JP7428097B2 - Method for producing latex for foam rubber and method for producing foam rubber - Google Patents

Description

The present invention relates to a method for producing latex for foam rubber and a method for producing foam rubber.

Generally, latexes with relatively high concentrations (for example, solid content concentrations of 60% by weight or more) are preferably used for producing foam rubber. On the other hand, latex produced by ordinary emulsion polymerization has a relatively low concentration, so it needs to be concentrated in order to be used for foam rubber production. However, latex produced by ordinary emulsion polymerization has small latex particles (for example, a number average particle diameter of 0.2 μm or less), and if it is concentrated as it is, the fluidity of the latex decreases. Therefore, methods of enlarging the rubber particles in latex are being considered.

Methods for enlarging rubber particles in latex are broadly divided into (A) a method of enlarging the particles during the polymerization process, and (B) a method of enlarging the particles by post-processing the latex produced by polymerization. be done. Method (A) had problems such as a long polymerization time and low productivity. As an alternative to these methods, method (B) has been actively studied.

For example, in Patent Document 1, an acid is added to a small particle diameter polymer latex obtained by an emulsion polymerization method using an emulsifier mainly containing an emulsifier whose surfactant ability decreases when it is acidic, and the latex particles are agglomerated and enlarged. In the method of After obtaining the latex, the acid is neutralized with an alkaline substance to stabilize the medium particle size polymer latex, and in the second step, an acid and a polymer flocculant are added to this medium particle size polymer latex again to coagulate it into particles. A method for aggregating polymer latex is described, which is characterized by obtaining a polymer latex with a large particle size of 5,000 Å or more in diameter.

Japanese Patent Application Publication No. 7-188329

However, in the conventional method, it is necessary to go through many steps to enlarge the rubber particles in the latex, and there is a need for a technology that can produce latex for foam rubber with higher productivity. Furthermore, latexes for foam rubber are required to have excellent gelling properties and also to suppress shrinkage during foam rubber molding.

The present invention was made in view of the above circumstances, and is a foam rubber latex that can be produced with high productivity and has excellent gelling properties and shrinkage control properties during foam rubber molding. The object of the present invention is to provide a method for producing latex for rubber.

As a result of intensive research to achieve the above object, the present inventors have found that the solid content concentration can be adjusted to a specific range by incorporating a specific amount of a surfactant having a sulfonate structure into rubber latex. The present inventors have discovered that the above object can be achieved by a production method in which an acidic substance having a pKa of 0 to 4 is added to a stabilized latex, and the present invention has been completed.

That is, according to the present invention, there is a preparatory step of preparing a rubber latex in which rubber particles having a number average particle diameter of 0.40 μm or less are dispersed in water, and a surfactant having a sulfonate structure is added to the rubber latex. is added at a ratio of 0.15 to 2.0 parts by weight per 100 parts by weight of the rubber component in the rubber latex, and the stabilized latex has a solid content concentration of 40% by weight or more and less than 60% by weight. and a stabilization step of adding an acidic substance having a pKa of 0 to 4 to the stabilized latex to enlarge the rubber particles to obtain an enlarged rubber having a number average particle diameter of 0.7 μm or more. A method for producing a latex for foam rubber is provided, which includes a step of increasing the particle size to obtain an enlarged latex in which particles are dispersed in water.

In the production method of the present invention, it is preferable that the rubber particles are particles of a nitrile group-containing conjugated diene copolymer.
In the production method of the present invention, it is preferable that the surfactant having a sulfonate structure is β-naphthalene sulfonic acid formalin condensate sodium salt.
Preferably, the preparatory step is a step of emulsion polymerizing the monomer in water in the presence of a fatty acid salt.
The production method of the present invention preferably includes a concentration step of concentrating the particle size enlarged latex to a solid content concentration of 60% by weight or more.
Furthermore, a method for producing foam rubber is provided in which the latex for foam rubber obtained by the production method of the present invention is foamed and solidified.

According to the production method of the present invention, a latex for foam rubber that is excellent in gelling properties and shrinkage suppressing properties during foam rubber molding can be produced with high productivity.

As a method for enlarging the particle size of latex, a method of coalescing rubber particles by destabilizing the dispersibility of latex, such as by adding a particle size enlarging agent to latex, has been studied.

For example, the dispersibility of the latex can be destabilized by adding a monomer that forms latex particles to the latex and vigorously stirring the mixture. However, this method requires an additional step of treating monomers remaining in the latex, and therefore there is a need for a technology that can produce latex for foam rubber with higher productivity.

Further, the dispersibility of the latex can be destabilized by adding a latex of a carboxyl group-modified polymer to the latex and stirring or the like. However, the latex obtained by this method requires a long time to gel, that is, has poor gelling properties. Therefore, when the latex obtained by this method was used to produce foam rubber, the productivity of the foam rubber was poor, and it was not suitable for use as a latex for foam rubber.

Furthermore, in order to increase the particle size enlarging efficiency, it is also being considered to increase the latex concentration when adding a particle size enlarging agent. However, there has been a problem in that the higher the latex concentration when adding the particle size enlarger, the more likely the latex is to coagulate.

In contrast, an acidic substance with a pKa of 0 to 4 is added to stabilized latex, which is obtained by adding a specific amount of a surfactant having a sulfonate structure to rubber latex and whose solid content concentration is adjusted to a specific range. According to the production method of the present invention, it is possible to increase the particle size with high efficiency, and produce foam rubber latex with excellent gelling properties and shrinkage control properties during foam rubber molding with high productivity. can be produced.

<Preparation process>
The manufacturing method of the present invention includes a preparation step of preparing rubber latex in which rubber particles having a number average particle diameter of 0.40 μm or less are dispersed in water.

The rubber constituting the rubber particles is not particularly limited, but includes, for example, a nitrile group-containing conjugated diene copolymer; a styrene-butadiene copolymer; a synthetic polyisoprene; a styrene-isoprene-styrene block copolymer (SIS); Examples include polybutadiene; natural rubber; deproteinized natural rubber, and the like. Among these, nitrile group-containing conjugated diene copolymers or styrene-butadiene copolymers are preferred. A nitrile group-containing conjugated diene copolymer is more preferred because the resulting foam rubber can have excellent texture and durability such as oil resistance and abrasion resistance. The rubber particles constituting the rubber particles may be used singly or in combination of two or more types.

The rubber constituting the rubber particles may have a modified group such as a carboxyl group, but from the viewpoint of gelling properties, a rubber that does not have a carboxyl group is preferable. Note that the rubber without carboxyl groups may be any rubber that does not substantially have carboxyl groups; A carboxyl group may be included as long as the amount is less than 10%.

The nitrile group-containing conjugated diene copolymer is a copolymer formed by copolymerizing a conjugated diene monomer and an ethylenically unsaturated nitrile monomer, and in addition to these, it is used as necessary. , a copolymer formed by copolymerizing other ethylenically unsaturated monomers copolymerizable with these may also be used.

Examples of the conjugated diene monomer include 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, 1,3-pentadiene, and chloroprene. It will be done. Among these, 1,3-butadiene and isoprene are preferred, and 1,3-butadiene is more preferred. These conjugated diene monomers can be used alone or in combination of two or more. The content of conjugated diene monomer units formed by conjugated diene monomers in the nitrile group-containing conjugated diene copolymer is preferably 20 to 95% by weight, more preferably 30 to 85% by weight. , more preferably 40 to 80% by weight. By setting the content of the conjugated diene monomer unit within the above range, the resulting foam rubber can have excellent feel and durability.

