Polymer latex

Abstract

The present invention provides a polymer latex that, when used particularly as a binder for paper coating compositions, exhibits excellent coating operability in the coated paper manufacturing process, as well as excellent dry pick strength and blister resistance of coated paper. [Solution] A polymer latex characterized in that the polymer latex contains 13.0% or less of a component in the tetrahydrofuran solubility of the polymer latex whose molecular weight (in polystyrene terms) is greater than 200 and less than or equal to 3000.

Description

Technical Field

[0001] The present invention relates to polymer latex, preferably to its use as a paper coating.

Background Art

[0002] Polymer latex is widely used as a binder in fields such as paper coating, carpet back sizing, wooden products such as plywood and veneer, battery electrodes, and tire cord. Coated paper is manufactured by applying a paper coating composition to the surface of a base paper for coating and drying. Coated paper is widely used for printed matter, and research and improvement of a paper coating composition mainly composed of a pigment and an aqueous binder have been advanced to obtain high-quality coated paper. As the aqueous binder, natural binders such as starch and synthetic rubber binders such as styrene-butadiene copolymer latex are widely used. Among them, conjugated diene latex and acrylic latex have a large degree of freedom in quality design and are widely used today as the most suitable binder for paper coating compositions, and it is known that the performance of the binder affects the performance of the paper coating composition, the operability during the production of coated paper, or the quality of the final coated paper product such as surface strength and printing gloss. In recent years, while high-speed coating and high production have been advanced in the paper processing field, various studies have been conducted on the quality design and manufacturing method of synthetic rubber binders, and technical improvements have been introduced.

[0003] To achieve a high balance of the above-mentioned physical properties, conjugated diene latex has been proposed using a method to control the molecular weight of the solvent-dissolved portion of the copolymer. For example, Patent Document 1 discloses that in polymer latex, by setting the polystyrene-equivalent weight-average molecular weight (Mw) of the tetrahydrafuran-dissolved portion of the latex film to 2,000 or more and less than 100,000, a coated paper can be obtained that has excellent coating workability for paper coating compositions and good dry pick strength and blister resistance. Furthermore, Patent Document 2 discloses that, regarding conjugated diene latex, by first defining the composition of the monomer mixture used when producing the polymer to a specific range, and second, by obtaining a conjugated diene latex by emulsion polymerization of the monomer mixture in the presence of a specific component, a paper coating binder can be provided that has excellent coating workability, exhibits high print gloss, and simultaneously satisfies the viscosity of the conjugated diene latex and good tackiness resistance of the coated paper surface. However, these various improvement techniques did not meet the high level of quality required for polymer latex for paper coating, and further improvements were strongly needed. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2012-140518

[0005] [Patent Document 2] Japanese Patent Publication No. 2011-225716 [Overview of the project] [Problems that the invention aims to solve]

[0006] The present invention aims to provide a polymer latex that, when used particularly as a binder in paper coating compositions, exhibits excellent coating operability in the coated paper manufacturing process, as well as excellent dry pick strength and blister resistance of coated paper. [Means for solving the problem]

[0007] As a result of diligent research, the inventors of the present invention discovered that the above problem can be solved by ensuring that the tetrahydrofuran dissolved in the polymer latex contains 13.0% or less of components with a molecular weight (polystyrene equivalent) between 200 and 3000, and thus completed the present invention.

[0008] In other words, the present invention is comprised of the following: [1] A polymer latex characterized in that the tetrahydrofuran solubility of the polymer latex contains 13.0% or less of a component with a molecular weight (polystyrene equivalent) of more than 200 and 3000 or less. [2] The polymer latex described in [1] for paper coating. [Effects of the Invention]

[0009] The polymer latex of the present invention, when used as a binder in coating compositions for various applications, yields coatings with excellent operability, adhesive strength, and surface appearance of the coated layer. In particular, when used as a binder in paper coating compositions, it yields coated paper with excellent operability in the coated paper manufacturing process, as well as excellent dry pick strength and blister resistance. [Modes for carrying out the invention]

[0010] The composition of the polymer latex of the present invention is not particularly limited, but for example, one or more types of conjugated diene latex such as styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), methyl methacrylate-butadiene rubber (MBR), and butadiene rubber (BR) can be used; acrylic latex with (meth)acrylic acid ester as the main monomer; natural rubber latex; chloroprene rubber latex; vinyl acetate latex; and silicone latex. Among these, conjugated diene latex and acrylic latex are preferred. The acrylic latex referred to here is one in which structural units derived from aliphatic conjugated diene monomers are less than 10% by weight.

