effective
manner. In this respect, LLDPE has significant advantages over LDPE
which, due to both its behaviour in extension and its mechanical
performance, is not normally regarded as a product of choice for
stretch wrapping applications.
[0021] Application of stretch wrap films may be either by hand or
by machine. The film may be either wrapped directly onto the
article or articles to be packaged, or it may undergo a
pre-stretching operation prior to wrapping. Pre-stretching
typically enhances the mechanical property of the film and provides
a more effective packaging and more efficient coverage for a given
unit mass of film. Hence the response of the film to either a
pre-stretch or the stretch applied during wrapping is an important
parameter affecting film performance. In particular for a given
film width and thickness the efficiency with which an object is
wrapped is affected by the degree to which the film can be thinned
during the stretching and the loss of film width which may occur at
the same time. The resistance to sudden impact events, puncture by
sharp objects and the ability to maintain a tension sufficient to
maintain the package in the desired shape and configuration are
also important parameters.
[0022] A further requirement in many stretch wrapping applications
is that the film displays a certain degree of adhesive or cling
behaviour enabling a firm closure of the package to be achieved
without resort to use of additional securing measures such as
straps, glues or heat sealing operations. For monolayer films, such
adhesion may be provided by the intrinsic film properties or by
using a "cling" additive in the film formulation. An example of a
cling additive which is widely used is poly(isobutene) (PIB) which
term is taken to include polybutenes produced from mixed isomers of
butene. For multi-layer films, it is relatively easy to provide one
or more surface layers which are specifically formulated to provide
cling. In general this method allows a more flexible approach to
film manufacture as choice of product for the main body of the film
may be made on the basis of mechanical performance and the surface
layers can be specially formulated for adhesion. Those skilled in
the art will appreciate the multiplicity and flexibility of the
choices of possible film structures.
[0023] A further requirement for the film producer is that the
fabrication of the film is made as easy as possible by the use of
polyethylenes having processing characteristics which allow film
extrusion to be carried out as easily as possible. The use of a
product of lower molecular weight or broader molecular weight
distribution provides easier processability, but normally at the
expense of a reduction in mechanical performance of the film.
Similarly the use of products such an LDPE containing long chain
branches (LCB) may assist processability but at the expense of
stretchability in the subsequent wrapping process.
[0024] We now found that a particularly advantageous combination of
film properties may be obtained by producing a stretch film from
the novel copolymers described in the aforementioned WO 00/68285.
The films have a particularly advantageous combination of
properties, combining high impact resistance with easy
processability and good performance in stretch wrapping and when
combined with polyisobutene as a cling enhancer, the films show a
particularly advantageous control of cling force.
[0025] Thus according to the present invention there is provided a
stretch film comprising a cling additive in amount >0.5% and
having [0026] (a) dart impact of >450 g [0027] (b) MD tear
strength of >190 g/25 .mu.m [0028] (c) MD elongation at break of
>450%
[0029] said film comprising a copolymer of ethylene and an
alpha-olefin having from 3 to 10 carbons atoms, said copolymer
having [0030] (a) a density in the range 0.900 to 0.940 [0031] (b)
an apparent Mw/Mn of 2-3.4 [0032] (c) I.sub.21/I.sub.2 from 16 to
24 [0033] (d) activation energy of flow from 28 to 45 kJ/mol [0034]
(e) a ratio Ea(HMW)/Ea(LMW) >1.1, and [0035] (f) a ratio
g'(HMW)/g'(LMW) from 0.85 to 0.95.
[0036] The preferred stretch films according to the present are
those having a dart impact of >600 g and most preferably
>1100 g.
[0037] The preferred films show an elongation of >500%.
[0038] The cling additive may be present in amount >2% and most
preferably in amount of greater than or equal to 4%.
[0039] The preferred cling additive is polyisobutene (PIB).
[0040] The novel stretch films of the present invention may also be
utilised in multi layer films, for example in 3-layer films wherein
the other layers comprise polymers of lower density or copolymers
as described above.
[0041] When extruded into a stretch film by film blowing, the
products of the invention give produce films with a particularly
advantageous balance of properties. The processability of the
ethylene copolymers during the film production process is typically
comparable if not better than an LLDPE type polymer produced from a
ziegler catalyst. The processability is assessed from measures such
as the melt pressure in extrusion, the output rate for a given set
of extruder conditions and the motor load. Such processing
performance allows these products to be a "drop-in" for existing
LLDPE grades of similar specification without having to make
expensive changes to extrusion machinery or suffering a handicap in
terms of extrusion performance.
