Note: Descriptions are shown in the official language in which they were submitted.
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FOAMABLE POLYPROPYLENE POLYMERS
AND THEIR USE
FIELD OF THE INVENTION
This invention relates generally to polypropylene polymers, their uses, and to
methods of their production.
BACKGROUND
l0 Among the three most versatile commodity plastics, which are polyethylene
(PE), polystyrene (PS) and polypropylene (PP), polypropylene is considered to
possess
the most favorable properties profile of the three for a variety of
applications. These
applications include, for example, oriented and non-oriented films, textile
fibers,
nonwovens and a variety of injection molded parts. Comparing the properties,
it is
15 well known that polypropylene has a higher modulus and heat deflection
temperature
(HDT) than polyethylene. The higher the modulus and HDT, the more suited the
polymer is for durable applications in the appliance and automotive segments.
Additionally, because polypropylene is nonpolar, it resists degradation by
common
solvents, such as acids and alkalis. Compared to polystyrene, polypropylene is
20 preferred in applications requiring good organoleptic performance, high
burner
properties and the living hinge property. Finally, polypropylene blends well
with a
variety of other polymers, and in impact-modified form occupies a dominant
position in
the automotive industry in the areas of bumpers, side panels, floor mats,
dashboards
and instrument panels.
25 However, there exist some polymer applications where polypropylene is not
the
preferred plastic of choice. Examples of such polymer application areas
include
thermoforming and foaming. Foamed polymers find usage in automotive, marine,
appliance and packaging applications because of their good insulating and
structural
properties at low added weight. Thermoforming is a popular fabricating mode
that
3o competes favorably with injection molding in the making of thin-walled
containers.
Polypropylene's deficiencies in foaming and thermoforming are believed
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to be related to its generally poor melt strength and rapid melt viscosity
drop, poor
sheet sag and comparatively slow crystallization kinetics. For example, to
successfully
foam an article formed from a polyolefin, it is desirable that the polyolefin
selected for
foaming possess high melt strength. With high melt strength, the bubble growth
rate
within the polyolefin can be controlled without premature bursting.
Controlling bubble
growth rate is also important for ensuring a uniform distribution of cell
sizes, which
leads to greater product uniformity. Additionally, broader polymer processing
temperature windows are desirable so that when the polymer is used in an
article
forming process, the temperature variances along the process line are less
disruptive to
1 o the fabrication of a quality product.
So that manufactures of plastic articles and the consuming public may more
fully benefit from the use of polypropylene in a broader array of
applications, further
development and investigation is needed in the area of polypropylene
compositions and
methods of manufacturing. This is particularly so, as described above, when
the article
manufacturing process requires that the polymer be foamed.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a photomicrograph of a foamed homopolymer described in Example 1.
Fig. 2 is a photomicrograph of a foamed homopolymer of the present invention
described in Example 2.
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SUMMARY OF THE INVENTION
It has been discovered that articles formed from polypropylene polymer foams
and desirable polypropylene homopolymer foams, prepared in multistage reactors
that
exhibit broad molecular weight distribution possess superior bubble control
compared
to foamed articles formed from traditional polypropylene polymers.
More particularly, the invention relates to a foamed polypropylene polymer
which includes an isotactic propylene homopolymer with a molecular weight
distribution in the range from 2.5 to 20.0, hexane extractables of less than
1.0 weight
percent, a melt flow rate in the range of 0.2 dg/min to 30.0 dg/min and a foam
density
1 o in the range of from and including 0.1 to and including 1.0 g/cm3. This
foamed
polypropylene polymer may further include a blend of first and second
propylene
homopolymers. The first propylene homopolymer has a melt flow rate in the
range of
0.15 dg/min to 4.0 dg/min and a molecular weight distribution in the range of
1.8 to
2.5. The second propylene homopolymer has a melt flow rate in the range of 5
dg/min
15 to 1000 dg/min and a molecular weight distribution in the range of 1.8 to
2.5.
In another embodiment, a process for forming a foamed isotactic polypropylene
polymer is provided having the following steps. Propylene is homopolymerized
in the
presence of a metallocene based catalyst, and desirably a single metallocene
based
catalyst, and a first concentration of chain transfer agent sufficient to
produce a first
2o propylene homopolymer having a melt flow rate in the range from 0.15 dg/min
to 4.0
dg/min and a molecular weight distribution in the range of 1.8 to 2.5. The
first
propylene homopolymer and the metallocene based catalyst is homopolymerized
with
propylene in the presence of a second concentration of chain transfer agent
sufficient to
produce a second propylene homopolymer having a molecular weight distribution
in
25 the range of 1.8 to 2.5 and a melt flow rate in the range from 5 dg/min to
1000 dg. The
isotactic polypropylene is a blend of the first and second homopolymer having
a
molecular weight distribution in the range of from 2.5 to 20 and wherein the
first
homopolymer includes from 40 weight percent (wt%) to 80 weight percent of the
isotactic polypropylene polymer and the second homopolymer includes from 20
weight
30 percent to 60 weight percent of the isotactic polypropylene polymer.
Desirably, the
chain transfer agent in at least one of the steps is hydrogen.
