TITLE: Linear low density polyethylene cast film.
European Patent Application EP0292336 A1

ABSTRACT:
Draw resonance problems previously encountered when extrusion casting
or extrusion coating linear low density polyethylene, LLDPE, are
substantially alleviated by use of a polymer designed to have an I2
melt index of from 0.1 to 25 gm/10 minutes, and an I10/I2 ratio from
4.0 to 8.3. Line speeds approaching and even surpassing those attained
with LDPE are made possible. This invention can even be used in
conjunction with known prior methods for alleviating draw resonance
problems with LLDPE and obtain the benefits of the combination.

INVENTORS:
Dohrer, Katryn K.
Niemann, Debra H.

APPLICATION NUMBER: EP19880304667
PUBLICATION DATE: 11/23/1988
FILING DATE: 05/23/1988

ASSIGNEE: DOW CHEMICAL CO (US)
INTERNATIONAL CLASSES: B29C41/26; B29C47/00; B29C47/02; B29C47/88; C08F210/16; C08J5/18;
B29K23/00; (IPC1-7): B29C47/00; B29D7/00; B29C55/06
EUROPEAN CLASSES: B29C47/00B; B29C47/88C4

DOMESTIC PATENT REFERENCES:
EP0107076 Process for reducing draw resonance in polymeric film.

FOREIGN REFERENCES:
4608221 Process for reducing draw resonance in polymeric film
GB2124139A

OTHER REFERENCES:
JOURNAL OF APPLIED POLYMER SCIENCE, vol. 24, no. 1, July 1979, pages
61-87, John Wiley & Sons, Inc., New York, US; C.D. HAN et al.: "Studies
on melt spinning. VIII. The effects of molecular structure and cooling
conditions on the severity of draw resonance"
M. AHMED: "POLYPROPYLENE FIBERS - SCIENCE AND TECHNOLOGY", 1982, pages
237-254, Elsevier Scientific Publishing Co., Amsterdam, NL
CHEMICAL ABSTRACTS, vol. 94, no. 2, 12th January 1981, page 45, no.
4850r, Columbus, Ohio, US; A. GHIJSELS et al.: "Draw resonance studies
on polypropylene melts" & RHEOL., ÄPROC. INT. CONGR.Ü, 8TH 1980, 3,
15-24

CLAIMS:
1. A process in which LLDPE polymer is meltspun into a film by an
extrusion-casting or an extrusion-coating technique, and in which the
LLDPE polymer consists essentially of ethylene inter-polymerized with
from 1 percent to 60 percent by weight of at least one alkene of C3-C12
to form an inter-polymer having a density in the range of from 0.87 to
0.955 gm/cc and has a melt index, I2, in the range of 0.1 to 25 gm/10
minutes as determined by ASTM D-1238-E; characterized by using LLDPE
polymer having an I10/I2 ratio in the range of 4.0 to 8.3, where the
value of I10 is determined by ASTM D-1238-N; whereby faster line speeds
are obtained without onset of draw resonance.
2. A process as claimed in Claim 1, wherein said ratio is less than
8.0.
3. A process as claimed in Claim 2, wherein said ratio is less than
7.0.
4. A process as claimed in any one of the preceding claims, wherein I2
is in the range of 1 to 10 gm/10 minutes and the line speed is greater
than 4 m/s (800 feet per minute).
5. A process as claimed in any one of the preceding claims, wherein the
LLDPE comprises an ethylene interpolymer containing from 1 percent to
40 percent of at least one comonomer selected from the group
represented by the formula H2C=CH-R, where R represents a hydrocarbon
moiety containing from 1 to 10 carbon atoms.
6. A process as claimed in any one of the preceding claims, wherein the
LLDPE comprises an interpolymer of ethylene with at least one comonomer
of the group consisting of butene, 4-methyl pentene-1, hexene, and
octene.
7. A process as claimed in Claim 6, wherein the LLDPE comprises an
interpolymer of ethylene and octene.
8. A process as claimed in any one of the preceding claims, wherein the
LLDPE has a density in the range of 0.88 to 0.945 gm/cc.
9. A process as claimed in any one of the preceding claims, wherein I2
is in the range of 1.5 to 6 gm/10 minutes.
10. A process as claimed in any one of the preceding claims, wherein
the said LLDPE is used in conjunction with at least one of the
following previously known techniques for improving draw resonance
resistance and increasing line speed: (a) the use of a draw resonance
eliminator known as the DRE; (b) the use of a coolant gas which is
directed onto the film between the extrusion die and the nip roll as
the film is extruded and drawn; (c) the use of an extruder wherein the
draw roller is located not more than 15 cm (6 inches) from the
extrusion die; and (d) the use of an olefin polymer blended in the
LLDPE.

