JP2008231262A - Method for producing copolymer latex for paper coating and composition for paper coating containing the copolymer latex - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a copolymer latex having good stickiness which causes smearing during operation, and excellent adhesive strengths. <P>SOLUTION: The method for producing the copolymer latex for paper coating is, in a copolymer latex prepared by polymerizing an aliphatic conjugate diene, an ethylene-based unsaturated carboxylic acid monomer and another copolymerizable monomer, to perform polymerization by adding the monomer separately in ≥2 steps so that a theoretical Tg derived by the Fox equation from the Tg of a homopolymer of the monomer added in each step is in a range of -30 to 30°C in a copolymer composition of the first step, and adding the monomer to have ≥±10°C difference in a copolymer composition after the second step in comparison with the theoretical Tg of the proceeding step. The obtained latex has one glass transition temperature (Tg) of the prepared latex measured by a differential scanning calorimeter, and the difference between T1 and T2 in the endothermic reaction in a chart is <20°C. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a method for producing a paper coating copolymer latex and a paper coating composition comprising the paper coating copolymer latex.
Specifically, the present invention relates to a method for producing a copolymer latex for paper coating having an excellent balance between dry pick strength and operability.

  In recent years, coated paper has been used in a large number of printed materials because of its high printing effect, and coated paper has whiteness, gloss, smoothness, and printability compared to non-coated paper. However, since a process for coating a paper coating composition using a blade coater or a roll coater after making a base paper is necessary, As the speed increases, a high level of operability is required for a paper coating composition.

  In general, a composition for paper coating is 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 water-based 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 today as the most suitable binder for paper coating compositions. However, it greatly affects the adhesive strength such as the dry pick strength which is the performance of the paper coating composition or the final coated paper product.

  However, in general, increasing the adhesive strength deteriorates the stickiness of latex, which is one of the indicators of operability, and the strength and operability are contradictory elements.

  Therefore, in order to achieve both strength and operability for the quality design and manufacturing method of styrene-butadiene copolymer latex for paper coating, various improved technologies such as core / shell technology have been introduced. For example, in Japanese Patent Application Laid-Open No. 11-350389 (Patent Document 1), a paper having excellent adhesion strength and blocking resistance is obtained by using a copolymer latex polymerized by specifying a ratio of butadiene added in the first half of polymerization and the second half of polymerization. There is an introduction that a composition for coating can be obtained.

However, these various improvement techniques have not yet reached a level that sufficiently satisfies the performance required for the copolymer latex for paper coating, and further improvement has been strongly demanded. In particular, in a heterogeneous structure where the outer side of the latex particles is hard and the inner side is soft, the outer hard layer (also referred to as the shell part) provides blocking resistance, and the inner soft layer (also referred to as the core part) provides strength. The balance between strength and operability is superior to particles with a uniform composition structure, but there are layers with hard components in the particles, so sufficient performance in terms of strength is obtained. There is a tendency that is not possible.
Japanese Patent Laid-Open No. 11-350389 JP 2005-139309 A

  An object of the present invention is to provide a copolymer latex for paper coating that has good stickiness that causes operational stains and has excellent adhesion.

  That is, the present invention relates to an aliphatic conjugated diene monomer 20 to 65% by weight, an ethylenically unsaturated carboxylic acid monomer 0.5 to 10% by weight and other monomers 25 to 79 copolymerizable therewith. In copolymer latex obtained by polymerizing 5% by weight (total monomer 100% by weight), when performing multistage polymerization in which monomers are added in two or more stages, a single amount added in each stage The theoretical glass transition temperature (hereinafter referred to as the theoretical Tg) derived from the Tg of the homopolymer of the body by the Fox equation is in the range of −30 to 30 ° C. in the first-stage copolymer composition, The copolymer composition of the second and subsequent stages is polymerized by adding a difference of ± 10 ° C. or more compared to the theoretical Tg of the previous stage, and the glass transition by the differential scanning calorimeter of the actually obtained latex The temperature (Tg) is one and the indication There is provided a method for producing a paper coating copolymer latex, wherein the difference between the start point T1 and end T2 of the endothermic reaction is less than 20 ° C. in the chart obtained from investigation calorimeter.

  The present invention has an effect that, by adopting the above-described configuration, a copolymer latex for paper coating having good stickiness that causes operational stains and excellent adhesion can be obtained. .

The present invention is described in detail below.
Examples of the aliphatic conjugated diene monomer in the present invention include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 2-chloro-1,3-butadiene. Examples include butadiene, substituted linear conjugated pentadienes, substituted and side chain conjugated hexadienes, and the like, and one or more of them can be used. In particular, the use of 1,3-butadiene is preferred.

