JP2574186B2 - Composition for paper coating - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a paper coating composition. More specifically,
The present invention relates to a paper coating composition which is excellent in coating operability and provides coated paper having excellent printability such as coated paper surface smoothness and coated paper surface strength.

BACKGROUND OF THE INVENTION Coating paper and paperboard with an aqueous pigment composition provides various advantages, the main one being commercial value by improving the optical properties of gloss, whiteness and opacity. And improving printability by improving surface smoothness and ink acceptability.

In the conventional aqueous pigment composition, clay, calcium carbonate, satin white, titanium oxide and the like as a pigment,
A styrene-butadiene copolymer latex and a water-soluble natural polymer such as starch, casein, carboxymethyl cellulose and the like were used in combination as a binder. However, since these water-soluble natural polymers are rigid polymers, there is a problem that it is difficult to improve the surface smoothness of coated paper.

Therefore, in recent years, studies have been made to impart paint viscosity by using an alkali-thickened styrene-butadiene copolymer latex instead of these rigid water-soluble natural polymers.

[Problems to be Solved by the Invention] However, when an alkali-soluble latex is used in place of natural polymers such as starch, kazenin, carboxymethylcellulose, etc. to impart viscosity of the paint, the water retention of the paint is improved. However, the viscosity of the paint greatly changed with the change in pH, causing a problem in workability. Further, when the alkali swelling copolymer latex was used, a sufficient coating viscosity was not obtained, and it was difficult to control a predetermined coating amount. Therefore, without causing these problems,
There has been a demand for a method capable of imparting viscosity to a paint.

The present inventors have conducted intensive studies to achieve the above object, and as a result of using as a binder an alkali-thickened latex mixed with a specific ratio of a latex having a specific composition as a binder, the present invention depends on the paint pH. Without this, it was found that a stable coating aging was obtained, and that the coating workability was remarkably improved, and the present invention was completed based on this finding.

[Means for Solving the Problems] Thus, according to the present invention, in a paper coating composition containing an aqueous dispersion containing a binder as a main component and a pigment, the aqueous dispersion comprises a solid content weight ratio And a mixture comprising 10 to 60% of an alkali-insoluble copolymer latex (A), 30 to 88% of an alkali swellable copolymer latex (B) and 2 to 10% of an alkali-soluble copolymer latex (C). A latex (A) containing an ethylenically unsaturated carboxylic acid monomer; 0.5 to 3.5% by weight of an aliphatic conjugated diolefin monomer; 20 to 60% by weight of an aromatic vinyl monomer. 20 to 60% by weight of other monomeric monomer; 0 to 4% by weight of emulsion-polymerized monomer mixture, wherein the alkali-swellable copolymer latex (B) is ethylene-free. Saturated carboxylic acid monomer; 3.5-10 weight % Aliphatic conjugated diolefin monomer; 20-60% by weight Aromatic vinyl monomer; 20-60% by weight Other monoolefin monomer; Emulsifies a monomer mixture consisting of 0-40% by weight The alkali-soluble copolymer latex (C) is obtained by polymerization, and the ethylenically unsaturated carboxylic acid monomer; 5 to 30% by weight aromatic vinyl monomer; 0 to 50% by weight other monoolefin monomer The present invention provides a paper coating composition obtained by emulsion-polymerizing a monomer mixture composed of 20 to 80% by weight.

 Hereinafter, the present invention will be described in detail.

(Alkali-insoluble copolymer latex (A)) The alkali-insoluble copolymer latex (A) used in the present invention is a latex that does not dissolve or swell with alkali. That is, the absorbance change rate defined by the following equation is
Latex of 80% or more and 140% or less. The particle size of the alkali-insoluble copolymer latex (A) does not change with pH, and the change in absorbance with pH is small.

(Note that the absorbance is 0.03% for latex concentration, wavelength
Measure at 500 nm. The ethylenically unsaturated carboxylic acid monomer used for the synthesis of the alkali-insoluble copolymer latex (A) includes:
Monocarboxylic acids such as acrylic acid and methacrylic acid layers; polycarboxylic acids such as fumaric acid, maleic acid and itaconic acid; and polycarboxylic acid partial esters such as monoethyl maleate can be exemplified. Two or more of these ethylenically unsaturated carboxylic acid monomers may be used in combination. The ethylenically unsaturated carboxylic acid monomer is used in an amount of 0.5 to 3.5% by weight in the monomer mixture. If it is less than 0.5% by weight, the adhesive strength and the mechanical stability of the latex decrease, while if it exceeds 3.5% by weight, the viscosity of the latex becomes too high.

