JP2011195645A - Copolymer latex - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a copolymer latex having little generation of aggregates in the production process and a low latex viscosity.SOLUTION: The copolymer latex is obtained by polymerizing a monomer composition material prepared by at least two stages of polymerization of monomer compositions different from each other, with the monomer compositions in the first and second stages being within specified ranges. The latex shows a shear rate dependence of the E-type viscosity satisfying an expression (1): B/A=0.50 to 1.1, when the latex is controlled to have a solid content of 50% and a liquid temperature of 25°C. A is the E-type viscosity at shear rate of 2 secand B is the E-type viscosity at shear rate of 10 sec.

Description

  The present invention relates to a copolymer latex. Specifically, the present invention relates to a copolymer latex that is less likely to generate aggregates and deposits in the production process, has a low latex viscosity, and has good transportability, storage and storage stability.

  It is well known that carboxy-modified butadiene copolymer latex is widely used as various adhesives (binders) including the paper processing field and carpet backsizing. The performance required of binders varies according to each application, and in order to respond to it, copolymer latex can improve the adhesive strength and reduce stickiness (tackiness) by changing the latex composition and structure. The device is devised so that it can be demonstrated. On the other hand, since it is a binder, that is, an “adhesive”, there is a trouble in the manufacturing process. For example, there are adhesion problems in each facility, an increase in aggregates during transportation, solidification in piping, and the like.

  In particular, in the paper processing field, cost reduction has been promoted in the production of coated papers in recent years, and the latex performance of a relatively expensive copolymer latex among the paint formulations is improved, and the number of blended parts is increased accordingly. Since the reduction greatly contributes to the reduction of the manufacturing cost, the influence of the latex on the coated paper quality and the manufacturing cost of the coated paper, such as improving the strength at the time of printing the latex, is increasingly regarded as important. In general, it is important to reduce the latex particle size and increase the adhesive surface area, or increase the amount of functional groups to ensure the stability of the paint and to fully bring out the adhesive effect in order to improve the printing strength. However, contrary to these, the latex viscosity increases, and it becomes difficult to handle the copolymer latex in the production stage, the transport process, the storage process, and the preparation process of the paper coating composition. The number of cases is increasing, and the problem is how to lower the latex viscosity while maintaining high quality.

In order to solve the above problems, for example, in Japanese Patent Laid-Open No. 3-182504 (Patent Document 1), a manufacturing method of polymerizing a specific composition monomer composition in the presence of a seed latex having a specific composition and an α-methylstyrene dimer is used. The technology that the obtained latex is excellent in stability with less generation of fine coagulum has been introduced. In JP-A-11-117189 (Patent Document 2), a copolymer latex for paper coating obtained by properly using two different types of emulsifiers in a polymerization process is produced stably and efficiently with few polymerization residues. A technique is disclosed in which latex using this is excellent in physical properties of coated paper.
However, these various improved technologies are still insufficient to achieve both the good printing strength required for coated paper and the handleability of the copolymer latex. Reducing the viscosity is a major issue.

Japanese Patent Laid-Open No. 3-182504

JP 11-117189 A

  It is an object of the present invention to provide a copolymer latex that is excellent in production efficiency and has low latex viscosity and excellent transportability due to less adhesion and aggregate generation in the production process and excellent particle stability. To do.

That is, the present invention has at least two stages of polymerization steps composed of different monomer compositions, and 1.5 to 24% by weight of aliphatic conjugated diene monomer in the first stage, vinyl cyanide It is composed of 5.1 to 25% by weight of a monomer, 3.1 to 20% by weight of an ethylenically unsaturated carboxylic acid monomer, and 0 to 22.3% by weight of another monomer copolymerizable therewith. The total amount of monomers is 13 to 32% by weight (total monomer total 100% by weight), and the second and subsequent stages are 10 to 60% by weight aliphatic conjugated diene monomer, vinyl cyanide Monomer total 5 to 30 wt%, ethylenically unsaturated carboxylic acid monomer 0 to 13 wt% and other monomers copolymerizable with these monomers 0 to 72 wt% total 68 to 87 wt% % Copolymer latex obtained by polymerizing 100% (total monomer total 100% by weight) Te, the solids of the copolymer latex 50%, shear rate dependence of the E-type viscosity when adjusted to a liquid temperature 25 ° C. is characterized by satisfying the following formula (1).
B / A = 0.50-1.1 (1)
Here, A is an E-type viscosity at shear rate a time of 2 sec ^-1, B denotes an E type viscosity at shear rate of 10 sec ^-1.