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The isotactic polypropylene polymer so formed is then processed so that when
the
isotactic polypropylene polymer is contacted with a foaming agent the foamed
isotactic
polypropylene polymer is formed.
s DETAILED DESCRIPTION OF THE INVENTION/
Polypropylene Polymer
The polypropylene polymers, and desirably the isotactic polypropylene
polymers, include a reactor blend of two or more isotactic propylene
homopolymers
having differing weight average molecular weights such that the overall
polymer has a
1o molecular weight distribution that is in the range of from 2.5 to 20.0,
desirably from 2.8
to 12.0, even more desirably from 3.0 to 8Ø
Each isotactic propylene homopolymer desirably has a different melt flow rate.
As such, the polypropylene polymer includes one or more isotactic propylene
homopolymers having a low melt flow rate, i.e. the low melt flow rate polymer
species
15 (in the range of from 0.15 dg/min. to 4.0 dg/min) that was prepared in the
low melt
flow rate stage and one or more isotactic propylene homopolymers having a high
melt
flow rate, i.e. the high melt flow rate polymer species (in the range of from
5 dg/min to
1000 dg/min) that was prepared in the high melt flow rate stage. In this way,
the
polypropylene polymers desirably have a melt flow rate in the range of from
0.2 dg/min
2o to 30 dg/min, desirably from 0.5 dg/min to 20.0 dg/min, even more desirably
from 1.0
dg/min to 10.0 dg/min. The melting point of the polypropylene polymer is
desirably
greater than 145°C, more desirably greater than 150°C, and even
more desirably greater
than 155°C. Upper limits for melting point depend on the specific
application and
metallocene used but would typically not be higher than 170°C. The
hexane
25 extractables level (as measured by 21 CFR 177.1520(d)(3)(i)) of the
polypropylene
polymer is desirably less than 2.0 wt%, more desirably less than 1.0 wt%,
despite the
broad MWD.
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The polypropylene polymers of this invention desirably have a weight average
molecular weight (MVO that is in the range of from 140,000 to 750,000 more
desirably
from 150,000 to 500,000, and most desirably from 200,000 to 400,000.
Desirably, the polypropylene polymers include from 40 wt% to 80 wt% of the
low melt flow rate polymer species based on the total weight of the
polypropylene
polymer and from 20 wt% to 60 wt% of the high melt flow rate polymer species
based
on the total weight of the polypropylene polymer, and more desirably from 55
wt% to
65 wt% of the low melt flow polymer species based on the total weight of the
polypropylene polymer and from 35 wt% to 45 wt% of the high melt flow rate
species
1o based on the total weight of the polypropylene polymer.
Although the focus here is on homopolymers with a unique combination of
molecular weight distribution, good physical properties, and low extractables
levels, it
will be clear to persons skilled in the art that similarly unique combinations
of
properties will also be possible with copolymers, where controlled levels of
comonomer(s), such as ethylene and alpha olefins, such as for example, 1-
butene, 1-
pentene, 1-hexene, and 1-octene are additionally employed.
Polypropylene Polymer Polymerization Process
The polypropylene polymer polymerization process involves the use of
2o metallocene catalyst systems that comprise a metallocene component and at
least one
activator. Desirably, these catalyst system components are supported on
support
material.
The polypropylene polymers of this invention are generally prepared in a
multiple stage process wherein homopolymerization is conducted in each stage
separately in parallel or, desirably in series. Individually, each stage may
involve any
process including gas, slurry or solution phase or high pressure autoclave
processes.
Desirably, the polypropylene polymer is prepared in a multiple stage, series,
slurry
loop reactor using propylene as the polymerization diluent. The polymerization
is
carried out using a pressure of from 200 kPa to 7,000 kPa at a temperature in
the range
of from 50°C to 120°C. In each stage, propylene may be
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homopolymerized with the same catalyst system, which desirably includes a
metallocene catalyst, but with a different concentration of chain termination
agent in at
least two of the stages.
Examples of chain termination agents are those commonly used to terminate
chain growth in Ziegler-Natta polymerization, a description of which can be
found in
Ziegler-Natta Catalyst and Polymerization Hydrogen; J. Boor (Academic Press,
1979).
Hydrogen and diethyl zinc are examples of agents that are very effective in
the control
of polymer molecular weight in olefin polymerization. Hydrogen is the more
desirable
agent.
to Desirably, the concentration of chain termination agent in one stage is
sufficient
to produce a propylene homopolymer having a melt flow rate in the range of
from 0.15
dg/min. to 4.0 dg/min, desirably from 0.2 dg/min to 2.0 dg/min, even more
desirably
from 0.2 dg/min to 1.0 dg/min and a molecular weight distribution (Mw/Mn) in
the
range from 1.8 to 2.5 and desirably from 1.8 to 2.3. Desirably, the
concentration of
15 chain termination agent in a separate, either earlier or later stage, is
sufficient to
produce a propylene homopolymer having a melt flow rate in the range of from 5
dg/min to 1000 dg/min, desirably from 20 dg/min to 200 dg/min and more
desirably
from 30 dg/min to 100 dg/min and a molecular weight distribution (Mw/Mn) in
the
range from 1.8 to 2.5 and desirably from 1.8 to 2.3.
