阅读说明：本技术 含羧基腈橡胶的胶乳中包含的、未反应的α,β-烯属不饱和腈单体的回收方法 (Method for recovering unreacted alpha, beta-ethylenically unsaturated nitrile monomer contained in a latex of a carboxyl-containing nitrile rubber ) 是由 吉村务 山之上智士 于 2019-03-13 设计创作，主要内容包括：本发明提供了一种未反应的α,β-烯属不饱和腈单体的回收方法,为从将包含α,β-烯属不饱和腈单体和含羧基单体的单体混合物乳液聚合而得到的腈橡胶的胶乳中回收该胶乳中包含的未反应的α,β-烯属不饱和腈单体的方法,具有如下工序：将上述腈橡胶的胶乳的pH调节为6.5以上的工序；在加热条件下对pH调节为6.5以上的上述腈橡胶的胶乳进行减压的工序；以及回收通过在上述加热条件下的减压而蒸发的成分的工序。(The present invention provides a method for recovering an unreacted α, β -ethylenically unsaturated nitrile monomer, which is a method for recovering an unreacted α, β -ethylenically unsaturated nitrile monomer contained in a latex of a nitrile rubber obtained by emulsion-polymerizing a monomer mixture containing an α, β -ethylenically unsaturated nitrile monomer and a carboxyl group-containing monomer, comprising the steps of: adjusting the pH of the nitrile rubber latex to 6.5 or more; a step of decompressing the nitrile rubber latex having a pH adjusted to 6.5 or more under heating; and recovering the component evaporated by the pressure reduction under the heating condition.)

1. A method for recovering an unreacted alpha, beta-ethylenically unsaturated nitrile monomer, which comprises recovering an unreacted alpha, beta-ethylenically unsaturated nitrile monomer contained in a latex of a carboxyl group-containing nitrile rubber obtained by emulsion-polymerizing a monomer mixture comprising an alpha, beta-ethylenically unsaturated nitrile monomer and a carboxyl group-containing monomer,

the method for recovering an unreacted α, β -ethylenically unsaturated nitrile monomer comprises the steps of:

adjusting the pH of the latex of the carboxyl group-containing nitrile rubber to 6.5 or more;

a step of decompressing the latex of the carboxyl group-containing nitrile rubber with the pH adjusted to 6.5 or more under heating; and

and recovering the component evaporated by the pressure reduction under the heating condition.

2. The method for recovering an unreacted α, β -ethylenically unsaturated nitrile monomer according to claim 1, wherein the heating temperature of the latex of the carboxyl group-containing nitrile rubber under reduced pressure under heating is 40 to 80 ℃.

3. The method for recovering an unreacted α, β -ethylenically unsaturated nitrile monomer according to claim 1 or 2, wherein the decompression condition when the latex of the carboxyl group-containing nitrile rubber is decompressed under heating is-85 kPa or less in gauge pressure.

4. The method for recovering an unreacted α, β -ethylenically unsaturated nitrile monomer according to any one of claims 1 to 3, wherein the latex of the carboxyl group-containing nitrile rubber is obtained by emulsion polymerization of a monomer mixture comprising acrylonitrile and methacrylic acid.

5. The method for recovering an unreacted α, β -ethylenically unsaturated nitrile monomer according to any one of claims 1 to 4, wherein the polymerization conversion in the emulsion polymerization is 60 to 95%.

6. The method for recovering an unreacted α, β -ethylenically unsaturated nitrile monomer according to any one of claims 1 to 5, wherein the latex of the carboxyl group-containing nitrile rubber is obtained by emulsion polymerization of a monomer mixture further comprising a diene monomer,

the recovery method further comprises a step of removing unreacted diene monomer before or after the step of adjusting the pH of the latex of the carboxyl group-containing nitrile rubber to 6.5 or more,

in the step of reducing the pressure under heating, a latex of a carboxyl group-containing nitrile rubber obtained by removing an unreacted diene monomer and having a pH adjusted to 6.5 or more is used.

7. The method for recovering an unreacted α, β -ethylenically unsaturated nitrile monomer according to claim 6, wherein the temperature and pressure in the treatment for removing an unreacted diene monomer are lower than the heating temperature and the degree of vacuum when the latex of the carboxyl group-containing nitrile rubber is subjected to decompression under heating, respectively.

Technical Field

The present invention relates to a process for recovering unreacted alpha, beta-ethylenically unsaturated nitrile monomer contained in a latex of a carboxyl-containing nitrile rubber.

Background

Nitrile rubber (acrylonitrile-butadiene copolymer rubber) has been conventionally used as industrial belts, hoses, seals, fillers, gaskets, and the like by effectively utilizing oil resistance, mechanical properties, chemical resistance, and the like.

