Method for producing porous membrane and drying device of porous membrane

1. A method for producing a porous membrane comprising:

a solidification step of forming a porous membrane by solidifying a membrane-forming solution by a solidification solution; and

a heating step of drying the porous membrane,

wherein the heating step includes at least a water reduction step of reducing moisture contained in the porous membrane,

the water reduction step uses a heating medium that heats the porous membrane, and

the temperature of the heating medium used in the water reduction step satisfies a relation of the following formula (a):

t gh >T d   (a)

In formula (a), t gh represents the temperature of the heating medium used in the water reduction step and T d represents the heat deformation temperature of a membrane material.

2. The method for producing a porous membrane according to claim 1 ,

wherein the heating step includes a final drying step of drying the porous membrane after the water reduction step,

the final drying step uses a heating medium that heats the porous membrane, and

the temperature of the heating medium used in the final drying step satisfies a relation of the following formula (b):

t sh ≦T d   (b)

In formula (b), t sh represents the temperature of the heating medium used in the final drying step and T d represents the heat deformation temperature of the membrane material.

3. The method for producing a porous membrane according to claim 1 ,

wherein the temperature of the heating medium used in the water reduction step is equal to or higher than a melting point of the membrane material.

4. The method for producing a porous membrane according to claim 1 ,

wherein the maximum surface temperature of the porous membrane in the water reduction step is lower than the heat deformation temperature (T d ) of the membrane material.

5. The method for producing a porous membrane according to claim 1 ,

wherein the water reduction step reduces moisture contained in the porous membrane until the moisture content of the porous membrane reaches at least 70% or less of initial moisture content.

6. The method for producing a porous membrane according to claim 2 ,

wherein the maximum surface temperature of the porous membrane in the final drying step satisfies a relation of the following formula (c):

T gh <T sh <T d   (c)

In formula (c), T gh represents the maximum surface temperature of the porous membrane in the water reduction step, T sh represents the maximum surface temperature of the porous membrane in the final drying step, and T d represents the heat deformation temperature of the membrane material.

7. The method for producing a porous membrane according to claim 6 ,

wherein the temperature of the heating medium used in the final drying step is 120° C. or less.

8. The method for producing a porous membrane according to claim 1 ,

wherein the moisture content of the porous membrane after the heating step is 1% or less, and a reduction rate of the amount of permeated water is 30% or less.

9. The method for producing a porous membrane according to claim 1 ,

wherein the surface temperature of the porous membrane after performing the water reduction step is measured, and the temperature and/or the flow speed of the heating medium used in the water reduction step are controlled according to the surface temperature of the porous membrane.

10. The method for producing a porous membrane according to claim 2 ,

wherein the surface temperature of the porous membrane after performing the final drying step is measured, and the temperature and/or the flow speed of the heating medium used in the final drying step are controlled according to the surface temperature of the porous membrane.

11. The method for producing a porous membrane according to claim 2 ,

wherein the final drying step dries the porous membrane while regulating the tension of the porous membrane.

12. A drying device of a porous membrane including a water reduction unit that reduces moisture contained in a porous membrane with a heating medium, and a final drying unit that is provided in the downstream of the water reduction unit and dries the porous membrane with the heating medium, wherein the drying device of a porous membrane comprises a membrane-surface-temperature measurement means for measuring the surface temperature of the porous membrane and provided between the water reduction unit and the final drying unit; and

a control means for controlling the temperature and/or the flow speed of the heating medium of the water reduction unit according to a measured result of the surface temperature.

13. The drying device of a porous membrane according to claim 12 , further comprising:

a membrane-surface-temperature measurement means for measuring the surface temperature of the porous membrane and provided in the downstream of the final drying unit; and

a control means for controlling the temperature and/or the flow speed of the heating medium of the final drying unit according to a measured result of the surface temperature.

14. A drying device comprising:

a water reduction unit that reduces moisture contained in a porous membrane with use of a heating medium; and

a final drying unit that is provided in the downstream of the water reduction unit and dries the porous membrane,

wherein the final drying unit includes a control mechanism that regulates the tension of the porous membrane,

a temperature of the heating medium used in the water reduction step satisfies a relation of the following formula (a):

t gh >T d   (a)

wherein, in formula (a), t gh represents the temperature of the heating medium used in the water reduction step and T d represents a heat deformation temperature of a membrane material.

15. The method for producing a porous membrane according to claim 9 ,

wherein the surface temperature of the porous membrane after performing the final drying step is measured, and the temperature and/or the flow speed of the heating medium used in the final drying step are controlled according to the surface temperature of the porous membrane.