JP7816166B2 - Carboxyl group-containing acrylic rubber, rubber composition, and cross-linked rubber - Google Patents

Description

The present invention relates to a carboxyl group-containing acrylic rubber, a rubber composition, and a cross-linked rubber product. More specifically, the present invention relates to a carboxyl group-containing acrylic rubber that can provide a cross-linked rubber product that has excellent processability and excellent resistance to compression set, as well as a rubber composition and a cross-linked rubber product obtained using such a carboxyl group-containing acrylic rubber.

Acrylic rubber is a polymer whose main component is units derived from (meth)acrylic acid ester monomers. It is generally known as a rubber with excellent heat resistance, oil resistance, and ozone resistance, and is widely used in fields such as automotive.

For example, Patent Document 1 discloses a technology in which the acrylic rubber contains carboxyl groups and the content of anionic emulsifier contained in such carboxyl group-containing acrylic rubber is set within a specific range, with the aim of achieving excellent compression set resistance while also improving water resistance.

International Publication No. 2018/079787

However, the carboxyl group-containing acrylic rubber disclosed in Patent Document 1 does not necessarily have sufficient processability when compounded with various compounding ingredients. Therefore, there was a need to improve processability when compounding with compounding ingredients while maintaining compression set resistance when made into a cross-linked rubber product.

The present invention was made in consideration of these circumstances, and aims to provide a carboxyl group-containing acrylic rubber that can produce a cross-linked rubber product that has excellent processability and resistance to compression set.

As a result of intensive research conducted by the inventors to achieve the above-mentioned objective, they discovered that the above-mentioned objective can be achieved by setting the carboxyl group content and the amount of aqueous ethanol extractable material extracted by an aqueous ethanol extraction operation within specific ranges for a carboxyl group-containing acrylic rubber containing (meth)acrylic acid ester monomer units and carboxyl group-containing monomer units, and by setting the amount of change in carboxyl group content during extraction by an aqueous ethanol extraction operation within specific ranges, thereby completing the present invention.

That is, according to the present invention, there is provided a carboxyl group-containing acrylic rubber containing (meth)acrylic acid ester monomer units and carboxyl group-containing monomer units,
the carboxyl group content is in the range of 5.0×10 ⁻³ to 1.5×10 ⁻² mephhr in terms of the number of moles of carboxyl groups per 100 g of the carboxyl group-containing acrylic rubber,
the amount of the aqueous ethanol solution extracted by an extraction procedure using an aqueous ethanol solution containing 75% by volume of ethanol and 25% by volume of water is 3.0 to 8.0% by weight;
There is provided a carboxyl group-containing acrylic rubber in which the change in carboxyl group content when extracted by the aqueous ethanol extraction procedure is in the range of 0.2×10 ⁻³ to 1.2×10 ⁻³ mephhr.

In the carboxyl group-containing acrylic rubber of the present invention, the carboxyl group-containing monomer unit is preferably a butenedioic acid monoester monomer unit.
In the carboxyl group-containing acrylic rubber of the present invention, the carboxyl group-containing monomer unit is preferably a fumaric acid monoester monomer unit.
In the carboxyl group-containing acrylic rubber of the present invention, the amount extracted with the aqueous ethanol solution is preferably 4.0 to 6.5% by weight.
In the carboxyl group-containing acrylic rubber of the present invention, it is preferable that the amount of change in carboxyl group content when extraction is carried out by the aqueous ethanol extraction procedure is in the range of 0.5×10 ⁻³ to 0.8×10 ⁻³ mephhr.

Furthermore, according to the present invention, there is provided a rubber composition comprising a rubber component containing the above-described carboxyl group-containing acrylic rubber and a crosslinking agent.
Furthermore, according to the present invention, there is provided a cross-linked rubber product obtained by cross-linking the above-described rubber composition.

The present invention provides a carboxyl group-containing acrylic rubber that can produce a cross-linked rubber product that has excellent processability and resistance to compression set.

<Carboxyl group-containing acrylic rubber>
The carboxyl group-containing acrylic rubber of the present invention contains (meth)acrylic acid ester monomer units and carboxyl group-containing monomer units,
the carboxyl group content is in the range of 5.0×10 ⁻³ to 1.5×10 ⁻² mephhr in terms of the number of moles of carboxyl groups per 100 g of the carboxyl group-containing acrylic rubber,
the amount of the aqueous ethanol solution extracted by an extraction procedure using an aqueous ethanol solution containing 75% by volume of ethanol and 25% by volume of water is 3.0 to 8.0% by weight;
When extraction is carried out by the aqueous ethanol extraction procedure, the amount of change in carboxyl group content is in the range of 0.2×10 ⁻³ to 1.2×10 ⁻³ mephhr.

The carboxyl group-containing acrylic rubber of the present invention is a rubbery polymer containing (meth)acrylic acid ester monomer units (meaning acrylic acid ester monomers and/or methacrylic acid ester monomers; hereinafter, the same applies to methyl (meth)acrylate, etc.) as the main component (in this invention, this refers to units that account for 50% by weight or more of all rubber monomer units) and carboxyl group-containing monomer units in the molecule.

The (meth)acrylic acid ester monomer that forms the (meth)acrylic acid ester monomer unit is not particularly limited, but examples include (meth)acrylic acid alkyl ester monomers and (meth)acrylic acid alkoxyalkyl ester monomers.

The (meth)acrylic acid alkyl ester monomer is not particularly limited, but is preferably an ester of an alkanol having 1 to 12 carbon atoms and (meth)acrylic acid (a (meth)acrylic acid ester having an alkyl group having 1 to 12 carbon atoms), more preferably an ester of an alkanol having 1 to 8 carbon atoms and (meth)acrylic acid (a (meth)acrylic acid ester having an alkyl group having 1 to 8 carbon atoms), and even more preferably an ester of an alkanol having 2 to 6 carbon atoms and (meth)acrylic acid (a (meth)acrylic acid ester having an alkyl group having 2 to 6 carbon atoms).

Specific examples of (meth)acrylic acid alkyl ester monomers include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and cyclohexyl (meth)acrylate. Of these, ethyl (meth)acrylate and n-butyl (meth)acrylate are preferred, with ethyl acrylate and n-butyl acrylate being more preferred. These can be used alone or in combination of two or more.

The (meth)acrylic acid alkoxyalkyl ester monomer is not particularly limited, but is preferably an ester of an alkoxyalkyl alcohol having 2 to 12 carbon atoms and (meth)acrylic acid (a (meth)acrylic acid ester having an alkoxyalkyl group having 2 to 12 carbon atoms), more preferably an ester of an alkoxyalkyl alcohol having 2 to 8 carbon atoms and (meth)acrylic acid (a (meth)acrylic acid ester having an alkoxyalkyl group having 2 to 8 carbon atoms), and even more preferably an ester of an alkoxyalkyl alcohol having 2 to 6 carbon atoms and (meth)acrylic acid (a (meth)acrylic acid ester having an alkoxyalkyl group having 2 to 6 carbon atoms).

Specific examples of (meth)acrylic acid alkoxyalkyl ester monomers include methoxymethyl (meth)acrylate, ethoxymethyl (meth)acrylate, 2-methoxyethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, 2-propoxyethyl (meth)acrylate, 2-butoxyethyl (meth)acrylate, 3-methoxypropyl (meth)acrylate, and 4-methoxybutyl (meth)acrylate. Of these, 2-ethoxyethyl (meth)acrylate and 2-methoxyethyl (meth)acrylate are preferred, with 2-ethoxyethyl acrylate and 2-methoxyethyl acrylate being particularly preferred. These can be used alone or in combination of two or more.

Of all the monomer units constituting the carboxyl group-containing acrylic rubber of the present invention, the content of (meth)acrylic acid ester monomer units is preferably 70 to 99.9% by weight, more preferably 80 to 99.5% by weight, even more preferably 90 to 99% by weight, and particularly preferably 97 to 99% by weight. By ensuring that the content of (meth)acrylic acid ester monomer units falls within the above range, the weather resistance, heat resistance, and oil resistance of the resulting cross-linked rubber can be ensured.

In the carboxyl group-containing acrylic rubber of the present invention, the (meth)acrylic acid ester monomer units preferably consist of 30 to 100% by weight of (meth)acrylic acid alkyl ester monomer units and 70 to 0% by weight of (meth)acrylic acid alkoxyalkyl ester monomer units, more preferably 90 to 100% by weight of (meth)acrylic acid alkyl ester monomer units and 10 to 0% by weight of (meth)acrylic acid alkoxyalkyl ester monomer units, and even more preferably 100% by weight of (meth)acrylic acid alkyl ester monomer units and 0% by weight of (meth)acrylic acid alkoxyalkyl ester monomer units. By ensuring that the content ratios of (meth)acrylic acid alkyl ester monomer units and (meth)acrylic acid alkoxyalkyl ester monomer units fall within the above ranges, the resulting cross-linked rubber can be made to have sufficient oil resistance and cold resistance.

The carboxyl group-containing monomer that forms the carboxyl group-containing monomer unit is not particularly limited, but examples include α,β-ethylenically unsaturated monocarboxylic acids, α,β-ethylenically unsaturated dicarboxylic acids, and α,β-ethylenically unsaturated dicarboxylic acid monoesters. By including carboxyl group-containing monomer units, the acrylic rubber can be made into a carboxyl group-containing acrylic rubber that has carboxyl groups as crosslinking points, thereby appropriately improving the compression set resistance of the crosslinked rubber.

The α,β-ethylenically unsaturated monocarboxylic acid is not particularly limited, but is preferably an α,β-ethylenically unsaturated monocarboxylic acid having 3 to 12 carbon atoms, and specific examples thereof include acrylic acid, methacrylic acid, α-ethylacrylic acid, crotonic acid, and cinnamic acid.
The α,β-ethylenically unsaturated dicarboxylic acid is not particularly limited, but is preferably an α,β-ethylenically unsaturated dicarboxylic acid having 4 to 12 carbon atoms, and specific examples thereof include butenedioic acids such as fumaric acid and maleic acid; itaconic acid; citraconic acid; and chloromaleic acid.
The α,β-ethylenically unsaturated dicarboxylic acid monoester is not particularly limited, but is preferably a monoester of an α,β-ethylenically unsaturated dicarboxylic acid having 4 to 12 carbon atoms and an alkanol, more preferably a monoester of an α,β-ethylenically unsaturated dicarboxylic acid having 4 to 6 carbon atoms and an alkanol, and even more preferably a monoester of butenedioic acid having 4 carbon atoms and an alkanol. That is, butenedioic acid monoester is preferred. The alkanol preferably has 1 to 12 carbon atoms, more preferably has 2 to 8 carbon atoms, and even more preferably has 2 to 6 carbon atoms. Specific examples of α,β-ethylenically unsaturated dicarboxylic acid monoesters include butenedioic acid mono-chain alkyl esters such as monomethyl fumarate, monoethyl fumarate, mono-n-butyl fumarate, monomethyl maleate, monoethyl maleate, and mono-n-butyl maleate; butenedioic acid monoesters having an alicyclic structure such as monocyclopentyl fumarate, monocyclohexyl fumarate, monocyclohexenyl fumarate, monocyclopentyl maleate, monocyclohexyl maleate, and monocyclohexenyl maleate; and itaconic acid monoesters such as monomethyl itaconate, monoethyl itaconate, mono-n-butyl itaconate, and monocyclohexyl itaconate. Among these, fumaric acid monoesters are preferred, and mono-n-butyl fumarate is more preferred. These α,β-ethylenically unsaturated carboxylic acid monomers can be used alone or in combination of two or more. Among the above-mentioned monomers, the dicarboxylic acids also include those present as anhydrides.

The content of carboxyl group-containing monomer units in all monomer units constituting the carboxyl group-containing acrylic rubber of the present invention may be an amount such that the carboxyl group content in the carboxyl group-containing acrylic rubber falls within the above-mentioned range, but is preferably 0.01% by weight or more, more preferably 0.01 to 20% by weight, even more preferably 0.1 to 10% by weight, even more preferably 0.5 to 5% by weight, and particularly preferably 1 to 3% by weight.

