[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.

It is an object of the present invention to provide a cellulose-containing composition having excellent coating suitability, a production method thereof, and a film. According to the present invention, provided is a cellulose-containing composition comprising cellulose fibers having a fiber width of 1000 nm or less and protein, wherein the protein includes an enzyme, the content of the protein is 1×10.sup.−3 parts by mass or less with respect to 1 part by mass of the cellulose fibers, and when the cellulose-containing composition having a solid concentration of 0.4% by mass is obtained, the viscosity of the cellulose-containing composition measured under conditions of 25° C. and a rotation number of 3 rpm is 10 mPa.Math.s or more and 11000 mPa.Math.s or less.

It is an object of the present invention to provide a cellulose-containing composition having excellent coating suitability, a production method thereof, and a film. According to the present invention, provided is a cellulose-containing composition comprising cellulose fibers having a fiber width of 1000 nm or less and protein, wherein the protein includes an enzyme, the content of the protein is 1×10.sup.−3 parts by mass or less with respect to 1 part by mass of the cellulose fibers, and when the cellulose-containing composition having a solid concentration of 0.4% by mass is obtained, the viscosity of the cellulose-containing composition measured under conditions of 25° C. and a rotation number of 3 rpm is 10 mPa.Math.s or more and 11000 mPa.Math.s or less.

It is an object of the present invention to provide a resin composite comprising ultrafine cellulose fibers, in which deterioration of a resin component is suppressed. The present invention relates to a cellulose fiber-containing resin composition comprising cellulose fibers with a fiber width of 1000 nm or less having phosphoric acid groups or phosphoric acid group-derived substituents, and a resin, wherein the counterions of the phosphoric acid groups or phosphoric acid group-derived substituents are organic onium ions, and the conjugate bases of the organic onium ions have a pKb value of 1.0 or more.

It is an object of the present invention to provide a resin composite comprising ultrafine cellulose fibers, in which deterioration of a resin component is suppressed. The present invention relates to a cellulose fiber-containing resin composition comprising cellulose fibers with a fiber width of 1000 nm or less having phosphoric acid groups or phosphoric acid group-derived substituents, and a resin, wherein the counterions of the phosphoric acid groups or phosphoric acid group-derived substituents are organic onium ions, and the conjugate bases of the organic onium ions have a pKb value of 1.0 or more.

A fine cellulose fiber in which when the fine cellulose fiber is formed into a dispersion, the dispersion has very high light transmittance and viscosity, and a method for producing the fine cellulose fiber. The fine cellulose fiber has a fiber width of 1 to 200 nm. A part of hydroxy groups of the cellulose fiber are substituted with a predetermine functional group to introduce an ester of phosphorus-oxo acid, and the introduction amount of the functional group is more than 2.0 mmol per 1 g of the cellulose fiber. For producing the fine cellulose fiber, to cellulose fiber is added a solution having a pH of less than 3.0 and including an additive (A) containing at least one of a phosphorus-oxo acid and a phosphorus-oxo acid metal salt and an additive (B) containing at least one of urea and a urea derivative, and the mixture is heated to perform fibrillation.

A fine cellulose fiber in which when the fine cellulose fiber is formed into a dispersion, the dispersion has very high light transmittance and viscosity, and a method for producing the fine cellulose fiber. The fine cellulose fiber has a fiber width of 1 to 200 nm. A part of hydroxy groups of the cellulose fiber are substituted with a predetermine functional group to introduce an ester of phosphorus-oxo acid, and the introduction amount of the functional group is more than 2.0 mmol per 1 g of the cellulose fiber. For producing the fine cellulose fiber, to cellulose fiber is added a solution having a pH of less than 3.0 and including an additive (A) containing at least one of a phosphorus-oxo acid and a phosphorus-oxo acid metal salt and an additive (B) containing at least one of urea and a urea derivative, and the mixture is heated to perform fibrillation.

[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.Math.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.

The invention relates to cellulose esters, the ester groups of which are at least partially containing phosphorus and are based on unsaturated carboxylic acids and reactive phosphorus components, wherein the latter are preferably phosphorus derivatives of sugar alcohols or of tartaric acid derivatives. The invention also relates to methods for preparing novel phosphorus-containing cellulose esters and to their use as flame retardants for plastics.

The invention relates to cellulose esters, the ester groups of which are at least partially containing phosphorus and are based on unsaturated carboxylic acids and reactive phosphorus components, wherein the latter are preferably phosphorus derivatives of sugar alcohols or of tartaric acid derivatives. The invention also relates to methods for preparing novel phosphorus-containing cellulose esters and to their use as flame retardants for plastics.