U.S. patent application number 10/053301 was filed with the patent office on 2002-07-25 for methods of stretching films and such films.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Ferguson, Anthony B., Hanschen, Thomas P., Jackson, Jeffery N., Merrill, William W., Roska, Fred J., Wong, Chiu Ping.
Application Number | 20020098372 10/053301 |
Document ID | / |
Family ID | 22708060 |
Filed Date | 2002-07-25 |
United States Patent
Application |
20020098372 |
Kind Code |
A1 |
Wong, Chiu Ping ; et
al. |
July 25, 2002 |
Methods of stretching films and such films
Abstract
A method of biaxially stretching a polymeric film along an
overbias stretch profile. The method comprises the steps of: )
imparting a sufficiently high temperature to the film to allow a
significant amount of biaxial stretch; and b) biaxial tenter
stretching the film to a final first direction stretch parameter
and a final second direction stretch parameter, wherein at least
75% of the final first direction stretch parameter is attained
before no more than 50% of the final second direction stretch
parameter is attained, and wherein the final first direction
stretch parameter is no greater than the final second direction
stretch parameter. An alternative method comprises a method of
biaxially stretching a polymeric film along an overbias stretch
profile. The method comprising the steps of: a) imparting a
sufficiently high temperature to the film to allow a significant
amount of biaxial stretch; and b) biaxial tenter stretching the
film according to a stretch profile to a final first direction
stretch parameter and a final second direction stretch parameter,
wherein the final first direction stretch parameter is no greater
than the final second direction stretch parameter. In such a
method: i) a straight line between the point defining zero stretch
parameter and the point defining the final first and second
direction stretch parameters represents a proportional stretch
profile and defines a proportional stretch area; and ii) the curve
representing the stretch profile between the point defining zero
stretch parameter and the point defining the final first and second
direction stretch parameters defines an area at least 1.4 times the
proportional stretch area.
Inventors: |
Wong, Chiu Ping; (Vadnais
Heights, MN) ; Hanschen, Thomas P.; (St. Paul,
MN) ; Ferguson, Anthony B.; (Lake Elmo, MN) ;
Merrill, William W.; (White Bear Lake, MN) ; Roska,
Fred J.; (Woodbury, MN) ; Jackson, Jeffery N.;
(Woodbury, MN) |
Correspondence
Address: |
3M Innovative Properties Company
Office of Intellectual Property Counsel
PO Box 33427
St. Paul
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
22708060 |
Appl. No.: |
10/053301 |
Filed: |
January 17, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10053301 |
Jan 17, 2002 |
|
|
|
09192059 |
Nov 13, 1998 |
|
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|
Current U.S.
Class: |
428/523 ;
528/502B |
Current CPC
Class: |
B29C 55/16 20130101;
C09J 2423/006 20130101; C08J 5/18 20130101; Y10T 428/2848 20150115;
C09J 7/243 20180101; C08J 2323/02 20130101; B29K 2023/12 20130101;
C08J 2323/10 20130101; Y10T 428/31938 20150401; Y10T 428/2839
20150115; B29C 55/12 20130101; Y10T 428/28 20150115 |
Class at
Publication: |
428/523 ;
528/502.00B |
International
Class: |
B32B 027/32 |
Claims
What is claimed is:
1. A method of biaxially stretching a polymeric film, the method
comprising the steps of: a) imparting a sufficiently high
temperature to the film to allow a significant amount of biaxial
stretch; and b) biaxial tenter stretching the film to a final first
direction stretch parameter and a final second direction stretch
parameter, wherein at least 75% of the final first direction
stretch parameter is attained before no more than 50% of the final
second direction stretch parameter is attained, and wherein the
final first direction stretch parameter is no greater than the
final second direction stretch parameter.
2. The method of claim 1, wherein step b) comprises biaxial tenter
stretching the film such that a substantial portion of the first
direction stretch and the second direction stretch is performed
simultaneously.
3. The method of claim 1, wherein at least 90% of the final first
direction stretch parameter is attained before no more than 50% of
the final second direction stretch parameter is attained.
4. The method of claim 1, wherein the first direction is the MD and
the second direction is the TD.
5. The method of claim 1, wherein the final first direction stretch
parameter is less than the natural stretch parameter for a
proportional stretch profile.
6. The method of claim 1, wherein the final first direction stretch
parameter is less than the uniaxial natural stretch parameter.
7. The method of claim 1, wherein the film comprises a
thermoplastic film.
8. The method of claim 7, wherein the film comprises a
semi-crystalline film.
9. The method of claim 8, wherein the film comprises a
polyolefin.
10. The method of claim 9, wherein the film comprises
polypropylene.
11. The method of claim 1, wherein step b) further comprises
grasping the film with a plurality of clips along the opposing
edges of the film and propelling the clips at varying speeds in the
machine direction along clip guide means that diverge in the
transverse direction.
12. The method of claim 1, wherein step b) further includes
stretching the film to more than 100% of the final first direction
stretch parameter before no more than 50% of the final second
direction stretch parameter is attained, and thereafter retracting
the film in the machine direction to the final first direction
stretch parameter.
13. The method of claim 12, wherein a significant portion of the
retraction is performed simultaneously with a portion of the second
direction stretch.
14. The method of claim 1, wherein step b) further includes
stretching the film to a peak first direction stretch parameter
that is at least 1.2 times the final first direction stretch
parameter, and thereafter retracting the film in the first
direction to the final first direction stretch parameter.
15. The method of claim 14, wherein a significant portion of the
retraction is performed simultaneously with a portion of the second
direction stretch.
16. The method of claim 14, wherein step b) further includes
stretching the film to the peak first direction stretch parameter
before no more than 50% of the final second direction stretch
parameter is attained.
17. A film obtained by the method of claim 1.
18. A tape comprising a backing including a fist major surface and
a layer of adhesive on said first major surface, wherein said
backing comprises the film of claim 17.
19. A method of biaxially stretching a polypropylene film, the
method comprising the steps of: a) imparting a sufficiently high
temperature to the film to allow a significant amount of biaxial
stretch; and b) biaxial tenter stretching the film to a final first
direction stretch parameter and a final second direction stretch
parameter, wherein: i) a substantial portion of the first direction
stretch and second direction stretch is performed simultaneously;
ii) at least 90% of the final first direction stretch parameter is
attained before no more than 50% of the final second direction
stretch parameter is attained iii) the final first direction
stretch parameter is not greater than the final second direction
stretch parameter; and iv) the final first direction stretch
parameter less than the natural stretch parameter for a
proportional stretch profile.
20. A film obtained by the method of claim 19.
21. A tape comprising a backing including a first major surface and
a layer of adhesive on said first major surface, wherein said
backing comprises the film of claim 20.
22. A method of biaxially stretching a polymeric film, the method
comprising the steps of: a) imparting a sufficiently high
temperature to the film to allow a significant amount of biaxial
stretch; and b) biaxial tenter stretching the film according to a
stretch profile to a final first direction stretch parameter and a
final second direction stretch parameter, wherein the final first
direction stretch parameter is no greater than the final second
direction stretch parameter, and wherein: i) a straight line
between the point defining zero stretch parameter and the point
defining the final first and second direction stretch parameters
represents a proportional stretch profile and defines a
proportional stretch area; and ii) the curve representing the
stretch profile between the point defining zero stretch parameter
and the point defining the final first and second direction stretch
parameters defines an area at least 1.4 times the proportional
stretch area.
