Aqueous dispersion of nitrile rubber particles

Abstract

Provided is an aqueous dispersion of nitrile rubber particles obtained through water-based polymerization. The aqueous dispersion of the nitrile rubber particles is characterized in that value A represented by formula (1) is 0.8 or more.　(1): A=-0.065×TS+0.003×C(K)+0.009×C(Na) In formula (1), TS represents the solid content concentration [wt%] of the aqueous dispersion of the nitrile rubber particles, C(K) represents the K-concentration [ppm] of the aqueous dispersion of the nitrile rubber particles as measured through ICP emission spectroscopic analysis, and C(Na) represents the Na-concentration [ppm] of the aqueous dispersion of the nitrile rubber particles as measured through ICP emission spectroscopic analysis.

Description

Aqueous dispersion of nitrile rubber particles

[0001] The present invention relates to an aqueous dispersion of nitrile rubber particles.

[0002] In relation to the production of an aqueous dispersion of particles by aqueous polymerization, a method of heating using microwaves has been known as a method for removing unreacted monomers remaining in the liquid (for example, Patent Document 1).

[0003] Japanese Unexamined Patent Publication No. 51-119785

[0004] However, as in the above-mentioned technology, there are cases where the unreacted monomer cannot be sufficiently removed by simply irradiating the aqueous dispersion of polymer particles with microwaves, and therefore there has been a demand for further improvement in the performance of removing the unreacted monomer by microwave heating.The problem to be solved by the present invention is to provide an aqueous dispersion of nitrile rubber particles that is suitable for removing the unreacted monomer using microwave heating.

[0005] As a result of intensive research to solve the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved by an aqueous dispersion of nitrile rubber particles obtained by aqueous polymerization, in which the solid content concentration TS [wt %], the K concentration C(K) [ppm] measured by ICP, and the Na concentration C(Na) [ppm] measured by ICP satisfy specific relationships, and have completed the present invention based on this finding.

[0006] [1] According to a first aspect of the present invention, there is provided an aqueous dispersion of nitrile rubber particles obtained by aqueous polymerization, characterized in that the value A represented by the following formula (1) is 0.8 or more: A = -0.065 x TS + 0.003 x C(K) + 0.009 x C(Na) (1) In the above formula (1), TS is the solids concentration [wt %] of the aqueous dispersion of nitrile rubber particles, C(K) is the K concentration [ppm] of the aqueous dispersion of nitrile rubber particles measured by ICP emission spectroscopy, and C(Na) is the Na concentration [ppm] of the aqueous dispersion of nitrile rubber particles measured by ICP emission spectroscopy.

[0007] [2] According to a second aspect of the present invention, there is provided the aqueous dispersion of nitrile rubber particles according to the first aspect, characterized in that the aqueous polymerization is emulsion polymerization.

[0008] [3] According to a third aspect of the present invention, there is provided an aqueous dispersion of nitrile rubber particles according to the first or second aspect, characterized in that it contains a dispersant.

[0009] [4] According to a fourth aspect of the present invention, there is provided an aqueous dispersion of nitrile rubber particles according to any one of the third aspects, characterized in that the dispersant is an anionic emulsifier.

[0010] [5] According to a fifth aspect of the present invention, there is provided the aqueous dispersion of nitrile rubber particles according to the fourth aspect, characterized in that the anionic emulsifier is beef tallow fatty acid potassium soap.

[0011] [6] According to a sixth aspect of the present invention, there is provided an aqueous dispersion of nitrile rubber particles according to any one of the first to fifth aspects, which contains sodium carbonate as a builder.

[0012] According to the present invention, it is possible to provide an aqueous dispersion of nitrile rubber particles that is suitable for removing unreacted monomers using microwave heating.

[0013] <Aqueous Dispersion of Nitrile Rubber Particles> The aqueous dispersion of nitrile rubber particles of the present invention is obtained by aqueous polymerization, and is characterized in that the value A represented by the following formula (1) is 0.8 or more: A = -0.065 x TS + 0.003 x C(K) + 0.009 x C(Na) (1) In the above formula (1), TS is the solids concentration [wt %] of the aqueous dispersion of nitrile rubber particles, C(K) is the K concentration [ppm] of the aqueous dispersion of nitrile rubber particles measured by ICP emission spectroscopy, and C(Na) is the Na concentration [ppm] of the aqueous dispersion of nitrile rubber particles measured by ICP emission spectroscopy.

[0014] <Nitrile Rubber Particles> Nitrile rubber is a rubber that can be obtained, for example, by copolymerizing an α,β-ethylenically unsaturated nitrile monomer, a diene monomer, and, if necessary, a monomer copolymerizable therewith.

[0015] The α,β-ethylenically unsaturated nitrile monomer is not particularly limited as long as it is an α,β-ethylenically unsaturated compound having a nitrile group, and examples thereof include acrylonitrile; α-halogenoacrylonitriles such as α-chloroacrylonitrile and α-bromoacrylonitrile; and α-alkylacrylonitriles such as methacrylonitrile and ethacrylonitrile. Of these, acrylonitrile and methacrylonitrile are preferred, and acrylonitrile is particularly preferred. The α,β-ethylenically unsaturated nitrile monomer may be used alone or in combination.