Among nitrile rubbers, carboxyl group-containing nitrile rubbers are excellent in abrasion resistance, adhesiveness, and the like, and therefore are preferably used in applications requiring excellent abrasion resistance, high adhesiveness, and the like.

Such a carboxyl group-containing nitrile rubber is usually produced by polymerizing a monomer mixture containing an α, β -ethylenically unsaturated nitrile monomer such as acrylonitrile and a carboxyl group-containing monomer by emulsion polymerization using an emulsifier, and then adding a polymerization terminator to the polymerization system at a predetermined polymerization conversion rate to terminate the polymerization, thereby obtaining a latex of the carboxyl group-containing nitrile rubber (for example, refer to patent document 1).

The latex of the carboxyl group-containing nitrile rubber obtained in this way usually contains unreacted monomers, and from the viewpoint of reducing the content of the unreacted monomers in the coagulation waste water generated when the latex is coagulated, an operation of removing the unreacted monomers from the latex obtained by polymerization is usually performed. In patent document 1, an operation of removing an unreacted monomer from a latex obtained by polymerization is also performed.

Disclosure of Invention

Problems to be solved by the invention

On the other hand, from the viewpoint of contributing to further improvement in productivity, it is desired to remove not only unreacted monomers contained in the latex obtained by polymerization but also to reuse the removed monomers, and from such a viewpoint, it is desired to be able to recover a specific monomer with high purity from among a plurality of monomers contained in the monomer mixture.

The present invention has been made in view of such circumstances, and an object thereof is to provide a method for recovering an unreacted α, β -ethylenically unsaturated nitrile monomer contained in a latex of a carboxyl group-containing nitrile rubber with high purity.

Means for solving the problems

As a result of intensive studies to achieve the above object, the present inventors have found that, in the method for removing unreacted monomers described in patent document 1, it is difficult to selectively recover α, β -ethylenically unsaturated nitrile monomers, which are important raw materials for carboxyl group-containing nitrile rubbers, from among the unreacted monomers contained in the latex of carboxyl group-containing nitrile rubbers, and that unreacted α, β -ethylenically unsaturated nitrile monomers can be recovered at high purity by performing pressure reduction under heating conditions in a state where the pH of the latex is adjusted to 6.5 or more, and have completed the present invention based on such findings.

That is, according to the present invention, there is provided a method for recovering an unreacted α, β -ethylenically unsaturated nitrile monomer from a latex of a carboxyl group-containing nitrile rubber obtained by emulsion polymerization of a monomer mixture containing an α, β -ethylenically unsaturated nitrile monomer and a carboxyl group-containing monomer, the method comprising the steps of:

adjusting the pH of the latex of the carboxyl group-containing nitrile rubber to 6.5 or more;

decompressing the latex of the carboxyl group-containing nitrile rubber with the pH adjusted to 6.5 or more under heating; and

and recovering the component evaporated by the reduced pressure under the heating condition.

In the recovery method of the present invention, the heating temperature of the latex of the carboxyl group-containing nitrile rubber under reduced pressure under heating is preferably 40 to 80 ℃.

In the recovery method of the present invention, it is preferable that the decompression condition when the latex of the carboxyl group-containing nitrile rubber is decompressed under heating is-85 kPa or less in gauge pressure.

In the recovery method of the present invention, it is preferable that the latex of the carboxyl group-containing nitrile rubber is obtained by emulsion polymerization of a monomer mixture containing acrylonitrile and methacrylic acid.

In the recovery method of the present invention, the polymerization conversion rate in the emulsion polymerization is preferably 60 to 95%.

In the recovery method of the present invention, it is preferable that: the latex of the carboxyl group-containing nitrile rubber is obtained by emulsion polymerization of a monomer mixture further containing a diene monomer, and in the recovery method, the latex of the carboxyl group-containing nitrile rubber further contains a step of performing a treatment for removing an unreacted diene monomer before or after the step of adjusting the pH of the latex of the carboxyl group-containing nitrile rubber to 6.5 or more, and in the step of performing pressure reduction under heating, the latex of the carboxyl group-containing nitrile rubber obtained by performing a treatment for removing an unreacted diene monomer and having a pH adjusted to 6.5 or more is used.

In the recovery method of the present invention, the temperature and pressure in the treatment for removing the unreacted diene monomer are preferably lower than the heating temperature and the degree of vacuum when the above-mentioned latex of the carboxyl group-containing nitrile rubber is decompressed under heating, respectively.

Effects of the invention

According to the present invention, unreacted α, β -ethylenically unsaturated nitrile monomer contained in a latex of a carboxyl group-containing nitrile rubber can be recovered at high purity.

