Note: Descriptions are shown in the official language in which they were submitted.
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METEIOD FOR THE MANUFACT~RE OF POLYOLEFIN
FILM FROM BLENDS OF POLYOLEFINS_ __
The present invention relates to a method
for the manufacture of polyolefin film from blends of
polyolefins and especially to the manufacture of such
film from a physical admixture of a polymer of
ethylene and a grafted polymer of ethylene.
Polymers of ethylene, for example
homopolymers of ethylene and copolymers of ethylene
and hydrocarbon alpha-olefins, are used in large
volumes for a variety of end-uses, including the
manufacture of film. Such film is used in, for
example, the packaging industry.
In the manufacture of film from polymers of
ethylene, it may be found that the process for the
manufacture of film is capable of improvement,
especially with respect to the manufacture of film of
acceptable quality in a commercially-viable manner.
The improvement may be capable of being made with
respect to the apparatus, the processing technique
and/or the polymer composition fed to the process.
In some circumstances, the apparatus may not have
been intended for use with the particular type of
polymer that is fed to the apparatus. For instance,
apparatus designe~ for use with so-called high
pressure polyethylene may not be readily adaptable
for use with linear polymers of ethylene and/or may
only be capable of producing film that is not of
commercially-acceptable quality~
European patent publication 0 172 650 of
G. White, published 1986 February 26, describes an
improved method for the extrusion of homopolymers of
ethylene and copolymers of ethylene and hydrocarbon
alpha-olefins, in which a physical admixture of a
major portion of particles of polymer is admixed with
a minor portion of particles of a composition of a
polymer and a cross-linking agent. Such a method is
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particularly useful in the extrusion of linear
polymers of ethylene in apparatus that might not have
been designed for use with such polymers and/or is
capable of improvement when used with such polymers.
However, the method is capable of further
improvement, especially with regard to some physical
properties of the resultant film, especially tear
strength and optional properties.
Processes for the grafting of ethylenically
unsaturated carboxylic acids and anhydrides thereof
onto polymers of ethylene are known in the art. A
preferred example of such a process is described in
the U.S. Patent 4 612 155 of C.S. Wong and
R.A. ZeIonka, issued 1986 September 16.
U.S. Patent 3 804 926 of E.C.A. Schwarz et
al., issued 1974 April 16, describes an improvement
in the extrusion performance and physical properties
of isotactic poIypropylene, low density polyethylene,
ethylene/propylene copolymers and polybutene-l by
melt blending lO0 parts by weight of such polymers
with 0.05-5 parts by weight of maleic anhydride and
0.2-10 parts by weight of an aromatic compound.
Examples of aromatic compounds are polystyrene,
cumene, m-di-isopropyl benzene and
l,l-diphenylethane.
Japanese patent application 59 215 828 of
~ Gunz K.K., published 1984 December 04, discloses
!, stretching film formed from linear low density
`;! ~ polyethylene and modified polyethylene. In an
example, a composition of lO0 parts by weight of
linear low density polyethylene of density 0.920
g/cm3 and a melt index of 2.1 dg/min. and 40 parts
by weight of a modified polyethylene prepared by
graft polymerizing polyethylene with maleic
anhydride, was extruded and the resultant Eilm was
stretched.
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Compositions of high melt tension and formed
from, for example, polyethylene having, in
particular, maleic anhydride grafted onto the
polyethylene are disclosed in Japanese patent
application 59 084 940 of Showa Denko K.K., published
1984 May 16. The compositions are stated to be
useful for blow moulding and moulding of sheets and
laminated films
A method for the improvement of a process
for the manufacture of film from polyethylene has now
been found, in which the melt strength of the polymer
may be increased significantly while substantially
: retaining certain physical properties in the
resultant film.
