Pulp, slurry, sheet, laminate, and method for producing pulp

Abstract

An object is to optimize a bleaching process in a step of producing phosphorylated cellulose fibers. A pulp comprises cellulose fibers having 0.5 mmol/g or more of phosphoric acid groups or phosphoric acid group-derived substituents, when the pulp is processed into a sheet and four sheets are laminated on one another, the ISO whiteness of the laminate is 82% or more. Moreover, when the pulp is processed into a sheet and four sheets are then laminated on one another, the b* value of the obtained laminate according to the L*a*b* color system is 5.5 or less.

1 . A pulp comprising cellulose fibers having 0.5 mmol/g or more of phosphoric acid groups or phosphoric acid group-derived substituents, wherein when the pulp is processed into a sheet under the following condition A and four sheets are then laminated on one another, the ISO whiteness of the obtained laminate measured in accordance with JIS P 8148, with the exception that the test piece defined by JIS P 8148 is set to be the obtained laminate, is 82% or more: 
       (Condition A)
 ion exchange water is added to the pulp to prepare a suspension in which the concentration of the pulp comprising phosphorylated cellulose fibers is 0.3% by mass, and then, a wet sheet having an absolute dry basis weight of 200 g/m 2  is formed from the suspension; the wet sheet is peeled off from a filter, is then placed on a stainless steel tray, and is then dried under conditions of 23° C. and a relative humidity of 50% for 3 days; and 
 both surfaces of the dried sheet are sandwiched by papers and metal plates, and the sheet is then pressed by a pressure of 7.7 MPa for 1 minute to obtain a pulp sheet. 
 
     
     
         2 . The pulp according to  claim 1 , wherein when the pulp is processed into a sheet under the condition A and four sheets are then laminated on one another, the b* value of the obtained laminate according to the L*a*b* color system is 5.5 or less. 
     
     
         3 . The pulp according to  claim 1 , wherein
 when a fibrillation treatment and a centrifugation treatment are carried out under the following condition a, a supernatant yield calculated according to the following Equation b is 50% or more:   
       (Condition a)
 the pulp is diluted with ion exchange water to a concentration of 0.5% by mass to obtain a slurry, and the slurry is then subjected to a fibrillation treatment using CLEARMIX-2.2S manufactured by M Technique Co., Ltd., at a rotation number of 21500 rpm for 30 minutes; and thereafter, the obtained slurry is diluted with ion exchange water to a solid concentration of 0.1% by mass, and the resulting slurry is then subjected to a centrifugation treatment at 12000 G for 10 minutes,
   Supernatant yield (%)=solid concentration (% by mass) in supernatant obtained after centrifugation treatment/solid concentration (% by mass) in slurry before centrifugation treatment×100.   (b)
 
 
 
     
     
         4 . A slurry comprising phosphorylated ultrafine cellulose fibers, which have 0.5 mmol/g or more of phosphoric acid groups or phosphoric acid group-derived substituents and have a fiber width of 1000 nm or less, wherein
 when a sheet is formed using the slurry under the following condition c, the yellowness (YI 30 ) of the sheet relative to a film thickness of 30 μm calculated according to the following Equation d is 0.57 or less:   
       (Condition c)
 the concentration of the phosphorylated ultrafine cellulose fibers in the slurry is adjusted to be 0.5% by mass, and the slurry is then dehydrated by suction filtration using, as a filter medium, a PVDF membrane filter having a pore diameter of 650 nm, until the solid content in the phosphorylated ultrafine cellulose fibers becomes 4% by mass or more; and thereafter, the resulting slurry is subjected to drying under tension for 2 days in a humidity conditioning chamber at 23° C. and a relative humidity of 50%, so as to obtain a sheet having an absolute dry basis weight of 40 g/m 2 ,
   Yellowness (YI 30 ) of sheet relative to film thickness of 30 μm =yellowness (YI) of sheet×(30 (μm))/(film thickness of sheet (μm)),   (d)
 
 
 wherein, in the above equation, the yellowness (YI) of a sheet is measured in accordance with JIS K 7373.