Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BLOWN FILM EXTRUSION
Cross-Reference to Related Application
This application is related to Canadian Serial
Number 469,747 filed December 10, 1984.
BACKGROUND OF THE INVENTION
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Field of the Invention
This invention relates to blown film extrusion and,
more particularly, this invention relates to an improved method
for producing polyolefin films by blown film extrusion.
Descrlption of the Prior Art
Production of polyolefin film by blown film extrusion
is well known. In a typical blown film extrusion process, poly-
olefin resin is melted in a screw extruder wherein pressure is
developed on the molten resin, causing the molten resin to pass
through a die having a circular orifice to form a tubular film
or sleeve, also known as a "bubble".
Gas, usually air, is provided to the interior of
the bubble to inflate it to a desired diameter. The yas is
contained within the bubble by the die and by a pair of nip
rolls disposed downstream from the die. The nip rolls provide
the force to pull the bubble away from the die in a machine
direction ("MD") at a d~sired speed.
The rate o~ extrusion of the mel-t, the rate of speed
of the nip rolls, and the degree of inflation of the bubble
together determine the final thickness of the film.
Between the die and the nip rolls, the melt cools,
and undergoes a phase change to the crystalline s-tate. A so-
called "frost line" is observable at the point of the bubble at
which the phase change occurs.
Conventional blown film extrusion can be generally
classified as either a "stalk" process or a "pocket" process.
In stalk extrusion, an air ring, usually a single
lip air ring, is disposed adjacent the die and provides stabi-
lizing air flow generally parallel to the machine direction.
Thus, the bubble maintains a relatively uniform diameter
approximately equal to that of the annular die for a significant
distance from the die, and eventually expands in the transverse
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direction ("TD") to the desired diameter due to the gas
pressure within the bubble.
In pocket extrusion, a force is applied by an air
ring disposed adjacent the die to cause the bubble leaving
the die to irnmediately expand in the transverse direction at
a rate dependent on the design of the air ring. This type of
rapid expansion is achieved with a so-called "dual lip" air
ring designed to create a vacuum to pull the bubble in the
transverse direction.
Single lip air rings are also useful in pocket
extrusion, and allow slower expansion of the bubble. Such
air rings do not exert as strong an outward force as do dual
lip air rings.
Though useful and widely accepted, prior blown film
extrusion processes, including prior stalk and pocket extrusion
methods, do exhibit disadvantages. Stalk extrusion methods are
inherently unstable, with limited output potential, and gauge
control is difficult with such methods. Polyethylene films
made by pocket extrusion methods are generally not as strong
as desired, and sometimes have undesirable optical proper-ties.
Also, the effectiveness of prior methods varies
depending on the type of polyethylene resin employed. For
example, with high molecular weight, low density polyethylene,
the film reacts adversely to pocket extrusion methods because
of the resin's high melt viscosity and elasticity. While these
properties contribute to stability in stalk extrusion, the
inherent instability and limited output potential of stalk
extrusion methods make them undesirable with some resins. Also,
the high degree of molecular orientation that can be attained
by stalk extrusion can reduce product tear resistance and
stiffness.
SUMMARY OF_THE I ENTION
Accordingly, the invention seeks to overcome one
or more of the problems described above.
The invention pertains to a method of making poly-
olefin film including the steps of extruding molten polyolefin
material through an annular die to form a tubular film, with-
drawing the tubular film from the die in a machine direction,
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and expanding the tubular film in a direction transverse to
the machine direction by maintaining a positive gas pressure
therein. The tubular film defines first and second regions
generally coaxial with the die, the first region being
adjacent the die and extending therefrom in the machine
direction and having a maximum diameter substantially equal to
the diameter of the die. The second region extends from the
first region and has a diameter substantially greater than those
of the die and the first region. The improvement comprehends
the polyolefin material being selected from homopolymers of
ethylene made under high pressure conditions, copolymers of
ethylene and one or more members of the group consisting of
vinyl acetate and butyl acrylate made under high pressure
conditions, and high density copolymers of ethylene and
l-olefins having three to eight carbon atoms made under low
pressure conditions. The polyolefin material has an elongational
viscosity of at least about 350,000 poise at 190C substantially
independent of applied stress at the temperature of extrusion.