DESCRIPTION:
LINEAR LOW DENSITY POLYETHYLENE CAST FILM
Films of linear low density polyethylene (LLDPE), having high
resistance to draw resonance, are cast using fast line speeds.
It is known that low density polyethylene (LDPE) made by the high
pressure polymerization of ethylene, using a free radical initiator,
can be cast into films using fast line speeds. The films so-produced
lack sufficient impact strength and tensile properties for many of the
applications for which a polymer film is desired. These LDPE polymers
are characterized by the presence of long chain-branching along the
polymer chains.
Linear low density polyethylene (LLDPE), which is known to be a linear
polymer prepared using a coordination catalyst, contains a higher
olefin comonomer (C3-C12) interpolymerized into the polymer chain, but
because of the substantial absence of long chain-branching, the
interpolymer exhibits appreciably different properties when molten, and
when solid, as compared to LDPE and "HDPE". HDPE (a linear high
density@thylene homopolymer) is made in substantially the same manner
as LLDPE, but without the comonomer; it is the presence of the
comonomer in the copolymer which causes the density of LLDPE to be
lower than the HDPE homopolymer.
In the presently described invention, the density of the LLDPE may be
in the same density range as LDPE, or even lower, if enough of a given
comonomer is used in the copolymer, or may be in an intermediate range
(i. e. about 0.94 to about 0.955 gm/cc) when a lesser amount of the
given comonomer is used. These linear polymers are made using a
coordination catalyst, such as the Ziegler-type or Phillips-type
catalyst, not with a free radical initiator such as used in making
LDPE. The LLDPE polymers may be made in substantial accordance with,
e.g. U.S. 4,302,566 or U.S. 4,076,698, especially the latter, so long
as the catalyst and/or process conditions used are those which will
produce the low ratio of I10/I2 required for the present invention,
such as by using the catalyst described in U.S. 4,612,300.
Whereas it is known that LDPE can be cast into films using fast line
speeds, it is also known that stronger films are obtained with LLDPE.
Persons attempting to cast LLDPE, in order to obtain stronger films,
have encountered problems when trying to cast the films at line speeds
as fast as that used with LDPE. The problems with using fast line
speeds when casting films of LLDPE have been attributed to the onset to
"draw resonance".
The expression "draw resonance" or "surging" is defined in, e.g., U.S.
Patent 4,339,507 as "... a limit cycle corresponding to a sustained
periodic oscillation in the velocity and cross-sectional area of a
drawing process when the boundary conditions are a fixed velocity at
the exit of an extruder and a fixed velocity at the take-off position.
It occurs when the draw ratio exceeds a critical value. Draw resonance
or surging can be thought of as an instability in take-off phenomenon
of a material coming from an extruder, more or less homogeneously. The
instability manifests itself in the occurrence of periodic fluctuations
in the extrudate dimensions such as film thickness when a critical
value of the take-up speed is exceeded.
Surging may be so extreme as to actually break a web or film that
extrudes from a die and totally shut down an extrusion coating
process." The expressions "draw-down", "draw ratio", "melt strength",
and "neck-in" are also defined in the patent. The patent discloses that
LLDPE suffers from deficiencies in processing which include problems
with draw resonance and with high neck-in. The invention claimed in the
patent is an extrusion coating process which uses a blend of LDPE
polymer and LLDPE polymer to obtain the desired extrusion properties.
In U.S. Patent 4,486,377 the draw resonance problem, found in drawing
films of LLDPE, is said to be reduced by using a cooling gas between
the die and the nip roll.
In an article titled "Reducing Draw Resonance in LLDPE Film Resins" by
Peter J. Lucchesi et al, published in PLASTICS ENGINEERING, May, 1985,
it is reported that LLDPE experiences resonance even at very low draw