[0016] The content of the α,β-ethylenically unsaturated nitrile monomer in the nitrile rubber is preferably 10 to 65% by weight, more preferably 15 to 55% by weight, and even more preferably 20 to 40% by weight, based on the total monomer units.

[0017] The diene monomer is not particularly limited, but examples thereof include conjugated dienes having 4 or more carbon atoms, such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene; and non-conjugated dienes having 5 to 12 carbon atoms, such as 1,4-pentadiene and 1,4-hexadiene. Of these, conjugated dienes are preferred, with 1,3-butadiene and isoprene being more preferred, and 1,3-butadiene being even more preferred.

[0018] The content of diene monomer units in the nitrile rubber is preferably 35 to 90% by weight, more preferably 45 to 85% by weight, and even more preferably 60 to 80% by weight, based on the total monomer units.

[0019] The other monomer copolymerizable with the α,β-ethylenically unsaturated nitrile monomer and the diene monomer is not particularly limited, and examples thereof include a carboxyl group-containing monomer, an α,β-ethylenically unsaturated monocarboxylic acid ester monomer, an α,β-ethylenically unsaturated dicarboxylic acid diester monomer, an aromatic vinyl monomer, a polyfunctional ethylenically unsaturated monomer, a self-crosslinking monomer, and a copolymerizable antioxidant.

[0020] Examples of carboxyl group-containing monomers include α,β-ethylenically unsaturated monocarboxylic acid monomers, α,β-ethylenically unsaturated polycarboxylic acid monomers, and α,β-ethylenically unsaturated dicarboxylic acid monoester monomers. Also included as carboxyl group-containing monomers are monomers in which the carboxyl groups of these monomers form carboxylates. Furthermore, anhydrides of α,β-ethylenically unsaturated polycarboxylic acids can also be used as carboxyl group-containing monomers because they form carboxyl groups by cleaving the acid anhydride group after copolymerization.

[0021] Examples of the α,β-ethylenically unsaturated monocarboxylic acid monomer include (meth)acrylic acid, ethylacrylic acid, crotonic acid, and cinnamic acid.

[0022] Examples of the α,β-ethylenically unsaturated polycarboxylic acid monomer include butenedioic acids such as fumaric acid and maleic acid, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid, allylmalonic acid, and teraconic acid. Examples of the anhydrides of α,β-unsaturated polycarboxylic acids include maleic anhydride, itaconic anhydride, and citraconic anhydride.

[0023] Examples of the α,β-ethylenically unsaturated dicarboxylic acid monoester monomer include monoalkyl maleates such as monomethyl maleate, monoethyl maleate, mono-n-propyl maleate, and mono-n-butyl maleate; monocycloalkyl maleates such as monocyclopentyl maleate, monocyclohexyl maleate, and monocycloheptyl maleate; monoalkylcycloalkyl maleates such as monomethylcyclopentyl maleate and monoethylcyclohexyl maleate; monoalkyl fumarate esters such as monomethyl fumarate, monoethyl fumarate, mono-n-propyl fumarate, and mono-n-butyl fumarate; monocycloalkyl fumarate esters such as monocyclopentyl fumarate, monocyclohexyl fumarate, and monocycloheptyl fumarate; monoalkylcycloalkyl fumarate esters such as monomethylcyclopentyl fumarate and monoethylcyclohexyl fumarate; monoalkyl citraconic acid esters such as monomethyl citraconic acid, monoethyl citraconic acid, mono-n-propyl citraconic acid, and mono-n-butyl citraconic acid; monocycloalkyl citraconic acid esters such as monocyclopentyl citraconic acid, monocyclohexyl citraconic acid, and monocycloheptyl citraconic acid; monoalkyl citraconic acid esters such as monomethylcyclopentyl citraconic acid and monoethylcyclohexyl citraconic acid; monoalkyl itaconic acid esters such as monomethyl itaconic acid, monoethyl itaconic acid, mono-n-propyl itaconic acid, and mono-n-butyl itaconic acid; monocycloalkyl itaconic acid esters such as monocyclopentyl itaconic acid, monocyclohexyl itaconic acid, and monocycloheptyl itaconic acid; and monoalkyl itaconic acid esters such as monomethylcyclopentyl itaconic acid and monoethylcyclohexyl itaconic acid.

