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United States Patent 7,658,875
Cooper February 9, 2010
Biaxially-oriented metallocene-based polypropylene films
having reduced thickness
Abstract
A polymer film having a thickness of 10 microns or less and improved
barrier characteristics to both water vapor and oxygen is formed from
a biaxially-oriented polypropylene film of isotactic polypropylene.
The polypropylene is prepared by the polymerization of propylene in
the presence of an isospecific metallocene catalyst. The film has a
permeability to water vapor of less than about 2.5 g/m.sup.2
day/25 .mu.m and a permeability to oxygen of less than about 2200
cc/m.sup.2 day/25 .mu.m. The film also has a haze properties of less
than about 1%. The polypropylene contains 0.1 to 1% 2,1 insertions
and has an isotacticity of at least 96% meso pentads. The isotactic
polypropylene thus produced is then extruded into a sheet that is
biaxially-oriented by stressing the sheet in the transverse and
longitudinal directions to a draw ratio of at least about 6:1 in the
transverse direction, and at least about 4:1 in the longitudinal
direction.
Inventors: Cooper; Scott D. (Humble, TX)
Assignee: Fina Technology, Inc. (Houston, TX)
Appl. No.: 12/277,619
Filed: November 25, 2008
Related U.S. Patent Documents
Application Number Filing Date Patent Number Issue Date
11656247 Jan., 2007 7473751
Current U.S. Class: 264/210.1 ; 264/331.17; 526/126; 526/131;
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526/160; 526/348.1
Current International
Class:
B29C 55/02 (20060101); C08J 5/18
(20060101); C08F 210/06 (20060101)
Field of Search: 526/348.1,126,131,160 264/210.1,331.17
References Cited [Referenced By]
U.S. Patent Documents
5795946 August 1998 Agarwal et al.
6090325 July 2000 Wheat et al.
2007/0235896 October 2007 McLeod et al.
Primary Examiner: Teskin; Fred M
Attorney, Agent or Firm: Kilpatrick-Lee; Diane L.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional application of U.S. patent
application Ser. No. 11/656,247, filed on Jan. 22, 2007, now issued
as U.S. Pat. No. 7,473,751.
Claims
The invention claimed is:
1. A method of producing a biaxially-oriented film comprising
providing an isotactic propylene polymer prepared by polymerization
of propylene in the presence of an isospecific metallocene catalyst
characterized by the formula: R''(C.sub.5(R').sub.4).sub.2MeQ.sub.p
wherein each (C.sub.5(R').sub.4) is a substituted cyclopentadienyl
ring, each R' is the same or different and is a hydrogen or
hydrocarbyl radical having 1-20 carbon atoms, R'' is a structural
bridge between the two (C.sub.5(R').sub.4) rings imparting
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stereorigidity to the catalyst with the two (C.sub.5(R').sub.4) rings
being in a racemic configuration relative to Me, and R'' is selected
from the group consisting of an alkylene radical having 1-4 carbon
atoms, a silicon hydrocarbyl radical, a germanium hydrocarbyl radical,
a phosphorus hydrocarbyl radical, a nitrogen hydrocarbyl radical, a
boron hydrocarbyl radical, and an aluminum hydrocarbyl radical; Me is
a group 4b, 5b or 6b metal as designated in the Periodic Table of
Elements; each Q is a hydrocarbyl radical having 1-20 carbon atoms or
is a halogen; and 0.ltoreq.p.ltoreq.3; (a) extruding the isotactic
polypropylene into a sheet; and (b) stressing the extruded sheet in
the transverse and longitudinal directions to form a biaxially-
oriented film having a thickness of 10 microns or less and a secant
modulus in the longitudinal direction of greater than about 1400 MPa
and a secant modulus in the transverse direction of greater than
about 2200 MPa, the polymer film having a permeability to water vapor
of less than about 2.5 g/m.sup.2 day/25 .mu.m and a permeability to
O.sub.2 of less than about 2200 cc/m.sup.2 day/25 .mu.m.
2. The method of claim 1 wherein the extruded sheet is stressed in
the transverse direction at a draw ratio of at least 6:1 and in the
longitudinal direction at a draw ratio of at least 4:1.
3. The method of claim 1 wherein the isotactic polypropylene has a
meso pentad content of greater than about 96%.
4. The method of claim 1 wherein isotactic polypropylene has 2,1-
insertions of between about 0.1 to 1%.
5. The method of claim 1 wherein the isotactic polypropylene has a
melt flow rate of between about 1 to 15 g/10 min.