[0011] The conjugated diene latex is preferably obtained by polymerizing 10 to 80% by weight of an aliphatic conjugated diene monomer, 0.1 to 15% by weight of an ethylene-based unsaturated carboxylic acid monomer, and 5 to 89.9% by weight of other copolymerizable monomers.

[0012] Examples of aliphatic conjugated diene monomers include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chlor-1,3-butadiene, substituted linear conjugated pentadienes, and substituted and side-chain conjugated hexadienes, and one or more of these can be used. The use of 1,3-butadiene is particularly preferred.

[0013] The content of aliphatic conjugated diene monomers is preferably 10 to 80% by weight, and more preferably 25 to 60% by weight. By adjusting the content within this range, a product with an excellent balance between adhesive strength and operability can be obtained.

[0014] As the ethylene-based unsaturated carboxylic acid monomer, one or more monobasic or dibasic acids (anhydrides) such as itaconic acid, acrylic acid, methacrylic acid, crotonic acid, maleic acid, and fumaric acid can be used.

[0015] The content of ethylene-based unsaturated carboxylic acid monomers is preferably 0.1 to 15% by weight, and more preferably 2 to 10% by weight. By adjusting within this range, a polymer latex with an excellent balance of dispersibility in water and viscosity can be obtained.

[0016] Other copolymerizable monomers include alkenyl aromatic monomers, vinyl cyanide monomers, unsaturated alkyl carboxylate monomers, unsaturated monomers containing hydroxyalkyl groups, unsaturated carboxylic acid amide monomers, and monomers containing sulfonic acid groups.

[0017] Examples of the alkenyl aromatic monomer include styrene, α-methylstyrene, methyl-α-methylstyrene, vinyltoluene, divinylbenzene, etc. These can be used alone or in combination of two or more. In particular, the use of styrene is preferred.

[0018] Examples of the vinyl cyanide monomer include monomers such as acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, α-ethylacrylonitrile, etc. These can be used alone or in combination of two or more. In particular, the use of acrylonitrile or methacrylonitrile is preferred.

[0019] Examples of the unsaturated carboxylic acid alkyl ester monomer include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, glycidyl methacrylate, dimethyl fumarate, diethyl fumarate, dimethyl maleate, diethyl maleate, dimethyl itaconate, monomethyl fumarate, monoethyl fumarate, 2-ethylhexyl acrylate, etc. These can be used alone or in combination of two or more. In particular, the use of methyl methacrylate is preferred.

[0020] Examples of the unsaturated monomer containing a hydroxyalkyl group include β-hydroxyethyl acrylate, β-hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, 3-chloro-2-hydroxypropyl methacrylate, etc. These can be used alone or in combination of two or more.

[0021] Examples of the unsaturated carboxylic acid amide monomer include acrylamide, methacrylamide, N-methylol acrylamide, N-methylol methacrylamide, N,N-dimethylacrylamide, etc. These can be used alone or in combination of two or more.

[0022] Examples of the monomer containing a sulfonic acid group include 2-acrylamido-2-methylpropanesulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, vinylsulfonic acid, styrenesulfonic acid, α-methylstyrenesulfonic acid, 2-sulfoethyl methacrylate, and salts thereof. These can be used alone or in combination of two or more kinds.

[0023] In addition to the above monomers, any monomer used in emulsion polymerization such as ethylene, propylene, vinyl acetate, vinyl propionate, vinyl chloride, vinylidene chloride, etc. can be used. Furthermore, it can also be polymerized in the presence of polymers containing a hydroxyl group such as polyvinyl alcohol (PVA); saponified ethylene-vinyl acetate copolymer, vinyl alcohol resin typified by poly-α-hydroxyvinyl alcohol; cellulose derivatives typified by carboxymethyl cellulose.

[0024] The content of other copolymerizable monomers is preferably 5 to 89.9% by weight, and more preferably 30 to 73% by weight.