Examples of the aliphatic conjugated diolefin monomer include 1,3-
Butadiene, 2-methyl-1,3-butadiene, and 2-
Chloro-1,3-butadiene and the like. These proportions are in the range from 20 to 60% by weight in the monomer mixture.
If it is less than 20% by weight, the copolymer becomes too hard, and the pigment bonding strength is reduced. If it exceeds 60% by weight, the water resistance will decrease. The preferred range is 30-55% by weight.

Specific examples of the aromatic vinyl monomer include styrene, α
-Methylstyrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, 2,4-diisopropylstyrene, 2,4-dimethylstyrene, 4-t-butylstyrene, 5-t-butyl-2-methylstyrene And monochlorostyrene, dichlorostyrene, monofluorostyrene, hydroxymethylstyrene and the like, and styrene is particularly preferred. The proportion of the aromatic vinyl monomer used is in the range of 20 to 60% by weight, preferably 25 to 55% by weight. If it is less than 20% by weight, the water resistance decreases, and if it exceeds 60% by weight, the pigment bonding strength decreases.

For the synthesis of the alkali-insoluble copolymer latex (A), various monoolefin monomers copolymerizable with the above monomers can be used. Examples thereof include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, hydroxyethyl acrylate, and the like of acrylic acid or methacrylic acid such as hydroxyethyl methacrylate. Esters; Ethylene nitrile compounds such as acrylonitrile and methacrylonitrile; unsaturated amide compounds such as acrylamide, methacrylamide, N-methylolacrylamide and diacetone acrylamide; vinyl monomers such as acrolein, vinyl chloride and vinyl acetate. Can be These can be used in a range that does not impair the properties of the finally obtained copolymer latex.
It is in the range of 40% by weight. .

(Alkali-swellable copolymer latex (B)) The alkali-swellable copolymer latex (B) used in the present invention is a latex in which particles swell and enlarge due to alkali, and the absorbance change rate is 140%. Is more than

For the synthesis of the alkali-swellable copolymer latex (B), the same ethylenically unsaturated carboxylic acid monomer and aliphatic conjugated diolefin as those used for the synthesis of the alkali-insoluble copolymer latex (A) are used. Body and aromatic vinyl monomers can be used.

The amount of the ethylenically unsaturated carboxylic acid monomer used is 3.5 to 10% by weight in the monomer mixture. If the amount is less than 3.5% by weight, the mechanical stability of the latex decreases, and a sufficient paint viscosity cannot be obtained due to insufficient swelling of the latex due to alkali. On the other hand, if it exceeds 10% by weight, the water resistance decreases.

The amount of the aliphatic conjugated diolefin monomer used is 20 to 60% by weight in the monomer mixture. If it is less than 20% by weight, the copolymer becomes too hard, and the pigment bonding strength is reduced. On the other hand, if it exceeds 6% by weight, the water resistance is reduced. Preferably it is in the range of 30 to 50% by weight.

The amount of the aromatic vinyl monomer used is 20 to 60 in the monomer mixture.
%, Preferably in the range of 25-55% by weight. If it is less than 20% by weight, the water resistance decreases, and if it exceeds 60% by weight, the pigment binding strength decreases.

In the synthesis of the alkali swellable copolymer latex (B), various monoolefin monomers copolymerizable with the above monomers can be used. Examples thereof include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, hydroxyethyl acrylate, and the like of acrylic acid or methacrylic acid such as hydroxyethyl methacrylate. Esters; Ethylene nitrile compounds such as acrylonitrile and methacrylonitrile; unsaturated amide compounds such as acrylamide, methacrylamide, N-methylolacrylamide and diacetone acrylamide; vinyl monomers such as acrolein, vinyl chloride and vinyl acetate. Can be These monomers can be used in a range that does not impair the properties of the finally obtained copolymer latex, but are usually in the range of 0 to 40% by weight in the monomer mixture.