6. The method of claim 1 wherein the isotactic polypropylene having a
xylene solubles in the amount of between about 0.1 to 1% by weight of
polypropylene.
7. The method of claim 1 wherein the isotactic polypropylene is
extruded into a sheet having a thickness from about 160 to 400
microns.
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8. The method of claim 1 wherein Me is selected from a group
consisting of titanium, zirconium, hafnium and vanadium.
Description
FIELD OF THE INVENTION
This invention relates to biaxially-orientated polypropylene films,
and more particularly to films having a reduced thickness and
processes for their preparation from metallocene-based isotactic
polypropylene.
BACKGROUND OF THE INVENTION
Isotactic polypropylene is one of a number of crystalline polymers
which can be characterized in terms of the stereoregularity of the
polymer chain, Various stereo specific structural relationships
denominated primarily in terms of syndiotacticity and isotacticity
may be involved in the formation of stereoregular polymers from
various monomers. Stereospecific propagation may be applied in the
polymerization of ethylenically unsaturated monomers such as C.sub.3+
alpha olefins, 1-dienes such as 1,3-butadiene, substituted vinyl
compounds such as vinyl aromatics, e.g. styrene or vinyl chloride,
vinyl chloride, vinyl ethers such as alkyl vinyl ethers, e.g.,
isobutyl vinyl ether, or even aryl vinyl ethers. Stereospecific
polymer propagation is probably of most significance in the
production of polypropylene of isotactic or syndiotactic structure.
Isotactic polypropylene is conventionally used in the production of
relatively thin films in which the polypropylene is heated and then
extruded through dies and subjected to biaxial orientation by
stressing the film in both a longitudinal direction (referred to as
the machine direction) and in a transverse or lateral direction
sometimes referred to as the "tenter" direction. The structure of
isotactic polypropylene is characterized in terms of the methyl group
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attached to the tertiary carbon atoms of the successive propylene
monomer units lying on the same side of the main chain of the polymer.
That is, the methyl groups are characterized as being all above or
below the polymer chain. Isotactic polypropylene can be illustrated
by the following chemical formula:
##STR00001## Stereoregular polymers, such as isotactic and
syndiotactic polypropylene can be characterized in terms of the
Fisher projection formula. Using the Fisher projection formula, the
stereochemical sequence of isotactic polypropylene as shown by
formula (2) is described as follows:
##STR00002## Another way of describing the structure is through the
use of NMR. Bovey's NMR nomenclature for an isotactic pentad is . . .
mmmm . . . with each "m" representing a "meso" dyad, or successive
methyl groups on the same side of the plane of the polymer chain. As
is known in the art, any deviation or inversion in the structure of
the chain lowers the degree of isotacticity and crystallinity of the
polymer. Syndiotactic propylene polymers are those in which the
methyl groups attached to the tertiary carbon atoms of successive
monomeric units in the polymer chain lie on alternate sides of the
plane of the polymer. Using the Fisher projection formula, the
structure of syndiotactic polypropylene can be shown as follows:
##STR00003## Syndiotactic polymers are semi-crystalline and, like the
isotactic polymers, are insoluble in xylene. This crystallinity
distinguishes both syndiotactic and isotactic polymers from an
atactic polymer, which is non-crystalline and highly soluble in
xylene. An atactic polymer exhibits no regular order of repeating
unit configurations in the polymer chain and forms essentially a waxy
product.
In most cases, the polymer configuration will be a predominantly
isotactic or syndiotactic polymer with very little atactic polymer
Catalysts that produce isotactic polyolefins are disclosed in U.S.
Pat. Nos. 4,794,096 and 4,975,403. These patents disclose chiral,
stereorigid metallocene catalysts that polymerize olefins to form
isotactic polymers such as highly isotactic polypropylene.
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Metallocene catalysts, such as those described above, can be used
either as so-called "neutral metallocenes" in which case an alumoxane,
such as methylalumoxane, is used as a co-catalyst, or they can be
employed as so-called "cationic metallocenes" which incorporate a
stable non-coordinating anion and normally do not require the use of
an alumoxane. For example, syndiospecific cationic metallocenes are
disclosed in U.S. Pat. No. 5,243,002 to Razavi.