[0042] As regards mechanical performance the dart impact of the
films is very high compared to a ziegler product of similar
specification, being typically more than 600 g and preferably more
than 1100 g for a film of thickness 25 .mu.m for a product of melt
index about 1 and density 917. Film elongation is maintained at
more than 500% despite the presence of LCB. It is important that
the film can be stretched to 300% or more without fracturing.
[0043] Due to their unique structure, the films of the invention
show an advantageous behaviour whilst undergoing stretching that
the film width is not unduly reduced. For a pre-stretch of 70% the
films retain over 75% of their initial width, this property being
retained during storage of the film roll for up to one month or
more.
[0044] The films of the invention show a hi-cling force as assessed
by a Thimon stretch wrapping machine. A particularly advantageous
behaviour is that the cling force varies only weakly with the
amount of PIB cling agent added to the film. Hence there is a wide
latitude for addition levels of PIB to vary without causing either
too much or too little cling to develop in the film.
[0045] Good elongation combined with outstanding impact resistance
provides significant advantages in wrapping applications.
[0046] In the application of polyethylene copolymer products in
blown films, a key performance compromise is the balance between
the modulus of the film and its impact performance. In general,
alterations to the polymer structure such as increasing the
crystallinity lead to increased modulus but at the expense of
reduced impact performance. The advent of metallocene catalysed
products has lead to a redefinition of this performance compromise.
It is generally acknowledged that blown films from copolymers
produced from metallocene catalysts have a different balance of
properties when compared to LLDPE type products produced by the
more well established ziegler catalysts. When comparing products of
the same basic specification in melt index and density, the
metallocene products tend to have very high impact properties due
to narrow molecular weight distribution and reduced modulus due to
homogeneity of comonomer distribution.
[0047] We have found that the copolymers of the present invention
can offer increased modulus and impact when compared to more
conventional ziegler products while at the same time having no
penalty in extrusion performance. For a given balance of
performance in impact and modulus, the creep performance of the
inventive resins is also better than conventional Ziegler products,
as are the film optical properties. Sealing is also improved. Hence
the resins of the invention show many advantages without displaying
any disadvantage in processing.
[0048] A particular application of blown films is for use in heavy
duty sacks for example for use for fertilisers, plastic pellets,
etc. The mechanical properties of stiffness, impact and creep
resistance are of prime importance for the suitability of the
copolymer product. Because of the intrinsic high impact resistance,
the stiffness of the copolymers can be increased while maintaining
a better impact resistance compared with conventional products.
Also due to the superior SCBD of the copolymers the cereep
resistance (creep elongation) is significantly improved leading to
advantages in handling of the filled bags and provides a potential
for significant downgauging while maintaining similar performance
to reference proprietary products.
[0049] For this application the films of the present invention
suitably comprise copolymers of density >0.920.
[0050] Thus according to another aspect of the present invention
there is provided a blown film having [0051] (a) dart impact of
>450 g [0052] (b) MD tear strength >190 g/25 .mu.m [0053] (c)
MD elongation >450% said film comprising a copolymer of ethylene
and an alpha-olefin having from 3 to 10 carbons atoms, said
copolymer having [0054] (a) a density >0.920; [0055] (b) an
apparent Mw/Mn of 2-3.4 [0056] (c) I.sub.21/I.sub.2 from 16 to 24
[0057] (d) activation energy of flow from 28 to 45 kJ/mol [0058]
(e) a ratio Ea(HMW)/Ea(LMW) >1.1, and [0059] (g) a ratio
g'(HMW)/g'(LMW) from 0.85 to 0.95.
[0060] The preferred blown films according to this aspect of the
present invention are those having a dart impact of >600 g and
most preferably >1100 g.
[0061] The preferred blown films show an elongation of
>500%.
[0062] The novel blown films of the present invention may suitably
be utilised in blends, for example with medium density
polyethylenes.
[0063] The most preferred copolymers for use in the novel stretch
films of the present invention are those having [0064] (a) a
density in the range 0.900 to 0.940 [0065] (b) an apparent Mw/Mn in
the range 2.5 to 3 [0066] (c) I.sub.21/I.sub.2 from 18-24 [0067]
(d) activation energy of flow from 30 to 35 kJ/mol [0068] (e) a
ratio Ea(HMW)/Ea(LMW) >1.2, and [0069] (f) a ratio
g'(HMW)/g'(LMW) from 0.85 to 0.95.