The ethylenically unsaturated nitrile monomer is not particularly limited as long as it is an ethylenically unsaturated monomer containing a nitrile group, but examples include acrylonitrile, methacrylonitrile, fumaronitrile, α-chloroacrylonitrile, α-cyanoethyl acrylonitrile. Examples include. Among them, acrylonitrile and methacrylonitrile are preferred, and acrylonitrile is more preferred. These ethylenically unsaturated nitrile monomers can be used alone or in combination of two or more. The content of ethylenically unsaturated nitrile monomer units formed by ethylenically unsaturated nitrile monomers in the nitrile group-containing conjugated diene copolymer is preferably 5 to 80% by weight, more preferably is 15 to 70% by weight, more preferably 20 to 60% by weight. By setting the content of the ethylenically unsaturated nitrile monomer unit within the above range, the resulting foam rubber can have excellent feel and durability.

Examples of other ethylenically unsaturated monomers that can be copolymerized with the conjugated diene monomer and the ethylenically unsaturated nitrile monomer include (meth)acrylic acid, (anhydrous) maleic acid, fumaric acid, and itaconic acid. Ethylenically unsaturated carboxylic acids such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, mono- or dimethyl maleate, mono-fumarate or mono- or dialkyl esters of ethylenically unsaturated carboxylic acids such as diethyl, mono- or di-n-butyl fumarate, mono- or di-n-butyl itaconate; ethylenic acids such as methoxyacrylate, ethoxyacrylate, methoxyethoxyethyl acrylate; Alkoxyalkyl ester of unsaturated carboxylic acid; (meth)acrylate having a hydroxyalkyl group such as 2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate; glycidyl (meth)acrylate; ( (meth)acrylic acid amides and their derivatives such as meth)acrylamide, N-methylol(meth)acrylamide, and N-butoxymethyl(meth)acrylamide; acrylates having amino groups such as dimethylaminomethyl acrylate and diethylaminomethyl acrylate; styrene; Examples include aromatic vinyl monomers such as α-methylstyrene, vinyltoluene, and chlorostyrene; α-olefins such as ethylene and propylene; and non-conjugated diene monomers such as dicyclopentadiene and vinylnorbornene. These monomers can be used alone or in combination of two or more. The content of other monomer units formed by other ethylenically unsaturated monomers in the nitrile group-containing conjugated diene copolymer is preferably 40% by weight or less, more preferably 30% by weight. % or less, more preferably 20% by weight or less. By controlling the content of other monomer units within the above range, the resulting foam rubber can have excellent feel and durability.

In addition, styrene-butadiene copolymer is a copolymer obtained by copolymerizing 1,3-butadiene as a conjugated diene monomer with styrene, and in addition to these, these may be used as necessary. A copolymer formed by copolymerizing other copolymerizable ethylenically unsaturated monomers may also be used.

The method for preparing rubber latex in which rubber particles are dispersed in water is not particularly limited, and conventionally known methods can be employed. Methods for preparing rubber latex in which rubber particles are dispersed in water include, for example, a method of emulsion polymerization of monomers, and a method of polymerizing a rubber solution obtained by solution polymerizing monomers in the presence of an emulsifier. , a method of emulsifying in water and removing the organic solvent if necessary, emulsifying a rubber solution or an aqueous rubber dispersion obtained by dissolving or finely dispersing solid rubber in an organic solvent in water in the presence of an emulsifier. , a method of removing the organic solvent if necessary. Among these, from the viewpoint of productivity, a method of obtaining rubber latex by emulsion polymerization of monomers is preferred. That is, in the preparation step, it is preferable to emulsion polymerize the monomers.

The method for obtaining rubber latex by emulsion polymerization of monomers may be any method in which monomers are emulsion polymerized in water in the presence of an emulsifier, and conventionally known methods can be employed. For example, when emulsion polymerizing monomers, commonly used polymerization auxiliary materials such as a polymerization initiator, chelating agent, oxygen scavenger, molecular weight regulator, and pH regulator can be used. The method of adding these polymerization auxiliary materials is not particularly limited, and any method such as an initial batch addition method, a divided addition method, or a continuous addition method may be used.

Examples of the emulsifier include, but are not limited to, anionic emulsifiers, nonionic emulsifiers, and the like. Examples of anionic emulsifiers include fatty acid salts such as potassium tallow fatty acid, partially hydrogenated potassium tallow fatty acid, potassium oleate, and sodium oleate; potassium rosinate, sodium rosinate, hydrogenated potassium rosinate, and hydrogenated sodium rosinate. and alkylbenzenesulfonates such as sodium dodecylbenzenesulfonate. Examples of nonionic emulsifiers include emulsifiers of polyethylene glycol ester type and Pluronic (registered trademark) type such as block copolymers of ethylene oxide and propylene oxide. Among these, fatty acid salts are preferred, potassium oleate and sodium oleate are more preferred, and potassium oleate is even more preferred. Further, these emulsifiers can be used alone or in combination of two or more. The amount of emulsifier used is preferably 0.5 to 5 parts by weight, more preferably 1.5 to 4 parts by weight, based on 100 parts by weight of the monomer used for polymerization.

Examples of the polymerization initiator include, but are not limited to, inorganic peroxides such as sodium persulfate, potassium persulfate, ammonium persulfate, potassium perphosphate, and hydrogen peroxide; diisopropylbenzene hydroperoxide, cumene hydroperoxide, t-Butyl hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide, 2,5-dimethylhexane-2,5-dihydroperoxide, di-t-butyl peroxide, di-α- Organic peroxides such as cumyl peroxide, acetyl peroxide, isobutyryl peroxide, benzoyl peroxide, octanoyl peroxide, 3,5,5-trimethylhexanoyl peroxide; azobisisobutyronitrile, azobis-2 , 4-dimethylvaleronitrile, azo compounds such as methyl azobisisobutyrate; and the like. These polymerization initiators can be used alone or in combination of two or more. The amount of the polymerization initiator used is preferably 0.01 to 10 parts by weight, more preferably 0.01 to 2 parts by weight, based on 100 parts by weight of the monomer used for polymerization.

Further, a peroxide initiator can be used as a redox polymerization initiator in combination with a reducing agent. Examples of this reducing agent include, but are not limited to, compounds containing metal ions in a reduced state such as ferrous sulfate and cuprous naphthenate; sulfonic acid compounds such as sodium methanesulfonate; amine compounds such as dimethylaniline. ; etc. These reducing agents can be used alone or in combination of two or more. The amount of reducing agent used is preferably 3 to 1000 parts by weight per 100 parts by weight of peroxide.

Examples of molecular weight regulators include alkyl mercaptans such as n-hexyl mercaptan, n-octyl mercaptan, t-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, and n-stearyl mercaptan; dimethylxanthogen disulfide, diisopropyl xanthogen disulfide, etc. Xanthogen compounds such as sulfides; thiuram compounds such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, and tetramethylthiuram monosulfide; phenolic compounds such as 2,6-di-t-butyl-4-methylphenol and styrenated phenol; Allyl compounds such as allyl alcohol; halogenated hydrocarbon compounds such as dichloromethane, dibromomethane, and carbon tetrabromide; α-benzyloxystyrene, α-benzyloxyacrylonitrile, α-benzyloxyacrylamide, triphenylethane, pentaphenylethane, Examples include acrolein, metaacrolein, thioglycolic acid, thiomalic acid, 2-ethylhexylthioglycolate, α-methylstyrene dimer, and terpinolene. These molecular weight regulators can be used alone or in combination of two or more. The amount of the molecular weight regulator to be used is preferably 0.1 to 3 parts by weight, more preferably 0.2 to 2 parts by weight, particularly preferably 0.3 to 1. It is 5 parts by weight. By setting the amount of the molecular weight regulator within the above range, the strength of the resulting foam rubber can be improved.