Catalysts employed in the polymerization of alpha-olefins may be
characterized as supported catalysts or unsupported catalysts, the
later sometimes referred to as homogeneous catalysts. Metallocene
catalysts are often employed as unsupported or homogeneous catalysts,
although, as described below, they also may be employed in supported
catalyst components. Traditional supported catalysts are the so-
called "conventional" Ziegler-Natta catalysts, such as titanium
tetrachloride supported on an active magnesium dichloride as
disclosed, for example, in U.S. Pat. Nos. 4,298,718 and 4,544,717,
both to Mayr et al.
While metallocene catalysts are generally proposed for use as
homogeneous catalysts, it is also known in the art to provide
supported metallocene catalysts. As disclosed in U.S. Pat. Nos.
4,701,432 and 4,808,561, both to Welborn, a metallocene catalyst
component may be employed in the form of a supported catalyst. As
described in the Welborn '432 patent, the support may be any support
such as talc, an inorganic oxide, or a resinous support material such
as a polyolefin. Specific inorganic oxides include silica and alumina,
used alone or in combination with other inorganic oxides such as
magnesia, zirconia and the like. The Welborn '561 patent discloses a
heterogeneous catalyst which is formed by the reaction of a
metallocene and an alumoxane in combination with the support material.
A catalyst system embodying both a homogeneous metallocene component
and a heterogeneous component, is disclosed in U.S. Pat. No.
5,242,876 to Shamshoum et al., Various other catalyst systems
involving supported metallocene catalysts are disclosed in U.S. Pat.
No. 5,308,811 to Suga et al. and U.S. Pat. No. 5,444,134 to Matsumoto.
7 / 43
Processes for the preparation of biaxially-oriented polypropylene
films employing polymers produced by the use of isospecific
metallocenes involving di- or tri-substituted indenyl groups are
disclosed in Canadian Patent Application No. 2,178,104. Four
isotactic polymers disclosed there were based upon the polymerization
of propylene in the presence of heavily substituted
bis(indenyl)ligand structures. In each case, the metallocene used was
a silicon-bridged di- or tri-substituted bis(indenyl)zirconium
dichloride.
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a polymer
film product of a polypropylene film formed from isotactic
polypropylene. The isotactic polypropylene is prepared by the
polymerization of propylene in the presence of an isospecific
metallocene catalyst. The polymer film is biaxially oriented and has
a thickness of about 10 microns or less. The polymer film has a
permeability to water vapor of less than about 2.5 g/m.sup.2
day/25 .mu.m and permeability to O.sub.2 of less than about 2200
cc/m.sup.2 day/25 .mu.m. In one embodiment of the invention the
polypropylene film has haze properties of less than about 1%. The
polypropylene film may be oriented in the transverse direction to a
draw ratio of at least about 6:1 and may be oriented in the
longitudinal direction to a draw ratio of at least about 4:1 and may
have a secant modulus in the longitudinal direction of greater than
about 1400 MPa and a secant modulus in the transverse direction of
greater than about 2200 MPa.
Additionally, the polypropylene polymer used in forming the
polypropylene film may be formed from an isospecific metallocene
catalyst characterized by the formula
R''(C.sub.5(R').sub.4).sub.2MeQ.sub.p (5) wherein each
(C.sub.5(R').sub.4) is a substituted cyclopentadienyl ring, each R'
is the same or different and is a hydrogen or hydrocarbyl radical
having 1-20 carbon atoms, R'' is a structural bridge between the two
(C.sub.5(R').sub.4 rings imparting stereorigidity to the catalyst
with the two (C.sub.5(R').sub.4) rings being in a racemic
8 / 43
configuration relative to Me, and R'' is selected from the group
consisting of an alkylene radical having 1-4 carbon atoms, a silicon
hydrocarbyl radical, a germanium hydrocarbyl radical, a phosphorus
hydrocarbyl radical, a nitrogen hydrocarbyl radical, a boron
hydrocarbyl radical, and an aluminum hydrocarbyl radical; Me is a
group 4b, 5b or 6b metal as designated in the Periodic Table of
Elements. More specifically, Me is titanium, zirconium, hafnium or
vanadium; each Q is a hydrocarbyl radical having 1-20 carbon atoms or
is a halogen; and 0.ltoreq.p.ltoreq.3. The isotactic polypropylene
used in forming the polypropylene film may be further characterized
by having a meso pentad content of greater than about 96%, 2,1-
insertions of about 0.1 to 1%, a xylene solubles of less than about 1%
by weight of polypropylene, and a melt flow rate of between about 1
to 15 g/10 min.
In a further aspect of the invention, there is provided a process for
the production of biaxially-oriented polypropylene film. In the
process of the present invention there is provided an isotactic
propylene polymer produced by the polymerization of propylene in the
presence of a metallocene catalyst characterized by formula (5) above.