23. The method of claim 22, wherein step b) comprises stretching
the film such that: the curve representing the stretch profile
between the point defining zero stretch parameter and the point
defining the final first and second direction stretch parameters
defines an area at least 1.7 times the proportional stretch
area.
24. The method of claim 22, wherein step b) comprises stretching
the film such that a substantial portion of the first direction
stretch and second direction stretch is performed
simultaneously.
25. The method of claim 22, wherein the first direction is the MD
and the second direction is the TD.
26. The method of claim 22, wherein step b) comprises stretching
the film to a final first direction stretch parameter less than the
natural stretch parameter for a proportional stretch profile.
27. The method of claim 22, wherein step b) comprise stretching the
film to a final first direction stretch parameter less than the
uniaxial natural stretch parameter.
28. The method of claim 22, wherein the film comprises a
thermoplastic film.
29. The method of claim 28, wherein the film comprises a
semi-crystalline film.
30. The method of claim 29, wherein the films comprises a
polyolefin.
31. The method of claim 30, wherein the film comprises
polypropylene.
32. The method of claim 22, wherein step b) further comprises
grasping the film with a plurality of clips along the opposing
edges of the film and propelling the clips in the machine direction
along clip guide means that diverge in the transverse
direction.
33. The method of claim 22, wherein step b) further includes
stretching the film to more than 100% of the final first direction
stretch parameter before no more than 50% of the final second
direction stretch parameter is attained and thereafter retracting
the film in the first direction to the final machine direction
stretch parameter.
34. The method of claim 33, wherein a significant portion of the
retraction is performed simultaneously with a portion of the second
direction stretch.
35. The method of claim 22, wherein step b) further includes
stretching the film to a peak first direction stretch parameter
that is at least 1.2 times the final first direction stretch
parameter, and thereafter retracting the film in the first
direction to the final first direction stretch parameter.
36. The method of claim 35, wherein a significant portion of the
retraction is performed simultaneously with a portion of the second
direction stretch.
37. The method of claim 35, wherein step b) further includes
stretching the film to the peak first direction stretch parameter
before no more than 50% of the final second direction stretch
parameter is attained.
38. A film obtained by the method of claim 22.
39. A tape comprising a backing including a fist major surface and
a layer of adhesive on said first major surface, wherein said
backing comprises the film of claim 38.
40. A method of biaxially stretching a polypropylene film, the
method comprising the steps of: a) imparting a sufficiently high
temperature to the film to allow a significant amount of biaxial
stretch; and b) biaxial tenter stretching the film according to a
stretch profile to a final first direction stretch parameter and a
final second direction stretch parameter, wherein: i) a substantial
portion of the first direction stretch and second direction stretch
is performed simultaneously; ii) a straight line between the point
defining zero stretch parameter and the point defining the final
first and second direction stretch parameters represents a
proportional stretch profile and defines a proportional stretch
area; and iii) the curve representing the stretch profile between
the point defining zero stretch parameter and the point defining
the final first and second direction stretch parameters defines an
area at least 1.4 times the proportional stretch area; iv) the
final first direction stretch parameter is no greater than the
final second direction stretch parameter; and v) the final first
direction stretch parameter is less than the natural stretch
parameter for a proportional stretch profile.
41. A film obtained by the method of claim 40.
42. A tape comprising a backing including a fist major surface and
a layer of adhesive on said first major surface, wherein said
backing comprises the film of claim 41.
43. The method of claim 1, wherein the final second direction
stretch parameter is greater than the natural stretch parameter for
a proportional stretch profile.
44. The method of claim 1, wherein the final second direction
stretch parameter is greater than the uniaxial natural stretch
parameter.
45. The method of claim 22, wherein step b) comprises stretching
the film to a final second direction stretch parameter greater than
the natural stretch parameter for a proportional stretch
profile.
46. The method of claim 22, wherein step b) comprise stretching the
film to a final second direction stretch parameter greater than the
uniaxial natural stretch parameter.
Description
TECHNICAL FIELD
[0001] The present invention relates generally to methods of
biaxially stretching films and such films, and more particularly to
methods of stretching films in two directions simultaneously and
such films.
BACKGROUND OF THE INVENTION
[0002] 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. The variability in
stretch profiles available with some of these methods and apparatus
has also been described.
[0003] For example, U.S. Pat. No. 3,890,421 illustrates in its FIG.
1 what the text describes as: Curve I representing normal
sequential drawing with lateral drawing following longitudinal
drawing; Curve II corresponding to reverse sequential drawing with
longitudinal drawing following transverse drawing; and diagonal
Curve II (sic, Curve III) representing a regularly progressive
simultaneous biaxial drawing in both lateral and longitudinal
directions. The '421 patent also states that simultaneous drawing
can be performed along an indefinite number of curves between
curves I and II with the methods and apparatus described therein
(column 4, lines 14-31). Without providing detailed descriptions of
stretch profiles to achieve the stated objects, the '421 patent
states that the object of the method and apparatus described
therein is to regulate the resistance, tensile strength, modulus of
elasticity. shrinkage, and flatness of biaxially drawn film by
controlling drawing and slack tension throughout the drawing
process while avoiding the limiting factors from successive biaxial
drawing (column 3, lines 34-39).
[0004] U.S. Pat. No. 4,853,602 states that with the method and
apparatus described therein, sequential drawing may be performed
with lateral preceding longitudinal or with longitudinal preceding
lateral (column 34, lines 35-55). This patent also states that for
simultaneous stretching, any desired drawing of the film can be
achieved (column 35, lines 17 et seq.).
[0005] Stretch profiles which include relaxing the film in one or
more directions after achieving a higher intermediate stretch are
also known. For example, U.S. Pat. No. 4,330,499 states that
shrinking of the film occurs in the longitudinal direction at up to
10% of the previous produced longitudinal stretching, over the last
5 to 10% of the stretch apparatus length, preferably while the film
is further stretched in the transverse direction (see
Abstract).
[0006] Uniform thickness is important in adhesive tape
manufacturing because it is an indication of the uniformity of the
film properties and because non-uniform thickness leads to gapping
or telescoping of tape rolls.
[0007] The majority of commercially available biaxially oriented
polypropylene films are produced by the flat film or tenter
stretching process. Typical tenter processes serve to biaxially
stretch films either predominately simultaneously or predominately
sequentially. Currently, simultaneously tenter stretched films
comprise a minor part of the film backing market because, although
such processes can continuously stretch films in both longitudinal
and transverse directions, they have historically proven costly,
slow, and inflexible regarding allowable stretching ratios.
SUMMARY OF THE INVENTION
[0008] One aspect of the present invention provides a method of
biaxially stretching a polymeric film. The method comprises the
steps of:
[0009] a) imparting a sufficiently high temperature to the film to
allow a significant amount of biaxial stretch; and
[0010] b) biaxial tenter stretching the film to a final first
direction stretch parameter and a final second direction stretch
parameter, wherein at least 75% of the final first direction
stretch parameter is attained before no more than 50% of the final
second direction stretch parameter is attained, and wherein the
final first direction stretch parameter is no greater than the
final second direction stretch parameter.
[0011] In one preferred embodiment of the above method of claim 1,
step b) comprises biaxial tenter stretching the film such that a
substantial portion of the first direction stretch and the second
direction stretch is performed simultaneously.