[0024] Examples of the α,β-ethylenically unsaturated monocarboxylic acid ester monomer include (meth)acrylic acid esters having an alkyl group having 1 to 18 carbon atoms, such as methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, n-dodecyl acrylate, methyl methacrylate, and ethyl methacrylate (abbreviation for "methacrylic acid esters and acrylic acid esters"; the same applies hereinafter); methoxymethyl acrylate, 2-methoxyethyl acrylate, 2-methoxyethyl methacrylate, 3-methoxypropyl acrylate, 4-ethoxybutyl methacrylate, 6-methoxyhexyl acrylate, 4-ethoxyheptyl methacrylate, 8- (meth)acrylic acid esters having an alkoxyalkyl group having 2 to 12 carbon atoms, such as methoxyoctyl; (meth)acrylic acid esters having a cyanoalkyl group having 2 to 12 carbon atoms, such as α-cyanoethyl acrylate, α-cyanoethyl methacrylate, and cyanobutyl methacrylate; (meth)acrylic acid esters having a hydroxyalkyl group having 1 to 12 carbon atoms, such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and 2-hydroxyethyl methacrylate; and (meth)acrylic acid esters having a fluoroalkyl group having 1 to 12 carbon atoms, such as trifluoroethyl acrylate and tetrafluoropropyl methacrylate.

[0025] Examples of the α,β-ethylenically unsaturated dicarboxylic acid diester monomer include maleic acid diesters such as diethyl maleate; fumaric acid diesters such as dimethyl fumarate; citraconic acid diesters such as dimethyl citraconic acid; and itaconic acid diesters such as dibutyl itaconate.

[0026] Examples of aromatic vinyl monomers include styrene, α-methylstyrene, and vinylpyridine.

[0027] Examples of polyfunctional ethylenically unsaturated monomers include divinyl compounds such as divinylbenzene; di(meth)acrylic acid esters such as diethylene glycol di(meth)acrylate and ethylene glycol di(meth)acrylate; and trimethacrylic acid esters such as trimethylolpropane tri(meth)acrylate.

[0028] Examples of the self-crosslinking monomer include N-methylol(meth)acrylamide and N,N'-dimethylol(meth)acrylamide.

[0029] Examples of copolymerizable antioxidants include N-(4-anilinophenyl)acrylamide, N-(4-anilinophenyl)methacrylamide, N-(4-anilinophenyl)cinnamamide, N-(4-anilinophenyl)crotonamide, N-phenyl-4-(3-vinylbenzyloxy)aniline, and N-phenyl-4-(4-vinylbenzyloxy)aniline.

[0030] A content of the copolymerizable other monomer in the nitrile rubber is preferably 50% by weight or less, more preferably 30% by weight or less, and even more preferably 10% by weight or less, based on the total monomer units.

[0031] In the aqueous dispersion of nitrile rubber particles of the present invention, the value A represented by the following formula (1) is 0.8 or more: A = -0.065 x TS + 0.003 x C(K) + 0.009 x C(Na) (1) In the above formula (1), TS is the solids concentration [wt %] of the aqueous dispersion of nitrile rubber particles, C(K) is the K concentration [ppm] of the aqueous dispersion of nitrile rubber particles measured by ICP emission spectroscopy, and C(Na) is the Na concentration [ppm] of the aqueous dispersion of nitrile rubber particles measured by ICP emission spectroscopy.

[0032] The aqueous dispersion of nitrile rubber particles of the present invention has a value A represented by the above formula (1) of 0.8 or more, and therefore easily absorbs microwaves and is easily heated by heating using microwaves (hereinafter also referred to as microwave heating). Therefore, the aqueous dispersion of nitrile rubber particles of the present invention is suitable for removing unreacted monomers using microwave heating. Microwave heating is heating based on the principle of dielectric heating, and is a method of heating the aqueous dispersion by irradiating the aqueous dispersion of nitrile rubber particles with microwaves to generate heat from within the aqueous dispersion.

[0033] The value A expressed by the above formula (1) represents the degree of ease of temperature rise when an aqueous dispersion of nitrile rubber particles is subjected to microwave heating. First, molecules and charged particles cannot move freely in the solid content of the aqueous dispersion, so even when microwave heating is applied, they penetrate and are hardly heated. Therefore, it is thought that the higher the solid content concentration TS [wt %], the more difficult it is to heat by microwave heating. On the other hand, when microwaves are applied to charged particles (ions) contained in the liquid and capable of moving freely, the charged particles vibrate and generate thermal energy through collisions with surrounding molecules, etc. Therefore, it is thought that the higher the concentration of charged particles present in the aqueous dispersion, the greater the thermal energy caused by the vibration of the charged particles, making it easier to raise the temperature by microwave heating. The inventors have found that, among charged particles, the concentrations of potassium and sodium in particular contribute significantly to microwave heating. Furthermore, the inventors have found that an aqueous dispersion of nitrile rubber particles can be made suitable for microwave heating by appropriately controlling the K concentration C(K) and the Na concentration C(Na) according to the solid content concentration TS. From the above, the inventors have concluded that the value A expressed by the above formula (1) should be 0.8 or more.

[0034] The value A may be 0.8 or more, preferably 3, and more preferably 6 or more. By setting the value A within the above range, the aqueous dispersion of nitrile rubber particles can be more easily heated by microwave heating, and unreacted monomers present in the aqueous dispersion can be more efficiently removed by microwave heating. The upper limit of the value A is not particularly limited, but is usually 10 or less. If the value A is too large, most of the microwaves will be absorbed by the surface of the aqueous dispersion during microwave heating, and the microwaves will not penetrate deep into the aqueous dispersion, which may make the aqueous dispersion difficult to heat by microwave heating.