  According to the present invention, it is possible to obtain a copolymer latex with less adhesion and aggregate generation in the production process and low viscosity.

The present invention is described in detail below.
The copolymer latex of the present invention is obtained by emulsion polymerization of an aliphatic conjugated diene monomer, a vinyl cyanide monomer, an ethylenically unsaturated carboxylic acid monomer, and other monomers copolymerizable therewith. Obtained.
Aliphatic conjugated diene monomers include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, substituted Examples thereof include linear conjugated pentadienes, substituted and side chain conjugated hexadienes and the like, and these can be used alone or in combination. In particular, the use of 1,3-butadiene is preferred.

  Examples of the vinyl cyanide monomer include acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, α-ethylacrylonitrile, and the like, and one or more of these can be used. In particular, the use of acrylonitrile or methacrylonitrile is preferred.

  Examples of the ethylenically unsaturated carboxylic acid monomer include monobasic acids or dibasic acids (anhydrides) such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, and itaconic acid. Or 2 or more types can be used.

  Examples of other monomers copolymerizable with the above aliphatic conjugated diene monomers, vinyl cyanide monomers and ethylenically unsaturated carboxylic acid monomers include alkenyl aromatic monomers and unsaturated carboxylic acids. Examples thereof include alkyl ester monomers, unsaturated monomers containing a hydroxyalkyl group, and unsaturated carboxylic acid amide monomers.

  Examples of the alkenyl aromatic monomer include styrene, α-methylstyrene, methyl α-methylstyrene, vinyltoluene and divinylbenzene, and these can be used alone or in combination. In particular, use of styrene is preferable.

  Examples of unsaturated carboxylic acid alkyl ester monomers include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, glycidyl methacrylate, dimethyl fumarate, diethyl fumarate, dimethyl maleate, diethyl maleate, dimethyl itaconate, Examples thereof include monomethyl fumarate, monoethyl fumarate, 2-ethylhexyl acrylate, and the like can be used alone or in combination. In particular, the use of methyl methacrylate is preferred.

  Examples of unsaturated monomers containing a hydroxyalkyl group include β-hydroxyethyl acrylate, β-hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, and 3-chloro-2-hydroxypropyl. Methacrylate, di- (ethylene glycol) maleate, di- (ethylene glycol) itaconate, 2-hydroxyethyl maleate, bis (2-hydroxyethyl) maleate, 2-hydroxyethyl methyl fumarate, and the like. Or 2 or more types can be used. In particular, the use of β-hydroxyethyl acrylate is preferred.

  Examples of the unsaturated carboxylic acid amide monomer include acrylamide, methacrylamide, N-methylol acrylamide, N-methylol methacrylamide, N, N-dimethylacrylamide, and the like, and one or more of these may be used. it can. Particularly preferred is the use of acrylamide or methacrylamide.

  Further, in addition to the above monomers, any of the monomers used in ordinary emulsion polymerization such as ethylene, propylene, vinyl acetate, vinyl propionate, vinyl chloride, vinylidene chloride can be used.

The copolymer latex of the present invention has a solid content adjusted to 50%, a liquid temperature adjusted to 25 ° C., and the E-type viscosity measured by changing the shear rate satisfies the following formula (1). is required.
B / A = 0.50-1.1 (1)
Here, A is, E-type viscosity at shear rate a time of 2 sec ^-1, B represents an E-type viscosity at shear rate of 10 sec ^-1.
If the value of B / A is outside the range of 0.50 to 1.1, the generation of deposits and aggregates in the production process increases, and the latex viscosity also increases.
Also, for example, when copolymer latex is used as a binder for a paper coating composition, the latex particle size can be reduced or functional groups (for example, ethylenically unsaturated) can be used to improve the strength of the coated paper during printing. The amount of the carboxylic acid monomer etc.) may be increased, but even in such a case, if the latex shear rate dependency satisfies the formula (1) shown above, the latex viscosity is difficult to increase, preferable.