[0012] In another preferred embodiment of the above method, at
least 90% of the final first direction stretch parameter is
attained before no more than 50% of the final second direction
stretch parameter is attained.
[0013] In another preferred embodiment of the above method, the
first direction is the MD and the second direction is the TD.
[0014] In another preferred embodiment of the above method, the
final first direction stretch parameter is less than the natural
stretch parameter for a proportional stretch profile.
[0015] In another preferred embodiment of the above method, the
final first direction stretch parameter is less than the uniaxial
natural stretch parameter.
[0016] In another preferred embodiment of the above method, the
final second direction stretch parameter is greater than the
natural stretch parameter for a proportional stretch profile.
[0017] In another preferred embodiment of the above method, the
final second direction stretch parameter is greater than the
uniaxial natural stretch parameter.
[0018] In another preferred embodiment of the above method, the
film comprises a thermoplastic film. More preferably, the film
comprises a semi-crystalline film. Still more preferably, the film
comprises a polyolefin. In a particularly preferred embodiment, the
film comprises polypropylene.
[0019] In another preferred embodiment of the above method, step b)
further comprises grasping the film with a plurality of clips along
the opposing edges of the film and propelling the clips at varying
speeds in the machine direction along clip guide means that diverge
in the transverse direction.
[0020] In another preferred embodiment of the above method, step b)
further includes stretching the film to more than 100% of the final
first direction stretch parameter before no more than 50% of the
final second direction stretch parameter is attained, and
thereafter retracting the film in the machine direction to the
final first direction stretch parameter. A significant portion of
the retraction may be performed simultaneously with a portion of
the second direction stretch.
[0021] In another preferred embodiment of the above method, step b)
further includes stretching the film to a peak first direction
stretch parameter that is at least 1.2 times the final first
direction stretch parameter, and thereafter retracting the film in
the first direction to the final first direction stretch parameter.
A significant portion of the retraction may be performed
simultaneously with a portion of the second direction stretch.
Furthermore, step b) further may include stretching the film to the
peak first direction stretch parameter before no more than 50% of
the final second direction stretch parameter is attained.
[0022] In another aspect, the present invention provides a method
of biaxially stretching a polypropylene film. The method comprising
the steps of: a) imparting a sufficiently high temperature to the
film to allow a significant amount of biaxial stretch; and b)
biaxial tenter stretching the film to a final first direction
stretch parameter and a final second direction stretch parameter.
In such a method: i) a substantial portion of the first direction
stretch and second direction stretch is performed simultaneously;
ii) at least 90% of the final first direction stretch parameter is
attained before no more than 50% of the final second direction
stretch parameter is attained; iii) the final first direction
stretch parameter is not greater than the final second direction
stretch parameter; and iv) the final first direction stretch
parameter less than the natural stretch parameter for a
proportional stretch profile.
[0023] In a further aspect, the present invention provides another
method of biaxially stretching a polymeric film. The method
comprising the steps of: a) imparting a sufficiently high
temperature to the film to allow a significant amount of biaxial
stretch; and b) biaxial tenter stretching the film according to a
stretch profile to a final first direction stretch parameter and a
final second direction stretch parameter, wherein the final first
direction stretch parameter is no greater than the final second
direction stretch parameter. In such a method: i) a straight line
between the point defining zero stretch parameter and the point
defining the final first and second direction stretch parameters
represents a proportional stretch profile and defines a
proportional stretch area; and ii) the curve representing the
stretch profile between the point defining zero stretch parameter
and the point defining the final first and second direction stretch
parameters defines in area at least 1.4 times the proportional
stretch area.
[0024] In one preferred embodiment of the above method, step b)
comprises stretching the film such that the curve representing the
stretch profile between the point defining zero stretch parameter
and the point defining the final first and second direction stretch
parameters defines an area at least 1.7 times the proportional
stretch area.
[0025] In another preferred embodiment of the above method, step b)
comprises stretching the film such that a substantial portion of
the first direction stretch and second direction stretch is
performed simultaneously.
[0026] In another preferred embodiment of the above method, the
first direction is the MD and the second direction is the TD.
[0027] In another preferred embodiment of the above method, step b)
comprises stretching the film to a final first direction stretch
parameter less than the natural stretch parameter for a
proportional stretch profile.
[0028] In another preferred embodiment of the above method, step b)
comprise stretching the film to a final first direction stretch
parameter less than the uniaxial natural stretch parameter.
[0029] In another preferred embodiment of the above method, the
final second direction stretch parameter is greater than the
natural stretch parameter for a proportional stretch profile.
[0030] In another preferred embodiment of the above method, the
final second direction stretch parameter is greater than the
uniaxial natural stretch parameter.
[0031] In another preferred embodiment of the above method, the
film comprises a thermoplastic film. More preferably, the film
comprises a semi-crystalline film. Still more preferably, the film
comprises a polyolefin. In a particularly preferred embodiment, the
film comprises polypropylene.
[0032] In another preferred embodiment of the above method; step b)
further comprises grasping the film with a plurality of clips along
the opposing edges of the film and propelling the clips in the
machine direction along clip guide means that diverge in the
transverse direction.
[0033] In another preferred embodiment of the above method, step b)
further includes stretching the film to more than 100% of the final
first direction stretch parameter before no more than 50% of the
final second direction stretch parameter is attained and thereafter
retracting the film in the first direction to the final machine
direction stretch parameter. A significant portion of the
retraction may be performed simultaneously with a portion of the
second direction stretch.
[0034] In another preferred embodiment of the above method, step b)
further includes stretching the film to a peak first direction
stretch parameter that is at least 1.2 times the final first
direction stretch parameter, and thereafter retracting the film in
the first direction to the final first direction stretch parameter.
A significant portion of the retraction may be performed
simultaneously with a portion of the second direction stretch.
[0035] In another preferred embodiment of the above method, step b)
further includes stretching the film to the peak first direction
stretch parameter before no more than 50% of the final second
direction stretch parameter is attained.
[0036] In yet another aspect, the present invention provides a
method of biaxially stretching a polypropylene film. The method
comprising the steps of: a) imparting a sufficiently high
temperature to the film to allow a significant amount of biaxial
stretch; and b) biaxial tenter stretching the film according to a
stretch profile to a final first direction stretch parameter and a
final second direction stretch parameter. In such a method: i) a
substantial portion of the first direction stretch and second
direction stretch is performed simultaneously; ii) a straight line
between the point defining zero stretch parameter and the point
defining the final first and second direction stretch parameters
represents a proportional stretch profile and defines a
proportional stretch area; and iii) the curve representing the
stretch profile between the point defining zero stretch parameter
and the point defining the final first and second direction stretch
parameters defines an area at least 1.4 times the proportional
stretch area; iv) the final first direction stretch parameter is no
greater than the final second direction stretch parameter; and v)
the final first direction stretch parameter is less than the
natural stretch parameter for a proportional stretch profile.
[0037] The present invention also provides a film obtained by any
of the methods described above. The present invention also provides
a tape comprising a backing including a fist major surface and a
layer of adhesive on said first major surface, wherein said backing
comprises a the film a film obtained by any of the methods
described above.
[0038] 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."
[0039] 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 ("MDR") 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 stretch. In a stretched film having a
TD stretch ratio ("TDR") 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
stretch.
