Methods and devices for stretching polymer films

• US 20040099992A1
• Filed: 11/27/2002
• Published: 05/27/2004
• Est. Priority Date: 11/27/2002
• Status: Active Grant

• Current Assignee: 3M Innovative Properties Company (3M Company)
• Original Assignee: 3M Innovative Properties Company (3M Company)
• Inventors: Jackson, Jeffrey N., Wheatley, John A., Ruff, Andrew T., Merrill, William W., Harvey, Curtis A., Thompson, Desiree L., Amundson, Jeffrey D.
• Granted Patent: US 6,949,212 B2
• Time in Patent Office: Days
• US Class Current: 264/290.2
• CPC Class Codes: B29C 55/08 transverse to the direction of feed
• CPC Class Codes: B29C 55/165 Apparatus therefor
• CPC Class Codes: Y10S 425/041 Reshape

Abstract

Stretched polymeric films can be used in a variety of applications, including optical applications. The stretching conditions and shape of the stretching tracks in a stretching apparatus can determine or influence film properties. Methods and stretching apparatuses can include adjustable or zone-defined stretching regions.

Claims

1. An apparatus for stretching a film, the apparatus comprising:

a plurality of gripping elements to hold opposing ends of a film;

opposing tracks along which the plurality of gripping elements travel, the opposing tracks defining a primary stretching region in which the tracks generally diverge to stretch a film held by the gripping elements, wherein each of the opposing tracks in the primary stretching region comprises at least one continuous rail extending the length of the track through the primary stretching region; and

at least one track shape controller coupled to at least one of the continuous rails within the primary stretching region, wherein the track shape controller is capable of applying a force to the continuous rail to modify a shape of the track in the primary stretching region.

2. The apparatus of claim 1, wherein at least one track shape controller is coupled to each continuous rail of at least one of the opposing tracks.

3. The apparatus of claim 1, wherein a plurality of track shape controllers is coupled to each of the continuous rails.

4. The apparatus of claim 1, wherein at least one of the tracks comprises a fixed control point that does not move when the track shape controllers apply force to modify the shape of the track.

5. The apparatus of claim 4, wherein the fixed control point is positioned on the track prior, relative to a directional flow of film through the apparatus, to any track shape controller coupled to the track.

6. The apparatus of claim 4, wherein the fixed control point is positioned on the track after, relative to the directional flow of film through the apparatus, at least one track shape controller coupled to the track.

7. The apparatus of claim 1, wherein the continuous rail has a cross-sectional area that varies along a length of the track.

8. The apparatus of claim 7, wherein the continuous rail is monotonically tapered along the length of the track.

9. The apparatus of claim 7, wherein the cross-sectional area of the continuous rail decreases along the length of the track.

10. The apparatus of claim 1, wherein an equilibrium shape of the continuous rail corresponds to a shape in which no track shape controller applies force to modify the shape of the continuous rail and the equilibrium shape of the continuous rail is parabolic.

11. A method of stretching a film, the method comprising:

providing a stretcher comprising

a plurality of gripping elements,

opposing tracks along which the plurality of gripping elements travel, the opposing tracks defining a primary stretching region in which the tracks generally diverge to stretch a film held by the gripping elements, wherein each of the opposing tracks in the primary stretching region comprises at least one continuous rail extending the length of the track through the primary stretching region; and

at least one track shape controller coupled to at least one of the continuous rails within the primary stretching region;

activating at least one of the track shape controllers to apply a force to at least one of the continuous rails to modify a shape of at least one of the tracks;

gripping opposing ends of a film using the gripping elements;

conveying the film along the opposing tracks to the primary stretching region; and

stretching the film within the primary stretching region.

12. An apparatus for stretching a film, the apparatus comprising:

a plurality of gripping elements to hold opposing ends of a film;

opposing tracks along which the plurality of gripping elements travel, the opposing tracks defining a primary stretching region in which the tracks generally diverge to stretch a film held by the gripping elements, wherein the primary stretching region comprises

(i) an initial stretching zone configured and arranged so that the machine direction draw ratio (MDDR) and the transverse direction draw ratio (TDDR) based on the tracks have the following relationship;

MDDR<

(TDDR)<sup>−

1/2</sup>

TDDR is increased by at least 0.5; and

(ii) a later stretching zone configured and arranged so that MDDR is approximately equal to (TDDR)<sup>−

1/2</sup> and TDDR is increased by at least 0.5.