A first air ring coaxial with the die and the tubular film is
positioned about the tubular film adjacent the die to provide
air flow in a direction parallel to the machine direction to
stabilize the film without allowing substantial expansion there-
of. A second air ring is positioned about the tubular film at
the point of initial expansion to assist in the expansion and
cooling and to maintain the stability of the film.
More particularly polyolefin resins having high melt
viscosities which are substantially independent of applied
stress at processing temperatures may be extruded by a blown
film process using two air rings. A first ring, which is
preferably a single lip air ring, is disposed adjacent -the die
and provides a stabilizing flow of air generally parallel to
the machine direction. The film bubble is maintained in a
stalk configuration having a maximum diameter substantially
e~ual to that of the die.
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A second air ring, which is preferably a dual
lip alr ring, is positioned downstream from the first air ring
at the point where the bubble naturally expands to provide a
suction force in the transverse direction to assist in
expansion. The second air ring's position is selected such
that the resin's melt elasticity has dissipated.
The first air ring stabilizes the bubble without
causing expansion, while the second air ring assures that
expansion occurs under stable conditions.
Surprisingly, blown film extrusion utilizing two
air rings according to the invention provides product films
having improved optical properties (i.e., lower haze and higher
gloss), improved strength characteristics, and higher output
and improved drawdown as compared to prior processes.
The invention is especially useful with those poly-
ethylene resins which have high viscosity and high melt
elasticity which will naturally form a long stalk configuration.
Other aspects and advantages will be apparen-t to
those skilled in the art from the following detailed description
taken in conjunction with the drawings and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a sectional elevation of a blown film
extrusion apparatus suitable for carrying out the method of
the invention;
Fig. 2 is a sectional view of a single lip air
ring disposed adjacent the die of the apparatus of Fig. 1,
taken generally along line 2-2 of Fig. l; and,
Fig. 3 is a sectional view of a dual lip air
ring disposed downstream of the die of Fig. 1, taken generally
along line 3-3 of Fig. 1.
s~o;~
DETAILED DESCRIPTION OF ~HE INVENTION
. ~
Referring to Fi~s. 1-3, an apparatu6 ~uitable
for carrying out the method of the invention i6
illustrated. In Pig. 1, a thermoplastic resin ~uitable
for ~orming a film i~ fed to an extruder 2 by a hopper 4.
In the extruder 2, the resin i5 heated to a molten
condition and passed under pressure through a conduit 6
to a die 10 having a circular annular orifice 12. A
conduit 14 supplies a gas, typically air, at a desired
positive pressure to the inside of a bubble, generally
designated 16, formed by the molten resin extruded from
the die 10. The bubble i~ pulled through a collapsing
frame 20 by nip rollers 22 which flatten the bubble to a
film 24.
The bubble 16 comprises an elongate stalk 30
having a maximum diameter which is ~ubstantially equal to
that of the die orifice 12 and which extends therefrom.
With the high melt ~trength, high viscosity resins of the
invention, the diameter of the stalk gradually decreases
in the machine direction and reaches a minimum before
expanding. The stalk is generally 20 to 60 inches in
length, but may be longer under certain conditions. As
i5 well known in the art, the length of the stalk is a
function of numerous variables, including the viscosity
and elasticity of the resin, the design and temperature
of the passages in the die, the meltang temperature of
the extruder, the output rate of the extruder, the length
and gap of the die lands which define the die orifice,
take off ~peed, and the degree of drawing l"drawdown n
among other variables.