Furthermore, the carboxyl group-containing acrylic rubber of the present invention may contain, in addition to the (meth)acrylic acid ester monomer units and the carboxyl group-containing monomer units, units of other monomers copolymerizable with these. Examples of such copolymerizable other monomers include, but are not limited to, monomers having an epoxy group, monomers having a halogen atom, diene monomers, aromatic vinyl monomers, α,β-ethylenically unsaturated nitrile monomers, acrylamide monomers, α,β-ethylenically unsaturated dicarboxylic acid diester monomers, and other olefin-based monomers.

Monomers having an epoxy group are not particularly limited, but examples include epoxy group-containing (meth)acrylic acid esters such as glycidyl (meth)acrylate; epoxy group-containing ethers such as allyl glycidyl ether and vinyl glycidyl ether; and the like.

Halogen-containing monomers include, but are not limited to, unsaturated alcohol esters of halogen-containing saturated carboxylic acids, (meth)acrylic acid haloalkyl esters, (meth)acrylic acid haloacyloxyalkyl esters, (meth)acrylic acid (haloacetylcarbamoyloxy)alkyl esters, halogen-containing unsaturated ethers, halogen-containing unsaturated ketones, halomethyl group-containing aromatic vinyl compounds, halogen-containing unsaturated amides, and haloacetyl group-containing unsaturated monomers. It is preferable that the halogen-containing monomer contains a chlorine atom as the halogen atom.

Specific examples of unsaturated alcohol esters of halogen-containing saturated carboxylic acids include vinyl chloroacetate, vinyl 2-chloropropionate, and allyl chloroacetate.
Specific examples of (meth)acrylic acid haloalkyl esters include chloromethyl (meth)acrylate, 1-chloroethyl (meth)acrylate, 2-chloroethyl (meth)acrylate, 1,2-dichloroethyl (meth)acrylate, 2-chloropropyl (meth)acrylate, 3-chloropropyl (meth)acrylate, and 2,3-dichloropropyl (meth)acrylate.
Specific examples of the (meth)acrylic acid haloacyloxyalkyl ester include 2-(chloroacetoxy)ethyl (meth)acrylate, 2-(chloroacetoxy)propyl (meth)acrylate, 3-(chloroacetoxy)propyl (meth)acrylate, and 3-(hydroxychloroacetoxy)propyl (meth)acrylate.
Specific examples of (meth)acrylic acid (haloacetylcarbamoyloxy)alkyl esters include 2-(chloroacetylcarbamoyloxy)ethyl (meth)acrylate and 3-(chloroacetylcarbamoyloxy)propyl (meth)acrylate.

Specific examples of halogen-containing unsaturated ethers include chloromethyl vinyl ether, 2-chloroethyl vinyl ether, 3-chloropropyl vinyl ether, 2-chloroethyl allyl ether, and 3-chloropropyl allyl ether.
Specific examples of halogen-containing unsaturated ketones include 2-chloroethyl vinyl ketone, 3-chloropropyl vinyl ketone, and 2-chloroethyl allyl ketone.
Specific examples of halomethyl group-containing aromatic vinyl compounds include p-chloromethylstyrene, m-chloromethylstyrene, o-chloromethylstyrene, and p-chloromethyl-α-methylstyrene.

Specific examples of halogen-containing unsaturated amides include N-chloromethyl(meth)acrylamide.
Specific examples of haloacetyl group-containing unsaturated monomers include 3-(hydroxychloroacetoxy)propyl allyl ether and p-vinylbenzyl chloroacetic acid ester.

The diene monomer includes a conjugated diene monomer and a non-conjugated diene monomer.
Specific examples of conjugated diene monomers include 1,3-butadiene, isoprene, and piperylene.
Specific examples of the non-conjugated diene monomer include ethylidene norbornene, dicyclopentadiene, dicyclopentadienyl (meth)acrylate, and 2-dicyclopentadienylethyl (meth)acrylate.

Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, and divinylbenzene.
Examples of the α,β-ethylenically unsaturated nitrile monomer include acrylonitrile and methacrylonitrile.
Examples of the acrylamide monomer include acrylamide and methacrylamide.
Examples of the α,β-ethylenically unsaturated dicarboxylic acid diester monomer include dialkyl maleates such as dimethyl maleate and di-n-butyl maleate, wherein the alkyl group has 1 to 18 carbon atoms; dialkyl fumarate esters such as dimethyl fumarate and di-n-butyl fumarate, wherein the alkyl group has 1 to 18 carbon atoms; dicycloalkyl maleates such as dicyclopentyl maleate and dicyclohexyl maleate, wherein the cycloalkyl group has 4 to 16 carbon atoms; dicycloalkyl fumarate esters such as dicyclopentyl fumarate and dicyclohexyl fumarate, wherein the cycloalkyl group has 4 to 16 carbon atoms; dialkyl itaconate esters such as dimethyl itaconate and di-n-butyl itaconate, wherein the alkyl group has 1 to 18 carbon atoms; and dicycloalkyl itaconate esters such as dicyclohexyl itaconate, wherein the cycloalkyl group has 4 to 16 carbon atoms.
Other olefinic monomers include ethylene, propylene, vinyl chloride, vinylidene chloride, vinyl acetate, ethyl vinyl ether, and butyl vinyl ether.

The copolymerizable other monomers can be used alone or in combination of two or more. The content of these copolymerizable other monomer units in the monomer units constituting the carboxyl group-containing acrylic rubber of the present invention is preferably 29.9% by weight or less, more preferably 15% by weight or less, even more preferably 9% by weight or less, and particularly preferably 4.5% by weight or less.

When the carboxyl group-containing acrylic rubber of the present invention contains units of other copolymerizable monomers, the content thereof may be within the above-mentioned range. In particular, when the carboxyl group-containing acrylic rubber of the present invention contains ethylene units, the content of ethylene units in the monomer units constituting the carboxyl group-containing acrylic rubber of the present invention is preferably 9% by weight or less, and more preferably 4.5% by weight or less. Similarly, when the carboxyl group-containing acrylic rubber of the present invention contains acrylonitrile units or methacrylonitrile units, the total content of acrylonitrile units and methacrylonitrile units is preferably 9% by weight or less, and more preferably 4.5% by weight or less.

The carboxyl group-containing acrylic rubber of the present invention has a carboxyl group content, in terms of the number of moles of carboxyl groups per 100 g of carboxyl group-containing acrylic rubber, in the range of 5.0×10 ⁻³ to 1.5×10 ⁻² mephhr (molar equivalent per hundred rubber). According to the present invention, by setting the carboxyl group content in the carboxyl group-containing acrylic rubber in the above range, and setting the amount of aqueous ethanol extractable material extracted by the aqueous ethanol extraction procedure described below in the range of 3.0 to 8.0% by weight, and setting the change in carboxyl group content during extraction in the aqueous ethanol extraction procedure in the range of 0.2×10 ⁻³ to 1.2×10 ⁻³ mephhr, it is possible to provide the carboxyl group-containing acrylic rubber with excellent processability and excellent compression set resistance when made into a cross-linked rubber.

The carboxyl group content of the carboxyl group-containing acrylic rubber of the present invention may be in the range of 5.0×10 ⁻³ to 1.5×10 ⁻² mephhr, preferably 7.0×10 ⁻³ to 1.2×10 ⁻² mephhr, and more preferably 9.0×10 ⁻³ to 1.05×10 ⁻² mephhr. If the carboxyl group content is too low or too high, the resulting cross-linked rubber will have poor compression set resistance. The carboxyl group content of the carboxyl group-containing acrylic rubber can be measured, for example, by dissolving the carboxyl group-containing acrylic rubber in acetone and adding water to the resulting solution to prepare a measurement sample, titrating the measurement sample with alcoholic KOH to the neutralization point, and calculating the molar equivalent determined from the amount of alcoholic KOH used in the titration.

The carboxyl group content of the carboxyl group-containing acrylic rubber can be adjusted, for example, by adjusting the amount of carboxyl group-containing monomer units contained in the carboxyl group-containing acrylic rubber. More specifically, when obtaining the carboxyl group-containing acrylic rubber, it can be adjusted by adjusting the content of carboxyl group-containing monomers among all the monomers used in the polymerization, or by adjusting the final polymerization conversion rate.

In addition, from the viewpoint of improving the compression set resistance of the resulting cross-linked rubber product, the carboxyl group-containing acrylic rubber of the present invention preferably contains a monomer forming the carboxyl group-containing monomer units that is an α,β-ethylenically unsaturated dicarboxylic acid monoester, more preferably a butenedioic acid monoester, even more preferably a fumaric acid monoester, and particularly preferably mono-n-butyl fumarate. Therefore, the above-mentioned carboxyl group content is preferably the content of carboxyl groups derived primarily from α,β-ethylenically unsaturated dicarboxylic acid monoester monomer units, more preferably the content of carboxyl groups derived from butenedioic acid monoester monomer units, even more preferably the content of carboxyl groups derived from fumaric acid monoester monomer units, and particularly preferably the content of carboxyl groups derived from mono-n-butyl fumarate units.

Furthermore, the carboxyl group-containing acrylic rubber of the present invention has an ethanol aqueous solution extraction amount in the range of 3.0 to 8.0 wt % when extracted using an ethanol aqueous solution containing 75 vol % ethanol and 25 vol % water (volume ratio at a temperature of 25°C). The ethanol aqueous solution extraction amount is preferably 4.0 to 6.5 wt %, and more preferably 4.4 to 6.0 wt %. The component extracted by the ethanol aqueous solution extraction operation is a relatively low molecular weight polymer component contained in the carboxyl group-containing acrylic rubber of the present invention, typically a component with a molecular weight of 5,000 or more and a relatively low molecular weight that can be extracted with an ethanol aqueous solution. In the present invention, by setting the amount of the component extracted by the ethanol aqueous solution extraction operation, i.e., the amount of the relatively low molecular weight polymer component, within the above range, excellent processability can be achieved due to the action of this relatively low molecular weight polymer component. If the amount of aqueous ethanol solution extractable is too small, the carboxyl group-containing acrylic rubber will have poor processability, while if the amount of aqueous ethanol solution extractable is too large, when a cross-linked rubber is produced, the compression set resistance will be poor.

In the present invention, the aqueous ethanol extraction operation is carried out as follows. That is, 1.5 g of carboxyl group-containing acrylic rubber is first immersed in 150 ml of an aqueous ethanol solution containing 75 vol % ethanol and 25 vol % water, and the solution is heated at a temperature of 90°C or higher for 1.5 hours in a heating device equipped with a reflux condenser. This operation is repeated twice (i.e., 1.5-hour heating operations are repeated twice). The carboxyl group-containing acrylic rubber after the aqueous ethanol extraction operation is then dried and weighed to determine the weight _W1 of the carboxyl group-containing acrylic rubber after the aqueous ethanol extraction operation. The difference ( _W0 - _W1 ) between this and the weight _W0 before the aqueous ethanol extraction operation is calculated, and the ratio of this to the weight _W0 before the aqueous ethanol extraction operation ({( _W0 - _W1 )/ _W0 } x 100, units: wt%) can be calculated to determine the amount of aqueous ethanol extract.

The amount of carboxyl group-containing acrylic rubber extracted with an aqueous ethanol solution can be adjusted, for example, in the method for producing carboxyl group-containing acrylic rubber of the present invention described below, by adjusting the polymerization conversion rate at the start of the post-reaction step or by adjusting the final polymerization conversion rate, but is not particularly limited to these methods.