The amount of water used in emulsion polymerization is preferably 80 to 600 parts by weight, more preferably 100 to 200 parts by weight, based on 100 parts by weight of the monomer used in the polymerization.

The emulsion polymerization reaction may be continuous or batchwise, and the polymerization time and the like are not particularly limited. Methods for adding monomers include, for example, adding the monomers used in the reaction vessel all at once, adding them continuously or intermittently as the polymerization progresses, and adding a portion of the monomers. Examples include a method in which the reaction is carried out to a specific conversion rate, and then the remaining monomers are added continuously or intermittently for polymerization, and any method may be employed. When the monomers are mixed and added continuously or intermittently, the composition of the mixture may be constant or variable. Further, each monomer may be added to the reaction vessel after mixing the various monomers used in advance, or may be added to the reaction vessel separately. In addition, when producing a latex of a nitrile group-containing conjugated diene copolymer as a rubber latex, a method is used in which after starting the polymerization reaction, a part of the monomer is added to the reactor to continue the polymerization. In some cases, for example, after adding a portion of the ethylenically unsaturated nitrile monomer and the conjugated diene monomer to the reactor to start the polymerization reaction, the polymerization reaction rate in the reactor is 20 to 65%. During this period, the remainder of the conjugated diene monomer is added to the reactor all at once or in portions, and the polymerization reaction is further continued. In this case, the proportion of the conjugated diene monomer added after starting the polymerization reaction is preferably 20 to 60% by weight of the total amount of conjugated diene monomer used in the polymerization.

The monomer mixture is emulsion polymerized as described above, and when a predetermined polymerization conversion rate is reached, the polymerization reaction is stopped by cooling the polymerization system or adding a polymerization terminator. The polymerization conversion rate at the time of stopping the polymerization is not particularly limited, but is preferably 75% by weight or more. If the polymerization conversion rate is too low or too high, productivity tends to decrease. The polymerization temperature is not particularly limited, but is preferably 0 to 50°C, more preferably 5 to 35°C.

Examples of polymerization terminators include, but are not limited to, hydroxylamine, hydroxyamine sulfate, diethylhydroxylamine, hydroxyamine sulfonic acid and its alkali metal salts, sodium dimethyldithiocarbamate, hydroquinone derivatives, catechol derivatives, and hydroxydimethyl. Examples include aromatic hydroxydithiocarboxylic acids such as benzenethiocarboxylic acid, hydroxydiethylbenzenedithiocarboxylic acid, and hydroxydibutylbenzenedithiocarboxylic acid, and their alkali metal salts. The amount of the polymerization terminator used is preferably 0.05 to 2 parts by weight based on 100 parts by weight of the monomer used for polymerization.

By carrying out the polymerization reaction as described above, a rubber latex in which rubber particles are dispersed in water can be obtained. If necessary, unreacted monomers may be removed from the rubber latex.

The number average particle diameter of the rubber particles in the rubber latex is 0.40 μm or less. The number average particle diameter of the rubber particles is preferably 0.10 μm or more and 0.30 μm or less, more preferably 0.15 μm or more and 0.20 μm or less, from the viewpoint of productivity of latex for foam rubber. The number average particle size of the rubber particles can be measured using a light scattering diffraction particle size measuring device (model "LS-13320", manufactured by Beckman Coulter).

When obtaining rubber latex by emulsion polymerization of monomers, the number average particle diameter of rubber particles is determined by adjusting polymerization conditions such as the type and amount of emulsifier, type and amount of initiator, amount of water, and polymerization time. This allows for control. In addition, when obtaining rubber latex by emulsifying a rubber solution or rubber aqueous dispersion in water in the presence of an emulsifier, the type and amount of the emulsifier, the amount of water, the rubber solution or rubber aqueous dispersion, etc. It can be controlled by adjusting emulsification conditions such as mixing conditions of and water.

The solid content concentration of rubber latex is usually 10% to 50% by weight. The solids concentration of the rubber latex may be less than 40% by weight. The solid content concentration of the rubber latex can be measured by the method described below as a method for measuring the solid content concentration of the stabilized latex.

The rubber latex may contain a chelating agent, an oxygen scavenger, an anti-aging agent, a preservative, an antibacterial agent, an antifoaming agent, etc. as appropriate.

<Stabilization process>
In the production method of the present invention, a surfactant having a sulfonate structure is added to the rubber latex obtained in the above-described preparatory step at a ratio of 0.15 to 2.0 parts by weight per 100 parts by weight of the rubber component. and adjusting the solid content concentration as necessary to obtain a stabilized latex with a solid content concentration in the range of 40% by weight or more and less than 60% by weight.

A surfactant having a sulfonate structure is a surfactant having at least one structure consisting of a salt of a sulfonate anion group (-SO3 ^- ) and a counter cation. Examples of surfactants having a sulfonate structure include naphthalene sulfonic acid formalin condensate salts such as β-naphthalene sulfonic acid formalin condensate sodium salt (NASF); sodium dodecylbenzenesulfonate, potassium dodecylbenzenesulfonate, decyl Alkylbenzenesulfonates such as sodium benzenesulfonate, potassium decylbenzenesulfonate, sodium cetylbenzenesulfonate, potassium cetylbenzenesulfonate; sodium di(2-ethylhexyl)sulfosuccinate, potassium di(2-ethylhexyl)sulfosuccinate, dioctyl Alkyl sulfosuccinates such as sodium sulfosuccinate; alkyl sulfate ester salts such as sodium lauryl sulfate and potassium lauryl sulfate; polyoxyethylene alkyl ether sulfate salts such as sodium polyoxyethylene lauryl ether sulfate and potassium polyoxyethylene lauryl ether sulfate, etc. can be mentioned. Surfactants having a sulfonate structure can be used alone or in combination of two or more.

The alkylbenzene sulfonate may be linear or branched, but is preferably linear.

As a surfactant with a sulfonate structure, naphthalene sulfonic acid formalin is used because it can produce foam rubber latex with excellent gelling properties and shrinkage suppressing properties during foam rubber molding with higher productivity. Condensate salts or linear alkylbenzene sulfonates are preferred, naphthalene sulfonic acid formalin condensate sodium salts or linear alkylbenzene sulfonate sodium salts are more preferred, β-naphthalene sulfonic acid formalin condensate sodium salts (NASF) or linear Chained dodecylbenzenesulfonic acid sodium salt is more preferred, and β-naphthalenesulfonic acid formalin condensate sodium salt (NASF) is particularly preferred.

The addition ratio of the surfactant having a sulfonate structure is 0.15 to 2.0 parts by weight based on 100 parts by weight of the rubber component contained in the rubber latex. If the amount of the surfactant having a sulfonate structure added is too small, it will tend to coagulate when an acidic substance with a pKa of 0 to 4 is added in the particle size enlargement step described below. On the other hand, if the amount of the surfactant having a sulfonate structure is too large, the obtained latex for foam rubber will have poor gelling properties and shrinkage suppressing properties during foam rubber molding. The addition ratio of the surfactant having a sulfonate structure makes it possible to produce foam rubber latex with better gelling properties and shrinkage control properties during foam rubber molding with higher productivity. The amount is 0.15 to 2.0 parts by weight, preferably 0.20 to 2.0 parts by weight, and more preferably 0.20 to 1.9 parts by weight based on 100 parts by weight of the rubber component. .