[0035] There are no particular limitations on the method for controlling the K concentration C(K) and the Na concentration C(Na) so that the value A is 0.8 or greater, but examples include a method of adjusting the type and amount of dispersant and builder used in the polymerization, as described below. The K concentration C(K) is not particularly limited as long as the value A is 0.8 or greater, but is preferably 1000 to 3000 ppm, and more preferably 1500 to 2500 ppm. The Na concentration C(Na) is not particularly limited as long as the value A is 0.8 or greater, but is preferably 200 to 500 ppm, and more preferably 250 to 400 ppm. If the K concentration C(K) or the Na concentration C(Na) is too high, the pH of the aqueous dispersion of nitrile rubber particles will increase, which may increase the viscosity or make the nitrile rubber particles more susceptible to aggregation. In the above formula (1), the K concentration C(K) [ppm] and the Na concentration C(Na) [ppm] can be measured by ICP emission spectrometry using the method described in the Examples.

[0036] From the viewpoint of being able to suitably control the value A represented by the above formula (1), the aqueous dispersion of nitrile rubber particles of the present invention preferably contains a dispersant. The dispersant is not particularly limited, but examples thereof include nonionic emulsifiers such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenol ethers, polyoxyethylene alkyl esters, and polyoxyethylene sorbitan alkyl esters; anionic emulsifiers such as salts of fatty acids such as myristic acid, palmitic acid, oleic acid, linolenic acid, and beef tallow fatty acid, alkylbenzenesulfonates such as sodium dodecylbenzenesulfonate, sodium β-naphthalenesulfonate-formalin condensate, higher alcohol sulfates, and alkyl sulfosuccinates; and copolymerizable emulsifiers such as sulfoesters of α,β-unsaturated carboxylic acids, sulfate esters of α,β-unsaturated carboxylic acids, and sulfoalkylaryl ethers. Among these, from the viewpoint of being able to suitably control the value A represented by the above formula (1) by controlling the potassium concentration C(K) [ppm] and the sodium concentration C(Na), anionic emulsifiers are preferred, salts of fatty acids such as oleic acid, linolenic acid, and tallow fatty acid are more preferred, and potassium oleate and tallow fatty acid potassium soap are even more preferred. These dispersants may be used alone or in combination of two or more. The amount of dispersant added is preferably 0.1 to 10 parts by weight, more preferably 0.5 to 5 parts by weight, and even more preferably 1 to 4 parts by weight, per 100 parts by weight of the monomer mixture used in the polymerization of the nitrile rubber particles.

[0037] <Method for producing aqueous dispersion of nitrile rubber particles> The aqueous dispersion of nitrile rubber particles of the present invention can be obtained by aqueous polymerization of a monomer mixture containing the above-mentioned monomers that constitute the rubber. Either emulsion polymerization or suspension polymerization can be used for the aqueous polymerization, but emulsion polymerization is preferred because it is easier to control the polymerization reaction. During emulsion polymerization, in addition to the above-mentioned dispersant, a polymerization initiator, a molecular weight modifier, and commonly used secondary polymerization materials can be used.

[0038] The polymerization initiator is not particularly limited as long as it is a radical initiator, and examples thereof include inorganic peroxides such as potassium persulfate, sodium persulfate, ammonium persulfate, potassium perphosphate, and hydrogen peroxide; organic peroxides such as t-butyl peroxide, cumene hydroperoxide, p-menthane hydroperoxide, di-t-butyl peroxide, t-butylcumyl peroxide, acetyl peroxide, isobutyryl peroxide, octanoyl peroxide, dibenzoyl peroxide, 3,5,5-trimethylhexanoyl peroxide, and t-butylperoxyisobutyrate; and azo compounds such as azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, azobiscyclohexanecarbonitrile, and methyl azobisisobutyrate. These polymerization initiators can be used alone or in combination of two or more. Inorganic or organic peroxides are preferred as the polymerization initiator. When a peroxide is used as a polymerization initiator, it can also be used as a redox polymerization initiator in combination with a reducing agent such as sodium bisulfite, ferrous sulfate, sodium formaldehyde sulfoxylate, and ethylenediaminetetraacetic acid ferric sodium salt. The amount of the polymerization initiator added is preferably 0.01 to 2 parts by weight based on 100 parts by weight of the monomer mixture used in the polymerization.

[0039] The molecular weight modifier is not particularly limited, but examples thereof include mercaptans such as t-dodecyl mercaptan, n-dodecyl mercaptan, and octyl mercaptan; halogenated hydrocarbons such as carbon tetrachloride, methylene chloride, and methylene bromide; α-methylstyrene dimer; and sulfur-containing compounds such as tetraethylthiuram disulfide, dipentamethylenethiuram disulfide, and diisopropylxanthogen disulfide. These may be used alone or in combination of two or more. Of these, mercaptans are preferred, and t-dodecyl mercaptan is more preferred. The amount of molecular weight modifier used is preferably 0.1 to 0.8 parts by weight per 100 parts by weight of all monomers.