Furthermore, the carboxyl group-containing acrylic rubber of the present invention, when extracted by an aqueous ethanol extraction procedure, has a change in carboxyl group content, in terms of the number of moles of carboxyl groups per 100 g of carboxyl group-containing acrylic rubber, in the range of 0.2×10 ⁻³ to 1.2×10 ⁻³ mephhr (molar equivalent per hundred rubber). The change in carboxyl group content when extracted by an aqueous ethanol extraction procedure is the difference (A ₀ −A ₁ ) between the carboxyl group content A ₀ of the carboxyl group-containing acrylic rubber before the above-mentioned aqueous ethanol extraction procedure and the carboxyl group content A ₁ of the carboxyl group-containing acrylic rubber after the above-mentioned aqueous ethanol extraction procedure, and is affected by the carboxyl group content in the component extracted by the aqueous ethanol extraction procedure, i.e., the carboxyl group content in the polymer component having a relatively low molecular weight. More specifically, the change in the carboxyl group content when extraction is performed using an aqueous ethanol solution tends to increase as the proportion of carboxyl groups contained in the polymer component with a relatively low molecular weight increases.

According to the present invention, by setting the carboxyl group content and aqueous ethanol extractable amount of the carboxyl group-containing acrylic rubber within the above-mentioned ranges and setting the change in carboxyl group content during extraction by aqueous ethanol extraction within the above-mentioned ranges, it is possible to obtain a cross-linked rubber product with excellent compression set resistance while achieving excellent processability. In particular, the inventors of the present invention have conducted extensive research focusing on relatively low molecular weight polymer components such as those extracted by aqueous ethanol extraction, and have discovered the following: By setting the amount of relatively low molecular weight polymer components such as those extracted by aqueous ethanol extraction within a specific range, it is possible to improve processability. On the other hand, they have discovered that if the amount of carboxyl groups contained in relatively low molecular weight polymer components such as those extracted by aqueous ethanol extraction is too low or too high, compression set resistance will decrease. Based on these findings, the present invention was completed. In particular, the present inventors have discovered that in carboxyl group-containing acrylic rubber, the carboxyl groups act as crosslinkable groups, thereby contributing to improved compression set resistance, while controlling the amount of carboxyl groups contained in polymer components with relatively low molecular weights, such as components extracted by an ethanol aqueous solution extraction operation, within a specific range also contributes to improved compression set resistance.

The change in carboxyl group content when the carboxyl group-containing acrylic rubber is extracted with an aqueous ethanol solution may be in the range of 0.2×10 ⁻³ to 1.2×10 ⁻³ mephhr, preferably in the range of 0.4×10 ⁻³ to 1.0×10 ⁻³ mephhr, and more preferably in the range of 0.5×10 ⁻³ to 0.8×10 ⁻³ mephhr. If the change in carboxyl group content when the carboxyl group-containing acrylic rubber is extracted with an aqueous ethanol solution is too small, the carboxyl group-containing acrylic rubber will have poor processability, while if the change in carboxyl group content is too large, the resulting cross-linked rubber will have poor compression set resistance and the carboxyl group-containing acrylic rubber will also have poor processability.

The carboxyl group content of the carboxyl group-containing acrylic rubber after the aqueous ethanol extraction operation can be measured in the same manner as described above.

The change in carboxyl group content when extracting carboxyl group-containing acrylic rubber with an aqueous ethanol solution can be adjusted, for example, in the method for producing carboxyl group-containing acrylic rubber of the present invention described below, by adjusting the polymerization conversion rate at the start of the post-reaction step, by adjusting the proportion of carboxyl group-containing monomers among all monomers contained in the polymerization reaction system at the start of the post-reaction step, or by adjusting the final polymerization conversion rate, but is not particularly limited to these methods.

Furthermore, the change in carboxyl group content is primarily derived from the carboxyl group-containing monomer units that form the carboxyl group-containing acrylic rubber. Therefore, from the perspective of improving the compression set resistance of the resulting cross-linked rubber, the change in carboxyl group content is preferably primarily the change in the content of carboxyl groups derived from α,β-ethylenically unsaturated dicarboxylic acid monoester, more preferably the change in the content of carboxyl groups derived from butenedioic acid monoester monomer units, even more preferably the change in the content of carboxyl groups derived from fumaric acid monoester monomer units, and particularly preferably the change in the content of carboxyl groups derived from fumaric acid mono-n-butyl units.

The carboxyl group content of the carboxyl group-containing acrylic rubber after extraction with an aqueous ethanol solution is not particularly limited, but is preferably 4.0×10 ⁻³ to 1.4×10 ⁻² mephhr, more preferably 6.0×10 ⁻³ to 1.1×10 ⁻² mephhr, and even more preferably 8.0×10 ⁻³ to 9.05×10 ⁻³ mephhr. By ensuring that the carboxyl group content after extraction with an aqueous ethanol solution is within the above range, the processability of the carboxyl group-containing acrylic rubber and the compression set resistance when made into a cross-linked rubber can be further improved.

The carboxyl group-containing acrylic rubber of the present invention has a Mooney viscosity (ML1+4, 100°C) in the range of 10 to 150, preferably in the range of 20 to 100, and more preferably in the range of 25 to 70. By keeping the Mooney viscosity in this range, it is possible to achieve a high level of balance between the processability and strength properties of the carboxyl group-containing acrylic rubber.

The weight-average molecular weight (Mw) of the carboxyl group-containing acrylic rubber of the present invention is not particularly limited, but from the viewpoint of achieving good mechanical strength and processability, it is preferably 800,000 or more, more preferably 800,000 to 2,600,000, even more preferably 900,000 to 2,300,000, and particularly preferably 1,000,000 to 2,000,000. The molecular weight distribution of the carboxyl group-containing acrylic rubber is also not particularly limited, but the Mw/Mn value is preferably 1.50 to 10.0, more preferably 1.70 to 8.00, and even more preferably 2.00 to 6.00, and the Mz/Mw value is preferably 1.30 to 3.00, more preferably 1.45 to 2.70, and even more preferably 1.60 to 2.50. The weight average molecular weight and molecular weight distribution of the carboxyl group-containing acrylic rubber can be determined as the absolute molecular weight and absolute molecular weight distribution, for example, by a GPC-MALS method using a GPC (Gel Permeation Chromatography) device incorporating a multi-angle laser light scattering photometer (MALS).

<Method of manufacturing carboxyl group-containing acrylic rubber>
The method for producing the carboxyl group-containing acrylic rubber of the present invention is not particularly limited, and any method may be used, but for example, the following method is preferred.
a monomer emulsion preparation step of emulsifying a monomer component to be used in polymerization, including a (meth)acrylic acid ester monomer and a carboxyl group-containing monomer, with an emulsifier and water to obtain a monomer emulsion;
an initial polymerization step of initiating a polymerization reaction by adding a polymerization initiator to a part of the total amount of the monomer emulsion prepared in the monomer emulsion preparation step;
a monomer emulsion dropping step of continuously or successively dropping the remainder of the monomer emulsion prepared in the monomer emulsion preparation step into a polymerization reaction system, thereby progressing the polymerization reaction;
A suitable production method includes a post-reaction step in which, after the dropwise addition of the monomer emulsion is completed, a polymerization initiator and/or a molecular weight modifier is added to the polymerization reaction system to continue the polymerization reaction.

(Monomer emulsion preparation step)
The monomer emulsion preparation step is a step of obtaining a monomer emulsion by emulsifying the monomer components to be used in polymerization, including a (meth)acrylic acid ester monomer and a carboxyl group-containing monomer, using an emulsifier and water.

The monomer components used in preparing the monomer emulsion can include the monomers listed above, and suitable monomers are also listed above. The amount of each monomer used can be selected appropriately so as to fall within the composition ranges listed above.

The emulsifier is not particularly limited, and examples include nonionic emulsifiers, anionic emulsifiers, cationic emulsifiers, etc.

Nonionic emulsifiers are not particularly limited, but examples include polyoxyalkylene fatty acid esters such as polyoxyethylene stearic acid ester and polyoxyethylene sorbitan alkyl ester; polyoxyalkylene alkyl ethers such as polyoxyethylene dodecyl ether; and polyoxyalkylene alkyl phenyl ethers such as polyoxyethylene nonylphenyl ether. Among these, polyoxyalkylene alkyl ethers and polyoxyalkylene alkyl phenyl ethers are preferred, with polyoxyethylene alkyl ethers and polyoxyethylene alkyl phenyl ethers being more preferred. The weight-average molecular weight (weight-average molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC)) of the nonionic emulsifier is not particularly limited, but is typically in the range of 300 to 50,000, preferably 500 to 30,000, and more preferably 1,000 to 15,000. These nonionic emulsifiers can be used alone or in combination of two or more.

Anionic emulsifiers include, but are not limited to, salts of fatty acids such as myristic acid, palmitic acid, oleic acid, and linolenic acid; alkylbenzenesulfonates such as sodium dodecylbenzenesulfonate; higher alcohol sulfates such as sodium lauryl sulfate; phosphate ester salts such as sodium alkyl phosphate esters, preferably higher alcohol phosphate ester salts such as sodium phosphate esters of alcohols with a hydrophobic group containing 6 or more carbon atoms; and alkyl sulfosuccinates. Among these anionic emulsifiers, phosphate ester salts and higher alcohol sulfate ester salts are preferred, with higher alcohol phosphate ester salts and higher alcohol sulfate ester salts being more preferred, and higher alcohol phosphate ester salts being even more preferred. These anionic emulsifiers can be used alone or in combination of two or more.

Examples of cationic emulsifiers include alkyltrimethylammonium chloride, dialkylammonium chloride, and benzylammonium chloride.

These emulsifiers can be used alone or in combination of two or more. Among these, nonionic emulsifiers and anionic emulsifiers are preferred, with a combination of a nonionic emulsifier and an anionic emulsifier being even more preferred. Using a combination of a nonionic emulsifier and an anionic emulsifier effectively suppresses the formation of stains due to adhesion of polymers and other substances to the polymerization equipment (e.g., polymerization tank) during emulsion polymerization, while reducing the amount of coagulant used in the coagulation step described below. This reduces the amount of coagulant in the final carboxyl group-containing acrylic rubber, thereby improving the water resistance of the resulting cross-linked rubber. Furthermore, using a combination of a nonionic emulsifier and an anionic emulsifier enhances the emulsifying effect, thereby reducing the amount of emulsifier used. This reduces the amount of emulsifier remaining in the final carboxyl group-containing acrylic rubber, thereby further improving the water resistance of the resulting carboxyl group-containing acrylic rubber.

The amount of emulsifier used, in terms of the total amount of emulsifier used per 100 parts by weight of the monomer components used in the polymerization, is typically 0.01 to 10 parts by weight, preferably 0.1 to 5 parts by weight, and more preferably 1 to 3 parts by weight. Furthermore, when a nonionic emulsifier and anionic emulsifier are used in combination, the weight ratio of nonionic emulsifier/anionic emulsifier is typically 1/99 to 99/1, preferably 10/90 to 80/20, more preferably 13/87 to 40/60, and even more preferably 15/85 to 35/65.

In the monomer emulsion preparation process, the monomer components used in polymerization, including the (meth)acrylic acid ester monomer and the carboxyl group-containing monomer, are emulsified using water and an emulsifier. While there are no particular limitations on the method, a method of mixing the monomer components, water, and an emulsifier is preferred, and a method of stirring the monomer components, water, and an emulsifier using a stirrer such as a homogenizer or disc turbine is more preferred. The monomer emulsion may also contain secondary polymerization materials such as particle size regulators, chelating agents, and oxygen scavengers, as needed.

The amount of water used in the monomer emulsion preparation process is preferably 5 to 500 parts by weight, more preferably 10 to 300 parts by weight, and even more preferably 20 to 200 parts by weight per 100 parts by weight of the monomer components used in the polymerization.

In the manufacturing method of the present invention, the entire amount of the monomer components used in the polymerization may be in the form of a monomer emulsion, and the carboxyl group-containing acrylic rubber may be obtained by going through the initial polymerization step, monomer emulsion dropping step, and post-reaction step described below. However, an alternative embodiment is also possible in which, of the total 100% by weight of the monomer components used in the polymerization, preferably 80 to 100% by weight, and more preferably 90 to 100% by weight, is in the form of a monomer emulsion, and the remainder is not in the form of a monomer emulsion but is added separately in the initial polymerization step or post-reaction step described below.