(Alkali-soluble copolymer latex (C)) The alkali-soluble copolymer latex (C) used in the present invention is a latex in which particles are dissolved by alkali. That is, the rate of change in absorbance is less than 80%.

For the synthesis of the alkali-soluble copolymer latex (C), the same ethylenically unsaturated carboxylic acid monomer and aromatic vinyl monomer as those used for the synthesis of the alkali-insoluble copolymer latex (A) are used. can do.

The amount of the ethylenically unsaturated carboxylic acid monomer used is 5 to 30% by weight, preferably 10 to 25% by weight in the monomer mixture. If it is less than 5% by weight, the latex will not be soluble in alkali, and if it exceeds 30% by weight, the polymerization stability during latex production will be reduced.

The amount of the aromatic vinyl monomer used is 0 to 50 in the monomer mixture.
% By weight. If it exceeds 50% by weight, the pigment bonding strength will be reduced.

For the synthesis of the alkali-soluble copolymer latex (C), various monoolefin monomers copolymerizable with the above monomers can be used. Examples thereof include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, hydroxyethyl acrylate, and the like of acrylic acid or methacrylic acid such as hydroxyethyl methacrylate. Esters; Ethylene nitrile compounds such as acrylonitrile and methacrylonitrile; unsaturated amide compounds such as acrylamide, methacrylamide, N-methylolacrylamide and diacetone acrylamide; vinyl monomers such as acrolein, vinyl chloride and vinyl acetate. Can be These monomers can be used in a range that does not impair the properties of the finally obtained copolymer latex, but are usually in the range of 20 to 70% by weight in the monomer mixture.

(Method for Producing Each Latex) Each copolymer latex used in the present invention can be obtained by emulsion polymerization of the monomer mixture according to a known method. Incidentally, the alkali swellable copolymer latex is prepared, for example, by a method of polymerizing a component having a high content of ethylenically unsaturated carboxylic acid monomer in the latter half of the polymerization (Japanese Patent Application Laid-Open No.
No. 15498), a method of polymerizing a component having a high content of an ethylenically unsaturated carboxylic acid monomer in the first half of the polymerization (JP-A-60-1985).
162897), but the latter method is preferred from the viewpoint of the pH dependence of the viscosity of the paint.

The polymerization initiator is not particularly limited, but a persulfate, a peroxide, an azo compound, a redox catalyst, or the like can be used. Preferably, water-soluble persulfates such as sodium persulfate, potassium persulfate and ammonium persulfate are used alone or in a redox system.

As the emulsifier, an anionic emulsifier and / or a nonionic emulsifier is used, but is not limited thereto.

In order to adjust the molecular weight of the copolymer, a chain transfer agent, for example, mercaptans such as n-dodecyl mercaptan; carbon tetrachloride; thioglycolic acid; diterpenes, terpinolene, and γ-terpinenes can be used.

(Aqueous Dispersion) The aqueous dispersion used in the present invention comprises the alkali-insoluble copolymer latex (A): 10 to 60%, the alkali-swellable copolymer latex (B): 30 to 80%, and the alkali-soluble copolymer. Polymer latex (C): A mixed latex containing 2 to 10% by weight of solid content.

In this mixed latex, if the mixing ratio of the alkali-insoluble copolymer latex (A) is less than 10%, the surface strength of the coated paper becomes too low, and if it exceeds 60%, the alkali thickening is low and there is a problem in coating suitability. Occurs. On the other hand, if the mixing ratio of the alkali swellable copolymer latex (B) is less than 30%, the change in paint viscosity due to a change in paint pH is too large, and if it exceeds 88%, the coated paper surface strength is too low.
Further, if the mixing ratio of the alkali-soluble copolymer latex (C) is less than 2%, sufficient water retention of the paint cannot be obtained,
If it exceeds 10%, the coated paper surface strength will be too low.

The method of mixing the alkali-insoluble copolymer latex (A), alkali-swellable copolymer latex (B) and alkali-soluble copolymer latex (C) is not particularly limited.

If necessary, a water-proofing agent, a dispersant, an antifoaming agent, and the like can be added to the aqueous dispersion used in the present invention.