20 Non-limiting examples of metallocenes suitable for use in the homopolymer
polymerization process as well as the copolymer polymerization process
include:
Dimethylsilandiylbis (2-methyl-4-phenyl-1-indenyl)Zirconium dimethyl;
Dimethylsilandiylbis(2-methyl-4,5-benzoindenyl) Zirconium dimethyl;
Dimethylsilandiylbis(2-methyl-4,6-diisopropylindenyl) Zirconium dimethyl;
25 Dimethylsilandiylbis(2-ethyl-4-phenyl-1-indenyl) Zirconium dimethyl;
Dimethylsilandiylbis (2-ethyl-4-naphthyl-1-indenyl) Zirconium dimethyl,
Phenyl(methyl)silandiylbis(2-methyl-4-phenyl-1-indenyl) Zirconium dimethyl,
Dimethylsilandiylbis(2-methyl-4-(1-naphthyl)-1-indenyl) Zirconium dimethyl,
Dimethylsilandiylbis(2-methyl-4-(2-naphthyl)-1-indenyl) Zirconium dimethyl,
3o Dimethylsilandiylbis(2-methyl-indenyl) Zirconium dimethyl,
Dimethylsilandiylbis(2-methyl-4,5-diisopropyl-1-indenyl) Zirconium dimethyl,
Dimethylsilandiylbis(2,4,6-trimethyl-1-indenyl) Zirconium dimethyl,
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Phenyl(methyl)silandiylbis(2-methyl-4,6-diisopropyl-1-indenyl)Zirconium
dimethyl,
1,2-Ethandiylbis(2-methyl-4,6-diisopropyl-1-indenyl) Zirconium dimethyl,
1,2-Butandiylbis(2-methyl-4,6-diisopropyl-1-indenyl) Zirconium dimethyl,
Dimethylsilandiylbis(2-methyl-4-ethyl-1-indenyl) Zirconium dimethyl,
Dimethylsilandiylbis(2-methyl-4-isopropyl-1-indenyl) Zirconium dimethyl,
Dimethylsilandiylbis(2-methyl-4-t-butyl-1-indenyl) Zirconium dimethyl,
Phenyl(methyl)silandiylbis(2-methyl-4-isopropyl-1-indenyl)Zirconium dimethyl,
Dimethylsilandiylbis(2-ethyl-4-methyl-1-indenyl) Zirconium dimethyl,
Dimethylsilandiylbis(2,4-dimethyl-1-indenyl) Zirconium dimethyl,
l0 Dimethylsilandiylbis(2-methyl-4-ethyl-1-indenyl) Zirconium dimethyl,
Dimethylsilandiylbis(2-methyl-a-acenaphth-1-indenyl) Zirconium dimethyl,
Phenyl(methyl)silandiylbis(2-methyl-4,5-benzo-1-indenyl) Zirconium dimethyl,
Phenyl(methyl)silandiylbis(2-methyl-4,5-(methylbenzo)-1-indenyl)Zirconium
dimethyl,
Phenyl(methyl)silandiylbis(2-methyl-4,5-(tetramethylbenzo)-1-indenyl)Zirconium
dimethyl,
Phenyl(methyl)silandiylbis(2-methyl-a-acenaphth-1-indenyl)Zirconium dimethyl,
1,2-Ethandiylbis(2-methyl-4,5-benzo-1-indenyl) Zirconium dimethyl,
1,2-Butandiylbis(2-methyl-4,5-benzo-1-indenyl) Zirconium dimethyl,
2o DimetHylsilandiylbis(2-methyl-4,5-benzo-1-indenyl) Zirconium dimethyl,
1,2-Ethandiylbis(2,4,7-trimethyl-1-indenyl) Zirconium dimethyl,
Dimethylsilandiylbis(2-methyl-1-indenyl) Zirconium dimethyl,
1,2-Ethandiylbis(2-methyl-1-indenyl) Zirconium dimethyl,
Phenyl(methyl)silandiylbis(2-methyl-1-indenyl) Zirconium dimethyl,
Diphenylsilandiylbis(2-methyl-1-indenyl) Zirconium dimethyl,
1,2-Butandiylbis(2-methyl-1-indenyl) Zirconium dimethyl,
Dimethylsilandiylbis(2-ethyl-1-indenyl) Zirconium dimethyl,
Dimethylsilandiylbis(2-methyl-5-isobutyl-1-indenyl) Zirconium dimethyl,
Phenyl(methyl)silandiylbis(2-methyl-5-isobutyl-1-indenyl) Zirconium dimethyl,
3o Dimethylsilandiylbis(2-methyl-5-t-butyl-1-indenyl) Zirconium dimethyl,
Dimethylsilandiylbis(2,5,6-trimethyl-1-indenyl) Zirconium dimethyl,
Dimethylsilandiylbis (2-methyl-4-phenyl-1-indenyl)Zirconium dichloride
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_g_
Dimethylsilandiylbis(2-methyl-4,5-benzoindenyl) Zirconium dichloride,
Dimethylsilandiylbis(2-methyl-4,6-diisopropylindenyl) Zirconium dichloride,
Dimethylsilandiylbis(2-ethyl-4-phenyl-1-indenyl) Zirconium dichloride,
