Processes and apparatus for making transversely drawn films with substantially uniaxial character

• US 20020180107A1
• Filed: 05/28/2002
• Published: 12/05/2002
• Est. Priority Date: 05/31/2001
• Status: Active Grant

• Current Assignee: 3M Innovative Properties Company (3M Company)
• Original Assignee: 3M Innovative Properties Company (3M Company)
• Inventors: Merrill, William Ward, Ruff, Andrew T., Jackson, Jeffery N., Krueger, Dennis L., Thompson, Desiree L.
• Granted Patent: US 6,916,440 B2
• Time in Patent Office: Days
• US Class Current: 264/288.4
• CPC Class Codes: B29C 2793/009 after shaping
• CPC Class Codes: B29C 55/08 transverse to the direction of feed
• CPC Class Codes: B29C 55/20 Edge clamps
• CPC Class Codes: B29C 69/001 a shaping technique combined with cutting, e.g. in parts or slices combined with rearranging and joining the cut parts for reinforced material B29C70/545; B29C49/4278, B29C51/268 take precedence
• CPC Class Codes: B29K 2067/003 PET, i.e. poylethylene terephthalate
• CPC Class Codes: B29K 2995/0051 mono-axially
• CPC Class Codes: B29L 2011/00 Optical elements, e.g. lenses, prisms
• CPC Class Codes: G02B 5/3033 in the form of a thin sheet or foil, e.g. Polaroid
• CPC Class Codes: Y10S 425/041 Reshape

Abstract

A process for stretching films is described. The process preferably stretches films in a uniaxial fashion. Preferably, optical films are stretched including multilayer optical films. Other aspects of the invention include a roll of stretched film and an apparatus for stretching films.

Claims

1. A method of processing a film, the method comprising:

conveying a film within a stretcher along a machine direction while holding opposing edge portions of the film;

stretching the film within a stretching region of the stretcher by moving the opposing edge portions along diverging paths to form a stretched region of the film;

releasing the opposing edge portions of the stretched region of the film; and

grasping the stretched region of the film at opposing take-away regions positioned inside each of the opposing edge portions prior to releasing the opposing edge portions to take the stretched film away from stretching region.

2. The method of claim 1, wherein the step of grasping the stretched region of the film further comprises slitting the stretched region of the film between the opposing edge regions and the opposing take-away regions to form selvages that continue along the diverging paths.

3. The method of claim 1, wherein, after grasping the stretched region of the film at opposing take-away regions, the portion of the stretched region of the film between the opposing take-away regions is not subject to substantial additional stretching.

4. The method of claim 1, wherein stretching the film within a stretching region of the stretcher comprising stretching the film within the stretching region of the stretcher by moving the opposing edge portions along diverging, substantially parabolic paths to form the stretched region of the film.

5. The method of claim 4, wherein the diverging, substantially parabolic paths have a functional form:

x/x₀=±

((¼

)(y/x₀)²+1)

wherein an x-axis is defined in a plane of the film and orthogonal to the machine direction with x=0 at a center axis of the film, a y-axis is defined in the plane of the film and along the machine direction with y=0 where stretching of the film commences within the stretcher, and x₀ is a distance of the edge portions from the center axis of the film when y=0.

6. The method of claim 4, wherein at least a portion of the diverging, substantially parabolic paths have a functional form:

(x+c)/(x₁+c)=(¼

)(y/(x₁+c))²+1

wherein an x-axis is defined in a plane of the multilayer film and orthogonal to the machine direction with x=0 at a center axis of the multilayer film, a y-axis is defined in the plane of the multilayer film and along the machine direction with y=0 where stretching of the multilayer film commences within the stretcher, c is any number, and x₁ is a distance of the edge portions from the center axis of the multilayer film when y=0.

7. The method of claim 1, further comprising providing the film in a continuous manner to the stretcher.

8. The method of claim 7, wherein providing the film comprises providing the film in a continuous manner to the stretcher from a roll of film.

9. The method of claim 7, wherein providing the film comprises extruding or coextruding the film in-line with the stretcher.

