U.S. patent application number 11/292630 was filed with the patent office on 2006-04-20 for method of stretching film and such film.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Joseph T. Bartusiak, Martin E. Denker, Anthony B. Ferguson, Thomas P. Hanschen, Jeffery N. Jackson, William Ward Merrill, Susan J. Newhouse, Fred J. Roska, Richard J. Thompson, Chiu Ping Wong.
Application Number | 20060082022 11/292630 |
Document ID | / |
Family ID | 23865765 |
Filed Date | 2006-04-20 |
United States Patent
Application |
20060082022 |
Kind Code |
A1 |
Denker; Martin E. ; et
al. |
April 20, 2006 |
Method of stretching film and such film
Abstract
A method of stretching films in which all or a portion of the
width of the film is cooled during or just after stretching so as
to improve the uniformity of the film. The includes stretching a
polymeric film in a tenter that grasps the film with a plurality of
clips along the opposing edges of the film and propels the clips to
thereby stretch the film. The tenter includes driven clips and
idler clips, with at least one idler clip between respective pairs
of driven clips. The cooling is done so as to improve the
uniformity of the clip spacing relative to the spacing obtained at
otherwise identical process conditions without such cooling.
Inventors: |
Denker; Martin E.; (Vadnais
Heights, MN) ; Bartusiak; Joseph T.; (Osseo, MN)
; Ferguson; Anthony B.; (Lake Elmo, MN) ;
Hanschen; Thomas P.; (St. Paul, MN) ; Jackson;
Jeffery N.; (Woodbury, MN) ; Merrill; William
Ward; (White Bear Lake, MN) ; Newhouse; Susan J.;
(Houlton, WI) ; Roska; Fred J.; (Woodbury, MN)
; Thompson; Richard J.; (Lino Lakes, MN) ; Wong;
Chiu Ping; (Vadnais Heights, MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
23865765 |
Appl. No.: |
11/292630 |
Filed: |
December 2, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09469972 |
Dec 21, 1999 |
|
|
|
11292630 |
Dec 2, 2005 |
|
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|
Current U.S.
Class: |
264/290.2 |
Current CPC
Class: |
B29C 55/165
20130101 |
Class at
Publication: |
264/290.2 |
International
Class: |
B29C 55/00 20060101
B29C055/00 |
Claims
1. In a method of stretching a polymeric film comprising the steps
of grasping the film with a plurality of clips along the opposing
edges of the film and propelling the clips to thereby stretch the
film, wherein the plurality of clips includes driven clips and
idler clips, with at least one idler clip between respective pairs
of driven clips, the improvement comprising: a) heating the
polymeric film to a sufficiently high temperature to allow a
significant amount of stretching without breaking; and b) imparting
a machine direction cooling gradient to at least a portion of the
width of the stretched film in an effective amount to reduce the
value of idler clip lag from the value of idler clip lag in the
absence of said cooling.
2. The method of claim 1, wherein step b) includes actively cooling
the opposed edge portions of the film.
3. The method of claim 1, wherein step b) includes actively cooling
the center portion of the film.
4. The method of claim 1, wherein step b) includes actively cooling
substantially the entire width of the film.
5. The method of claim 1, wherein step b) includes cooling at least
a portion of the film by at least 3.degree. C.
6. The method of claim 1, wherein the method further includes
propelling the clips through a stretch section in which the film is
stretched and subsequently through a post-stretch treatment
section, and wherein step b) is performed in at least one of the
stretch section and the treatment section.
7. The method of claim 1, wherein the method includes biaxially
stretching the film.
8. The method of claim 1, wherein the method includes
simultaneously biaxially stretching the film by propelling the
clips at varying speeds in the machine direction along clip guide
means that diverge in the transverse direction.
9. The method of claim 1, wherein the film comprises
polypropylene.
10. The method of claim 9, wherein the method includes stretching
the film to a final area stretch ratio of from 16:1 to 100:1.
11. The method of claim 9, wherein step a) comprises heating the
film to from 120 to 165.degree. C.
12. The method of claim 11, wherein step b) includes forcing
cooling air onto the film, wherein the cooling air is at least
5.degree. C. cooler than the film.
13. In a method of stretching a polymeric film comprising the steps
of grasping the film with a plurality of clips along the opposing
edges of the film and propelling the clips to thereby stretch the
film, wherein the plurality of clips includes driven clips and
idler clips, with at least one idler clip between respective pairs
of driven clips, the improvement comprising: a) heating the
polymeric film to a sufficiently high temperature to allow a
significant amount of stretching without breaking; and b) imparting
a machine direction cooling gradient to at least a portion of the
width of the stretched film in an effective amount to improve the
downweb caliper uniformity relative to the downweb caliper
uniformity in the absence of said cooling.
14. The method of claim 13, wherein step b) includes actively
cooling the opposed edge portions of the film.
15. The method of claim 13, wherein step b) includes actively
cooling the center portion of the film.
16. The method of claim 13, wherein step b) includes actively
cooling substantially the entire width of the film.
17. The method of claim 13, wherein the method further includes
propelling the clips through a stretch section in which the film is
stretched and subsequently through a post-stretch treatment
section, and wherein step b) is performed in at least one of the
stretch section and the treatment section.
18. The method of claim 13, wherein the method includes
simultaneously biaxially stretching the film by propelling the
clips at varying speeds in the machine direction along clip guide
means that diverge in the transverse direction.
19. The method of claim 13, wherein the film comprises
polypropylene.
20. The method of claim 19, wherein the method includes stretching
the film to a final area stretch ratio of from 16:1 to 100:1.
21. The method of claim 19, wherein step a) comprises heating the
film to from 120 to 165.degree. C.
22. The method of claim 19, wherein step b) includes forcing
cooling air onto the film, wherein the cooling air is at least
5.degree. C. cooler than the film.
Description
[0001] This application is a divisional of U.S. Ser. No.09/469,972,
filed Dec. 21, 1999, now allowed, the disclosure of which is herein
incorporated by reference.
TECHNICAL FIELD
[0002] The present invention generally relates to methods of
stretching films and to the resulting films, and more particularly
to methods of stretching films in which all or a portion of the
width of the film is cooled during or just after stretching so as
to improve the uniformity of the film and to the resulting
films.
BACKGROUND OF THE INVENTION
[0003] It has been known in the art to biaxially stretch films.
Additionally, several methods and apparatuses have been described
for biaxially stretching films simultaneously in two directions.
See, e.g., U.S. Pat. Nos. 2,618,012; 3,046,599; 3,502,766;
3,890,421; 4,330,499; 4,525,317; and 4,853,602.
[0004] Tenters have been used for the transverse direction
stretching in sequential biaxial film stretching processes. For a
simultaneous biaxial stretching process, tenter stretching is
performed on a tenter apparatus that has grips or clippers that
grasp the film along the opposing edges of the film and propels the
grasping means at varying speeds along guiding means, which
typically are rails. As used herein, "grippers" and "clips" include
other film-edge grasping means, and the word "rails" includes other
clip guide means. By increasing clip speed in the machine
direction, stretching in the machine direction occurs. By using
such means as diverging rails, transverse direction stretching
occurs. Such stretching can be accomplished, for example, by the
methods and apparatus disclosed in U.S. Pat. Nos. 4,330,499 and
4,595,738, in which each of the clips is mechanically driven in the
tenter apparatus. More recently, tenter frames for stretching films
have been described in which the clips that propel the film through
the tenter apparatus are driven by linear motors. See, e.g., the
methods and tenter apparatus disclosed in U.S. Pat. Nos. 4,675,582;
4,825,111; 4,853,602; 5,036,262; 5,051,225; and 5,072,493.
[0005] In the simultaneous biaxial stretching apparatus described
in U.S. Pat. No. 5,051,225, tenter clips are driven by linear
electric motors. For reasons of spacing and cost, tenters such as
described in the '225 patent may not have every clip driven by a
linear motor. For example, every third clip on each rail may be
driven by a linear motor with the intervening two clips being
nondriven, and thus propelled forward only by the film itself. Such
nondriven clips are referred to as idler clips. It has been
observed that the relative position of the idler clips to the
driven clips is not necessarily the ideal position of being equally
spaced between driven clips. Any inequality in the clip-to-clip
spacing among two nearest-neighbor driven clips on a rail and their
intervening idler clips may be referred to using such terms as
idler non-uniformity, uneven clip spacing, non-uniform clip
spacing, and the like. Two special cases, however, are important.
The case in which the first and last (or only) idler clip(s)
between a pair of driven clips on a rail are propelled forward by
the film in an amount less than would be necessary for equal
spacing among clips is referred to as idler lag or lagging. The
case in which the first and last (or only) idler clip(s) between a
pair of driven clips on a rail are propelled forward by the film in
an amount greater than would be necessary for equal spacing among
clips is referred to as idler lead or leading. In the case where
there is more than one idler clip between each pair of driven clips
on each rail, it is possible to have one propelled forward by the
film in an amount less than would be necessary for equal spacing
among clips and, simultaneously, to have the other propelled
forward by the film in an amount greater than would be necessary
for equal spacing among clips. This situation results in an uneven
clip spacing, or idler non-uniformity, which is neither an idler
lag nor an idler lead.