[0040] "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 overall 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 stretch.
[0041] The mechanical stretch ratio, also know as nominal stretch
ratio, is determined by the unstretched and stretched dimensions of
the overall film, 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 draw 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 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 stretch. When stretch is not uniform over substantially
the entire edge-trimmed film, then the local ratio can be different
from the global ratio. When stretch 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 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.
[0042] The term "stretch parameter" is used to indicate the value
of the stretch ratio minus 1. For example "first direction stretch
parameter" and "second direction stretch parameter" are used herein
to indicate the value of first direction stretch ratio minus 1, and
second direction stretch ratio minus 1, respectively. Likewise, the
terms "MD stretch parameter" and "TD stretch parameter" are used
herein to indicate the value of MD stretch ratio minus 1, and TD
stretch ratio minus 1, respectively. For example, a film that has
not been stretched in the machine direction would have an MD
stretch ratio of 1 (i.e., dimension after stretch is equal to
dimension before stretch). Such a film would have an MD stretch
parameter of 1 minus 1, or zero (i.e., the film has not been
stretched). Likewise, a film having an MD stretch ratio of 7 would
have an MD stretch parameter of 6.
[0043] In reference to simultaneous biaxial stretching, the term
"proportional stretch profile" is a stretch profile in which the
ratio of the first direction stretch parameter to the second
direction stretch parameter is kept substantially constant
throughout the stretch process. A particular example of this would
be the case where the ratio of the MD stretch parameter to the TD
stretch parameter is kept substantially constant throughout the
stretch process. As illustrated in FIG. 1, a plot of MD stretch
parameter (y-axis) vs. TD stretch parameter (x-axis) for a
proportional stretch profile provides a straight line 10 between
the point 12 representing zero MD stretch parameter (or an MD
stretch ratio of 1) and zero TD stretch parameter (or a TD stretch
ratio of 1) to the point 14 representing the final MD stretch
parameter and the final TD stretch parameter. For a proportional
stretch profile, this line 10 is straight whether the final MD and
TD stretch parameters are equal (a "balanced" stretch) or unequal.
Also identified on FIG. 1 is the area A under the curve 10 for the
proportional stretch profile.
[0044] The term "MD overbias" refers to a stretch profile in which
the MD stretch ratio during a significant portion of the stretching
process is greater than it would be for the proportional stretch
profile having the same final MD and TD stretch ratios. One
representative MD overbias curve is represented as 16 on FIG. 1.
Another way to identify an overbias stretch profile is that the
area B under the curve 16 is greater than area A for a proportional
stretch profile ending at the same final MD and TD stretch
parameter values. An MD overbias profile does not necessarily
exclude having some portion of the profile under the proportional
stretch profile line 10.
[0045] When many films are stretched uniaxially or biaxially at a
temperature below the melting point of the polymer, particularly at
a temperature below the line drawing temperature of the film, the
film stretches non-uniformly, and a clear boundary is formed
between stretched and unstretched parts. This phenomenon is
referred to as necking or line drawing. Substantially the entire
film is stretched uniformly when the film is stretched to a
sufficiently high degree. The stretch ratio at which this occurs is
referred to as the "natural stretch ratio" or "natural draw ratio."
The necking phenomenon and the effect of natural stretch ratio is
discussed, for example, in U.S. Pat. Nos. 3,903,234; 3,995,007; and
4,335,069. Most discussions of natural draw ratio for biaxial
orientation processes are with respect to sequential stretching
processes. In such a process, for either a natural draw ratio in
the first stretching direction or a natural draw ratio in the
second stretching direction, the natural draw ratio in question is
substantially analogous to that for a uniaxial stretch. When
stretching is done at temperatures near the melting point, or when
simultaneous equal biaxial stretching (also referred to a square
stretching) is performed, the necking phenomena can be less
pronounced, resulting in stretched areas having different local
stretch ratios, rather than strictly stretched and unstretched
parts. In such a situation, and in any simultaneous biaxial
stretching process, the "natural stretch ratio" for a given
direction is defined as that global stretch ratio at which the
relative standard deviation of the local stretch ratios measured at
a plurality of locations upon the film is below about 15%.
Stretching above the natural stretch ratio is widely understood to
provide significantly more uniform properties or characteristics
such as thickness, tensile strength, and modulus of elasticity. For
any given film and stretch conditions, the natural stretch ratio is
determined by factors such as the polymer composition, morphology
due to cast web quenching conditions and the like, and temperature
and rate of stretching. Furthermore, for biaxially stretched films,
the natural stretch ratio in one direction will be affected by the
stretch conditions, including final stretch ratio, in the other
direction. Thus, there may be said to be a natural stretch ratio in
one direction given a fixed stretch ratio in the other, or,
alternatively, there may be said to be a pair of stretch ratios
(one in MD and one in TD) which result in the level of local
stretch uniformity by which the natural stretch ratio is defined
above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] The present invention will be further explained with
reference to the appended FIGS., wherein like structure is referred
to by like numerals throughout the several views, and wherein:
[0047] FIG. 1 is a plot of a proportional stretch profile and a
representative MD overbias stretch profile;
[0048] FIG. 2 is an isometric view of a preferred tape according to
the present invention;
[0049] FIG. 3 is a plot of a preferred overbias stretch profile
according to the present invention;
[0050] FIG. 4 is a plot of an alternative preferred overbids
stretch profile according to the present invention; and
[0051] FIG. 5 is a plot of a preferred overstretch profile
according to the present invention.
[0052] FIG. 6 is a plot of the time-dependent component stretching
profiles of Example C1.
[0053] FIG. 7 is a plot of the stretching profile of Example C
1.
[0054] FIG. 8 is a plot of the time-dependent component stretching
profiles of Example C2.
[0055] FIG. 9 is a plot of the stretching profile of Example
C2.
[0056] FIG. 10 is a plot of the stretching profile of Example
3.
[0057] FIG. 11 is a plot of the time-dependent component stretching
profiles of Example 4.
[0058] FIG. 12 is a plot of the stretching profile of Example
4.
[0059] FIG. 13 is a plot of the stretching profile of Example
5.
[0060] FIG. 14 is a plot of the stretching profile of Example
6.
[0061] FIG. 15 is a plot of the stretching profile of Example
7.
[0062] FIG. 16 is a plot of the time-dependent component stretching
profiles of Example 8.
[0063] FIG. 17 is a plot of the stretching profile of Example
8.
[0064] FIG. 18 is a plot of the stretching profile of Example
9.
[0065] FIG. 19 is a plot of the stretching profile of Example
10.
[0066] FIG. 20 is a plot of the stretching profile of Example
12.
[0067] FIG. 21 is a plot of the stretching profile of Example
13.
DETAILED DESCRIPTION OF THE INVENTION
[0068] Referring to FIG. 2, there is shown a length of tape 20
according to one preferred embodiment of the present invention.
Tape 20 comprises a film backing 22 which includes first major
surface 24 and second major surface 26. Preferably, backing 22 has
a thickness in the range of about 0.020 to about 0.064 mm. Backing
22 of tape 20 is coated on first major surface 24 with a layer of
adhesive 28. Adhesive 28 may be any suitable adhesive as is known
in the art. Backing 22 may have an optional release or low adhesion
backsize layer 30 coated on the second major surface 26 as is known
in the art. In one preferred embodiment, backing 22 comprises a
biaxially stretched monolayer film as described herein. Backing 22
alternatively may comprise a bilayer, trilayer or other multilayer
backing, one of which layers comprises a biaxially stretched film
as described herein.
