JP2013189497A - Copolymer latex and paper coating composition containing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a copolymer latex which shows given elongation and stress in a latex film state in a high-speed tension test, and particularly which gives excellent high-speed offset printing property (printing durability) to coated paper when the composition is used as a binder for a paper coating composition.SOLUTION: This copolymer latex is obtained by emulsion polymerizing 35 to 65 wt.% of an aliphatic conjugated diene-based monomer, 5 to 35 wt.% of a vinyl cyanide monomer, 2.5 to 10 wt.% of an ethylene-based unsaturated carboxylic acid monomer, and 0 to 57.5 wt.% of other monomers copolymerizable with the above monomers (in total 100 wt.% of monomers); and the latex has a glass transition temperature (Tg) of +10°C or lower and a gel content of 75% to 100%. A film formed by using the above copolymer latex shows 100% or more elongation of the film in a high-speed tension test at 15 m/s, and 12 MPa or more of stress in the film at 100% elongation.

Description

  The present invention relates to a copolymer latex and a paper coating composition. More specifically, when used as a binder in a paper coating composition, a copolymer latex excellent in piling resistance, particularly edge piling resistance in actual printing of coated paper, and paper containing the copolymer latex The present invention relates to a coating composition.

In recent years, coated paper has been used in a large number of printed materials because of its high printing effect. Even in periodicals such as quarterly and monthly papers, the use of coated paper on all pages has increased considerably. In particular, most direct mails and product catalogs in the mail order business use coated paper for all pages.
In general, a paper coating composition is a water dispersion composed of a pigment dispersion in which a white pigment such as clay or calcium carbonate is dispersed in water, a binder for adhering and fixing the pigments to each other and a base paper, and other additives. It is a paint. As the binder, a synthetic emulsion binder represented by styrene-butadiene copolymer latex and a natural binder represented by starch and casein are used. Among them, styrene-butadiene copolymer latex has a large degree of freedom in quality design, and is widely used as the most suitable binder for paper coating compositions today, and the performance of styrene-butadiene copolymer latex. However, it is known that it affects the performance of the composition for paper coating, the operability at the time of preparing the coated paper, or the quality of the final coated paper product, such as the surface strength and printing gloss.

  On the other hand, in recent years, the cost of pigments has been reduced due to soaring pigment costs, the blending ratio of expensive kaolin to inexpensive calcium carbonate has increased, and the amount of copolymer latex that accounts for a large proportion of paint costs There is a move to reduce. Thus, in a composition for paper coating having a relatively high calcium carbonate blending ratio, there is a demand for a latex having sufficient strength even if the blending amount of the latex is reduced and the coating cost is rationalized.

  Especially in recent high-speed offset printing, even if there is no problem in the comparative test using the RI printer, which is a laboratory test machine that has been used as an indicator of the coating layer surface strength, piling occurs during high-speed offset printing. In actual printing, troubles derived from the surface strength of the coating layer have not been solved yet, and how to improve the piling that occurs during high-speed offset printing has become an important issue.

In order to solve the above-mentioned problems, for example, JP 2010-070899 A (Patent Document 1) is a copolymer latex used in a paper coating composition containing high calcium carbonate, and the composition of each monomer. And by specifying the ratio of monobasic acid and dibasic acid in the ethylenically unsaturated carboxylic acid monomer, a copolymer latex excellent in coating operability and strength during printing of the coated paper can be obtained. There is a description.
In JP 2009-013541 A (Patent Document 2), a small amount of only aromatic vinyl and / or unsaturated carboxylic acid alkyl ester is emulsion-polymerized in the latter half of the polymerization, and then potassium hydroxide is added to the pH adjuster by 1/3 or more. There are introductions of copolymer latex for paper coating that has excellent vessel picking properties, mechanical stability, stickiness, and dry pick strength of coated paper.
However, these various improved technologies have not yet reached a level that sufficiently satisfies the printing durability required for a paper coating composition in actual high-speed offset printing. It was sought after.

JP 2010-070899 A

JP 2009-013541 A

  The present invention is a copolymer latex that exhibits a certain elongation and stress when a latex film is subjected to a tensile test at a high speed, and particularly when used as a binder in a paper coating composition, It is an object of the present invention to provide a copolymer latex having excellent high-speed offset printing suitability (printing durability).