[0006] U.S. Pat. No. 5,753,172 describes a process for the
simultaneous biaxial stretching in a tenter frame of a
thermoplastic polymer film having beaded edges, comprising gripping
the beaded edges of the film with tenter clips and increasing the
temperature of the beaded edges to within the film orientation
temperature range prior to or during simultaneous stretching, and
in subsequent stretching or heat-setting steps, by focusing heat on
the beaded edges of the film. The '172 patent states that bead
temperatures that are either too high or too low or beads that are
too thin can cause the spacing of the idler clips to be
non-uniform. Column 3, lines 30-33; column 11, lines 58-62. The
'172 patent further states that it is generally desirable for the
temperature of the beads to be approximately equal to, or higher
than, the temperature of the central film web. Column 5, lines
27-29. The '172 patent also states that the need for separate
control of bead temperatures is driven in part by the unequal
heating applied to the beads compared to the film in typical
stretcher heating zones. Col. 5, lines 33-35. It is both well-known
in the art and demonstrated in the '172 patent (Col. 11, lines
35-40) that such unequal heating in typical stretcher heating zones
leads to the beads being cooler than the central film web. U.S.
Pat. Nos. 3,231,642; 3,510,552; and 5,429,785 also discuss certain
effects of temperature control in various film stretching
processes.
SUMMARY OF THE INVENTION
[0007] The present inventors have discovered that by cooling all or
a portion of the width of the film by an effective amount during
and/or just after stretching, clip spacing non-uniformity,
particularly idler clip lagging, can be minimized to provide more
uniformly spaced idler clips, and to provide a final film with more
uniform properties and characteristics. Cooling can also be used to
cause idler clip leading, if desired.
[0008] In the simultaneous biaxial stretching apparatus of the type
described in the '225 patent discussed above, tenter clips are
driven by linear electric motors. For spacing or cost reasons, not
every clip is driven by a linear motor. For example, every second
or every third clip on each rail may be driven with the intervening
idler clip(s) being nondriven, and thus propelled forward only by
the film itself. The relative position of the idler clips to the
driven clips is a complex result of the interactions of film and
process variables, such as the film's visco-elastic properties
(e.g. stress as a function of strain rate history) and caliper
profile, and the stretching and temperature profiles as functions
of position along the tenter. Idler clips are propelled forward
through the tenter by force imparted by the driven clip in front of
the idler(s) and the film material between them. At the same time,
the forward motion of each idler clip may be resisted by force
imparted by the driven clip and film material behind it. As the
film is stretched in the machine and transverse directions downweb,
a complex interaction among the film material, the idler and driven
clips, and the bearing frictions within the clips usually results
in a net backward force on an idler clip, when viewed in a frame of
reference which is moving with the forward driven clip. Since there
is no linear motor force on the idler clips to counter this force,
the idler clips lag behind their ideal positions. At the exit end
of the tenter, where the film has been cooled, the idler lagging
may be accompanied by permanent downweb variations in machine
direction draw ratio that extend across the width of the film.
Idler clip lagging is a result of processing conditions which also
adversely affect the uniformity of the film properties such as
caliper, mechanical properties, and optical properties. Idler clip
lagging occurs at different locations in the process and to greater
or lesser extents depending upon the material and the stretching
conditions. Thus, it would be most advantageous to control the clip
lagging throughout the process (lag history), though we believe
there will be considerable advantage to controlling the magnitude
of the overall, or final, clip lagging.
[0009] The present invention provides methods to reduce clip
lagging to cause the idler clips to be closer to or at their ideal
positions relative to adjacent driven clips, and in some cases to
reverse clip lagging, causing the idler clips to be in front of
their ideal positions (idler lead). One method is edge cooling. In
edge cooling, the edge portions of the film are cooled an effective
amount at effective locations in the stretch section of the tenter
and/or in the section immediately after the stretch section,
referred to herein as the post-stretch treatment section. Edge
cooling is believed to increase the modulus of elasticity of the
material at the edge portions in a controlled fashion so that an
idler clip is pulled forward more than would be the case without
edge cooling by the driven clip and stiffer (cooler) edge bead in
front of it, resulting in a decrease in clip lagging. As a result,
the idler clip lagging is reduced, eliminated, or reversed (idler
lead). A second method is zone cooling, in which substantially the
entire width of the web is cooled an effective amount at effective
locations, or zones, in the stretch section of the tenter and/or in
the post-stretch treatment section. Zone cooling is believed to
increase the modulus of elasticity of the film across substantially
the entire width of the web in a controlled fashion, so that an
idler clip is pulled forward by the driven clip and film in front
of it more than would be the case without zone cooling, resulting
in a decrease of the backward force that causes clip lagging
without zone cooling.
[0010] One aspect of the present invention provides an improvement
to the method of stretching a polymeric film comprising the steps
of grasping the film with a plurality of clips along the opposing
edges of the film and propelling the clips to thereby stretch the
film. The plurality of clips includes driven clips and idler clips,
with at least one idler clip between respective pairs of driven
clips. The improvement comprises heating the polymeric film to a
sufficiently high temperature to allow a significant amount of
stretching without breaking, and actively imparting a machine
direction cooling gradient to at least a portion of the width of
the stretched film in an effective amount to improve the uniformity
of spacing of the driven and idler clips.
[0011] In another aspect, the present invention provides an
improvement to the method of stretching a polymeric film comprising
the steps of grasping the film with a plurality of clips along the
opposing edges of the film and propelling the clips to thereby
stretch the film. The plurality of clips includes driven clips and
idler clips, with at least one idler clip between respective pairs
of driven clips. The improvement comprises heating the center
portion and edge portions of the polymeric film to a sufficiently
high temperature to allow a significant amount of stretching
without breaking, maintaining, at the onset of stretching, the edge
portions of the film no hotter than the center portion of the film,
and imparting a machine direction cooling gradient to at least a
portion of the width of the stretched film in an effective amount
to improve the uniformity of spacing of the driven and idler
clips.
[0012] In one preferred embodiment of the above method, maintaining
the edge portions of the film no hotter than the center portion of
the film includes actively cooling the opposed edge portions of the
film.
[0013] In still another aspect, the present invention provides an
improvement to the method of stretching a polymeric film comprising
the steps of grasping the film with a plurality of clips along the
opposing edges of the film and propelling the clips to thereby
stretch the film. The plurality of clips includes driven clips and
idler clips, with at least one idler clip between respective pairs
of driven clips. The improvement comprises heating the polymeric
film to a sufficiently high temperature to allow a significant
amount of stretching without breaking, and imparting a machine
direction cooling gradient to at least a portion of the width of
the stretched film in an effective amount to reduce the value of
idler clip lag from the value of idler clip lag in the absence of
said cooling.
[0014] In yet another aspect, the present invention provides an
improvement to the method of stretching a polymeric film comprising
the steps of grasping the film with a plurality of clips along the
opposing edges of the film and propelling the clips to thereby
stretch the film. The plurality of clips includes driven clips and
idler clips, with at least one idler clip between respective pairs
of driven clips. The improvement comprises heating the polymeric
film to a sufficiently high temperature to allow a significant
amount of stretching without breaking, and imparting a cooling
gradient to at least a portion of the width of the stretched film
in an effective amount to improve the downweb caliper uniformity
relative to the downweb caliper uniformity in the absence of said
cooling.
[0015] In still another aspect, the present invention provides an
improvement to the method of stretching a pre-crystallized
polymeric film comprising the steps of grasping the film with a
plurality of clips along the opposing edges of the film and
propelling the clips to thereby stretch the film. The plurality of
clips includes driven clips and idler clips, with at least one
idler clip between respective pairs of driven clips. The
improvement comprises heating the polymeric film to a sufficiently
high temperature to allow a significant amount of stretching
without breaking, and imparting a cooling gradient to at least a
portion of the width of the stretched film in an effective amount
to improve the uniformity of spacing of the driven and idler
clips.
[0016] In yet another aspect, the present invention provides an
improvement to the method of stretching a vinyl polymer film
comprising the steps of grasping the film with a plurality of clips
along the opposing edges of the film and propelling the clips to
thereby stretch the film. The plurality of clips includes driven
clips and idler clips, with at least one idler clip between
respective pairs of driven clips. The improvement comprises heating
the polymeric film to a sufficiently high temperature to allow a
significant amount of stretching without breaking, and imparting a
cooling gradient to at least a portion of the width of the
stretched film in an effective amount to improve the uniformity of
spacing of the driven and idler clips.
[0017] In one preferred embodiment of the any of the above methods,
the opposed edge portions of the film are cooled.
[0018] In another preferred embodiment of any of the above methods,
the center portion of the film is cooled.
[0019] In another preferred embodiment of any of the above methods,
substantially the entire width of the film is cooled.
[0020] In another preferred embodiment of any of the above methods,
at least a portion of the film is cooled by at least 3.degree.
C.
[0021] In another preferred embodiment of any of the above methods,
the clips are propelled through a stretch section in which the film
is stretched and subsequently through a post-stretch treatment
section, and the cooling is performed in at least one of the
stretch section and the treatment section.
[0022] In another preferred embodiment of any of the above methods,
the film is biaxially stretched. More preferably, the film is
simultaneously biaxially stretched by propelling the clips at
varying speeds in the machine direction along clip guide means that
diverge in the transverse direction. Still more preferably, the
film is stretched to a final stretch ratio of at least 2:1 in the
machine direction and at least 2:1 in the transverse direction.