Detailed Description

The recovery method of the present invention is a method for recovering an unreacted α, β -ethylenically unsaturated nitrile monomer contained in a latex of a carboxyl group-containing nitrile rubber obtained by emulsion-polymerizing a monomer mixture containing an α, β -ethylenically unsaturated nitrile monomer and a carboxyl group-containing monomer, and comprises the steps of:

adjusting the pH of the latex of the carboxyl group-containing nitrile rubber to 6.5 or more;

decompressing the latex of the carboxyl group-containing nitrile rubber with the pH adjusted to 6.5 or more under heating; and

and recovering the component evaporated by the reduced pressure under the heating condition.

< latex of carboxyl group-containing nitrile rubber >

First, the latex of the carboxyl group-containing nitrile rubber used in the present invention will be described.

The latex of the carboxyl group-containing nitrile rubber used in the present invention is a latex of a nitrile rubber containing α, β -ethylenically unsaturated nitrile monomer units and carboxyl group-containing monomer units, obtained by emulsion-polymerizing a monomer mixture containing α, β -ethylenically unsaturated nitrile monomers and carboxyl group-containing monomers.

The α, β -ethylenically unsaturated nitrile monomer is not particularly limited as long as it is an α, β -ethylenically unsaturated compound having a nitrile group, and examples thereof include: acrylonitrile; α -halogenated acrylonitrile such as α -chloroacrylonitrile and α -bromoacrylonitrile; and alpha-alkylacrylonitrile such as methacrylonitrile and ethacrylonitrile. Among them, acrylonitrile and methacrylonitrile are preferred, and acrylonitrile is particularly preferred. The alpha, beta-ethylenically unsaturated nitrile monomer may be used singly or in combination of two or more.

The amount of the α, β -ethylenically unsaturated nitrile monomer in the monomer mixture may be determined as appropriate depending on the composition of the finally obtained carboxyl group-containing nitrile rubber, and is preferably 4.9 to 59.9% by weight, more preferably 9.5 to 50.9% by weight, still more preferably 9.5 to 50.5% by weight, particularly preferably 14 to 44.9% by weight, and most preferably 14 to 44.0% by weight. When the amount of the α, β -ethylenically unsaturated nitrile monomer used is within the above range, the obtained carboxyl group-containing nitrile rubber can have an excellent balance between oil resistance and cold resistance.

The carboxyl group-containing monomer is not particularly limited as long as it is a monomer that is copolymerizable with the α, β -ethylenically unsaturated nitrile monomer and the diene monomer and has 1 or more unsubstituted (free) carboxyl groups that are not esterified or the like.

Examples of the carboxyl group-containing monomer include α, β -ethylenically unsaturated monocarboxylic acid monomers, α, β -ethylenically unsaturated polycarboxylic acid monomers, α, β -ethylenically unsaturated dicarboxylic acid monoester monomers, and the like. In addition, among the carboxyl group-containing monomers, monomers in which the carboxyl groups of these monomers form carboxylate salts are also included. Further, an acid anhydride of an α, β -ethylenically unsaturated polycarboxylic acid can be used as a carboxyl group-containing monomer since the acid anhydride group is dissociated after copolymerization to form a carboxyl group.

Examples of the α, β -ethylenically unsaturated monocarboxylic acid monomer include acrylic acid, methacrylic acid, ethylene acrylic acid, crotonic acid, cinnamic acid, and the like.

As α, β -ethylenically unsaturated polycarboxylic acid monomers, there may be mentioned: butenedioic acids such as fumaric acid and maleic acid; itaconic acid; citraconic acid; mesaconic acid; glutaconic acid; allylmalonic acid; mesaconic acid, and the like. Examples of the acid anhydride of the α, β -unsaturated polycarboxylic acid include maleic anhydride, itaconic anhydride, and citraconic anhydride.

As α, β -ethylenically unsaturated dicarboxylic acid monoester monomers, there can be mentioned: monoalkyl maleates such as monomethyl maleate, monoethyl maleate, mono-n-propyl maleate, and mono-n-butyl maleate; monocyclic alkyl maleates such as monocyclic amyl maleate, monocyclic hexyl maleate, and monocyclic heptyl maleate; monoalkylcycloalkyl maleates such as monomethylcyclopentyl maleate and monoethylcyclohexyl maleate; monoalkyl fumarates such as monomethyl fumarate, monoethyl fumarate, mono-n-propyl fumarate and mono-n-butyl fumarate; monocyclic alkyl fumarates such as monocyclic amyl fumarate, monocyclic hexyl fumarate and monocyclic heptyl fumarate; monoalkylcycloalkyl fumarates such as monomethylcyclopentyl fumarate and monoethylcyclohexyl fumarate; monoalkyl citraconates such as monomethyl citraconate, monoethyl citraconate, mono-n-propyl citraconate, and mono-n-butyl citraconate; citraconic acid monocyclic alkyl esters such as citraconic acid monocyclic pentyl ester, citraconic acid monocyclic cyclohexyl ester, and citraconic acid monocyclic heptyl ester; monoalkyl cycloalkyl citraconates such as monomethyl cyclopentyl citraconate and monoethyl cyclohexyl citraconate; monoalkyl itaconates such as monomethyl itaconate, monoethyl itaconate, mono-n-propyl itaconate and mono-n-butyl itaconate; monocyclic alkyl itaconates such as monocyclic amyl itaconate, monocyclic hexyl itaconate and monocyclic heptyl itaconate; monoalkylcycloalkyl itaconates such as monomethylcyclopentyl itaconate and monoethylcyclohexyl itaconate.