Dimethylsilandiylbis (2-ethyl-4-naphthyl-1-indenyl) Zirconium dichloride,
Phenyl(methyl)silandiylbis(2-methyl-4-phenyl-1-indenyl) Zirconium dichloride,
Dimethylsilandiylbis(2-methyl-4-(1-naphthyl)-1-indenyl) Zirconium dichloride,
Dimethylsilandiylbis(2-methyl-4-(2-naphthyl)-1-indenyl) Zirconium dichloride,
Dimethylsilandiylbis(2-methyl-indenyl) Zirconium dichloride,
Dimethylsilandiylbis(2-methyl-4,5-diisopropyl-1-indenyl) Zirconium dichloride,
to Dimethylsilandiylbis(2,4,6-trimethyl-1-indenyl) Zirconium dichloride,
Phenyl(methyl)silandiylbis(2-methyl-4,6-diisopropyl-1-indenyl)Zirconium
dichloride,
1,2-Ethandiylbis(2-methyl-4,6-diisopropyl-1-indenyl) Zirconium dichloride,
1,2-Butandiylbis(2-methyl-4,6-diisopropyl-1-indenyl) Zirconium dichloride,
Dimethylsilandiylbis(2-methyl-4-ethyl-1-indenyl) Zirconium dichloride,
Dimethylsilandiylbis(2-methyl-4-isopropyl-1-indenyl) Zirconium dichloride,
Dimethylsilandiylbis(2-methyl-4-t-butyl-1-indenyl) Zirconium dichloride,
Phenyl(methyl)silandiylbis(2-methyl-4-isopropyl-1-indenyl) Zirconium
dichloride,
Dimethylsilandiylbis(2-ethyl-4-methyl-1-indenyl) Zirconium dichloride,
Dimethylsilandiylbis(2,4-dimethyl-1-indenyl) Zirconium dichloride,
2o Dimethylsilandiylbis(2-methyl-4-ethyl-1-indenyl) Zirconium dichloride,
Dimethylsilandiylbis(2-methyl-a-acenaphth-1-indenyl) Zirconium dichloride,
Phenyl(methyl)silandiylbis(2-methyl-4,5-benzo-1-indenyl) Zirconium dichloride,
Phenyl(methyl)silandiylbis(2-methyl-4,S-(methylbenzo)-1-indenyl)Zirconium
dichloride,
Phenyl(methyl)silandiylbis(2-methyl-4,5-(tetramethylbenzo)-1-indenyl)Zirconium
dichloride,
Phenyl(methyl)silandiylbis (2-methyl-a-acenaphth-1-indenyl) Zirconium
dichloride,
1,2-Ethandiylbis(2-methyl-4,5-benzo-1-indenyl) Zirconium dichloride,
1,2-Butandiylbis(2-methyl-4,5-benzo-1-indenyl) Zirconium dichloride,
Dimethylsilandiylbis(2-methyl-4,5-benzo-1-indenyl) Zirconium dichloride,
1,2-Ethandiylbis(2,4,7-trimethyl-1-indenyl) Zirconium dichloride,
Dimethylsilandiylbis(2-methyl-1-indenyl) Zirconium dichloride,
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1,2-Ethandiylbis(2-methyl-1-indenyl) Zirconium dichloride,
Phenyl(methyl)silandiylbis(2-methyl-1-indenyl) Zirconium dichloride,
Diphenylsilandiylbis(2-methyl-1-indenyl) Zirconium dichloride,
1,2-Butandiylbis(2-methyl-1-indenyl) Zirconium dichloride,
Dimethylsilandiylbis(2-ethyl-1-indenyl) Zirconium dichloride,
Dimethylsilandiylbis(2-methyl-5-isobutyl-1-indenyl) Zirconium dichloride,
Phenyl(methyl)silandiylbis(2-methyl-5-isobutyl-1-indenyl) Zirconium
dichloride,
Dimethylsilandiylbis(2-methyl-5-t-butyl-1-indenyl) Zirconium dichloride,
Dimethylsilandiylbis(2,5,6-trimethyl-1-indenyl) Zirconium dichloride,
l0 and the like.
Many of these desirable transition metal compound components are described
in detail in U.S. Patent Nos. 5,145,819; 5,243,001; 5,239,022; 5,329,033;
5,296,434;
5,276,208; 5,672,668, 5,304,614 and 5,374,752; and EP 549 900 and 576 970 all
of
which are herein fully incorporated by reference.
Additionally, metallocenes such as those described in U. S. Patent No.
5,510,502, U. S. Patent No. 4,931,417, U. S. Patent No. 5,532,396, U.S. Patent
No.
5,543,373, WO 98/014585, EP611 773 and WO 98/22486 (each fully incorporated
herein by reference) are suitable for use in this invention.
The above polymer and polymerization process are further described in U.S.