[0023] In another preferred embodiment of any of the above methods,
there are at least two idler clips between each respective pair of
driven clips.
[0024] In another preferred embodiment of any of the above methods,
the film comprises a thermoplastic film. More preferably, the film
comprises a semi-crystalline film. Of the semi-crystalline
embodiments, one preferred film has a degree of crystallinity
greater than about 1% prior to the heating. Still more preferably,
the degree of crystallinity is greater than about 7% prior to the
heating. Still more preferably, the degree of crystallinity is
greater than about 30% prior to the heating.
[0025] In another preferred embodiment of any of the first four or
the sixth of the above methods, the film comprises a thermoplastic
film which is an amorphous film.
[0026] In another preferred embodiment of any of the above methods,
the film comprises a vinyl polymer. More preferably, the film
comprises a polyolefin. Still more preferably, the film comprises
polyethylene or polypropylene.
[0027] In another preferred embodiment of any of the above methods,
the film comprises polypropylene, and the film is stretched to a
final area stretch ratio of at least 16:1. More preferably, the
film is stretched to a final area stretch ratio of from 25:1 to
100:1.
[0028] In another preferred embodiment of any of the above methods,
the film comprises polypropylene, and the film is heated to from
120 to 165.degree. C. More preferably, the film is heated to from
150 to 165.degree. C.
[0029] In another preferred embodiment of any of the above methods,
the film comprises polypropylene, the film is heated to from 120 to
165.degree. C., and the cooling includes forcing cooling air onto
the film. The cooling air is at least 5.degree. C. cooler than the
film.
[0030] Certain terms are used in the description and the claims
that, while for the most part are well known, may require some
explanation. "Biaxially stretched," when used herein to describe a
film, indicates that the film has been stretched in two different
directions, a first direction and a second direction, in the plane
of the film. Typically, but not always, the two directions are
substantially perpendicular and are in the machine direction ("MD")
of the film and the transverse direction ("TD") of the film.
Biaxially stretched films may be sequentially stretched,
simultaneously stretched, or stretched by some combination of
simultaneous and sequential stretching. "Simultaneously biaxially
stretched," when used herein to describe a film, indicates that
significant portions of the stretching in each of the two
directions are performed simultaneously. Unless context requires
otherwise, the terms "orient," "draw," and "stretch" are used
interchangeably throughout, as are the terms "oriented," "drawn,"
and "stretched," and the terms "orienting," "drawing," and
"stretching."
[0031] The term "stretch ratio," as used herein to describe a
method of stretching or a stretched film, indicates the ratio of a
linear dimension of a given portion of a stretched film to the
linear dimension of the same portion prior to stretching. For
example, in a stretched film having an MD stretch ratio of 5:1, a
given portion of unstretched film having a 1 cm linear measurement
in the machine direction would have 5 cm measurement in the machine
direction after stretching. In a stretched film having a TD stretch
ratio of 5:1, a given portion of unstretched film having a 1 cm
linear measurement in the transverse direction would have 5 cm
measurement in the transverse direction after stretching.
[0032] "Area stretch ratio," as used herein, indicates the ratio of
the area of a given portion of a stretched film to the area of the
same portion prior to stretching. For example, in a biaxially
stretched film having an area stretch ratio of 50:1, a given 1
cm.sup.2 portion of unstretched film would have an area of 50
cm.sup.2 after stretching.
[0033] The mechanical stretch ratio, also known as nominal stretch
ratio, is determined by the unstretched and stretched dimensions,
and can typically be measured at the film grippers at the edges of
the film used to stretch the film in the particular apparatus being
used. Global stretch ratio refers to the overall stretch ratio of
the film after the portions that lie near the grippers, and thus
are affected during stretching by the presence of the grippers,
have been removed from consideration. The global stretch ratio can
be equivalent to the mechanical stretch ratio when the input
unstretched film has a constant thickness across its full width
(from gripper to gripper, crossweb) and when the effects of
proximity to the grippers upon stretching are small. More
typically, however, the thickness of the input unstretched film is
adjusted so as to be thicker or thinner near the grippers than at
the center of the film. When this is the case, the global stretch
ratio will differ from the mechanical or nominal stretch ratio.
These global or mechanical ratios are both to be distinguished from
a local stretch ratio. The local stretch ratio is determined by
measuring a particular portion of the film (for example a 1 cm
portion) before and after stretching. When stretching is not
uniform over substantially the entire edge-trimmed film, then the
local ratio can be different from the global ratio. When stretching
is substantially uniform over substantially the entire film
(excluding the area immediately near the edges and surrounding the
grippers along the edges), then the local ratio everywhere will be
substantially equal to the global ratio. Unless the context
requires otherwise, the terms first direction stretch ratio, second
direction stretch ratio, MD stretch ratio, TD stretch ratio, and
area stretch ratio are used herein to describe the global stretch
ratio.
[0034] The term "stretch profile" is meant to refer collectively to
the values of all the variables of stretching the film, including
overall throughput rate of the tenter, and the stretch ratios and
temperatures as a function of position in the process, and to the
techniques used to attain these values, such as air impingement
velocities, clip accelerations and the like.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] The present invention will be further explained with
reference to the appended Figures, wherein:
[0036] FIG. 1 is a top schematic view of a tenter apparatus for use
with the present invention.
[0037] FIG. 2 is a plot of the caliper variation as a function of
MD position for a center-sample and an edge-sample of the film of
Example 11.
[0038] FIG. 3 is a plot of the caliper variation as a function of
MD position for the center-samples of the film of Example 11 and
the film of Example 10.
DETAILED DESCRIPTION OF THE INVENTION
[0039] FIG. 1 illustrates a top schematic view of a tenter
apparatus for carrying out the methods of the present invention.
The tenter is preferably of the type disclosed in U.S. Pat. No.
5,051,225, "Method of Drawing Plastic Film in a Tenter Frame,"
Hommes et al., the entire contents of which are incorporated
herein. Tenter apparatus 10 includes a first side rail 12 and a
second side rail 14 on which the driven clips 22 and idler clips 24
ride. In FIG. 1, the driven clips 22 are illustrated schematically
as boxes marked "X" while the idler clips 24 are illustrated
schematically as open boxes. Between pairs of driven clips 22 on a
given rail, there are one or more idler clips 24. As illustrated,
there are two idler clips 24 between each pair of clips 22 on a
given rail. One set of clips 22, 24 travels in a closed loop about
first rail 12 in the direction indicated by the arrows at the ends
of the rail. Similarly, another set of clips 22, 24 travels in a
closed loop about second rail 14 in the direction indicated by the
arrows at the ends of the rail. The clips 22, 24 hold the film
edges and propel film 26 in the direction shown by the arrow at the
center of the film. At the ends of the rails 12, 14, the clips 22,
24 release the film 26. The clips then return along the outside of
the rails to the entrance of the tenter to grip the cast web to
propel it through the tenter. (For clarity of illustration, the
clips returning to the entrance on the outside of the rails have
been omitted from FIG. 1.) The stretched film 26 exiting the tenter
may be wound up for later processing or use, or may be further
processed before winding.
[0040] The polymer can be cast into sheet form as is known in the
art, to prepare a web suitable for stretching to arrive at the
preferred film described herein. The web can be a homopolymer,
copolymer, blend, monolayer, or multilayer, as is known in the art.
When making polypropylene films, a suitable method for casting a
web is to feed the resin into the feed hopper of a single screw,
twin screw, cascade, or other extruder system having extruder
barrel temperatures adjusted to produce a stable homogeneous melt.
The polypropylene melt can be extruded through a sheet die onto a
rotating cooled metal casting wheel. Optionally, the casting wheel
can be partially immersed in a fluid-filled cooling bath, or, also
optionally, the cast web can be passed through a fluid-filled
cooling bath after removal from the casting wheel. The web is then
biaxially stretched according to the preferred methods described
herein. The extruded web is typically quenched, optionally
re-heated by passing through an infrared heater, and fed to the
clips 22, 24 on the first and second rails 12, 14, to be propelled
through the tenter apparatus 10. The optional infrared heating and
the gripping by the clips 22, 24 may occur in any order or
simultaneously.
[0041] The rails 12, 14 pass through three sections: preheat
section 16; stretch section 18; and post-stretch treatment section
20. In the preheat section 16, the film is heated to within an
appropriate temperature range to allow a significant amount of
stretching without breaking. The three functional sections 16, 18,
20 may be broken down further into zones. For example, in one
preferred embodiment of a tenter, the preheat section 16 includes
zones Z1, Z2, and Z3, the stretch section 18 includes zones Z4, Z5,
and Z6, and the post-stretch treatment section 20 includes zones
Z7, Z8, and Z9. It is understood that the preheat, stretch, and
post-treatment sections may each include fewer or more zones than
illustrated. Further, within the stretch section 18, the TD
component of stretch or the MD component of stretch may be
performed in the same or in different zones. For example, MD and TD
stretch each may occur in any one, two or three of the zones Z4,
Z5, and Z6. Further, one component of stretch may occur before the
other, or may begin before the other and overlap the other. Still
further, either component of stretch may occur in more than one
discrete step. For example, MD stretch may occur in Z4 and Z6
without any MD stretch occurring in Z5.