[0040] Water is usually used as the medium for emulsion polymerization, and the amount of water is preferably 80 to 500 parts by weight, more preferably 80 to 300 parts by weight, per 100 parts by weight of the monomers used in the polymerization.

[0041] From the viewpoint of being able to suitably control the value A represented by the above formula (1), it is preferable to use a builder together with the dispersant during emulsion polymerization. The builder is not particularly limited, but examples thereof include carbonates and silicates, with sodium carbonate being preferred. The amount of the builder used is preferably 0.01 to 0.5 parts by weight, more preferably 0.05 to 0.2 parts by weight, and even more preferably 0.1 to 0.2 parts by weight, per 100 parts by weight of the total monomers.

[0042] From the viewpoint of being able to suitably control the value A represented by the above formula (1), the ratio of the amounts of dispersant and builder used is preferably 100:0.1 to 100:500, more preferably 100:1 to 100:40, and even more preferably 100:5 to 100:20, in terms of the weight ratio of "dispersant:builder".

[0043] In emulsion polymerization, if necessary, polymerization sub-materials such as a polymerization terminator, a chelating agent, a stabilizer, a pH adjuster, an oxygen scavenger, a particle size adjuster, etc. When these are used, the type and amount used are not particularly limited as long as the value A represented by the above formula (1) satisfies 0.8 or more.

[0044] The polymerization conversion rate in emulsion polymerization is not particularly limited, but is preferably 60 to 95%, more preferably 65 to 90%, and even more preferably 75 to 90%. By setting the polymerization conversion rate within the above range, high productivity can be achieved while appropriately suppressing problems caused by an excessively high polymerization conversion rate, such as the generation of gel during polymerization, poor processability of the resulting polymer, and deterioration of the properties of the resulting polymer.

[0045] After the polymerization reaction is terminated, the solid content concentration may be adjusted as necessary. The solid content concentration of the aqueous dispersion of nitrile rubber particles is preferably 10 to 70% by weight, more preferably 15 to 65% by weight, and even more preferably 15 to 30% by weight. When adjusting the solid content concentration after the polymerization reaction is terminated, it is preferable that the value A represented by the above formula (1) after adjusting the solid content concentration be 0.8 or more.

[0046] Furthermore, after the polymerization reaction is terminated, if necessary, some of the unreacted monomers may be removed by a method other than microwave heating, which will be described later. For example, after the polymerization reaction is terminated and before microwave heating is performed, the aqueous dispersion of nitrile rubber may be heated under reduced pressure to remove some of the unreacted monomers. The unreacted monomers removed by a method other than microwave heating may be the same monomer as the unreacted monomers removed by microwave heating, or may be a different monomer. When some of the unreacted monomers are removed by a method other than microwave heating, it is preferable that the value A represented by the above formula (1) after removal of the unreacted monomers is 0.8 or more.

[0047] <Removal of Unreacted Monomer by Heating Using Microwaves> The aqueous dispersion of nitrile rubber particles of the present invention has a value A represented by the above formula (1) of 0.8 or more, so that unreacted monomer contained in the aqueous dispersion can be efficiently removed by heating using microwaves. By heating the aqueous dispersion of nitrile rubber particles of the present invention using microwave heating, the unreacted monomer is removed as an evaporated component.

[0048] The monomer removed by microwave heating is not particularly limited as long as it is a monomer that constitutes the above-mentioned rubber, but specifically, at least one selected from the group consisting of α,β-ethylenically unsaturated nitrile monomers such as acrylonitrile, diene monomers such as 1,3-butadiene, aromatic vinyl monomers, α,β-ethylenically unsaturated monocarboxylic acid ester monomers such as meth(acrylic) acid esters, α,β-ethylenically unsaturated monocarboxylic acid monomers such as meth(acrylic acid), and self-crosslinking monomers is preferred. One type of monomer may be removed alone, or multiple types of monomers may be removed.

[0049] The frequency (center frequency) of the microwave is not particularly limited, but a frequency included in the ISM (Industrial Scientific and Medical) band is preferable, and among these, 915 MHz or 2450 MHz is more preferable.

[0050] The method of microwave irradiation is not particularly limited, and examples thereof include a method of directly irradiating the aqueous dispersion of nitrile rubber particles with microwaves, a method of indirectly irradiating the aqueous dispersion with microwaves reflected from the wall of a container, etc. During heating with microwaves, it is preferable to stir the aqueous dispersion using a stirring blade.

[0051] The removal of unreacted monomers by microwave heating is preferably carried out under reduced pressure, although not particularly limited thereto. Specifically, the degree of reduced pressure is preferably 0 kPa (gauge pressure) or less, more preferably −20 kPa (gauge pressure) or less, and even more preferably −40 kPa (gauge pressure) or less.

[0052] The heating temperature by microwave heating is not particularly limited, but is preferably 40 to 90° C., more preferably 45 to 85° C., and even more preferably 50 to 80° C. The microwave output in microwave heating can be appropriately set depending on the heating temperature, and is not particularly limited.