The carboxyl group-containing monomer may be used alone or in combination of two or more. For example, when the carboxyl group-containing nitrile rubber is used in applications where excellent abrasion resistance and adhesiveness are required, the α, β -ethylenically unsaturated monocarboxylic acid monomer is preferable, acrylic acid and methacrylic acid are more preferable, and methacrylic acid is particularly preferable, from the viewpoint that the abrasion resistance and adhesiveness can be further improved. Alternatively, when the carboxyl group-containing nitrile rubber is used in applications requiring further excellent abrasion resistance, the α, β -ethylenically unsaturated dicarboxylic acid monoester monomer is preferable, the monoalkyl maleate is more preferable, and the mono-n-butyl maleate is particularly preferable, from the viewpoint of further improving the abrasion resistance.

The amount of the carboxyl group-containing monomer in the monomer mixture may be determined as appropriate depending on the composition of the finally obtained carboxyl group-containing nitrile rubber, and is preferably 0.1 to 20% by weight, more preferably 0.5 to 15% by weight, and still more preferably 1.0 to 10% by weight. When the amount of the carboxyl group-containing monomer used is within the above range, the effects of carboxyl group introduction, such as high adhesion and excellent abrasion resistance, can be further suitably improved.

In addition, from the viewpoint of being able to improve the rubber elasticity of the carboxyl group-containing nitrile rubber finally obtained, it is preferable that the monomer mixture that can be used to obtain the latex of the carboxyl group-containing nitrile rubber used in the present invention contains a diene monomer in addition to the above α, β -ethylenically unsaturated nitrile monomer and carboxyl group-containing monomer.

The diene monomer is not particularly limited, and examples thereof include: conjugated dienes having 4 or more carbon atoms such as 1, 3-butadiene, isoprene, 2, 3-dimethyl-1, 3-butadiene, and 1, 3-pentadiene; non-conjugated dienes having 5 to 12 carbon atoms such as 1, 4-pentadiene and 1, 4-hexadiene. Among them, conjugated dienes are preferable, 1, 3-butadiene and isoprene are more preferable, and 1, 3-butadiene is further preferable.

The amount of the diene monomer in the monomer mixture may be determined as appropriate depending on the composition of the finally obtained carboxyl group-containing nitrile rubber, and is preferably 40 to 95% by weight, more preferably 49 to 90% by weight, and still more preferably 55 to 85% by weight. When the amount of the diene monomer used is within the above range, the obtained carboxyl group-containing nitrile rubber can maintain good heat resistance and chemical resistance and has excellent rubber elasticity.

As the monomer mixture for obtaining the latex of the carboxyl group-containing nitrile rubber used in the present invention, other monomers copolymerizable with the above-mentioned α, β -ethylenically unsaturated nitrile monomer and carboxyl group-containing monomer and, if necessary, diene monomer may be contained. Such other monomers are not particularly limited, and examples thereof include an α, β -ethylenically unsaturated monocarboxylic acid ester monomer, an α, β -ethylenically unsaturated dicarboxylic acid diester monomer, an aromatic vinyl monomer, a polyfunctional ethylenically unsaturated monomer, a self-crosslinkable monomer, and a copolymerization inhibitor.

Examples of α, β -ethylenically unsaturated monocarboxylic acid ester monomers include: (meth) acrylates having an alkyl group having 1 to 18 carbon atoms (such as methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, n-dodecyl acrylate, methyl methacrylate, and ethyl methacrylate (abbreviated as "methacrylate and acrylate" hereinafter the same); (meth) acrylates having an alkoxyalkyl group having 2 to 12 carbon atoms such as methoxymethyl acrylate, 2-methoxyethyl methacrylate, 3-methoxypropyl acrylate, 4-ethoxybutyl methacrylate, 6-methoxyhexyl acrylate, 4-ethoxyheptyl methacrylate, and 8-methoxyoctyl acrylate; (meth) acrylates having a cyanoalkyl group having 2 to 12 carbon atoms such as α -cyanoethyl acrylate, α -cyanoethyl methacrylate, and cyanobutyl methacrylate; (meth) acrylates having a hydroxyalkyl group having 1 to 12 carbon atoms such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and 2-hydroxyethyl methacrylate; (meth) acrylates having a fluoroalkyl group having 1 to 12 carbon atoms such as trifluoroethyl acrylate and tetrafluoropropyl methacrylate.