(Initial polymerization step)
The initial polymerization step is a step of initiating a polymerization reaction by adding a polymerization initiator to a part of the total amount of the monomer emulsion prepared in the monomer emulsion preparation step.

In the initial polymerization step, a portion of the total amount of the monomer emulsion prepared in the monomer emulsion preparation step is used to initiate the polymerization reaction, primarily to form seeds. From the standpoint of polymerization stability, in the initial polymerization step, it is preferable to use 1.0 to 25.0 wt.% of the total amount of the monomer emulsion (100 wt.%), more preferably 1.5 to 20.0 wt.%, and even more preferably 2.0 to 15.0 wt.%.

There are no particular restrictions on the polymerization initiator, and any initiator commonly used in emulsion polymerization can be used without restriction. For example, it is preferable to use a peroxide, an azo compound, or a redox polymerization initiator consisting of a peroxide and a reducing agent.

Either inorganic or organic peroxides may be used as peroxides.

Examples of inorganic peroxides include sodium persulfate, potassium persulfate, hydrogen peroxide, and ammonium persulfate. Among these, potassium persulfate, hydrogen peroxide, and ammonium persulfate are preferred, with potassium persulfate being particularly preferred.

Examples of organic peroxides include 2,2-di(4,4-di-(t-butylperoxy)cyclohexyl)propane, 1-di-(t-hexylperoxy)cyclohexane, 1,1-di-(t-butylperoxy)cyclohexane, 4,4-di-(t-butylperoxy)n-butyl valerate, 2,2-di-(t-butylperoxy)butane, t-butyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, paramenthane hydroperoxide, benzoyl peroxide, and 1,1,3,3-tetramethylpropane. Triethylbutyl hydroperoxide, t-butylcumyl peroxide, di-t-butyl peroxide, di-t-hexyl peroxide, di(2-t-butylperoxyisopropyl)benzene, dicumyl peroxide, diisobutyryl peroxide, di(3,5,5-trimethylhexanoyl) peroxide, dilauroyl peroxide, disuccinic acid peroxide, dibenzoyl peroxide, di(3-methylbenzoyl) peroxide, benzoyl(3-methylbenzoyl) peroxide, diisobutyryl peroxide Carbonate, di-n-propyl peroxydicarbonate, di(2-ethylhexyl)peroxydicarbonate, di-sec-butyl peroxydicarbonate, 1,1,3,3-tetramethylbutylperoxyneodecanate, t-hexyl peroxypivalate, t-butyl peroxyneodecanate, t-hexyl peroxypivalate, t-butyl peroxypivalate, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanate, t-hexyl Examples of the peroxybenzoate include 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, t-butylperoxyacetate, t-hexylperoxybenzoate, t-butylperoxybenzoate, and 2,5-dimethyl-2,5-di(t-butylperoxy)hexane. Among these, diisopropylbenzene hydroperoxide, cumene hydroperoxide, paramenthane hydroperoxide, and benzoyl peroxide are preferred.

Examples of azo compounds include azobisisobutyronitrile, 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis[2-(2-imidazolin-2-yl)propane, 2,2'-azobis(propane-2-carboxamidine), 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropanamide], 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane}, 2,2'-azobis(1-imino-1-pyrrolidino-2-methylpropane), and 2,2'-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propanamide}.

These peroxides and azo compounds can be used alone or in combination of two or more. The amount of peroxide and azo compound used in the initial polymerization step is preferably 0.001 to 0.020 parts by weight, more preferably 0.002 to 0.015 parts by weight, and even more preferably 0.003 to 0.010 parts by weight, per 100 parts by weight of the monomer components used in the polymerization.

Any reducing agent that can be used in combination with the peroxide can be used as long as it is used as a redox catalyst for emulsion polymerization. It is preferable to use at least two types of reducing agents, and particularly, a combination of a metal ion compound in a reduced state and another reducing agent is preferred.

The reduced metal ion compound is not particularly limited, but examples include ferrous sulfate, sodium iron hexamethylenediaminetetraacetate, and cuprous naphthenate. Of these, ferrous sulfate is preferred.

The reduced metal ion compound can be used alone or in combination of two or more. The amount of reduced metal ion compound used in the initial polymerization step is preferably 0.0005 to 0.0030 parts by weight, more preferably 0.0007 to 0.0025 parts by weight, and even more preferably 0.0010 to 0.0020 parts by weight per 100 parts by weight of the monomer components used in the polymerization.

Reducing agents other than reduced metal ion compounds include, but are not limited to, ascorbic acid or its salts such as ascorbic acid, sodium ascorbate, and potassium ascorbate; erythorbic acid or its salts such as erythorbic acid, sodium erythorbate, and potassium erythorbate; sulfinates such as sodium formaldehyde sulfoxylate; sulfites such as sodium sulfite, potassium sulfite, sodium bisulfite, aldehyde sodium bisulfite, and potassium bisulfite; pyrosulfites such as sodium pyrosulfite, potassium pyrosulfite, sodium bisulfite, and potassium pyrosulfite; thiosulfates such as sodium thiosulfate and potassium thiosulfate; phosphorous acid or its salts such as phosphorous acid, sodium phosphite, potassium phosphite, sodium bisphosphite, and potassium bisphosphite; pyrophosphorous acid or its salts such as pyrophosphorous acid, sodium pyrophosphite, potassium pyrophosphite, sodium bisphosphite, and potassium bisphosphite; and the like. Among these, ascorbic acid or a salt thereof, and sodium formaldehyde sulfoxylate are preferred.

Reducing agents other than reduced metal ion compounds can be used alone or in combination of two or more. The amount of reducing agent other than reduced metal ion compounds used in the initial polymerization step is preferably 0.005 to 0.080 parts by weight, more preferably 0.010 to 0.060 parts by weight, and even more preferably 0.020 to 0.040 parts by weight per 100 parts by weight of the monomer components used in the polymerization.

Preferred combinations of reduced metal ion compounds and reducing agents other than reduced metal ion compounds include ferrous sulfate and ascorbic acid or a salt thereof and/or sodium formaldehyde sulfoxylate, with a more preferred combination being ferrous sulfate and an ascorbate salt and/or sodium formaldehyde sulfoxylate, and a particularly preferred combination being ferrous sulfate and sodium formaldehyde sulfoxylate.

The amount of water used in the initial polymerization step is preferably 5 to 500 parts by weight, more preferably 10 to 300 parts by weight, and even more preferably 20 to 200 parts by weight per 100 parts by weight of the monomer components used in the polymerization.

The polymerization temperature and polymerization time in the initial polymerization step are not particularly limited, but the polymerization temperature is preferably 0 to 100°C, more preferably 5 to 80°C, and even more preferably 10 to 50°C. The polymerization time is preferably 0.3 to 2.0 hours, and more preferably 0.5 to 1.0 hour.

In addition, in the initial polymerization step, some of the monomer components used in the polymerization may be added to the polymerization reaction system directly, rather than in the form of a monomer emulsion, to initiate the polymerization reaction in the initial polymerization step.

(Monomer emulsion dropping step)
The monomer emulsion dropping step is a step of continuously or continuously dropping the remainder of the monomer emulsion prepared in the monomer emulsion preparation step into the polymerization reaction system that has undergone the initial polymerization step, thereby progressing the polymerization reaction.

In the monomer emulsion dripping step, the remainder of the monomer emulsion prepared in the monomer emulsion preparation step is dripped into the polymerization reaction system continuously or continuously, preferably continuously, to allow the polymerization reaction to proceed. The dripping time is not particularly limited, but is preferably 2.0 to 20 hours, more preferably 3.0 to 10.0 hours, from the perspective of controlling the polymerization conversion rate at the start of the post-reaction step described below when the resulting carboxyl group-containing acrylic rubber is extracted using an aqueous ethanol extraction procedure. Furthermore, in the monomer emulsion dripping step, the polymerization reaction may be continued after the dripping of the monomer emulsion is completed until the polymerization conversion rate required to start the post-reaction in the post-reaction step described below is reached.

In the monomer emulsion dropping step, the remaining monomer emulsion prepared in the monomer emulsion preparation step can be dropped into the polymerization reaction system continuously or continuously to allow the polymerization reaction to proceed. However, it is preferable that the polymerization initiator, in addition to the monomer emulsion, is also dropped into the polymerization reaction system continuously or continuously, and more preferably dropped continuously. In this case, it is preferable to use a peroxide, an azo compound, or a redox polymerization initiator consisting of a peroxide and a reducing agent as the polymerization initiator.

In the monomer emulsion dripping step, when the polymerization initiator is dripped continuously or continuously into the polymerization reaction system together with the remainder of the monomer emulsion, they may be dripped into the polymerization reaction system using separate dripping devices. Alternatively, when a redox polymerization initiator consisting of a peroxide and a reducing agent is used, at least the peroxide and reducing agent may be mixed in advance and, if necessary, dripped into the polymerization reaction system in the form of an aqueous solution from the same dripping device.

The amount of peroxide and azo compound used in the monomer emulsion dropping step is preferably 0.003 to 0.020 parts by weight, more preferably 0.005 to 0.015 parts by weight, and even more preferably 0.006 to 0.012 parts by weight, per 100 parts by weight of the monomer components used in the polymerization. Furthermore, the amount of reducing agent used in the monomer emulsion dropping step is preferably 0.05 to 0.50 parts by weight, more preferably 0.10 to 0.40 parts by weight, and even more preferably 0.15 to 0.35 parts by weight, per 100 parts by weight of the monomer components used in the polymerization.

The polymerization temperature in the monomer emulsion dropping process is not particularly limited, but is preferably 0 to 100°C, more preferably 5 to 80°C, and even more preferably 10 to 50°C.

(Post-reaction step)
The post-reaction step is a step in which, after the dropping of the monomer emulsion in the monomer emulsion dropping step is completed, a polymerization initiator and/or a molecular weight modifier is added to the polymerization reaction system to continue the polymerization reaction.

In the post-reaction step, it is sufficient that a polymerization initiator and/or a molecular weight modifier is added to continue the polymerization reaction after the dripping of the monomer emulsion in the above-mentioned monomer emulsion dripping step is completed. However, from the perspective of ensuring that the amount of aqueous ethanol extractable carboxyl group-containing acrylic rubber obtained falls within the above-mentioned range, it is preferable that the polymerization initiator and/or molecular weight modifier is added to continue the polymerization reaction after the dripping of the monomer emulsion in the above-mentioned monomer emulsion dripping step is completed, when the polymerization conversion rate reaches preferably 80 to 94%, more preferably 81 to 93%, even more preferably 83 to 92%, and even more preferably 85 to 91%. The polymerization conversion rate at the end of the dropping of the monomer emulsion in the monomer emulsion dropping step can be adjusted, for example, by adjusting the amount of polymerization initiator (e.g., peroxide, azo compound, or reducing agent used in combination with peroxide) used in the initial polymerization step, by adjusting the amount of polymerization initiator (e.g., peroxide, azo compound, or reducing agent used in combination with peroxide) used in the monomer emulsion dropping step, by adjusting the dropping time in the monomer emulsion dropping step, etc. Then, in the post-reaction step, after the dropwise dropping of the monomer emulsion in the monomer emulsion dropping step is completed, when the polymerization conversion rate reaches the above-mentioned range, it is preferable to add a polymerization initiator and/or a molecular weight modifier to continue the polymerization reaction.

In the post-reaction step, after the monomer emulsion has been dropped in the monomer emulsion dropping step described above, a polymerization initiator and/or molecular weight modifier is added to continue the polymerization reaction. This allows the action of the polymerization initiator and/or molecular weight modifier to promote the production of polymer components with relatively low molecular weights, thereby controlling the amount of carboxyl group-containing acrylic rubber extracted with an aqueous ethanol solution. Therefore, in the post-reaction step, it is preferable to add a polymerization initiator and/or molecular weight modifier once the polymerization conversion rate reaches the above-mentioned range and continue the polymerization reaction.