[0069] Preferably, the film backing 22 comprises a polymeric film.
More preferably, the film backing 22 comprises a thermoplastic
polymer. For a film comprising more than one layer, the description
of suitable materials which follows need apply only to one of said
layers. Suitable polymeric film materials for use in the current
invention include all 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, 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 comprising any
of the above-named polymers, and copolymers comprising 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.
[0070] Still more preferred are semi-crystalline, thermoplastic,
polymeric films. Semi-crystalline themoplastics include, but are
not limited to, polyesters, polyamides, thermoplastic polyimides,
polyarylether ketones, aliphatic polyketones, polyphenylene
sulfide, isotactic or syndiotactic polystyene and their
derivatives, polyacrylates, polymethacrylates, cellulose
derivatives, polyethylene, polyolefins, fluorinated polymers and
copolymers, polyvinylidene chloride, polyacrylonitrile,
polyvinylacetate, and polyethers. Still more preferred are
semi-crystalline thermoplastics which can be stretched to form a
biaxially oriented film from the semi-crystalline state. These
include, but are not limited to, certain polyesters and polyamides,
certain fluorinated polymers, syndiotactic polystyrene,
polyethylenes, and polyolefins. Still more preferred are
polyethylenes and polypropylenes. Predominantly isotactic
polypropylene is most preferred.
[0071] For the purposes of the present invention, the term
"polypropylene" is meant to include copolymers comprising at least
about 90% propylene monomer units, by weight. "Polypropylene" is
also meant to include polymer mixtures comprising at least about
75% 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 D 1505-96 ("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 and 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 in 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. The polypropylene may be intentionally partially degraded
during processing by addition of organic peroxides such is dialkyl
peroxides having alkyl groups having up to six carbon atoms,
2,5-dimethyl-2,5-di(tert-butylperoxy)hexane and di-tert-butyl
peroxide. A degradation factor between about 2 and 15 is suitable.
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.
[0072] As already mentioned, mixtures having at least about 75%
isotactic polypropylene and at most about 25% 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 comprising monomers having from four to
eight carbon atoms, and other polypropylene resins.
[0073] 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 thee
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.
[0074] 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 methacrylatc, 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.
[0075] 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.
[0076] The polymer can be cast into sheet form as is known in the
art, to prepare a sheet suitable for stretching to arrive at the
preferred film described herein. When making polypropylene films, a
suitable method for casting a sheet is to feed the resin into the
feed hopper of a single screw, twin screw, cascade, or other
extruder system having an extruder barrel temperature 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 sheet can
be passed through a fluid-filled cooling bath after removal from
the casting wheel.
[0077] The sheet is then biaxially stretched according to the
preferred profiles described herein to provide backing film 22. Of
all stretching methods, the methods most preferred for commercial
manufacture of films for tape backings include biaxial stretching
by a flat film tenter apparatus. Such a stretch method is referred
to herein as biaxial tenter stretching. This process is distinct
from conventional sequential biaxial stretch apparatus in which the
film is stretched in the MD by being propelled over rollers of
increasing speed. Biaxial tenter stretching is preferred because it
avoids contacting the full surface of the film with a roller during
stretch. Biaxial tenter stretching is performed on a tenter
apparatus that grasps the film (employing such means as a plurality
of clips) along the opposing edges of the film and propels the
grasping means at varying speeds along divergent rails. Throughout
this document, the words grippers and clips are meant to be
inclusive of other film-edge grasping means. By increasing clip
speed in the MD, stretch in the MD occurs. By using such means as
diverging rails, TD stretch 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, and more preferably by
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. Such a biaxial tenter apparatus is capable of sequential
and simultaneous biaxial stretch processes, and the present
invention includes either process. When the preferred stretch
profiles described and claimed herein are referred to as including
a substantial portion that is simultaneous, this means more than an
incidental amount, preferably at least 10% of the final stretch in
each direction being performed simultaneously, more preferably at
least 25%, and still more preferable at least 40%. Although
biaxially stretched films can be made by tubular blown film
stretching processes, it is preferable that the films of this
invention, when used as tape backings, be made by the preferred
flat film tenter stretching processes just described to minimize
thickness variations and avoid processing difficulties typically
associated with tubular blown film processes.
[0078] One class of preferred stretch profiles according to the
present invention is the class of MD overbias stretch profiles. In
an MD overbias stretch profile, the MD stretch parameter attains a
higher value over a significant portion of the stretching process
than it would attain in the case of the proportional stretch
profile having the same final MD and TD stretch ratios. One
illustrative MD overbias curve is represented as 16 on FIG. 1. One
preferred MD overbias stretch profile is one in which at least 75%
of the final MD stretch parameter is attained before no more than
50% of the final TD stretch parameter is attained. A more preferred
MD overbias stretch profile is one in which at least 90% of the
final MD stretch parameter is attained before no more than 50% of
the final TD stretch parameter is attained. An example of such a
profile 16 is illustrated in FIG. 3. For a film having a final MD
stretch ratio of 5.4 and a final TD stretch ratio of 8.5 (commonly
referred to as a 5.4 x 8.5 film), the final MD stretch parameter
equals 4.4 and the final TD stretch parameter equals 7.5 and is
identified as point 14 on FIG. 3. For the preferred MD overbias
profile of FIG. 3, at least 90% of the final MD stretch parameter
is (0.9.times.5.4)=4.86, illustrated as point 40 on the y-axis.
Illustrated as point 42 on the x-axis is 50% of the final TD
stretch parameter is (0.5.times.7.5)=3.75. Therefore, for the
illustrated preferred profile, an MD stretch parameter of 4.86
attained before a TD stretch parameter of no more than 3.75 is
attained as illustrated at point 44 on the profile. The illustrated
MD overbias profile 16 does not include any portion that is below
the proportional stretch profile line 10. However, it is within the
scope of the present invention to include a portion of the profile
under the proportional stretch profile line in an MD overbias
profile that attains preferably at least 75%, more preferably at
least 90% of the final MD stretch parameter before no more than 50%
of the final TD stretch parameter is attained. This is illustrated
as profile 16a in FIG. 3.
[0079] Another way to identify an MD overbias stretch profile is
that the area B under the curve 16 is greater than area A for a
proportional stretch profile ending at the same final MD and TD
stretch parameters as illustrated in FIG. 4. One preferred
embodiment of an MD overbias stretch profile 16 is one in which the
area B under the stretch profile curve 16 is at least 1.4 times the
area A under the line 10 defining a proportional stretch profile.
In another preferred profile, the area B is at least 1.7 times area
A. In still another preferred profile, area B is at least 2.0 times
area A. In yet another preferred profile, area B is at least 2.5
times area A. And in another preferred profile, area B is
approximately 2.5 times area A. In the profile illustrated in FIG.
4, the MD overbias stretch profile 16 does not include a portion
under the proportional stretch profile line 10. However, it is
within the scope of the invention to include a portion of the
profile under the proportional stretch profile line in an MD
overbias profile that has area B larger than proportional area A by
the specified amount as illustrated by profile 16b in FIG. 4.
