JPS6229558B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a paper coating composition comprising an aqueous dispersion of a binder and inorganic and organic pigments. More specifically, the present invention contains an organic pigment which is a polymer particle mainly composed of styrene, and has an intrinsic viscosity of 0.1 to 0.8 dl/g, and has excellent adhesive properties and white paper gloss. The present invention relates to a paper coating composition. Generally, the surface of the paper is coated with a coating composition consisting of an aqueous dispersion of a binder and a pigment to improve printability and improve optical performance. Mineral inorganic pigments are mainly used as pigments, but it has been proposed to replace some or all of these inorganic pigments with organic pigments. The use of polystyrene polymer latex as an organic pigment for coating paper is known from Japanese Patent Publication No. 46-6524. Coated paper using organic pigments as some or all of the pigments is lighter and whiter with excellent gloss than coated paper using only inorganic pigments. However, organic pigments have the disadvantage that they have lower adhesion to paper than inorganic pigments, and organic pigments have not been fully utilized despite their many advantages. In order to improve this drawback, it is possible to increase the amount of binder, but increasing the amount of binder often not only deteriorates the optical performance of the coated paper, but also reduces the air permeability of the paper, resulting in poor blister resistance. have a negative impact on It is also possible to soften the organic pigment itself so that it has adhesive properties, but if the organic pigment is made soft enough to have adhesive properties, it will form a film at room temperature and its performance as a pigment will deteriorate. The present inventors conducted intensive research to obtain an organic pigment that has features such as light weight, gloss, and whiteness, and also has excellent adhesive performance. The inventors have discovered that the above object can be achieved by reducing the intrinsic viscosity of the copolymer, and have arrived at the present invention. The present invention comprises (A) a binder, (B) an inorganic pigment, and (C) an organic pigment, namely, 60 to 99.5% by weight of styrene, 0 to 30% by weight of an alkyl (meth)acrylic acid ester, and 0.5 to 0.5% by weight of an unsaturated carboxylic acid. Intrinsic viscosity of a polymer obtained by copolymerizing 10% by weight with no film-forming properties at room temperature (measured at 25℃ after dissolving in tetrahydrofuran)
is 0.1-0.8dl/g, average particle size is 0.1-1.0
consisting of an aqueous dispersion of polymer particles of μ;
A paper coating composition characterized in that the composition ratio of (C) is 99.5-50/0.5-50 on a dry weight basis. Examples of unsaturated carboxylic acid monomers include methacrylic acid, acrylic acid, itaconic acid, maleic acid, and fumaric acid, and these monomers may be used alone or in combination of two or more. Particularly preferred are methacrylic acid and acrylic acid. As the (meth)acrylic acid alkyl ester, alkyl esters of acrylic acid or methacrylic acid such as ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, etc. are used. These monomers are used within 30% by weight of the total monomers. Among these, particularly preferred are ethyl acrylate, butyl acrylate, and methyl methacrylate, and when a small amount of these monomers is polymerized together with styrene, stability during polymerization is improved. Specifically, preferred combinations include styrene/methacrylic acid, styrene/methyl methacrylate/methacrylic acid, styrene/ethyl acrylate/methacrylic acid, styrene/butyl acrylate/methacrylic acid, and styrene/acrylic acid. The composition ratio of the monomers is preferably in the range of 90 to 99.5% by weight of monomers mainly composed of styrene (0 to 30% by weight of monomers other than styrene) and 0.5 to 10% by weight of unsaturated carboxylic acids. . If the amount of unsaturated carboxylic acid is more than 10% by weight, the polymerization stability will deteriorate, and if such an organic pigment is used, the water resistance of the coated paper will decrease. Moreover, at least polymerization stability becomes worse than 0.5% by weight. Among these compositions, particularly preferred are 73 to 98% by weight of styrene, 0 to 20% by weight of at least one monomer selected from ethyl acrylate, butyl acrylate, and methyl methacrylate, and methacrylic acid and/or methacrylic acid. Or 2 to 7% by weight of acrylic acid. To adjust the intrinsic viscosity, long-chain alkyl mercaptans such as tert-dodecyl mercaptan, n-dodecyl mercaptan, octyl mercaptan, and stearyl mercaptan, alkyl esters of thioglycolic acid such as 2-ethylhexyl thioglycolate,
Commonly used chain transfer agents such as halogenated hydrocarbons such as tetrachloromethane, trichloromethane, and trichlorobromomethane, or lower alkylxanthogens are used. Among these, particularly preferred are long-chain alkyl mercaptans such as tert-dodecyl mercaptan and n-dodecyl mercaptan. When using these long-chain alkyl mercaptans, the amount is approximately 0.1 parts by weight per 100 parts by weight of the monomer.
At least 0.5 to 2.0 parts by weight (not more than 0.5 to 2.0 parts by weight)
(abbreviated as PHM) is used. The polymer particles used in the present invention are prepared by adding a monomer mixture, a chain transfer agent, a polymerization initiator, and any necessary emulsifier to water and heating the mixture to approximately 50 to 100°C while stirring.
It is obtained by heating and polymerizing. However, particularly suitable is a method in which a predetermined monomer mixture mixed with a chain transfer agent is added intermittently or continuously to carry out the polymerization while maintaining the added water at a predetermined temperature. According to this polymerization method, stable polymerization can be performed with little generation of coagulated substances, and the amount of emulsifier may be relatively small. When an emulsifier is used, it may be added before adding the monomer, during or after the addition. However, if a large amount of emulsifier is used, the water resistance of the coated paper will deteriorate, so it is necessary to use the emulsifier in as little amount as possible. The emulsifier used in the production of polymer particles is 2 parts by weight or less per 100 parts by weight of the monomer,