Accordlngly, the present invention provides
in a process for the manufacture of film from
polyethylene in which the polyethylene in a molten
state is extruded through an annular die in the form
of a tube and the resultant tube is expanded and
cooled, the improvement comprising feeding to the
extruder a blend comprisin~, in physical admixture, a
:~ major portion of particles of a first polyethylene
: and a minor portion of particles of a second
polyethylene, each of said polyethylenes being
selected from the group consisting of homopolymers of
~: ethylene and copolymers of ethylene and at least one
C4-Clo hydrocarbon alpha-olefin, said first
polyethylene being at least 80~ by weight of the
physical admixture and consisting of 75-100% by
weight of linear polyethylene and 0-25% by weight of
high pressure low density polyethylene, said second
polyethylene having been grafted with 0.5 to 2.0~ by
weight of at least one of an ethylenically
unsaturated carboxylic acid and an anhydride thereof.
In a preferred embodiment of the method of
. the present invention, the first and second
polyethylenes are linear polymers.
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In another embodiment, the first
polyethylene is a blend of linear low density
polyethylene and low density high pressure
polyethylene.
The present invention also provides a blend
of polymers, said blend comprising a physical
admixture of a major portion of particles of a first
polyethylene and a minor portion of particles of a
second polyethylene, each of said polyethylenes being
selected from the group consisting of homopolymers of
ethylene and copolymers of ethylene and at least one
C4-Clo hydrocarbon alpha-olefin, said first
polyethylene being at least 80~ by weight of the
physical admixture and consisting of 75-100% by
weight of linear polyethylene and 0-25~ by weight of
high pressure low density polyethylene, said second
polyethylene having been grafted with 0.5 to 2.0% by
weight of at least one of an ethylenically
unsaturated carboxylic acid and an anhydride thereof.
The present invention relates to a method
for the manufacture of fîlm from polyethylene.
Techniques for the manufacture of such film are known
in the art and, in particular, include the so-called
~1 blown film process. In that process, polyethylene in
comminuted form, usually in the form of pellets or
granules, is fed to an extruder. The polyethylene is
heated to a molten state, subjected to a mixing
action by means of the screw in the extruder so as to
provide molten polyethylene of adequate uniformity,
and extruded through an annular die. The resultant
tube of molten polyethylene is then expanded by means
of pressure of gas e.g. nitrogen, air or helium
supplied-to the inside of the tube, so that a
combination o~ the pressure, the geometry of the
annular die and the rate at which the tube is removed
from the die results in the manufacture of film of
the desired thickness. The tube of molten
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polyethylene is cooled to solidify the polymer and
the resultant film is usually slit and wound up on a
roll.
Polyethylene îs commercially available and,
as will be appreciated by those skilled in the art,
there are a number of methods for the manufacture of
polyethylene from monomer~s). For example, ethylene
may be homopolymerized in the presence of free
radical catalysts and under high pressure to form
so-called high-pressure po]yethylene. Alternatively,
polyethylene may be manufactured by the
homopolymerization or copolymerization of ethylene at
relatively low pressures and in the presence of a
coordination catalyst. The latter polymers may be
referred to as linear polyethylene, and may be
homopolymers of ethylene or copolymers of ethylene
and at least one C4-C10 hydrocarbon alpha-olefin,
especially butene-l, hexene-l and/or octene-l. While
such polymers may have densities in the range of
about 0.910 to 0.965 g/cm3, the polymers preferahly
have densities in the range of about 0.910 to 0.930,
especially 0.915 to 0.925, g/cm3; the preferred
polymers may be referred to herein as linear low
density polyethylene. As will be appreciated by
those skilled in the art, polymers useful for the
manufacture of film usually have a meIt index of less
than 10 dg/mint especially in the range of 0.5 to 5
dg/min; melt index is measured by the procedure of
ASTM D-1238 ~condition E).
In embodiments of the present invention, the
polyethylene is linear low density polyethylene or a
blend of linear low density polyethylene and a low
density polyethylene manufacture by a so-called high
pressure process. Such low density polyethylene
should be not more than 25% by weight of the blend.
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In the improvement of the method of the
present invention, a blend of polyethylenes in the
form of a physical admixture is fed to the extruder.
The polyethylenes of the physical admixture are each
selected from homopolymers of ethylene and copolymers
of ethylene and hydrocarbon alpha-olefins, as defined
above. The polyethylene of the major portion may be
the same or different from the polyethylene of the
minor portion. The latter polyethylene does7 however,
differ from the polyethylene of the major portion in
that the second polyethylene is a grafted polymer.