[0025] As the acrylic latex, it is preferably composed of 15 to 99.9% by weight of an unsaturated carboxylic acid alkyl ester monomer, 0.1 to 70% by weight of an ethylenically unsaturated carboxylic acid monomer, and 0 to 84.9% by weight of other copolymerizable monomers.

[0026] As the unsaturated carboxylic acid alkyl ester monomer, those described above can be used. In particular, the use of butyl acrylate is preferred.

[0027] The content of the unsaturated carboxylic acid alkyl ester monomer is preferably 15 to 99.9% by weight, and more preferably 20 to 90% by weight.

[0028] As the ethylenically unsaturated carboxylic acid monomer, those described above can be used.

[0029] The content of ethylene-based unsaturated carboxylic acid monomers is preferably 0.1 to 70% by weight, and more preferably 10 to 50% by weight.

[0030] Other copolymerizable monomers include aliphatic conjugated diene monomers, alkenyl aromatic monomers, vinyl cyanide monomers, unsaturated monomers containing hydroxyalkyl groups, unsaturated carboxylic acid amide monomers, and monomers containing sulfonic acid groups, and those described above can be used. Furthermore, polymerization can also be carried out in the presence of polymers containing hydroxyl groups, such as polyvinyl alcohol (PVA); saponified ethylene-vinyl acetate copolymers; vinyl alcohol resins represented by poly-α-hydroxyvinyl alcohol; and cellulose derivatives represented by carboxymethylcellulose.

[0031] The content of other copolymerizable monomers is preferably 0 to 84.9% by weight, and more preferably 5 to 60% by weight.

[0032] The polymer latex of the present invention can be produced by known emulsion polymerization methods using the monomers described above. When carrying out emulsion polymerization, in addition to the monomers, emulsifiers (surfactants), polymerization initiators, and, if necessary, chain transfer agents, reducing agents, etc., can be used.

[0033] As emulsifiers (surfactants), one or more types of anionic emulsifiers such as sulfate ester salts of higher alcohols, alkylbenzene sulfonates, alkyl diphenyl ether sulfonates, aliphatic sulfonates, aliphatic carboxylates, and sulfate ester salts of nonionic surfactants, and nonionic emulsifiers such as alkyl ester type, alkylphenyl ether type, and alkyl ether type polyethylene glycol can be used in combination. The amount of emulsifier is not particularly limited, but 0.2 to 3.5 parts by weight per 100 parts by weight of monomer is preferred.

[0034] Examples of polymerization initiators include water-soluble polymerization initiators such as lithium persulfate, potassium persulfate, sodium persulfate, and ammonium persulfate; and oil-soluble polymerization initiators such as cumene hydroperoxide, benzoyl peroxide, t-butyl hydroperoxide, acetyl peroxide, diisopropylbenzene hydroperoxide, and 1,1,3,3-tetramethylbutyl hydroperoxide. These can be used individually or in combination of two or more. It is particularly preferable to select from potassium persulfate, sodium persulfate, cumene hydroperoxide, and t-butyl hydroperoxide. The amount of polymerization initiator to be added is not particularly limited, but should be adjusted as appropriate considering the monomer composition, pH of the polymerization reaction system, and combinations with other additives.

[0035] Examples of chain transfer agents include alkyl mercaptans such as n-hexyl mercaptan, n-octyl mercaptan, t-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, and n-stearyl mercaptan; xanthogene compounds such as dimethyl xanthogene disulfide and diisopropyl xanthogene disulfide; thiram compounds such as tetramethyl thiuram disulfide, tetraethyl thiuram disulfide, and tetramethyl thiuram monosulfide; 2,6-di-t-butyl-4-methylphenol, and Examples of chain transfer agents include phenolic compounds such as bistyrene-modified phenol; allyl compounds such as allyl alcohol; halogenated hydrocarbon compounds such as dichloromethane, dibromomethane, and carbon tetrabromide; vinyl ethers such as α-benzyloxystyrene, α-benzyloxyacrylonitrile, and α-benzyloxyacrylamide; and chain transfer agents such as triphenylethane, pentaphenylethane, acrolein, methacrolein, thioglycolic acid, thiomalic acid, 2-ethylhexyl thioglycolate, terpinolene, and α-methylstyrene dimer. These can be used individually or in combination of two or more. The amount of chain transfer agent can be adjusted as appropriate, taking into account the combination with other additives.

