Ultrafine fibers and liquid fiber dispersion

Abstract

A fiber dispersion liquid includes ultrafine fibers having a fiber diameter of 100 to 5000 nm dispersed in an aqueous medium, wherein the fiber dispersion liquid has a solid content concentration of 0.01% to 10% by weight and a dispersion index of 20 or less, in which the dispersion index is measured by preparing a fiber dispersion liquid such that a solid content concentration is 0.01% by weight with respect to a total amount of the fiber dispersion liquid, an image of the obtained fiber dispersion liquid at a magnification of 50 times is captured with a microscope under transmitted illumination, the image is converted into a monochrome image by using an image processing software, then a luminance histogram is formed with 256 grades, and a standard deviation obtained from the luminance histogram is used as the dispersion index.

What is claimed is: 
     
         1 . A fiber dispersion liquid comprising ultrafine fibers having a fiber diameter of 100 to 5000 nm dispersed in an aqueous medium, wherein the fiber dispersion liquid has a solid content concentration of 0.01% to 10% by weight and a dispersion index of 20 or less, in which the dispersion index is measured by:
 preparing a fiber dispersion liquid such that a solid content concentration is 0.01% by weight with respect to a total amount of the fiber dispersion liquid, an image of the obtained fiber dispersion liquid at a magnification of 50 times is captured with a microscope under transmitted illumination, the image is converted into a monochrome image by using an image processing software, then a luminance histogram is formed with 256 grades, and a standard deviation obtained from the luminance histogram is used as the dispersion index.   
     
     
         2 . The fiber dispersion liquid according to  claim 1 , wherein a dispersion stability index defined by formula (1) is 0.70 or more: 
       
         
           
             
               
                 
                   
                     
                       Dispersion 
                       ⁢ 
                           
                       stability 
                       ⁢ 
                           
                       index 
                     
                     = 
                     
                       
                         H 
                         0 
                       
                       / 
                       
                         H 
                         1 
                       
                     
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
         wherein H 0  is a height of a fiber dispersion liquid in a container after standing for 10 minutes, and H 1  is a dispersion height of the fiber dispersion liquid in the container after standing for 7 days. 
       
     
     
         3 . The fiber dispersion liquid according to  claim 1 , wherein a thixotropy index (TI) defined by formula (2) is 7.0 or more: 
       
         
           
             
               
                 
                   
                     
                       Thixotropy 
                       ⁢ 
                           
                       index 
                       ⁢ 
                           
                       
                         ( 
                         TI 
                         ) 
                       
                     
                     = 
                     
                       
                         η 
                         6 
                       
                       / 
                       
                         η 
                         60 
                       
                     
                   
                 
                 
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
         wherein η 6  is viscosity (at 25° C.) of a fiber dispersion liquid prepared such that solid content concentration is 0.5% by weight with respect to a total amount of the fiber dispersion liquid measured at a rotation speed of 6 rpm, and η 60  is viscosity (at 25° C.) of the fiber dispersion liquid measured at a rotation speed of 60 rpm. 
       
     
     
         4 . The fiber dispersion liquid according to  claim 1 , wherein the ultrafine fiber is formed of polyester. 
     
     
         5 . The fiber dispersion liquid according to  claim 1 , further comprising a dispersant.