[0042] Some stretching in the MD and/or TD may also occur in the
preheat section or post-stretch treatment section. For example, in
the embodiment illustrated, stretching may begin in Zone 3.
Stretching may continue into Zone 7 or beyond. Stretching may
resume in any of the Zones after Zones Z4, Z5, or Z6.
[0043] In one preferred stretch profile, the film is stretched to
an MD stretch ratio of at least 2:1 and a TD stretch ratio of at
least 2:1. The final stretch ratios may be selected to provide
films having desired characteristics and properties.
[0044] In one preferred stretch profile, simultaneous biaxial
stretching occurs in the stretch section 18. For example, TD
stretch occurs throughout zones Z4, Z5 and Z6. For this to occur,
the first and second rails 12, 14 are configured to diverge through
each of these zones. In this stretch profile, MD stretch preferably
occurs only in zone Z4. For this to occur, the driven clips 22 are
accelerated through zone Z4 so as to induce MD stretch, and then
the spacing of the driven clips 22 is maintained substantially
constant in the MD through zones Z5 and Z6. In another preferred
stretch profile, MD stretch occurs in zones Z4 and Z5, while TD
stretch occurs in zones Z4, Z5, Z6. In yet another preferred
stretch profile, both MD and TD stretch occur in zones Z4, Z5, and
Z6.
[0045] In another preferred stretch profile, sequential biaxial
stretching occurs. For MD stretch to precede TD, the rails 12, 14
can remain parallel in zone Z4 while the driven clips 22 accelerate
in the MD. The rails 12, 14 then diverge in either or both of zones
Z5 and Z6 for TD stretch while the MD spacing of the driven clips
22 remains substantially constant in these zones. For TD to precede
MD, the rails 12, 14 diverge initially with no or little MD
stretch, and then remain parallel while MD stretch occurs.
[0046] Usually the film 26 is then propelled through the
post-stretch treatment section 20. In this section, the film 26
typically is maintained at a desired temperature while no
significant stretching occurs. This treatment is often referred to
as a heat set or anneal, and is performed to improve the properties
of the final film, such as the dimensional stability.
Alternatively, a small amount of relaxation in either or both of
the MD and TD may occur in the post-stretch treatment section 20.
Relaxation here refers to a convergence of the rails in the TD
and/or a convergence of the driven clips on each rail in the
MD.
[0047] Biaxial stretching of films is sensitive to many process
conditions, including but not limited to the composition of the
resin, film casting and quenching parameters, the time-temperature
history while preheating the film prior to stretching, the
stretching temperature used, the stretch profile used, and the
rates of stretching. With the benefits of the teachings herein, one
of skill in the art may adjust any or all of the parameters and
thereby obtain films having desired properties and
characteristics.
[0048] Some preferred stretching conditions are as follows for
polypropylene film. Cast web thickness is preferably from about 0.2
to 12 mm, more preferably from about 1 to 3 mm. The temperature of
the IR heat source is high enough to impart the desired pre-heating
to the cast web. The air temperature in the preheat section 16 is
preferably about 170 to 220.degree. C. The air temperature in the
stretch section 18 and post-stretch treatment section 20 is
preferably about 150 to 170.degree. C. The film itself in stretch
section 18 is preferably approximately 120 to 165.degree. C. to
allow significant stretching to occur without breaking, more
preferably approximately 150 to 165.degree. C. For polypropylene,
final area stretch ratio is at least 16:1; more preferably from
about 25:1 to 100:1. The MD stretch ratio and TD stretch ratio are
selected as desired, and may or may not be equal to each other.
[0049] The cooling of the present invention, whether edge cooling
or zone cooling, may begin before or after the onset of stretching
in the stretch section 18. If cooling begins before the onset of
stretching, it should continue after the onset of stretching into
the stretch section 18. As used herein, including the claims, the
phrase, "imparting a machine direction cooling gradient to at least
a portion of the width of the stretched film" means imparting a
temperature gradient such that the film is cooler at the forward
side of the cooled film portion and warmer at the rearward side of
the cooled film portion. "Forward" means the direction of film
travel in the machine direction and "rearward" is opposite to the
direction of film travel in the machine direction. By stating that
the gradient is applied to at least a portion of the "stretched
film," this means the gradient is present after stretching begins.
The gradient may in addition be present prior to the onset of
stretching provided the gradient continues to be imparted, or is
re-imposed, after stretching begins. The gradient may be imparted
to the stretched film at any location of the stretch section and/or
just after the stretch section. Preferably, the cooling, and
therefore the gradient, begins at, or continues at least until, the
end of the stretch section 18 or the beginning of the post-stretch
treatment section 20. In one preferred embodiment, the cooling
occurs at the latter portion of the stretch section 18 and in the
beginning of the post-stretch section 20. This would be, for
example, in zones Z6 and Z7 for the embodiment illustrated in FIG.
1. In another preferred embodiment, cooling occurs at the latter
portion of the stretch section 18. For example, cooling can occur
in either or both of zones Z5 and Z6 in the apparatus illustrated
in FIG. 1, or in the second half of zone Z4 and throughout zones Z5
and Z6. In another preferred embodiment, cooling occurs throughout
the stretch section 18, for example in zones Z4, Z5, and Z6 of the
tenter of FIG. 1. In another preferred embodiment, cooling can
occur at the beginning of the post-stretch section 20, such as in
either or both of zones Z7 and Z8. If the MD stretching and TD
stretching zones do not coincide with one another, then in one
preferred embodiment, cooling occurs in both the MD and the TD
stretching zones. In another preferred embodiment, cooling occurs
at the MD stretching zones only.
[0050] Cooling is provided to at least a portion of the width of
the film 26. Preferably, cooling is provided by actively cooling
either: i) the edge portions 28 of the film in a zone or zones; or
ii) the full width, including the edge portions 28 and the center
portion 30, of the film in a zone or zones. In one preferred
stretch profile, at the onset of stretching, the edge portions of
the film are maintained no hotter than the center portion of the
film. This may be continued throughout the stretching process.
[0051] Preferably, cooling is provided by forced air convection.
The cooling air must be cooler than the temperature of the film at
the location the air is provided. Preferably, the cooling air is
provided at a temperature and flow rate effective to cool the film
by at least 3.degree. C., more preferably 5.degree. C., and still
more preferably 10.degree. C. The difference of the temperature of
the cooling air and that of the film to be cooled is called the air
temperature differential and should be at least 5.degree. C., and
may be significantly greater. The difference of the temperature of
the film with and without cooling is called the target film
temperature differential. Usually, due to the nature of heat
transfer, the edge air or zone air temperature differential is
greater than the target film temperature differential. The cooling
imparts a temperature drop in the film in the machine direction
such that, when viewed from a location upon the film, the film is
cooler in the direction of film travel than in the direction
opposite film travel. The preferred temperature of the cooling air
will depend on factors such as film temperature, thickness, speed,
and heat transfer characteristics of the tenter. The temperature
and location of the cooling air can be selected by one of skill in
the art with reference to the teachings of the present invention to
obtain the desired improvements disclosed herein.
[0052] The cooling is provided at a location and temperature
effective to improve uniformity of the spacing of the idler clips
and driven clips compared to the spacing obtained at otherwise
identical conditions without such cooling. Spacing uniformity is
determined as follows. The spacing between the clips can be
determined, for example, by measurements on the stretched film 26.
The ideal clip spacing is defined, for a system with two idler
clips between each pair of driven clips on each rail, as one-third
of the spacing between successive driven clips D.sub.1
(forward--toward the tenter exit) and D.sub.2 (rearward--toward the
tenter entrance). If there are N idler clips between driven clips
D.sub.1 and D.sub.2, each nearest-neighbor pair of clips,
D.sub.1-I.sub.1, I.sub.1-I.sub.2, . . . through I.sub.N-D.sub.2,
should have an ideal spacing of 1/(N+1) of the distance D1-D2. A
numerical value for the non-uniformity of the spacing can be
obtained by measuring the actual pairwise spacings obtained,
subtracting from the measured spacing of each nearest-neighbor pair
the ideal spacing, taking the absolute value of each difference,
and summing. Ideal spacing, therefore, will give a value for
spacing non-uniformity of zero. Larger values represent increasing
spacing non-uniformity. An improvement in spacing uniformity will
manifest as a decrease in the value of the spacing non-uniformity.
Preferably, spacing non-uniformity is decreased by at least 5% of
what it would have been without the cooling. More preferably,
non-uniformity is decreased by at least 10%, and still more
preferably by at least 50%. Alternatively, cooling is provided at a
location and temperature effective to provide that the clip spacing
of each nearest-neighbor pair is within 20% of ideal, more
preferably within 10% of ideal, and most preferably within 5% of
ideal. In one preferred embodiment using polypropylene, when the
tenter temperature is set to approximately 160 to 165.degree. C.,
cooling air for edge cooling is approximately 30 to 140.degree. C.,
more preferably about 65 to 120.degree. C., and still more
preferably about 70 to 110.degree. C. In one preferred zone cooling
embodiment using polypropylene, when the tenter temperature is set
to approximately 160 to 165.degree. C., cooling air is
approximately 100 to 150.degree. C., more preferably about 120 to
140.degree. C., and still more preferably about 125 to 130.degree.
