TITLE: Linear low density polyethylene cast film
European Patent EP0292336
 B1

ABSTRACT:
Abstract of EP0292336
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. (33 Holly Chase, Clute, Texas, 77531, US)
Niemann, Debra H. (625 Milton, Angleton, Texas, 77515, US)

APPLICATION NUMBER: EP19880304667
PUBLICATION DATE: 03/24/1993
FILING DATE: 05/23/1988

ASSIGNEE: THE DOW CHEMICAL COMPANY (2030 Dow Center Abbott Road P.O. Box 1967,
Midland, Michigan, 48640-1967, US)
INTERNATIONAL CLASSES: B29C41/26; B29C47/00; B29C47/02; B29C47/88; C08F210/16; C08J5/18;
B29K23/00; (IPC1-7): B29C47/00; B29C55/06; B29D7/00; B29K23/00
EUROPEAN CLASSES: B29C47/00B; B29C47/88C4

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

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

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., [PROC. INT. CONGR.], 8TH 1980, 3,
15-24
Attorney, Agent or Firm:
Raynor, John (W.H. Beck, Greener & Co 7 Stone Buildings Lincoln's Inn,
London, WC2A 3SZ, GB)

CLAIMS:
1. A process for increasing the line speed of extrusion-casting or
extrusion-coating a LLDPE polymer film, said LLDPE polymer consisting
essentially of ethylene inter-polymerized with from 1 percent to 60
percent by weight of at least one alkene of C[3]-C[1][2] to form an
interpolymer having a density in the range of from 0.87 to 0.955 gm/cc
and having a melt index, I[2], in the range of 0.1 to 25 gm/10 minutes
as determined by ASTM D-1238-E;
      characterized by selecting the LLDPE polymer to have an
I[1][0]/I[2] ratio in the range of 4.0 to 8.3, where the value of
I[1][0] is determined by ASTM D-1238-N.
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
I[2]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 H[2]C=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
I[2]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: (a) a constant tension device; (b) 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) an extruder wherein the draw roller
is located not more than 15 cm (6 inches) from the extrusion die; and
(d) an olefin polymer blended in the LLDPE.
11. A process as claimed in any one of Claims 1 to 9, wherein the film
is extrusion-cast or extrusion-coated without the use of specific
equipment to improve draw resonance resistance, and the line speed is
above 1.5 m/s (300 feet per minute).
12. A process as claimed in any one of Claims 1 to 9, wherein the film
is extrusion-cast or extrusion-coated without the use of a draw
resonance eliminator or tension control device acting on the molten
film to improve draw resonance resistance, and the line speed is above
1.5 m/s (300 feet per minute).