In addition, in the stabilization process, the solid content concentration of the rubber latex is adjusted as necessary before or after adding the surfactant having a sulfonate structure to the rubber latex. The solid content concentration of the stabilized latex obtained in step 1 is in the range of 40% by weight or more and less than 60% by weight.

Adjustment of the solid content concentration of the rubber latex is not particularly limited, but can be performed by a method of concentrating or diluting the rubber latex depending on the solid content concentration of the rubber latex obtained in the preparation step.

Methods for concentrating rubber latex include, but are not particularly limited to, methods such as vacuum distillation, normal pressure distillation, centrifugation, and membrane concentration. As a method for concentrating rubber latex, distillation is preferable, and vacuum distillation is preferable because it can produce foam rubber latex with better gelation properties and shrinkage control properties during foam rubber molding with higher productivity. More preferred. Further, as a method of diluting the rubber latex, a method of adding water can be mentioned.

The temperature during distillation is preferably 30°C to 100°C, more preferably 40°C to 90°C. The pressure (gauge pressure) during distillation is preferably -0.1 to 0.0 MPa, more preferably -0.05 to -0.099 MPa.

Centrifugation can be carried out, for example, by using a continuous centrifuge, applying a centrifugal force of preferably 100 to 10,000 G, and adjusting the solid content concentration of the rubber latex before centrifugation to preferably 2 to 15% by weight. It is preferable that the flow rate is preferably 500 to 1,700 Kg/hr, and the back pressure (gauge pressure) of the centrifuge is preferably 0.03 to 1.6 MPa. , a stabilized latex can be obtained.

The solid content concentration of the stabilized latex is 40% by weight or more and less than 60% by weight. If the solid content concentration of the stabilized latex is too low, the effect of adding an acidic substance in the particle size enlargement step described below will not be sufficiently obtained, and the particle size cannot be enlarged appropriately. The solid content concentration of the stabilized latex is preferably 40 to 55% by weight, since it is possible to produce foam rubber latex with excellent gelling properties and shrinkage control properties during foam rubber molding with higher productivity. The content is more preferably 42 to 52% by weight.

The solid content concentration of the stabilized latex can be measured by the following method.
Precisely weigh 2 g of the sample (weight: X2) on an aluminum plate (weight: X1) and dry it in a hot air dryer at 105° C. for 2 hours. Next, after cooling in a desiccator, the weight of each aluminum plate is measured (weight: X3), and the solid content concentration is calculated according to the following formula.
Solid content concentration (weight%) = (X3-X1) x 100/X2

<Particle size enlargement process>
In the production method of the present invention, an acidic substance having a pKa of 0 to 4 is added to the stabilized latex obtained in the above-mentioned stabilization step, thereby enlarging the rubber particles so that the number average particle diameter is 0. A particle size enlarging step is provided in which a particle size enlarged latex is obtained by dispersing enlarged rubber particles having a diameter of 7 μm or more in water.

In the particle size enlargement step, an acidic substance with a pKa of 0 to 4 is used. An acidic substance is a substance that releases H ⁺ when ionized in water. If the pKa of the acidic substance is too small, there is a problem in that the rubber latex tends to coagulate. On the other hand, if the pKa of the acidic substance is too large, particle size enlargement will be difficult to proceed and it will be difficult to form enlarged rubber particles having a number average particle size of 0.7 μm or more. Furthermore, if the pKa of the acidic substance is too large, the obtained latex for foam rubber will have poor shrinkage suppressing properties during foam rubber molding.

In this specification, pKa represents a dissociation index in water at 25°C. The dissociation index is the equilibrium constant Ka when the acid represented by HA is ionized into H ⁺ and A ^- in water, resulting in ionization equilibrium:
Ka=[H ⁺ ][A ^- ]/[HA]
([ ] represents the molar concentration of each substance.)
is the common logarithm of the reciprocal of .

When an acidic substance is a substance that ionizes in multiple steps, the pKa of the acidic substance is determined by the ionization equilibrium between the acidic substance before ionization and the substance that has been ionized in one step (monovalent anion and H ⁺ ). It means pKa calculated from the equilibrium constant in the state. For example, in this specification, the pKa of phosphoric acid means the dissociation index pKa=2.12 calculated from the equilibrium constant in the ionization equilibrium state of H ₃ PO ₄ and H ₂ PO ₄ ⁻ and H ⁺ .

Examples of acidic substances having a pKa of 0 to 4 include oxalic acid, sulfite, phosphoric acid, salicylic acid, citric acid, formic acid, and lactic acid. Phosphoric acid or citric acid is used as an acidic substance with a pKa of 0 to 4, since it is possible to produce foam rubber latex with excellent gelling properties and shrinkage control properties during foam rubber molding with higher productivity. is preferred, and phosphoric acid is more preferred. Acidic substances having a pKa of 0 to 4 can be used alone or in combination of two or more.

The pKa of the acidic substance having a pKa of 0 to 4 is not particularly limited, but it is possible to produce a latex for foam rubber with excellent gelling properties and shrinkage control properties during foam rubber molding with higher productivity. 1.0 to 3.5 is preferable, 1.5 to 3.3 is more preferable, and 1.8 to 2.4 is even more preferable.

The amount of the acidic substance with a pKa of 0 to 4 added in the particle size enlargement step is not particularly limited, but from the viewpoint of stability of the obtained latex for foam rubber, it is 0.00 parts by weight per 100 parts by weight of the stabilized latex. It is preferably 1 to 50 parts by weight, more preferably 0.5 to 20 parts by weight.

Methods for adding acidic substances with pKa of 0 to 4 to stabilized latex include adding pure acidic substances with pKa of 0 to 4 to stabilized latex as they are, and adding acidic substances with pKa of 0 to 4 to stabilized latex. Examples include a method of preparing a solution or dispersion of and adding the resulting solution or dispersion to the stabilized latex. From the viewpoint of stability and productivity of latex for foam rubber, a method is preferred in which a solution or dispersion of an acidic substance having a pKa of 0 to 4 is prepared and the resulting solution or dispersion is added to the stabilized latex. The concentration of an acidic substance with a pKa of 0 to 4 in a solution or dispersion of an acidic substance with a pKa of 0 to 4 depends on the type of acidic substance with a pKa of 0 to 4, the solid content concentration of the stabilized latex, etc. Although it may be determined as appropriate, from the viewpoint of gelling properties, stability, and productivity of the latex for foam rubber, it is preferably 1% to 30% by weight, more preferably 2% to 20% by weight.

When adding an acidic substance with a pKa of 0 to 4 to the stabilized latex, the entire amount of the acidic substance with a pKa of 0 to 4 may be added at once, or the acidic substance with a pKa of 0 to 4 may be added multiple times. The acidic substance may be added in portions, or the acidic substance having a pKa of 0 to 4 may be added continuously. From the viewpoint of the stability of the obtained latex for foam rubber, there is a method in which an acidic substance with a pKa of 0 to 4 is added to the stabilized latex in multiple portions, or an acidic substance with a pKa of 0 to 4 is added continuously. A method of adding it to a stabilized latex is preferred.

A method in which an acidic substance with a pKa of 0 to 4 is added to the stabilized latex in multiple portions, or a method in which an acidic substance with a pKa of 0 to 4 is continuously added to the stabilized latex. The time from the start of addition to the end of addition of an acidic substance with a pKa of 0 to 4 is the time required to produce a foam rubber latex with better gelling properties and shrinkage control properties during foam rubber molding, and a higher production rate. The time is preferably 0.5 to 120 minutes, and more preferably 0.5 to 60 minutes, since production can be carried out at high speed.