  35 to 65% by weight of aliphatic conjugated diene monomer, 5 to 35% by weight of vinyl cyanide monomer, 2.5 to 10% by weight of ethylenically unsaturated carboxylic acid monomer, and copolymerizable therewith It is obtained by emulsion polymerization of 0 to 57.5% by weight of other monomers (a total of 100% by weight of monomers), and has a glass transition temperature Tg of + 10 ° C. or lower and a gel content of 75% to 100%. The film is a polymer latex, and the film produced using the copolymer latex has an elongation of 100% or more under a high-speed tensile test condition at 15 m / s, and the stress of the film when the elongation is 100% is 12 MPa or more. A copolymer latex is provided.

  According to the present invention, when a coated paper is used, a copolymer latex excellent in printing durability in which generation of piling in high-speed offset printing is suppressed can be obtained.

Film dumbbell-shaped as a sample for high-speed tensile testing.

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 or α-methylstyrene 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, use of methyl methacrylate, ethyl acrylate, or butyl acrylate is preferable.

  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 monomer composition is 35 to 65% by weight of aliphatic conjugated diene monomer, 5 to 35% by weight of vinyl cyanide monomer, and 2.5 to 10% by weight of ethylenically unsaturated carboxylic acid monomer. And 0 to 57.5% by weight of other monomers copolymerizable therewith (100% by weight of the total monomer).

  When the aliphatic conjugated diene monomer is less than 35% by weight, the latex film becomes hard and hardly stretches even if the stress of the film appears. On the other hand, if it exceeds 65% by weight, the stress of the latex film is inferior. More preferably, it is 37 to 55% by weight.

  If the vinyl cyanide monomer is less than 5% by weight, the solvent resistance of the latex film is inferior, so that it is easily affected by the ink solvent, and the inherent performance of the copolymer latex is difficult to be obtained. On the other hand, if it exceeds 35% by weight, the latex film becomes so hard that it is difficult to stretch even if the stress is generated.

  When the amount of the ethylenically unsaturated carboxylic acid monomer is less than 2.5% by weight, the stress of the latex film is inferior. Furthermore, the viscosity of the latex itself becomes too high, and the handling property deteriorates such as poor transportability of the latex by the liquid feed pump, which is not preferable.

  If the copolymerizable monomer exceeds 57.5% by weight, the latex film becomes too hard and is difficult to stretch, which is not 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 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-benzyloxyacrylonite 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. The amount of the polymerization initiator is not particularly limited, but is appropriately adjusted in consideration of a combination of the monomer composition, the pH of the polymerization reaction system, and other additives.

  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 the polymerization of the copolymer latex of the present invention, saturated hydrocarbons such as pentane, hexane, heptane, octane, cyclohexane, cycloheptane, pentene, hexene, heptene, cyclopentene, cyclohexene, cycloheptene, 4-methylcyclohexene, 1-methyl Hydrocarbon compounds such as unsaturated hydrocarbons such as cyclohexene 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.

  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.

  The gel content of the copolymer latex in the present invention needs to be 75% to 100%. When the gel content is less than 75%, the stress during high-speed tensioning of the latex film decreases. Preferably, it is 80% to 95%.

  The glass transition temperature (Tg) of the copolymer latex in the present invention is + 10 ° C. or lower. When Tg exceeds 10 ° C., the latex film becomes too hard and the elongation rate in the high-speed tensile test is small. Preferably, it is -45 degreeC-+8 degreeC, More preferably, it is -35 degreeC-+5 degreeC.

  The elongation of the latex film produced using the copolymer latex in the present invention is 100% or more when pulled at a high speed (15 m / s), and the stress of the film when the elongation is 100% is 12 MPa or more. is necessary.

In recent commercial offset printing, printing is performed at a very high speed in order to emphasize productivity. It is said that printing speed is 2 m / s or more for flat offset printing and 10 m / s or more for winding type offset printing. ing. In general, the surface strength of coated paper is often substituted by simulating actual machine printing using an RI printer, but in reality it is printed at a very fast speed. It is considered that the evaluation result of the RI printing machine is not reflected in the actual printing as it is, and it leads to the occurrence of piling (especially in many cases edge piling occurs) at the site of commercial offset printing.
Although it was very difficult to simulate the strength of coated paper in such a high-speed printing area, as a result of intensive studies, it was possible to control the latex film characteristics using a high-speed tensile tester. It has been found that it is possible to suppress the occurrence of piling during printing.

  If the elongation at the time of the high-speed tensile test at 15 m / s is less than 100%, the instantaneous impact mitigating action in high-speed printing is small, and the piling is inferior. Further, if the stress of the film at 100% elongation is less than 12 MPa, the material of the latex film is likely to be broken, and the pile is inferior.