The extruded sheet is stressed in the transverse and longitudinal
directions to form a biaxially-oriented film having a thickness of
about 10 microns or less. The polymer film has a permeability to
water vapor of less than about 2.5 g/m.sup.2 day/25 .mu.m and a
permeability to O.sub.2 of less than about 2200 cc/m.sup.2
day/25 .mu.m. In one embodiment, the polypropylene film may have haze
properties of less than about 1%. The polypropylene film may be
oriented in the transverse direction to a draw ratio of at least 6:1
and may be oriented in the longitudinal direction to a draw ratio of
at least 4:1. The isotactic polypropylene used in forming the
polypropylene film may be further characterized by having a meso
pentad content of greater than about 96%, 2,1-insertions of less than
about 1%, a xylene solubles in the amount of between 0.1 to 1% by
weight of polypropylene, and a melt flow rate ranging from about 1 to
15 g/10 min.
BRIEF DESCRIPTION OF THE DRAWINGS
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FIG. 1 is a schematic illustration in isometric view of a tenter
frame system which may be employed in carrying out the present
invention.
FIG. 2 is a graphical presentation of yield stress analysis
illustrating biaxial stretching for polymer films employing isotactic
polypropylenes produced by Ziegler-Natta catalysis and isotactic
polypropylenes produced by metallocene catalysis.
FIG. 3 is a comparative graphical presentation of differential
scanning calorimeter endotherms for polypropylene produced employing
a conventional Ziegler-Natta catalyst and polypropylene produced
employing a metallocene catalyst.
DETAILED DESCRIPTION OF THE INVENTION
The biaxially-oriented films of the present invention are
characterized in terms of certain well-defined characteristics
relating to their stereoregular structures and physical properties,
including melt temperatures, as well as in relatively low
coefficients of friction and relatively high tensile moduli and
relatively low permeation rates to oxygen and water. The biaxially-
oriented films of the present invention are formed using a
particularly configured polyolefin polymer as described in greater
detail below and by using any suitable oriented film production
technique, such as the conventionally-used tenter frame process.
In general, such biaxially-oriented film production can be of any
suitable technique, such as disclosed in the aforementioned Canadian
Patent Application No. 2,178,104 to Peiffer et al. As described in
the Peiffer et al application, the polymer or polymers used to make
the film are melted and then passed through an extruder to a slot die
mechanism after which it is passed over a first roller, characterized
as a chill roller, which tends to solidify the film. The film is then
oriented by stressing it in a longitudinal direction, characterized
as the machine direction, and in a transverse direction to arrive at
a film which can be characterized in terms of orientation ratios,
sometimes also referred to as stretch ratios, in both longitudinal
10 / 43
and transverse directions. The machine direction orientation is
accomplished through the use of two sequentially disposed rollers,
the second or fast roller operating at a speed in relation to the
slower roller corresponding to the desired orientation ratio. This
may alternatively be accomplished through a series of rollers with
increasing speeds, sometimes with additional intermediate rollers for
temperature control and other functions. After the film has been
stressed in the machine direction, it is again cooled and then pre-
heated and passed into a lateral stressing section, for example, a
tenter frame mechanism, where it is again stressed, this time in the
transverse direction. Orientation in the transverse direction is
often followed by an annealing section. Subsequently, the film is
then cooled and may be subjected to further treatment, such as corona
treatment or flame treatment, as described, for example, in the
aforementioned Canadian Patent Application No. 2178104 or in U.S. Pat.
No. 4,029,876 to Beatty, the entire disclosures of which are
incorporated herein by reference. The film may also be metallized as
described in U.S. Pat. No. 4,692,380 to Reid, the entire disclosure
of which is incorporated herein by reference. While corolla and flame
treatment typically occur immediately following orientation and prior
to the initial roll up, metallizing is typically performed at a
separate time and location.
Turning now to FIG. 1, there is shown a schematic illustration of a
suitable "Tenter Frame" orientation process pilot line which may be
employed in producing biaxially-oriented polypropylene film in
accordance with the present invention. More particularly and with
reference to FIG. 1, a source of molten polymer is supplied from a
hopper 10 to an extruder 12 and from there to a slot die 14 which
produces a flat, relatively thick film or sheet 16 at its output. The
isotactic polypropylene is extruded in a sheet, which may have a
thickness of from about 160 to 400 microns. This thickness may vary,
however, depending
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