In the particle size enlargement process, foam rubber latex with excellent gelling properties and shrinkage control properties during foam rubber molding can be produced with higher productivity, so while stirring the stabilized latex, the pKa It is preferable to add an acidic substance having a value of 0 to 4. Examples of methods for stirring the stabilized latex include a method of stirring at a rotational speed of 50 to 2,500 rpm using a stirring device such as a paddle-type stirring blade.

Further, in the particle size enlargement step, the stabilized latex may be stirred after adding an acidic substance having a pKa of 0 to 4 to the stabilized latex. The stirring conditions for the stabilized latex after adding an acidic substance with a pKa of 0 to 4 are as follows: A latex for foam rubber with excellent gelling properties and shrinkage control properties during foam rubber molding can be produced with higher productivity. The rotation speed is preferably 50 to 2,500 rpm, more preferably 50 to 1000 rpm, and the stirring time is preferably 0.5 to 12 hours, more preferably 0.5 to 5 hours.

In the particle size enlarging step, the rubber particles are enlarged to become enlarged rubber particles having a number average particle diameter of 0.7 μm or more. If the number average molecular weight of the enlarged rubber particles is too small, the obtained latex for foam rubber will have poor shrinkage suppressing properties during foam rubber molding. The number average particle size of the enlarged rubber particles in the particle size enlarged latex makes it possible to produce foam rubber latex with higher productivity and excellent gelling properties and shrinkage control properties during foam rubber molding. It is preferably 0.75 to 5 μm, more preferably 0.75 to 4 μm, since it can improve the quality of the foam rubber obtained. The number average particle diameter of the enlarged rubber particles in the enlarged latex can be measured in the same manner as the method for measuring the number average particle diameter of the rubber particles in the rubber latex.

The particle size enlarging step is not particularly limited, but is preferably a step of obtaining an enlarged particle latex having a pH of 2.0 to 6.0. Since a foam rubber latex with excellent gelling properties and shrinkage control properties during foam rubber molding can be produced with higher productivity, the pH of the particle size enlarged latex should be 3.0 to 5. 5 is more preferable, and 3.0 to 5.2 is even more preferable.

Preferably, the production method of the present invention includes a step of adding an alkaline aqueous solution to the particle size enlarged latex. By adding an alkaline aqueous solution to the particle size enlarged latex, a foam rubber latex with excellent stability can be produced with higher productivity.

Examples of alkaline aqueous solutions include aqueous solutions of alkali metal hydroxides such as aqueous sodium hydroxide and potassium hydroxide; aqueous solutions of alkali metal carbonates such as aqueous sodium carbonate and potassium carbonate; alkali metal hydrogen carbonates such as aqueous sodium hydrogen carbonate. Aqueous solutions of salts; aqueous ammonia solutions; aqueous solutions of organic amine compounds such as trimethylamine aqueous solutions and triethanolamine aqueous solutions; etc., but aqueous alkali metal hydroxide solutions or ammonia aqueous solutions are preferred, and potassium hydroxide aqueous solutions, sodium hydroxide Aqueous solutions and ammonia aqueous solutions are more preferred, and potassium hydroxide aqueous solutions and sodium hydroxide aqueous solutions are even more preferred.

The pH of the particle size enlarged latex after adding the alkaline aqueous solution is preferably 7.5 to 12.0, more preferably 8.0 to 11.0.

Preferably, the production method of the present invention includes a step of concentrating the particle size enlarged latex to a solid content concentration of 60% by weight or more.

In the particle size enlargement step, a particle size enlarged latex can be obtained in which enlarged rubber particles having a number average particle size of 0.7 μm or more are dispersed in water. Such a latex with enlarged particle size has excellent fluidity during concentration, and the solid content concentration can be easily made to be 60% by weight or more (solid content concentration suitable for use in foam rubber production). By concentrating the particle size enlarged latex to a solid content concentration of 60% by weight or more, the latex for foam rubber can be made to have better shrinkage suppressing properties during foam rubber molding. It is also possible to improve the quality (e.g. cell uniformity and strength) of the foam rubber produced.

As a method for concentrating the particle size enlarged latex, the method and conditions described above as a method for concentrating rubber latex can be appropriately employed. Distillation is preferred as a method for concentrating the latex with enlarged particle size, since it allows the production of foam rubber latex with better gelling properties and shrinkage suppressing properties during foam rubber molding with higher productivity. Distillation under reduced pressure is more preferred.

The solid content concentration of the particle size enlarged latex after concentration is preferably 60% by weight or more. It is possible to produce latex for foam rubber with excellent gelling properties and shrinkage control properties during foam rubber molding with higher productivity, and it is also possible to improve the quality of the resulting foam rubber. The solid content concentration of the subsequent particle size enlarged latex is more preferably 60 to 75% by weight, and even more preferably 60 to 70% by weight.

When the production method of the present invention includes the step of adding an alkaline aqueous solution to the particle size enlarged latex and the step of concentrating the particle size enlarged latex to a solid content concentration of 60% by weight or more, the particle size enlarged latex After adding an alkaline aqueous solution to the alkali aqueous solution, the particle size enlarged latex to which the alkaline aqueous solution has been added may be concentrated to a solid content concentration of 60% by weight or more, and after concentrating the particle size enlarged latex to a solid content concentration of 60% by weight or more. , an alkaline aqueous solution may be added to the particle size enlarged latex after concentration. From the viewpoint of producing foam rubber latex with excellent gelling properties and shrinkage control properties during foam rubber molding, and excellent stability with higher productivity, after adding an alkaline aqueous solution to the particle size enlarged latex. It is preferable to concentrate the particle size enlarged latex to which an alkaline aqueous solution has been added to a solid content concentration of 60% by weight or more.

Latex for foam rubber further contains foam stabilizers; chelating agents; oxygen scavengers; anti-aging agents; preservatives; antibacterial agents; antifoaming agents; colorants; thickeners; reinforcements such as carbon black, silica, and talc. Fillers such as calcium carbonate and clay; ultraviolet absorbers; plasticizers and the like may be added.

As the bubble stabilizer, those commonly used in the production of foam rubber can be used, such as ethyl chloride/formaldehyde/ammonia reaction products (for example, trade name "Trimenbase", manufactured by Crompton Corp.), hexafluorosilicic acid, etc. Examples include salt. The amount of the foam stabilizer is preferably 0.4 to 10 parts by weight, more preferably 0.4 to 6 parts by weight, based on 100 parts by weight of the rubber component in the latex for foam rubber. By setting the amount of the bubble stabilizer in the above range, the bubbles contained in the obtained foam rubber can be made fine and uniform, and the flexibility and strength can be improved.

<Latex composition for foam rubber>
A latex composition for foam rubber may be prepared by adding a crosslinking agent to the latex for foam rubber obtained by the production method of the present invention.

Examples of crosslinking agents include sulfur such as powdered sulfur, sulfur flower, precipitated sulfur, colloidal sulfur, surface-treated sulfur, and insoluble sulfur; sulfur chloride, sulfur dichloride, morpholine disulfide, alkylphenol disulfide, N,N'-dithio Examples include sulfur-containing compounds such as -bis(hexahydro-2H-azepinone-2), phosphorus-containing polysulfide, polymer polysulfide, and 2-(4'-morpholinodithio)benzothiazole. Among these, sulfur can be preferably used. The crosslinking agents can be used alone or in combination of two or more.

The amount of the crosslinking agent is not particularly limited, but it is preferably 0.1 to 10 parts by weight, more preferably 0.2 to 3 parts by weight, based on 100 parts by weight of the rubber component in the latex composition for foam rubber. It is. By setting the amount of the crosslinking agent within the above range, the strength of the resulting foam rubber can be increased.