DESCRIPTION:
The invention relates to the production of films of linear low density
polyethylene (LLDPE) and, in particular, provides for the use of fast
line speeds by selecting LLDPE of high resistance to draw resonance.
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 (C[3]-C[1][2]) 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 ethylene homopolymer) is made ln
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. 0.94 to 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. US-A-4,302,566 or
US-A-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
I[1][0]/I[2] required for the present invention, such as by using the
catalyst described in US-A-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.,
US-A-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 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.
Chang Dae Han et al (J. Appl. Polym. Sci., 24 (1979) 61-87) reported
the effects of molecular structure and cooling conditions on the draw
resonance of viscoelastic polymers including, inter alia, HDPE, during
melt spinning of fibers. They concluded that the severity of draw
resonance depends strongly on the distribution of molecular weight and
molecular structure and that cooling may increase the severity of draw
resonance of viscoelastic fluids in general but the degree varies from
material to material. A broad molecular weight distribution (BMWD) was
found to give rise to lower melt strength and to lower critical stretch
ratio for the onset of draw resonance than a narrow molecular weight
distribution (NMWD).
C.A. 94:4850r (1981) reports that the draw resonance in spinning of
isotactic polypropylene melts depends upon, inter alia, the molecular
structure of the polymer. In particular, the critical draw ratio for
the onset of draw resonance was found to increase with decreasing
molecular weight and with decreasing width of molecular weight
distribution.
M. Ahmed ("Polypropylene Fibers-Science and Technology", Elsevier
Scientific Publishing Co. (1982), 237-254) reports that the intensity
of draw resonance in polypropylene spinning is dependent, inter alia,
on molecular weight distribution. In particular, NMWD polypropylene was
found to have a lesser tendency for draw resonance than BMWD
polypropylene.
It is reported in EP-A-0107076 (1984) and US-A-4608221 (1986) that
extensional rheology characteristics are important in commercial
extrusion processes and can dominate over shear rheology
characteristics. LDPE was found to be soft in shear and stiff in
extension compared with LLDPE, which exhibits the opposite rheology.
Although this difference in strain hardening allows LLDPE to be drawn
down at greater rates than LDPE without melt breakage, the greater
susceptibility of LLDPE to draw resonance is believed to be due to
lower stress build-up with drawing. Extensional viscosity and shear
thinning are both reported to be higher in BMWD resins than NMWD resins
and accordingly this prior art would appear to teach that draw
resonance of LLDPE would decrease with increasing molecular weight
distribution.
GB-A-2,124,139 (published 2015084) 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.
EP-A-0107076 (corresponding to US-A-4,486,377) proposes that the draw
resonance problem, found in drawing films of LLDPE, is reduced by
directing a fluid medium, preferably air, against the molten film
between the die and a rapid cooling zone. The preferred LLDPE for use
in that invention has a molecular weight distribution (Mw/Mn) between
2.4 and 5.0, preferably between 2.8 and 3.4. These distributions
correspond to a melt flow ratio (i.e. Flow Rate as determined by ASTM
1238, Condition F divided by Melt Index as determined by ASTM D1238,
Condition E) of between 20 and 37 and between 25 and 32 respectively.
In the absence of the fluid medium, the onset of draw resonance of all
LLDPE tested in the Examples was recorded at 300 fpm (1.5 m/s) or less.
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 (see above acknowledgement of
EP-A-0107076/US-A-4486377).
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.
US-A-4,608,221 reports that the problem of draw resonance can be
overcome by extruding LLDPE film into a rapid cooling zone provided
with a tensioning device between the die and the nip of a pressure roll
and a chill roll. The tensioning device provides a substantially
friction free surface with respect to the moving film and is responsive
to tension variations in the moving film between the die and the rapid
cooling zone to maintain substantially constant tension in the moving
film. The same LLDPE is preferred as in EP-A-0107076/US-A-4486377. The
only exemplified process uses line speeds of 200 to 300 fpm (1 to 1.5
m/s) and is compared with a process according to EP-A-
0107076/US-A-4486377 operating at the same speeds.
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
formed 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 C[3]-C[1][2] to form an interpolymer having a
density in the range of from 0.87 to 0.955 gm/cc and has a melt index,
I[2], 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 I[1][0]/I[2]
ratio in the range of 4.0 to 8.3, where the value of I[1][0] 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 aforementioned US-A-4,608,221, been attained by
designing an LLDPE polymer which has an unusually narrow molecular
weight distribution as indicated by an I[1][0]/I[2] 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 C[3]-C[1][2], preferably at least one of those in the range of
C[4]-C[8], most especially octene-1; these olefins are represented by
the formula H[2]C=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 0.88 to 0.945
gm/cc (as measured by ASTM D-1248;
      It has an I[2] value in the range of 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 I[1][0]
value is determined by using ASTM D-1238-N;
      It is critical, for purposes of this invention, that within the
above described I[2] range, the LLDPE has an I[1][0]/I[2] 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 I[2] 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 US-A-4,612,300; the invention is, however, not limited to
only those in accordance with US-A-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 I[1][0]/I[2] 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
I[1][0]/I[2] 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® 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:
TABLE I
Resin I[2] gm/10 min I[1][0]/I[2] Ratio Density gm/cc
A 2.3 6.5 0.936
B 2.5 8.3 0.934
E 2.1 8.3 0.940
F 2.1 8.2 0.941
G 2.1 7.4 0.941
H 2.2 7.0 0.941
K 2.2 6.6 0.941
LDPE * 1.8 10.5 0.923 *The LDPE is used for comparisons only and does
not represent an example of the present invention.
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 fpm (7 m/s) and the
B sample exhibits a maximum line speed of 800 fpm (4 m/s).
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.
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 fpm (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).
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°F (290°C) and at an RPM of 51. Sample A, which has
the narrower I[1][0]/I[2] 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 I[2] melt index of 4.0 gm/10 minutes and an
I[1][0]/I[2] 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 I[1][0]/I[2] ratio of 6.8 could be
processed at a maximum line speed that was about 25 percent higher than
that obtainable at an I[1][0]/I[2] 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 I[2] melt index of 25 gm/10
minutes, would be increased by about ten percent by reducing the
I[1][0]/I[2] 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 I[2] melt index of 1.0 gm/10 minutes, would be increased by about
fifteen percent by reducing the I[1][0]/I[2]ratio 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 I[2] melt index of 6 gm/10
minutes, would be increased by about 10 percent by reducing the
I[1][0]/I[2] ratio from 8.5 to 7.5.