Wet-laid nonwoven fabric for semipermeable membrane supporting body, method for producing said wet-laid nonwoven fabric, and method for identifying low-density defect of wet-laid nonwoven fabric

Abstract

A uniform semipermeable membrane supporting body which does not cause any defect in the semipermeable membrane coating layer when a semipermeable membrane coating liquid is applied, and is free of low density defects. The wet-laid nonwoven fabric for a semipermeable membrane supporting body according to the present disclosure contains a synthetic fiber as a main constituent fiber, has been subjected to hot press processing, has a pressure loss at a face velocity of 5.3 cm/second of from 50 Pa to 3000 Pa and a sheet density of 0.5 g/cm 3 or greater, and satisfies the evaluation criterion of condition 1 or condition 2: (Condition 1) Spots are not observed when solid printing is carried out with a printing ink over the entire surface of one surface of the wet-laid nonwoven fabric for a semipermeable membrane supporting body (rolled product, cut to a length of 1800 mm in the length direction); and (Condition 2) When a wet-laid nonwoven fabric for a semipermeable membrane supporting body that has been cut to a size of 210 mm×297 mm is subjected to solid printing over the entire surface of one surface with a printing ink, is used as a sample, spots are not observed in 95% or more of the samples among such samples thus extracted.

What is claimed is: 
     
         1 . A wet-laid nonwoven fabric for a semipermeable membrane supporting body, the nonwoven fabric comprising a synthetic fiber as a main constituent fiber and having been subjected to hot press processing,
 wherein the wet-laid nonwoven fabric for a semipermeable membrane supporting body has a pressure loss at a face velocity of 5.3 cm/second of from 50 Pa to 3000 Pa, and a sheet density of 0.5 g/cm 3  or greater, and satisfies the evaluation criterion of condition 1 or condition 2:   (Condition 1) Spots are not observed when solid printing is carried out with a printing ink over the entire surface of one surface of the wet-laid nonwoven fabric for a semipermeable membrane supporting body (rolled product, cut to a length of 1800 mm in the length direction); and   (Condition 2) When a wet-laid nonwoven fabric for a semipermeable membrane supporting body that has been cut to a size of 210 mm×297 mm is subjected to solid printing over the entire surface of one surface with a printing ink, is used as a sample, spots are not observed in 95% or more of the samples among such samples thus extracted.   
     
     
         2 . The wet-laid nonwoven fabric for a semipermeable membrane supporting body according to  claim 1 , further comprising a binder fiber. 
     
     
         3 . The wet-laid nonwoven fabric for a semipermeable membrane supporting body according to  claim 1 , wherein the nonwoven fabric has been subjected to the hot press processing two or more times. 
     
     
         4 . The wet-laid nonwoven fabric for a semipermeable membrane supporting body according to  claim 3 , wherein the hot press processing temperature in the second and subsequent treatments is higher by 10° C. or more than the hot press processing temperature in the first treatment. 
     
     
         5 . A method for producing a wet-laid nonwoven fabric for a semipermeable membrane supporting body, the method comprising: performing papermaking according to a wet papermaking method by using a fiber slurry containing a synthetic fiber as a main constituent fiber; drying the fiber slurry; subsequently subjecting the dried sheet to hot press processing two or more times by using a heat calender apparatus, while treating the dried sheet by using a hard nip heat calender apparatus equipped with a combination of a metal roll and a metal roll for at least one time of the hot press processing; and thereby obtaining a wet-laid nonwoven fabric for a semipermeable membrane supporting body. 
     
     
         6 . The method for producing a wet-laid nonwoven fabric for a semipermeable membrane supporting body according to  claim 5 , wherein the hot press process temperature in the second and subsequent treatments is adjusted to be higher by 10° C. or more than the hot press processing temperature in the first treatment. 
     
     
         7 . The method for producing a wet-laid nonwoven fabric for a semipermeable membrane supporting body according to  claim 5 , wherein the hot press processing is carried out two or more times by using a hard nip heat calender apparatus equipped with a combination of a metal roll and a metal roll. 
     
     
         8 . A method for identifying a low density defect in a wet-laid nonwoven fabric for a semipermeable membrane supporting body, the nonwoven fabric comprising a synthetic fiber as a main constituent fiber and having been subjected to hot press processing,
 wherein when the wet-laid nonwoven fabric for a semipermeable membrane supporting body satisfies the evaluation criterion of condition 1 or condition 2, it is determined that the nonwoven fabric has no low density defect; and when the wet-laid nonwoven fabric does not satisfy the evaluation criterion of condition 1 or condition 2, it is determined that the nonwoven fabric has a low density defect:   (Condition 1) Spots are not observed when solid printing is carried out with a printing ink over the entire surface of one surface of the wet-laid nonwoven fabric for a semipermeable membrane supporting body (rolled product, cut to a length of 1800 mm in the length direction); and   (Condition 2) When a wet-laid nonwoven fabric for a semipermeable membrane supporting body that has been cut to a size of 210 mm×297 mm is subjected to solid printing over the entire surface of one surface with a printing ink, is used as a sample, spots are not observed in 95% or more of the samples among such samples thus extracted.   
     
     
         9 . The wet-laid nonwoven fabric for a semipermeable membrane supporting body according to  claim 2 , wherein the nonwoven fabric has been subjected to the hot press processing two or more times. 
     
     
         10 . The wet-laid nonwoven fabric for a semipermeable membrane supporting body according to  claim 9 , wherein the hot press processing temperature in the second and subsequent treatments is higher by 10° C. or more than the hot press processing temperature in the first treatment. 
     
     
         11 . The method for producing a wet-laid nonwoven fabric for a semipermeable membrane supporting body according to  claim 6 ,
 wherein the hot press processing is carried out two or more times by using a hard nip heat calender apparatus equipped with a combination of a metal roll and a metal roll.