A method of recovering nitrile rubber from water-containing crumbs includes crumb-like nitrile rubber using an extruder in which a screw is disposed inside a barrel so as to be capable of being rotationally driven. The barrel is formed with at least a washing zone. The method includes supplying washing water to the washing zone at a water pressure of 0.2 MPa or higher while mixing the crumb-like nitrile rubber and the supplied washing water in the washing zone, thereby washing the crumb-like nitrile rubber.

A method of recovering nitrile rubber from water-containing crumbs includes crumb-like nitrile rubber using an extruder in which a screw is disposed inside a barrel so as to be capable of being rotationally driven. The barrel is formed with at least a washing zone. The method includes supplying washing water to the washing zone at a water pressure of 0.2 MPa or higher while mixing the crumb-like nitrile rubber and the supplied washing water in the washing zone, thereby washing the crumb-like nitrile rubber.

A carboxyl group-containing nitrile rubber having an iodine value of 120 or less, wherein the content of sodium is 1500 ppm by weight or less, the total content of calcium, magnesium, and aluminum is 350 ppm by weight or less, and the storage elastic modulus at 140° C. is 350 kPa or more.

A carboxyl group-containing nitrile rubber having an iodine value of 120 or less, wherein the content of sodium is 1500 ppm by weight or less, the total content of calcium, magnesium, and aluminum is 350 ppm by weight or less, and the storage elastic modulus at 140° C. is 350 kPa or more.

A carboxyl group-containing nitrile rubber comprising an α,β-ethylenically unsaturated nitrile monomer unit in a content of 5 to 60 wt % and having an iodine value of 120 or less, wherein the carboxyl group-containing nitrile rubber has a polymer Mooney viscosity (ML1+4, 100° C.) of 60 or less and a polymer pH of 7 or less.

A carboxyl group-containing nitrile rubber comprising an α,β-ethylenically unsaturated nitrile monomer unit in a content of 5 to 60 wt % and having an iodine value of 120 or less, wherein the carboxyl group-containing nitrile rubber has a polymer Mooney viscosity (ML1+4, 100° C.) of 60 or less and a polymer pH of 7 or less.

Provided are a water-soluble film capable of inhibiting sticking between the films while maintaining excellent water solubility and a package using the same. A water-soluble film of the present invention includes a polyvinyl alcohol resin. When a crystallinity index calculated using a diamond prism is Fd1 and a crystallinity index calculated using a germanium prism is Fg1 for FT-IR measurement of a first surface of the water-soluble film by ATR technique, Fd1 and Fg1 are Fd1≤0.65 and Fd1/Fg1<1. Fd1 is preferably Fd1≥0.25.

Provided are a water-soluble film capable of inhibiting sticking between the films while maintaining excellent water solubility and a package using the same. A water-soluble film of the present invention includes a polyvinyl alcohol resin. When a crystallinity index calculated using a diamond prism is Fd1 and a crystallinity index calculated using a germanium prism is Fg1 for FT-IR measurement of a first surface of the water-soluble film by ATR technique, Fd1 and Fg1 are Fd1≤0.65 and Fd1/Fg1<1. Fd1 is preferably Fd1≥0.25.

It is an object of the present invention to provide an acrylic copolymer for providing a highly heat-resistant rubber material that shows a small rate of change in strength without loss of elongation, even under a high-temperature condition over a long period, and a composition containing the acrylic copolymer. The present invention provides an acrylic copolymer comprising a structural unit (A) derived from a monomer containing a structure having a maleimide skeleton, a structural unit (B) derived from an acrylic acid alkyl ester and/or a structural unit (B) derived from an acrylic acid alkoxyalkyl ester, and a structural unit (C) derived from an unsaturated monomer having a crosslinking group; a composition comprising the acrylic copolymer and a crosslinking group; and a rubber material produced from the composition.

It is an object of the present invention to provide an acrylic copolymer for providing a highly heat-resistant rubber material that shows a small rate of change in strength without loss of elongation, even under a high-temperature condition over a long period, and a composition containing the acrylic copolymer. The present invention provides an acrylic copolymer comprising a structural unit (A) derived from a monomer containing a structure having a maleimide skeleton, a structural unit (B) derived from an acrylic acid alkyl ester and/or a structural unit (B) derived from an acrylic acid alkoxyalkyl ester, and a structural unit (C) derived from an unsaturated monomer having a crosslinking group; a composition comprising the acrylic copolymer and a crosslinking group; and a rubber material produced from the composition.