  Examples of the ethylenically unsaturated carboxylic acid monomer include mono- or dicarboxylic acids (anhydrides) such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, and itaconic acid. It can be used above.

  Other monomers copolymerizable with the above aliphatic conjugated diene monomer and ethylenically unsaturated carboxylic acid monomer include alkenyl aromatic monomers, unsaturated carboxylic acid alkyl ester monomers, hydroxy Examples thereof include unsaturated monomers containing an alkyl group, vinyl cyanide monomers, 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 malate, dimethyl itaconate, Examples thereof include monomethyl fumarate, monoethyl fumarate, 2-ethylhexyl acrylate, and the like can be used alone or in combination of two or more. 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 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 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.

  The monomer composition is composed of 20 to 65% by weight of aliphatic conjugated diene monomer, 0.5 to 10% by weight of ethylenically unsaturated carboxylic acid monomer, and other monomers copolymerizable therewith. 25 to 79.5% by weight (however, the total monomer is 100% by weight)

  If the aliphatic conjugated diene monomer is less than 20% by weight, the adhesiveness such as dry pick strength required at the time of printing, and if it exceeds 65% by weight, the wet adhesiveness such as the wet pick strength required at the time of printing. Is inferior and is not preferred. More preferably, it is 30 to 60% by weight.

  When the amount of the ethylenically unsaturated carboxylic acid monomer is less than 0.5% by weight, the mechanical stability of the copolymer latex itself and the paper coating composition is inferior, and when it exceeds 10% by weight, the viscosity of the latex increases. This is not preferable because it may cause a problem in handling of the copolymer latex itself. More preferably, it is 1 to 7% by weight.

  If the other copolymerizable monomer is less than 25% by weight, the above-mentioned wet adhesiveness is not preferable, and if it exceeds 79.5% by weight, the adhesiveness such as dry pick strength is inferior. More preferably, it is 33 to 69% by weight.

  In the present invention, 0.1 to 4 parts of α-methylstyrene dimer and 1 to 40 parts of unsaturated cyclic hydrocarbon having one unsaturated bond in the ring are used in combination for polymerization of the monomer. Emulsion polymerization is preferable for obtaining a copolymer latex for paper coating which is an object of the present invention.

  The above α-methylstyrene dimer includes 2,4-diphenyl-4-methyl-1-pentene, 2,4-diphenyl-4-methyl-2-pentene and 1,1,3-trimethyl-3 as isomers. There is -phenylindane, but the α-methylstyrene dimer used in the present invention has a content of 2,4-diphenyl-4-methyl-1-pentene of 60% by weight or more, particularly 80% by weight or more. Preferably there is.

  Examples of the unsaturated cyclic hydrocarbon having one unsaturated bond in the ring include cyclopentene, cyclohexene, cycloheptene, 4-methylcyclohexene, 1-methylcyclohexene and the like, and cyclohexene is particularly preferable. In addition, hydrocarbon compounds such as saturated hydrocarbons such as pentane, hexane, heptane, octane, cyclohexane and cycloheptane, unsaturated hydrocarbons such as pentene, hexene and heptene, and aromatic hydrocarbons such as benzene, toluene and xylene. May be used in combination.

  In the present invention, known molecular weight modifiers other than α-methylstyrene dimer, such as n-hexyl mercaptan, n-octyl mercaptan, t-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, n-stearyl mercaptan, etc. Xanthogen compounds such as alkyl mercaptan, dimethylxanthogen disulfide, diisopropylxanthogen disulfide, thiuram compounds such as terpinolene, tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetramethylthiuram monosulfide, 2,6-di-t-butyl- Phenol compounds such as 4-methylphenol and styrenated phenol, allyl compounds such as allyl alcohol, dichloromethane, dibromomethane, carbon tetrabromide, etc. Halogenated hydrocarbon compounds, vinyl ethers such as α-benzyloxystyrene, α-benzyloxyacrylonitrile, α-benzyloxyacrylamide, triphenylethane, pentaphenylethane, acrolein, methacrolein, thioglycolic acid, thiomalic acid, 2-ethylhexyl A thioglycolate etc. are mentioned, These can be used 1 type or 2 or more types.

Furthermore, in emulsion polymerization, conventional emulsifiers, polymerization initiators, hydrocarbon solvents, electrolytes, polymerization accelerators, chelating agents, and the like can be used.
As an emulsifier, anionic surfactants such as sulfate esters of higher alcohols, alkylbenzene sulfonates, alkyl diphenyl ether sulfonates, aliphatic sulfonates, aliphatic carboxylates, sulfate esters of nonionic surfactants, etc. Alternatively, nonionic surfactants such as an alkyl ester type, an alkyl phenyl ether type, and an alkyl ether type of polyethylene glycol can be used, and one or more of these can be used.