C. With the benefits of the teachings herein, one of skill in the
art can select edge cooling and zone cooling parameters for other
materials, thicknesses, film speeds, tenter temperatures, and other
stretch profiles.
[0053] In another preferred stretch profile, cooling is provided to
at least a portion of the width of the film in an effective amount
to reduce the value of idler clip lag from the value of idler clip
lag obtained at otherwise identical conditions in the absence of
said cooling. Clip lag values are determined as follows. The
spacing between the clips can be determined, for example, by
measurements on the stretched film 26. The ideal clip spacing is
defined, for a system with two idler clips between each pair of
driven clips on each rail, as one-third of the spacing between
successive driven clips D.sub.1 (forward--toward the tenter exit)
and D.sub.2 (rearward--toward the tenter entrance). Idler clip
I.sub.1 is the forward of the two idler clips between driven clips,
and idler clip I.sub.2 is the rearward of the two. The values for
pairs D.sub.1-I.sub.1, I.sub.1-I.sub.2, and I.sub.2-D.sub.2, as
percent variations in spacing from ideal (with respect to the
ideal) are calculated, with positive numbers indicating spacings
farther than ideal, and negative numbers indicating spacings closer
than ideal. D.sub.1-I.sub.1 indicates the percent spacing variation
from ideal between the forward driven and forward idler clips,
I.sub.1-I.sub.2 is the percent spacing variation from ideal between
idler clips, and I.sub.2-D.sub.2 the spacing variation from ideal
between the rear idler clip and the rear driven clip. The total
clip lag value reported is calculated as the percent variation from
ideal spacing for D.sub.1-I.sub.1, minus the percent variation from
ideal for I.sub.2-D.sub.2. This calculation can be extended to
cases with differing numbers of idler clips between each pair of
driven clips. For the case of only one idler clip between each pair
of driven clips, I.sub.1 equals I.sub.2, and the calculation
outlined above may proceed on that basis. For the case of N>2
idler clips, I.sub.2 in the formulation above becomes I.sub.N, and
the calculation may proceed on that basis. Spacings between any two
idler clips are ignored in the calculation of idler clip lag
regardless of the number of idlers present.
[0054] Preferably, idler clip lag is decreased by at least 5% of
what it would have been at otherwise identical conditions without
the cooling. More preferably, idler clip lag is decreased by at
least 10%, and still more preferably by at least 50%.
Alternatively, cooling is provided at a location and temperature
effective to provide that the value of idler clip lag is less than
about 20%, more preferably less than about 10%, and most preferably
less than about 5%.
[0055] A negative value for clip lag, thus defined, is indicative
of clip lead. Preferably, clip lag approaches zero. In some cases,
it may be preferable to impart clip lead. As used herein, including
the claims, the phrase, "reduce the value of idler clip lag" is
meant to indicate that the value will be made either a smaller
positive number, zero, or any negative number (clip lead). To
denote specifically an approach toward the ideal (uniform) clip
separation, the phrase "reduce the absolute value of idler clip
lag" will be used.
[0056] In another preferred stretch profile, cooling is provided to
at least a portion of the width of the film in an effective amount
to improve the caliper uniformity relative to the caliper
uniformity obtained at otherwise identical conditions in the
absence of the cooling. Caliper uniformity may be measured either
across the web, e.g. from clip face to clip face, or down the web,
e.g. along the direction of film travel. Either or both of the
crossweb and downweb caliper uniformity may be improved. The
non-uniformity may be characterized by the standard deviation from
the mean of a caliper scan along a given direction. Alternatively,
the maximum peak to valley height of a caliper scan along a given
direction may be used. A perfectly uniform film would have a
non-uniformity of zero. A variety of caliper measuring techniques
may be used. Typically, the higher the resolution, the better. A
preferred measurement technique is to cut crossweb or downweb
strips and then scan the caliper using a PC 5000 Electronic
Thickness Gauge available from Electro-Gauge Inc., located in Eden
Prairie, Minn., USA. Crossweb uniformity may also be characterized
by comparing a series of downweb-cut strips cut along "lanes"
differing in crossweb position.
[0057] FIG. 2 presents such a pair of caliper scans. The marks on
the MD Position axis represent the positions of the driven clips
relative to the film samples. The data of FIG. 2 is taken from a
film made in a process with two idler clips between each pair of
driven clips according to Example 11 below. The edge lane (plot
E.sub.11) was located about 16% of the way across the film from a
clip face whereas the center lane (plot C.sub.11) was 50% of the
way across the film (centered). Total clip lag was measured as 58%.
FIG. 2 shows that there is a relationship between caliper
non-uniformity and clip lag. The caliper non-uniformity is periodic
with a "wavelength" roughly equal to the final separation of the
driven clips. FIG. 2 also shows that the magnitude of caliper
non-uniformity decreases from the edge of the film near the clips
towards the center of the film. A downweb strip cut along a lane
near the edge has higher non-uniformity than a downweb strip cut
along a lane near the center, though the periodic nature of the
caliper fluctuation remains. Increasing the initial web width may
increase the width of a central portion with relatively low
non-uniformity; nevertheless, clip lagging will occur in films
having lower yield (the fraction of the width which is usable
width).
[0058] FIG. 3 shows that the non-uniformity decreases for center
lanes with decreasing clip lagging. Therefore, reduced clip
lagging, or variation from the ideal clip spacing, is observed in
more uniform films and/or in films in which a larger portion of the
width has good uniformity, thereby increasing the yield for a given
caliper uniformity specification. Caliper traces shown represent
58% lagging (plot C.sub.11, of Example 11 below) and less than 2%
lagging (plot C.sub.10, of Example 10 below). Plot C.sub.10 of
Example 10 does not show the same periodicity based on driven-clip
separations. Clip position does not correlate strongly with caliper
non-uniformity in this example with low values of clip lag and
caliper non-uniformity.
[0059] It will be readily appreciated that idler clip lagging, or
any non-uniformity of clip spacing, occurs when there is downweb
caliper non-uniformity. Typical polymeric films drawn above the
glass transition temperature are nearly volume preserving, except
through voiding or via densification due to crystallization, so
that the decrease in thickness is approximately proportional to the
product of the local principal draw ratios, e.g. the local crossweb
and downweb draw ratios. The present invention also recognizes the
link between caliper and draw ratio nonuniformities and
non-uniformity of other properties both crossweb and downweb. These
physical, mechanical and optical properties include but are not
limited to elastic moduli, tensile strength, elongation at break,
energy-to-break per unit volume and other tear and dispensing
properties, surface characteristics, interlayer adhesion in
multilayer films, coefficients of thermal and hygroscopic
expansion, heat shrinkage, refractive indices, capacitance and
other dielectric properties, haze, transparency, color, spectral
band edges, and other optical measures of appearance and
performance. By dispensing properties is meant the properties
relating to the ease of severing and the quality of severed edge
when a film, converted into the form of a tape, is dispensed using
a dispenser having a cutting edge. The level of non-uniformity of
these various properties may be related to the caliper fluctuations
and clip lag, for example, through differing sensitivities of these
properties to the local caliper and local draw ratios. Thus,
lagging is symptomatic of a downweb draw ratio fluctuation which
causes both a downweb caliper fluctuation and a downweb modulus of
elasticity fluctuation. Caliper may fluctuate differently than
modulus because of corresponding partial compensation of the
thickness by concomitant crossweb draw ratio fluctuations under
certain conditions as well as the nonlinear relationship between
moduli and draw ratios.
[0060] Although the present invention is described herein with
particular applicability to methods of biaxially stretching films
and to resulting biaxially stretched films, the present invention
may also be applied advantageously to methods of stretching films
in a single direction under conditions in which the film is held by
clips, and the clips are separated along the machine direction, and
thus capable of producing idler clip lagging or leading. In one
such method, the film is stretched solely along the machine
direction thus separating the clips along the machine direction and
creating the possibility of clip lagging. In another example, the
clips begin the draw with some MD separation, and then stretching
in the transverse direction, for example, may create non-uniformity
in the MD clip spacing.
[0061] The methods of stretching with appropriate cooling described
herein are well suited for use on films including a polymeric film.
Preferably, the film comprises a thermoplastic polymer. For a film
having more than one layer, the description of suitable materials
which follows need apply only to one of the layers. Suitable
polymeric film materials for use in the current invention include
thermoplastics capable of being formed into biaxially oriented
films. Suitable thermoplastic polymer film materials include, but
are not limited to, polyesters, polycarbonates, polyarylates,
polyamides, polyimides, polyamide-imides, polyether-amides,
polyetherimides, polyaryl ethers, polyarylether ketones, aliphatic
polyketones, polyphenylene sulfide, polysulfones, polystyrenes and
their derivatives, polyacrylates, polymethacrylates, cellulose
derivatives, polyethylenes, aliphatic and cycloaliphatic
polyolefins, copolymers having a predominant olefin monomer,
fluorinated polymers and copolymers, chlorinated polymers,
polyacrylonitrile, polyvinylacetate, polyvinylalcohol, polyethers,
ionomeric resins, elastomers, silicone resins, epoxy resins, and
polyurethanes. Miscible or immiscible polymer blends including any
of the above-named polymers, and copolymers having any of the
constituent monomers of any of the above-named polymers, are also
suitable, provided a biaxially oriented film may be produced from
such a blend or copolymer.
