Hard sheet and method for producing the same

Abstract

Provided is a hard sheet including a non-woven fabric of ultrafine fibers and elastic polymer added into the non-woven fabric. JIS-D hardness of the sheet is 45 degrees or more. R % thereof calculated by R (%)=(D hardness maximum−D hardness minimum)/D hardness average×100 is 0 to 20%, when a sectional surface of the sheet extending in the thickness direction thereof is evenly divided into three parts corresponding to a first outer layer, an intermediate layer, and a second outer layer in order from any one surface side of the sheet; JIS-D hardness measurements are made at six points being three arbitrary points each on the first outer layer and on the intermediate layer; and then the JIS-D hardnesses obtained are used for the calculation. In the sheet, the total content of ions capable of changing the pH of water is 400 μg/cm 3 or less.

1 . A hard sheet comprising:
 a non-woven fabric comprising ultrafine fibers having a fineness of 0.0001 to 0.5 dtex; and   an elastic polymer added into the non-woven fabric,   the hard sheet having:   a JIS-D hardness of 45 degrees or more;   a R % calculated by an equation of R (%)=(D hardness maximum−D hardness minimum)/D hardness average×100 of 0 to 20%, when a sectional surface of the hard sheet extending in a thickness direction of the hard sheet is evenly divided into three parts corresponding to a first outer layer, an intermediate layer, and a second outer layer in order from any one surface side; JIS-D hardness measurements are made at a total of six points being three arbitrary points on the first outer layer and three arbitrary points on the intermediate layer; and then the JIS-D hardnesses obtained at the six points are used for the calculation; and   a total content of ions capable of causing a pH change in water, of 400 μg/cm 3  or less.   
     
     
         2 . The hard sheet of  claim 1 , wherein the total content of the ions is from 1 to 100 μg/cm 3 . 
     
     
         3 . The hard sheet of  claim 1 , wherein the ultrafine fibers are long fibers and form fiber bundles. 
     
     
         4 . The hard sheet of  claim 3 , wherein an apparent density of the non-woven fabric is from 0.35 to 0.90 g/cm 3 . 
     
     
         5 . The hard sheet of  claim 3 , wherein, on the sectional surface of the hard sheet extending in the thickness direction of the hard sheet, the ultrafine fibers forming the fiber bundles are, partially at least, bundled together by the elastic polymer. 
     
     
         6 . The hard sheet of  claim 5 , wherein, on the sectional surface of the hard sheet extending in the thickness direction of the hard sheet, the fiber bundles are, partially at least, bound together by the elastic polymer. 
     
     
         7 . The hard sheet of  claim 3 , wherein half or more of the ultrafine fibers forming the fiber bundles are bound together by the elastic polymer. 
     
     
         8 . The hard sheet of  claim 7 , wherein on the sectional surface of the hard sheet extending in the thickness direction of the hard sheet, half or more of the fiber bundles are bound together by the elastic polymer. 
     
     
         9 . The hard sheet of  claim 1 , wherein the elastic polymer is a non-porous elastic polymer. 
     
     
         10 . The hard sheet of  claim 1 , wherein a mass ratio of the non-woven fabric to the elastic polymer (non-woven fabric/elastic polymer) is from 90/10 to 55/45. 
     
     
         11 . The hard sheet of  claim 10 , wherein an apparent density is from 0.50 to 1.2 g/cm 3 . 
     
     
         12 . The hard sheet of  claim 1 ,
 wherein the second outer layer has a JIS-D hardness of 45 degrees or more, and   the R % calculated by the equation of R (%)=(D hardness maximum−D hardness minimum)/D hardness average×100 is 0 to 20%, when JIS-D hardness measurements are made at a total of six points being three arbitrary points on the second outer layer and three arbitrary points on the intermediate layer, and then the JIS-D hardnesses obtained at the six points are used for the calculation.   
     
     
         13 . A polishing pad comprising the hard sheet of  claim 1  as a polishing layer. 
     
     
         14 . A production method for producing a hard sheet, the method comprising:
 (1) preparing an entangled fiber sheet comprising long fibers of ultrafine-fiber-forming fibers, the entangled fiber sheet being capable of forming a non-woven fabric with an apparent density of 0.35 g/cm 3  or more comprising ultrafine fibers with a fineness of 0.5 dtex or less, by subjecting ultrafine-fiber-forming treatment;   (2) impregnating the entangled fiber sheet with a first emulsion comprising an elastic polymer and a gelling agent comprising ions capable of causing a pH change in water, then allowing the first emulsion to gelate, and then solidifying the elastic polymer by heating and drying;   (3) forming a first composite body comprising the non-woven fabric and the elastic polymer by subjecting the ultrafine-fiber-forming fibers to ultrafine-fiber-forming treatment;   (4) forming a second composite body by impregnating the first composite body with a second emulsion including an elastic polymer and a gelling agent and then solidifying the elastic polymer by heating and drying, the second composite body having a difference in porosity between a first outer layer and an intermediate layer of 5% or less, when the second composite body formed is evenly divided into three parts in a thickness direction of the second composite body, the three parts corresponding to the first outer layer, the intermediate layer, and a second outer layer in order from any one surface side;   (5) water washing the second composite body such that a total content of the ions becomes 400 μg/cm 3  or less to obtain a hard sheet; and   (6) hot pressing at least one selected from the group consisting of the first composite body, the second composite body, and the hard sheet, such that a surface hardness of the hard sheet becomes 45 degrees or more in JIS-D hardness.   
     
     
         15 . The production method of  claim 14 , wherein the total content of the ions is from 1 to 100 μg/cm 3 . 
     
     
         16 . The production method of  claim 14 ,
 wherein the ultrafine-fiber-forming fibers are sea-island-type conjugated fibers comprising a water-soluble thermoplastic polyvinyl alcohol-based resin as a sea component and a water-insoluble thermoplastic resin as island components; and   the ultrafine-fiber-forming treatment in the forming (3) is a process whereby the water-soluble thermoplastic polyvinyl alcohol-based resin is dissolved in hot water and selectively removed.