  The characteristics of the latex film during the high-speed tensile test at 15 m / s are as follows: latex gel content, latex Tg, type and amount of ethylenically unsaturated carboxylic acid monomer used during polymerization, temperature during polymerization Control is possible by control or the like.

In the production of the copolymer latex of the present invention, the method for adding the monomer and other components is not particularly limited, but a divided addition method, a continuous addition method, and a power feed method can be employed. In the present invention, any one-stage polymerization, two-stage polymerization, multi-stage polymerization, etc. can be employed, but in the one-stage polymerization, from the start of monomer addition until the end of monomer addition, By increasing the polymerization temperature by 3 ° C. or more, the stress during the high-speed tensile test can be controlled within the specified range of the present invention.
In multistage polymerization of two or more stages, it is preferable to provide at least one stage for continuously raising the polymerization temperature from the start of monomer addition to the end of monomer addition. At that time, the difference in polymerization temperature between the start of monomer addition and the end of monomer addition is preferably 3 ° C. or more, more preferably 5 ° C. to 30 ° C.

The paper coating composition containing the copolymer latex of the present invention uses kaolin clay and calcium carbonate as main pigments, but other pigments include, for example, talc, barium sulfate, titanium oxide, Inorganic pigments such as aluminum hydroxide, zinc oxide, and satin white, or organic pigments such as polystyrene latex can be used alone or in combination.
The content of the copolymer latex in the paper coating composition is preferably 3 to 20 parts by weight (solid content) with respect to 100 parts by weight (solid content) of the pigment. If the content of the copolymer latex is 3 parts by weight or less, the pigment cannot be sufficiently adhered, which is not preferable, and if it exceeds 20 parts by weight, the air permeability of the coated paper is lowered, which is not preferable.

  If necessary, modified starches such as starch, oxidized starch and esterified starch, natural binders such as soybean protein and casein, or water-soluble synthetic binders such as polyvinyl alcohol and carboxymethyl cellulose may be used. Further, synthetic latex such as polyvinyl acetate latex and acrylic latex may be used in combination with the copolymer latex of the present invention.

  In preparing a paper coating composition using the copolymer latex of the present invention, other auxiliary agents such as a dispersant (sodium pyrophosphate, sodium polyacrylate, sodium hexametaphosphate, etc.), antifoaming Agent (polyglycol, fatty acid ester, phosphate ester, silicone oil, etc.), leveling agent (funnel oil, dicyandiamide, urea, etc.), preservative, mold release agent (calcium stearate, paraffin emulsion, etc.), fluorescent dye, color water retention An improver (such as sodium alginate) may be added as necessary.

  Further, as a method for applying the paper coating composition to the coated paper, any known technique such as an air knife coater, blade coater, roll coater, bar coater, curtain coater or the like can be used. There is no problem. Also, after coating, the surface is dried and finished by calendaring or the like.

  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.

A latex film is prepared in an atmosphere at a measurement temperature of 40 ° C. and a humidity of 85% for the gel content of the copolymer latex . Thereafter, about 1 g of the latex film is weighed to obtain Xg. This is put into 400 ml of toluene and swelled and dissolved for 48 hours. Then, this is filtered through a weighed 300-mesh stainless steel wire net, and then toluene is evaporated to dryness. The mesh weight is subtracted from the weight after drying, and the weight after drying of the sample is weighed to obtain Yg.
Gel content (%) = Y / X * 100

Measurement of Tg of Copolymer Latex Copolymer latex was cast on a glass plate and dried at 70 ° C. for 4 hours to prepare a film. The film was packed in an aluminum pan, and a differential scanning calorimeter (DSC6200: Seiko Instruments Inc.). Set it to). The apparatus is cooled to a temperature about 50 ° C. lower than the expected Tg in advance, and then heated at a heating rate of 10 ° C./min to draw a DSC curve.
The glass transition temperature in the present invention refers to (a) the peak top temperature when there is one peak in the differential curve of the DSC measurement chart, and (b) the area of the peak when there are two or more peaks. The highest peak top temperature, (c) If the shape has a peak top but the shape is broad, the temperature of the peak top is (d) the broad (trapezoidal) shape without the peak top. In the case of (2), the temperature of the central portion is expressed as follows: (e) When there are two or more peaks, and the shape of the largest portion of the area corresponds to (c) and (d), It refers to the temperature obtained based on the criteria of c) and (d).