[0080] Another preferred stretch profile of the present invention
includes an MD overstretch in the profile, followed by a retraction
in the machine direction. As illustrated in FIG. 5, such a profile
46 includes reaching a peak MD stretch parameter at point 48
followed by a retraction in the machine direction to the final MD
stretch parameter at point 14. While it is possible to perform this
retraction in the absence of TD direction stretch, it is preferred
that for a significant amount of overstretch, a significant portion
of the retraction occur simultaneous with a portion of the TD
stretch as illustrated by segment 46a of the profile 46 of FIG. 5.
In one preferred embodiment, the peak MD stretch parameter 48
achieved during overstretch is at least 1.2 times the value of the
final MD stretch parameter 14. In another preferred embodiment, the
peak MD stretch parameter is at least 1.3 times the final MD
stretch parameter. In still another preferred embodiment, the peak
MD stretch parameter is at least 1.4 times the final MD stretch
parameter. In yet another preferred embodiment, the peak MD stretch
parameter is at least 1.5 times the final MD stretch parameter. And
in another preferred embodiment, the peak MD stretch parameter is
approximately 1.5 times the final MD stretch parameter.
[0081] The preferred MD overstretch profiles described herein may
also be combined with the preferred MD overbias stretch profiles
described herein. In other words, such a stretch profile would
achieve the desired amount of MD stretch parameter before no more
than the specified amount of TD stretch parameter is attained,
while also achieving the preferred peak MD stretch parameter and
subsequent machine direction retraction described above. Similarly,
for any of the MD overbias stretch profiles that include area B
sufficiently larger than area A, these profiles may also include
the attainment of preferred peak MD stretch parameter and
subsequent machine direction retraction described above.
[0082] Many of the preferred embodiments are described herein with
respect to the MD and TD of the film, as are the examples. However,
it is understood that any of the preferred stretch profiles herein
and examples reported herein can be described with reference to a
first direction and a second direction substantially perpendicular
to the first direction. This is so with respect to overbias stretch
profiles, overstretch profiles, and any of the parameters described
with respect to the profiles such as final stretch ratio, stretch
parameter, and natural stretch ratio. Thus, the preferred overbias
and/or overstretch profiles of the present invention may be
described with reference to a first direction in which the final
stretch ratio is no greater than the final stretch ratio in a
second direction. The first direction may be either the MD or the
TD. That is, the profile may be first direction overbias or first
direction overstretch, and these encompass profiles which may be MD
overbias, TD overbias, MD overstretch, and TD overstretch. Either
the first or second direction may correspond to the MD with the
other corresponding to the TD. It is also understood that the
improved properties of a film made with, for example, a TD overbias
stretching profile, would pertain to the opposite direction from
those of a film made with a MD overbias stretching profile.
[0083] In any of the overbias or overstretch profiles described
herein, it is sometimes preferred that the final stretch ratio in
the first direction be less than the natural stretch ratio measured
on the same film in a uniaxial stretching mode. For such a case,
the overbias or overstretch is in the same direction as the
direction for which the final stretch ratio is less than the
uniaxial natural stretch ratio. In one particularly preferred
overbias profile, the profile is MD overbias, and the final MD
stretch ratio is less than the uniaxial natural stretch ratio. In
another preferred profile, it is preferred that for the direction
that is not overbias, the final stretch ratio is greater than the
uniaxial natural draw ratio. In another preferred profile, it is
preferred that the final draw ratio in the first direction, having
overbias, be less than the uniaxial natural draw ratio and that the
final draw ratio in the second direction be greater than the
uniaxial natural draw ratio. An example of such a preferred profile
is one that is MD overbiased, the final MD stretch ratio is less
than the uniaxial natural stretch ratio, and the final TD stretch
ratio is greater than the uniaxial natural stretch ratio. As
described above, when the final stretch ratio in the first
direction is less than the uniaxial natural stretch ratio, it is
expected that the resulting film Would have in that direction
significantly non-uniform properties such as thickness and
uniformity of stretch. Surprisingly, by using the overbias and
overstretch stretch profiles; described herein, uniformity of
properties may be attained in a given direction despite stretching
the film to a final stretch ratio less than the uniaxial natural
stretch ratio.
[0084] Another way to describe this unexpected benefit is to
compare films that have been drawn along different stretch profiles
to the same final stretch ratio or parameter. When a proportional
stretch profile is used, uniform film properties will not be
obtained if the final draw ratio in the first direction is below
the natural draw ratio for that direction. When a film is stretched
to the same final stretch parameter or ratio along a stretch
profile have sufficient overbias, the film will exhibit uniform
properties. It can be said that the overbias stretch profile
reduces the value of the natural draw ratio in the direction in
which the overbias is present. This allows stretching the film
along an overbias stretch profile to a lower final draw ratio in
that direction than would have been possible for a proportional
stretch profile while nonetheless achieving a stretched film having
acceptable uniform properties and characteristics.
[0085] Sometimes it is preferred to have a film with a high
elongation to break and high toughness in a certain direction.
These properties can be achieved with a low final draw ratio in
that direction. Prior to the present invention, it was difficult to
obtain films with uniform thickness and properties by stretching to
a low final draw ratio. A low final draw ratio is conveniently
obtained with the overbias and/or overstretch profiles described
herein. These profiles also provide films with uniform properties
and thickness.
[0086] 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 employed, 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 improvements
which differ in magnitude, or may thereby be able to adjust the
precise levels of stretch profile overbias necessary to realize
said improvements.
[0087] The films useful in this invention, when used as a backing
22 for a tape 20, preferably have a final thickness between about
0.020 to 0.064 mm. Thicker and thinner films may be used, with the
understanding that the film should be thick enough to avoid
excessive flimsiness and difficulty in handling, while not being so
thick so as to be undesirably rigid or stiff and difficult to
handle or use. Variability in film thickness, as measured by the
standard deviation relative to the average, is preferably less than
10% down the web and across the interior width of the film
excluding its edge areas. This interior width varies depending on
the relative portion of the film edges to the entire width of the
film. Generally, film edge is not stretched biaxially, but rather
exhibits stretched characteristics that tend toward the uniaxial
even in a biaxial stretching operation. Therefore the film edges
are thicker. In some cases, a cast web of intentionally non-uniform
thickness is stretched. If a thicker edge is used in the cast web,
then the film edge width in the stretched film will be defined by
the original cast web thickness profile, in addition to the
localizing effect of the gripper.
[0088] For the preferred embodiment of film backing 22 comprising
isotactic polypropylene, the film backing 22 preferably has a
tensile elongation to break of at least 110% and a tensile
volumetric energy to break of at least 18,000 in-lb/in.sup.3.
[0089] Backing 22 may optionally include additives and other
components as is known in the art and described above, preferably
in an amount selected so as not to adversely affect the tensile
properties attained by the preferred embodiments described
herein.
[0090] In the case of films intended for use as adhesive tape
backings, stock rolls are typically slit from a wider input film
roll from the film maker. The stock rolls are typically coated with
adhesive on one surface and a release coating or low adhesion
backsize (LAB) on the other, slit to narrow widths and wound into
roll form.
[0091] The adhesive 28 coated on the first major surface 24 of tape
backing 22 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 28 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 according to the present invention include
all 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 adhesives
useful in the invention 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 its tackifiers, plasticizers, fillers, antioxidants,
stabilizers, pigments, diffusing materials, curatives, fibers,
filaments, and solvents. Also, the adhesive optionally can be cured
by any known method.