The copolymer latex of the present invention has at least two stages of polymerization steps composed of different monomer compositions, and the monomer composition must be within the following range. That is, in the first stage, aliphatic conjugated diene monomer 1.5 to 24% by weight, vinyl cyanide monomer 5.1 to 25% by weight, ethylenically unsaturated carboxylic acid monomer 3.1 to The total monomer composed of 20% by weight and 0-22.3% by weight of other monomers copolymerizable therewith must be 13-32% by weight. Further, after the second stage, the aliphatic conjugated diene monomer is 10 to 60% by weight, the vinyl cyanide monomer is 5 to 30% by weight, the ethylenically unsaturated carboxylic acid monomer is 0 to 13% by weight, It is necessary to polymerize a total of 68 to 87% by weight of monomers composed of 0 to 72% by weight of other polymerizable monomers.
Outside the ranges of the monomer composition range and the total monomer amount after the first stage and the second stage shown above, the shear rate dependency of the copolymer latex does not satisfy the above formula (1), and the latex In this manufacturing process, agglomerates and deposits are often generated, and the latex viscosity does not decrease, so handling properties such as transportability deteriorate.

  In addition, 60% by weight of the total amount of the ethylenically unsaturated carboxylic acid monomer is preferably added in the first stage polymerization step, and if it is less than 60% by weight, the generation of aggregates in the production process tends to be inferior. It is not preferable.

  More preferably, the polymerization process has three or more stages having different monomer compositions, and the first stage has an aliphatic conjugated diene monomer of 1.5 to 24% by weight, a vinyl cyanide monomer of 5.1 to A total of 13 to 32% by weight of a monomer comprising 25% by weight, 4.1 to 20% by weight of an ethylenically unsaturated carboxylic acid monomer and 0 to 21.3% by weight of another monomer copolymerizable therewith And after the second stage, the aliphatic conjugated diene monomer 10 to 60% by weight, vinyl cyanide monomer 5 to 30% by weight, ethylenically unsaturated carboxylic acid monomer 0 to 13% by weight and It is preferable to polymerize a total of 68 to 87% by weight of monomers composed of 0 to 72% by weight of other monomers copolymerizable therewith.

When the polymerization process has three or more stages, the latex viscosity decreases because 55% by weight or more of the vinyl cyanide monomer used in the second and subsequent stages is added in the polymerization process before the final stage. This is preferable because it tends to be easy.
Further, when the total monomer in the second stage is 10 to 66% by weight and the total monomer in the third and subsequent stages is 2 to 62% by weight, the generation of aggregates in the production process tends to be reduced. Therefore, it is preferable.

  A known emulsifier (surfactant) can be used for the polymerization of the copolymer latex of the present invention. For example, sulfate esters of higher alcohols, alkylbenzene sulfonates, alkyl diphenyl ether disulfonates, aliphatic sulfonates, aliphatic carboxylates, dehydroabietic acid salts, formalin condensates of naphthalene sulfonic acid, nonionic surface activity Nonionic surfactants such as anionic surfactants such as sulfuric acid ester salts, polyethylene glycol alkyl ester type, alkylphenyl ether type, alkyl ether type, etc., and one or more of these should be used Can do.

  For the polymerization of the copolymer latex of the present invention, a known chain transfer agent (molecular weight adjusting agent) can be used without limitation. For example, xanthogen compounds such as alkyl mercaptans such as n-hexyl mercaptan, n-octyl mercaptan, t-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, n-stearyl mercaptan, dimethylxanthogen disulfide, diisopropylxanthogen disulfide, etc. Thiuram compounds such as tetramethyl thiuram disulfide, tetraethyl thiuram disulfide, tetramethyl thiuram monosulfide, phenol compounds such as 2,6-di-t-butyl-4-methylphenol, styrenated phenol, and allyl compounds such as allyl alcohol , Halogenated hydrocarbon compounds such as dichloromethane, dibromomethane, carbon tetrabromide, α-benzyloxystyrene, α-benzyloxyacrylonitrile And vinyl ethers such as ril, α-benzyloxyacrylamide, triphenylethane, pentaphenylethane, acrolein, methacrolein, thioglycolic acid, thiomalic acid, 2-ethylhexyl thioglycolate, terpinolene, α-methylstyrene dimer, etc. One or more of these can be used.