[0036] Examples of reducing agents include reducing sugars such as dextrose and saccharose; amines such as dimethylaniline and triethanolamine; carboxylic acids and their salts such as L-ascorbic acid, erythorbic acid, tartaric acid, and citric acid; and sulfites, bisulfites, pyrosulfites, nithinites, nithinites, thiosulfates, formaldehyde sulfonates, and benzaldehyde sulfonates. It is particularly preferable to select from formaldehyde sulfonates. The amount of reducing agent can be adjusted as appropriate, taking into account the combination with other additives.

[0037] Furthermore, hydrocarbon compounds such as saturated hydrocarbons like pentane, hexane, heptane, octane, cyclohexane, and cycloheptane; unsaturated hydrocarbons like pentene, hexene, heptene, cyclopentene, cyclohexene, cycloheptene, 4-methylcyclohexene, and 1-methylcyclohexene; and aromatic hydrocarbons like benzene, toluene, and xylene can be used in the emulsion polymerization described above.

[0038] Furthermore, known additives such as oxygen scavengers, chelating agents, and dispersants may be used as needed, and there are no particular limitations on the type or amount used; they can be used in appropriate amounts as appropriate. Furthermore, known additives such as defoamers, anti-aging agents, preservatives, antibacterial agents, flame retardants, and UV absorbers may be used, and there are no particular limitations on the type or amount used; they can be used in appropriate amounts as appropriate.

[0039] The temperature during emulsion polymerization is preferably set in the range of 20 to 100°C, more preferably 25 to 85°C, even more preferably 30 to 70°C, and particularly preferably 30 to 60°C, from the viewpoint of safety, internal pressure, and productivity.

[0040] Methods for adding monomer components and other components during emulsion polymerization include, for example, a single-component addition method, a divided addition method, a continuous addition method, and a power feed method. Among these, the continuous addition method (hereinafter sometimes referred to as "continuous addition") is preferred. Furthermore, continuous addition may be performed multiple times.

[0041] Regarding the reaction time for emulsion polymerization, for example, from the viewpoint of productivity, it is preferably 1 to 15 hours, and more preferably 2 to 10 hours.

[0042] Furthermore, it is preferable to terminate the emulsion polymerization reaction after confirming that the polymer conversion rate exceeds 97%. In this way, polymer latex is obtained. The polymer conversion rate can be calculated from the amount of solids or from the amount of heat used to cool the polymerization tank.

[0043] From the viewpoint of dispersion stability, the resulting polymer latex is preferably adjusted to a pH of 5 to 9.5 using ammonia, potassium hydroxide, sodium hydroxide, etc., and more preferably to 5.5 to 8.5.

[0044] Furthermore, it is preferable that the obtained polymer latex has had unreacted monomers and other low-boiling point compounds removed by methods such as steam distillation.

[0045] The polymer latex of the present invention requires that the tetrahydrofuran soluble portion of the polymer latex contains 13.0% or less of a component with a molecular weight (polystyrene equivalent) between 200 and 3000, preferably 10.0% or less, and more preferably 5.0% or less. If the tetrahydrofuran soluble portion contains more than 13.0% of a component with a molecular weight (polystyrene equivalent) between 200 and 3000, for example, when used in a paper coating composition, the balance of coating operability, dry pick strength, and blister resistance tends to be poor.

[0046] Components in the tetrahydrofuran solubility of polymer latex with a molecular weight (polystyrene equivalent) between 200 and 3000 can be determined by the method described in the examples.

[0047] There are no particular limitations on the method for adjusting the amount of components with a molecular weight (polystyrene equivalent) between 200 and 3000 in the tetrahydrofuran solubility of polymer latex to 13.0% or less, but examples include lowering the polymerization temperature, lowering the polymerization conversion rate, using a chain transfer agent, and increasing the concentration of unreacted monomers during the polymerization reaction.

[0048] There are no particular restrictions on the average particle size of the polymer latex of the present invention, but it is preferably 40 nm to 300 nm, more preferably 150 nm or less, and particularly preferably 110 nm or less.