The polymerization initiator used in the post-reaction step is not particularly limited, but it is preferable to use a peroxide, an azo compound, or a redox polymerization initiator consisting of a peroxide and a reducing agent. The amount of peroxide and azo compound used in the post-reaction step is preferably 0.015 to 0.045 parts by weight, more preferably 0.020 to 0.040 parts by weight, and even more preferably 0.025 to 0.035 parts by weight, per 100 parts by weight of the monomer components used in the polymerization. Furthermore, the amount of reducing agent used in the post-reaction step is preferably 0.004 to 0.015 parts by weight, more preferably 0.005 to 0.012 parts by weight, and even more preferably 0.006 to 0.010 parts by weight, per 100 parts by weight of the monomer components used in the polymerization.

In addition, molecular weight adjusters used in the post-reaction process are not particularly limited, but include mercaptans such as n-butyl mercaptan and t-dodecyl mercaptan; sulfides such as tetraethylthiuram sulfide and dipentamethylenethiuram hexasulfide; α-methylstyrene dimer; carbon tetrachloride, etc.

The amount of molecular weight regulator used in the post-reaction process is preferably 0.006 to 0.020 parts by weight, more preferably 0.007 to 0.015 parts by weight, and even more preferably 0.008 to 0.012 parts by weight per 100 parts by weight of the monomer components used in the polymerization.

In the post-reaction step, a polymerization initiator and/or a molecular weight modifier is added to the polymerization reaction system, and the polymerization reaction is continued until the final polymerization conversion rate (the polymerization conversion rate at which the polymerization in the post-reaction step is terminated) is preferably 95% or higher, more preferably 97% or higher, and even more preferably 98% or higher. There is no particular upper limit to the final polymerization conversion rate, but it is preferably 99.5% or lower.

A polymerization terminator can be used to terminate the polymerization reaction. Examples of polymerization terminators include hydroxylamine, hydroxyamine sulfate, diethylhydroxyamine, hydroxyamine sulfonic acid and its alkali metal salts, sodium dimethyldithiocarbamate, and hydroquinone. There are no particular restrictions on the amount of polymerization terminator used, but it is preferably 0.1 to 2 parts by weight per 100 parts by weight of the monomer components used in the polymerization.

Of the monomers used to form the carboxyl group-containing acrylic rubber of the present invention, the carboxyl group-containing monomer has a relatively low polymerization reactivity compared to other monomers and tends to remain more easily than other monomers. Therefore, at the start of the post-reaction step, the content of the carboxyl group-containing monomer tends to be relatively high among all the monomers contained in the polymerization reaction system. Taking this tendency into consideration, it is preferable to control the conditions for initiating the polymerization in the post-reaction step, such as the polymerization conversion rate at the start of the post-reaction and the final polymerization conversion rate. This makes it possible to control the change in carboxyl group content (i.e., the amount of carboxyl groups contained in the polymer component with a relatively low molecular weight) during extraction using an aqueous ethanol extraction procedure.

As mentioned above, polymerization in the post-reaction step promotes the production of polymer components with relatively low molecular weights. Therefore, from the perspective of adjusting the change in carboxyl group content (i.e., the amount of carboxyl groups contained in polymer components with relatively low molecular weights) during extraction using an aqueous ethanol extraction procedure, it is possible to adjust the proportion of unreacted carboxyl group-containing monomers in the polymerization reaction system at the start of polymerization in the post-reaction step by, for example, adjusting the proportion of carboxyl group-containing monomers in all monomers used in polymerization, or by adding a portion of the total amount of carboxyl group-containing monomers used in polymerization in the initial polymerization step or in the post-reaction step.

For example, by adding a portion of the total amount of carboxyl group-containing monomer used in the polymerization in the initial polymerization step, a relatively large amount of carboxyl group-containing monomer can be present in the polymerization reaction system at the initial stage of polymerization, thereby promoting the consumption of carboxyl group-containing monomer in the polymerization reaction. As a result, when starting polymerization in the post-reaction step, the content of carboxyl group-containing monomer in all monomers contained in the polymerization reaction system can be made relatively low.

The polymerization temperature in the post-reaction step is not particularly limited, but the polymerization temperature is preferably 0 to 100°C, more preferably 5 to 80°C, and even more preferably 10 to 50°C.

(solidification process)
The production method of the present invention preferably further comprises a coagulation step in which the emulsion polymerization liquid obtained through the post-reaction step is brought into contact with a coagulant to produce water-containing crumbs.

The coagulant is not particularly limited, but examples include monovalent to trivalent metal salts. Monovalent to trivalent metal salts are salts containing metals that become monovalent to trivalent metal ions when dissolved in water, and examples include, but are not limited to, salts of inorganic acids selected from hydrochloric acid, nitric acid, sulfuric acid, etc., or organic acids such as acetic acid, with metals selected from sodium, potassium, lithium, magnesium, calcium, zinc, titanium, manganese, iron, cobalt, nickel, aluminum, tin, etc. Hydroxides of these metals can also be used.

Specific examples of monovalent, trivalent, and trivalent metal salts include metal chlorides such as sodium chloride, potassium chloride, lithium chloride, magnesium chloride, calcium chloride, zinc chloride, titanium chloride, manganese chloride, iron chloride, cobalt chloride, nickel chloride, aluminum chloride, and tin chloride; nitrates such as sodium nitrate, potassium nitrate, lithium nitrate, magnesium nitrate, calcium nitrate, zinc nitrate, titanium nitrate, manganese nitrate, iron nitrate, cobalt nitrate, nickel nitrate, aluminum nitrate, and tin nitrate; and sulfates such as sodium sulfate, potassium sulfate, lithium sulfate, magnesium sulfate, calcium sulfate, zinc sulfate, titanium sulfate, manganese sulfate, iron sulfate, cobalt sulfate, nickel sulfate, aluminum sulfate, and tin sulfate. Of these, calcium chloride, sodium chloride, aluminum sulfate, magnesium chloride, magnesium sulfate, zinc chloride, zinc sulfate, and sodium sulfate are preferred. These may be used alone or in combination.

The amount of coagulant used is preferably 0.01 to 100 parts by weight, more preferably 0.1 to 50 parts by weight, and even more preferably 1 to 30 parts by weight, per 100 parts by weight of the monomer components used in the polymerization. By using an amount of coagulant within this range, the carboxyl group-containing acrylic rubber can be sufficiently coagulated in the coagulation step, while the resulting carboxyl group-containing acrylic rubber has excellent water resistance.

The temperature of the aqueous solution containing the coagulant during the coagulation process (i.e., the coagulation temperature) is not particularly limited, but from the perspective of more appropriately controlling the particle size of the hydrous crumbs produced by coagulation, it is preferably 50°C or higher, more preferably 50 to 95°C, and even more preferably 60 to 90°C.

(Cleaning process)
In addition, the production method of the present invention preferably further comprises a washing step of washing the water-containing crumbs obtained in the coagulation step.

The washing method used in the washing step is not particularly limited, but examples include washing the hydrous crumbs obtained in the coagulation step described above with water, and preferably mixing the hydrous crumbs obtained in the coagulation step described above with water. The temperature during water washing is not particularly limited, but is preferably 5 to 60°C, more preferably 10 to 50°C, and the mixing time is 1 to 60 minutes, more preferably 2 to 30 minutes.

In addition, the amount of water to be mixed with the hydrous crumbs in the washing process is not particularly limited, but from the perspective of further increasing the efficiency of water washing, it is preferable to use an amount of 50 parts by weight or more per 100 parts by weight of the monomer components used in the polymerization, more preferably 50 to 15,000 parts by weight, even more preferably 100 to 10,000 parts by weight, and particularly preferably 500 to 5,000 parts by weight.

The washing time is not particularly limited, but is preferably 1 to 120 minutes, more preferably 2 to 60 minutes, and even more preferably 3 to 30 minutes.

The number of water washes in the washing step is not particularly limited, but is preferably 1 to 10 times, more preferably 1 to 5 times, and even more preferably 1 to 3 times. In the present invention, the number of water washes refers to the number of times when one water wash is defined as the operation of adding water to the wet crumbs, mixing for a predetermined time, and separating the wet crumbs from the water used for the water wash. That is, for example, two water washes means adding water to the wet crumbs, mixing for a predetermined time, and separating the wet crumbs from the water used for the water wash, followed by adding water to the wet crumbs, mixing for a predetermined time, and separating the wet crumbs from the water used for the water wash. When two or more water washes are performed, the temperature, amount, and time of the water used for the water washes may be the same or different.

In the present invention, after water washing, an acid wash may be performed using an acid as the washing solution. After acid washing, it is preferable to further perform water washing, and the water washing conditions may be the same as those described above.

(drying process)
The above-mentioned manufacturing method may further include a drying step of drying the hydrous crumbs washed in the washing step.

The method for drying the hydrous crumbs is not particularly limited and can be any conventional method, but examples include drying using a dryer such as a hot air dryer, a vacuum dryer, an expander dryer, a kneader-type dryer, or a screw-type extruder.

The drying temperature for the hydrous crumb is not particularly limited, but is preferably 80 to 250°C, more preferably 90 to 200°C, and even more preferably 100 to 180°C.

Methods for dehydrating the hydrous crumbs include, but are not limited to, methods of removing water from the hydrous crumbs using dehydrators such as centrifuges, squeezers, and screw extruders. From the perspective of further reducing the moisture content of the hydrous crumbs, methods using squeezers and screw extruders are preferred, with screw extruders being particularly preferred. Using a screw extruder allows the hydrous crumbs to be dehydrated and dried in a continuous process, thereby increasing productivity.

As a screw-type extruder, a twin-screw extruder having a barrel unit consisting of multiple barrel blocks and a pair of screws rotatably arranged inside the barrel unit can be suitably used.

According to the production method of the present invention, a carboxyl group-containing acrylic rubber can be produced as described above.
In the present invention, the carboxyl group-containing acrylic rubber may be obtained in the form of crumbs, or may be obtained as a baled rubber, i.e., a carboxyl group-containing acrylic rubber bale (a mass of carboxyl group-containing acrylic rubber in a predetermined shape).

<Rubber composition>
The rubber composition of the present invention contains a rubber component containing the above-described carboxyl group-containing acrylic rubber of the present invention and a crosslinking agent.
The content of the carboxyl group-containing acrylic rubber component of the present invention in the rubber component may be selected appropriately depending on the intended use, but is preferably 51% by weight or more, more preferably 71% by weight or more, even more preferably 86% by weight or more, and particularly preferably 100% by weight (i.e., an embodiment in which the rubber component is essentially composed of only the carboxyl group-containing acrylic rubber component).

Rubbers other than the carboxyl group-containing acrylic rubber of the present invention that make up the rubber component are not particularly limited, but include acrylic rubbers other than the carboxyl group-containing acrylic rubber of the present invention, natural rubber, polybutadiene rubber, polyisoprene rubber, styrene-butadiene rubber, acrylonitrile-butadiene rubber, silicone rubber, fluororubber, olefin-based elastomers, styrene-based elastomers, vinyl chloride-based elastomers, polyester-based elastomers, polyamide-based elastomers, polyurethane-based elastomers, and polysiloxane-based elastomers.

The rubbers other than the carboxyl group-containing acrylic rubber of the present invention can be used alone or in combination of two or more. The shape of the carboxyl group-containing acrylic rubber of the present invention and the rubbers other than the carboxyl group-containing acrylic rubber of the present invention are not particularly limited, but may be in the form of a veil, sheet, powder, or the like.

The crosslinking agent is not particularly limited, but conventional crosslinking agents such as polyamine compounds such as diamine compounds and their carbonates; and polyepoxy compounds can be used. These crosslinking agents can be used alone or in combination of two or more. Of these, it is preferable to use polyamine compounds and their carbonates, as this can further enhance the compression set resistance of the resulting crosslinked rubber product.

While there are no particular limitations on the polyamine compounds and carbonates thereof, polyamine compounds having 4 to 30 carbon atoms and carbonates thereof are preferred. Examples of such polyamine compounds and carbonates thereof include aliphatic polyamine compounds and carbonates thereof, and aromatic polyamine compounds.

