Rubber composition, crosslinked product thereof, and method for producing acrylic rubber

Abstract

The rubber composition contains an acrylic rubber and a nonionic emulsifier, wherein more than 0 mass% and not more than 80 mass% of the total mass of the nonionic emulsifier is fixed to the acrylic rubber.

Description

Rubber composition, crosslinked product thereof, and method for producing acrylic rubber

[0001] The present invention relates to a rubber composition, a crosslinked product thereof, and a method for producing an acrylic rubber.

[0002] Acrylic rubber and its crosslinked products have excellent physical properties such as heat resistance, oil resistance, and mechanical properties, and are therefore used, for example, as materials for hoses in automobile engine compartments, sealing parts, etc. For example, Patent Document 1 discloses an acrylic rubber containing, as monomer units, an alkyl acrylate and a crosslinked monomer, and in which the content of a nonionic emulsifier in the acrylic rubber is 0.5 to 2 mass %.

[0003] International Publication No. 2021 / 075128

[0004] As disclosed in Patent Document 1, studies have been conducted focusing on the content of nonionic emulsifier coexisting with acrylic rubber, but according to the studies of the present inventors, even if the content of nonionic emulsifier coexisting with acrylic rubber is the same, the water resistance and tensile strength at break of the cross-linked acrylic rubber may change, and in some cases the water resistance or tensile strength at break may deteriorate.

[0005] Therefore, one aspect of the present invention aims to provide a cross-linked acrylic rubber that has both good water resistance and good tensile strength at break.

[0006] The present inventors have found that, although nonionic emulsifiers that coexist with acrylic rubber are generally referred to as such, there can be nonionic emulsifiers that are fixed to the acrylic rubber and are relatively difficult to remove, and nonionic emulsifiers that are not fixed to the acrylic rubber and are relatively easy to remove.

[0007] It was also found that even when the same total amount of nonionic emulsifier is coexistent with acrylic rubber having the same monomer composition, if there is no nonionic emulsifier fixed to the acrylic rubber (only nonionic emulsifier not fixed to the acrylic rubber is present), the tensile strength at break of the cross-linked acrylic rubber decreases, while if there is more than the necessary amount of nonionic emulsifier fixed to the acrylic rubber, the water resistance of the cross-linked acrylic rubber deteriorates. Therefore, in order to achieve both the water resistance and tensile strength at break of the cross-linked acrylic rubber (without lowering or worsening either one) when the same total amount of nonionic emulsifier is coexistent with acrylic rubber having the same monomer composition, it is important to keep the proportion of nonionic emulsifier fixed to the acrylic rubber within a specific range.

[0008] The present invention includes the following aspects. [1] A rubber composition containing an acrylic rubber and a nonionic emulsifier, wherein more than 0 mass % and not more than 80 mass % of the total mass of the nonionic emulsifier is fixed to the acrylic rubber. [2] The rubber composition according to [1], wherein the total mass of the nonionic emulsifier is not more than 5 parts by mass per 100 parts by mass of the total mass of the acrylic rubber. [3] The rubber composition according to [1] or [2], wherein the acrylic rubber contains at least one monomer unit selected from the group consisting of alkyl acrylate esters, alkyl methacrylate esters, and alkoxyalkyl acrylate esters. [4] The rubber composition according to any one of [1] to [3], further containing a crosslinking agent. [5] A crosslinked product of the rubber composition according to any one of [1] to [4]. [6] A method for producing acrylic rubber, comprising the steps of polymerizing acrylic monomers in the presence of a nonionic emulsifier to obtain a latex of an acrylic polymer, and coagulating and washing the acrylic polymer to obtain an acrylic rubber, wherein more than 0% by mass but not more than 70% by mass of the total mass of the nonionic emulsifier in the latex is fixed to the acrylic polymer. [7] A method for producing acrylic rubber according to [6], wherein the total mass of the nonionic emulsifier in the latex is 6 parts by mass or less per 100 parts by mass of the total mass of the acrylic polymer. [8] A method for producing acrylic rubber according to [6] or [7], wherein the acrylic rubber contains at least one monomer unit selected from the group consisting of alkyl acrylates, alkyl methacrylates, and alkoxyalkyl acrylates.

[0009] According to one aspect of the present invention, it is possible to achieve both water resistance and tensile strength at break in a cross-linked acrylic rubber.

[0010] According to another aspect of the present invention, the toluene-insoluble content of the acrylic rubber can be reduced. According to another aspect of the present invention, the flex fatigue resistance of the cross-linked acrylic rubber can be improved. According to another aspect of the present invention, the copper damage resistance of the cross-linked acrylic rubber can be improved.

[0011] One embodiment of the present invention is a method for producing acrylic rubber, comprising: a step of polymerizing acrylic monomers in the presence of a nonionic emulsifier to obtain a latex of an acrylic polymer (hereinafter also referred to as "step S1"); and a step of coagulating the acrylic polymer and washing it to obtain an acrylic rubber (hereinafter also referred to as "step S2").

[0012] In step S1, for example, a monomer mixture containing an acrylic monomer and a polymerization initiator are added to a suspension of a nonionic emulsifier to polymerize the acrylic monomer, thereby obtaining a latex in which an acrylic polymer is dispersed in water.

[0013] The acrylic monomer may include at least one monomer selected from the group consisting of alkyl acrylates, alkyl methacrylates, and alkoxyalkyl acrylates.

[0014] The alkyl acrylate is represented by the following formula (1). In the formula, R １ represents an alkyl group.

[0015] The alkyl group (R １ The alkyl group (R ) in the alkyl acrylate ester may be linear or branched. １ The number of carbon atoms in the alkyl acrylate may be 1 or more and 16 or less. Specific examples of the alkyl acrylate include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-pentyl acrylate, isoamyl acrylate, n-hexyl acrylate, 2-methylpentyl acrylate, n-octyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, hexadecyl acrylate, 1-adamantyl acrylate, and cyclohexyl acrylate. These alkyl acrylates may be used alone or in combination of two or more.

[0016] The content of the alkyl acrylate may be 40% by mass or more, 50% by mass or more, or 60% by mass or more, and may be 99% by mass or less, 95% by mass or less, or 90% by mass or less, based on the total mass of the acrylic monomers.

[0017] The alkyl acrylate has an alkyl group having 3 or less carbon atoms (R １ an alkyl acrylate (first alkyl acrylate) having an alkyl group having 3 or less carbon atoms (R １ wherein the alkyl group has 4 or more carbon atoms.

[0018] The number of carbon atoms in the alkyl group in the first alkyl acrylate may be 1 or more, 2 or less, or even 2. The first alkyl acrylate is preferably ethyl acrylate. The number of carbon atoms in the alkyl group in the second alkyl acrylate may be 8 or less, 6 or less, or 5 or less, or even 4. The second alkyl acrylate is preferably n-butyl acrylate.

[0019] The content of the first alkyl acrylate may be 10% by mass or more, 20% by mass or more, or 30% by mass or more, and may be 70% by mass or less, 60% by mass or less, or 50% by mass or less, based on the total mass of the acrylic monomers.

[0020] The content of the second alkyl acrylate may be 20% by mass or more, 30% by mass or more, or 40% by mass or more, and may be 70% by mass or less, 60% by mass or less, or 50% by mass or less, based on the total mass of the acrylic monomers.

[0021] The alkyl methacrylate is represented by the following formula (2). In the formula, R ２ represents an alkyl group.

[0022] The alkyl group (R ２The alkyl group (R ) in the alkyl methacrylate ester may be linear or branched. ２ The number of carbon atoms in the alkyl methacrylate may be 1 or more, or 4 or less, preferably 2 or more or 3 or more, and may be 3. Specific examples of the alkyl methacrylate include methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, and isobutyl methacrylate. These alkyl methacrylates may be used alone or in combination of two or more. The alkyl methacrylate is preferably n-butyl methacrylate.

