Ion-exchange membrane and process for producing the same

Abstract

[Problems] To provide an ion-exchange membrane using an inexpensive nonwoven fabric sheet as a base sheet, featuring excellent properties such as strength, dimensional stability and shape stability, effectively suppressing undulation when it is brought into contact with the electrolyte, and having a low membrane resistance and stable properties. [Means for Solution] An ion-exchange membrane comprising a nonwoven fabric sheet 1 and an ion-exchange resin coating 3 formed on one surface of the nonwoven fabric sheet 1, the nonwoven fabric sheet 1 having a fiber layer structure that includes long filament layers 5 of a fiber diameter of 8 to 30 μm on both surfaces thereof and an extra-fine filament layer 7 of a fiber diameter of not more than 5 μm as an intermediate layer formed by melt-adhesion of fibers between the long filament layers 5.

1 . An ion-exchange membrane comprising a nonwoven fabric sheet and an ion-exchange resin coating formed on one surface of said nonwoven fabric sheet, said nonwoven fabric sheet having a fiber layer structure that includes long filament layers of a fiber diameter of 8 to 30 μm on both surfaces thereof and an extra-fine filament layer of a fiber diameter of not more than 5 μm as an intermediate layer formed by melt-adhesion of fibers between said long filament layers. 
     
     
         2 . The ion-exchange membrane according to  claim 1 , wherein said ion-exchange membrane has a burst strength of not less than 0.1 MPa, an electric resistance (25° C.) of not more than 10 Ωcm 2 , a rate of area change of not more than 2% and a rate of thickness change of not more than 10% in a dry state of after it is held in a dry atmosphere of 50° C. for 24 hours and in a wet state of after it is dipped in pure water of 25° C. for 24 hours, and a surface roughness Ra of not more than 10 μm on a surface of the ion-exchange resin coating of the ion-exchange membrane in said wet state. 
     
     
         3 . A process for producing an ion-exchange membrane comprising steps of:
 providing a nonwoven fabric sheet having a fiber layer structure that includes long filament layers of a fiber diameter of 8 to 30 μm on both surfaces thereof and an extra-fine filament layer of a fiber diameter of not more than 5 μm as an intermediate layer formed by melt-adhesion of fibers between said long filament layers, a strippable film, and an organic solvent solution of an ion-exchange resin or an ion-exchange resin precursor that is capable of introducing ion-exchange groups;   applying said organic solvent solution onto one surface of said strippable film to form an applied layer of the ion-exchange resin or the ion-exchange resin precursor;   overlapping said nonwoven fabric sheet on said applied layer;   forming a coating of the ion-exchange resin or the ion-exchange resin precursor by drying said applied layer in a state where said nonwoven fabric sheet has been overlapped thereon; and   stripping said strippable film.   
     
     
         4 . The process for producing the ion-exchange membrane according to  claim 3 , wherein when an organic solvent solution of said ion-exchange resin precursor is used as said organic solvent solution, provision is, further, made of a step of introducing ion-exchange groups into said ion-exchange resin precursor after the coating of the ion-exchange resin precursor has been formed but before stripping said strippable film. 
     
     
         5 . The process for producing the ion-exchange membrane according to  claim 3 , wherein said organic solvent solution has a viscosity (25° C.) in a range of 0.5 to 80 dPa·s. 
     
     
         6 . A nonwoven fabric sheet for use as a base sheet of an ion-exchange membrane that has a fiber layer structure that includes long filament layers of a fiber diameter of 8 to 30 μm on both surfaces thereof and an extra-fine filament layer of a fiber diameter of not more than 5 μm as an intermediate layer formed by melt-adhesion of fibers between said long filament layers.