[0070] By apparent Mw/Mn is meant a value of Mw/Mn uncorrected for
long chain branching.
[0071] The significance of the parameters Ea(HMW)/Ea(LMW) and
g'(HMW)/g'(LMW) is described below. The experimental procedures for
their measurements are described later in the text.
[0072] The polymers contain an amount of LCB which is clearly
visible by techniques such as GPC/viscometry and flow activation
energy. The content of LCB is lower than reported in many earlier
publications, but is still sufficient, when coupled with broadened
Mw/Mn, to give improved processability compared to linear polymers
of narrow MWD (Mw/Mn less than about 3), which do not contain
LCB.
[0073] For the measurement of LCB, we have found that the most
useful techniques are those which have a particular sensitivity to
the presence of LCB in the high molecular weight chains. For these
high molecular weight molecules, the physical effects of LCB on,
the solution and melt properties of the polymer are maximised.
Hence detection of LCB using methods based upon solution and melt
properties is facilitated.
[0074] Activation energy of flow is commonly used as an indicator
of the presence of LCB in polyethylenes as summarised in the
aforementioned WO 97/44371. For lower amounts of LCB, for which the
global activation energy is of the order of 28 to 45 kJ/mol, it is
found that the LCB has a strong effect upon the activation energy
as measured at low test rates ie the region in which the rheology
is dominated by the high molecular weight (HMW) species. Therefore,
the ratio- of activation energy derived from the low rate data
Ea(HMW) tends to exceed that derived from the high rate data,
Ea(LMW). Hence polymers containing LCB predominantly in the high
molecular-weight chains tend to show the ratio Ea(HMW)/Ea(LMW)
greater than unity.
[0075] A further well established method indicating the presence of
LCB is gel permeation chromatography with on-line detection of
viscosity (GPC/OLV). By combining the data from 2 detectors, the
ratio g' can be derived as a function of molecular weight; g' is
the ratio of the measured intrinsic viscosity [.eta.] divided by
the intrinsic viscosity [.eta.].sub.linear of a linear polymer
having the same molecular weight. In polymers containing LCB, the
g' measured at high molecular weights tends to be less than that
measured at low molecular weights. To quantify this effect, we have
used a simple ratio g'(HMW)/g'(LMW). g'(HMW) is the weighted mean
value of g' calculated for the 30% of the polymer having the
highest molecular weight, while g'(LMW) is the weighted mean value
of g' calculated for the 30% of the polymer having lowest molecular
weight. For linear polymers, g' is equal to 1 at all molecular
weights, and so g'(HMW)/g'(LMW) is also equal to 1 when there is no
LCB present. For polymers containing LCB, g'(HMW)/g'(LMW) is less
than 1. It should be noted that the g' data can be corrected for
the effect of short chain branching (SCB). This would normally be
done using a mean value of SCB content, the correction being
applied uniformly at all molecular weights. Such a correction has
not been applied here because in measuring the ratio
g'(HMW)/g'(LMW) the same correction would apply to both g' values
and there would be no net effect on the results reported here.
Another method to quantify LCB content in polyethylenes is by
carbon-13 Nuclear Magnetic Resonance (13C-NMR). For the low amounts
of LCB observed for polymers of the invention it is
generally-accepted that this technique can give a reliable
quantification of the number of LCB points present in the polymer
when the polymer is a homopolymer or a copolymer of ethylene and
propylene or butene-1. For the purposes of this specification, a
measurement of LCB by 13C-NMR is achieved in such polymers by
quantification of the isolated peak at about 38.3ppm corresponding
to the CH carbon of a tri-functional long chain branch. A
tri-functional long chain branch is taken to mean a structure for
which at least the first four carbon atoms of each of the 3 chains
radiating from the CH branch carbon are all present as CH2 groups.
Care must be exercised in making such measures to ensure that
sufficient signal:noise is obtained to quantify the resonance and
that spurious LCB structures are not generated during the sample
heating by oxidation induced free-radical reactions.
[0076] The above described analysis of LCB by 13C-NMR is much more
difficult when the copolymer contains hexene-1. This is because the
resonance corresponding to an LCB is very close to or overlapping
that for the CH carbon at the branch site of the n-butyl branch
obtained from this comonomer. Unless the two CH resonances can be
resolved, which is unlikely using NMR equipment currently
available, LCB could only be determined for an ethylene/hexene-1
copolymer using the above described technique if the amount of
n-butyl branches was so low, in comparison to the amount of LCB
present, that it could either be ignored or a reliable subtraction
carried out on the CH resonance at about 38.3ppm.