(Pigment) In the present invention, mineral pigments generally used for paper coating can be used. Specific examples thereof include clay, calcium carbonate, aluminum hydroxide, titanium oxide, barium sulfate, satin white, and talc. Further, organic pigments such as polystyrene and phenol resin can also be used.

(Paper Coating Composition) The paper coating composition of the present invention can be obtained by mixing the aqueous dispersion and a pigment by any method.

In the paper coating composition of the present invention, the amount of the water dispersion used is 5 to 30 parts by weight, preferably 5 to 25 parts by weight, based on 100 parts by weight of the pigment in terms of solid content. If the amount is less than 5 parts by weight, the surface strength of the coated paper decreases, and if it exceeds 30 parts by weight, the inking property is deteriorated.

In the paper coating composition of the present invention, if necessary, in addition to the pigment and the binder, various commonly used compounding agents, for example, a pigment dispersant, a pH adjuster, a waterproofing agent, a defoaming agent , A surfactant, a fluorescent dye, a lubricant, and the like.

Further, in the paper coating composition of the present invention, the use of water-soluble natural polymers such as starch and casein and carboxymethylcellulose, methylcellulose, sodium alginate and other synthetic thickeners are unnecessary, but if necessary. It is possible to use them together.

(Effects of the Invention) Thus, according to the present invention, a paper coating composition which provides a coated paper excellent in coating operability and excellent in printability such as coated paper surface smoothness and coated paper surface strength. Things can be provided.

(Example) Hereinafter, the present invention will be described in more detail with reference to examples.
The parts and percentages in the examples are on a weight basis unless otherwise specified. The amount of the latex used and the mixing ratio are in terms of solid content.

The composition of the paper coating composition in this example is as shown in Table 1.

The method for measuring each physical property in this example is as follows.

(1) Viscosity of latex and paper coating composition (cp) Measured at 20 ° C. using a BL type viscometer (60 rpm, No. 3 and No. 4 spindle).

(2) The latex and the composition for paper coating are measured at pH 25 ° C.

(3) Dry pick RI-type printing tester (manufactured by Akira Seisakusho) performs overprinting four times using offset ink, determines the degree of peeling with the naked eye, and uses the 5-point method (5: excellent → 1: poor).

(4) Using a wet topic RI type printing test machine (manufactured by Meisho Seisakusho), apply water to the test piece with a Molton roll, then perform solid printing using offset ink, and visually determine the degree of peeling. And evaluated according to the 5-point method (5: excellent → 1: poor).

(5) White paper gloss 75 degrees by gloss meter (Murakami Color Research Laboratory)
Measure the 75 degree reflectance.

(6) Gloss of printing Solid printing was performed using an offset ink with an RI type printing test machine (manufactured by Meiji Seisakusho), and the solid printing portion was measured with a gloss meter (manufactured by Murakami Color Research Laboratory).
Measure the 75-75 degree reflectance.

Weight Example 1 (Polymerization of alkali-insoluble copolymer latex) A tank equipped with a stirrer was charged with 200 parts of water, 1 part of sodium dodecylbenzenesulfonate and 0.1 part of potassium persulfate, and heated to 80 ° C. Then, in this, styrene 43
Parts, 20 parts of methyl methacrylate, 2 parts of methacrylic acid and 35 parts of butadiene were continuously added over 2 hours. After completion of the addition, the reaction was continued for 2 hours. As a result, the polymerization conversion was 99%. Latex A1 obtained
Had a particle size of 150 nm and a pH of 2.5. The rate of change in absorbance of this latex A1 was 105%. Similarly, an alkali-insoluble copolymer latex shown in Table 2 was obtained.

Polymerization Example 2 (Polymerization of alkali-swellable copolymer latex) Same as Polymerization Example 1 except that the composition of the monomer mixture was 40 parts of styrene, 20 parts of methyl methacrylate, 5 parts of methacrylic acid and 35 parts of butadiene. With a polymerization conversion of 99%,
Latex B1 having a particle size of 150 nm and a pH of 2.5 was obtained. The rate of change in absorbance of this latex B1 was 153%. Similarly, an alkali swellable copolymer latex shown in Table 3 was obtained.