rates, largely due to its lack of long-chain branching. The article
also states that "...The only known way to reduce the draw resonance of
LLDPE is to cool the extrudate gradually rather than expose it to the
shock-cooling of a water bath or chill-roll ". The article suggests the
use of a "draw resonance eliminator (DRE)" which involves the use of a
fluid medium (air) against the molten film between the die and the nip
and chill roll.
Authors E. H. Roberts et al in an article titled "New Processes For The
Reduction of Draw Resonance in Melt Embossing And Extrusion Coating"
published in 1985 in LAMINATIONS & COATINGS CONFERENCE/TAPPI
Proceedings, also reported on the "Draw Resonance Eliminator (DRE)".
The same authors also reported on the same subject in an article
published in ANTEC, 1985.
In AlChE Journal (Vol. 24, No. 3), May 1978, on page 418, there is an
article by Jae Chun Hyun titled "Part 1. Newtonian Fluids" which offers
theoretical considerations related to draw resonance in melt spinning
of fibers and films.
U.K. Patent G.B. 2,124,139 (published 2-15-84) discloses that LLDPE,
having a propensity for draw resonance at high draw speeds, is drawn
into a film using a draw roller located not more than 6 inches (15 cm)
from the die so as to have a short draw-gap.
U.S. Patent 4,608,221 is of particular interest because it predicts
that broad molecular weight distribution is desirable to prevent the
onset of draw resonance (see particularly the bottom of column 3, and
the top of column 4). Its only working example concerns a process
having a maximum line speed of only 300 ft/min. (1.5 m/s).
There is a perceived need for other means of avoiding draw resonance
problems when extruding LLDPE which avoids having to add other polymers
to it and avoids having to apply mechanical changes in the extrusion
equipment whereby the drawing process is altered. The present invention
substantially fills such need, as described hereinafter. However, if
one already has made modifications in a cast film or extrusion coating
process or equipment used therefor, or wishes to use a blend of LLDPE
with another polymer (such as LDPE and the like), then the present
invention can still be used to good advantage in such processes or
equipment, or with such blends, and obtain the benefits of the
combination of techniques.
A broad aspect of the invention is a process in which LLDPE polymer is
meltspun into a film by an extrusion-casting or an extrusion-coating
technique, and in which the LLDPE polymer consists essentially of
ethylene interpolymerized with from 1 percent to 60 percent by weight
of at least one alkene of C3-C12 to form an interpolymer having a
density in the range of from 0.87 to 0.955 gm/cc and has a melt index,
I2, in the range of 0.1 to 25 gm/10 minutes as determined by ASTM
D-1238-E; characterized by using LLDPE polymer having an I10/I2 ratio
in the range of 4.0 to 8.3, where the value of I10 is determined by
ASTM D-1238-N; whereby faster line speeds are obtained without onset of
draw resonance.
We have now found that LLDPE can be designed to have a high resistance
to draw resonance, allowing it to be drawn into films at line speeds
previously thought to be unattainable without using additives or by
using certain special equipment. This high resistance has, very
unexpectedly in view of afore-mentioned U.S. Patent No. 4,608,221, been
attained by designing an LLDPE polymer which has an unusually narrow
molecular weight distribution as indicated by an I10/I2 ratio of not
greater than 8.3. This permits the extrusion casting and extrusion
coating of LLDPE at line speeds approaching, and in some cases
surpassing, the line speeds which can be used with LDPE.
Furthermore, when such an LLDPE polymer is used in combination with an
additive (such as another polymer) which helps alleviate draw resonance
problems, or when used with a draw resonance eliminator (DRE), or when
used with a very short draw-gap, (all these known techniques being
referred to above), then the benefits of the combination are obtained.
The properties of the LLDPE polymer used in the present invention,