The stalk 30 expands to define a bubble region
32 of enlarged diameterO As is well known in the art,
the diameter ~f the region 32 may be several times that
of the ~talk 30, with blow up ratios (~BUR"~ of up to 6
~ZS(~4~
being c~mmon. The thickness o~ the bubble wall in the
region 32 i~ correspondingly ~maller than the wall
thickness in the stalk region 30.
In Fig. 1, the direction represented by the
arrow 34 is conventi~nally defined as the ~machine direc-
tion". In Fig. 1, the machine direction i8 depicted to
~e vertically directed away from the die. While this is
most common, it is not critical that the extrusion pro-
cess take place in the vertical direction.
The direction of expansion of the bubble away
from the stalk 30 is a direction which is transvex~e to
the machine direction, and is conventionally referred to
as the ~transverse directionn.
A visible line 36 is located downstream from
the die in the bubble region 32 and is conventionally
known as the "frost linen. This is the point in the
bubble 16 at which the molten resin has crystallized to
form a ~olid thermoplastic material. The distance of the
frost line from the die varies depending on a number of
variables as is well known in the art.
As noted above, the maximum diameter of the
stalk 30 is generally substantially equal to that of the
die orifice 12. It is to be understood for the purposes
of this disclosure, however, that the stalk diameter may
be as large as about 1.1 times the orifice diameter in
the practice of the inven`tion.
Two air rings, generally designated 40 and 42,
are di6posed about the bubble 16 at a point adjacent the
die 10 and downstream therefrom, respectively.
The air ring 40 i~ disposed adjacent the die 10
and i6 preferably the type conventionally referred to as
a ~ingle lip" air ring. The air ring 40 includes a body
44 with an air passage 45 communicating with a ~urce of
~2~V~
pressurized air (not shown) and with an annular orifice 48
designed to provide a flow of air in a direction which is
substantially parallel to the direction of movement of the
bubble 16, as seen in Fig. 2.
The air ring 42 disposed downstream of the air
ring 40 may be of the type conventionally known as a "dual
lip" air ring. Representative dual lip air rings suitable
for use in the invention are described in Cole U.S. patent
No. 4,259,047 (March 31, 1981).
The air ring 42 comprises a body 50 having an air
passage 52 communicating with a source of pressurized air
(not shown) and with an annular orifice 54. An annular body
member 56 is disposed in the orifice 54 about the bubble 16
to define inner and outer flow passages 60 and 62,
respectively. By design, the air stream flowing through
passage 60 is of relatively low volume and low velocity,
while that ~lowing through passage 62 is of relatively high
volume and high velocity. The cooperative effect o-f the two
air streams is to provide a partial vacuum by the venturi
efEect to assist in expansion of the bubble 16 in the
transverse direction.
The first and second air rings 40 and 42 are
coaxial with each other and with the die orifice 12. In
Fig. 1, the common axis is defined by the air conduit 14.
According to the invention, a thermoplastic resin
is extruded through the die 10 and through the air
rings 40 and 42 to provide controlled film formation.
The first air ring 40 provides stabilization of the
resin, allowing the molten resin to cool and relax
without expanding. The second air ring 42 provides
controlled expansion at a desired distance from the die.
l~S(14~
The position of t~e second air ring 42 is selected to
emphasize desired qualities Df the product film. The
cooperative effect of the air rings 40 and 42 in the
system shown in the Figures pro~ides relat~vely high
product output rates, a6 well ~s enhanced optical and
6trength characteri~tics of the film.
As the resin leaves the die, it has a degree of
melt elasticity that depends on the nature of the
polymer, the design of the die and the rate of extrusion.
This elasticity is a result of deformation of the xesin
as it passes through the die, and, in cooperation with
the viscosity of the resin, it acts as a force within the
resin to resist drawing. Drawing leads to the final
state, a ~olid, semi-crystalline polyethylene film having
a desired thickness and size.
