Oriented porous polypropylene films

• US 4,975,469 A
• Filed: 03/20/1989
• Issued: 12/04/1990
• Est. Priority Date: 03/20/1989
• Status: Expired due to Term

• Current Assignee: Amoco Corporation (BP PLC)
• Original Assignee: Amoco Corporation (BP PLC)
• Inventors: Jacoby, Philip, Bauer, Charles W.
• Primary Examiner(s): Hoke, Veronica P.
• Application Number:
  US07/325,680
• Time in Patent Office: 624 Days
• Field of Search: 521/90, 521/143, 521/84.1, 523/128
• US Class Current: 521/84.1
• CPC Class Codes: A01G 13/0275 Films
• CPC Class Codes: A01G 9/1438 Covering materials therefor; Materials for protective coverings used for soil and plants, e.g. films, canopies, tunnels or cloches
• CPC Class Codes: B29C 55/005 characterised by the choice of materials
• CPC Class Codes: B29C 67/202 comprising elimination of a solid or a liquid ingredient
• CPC Class Codes: B29K 2023/12 PP, i.e. polypropylene
• CPC Class Codes: C08J 2201/0424 containing halogen, nitrogen, sulphur or phosphorus atoms
• CPC Class Codes: C08J 2323/10 Homopolymers or copolymers of propene
• CPC Class Codes: C08J 5/18 Manufacture of films or sheets
• CPC Class Codes: C08J 9/26 by elimination of a solid phase from a macromolecular composition or article, e.g. leaching out
• CPC Class Codes: Y02A 40/25 Greenhouse technology, e.g. cooling systems therefor

Abstract

This invention relates to oriented porous films having cells with polygonal cross sections and average pore diameters of about 5 to about 30 microns and interconnecting pores between the cells which have a diameter of about 0.2 to about 20 microns which are formed from a polypropylene-based resin and a process for forming the film which comprises the steps of forming a film containing beta-spherulites, selectively extracting a portion of the beta-spherulites to form a porous film and stretching the porous film in at least one direction to form the oriented polymeric porous film.

Claims

1. An oriented polymeric porous film with polygonal cells having average diameters of about 5 to about 30 microns and interconnecting pores between the cells having average diameters of about 0.2 to about 20 microns prepared from a polypropylene-based resin having at least 40% crystallinity by a process comprising the steps of:

(a) dispersing in said polypropylene-based resin a nucleating agent capable of producing beta-spherulites;

(b) extruding the nucleating agent-containing polypropylene-based resin into a film;

(c) cooling said film below the crystallization temperature of said polypropylene-based resin to form at least 20 weight percent of beta-spherulites in the film;

(d) extracting selectively said beta-spherulites in an amount corresponding to at least 15 weight percent of the polypropylene-based resin from the cooled film with an extraction solvent to form a porous film; and

(e) orienting the porous film by heating the porous film at a temperature of about 115°

to about 135°

C. for a time period of about 2 to about 20 seconds and stretching the heated porous film in at least one direction at a stretch ratio of about 1.5 to about 7.5 to form said oriented polymeric porous film.

2. The oriented polymeric porous film of claim 1 wherein said nucleating agent has the structural formula:

##STR2##

3. The oriented polymeric porous film of claim 2 wherein said nucleating agent is dispersed in the polypropylene-based resin at a level of about 0.1 to about 10 ppm based on the weight of said resin.

4. The oriented polymeric porous film of claim 1 wherein said extracting solvent is selected from the group consisting of toluene, carbon tetrachloride or xylene.

5. The oriented polymeric porous film of claim 1 wherein said polypropylene-based resin contains about 5 to about 15 weight percent cornstarch based on the weight of said resin.

6. The oriented polymeric porous film of claim 1 wherein said step of extracting selectively said beta-spherulites from the cooled film is at a temperature of about 85°

to about 95°

C. for a time period of 10 minutes or less.

7. The oriented polymeric porous film of claim 5 wherein said step of extracting selectively said beta-spherulites from the cooled film is at a temperature of about 85°

to about 90°

C. for a time period of 10 minutes or less.

8. A process for preparing an oriented polymeric porous film with polygonal cells having average diameters of about 5 to about 30 microns and interconnecting pores between the cells having average diameters of about 0.2 to about 20 microns from a polypropylene-based resin having at least 40% crystallinity comprising the steps of:

(a) dispersing in said polypropylene-based resin a nucleating agent capable of producing beta-spherulites;

(b) extruding the nucleating agent-containing polypropylene-based resin into a film;

(c) cooling said film below the crystallization temperature of said polypropylene-based resin to form at least 20 weight percent of beta-spherulites in the film;

(d) extracting selectively said beta-spherulites in an amount corresponding to at least 15 weight percent of the polypropylene-based resin from the cooled film with an extraction solvent to form a porous film; and

(e) orienting the porous film by heating the porous film at a temperature of about 115°

to about 135°

C. for a time period of about 2 to about 20 seconds and stretching the heated porous film in at least one direction at a stretch ratio of about 1.5 to about 7.5 to form said oriented polymeric porous film.

9. The process of claim 8 wherein said nucleating agent has the structural formula:

##STR3##

10. The process of claim 8 wherein said nucleating agent is dispersed in the polypropylene-based resin at a level of about 0.1 to about 10 ppm based on the weight of said resin.

11. The process of claim 8 wherein said extracting solvent is selected from the group consisting of toluene, carbon tetrachloride or xylene.

12. The process of claim 8 wherein said polypropylene-based resin contains about 5 to about 15 weight percent cornstarch based on the weight of said resin.

13. The process of claim 8 wherein said step of extracting selectively said beta-spherulites from the cooled film is at a temperature of about 85°

to about 95°

C. for a time period of 10 minutes or less.

14. The process of claim 8 wherein the porous film is oriented by stretching the porous film uniaxially with a stretch ratio of about 2.0 to about 6.0.

15. The process of claim 8 wherein the porous film is oriented by stretching the porous film biaxially with a stretch ratio of about 2.0 to about 5.0 said stretch ratio being the same in a machine direction and a transverse direction.

16. The process of claim 8 wherein said oriented film has a thickness of about 0.005 to about 0.2 millimeters.

17. An oriented polymeric porous film having a thickness of about 0.005 to about 0.2 millimeters, a moisture vapor transmission rate as determined according to ASTM E-96, procedure E, in the range of about 2,500 to about 7,500 grams per square meter per 24 hours prepared from a polypropylene-based resin having at least 40% crystallinity by a process comprising the steps of:

(a) forming a homogeneous blend of a polypropylene-based resin, about 0.5 to about 10 ppm of a red quinacridone dye acting as a beta-spherulite nucleant and optionally a starch-containing material;

(b) extruding the blend into a film having a thickness of about 0.01 to about 0.4 millimeters and at least 20 percent beta-spherulites on a cast film line having a chill roll temperature of about 90°

to 130°

C.;

(c) extracting beta-spherulites corresponding to at least 15 weight percent of the blend from the cast film by immersing the cast film in a toluene bath at a temperature of about 85°

to about 95°

C. for a time period of 10 minutes or less to form a porous film;

(d) drying the porous film at a temperature of about 15°

to about 110°

C. for 20 minutes or less; and

(e) orienting the porous film by heating the porous film at a temperature of about 115°

to about 135°

C. for a time period of about 2 to about 20 seconds and stretching the heated porous film in at least one direction at a stretch ratio of about 1.5 to about 7.5 to form the oriented polymeric porous film.