Examples of the α, β -ethylenically unsaturated dicarboxylic acid diester monomer include: maleic acid diesters such as diethyl maleate; fumaric diesters such as dimethyl fumarate; citraconic acid diesters such as dimethyl citraconate; itaconic acid diesters such as dibutyl itaconate.

Examples of the aromatic vinyl monomer include styrene, α -methylstyrene, and vinylpyridine.

Examples of the polyfunctional ethylenically unsaturated monomer include: divinyl compounds such as divinylbenzene; di (meth) acrylates such as diethylene glycol di (meth) acrylate and ethylene glycol di (meth) acrylate; and trimethacrylates such as trimethylolpropane tri (meth) acrylate.

Examples of the self-crosslinkable monomer include N-methylol (meth) acrylamide and N, N' -dimethylol (meth) acrylamide.

Examples of the copolymerization inhibitor include N- (4-anilinophenyl) acrylamide, N- (4-anilinophenyl) methacrylamide, N- (4-anilinophenyl) cinnamamide, N- (4-anilinophenyl) crotonamide, N-phenyl-4- (3-vinylbenzyloxy) aniline, and N-phenyl-4- (4-vinylbenzyloxy) aniline.

These other monomers copolymerizable therewith may be used in combination in plural. The amount of the other copolymerizable monomer in the monomer mixture used in the production method of the present invention may be appropriately determined depending on the composition of the finally obtained carboxyl group-containing nitrile rubber, and is preferably 50% by weight or less, more preferably 30% by weight or less, and still more preferably 10% by weight or less.

The latex of the carboxyl group-containing nitrile rubber used in the present invention can be obtained by emulsion polymerization of the monomer mixture described above. In the emulsion polymerization, a commonly used polymerization auxiliary material can be used in addition to the emulsifier, the polymerization initiator, and the molecular weight regulator.

The emulsifier is not particularly limited, and examples thereof include: nonionic emulsifiers such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenol ethers, polyoxyethylene alkyl esters, and polyoxyethylene sorbitan alkyl esters; salts of fatty acids such as myristic acid, palmitic acid, oleic acid, and linolenic acid, alkyl benzene sulfonates such as sodium dodecylbenzenesulfonate, sodium β -naphthalenesulfonate formalin condensate, higher alcohol sulfate salts, and anionic emulsifiers such as alkyl sulfosuccinates; and copolymerizable emulsifiers such as sulfonic acid esters of α, β -unsaturated carboxylic acids, sulfuric acid esters of α, β -unsaturated carboxylic acids, and sulfoalkylaryl ethers. The amount of the emulsifier added is preferably 0.1 to 10 parts by weight, more preferably 0.5 to 5 parts by weight, based on 100 parts by weight of the monomer mixture for polymerization.

The polymerization initiator is not particularly limited as long as it is a radical initiator, and examples thereof include: inorganic peroxides such as potassium persulfate, sodium persulfate, ammonium persulfate, potassium perphosphate, and hydrogen peroxide; organic peroxides such as t-butyl peroxide, cumene hydroperoxide, p-menthane hydroperoxide, di-t-butyl peroxide, t-butylcumyl peroxide, acetyl peroxide, isobutyryl peroxide, octanoyl peroxide, dibenzoyl peroxide, 3,5, 5-trimethylhexanoyl peroxide, and t-butyl peroxyisobutyrate; azo compounds such as azobisisobutyronitrile, azobis-2, 4-dimethylvaleronitrile, azobiscyclohexanecarbonitrile, and methyl azobisisobutyrate. These polymerization initiators can be used alone or in combination of 2 or more. As the polymerization initiator, inorganic or organic peroxides are preferable. When a peroxide is used as the polymerization initiator, sodium hydrogen sulfite, ferrous sulfate, sodium formaldehyde sulfoxylate, a reducing agent such as sodium iron ethylenediaminetetraacetate, or the like can be used in combination as the redox polymerization initiator. The amount of the polymerization initiator added is preferably 0.01 to 2 parts by weight based on 100 parts by weight of the monomer mixture for polymerization.

The molecular weight regulator is not particularly limited, and examples thereof include: mercaptans such as dodecyl mercaptan, n-dodecyl mercaptan, and octyl mercaptan; halogenated hydrocarbons such as carbon tetrachloride, methylene chloride and methyl bromide; alpha-methylstyrene dimer; sulfur-containing compounds such as tetraethylthiuram disulfide, dipentamethylenethiuram disulfide, and diisopropylxanthogen disulfide. They can be used alone or in combination of 2 or more. Among them, mercaptans are preferable, and tert-dodecyl mercaptan is more preferable. The amount of the molecular weight modifier used is preferably 0.1 to 0.8 part by weight based on 100 parts by weight of the total monomers.