[0062] Preferred among thermoplastics are the vinyl polymers, by
which is meant all polymers of the general formula
--[CWX--CYZ].sub.n--, where W, X, Y, and Z are either hydrogen (H)
or any substituent atoms or groups. Thus within the preferred vinyl
polymer class we include the tetrasubstituted, trisubstituted,
1,2-disubstituted and 1,1-disubstituted polymers (including the
"vinylidene" polymers) as well as the more common monosubstituted
vinyl polymers. Examples include the polyolefins, polyvinyl
chloride, polyvinyl fluoride, polyvinylidene chloride,
polyvinylidene fluoride, polytrifluoroethylene,
polychlorotrifluoroethylene, polyvinyl acetate, polyvinyl alcohol,
polyacrylic acid and its esters, polyacrylonitrile, and
polymethacrylic acid and its esters (such as polymethyl
methacrylate).
[0063] More preferred are the polyolefins, by which is meant all
polymers of the general formula
--[CH.sub.2CR.sup.1R.sup.2].sub.n--, where R.sup.1 and R.sup.2 are
saturated or unsaturated, linear or branched alkyl, cycloalkyl, or
aryl groups, or hydrogen. Included are such polymers as
polyethylenes, polypropylenes, polybutene-1,
poly-(4-methylpentene-1), polyisobutene, poly-(vinylcyclohexane),
polybutadienes, and polystyrene and its ring- and alpha-substituted
derivatives.
[0064] Still more preferred are polyethylene and the saturated
alkyl or cycloalkyl polyolefins. Polypropylene is most
preferred.
[0065] The methods of stretching with appropriate cooling described
herein are well suited for use on films including amorphous or
semi-crystalline thermoplastic polymeric films. Amorphous
thermoplastics include, but are not limited to, polymethacrylates,
polycarbonates, atactic polyolefins and random copolymers.
Semi-crystalline thermoplastics include, but are not limited to,
polyesters, polyamides, thermoplastic polyimides, polyarylether
ketones, aliphatic polyketones, polyphenylene sulfide, isotactic or
syndiotactic polystyrene and their derivatives, polyacrylates,
polymethacrylates, cellulose derivatives, polyethylene,
polyolefins, fluorinated polymers and copolymers, polyvinylidene
chloride, polyacrylonitrile, polyvinylacetate, and polyethers.
[0066] Semicrystalline thermoplastics from which biaxially oriented
films may be produced are sometimes characterized in terms of their
degree of crystallinity at various stages in the film-making
process. Thus, polyethylene terephthalate (PET), a common polymer
for biaxially oriented film, is well-known to be quenchable when
cast into a film. That is, PET crystallizes slowly enough that it
can be extruded onto a chilled roll and thereby cooled below its
glass transition temperature sufficiently quickly to prevent the
formation of measurable amounts of crystallinity. It is well known
that such quenching is advantageous for the production of biaxially
oriented PET film, both because it enables the stretching step(s)
to take place at temperatures only slightly above the glass
transition, and because it allows a significant amount of
stretching without breaking, which breaking is prevalent if a more
brittle semicrystalline cast web is allowed to form.
[0067] The degree of crystallinity of a semicrystalline polymer
film is difficult to precisely quantify, as it depends not only on
the assumption of a two-phase model (crystalline and amorphous) for
polymer morphology which may or may not be precisely accurate, but
also on the assumption of the constancy of some measurable property
(density, for example) for each phase regardless of such variables
as degree of orientation. Different measurement techniques
frequently provide different results due to the inadequacies of
these assumptions. Thus, precise agreement among workers is not to
be expected, especially where different techniques have been
employed. Techniques well-known in the art for estimating the
degree of crystallinity include density, differential scanning
calorimetry (DSC), average refractive index (through its
relationship to the density), analysis of infrared bands, and X-ray
methods.
[0068] Usually, the degree of crystallinity of PET in the form of
unstretched cast film is reported to be undetectably low, or 0%, or
below 1%. This is typically referred to as an amorphous cast web.
In a simultaneous biaxial orientation process, film of this low
degree of crystallinity would be fed to the tenter. In the more
commonly employed sequential process, however, such an amorphous
film is first stretched in the machine direction using heated rolls
rotating at different speeds. Such "length orientation" imparts
some crystallinity to the film, the degree of which has been
reported at anywhere from 7% to 30%. See LeBourvellec and
Beautemps, J. Appl. Polym. Sci. 39, 329-39 (1990); and Faisant de
Champchesnel, et al., Polymer 35(19), 4092-4102 (1994). Typical
values in commercial practice range from 10-20%. See Encycl. Of
Polym. Sci. & Engrg., vol. 12, Wiley (NY) 1988, pg. 197. In a
sequential process, it is film of this degree of crystallinity
which would be fed to the tenter. Transverse direction stretching
in the tenter has been reported to increase the degree of
crystallinity to within the range of 17% to 40%. Subsequent
heat-setting or annealing under transverse constraint in the tenter
is reported to further increase the degree of crystallinity to
about 45% to 50%. The breadth of the range reported for pre-heatset
film is due both to the range of crystallinities of the
length-oriented films provided as input to that step of the
process, and to the experimental difficulty of decoupling the
transverse direction stretching step from the heat-setting step,
both of which occur within the tenter oven. Considerably less is
known regarding the behavior of PET in a simultaneous biaxial
orientation process, but the available data places the degrees of
crystallinity after stretching and after heat-setting in the same
ranges as those for the sequential process after TD stretching and
after heat-setting.
[0069] Another polyester suitable for use with the present
invention is polyethylene naphthalate (PEN). PEN is known to
crystallize somewhat more slowly than PET. Nonetheless, reports of
its behavior in tenter-film processes place the degrees of
crystallinity at the end of each process step in roughly the same
ranges as those reported for PET. Thus, when processed
conventionally, PEN too is an example of an amorphous cast web.
[0070] In contrast to the polyesters, polypropylene (PP)
crystallizes so rapidly that it is almost impossible to quench the
molten polymer to less than 50% crystallinity with any practical
commercial method. See The Science and Technology of Polymer Films,
Vol. II, by Orville J. Sweeting, Wiley (NY), 1971, pg. 223. As a
result, PP is stretched at temperatures just below the crystalline
melting point, rather than at temperature just above the glass
transition as is the case for the polyesters. Some additional
crystallinity develops during the process, but the amount is small.
One comprehensive study found the degrees of crystallinity of PP
cast (unstretched) film, length-oriented film, and sequentially
biaxially oriented film to be 58%, 62% and 70%, respectively. See
A. J. deVries, Pure Appl. Chem. 53, 1011-1037 (1981). The
Encyclopedia of Polym. Sci. & Engrg., Vol. 7, Wiley (NY), 1987,
pg. 80, reports the degree of crystallinity of typical biaxially
oriented PP films at 65-70%.
[0071] The methods of stretching with appropriate cooling described
herein are well suited for use on films including semicrystalline
thermoplastic polymer films. Preferred semicrystalline
thermoplastic polymers are those which can undergo a significant
amount of stretching without breaking when the film entering the
tenter inlet has a degree of crystallinity greater than about 1%.
Such films are referred to herein as pre-crystallized polymeric
films. More preferred semicrystalline thermoplastic polymers are
those which can be effectively biaxially stretched without breaking
when the film entering the tenter inlet has a degree of
crystallinity greater than about 7%. Still more preferred
semicrystalline thermoplastic polymers are those which can be
effectively biaxially stretched without breaking when the film
entering the tenter inlet has a degree of crystallinity greater
than about 30%. Even more preferred semicrystalline thermoplastic
polymers are those which can be effectively biaxially stretched
without breaking when the film entering the tenter inlet has a
degree of crystallinity greater than about 50%. Polypropylene is
most preferred.
[0072] For the purposes of the present invention, the term
"polypropylene" is meant to include copolymers having at least
about 90% propylene monomer units, by weight. "Polypropylene" is
also meant to include polymer mixtures having at least about 65%
polypropylene, by weight. Polypropylene for use in the present
invention is preferably predominantly isotactic. Isotactic
polypropylene has a chain isotacticity index of at least about 80%,
an n-heptane soluble content of less than about 15% by weight, and
a density between about 0.86 and 0.92 grams/cm.sup.3 measured
according to ASTM D1505-96 ("Density of Plastics by the
Density-Gradient Technique"). Typical polypropylenes for use in the
present invention have a melt flow index between about 0.1 and 15
grams/ten minutes according to ASTM D1238-95 ("Flow Rates of
Thermoplastics by Extrusion Plastometer") at a temperature of
230.degree. C. and force of 21.6 N, a weight-average molecular
weight between about 100,000 and 400,000, and a polydispersity
index between about 2 and 15. Typical polypropylenes for use in the
present invention have a melting point as determined using
differential scanning calorimetry of greater than about 130.degree.