[0077] Using the preferred catalyst system of the present invention
an ethylene/butene-1 copolymer containing 6.5 wt % butene-1 has
been prepared using a continuous gas phase reactor. This polymer
contained 0.12 LCB/10,000 total carbons using the 13C-NMR technique
described above. The spectrum was obtained from a 600 MHz NMR
spectrometer after 912,000 scans. The polymer also contained 0.25
n-butyl branches/10,000 total carbons. No detectable oxidation was
observed during this analysis with a limit of detection of
approximately 0.05/10,000 total carbons.
[0078] Despite a relatively low average LCB content, it would be
expected that such polymers would show distinctly modified
rheological behaviour in comparison with truly linear polymers. If
the LCB is concentrated in the molecules of higher molecular
weight, as is known to be the case, then an average value of 0.12
LCB/10,000 total carbons in the whole polymer could correspond to
about 0.3 or more LCB/10,000 for molecules of molecular weight
about one million. Hence these molecules would be expected to
contain at least 2 LCB points per molecule, equivalent to a
branched structure with 5 arms. Such molecules are known to display
very different rheological properties to linear molecules.
[0079] The preferred polymers of the invention also show quite low
amounts of vinyl unsaturation as determined by either infra-red
spectroscopy or preferably proton NMR. For a polymer of melt index
(2.16 kg) about 1, values are less than 0.05 vinyl groups per 1000
carbon atoms or even as low as less than 0.02 vinyl groups per 1000
carbon atoms. Again, for melt index (2.16 kg) about 1, total
unsaturations are also low compared to some other metallocene
polymers containing LCB, the total unsaturations as measured by
proton NMR to be the sum of vinyl, vinylidene, tri-substituted and
cis+trans di-substituted internal unsaturation being in the range
of less than 0.2 to 0.5 per 1000 carbon atoms. Products with higher
or lower melt index, and hence lower or higher number average
molecular weights, may show respectively higher or lower terminal
unsaturations, in proportion to the total number of chain ends
present. Hence the total unsaturations per 1000 carbon atoms are
less than 17500/Mn where Mn is the number average molecular weight
uncorrected for LCB and the vinyl unsaturations are less than
1750/Mn.
[0080] The comonomer present in the preferred polymers of the
invention is not randomly placed within the polymer structure. If
the comonomer was randomly placed, it would be expected that the
elution trace derived from temperature rising elution fractionation
(TREF) would show a single narrow peak, the melting endotherm as
measured by differential scanning calorimetry would also show a
substantially singular and narrow peak. It would also be expected
that little variation would be expected in either the amount of
comonomer measured as a function of molecular weight by techniques
such as GPC/FTIR, or the molecular weight of fractions measured as
a function of comonomer content by techniques such as TREF/DV.
These techniques for structure determination are also described in
the aforementioned WO 97/44371, the relevant parts of which are
incorporated herein by reference.
[0081] However, the comonomer may be placed in a way as to give a
distinct broadening of the TREF elution data, often with the
appearance of one or two or even three peaks. At a polymer density
of about 91.8 kg/m.sup.3 the TREF data typically show two main
peaks, one at about 87.degree. C. and another distinct but smaller
peak at about 72.degree. C., the latter being about 2/3 of the
height of the former. These peaks represent a heterogeneity in the
amount of comonomer incorporated in the polymer chains. A third
peak is often visible at about 100.degree. C. Without being bound
by any theory this peak is considered to be nothing other than a
consequence of the fact that the polymer molecules of low comonomer
content tend to crystallise into large chain folded crystals which
melt and dissolve in the TREF experiment in a narrow range of
temperatures at about 100.degree. C. The same peak is very clearly
visible in certain types of LLDPE polymers produced by ziegler
catalysts and it is present in TREF analysis of MDPE and HDPE type
polyethylenes. Thus, without being bound by any theory, the third
peak at about 100.degree. C. is more a result of the
crystallisation of linear or near-linear molecules, than a feature
which can be simply interpreted as representing a particular and
separate polymer species.
[0082] The CDBI (Composition Distribution Branch Index) of the
polymers is between 55 and 75%, preferably 60 to 75%, reflecting
the fact that the polymers are neither highly homogeneous
(CDBI>about 90%) nor highly heterogeneous (CDBI<about 40%).