The emulsion polymerization medium is generally water. The amount of water is preferably 80 to 500 parts by weight, more preferably 80 to 300 parts by weight, based on 100 parts by weight of the monomer for polymerization.

In the emulsion polymerization, polymerization aids such as a polymerization terminator, a stabilizer, a dispersant, a pH adjuster, a deoxidizer, and a particle size adjuster can be further used as necessary. When they are used, the kind and the amount of them are not particularly limited.

The polymerization conversion rate in the emulsion polymerization is not particularly limited, but is preferably 60 to 95%, more preferably 65 to 90%, and still more preferably 75 to 90%. By setting the polymerization conversion rate within the above range, it is possible to suitably suppress the occurrence of defects caused by an excessively high polymerization conversion rate, that is, defects such as the occurrence of gels during polymerization, deterioration in the processability of the obtained polymer, and deterioration in the properties of the obtained polymer, and to realize a high productivity.

< method for recovering unreacted alpha, beta-ethylenically unsaturated nitrile monomer >

Next, the recovery method of the present invention will be explained.

According to one embodiment of the present invention, a latex of a carboxyl group-containing nitrile rubber can be obtained as described above, and a part of monomers used for polymerization is generally contained in an unreacted state in the latex of the carboxyl group-containing nitrile rubber thus obtained.

In this respect, according to the present invention, selective recovery of α, β -ethylenically unsaturated nitrile monomers, which are important raw materials of carboxyl group-containing nitrile rubbers, among unreacted monomers contained in such a latex is achieved.

The recovery method of the present invention will be specifically described below.

That is, in the recovery method of the present invention, the latex of the carboxyl group-containing nitrile rubber obtained by the above-described method is adjusted to pH 6.5 or more, and the latex of the carboxyl group-containing nitrile rubber adjusted to pH 6.5 or more is decompressed under heating, whereby a component containing the α, β -ethylenically unsaturated nitrile monomer in high purity (the content of the monomer other than the α, β -ethylenically unsaturated nitrile monomer is suppressed to be low) can be recovered as an evaporation component.

Here, since the latex of the carboxyl group-containing nitrile rubber obtained by the above-mentioned method is obtained by emulsion-polymerizing a monomer mixture containing an α, β -ethylenically unsaturated nitrile monomer and a carboxyl group-containing monomer, the pH thereof is usually 5.5 or less after the emulsion polymerization due to the influence of the unreacted carboxyl group-containing monomer. On the other hand, in the recovery method of the present invention, unreacted monomers as an evaporation component can be recovered by decompressing the latex of the carboxyl group-containing nitrile rubber under heating in a state where the pH is 6.5 or more and the pH is 6.5 or more, and by adopting such a method, a component containing the α, β -ethylenically unsaturated nitrile monomer in high purity as the evaporation component can be recovered.

The present inventors have particularly studied and found that the following problems are present: when it is desired to recover unreacted monomers, particularly unreacted α, β -ethylenically unsaturated nitrile monomers, from a latex of a carboxyl group-containing nitrile rubber by evaporating the monomers under reduced pressure under heating, the unreacted carboxyl group-containing monomers are evaporated together with the α, β -ethylenically unsaturated nitrile monomers, and therefore a considerable amount of the carboxyl group-containing monomers are contained in the evaporation components, and as a result, the α, β -ethylenically unsaturated nitrile monomers cannot be obtained in high purity and cannot be reused. In particular, since the α, β -ethylenically unsaturated nitrile monomer cannot be easily separated from the carboxyl group-containing monomer, it is desirable to obtain the α, β -ethylenically unsaturated nitrile monomer in high purity in order to reuse the α, β -ethylenically unsaturated nitrile monomer. As a result of further studies, the present inventors have found that the pH of a latex of a carboxyl group-containing nitrile rubber after emulsion polymerization is usually 5.5 or less due to the influence of an unreacted carboxyl group-containing monomer, and that a component containing an α, β -ethylenically unsaturated nitrile monomer with high purity as an evaporation component, specifically, a component in which the content of a monomer other than the α, β -ethylenically unsaturated nitrile monomer is suppressed can be recovered by reducing the pressure in a state in which the pH is 6.5 or more and the pH is 6.5 or more under heating.

The pH of the latex of the carboxyl group-containing nitrile rubber may be adjusted to 6.5 or more, preferably 6.7 or more, more preferably 7.0 or more, still more preferably 7.05 or more, and particularly preferably 7.5 or more. The upper limit of the pH is not particularly limited, but is preferably 9.5 or less, and more preferably 9.0 or less, from the viewpoint of appropriately reducing the possibility that unreacted α, β -ethylenically unsaturated nitrile monomers react with each other to form a dimer or trimer. When the pH of the latex of the carboxyl group-containing nitrile rubber is too low, a considerable amount of the carboxyl group-containing monomer is contained in the evaporation components, and the α, β -ethylenically unsaturated nitrile monomer cannot be recovered in high purity.