The amount is preferably 1 part by weight or less. Examples of emulsifiers include anionic emulsifiers such as sodium dodecylbenzenesulfonate, sodium or ammonium salts of sulfuric esters of lauryl alcohol, sodium dodecyl diphenyl oxide disulfonate, and non-ionic emulsifiers such as polyoxyethylene lauryl ether and polyoxyethylene nonylphenol ether. Ionic emulsifiers and the like are preferably used. The polymerization initiator used in the production of polymer particles is not particularly limited, but persulfates such as potassium persulfate and ammonium persulfate are preferred. In addition, the pH of the aqueous dispersion of polymer particles can be adjusted with an alkali.
Adjusted to 6-11. Potassium hydroxide, sodium hydroxide, ammonia or a volatile organic amine is used as the alkali. When polymer particles are used as organic pigments, their average particle diameter is one of the important factors that influences the performance of coated paper. The average particle diameter of the polymer particles used in the present invention is
The thickness is in the range of 0.1 to 1.0μ, preferably 0.15 to 0.6μ, and particularly preferably 0.15 to 0.3μ. If the average particle size is smaller than 0.1μ, the white paper quality of the coated paper will deteriorate. Moreover, if it is larger than 1μ, the gloss of the coated paper may be reduced. It is also appropriate that the majority of the polymer particles of the present invention, for example at least 80%, have a particle size of 0.1 to 1 .mu.m. The intrinsic viscosity of the polymer particles, which is an essential component of the present invention, is 0.1 to 0.8 dl/g, preferably 0.1 to 0.5 dl/g.
It must be within the g range. Intrinsic viscosity is 0.1dl/
If it is smaller than g, the adhesive strength will be high, but the white gloss of the coated paper required as a pigment will be reduced. On the other hand, if the intrinsic viscosity is greater than 0.8 dl/g, the white gloss of the coated paper will be good, but the adhesive strength will be poor. Adhesive strength and paper gloss are particularly well-balanced and excellent when the intrinsic viscosity is in the range of 0.1 to 0.5 dl/g. Binders used in the paper coating composition of the present invention include starch, modified starch, soybean protein, natural binders such as casein, styrene-butadiene copolymer latex, and carboxy-modified styrene-based copolymer latex.
There are synthetic binders such as butadiene polymer latex, polyvinyl acetate latex, polychloroprene latex, and polyvinyl alcohol.
These may be used alone or in combination. Inorganic pigments include kaolin clay, talc, barium sulfate, titanium oxide (rutile, anatase), calcium carbonate, aluminum hydroxide,
Zinc oxide, sachin white, etc. are used. Other auxiliary agents, such as dispersants (sodium pyrophosphate, sodium hexametaphosphate, sodium tripolyphosphate, sodium polycarboxylate, etc.), antifoaming agents (polyglycol fatty acid esters,
Phosphate ester, silicone oil, etc.) Leveling agents (road oil, dicyandiamide, urea, etc.) Preservatives, water resistance agents (formalin, hexamine, melamine resin, urea resin, glyoxal,
Zirconium ammonium carbonate, etc.), mold release agents (calcium stearate, paraffin emulsion, etc.), fluorescent dyes, colors, water retention improvers (carboxymethylcellulose, sodium alginate, etc.), etc. are added as necessary. The mixing ratio of the inorganic pigment and the binder is usually 100:5 to 25 on a dry weight basis. The paper coating composition of the present invention contains an inorganic pigment of 0.5 to
Its characteristics are exhibited by replacing 50% by weight with the polymer particles (organic pigment). If the amount of polymer particles is less than 0.5% by weight, there is no benefit from using the polymer particles (benefits such as improved printability and optical properties of coated paper), and if more than 50% by weight, the printability is lost. Not only will it become inferior, but it will also be economically disadvantageous. The paper coating composition of the present invention can be applied to a paper substrate using known techniques such as an air knife coater,
This is done using a blade coater, roll coater, etc. After coating, the surface is dried and finished by calendering, etc. Next, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples unless the gist thereof is exceeded. In addition, in the present invention, the intrinsic viscosity of the polymer is determined by dissolving the dried polymer in tetrahydrofuran.
The concentration was determined to be 0.1 to 0.6% by weight and measured at 25°C using an Ubbelohde viscometer. Further, the particle diameter of the polymer was measured using an electron microscope, and was determined by number averaging. In addition, all parts and percentages showing proportions in the following examples are based on weight. Example 1 Production of polymer particles After stirring, a 1-separable 4-neck glass flask (reactor) equipped with a cooling tube and a thermometer was purged with nitrogen, and 170 parts of distilled water was added to raise the temperature to 80°C. . Next, 0.5 part of ammonium persulfate was dissolved in 5 parts of distilled water and charged. Subsequently, a monomer solution prepared by mixing 100 parts of the monomer mixture shown in Table 1 with a variable amount of tert-dodecyl mercaptan was continuously added to the reactor over a period of 4 hours. 90℃ after monomer addition
The temperature was raised to 1, and polymerization was continued for an additional 2 hours. The resulting emulsion was filtered through a 120-mesh stainless steel wire mesh, and the pH was adjusted to 8.0± with sodium hydroxide.
Adjusted to 0.2. The intrinsic viscosity was measured by drying the obtained emulsion and dissolving it in tetrahydrofuran at 25° C. according to a conventional method. The average particle diameter was determined using an electron microscope. The results are shown in Table 1.