10. The method of claim 1, wherein stretching the film within a stretching region of the stretcher comprising stretching the film within the stretching region of the stretcher by moving the opposing edge portions along diverging, substantially parabolic paths to form the stretched region of the film, wherein the film has an initial thickness and initial width when conveyed into the stretcher and the stretched region of the film has a stretched thickness and a stretched width;

and

wherein, after stretching the film to a ratio of stretched width/initial width defined as λ

, a ratio of stretched thickness/initial thickness is approximately λ

<sup>−

1/2</sup>.

26. The method of claim 1, wherein a take-away speed of the film while the film is grasped at the opposing take-away regions is greater than a speed of the film in the stretching region.

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

conveying a film within a stretcher along a machine direction while holding opposing edge portions of the film;

stretching the film within a stretching region of the stretcher by moving the opposing edge portions along diverging paths to form a stretched film;

slitting the stretched film between the opposing edge regions and opposing takeaway regions to form selvages that continue along the diverging paths while a remainder of the stretched film is conveyed in the machine direction; and

releasing the opposing edge portions of the stretched film.

12. The method of claim 11, further comprising grasping the film at opposing take-away regions inside each of the opposing edge portions prior toslitting the stretched film.

13. The method of claim 11, further comprising grasping the film at opposing take-away regions inside each of the opposing edge portions while slitting the stretched film.

14. The method of claim 11, wherein stretching the film within a stretching region of the stretcher comprising stretching the film within the stretching region of the stretcher by moving the opposing edge portions along diverging, substantially parabolic paths to form the stretched region of the film.

16. The apparatus of claim 15, wherein the isolated takeaway device is configured and arranged to convey the film without substantial stretching of the region of the film between the opposing take-away regions.

17. The apparatus of claim 15, wherein the conveyor comprises a plurality of pathways with a plurality of the gripping members disposed to follow along each of the pathways.

18. The apparatus of claim 15, further comprising a slitting member positioned within the apparatus to slit the stretched portion of the film between the opposing edge regions and the opposing takeaway regions to form selvages that continue along the diverging paths while a remainder of the stretched portion of the film is conveyed in the machine direction.

19. The apparatus of claim 15, wherein the slitting member is configured and arranged to slit the stretched portion of the film after the takeaway device has grasped the opposing takeway regions.

20. The apparatus of claim 15, wherein the diverging paths are substantially parabolic.

21. The apparatus of claim 20, wherein the diverging, substantially parabolic paths have a functional form:

x/x₀=±

(¼

)(y/x₀)²+1)

wherein an x-axis is defined in a plane of the film and orthogonal to the machine direction with x=0 at a center axis of the film, a y-axis is defined in the plane of the film and along the machine direction with y=0 where stretching of the film commences within the stretcher, and x₀ is a distance of the edge portions from the center axis of the film when y=0.

22. The apparatus of claim 20, wherein at least a portion of the diverging, substantially parabolic paths have a functional form:

(x+c)/(x₁+c)=±

((¼

)(y/(x₁+c))²+1)

wherein an x-axis is defined in a plane of the multilayer film and orthogonal to the machine direction with x=0 at a center axis of the multilayer film, a y-axis is defined in the plane of the multilayer film and along the machine direction with y=0 where stretching of the multilayer film commences within the stretcher, c is any number, and x₁ is a distance of the edge portions from the center axis of the multilayer film when y=0.

23. The apparatus of claim 15, further comprising an extruder in-line with the conveyor, wherein the extruder is configured and arranged to provide the film to the conveyor.

25. The apparatus of claim 24, further comprising slitting means for slitting a stretched portion of the film between the opposing edge regions and the opposing takeaway regions to form selvages.

15. An apparatus for processing a film, the apparatus comprising:

a conveyor comprising gripping members that hold opposing edge portions of the film and convey, under influence of a drive member, the film along a machine direction within a stretching region of the apparatus, wherein a portion of the conveyor is configured and arranged to provide diverging paths along which the gripping members move to stretch the film; and

an isolated takeaway device that receives the film from the conveyor and comprising gripping members to grasp opposing takeaway regions of the film after a desired amount of stretching and convey the film in the machine direction.

24. An apparatus for processing a film, the apparatus comprising:

conveying means for conveying a film along a machine direction within a stretching region of the apparatus, the conveying means comprising gripping means for holding opposing edge portions of the film, a portion of the conveying means providing diverging paths to stretch the film; and

isolated takeaway means for grasping opposing takeaway regions and taking the film from the conveyor after a desired amount of stretching and conveying the film further along in the machine direction.