[0023] The content of the alkyl methacrylate ester (preferably an alkyl methacrylate ester having an alkyl group having 3 or more carbon atoms) may be 3% by mass or more, 5% by mass or more, or 7% by mass or more, and is 40% by mass or less, 35% by mass or less, 30% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, or 13% by mass or less, based on the total mass of the acrylic monomer.

[0024] The alkoxyalkyl acrylate is represented by the following formula (3). In the formula, R ３ represents an alkylene group, and R ４ represents an alkyl group.

[0025] The alkylene group (R ３ ) and alkyl groups (R ４ The alkylene group (R ) in the alkoxyalkyl acrylate ester may be linear or branched. ３ The number of carbon atoms in the alkyl group (R ) may be 1 or more, or 2 or more, and may be 4 or less, or 3 or less. ４ ) may have 1 or more carbon atoms, and may have 4 or less, 3 or less, or 2 or less carbon atoms.

[0026] Specific examples of alkoxyalkyl acrylates include 2-methoxyethyl acrylate, 2-ethoxyethyl acrylate, 2-(n-propoxy)ethyl acrylate, 2-(n-butoxy)ethyl acrylate, 3-methoxypropyl acrylate, 3-ethoxypropyl acrylate, 2-(n-propoxy)propyl acrylate, and 2-(n-butoxy)propyl acrylate. These alkoxyalkyl acrylates may be used alone or in combination of two or more.

[0027] The content of the alkoxyalkyl acrylate may be 5% by mass or more, 10% by mass or more, or 12% by mass or more, and may be 30% by mass or less, 25% by mass or less, or 20% by mass or less, based on the total mass of the acrylic monomers.

[0028] The acrylic monomer may further include a crosslinking monomer. The crosslinking monomer is a monomer that is copolymerizable with an alkyl acrylate (and further with an alkyl methacrylate and an alkoxyalkyl acrylate) and has a crosslinkable group that forms a crosslinking site (also referred to as a crosslinking point). The crosslinking monomer has a polymerizable carbon-carbon double bond, such as an acryloyl group, a methacryloyl group, an allyl group, a methallyl group, a vinyl group, or an alkenylene group. Examples of the crosslinking group include a carboxyl group, an epoxy group, and an active chlorine group. The crosslinking monomer may have one or more of these functional groups.

[0029] Examples of cross-linkable monomers having a carboxyl group as a cross-linkable group include acrylic acid, methacrylic acid, crotonic acid, 2-pentenoic acid, maleic acid, fumaric acid, itaconic acid, and maleic acid monoalkyl esters.

[0030] Examples of cross-linking monomers having an epoxy group as a cross-linkable group include glycidyl acrylate, glycidyl methacrylate, allyl glycidyl ether, and methallyl glycidyl ether.

[0031] Examples of crosslinkable monomers having an active chlorine group as a crosslinkable group include 2-chloroethyl vinyl ether, 2-chloroethyl acrylate, vinylbenzyl chloride, vinyl chloroacetate, and allyl chloroacetate.

[0032] The content of the crosslinking monomer may be 0.5% by mass or more, 1% by mass or more, or 1.2% by mass or more, and may be 8% by mass or less, 5% by mass or less, or 3% by mass or less, based on the total mass of the acrylic monomers.

[0033] The acrylic monomer may further contain other monomers copolymerizable with the above-mentioned monomers, such as ethylene, methacrylic acid alkoxy esters, alkyl vinyl ketones, vinyl ethers, allyl ethers, vinyl aromatic compounds, vinyl nitriles, maleic acid dialkyl esters, fumaric acid dialkyl esters, itaconic acid dialkyl esters, citraconic acid dialkyl esters, mesaconic acid dialkyl esters, 2-pentenedioic acid dialkyl esters, and acetylenedicarboxylic acid dialkyl esters.

[0034] The nonionic emulsifier may include at least one selected from the group consisting of ether-type nonionic emulsifiers, ester-type nonionic emulsifiers, ether ester-type nonionic emulsifiers, nitrogen-containing nonionic emulsifiers, and polyvinyl alcohol-based emulsifiers.

[0035] Examples of ether-type nonionic emulsifiers include polyoxyethylene alkyl ethers, polyoxyethylene alkyl allyl ethers, polyoxyalkylene alkyl ethers, polyoxyethylene alkenyl ethers, polyoxyethylene alkylphenol ethers, polyoxyethylene polyoxypropylene copolymers, polyoxyethylene lauryl glycol, and polyoxypropylene glycol. The ether-type nonionic emulsifier preferably contains a polyoxyethylene alkyl ether. The number of carbon atoms in the alkyl group in the polyoxyethylene alkyl ether may be 1 or more and 15 or less. The molecular weight of the polyoxyethylene alkyl ether may be 1,000 or more and 2,000 or less.

[0036] Examples of ester-type nonionic emulsifiers include sorbitan fatty acid esters and glycerin fatty acid esters. The sorbitan fatty acid esters may be, for example, monoesters. The glycerin fatty acid esters may be, for example, monoesters or diesters.

[0037] Examples of ether ester-type nonionic emulsifiers include polyoxyethylene fatty acid esters and polyoxyethylene sorbitan fatty acid esters. The polyoxyethylene fatty acid esters may be, for example, monoesters or diesters. The polyoxyethylene sorbitan fatty acid esters may be, for example, monoesters.

[0038] Examples of nitrogen-containing nonionic emulsifiers include polyoxyethylene alkylamines, polyoxyethylene fatty acid monoethanolamides, and fatty acid diethanolamides.

[0039] The polyvinyl alcohol emulsifier may be a fully saponified polyvinyl alcohol or a partially saponified polyvinyl alcohol, preferably a partially saponified polyvinyl alcohol. The saponification degree of the polyvinyl alcohol may be 60 mol% or more, 70 mol% or more, or 80 mol% or more, and may be 100 mol% or less, 99 mol% or less, or 90 mol% or less. The saponification degree of the polyvinyl alcohol means a value measured in accordance with JIS K6726 "3.5 Saponification degree."

[0040] The amount of the nonionic emulsifier added may be 1 part by mass or more, 1.5 parts by mass or more, 2 parts by mass or more, 2.5 parts by mass or more, 3 parts by mass or more, 3.5 parts by mass or more, 4 parts by mass or more, or 4.5 parts by mass or more, relative to 100 parts by mass of the total mass of the acrylic monomers. From the viewpoint of further improving the water resistance of the crosslinked product, lowering the toluene-insoluble content of the acrylic rubber, and improving the flex fatigue resistance of the crosslinked product, the amount of the nonionic emulsifier added may be preferably 10 parts by mass or less, 9 parts by mass or less, 8 parts by mass or less, 7 parts by mass or less, 6 parts by mass or less, or 5 parts by mass or less, relative to 100 parts by mass of the total mass of the acrylic monomers.

[0041] In one embodiment, an anionic emulsifier may be used in addition to the nonionic emulsifier. That is, in step S1, an acrylic polymer latex may be obtained by polymerizing an acrylic monomer in the presence of a nonionic emulsifier and an anionic emulsifier. The anionic emulsifier may be a known anionic emulsifier such as sodium lauryl sulfate. In this case, the mass ratio of the nonionic emulsifier to the anionic emulsifier may be 6 / 4 or more and 8 / 2 or less.