  As the polymerization initiator, water-soluble polymerization initiators such as potassium persulfate, sodium persulfate, and ammonium persulfate, redox polymerization initiators, and oil-soluble polymerization initiators such as benzoyl peroxide can be appropriately used. In particular, the use of a water-soluble polymerization initiator is preferred.

In the present invention, it is necessary to perform multi-stage polymerization of two or more stages in emulsion polymerization.
That is, a part of the predetermined monomer (first stage) is first polymerized, and then the second and subsequent monomers are sequentially polymerized.
Specifically, the monomer composition in the first stage has a theoretical glass transition temperature (theoretical Tg) derived from the Tg of the homopolymer of the monomer to be added by the Fox equation from −30 ° C. to + 30 ° C. And the second stage monomer composition should have a similar theoretical Tg of ± 10 ° C. or more compared to the first stage theoretical Tg. It is necessary that the glass transition temperature (Tg) by the differential search calorimeter is one and the difference between the end point T1 and the end point T2 of the endothermic reaction in the chart obtained from the differential search calorimeter is less than 20 ° C. is there. If the difference in theoretical Tg between the first stage and the second stage is less than 10 ° C., a latex having an excellent balance of adhesive strength and stickiness, which is an object of the present invention, cannot be obtained. Even if the glass transition temperature (Tg) measured by the differential scanning calorimeter is 1 ° C. or higher and the difference between T1 and T2 exceeds 20 ° C., the object of the present invention is It is not preferable because a latex having an excellent balance between adhesive strength and stickiness cannot be obtained.
The theory Tg based on the above Fox formula is described in, for example, Introduction to Polymer Science, Second Edition, page 172 (published in 1981).

  In addition, in order to obtain the target latex of the present invention, the addition of the second-stage monomer is performed at a stage where the polymerization conversion of the first-stage monomer is 5% by weight or more and less than 40% by weight. In addition, as described above, when polymerizing the above monomer, 0.1 to 4 parts of α-methylstyrene dimer and 1 to 40 unsaturated cyclic hydrocarbons having one unsaturated bond in the ring. It is preferable to carry out emulsion polymerization using a part together.

  Examples of the pigment used when preparing the paper coating composition include inorganic pigments such as kaolin clay, talc, barium sulfate, titanium oxide, calcium carbonate, aluminum hydroxide, zinc oxide, and satin white, or Examples thereof include organic pigments such as polystyrene latex, and these are used alone or in combination.

  If necessary, modified starch such as starch, oxidized starch and esterified starch, natural binder such as soybean protein and casein, or water-soluble synthetic binder such as polyvinyl alcohol may be used. In addition, synthetic latex such as polyvinyl acetate latex and acrylic latex may be used in combination with the copolymer latex of the present invention. It is desirable to keep it below 50% by weight (solid content) of the polymer latex.

  Furthermore, other auxiliary agents such as dispersants (sodium pyrophosphate, sodium polyacrylate, sodium hexametaphosphate, etc.), antifoaming agents (polyglycol, fatty acid ester, phosphate ester, silicone oil, etc.), leveling agents (funnel oil, Dicyandiamide, urea, etc.), preservatives, mold release agents (calcium stearate, paraffin emulsion, etc.), fluorescent dyes, color water retention agents (carboxymethylcellulose, sodium alginate, etc.) may be added as necessary.

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

〔Example〕
EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated 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 on a sticky PET film of a latex film using a wire bar # 10 and dried in a hot air circulation oven at 110 ° C. for 1 minute to obtain a film. No. on the film sample. After placing 5 filter papers and pressure bonding with a pressure type hot roll device, remove the filter paper and judge the condition of the filter paper adhering to the film with the naked eye. From ◎ (best) to x (worst) shown below Was relatively evaluated. The results are shown in Table 1.
(Excellent) ◎>○>△> × (poor)

Evaluation of dry pick strength of coated paper Judge the degree of picking when each coated paper sample is printed at the same time with an RI printer, and (excellent) ◎>○>△> × (poor) relatively evaluated.

Measurement of Tg, T1, and T2 of copolymer latex Copolymer latex was cast on a glass plate and dried at 70C for 4 hours to prepare a film. Set on a scanning calorimeter (DSC6200: manufactured by Seiko Instruments Inc.).
The apparatus is cooled in advance to a temperature about 50 ° C. lower than the expected Tg, and then heated at a heating rate of 10 ° C./min to draw a DSC curve (see FIG. 1).
<< Glass transition temperature (Tg) >>
The glass transition temperature (Tg) is a temperature at a point where a straight line equidistant from the extended straight line of each base line in the vertical axis direction and the curve of the step-change portion of the glass transition intersect.
<< T1 (glass transition start temperature), T2 (glass transition end temperature) >>
T1 (glass transition start temperature) is a temperature at the intersection of a straight line obtained by extending the base line on the low temperature side to the high temperature side and a tangent drawn so that the slope of the step-like change portion of the glass transition is maximized.
T2 (glass transition end temperature) is a temperature at the intersection of a straight line obtained by extending the base line on the high temperature side to the low temperature side and a tangent drawn so that the slope of the step-like change portion of the glass transition is maximized.