Moreover, it is preferable that a crosslinking accelerator is further added to the latex composition for foam rubber.

As the crosslinking accelerator, those commonly used in the production of foam rubber can be used, such as diethyldithiocarbamic acid, dibutyldithiocarbamic acid, di-2-ethylhexyldithiocarbamic acid, dicyclohexyldithiocarbamic acid, diphenyldithiocarbamic acid, dibenzyldithiocarbamic acid, etc. dithiocarbamic acids and their zinc salts; 2-mercaptobenzothiazole, 2-mercaptobenzothiazole zinc, 2-mercaptothiazoline, dibenzothiazyl disulfide, 2-(2,4-dinitrophenylthio)benzothiazole, 2-( N,N-diethylthio carbylthio)benzothiazole, 2-(2,6-dimethyl-4-morpholinothio)benzothiazole, 2-(4'-morpholino dithio)benzothiazole, 4-morphonylyl-2-benzothiazyl disulfide , 1,3-bis(2-benzothiazyl mercaptomethyl) urea, etc., and zinc diethyldithiocarbamate, zinc 2-dibutyldithiocarbamate, and zinc 2-mercaptobenzothiazole are preferred. The crosslinking accelerator can be used alone or in combination of two or more.

The amount of the crosslinking accelerator is preferably 0.05 to 5 parts by weight, more preferably 0.1 to 2 parts by weight, based on 100 parts by weight of the rubber component in the latex composition for foam rubber. By setting the amount of the crosslinking accelerator within the above range, the strength of the resulting foam rubber can be increased.

Moreover, it is preferable that zinc oxide is further blended into the latex composition for foam rubber.

The content of zinc oxide is not particularly limited, but is preferably 0.1 to 5 parts by weight, more preferably 0.2 to 2 parts by weight, based on 100 parts by weight of the polymer contained in the polymer latex. By setting the amount of zinc oxide within the above range, the strength of the resulting foam rubber can be increased.

The method for preparing the latex composition for foam rubber is not particularly limited, but for example, using a dispersing machine such as a ball mill, kneader, or disper, the latex for foam rubber is mixed with a crosslinking agent and various compounds as necessary. Examples include a method of mixing agents, and a method of preparing an aqueous dispersion of ingredients other than the latex for foam rubber using the above-mentioned dispersing machine, and then mixing the aqueous dispersion with the latex for foam rubber.

The latex composition for foam rubber contains chelating agents; oxygen scavengers; anti-aging agents; preservatives; antibacterial agents; antifoaming agents; colorants; thickeners; reinforcing agents such as carbon black, silica, and talc; calcium carbonate; Fillers such as clay; ultraviolet absorbers; plasticizers; pH adjusters, etc. may be added.

Examples of pH adjusting agents include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide; alkali metal carbonates such as sodium carbonate and potassium carbonate; alkali metal hydrogen carbonates such as sodium hydrogen carbonate; ammonia ; organic amine compounds such as trimethylamine and triethanolamine; and the like, but alkali metal hydroxides or ammonia are preferred.

<Method for manufacturing foam rubber>
The latex for foam rubber obtained by the production method of the present invention and the above-mentioned latex composition for foam rubber have excellent gelling properties and shrinkage suppressing properties during foam rubber molding. Therefore, by using the latex for foam rubber obtained by the production method of the present invention or the above-mentioned latex composition for foam rubber, high-quality foam rubber with suppressed shrinkage can be produced with high productivity. As a method for manufacturing foam rubber, a manufacturing method in which the latex for foam rubber produced by the manufacturing method of the present invention or the above-mentioned latex composition for foam rubber is foamed and solidified is preferred.

Air is usually used when foaming latex for foam rubber or the above-mentioned latex composition for foam rubber. For example, the latex for foam rubber or the above-described latex composition for foam rubber can be stirred and foamed by drawing in air. At this time, for example, an Oaks foaming machine, an ultrasonic foaming machine, etc. can be used. In addition, carbonates such as ammonium carbonate and sodium bicarbonate; azo compounds such as azodicarboxylic acid amide and azobisisobutyronitrile; and gas generating substances such as benzenesulfonyl hydrazide may be added to latex for foam rubber or the latex composition for foam rubber described above. It can also be added to foam.

After foaming the latex for foam rubber or the above latex composition for foam rubber to obtain a foamed product, it is preferable to solidify the foamed product in order to fix the foamed state. The solidification method may be any method as long as it can gel and solidify the foam, and conventionally known methods can be used. For example, sodium hexafluorosilicate (sodium fluorosilicide), potassium hexafluorosilicate ( Dunlop method (cold coagulation method) in which a room temperature coagulant such as a silicon fluoride compound such as potassium silicate fluoride (potassium silicate fluoride) or a silicon fluoride compound such as titanium sodium silicate fluoride is added to the foam; organopolysiloxane, polyvinyl methyl ether, zinc ammonium sulfate complex salt, etc. A heat-sensitive coagulation method in which a heat-sensitive coagulant is added to the foam; a freeze-coagulation method, etc. are used. The amount of a coagulant such as a room temperature coagulant or a heat-sensitive coagulant to be used is not particularly limited, but is preferably 0.5 to 10 parts by weight, more preferably 0.5 parts by weight, based on 100 parts by weight of the rubber component in the foam. ~8 parts by weight.

Then, after adding a coagulant to the foam, it is transferred to a mold of a desired shape and coagulated to obtain foam rubber. Further, after solidification, heating may be performed in order to crosslink the foam. The conditions for crosslinking may be such that heat treatment is preferably performed at a temperature of 100 to 160° C., preferably for 15 to 120 minutes.

The obtained foam rubber is preferably washed after being removed from the mold. The washing method is not particularly limited, but includes, for example, a method of washing with water at about 20 to 70° C. for about 5 to 15 minutes using a washing machine or the like. After washing, it is preferably drained and dried at a temperature of about 30 to 90°C so as not to impair the feel of the foam rubber. The foam rubber thus obtained can be used, for example, as a puff (cosmetic sponge) by slicing it to a predetermined thickness, cutting it into a predetermined shape, and polishing the side surface with a rotating grindstone. can.

The foam rubber obtained using the latex for foam rubber of the present invention or the above-mentioned latex composition for foam rubber can be suitably used for various uses such as mattresses, puffs (cosmetic sponges), rolls, and shock absorbers. .

Hereinafter, the present invention will be explained based on more detailed examples, but the present invention is not limited to these examples. In addition, in the following, "parts" and "%" are based on weight unless otherwise specified. In addition, the tests and evaluations were conducted in accordance with the following.

<Solid content concentration>
2 g of the sample was accurately weighed (weight: X2) in an aluminum dish (weight: X1), and this was dried in a hot air dryer at 105° C. for 2 hours. Next, after cooling in a desiccator, the weight of each aluminum plate was measured (weight: X3), and the solid content concentration was calculated according to the following formula.
Solid content concentration (weight%) = (X3-X1) x 100/X2

<Number average molecular weight of rubber particles>
The number average particle size of the rubber particles was determined using a light scattering diffraction particle size measuring device (model "LS-13320", manufactured by Beckman Coulter).