Measurement of high-speed tensile test of copolymer latex The copolymer latex is allowed to stand at room temperature for 48 hours and dried to prepare a latex film having a thickness of 0.4 mm to 0.6 mm. Further, the obtained film is allowed to stand in an oven at 130 ° C. for 15 minutes, and then is cut into a dumbbell shape shown in FIG. 1 and stored in a desiccator.
Using a Shimadzu high-speed impact tester Hydroshot HITS-T10, the stress and displacement were measured at a tensile speed of 15 m / s.
In the test, measurement is performed immediately after the film is removed from the desiccator so that the moisture content of the film does not exceed 2%.
* Sample grip: Grip for polymer film * Specimen shape: Parallel part width 15mm dumbbell mold (die punching process) (see Fig. 1)
* Distance between chucks: 30 mm
* Test force capacity: 10kN
* Test temperature: Room temperature * Elongation rate (%) = Displacement / Distance between chucks (30 mm) × 100

Synthetic pressure resistant polymerization reactors of copolymer latex (A, B, G, H) were charged with 140 parts of polymerized water and 1.0 potassium persulfate. The respective monomers and other compounds shown in the eye were added, and polymerization was started at the temperature at the start of monomer addition shown in Tables 1 and 2. When the polymerization conversion rate in the first stage exceeded 60%, each monomer and other compounds in the second stage were then continuously added at the addition time shown in Table 1. The polymerization was continued as it was, and the polymerization was terminated when the final conversion rate exceeded 95%. In the second stage, G and H having a difference between the temperature at the start of addition of the monomers described in Table 1 and Table 2 and the temperature at the end of addition are continuously from the start to the end of continuous addition. The polymerization temperature was changed.
Next, the pH of the obtained copolymer latex is adjusted to 8 using sodium hydroxide, unreacted monomers and other low-boiling compounds are removed by steam distillation, and copolymer latex A, B, G and H were obtained.

Synthesis of copolymer latex (C, I) A polymerization reactor made of pressure-resistant polymer was charged with 140 parts of polymerized water and 1.0 potassium persulfate and stirred sufficiently. When the temperature at the start of addition was reached, continuous addition of each monomer and other compound in the first stage was started. The first stage is continuously added at the first stage addition time described in Table 1 and Table 2, and then each monomer and other compounds in the second stage are added to the second stage described in Table 1 and Table 2. At the time of continuous feeding The polymerization was continued as it was, and the polymerization was terminated when the final conversion rate exceeded 95%. Here, in the latex C, the polymerization temperature was continuously increased from the start of the second stage addition to the end, and for the latex I, the polymerization temperature was continuously increased from the start of the first stage addition to the end of the second stage addition. Changed.
Next, the pH of the obtained copolymer latex is adjusted to 8 using sodium hydroxide, unreacted monomers and other low-boiling compounds are removed by steam distillation, and copolymer latexes C and I are obtained. Obtained.

Synthetic pressure resistant polymerization reactors for copolymer latex (D, J, K) were charged with 140 parts of polymerized water and 1.0 part of potassium persulfate and stirred sufficiently. When the monomer addition start temperature was reached, the addition of each monomer and other compounds shown in Tables 1 and 2 was started. The addition was completed at the addition times shown in Tables 1 and 2. The polymerization was continued as it was, and the polymerization was terminated when the final polymerization conversion rate exceeded 95%. The polymerization temperature was continuously changed from the start to the end of the monomer addition so as to reach the temperature at the end of monomer addition shown in Tables 1 and 2.
Subsequently, the pH of the obtained copolymer latex is adjusted to 8 using sodium hydroxide, unreacted monomers and other low-boiling compounds are removed by steam distillation, and the copolymer latex D, J, K was obtained.

Synthesis of copolymer latex (E) 140 parts of polymerization water and potassium persulfate 1.0 were charged into a polymerization pressure-resistant polymerization reactor, and after stirring sufficiently, when the temperature reached 69 ° C., one step shown in Table 1 The addition of each monomer of the eye and other compounds was started and added in 35 minutes. Next, each monomer in the second stage shown in Table 1 and other compounds were continuously added over 120 minutes. Next, the respective monomers in the third stage shown in Table 1 were continuously added in 95 minutes, and the polymerization was continued thereafter, and the polymerization was terminated when the final polymerization conversion rate exceeded 95%. At this time, the polymerization temperature was continuously changed from the start of the second stage of addition (69 ° C.) to 73 ° C. at the end of the third stage of addition.
Subsequently, pH of the obtained copolymer latex was adjusted to 8 using sodium hydroxide, and unreacted monomers and other low-boiling compounds were removed by steam distillation to obtain copolymer latex E. .

