A method for producing a nitrile rubber, includes recovering a nitrile rubber from a latex of nitrile rubber by continuously feeding the latex of nitrile rubber and a coagulant into an extruder including a screw disposed inside a barrel to be rotatably driven, wherein the latex of nitrile rubber fed into the extruder contains 0.1 to 3 parts by weight of a hindered phenol-based antiaging agent having a molecular weight of 300 to 3000 relative to 100 parts by weight of the nitrile rubber.

A method for producing a nitrile rubber, includes recovering a nitrile rubber from a latex of nitrile rubber by continuously feeding the latex of nitrile rubber and a coagulant into an extruder including a screw disposed inside a barrel to be rotatably driven, wherein the latex of nitrile rubber fed into the extruder contains 0.1 to 3 parts by weight of a hindered phenol-based antiaging agent having a molecular weight of 300 to 3000 relative to 100 parts by weight of the nitrile rubber.

A water-soluble film is provided, which is less susceptible to curling even after long-term storage. The water-soluble film includes a polyvinyl alcohol resin (A) , and has a first surface, which has a crystal Unity index Xa, and a second surface, which is opposite to the first surface and has a crystallinity index Xb, where: Xa≥Xb; Xa−Xb is from 0.015 to 0.10; and the crystallinity indexes Xa and Xb are measured through an infrared spectroscopic analysis by an attenuated total reflection method and represented by: ABS.sub.1141/ABS.sub.1093, wherein ABS.sub.1141 and ABS.sub.1093 are absorbances at wavenumbers of 1141 cm.sup.−1 and 1093 cm.sup.−1, respectively.

A water-soluble film is provided, which is less susceptible to curling even after long-term storage. The water-soluble film includes a polyvinyl alcohol resin (A) , and has a first surface, which has a crystal Unity index Xa, and a second surface, which is opposite to the first surface and has a crystallinity index Xb, where: Xa≥Xb; Xa−Xb is from 0.015 to 0.10; and the crystallinity indexes Xa and Xb are measured through an infrared spectroscopic analysis by an attenuated total reflection method and represented by: ABS.sub.1141/ABS.sub.1093, wherein ABS.sub.1141 and ABS.sub.1093 are absorbances at wavenumbers of 1141 cm.sup.−1 and 1093 cm.sup.−1, respectively.

A crosslinking material, the crosslinking material comprising the reaction product of a reaction mixture comprising: (i) a cyclic unsaturated acid anhydride and/or diacid derivative thereof; (ii) an ethylenically unsaturated monomer; and (iii) an alcohol, amine and/or thiol,
wherein the cyclic unsaturated acid anhydride and/or diacid derivative thereof is partially-esterified by reaction with the alcohol, amine and/or thiol; and wherein the wherein the crosslinking material has an acid number of at least 100 mg KOH/g.

A crosslinking material, the crosslinking material comprising the reaction product of a reaction mixture comprising: (i) a cyclic unsaturated acid anhydride and/or diacid derivative thereof; (ii) an ethylenically unsaturated monomer; and (iii) an alcohol, amine and/or thiol,
wherein the cyclic unsaturated acid anhydride and/or diacid derivative thereof is partially-esterified by reaction with the alcohol, amine and/or thiol; and wherein the wherein the crosslinking material has an acid number of at least 100 mg KOH/g.

An acrylic rubber, including: 50 to 99.9% by weight of a bond unit derived from at least one (meth) acrylic acid ester selected from the group consisting of (meth) acrylic acid alkyl ester and (meth) acrylic acid alkoxyalkyl ester; 0.1 to 20% by weight of a bond unit derived from a monomer containing a reactive group; and 0 to 30% by weight of a bond unit derived from other monomer, wherein the acrylic rubber contains a phenolic anti-aging agent represented by a general formula (1),

##STR00001##

(R1 represents an isopropyl group or a t-butyl group, and R2 represents an alkyl group having 1 to 12 carbon atoms), and the weight average molecular weight (Mw) of the acrylic rubber is in the range of 100,000 to 5,000,000.

An acrylic rubber, including: 50 to 99.9% by weight of a bond unit derived from at least one (meth) acrylic acid ester selected from the group consisting of (meth) acrylic acid alkyl ester and (meth) acrylic acid alkoxyalkyl ester; 0.1 to 20% by weight of a bond unit derived from a monomer containing a reactive group; and 0 to 30% by weight of a bond unit derived from other monomer, wherein the acrylic rubber contains a phenolic anti-aging agent represented by a general formula (1),

##STR00001##

(R1 represents an isopropyl group or a t-butyl group, and R2 represents an alkyl group having 1 to 12 carbon atoms), and the weight average molecular weight (Mw) of the acrylic rubber is in the range of 100,000 to 5,000,000.

An acrylic rubber composition includes an acrylic rubber and a specific antioxidant, wherein the acrylic rubber includes 15 to 55% by weight of methacrylic acid alkyl ester monomer units (a), 44.5 to 84.5% by weight of acrylic acid ester monomer units (b), and 0.5 to 4% by weight of carboxyl group-containing monomer units (c), and the content of the antioxidant is 0.1 to 5 parts by weight relative to 100 parts by weight of the acrylic rubber.

An acrylic rubber composition includes an acrylic rubber and a specific antioxidant, wherein the acrylic rubber includes 15 to 55% by weight of methacrylic acid alkyl ester monomer units (a), 44.5 to 84.5% by weight of acrylic acid ester monomer units (b), and 0.5 to 4% by weight of carboxyl group-containing monomer units (c), and the content of the antioxidant is 0.1 to 5 parts by weight relative to 100 parts by weight of the acrylic rubber.