TECHNICAL FIELD

The present invention relates to a carboxyl group-containing nitrile rubber having an excellent formability when a cross-linking agent is added as well as being capable of giving a cross-linked rubber excellent in the original state physical properties, compression set resistance and water resistance, and a production method thereof, and to a cross-linkable nitrile rubber composition and a cross-linked rubber obtained by using such a carboxyl group-containing nitrile rubber.

BACKGROUND ART

Nitrile rubbers (acrylonitrile-butadiene copolymer rubbers) have hitherto been used, by taking advantage of the oil resistance, mechanical properties, chemical resistance and the like thereof, as the materials for vehicle rubber components such as hoses and tubes; hydrogenated nitrile rubbers (hydrogenated acrylonitrile-butadiene copolymer rubbers) obtained by hydrogenating the carbon-carbon double bonds in the polymer main chains of nitrile rubbers are more excellent in heat resistance, and accordingly are used as rubber components such as belts, hoses and diaphragms.

As such a nitrile rubber, for example, Patent Document 1 discloses a nitrile rubber which is obtained by coagulating the latex of a copolymer rubber using an aluminum-containing coagulant, a magnesium-containing coagulant or a calcium-containing coagulant, has a content of an α,β-ethylenically unsaturated nitrile monomer unit of 10 to 60 wt %, comprises an α,β-ethylenically unsaturated dicarboxylic acid monoester monomer unit, and has an iodine value of 120 or less. According to the nitrile rubber described in Patent Document 1, there is obtained a cross-linked rubber improved in compression set resistance to some extent; however, in terms of accomplishment of a recent request of high-performance, the further improvement of the compression set resistance has been demanded. Also, the nitrile rubber described in Patent Document 1 has a relatively high viscosity when a cross-linking agent is added (compound Mooney viscosity), therefore, in terms of enhancement of the formability, improvement of the formability has also been desired.

RELATED ART DOCUMENTS

Patent Documents

Patent Document 1: WO 2007/049651

SUMMARY OF THE INVENTION

Problem to be Solved by the Invention

The present invention was made in view of such a circumstance, and an object of the present invention is to provide a carboxyl group-containing nitrile rubber having an excellent formability when a cross-linking agent is added as well as being capable of giving a cross-linked rubber excellent in the original state physical properties, compression set resistance and water resistance. Also, an object of the present invention is to provide a cross-linkable nitrile rubber composition and a cross-linked rubber obtained by using such a carboxyl group-containing nitrile rubber.

Means for Solving the Problem

The present inventors engaged in a diligent study to achieve the above objects, and consequently have completed the present invention by discovering that the above object can be achieved by controlling the polymer Mooney viscosity and polymer pH within specific ranges for 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.

Specifically, the present invention provides 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.

The carboxyl group-containing nitrile rubber of the present invention preferably has a sodium content of 10,000 ppm by weight or less.

The carboxyl group-containing nitrile rubber of the present invention preferably comprises a carboxyl group-containing monomer unit in a content of 1 to 30 wt %. The carboxyl group-containing nitrile rubber of the present invention preferably comprises a conjugated diene monomer unit in a content of 20-89.9 wt %.

The present invention also provides a cross-linkable nitrile rubber composition comprising the above-mentioned carboxyl group-containing nitrile rubber and a cross-linking agent.

Moreover, the present invention provides a cross-linked rubber obtained by cross-linking the cross-linkable nitrile rubber composition.

Further, the present invention provides a method for producing the above-mentioned carboxyl group-containing nitrile rubber, comprising adding a salt of a monovalent cation to a latex of the carboxyl group-containing nitrile rubber to coagulate the carboxyl group-containing nitrile rubber.

In the method for producing the carboxyl group-containing nitrile rubber of the present invention, the salt is preferably sodium chloride, sodium nitrate or sodium sulfate.

In the method for producing the carboxyl group-containing nitrile rubber of the present invention, a pH of the latex is preferably 5.5 or less.

The method for producing the carboxyl group-containing nitrile rubber of the present invention preferably further comprises, after the coagulation of the carboxyl group-containing nitrile rubber, water-washing a water-containing crumb of the carboxyl group-containing nitrile rubber obtained by coagulation.

Effects of Invention

The present invention can provide a carboxyl group-containing nitrile rubber having an excellent formability when a cross-linking agent is added as well as being capable of giving a cross-linked rubber excellent in the original state physical properties, compression set resistance and water resistance, and a production method thereof, and a cross-linkable nitrile rubber composition and a cross-linked rubber obtained by using such a carboxyl group-containing nitrile rubber.