The method of adjusting the pH of the latex of the carboxyl group-containing nitrile rubber to 6.5 or more is not particularly limited, but the method of adding the basic compound is preferable because the pH of the latex of the carboxyl group-containing nitrile rubber after the emulsion polymerization reaction is usually as low as 5.5 or less. The basic compound is not particularly limited, and examples thereof include: hydroxides of alkali metals such as sodium hydroxide and potassium hydroxide; carbonates of alkali metals such as sodium carbonate and potassium carbonate; alkali metal hydrogen carbonates such as sodium hydrogen carbonate; ammonia; and organic amine compounds such as trimethylamine and triethanolamine. Among them, hydroxides of alkali metals are preferable, and potassium hydroxide is more preferable.

In the recovery method of the present invention, the heating temperature when the latex of the carboxyl group-containing nitrile rubber having a pH adjusted to 6.5 or more is decompressed under heating is not particularly limited, and is preferably 40 to 80 ℃, more preferably 45 to 75 ℃, and further preferably 50 to 70 ℃. By setting the heating temperature in the above range, it is possible to suppress deterioration of the carboxyl group-containing nitrile rubber contained in the latex and to increase the recovery amount of the unreacted α, β -ethylenically unsaturated nitrile monomer. In the recovery method of the present invention, when the latex of the carboxyl group-containing nitrile rubber having a pH adjusted to 6.5 or more is depressurized under heating, the temperature of the latex may be heated to a temperature higher than room temperature, preferably the above-mentioned temperature, and it is not necessary to continue heating from an external heat source. For example, the following method is possible: the latex is heated from an external heat source in advance to be in a state of being heated to a predetermined temperature, and then the heating from the external heat source is stopped, and the pressure is reduced in a state of not being heated from the outside.

In the recovery process of the present invention, the decompression conditions when the latex of the carboxyl group-containing nitrile rubber having been adjusted to pH 6.5 or more under heating conditions is decompressed are not particularly limited, but the degree of vacuum is preferably-85 kPa (gauge pressure) or less, more preferably-90 kPa (gauge pressure) or less, still more preferably less than-90 kPa (gauge pressure), particularly preferably-95 kPa (gauge pressure) or less, and the lower limit thereof is not particularly limited, and usually-100 kPa (gauge pressure) or more. When the degree of vacuum is within the above range, the recovery amount of the unreacted α, β -ethylenically unsaturated nitrile monomer can be increased while suppressing the occurrence of problems such as a decrease in the yield of the latex itself due to foaming at the liquid surface of the latex.

In the recovery method of the present invention, the time for reducing the pressure under heating when the latex of the carboxyl group-containing nitrile rubber having a pH adjusted to 6.5 or more is reduced under heating is not particularly limited, and is usually 0.5 to 10 hours, preferably 1.0 to 8 hours. In addition, in the case of adopting an embodiment in which the latex is heated to a predetermined temperature by heating with an external heat source in advance, then the heating with the external heat source is stopped, and the pressure is reduced in a state in which the heating is not performed from the outside, or an embodiment in which the pressure reduction under the heating condition is terminated when the temperature of the latex is reduced to a predetermined temperature after the pressure reduction under the heating condition is started.

In the recovery method of the present invention, the method for recovering the evaporated component generated by decompressing the latex under heating conditions is not particularly limited, and examples thereof include a method of liquefying the evaporated component by a cooling method such as condensation. In addition, in the case where the liquefied evaporation component contains water in addition to the α, β -ethylenically unsaturated nitrile monomer, the α, β -ethylenically unsaturated nitrile monomer can be recovered by performing an operation such as distillation.

In the recovery method of the present invention, a latex obtained by emulsion-polymerizing a monomer mixture containing an α, β -ethylenically unsaturated nitrile monomer and a carboxyl group-containing monomer may be used as the target of recovery, and from the viewpoint of further enhancing the effect of the present invention, particularly from the viewpoint of being able to appropriately prevent the carboxyl group-containing monomer from being mixed into the evaporation component and being able to recover the unreacted α, β -ethylenically unsaturated nitrile monomer at a higher purity, a latex obtained by emulsion-polymerizing a monomer containing acrylonitrile and methacrylic acid is preferably used.

In the recovery method of the present invention, a latex obtained by emulsion-polymerizing a monomer mixture containing a diene monomer in addition to an α, β -ethylenically unsaturated nitrile monomer and a carboxyl group-containing monomer as a latex of a carboxyl group-containing nitrile rubber may be used as the recovery target, and in this case, it is preferable to perform a treatment of removing an unreacted diene monomer from the latex before the decompression under heating conditions for recovering the α, β -ethylenically unsaturated nitrile monomer.