<Gelification time>
For 100 parts of the rubber component in the latex for foam rubber, add a vulcanizing aqueous dispersion (colloidal sulfur/dithiocarbamate vulcanization accelerator Noxela EZ (manufactured by Ouchi Shinko Kagaku Kogyo Co., Ltd.)/thiazole vulcanization accelerator Noxela MZ (manufactured by Ouchi Shinko Kagaku Kogyo Co., Ltd.) = 2/1/1 (weight ratio), solid content concentration 50%) 4 parts, zinc oxide aqueous dispersion (solid content concentration 50%) 3 parts, bubble stabilizer ( A latex composition for foam rubber was obtained by adding 1 part of trimene base (manufactured by Crompton Corp) and thoroughly dispersing it. The obtained latex composition for foam rubber was evaluated according to the above method.
The latex for foam rubber was stirred and foamed using a stand mixer (manufactured by Electrolux, ESM945) to obtain a foamed product having a volume 5.0 times the volume before stirring. A sodium silicofluoride aqueous dispersion (solid content concentration 20% by weight) is added to the obtained foam so that the amount of sodium silicofluoride is 1.5 parts per 100 parts of the rubber component in the obtained foam. A gelatinous foam was obtained by adding the mixture as follows and stirring for an additional minute. The time from when the gelatinable foam was obtained to when the gelatinable foam no longer adhered to the touch of the hand was determined, and the evaluation was made according to the following criteria.
○: Gelation time was within 5 minutes.
Δ: Gelation time was more than 5 minutes and less than 10 minutes.
x: Gelation time was more than 10 minutes.
The shorter the gelling time, the better the foam rubber latex has gelling properties.

<Shrinkage rate during foam rubber molding>
Petri dishes of the same shape were used in all Examples and Comparative Examples. A gelling foam was obtained at the same time as gelling time measurement. The gelatinous foam was poured into a petri dish to a predetermined height (i.e. the height of the foam before crosslinking), crosslinked at 110°C for 40 minutes, the height of the resulting crosslinked product was measured, and the following equation was calculated: The shrinkage rate was determined by
Shrinkage rate (%) = [(Height of cross-linked product) - (Height of foam before cross-linking)] / (Height of foam before cross-linking) x 100
The smaller the shrinkage rate, the more excellent the latex for foam rubber is in suppressing shrinkage during foam rubber molding.

<Example 1>
In a pressure-resistant reaction vessel, 200 parts of water, 3 parts of potassium oleate (OLK), 55 parts of acrylonitrile, 0.5 parts of t-dodecyl mercaptan, 0.03 parts of sodium formaldehyde sulfoxylate, and 0.003 parts of ferrous sulfate. , 0.008 parts of sodium ethylenediaminetetraacetic acid were added, and after thorough degassing, 25 parts of 1,3-butadiene was added. Next, 0.05 part of cumene hydroperoxide was added as a polymerization initiator, and emulsion polymerization was started at a reaction temperature of 5°C. When the polymerization conversion rate reached 40%, 10 parts of 1,3-butadiene was added to continue the polymerization reaction. Furthermore, when the polymerization conversion rate reached 60%, 10 parts of 1,3-butadiene was added to continue the polymerization reaction. When the polymerization conversion rate reaches 80%, a polymerization terminator solution consisting of 0.25 parts of diethylhydroxylamine and 5 parts of water is added to stop the polymerization reaction, unreacted monomers are removed, and rubber latex is formed. I got it. The number average molecular weight of the rubber particles in the rubber latex was 180 nm.

To the obtained rubber latex, a NASF aqueous solution (45% by weight aqueous solution of β-naphthalene sulfonic acid formalin condensate sodium salt (NASF), product name "Demol T-45", manufactured by Kao Corporation) was added to 100 parts of the rubber component. After adding NASF in an amount of 1.5 parts, a stabilized latex was obtained by heating at 70°C under reduced pressure conditions of -0.05 MPa (gauge pressure). The solids concentration of the stabilized latex was measured according to the method described above. The results are shown in Table 1.

To the obtained stabilized latex, 0.01 part of a mineral oil-based modified hydrocarbon oil (trade name "DF714S", manufactured by Seiko PMC Co., Ltd.) as an antifoaming agent was added, and the mixture was stirred using a paddle-type stirring blade. , and stirred at a rotational speed of 100 rpm. While stirring was continued, an aqueous phosphoric acid solution (phosphoric acid concentration: 10% by weight) was added such that the ratio of phosphoric acid to 100 parts of stabilized latex was 1.7 parts. Note that the addition of the phosphoric acid aqueous solution was carried out in multiple portions, and the time from the start of the addition of the phosphoric acid aqueous solution to the end of the addition was 30 minutes. After completing the addition of the phosphoric acid aqueous solution, stirring was continued for 1.0 hour to obtain a latex with enlarged particle size. Table 1 shows the pH of the obtained latex with enlarged particle size. Furthermore, the average particle size of the rubber particles of the obtained latex with enlarged particle size was measured according to the above method. The results are shown in Table 1.

A potassium hydroxide aqueous solution (potassium hydroxide concentration: 3% by weight) was added to the obtained particle size enlarged latex to adjust the pH of the particle size enlarged latex to 10.0. ) was heated at 70° C. under reduced pressure conditions to obtain a latex for foam rubber with a solid content concentration of 65% by weight. The latex for foam rubber had excellent fluidity. Regarding the latex for foam rubber, the solid content concentration, gelation time, and shrinkage rate during foam rubber molding were measured and evaluated according to the above method. The results are shown in Table 1.

<Example 2>
A latex for foam rubber was obtained in the same manner as in Example 1, except that the amount of the NASF aqueous solution added was changed to an amount such that the amount of NASF was 0.3 parts per 100 parts of the rubber component in the rubber latex. The latex for foam rubber had excellent fluidity. Table 1 shows the results of measurement and evaluation in the same manner as in Example 1.

<Example 3>
In a polymerization reactor, 30 parts of styrene, 70 parts of 1,3-butadiene, 0.3 parts of t-dodecylmercaptan, 132 parts of ion-exchanged water, 3 parts of sodium dodecylbenzenesulfonate, and β-naphthalenesulfonic acid formalin condensate sodium salt. 0.5 parts of potassium persulfate, 0.3 parts of potassium persulfate, and 0.05 parts of ethylenediaminetetraacetic acid sodium salt were added, and the polymerization temperature was maintained at 60°C to start polymerization.
When the polymerization conversion rate reached 60%, 0.15 part of t-dodecyl mercaptan was added and the polymerization temperature was raised to 70°C, and then, when the polymerization conversion rate reached 80%, t-dodecyl mercaptan was added. -0.15 parts of dodecyl mercaptan was added to continue the polymerization reaction until the polymerization conversion rate reached 95%. Thereafter, 0.1 part of sodium dimethyldithiocarbamate was added as a polymerization terminator to stop the polymerization reaction, and a copolymer latex was obtained.
Unreacted monomers were removed from the obtained copolymer latex to obtain rubber latex. The number average molecular weight of the rubber particles in the rubber latex was 180 nm.

A latex for foam rubber was obtained in the same manner as in Example 1, except that the rubber latex obtained above was used. The latex for foam rubber had excellent fluidity. Table 1 shows the results of measurement and evaluation in the same manner as in Example 1.

<Example 4>
Linear sodium dodecylbenzenesulfonate was added to the rubber latex obtained in the same manner as in Example 1 so that the amount of sodium linear dodecylbenzenesulfonate was 0.3 parts per 100 parts of the rubber component in the rubber latex. Thereafter, a stabilized latex was obtained by heating at 70° C. under reduced pressure conditions of −0.05 MPa (gauge pressure).