Aliphatic polyamine compounds and their carbonates are not particularly limited, but examples include hexamethylenediamine, hexamethylenediamine carbamate, and N,N'-dicinnamylidene-1,6-hexanediamine. Of these, hexamethylenediamine carbamate is preferred.

Aromatic polyamine compounds are not particularly limited, but examples include 4,4'-methylenedianiline, p-phenylenediamine, m-phenylenediamine, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 4,4'-(m-phenylenediisopropylidene)dianiline, 4,4'-(p-phenylenediisopropylidene)dianiline, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 4,4'-diaminobenzanilide, 4,4'-bis(4-aminophenoxy)biphenyl, m-xylylenediamine, p-xylylenediamine, and 1,3,5-benzenetriamine. Of these, 2,2'-bis[4-(4-aminophenoxy)phenyl]propane is preferred.

The content of the crosslinking agent in the rubber composition of the present invention is preferably 0.001 to 20 parts by weight, more preferably 0.1 to 10 parts by weight, even more preferably 0.1 to 5 parts by weight, and particularly preferably 0.2 to 4 parts by weight, per 100 parts by weight of the rubber component containing the carboxyl group-containing acrylic rubber of the present invention. By keeping the content of the crosslinking agent within the above range, it is possible to ensure sufficient rubber elasticity while also achieving excellent mechanical strength as a crosslinked rubber product.

The rubber composition of the present invention preferably further contains a crosslinking accelerator. The crosslinking accelerator is not particularly limited. However, when the crosslinking agent is a polyvalent amine compound or a carbonate thereof, guanidine compounds, diazabicycloalkene compounds, imidazole compounds, quaternary onium salts, tertiary phosphine compounds, aliphatic monovalent secondary amine compounds, and aliphatic monovalent tertiary amine compounds can be used. Among these, guanidine compounds, diazabicycloalkene compounds, and aliphatic monovalent secondary amine compounds are preferred, with guanidine compounds and diazabicycloalkene compounds being particularly preferred. These basic crosslinking accelerators can be used alone or in combination of two or more.

Specific examples of guanidine compounds include 1,3-di-o-tolylguanidine and 1,3-diphenylguanidine. Specific examples of diazabicycloalkene compounds include 1,8-diazabicyclo[5.4.0]undec-7-ene and 1,5-diazabicyclo[4.3.0]non-5-ene. Specific examples of imidazole compounds include 2-methylimidazole and 2-phenylimidazole. Specific examples of quaternary onium salts include tetra-n-butylammonium bromide and octadecyltri-n-butylammonium bromide. Specific examples of tertiary phosphine compounds include triphenylphosphine and tri-p-tolylphosphine.

Aliphatic monovalent secondary amine compounds are compounds in which two of the hydrogen atoms of ammonia are substituted with aliphatic hydrocarbon groups. The aliphatic hydrocarbon groups replacing the hydrogen atoms preferably have 1 to 30 carbon atoms. Specific examples of aliphatic monovalent secondary amine compounds include dimethylamine, diethylamine, dipropylamine, diallylamine, diisopropylamine, di-n-butylamine, di-t-butylamine, di-sec-butylamine, dihexylamine, diheptylamine, dioctylamine, dinonylamine, didecylamine, diundecylamine, didodecylamine, ditridecylamine, ditetradecylamine, dipentadecylamine, dicetylamine, di-2-ethylhexylamine, and dioctadecylamine.

Aliphatic monovalent tertiary amine compounds are compounds in which all three hydrogen atoms of ammonia have been substituted with aliphatic hydrocarbon groups. The aliphatic hydrocarbon groups substituting the hydrogen atoms preferably have 1 to 30 carbon atoms. Specific examples of aliphatic monovalent tertiary amine compounds include trimethylamine, triethylamine, tripropylamine, triallylamine, triisopropylamine, tri-n-butylamine, tri-t-butylamine, tri-sec-butylamine, trihexylamine, triheptylamine, trioctylamine, trinonylamine, tridecylamine, triundecylamine, and tridodecylamine.

The content of the crosslinking accelerator in the rubber composition of the present invention is preferably 0.1 to 10 parts by weight, more preferably 0.5 to 7.5 parts by weight, and particularly preferably 1 to 5 parts by weight, per 100 parts by weight of the rubber component containing the carboxyl group-containing acrylic rubber of the present invention. By keeping the content of the crosslinking accelerator within the above range, the tensile strength and compression set resistance of the resulting crosslinked rubber product can be further improved.

It is also preferable that the rubber composition of the present invention further contains a filler. The filler is not particularly limited, but examples include reinforcing fillers and non-reinforcing fillers, and of these, reinforcing fillers are preferred.

Reinforcing fillers include, for example, carbon blacks such as furnace black, acetylene black, thermal black, channel black, and graphite; silicas such as wet silica, dry silica, and colloidal silica; and non-reinforcing fillers include quartz powder, clays such as diatomaceous earth, zinc oxide, basic magnesium carbonate, activated calcium carbonate, magnesium silicate, aluminum silicate, titanium dioxide, talc, aluminum sulfate, calcium sulfate, and barium sulfate.

These fillers can be used alone or in combination of two or more. The amount of filler contained in the rubber composition of the present invention is not particularly limited, but is preferably 1 to 200 parts by weight, more preferably 10 to 150 parts by weight, and even more preferably 20 to 100 parts by weight per 100 parts by weight of the rubber component containing the carboxyl group-containing acrylic rubber of the present invention.

In addition to the above components, the rubber composition of the present invention can also contain compounding agents commonly used in the rubber processing field. Examples of such compounding agents include antioxidants, light stabilizers, scorch inhibitors, plasticizers, processing aids, adhesives, glidants, lubricants, flame retardants, mildew inhibitors, antistatic agents, colorants, and crosslinking retarders. The amount of these compounding agents is not particularly limited as long as it does not impair the objectives and effects of the present invention, and they can be compounded in amounts appropriate to the purpose of the compounding.

The rubber composition of the present invention is prepared by blending a rubber component containing the carboxyl group-containing acrylic rubber of the present invention described above with a crosslinking agent and various other compounding ingredients used as needed, mixing and kneading the mixture using an open roll, Banbury mixer, various kneaders, etc., and then further kneading the mixture using a kneading roll.

The order in which the components are mixed is not particularly limited, but it is preferable to thoroughly mix the components that are less likely to react or decompose when heated, and then mix components that are more likely to react or decompose when heated, such as crosslinking agents, in a short period of time at a temperature that will not cause reaction or decomposition.

<Rubber cross-linked product>
The cross-linked rubber product of the present invention is obtained by cross-linking the above-mentioned rubber composition of the present invention.
The cross-linked rubber product of the present invention can be produced by using the rubber composition of the present invention, molding it into a desired shape using a molding machine such as an extruder, injection molding machine, compressor, or roll, and then heating it to cause a cross-linking reaction and fix the shape as a cross-linked rubber product. In this case, cross-linking may be carried out after molding in advance or simultaneously with molding. The molding temperature is usually 10 to 200°C, preferably 25 to 120°C. The cross-linking temperature is usually 130 to 220°C, preferably 150 to 190°C, and the cross-linking time is usually 2 minutes to 10 hours, preferably 3 minutes to 5 hours. The heating method may be appropriately selected from methods commonly used for cross-linking rubber, such as press heating, steam heating, oven heating, and hot air heating.

Depending on the shape, size, etc. of the cross-linked rubber product, the cross-linked rubber product of the present invention may be further heated to perform secondary cross-linking. The secondary cross-linking time varies depending on the heating method, cross-linking temperature, shape, etc., but is preferably performed for 1 to 48 hours. The heating method and heating temperature may be selected as appropriate.

The cross-linked rubber product of the present invention obtained in this manner is suitable for use as sealing materials such as O-rings, packing, diaphragms, oil seals, shaft seals, bearing sheaths, mechanical seals, wellhead seals, seals for electrical and electronic equipment, and seals for air compressors; various gaskets such as cylinder head gaskets attached to the connection between a cylinder block and a cylinder head, rocker cover gaskets attached to the connection between a rocker cover and a cylinder head, oil pan gaskets attached to the connection between an oil pan and a cylinder head or a transmission case, fuel cell separator gaskets attached between a pair of housings sandwiching a unit cell comprising a positive electrode, an electrolyte plate, and a negative electrode, and gaskets for the top cover of a hard disk drive; cushioning materials, vibration-proofing materials; wire coating materials; industrial belts; tubes and hoses; sheets; etc.

The cross-linked rubber product of the present invention is also suitable for use in extrusion molded articles and cross-linked products for automotive applications, such as various hoses including fuel oil hoses such as fuel hoses, filler neck hoses, vent hoses, and vapor hoses, air hoses such as turbo air hoses and mission control hoses, radiator hoses, heater hoses, brake hoses, and air conditioning hoses.

The present invention will be described in more detail below with reference to examples and comparative examples. In each example, "parts" is by weight unless otherwise specified.
Various physical properties were evaluated according to the following methods.

(1) Measurement of Carboxyl Group Content of Carboxyl Group-Containing Acrylic Rubber 0.5 g of carboxyl group-containing acrylic rubber was weighed out, dissolved in 100 ml of acetone, and then 10 ml of pure water was added to prepare a measurement sample. Next, the measurement sample was titrated to the neutralization point using alcoholic KOH, and the molar equivalent was calculated from the amount of alcoholic KOH used in the titration, thereby determining the carboxyl group content of the carboxyl group-containing acrylic rubber in moles (unit: mephr) of carboxyl groups per 100 g of carboxyl group-containing acrylic rubber.

(2) Measurement of the Amount of Aqueous Ethanol Extract of Carboxylic Acrylic Rubber 1.5 g of carboxyl group-containing acrylic rubber was weighed out, and this was designated as the weight of the carboxyl group-containing acrylic rubber before the aqueous ethanol extraction procedure, _W0 . Next, the weighed carboxyl group-containing acrylic rubber was cut into 2 mm squares and immersed in 150 ml of an aqueous ethanol solution containing 75 vol% ethanol and 25 vol% water. The sample was then heated at a temperature of 90°C or higher for 1.5 hours in a heating device equipped with a reflux condenser. This operation was repeated twice to perform the aqueous ethanol extraction procedure. The carboxyl group-containing acrylic rubber sample after the aqueous ethanol extraction procedure was then dried and weighed, and this was designated as the weight of the carboxyl group-containing acrylic rubber after the aqueous ethanol extraction procedure, _W1 . The amount of the carboxyl group-containing acrylic rubber extracted with an aqueous ethanol solution was then calculated according to the following formula:
Amount of carboxyl group-containing acrylic rubber extracted with aqueous ethanol solution (wt %)={(W ₀ −W ₁ )/W ₀ }×100

(3) Measurement of Change in Carboxyl Group Content After Aqueous Ethanol Extraction The carboxyl group content of the carboxyl group-containing acrylic rubber after aqueous ethanol extraction, obtained in the above-mentioned (2) Measurement of Amount of Extracted Aqueous Ethanol of Carboxyl Group-Containing Acrylic Rubber, was measured using the same method as the above-mentioned (1) Measurement of the Carboxyl Group Content of the Carboxyl Group-Containing Acrylic Rubber. Then, the difference (A 0 -A 1 ) between the carboxyl group content A ₀ obtained in the above-mentioned (1) Measurement of the carboxyl group content of the carboxyl group-containing acrylic rubber (the carboxyl group content A ₀ of the carboxyl group-containing acrylic rubber before aqueous ethanol extraction) and the carboxyl group content _{A 1} of _the carboxyl group-containing acrylic rubber after aqueous ethanol extraction was determined, and this was taken as the change in carboxyl group content (unit: mephr) when extraction was performed by aqueous ethanol extraction.