As the molten resin cools and crystallizes to
form a solid film, the degree of drawing and the state of
the melt at the time of drawing determine the final
physical dimensions and film properties.
When the elastic forces have dissipated to the
degree that they are overbalanced by the pressure of gas
within the bubble, as well as the drawing force of the
nip rDlls, the bubble simultaneously expands in the
transverse direction and draws in the machine direction.
The melt reaches the tem~erature of crystallization, and
changes to a semi-cry~talline solid at the frost line
36. This increases the tensile strength of the bubble to
offset the forces of drawing and expansion.
According to the invention, the molten resin is
allowed to relax (i~e. the melt elasticity is allowed to
dissipate) before drawing. When drawing occurs in a re-
laxed state, i.e. with little if any remaining elasti-
city, the resin is better able to respond to drawing
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f~rce~, and thu~ can be drawn to lower gauges. Impact
~trength i~ impr~ved ~ecau~e drawing takes place with a
relaxed melt at a temperature just above temperature of
cry~tallization. Thus, proper conditions ~or development
of crystalline structure and orientation are provided.
For various rea~ons, the method of the inven-
tion i5 not suitable ~or the type of ethylene/l-olefin
copolymer known in the art as ~linear low density poly-
ethylene" or for other sensitive resins which do not have
sufficiently high viscosities and melt 6trength necessary
for the formation of a long stalk. However, the inven-
tion is very desirable for use with those resins having
high elongational viscosity and high melt elasticity.
These include high density polyethylene and medium
density polyethylene, and especially high molecular
weight, low density polyethylene (i.e., ethylene
homopolymers having densities less than about 0.930 g/cc)
and ethylene/vinyl acetate or ethylene/butyl acrylate
' copolymers. Suitable resins can qenerally he
characterized as ethylene homopolymers made under
conditions of high pressure (i.e., at least about 5000
psi, and preferably at least about 15,000 psi, up to
about 60,000 psi), ethylene/vinyl acetate or
ethylene/butyl acrylate copolymers made under conditions
of high pressure, and copolymers of ethylene and
1-olefins having 3 to 8 carbon atoms made at conditions
of low pressure (less than about 5,000 psi) and having
high densities (i.e. about 0.940 g/cc or above)~
alends and coextruded combinations of suitable
materials including but not necessarily limited to those
of the type identified ~bove are suitable for production
according to the method of this invention.
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~ he resin must have a sufficiently high
elongational viscosity which is substantially independent
of applied ~tress (i.e. does not decrease with an
increase in applied stress) at the temperature and
pressure of extrusion in order to be processed accordin~
to the invention. It has been determined that a minimum
elongational viscosity of about 350,000 poise at normal
extrusion conditions (i.e. 190C) is sufficient.
For example, a high pressure ethylene
homopolymer (1.0 MI, density = 0.922) sold under the
trademark Norchem 3401 by Norchem, Inc. has a
substantially consistant elongational viscosity of
350,000-400,000 poise at 190C at an applied elongational
stress of about 1.8 X 104 to about 7.2 X 105 dyne/cm~.
Norchem, Inc. product 3503 is a high mol~cular weight,
low density (0.3 MI, 0.925 density) ethylene homopolymer
having a substantially constant elongational viscosity
of about 1,100,000-1,500,000 poise at 190C and about 2.5
X 105 to about 1.5 X 106 dyne/cm2 applied elongational
stress.
A typical LLDPE having a melt index of 1.0, on
the other hand, exhibits a decrease in elongational
viscosity from about 340,000 poise to about 100,000 poise
at 190C as applied elongational stress is increased from
about 0.9 X 105 to about 3.5 X 106 dyne/cm2. Such a
resin is unsuitable for use in the inventive method.