  For the polymerization of the copolymer latex of the present invention, known polymerization initiators include water-soluble polymerization initiators such as lithium persulfate, potassium persulfate, sodium persulfate, and ammonium persulfate, cumene hydroperoxide, benzoyl peroxide, Oil-soluble polymerization initiators such as t-butyl hydroperoxide, acetyl peroxide, diisopropylbenzene hydroperoxide, 1,1,3,3-tetramethylbutyl hydroperoxide can be appropriately used. In particular, it is preferable to use potassium persulfate, sodium persulfate, cumene hydroperoxide, or t-butyl hydroperoxide.

  Specific examples of the reducing agent preferably used in the present invention include sulfite, bisulfite, pyrosulfite, nitrite, nithionate, thiosulfate, formaldehyde sulfonate, benzaldehyde sulfonate, Examples thereof include carboxylic acids such as L-ascorbic acid, erythorbic acid, tartaric acid and citric acid and salts thereof, reducing sugars such as dextrose and saccharose, and amines such as dimethylaniline and triethanolamine. In particular, L-ascorbic acid and erythorbic acid are preferable.

  For polymerization of the copolymer latex of the present invention, if necessary, saturated hydrocarbons such as pentane, hexane, heptane, octane, cyclohexane, cycloheptane, pentene, hexene, heptene, cyclopentene, cyclohexene, cycloheptene, 4-methylcyclohexene, Hydrocarbon compounds such as unsaturated hydrocarbons such as 1-methylcyclohexene and aromatic hydrocarbons such as benzene, toluene and xylene can be used. In particular, cyclohexene and toluene, which have a moderately low boiling point and can be easily recovered and reused by steam distillation after the completion of polymerization, are suitable from the viewpoint of environmental problems, although they are different from the object of the present invention.

  The particle diameter of the copolymer latex in the present invention is not particularly limited, but is preferably 50 to 200 nm. If the number average particle diameter is less than 50 nm, the stability of the particles tends to be inferior. If the number average particle diameter exceeds 200 nm, the number of particles per unit weight is reduced, and the adhesion area is reduced, which is not preferable. More preferably, it is 50-150 nm.

  Although there is no restriction | limiting in the gel content of the copolymer latex in this invention, 60-100 weight% of gel content is preferable. If the gel content is less than 60% by weight, the film strength of the latex tends to decrease, and the adhesive strength of various adhesives tends to decrease. Particularly preferred is 65 to 95% by weight.

  For the polymerization of the copolymer latex of the present invention, known additives such as oxygen scavengers, chelating agents, and dispersing agents may be used as necessary, and these are not particularly limited in both type and amount used. An appropriate amount can be used as appropriate. Furthermore, known additives such as antifoaming agents, anti-aging agents, antiseptics, antibacterial agents, flame retardants, and UV absorbers may be used. I can do it. Further, the copolymer latex of the present invention can be appropriately blended with other latexes depending on the purpose of use.

  In the production of the copolymer latex of the present invention, the method for adding the monomer and other components is not particularly limited, and any of a divided addition method, a continuous addition method, and a power feed method can be adopted. However, in the present invention, it is necessary to carry out multistage polymerization of two or more stages as a polymerization method.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not limited to these Examples, unless the summary is exceeded. In the examples, parts and percentages indicating percentages are based on weight unless otherwise specified. In addition, various physical properties in the examples were evaluated by the following methods.

Measurement of number average particle size of copolymer latex The number average particle size of the copolymer latex was measured by a dynamic light scattering method. In the measurement, FPAR-1000 (manufactured by Otsuka Electronics) was used.

E-type viscosity of copolymer latex The E-type viscosity was measured by adjusting the solid content of the copolymer latex to 50%. The liquid temperature at the time of measurement was kept constant at 25 ° C. using a thermostatic water bath, and measurement was performed using a 3 ° cone. The measuring instrument used was an E-type viscometer manufactured by Tokyo Keiki.