(4) Measurement of Mooney viscosity (ML1+4, 100°C) of rubber composition The Mooney viscosity (ML1+4, 100°C, compound Mooney) of the rubber composition was measured in accordance with JIS K6300:2013, and the processability was evaluated according to the following criteria. In the following criteria, the higher the score, the better the processability.
5 points: The Mooney viscosity of the rubber composition is 35 or more and less than 55. 4 points: The Mooney viscosity of the rubber composition is 30 or more and less than 35, or 55 or more and less than 65. 3 points: The Mooney viscosity of the rubber composition is 25 or more and less than 30, or 65 or more and less than 75. 2 points: The Mooney viscosity of the rubber composition is 20 or more and less than 25, or 75 or more and less than 85. 1 point: The Mooney viscosity of the rubber composition is less than 20, or 85 or more.

(5) Measurement of Surface Smoothness of Rubber Composition The rubber composition was extrusion molded using a single-screw extruder and a Garvey die, and the surface smoothness was evaluated in accordance with ASTM D2230. The surface smoothness was evaluated according to the following criteria. In the following criteria, the higher the score, the better the extrusion processability.
5 points: The sheet surface is smooth and glossy, and there are no air bubbles trapped in it. 4 points: The sheet surface is smooth and glossy, and there are some air bubbles trapped in it. 3 points: The sheet surface is smooth but not glossy. 2 points: There are rough areas on the sheet surface, such as unevenness or small wrinkles, but there are no holes in the sheet. 1 point: There are rough areas on the sheet surface, such as unevenness or small wrinkles, and there are holes in the sheet.

(6) Measurement of Roll Adhesion of Rubber Composition When a rubber composition was prepared using a roll, the adhesion to the roll was evaluated according to the following criteria. In the following criteria, the higher the score, the better the processability.
5 points: No sticking to the roll surface, good operability 4 points: Some sticking to the roll surface occurred, but it was possible to recut and thin-pass the material 3 points: Some sticking to the roll surface occurred, but it was possible to recut, but thin-passing was impossible 2 points: There was significant sticking to the roll surface, and it was impossible to recut or thin-pass the material 1 point: There was significant sticking to the roll surface, and the rubber stuck to both the front and back of the roll, making kneading impossible

(7) Overall Evaluation of Processability The average value of the results of (4) measurement of the Mooney viscosity (ML1+4, 100°C) of the rubber composition, (5) measurement of the surface smoothness of the rubber composition, and (6) measurement of the roll adhesion of the rubber composition was calculated, and a score of 4 or more was determined to be good processability.

(8) Measurement of Compression Set Rate The rubber composition was pressed using a mold at a temperature of 170°C for 20 minutes to effect primary crosslinking, thereby obtaining a cylindrical primary crosslinked product having a diameter of 29 mm and a height of 12.7 mm. The obtained primary crosslinked product was then further heated in a gear oven at 170°C for 4 hours to effect secondary crosslinking, thereby obtaining a cylindrical crosslinked rubber product. The obtained crosslinked rubber product was then placed in an environment of 175°C for 70 hours in a state compressed by 25%, and the compression set rate was measured according to JIS K6262:2006. The smaller this value, the better the compression set resistance.

[Example 1]
(Production of Carboxyl Group-Containing Acrylic Rubber (A-1))
Monomer emulsion preparation step: 48.2 parts of pure water, 45 parts of ethyl acrylate, 53.2 parts of n-butyl acrylate, 1.8 parts of mono-n-butyl fumarate, 1.5 parts of sodium lauryl sulfate (trade name "EMAL 2FG", manufactured by Kao Corporation) as an anionic surfactant, 0.3 parts of polyoxyethylene dodecyl ether (trade name "EMULGEN 105", weight average molecular weight in terms of polystyrene measured by GPC: approximately 1500, manufactured by Kao Corporation) as a nonionic surfactant, and 0.01 parts of ethylenediaminetetraacetic acid (EDTA) were charged into a mixing vessel equipped with a homomixer and stirred to obtain 150 parts of a monomer emulsion.

Initial Polymerization Step Next, 150 parts of pure water and 15.0 parts of the monomer emulsion obtained above were charged into a polymerization reaction tank equipped with a thermometer and a stirrer, and the mixture was cooled to a temperature of 15° C. under a nitrogen stream. Next, 0.0015 parts of ferrous sulfate (reducing agent), 0.030 parts of sodium formaldehyde sulfoxylate (reducing agent), and 0.005 parts of cumene hydroperoxide (peroxide) were added dropwise to the polymerization reaction tank to initiate the reaction, which was continued for 1 hour.

Monomer emulsion dropping step: After the reaction was carried out for 1 hour, 135 parts of the monomer emulsion obtained above (the remainder excluding the amount used in the initial polymerization step), 0.003 parts of ferrous sulfate, 0.20 parts of sodium formaldehyde sulfoxylate, and 0.008 parts of cumene hydroperoxide were continuously dropped into the polymerization reaction tank over 9 hours. The monomer emulsion, reducing agent, and peroxide were each dropped from separate dropping devices.

Post-reaction step After completion of the dropwise addition in the monomer emulsion dropping step (the polymerization conversion rate at the time of completion of the dropwise addition in the monomer emulsion dropping step was 88.7%), the temperature inside the polymerization reaction tank was raised to 35°C, and while maintaining the temperature at 35°C, 0.003 part of ferrous sulfate, 0.005 part of sodium formaldehyde sulfoxylate, and 0.03 part of cumene hydroperoxide were added, and the reaction was continued for 1 hour to carry out a post-reaction. When the polymerization conversion rate reached 98.2%, hydroquinone was added as a polymerization terminator to terminate the polymerization reaction, and a polymer emulsion was obtained.

Coagulation step, washing step, drying step Next, the obtained emulsion polymerization liquid was transferred to a coagulation tank, and 60 parts of industrial water was added to 100 parts of this emulsion polymerization liquid. The temperature was raised to 85°C, and then 10 parts of magnesium sulfate as a coagulant was continuously added while stirring the mixed liquid at a temperature of 85°C, thereby coagulating the polymer and filtering it to obtain hydrous crumbs.
Next, 3,000 parts of industrial water was added to 100 parts of the solid content of the obtained hydrous crumbs, and the mixture was stirred at room temperature for 5 minutes in a coagulation tank, and then the water was discharged from the coagulation tank, thereby washing the hydrous crumbs with water. The washed hydrous crumbs were then dried for 1 hour in a hot air dryer at 110°C, thereby obtaining a solid carboxyl group-containing acrylic rubber (A-1).

The carboxyl group content of the resulting carboxyl group-containing acrylic rubber (A-1), the amount of extraction with an aqueous ethanol solution, and the change in the carboxyl group content after the extraction with an aqueous ethanol solution were measured according to the above-mentioned methods. The results are shown in Table 1.
In addition, in the components extracted with an aqueous ethanol solution of the obtained carboxyl group-containing acrylic rubber (A-1), components having a molecular weight of 5,000 or more accounted for 85% by weight or more. Therefore, it can be determined that the amount extracted with an aqueous ethanol solution was mainly composed of polymer components having a relatively low molecular weight, with the remainder being polymerization secondary materials and the like (the same applies to Examples 2 to 5 and Comparative Examples 1 to 5 described below).
Furthermore, the monomer composition of the resulting carboxyl group-containing acrylic rubber (A-1) was almost equal to the ratio of the monomers used (the same applies to Examples 2 to 5 and Comparative Examples 1 to 5 described later).

(Preparation of Rubber Composition)
Using a Banbury mixer, 100 parts of the carboxyl group-containing acrylic rubber (A-1) obtained above was mixed with 60 parts of FEF carbon black (product name "SEAST SO", manufactured by Tokai Carbon Co., Ltd., filler, "SEAST" is a registered trademark), 2 parts of stearic acid, 1 part of ester wax (product name "GREG G-8205", manufactured by Dainippon Ink and Chemicals, Inc., lubricant), and 2 parts of 4,4'-bis(α,α-dimethylbenzyl)diphenylamine (product name "NOCRAC CD", manufactured by Ouchi Shinko Chemical Industry Co., Ltd., antioxidant, "NOCRAC" is a registered trademark) at 80°C for 5 minutes. The resulting mixture was then transferred to a roll at 50°C, and 0.60 parts of hexamethylenediamine carbamate (trade name "Diak#1", manufactured by DuPont Elastomers, a crosslinking agent) and 2 parts of 1,8-diazabicyclo[5.4.0]undec-7-ene (trade name "RHENOGRAN XLA-60", manufactured by LANXESS, a crosslinking accelerator) were blended and kneaded to obtain a rubber composition.
The rubber compositions thus obtained were used to evaluate processability and measure compression set according to the methods described above. The results are shown in Table 1.

[Example 2]
(Production of Carboxyl Group-Containing Acrylic Rubber (A-2))
The monomer emulsion preparation step, the initial polymerization step, and the monomer emulsion dropping step were carried out in the same manner as in Example 1, except that the blending amounts of ethyl acrylate and n-butyl acrylate in the monomer emulsion preparation step were changed to 42.5 parts and 50.7 parts, respectively, and the amounts of ferrous sulfate, sodium formaldehyde sulfoxylate, and cumene hydroperoxide used in the initial polymerization step and the monomer emulsion dropping step were changed to the amounts shown in Table 1.

Next, after the monomer emulsion dropping step was completed (the polymerization conversion rate at the end of the monomer emulsion dropping step was 92.0%), the temperature inside the polymerization reactor was raised to 35°C. While maintaining the temperature at 35°C, 2.5 parts of ethyl acrylate, 2.5 parts of n-butyl acrylate, 0.003 parts of ferrous sulfate, 0.005 parts of sodium formaldehyde sulfoxylate, and 0.03 parts of cumene hydroperoxide were added. The reaction was continued for one hour to carry out a post-reaction. When the polymerization conversion rate reached 98.1%, hydroquinone was added as a polymerization terminator to terminate the polymerization reaction, yielding an emulsion polymerization liquid. The solidified, washed, and dried carboxyl group-containing acrylic rubber (A-2) was obtained in the same manner as in Example 1, and measurements were performed in the same manner. The results are shown in Table 1.

(Preparation of Rubber Composition)
Except for using the carboxyl group-containing acrylic rubber (A-2) obtained above, a rubber composition was prepared and each measurement was carried out in the same manner as in Example 1. The results are shown in Table 1.

[Example 3]
(Production of Carboxyl Group-Containing Acrylic Rubber (A-3))
The monomer emulsion preparation step, the initial polymerization step, and the monomer emulsion dropping step were carried out in the same manner as in Example 1, except that the amount of mono-n-butyl fumarate blended in the monomer emulsion preparation step was changed to 1.44 parts, that 0.36 parts of mono-n-butyl fumarate was further blended separately in the initial polymerization step, and that the amounts of ferrous sulfate, sodium formaldehyde sulfoxylate, and cumene hydroperoxide used in the initial polymerization step and the monomer emulsion dropping step were changed to the amounts shown in Table 1.

Next, after the monomer emulsion dropping step was completed (the polymerization conversion rate at the end of the monomer emulsion dropping step was 83.1%), the temperature inside the polymerization reactor was raised to 35°C. While maintaining the temperature at 35°C, 0.003 parts of ferrous sulfate, 0.005 parts of sodium formaldehyde sulfoxylate, and 0.03 parts of cumene hydroperoxide were added, and the reaction was continued for one hour to carry out a post-reaction. When the polymerization conversion rate reached 97.5%, hydroquinone was added as a polymerization terminator to terminate the polymerization reaction, yielding an emulsion polymerization liquid. The solidified, washed, and dried products were then obtained in the same manner as in Example 1, yielding a solid carboxyl group-containing acrylic rubber (A-3), which was then subjected to the same measurements. The results are shown in Table 1.

(Preparation of Rubber Composition)
Except for using the carboxyl group-containing acrylic rubber (A-3) obtained above, a rubber composition was prepared and each measurement was carried out in the same manner as in Example 1. The results are shown in Table 1.