C., preferably greater than about 140.degree. C., and most
preferably greater than about 150.degree. C. Further, the
polypropylenes useful in this invention may be copolymers,
terpolymers, quaterpolymers, etc., having ethylene monomer units
and/or alpha-olefin monomer units having between 4-8 carbon atoms,
said comonomer(s) content being less than 10% by weight. Other
suitable comonomers include, but are not limited to, 1-decene,
1-dodecene, vinylcyclohexene, styrene, allylbenzene, cyclopentene,
norbornene, and 5-methylnorbornene. One suitable polypropylene
resin is an isotactic polypropylene homopolymer resin having a melt
flow index of 2.5 g/10 minutes, commercially available under the
product designation 3374 from FINA Oil and Chemical Co., Dallas,
Tex. Recycled or reprocessed polypropylene in the form, for
example, of scrap film or edge trimmings, may also be incorporated
into the polypropylene in amounts less than about 60% by
weight.
[0073] As already mentioned, mixtures having at least about 65%
isotactic polypropylene and at most about 35% of another polymer or
polymers may also be advantageously used in the process of the
present invention. Suitable additional polymers in such mixtures
include, but are not limited to, propylene copolymers,
polyethylenes, polyolefins having monomers having from four to
eight carbon atoms, and other polypropylene resins.
[0074] Polypropylene for use in the present invention may
optionally include 1-40% by weight of a resin, of synthetic or
natural origin, having a molecular weight between about 300 and
8000, and having a softening point between about 60.degree. C. and
180.degree. C. Typically, such a resin is chosen from one of four
main classes: petroleum resins, styrene resins, cyclopentadiene
resins, and terpene resins. Optionally, resins from any of these
classes may be partially or fully hydrogenated. Petroleum resins
typically have, as monomeric constituents, styrene, methylstyrene,
vinyltoluene, indene, methylindene, butadiene, isoprene,
piperylene, and/or pentylene. Styrene resins typically have, as
monomeric constituents, styrene, methylstyrene, vinyltoluene,
and/or butadiene. Cyclopentadiene resins typically have, as
monomeric constituents, cyclopentadiene and optionally other
monomers. Terpene resins typically have, as monomeric constituents,
pinene, alpha-pinene, dipentene, limonene, myrcene, and
camphene.
[0075] Polypropylene for use in the present invention may
optionally include additives and other components as is known in
the art. For example, the films of the present invention may
contain fillers, pigments and other colorants, antiblocking agents,
lubricants, plasticizers, processing aids, antistatic agents,
nucleating agents, antioxidants and heat stabilizing agents,
ultraviolet-light stabilizing agents, and other property modifiers.
Fillers and other additives are preferably added in an effective
amount selected so as not to adversely affect the properties
attained by the preferred embodiments described herein. Typically
such materials are added to a polymer before it is made into an
oriented film (e.g., in the polymer melt before extrusion into a
film). Organic fillers may include organic dyes and resins, as well
as organic fibers such as nylon and polyimide fibers, and
inclusions of other, optionally crosslinked, polymers such as
polyethylene, polyesters, polycarbonates, polystyrenes, polyamides,
halogenated polymers, polymethyl methacrylate, and cycloolefin
polymers. Inorganic fillers may include pigments, fumed silica and
other forms of silicon dioxide, silicates such as aluminum silicate
or magnesium silicate, kaolin, talc, sodium aluminum silicate,
potassium aluminum silicate, calcium carbonate, magnesium
carbonate, diatomaceous earth, gypsum, aluminum sulfate, barium
sulfate, calcium phosphate, aluminum oxide, titanium dioxide,
magnesium oxide, iron oxides, carbon fibers, carbon black,
graphite, glass beads, glass bubbles, mineral fibers, clay
particles, metal particles and the like. In some applications it
may be advantageous for voids to form around the filler particles
during the biaxial orientation process of the present invention.
Many of the organic and inorganic fillers may also be used
effectively as antiblocking agents. Alternatively, or in addition,
lubricants such as polydimethyl siloxane oils, metal soaps, waxes,
higher aliphatic esters, and higher aliphatic acid amides (such as
erucamide, oleamide, stearamide, and behenamide) may be
employed.
[0076] Antistatic agents may also be employed, including aliphatic
tertiary amines, glycerol monostearates, alkali metal
alkanesulfonates, ethoxylated or propoxylated
polydiorganosiloxanes, polyethylene glycol esters, polyethylene
glycol ethers, fatty acid esters, ethanol amides, mono- and
diglycerides, and ethoxylated fatty amines. Organic or inorganic
nucleating agents may also be incorporated, such as
dibenzylsorbitol or its derivatives, quinacridone and its
derivatives, metal salts of benzoic acid such as sodium benzoate,
sodium bis(4-tert-butyl-phenyl)phosphate, silica, talc, and
bentonite. Antioxidants and heat stabilizers, including phenolic
types (such as pentaerythrityl tetrakis
[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and
1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene),
and alkali and alkaline earth metal stearates and carbonates may
also be advantageously used. Other additives such as flame
retardants, ultraviolet-light stabilizers, compatibilizers,
antimicrobial agents (e.g., zinc oxide), electrical conductors, and
thermal conductors (e.g., aluminum oxide, boron nitride, aluminum
nitride, and nickel particles) may also be blended into the polymer
used to form the film.
[0077] Resulting films have desirably uniform properties and are
well suited for many applications. One preferred application for
the film of the present application is as a tape backing.
Preferably, the tape backing has a thickness in the range of about
0.020 to about 0.064 mm. The backing is coated with a layer of any
suitable adhesive as is known in the art. The backing may have an
optional release or low adhesion backsize layer as is known in the
art.
[0078] The adhesive may be any suitable adhesive as is known in the
art. Preferred adhesives are those activatable by pressure, heat or
combinations thereof. Suitable adhesives include those based on
acrylate, rubber resin, epoxies, urethanes or combinations thereof.
The adhesive may be applied by solution, water-based or hot-melt
coating methods. The adhesive can include hot melt-coated
formulations, transfer-coated formulations, solvent-coated
formulations, and latex formulations, as well as laminating,
thermally-activated, and water-activated adhesives and bonding
agents. Useful adhesives include pressure sensitive adhesives.
Pressure sensitive adhesives are well known to possess properties
including: aggressive and permanent tack, adherence with no more
than finger pressure, and sufficient ability to hold onto an
adherend. Examples of useful adhesives include those based on
general compositions of polyacrylate; polyvinyl ether; diene rubber
such as natural rubber, polyisoprene, and polybutadiene;
polyisobutylene; polychloroprene; butyl rubber;
butadiene-acrylonitrile polymer; thermoplastic elastomer; block
copolymers such as styrene-isoprene and styrene-isoprene-styrene
(SIS) block copolymers, ethylene-propylene-diene polymers, and
styrene-butadiene polymers; poly-alpha-olefin; amorphous
polyolefin; silicone; ethylene-containing copolymer such as
ethylene vinyl acetate, ethylacrylate, and ethyl methacrylate;
polyurethane; polyamide; epoxy; polyvinylpyrrolidone and
vinylpyrrolidone copolymers; polyesters; and mixtures or blends
(continuous or discontinuous phases) of the above. Additionally,
the adhesives can contain additives such as tackifiers,
plasticizers, fillers, antioxidants, stabilizers, pigments,
diffusing materials, curatives, fibers, filaments, and solvents.
Also, the adhesive optionally can be cured by any known method.
[0079] A general description of useful pressure sensitive adhesives
may be found in Encyclopedia of Polymer Science and Engineering,
Vol. 13, Wiley-Interscience Publishers (New York, 1988). Additional
description of useful pressure sensitive adhesives may be found in
Encyclopedia of Polymer Science and Technology, Vol. 1,
Interscience Publishers (New York, 1964).
[0080] The film for tape backing may be optionally treated by
exposure to flame or corona discharge or other surface treatments
including chemical priming to improve adhesion of subsequent
coating layers.
[0081] The operation of the present invention will be further
described with regard to the following detailed examples. These
examples are offered to further illustrate the various specific and
preferred embodiments and techniques. It should be understood,
however, that many variations and modifications may be made while
remaining within the scope of the present invention.
EXAMPLES 1-6
[0082] The following examples were prepared on a linear motor
tenter generally as described in the '225 patent discussed above,
which had two idler clips between each pair of driven clips. A
continuous polypropylene cast sheet (Fina 3374x, from Fina
Chemical, Houston, Tex.) was extruded at a thickness of
approximately 0.054 inches (1.4 mm) and a width of 9.6 inches (244
mm), and quenched on a chill roll/water bath system. The film was
passed between a set of infrared heaters (IR heater), then into a
linear motor tenter oven. The IR heat temperature, oven preheat
section temperatures (Zones 1-3), and stretch section temperatures
(Zones 4-6) are set forth in Table 1. Web temperature as measured
by an IR pyrometer at the entrance to Zone 4 at the beginning of
the stretch section is also reported in Table 1. For Examples 1-6,
the post-stretch treatment temperatures were as follows: Zone 7:
160.degree. C.; Zones 8 and 9: 165.degree. C. Also, in each of
these examples, the final stretch ratios were 7:1 in the MD and 7:1
in the TD. The Zones in which MD stretch occurred (4, 4-5, or 4-6)
are reported below. In each of these examples, TD stretch was
performed in Zones 4 through 6. All of these stretch profiles were
linear with respect to machine position, and include a 10% stretch
relaxation in both directions that occurred in Zones 8 and 9.