2o Patent Application Serial No. 09/293,656 (98B030) filed April 16, 1999
which is
incorporated in its entirety by reference herein.
Activators
Metallocenes are generally used in combination with some form of activator in
order to create an active catalyst system. The term "activator" is defined
herein to be
any compound or component, or combination of compounds or components, capable
of
enhancing the ability of one or more metallocenes to
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polymerize olefins to polyolefins. Alklyalumoxanes such as methylalumoxane
(MAO) are commonly used as metallocene activators. Generally alkylalumoxanes
contain 5 to 40 of the repeating units:
R(A1R0 )xAlR2 for linear species and
(A1R0 )x for cyclic species
where R is a C1-Cg alkyl including mixed alkyls. Particularly desirable are
the
compounds in which R is methyl or other lower alkyls such as C3-Cg, now
commercial
sold by AKZO as "modified Alumoxane". Alumoxane solutions, particularly
methylalumoxane solutions, may be obtained from commercial vendors as
solutions
having various concentrations. There are a variety of methods for preparing
alumoxane, non-limiting examples of which are described in U.S. Patent No.
is 4,665,208, 4,952,540, 5,091,352, 5,206,199, 5,204,419, 4,874,734,
4,924,018,
4,908,463, 4,968,827, 5,308,815, 5,329,032, 5,248,801, 5,235,081, 5,103,031
and EP-
A-0 561 476, EP-B1-0 279 586, EP-A-0 594-218 and WO 94/10180, each fully
incorporated herein by reference. (As used herein unless otherwise stated
"solution"
refers to any liquid containing mixture including suspensions.)
Discrete ionizing activators may also be used to activate metallocenes. These
activators are neutral or ionic, or are compounds such as tri(n-butyl)ammonium
tetrakis(pentaflurophenyl)borate, which ionize the neutral metallocene
compound.
Such ionizing compounds may contain an active proton, or some other cation
associated with, but not coordinated or only loosely coordinated to, the
remaining ion
of the ionizing compound. Combinations of activators may also be used, for
example,
alumoxane and ionizing activators in combinations, see for example, WO
94/07928.
Descriptions of ionic catalysts for coordination polymerization comprised of
metallocene cations activated by non-coordinating anions appear in the early
work in
EP-A-0 277 003, EP-A-0 277 004 and US patent 5,198,401 and WO-A-92/00333
(incorporated herein by reference). These teach a desirable method of
preparation
wherein metallocenes (bisCp and monoCp) are protonated by an anion precursor
such
that an alkyl/hydride group is abstracted from a transition metal to make it
both
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cationic and charge-balanced by the non-coordinating anion. Suitable ionic
salts
include tetrakis-substituted borate or aluminum salts having fluorided aryl-
constituents
such as phenyl, biphenyl and napthyl.
The term "noncoordinating anion" (NCA) means an anion which either does not
coordinate to said cation or which is only weakly coordinated to said cation
thereby
remaining sufficiently labile to be displaced by a neutral Lewis base.
"Compatible"
noncoordinating anions are those which are not degraded to neutrality when the
initially formed complex decomposes. Further, the anion will not transfer an
anionic
substituent or fragment to the canon so as to cause it to form a neutral four
coordinate
1 o metallocene compound and a neutral by-product from the anion.
Noncoordinating
anions useful in accordance with this invention are those which are
compatible,
stabilize the metallocene cation in the sense of balancing its ionic charge in
a +1 state,
yet retain sufficient lability to permit displacement by an ethylenically or
acetylenically
unsaturated monomer during polymerization.
The use of ionizing ionic compounds not containing an active proton but
capable of producing both the active metallocene canon and a noncoordinating
anion is
also known. See, for example, EP-A-0 426 637 and EP-A- 0 573 403 (incorporated
herein by reference). An additional method of making the ionic catalysts uses
ionizing
anion precursors which are initially neutral Lewis acids but form the cation
and anion
upon ionizing reaction with the metallocene compounds, for example the use of
tris(pentafluorophenyl) borane. See EP-A-0 520 732 (incorporated herein by
reference). Ionic catalysts for addition polymerization can also be prepared
by
oxidation of the metal centers of transition metal compounds by anion
precursors
containing metallic oxidizing groups along with the anion groups, see EP-A-0
495 375
(incorporated herein by reference).
Where the metal ligands include halogen moieties (for example, bis-
cyclopentadienyl zirconium dichloride) which are not capable of ionizing
abstraction
under standard conditions, they can be converted via known alkylation
reactions with
organometallic compounds such as lithium or aluminum hydrides or alkyls,
alkylalumoxanes, Grignard reagents, etc. See EP-A-0 500 944 and EP-A1-0 570
982
(incorporated herein by reference) for in situ processes describing the
reaction of alkyl
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aluminum compounds with dihalo-substituted metallocene compounds prior to or
with
the addition of activating anionic compounds.
Desirable methods for supporting ionic catalysts comprising metallocene
canons and NCA are described in U.S. Patent No. 5,643,847, U.S. Patent
Application
No. 09184358, filed November 2, 1998 and U.S. Patent Application No. 09184389,
filed November 2, 1998 (all fully incorporated herein by reference).