The CDBI of a polymer is readily-calculated from techniques known
in the art, such as, for example, temperature rising elution
fractionation (TREF) as described, for example, in Wild et al.,
Journal of Polymer Science, Polymer Phys. Ed., Vol 20, p 441
(1982), or in U.S. Pat. No. 4,798,081.
[0083] The behaviour seen in melting endotherms by DSC reflects the
behaviour in TREF in that one, two or three peaks are typically
seen. For example three peaks are often seen for the preferred
polymers of density about 918 kg/m.sup.3, when heated at 10.degree.
C./min. after crystallisation at the same rate. As is usual, it
would be expected that the peaks seen in TREF and DSC would move to
lower temperatures for polymers of lower density and to higher
temperatures for polymer of higher density. The peak melting
temperature Tp (the temperature in .degree. C. at which the maximum
heat flow is observed during the second heating of the polymer) can
be approximated by the following expression within normal
experimental errors: Tp=462.times.density-306 The amount of
comonomer measured as a function of molecular weight by GPC/FTIR
for the preferred polymers shows an increase as molecular weight
increases. The associated parameter C.sub.pf is greater than 1.1.
The measurement of C.sub.pf is described in WO 97/44371.
[0084] The preferred copolymers exhibit extensional rheological
behaviour, in particular strain-hardening properties, consistent
with the presence of long chain branching.
[0085] The copolymers may suitably be prepared by use of a
metallocene catalyst system comprising, for example a traditional
bisCp metallocene complex or a complex having a `constrained
geometry` configuration together with a suitable activator.
[0086] Suitable complexes, for example, are those disclosed in WO
95/00526 the disclosure of which is incorporated herein by
reference.
[0087] Suitable activators may comprise traditional aluminoxane or
boron compounds for example borates again disclosed in the
aforementioned WO 95/00526.
[0088] Preferred metallocene complexes for use in the preparation
of the copolymers may be represented by the general formula:
##STR1## wherein:-- [0089] R' each occurrence is independently
selected from hydrogen, hydrocarbyl, silyl, germyl, halo, cyano,
and combinations thereof, said R' having up to 20 nonhydrogen
atoms, and optionally, two R' groups (where R' is not hydrogen,
halo or cyano) together form a divalent derivative thereof
connected to adjacent positions of the, cyclopentadienyl ring to
form a fused ring structure; [0090] X is a neutral .fwdarw..sup.4
bonded diene group having up to 30 non-hydrogen atoms, which forms
a complex with M; [0091] Y is --O--, --S--, --NR*--, --PR*--,
[0092] M is titanium or zirconium in the +2 formal oxidation state;
[0093] Z* is SiR*.sub.2, CR*.sub.2, SiR*.sub.2SIR*.sub.2,
CR*.sub.2CR*.sub.2, CR*.dbd.CR*, CR*.sub.2SIR*.sub.2, or
GeR*.sub.2, wherein: [0094] R* each occurrence is independently
hydrogen, or a member selected from hydrocarbyl, silyl, halogenated
alkyl, halogenated aryl, and combinations thereof, said R* having
up to 10 non-hydrogen atoms, and optionally, two R* groups from Z*
(when R* is not hydrogen), or an R* group from Z* and an R* group
from Y form a ring system.
[0095] Examples of suitable X groups include
s-trans-.fwdarw..sup.4-1,4-diphenyl-1,3-butadiene,
s-trans-.fwdarw..sup.4-3-methyl-1,3-pentadiene;
s-trans-.fwdarw..sup.4-2,4-hexadiene;
s-trans-.fwdarw..sup.4-1,3-pentadiene;
s-trans-.fwdarw..sup.4-1,4-ditolyl-1,3-butadiene;
s-trans-.fwdarw..sup.4-1,4-bis(trimethylsilyl)i1,3-butadiene;
s-cis-_.sup.4-3-methyl-1,3-pentadiene; s-cis-;.sup.4-1,4
dibenzyl-1,3-butadiene; s-cis-.fwdarw..sup.4-1,3-pentadiene;
s-cis-.fwdarw..sup.4-1,4-bis(trimethylsilyl)-1,3-butadiene, said
s-cis diene group forming a .pi.-complex as defined herein with the
metal.
[0096] Most preferably R' is hydrogen, methyl, ethyl, propyl,
butyl, pentyl, hexyl, benzyl, or phenyl or 2 R' groups (except
hydrogen) are linked together, the entire C.sub.5R'.sub.4 group
thereby being, for example, an indenyl, tetrahydroindenyl,
fluorenyl, terahydrofluorenyl, or octahydrofluorenyl group.