[0042] The polymerization initiator may be, for example, an azo compound such as azobisisobutyronitrile, an organic peroxide such as tert-butyl hydroperoxide, cumene hydroperoxide, or benzoyl peroxide, or an inorganic peroxide such as sodium persulfate or ammonium persulfate, and is preferably tert-butyl hydroperoxide.

[0043] When polymerizing the acrylic monomer, a pH adjuster may be further used. The pH adjuster may be, for example, an alkali metal salt such as sodium acetate, sodium hydroxide, potassium hydroxide, sodium phosphate, or sodium citrate, and is preferably sodium acetate. The amount of the pH adjuster added may be, for example, 0.1 parts by mass or more and 5 parts by mass or less relative to 100 parts by mass of the total amount of the monomers.

[0044] The acrylic polymer in the latex obtained in step S1 may contain the above-mentioned acrylic monomer as a monomer unit, i.e., the acrylic polymer may contain at least one monomer unit selected from the group consisting of an alkyl acrylate ester, an alkyl methacrylate ester, and an alkoxyalkyl acrylate ester.

[0045] In this latex, more than 0% by mass and not more than 70% by mass of the total mass of the nonionic emulsifier is fixed to the acrylic polymer. In this specification, the amount of nonionic emulsifier fixed to the acrylic polymer is determined by subtracting the amount of nonionic emulsifier not fixed to the acrylic polymer from the total amount (charge amount) of nonionic emulsifier in the latex. The amount of nonionic emulsifier not fixed to the acrylic polymer is defined as the amount of nonionic emulsifier contained in the supernatant after centrifuging the latex at 10,000 rpm for 30 minutes and separating the precipitated solids. The amount of nonionic emulsifier contained in the supernatant is measured by a method appropriately selected depending on the type of nonionic emulsifier. For example, when the nonionic emulsifier is polyvinyl alcohol, the amount of polyvinyl alcohol contained in the supernatant is measured by the method described in the Examples.

[0046] That is, in this specification, a nonionic emulsifier fixed to an acrylic polymer is defined as a nonionic emulsifier contained in the precipitated solids when latex is centrifuged at 10,000 rpm for 30 minutes (a nonionic emulsifier that does not detach from the acrylic polymer). The nonionic emulsifier fixed to an acrylic polymer may be chemically bound to the acrylic polymer or may be physically bound (adsorbed) to the acrylic polymer. Furthermore, in this specification, a nonionic emulsifier not fixed to an acrylic polymer is defined as a nonionic emulsifier contained in the supernatant when latex is centrifuged at 10,000 rpm for 30 minutes (a nonionic emulsifier that has detached from the acrylic polymer).

[0047] The lower limit of the amount of the nonionic emulsifier fixed to the acrylic polymer may be preferably 0.1% by mass or more, 0.5% by mass or more, 1% by mass or more, 1.5% by mass or more, 2% by mass or more, 3% by mass or more, 4% by mass or more, 5% by mass or more, 6% by mass or more, 7% by mass or more, 8% by mass or more, 9% by mass or more, 10% by mass or more, 12% by mass or more, 14% by mass or more, 16% by mass or more, 18% by mass or more, 20% by mass or more, 25% by mass or more, 30% by mass or more, 40% by mass or more, or 45% by mass or more, based on the total mass of the nonionic emulsifier, from the viewpoint of further improving the tensile strength at break of the crosslinked product and improving the copper damage resistance of the crosslinked product. The upper limit of the amount of nonionic emulsifier fixed to the acrylic polymer may be preferably 68% by mass or less, 65% by mass or less, 60% by mass or less, 55% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, 25% by mass or less, 20% by mass or less, 15% by mass or less, or 10% by mass or less, based on the total mass of the nonionic emulsifier, from the viewpoints of further improving the water resistance of the crosslinked product, lowering the toluene-insoluble content of the acrylic rubber, and improving the flex fatigue resistance of the crosslinked product.

[0048] The amount of the nonionic emulsifier that is not fixed to the acrylic polymer may be preferably 30% by mass or more, 32% by mass or more, 35% by mass or more, 40% by mass or more, 45% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, 75% by mass or more, 80% by mass or more, 85% by mass or more, or 90% by mass or more, based on the total mass of the nonionic emulsifier, from the viewpoints of further improving the water resistance of the crosslinked product, lowering the toluene-insoluble content of the acrylic rubber, and improving the flex fatigue resistance of the crosslinked product. From the viewpoint of further improving the tensile strength at break of the crosslinked product and improving the copper damage resistance of the crosslinked product, the amount of the nonionic emulsifier that is not fixed to the acrylic polymer, based on the total mass of the nonionic emulsifier, may be preferably less than 100% by mass, 99.9% by mass or less, 99.5% by mass or less, 99% by mass or less, 98.5% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 94% by mass or less, 93% by mass or less, 92% by mass or less, 91% by mass or less, 90% by mass or less, 88% by mass or less, 86% by mass or less, 84% by mass or less, 82% by mass or less, 80% by mass or less, 75% by mass or less, 70% by mass or less, 60% by mass or less, or 55% by mass or less.

[0049] The total mass of the nonionic emulsifiers in the latex may be 1 part by mass or more, 1.5 parts by mass or more, 2 parts by mass or more, 2.5 parts by mass or more, 3 parts by mass or more, 3.5 parts by mass or more, 4 parts by mass or more, or 4.5 parts by mass or more, relative to 100 parts by mass of the total mass of the acrylic polymer. From the viewpoints of further improving the water resistance of the crosslinked product, lowering the toluene-insoluble content of the acrylic rubber, and improving the flex fatigue resistance of the crosslinked product, the total mass of the nonionic emulsifiers in the latex may be preferably 10 parts by mass or less, 9 parts by mass or less, 8 parts by mass or less, 7 parts by mass or less, 6 parts by mass or less, or 5 parts by mass or less, relative to 100 parts by mass of the total mass of the acrylic polymer.

[0050] In order for a specific amount of the nonionic emulsifier in the latex to be fixed to the acrylic polymer and for a specific amount of the nonionic emulsifier in the rubber composition described below to be fixed to the acrylic rubber, it is important in step S1 to separately add a monomer mixture containing an acrylic monomer and a polymerization initiator to a suspension of the nonionic emulsifier, to adjust the amount of the polymerization initiator added within a specific range, and to adjust the temperature during polymerization (polymerization temperature) within a specific range.

[0051] Specifically, for example, the amount of polymerization initiator added tends to increase as the amount of nonionic emulsifier fixed to the acrylic polymer in the latex and the amount of nonionic emulsifier fixed to the acrylic rubber in the rubber composition increases, so it is necessary to adjust the amount of polymerization initiator added taking this tendency into consideration.Furthermore, the amount of nonionic emulsifier fixed to the acrylic polymer in the latex and the amount of nonionic emulsifier fixed to the acrylic rubber in the rubber composition tends to increase as the polymerization temperature increases, so it is necessary to adjust the polymerization temperature taking this tendency into consideration.

[0052] Furthermore, in step S1, the amount of nonionic emulsifier fixed to the acrylic polymer and the amount of nonionic emulsifier fixed to the acrylic rubber in the rubber composition can be adjusted by increasing or decreasing the amount of acrylic monomer added at the beginning of polymerization of the acrylic monomer or by adjusting the polymerization time. Specifically, for example, even if the total amount of acrylic monomer added is the same, if the amount (addition rate) of acrylic monomer added is changed in two stages, with the amount of acrylic monomer added in the first half being relatively small and the amount of acrylic monomer added in the second half being relatively large, the amount of nonionic emulsifier fixed to the acrylic polymer and the amount of nonionic emulsifier fixed to the acrylic rubber in the rubber composition will tend to decrease.