[0092] A general description of useful pressure sensitive adhesives
may be found in Encyclopedia of Polymer Science and Engineering,
Vol. 13, Wiley-Interscience Publishers (N.Y., 1988). Additional
description of useful pressure sensitive adhesives may be found in
Encyclopedia of Polymer Science and Technology, Vol. 1,
Interscience Publishers (N.Y., 1964).
[0093] The film backing 22 of the tape 20 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. In addition, the second surface 26 of
the film backing 22 may be coated with optional low adhesion
backsize materials 30 to restrict adhesion between the opposite
surface adhesive layer 28 and the film 22, thereby allowing for
production of adhesive tape rolls capable of easy unwinding, as is
well known in the adhesive coated tape-making art.
[0094] 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
[0095] For all Examples 1-13, the unstretched cast film was
obtained as follows. A film-grade isotactic polypropylene copolymer
resin having a nominal melt flow index of 2.5 g/10 minutes and
having an ethylene comonomer content of 0.3%, obtained from Exxon
Chemical Co. (Houston, Tex.), and having the commercial designation
Escorene 4792, was fed to a cascade extrusion system, comprising a
17.5 cm single screw extruder and a 22.5 cm single screw extruder,
manufactured by Barmag AG (Remscheid, Germany), having an extruder
barrel temperature of about 250.degree. C., which was adjusted to
produce a stable homogeneous melt. The polypropylene melt was
extruded through a 91.4 cm single manifold sheet die onto a
rotating cooled steel casting wheel maintained at about 38.degree.
C. The casting wheel was mounted in such a way as to be immersed to
a high level in a water bath, which was maintained at 20.degree. C.
The cast film thus traveled through the water bath while still in
contact with the casting wheel. The unstretched cast film had a
thickness of about 0.13 cm.
[0096] Specimens of the cast film were then stretched
simultaneously in their two orthogonal in-plane directions to in
Ml) mechanical stretch ratio ("MDR") of 5.4 and(i a TD mechanical
stretch ratio ("TDR") of 8.5. Independent measurements in uniaxial
mode on the same unstretched cast film at similar temperatures and
stretch rates indicated that the uniaxial natural stretch ratio for
this material was between about 6 and about 7, thus the MDR is
smaller than the uniaxial natural stretch ratio and the TDR is
larger than the uniaxial natural stretch ratio in all the Examples.
Stretching was performed on a hydraulically-driven laboratory
biaxial film stretching device having a programmable
temperature-controlled oven. The positions of two orthogonal
stretching subsystems within the oven, and hence the stretch ratios
of the film specimen, were also programmable as a function of time.
The MD and TD were defined for each specimen in terms of the
original MD and TD of the film extrusion-casting process. It should
be clearly understood that the laboratory biaxial film stretching
device, itself, has no inherent "machine" and "transverse"
directions, since it is a batchwise, rather than a continuous
processing, device. In all Examples, stretching began and ended
simultaneously for each of the two orthogonal directions. Other
parts of the procedure common to all Examples were as follows.
[0097] The cast film sheet of about 0.13 cm thickness was cut into
square specimens. The specimens were cut to a size which resulted
in the gripped specimens having a stretchable dimension of about
4.6 cm in each of the two planar directions, after edgewise
gripping by the jaws of a film stretching frame within the oven
chamber of the device. Each specimen was pre-heated for 45 seconds
at 130.degree. C., followed by an additional 45 seconds at
160.degree. C. Each specimen was then simultaneously biaxially
stretched using pre-programmed stretching profiles which were
computed to simulate the workings of a film line capable of
simultaneous biaxially orientation within its tenter oven. After
the completion of the stretching, specimens were rapidly cooled and
then quickly removed from the film stretching device. At least
three specimens were stretched at the conditions of each Example,
and the resultant replicate specimen films were examined visually
for consistency of stretching behavior. Occasional specimens which
behaved anomalously (tearing at or near a gripper, for example)
were discarded. One specimen from the three at a given set of
conditions was used for stretch uniformity measurements, while the
other two were used for tensile testing.
[0098] In each Example, the two component (MD and TD)
time-dependent stretching profiles were combined into a plot of MD
stretch parameter vs. TD stretch parameter by pairing the points
from the two component time-dependent stretching profiles at
identical times. This plot is hereafter referred to as the Stretch
Profile. From such a plot, the following parameters may be
calculated, either graphically or numerically:
[0099] "% MD stretch parameter at 25% TD stretch parameter." This
represents what percent of the final MD stretch parameter was
attained when 25% of the final TD stretch parameter was
attained.
[0100] "% MD stretch parameter at 50% TD stretch parameter." This
represents what percent of the final MD stretch parameter was
attained when 50% of the TD stretch parameter was attained.
[0101] "Stretch Profile Area Ratio." This parameter represents the
ratio of:
[0102] the area bounded by the Stretch Profile, the axis at which
the MD Stretch Parameter equals zero, and the vertical line drawn
at the final TD Stretch Parameter; to
[0103] the area bounded by a straight line connecting the starting
point to the final point (i.e., the proportional stretch profile),
the axis at which the MD Stretch Parameter equals zero, and the
vertical line drawn at the final TD Stretch Parameter.
[0104] This is represented by the ratio of area B to area A in FIG.
1.
[0105] Test Methods
[0106] Stretch Uniformity:
[0107] Prior to stretching, grids having reference lines along the
MD and TD at one centimeter spacings were drawn on the cut-square
cast film specimens in such a way that two of the reference lines
were positioned to cross at the exact film center. After
stretching, the separation of these reference markings was measured
to determine the local stretch ratios. To exclude the edge effects
due to scalloping of edges between adjacent pairs of film grippers,
measurements were made using only the central three reference lines
running in each of the machine and transverse directions. Further,
reference line displacements were measured only along the
perpendicular reference lines. Thus, reference line displacements
in the MD were measured between the central reference line running
along the TD and the adjacent reference line to either side, and
were measured along only the central reference line running along
the MD and the adjacent reference lines to either side, for a total
of six measurements. Measurements of displacements in the TD were
performed analogously.
[0108] The local stretch ratios of films, measured in this way, can
vary significantly within one specimen due to necking or
line-drawing in one or both of the stretch directions. For the case
of simultaneous biaxial stretching, line drawing usually manifests
itself as a band or bands on the film, arranged substantially
perpendicular to a stretch direction for which the stretch ratio is
less than the natural stretch ratio in that direction, wherein such
bands are substantially less highly stretched than the remainder of
the film. Such non-uniformity was quantified for Examples 1- 13 by
calculating the Relative Standard Deviation of the MDR, expressed
as the ratio of the standard deviation of the six local MDR
measurements to the mean value of the six local MDR measurements.
It will be readily appreciated that, when an unstretched cast film
of uniform thickness is employed as a starting material, the
Relative Standard Deviation of the MDR stands also as an indirect
qualitative measure of the finished film thickness uniformity, as a
relatively large local stretch ratio will result in a local thin
spot, all else being equal. It will also be appreciated that other
direct and indirect measurement methods exist for quantifying
nonuniformity of the film. The method used herein is meant to be
illustrative and should not be regarded as limiting.