Measurement of gel content of copolymer latex A latex film is prepared at room temperature. Thereafter, about 1 g of the latex film is weighed and placed in 400 cc of toluene to be dissolved for 48 hours. Thereafter, this was filtered through a 300-mesh wire mesh, and the toluene insoluble portion captured by the wire mesh was dried and weighed, and the ratio of the weight to the weight of the first latex film was calculated as a gel content in wt%.

Measurement of particle diameter of copolymer latex The number average particle diameter of the copolymer latex was measured by a dynamic light scattering method. In the measurement, LPA-3000 / 3100 (manufactured by Otsuka Electronics) was used.

Preparation of copolymer latex In a pressure-resistant polymerization reactor, 150 parts of polymerization water, 1.0 part of sodium dodecylbenzenesulfonate as an emulsifier, 0.2 part of sodium carbonate and 1 part of potassium persulfate were charged and stirred sufficiently. After each monomer corresponding to the first stage shown in Table 1 and Table 2 was added to initiate polymerization, the polymerization conversion rate of the first stage monomer reached the values shown in Table 1 and Table 2. Polymerization was continued by adding the second-stage monomer, and the polymerization was terminated when the final polymerization conversion rate exceeded 96%.
Next, after adjusting the pH of these copolymer latexes to 7 with an aqueous caustic soda solution, unreacted monomers and other low-boiling compounds were removed by steam distillation to obtain copolymer latexes A to E.

Production and Evaluation of Paper Coating Composition A paper coating composition was produced using copolymer latexes A to D and copolymer latexes E to F in accordance with the formulation shown below. The evaluation results of each paper coating composition are shown in Tables 1 and 2.
(Formulation formulation of paper coating composition)
Kaolin clay 50 parts Heavy calcium carbonate 50 parts Modified starch 3 parts Copolymer latex 9 parts ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・
Solid content 65%

Preparation and Evaluation of Coated Paper Coated paper (basis weight 67 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 10 g / m ^2. After being worked and dried, a super calender treatment was performed under the conditions of a linear pressure of 70 kg / cm and a temperature of 50 ° C. to obtain a coated paper. The obtained coated paper was evaluated for adhesive strength, and the results are shown in Tables 1 and 2.

  As described above, by using the copolymer latex for paper coating obtained in the present invention, it has good stickiness that causes operational stains and can exhibit excellent adhesive strength. It can be suitably used for paper coating applications.

It is a DSC curve by a differential scanning calorimeter.

Explanation of symbols

T1: Glass transition start temperature T2: Glass transition end temperature Tg: Glass transition temperature

Claims (5)

  20 to 65% by weight of aliphatic conjugated diene monomer, 0.5 to 10% by weight of ethylenically unsaturated carboxylic acid monomer and 25 to 79.5% by weight of other monomers copolymerizable therewith ( In the copolymer latex obtained by polymerizing a total of 100% by weight of the monomers, when performing multi-stage polymerization in which the monomer is added in two or more stages, the homopolymer of the monomer added in each stage The theoretical glass transition temperature (hereinafter referred to as the theoretical Tg) derived from the Tg by the Fox equation is in the range of −30 to 30 ° C. in the first stage copolymer composition, and the second and subsequent stages. The copolymer composition is added and polymerized so as to have a difference of ± 10 ° C. or more compared with the theoretical Tg of the previous stage, and the glass transition temperature (Tg) of the actually obtained latex by the differential scanning calorimeter is One and the differential search calorimeter The method of manufacturing the paper coating copolymer latex, wherein the difference between the start point T1 and end T2 of the endothermic reaction in the resulting chart is less than 20 ° C..   The method for producing a copolymer latex for paper coating according to claim 1, wherein the polymerization of the second and subsequent stages is started after the polymerization conversion in the first stage of polymerization is 5% by weight or more and less than 40% by weight.   The method for producing a copolymer latex for paper coating according to claim 1 or 2, wherein a vinyl cyanide monomer is included as a monomer to be added in each stage.   4. Emulsion polymerization using 0.1 to 4 parts of α-methylstyrene dimer and 1 to 40 parts of unsaturated cyclic hydrocarbon having one unsaturated bond in the ring. Of Copolymer Latex for Paper Coating as described in   A paper coating composition comprising the copolymer latex for paper coating according to any one of claims 1 to 4.

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