Resins having elongational viscosities of substantially
less than 350,000 poise at 190C are also
unsuitable, even if the viscosity does not vary
with applied stress. An example of ~uch a resin is
Norchem, Inc. product 3404 which is a low density (1.8
~2SV4V;~
,
MI, density 0.923) ethylene homopolymer which exhibits an
elongational viscosity at 190C of about 2 X 105 poise at
an applied elongational stress of about 1.2 X 105 to about
5.5 X 106 dyne/cm2.
It will be understood by those skilled in the
art that the viscosity of the polyolefin material may be
less than 350,000 poise at the actual temperature of
extrusion, which may range up to 220C or even higher in
some cases.
Because of the varying properties of suitable
resins, the spacing between the air rings 40 and 42 is
adjustable. Al60, operation with a ~ingle resin under
varying conditions allows the production of products
having different properties by varying the spacing of the
air rings. As noted above, the spacing is generally
within the 20 to 60 inch range, although greater spacings
are entirely operable.
Die lip gaps and diameters can vary over a wide
range. Generally, the die lip gap will be in the range
of 25-llO mils, with a gap of 40 to 70 mils being
preferred. A die diameter of about 8 inches is typical.
If desired, a stabilizing iris may be posi-
tioned downstream from the second air ring. The space
between the air rings 40 and 42 need not be sealed. It
should be noted, however, that the effectiveness of the
invention is not dependent upon the spacing of the air
rings or the use of a sizing cage, as is characteristic
of some prior art processes. If desired, the inventive
process can be started with the upper air ring close to
the lower air ring, with~the operator raising the upper
air ring to its desired ultimate position once the stalk
has been established.
Since the second air ring 42 is positioned at a
point selected to coincide at the point at which the
bubble 16 naturally expands, expansion is stabilized and
occurs after the melt elasticity of the resin has
dissipated. As a result, strength is improved ~nd higher
output rates are achieved. Compared with prior ~talk
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extrusion ~ethods, the second air xing improves bubble
~tability, gauge uniformity, and extrusion rates. Also,
optical properties are improved.
For example, a comparison of properties
obtainable by means of the invention compared to film
products obtained by means of prior pocket and stalk
extrusion methods is shown below.
Pocket
Dual-lip Sinqle-lip Stalk Invention
Haze,~ . 6.9 6.3 6.0 4.8
~loss 59 64 68 72
TEDD*, ft.-lbØ6 l.0 2.0 1.0
Thickness, mil 1.3 1.3 l.0 1.0
Output, pph 280 280 275 3B0
*Total energy dart drop
The foregoing values were obtained using a high
molecular weight, low density (0.924 g/cc) ethylene
homopolymer having a melt index (MI) of 0.3, marketed by
Norchem, Inc., Rolling Meadows, Illinois under the trade
designation "3503".
The foregoing data illustrate another important
advantage of the invention over prior pocket extrusion
processes, namely the improvement in drawdown o~tainable
due to the relaxation of the melt in the invention. In
the foregoing, l.3 mil film was produced in the pocket
extrusion examples due to the inabili~y to draw the
particular resin to a lower gauge in pocket extrusion.
According to the invention, on the other hand, a 1.0 mil
film was obtained.
The economies of production according to the
invention reflect two advantages thereof over prior
product extrusion processes. Fir~tly, drawing of film to
a lower gauqe results in ~ignificant material ~avings.
~For example, ~ 23% ~avings is reflected in the ~oregoing
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data.~ Secondly, aowngauging of *he film in combination
with increa~ed output results in a 6ignificant increase
in the production ~te of useable ~ilm. IFor example,
the foregoing data reflect ~ production increa~e of about
764 1380/280 x 1.3 c 1~76).)
Examples
The following specific examples will illustrate
various advantages of the invention as compared to the
prior art.
Example 1
Using an apparatus as shown in Fig. l, a
~loucester Engineering Corp. (Gloucester, MA) ~GEC)
6ingle-lip air ring is mounted adjacent the die. A
Uni-Flo Design, Inc. (Brampton, Ontario, Canada) dual-lip
air ring is mounted on an iris frame for adjustable
movement above the die. Pilm samples designated A
through J of various gauges were produced, using Chemplex
(now Norchem) 3503C polyethylene in Samples A-C, and
Chemplex (Norchem) 3503A polyethylene for Samples D-J.