[0109] Tensile Properties:
[0110] Tensile test specimens were cut from the stretched film
specimens of each of the Examples and tested in a Sintech tensile
tester (Stoughton, Mass.). Each tensile test specimen was 1.25 cm
in width and 14 cm in length. An initial jaw separation, or gauge
length, of 5.08 cm and an initial crosshead speed of 2.54 cm/min
was used. A secondary speed of 50.8 cm/min was used after a
deformation of 3% strain was reached. Ten tensile test specimens,
all cut along the film MD, were taken from one stretched film
specimen and tested, for each stretched film Example. Analogous
measurements were performed in the TD, with the exception that only
7, rather than 10, tensile specimens could be cut from each film
specimen, due to the smaller dimensions of the stretched film
specimens in the machine direction. The Tensile Elongation-to-Break
values based on the initial gauge length of the tensile specimen
were reported. In addition, the area under the tensile
stress-strain curves was reported as the Volumetric Tensile Energy
to Break. All reported tensile values are the averages of the 10
(MD) or 7 (TD) tensile specimens.
Comparative examples are designated by numbers having the prefix
"C"
Example C1
MD-Under-Biased Stretching
[0111] Stretching was done at an oven temperature of 160.degree. C.
The time-dependent component stretching profiles describing the
progression of the global MDR and TDR with time for Example C1 are
shown in FIG. 6 and the Stretch Profile is shown in FIG. 7. The
values of the parameters of the stretch profile and the results of
the Stretch Uniformity and Tensile tests are shown in Table 1. This
is a case of MD-Under-Biased stretching.
Example C2
Near-Proportional Stretching
[0112] Stretching was done at an oven temperature of 160.degree. C.
The time-dependent component stretching profiles describing the
progression of the global MDR and TDR with time for Example C2 are
shown in FIG. 8. and the Stretch Profile is shown in FIG. 9.
Example 3
MD-Over-Biased Stretching
[0113] Stretching was done at an oven temperature or 160.degree. C.
The Stretch Profile describing the progression of the global MDR
and TDR for Example 3 is shown in FIG. 10.
Example 4
MD-Over-Biased Stretching
[0114] Stretching was done at an oven temperature of 160.degree. C.
The time-dependent component stretching profiles describing the
progression of the global MDR and TDR with time for Example 4 are
shown in FIG. 11 and the Stretch Profile is shown in FIG. 12.
Example 5
MD-Over-Biased Stretching
[0115] Stretching was done at an oven temperature of 160.degree. C.
The Stretch Profile describing the progression of the global MDR
and TDR for Example 5 is shown in FIG. 13.
Example 6
MD-Over-Stretch Stretching
[0116] Stretching was done at an oven temperature of 160.degree. C.
The Stretch Profile describing the progression of the global MDR
and TDR for Example 6 is shown in FIG. 14.
Examples 7-10
MD-Over-Stretch Stretching
[0117] Stretching was done at an oven temperature of 160.degree. C.
The Stretch Profiles describing the progression of the global MDR
and TDR for Examples 7-10 are shown in FIGS. 15, 17, 18, and 19,
respectively. For illustrative purposes, the corresponding
time-dependent component stretching profiles describing the
progression of the global MDR and TDR with time for Example 8 are
shown in FIG. 16.
Example 11
Stretching at a Different Temperature
[0118] Example 11 was performed identically to Example 7, except
that the stretching was done at an oven temperature of 155.degree.
C.
Examples 12-13
Alternative Profiles
[0119] Example 12 was performed similarly to Example 11, at an oven
temperature of 155.degree. C. and with equivalent final MD stretch
parameter, final TD stretch parameter, and attaining the same
percent MD stretch parameter at 50% TD stretch parameter. However,
Example 12 differed from Example 11 in the ratio of the area B of
the Stretch Profile to the area A of the proportional stretch
profile. The Stretch Profile describing the relative progression of
the global MDR and TDR is shown in FIG. 20.
[0120] Example 13 was performed similarly to Example 9, at an oven
temperature of 160.degree. C. and with equivalent final MD stretch
parameter, final TD stretch parameter, and attaining the same
percent MD stretch parameter at 50% TD stretch parameter. However,
Example 13 differed from Example 9 in the ratio of the area B of
the Stretch Profile to the area A of the proportional stretch
profile. The Stretch Profile describing the relative progression of
the global MDR and TD)R is shown in FIG. 21.
[0121] Details regarding the stretch profiles and conditions of the
Examples, along with results indicating stretch uniformity,
elongation to break, and energy to break are reported in Table
1.
1TABLE 1 (MD). % MD Stretch Parameter at: MD MD 25% TD 50% TD MDR
Relative Elong. Energy to Temp. Stretch Stretch Stretch Profile
Std. Dev. to Break Break Ex. (.degree. C.) Parameter Parameter Area
Ratio (%) (%) (in-lb/in.sup.3) C1 160 7 30 0.78 66.0 61 13,900 C2
160 18 57 1.01 47.0 71 15,500 3 160 57 73 1.39 41.5 112 22,300 4
160 74 91 1.69 5.0 134 28,200 5 160 82 100 1.82 4.2 134 20,100 6
160 93 114 2.02 4.7 132 28,100 7 160 104 125 2.23 8.5 134 19,800 8
160 116 136 2.33 2.6 137 25,600 9 160 125 148 2.58 9.4 122 18,500
10 160 135 159 2.74 2.4 142 27,400 11 155 104 125 2.23 7.7 164
25,800 12 155 72 125 1.90 7.2 140 20,800 13 160 126 148 2.33 6.7
142 20,500
[0122]
2TABLE 1 (TD) % MD Stretch Parameter at: TD TD 25% TD 50% TD TDR
Relative Elong. Energy to Temp. Stretch Stretch Stretch Profile
Std. Dev. to Break Break Ex. (.degree. C.) Parameter Parameter Area
Ratio (%) (%) (in-lb/in.sup.3) C1 160 7 30 0.78 3.2 53 15,400 C2
160 18 57 1.01 7.7 34 6,970 3 160 57 73 1.39 6.5 49 14,700 4 160 74
91 1.69 4.7 50 16,100 5 160 82 100 1.82 5.4 39 10,900 6 160 93 114
2.02 3.5 55 17,400 7 160 104 125 2.23 2.2 47 14,900 8 160 116 136
2.33 3.9 47 15,700 9 160 125 148 2.58 5.4 43 13,800 10 160 135 159
2.74 4.1 34 9,200 11 155 104 125 2.23 5.2 43 12,800 12 155 72 125
1.90 5.5 50 15,400 13 160 126 148 2.33 8.1 44 14,600
[0123] It can be seen from the results that a marked improvement in
the values of MD elongation to break and MD energy to break occurs
at stretch profiles in which the ratio of the area under the
stretch profile curve to the area under the proportional stretch
profile is at least approximately 1.4; and at which at least
approximately 75% or more of the final MD stretch parameter is
attained before 50% of the final TD stretch parameter is attained.
It is also seen from the results that a marked increase in MD
stretch uniformity occurs at stretch profiles in which the ratio of
the area under the stretch profile curve to the area under the
proportional stretch profile is at least approximately 1.7; and at
which at least approximately 90% or more of the final MD stretch
parameter is attained before 50% of the final TD stretch parameter
is attained. It is expected that uniformity of stretch provides
uniformity of film properties and characteristics.
[0124] 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
numerical results.
[0125] 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.
* * * * *