The polyethylene has been grafted with 0.5 to 2.0% by
weight of at least one ethylenically unsaturated
carboxylic acid or anhydride thereof. Methods for
the grafting of polymers are known in the art. A
preferred method is that of C.S. Wong and R.A.
Zelonka referred to above. In such a method, the
polyethylene to be grafted is fed, as a physical
admixture, to an extruder with compositions of a
polymer and the grafting monomer and an organic
peroxide. The resultant blend is admixed in a molten
state in the extruder such that only a minor amount
o the peroxide is decomposed, and then heated in the
extruder for a ~urther period of time, usually at a
higher temperature. The resultant grafted polymer is
then extruded from the extruder.
In preferred embodiments of the invention,
the grafting monomer is maleic acid or, especially,
maleic anhydride.
The first and second polyethylenes are
chosen so that a homogeneous melt blend is formed in
the extruder used in the method. Thus, the selection
of the first and second polyethylene, with respect to
melt viscosity characteristics, will depend to a
large extent on the extrusion apparatus to be used,
especially the design of the screw, as will be
understood by those skilled in the art.
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The first polyethylene comprises at least
30% by weight of the physical admi~ture, and
especially 95~ to 99~ by weight of the physical
admixture.
In an embodiment of the present invention,
; the second polyethylene of the physical admixture fed
to the extruder is in the form of a composition of a
grafted polymer and an organic peroxide cross-linking
agent. The preferred organic peroxides are bis~tert.
alkyl peroxy alkyl) benzenes, dicumyl peroxide and
acetylenic diperoxy compounds, Examples of such
;~ peroxides include 2,5-dimethyl-2,5 bis(tert. butyl
peroxyisopropyl) benzene, which is available
commercially under the trademark Vulcup, and
2,5-dimethyl- 2l5-di~tert. butylperoxy) hexyne-3,
which is available commercially under the trademark
Iupersol. Preferably, the composition of polymer and
organic peroxide contains less than 4%, and
especially 0.05 to 1.0~, by weight of peroxide. The
composition of polymer and peroxide is admixed into
the admixtures of the invention in amounts of less
than 20% by weight of the admixture, and especially 1
to 5% by weight of the composition. The addition of
organic peroxide to the grafted polymer may have a
synergistic e~fect on the melt strength in the method
of the present invention.
The present invention also relates to the
blends that have been described herein with reference
to the method of the invention~
As will be appreciated by those skilled in
the art, stabilizers and other additives may be
incorporated into the blends of the method of the
invention. In particular, antioxidants and ultra
violet stabilizers may be added. Moreover, the
; 35 blends may contain slip agents, anti-static agents,
pigments, nucleating or other processing aids or the
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like. Examples of nucleating or other processing
aids include talc, silical fluorinated elastomers and
polyolefin waxes. The usç of fluorinated elastomers
as processing aids is known in the art, bein~
described in, for instance, U.S. Patent 3 125 547 o
P.S. Blatz, which issued 1954 March 17.
As will be understood by those skilled in
the art, stabilizing or other additives may have
detrimental effects on the reaction of cross-linking
agents, if present, and for that reason it may not be
desirable to use certain combinations of additives in
the polyethylene.
The particles of the first and second
polyethylenes may be any convenient size and shape
and may for example be granulesl powder, pellets or
the like. Such forms are commercially available
forms of polyethylene and/or may be obtained by known
techniques e.g. by grinding, melt pelletization and
the like. However, it is preferred that the
particles of the first and second polyethylene be of
substantially the same size. As the difference in
size between the particles increases, so does the
possibility that the two types of particle will
separate from one another during storage,
2S transportation or other handling of the admixtures;
such differences may be less critical if the
admixtures are to be fed to an extruder shortly after
admixing of the components.
The physical admixture is fed to extrusion
apparatus, especially an extruder of apparatus
intended for the manufacture of film by the so-called
blown film method. Such apparatus is known to those
skilled in the art. The admixture is fed to the
extruder, heated and mixed within the extruder to
form a molten polymer composition of acceptable
uniformity. The resultant molten composition is then
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extruded through an annular die to form a tube of
molten polymer. The tube is expanded and cooled so
as to forrn film. The tubular film thus obtained is
then slit and wound up on rolls.