The treatment for removing the unreacted diene monomer may be carried out before the step of reducing the pressure under the heating conditions described above, before the pH of the latex of the carboxyl group-containing nitrile rubber is adjusted to 6.5 or more, or after the pH is adjusted to 6.5 or more. From the viewpoint of enabling recovery of the unreacted α, β -ethylenically unsaturated nitrile monomer with higher purity, it is preferable to perform a treatment of removing the unreacted diene monomer after adjusting the pH of the latex of the carboxyl group-containing nitrile rubber to 6.5 or more.

The treatment for removing the unreacted diene monomer is not particularly limited, and the following methods may be mentioned: before the pH of the latex of the carboxyl group-containing nitrile rubber is adjusted to 6.5 or more, or after the pH is adjusted to 6.5 or more, the latex is depressurized under a vacuum degree of preferably-30 to-100 kPa (gauge pressure), more preferably-40 to-95 kPa (gauge pressure), and still more preferably-50 to-90 kPa (gauge pressure) under a relatively low temperature condition of preferably 20 to 50 ℃, more preferably 25 to 45 ℃.

The treatment for removing the unreacted diene monomer is preferably carried out under conditions more stable than the reduced pressure under the heating conditions for evaporating the unreacted α, β -ethylenically unsaturated nitrile monomer. Thus, in the treatment for removing the unreacted diene monomer, the evaporation of the unreacted α, β -ethylenically unsaturated nitrile monomer can be suppressed, and the unreacted diene monomer can be sufficiently removed mainly, and then, the unreacted α, β -ethylenically unsaturated nitrile monomer can be recovered at a high recovery rate and a higher purity by subjecting the latex from which the unreacted diene monomer has been sufficiently removed in advance to pressure reduction under heating.

Therefore, in the recovery method of the present invention, it is preferable that the temperature and pressure of the treatment for removing the unreacted diene monomer are lower than the heating temperature and the degree of vacuum for evaporating the unreacted diene monomer, respectively. The vacuum degree is low, which means that the pressure achieved by the reduced pressure is relatively close to the normal pressure. For example, in the recovery method of the present invention, the treatment for mainly removing the unreacted diene monomer can be performed under a temperature condition of 25 to 45 ℃ and a vacuum degree of-50 to-90 kPa (gauge pressure), and then, when mainly evaporating the unreacted α, β -ethylenically unsaturated nitrile monomer, the pressure can be reduced under a temperature condition of 50 to 70 ℃ and a vacuum degree of less than-90 kPa (gauge pressure).

Further, in the recovery method of the present invention, as the latex of the carboxyl group-containing nitrile rubber, a latex obtained by emulsion polymerization of a monomer mixture containing the above-mentioned copolymerizable other monomer in addition to the α, β -ethylenically unsaturated nitrile monomer, the carboxyl group-containing monomer and the diene monomer may be used as the recovery object. On the other hand, from the viewpoint of further making the effect of the present invention more remarkable, particularly from the viewpoint of more appropriately preventing the monomers other than the α, β -ethylenically unsaturated nitrile monomer from being mixed into the evaporation component and recovering the unreacted α, β -ethylenically unsaturated nitrile monomer with higher purity, a latex obtained by emulsion-polymerizing a monomer mixture substantially consisting of only the α, β -ethylenically unsaturated nitrile monomer, the carboxyl group-containing monomer and the diene monomer is preferable, and a latex obtained by emulsion-polymerizing a monomer mixture substantially consisting of only acrylonitrile, methacrylic acid and 1, 3-butadiene is particularly preferable.

As described above, according to the recovery method of the present invention, it is possible to recover the unreacted α, β -ethylenically unsaturated nitrile monomer contained in the latex of the carboxyl group-containing nitrile rubber obtained by emulsion-polymerizing the monomer mixture containing the α, β -ethylenically unsaturated nitrile monomer and the carboxyl group-containing monomer, in the form of an evaporation component such as condensed water with high purity. Specifically, the content ratio of the monomer other than the α, β -ethylenically unsaturated nitrile monomer containing the carboxyl group-containing monomer contained in the evaporation component from the latex can be suppressed to be as low as preferably less than 0.5 part by weight, more preferably less than 0.3 part by weight, and further preferably less than 0.1 part by weight with respect to 100 parts by weight of the α, β -ethylenically unsaturated nitrile monomer. Therefore, according to the recovery method of the present invention, the recovered α, β -ethylenically unsaturated nitrile monomer can be suitably reused as a raw material for various nitrile rubbers including carboxyl group-containing nitrile rubbers, thereby contributing to an improvement in productivity.