When the activator for the metallocene supported catalyst composition is a
NCA, desirably the NCA is first added to the support composition followed by
the
addition of the metallocene catalyst. When the activator is MAO, desirably the
MAO
1o and metallocene catalyst are dissolved together in solution. The support is
then
contacted with the MAO/metallocene catalyst solution. Other methods and order
of
addition will be apparent to those skilled in the art.
Support Materials
15 The catalyst systems used in the process of this invention are desirably
supported using a porous particulate material, such as for example, talc,
inorganic
oxides, inorganic chlorides and resinous materials such as polyolefin or
polymeric
compounds.
Desirably, the support materials are porous inorganic oxide materials, which
2o include those from the Periodic Table of Elements of Groups 2, 3, 4, 5, 13
or 14 metal
oxides. Silica, alumina, silica-alumina, and mixtures thereof are particularly
desirable.
Other inorganic oxides that may be employed either alone or in combination
with the
silica, alumina or silica-alumina are magnesia, titania, zirconia, and the
like.
Desirably the support material is porous silica which has a surface area in
the
25 range of from 10 to 700 m2/g, a total pore volume in the range of from 0.1
to 4.0 cc/g
and an average particle size in the range of from 10 to 500 pm. More
desirably, the
surface area is in the range of from 50 to 500 m2/g, the pore volume is in the
range of
from 0.5 to 3.5 cc/g and the average particle size is in the range of from 20
to 200 Vim.
Most desirably the surface area is in the range of from 100 to 400 m2/g, the
pore
3o volume is in the range of from 0.8 to 3.0 cc/g and the average particle
size is in the
range of from 30 to 100 ~.m. The average pore size of typical porous support
materials
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is in the range of from 10 to 1000A. Desirably, a support material is used
that has an
average pore diameter of from 50 to 500, and most desirably from 75 to 350. It
may
be particularly desirable to dehydrate the silica at a temperature of from
100°C to
800°C anywhere from 3 to 24 hours.
The metallocenes, activator and support material may be combined in any
number of ways. Suitable support techniques are described in U. S Patent No.s
4,808,561 and 4,701,432 (each fully incorporated herein by reference.).
Desirably the
metallocenes and activator are combined and their reaction product supported
on the
porous support material as described in U. S. Patent No. 5,240,894 and WO 94/
28034,
1o WO 96/00243, and WO 96/00245 (each fully incorporated herein by reference.)
Alternatively, the metallocenes may be preactivated separately and then
combined with
the support material either separately or together. If the metallocenes are
separately
supported, then desirably, they are dried then combined as a powder before use
in
polymerization.
15 Regardless of whether the metallocenes and their activator are separately
precontacted or whether the metallocenes and activator are combined at once,
the total
volume of reaction solution applied to porous support is desirably less than 4
times the
total pore volume of the porous support, more desirably less than 3 times the
total pore
volume of the porous support and even more desirably in the range of from more
than 1
2o to less than 2.5 times the total pore volume of the porous support.
Procedures for
measuring the total pore volume of porous support are well known in the art.
One such
method is described in Volume 1, Experimental Methods in Catalyst Research,
Academic Press, 1968, pages 67-96.
Methods of supporting ionic catalysts comprising metallocene cations and
25 noncoordinating anions are described in WO 91/09882, WO 94/03506, U.S.
Patent
5,072,823, WO 96/04319 and WO 95/06343, all of which are incorporated herein
by
reference. The methods generally comprise either physical adsorption on
traditional
polymeric or inorganic supports that have been largely dehydrated and
dehydroxylated,
or using neutral anion precursors that are sufficiently strong Lewis acids to
activate
3o retained hydroxy groups in silica containing inorganic oxide supports such
that the
Lewis acid becomes covalently bound and the hydrogen of the hydroxy group is
available to protonate the metallocene compounds.
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The supported catalyst system may be used directly in polymerization or the
catalyst system may be prepolymerized using methods well known in the art. For
details regarding prepolymerization, see United States Patent No.s 4,923,833
and
4,921,825, EP 0 279 863 and EP 0 354 893 each of which is fully incorporated
herein
by reference.
Additives
Additives may be included in the polypropylene polymers of this invention.
Such additives and their use are generally well known in the art. These
include those
commonly employed with plastics such as heat stabilizers or antioxidants,
plasticizers,
l0 neutralizers, slip agents, antiblock agents, pigments, antifogging agents,
antistatic
agents, clarifiers, nucleating agents, ultraviolet absorbers or light
stabilizers, fillers and
other additives in conventional amounts. Effective levels are known in the art
and
depend on the details of the base polymers, the fabrication mode and the end
application.
Foaming Agents
Foaming agents or additives may generally be divided into two classes:
physical
foaming agents and chemical foaming agents.
Physical foaming or blowing agents are generally gases such as carbon dioxide
or nitrogen. Hydrocarbon gases, such as butane or pentane and fluorocarbon
gases,
such as trichlorofluromethane and dichlorodifluromethane are effective as
physical
blowing agents producing good quality foams. Because hydrocarbon and
flurocarbon
gases are viewed as presenting certain health and environmental concerns, the
use of
these gases is generally not the most desirable. More desirable physical
blowing agents
are carbon dioxide, nitrogen and argon. Physical blowing agents are utilized
when low
foam densities L0.5 g/cm3) are required.