Polymerization Example 3 (Polymerization of alkali-soluble copolymer latex) A tank equipped with a stirrer was charged with 200 parts of water, 1 part of sodium dodecylbendensulfonate and 0.1 part of potassium persulfate, and heated to 80 ° C. Then, in this, styrene 10
Parts, 50 parts of methyl methacrylate, 20 parts of vinyl acetate and 20 parts of acrylic acid were continuously added over 1 hour. After the completion of the addition, the reaction was continued for 1 hour. As a result, the polymerization conversion was 99%. The particle size of the obtained latex C1 was 150 nm, and the pH was 2.5. The rate of change in absorbance of this latex C1 was 20%. Similarly, an alkali-soluble copolymer latex shown in Table 4 was obtained.

Example 1 Latex A1, Latex B1, and Latex C1
The mixed latex was prepared by mixing at the ratios shown in the table. Using this mixed latex, a paper coating composition having a total solid content of 45% was prepared. The pH of this composition is adjusted with sodium hydroxide
Table 5 shows the viscosity behavior when adjusted to 7.5 to 13.0 and the physical properties of coated paper obtained using this composition.

From the results in Table 5, when the composition of the mixed latex is out of the range specified in the present invention, the viscosity of the composition does not increase (Experiment No. 4) or the viscosity change is too large (Experiment No. 5). Is extremely difficult, and in any case, sufficient physical properties of coated paper cannot be obtained. On the other hand, it can be seen that the composition for paper coating of the present invention provides good physical properties of coated paper.

Example 2 The same experiment as in Example 1 was performed, except that the alkali-insoluble copolymer latex having the composition shown in Table 2 was used. The results are shown in Table 6.

From the results in Table 6, it can be seen that, according to the present invention, excellent coated paper properties can be obtained, but when the copolymer composition of the alkali-insoluble copolymer latex is out of the range specified in the present invention, It can be seen that the physical properties of the paper are inferior.

Example 3 The same experiment as in Example 1 was performed except that the alkali swellable copolymer latex having the composition shown in Table 3 was used. The results are shown in Table 7.

From the results in Table 7, while excellent coated paper properties are obtained according to the present invention, when the copolymer composition of the alkali swellable copolymer latex is out of the range specified in the present invention, It can be seen that the physical properties of the coated paper are inferior.

Example 4 The same experiment as in Example 1 was performed, except that the alkali-soluble copolymer latex having the composition shown in Table 4 was used. The results are shown in Table 8.

From the results in Table 8, it can be seen that excellent coated paper properties can be obtained according to the present invention, whereas when the copolymer composition of the alkali-soluble copolymer latex is out of the range specified in the present invention, It can be seen that the physical properties of the paper are inferior.

Claims (2)

(57) [Claims] 1. A paper coating composition comprising an aqueous dispersion containing a binder as a main component and a pigment, wherein the aqueous dispersion has an alkali water-soluble copolymer having a solid content of 10 to 60% by weight. A mixed latex consisting of a combined latex (A), 30 to 88% of an alkali-swellable copolymer latex (B) and 2 to 10% of an alkali-soluble copolymer latex (C), wherein the alkali-insoluble copolymer is Latex (A) is ethylenically unsaturated carboxylic acid monomer; 0.5 to 3.5% by weight Aliphatic conjugated diolefin monomer; 20 to 60% by weight Aromatic vinyl monomer; 20 to 60% by weight Other monoolefin A monomer mixture composed of 0-4% by weight of the monomer; emulsion-polymerized, and the alkali-swellable copolymer latex (B) is an ethylenically unsaturated carboxylic acid monomer; % Aliphatic conjugated diolefin monomer; 20-60% by weight Emulsion polymerization of a monomer mixture consisting of an aromatic vinyl monomer; 20 to 60% by weight of another monoolefin monomer; 0 to 40% by weight; the alkali-soluble copolymer latex (C) Is a monomer mixture composed of an ethylenically unsaturated carboxylic acid monomer; 5 to 30% by weight of an aromatic vinyl monomer; 0 to 50% by weight of another monoolefin monomer; A paper coating composition obtained by emulsion polymerization. 2. A paper coating composition according to claim 1, which contains 100 parts by weight of a paper coating paper pigment and 5 to 30 parts by weight of an aqueous dispersion in terms of solid content. .