0.01 part of a mineral oil-based modified hydrocarbon oil as an antifoaming agent was added to the obtained stabilized latex, and the mixture was stirred at a rotational speed of 100 rpm using a paddle type stirring blade. While stirring was continued, an aqueous citric acid solution (citric acid concentration: 10% by weight) was added such that the ratio of citric acid to 100 parts of stabilized latex was 2.5 parts. Note that the addition of the citric acid aqueous solution was carried out in multiple portions, and the time from the start of the addition of the citric acid aqueous solution to the end of the addition was 30 minutes. After completing the addition of the citric acid aqueous solution, stirring was continued for 1.0 hour to obtain a latex with enlarged particle size.

The obtained latex with enlarged particle size was heated at 70° C. under reduced pressure conditions of -0.05 MPa (gauge pressure) to obtain a latex for foam rubber with a solid content concentration of 65% by weight. The latex for foam rubber had excellent fluidity. Table 1 shows the results of measurement and evaluation in the same manner as in Example 1.

<Comparative example 1>
An aqueous NASF solution was added to the rubber latex obtained in the same manner as in Example 1 so that the amount of NASF was 1.5 parts per 100 parts of the rubber component in the rubber latex, and the stabilized latex was prepared without heating. I got it.

A latex with enlarged particle size was obtained in the same manner as in Example 1 except that the obtained stabilized latex was used. Furthermore, when the obtained latex with enlarged particle size was heated at 70°C under reduced pressure conditions of -0.05 MPa (gauge pressure), the fluidity of the latex was significantly reduced and the solid content concentration was 52% by weight. At this point, it became difficult to concentrate, so heating was stopped and latex for foam rubber was obtained. Table 1 shows the results of measurement and evaluation in the same manner as in Example 1.

<Comparative example 2>
After adding 1.5 parts of potassium oleate to 100 parts of the rubber component in the rubber latex to the rubber latex obtained in the same manner as in Example 1, the mixture was heated under reduced pressure conditions of -0.05 MPa (gauge pressure). , and heated at 70° C. to obtain a stabilized latex. The obtained stabilized latex was measured in the same manner as in Example 1. The results are shown in Table 1.

To the obtained stabilized latex, 0.01 part of a mineral oil-based modified hydrocarbon oil as an antifoaming agent was added, and the mixture was stirred at a rotational speed of 100 rpm using a paddle type stirring blade. When an aqueous phosphoric acid solution was added to the stabilized latex at a ratio of 1.7 parts to 100 parts of the stabilized latex while stirring continued, the rubber component coagulated in the stabilized latex, causing the latex state to change. It became unsustainable.

<Comparative example 3>
A stabilized latex was obtained in the same manner as in Example 1, except that the amount of the NASF aqueous solution added was changed to an amount such that the amount of NASF was 0.1 part per 100 parts of the rubber component in the rubber latex. The obtained stabilized latex was measured in the same manner as in Example 1. The results are shown in Table 1.

To the obtained stabilized latex, 0.01 part of a mineral oil-based modified hydrocarbon oil as an antifoaming agent was added, and the mixture was stirred at a rotational speed of 100 rpm using a paddle type stirring blade. When an aqueous phosphoric acid solution was added to the stabilized latex at a ratio of 1.7 parts to 100 parts of the stabilized latex while stirring continued, the rubber component coagulated in the stabilized latex, causing the latex state to change. It became unsustainable.

<Comparative example 4>
A foam rubber latex was prepared in the same manner as in Example 1, except that instead of the NASF aqueous solution, linear sodium dodecylbenzenesulfonate was added at a ratio of 5.0 parts to 100 parts of the rubber component in the rubber latex. I got it. Table 1 shows the results of measurement and evaluation in the same manner as in Example 1.

<Comparative example 5>
A stabilized latex was obtained in the same manner as in Example 1. To the obtained stabilized latex, 0.01 part of a mineral oil-based modified hydrocarbon oil as an antifoaming agent was added, and the mixture was stirred at a rotational speed of 100 rpm using a paddle type stirring blade. While stirring, an aqueous hydrochloric acid solution (concentration 3% by weight) was added so that the ratio of hydrochloric acid to 100 parts of the stabilized latex was 0.3 parts, and coagulation of the rubber component occurred in the stabilized latex. , it became impossible to maintain the latex state.

<Comparative example 6>
A stabilized latex was obtained in the same manner as in Example 1. 0.01 part of a mineral oil-based modified hydrocarbon oil as an antifoaming agent was added to the obtained stabilized latex, and the mixture was stirred at a rotational speed of 100 rpm using a paddle type stirring blade. While stirring was continued, an acetic acid aqueous solution (concentration 10% by weight) was added such that the ratio of acetic acid to 100 parts of stabilized latex was 80 parts. Note that the addition of the acetic acid aqueous solution was carried out in multiple portions, and the time from the start of the addition of the acetic acid aqueous solution to the end of the addition was 30 minutes. After completing the addition of the acetic acid aqueous solution, stirring was continued for 1.0 hour to obtain a latex with enlarged particle size.

When the obtained latex with enlarged particle size was heated at 70°C under reduced pressure conditions of -0.05 MPa (gauge pressure), the fluidity of the latex decreased significantly and the solid content concentration reached 55% by weight. As it became difficult to concentrate, heating was stopped and latex for foam rubber was obtained. Table 1 shows the results of measurement and evaluation in the same manner as in Example 1.

As shown in Table 1, a stabilized latex obtained by adding a specific amount of a surfactant having a sulfonate structure to rubber latex and having a solid content concentration adjusted to a specific range has a pKa of 0 to 4. According to the production method of adding an acidic substance, it was possible to produce foam rubber latex with high productivity and excellent gelling properties and shrinkage suppressing properties during foam rubber molding (Examples 1 to 4). ).

On the other hand, when the solid content concentration of the stabilized latex is less than 40% (Comparative Example 1) or when an acidic substance with a pKa of more than 4 is used (Comparative Example 6), the resulting latex for foam rubber is The shrinkage suppression properties during foam rubber molding were poor.

Furthermore, when the amount of the surfactant having a sulfonate structure added was less than 0.2 parts (Comparative Examples 2 and 3), or when an acidic substance with a pKa of less than 0 was used (Comparative Example 5), ), coagulation occurred and it was not possible to produce latex for foam rubber.

Furthermore, when the amount of the surfactant having a sulfonate structure was excessive, the resulting latex for foam rubber had poor gelling properties (Comparative Example 4).

Claims (6)

a preparation step of preparing rubber latex in which rubber particles having a number average particle diameter of 0.40 μm or less are dispersed in water;
A surfactant having a sulfonate structure is added to the rubber latex at a ratio of 0.15 to 2.0 parts by weight per 100 parts by weight of the rubber component in the rubber latex, so that the solid content concentration is 40 parts by weight. % to less than 60% by weight, and
By adding an acidic substance having a pKa of 0 to 4 to the stabilized latex, the rubber particles are enlarged, and enlarged rubber particles having a number average particle diameter of 0.7 μm or more are dispersed in water. A method for producing latex for foam rubber, comprising a particle size enlarging step to obtain latex with enlarged particle size. The method for producing a latex for foam rubber according to claim 1, wherein the rubber particles are particles of a nitrile group-containing conjugated diene copolymer. The method for producing a latex for foam rubber according to claim 1 or 2, wherein the surfactant having a sulfonate structure is β-naphthalene sulfonic acid formalin condensate sodium salt. The method for producing a latex for foam rubber according to any one of claims 1 to 3, wherein the preparation step is a step of emulsion polymerizing the monomer in water in the presence of a fatty acid salt. The method for producing latex for foam rubber according to any one of claims 1 to 4, comprising a concentration step of concentrating the particle size enlarged latex to a solid content concentration of 60% by weight or more. A method for producing foam rubber, which comprises foaming and solidifying a latex for foam rubber obtained by the production method according to any one of claims 1 to 5.