[0053] Subsequently, in step S2, a coagulant is added to the latex obtained in step S1 to coagulate the acrylic polymer. Examples of coagulants include ammonium salts, monovalent to trivalent metal salts, inorganic acids, and organic acids. Examples of ammonium salts include ammonium borate, ammonium sulfate, and ammonium chloride. Examples of monovalent to trivalent metal salts include sodium salts, magnesium salts, calcium salts, aluminum salts, and zinc salts. Examples of sodium salts include sodium borate, sodium sulfate, and sodium chloride. Examples of magnesium salts include magnesium sulfate, magnesium chloride, and magnesium nitrate. Examples of calcium salts include calcium sulfate, calcium chloride, and calcium nitrate. Examples of aluminum salts include aluminum sulfate and aluminum chloride. Examples of zinc salts include zinc chloride and zinc acetate. Examples of inorganic acids include hydrochloric acid, sulfuric acid, and nitric acid. Examples of organic acids include formic acid and acetic acid.

[0054] The amount of coagulant added may be 0.5 parts by mass or more, 1 part by mass or more, or 2 parts by mass or more, and may be 150 parts by mass or less, 50 parts by mass or less, or 20 parts by mass or less, relative to 100 parts by mass of the total mass of the acrylic polymer.

[0055] In step S2, the coagulated acrylic polymer is then washed with water and dried. During the washing with water, a portion of the nonionic emulsifier that is not bound to the acrylic polymer in the latex (the nonionic emulsifier that is detached by washing with water even without the above-mentioned centrifugation) is removed. Therefore, by undergoing step S2, the amount of nonionic emulsifier that is not bound to the acrylic rubber in the obtained acrylic rubber (rubber composition) can be less than the amount of nonionic emulsifier that is not bound to the acrylic polymer in the latex.

[0056] The above-described manufacturing method provides an acrylic rubber coexisting with a nonionic emulsifier (a rubber composition containing an acrylic rubber and a nonionic emulsifier). That is, another embodiment of the present invention is a rubber composition containing an acrylic rubber and a nonionic emulsifier. When an anionic emulsifier is used in the above-described step S1, the rubber composition may further contain an anionic emulsifier.

[0057] The acrylic rubber contains the above-mentioned acrylic monomer as a monomer unit. That is, the acrylic polymer may contain at least one monomer unit selected from the group consisting of an alkyl acrylate ester, an alkyl methacrylate ester, and an alkoxyalkyl acrylate ester. The acrylic rubber may further contain a crosslinking monomer as a monomer unit, or may further contain other monomers.

[0058] The nonionic emulsifier contained in the rubber composition may be derived from the nonionic emulsifier used in step S1. Details of the nonionic emulsifier are as described above. In the rubber composition, more than 0% by mass and 80% by mass or less of the total mass of the nonionic emulsifier is fixed to the acrylic rubber.

[0059] In this specification, the amount of nonionic emulsifier fixed to the acrylic rubber is determined by subtracting the amount of nonionic emulsifier not fixed to the acrylic rubber (and, if the acrylic rubber is washed, the amount of nonionic emulsifier removed by washing) from the total amount (charge amount) of nonionic emulsifier. The amount of nonionic emulsifier not fixed to the acrylic rubber is defined as the amount of nonionic emulsifier contained in the supernatant after centrifuging a solution in which the acrylic rubber is dissolved (see Examples for details) at 10,000 rpm for 30 minutes. The amount of nonionic emulsifier contained in the supernatant is measured by a method appropriately selected depending on the type of nonionic emulsifier. For example, if the nonionic emulsifier is polyvinyl alcohol, the amount of polyvinyl alcohol contained in the supernatant is measured by the method described in the Examples.

[0060] That is, in this specification, a nonionic emulsifier fixed to an acrylic rubber is defined as a nonionic emulsifier (a nonionic emulsifier that does not detach from the acrylic rubber) contained in the precipitated solids when an acrylic rubber solution is centrifuged at 10,000 rpm for 30 minutes. The nonionic emulsifier fixed to the acrylic rubber may be chemically bound to the acrylic rubber or may be physically bound (adsorbed) to the acrylic rubber. Furthermore, in this specification, a nonionic emulsifier that is not fixed to the acrylic rubber is defined as a nonionic emulsifier (a nonionic emulsifier that has detached from the acrylic rubber) contained in the supernatant when an acrylic rubber solution is centrifuged at 10,000 rpm for 30 minutes.

[0061] The lower limit of the amount of the nonionic emulsifier fixed to the acrylic rubber may be preferably 0.1% by mass or more, 0.5% by mass or more, 1% by mass or more, 1.5% by mass or more, 2% by mass or more, 3% by mass or more, 4% by mass or more, 5% by mass or more, 6% by mass or more, 7% by mass or more, 8% by mass or more, 9% by mass or more, 10% by mass or more, 12% by mass or more, 14% by mass or more, 16% by mass or more, 18% by mass or more, 20% by mass or more, 25% by mass or more, 30% by mass or more, 40% by mass or more, 45% by mass or more, or 50% by mass or more, based on the total mass of the nonionic emulsifier, from the viewpoint of further improving the tensile strength at break of the crosslinked product and improving the copper damage resistance of the crosslinked product. The upper limit of the amount of nonionic emulsifier fixed to the acrylic rubber may be preferably 75% by mass or less, 70% by mass or less, 68% by mass or less, 65% by mass or less, 60% by mass or less, 55% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, 25% by mass or less, 20% by mass or less, or 15% by mass or less, based on the total mass of the nonionic emulsifier, from the viewpoints of further improving the water resistance of the cross-linked product, lowering the toluene-insoluble content of the acrylic rubber, and improving the flex fatigue resistance of the cross-linked product.

[0062] The amount of the nonionic emulsifier that is not fixed to the acrylic rubber may be preferably 20% by mass or more, 25% by mass or more, 30% by mass or more, 32% by mass or more, 35% by mass or more, 40% by mass or more, 45% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, 75% by mass or more, 80% by mass or more, or 85% by mass or more, based on the total mass of the nonionic emulsifier, from the viewpoints of further improving the water resistance of the cross-linked product, lowering the toluene-insoluble content of the acrylic rubber, and improving the flex fatigue resistance of the cross-linked product. From the viewpoint of further improving the tensile strength at break of the cross-linked product and improving the copper damage resistance of the cross-linked product, the amount of the nonionic emulsifier that is not fixed to the acrylic rubber, based on the total mass of the nonionic emulsifier, may be preferably less than 100% by mass, 99.9% by mass or less, 99.5% by mass or less, 99% by mass or less, 98.5% by mass or less, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 94% by mass or less, 93% by mass or less, 92% by mass or less, 91% by mass or less, 90% by mass or less, 88% by mass or less, 86% by mass or less, 84% by mass or less, 82% by mass or less, 80% by mass or less, 75% by mass or less, 70% by mass or less, 60% by mass or less, 55% by mass or less, or 50% by mass or less.

[0063] The total mass of the nonionic emulsifier in the rubber composition may be 0.5 parts by mass or more, 1 part by mass or more, 1.5 parts by mass or more, 2 parts by mass or more, 2.5 parts by mass or more, 3 parts by mass or more, 3.5 parts by mass or more, or 4 parts by mass or more, relative to 100 parts by mass of the total mass of the acrylic rubber. From the viewpoint of further improving the water resistance of the cross-linked product, lowering the toluene-insoluble content of the acrylic rubber, and improving the flex fatigue resistance of the cross-linked product, the total mass of the nonionic emulsifier in the rubber composition may be preferably 10 parts by mass or less, 9 parts by mass or less, 8 parts by mass or less, 7 parts by mass or less, 6 parts by mass or less, 5 parts by mass or less, 4.5 parts by mass or less, or 4 parts by mass or less, relative to 100 parts by mass of the total mass of the acrylic rubber.

