Abstract: [Object] An object is to provide a sulfonated fine cellulose fiber having excellent transparency, a sulfonated pulp fiber suitable for the sulfonated fine cellulose fiber, a method for producing these fibers, and a derivative pulp containing the sulfonated pulp fiber. [Means for achieving the object] The sulfonated pulp fiber of the present invention is a pulp fiber comprising a plurality of cellulose fibers, wherein a part of hydroxyl groups of the cellulose fibers of the pulp fiber is substituted with sulfo groups, an introduction amount of sulfur attributable to the sulfo groups is adjusted to be higher than 0.42 mmol/g while maintaining a fiber shape, and a water retention value of the pulp fiber is adjusted to 150% or more. Since the sulfonated pulp fiber has a predetermined water retaining property or more while maintaining the fiber shape, the degree of flexibility in handling can be improved.

Abstract: [Object] An object is to provide a sulfonated fine cellulose fiber having excellent transparency a sulfonated pulp fiber suitable for the sulfonated fine cellulose fiber, a method for producing these fibers, and a derivative pulp containing the sulfonated pulp fiber. [Means for achieving the object] The sulfonated pulp fiber of the present invention is a pulp fiber comprising a plurality of cellulose fibers, wherein a part of hydroxyl groups of the cellulose fibers of the pulp fiber is substituted with sulfo groups, an introduction amount of sulfur attributable to the sulfo groups is adjusted to be higher than 0.42 mmol/g while maintaining a fiber shape, and a water retention value of the pulp fiber is adjusted to 150% or more. Since the sulfonated pulp fiber has a predetermined water retaining property or more while maintaining the fiber shape, the degree of flexibility in handling can be improved.

Abstract: [Object] The present invention provides a sulfonated fine cellulose fiber having excellent viscosity and the like, as well as a sulfonated fine cellulose fiber production method capable of efficiently producing the sulfonated fine cellulose fibers. [Method for Achieving the Object] A fine cellulose fiber obtained by fibrillating a cellulose fiber wherein a part of the hydroxyl groups of the fine cellulose fiber is substituted with sulfo groups, and an introduction amount of sulfur attributable to the sulfo groups is higher than 0.42 mmol/g. The fine cellulose fiber comprises a plurality of unit fibers and has an average fiber width of 30 nm or less. A dispersion liquid in which the fine cellulose fiber is dispersed in a water-soluble solvent at a solid concentration of 0.5% by mass has a viscosity of 5,000 mPa·s or more at 25° C. This makes it possible to obtain the desired viscosity when the fine cellulose fibers are dispersed in a dispersion liquid.

Abstract: [Object] The present invention provides a dry solid containing fine cellulose fibers having excellent redispersibility in a solvent such as water, and a simple production method for producing a dry solid containing fine cellulose fibers. Another object of the present invention is to provide a fine cellulose fiber redispersion liquid having excellent transparency and viscosity property. [Method for Achieving the Object] The dry solid containing fine cellulose fibers according to one embodiment of the present invention contains fine cellulose fibers having a sulfur introduction amount attributable to sulfo groups being 0.5 mmol/g or more. The production method of the dry solid containing fine cellulose fibers of the present invention comprises a drying step in which drying is performed at a relatively low temperature under a low pressure.

Abstract: [Object] An object is to provide a sulfonated fine cellulose fiber having excellent transparency, a sulfonated pulp fiber suitable for the sulfonated fine cellulose fiber, a method for producing these fibers, and a derivative pulp containing the sulfonated pulp fiber. [Means for achieving the object] The sulfonated pulp fiber of the present invention is a pulp fiber comprising a plurality of cellulose fibers, wherein a part of hydroxyl groups of the cellulose fibers of the pulp fiber is substituted with sulfo groups, an introduction amount of sulfur attributable to the sulfo groups is adjusted to be higher than 0.42 mmol/g while maintaining a fiber shape, and a water retention value of the pulp fiber is adjusted to 150% or more. Since the sulfonated pulp fiber has a predetermined water retaining property or more while maintaining the fiber shape, the degree of flexibility in handling can be improved.