[Example 4]
(Production of Carboxyl Group-Containing Acrylic Rubber (A-4))
In the same manner as in Example 1, the monomer emulsion preparation step, initial polymerization step, and monomer emulsion dropping step were performed (the polymerization conversion rate at the end of the monomer emulsion dropping step was 89.2%), and then the temperature inside the polymerization reaction vessel was raised to 35°C. While maintaining the temperature at 35°C, 0.01 parts of t-dodecyl mercaptan was added, and the reaction was continued for 4 hours to carry out a post-reaction. When the polymerization conversion rate reached 95.3%, hydroquinone was added as a polymerization terminator to terminate the polymerization reaction, thereby obtaining a polymer emulsion. Then, in the same manner as in Example 1, the coagulation step, washing step, and drying step were performed to obtain a solid carboxyl group-containing acrylic rubber (A-4), and each measurement was performed in the same manner. The results are shown in Table 1.

(Preparation of Rubber Composition)
Except for using the carboxyl group-containing acrylic rubber (A-4) obtained above, a rubber composition was prepared and each measurement was carried out in the same manner as in Example 1. The results are shown in Table 1.

[Example 5]
(Production of Carboxyl Group-Containing Acrylic Rubber (A-5))
The monomer emulsion preparation step, the initial polymerization step, and the monomer emulsion dropping step were carried out in the same manner as in Example 1, except that the blending amount of ethyl acrylate in the monomer emulsion preparation step was changed to 55 parts and the blending amount of n-butyl acrylate was changed to 43.2 parts.

Next, after the monomer emulsion dropping step (polymerization conversion rate at the end of the monomer emulsion dropping step was 88.0%), the temperature inside the polymerization reactor was raised to 35°C. While maintaining the temperature at 35°C, 0.003 parts of ferrous sulfate, 0.005 parts of sodium formaldehyde sulfoxylate, and 0.03 parts of cumene hydroperoxide were added, and the reaction was continued for one hour to carry out a post-reaction. When the polymerization conversion rate reached 97.9%, hydroquinone was added as a polymerization terminator to terminate the polymerization reaction, yielding an emulsion polymerization liquid. The solidified, washed, and dried acrylic rubber (A-5) was obtained in the same manner as in Example 1, and each measurement was performed in the same manner. The results are shown in Table 1.

(Preparation of Rubber Composition)
Except for using the carboxyl group-containing acrylic rubber (A-5) obtained above, a rubber composition was prepared and each measurement was carried out in the same manner as in Example 1. The results are shown in Table 1.

[Comparative Example 1]
(Production of Carboxyl Group-Containing Acrylic Rubber (A-6))
The monomer emulsion preparation step, the initial polymerization step, and the monomer emulsion dropping step were carried out in the same manner as in Example 1, except that the amounts of ferrous sulfate, sodium formaldehyde sulfoxylate, and cumene hydroperoxide used in the initial polymerization step and the monomer emulsion dropping step were set to the amounts shown in Table 1.

Next, after the monomer emulsion dropping step (polymerization conversion rate at the end of the monomer emulsion dropping step was 93.2%), the temperature inside the polymerization reactor was raised to 35°C. While maintaining the temperature at 35°C, 0.003 parts of ferrous sulfate, 0.005 parts of sodium formaldehyde sulfoxylate, and 0.03 parts of cumene hydroperoxide were added, and the reaction was continued for one hour to carry out a post-reaction. When the polymerization conversion rate reached 97.8%, hydroquinone was added as a polymerization terminator to terminate the polymerization reaction, yielding an emulsion polymerization liquid. The solidified, washed, and dried products were then obtained in the same manner as in Example 1, yielding a solid carboxyl group-containing acrylic rubber (A-6), which was then subjected to the same measurements. The results are shown in Table 1.

(Preparation of Rubber Composition)
Except for using the carboxyl group-containing acrylic rubber (A-6) obtained above, a rubber composition was prepared and each measurement was carried out in the same manner as in Example 1. The results are shown in Table 1.

[Comparative Example 2]
(Production of Carboxyl Group-Containing Acrylic Rubber (A-7))
The monomer emulsion preparation step, the initial polymerization step, and the monomer emulsion dropping step were carried out in the same manner as in Example 1, except that the amounts of ferrous sulfate, sodium formaldehyde sulfoxylate, and cumene hydroperoxide used in the initial polymerization step and the monomer emulsion dropping step were set to the amounts shown in Table 1.

Next, after the monomer emulsion dropping step (polymerization conversion rate at the end of the monomer emulsion dropping step was 83.2%), the temperature inside the polymerization reactor was raised to 35°C. While maintaining the temperature at 35°C, 0.003 parts of ferrous sulfate, 0.005 parts of sodium formaldehyde sulfoxylate, and 0.03 parts of cumene hydroperoxide were added, and the reaction was continued for one hour to carry out a post-reaction. When the polymerization conversion rate reached 98.1%, hydroquinone was added as a polymerization terminator to terminate the polymerization reaction, yielding an emulsion polymerization liquid. The solidified, washed, and dried acrylic rubber (A-7) was obtained in the same manner as in Example 1, and measurements were performed in the same manner. The results are shown in Table 1.

(Preparation of Rubber Composition)
Except for using the carboxyl group-containing acrylic rubber (A-7) obtained above, a rubber composition was prepared and each measurement was carried out in the same manner as in Example 1. The results are shown in Table 1.

[Comparative Example 3]
(Production of Carboxyl Group-Containing Acrylic Rubber (A-8))
The monomer emulsion preparation step, the initial polymerization step, and the monomer emulsion dropping step were carried out in the same manner as in Example 1, except that the amounts of ferrous sulfate, sodium formaldehyde sulfoxylate, and cumene hydroperoxide used in the initial polymerization step and the monomer emulsion dropping step were set to the amounts shown in Table 1.

Next, after the monomer emulsion dropping step (polymerization conversion rate at the end of the monomer emulsion dropping step was 88.6%), the temperature inside the polymerization reactor was raised to 35°C and the reaction was continued for 1 hour while maintaining the temperature at 35°C to carry out the post-reaction. When the polymerization conversion rate reached 94.0%, hydroquinone was added as a polymerization terminator to terminate the polymerization reaction, yielding an emulsion polymerization liquid. Furthermore, by carrying out the coagulation step, washing step, and drying step in the polymerization reaction system at the start of the post-reaction and in the same manner as in Example 1, a solid carboxyl group-containing acrylic rubber (A-8) was obtained, and each measurement was carried out in the same manner. The results are shown in Table 1.

(Preparation of Rubber Composition)
Except for using the carboxyl group-containing acrylic rubber (A-8) obtained above, a rubber composition was prepared and each measurement was carried out in the same manner as in Example 1. The results are shown in Table 1.

[Comparative Example 4]
(Production of Carboxyl Group-Containing Acrylic Rubber (A-9))
A polymerization reactor equipped with a thermometer and a stirrer was charged with 200 parts of pure water, 45 parts of ethyl acrylate, 53.2 parts of n-butyl acrylate, 1.8 parts of mono-n-butyl fumarate, 1.5 parts of sodium lauryl sulfate (trade name "EMAL 2FG", manufactured by Kao Corporation) as an anionic surfactant, 0.3 parts of polyoxyethylene dodecyl ether (trade name "EMULGEN 105", manufactured by Kao Corporation) as a nonionic surfactant, and 0.01 parts of ethylenediaminetetraacetic acid (EDTA), followed by stirring and cooling to a temperature of 15°C under a nitrogen stream. Next, 0.003 parts of ferrous sulfate, 0.02 parts of sodium formaldehyde sulfoxylate, and 0.008 parts of cumene hydroperoxide were added dropwise to the polymerization reactor to initiate the reaction. Thereafter, the reaction was carried out over 5 hours, and when the polymerization conversion rate reached 90.5%, hydroquinone was added as a polymerization terminator to terminate the polymerization reaction, thereby obtaining an emulsion polymerization liquid. Then, the solidification step, washing step, and drying step were carried out in the same manner as in Example 1, thereby obtaining a solid carboxyl group-containing acrylic rubber (A-9), which was then subjected to the same measurements. The results are shown in Table 1.

(Preparation of Rubber Composition)
Except for using the carboxyl group-containing acrylic rubber (A-9) obtained above, a rubber composition was prepared and each measurement was carried out in the same manner as in Example 1. The results are shown in Table 1.

[Comparative Example 5]
(Production of Carboxyl Group-Containing Acrylic Rubber (A-10))
A solid carboxyl group-containing acrylic rubber (A-10) was obtained in the same manner as in Comparative Example 4, except that the amount of cumene hydroperoxide used was changed to 0.01 parts, the reaction time was changed to 12 hours, and the polymerization reaction was carried out until the polymerization conversion rate reached 95.1%, and each measurement was similarly carried out. The results are shown in Table 1.

(Preparation of Rubber Composition)
Except for using the carboxyl group-containing acrylic rubber (A-10) obtained above, a rubber composition was prepared and each measurement was carried out in the same manner as in Example 1. The results are shown in Table 1.

In Table 1, Comparative Examples 4 and 5 were cases in which the polymerization reaction was carried out in one batch, but the amounts of each monomer, emulsifier, polymerization initiator, etc. used are shown in the columns for the monomer emulsion preparation step and the initial polymerization step.

As shown in Table 1, when the carboxyl group content was in the range of 5.0×10 ⁻³ to 1.5×10 ⁻² mephhr, the amount of aqueous ethanol extracted by the aqueous ethanol extraction operation was 3.0 to 8.0% by weight, and the change in carboxyl group content when extraction was performed by the aqueous ethanol extraction operation was in the range of 0.2×10 ⁻³ to 1.2×10 ⁻³ mephhr, the carboxyl group-containing acrylic rubber was excellent in processability, and the cross-linked rubber obtained using such carboxyl group-containing acrylic rubber was excellent in compression set resistance (Examples 1 to 5).

On the other hand, when the amount of aqueous ethanol solution extracted in the ethanol aqueous solution extraction operation is too small, the carboxyl group-containing acrylic rubber becomes inferior in processability (Comparative Examples 1, and 3 to 5). In particular, in Comparative Examples 4 and 5, in which the amount of aqueous ethanol solution extracted was extremely small at 0.70% by weight and 1.10% by weight, the compression set resistance was also poor when made into a cross-linked rubber.
Furthermore, even when the amount of aqueous ethanol extractable solution extracted by the aqueous ethanol extracting operation is in the range of 3.0 to 8.0% by weight, if the amount of change in the carboxyl group content when extraction is carried out by the aqueous ethanol extracting operation is too large, the result is that the compression set resistance when made into a cross-linked rubber product is also poor (Comparative Example 2).

Claims (7)

A carboxyl group-containing acrylic rubber containing (meth)acrylic acid ester monomer units and carboxyl group-containing monomer units,
the carboxyl group content is in the range of 5.0×10 ⁻³ to 1.5×10 ⁻² mephhr in terms of the number of moles of carboxyl groups per 100 g of the carboxyl group-containing acrylic rubber,
the amount of the aqueous ethanol solution extracted by an extraction procedure using an aqueous ethanol solution containing 75% by volume of ethanol and 25% by volume of water is 3.0 to 8.0% by weight;
A carboxyl group-containing acrylic rubber in which the amount of change in carboxyl group content when extracted by the aqueous ethanol extraction procedure is in the range of 0.2×10 ⁻³ to 1.2×10 ⁻³ mephhr. The carboxyl group-containing acrylic rubber described in claim 1, wherein the carboxyl group-containing monomer unit is a butenedioic acid monoester monomer unit. The carboxyl group-containing acrylic rubber described in claim 2, wherein the carboxyl group-containing monomer unit is a fumaric acid monoester monomer unit. A carboxyl group-containing acrylic rubber according to any one of claims 1 to 3, wherein the amount extracted with the aqueous ethanol solution is 4.0 to 6.5% by weight. 5. The carboxyl group-containing acrylic rubber according to claim 1, wherein the amount of change in carboxyl group content when the extraction with the aqueous ethanol solution is carried out is in the range of 0.5×10 ⁻³ to 0.8×10 ⁻³ mephhr. A rubber composition comprising a rubber component containing the carboxyl group-containing acrylic rubber according to any one of claims 1 to 5 and a crosslinking agent. A cross-linked rubber product obtained by cross-linking the rubber composition described in claim 6.