Method and apparatus for uniformly stretching thermoplastic film and products produced thereby
View Patent ↗Thermoplastic films and laminates are made by extrusion and stretching using differential speed rollers employing a short stretching gap of no more than one inch immediately after the extruded film product is formed. The method and apparatus control the thickness of the stretched film and impart desirable mechanical properties thereto such as film products with a high modulus for handling and suitable tensile for softness.
1. A method of making thermoplastic film, comprising
extruding a thermoplastic extrudate in the form of a web in its molten state,
locating a chill roller operating at a peripheral velocity (V 1 ) and at a temperature (T 1 ) to receive and cool said web thereby forming a film,
locating a second roller operating at a peripheral velocity (V 2 ) greater than said V 1 to receive said film at a temperature (T 2 ),
spacing said second roller from said chill roller to provide a roll gap of no more than one inch between said chill roller and said second roller for stretching said film,
uniformly stretching said film in said roll gap at a stretch temperature between said T 1 and T 2 at a speed between said V 1 and V 2 to form stretched thermoplastic film having a substantially uniform gauge.
2. The method of claim 1 wherein the roll gap is about 0.005 to about 0.05 inch.
3. The method of claim 1 wherein the roll gap is about 0.01 inch.
4. The method of claim 1 wherein the ratio of said V2 to V1 provides a stretching ratio between about 1.25:1 and about 4:1.
5. The method of claim 1 wherein said chill roller is a metal roll which cooperates with a backing roll to form a first nip for receiving said web.
6. The method of claim 5 wherein said chill roller is selected from the group of an embossing metal roll and a flat chrome roll.
7. The method of claim 5 wherein said backing roll is selected from the group of a rubber roll and a metal roll.
8. The method of claim 5 wherein said second roller cooperates with another backing roller to form a second nip for receiving said film from said first nip.
9. The method of claim 8 wherein said second roller is a metal roll.
10. The method of claim 8 wherein said backing roller is a rubber roll.
11. The method of claim 1 wherein a vacuum box is located adjacent to said chill roller.
12. The method of claim 11 wherein said chill roller is selected from the group of an embossing metal roll and a flat chrome roll.
13. The method of claim 11 wherein said second roller cooperates with a backing roller to form a first nip for receiving said film from said chill roller.
14. The method of claim 13 wherein said second roller is a metal roll and said backing roller is a rubber roll.
15. The method of claim 1 where said V 1 is slightly below that where the onset of draw resonance in the extruded web occurs and V 2 is in a range of up to about 4 or more times V 1 .
16. The method of claim 15 where V 2 is about 2 to about 4 times V 1 .
17. The method of claim 1 wherein the extruded film at the chill roller has an original gauge and the stretched film is up to about 4 or more times thinner than the original gauge.
18. The method of claim 1 wherein said film is uniformly stretched at speeds up to 4000 or more linear feet per minute.
19. The method of claim 1 wherein said thermoplastic extrudate is a thermoplastic polymer containing a dispersed phase of pore-forming particles selected from the group consisting of inorganic filler and organic material and said stretched thermoplastic film is microporous having a moisture vapor transmission rate (MVTR) and being a barrier to the passage of liquid.
20. The method of claim 19 wherein said MVTR is greater than about 1,000 g/m2/day, according to ASTM E96(E).
21. The method of claim 19 wherein said MVTR is greater than about 1,000 to 4,000 g/m2/day, according to ASTM E96(E).
22. The method of claim 1 wherein said thermoplastic extrudate comprises a polymer wherein said polymer is selected from the group consisting of polyolefin, polyester, nylon, and blends or coextrusions of two or more of such polymers.
23. The method of claim 22 wherein said polyolefin is selected from the group consisting of polyethylene, polypropylene, copolymers thereof, and blends thereof.
24. The method of claim 23 wherein said polyolefin contains a dispersed phase of pore-forming particles selected from the group consisting of inorganic filler and an organic material, and said stretched thermoplastic film is microporous.
25. The method of claim 24 wherein said pore-forming particle filler is selected from the group consisting of calcium carbonate, barium sulfate, silica, talc, and mica.
26. The method of claim 23 wherein the temperature of the film in said roll gap is in the range of about 20° C. to 100° C. (68° F. to about 212° F.).
27. The method of claim 26 wherein the temperature of the film in said roll gap in the range of about 30° C. to 80° C. (86° F. to 176° F.).
28. The method of claim 26 wherein the temperature of said second roller is from about 21° C. to 82° C. (70° F. to 180° F.).
