Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Reshaping running or indefinite-length work
Reexamination Certificate
1998-11-13
2001-10-16
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Reshaping running or indefinite-length work
C264S210700, C264S290200, C264S3420RE, C264S346000
Reexamination Certificate
active
06303067
ABSTRACT:
TECHNICAL FIELD
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
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.
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).
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.).
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).
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.
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
In one aspect, the present invention provides a method of biaxially stretching a polymeric film according to a stretch profile to a final first direction stretch parameter and a final second direction stretch parameter. The method comprising the steps of:
a) imparting a sufficiently high temperature to the film to allow a significant amount of biaxial stretch;
b) biaxial tenter stretching the film to a peak first direction stretch parameter that is at least 1.2 times the final first direction stretch parameter, wherein the final first direction stretch parameter is no larger than the final second direction stretch parameter; and
c) subsequent to step b), retracting the film in the first direction to the final first direction stretch parameter.
In one preferred embodiment of the above method, a significant portion of the first direction stretch is performed simultaneously with a portion of the second direction stretch. A significant portion of the retraction may be performed simultaneously with a portion of the second direction stretch.
In another preferred embodiment of the above method, a significant portion of the retraction is performed simultaneously with a portion of the second direction stretch.
In another preferred embodiment of the above method, the peak first direction stretch parameter is at least 1.3 times the final first direction stretch parameter. The peak first direction stretch parameter may at least 1.4 times the final first direction stretch parameter. The peak first direction stretch parameter may at least 1.5 times the final first direction stretch parameter.
In another preferred embodiment of the above method, the first direction is the MD and the second direction is the TD.
In another preferred embodiment of the above method, the final first direction stretch parameter is less than the uniaxial natural stretch parameter.
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.
In another preferred embodiment of the above method, the final second direction stretch parameter is greater than the uniaxial natural stretch parameter.
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.
In another preferred embodiment of the above method, the film comprises a thermoplastic film. Preferably, the film comprises a semi-crystalline film. More preferably, the film comprises polyolefin. In a particularly preferred embodiment, the film comprises polypropylene.
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.
In another preferred embodiment of the above method, step b) further includes stretching the film to at least 75% of the final first direction stretch parameter before no more than 50% of the final second direction stretch parameter is attained. Alternatively, step b) further includes stretching the film to at least 90% of the final first direction stretch parameter before no more than 50% of the final second direction stretch parameter is attained.
In another preferred embodiment of the above method, step
1
)) 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.
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.
In another preferred embodiment of the above method: i) a straight line between the point defining zero stretch parameter and the point defining the final first direction stretch parameter and the final second direction stretch parameter 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 direction stretch parameter and the final second direction stretch parameter defines an area at least 1.4 times the proportional stretch area. The ratio may be instead be at least 1.7.
Another aspect of the present provides a method of biaxially stretching a polypropylene film according to a stretch profile to a final first direction stretch parameter and a final second direction stretch parameter,
Ferguson Anthony B.
Hanschen Thomas P.
Jackson Jeffery N.
Merrill William W.
Roska Fred J.
3M Innovative Properties Company
Peters Carolyn V.
Poe Michael I.
Silbaugh Jan H.
Trussell James J.
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