which we refer to at times as a "fast-draw" LLDPE, are usually as
follows:
It is a copolymer of ethylene containing from 1 to 60 percent,
preferably from 1 to 40 percent by weight of at least one higher olefin
of C3-C12, preferably at least one of those in the range of C4-C8, most
especially octene-1; these olefins are represented by the formula
H2C=CH-R, where R represents a hydrocarbon moiety containing from 1 to
10 carbon atoms, especially butene, pentene, 4-methyl pentene-1,
hexene, or octene, or a mixture of such olefins;
It has a density in the range of 0.87 to 0.955 gm/cc, preferably in the
range of 0.88 to 0.950 gm/cc, most preferably about 0.88 to 0.945 gm/cc
(as measured by ASTM D-1248);
It has an I2 value in the range of about 0.1 to 25 gm/10 minutes,
preferably 1.0 to 10, most preferably 1.5 to 6 (as determined by ASTM
D-1238-E,which is the ASTM method used in determining "standard melt
flow", also known as "melt index"); on the other hand, the I10 value is
determined by using ASTM D-1238-N;
It is critical, for purposes of this invention, that within the above
described I2 range, the LLDPE has an I10/I2 ratio of 4.0 to 8.3,
preferably less than 8.0, more preferably less than 7.5, especially
less than 7.0. In this invention,the lower the said ratio at a given I2
value, the better, so far as attaining fast line speeds in
extrusion-casting or extrusion-coating is concerned.
The polymer used in this invention has a narrow molecular weight
distribution (MWD). It can be prepared using a coordination catalyst
which produces narrow molecular weight distribution LLDPE, such as is
described in U.S. 4,612,300; the invention is, however, not limited to
only those in accordance with U.S. 4,612,300, but any other catalyst
and/or process found to produce such narrow MWD may be used. It will be
understood that the smaller the I10/I2 ratio, the narrower will be the
MWD.
Considering that the cast film market and the extrusion coating market,
for which the presently claimed inventive process is especially
designed, is currently dominated by LDPE, then some discussion of the
differences between LDPE and LLDPE is appropriate here.
LDPE has been used for many years in cast applications and has been
found to draw at adequate line speeds for large scale operations.
However, it has its deficiencies such as in the more recently designed
disposable diapers which employ refastenable tapes. These refastenable
tapes require that the polymer film be strong enough to resist tearing
when the tape is peeled off; LDPE was found to be deficient in the
necessary tensile strength and tear strength. This change in the
diapers brought about a need for a polymer film which would have the
strength to withstand the peeling off of the tape, yet which would not
appreciably slow down the manufacturing process.
LLDPE not only is found to have the strength needed to withstand the
peeling off of the tape, but we have found that one can obtain fast
line speeds when extrusion-casting or extrusion-coating films of LLDPE
without encountering the draw resonance problems for which LLDPE is
well known in the industry, by using LLDPE polymers designed to have an
I10/I2 ratio as described in this disclosure.
It will be understood that the LLDPE polymers which are used in the
practice of the present invention will have applications other than in
making diapers, and the making of diapers is just one application in
which LLDPE can be used to good advantage if the problem of draw
resonance is alleviated in order to cast the films at fast line speeds.
As used in this disclosure, the expressions "fast draw" and "fast line
speeds" are in reference to the speeds obtainable with LLDPE and
indicate speeds which appreciably approach, or even surpass, the speeds
obtainable with LDPE using the same extrusion equipment and conditions.
The following examples illustrate some particular embodiments of the
present invention, but the invention is not limited to the particular
embodiments shown. It will be appreciated by practitioners of these
arts that the exact maximum line speeds one can attain with one type of