13. The apparatus of claim 12, wherein the initial stretching zone is configured and arranged so that TDDR is increased by at least 1.

14. The apparatus of claim 12, wherein the later stretching zone is configured and arranged so that TDDR is increased by at least 1.

15. The apparatus of claim 12, wherein the initial stretching zone is configured and arranged to achieve a TDDR of at least 4.

16. The apparatus of claim 12, further comprising an intermediate zone between the initial stretching zone and the later stretching zone.

17. A method for stretching a film, the method comprising:

conveying the film into a stretching region;

stretching the film by conveying the opposing edges of the film along generally diverging paths, wherein the generally diverging paths are configured and arranged to comprise

(i) an initial stretching zone configured and arranged so that the machine direction draw ratio (MDDR) and the transverse direction draw ratio (TDDR) based on the paths have the following relationship;

MDDR<

(TDDR)<sup>−

1/2</sup>

and TDDR is increased by at least 0.5; and

(ii) a later stretching zone configured and arranged so that

MDDR=(TDDR)<sup>−

1/2</sup>

and TDDR is increased by at least 0.5.

18. A method for stretching a film, the method comprising:

conveying the film into a stretching region;

stretching the film to increase a transverse dimension of the film by greater than a factor of 4 by conveying the opposing edges of the film along generally diverging paths, wherein the generally diverging paths are configured and arranged to provide a machine direction draw ratio (MDDR) and a transverse direction draw ratio (TDDR) based on the paths having the following relationship;

MDDR<

(TDDR)<sup>−

1/2</sup>

during the stretching.

19. The method of claim 18, wherein the diverging paths are substantially parabolic.

20. The method of claim 18, wherein the diverging paths are linear approximations of substantially parabolic paths.

21. The method of claim 18, wherein stretching the film comprises stretching the film to increase a transverse dimension of the film by greater than a factor of 6.8.

22. The method of claim 18, wherein the diverging paths are coplanar.

23. A method for stretching a film, the method comprising:

conveying the film into a stretching region;

stretching the film to increase a transverse dimension of the film by at least a factor of 2.5 by conveying the opposing edges of the film along generally diverging, coplanar paths, wherein the generally diverging paths are configured and arranged to provide a machine direction draw ratio (MDDR) and a transverse direction draw ratio (TDDR) based on the paths having the following relationships;

MDDR<

(TDDR)<sup>−

1/2</sup> 1/(MDDR×

TDDR^1/2)<

2

during the stretching.

24. The method of claim 23, wherein stretching the film comprises stretching the film to increase a transverse dimension of the film by greater than a factor of 5.3.

25. A method for stretching a film, the method comprising:

conveying the film into a stretching region;

stretching the film to increase a transverse dimension of the film by at least a factor of 4.6 by conveying the opposing edges of the film along generally diverging, coplanar paths, wherein the generally diverging paths are configured and arranged to provide a machine direction draw ratio (MDDR) and a transverse direction draw ratio (TDDR) based on the paths having the following relationships;

0.9*MDDR<

(TDDR)<sup>−

1/2</sup> 1/(MDDR×

TDDR^1/2)<

2

during the stretching.

26. The method of claim 25, wherein stretching the film comprises stretching the film to increase a transverse dimension of the film by greater than a factor of 6.8.

27. A method for stretching a film, the method comprising:

conveying the film into a stretching region;

stretching the film by conveying the opposing edges of the film along generally diverging paths, wherein the generally diverging paths are configured and arranged to comprise

(i) an initial stretching zone wherein the generally diverging paths have a functional form corresponding to

±

(x)/(x₁)=(¼

)(x₁/x₀)(y/x₁)²+1, and

(ii) a later stretching zone wherein the generally diverging paths have a functional form corresponding to

±

(x)/(x₂)=(¼

)(x₂/x₀)((y−

A)/x₂)²+1,

wherein x₂ and x₁ are different and A is a y-axis offset.