Examples 1-3 had edge cooling air turned off. Examples 4-6 included
edge cooling air, and otherwise correspond to Examples 1-3,
respectively. TABLE-US-00001 TABLE 1 Stretching Conditions Preheat
Section Temp. (C.) Stretch Section Temp. (C.) Web Temp. MD Stretch
Edge Cool Air Temp. (C.) Ex. IR Heat (C.) Z1 Z2 Z3 Z4 Z5 Z6 (C.)
Zones Z6 Z7 1 700 185 178 178 165 164 163 144.5 4 -- -- 2 740 192
186 176 162 161 160 -- 4-5 -- -- 3 680 193 185 180 166 164 163 148
4-6 -- -- 4 700 185 178 178 165 164 163 144.1 4 73.2 75.3 5 740 192
186 176 162 161 160 -- 4-5 140.5 85.1 6 680 193 185 180 166 164 163
148 4-6 74.4 77.8
[0083] The spacing between the clips was measured on the output
film and the idler clip lagging calculated and reported in Table 2.
The ideal idler clip spacing is defined as one-third of the spacing
between successive driven clips D.sub.1 (forward) and D.sub.2
(rearward). Idler clip I.sub.1 is the forward of the two idler
clips between driven clips, and idler clip I.sub.2 is the rearward
of the two. The values for D.sub.1-I.sub.1, I.sub.1-I.sub.2, and
I.sub.2-D.sub.2 in Table 2 are the percent variation in spacing
from ideal, with positive numbers indicating a spacing farther than
ideal, and negative numbers indicating spacing closer than ideal.
D.sub.1-I.sub.1 indicates the percent spacing variation between the
forward driven and forward idler clips, I.sub.1-I.sub.2 is the
percent spacing variation between idler clips, and I.sub.2-D.sub.2
the spacing variation between the rear idler clip and the rear
driven clip. The Total Lag reported is the percent variation from
ideal spacing of D.sub.1 to I.sub.1, minus the percent variation
from ideal for I.sub.2-D.sub.2. The effects of rounding cause some
of the values in the "Total" columns In Table 2 to deviate from the
differences of the D.sub.1-I.sub.1 and I.sub.2-D.sub.2 columns by
one unit in the last decimal place. All values are reported for
both the set of clips on the First Side of the tenter and on the
opposite Second Side of the tenter. TABLE-US-00002 TABLE 2 First
Side Second Side Ex. D.sub.1-I.sub.1 I.sub.1-I.sub.2
I.sub.2-D.sub.2 Total D.sub.1-I.sub.1 I.sub.1-I.sub.2
I.sub.2-D.sub.2 Total 1 3.5 -1.0 -2.5 6.1 2.1 -1.1 -1.0 3.1 2 12.8
-1.7 -11.1 23.9 8.6 -1.5 -7.1 15.7 3 5.3 -0.3 -5.0 10.3 3.7 0.0
-3.7 7.5 4 2.7 -0.7 -2.1 4.8 0.4 -0.5 0.1 0.3 5 11.0 -1.4 -9.5 20.5
6.4 -1.0 -5.4 11.8 6 -1.7 -1.0 2.7 -4.4 -2.5 -0.9 3.4 -5.9
[0084] From the results presented in Table 2, it can be seen that
idler clip lagging in Example 1 of 6.1 on one side and 3.1 on the
other side can be reduced to 4.8 and 0.3 respectively, with the
addition of edge cooling in Example 4. Furthermore, idler clip
lagging in Example 2 of 23.9 and 15.7, can be reduced to 20.5 and
11.8, respectively, with the addition of edge cooling in Example 5.
Also, idler clip lagging of 10.3 and 7.5 of Example 3 can be
changed to idler clip lead of -4.4 and -5.9 with the addition of
edge cooling in Example 6. The Examples also suggest that, if idler
clip lag can be reduced (examples 4 and 5), or changed to idler
clip lead (example 6), a set of edge cooling conditions can be
found which would lead to ideal idler clip spacing.
EXAMPLES 7-10
[0085] The following examples were prepared on a linear motor
tenter generally as described in the '225 patent discussed above,
which had two idler clips between each pair of driven clips. A
continuous polypropylene cast sheet (Fina 3374x, from Fina
Chemical, Houston, Texas) was extruded at a thickness of
approximately 0.054 inches (1.36 mm) and 13.8 inches wide (350 mm),
and quenched on a chill roll/water bath system. The film was passed
between a set of infrared heaters (IR heater), then into a linear
motor tenter oven. For examples 7-10, the IR heat temperature was
set at 500.degree. C., oven preheat zone temperatures (Zones 1-3)
were set at 207.degree. C., 205.degree. C., and 193 .degree. C.
respectively, and the stretch zone temperatures (Zones 4-5) were
set at 160.degree. C. and 155.degree. C. respectively. The
relaxation (Zone 6) and the post-stretch treatment (Zones 7-9)
temperatures were set as listed in Table 3. In each of these
examples, the final stretch ratios were 6.3:1 in the MD and 6.3:1
in the TD. The MD and TD stretches were performed simultaneously in
Zones 4 and 5. All of these stretch profiles were linear with
respect to machine position, and include a 10% stretch relaxation
in both MD and TD that occurred in Zone 6.
EXAMPLE 7
[0086] Example 7 included cooling air in Zone 6 that was 5.degree.
C. cooler than the temperature of Zone 5.
EXAMPLE 8
[0087] Example 8 was prepared according to Example 7, with the
exception of the use of 15.degree. C. cooling in Zone 6.
EXAMPLE 9
[0088] Example 9 was prepared according to Example 7, with the
exception of the use of 20.degree. C. cooling in Zone 6.
EXAMPLE 10
[0089] Example 10 was prepared according to Example 7, with the
exception of the use of 25.degree. C. cooling in Zone 6 and an
additional 5.degree. C. in zone 7. TABLE-US-00003 TABLE 3
Stretching Conditions Relax Zone Temp.(.degree. C.) Anneal Zone
Temperature (.degree. C.) Example Zone 6 Zone 7 Zone 8 Zone 9 7 150
150 140 130 8 140 140 140 130 9 135 135 135 130 10 130 125 125
125
[0090] The spacing between the clips was measured on the output
film and the idler clip lagging calculated and reported in Table 4
as discussed earlier. All values are reported for both the sets of
clips on the First Side of the tenter and on the opposite Second
Side of the tenter. TABLE-US-00004 TABLE 4 First Side Second Side
Ex. D.sub.1-I.sub.1 I.sub.1-I.sub.2 I.sub.2-D.sub.2 Total
D.sub.1-I.sub.1 I.sub.1-I.sub.2 I.sub.2-D.sub.2 Total 7 7.9 -1.4
-6.3 14.2 4.8 -1.1 -3.7 8.5 8 3.7 -1.6 -2.2 5.9 2.0 -1.6 -.4 2.4 9
0.6 0.6 -1.2 1.8 2.6 0.2 -1.6 4.1 10 1.3 -1.7 0.4 0.9 -0.7 -1.3 1.9
-2.6
[0091] From the results presented in Table 4, it can be seen that
idler clip lagging in Example 7 of 14.2 on one side and 8.5 on the
other side can be decreased with the addition of a sufficient
amount of zone cooling, after the onset of stretching, as shown in
Examples 8-10. In particular, as shown with Examples 9 and 10, the
amount of overall lagging is less than 5%.
EXAMPLE 11
[0092] Example 11 was prepared on a linear motor tenter generally
as described in the '225 patent discussed above, which had two
idler clips between each pair of driven clips. A continuous
polypropylene cast sheet (Fina 3374x, from Fina Chemical, Houston,
Tex.) was extruded at a thickness of approximately 0.055 inches
(1.39 mm) and 13.8 inches wide (350 mm), and quenched on a chill
roll/water bath system. The film was passed between a set of
infrared heaters (IR heater), then into a linear motor tenter oven.
The IR heat temperature was set at 600.degree. C., oven preheat
zone temperatures (Zones 1-3) were set at 184.degree. C.,
177.degree. C., and 156.degree. C. respectively, and the stretch
zone temperatures (Zones 4-5-6-7 were set at 152.degree. C.,
170.degree. C., 170.degree. C., and 170.degree. C. respectively.
The relaxation (Zone 8) and the post-stretch treatment (Zone 9)
temperatures were both set at 158C. In this example, the final
stretch ratios were 5.8:1 in the MD and 9.0:1 in the TD. The MD
stretch was performed in Zones 4 and 5 and the TD stretch was
performed in Zones 4 through 7. The stretch profile includes a 10%
stretch relaxation in both directions in Zone 8.
[0093] The tests and test results described above are intended
solely to be illustrative, rather than predictive, and variations
in the testing procedure can be expected to yield different
results.
[0094] The present invention has now been described with reference
to several embodiments thereof. The foregoing detailed description
and examples have been given for clarity of understanding only. No
unnecessary limitations are to be understood therefrom. All patents
and patent applications cited herein are hereby incorporated by
reference. It will be apparent to those skilled in the art that
many changes can be made in the embodiments described without
departing from the scope of the invention. Thus, the scope of the
present invention should not be limited to the exact details and
structures described herein, but rather by the structures described
by the language of the claims, and the equivalents of those
structures.
* * * * *