Evaluation of Deposits For each copolymer latex, the dirt adhered to the reactor during the polymerization was observed with the naked eye and evaluated in the following three stages.
○: Very little deposit
Δ: Less deposit
×: Many deposits

Measurement and Evaluation of Aggregate Generation Rate Copolymer latex after completion of polymerization: A 1 kg sample is collected and filtered through a 300 mesh wire net. After drying the agglomerates remaining on the wire mesh, the weight was measured to determine the ratio (aggregate generation rate) to the sample (in terms of solid content) and evaluated in the following three stages.
○: Less than 0.05% (less)
Δ: 0.05-less than 0.10% (slightly more)
×: 0.10% or more (very much)

Measurement and Evaluation of Latex Viscosity The viscosity of the latex when the solid content of the copolymer latex was adjusted to 50% was measured according to the measurement method of JIS K6838. About the obtained viscosity, it determined as follows.
◎: 300 mPa · s or less ○: 301-800 mPa · s
Δ: 801 to 2000 mPa · s
×: 2001 mPa · s or more

Synthesis of copolymer latex (A) Into a pressure-resistant polymerization reactor, 160 parts of polymerization water and each monomer and other compounds described in the first stage shown in Table 1 were added, and polymerization was started at 70 ° C. When the polymerization conversion rate exceeds 80%, the monomers and other compounds in the second stage are added and polymerized at 70 ° C., and the polymerization is completed when the final polymerization conversion rate exceeds 96%. Ended.
Subsequently, pH of the obtained copolymer latex was adjusted to 7 using sodium hydroxide, and unreacted monomers and other low-boiling compounds were removed by steam distillation to obtain copolymer latex A. .
Synthesis of copolymer latex (B, G to J) The monomer composition at each stage was changed to the contents shown in Table 1 and Table 2, and polymerization was performed in the same manner as Latex A. Latex B and G to J Got.
Synthesis of copolymer latex (C to E, K, M to N) In a polymerization reactor made of pressure-resistant polymer, 140 parts of polymerization water and each monomer described in the first stage shown in Tables 1 and 2 and other The compound is added and polymerization is started at 70 ° C. When the polymerization conversion rate exceeds 50%, the monomers and other compounds in the second stage are added and polymerization is performed at 70 ° C. After the second stage addition, when the polymerization conversion rate exceeded 80%, each monomer and other compounds in the third stage were added, and the polymerization was terminated when the final polymerization conversion rate exceeded 96%. .
Subsequently, the pH of the obtained copolymer latex was adjusted to 7 using sodium hydroxide, unreacted monomers and other low-boiling compounds were removed by steam distillation, and copolymer latexes C to E, K, M to N were obtained.
Synthesis of copolymer latex (F) Into a pressure-resistant polymerization reactor, 180 parts of polymerization water and each monomer and other compounds described in the first stage shown in Table 1 were added, and polymerization was started at 70 ° C. When the polymerization conversion rate exceeds 50%, the respective monomers and other compounds in the second stage are then added and polymerization is carried out at 70 ° C. After the addition of the second stage monomer, when the polymerization conversion rate exceeds 80%, each of the third stage monomers and other compounds are added, and when the polymerization conversion rate exceeds 85%, 4 is reached. The polymerization was terminated when the monomers and other compounds in the stage were added and the final polymerization conversion rate exceeded 96%.
Subsequently, pH of the obtained copolymer latex was adjusted to 7 using sodium hydroxide, and unreacted monomers and other low-boiling compounds were removed by steam distillation to obtain copolymer latex F. .
Synthesis of copolymer latex (L) Into a polymerization reactor made of pressure-resistant polymer, 140 parts of polymerization water and each monomer and other compounds described in the first stage shown in Table 2 were added, and polymerization was started at 70 ° C. The polymerization was terminated when the polymerization conversion rate exceeded 96%.
Subsequently, pH of the obtained copolymer latex was adjusted to 7 using sodium hydroxide, and unreacted monomers and other low-boiling compounds were removed by steam distillation to obtain copolymer latex L. .

  As shown in Table 1, all of the copolymer latexes A to F according to the present invention satisfy the above-described formula (1) because of the shear rate dependence of the E-type viscosity of the latex, and the generation of deposits and aggregates in the production process is small. The viscosity of the obtained latex is also low.