[0064] The rubber composition may further contain a cross-linking agent. The rubber composition may further contain a cross-linking accelerator. In this case, a cross-linked product can be obtained by kneading the rubber composition at a temperature equal to or lower than the cross-linking temperature and then heating it at a predetermined cross-linking temperature. Another embodiment of the present invention is a cross-linked product of the above rubber composition.

[0065] The heating conditions for crosslinking can be appropriately set depending on the compounding of the rubber composition and the type of crosslinking agent. The heating temperature may be 100°C or higher and 200°C or lower. The heating time may be 1 hour or higher and 10 hours or lower. As the heating method, a method used for crosslinking rubber, such as hot press heating, steam heating, or oven heating, can be used.

[0066] The apparatus for kneading, molding, and crosslinking the rubber composition and the apparatus for kneading and molding the crosslinked product of the rubber composition can be any apparatus typically used for rubber compositions, such as a roll, kneader, Banbury mixer, internal mixer, or twin-screw extruder.

[0067] The crosslinking agent is not particularly limited as long as it is one that is normally used for crosslinking acrylic rubber. For example, when the acrylic rubber contains a crosslinkable monomer having a carboxyl group as a monomer unit, the crosslinking agent is preferably a polyamine compound or a carbonate of a polyamine compound, more preferably a polyamine compound having 4 to 30 carbon atoms or a carbonate thereof.

[0068] Specific examples of polyvalent amine compounds include 4,4'-bis(4-aminophenoxy)biphenyl, 4,4'-diaminodiphenyl sulfide, 1,3-bis(4-aminophenoxy)-2,2-dimethylpropane, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)pentane, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(4-aminophenoxy)phenyl]sulfone, 4,4'-diaminodiphenyl sulfone, bis(4-aminophenoxy)-3,3-dimethylpropane, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)pentane, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(4-aminophenoxy)phenyl]sulfone, 4,4'-diaminodiphenyl sulfone, and bis(4-aminophenoxy)-3,3-dimethylpropane. (i) Aromatic polyamine compounds such as phenyl sulfone, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 4,4'-diaminobenzanilide, and bis[4-(4-aminophenoxy)phenyl]sulfone; and aliphatic polyamine compounds such as hexamethylenediamine, hexamethylenediamine carbamate, N,N'-dicinnamylidene-1,6-hexanediamine, diethylenetriamine, triethylenetetramine, and tetraethylenepentamine.

[0069] The content of the crosslinking agent in the rubber composition may be 0.1 parts by mass or more, 0.2 parts by mass or more, or 0.3 parts by mass or more, and may be 5 parts by mass or less, 4 parts by mass or less, or 3 parts by mass or less, relative to 100 parts by mass of the acrylic rubber.

[0070] The crosslinking accelerator is not particularly limited, but when the crosslinking agent is a polyvalent amine compound or a carbonate thereof, examples thereof include aliphatic monovalent secondary amine compounds, aliphatic monovalent tertiary amine compounds, guanidine compounds, imidazole compounds, quaternary onium salts, tertiary phosphine compounds, alkali metal salts of weak acids, and diazabicycloalkene compounds. The crosslinking accelerators can be used alone or in combination of two or more.

[0071] Examples of the aliphatic monovalent secondary amine compound include dimethylamine, diethylamine, di-n-propylamine, 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, dioctadecylamine, di-cis-9-octadecenylamine, and dinonadecylamine.

[0072] Examples of the aliphatic monovalent tertiary amine compounds include trimethylamine, triethylamine, tri-n-propylamine, triallylamine, triisopropylamine, tri-n-butylamine, tri-t-butylamine, tri-sec-butylamine, tripentylamine, trihexylamine, triheptylamine, trioctylamine, trinonylamine, tridecylamine, triundecylamine, tridodecylamine, tridecylamine, tritetradecylamine, tripentadecylamine, tricetylamine, tri-2-ethylhexylamine, trioctylamine, tridecylamine, tripentadecylamine, tri ... octadecylamine, tri-cis-9-octadecenylamine, trinonadecylamine, N,N-dimethyldecylamine, N,N-dimethyldodecylamine, N,N-dimethyltetradecylamine, N,N-dimethylcetylamine, N,N-dimethyloctadecylamine, N,N-dimethylbehenylamine, N-methyldidecylamine, N-methyldidodecylamine, N-methylditetradecylamine, N-methyldicetylamine, N-methyldioctadecylamine, N-methyldibehenylamine, and dimethylcyclohexylamine.

[0073] Examples of the guanidine compound include 1,3-di-o-tolylguanidine and 1,3-diphenylguanidine.

[0074] Examples of the imidazole compound include 2-methylimidazole and 2-phenylimidazole.

[0075] The quaternary onium salt is not particularly limited, but examples thereof include tetra-n-butylammonium chloride, trimethylphenylammonium chloride, trimethylstearylammonium chloride, trimethyllaurylammonium chloride, trimethylcetylammonium chloride, dimethyldistearylammonium chloride, tributylbenzylammonium chloride, tetra-n-butylammonium bromide, methyltriphenylammonium bromide, ethyltriphenylammonium bromide, trimethylphenylammonium bromide, trimethylbenzylammonium bromide, trimethylstearylammonium bromide, and tetrabutylammonium thiocyanate. and phosphonium salts such as tetra-n-butylphosphonium chloride, tetra-n-butylphosphonium bromide, methyltriphenylphosphonium bromide, ethyltriphenylphosphonium bromide, butyltriphenylphosphonium bromide, hexyltriphenylphosphonium bromide, benzyltriphenylphosphonium bromide, tetraphenylphosphonium chloride, tetraphenylphosphonium bromide, 4-butoxybenzyltriphenylphosphonium bromide, allyltributylphosphonium chloride, 2-propynyltriphenylphosphonium bromide, and methoxypropyltributylphosphonium chloride.

[0076] Tertiary phosphine compounds include triphenylphosphine, tri-p-tolylphosphine, and the like.

[0077] Examples of alkali metal salts of weak acids include inorganic weak acid salts such as sodium and potassium phosphates and carbonates, and organic weak acid salts such as sodium and potassium stearates and laurates.

[0078] Examples of diazabicycloalkene compounds include 1,8-diazabicyclo[5.4.0]undecene-7 (DBU), 1,5-diazabicyclo[4.3.0]nonene-5 (DBN), and 1,4-diazabicyclo[2.2.2]octane (DABCO). These diazabicycloalkene compounds may form salts with, for example, hydrochloric acid, sulfuric acid, carboxylic acid, sulfonic acid, phenol, or the like. Examples of carboxylic acids include octylic acid, oleic acid, formic acid, orthophthalic acid, and adipic acid. Examples of sulfonic acids include benzenesulfonic acid, toluenesulfonic acid, dodecylbenzenesulfonic acid, and naphthalenesulfonic acid.

[0079] The content of the crosslinking accelerator may be 0.1 parts by mass or more, 0.2 parts by mass or more, or 0.3 parts by mass or more, and 5 parts by mass or less, 4 parts by mass or less, or 3 parts by mass or less, relative to 100 parts by mass of the acrylic rubber.

[0080] The rubber composition may further contain other additives, such as a filler (reinforcing agent), a plasticizer, a lubricant, an antioxidant, a stabilizer, and a silane coupling agent.

[0081] The total content of the other additives in the rubber composition may be 0.1 parts by mass or more or 0.2 parts by mass or more, and 90 parts by mass or less or 80 parts by mass or less, per 100 parts by mass of the acrylic rubber.