[0049] There are no particular restrictions on the toluene-insoluble content of the polymer latex of the present invention, but it is preferably 70% or more, more preferably 80% or more, and particularly preferably 85% or more.

[0050] The polymer latex of the present invention functions as a binder and can therefore be used in a wide range of applications, including paper coating, battery electrodes, tire cords, carpet backing, and wood bonding. In particular, it is preferably used as a binder in coating compositions for paper coating and battery electrodes, and more preferably for paper coating.

[0051] When used for paper coating, polymer latex is blended with pigments and used as a paper coating composition.

[0052] As pigments to be incorporated into the paper coating composition, known pigments such as kaolin clay, calcium carbonate, talc, barium sulfate, titanium dioxide, aluminum hydroxide, zinc oxide, satin white, and other inorganic pigments, or organic pigments such as polystyrene latex, can be used individually or in combination. Furthermore, the polymer latex content in the paper coating composition is preferably 2 to 20 parts by weight (solids) per 100 parts by weight (solids) of pigment, and more preferably 4 to 15 parts by weight. By adjusting to the above range, an excellent balance between adhesive strength and blister resistance can be obtained.

[0053] Furthermore, if necessary, modified starches such as starch, oxidized starch, and esterified starch, natural binders such as soy protein and casein, or water-soluble synthetic binders such as polyvinyl alcohol and carboxymethylcellulose may be used.

[0054] Furthermore, when preparing paper coating compositions, other auxiliary agents, such as dispersants (e.g., sodium pyrophosphate, sodium polyacrylate, sodium hexametaphosphate), defoamers (e.g., polyglycol, fatty acid esters, phosphate esters, silicone oils), leveling agents (e.g., funnel oil, dicyandiamide, urea), preservatives, release agents (e.g., calcium stearate, paraffin emulsion), fluorescent dyes, and color water retention enhancers (e.g., carboxymethylcellulose, sodium alginate), may be added as needed.

[0055] Any known coating machine, such as an air knife coater, blade coater, roll coater, or bar coater, may be used to apply the paper coating composition to the coated paper. After coating, the surface is dried and finished by calendering or the like. [Examples]

[0056] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples unless it exceeds the gist of the invention. In the examples, parts and percentages are based on weight unless otherwise specified. The physical properties in the examples were evaluated by the following method.

[0057] Example 1 In a pressure-resistant polymerization reactor equipped with a stirrer, 140 parts of polymerization water, 3.0 parts of sodium dodecylbenzenesulfonate, 1.2 parts of potassium persulfate, and the first-stage polymerization components shown in Table 1 were charged together. The temperature was raised to 30°C, and the second-stage polymerization components shown in Table 1 were continuously added for 6 hours to allow the polymerization reaction to proceed. Polymerization was terminated when the polymerization conversion rate exceeded 98%. Next, the pH was adjusted to 7 using aqueous ammonia, and steam distillation was performed to remove unreacted monomers and other low-boiling point compounds to obtain polymer latex A.

[0058] Example 2 Polymer latex B was obtained by performing the same procedure as in Example 1, except that the polymerization temperature was set to 40°C.

[0059] Example 3 In a pressure-resistant polymerization reactor equipped with a stirrer, 140 parts of polymerization water, 1.4 parts of sodium dodecylbenzenesulfonate, 1.2 parts of potassium persulfate, and the first-stage polymerization components shown in Table 1 were charged together. The temperature was raised to 60°C and maintained at 60°C while the second-stage polymerization components shown in Table 1 were continuously added for 6 hours. After the addition of the second-stage polymerization components was completed, the temperature was raised to 85°C and polymerization was continued until the polymerization conversion rate exceeded 98%, at which point polymerization was terminated. Next, the pH was adjusted to 7 using aqueous ammonia, and steam distillation was performed to remove unreacted monomers and other low-boiling point compounds to obtain polymer latex C.