Synthesis of copolymer latex (F) Into a polymerization reactor made of pressure-resistant polymer, 140 parts of polymerization water and 1.0 potassium persulfate were charged and stirred sufficiently, and then each monomer and other compounds shown in Table 1 were added. Polymerization was started at 67 ° C. When the polymerization conversion rate in the first stage exceeded 60%, the addition of each monomer and other compounds in the second stage shown in Table 1 was started and continued over 185 minutes. Next, each monomer in the third stage shown in Table 1 and other compounds were continuously added over 135 minutes. Next, each monomer in the fourth stage shown in Table 1 and other compounds were continuously added over 10 minutes. Thereafter, the polymerization was continued, and the polymerization was terminated when the final polymerization conversion rate exceeded 95%. Here, in the third stage, the addition was started at 67 ° C., and then the polymerization temperature was continuously changed so as to reach 80 ° C. at the end of the fourth stage of addition.
Subsequently, pH of the obtained copolymer latex was adjusted to 8 using sodium hydroxide, and unreacted monomers and other low-boiling compounds were removed by steam distillation to obtain copolymer latex F. .

Preparation and Evaluation of Paper Coating Composition According to the formulation shown below, copolymer latexes A to F and G to K were used and adjusted to pH 9.5 with NaOH to prepare a paper coating composition. .
(Formulation formulation of paper coating composition)
Formulated kaolin 30 parts Heavy calcium carbonate 70 parts Modified starch 2 parts Copolymer latex 7 parts ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・
Solid content 65%

Preparation and Evaluation of Coated Paper Coated paper (basis weight 55 g / m ² ) is coated with the above-mentioned composition for paper coating using a wire bar so that the coating amount per side is 12 g / m ^2. After being worked and dried, a calendar treatment was performed under conditions of a linear pressure of 60 kg / cm and a temperature of 50 ° C. to obtain a coated paper. The obtained coated paper was subjected to an actual machine printing test and evaluated on the blanket after printing 5000 sheets, and the results are shown in Tables 1 and 2.
A: No occurrence of piling is observed.
○: Slight edge piling is observed.
Δ: Edge piling is conspicuous
X: Piling is observed on the entire blanket.

  As shown in Table 1, all of the copolymer latexes A to F according to the present invention are excellent in elongation and stress in the high-speed tensile test. Moreover, when coated paper was prepared using these latexes and printing durability was confirmed by actual printing, the occurrence of piling was hardly observed and good printing durability was exhibited.

In Comparative Example 1, since the gel is less than 75%, the elongation of the latex film is excellent, but the stress is inferior, and the printing durability of the coated paper is inferior.
In Comparative Example 2, since the amount of the aliphatic conjugated diene monomer is small and the glass transition temperature of the latex is high, the elongation of the latex film is inferior and the printing durability of the coated paper is inferior.
In Comparative Example 3, since the amount of the ethylenically unsaturated carboxylic acid monomer is small, the elongation of the latex film is excellent, but the stress is inferior. For this reason, the printing durability of the coated paper is inferior.
In Comparative Examples 4 and 5, the stress at 100% elongation in the high-speed tensile test is outside the specified range of the present invention, and the printing durability of the coated paper is inferior.

  As described above, the copolymer latex of the present invention has a certain elongation and stress when the latex film is pulled at a high speed, and thus has less printing and less printing durability in actual offset printing. Since a coated paper can be obtained, it is particularly useful as a binder in a paper coating composition.

Claims (2)

  35 to 65% by weight of aliphatic conjugated diene monomer, 5 to 35% by weight of vinyl cyanide monomer, 2.5 to 10% by weight of ethylenically unsaturated carboxylic acid monomer, and copolymerizable therewith It is obtained by emulsion polymerization of 0 to 57.5% by weight of other monomers (monomer total 100% by weight), glass transition temperature (Tg) is + 10 ° C. or lower, and gel content is 75% to 100%. It is a certain copolymer latex, and the elongation of the film under a high-speed tensile test condition at 15 m / s of the film prepared using the copolymer latex is 100% or more, and the stress of the film when the elongation is 100% is 12 MPa or more. A copolymer latex characterized by being.   A composition for paper coating containing the copolymer latex according to claim 1.