[0097] Highly preferred Y groups are nitrogen or phosphorus
containing groups containing a group corresponding to the formula
--N(R'') or --P(R'')-- wherein R'' is C.sub.1-10 hydrocarbyl.
[0098] Most preferred complexes are amidosilane- or
amidoalkcanediyl complexes.
[0099] Most preferred complexes are those wherein M is
titanium.
[0100] Specific complexes suitable for use in the preparation of
the novel copolymers of the present invention are those disclosed
in the aforementioned WO 95/00526 and are incorporated herein by
reference.
[0101] A particularly preferred complex for use in the preparation
of the novel copolymers of the present invention is (t-butylamido)
(tetramethyl-.fwdarw..sup.5-cyclopentadienyl) dimethyl
silanetitanium-.fwdarw..sup.4-1,3-penatadiene.
[0102] The activator may preferably be a boron compound for example
a borate such as ammonium salts, in particular. [0103]
triethylammonium tetraphenylborate [0104] triethylammonium
tetraphenylborate, [0105] tripropylammonium tetraphenylborate,
[0106] tri(n-butyl)ammonium tetraphenylborate, [0107]
tri(t-butyl)ammonium tetraphenylborate, [0108]
N,N-dimethylanilinium tetraphenylborate, [0109]
N,N-diethylanilinium tetraphenylborate, [0110] trimethylammonium
tetrakis(pentafluorophenyl) borate, [0111] triethylammonium
tetrakis(pentafluorophenyl) borate, [0112] tripropylammonium
tetrakis(pentafluorophenyl) borate, [0113] tri(n-butyl)ammonium
tetrakis(pentafluorophenyl) borate, [0114] N,N-dimethylanilinium
tetrakis(pentafluorophenyl) borate, [0115] N,N-diethylanilinium
tetrakis(pentafluorphenyl) borate.
[0116] Another type of activator suitable for use with the
metallocene complexes are the reaction products of (A) ionic
compounds comprising a cation and an anion wherein the anion has at
least one substituent comprising a moiety having an active hydrogen
and (B) an organometal or metalloid compound wherein the metal or
metalloid is from Groups 1-14 of the Periodic Table.
[0117] Suitable activators of this type are described in WO
98/27119 the relevant portions of which are incorporated herein by
reference.
[0118] A particular preferred activator of this type is the
reaction product obtained from alkylammonium
tris(pentafluorophenyl) 4-(hydroxyphenyl) borates and
trialkylaluminium. For example a preferred activator is the
reaction product of bis(hydrogenated tallow alkyl) methyl ammonium
tris (pentafluorophenyl) (4-hydroxyphenyl) borate and
triethylaluminium.
[0119] The molar ratio of metallocene complex to activator employed
in the process of the present invention may be in the range 1:10000
to 100:1. A preferred range is from 1:5000 to 10:1 and most
preferred from 1:10 to 10:1.
[0120] The metallocene catalyst system is most suitably supported.
Typically the support can be an organic or inorganic inert solid.
However particularly porous supports such as talc, inorganic oxides
and resinous support materials such as polyolefins which have
well-known advantages in catalysis are preferred. Suitable
inorganic oxide materials which maybe used include Group 2, 13 14
or 15 metal oxides such as silica, alumina, silica-alumina and
mixtures thereof.
[0121] Other inorganic oxides that may be employed either alone or
in combination with the silica, alumina or silica-alumina are
magnesia, titania or zirconia. Other suitable support materials may
be employed such as finely divided polyolefins such as
polyethylene.
[0122] The most preferred support material for use with the
supported catalysts is silica. Suitable silicas include Crosfield
ES70 and Grace Davison 948 silicas.
[0123] The support material may be subjected to a heat treatment
and/or chemical treatment to reduce the water content or the
hydroxyl content of the support material. Typically chemical
dehydration agents are reactive metal hydrides, aluminium alkyls
and halides. Prior to its use the support material may be subjected
to treatment at 100.degree. C. to 1000.degree. C. and preferably at
200 to 850.degree. C. in an inert atmosphere under reduced
pressure, for example, for 5 hrs.
[0124] The support material may be pretreated with an aluminium
alkyl at a temperature of -20.degree. C. to 150.degree. C. and
preferably at 20.degree. C. to 100.degree. C.
[0125] The pretreated support is preferably recovered before use in
the preparation of the supported catalysts.