[0060] Example 4 In a pressure-resistant polymerization reactor equipped with a stirrer, 140 parts of polymerization water, 3.0 parts of sodium dodecylbenzenesulfonate, 1.2 parts of potassium persulfate, and the first-stage polymerization components shown in Table 1 were charged together. The temperature was raised to 70°C and maintained at 70°C while the second-stage polymerization components shown in Table 1 were continuously added for 6 hours. After the addition of the second-stage polymerization components was completed, the temperature was raised to 85°C and polymerization was continued until the polymerization conversion rate exceeded 98%, at which point polymerization was terminated. Next, the pH was adjusted to 7 using ammonia water, and steam distillation was performed to remove unreacted monomers and other low-boiling point compounds to obtain polymer latex D.

[0061] Comparative Example 1 The polymerization latex E was obtained in the same manner as in Example 3 except that the reaction temperature was changed from 60°C to 70°C.

[0062] The proportion of components in the tetrahydrofuran solubility of polymer latex with a molecular weight (polystyrene equivalent) between 200 and 3000. <Sample preparation method> The polymer latex was dried at room temperature for 24 hours and then vacuum dried at room temperature for another 24 hours to prepare a film. After cutting the obtained film into about 5 mm squares, the water-soluble components were removed by refluxing with water for 16 hours and then vacuum dried at room temperature for two nights. Thereafter, it was refluxed with tetrahydrofuran (without stabilizer) for 16 hours, the obtained tetrahydrofuran solution was filtered, concentrated, dried to dryness, and further vacuum dried at room temperature for two nights to obtain the tetrahydrofuran-soluble components of the polymer latex. After dissolving the obtained tetrahydrofuran-soluble components of the polymer latex to about 0.03 g in 10 ml of tetrahydrofuran (for liquid chromatography), it was filtered through a disposable filter (specification: Shimadzu Corporation, liquid chromatography - non-aqueous system 13N pore size 0.45 μm) to obtain a sample for gel permeation chromatography (GPC) measurement. <GPC conditions> Measuring device: Agilent Technologies LC - 1260 infinity Data processing device: System Instruments Micro 7 Plus Data Station Analysis column: Agilent Technologies: PL gel 10 μm Mixed - B (300 mm × 7.5 mm) × 3 columns Guard column: Agilent Technologies PL gel 10 μm Mixed - B (50 mm × 7.5 mm) Column oven: 50°C Carrier liquid: Tetrahydrofuran (for liquid chromatography) Flow rate: 1 ml / min Detector: RI Sample injection volume: 100 μl <Calculation method> In the obtained chromatograph, the time at which the sample was injected was set to 0 minutes, and the points at detection times of 5 minutes and 40 minutes were connected by a line to establish the baseline. The portion with a detection intensity greater than this baseline was defined as the overall peak, and the area of ​​the total peak was calculated. Next, the area of ​​the components with a molecular weight (polystyrene equivalent) of 200 or less was calculated from the overall peak. Similarly, the area of ​​the components with a molecular weight (polystyrene equivalent) of 3000 or less was calculated from the overall peak. From the obtained areas, the percentage of components with a molecular weight (polystyrene equivalent) between 200 and 3000 was calculated using the following formula. (Area of ​​components with a molecular weight (polystyrene equivalent) of 200 or less) ÷ (Area of ​​the entire peak) × 100 = (Percentage of components with a molecular weight (polystyrene equivalent) of 200 or less) (Area of ​​components with a molecular weight (polystyrene equivalent) of 3000 or less) ÷ (Area of ​​the entire peak) × 100 = (Percentage of components with a molecular weight (polystyrene equivalent) of 3000 or less) (Components with a molecular weight (polystyrene equivalent) of 3000 or less (%)) - (Components with a molecular weight (polystyrene equivalent) of 200 or less (%)) = (Components with a molecular weight (polystyrene equivalent) exceeding 200 and 3000 or less (%))

[0063] Evaluation of the stickiness resistance of polymer latex As an indicator of how easily polymer latex adheres to backing rolls and the like, tests were conducted on the tackiness (adhesion) of polymer latex films. The better the tackiness, the less likely the polymer latex is to adhere to backing rolls and the like, indicating superior operability in the coated paper manufacturing process. Each polymer latex was coated onto a polyester film at a rate of 12 g / m2, dried in a 120°C oven for 1 minute, and then cut into 1 cm wide strips. All the latex film strips were laid out and attached to a black cardboard base. Filter paper was placed on top and pressed together using a laboratory thermal calender, passing it between hot rolls heated to 110°C. After removing the filter paper, the adhesion of the filter paper fibers to the surface of each latex film was visually assessed, and the tackiness resistance of each latex film was compared. Films with less fiber adhesion were considered to have superior tackiness resistance, while those with more fiber adhesion had inferior tackiness resistance, and were evaluated relatively from Grade 5 (Excellent) to Grade 1 (Poor).