The operation of a blown film process using
the method of the present invention may result in
significant increases in the rate of production of
film of acceptable quality. Moreover, the admixtures
may permit the manufacture of film from linear low
density polyethylene using apparatus that was
intended for use with so-called conventional or high
pressure polyethylene. In some circumstances, the
method of the present invention will result in
improvements in film production with linear low
density polyethylene compared with the method
disclosed in the aforementioned European publication
of G. White.
The process of the present invention is
further illustrated by the following examples:
Example I
-~ Using a blown film extrusion apparatus
;`~ equipped with a 6.5cm screw, an admixture in which
the major portion was SCLAIR 13J4 polyethylene was
extruded into film having a thickness of about 25
microns. SCLAIR 13J4 polyethylene is an
ethylene/octene-l copolymer having a density of 0.926
g/cm3 and a melt ;ndex of 1.0 dg/min. The minor
portion of the admixture was a grafted polymer,
~i formed by grafting about 0.6% by weight of maleic
anhydride onto SCLAIR 2113 polyethylene using the
melt grafting process of the aforementioned patent of
` C.S. Wong and R.A. 2elonka; SCLAIR 2113 is an
ethylene/butene-l copolymer having a density of 0.924
g/cm3 and a melt index of 20 dg/min. The minor
portion optionally also contained 0.3~ by weight of
Lupersol 130 organic peroxide.
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Further details and the results obtained are
given in Table I.
TABLE I
: 5 Production
Run Graft Peroxide in MD TearRate
No~ Polymer &raft Polymer g/mm Increase
(wt %) (~m) (%)
-- . _
1 0 0 43
: 2 2 0 55 ~5
:
3 2 3000 31 35
4 0 27 32
~B MD tear is the tear strength of the film in
the machine direction, as measured by the procedure
of ASTM D 1922.
Rate increase is the increase in rate of
extrusion of film of acceptable quality, compared to
polyethylene without added second polymer (Run 1).
Run 1 is a comparison run using the
polyethylene of the major portion without second
(graft) polymerO In Run 4r 60 ppm of peroxide was
incorporated into the first polyethylene.
From a comparison of Runs 1 and 2, it will
be noted that an increase in production rate was
achieved without a decrease in the tear strength of
the resultant film. Addition of organic peroxide to
: either the first polyethylene or the graft copolymer
resulted in an increase in production rate but a
decrease in the tear strength of the resultant film.
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Example II
; The procedure of Example I was repeated,
except that the film that was produced had a
; thickness of 150 microns.
Further details and the results obtained are
given in Table II.
Table II
10 Run ~raftPeroxide in MD TearRate
No. PolymerGraft Polymer g/mmIncrease
~) ppm (%~
0 - 0 75
lS 6 1.5 3000 59 85
7 0 0 75 22
8 1.5 0 75 10
Runs 5 and 7 are comparison runs without
graft copolymer; 50 ppm of peroxide was incorporated
into the first polyethylene of Run 7~
l From a comparison of Run 6 with Runs 7 and
-~ 25 8, it wilI be noted that a synergistic increase in
production rate was achieved when the graft copolymer
~' also contained peroxide.
~, Example III
The procedure of Example I was repeated
using CF-0218A polyethylene obtained from NOVA, an
Alberta Corporation; this polyethylene is an
ethylene/butene-l copolymer having a density o~ 0.920
g/cm3 and a melt index of 2 dg/min. The film that
was produced had a thickness of 12.5 microns.
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Further details and the results obtained are
given in Table III,
I
Table III
Run Graft Rate
No. PolymerMD Tear Increase
~ % ) g/mm ( ~ )
0 62
2 39 33
11 0 29 33
Runs 9 and 11 are comparative runs. In Run
ll, 50 ppm of peroxide was incorporated into the
first polyethylene.
The increase in production rate obtained in
Run 10 and 11 was the same, but the film obtained in
Run 10 had a higher tear strength.
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