An 8" diameter coextrusion die, fed by one 2.5" and two
2.0" extruders was used.
The data for 1.0 mil films is shown below in
Table I, the data for 2.0 mil films is shown in Table II,
~nd the data for ~ther gauges is shown in Table III. For
comparison (in Table I) data for 3503A extruded at a high
rate for each air ring alone is ~hown. The single air
ring data is for 280 pph, 2/1 BUR, with a target gauge of
1.25 mil.
The compaxisons clearly show the advantages of
using the tw~ air rings in the tandem configuration of
the invention. The 6ame die~ the same three extruders,
and the same tot of resin was used with each air ring
configuration. The two air rings used in andem were the
~zso~o~
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~ame two as were run 6eparately. The advantages of the
tandem configuration apparent from the results are:
1. Substantially improved optical propertie~ -
Sinqle Air Rinq Tandem Air Rin~
~aze 6.9~ dual-lip Uni-Flo 3.8~ ~ I.7/l BUR
6.3~ ~ingle-lip GEC 4.0% ~ 2.6~l BUR
Gloss 59 dual-lip Uni-Flo 77 ~ l.7/l BUR
64 single-lip GEC 72 ~ 2.6/l BVR
2. Improved impact ~trength (TEDD-ft.lb.) at lower
gauge -
Sinqle Air Rinq-1.3 mil Tandem Air Rin
0.6 dual-lip Uni-Flo 1.2 ~ 1.7/l BUR, l.l mil
l.0 single-lip GEC 1.0 @ 2.6/1 BUR, 1.0 mil
3. Output increased from 280 pph to 380 pph.
~2~J 4~
15 -
TABLE 1
COMPARISON OP 1 MIL FILM P~PERTIES
Uni-Flo GEC
Dual Single
Tandem Air Rinqs Lip
Sample ~ G J
Gauge, Mil 1.03 1.10 0.97 1.28 1.31
Output, pph 180 315 380 280 280
Blow-up Ratio 1.55/1 1.7/1 2.6/1 2/1 2/1
Haze, % 3.8 3.8 4.8 6O9 6.B
Gloss, 45 77 77 72 59 64
NAS, % 56 66 63
TEDD, ft.-lb.
Flat 1.4 1.2 1.0 0.6 1.0
Creased 0.4 0.5 0.6 0.2 0.3
TAgLE II
COMPARISON OF 2 MIL FILM PROPERTIES
-
Sample A F H
Gauge, mil 1.98 2.16 1.93
Output, pph 190 315 314
Blow-up Ratio 1.55/1 1.65/1 2/1
~aze, % 4.5 5.1 4.4
Gloss, 45 82 81 80
NAS, % 52 58 60
TEDD, ft.-lb.
Flat 1.8 1.6 1.6
Creased 0.7 0.6 0.8
14 Secant Modulus, psi
MD ~28100
TD 31200
:~.2S(3~
TABLE III
FILM P~PERTIES AT ~THER GAVGES
Sample I C D E
Gauge, mil 1.45 2.93 3.07 ~.03
Output, pph 380 190 315 315
Blow-up Ratio 2.5/1 1.55/1 1.65/1 1.65/1
Haze, % 4.3 6.1 6.4 8.3
Gloss, 45 78 82 78 75
NAS, % 61 45 55 45
TEDD, ~t.-lb.
Flat 1.3 2.0 2.1 2.8
Creased 1.1 1.4 1.0 1.5
The foregoing detailed description is given for
clearness of understandinq only, and no unnecessary
limitations should be inferred therefrom, as
modifications within the scope of the invention will be
obvious to those skilled in the art.