[Table] Performance evaluation of polymer particles Performance evaluation of polymer particles was performed under the compounding conditions shown in Table 2. The solid content concentration of these blended compositions was adjusted to 60%, and coated with base paper for coating (basis weight 100 g/
m ² ) was coated at a weight of 19.0±1 g/m ² . Then, it was dried at 120°C for 10 seconds and calendered twice at 70°C and 40 kg/cm. Table 3 shows the results of testing according to the usual coated paper evaluation method.

【table】

【table】

【table】

[Table] These results show that when the intrinsic viscosity of the polymer particles is large (Samples A, F, O, P, and Q), the adhesive strength (RI, WP) is inferior, and when the intrinsic viscosity is too small (Sample E) It can be seen that although the adhesive strength is strong, the white paper gloss, which shows the characteristics of a pigment, deteriorates. Example 2 Using the polymer particles (a) Sample F (comparative example) and (b) Sample K of Example 1, the performance of coated paper was evaluated by changing the blending ratio of the polymer particles. The evaluation method was the same as in Example 1. The results are shown in Table 4.

[Table] From the results in Table 4, when the intrinsic viscosity of the polymer particles is large ((a) Sample F), the adhesive strength (RI, WP) decreases significantly as the blending ratio of the polymer particles increases. However, it can be seen that when the intrinsic viscosity is low ((b) Sample K), the decrease in adhesive strength is small. Example 3 Polymer particles of Example 1 (c) Sample Q (comparative example)
(d) Using Sample N, the performance of coated paper was evaluated by changing the blending ratio of polymer particles. The evaluation method was the same as in Example 1. The blending ratios are shown in Table 5, and the performance evaluation results of the coated paper are shown in Table 6.

【table】

【table】

[Table] From these results, when the intrinsic viscosity of the polymer particles is large ((c) Sample Q), the adhesive strength (especially water resistance strength) decreases significantly as the blending ratio of the polymer particles increases. , when the intrinsic viscosity is small ((d) sample N), the adhesive strength (especially water resistance strength)
It can be seen that there is little decrease in Comparative Example 1 Polymer particle samples R and S were produced according to Example 1, and their performance was evaluated using the formulations () and () in Table 2. Table 7 shows the properties of the polymer particles, and Table 8 shows the performance evaluation results of the coated paper.

【table】

[Table] This result shows that when the (meth)acrylic acid alkyl ester in the polymer particles is replaced with butadiene, the whiteness of the coated paper, especially the white paper gloss, is greatly reduced.

Claims (1)

[Scope of Claims] 1 (A) binder, (B) inorganic pigment, and (C) organic pigment, namely 60 to 99.5% by weight of styrene, 0 to 30% by weight of alkyl (meth)acrylic acid ester, and unsaturated carboxylic acid. A polymer obtained by copolymerizing 0.5 to 10% by weight, which does not form a film at room temperature and has an intrinsic viscosity (dissolved in tetrahydrofuran and measured at 25°C) of 0.1 to 0.8.
dl/g, consisting of an aqueous dispersion of polymer particles with an average particle diameter of 0.1 to 1.0μ, and the composition ratio of (B)/(C) above on dry weight is 99.5 to 50/0.5 to 50. A paper coating composition characterized by: 2. The composition according to claim 1, wherein the polymer particles have an intrinsic viscosity of 0.1 to 0.5 dl/g. 3. The composition according to claim 1 or 2, wherein the average particle diameter of the polymer particles is 0.15 to 0.3 μ. 4 Claims in which the polymer particles are obtained by polymerization by adding monomers and a chain transfer agent intermittently or continuously to an aqueous solution of a polymerization initiator in the presence or absence of an emulsifier. The composition according to item 1, item 2 or item 3. 5. The (meth)acrylic acid alkyl ester of the polymer particles is at least one selected from methyl methacrylate, ethyl acrylate, and butyl acrylate, and the unsaturated carboxylic acid is methacrylic acid and/or acrylic acid. Range 1
The composition according to item 2, item 3, or item 4.