extruder or coater will not necessarily be the same as one may attain
with another extruder or coater. For purposes of making useful
comparisons of results, one should use the same extruder or coater with
all the samples being tested.
In the following Examples 1-3, the temperature settings are summarized
in Table II. The equipment consists of an Egan coextrusion cast film
line with the identical sample being fed through both extruders. The
extruders are a 2.5" (6.5 cm) 24:1 L/D Egan extruder with a Maddock
mixing section in parallel with and a 3.5" (9 cm) 32:1 L/D Egan
extruder, also with a Maddock mixing section. A 30-inch (75 cm) Johnson
"coat hanger" die with flex lips providing a die gap of about 22 mils
(0.56 mm) is attached. The film contacts two chrome plated chill rolls
that have a mirror finish. A CMR TM 1000 Microprocessor computer is an
integral part of the system for control and monitoring the equipment
conditions. A Fife Model OSP-2-40 Beta thickness gauge is used to
monitor thickness.
Also in the following examples, the LLDPE polymers comprise ethylene
interpolymerized with a sufficient amount of octene-1 to produce the
density shown for each resin sample. The densities of the LLDPE samples
are quite close to each other, which makes comparisons more readily
evaluated. The following Table I describes the polymers: EMI11.1
Example 1
Resins A and B are tested on the above described equipment. The test
consists of increasing the rpm and the line speed concurrently to
maintain 1.2 mils (0.03 mm) thickness. The point of draw resonance is
determined visually when the web becomes unstable.
The A sample exhibits a maximum line speed of 1400 fmp (7 m/s) and the
B sample exhibits a maximum line speed of 800 fpm (4 m/s). EMI12.1
Example 2
Another cast film trial is conducted using a greater number of resins,
i.e. LLDPE E-K. Table III presents data for samples E-K compared with
LDPE. EMI13.1 Example 3
Samples E-K, fabricated substantially as above in comparison with LDPE
, with the extruder rpm adjusted to maintain a film thickness of 1.2
mils (0.03 mm) at a line speed of 450 fmp (2.3 m/s), exhibit
significantly greater ultimate tensile strength, tensile yield, percent
elongation and toughness (ASTM D-882) than the LDPE as illustrated in
Table IV below, in both "machine direction" (MD) and "cross direction"
(CD). EMI15.1 Example 4
Samples A and B are evaluated on a 3.5" (9 cm) Black Clawson Model 435
30:1 L/D extrusion coating equipment with a 150 HP (110 kW)
Electro;Flyte drive system; the die is a 30" (75 cm) Black Clawson,
Model 300XLHL. The resin is coated onto Kraft paper at a melt
temperature of 550 DEG F (290 DEG C) and at an RPM of 51. Sample A,
which has the narrower I10/I2 ratio also achieves the faster line
speed. Examples 5A - 5B
Two different ethylene/octene copolymers (LLDPE) having a density of
0.941 gm/cc and an I2 melt index of 4.0 gm/10 minutes and an I10/I2
ratio of (i) 7.2; and (ii) 6.8 gm/10 minutes were extrusion-cast on the
same equipment under the same processing conditions. The copolymer
having an I10/I2 ratio of 6.8 could be processed at a maximum line
speed that was about 25 percent higher than that obtainable at an
I10/I2 ratio of 7.2. Examples 6A - 6B
From other experimental work it has been predicted that line speed
during extrusion-coating onto Kraft paper, using a nip-draw, of LLDPE
having a density of 0.916 gm/cc and an I2 melt index of 25 gm/10
minutes, would be increased by about ten percent by reducing the I10/I2
ratio from 8.5 to 7.0. Examples 7A - 7B
From other experimental work it is predicted that line speed during
extrusion-casting of LLDPE polymer having a density of 0.920 gm/cc and
an I2 melt index of 1.0 gm/10 minutes, would be increased by about
fifteen percent by reducing the I10/I2ratio from 8.1 to 7.1.
Comparative Example 8A AND Example 8B
From other experimental work it is predicted that maximum line speed
during extrusion-coating onto kraft paper, using a nip-draw, of LLDPE
having a density of 0.913 gm/cc and an I2 melt index of 6 gm/10
minutes, would be increased by about 10 percent by reducing the I10/I2
ratio from 8.5 to 7.5.