[0082] The rubber composition described above is suitably used as a rubber composition for a seal (also referred to as a sealing member) or a hose (also referred to as a hose member). The rubber composition can also be used as a rubber composition for vibration-proof rubber (also referred to as a vibration-proof rubber member). A cross-linked product of the rubber composition described above is suitably used as a seal or a hose. That is, another embodiment of the present invention is a seal or a hose containing the cross-linked product. The cross-linked product can also be used as vibration-proof rubber. That is, another embodiment of the present invention is a vibration-proof rubber containing the cross-linked product. Examples of hoses (hose members) include rubber hoses. Examples of seals (sealing members) include gaskets and packings. These components may consist solely of a cross-linked product of the rubber composition, or may include the cross-linked product in addition to other components.

[0083] Specific examples of hoses (hose members) include transmission oil cooler hoses, engine oil cooler hoses, air duct hoses, turbo intercooler hoses, hot air hoses, radiator hoses, power steering hoses, fuel system hoses, drain system hoses, etc. The hose member may have a reinforcing yarn or wire in an intermediate layer or the outermost layer of the hose.

[0084] Specific examples of seals (sealing members) include engine head cover gaskets, oil pan gaskets, oil seals, lip seal packings, O-rings, transmission seal gaskets, crankshafts, camshaft seal gaskets, valve stems, power steering seals, belt cover seals, boot materials for constant velocity joints, and rack and pinion boot materials.

[0085] Specific examples of vibration-isolating rubber (vibration-isolating rubber member) include a damper pulley, a center support cushion, and a suspension bush.

[0086] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0087] Example 1: 16.8 kg of a 4% by weight aqueous solution of a nonionic emulsifier (polyvinyl alcohol) and 22 g of sodium acetate were charged into a 40-liter pressure-resistant reactor and thoroughly mixed in advance with a stirrer to prepare a uniform suspension. After replacing the air in the upper part of the vessel with nitrogen, stirring was continued, and the vessel was maintained at 45°C. Then, 11.2 kg of a monomer mixture consisting of (meth)acrylic monomers containing ethyl acrylate and n-butyl acrylate and monobutyl maleate and 2 L of an aqueous t-butyl hydroperoxide solution (0.25% by weight) (the amount of t-butyl hydroperoxide added per 100 parts by weight of the monomer mixture) were separately added under pressure to initiate polymerization. The temperature inside the vessel (polymerization temperature) was maintained at 45°C, and the entire amount of the monomer mixture and the aqueous t-butyl hydroperoxide solution was added under pressure at a constant rate over 6 hours, and the reaction was terminated 6.5 hours after the start of polymerization. This resulted in a latex containing an acrylic polymer dispersed in water and a nonionic emulsifier. 20 L of a sodium borate aqueous solution (3.5% by mass) was added to the resulting latex as a coagulant to coagulate the acrylic polymer. The coagulated acrylic polymer was then washed with water, dehydrated, and dried to obtain an acrylic rubber (a rubber composition containing an acrylic rubber and a nonionic emulsifier).

[0088] The acrylic polymer and acrylic rubber had a monomer composition of 50.0% by mass of ethyl acrylate, 48.5% by mass of n-butyl acrylate, and 1.5% by mass of monobutyl maleate (hereinafter referred to as "monomer composition A").

[0089] Example 2 An acrylic rubber (rubber composition) was obtained in the same manner as in Example 1, except that the amount of the t-butyl hydroperoxide aqueous solution (0.25 mass%) added was changed to 0.75 times (the amount of t-butyl hydroperoxide added relative to 100 mass parts of the monomer mixed solution was approximately 0.033 mass parts).

[0090] Example 3 An acrylic rubber (rubber composition) was obtained in the same manner as in Example 1, except that the amount of t-butyl hydroperoxide aqueous solution (0.25% by mass) added was changed to 0.75 times (the amount of t-butyl hydroperoxide added relative to 100 parts by mass of the monomer mixture was approximately 0.033 parts by mass), and the amount of the monomer mixture added at the initial stage of polymerization was reduced by approximately 20%. More specifically, with regard to the amount of the monomer mixture added, the injection rate of the monomer mixture from the start of polymerization until two hours had elapsed was set to approximately 0.8 times the injection rate in Example 1, and the injection rate of the monomer mixture from two hours after the start of polymerization until the end of polymerization was adjusted so that the total amount of the monomer mixture added was the same as the total amount of the monomer mixture added in Example 1 (11.2 kg).

[0091] Example 4 An acrylic rubber (rubber composition) was obtained in the same manner as in Example 1, except that the amount of t-butyl hydroperoxide aqueous solution (0.25% by mass) added was changed to 0.75 times (the amount of t-butyl hydroperoxide added relative to 100 parts by mass of the monomer mixture was approximately 0.033 parts by mass), the amount of the monomer mixture added at the beginning of the polymerization was reduced by about 40%, and the polymerization temperature was changed to 35° C. More specifically, with regard to the amount of the monomer mixture added, the injection rate of the monomer mixture from the start of polymerization until two hours had elapsed was set to approximately 0.6 times the injection rate in Example 1, and the injection rate of the monomer mixture from two hours after the start of polymerization until the end of polymerization was adjusted so that the total amount of the monomer mixture added was the same as the total amount of the monomer mixture added in Example 1 (11.2 kg).

[0092] Example 5 An acrylic rubber (rubber composition) was obtained in the same manner as in Example 1, except that the amount of t-butyl hydroperoxide aqueous solution (0.25% by mass) added was changed to 0.5 times (the amount of t-butyl hydroperoxide added relative to 100 parts by mass of the monomer mixture was approximately 0.022 parts by mass), the amount of the monomer mixture added at the initial stage of polymerization was reduced by about 75%, and the polymerization temperature was changed to 35°C and the reaction time to 12 hours. More specifically, with regard to the amount of the monomer mixture added, the injection rate of the monomer mixture from the start of polymerization until 4 hours had elapsed was set to approximately 0.25 times the injection rate in Example 1, and the injection rate of the monomer mixture from 4 hours after the start of polymerization until the end of polymerization (12 hours after the start of polymerization) was adjusted so that the total amount of the monomer mixture added was the same as the total amount of the monomer mixture added in Example 1 (11.2 kg).

[0093] Comparative Example 1 An acrylic rubber (rubber composition) was obtained in the same manner as in Example 1, except that the polymerization temperature was changed to 55°C.

[0094] Comparative Example 2 An acrylic rubber (rubber composition) was obtained in the same manner as in Example 1, except that the amount of the t-butyl hydroperoxide aqueous solution (0.25 mass%) added was doubled (the amount of t-butyl hydroperoxide added relative to 100 mass parts of the monomer mixed solution was approximately 0.089 mass parts) and the polymerization temperature was changed to 55°C.

[0095] Comparative Example 3 An acrylic rubber (rubber composition) was obtained in the same manner as in Example 1, except that the nonionic emulsifier was changed to an anionic emulsifier.

[0096] Example 6 An acrylic rubber (rubber composition) was obtained in the same manner as in Example 2, except that the concentration of the aqueous solution of the nonionic emulsifier was changed from 4% by mass to 8% by mass.

[0097] Example 7 An acrylic rubber (rubber composition) was obtained in the same manner as in Example 1, except that polyoxyethylene dodecyl ether (molecular weight 1500) was used as the nonionic emulsifier instead of polyvinyl alcohol.

[0098] (Example 8) An acrylic rubber (rubber composition) was obtained in the same manner as in Example 1, except that a mixture of polyoxyethylene dodecyl ether (molecular weight 1500), which is a nonionic emulsifier, and sodium lauryl sulfate, which is an anionic emulsifier (mass ratio of polyoxyethylene dodecyl ether / sodium lauryl sulfate = 7 / 3), was used instead of the nonionic emulsifier (polyvinyl alcohol), and an aqueous sodium sulfate solution (2 mass%) was used as the coagulant.