[0064] <Production of coated paper> A paper coating composition was prepared according to the formulation shown below. The paper coating composition was adjusted to a pH of 9.5 with sodium hydroxide, and the solid content concentration was adjusted to 67% by weight by adding the required amount of pure water.

[0065] (Formulation) Kaolin (manufactured by Shiraishi Calcium Co., Ltd., Kaofine): 30 parts by weight Heavy calcium carbonate (FMT-90, manufactured by Fimatec Co., Ltd.): 70 parts by weight Modified starch (manufactured by Nippon Shokuhin Kako Co., Ltd., MS4600): 2 parts by weight Polymer latex: 8 parts by weight (based on solid content)

[0066] Coated base paper (basis weight 65 g / m2) was coated with a paper coating composition using a wire bar so that the coating amount per side was 10 g / m2, and after drying, coated paper was obtained by calendering under conditions of a linear pressure of 60 kg / cm and a temperature of 50°C.

[0067] Evaluation of Dry Pick Strength of Coated Paper Using an RI printing press, picking test ink (manufactured by DIC Graphics Co., Ltd.) was simultaneously printed onto each coated paper. The resulting printouts were pressed onto coated fine paper to transfer the ink, and areas where the ink was not transferred (white areas) were considered to be areas where picking occurred. The degree of picking at this time was judged visually and evaluated relatively from Grade 5 (excellent) to Grade 1 (poor).

[0068] Evaluation of blister resistance of coated paper For each coated paper sample, test pieces (4cm x 5cm) were printed on both sides using an RI printing press with offset printing ink (Toyo Ink Co., Ltd., WD Leoex). After printing, the test pieces were conditioned for 24 hours in a constant temperature and humidity chamber at 23°C x 50%RH. A blister tester (Osugi Co., Ltd.) was used to heat the test plates with hot air, and the lowest temperature at which the test piece developed a blister was determined. The higher the temperature at which blistering occurred, the better the blister resistance.

[0069] The components in Table 1 below are indicated by the following abbreviations. (monomer) BDE: Butadiene STY: Styrene MMA: Methyl methacrylate ACN: Acrylonitrile AA: Acrylic acid (Other compounds) EML: Sodium dodecylbenzenesulfonate (manufactured by Kao Corporation, Neoperex G-15) (anionic emulsifier) KPS: Potassium persulfate (polymerization initiator) EDTA-4Na: Sodium ethylenediaminetetraacetate SFS: Sodium formaldehyde sulfoxylate NaHCO3: Sodium bicarbonate TMTM: Tetramethylthiuram monosulfide CHX: Cyclohexene (unsaturated hydrocarbon) TDM: t-dodecyl mercaptan (chain transport agent) Molecular weight 200-3000: Components in the tetrahydrofuran solubility of polymer latex with a molecular weight (polystyrene equivalent) greater than 200 and less than or equal to 3000 (%)

[0070] [Table 1]

[0071] As shown in Table 1, using polymer latex in which the tetrahydrofuran solubility of the polymer latex contains 13.0% or less of components with a molecular weight (polystyrene equivalent) between 200 and 3000, it is clear that a good balance of coating operability, dry pick strength, and blister resistance can be achieved.

[0072] As described above, the polymer latex of the present invention is particularly useful as a binder for paper coating compositions because it exhibits excellent coating operability in the coated paper manufacturing process, as well as excellent dry pick strength and blister resistance of the coated paper. Furthermore, the binder of the present invention has a good balance of tack resistance, adhesive strength, and ease of deformation when heat is applied, making it useful in other applications requiring these effects.

Claims

1. A polymer latex characterized in that the polymer latex contains 13.0% or less of a component in the tetrahydrofuran solubility of the polymer latex whose molecular weight (in polystyrene terms) is greater than 200 and less than or equal to 3000.

2. The polymer latex according to claim 1, for use in paper coating.

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