As shown in Table 2, in Comparative Example 1, the vinyl cyanide monomer and ethylenically unsaturated carboxylic acid monomer to be added in the first stage are out of the range described in claim 1, and Comparative Example 2 Further, the aliphatic conjugated diene monomer added to the first stage from Comparative Example 1 is outside the range described in claim 1, and the shear rate dependency of the E-type viscosity is outside the range, and the aggregate There are many deposits and latex viscosity is high.
In Comparative Example 3, the total monomer in the first stage exceeds 32% by weight. In Comparative Example 4, the vinyl cyanide monomer added in the second and subsequent stages exceeds 30% by weight. , The shear rate dependence of E-type viscosity is out of the range, aggregates and deposits are slightly generated, and latex viscosity is high.
In Comparative Example 5, the ethylenically unsaturated carboxylic acid monomer added to the first stage exceeds 20% by weight, the shear rate dependence of the E-type viscosity is out of the range, and the generation of aggregates and deposits And latex viscosity is high.
In Comparative Example 6, there is only one polymerization step, the shear rate dependency of the E-type viscosity is out of the range, the generation of aggregates and deposits is somewhat more, and the latex viscosity is high.
In Comparative Example 7, the aliphatic conjugated diene monomer used in the second and subsequent stages exceeds 60% by weight. In Comparative Example 8, the vinyl cyanide monomer added in the second and subsequent processes is used. The body is less than 5% by weight, the shear rate dependence of the E-type viscosity is out of the range, many aggregates and deposits are generated, and the latex viscosity is high.

As described above, the copolymer latex of the present invention is less prone to deposits and agglomerates in the production process, is excellent in production efficiency, and has a low latex viscosity. It has good handleability in the process and is extremely useful.
The copolymer latex of the present invention is particularly useful when used as a binder in a paper coating composition because the viscosity of the latex can be kept low even when the strength of the coated paper is improved.

Claims (4)

It has at least two stages of polymerization steps comprising different monomer compositions, and the first stage is 1.5 to 24% by weight of aliphatic conjugated diene monomer, and vinyl cyanide monomer 5.1 to The total of monomers composed of 25% by weight, 3.1 to 20% by weight of ethylenically unsaturated carboxylic acid monomer and 0 to 22.3% by weight of other monomers copolymerizable therewith is 13 To 32% by weight (total monomer total 100% by weight), and further from the second stage, the aliphatic conjugated diene monomer is 10 to 60% by weight, the vinyl cyanide monomer is 5 to 30% by weight, ethylene 68 to 87% by weight of total monomers composed of 0 to 13% by weight of unsaturated carboxylic acid monomers and 0 to 72% by weight of other monomers copolymerizable therewith (total of 100 total monomers) Copolymer latex obtained by polymerizing the copolymer latex, and the copolymer latex 50% solids, the copolymer latex shear rate dependence of the E-type viscosity when adjusted to a liquid temperature 25 ° C. is characterized by satisfying the following formula (1).
B / A = 0.50-1.1 (1)
In this case, A is, E-type viscosity at shear rate a time of 2 sec ^-1, B represents an E-type viscosity at shear rate of 10 sec ^-1.   It has at least three stages of polymerization processes comprising different monomer compositions, and the first stage is 1.5 to 24% by weight of aliphatic conjugated diene monomer and 5.1 to 25% by weight of vinyl cyanide monomer. A total of 13 to 32% by weight of a monomer composed of 4.1 to 20% by weight of an ethylenically unsaturated carboxylic acid monomer and 0 to 21.3% by weight of another monomer copolymerizable therewith. After the second stage, 10 to 60% by weight of aliphatic conjugated diene monomer, 5 to 30% by weight of vinyl cyanide monomer, 0 to 13% by weight of ethylenically unsaturated carboxylic acid monomer and the like The copolymer latex according to claim 1, wherein a total of 68 to 87 wt% of monomers composed of 0 to 72 wt% of other monomers copolymerizable with the copolymer is polymerized.   The co-polymer according to claim 1 or 2, wherein 60% by weight or more of the total amount of the ethylenically unsaturated carboxylic acid monomer is added in the first stage polymerization step and not added in the last stage polymerization step. Polymer latex.   The copolymer latex according to any one of claims 1 to 3, which is used as a copolymer latex for paper coating.