[0099] Example 9 An acrylic rubber (rubber composition) was obtained in the same manner as in Example 1, except that the monomer composition A for the acrylic polymer and acrylic rubber was changed to monomer composition B (ethyl acrylate 68.5% by mass, n-butyl acrylate 30.0% by mass, and monobutyl maleate 1.5% by mass) by adjusting the compounding ratios of ethyl acrylate, n-butyl acrylate, and monobutyl maleate in the monomer mixture, and the amount of the t-butyl hydroperoxide aqueous solution (0.15% by mass) added was changed to 1.8 liters (the amount of t-butyl hydroperoxide added relative to 100 parts by mass of the monomer mixture was approximately 0.024 parts by mass).

[0100] (Example 10) The monomer mixture was changed to a monomer mixture consisting of ethyl acrylate, n-butyl acrylate, n-butyl methacrylate, 2-methoxyethyl acrylate, and monobutyl maleate, and the monomer composition A of the acrylic polymer and acrylic rubber was changed to monomer composition C (ethyl acrylate 35.0 mass%, n-butyl acrylate 39.5 mass%, n-butyl methacrylate 9.5 mass%, 2-methoxyethyl acrylate 14.5 mass%, and monobutyl maleate 1.5 mass%). In addition, the amount of t-butyl hydroperoxide aqueous solution (0.25 mass%) added was changed to 1.5 liters (the amount of t-butyl hydroperoxide added relative to 100 mass parts of the monomer mixture was about 0.033 mass parts), and the polymerization temperature was changed to 45 ° C., respectively. An acrylic rubber (rubber composition) was obtained in the same manner as in Example 1.

[0101] Example 11 An acrylic rubber (rubber composition) was obtained in the same manner as in Example 8, except that the amount of the t-butyl hydroperoxide aqueous solution (0.25 mass%) added was changed to 0.7 times (the amount of t-butyl hydroperoxide added per 100 mass parts of the monomer mixture was approximately 0.023 mass parts).

[0102] (Example 12) The monomer mixture was changed to a monomer mixture consisting of ethyl acrylate, n-butyl acrylate, n-butyl methacrylate, and monobutyl maleate, and the monomer composition A of the acrylic polymer and acrylic rubber was changed to monomer composition D (ethyl acrylate 25.0 mass%, n-butyl acrylate 43.5 mass%, n-butyl methacrylate 30.0 mass%, and monobutyl maleate 1.5 mass%). An acrylic rubber (rubber composition) was obtained in the same manner as in Example 1, except that the monomer mixture was changed to a monomer mixture consisting of ethyl acrylate, n-butyl acrylate, n-butyl methacrylate, and monobutyl maleate 1.5 mass%).

[0103] (Example 13) The monomer mixture was changed to a monomer mixture consisting of ethyl acrylate, n-butyl acrylate, n-butyl methacrylate, and monobutyl maleate, and the monomer composition A of the acrylic polymer and acrylic rubber was changed to monomer composition E (ethyl acrylate 35.0 mass%, n-butyl acrylate 48.5 mass%, n-butyl methacrylate 15.0 mass%, and monobutyl maleate 1.5 mass%). An acrylic rubber (rubber composition) was obtained in the same manner as in Example 1.

[0104] Example 14 An acrylic rubber (rubber composition) was obtained in the same manner as in Example 1, except that the monomer mixture was changed to a monomer mixture consisting of ethyl acrylate, n-butyl acrylate, 2-methoxyethyl acrylate, and monobutyl maleate, and the monomer composition A of the acrylic polymer and acrylic rubber was changed to monomer composition F (ethyl acrylate 70.0 mass%, n-butyl acrylate 13.5 mass%, 2-methoxyethyl acrylate 15.0 mass%, and monobutyl maleate 1.5 mass%).

[0105] Example 15 An acrylic rubber (rubber composition) was obtained in the same manner as in Example 1, except that the monomer mixture was changed to a monomer mixture consisting of ethyl acrylate, n-butyl acrylate, 2-methoxyethyl acrylate, and monobutyl maleate, and the monomer composition A of the acrylic polymer and acrylic rubber was changed to monomer composition G (ethyl acrylate 5.0 mass%, n-butyl acrylate 68.5 mass%, 2-methoxyethyl acrylate 25.0 mass%, and monobutyl maleate 1.5 mass%).

[0106] Tables 1 and 2 show the monomer composition, the total mass of emulsifier in the latex (the total mass (parts by mass) of nonionic emulsifier in the latex relative to 100 parts by mass of the total mass of the acrylic polymer), the amount of fixed emulsifier in the latex (the proportion of nonionic emulsifier fixed to the acrylic polymer (% by mass based on the total mass of nonionic emulsifier in the latex)), the total mass of emulsifier in the rubber composition (the total mass (parts by mass) of nonionic emulsifier in the rubber composition relative to 100 parts by mass of the total mass of the acrylic rubber), and the amount of fixed emulsifier in the rubber composition (the proportion of nonionic emulsifier fixed to the acrylic rubber (% by mass based on the total mass of the nonionic emulsifier in the rubber composition)).

[0107] The proportion of the nonionic emulsifier fixed to the acrylic polymer in the latex was measured by the following procedure. Ｅ (g) Latex M containing a nonionic emulsifier Ｌ 50 g of a sample diluted 20 times with pure water was placed in a centrifuge tube and centrifuged at 10,000 rpm for 30 minutes using a centrifuge (Hitachi "CT15D"). The supernatant after centrifugation was placed in a 100 mL plastic bottle, and the mass M Ｓ (g) was weighed. 1 mL of this supernatant, 15 mL of 4 mass% boric acid aqueous solution, and 3 mL of 0.05 mol iodine solution were added, and pure water was added to make up to 50 mL to obtain a measurement sample (a sample in which the latex was diluted 1000 times). The absorbance Abs of this measurement sample at a wavelength of 650 nm was measured using a spectrophotometer. Next, the concentration C of the nonionic emulsifier not fixed to the acrylic polymer in the latex was measured.ｆｒｅｅ (g / kg) is calculated using the following formula: ｆｒｅｅ (g / kg) = α (mg / L) x Abs x M Ｓ (g) / 50(g).

[0108] In the above formula, the density of the supernatant is converted to 1 g / mL, and the measurement sample is a latex diluted 1000 times. ｆｒｅｅ The unit of is g / kg. The concentration coefficient α (mg / L) was calculated by preparing measurement samples and measuring the absorbance at a wavelength of 650 nm for aqueous solutions of the nonionic emulsifier (the same polyvinyl alcohol as above) at concentrations of 2, 10, 20, 30, and 40 mg / L in the same manner as for the supernatant liquid, and approximating the relationship of concentration of each aqueous solution = α × absorbance.

[0109] And the above C ｆｒｅｅ (g / kg) to calculate the mass M of the nonionic emulsifier that is not fixed to the acrylic polymer in the latex. ｆｒｅｅ (g) = C ｆｒｅｅ (g / kg) x M Ｌ Finally, the ratio (mass%) of the nonionic emulsifier fixed to the acrylic polymer in the latex was calculated as follows: Ｅ (g)-M ｆｒｅｅ (g)] / M Ｅ (g) x 100 was calculated.