[0126] The copolymers comprise copolymers of ethylene and
alpha-olefins having 3 to 10 carbon atoms. Preferred alpha olefins
comprise 1-butene, 1-hexene and 4-methyl-1-pentene. A particularly
preferred alpha olefin is 1-hexene.
[0127] The copolymers are most suitably prepared in the gas phase
in particular in a continuous process operating at a temperature
>60.degree. C. and most preferably at a temperature of
75.degree. C. or above. The preferred process is one comprising a
fluidised bed reactor. A particularly suitable gas phase process is
that disclosed in EP 699213 incorporated herein by reference.
[0128] When prepared by use of the preferred catalyst systems
described above the copolymers have a titanium content in the range
0.1 to 2.0 ppm.
EXAMPLES
Catalyst Preparation
[0129] (i) Treatment of Silica
[0130] A suspension of Grace 948 silica (13 kg, previously calcined
at 250.degree. C. for 5 hours) in 110 litres (L) of hexane was made
up in a 240 L vessel under nitrogen. 1 L of a hexane solution
containing 2 g/L of Stadis 425 was added and stirred at room
temperature for 5 minutes. 29.1 L of a 892mmol Al/L solution of
triethylaluminium (TEA) in hexane was added slowly to the stirred
suspension over 30 minutes, while maintaining the temperature of
the suspension at 30.degree. C. The suspension was stirred for a
further 2 hours. The hexane was filtered, and the silica washed
with hexane, so that the aluminium content in the final washing was
less than 0.5 mmol Al/litre. Finally the suspension was dried in
vacuo at 60.degree. C. to give a free flowing treated silica powder
with residual solvent less than 0.5 wt %.
[0131] (ii) Catalyst Fabrication
[0132] All steps, unless otherwise stated, of the catalyst
fabrication were carried out at 20.degree. C. 3 L of toluene was
added to a 24 L vessel equipped with a turbine stirrer, and stirred
at 300 rpm. 5.01 L of a 9.5 wt % solution in toluene of
bis(hydrogenated tallow alkyl) methyl ammonium
tris(pentafluorophenyl)(4-hydroxyphenyl)borate was added during 15
minutes. Then 1.57 L of a 250mmolAl/L solution in toluene of
triethylaluminium was added during 15 minutes and mixture stirred
for 30 minutes. The solution obtained was then transferred under
nitrogen, with stirring during 2 hours, to an 80 L vessel
containing 10 kg of the TEA treated silica described above. 60 L of
hexane was then rapidly introduced and mixed for 30 minutes. 1.83
kg of a 7.15 wt % solution in heptane of
(t-butylamido)(tetramethyl-.eta..sup.5-cyclopentadienyl)
dimethylsilanetitanium-.eta..sup.4-1,3-pentadiene was added during
15 minutes. Mixing was continued for 1 hour and 1 L of a 2 g/L
hexane solution of stadis 425 was added. The catalyst slurry was
then transferred to a vessel of volume 240 L and 70 L of hexane
added. Excess solvent was removed by decantation, and a further 130
L of hexane added. This process was repeated until less than 0.2 L
of toluene remained in the solvent. 1 L of a 2 g/L hexane solution
of stadis 425 was then added and the catalyst dried under vacuum at
40.degree. C. to a residual solvent level of 1 wt %.
[0133] (iii) Polymerisation Using Continuous Fluidised Bed
Reactor
Example 1
[0134] Ethylene, 1-hexene, hydrogen and nitrogen were fed into a
continuous fluidised bed reactor of diameter 45 cm. Polymerisation
was performed in the presence of a catalyst similar to that
prepared above. Polymer product was continuously removed from the
reactor. Operating conditions are given in Table 1.
Example 2
[0135] The procedure for example 1 was scaled up to produce a
catalyst of batch size approximately 75 kg. This catalyst was used
to produce a copolymer in a commercial gas phase scale reactor of
diameter 5 metres again using the conditions shown in Table 1.
TABLE-US-00001 TABLE 1 Example 1 2 total pressure (bar) 20.0 19.8
temperature (.degree. C.) 80 75 ethylene pressure (bar) 7.5 8.1
H.sub.2/C.sub.2 ratio 0.0025 0.0023 C.sub.6/C.sub.2 ratio 0.0055
0.0050 production (kg/hr) 74 8700
Comparative Example 1
[0136] A film from Dowlex 2045 was used for comparison.