[0053] The heating time by microwave heating is not particularly limited, but is preferably 2 to 48 hours, more preferably 4 to 36 hours, and even more preferably 6 to 24 hours.

[0054] In addition to the unreacted monomer, water may also be removed. The heating temperature by microwave heating when removing the unreacted monomer and water is preferably 40 to 90°C, more preferably 45 to 85°C, and even more preferably 50 to 80°C. The heating time is preferably 2 to 48 hours, more preferably 4 to 36 hours, and even more preferably 6 to 24 hours. By removing water in addition to the unreacted monomer, the solids concentration of the aqueous dispersion of nitrile rubber particles after the removal of the unreacted monomer can be adjusted.

[0055] When microwave heating is performed, an antifoaming agent may be added to the aqueous dispersion of nitrile rubber particles, if necessary. Addition of an antifoaming agent can suppress foaming during microwave heating.

[0056] When an aqueous dispersion of nitrile rubber particles is heated by microwaves, agglomerates of the nitrile rubber particles may form and adhere to the walls, bottom, and stirring blades of a pressure vessel containing the aqueous dispersion. The rate of agglomerates generated by microwave heating is preferably less than 1 wt %, more preferably less than 0.8 wt %, and even more preferably less than 0.5 wt %, of the total weight of the aqueous dispersion of nitrile rubber particles. There is no particular restriction on the lower limit of the rate of agglomerates generated, and it is preferable that no agglomerates are generated after microwave heating. When water, an antifoaming agent, or the like is added to the aqueous dispersion of nitrile rubber particles before microwave heating, it is preferable that the rate of agglomerates generated relative to the total weight of the aqueous dispersion of nitrile rubber particles and the additives is within the above range.

[0057] The content of unreacted monomer remaining in the aqueous dispersion of nitrile rubber particles after microwave heating, i.e., the aqueous dispersion of nitrile rubber particles after irradiation with microwaves of 915 MHz or 2450 MHz, is not particularly limited, but is preferably 10,000 ppm or less, more preferably 5,000 ppm or less, even more preferably 1,000 ppm, still more preferably 200 ppm or less, particularly preferably 100 ppm or less, and most preferably 50 ppm or less. The lower limit of the content of unreacted monomer is not particularly limited, and it is preferable that the aqueous dispersion of nitrile rubber particles contains substantially no unreacted monomer.

[0058] The present invention will be described below in more detail with reference to examples, but the present invention is not limited to these examples. In the following, "parts" are by weight unless otherwise specified. Various tests and evaluations were carried out according to the following methods.

[0059] <Solid content concentration TS of aqueous dispersion of nitrile rubber particles> Approximately 2 g of a sample of the aqueous dispersion of nitrile rubber particles was precisely weighed (weight: X2) into an aluminum dish (weight: X1), and this was dried for 2 hours in a hot air dryer at 105°C. Next, after cooling in a desiccator, the weight of the aluminum dish was measured (weight: X3), and the solid content concentration TS was calculated according to the following formula. Note that weights X1 to X3 were all weighed to an accuracy of 0.0001 g. Solid content concentration TS [wt %] = (X3 - X1) × 100 / X2

[0060] <K Concentration C(K) and Na Concentration C(Na) of Aqueous Dispersions of Nitrile Rubber Particles> As a pretreatment, 0.3 g of the aqueous dispersions of nitrile rubber particles obtained in the Examples and Comparative Examples were weighed into a platinum crucible and placed on an electric heater to remove moisture. The crucible was heated with a gas burner to carbonize the polymer, then covered and placed in an electric furnace set at 550°C for 2 hours. The sample was removed and returned to room temperature, and 0.1 mL of concentrated sulfuric acid was added to the sample. A small amount of ultrapure water was then added to dissolve the ash. Ultrapure water was then added until the total volume reached 5 mL, and a measurement sample was prepared. A calibration curve for K and Na was created using an ICP-AES (product name "SPS5100", manufactured by Agilent), and the K and Na contents contained in the measurement sample were measured. The measured K and Na contents were used to calculate the K concentration C(K) [ppm] and the Na concentration C(Na) [ppm] of the aqueous dispersions of nitrile rubber particles.

[0061] <Content of Unreacted Acrylonitrile> In an unreacted monomer removal experiment described below, the content of unreacted acrylonitrile in the latex before and after the microwave heating treatment was measured by gas chromatography.

[0062] <Proportion of Aggregates Generated> In the unreacted monomer removal experiment described below, after the micro-heat treatment, aggregates adhering to the pressure vessel wall, bottom, and stirring blades were collected, and the proportion of aggregates generated (unit: weight %) was calculated based on the following formula: Proportion of aggregates generated (weight %) = α / β × 100 Here, α represents the weight of the collected aggregates, and β represents the total weight of the aqueous dispersion of nitrile rubber particles and other additives charged in the pressure vessel.

