Fluororesin film having excellent transparency

1. A film comprising a copolymer, the copolymer containing an ethylene unit, a tetrafluoroethylene unit, and a (fluoroalkyl)ethylene unit represented by Formula (1):

CH 2 ═CX—Rf  (1)

wherein X represents H or F, and Rf represents a fluoroalkyl group having 2 or more carbon atoms,

the copolymer containing the (fluoroalkyl)ethylene unit in an amount of 0.8 to 2.5 mol % relative to the amount of all the monomer units and containing the ethylene unit and the tetrafluoroethylene unit at a molar ratio of 30.0/70.0 to 50.0/50.0,

the film having a crystallinity of 68% or less, the crystallinity being calculated by the following equation on the basis of a diffraction intensity curve of the film resulting from X-ray diffraction measurement:

Crystallinity (%)=( S 19 +S 20 )/( S 17 +S 19 +S 20 )×100

wherein S 20 represents a peak area around 2θ=20°,

S 17 represents a peak area around 2θ=17°, and

S 19 represents a peak area around 2θ=19°.

2. The film according to claim 1 ,

wherein a peak area ratio around 2θ=20° calculated by the following equation is 10.0% or less:

Peak area ratio (%)= S 20 /( S 19 +S 20 )×100.

3. The film according to claim 1 ,

wherein a half-width of the peak around 2θ=19° is 3.0 or less.

4. The film according to claim 1 ,

wherein the copolymer has a melt flow rate of 40 (g/10 min) or less at 297° C.

5. The film according to claim 1 ,

wherein transmittance of light having a wavelength of 300 to 1100 nm is 75% or more.

6. The film according to claim 1 ,

wherein an arithmetic average inclination is 0.2° or less.

7. A solar battery including the film according to claim 1 .

8. A method for producing a film comprising a copolymer, the copolymer containing an ethylene unit, a tetrafluoroethylene unit, and a (fluoroalkyl)ethylene unit represented by Formula (1):

CH 2 ═CX—Rf  (1)

wherein X represents H or F, and Rf represents a fluoroalkyl group having 2 or more carbon atoms,

the copolymer containing the (fluoroalkyl)ethylene unit in an amount of 0.8 to 2.5 mol % relative to the amount of all the monomer units and containing the ethylene unit and the tetrafluoroethylene unit at a molar ratio of 30.0/70.0 to 50.0/50.0,

the film having a crystallinity of 68% or less, the crystallinity being calculated by the following equation on the basis of a diffraction intensity curve of the film resulting from X-ray diffraction measurement:

Crystallinity (%)=( S 19 +S 20 )/( S 17 +S 19 +S 20 )×100

wherein S 20 represents a peak area around 2θ=20°,

S 17 represents a peak area around 2θ=17°, and

S 19 represents a peak area around 2θ=19°,

the method comprising the steps of

melting the copolymer by heating to a melting point or higher,

molding the molten copolymer into a film,

rapidly cooling the film by contacting the film maintained at a temperature not lower than the crystallization temperature of the copolymer, with a cooling roll set to a temperature not exceeding a temperature that is higher than the glass transition temperature of the film by 10° C., and

collecting the rapidly cooled film.

9. The method according to claim 8 ,

wherein, a method of contacting the film with the cooling roll is a method of charging the cooling roll with static electricity for firm contact of the film with the cooling roll or a method of using a pressing roll to press the film to the cooling roll for firm contact of the film with the cooling roll.

10. The method according to claim 8 ,

wherein the melting and the molding are performed using an extruder equipped with a die, the die having an outlet set at a distance of 50 mm or less from a contact point of the film with the cooling roll.