[0137] 3 layer films were produced on a coextrusion operating line
at about 100 kg/hr. This line was equipped with 4 25 L/D LLDPE
extruders and a 300 mm diameter die with 1.2 mm die gap. The film
was of thickness 25 mm and the blow up ratio 2.5:1. The inner cling
layer was formed from an EVA copolymer containing TAC 100 (50%
PIB). The other layers were formed from the test polymer containing
TAC 100.
[0138] Details of the copolymers prepared and films produced are
given in Table 2. TABLE-US-00002 TABLE 2 Film properties Example
Comp Comp Comp 1a 1b 1c 1a 1b 1c 2a MI/2.16 g/10 mn 0.91 0.91 0.91
1.18 1.18 1.18 1.3 HLMI g/10 mn 25.8 25.8 25.8 23.70 23.70 23.70
25.80 MFR 28.4 28.4 28.4 20.1 20.1 20.1 19.8 Density kg/m.sup.3
919.4 919.4 919.4 916.6 916.6 916.6 916.9 EXTRUSION CONDITIONS Melt
pressure bar 533 494 460 508 496 467 454 Melt temperature .degree.
C. 232 232 231 229 233 230 228 Output kg/h 95 95 95 110 110 110 110
Motor Load A 55 50 50 54 51 49 49 Blend 4% PIB 5% PIB 6% PIB 4% PIB
5% PIB 6% PIB 5% PIB MECHANICAL PROPERTIES Dart Impact g 265 350
310 >1100 >1100 >1100 >1100 Elmendorf tear MD g/25
.mu.m 255 207 196 str. TD g/25 .mu.m 656 577 572 Elongation at MD %
670 640 600 break TD % 780 660 680
Example 3
[0139] A resin was produced in the gas phase using a similar
catalyst system to that described above with melt index 1 and
density 923.6 kg/m.sup.3. This was extruded into film 150 .mu.m
thick on a Reifenhauser blown film line equipped with a die of
diameter 150 mm and die gap 2.3 mm. The product was extruded both
pure and blended with 20% of a medium density polyethylene of
density about 938 kg/m.sup.3, melt index about 0.2 produced using a
chromium catalyst system.
Comparative Example 2
[0140] Dowlex 2045 was used as a comparative example.
[0141] The blown film properties are given in Table 3 below. The
films were also tested in creep at 60.degree. C. under 5Mpa load.
After 200 minutes, the deformation of the film of example 1b was
57% compared to 63% for comparative example 2 TABLE-US-00003 TABLE
3 Example 1a 1b Comp 1 MI/2.16 g/10 mn 1.00 1.00 0.94 HLMI g/10 mn
23.46 23.46 26.8 MFR 23.5 23.5 28.5 Density kg/m.sup.3 923.6 923.6
919.7 EXTRUSION Die mm 150 150 150 Die gap mm 2.3 2.3 2.3 Screw
speed rpm 83.4 85 89.2 Melt pressure bar 267 283 268 Melt
temperature .degree. C. 216.7 217 217.1 Output kg/h 50 50 50 BUR
2:1 2:1 2:1 Motor Load A 62 65 61 Specific energy KWh/Kg 0.22 0.23
0.23 Thickness .mu.m 150 150 150 Blend pure +20% MDPE +20% MDPE
MECHANICAL PROPERTIES Dart Impact g 1295 1084 890 Edge fold impact
(Staircase Method) (g) 805 735 650 Elmendorf tear str. MD g/25
.mu.m 260 210 341 TD g/25 .mu.m 418 471 573 Tensile str. at yield
MD MPa 12.9 14.4 12.5 TD MPa 14 14.6 13.4 Tensile str. at break MD
MPa 48 45.6 43.9 TD MPa 47.5 41.6 42.5 Elongation at break MD %
1250 862 930 TD % 1000 917 1000 Secant modulus 1% MD MPa 235 263
208 TD MPa 285 298 239 Haze % 23.8 22.5 19.8 Gloss 45.degree.
.Salinity. 57.7 49.4 47.9
Methods of Test
[0142] Melt index (190/2.16) was measured according to ISO
1133.
[0143] Melt flow ratio (MFR) was calculated from the ratio of flow
rates determined according to ISO 1133 under condition (190/21.6)
and condition (190/2.16).
[0144] Density was measured using a density column according to ISO
1872/1-1986, except that the melt index extrudates were not
annealed but were left to cool on a sheet of polymeric material for
30 minutes.
[0145] Dart impact was measured by ASTM D1709, tear strength by
ASTM D1922, and haze by ASTM D1003.
* * * * *