Chemical blowing agents allow the production of foamed produces having a
density of generally greater than 0.5 g/cm3. Examples of chemical blowing
agents
include bicarbonate of soda (used typically in combination with citric acid),
azodicarbonamide, sulfonyl hydrazide, sulfonyl semicarbazide. Bicarbonate of
soda
(endothermic agent) and azodicarbonamide (exothermic agent) are perhaps the
most
widely used chemical blowing agents.
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When used at low levels, generally less than 1 wt%, and desirably around 0.25
wt% based on the weight of the polymer being foamed, chemical blowing agents
may
function as bubble nucleating agents and facilitate the formation of more
uniformly
sized bubble. This function is often utilized even when the primary foaming
medium is
a physical blowing agent, such as carbon dioxide gas. Talc can also be
utilized for
bubble nucleation.
The resulting foamed articles made from these multistage processes have a
more uniform foam cell morphology and good extrudate skin surface. More
particularly, the foamable polypropylene polymers of the present invention are
useful in
applications such as, for example, sheet extrusion and molded articles, such
as molded
automotive parts. In those instances where the application is sheet extrusion,
the
foamed sheets may be subsequently thermoformed into packaging containers. In
those
instances where the application is for molded articles, the molded articles
may include a
variety of molded parts, particularly molded parts related to and used in the
automotive
industry, such as for example bumpers, side panels, floor mats, dashboards and
instrument panels. Examples of other applications for which foamed plastic,
such as
foamed polypropylene, are useful may be found in Encyclopedia of Chemical
Technology, by Kirk-Othmer, Fourth Edition, vol. 1 l, at pages 730-783, which
are
incorporated by reference herein.
EXAMPLES
The following example is presented to illustrate the foregoing discussion.
Although the example may be directed to certain embodiments of the present
invention, it is not to be viewed as limiting the invention in any specific
respect. The
equipment used and the experimental procedure employed to obtain the data in
the
following tables are outlined below.
Various foaming tests/trials on polypropylene polymers, were conducted at the
Polymer Processing Institute of the New Jersey Institute of Technology,
Newark, NJ. A
Killion-segmented single screw extruder of 1.25 inch diameter with 40 L/D
(length/diameter), equipped with a high-pressure gas injection port located at
19 L/D
length, and with a 1 /8 inch rod die was used to produce extruded foams via
physical
blowing agents. Carbon dioxide gas was used typically as blowing agent at
various
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pressures ranging from approximately 250 psi to upwards of 1000 psi. The
polymer
materials examined were plasticiated within the first 19 diameters of the
screw.
Beyond the gas injection port, the remaining extruder length was used for gas
mixing,
compression and cooling of the gas-laden polymer. At the die exit the
temperature was
approximately 150°C to 160°C. An auger feeder was used to
control the flow of resin
to the line. The extrusion line is typically operated in a near starve-fed
mode, to
optimize residence time and maximize foam formation. Under steady state
conditions,
the extrudate emerging from the die swells up to a diameter > 1/8 inch,
reflecting the
bubble formation of the gas as it comes out of solution from the extruded
polymer melt.
to Controllable line parameters include the temperature profile settings (feed
section
through to the die), the extruder speed and the speed of the auger screw feed,
the
blowing agent gas flow rate and gas pressure and the die pressure.
Sample 1 (Comparative)
Sample 1 is a commercial Ziegler-Natta-catalyzed, isotactic,
homopolypropylene polymer product available from Montell Polyolefins (product
number PP 6523) having an MFR around 4. Some of its physical properties are
listed in
Table 1. (See Figure 1 )
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Sample 2 (Invention)
The data for Sample 2 were taken on an isotactic polypropylene homopolymer.
This homopolymer was prepared in a manner similar to the homopolymer
polymerization process described above.
A catalyst system was prepared involving dimethylsilylbis (2-methyl-4-phenyl-
indenyl) zirconium dichloride metallocene, methylalumoxane (in toluene) as
activator,
styrene as modifier and Davison XPO 2407 silica (W.R. Grace, Davision Chemical
Division, Baltimore, MD) as support material. The catalyst preparation
procedure is
to described in USSN 09/293,656 which is incorporated by reference herein.
Specific
catalyst formulation details for the catalyst system prepared, are as follows:
Zr loading 0.028
(mmole/gSi02)
Al loading 2.9
(mmole/gSi02)
Styrene/Zr loading 5.7
Several batches of the catalyst system were combined to provide the charge for
the polymerization run. The catalyst system was oil slurried with DrakeolTM
white
mineral oil (Witco Chemical) for ease of addition to the reactor.
The polymerization was conducted in a pilot scale, two reactor, continuous,
stirred tank, bulk liquid phase process. The reactors were equipped with
jackets for
removing the heat of polymerization. The reactor temperatures were 70°C
in the first
reactor and 64.5°C in the second reactor. Catalyst was fed at a rate of
5.2 g/hr. TEAL
(2.0 wt% in hexane) was used as a scavenger and added at a rate of 17.3 wppm.
