Film-stretching apparatus and method of producing film

1. A method of producing a film having a porous structure, the film being configured to be used as separator for a nonaqueous electrolyte secondary battery, the method comprising:

a film stretching step of stretching a film by use of a film-stretching apparatus which includes a stretching furnace having (i) an entrance through which the film is transferred into the stretching furnace and (ii) an exit through which the film is transferred out of the stretching furnace; and

a transfer step of holding, on the outside of the stretching furnace, both sides of the film with use of chucks and transferring the film into the stretching furnace through the entrance,

wherein, in the film stretching step, an airflow in the entrance is controlled so as to be directed toward an outside of the stretching furnace, the airflow containing a plasticizing agent volatilized from the film,

wherein, in the transfer step, while the chucks are moving outside of the stretching furnace, a temperature of the chucks is kept at not lower than 90° C. and not higher than 150° C.,

wherein the stretching furnace is divided, along a direction in which the film is transferred, into a plurality of air amount control zones, one of the plurality of air amount control zones being a zone connected to the entrance,

wherein the method further comprises independently controlling an amount of air in each of the plurality of air amount control zones,

wherein the plurality of air amount control zones include at least a pre-heating chamber and a stretching chamber,

wherein each of the pre-heating chamber and the stretching chamber supplies and discharges air, and

wherein a pressure in at least the zone connected to the entrance is controlled so as to be a positive pressure, and wherein the produced film has the porous structure.

2. The method as set forth in claim 1 , further comprising controlling a pressure in the entire stretching furnace to be a positive pressure.

3. The method as set forth in claim 2 , wherein a difference between the pressure in the stretching furnace and atmospheric pressure is not smaller than 2 Pa.

4. The method as set forth in claim 3 , wherein the airflow directed toward the outside of the stretching furnace has an airflow rate of not less than 0.5 m/sec.

5. The method as set forth in claim 1 , further comprising, in the film stretching step, controlling an airflow in the exit so as to be directed toward the outside of the stretching furnace.

6. The method as set forth in claim 5 , further comprising controlling a pressure in the entire stretching furnace to be a positive pressure.

7. The method as set forth in claim 6 , wherein a difference between the pressure in the stretching furnace and atmospheric pressure is not smaller than 2 Pa.

8. The method as set forth in claim 7 , wherein the airflow in the exit directed toward the outside of the stretching furnace has an airflow rate of not less than 0.5 m/sec.

9. The method as set forth in claim 1 , further comprising independently controlling an operating temperature of each of the air amount control zones.

10. The method as set forth in claim 1 , wherein the plurality of air amount control zones includes a pre-heating chamber and at least a first stretching chamber.

11. The method as set forth in claim 10 , wherein a temperature in the pre-heating chamber is controlled within a range of 115° C. to 125° C. and wherein a temperature in the at least first stretching chamber is controlled within a range of 100° C. to 115° C.

12. The method as set forth in claim 10 , wherein the plurality of air amount control zones includes a heat-fixing chamber in which the film which has been stretched is heated while a width of the film which has been stretched is maintained.

13. The method as set forth in claim 12 , wherein a temperature in the heat-fixing chamber is controlled within a range of 115° C. to 130° C.

14. The method as set forth in claim 1 , further comprising pre-heating the film in the stretching furnace prior to stretching the film.

15. The method as set forth in claim 14 , wherein the film comprises a polyethylene-based resin.

16. The method as set forth in claim 15 , wherein the film stretching step is carried out at a temperature which is lower than a temperature to which the film has been pre-heated.

17. The method as set forth in claim 1 , wherein the film is uniaxially or biaxially stretched in the stretching furnace.

18. The method as set forth in claim 1 , wherein a speed [%/sec] at which the film is stretched is proportional to a speed at which the film is transferred through the film-stretching apparatus.