[0063] Example 1 A reactor was charged with 2 parts of beef tallow fatty acid potassium soap per 100 parts of monomer as an emulsifier, 0.13 parts of sodium carbonate as a builder, and 200 parts of ion-exchanged water. Next, 37 parts of acrylonitrile and 63 parts of 1,3-butadiene as monomers, and 0.5 parts of t-dodecyl mercaptan as a molecular weight modifier, were added to the reactor. Furthermore, 0.01 parts of cumene hydroperoxide and 0.01 parts of diisopropylbenzene hydroperoxide as polymerization initiators, and appropriate amounts of a reducing agent and a chelating agent were added to the reactor, and emulsion polymerization was initiated at 10°C. When the polymerization conversion rate reached 80%, 0.1 parts of a 5% aqueous hydroquinone solution (polymerization terminator) per 100 parts of monomer was added to terminate the polymerization, yielding an aqueous dispersion of nitrile rubber particles. The aqueous dispersion of nitrile rubber particles obtained by the polymerization reaction was heated under reduced pressure at 30°C, and then heated under reduced pressure at 60°C to remove unreacted monomers. The solid content concentration TS, K concentration C(K), and Na concentration C(Na) of this aqueous dispersion of nitrile rubber particles were measured using the methods described above. Using the measured solid content concentrations TS, K concentration C(K), and Na concentration C(Na), the value A was calculated based on the above formula (1). The results are shown in Table 1.

[0064] <Heating Experiment> 10 mL of the obtained aqueous dispersion sample of nitrile rubber particles was placed in a quartz tube, and with a thermocouple (model number "KE-221128A", manufactured by Okazaki Seisakusho Co., Ltd.) inserted, the quartz tube was placed in a heating furnace at room temperature. While monitoring the temperature of the sample with the thermocouple, the sample was heated by irradiating it with 2450 MHz microwaves at an output of 200 W using a semiconductor oscillator (manufactured by Tokyo Keiki Co., Ltd.). When the sample temperature reached 78°C, the microwave output was stopped, and the time required for the temperature to rise from the start of heating (room temperature) to 78°C was measured to evaluate microwave absorbency. The shorter the time required for the temperature to rise to 78°C, the higher the microwave absorbency of the aqueous dispersion of nitrile rubber particles can be evaluated. The results are shown in Table 1.

[0065] <Unreacted Monomer Removal Experiment> 80 kg of the resulting aqueous dispersion sample of nitrile rubber particles, 2 kg of acrylonitrile as the unreacted monomer, 20 kg of distilled water, and 1 kg of antifoaming agent to suppress foaming were placed in a pressure vessel and stirred until homogeneous. The liquid in the pressure vessel was irradiated with 915 MHz microwaves, and the temperature was raised from room temperature to a maximum of 70°C over approximately one hour. After reaching 70°C, the pressure inside the pressure vessel was reduced from ambient pressure to -70 kPa over approximately two hours. Because the reduced pressure caused the acrylonitrile and water in the pressure vessel to evaporate and remove latent heat, lowering the liquid temperature, microwave irradiation was performed using feedback control to heat the liquid, maintaining 70°C. The evaporated acrylonitrile and water were cooled and liquefied in a condenser connected to the pressure vessel and collected in a catch tank installed below the condenser. After heating for approximately nine hours at 70°C and -70 kPa, the content of unreacted acrylonitrile and the rate of aggregate formation were measured using the method described above. The results are shown in Table 1. In Table 1, the solid content concentration TS, K concentration C(K), and Na concentration C(Na) in the unreacted monomer removal experiment were all calculated based on the measured values ​​after unreacted monomer removal by heating under reduced pressure and the amount charged into the pressure vessel in the unreacted monomer removal experiment. Using the solid content concentration TS, K concentration C(K), and Na concentration C(Na) calculated in this manner, the value A was calculated based on the above formula (1).

[0066] Example 2 An aqueous dispersion of nitrile rubber particles was obtained in the same manner as in Example 1, and the solids concentration TS, K concentration C(Na), and Na concentration C(Na) were measured in the same manner. A temperature rise experiment was also carried out in the same manner as in Example 1. Furthermore, an unreacted monomer removal experiment was carried out in the same manner as in Example 1, except that the experimental conditions were changed to those shown in Table 1. The results are shown in Table 1.