29. The method of claim 1 wherein the stretching temperature is controlled by the chill roller.
30. The method of claim 29 wherein said second roller controls T 2 at ambient temperature or a higher temperature.
31. The method of claim 1 wherein said T 2 is maintained at or below T 1 .
32. The method of claim 1 wherein said film has a ratio of MD tensile strength at 25% elongation to CD tensile strength at 25% elongation of greater than 2 which provides said film with a modulus for web handling and CD tensile for softness.
33. The method of claim 1 wherein said thermoplastic polymer is an elastomeric polymer.
34. The method of claim 33 wherein said elastomeric polymer is selected from the group consisting of poly(ethylene-butene), poly(ethylene-hexene), poly(ethylene-octene), poly(ethylene-propylene), poly(styrene-butadiene-styrene), poly(styrene-isoprene-styrene), poly(styrene-ethylene-butylene-styrene), poly(ester-ether), poly(ether-amide), poly(ethylene-vinylacetate), poly(ethylene-methylacrylate), poly(ethylene-acrylic acid), poly(ethylene butylacrylate), polyurethane, poly(ethylene-propylene-diene), and ethylene-propylene rubber.
35. The method of claim 34 wherein said polymer contains pore-forming inorganic filler particles which provide microporosity to said film upon stretching.
36. The method of claim 1 wherein said stretched thermoplastic film has a thickness on the order of about 0.25 (about 6 gsm) to about 10 (about 250 gsm) mils.
37. The method of claim 1 wherein said thermoplastic extrudate comprises
about 30% to about 45% by weight of a linear low density polyethylene or polypropylene,
about 1% to about 10% by weight of a low density polyethylene, and
about 40% to about 60% by weight of calcium carbonate filler particles.
38. The method of claim 37 wherein the composition further contains a component selected from the group consisting of high density polyethylene, titanium dioxide, and mixtures thereof.
39. The method of claim 1 wherein said thermoplastic extrudate is a coextruded structure of one or more layers of different polymer compositions to produce a thermoplastic film selected from the group consisting of a
(a) breathable microporous film,
(b) nonbreathable microporous film, and
(c) nonmicroporous film.
40. The method of claim 39 wherein the coextruded structure has one or more layers of a polymer selected from the group consisting of polypropylene, linear low density polyethylene (LLDPE), low density polyethylene (LDPE), and blends thereof.
41. The method of claim 1 comprising the further step of introducing said thermoplastic film into incremental stretching rollers to incrementally stretch said film.
42. The method of claim 41 wherein said incremental stretching rollers comprise a first section and a second section and said film is incrementally stretched in a first direction by said first section followed by incremental stretching in a second direction by said second section.
43. The method of claim 42 wherein said first and second stretching directions are substantially perpendicular to one another.
44. The method of claim 37 wherein said stretched thermoplastic film has a film thickness of about 0.25 to 2 mils.
45. A method of making a breathable and liquid barrier microporous thermoplastic film comprising
extruding a microporous formable thermoplastic molten extrudate selected from the group consisting of polypropylene, polyethylene, copolymers thereof and blends thereof, said extrudate containing a pore-forming filler,
locating a chill roller operating at a peripheral velocity (V 1 ) and at a temperature (T 1 ) to cool said web thereby forming a microporous formable film,
locating a second roller operating at a peripheral velocity (V 2 ) greater than said V 1 to receive said film at a temperature (T 2 ),
spacing said second roller from said chill roller to provide a roll gap of no more than one inch between said chill roller and said second roller for stretching said film,
stretching said film in said roll gap at a stretch temperature between said T 1 and T 2 at a speed between said V 1 and V 2 to form a microporous thermoplastic film having a film thickness of about 0.25 to about 10 mils.
46. The method of claim 45 wherein the microporous film has a moisture vapor transmission rate (MVTR) greater than about 1,000 g/m2/day according to ASTM E96(E).
47. The method of claim 45 wherein said MVTR is about 1,000 to about 4,000 g/m2/day, according to ASTM E96(E).
48. The method of claim 45 wherein said microporous film has a ratio of MD tensile strength at 25% elongation to CD tensile strength at 25% elongation of greater than 2 which provides said microporous film with a modulus for web handling and CD tensile for softness.
49. The method of claim 1 wherein the film is annealed.
50. The method of claim 1 comprising the further step of laminating said extrudate to an extensible nonwoven fibrous sheet during said extrusion to form a laminate of said web and sheet before said uniform stretching.
51. The method of claim 1 comprising the further step of laminating said stretched thermoplastic film to a nonwoven fibrous sheet to form a laminate of said film and said sheet after said uniform stretching.