The
catalyst system prepared above was fed as a 20% slurry in mineral oil and was
flushed
into the first reactor with propylene. Total propylene monomer feed to the
first reactor
was 80Kg /hr . Propylene monomer feed to the second reactor was 30 Kg/hr.
Hydrogen was added for molecular weight control at a rate of 530 mppm to the
first
reactor and 8450 mppm to the second reactor. Reactor residence times were 2.5
hours
in the first reactor and 1.8 hours in the second reactor. Polymer production
rates were
20 Kg/hr from the first reactor and 11 Kg/hr from the second reactor. 65% of
the final
polymer product was derived from the first reactor and 35% from the second
reactor.
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Polymer was discharged from the reactors as a granular product having an MFR
of 3.6
dl/g. Product produced in the first reactor was estimated to be of 1 MFR and
product
produced in the second reactor of 78 MFR. Following pelletization of the
product with
stabilizers, the pellet MFR was 1.8.
Table 1
Physical Property Sample 1 Sample 2
MFR (dg/min on pellets) 4 1-8
Mw 360k 392k
Mz 1050k 870k
MWD(Mw/Mn) 5.4 8.3
Young's modulus 53,OOOpsi 66,OOOpsi
Yield Stress 4200psi 4900psi
Elongation at Break 940% 760%
Tm 162.3C 153.1 C
Tc 114.2C 120.0C
Heat of Fusion 111.2J/g 108.OJ/g
Foam Density 0.7 g/cm3 0.3-0.7 g/cm3
Table 1 Notes: - Molecular weights are via standard GPC procedures
- polypropylene polymer Tensile properties are via Instron testing at room
temperature at 2 in/min (equivalent to ASTM method D-524)
- Tm, Tc and Heat of Fusion are from standard DSC procedures using a
10°C/min heating and cooling rate.
Results
Experimentation on the two samples revealed clear and distinct advantages in
favor of Sample 1. These advantages were primarily in the area of foam
production and
in the quality of the final foamed product.
More specifically, it was observed that at generally any given gas pressure
foam formation via the described process was much easier during the foaming of
Sample 2 as compared to Sample 1. For instance, during the foaming of Sample
1,
excessive bubble collapse was observed in the gas pressure ranges of 250 to
750 psi.
Additionally, the final foamed Sample 1 product possessed undesirable large
bubbles
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and a poor texture as illustrated in Fig. 1. The presence of large bubbles
generally
reduces the structural integrity of the final foamed product and the lack of
bubble
uniformity generally results in a final product having a rough or coarse
surface.
Reduced structural integrity and coarse surfaces are undesirable
characteristics in a
foamed article. It was also observed, in the case of Sample 1, that the
extrudate spiraled
excessively and coiled at the die exit due to bubble bursting. This
illustrates
poor/undesirable processability and bubble control. The density of the foamed
product
(see Table 1) of Sample 1 was comparatively higher than that of inventive
Sample 2,
under similar foaming conditions.
l0 In contrast to Sample 1, as illustrated in Figure 1, Sample 2 exhibited a
wider
foam processing window. As illustrated in Fig. 2, the texture of the final
foamed
product was smoother, reflecting the more uniformly sized bubble, which is
desirable.
The cell structure showed little, if any, bursting of cells upon exiting the
extruder die.
Foams of a broader density range (see Table 1 ) down to 0.3 g/cm3 could be
made with
Sample 2.
It should be noted that the final value of foamed density is generally
dependent
on the equipment used. To achieve low levels (<0.5 g/cm3) of foamed density,
physical blowing agents are usually employed. The attainment of very low
foamed
densities (<0.1 g/cm3) is generally performed on lines employing tandem
extruders,
where the focus in the second extruder is primarily optimum mixing and
dissolution of
the gas and effective cooling of the polymer/gas mixture, prior to cell
formation when
the extrudate leaves the die. The fact that inventive Sample 2 provided a
lower foam
density than the standard Ziegler-Natta homopolypropylene Sample l, under the
same
foam fabrication set-up and conditions, reflects the benefits of the product
tailoring
described earlier. Furthermore, tandem extrusion of the inventive polymer may
enable
foamed articles having densities as low as 0.1 g/cm3. Among the other areas,
the
benefits provided by the inventive polymer are believed to be improved gas
solubility
and diffusion, which are desirable parameters for producing quality foamed
products.
While carbon dioxide was used as the foaming agent in the present case, other
3o gases, such as heptane, nitrogen, helium, butane, isobutane and isopentane
may also be
used to foam polymers of this invention.
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While the present invention has been described and illustrated by reference to
particular embodiments, it will be appreciated by those of ordinary skill in
the art, that
the invention lends itself to many different variations not illustrated
herein. For these
reasons, then, reference should be made solely to the appended claims for
purposes of
determining the true scope of the present invention.
Although the appendant claims have single appendencies in accordance with
U.S. patent practice, each of the features in any of the appendant claims can
be
combined with each of the features of other appendant claims or the main
claim.