[0067] Example 3 A reactor was charged with 3 parts of tallow fatty acid potassium soap as an emulsifier, 0.18 parts of sodium carbonate as a builder, and 215 parts of ion-exchanged water per 100 parts of monomer. Next, 10 parts of acrylonitrile, 23 parts of 1,3-butadiene, and 40 parts of n-butyl acrylate as monomers, as well as 0.1 parts of t-dodecyl mercaptan as a molecular weight modifier, were added to the reactor. Furthermore, 0.1 parts of cumene hydroperoxide as a polymerization initiator, appropriate amounts of a reducing agent, and a chelating agent were added to the reactor, and the first-stage emulsion polymerization was initiated at 5°C. When the polymerization conversion rate reached 40%, 4 parts of acrylonitrile, 9.5 parts of 1,3-butadiene, and 0.1 parts of t-dodecyl mercaptan as a molecular weight modifier were added to carry out the second-stage polymerization. Furthermore, when the polymerization conversion rate reached 70%, 4 parts of acrylonitrile, 9.5 parts of 1,3-butadiene, and 0.2 parts of t-dodecyl mercaptan as a quantity adjuster were added to conduct a third-stage polymerization. When the polymerization conversion rate reached 85%, 0.1 parts of a 5% aqueous solution of hydroquinone (polymerization terminator) per 100 parts of monomer was added to terminate the polymerization, yielding an aqueous dispersion of nitrile rubber particles. The aqueous dispersion of nitrile rubber particles obtained by the polymerization reaction was heated under reduced pressure at 30°C, and then heated under reduced pressure at 80°C to remove unreacted monomer. The solids concentration TS, K concentration C(Na), and Na concentration C(Na) of this aqueous dispersion of nitrile rubber particles were measured using the methods described above, and a temperature rise experiment was conducted in the same manner as in Example 1.

[0068] Furthermore, the aqueous dispersion of nitrile rubber particles after polymerization termination was heated at 30°C under reduced pressure to remove some of the unreacted monomers in advance. An experiment to remove unreacted monomers was carried out on this aqueous dispersion of nitrile rubber particles in the same manner as in Example 1, except that acrylonitrile was not added and the experimental conditions were changed to those shown in Table 1. The results are shown in Table 1.

[0069] Example 4 A reactor was charged with 2 parts of tallow fatty acid potassium soap as an emulsifier, 0.17 parts of sodium carbonate as a builder, and 160 parts of ion-exchanged water per 100 parts of monomer. Next, 60 parts of acrylonitrile and 20 parts of 1,3-butadiene as monomers, and 0.6 parts of t-dodecyl mercaptan as a molecular weight modifier, were added to the reactor. Furthermore, 0.04 parts of cumene hydroperoxide as a polymerization initiator, appropriate amounts of a reducing agent, and a chelating agent were added to the reactor, and the first-stage emulsion polymerization was initiated at 5°C. When the polymerization conversion rate reached 42%, 10 parts of 1,3-butadiene was added to carry out the second-stage polymerization. Furthermore, when the polymerization conversion rate reached 60%, 10 parts of 1,3-butadiene was added to carry out the third-stage polymerization. When the polymerization conversion rate reached 75%, 0.1 parts of a 5% aqueous solution of hydroquinone (polymerization terminator) per 100 parts of monomer was added to terminate the polymerization, yielding an aqueous dispersion of nitrile rubber particles. The aqueous dispersion of nitrile rubber particles obtained by the polymerization reaction was heated under reduced pressure at 30°C, and then heated under reduced pressure at 60°C to remove unreacted monomer. The solids concentration TS, K concentration C(Na), and Na concentration C(Na) of this aqueous dispersion of nitrile rubber particles were measured using the methods described above. A temperature rise experiment was also conducted in the same manner as in Example 1. The results are shown in Table 1.

[0070] Comparative Example 1 Ion-exchanged water was prepared, and the solids concentration TS, K concentration C(Na), and Na concentration C(Na) were measured using the methods described above. Furthermore, a temperature-raising experiment was carried out on the ion-exchanged water in the same manner as in Example 1. The results are shown in Table 1.

[0071]

[0072] As shown in Table 1, aqueous dispersions of nitrile rubber particles in which the value A represented by the above formula (1) is 0.8 or more took a relatively short time to heat up to a predetermined temperature by microwave heating. Furthermore, the content of unreacted acrylonitrile was sufficiently reduced by microwave heating, and the rate of formation of aggregates after microwave heating was sufficiently small. From the above, aqueous dispersions of nitrile rubber particles in which the value A represented by the above formula (1) is 0.8 or more were suitable for removing unreacted monomers by microwave heating.

Claims

1. An aqueous dispersion of nitrile rubber particles obtained by aqueous polymerization, characterized in that the value A represented by the following formula (1) is 0.8 or more: A = -0.065 x TS + 0.003 x C(K) + 0.009 x C(Na) ... (1) In the above formula (1), TS is the solids concentration [wt %] of the aqueous dispersion of nitrile rubber particles, C(K) is the K concentration [ppm] of the aqueous dispersion of nitrile rubber particles measured by ICP emission spectrometry, and C(Na) is the Na concentration [ppm] of the aqueous dispersion of nitrile rubber particles measured by ICP emission spectrometry.

2. The aqueous dispersion of nitrile rubber particles according to claim 1, wherein the aqueous polymerization is emulsion polymerization.

3. An aqueous dispersion of nitrile rubber particles according to claim 1 or 2, characterized in that it contains a dispersing agent.

4. The aqueous dispersion of nitrile rubber particles according to claim 3, wherein the dispersant is an anionic emulsifier.

5. The aqueous dispersion of nitrile rubber particles according to claim 4, wherein the anionic emulsifier is potassium tallow fatty acid soap.

6. The aqueous dispersion of nitrile rubber particles according to any one of claims 1 to 5, characterized in that it contains sodium carbonate as a builder.

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