[0110] The proportion of the nonionic emulsifier fixed to the acrylic rubber in the rubber composition was measured by the following procedure. First, 0.3 g of the rubber composition was prepared and precisely weighed into a 100 mL Erlenmeyer flask. This rubber composition was mixed with chloroform (CHCl ３Approximately 20 mL of dimethyl sulfoxide (DMSO) was added and the mixture was shaken to dissolve the rubber composition. Then, 10 mL of dimethyl sulfoxide (DMSO) was added and the mixture was heated in a water bath at 90°C or higher for 1 hour. Next, 20 mL of hot water at 80°C or higher was gradually added, and the mixture was heated in a water bath for approximately 30 minutes to remove the chloroform, after which the mixture was allowed to cool. After cooling, the sample was centrifuged at 10,000 rpm for 30 minutes in a centrifuge (Hitachi "CT15D"). The supernatant after centrifugation was diluted to 100 mL and filtered through a 200-mesh wire mesh to obtain a sample containing nonionic emulsifier not fixed to the acrylic rubber. The concentration C of the nonionic emulsifier not fixed to the acrylic polymer in the latex was then determined for this sample. ｆｒｅｅ The proportion of the nonionic emulsifier not fixed to the acrylic rubber in the rubber composition was determined in the same manner as in Example 1 (g / kg). The amount of the nonionic emulsifier not fixed to the acrylic rubber thus determined and the amount of the nonionic emulsifier removed by washing with water were subtracted from the amount of the nonionic emulsifier charged, thereby determining the amount of the nonionic emulsifier fixed to the acrylic rubber.

[0111] The Mooney viscosity ML(1+4) at 100°C of the acrylic rubber (rubber composition) obtained in each of the Examples and Comparative Examples was measured according to the method specified in JIS K 6300. The results are shown in Tables 1 and 2.

[0112] In addition, 100 parts by mass of the acrylic rubber (rubber composition) obtained in each of the examples and comparative examples was blended with the components shown below, and kneaded with an 8-inch open roll. Filler: Carbon black (Tokai Carbon Co., Ltd., SEAT SO) 55 parts by mass Lubricant a: Stearic acid (NOF Corporation, Beads Stearic Acid Tsubaki) 1 part by mass Lubricant b: Stearylamide (Mitsubishi Chemical Corporation, Amide AP-1) 0.3 parts by mass Lubricant c: Liquid paraffin (Kaneda Co., Ltd., High Cold K-230) 1 part by mass Mold release agent: Fatty acid, fatty acid ester, amine, wetting agent mixture (Tomoe Engineering Co., Ltd., MoldWiz INT-21G) 0.5 parts by mass Antioxidant: 4,4'-bis(α,α-dimethylbenzyl)diphenylamine (Addivant Co., Ltd., Naugard #445) 0.5 parts by mass Crosslinking agent: Hexamethylenediamine carbamate (DuPont, Diak #1) 0.5 parts by mass Crosslinking accelerator: Synthetic mixture of active amine and retarder (Lanxess) XLA-60) 1.0 parts by mass

[0113] The kneaded rubber composition was dispensed into a sheet having a thickness of 2.4 mm, and then heated and pressed at 170°C and a pressure of 10 MPa for 20 minutes using a press vulcanizer. Subsequently, the sheet was heated in a gear oven at 170°C for 4 hours to obtain a crosslinked product of the rubber composition.

[0114] (Evaluation of Water Resistance) In accordance with the "immersion test" of JIS K6258:2016, the crosslinked product was subjected to an immersion test in distilled water (80°C, 72 hours), and the volume change rate of the crosslinked product before and after the test was calculated based on the following formula: Volume change rate (%) = (volume after test - volume before test) / volume before test x 100 The results are shown in Tables 1 and 2. The smaller the volume change rate, the more excellent the water resistance.

[0115] (Measurement of tensile strength at break) The tensile strength at break Tb of the crosslinked product was measured in accordance with JIS K6251: 2017. The results are shown in Tables 1 and 2.

[0116]

[0117]

[0118] For Examples 1 to 15, the toluene insoluble matter, flex fatigue resistance, and copper damage resistance were also evaluated as follows. The results are shown in Tables 3 to 5.

[0119] (Measurement of Toluene-Insoluble Content) 1 g of acrylic rubber was precisely weighed and dissolved in 100 mL of toluene at 25°C for 48 hours. The solution was then transferred to a 250 mL centrifuge tube whose mass (referred to as X (g)) had been measured in advance. The solution was then centrifuged using an angle rotor with a maximum centrifugal radius of 13.8 cm at 10°C, 8500 rpm, and for 60 minutes, after which the non-precipitate was removed by decantation. The precipitate in the centrifuge tube was dried in a vacuum dryer at 70°C for 24 hours, and the mass (referred to as Y (g)) of the centrifuge tube after drying was measured. The toluene-insoluble content was calculated from the measured X and Y using the following formula: Toluene-insoluble content (mass%) = (Y - X) × 100. The lower the toluene-insoluble content, the better.

[0120] (Evaluation of flexural fatigue resistance) In accordance with JIS K6260:2010, the number of flexes until a 0.5 mm crack occurred in the crosslinked product was measured in an environment of 100°C. The number of flexes was taken as the average value of the results of similar measurements performed on five samples. The greater the number of flexes, the better the flexural fatigue resistance.

[0121] (Evaluation of Copper Damage Resistance) Cross-linked materials molded into No. 3 dumbbell shapes were used as test specimens. Five grams of a slurry prepared by mixing engine oil (Mobil 15W-30, manufactured by EMG Lubricants LLC) and copper powder (CE-1110, manufactured by Fukuda Metal Foil & Powder Co., Ltd.) at a mass ratio of 3:1 was applied to the test specimen using a brush so as to completely cover the area between the gauge lines, and the specimen was allowed to dry at room temperature for 12 hours. Subsequently, a copper damage resistance test was conducted by heating the test specimen in a gear oven at 150°C for 500 hours. The copper paste was then peeled off the test specimen using a spatula, and the elongation of the test specimen was measured according to JIS K6251:2010. The percent change in elongation (%) before and after the test was calculated using the following formula: percent change in elongation (%) = (elongation after test - elongation before test) / elongation before test × 100. The smaller the absolute value of the percent change in elongation, the better the copper damage resistance.

[0122]

[0123]

[0124]

Claims

1. A rubber composition comprising an acrylic rubber and a nonionic emulsifier, wherein more than 0 mass % and 80 mass % or less of the total mass of the nonionic emulsifier is fixed to the acrylic rubber.

2. The rubber composition according to claim 1, wherein the total mass of the nonionic emulsifier is 5 parts by mass or less per 100 parts by mass of the total mass of the acrylic rubber.

3. The rubber composition according to claim 1, wherein the acrylic rubber contains at least one monomer unit selected from the group consisting of alkyl acrylates, alkyl methacrylates, and alkoxyalkyl acrylates.

4. The rubber composition according to any one of claims 1 to 3, further comprising a crosslinking agent.

5. A crosslinked product of the rubber composition according to claim 4.

6. A method for producing acrylic rubber, comprising: a step of polymerizing an acrylic monomer in the presence of a nonionic emulsifier to obtain a latex of an acrylic polymer; and a step of coagulating and washing the acrylic polymer to obtain an acrylic rubber, wherein more than 0% by mass and not more than 70% by mass of the total mass of the nonionic emulsifier in the latex is fixed to the acrylic polymer.

7. A method for producing acrylic rubber as described in claim 6, wherein the total mass of the nonionic emulsifier in the latex is 6 parts by mass or less per 100 parts by mass of the total mass of the acrylic polymer.

8. The method for producing acrylic rubber according to claim 6 or 7, wherein the acrylic polymer contains at least one monomer unit selected from the group consisting of alkyl acrylates, alkyl methacrylates, and alkoxyalkyl acrylates.

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