GB2028832A - Aqueous coating color compositions - Google Patents

Abstract

An aqueous coating color composition comprises pigment, dispersant and binder, 5 to 95% by weight of the pigment being uniform cubic particles of calcium carbonate 0.1 to 1.0 mu m in average size. This composition is applied to a coating base to produce a pigment-coated paper or board having good printability, whiteness, gloss and opacity.

Description

SPECIFICATION Aqueous coating color compositions This invention relates to improvements in aqueous coating color compositions comprising pigments, a dispersant and a binder, and more particularly to aqueous coating color compositions useful for preparing coated papers and coated boards which are outstanding in printability, whiteness, gloss and opacity.

Coated papers and boards are prepared by coating bases with an aqueous dispersion containing a pigment, dispersant, binder and other additives. The pigments which have heretofore been mainly used include clays such as kaolin and calcium carbonate which is usually used in combination with clays.

Other pigments which are used conjointly with clays or with the combination of a clay and calcium carbonate include satin white for giving especially high gloss as required of converted papers, and titanium dioxide for affording opacity. Calcium carbonate particles heretofore used for coating papers are those of non-cubic shape exceeding 0.1 ssm in average size, and fine cubic particles less than 0.1 ym in average size. Such calcium carbonate particles, however, have various drawbacks.Non-cubic calcium carbonate particles larger than 0.1 ym in average size are substantially lower in gloss, whiteness and opacity than those less than 0.1 ssm in average size and are therefore used merely for forming the under coat of multi-layer coatings except for special coating purposes. On the other hand, fine calcium carbonate particles smaller than 0.1 ym in average size are used for high-quality art papers and coated papers because the particles have high whiteness and good amenability to ink which are inherent characteristics of calcium carbonate and are also comparable to kaolin of second grade in gloss.The fine calcium carbonate particles nevertheless are low in opacity and involve serious difficulties in handleability and dispersibility because when such particles are formulated into an aqueous dispersion or coating color composition along with clays, the dispersion or composition has a high yield value and high apparent viscosity. Thus the fine particles are unable to give aqueous dispersions and coating color compositions of high concentration.Further with an increase in the speed of coating operation, there arises the necessity of providing coating color compositions which exhibit improved flow characteristics when subjected to a high shearing force so as to be usable for the blade coating process for forming coatings of compact and smooth texture, whereas compositions incorporating an increased amount of fine calcium carbonate particles have a higher yield value and an enhanced viscosity and exhibit impaired flow characteristics when subjected to a high shearing force, while permitting fine particles to agglomerate into secondary particles which tend to produce faults or irregularities in the coating.For use with clays, therefore, the amount of the calcium carbonate of this type, if largest, is limited to about 30 parts by weight per 100 parts by weight of clay, so that it is difficult to make full use of the outstanding properties of the calcium carbonate.

The invention provides aqueous coating color components comprising pigments, a dispersant and a binder, from 5 to 95% by weight of the pigments being uniform cubic particles of calcium carbonate of from 0.1 to 1.0 ym in average size.

The invention further provides coated paper comprising a base and a coating of pigments formed over the base, from 5 to 95% by weight of the pigments are uniform cubic particles of calcium carbonate of from 0.1 to 1.0 ,um in average size.

The cubic calcium carbonate particles of from 0.1 to 1.0 ,um in average size used in this invention are preferably prepared by the process described and claimed in our British Patent Specification No.

1,545,412.

This process made it possible, for the first time, to produce uniform particles of calcium carbonate in specified sizes of about 0.1 to about 1.0 Mm. The calcium carbonate particles of the specified sizes have the unique characteristics that they possess stable surface properties and less lattice defects and are therefore very unlikely to agglomerates into secondary particles. We have made extensive research on the useful application of the calcium carbonate and found that in addition to inherent characteristics of giving high whiteness and printability to coated papers, the calcium carbonate has satisfactory gloss comparable or superior to that of kaolin of first grade, good opacity and very high dispersibility in water..

We have also found that the calcium carbonate is usable in greatly increased amounts in combination with clays, giving coating color compositions which exhibit outstanding flow characteristics even when subjected to a high shearing force. This invention has been accomplished based on these findings.

Examples of other pigments usuable conjointly with cubic calcium carbonate particles 0.1 to 1.0 ,am in average size are clays such as kaolin, talc, aluminium hydroxide, titanium dioxide, zinc oxide, barium sulfate, silica, silicate, barium carbonate, satin white, lake, carbon black, plastic pigment, ultramarine, prussian blue, cubic calcium carbonate particles less than 0.1 Mm in average size, noncubic calcium carbonate particles (rice grain-like, spindle-shaped, pillar-like, needle-like and other particles, or a mixture of such particles) having an average size of more than 0.1 ,t4m, and various other pigments.The amount of the cubic calcium carbonate particles about 0.1 to about 1.0 ,um to be contained in the whole pigment component is usually about 5 to about 95% by weight, preferably about 7 to about 90% by weight, the remainder being at least one of the above-mentioned pigments. With less than 5% by weight of the calcium carbonate present, the resulting composition fails to produce the desired effect to be achieved with the use of the specified calcium carbonate, whereas use of more than 95% by weight of the carbonate tends to impart reduced pick strength to the coated paper obtained.

The dispersants and binders useful for known coating compositions may be incorporated into the coating compositions of this invention in usual amounts. Examples of suitable dispersants are sodium polyacrylate, ammonium polyacrylate, sodium lignin sulfonic acid and various phosphates which are already known. Especially preferable to use are alkali metal salts and ammonium salts of a copolymer having a molecular weight of about 1000 to about 20000 and prepared by polymerizing about 5 to about 200 moles of maleic acid with 100 moles of acrylic acid. Such copolymer is disclosed in Published Unexamined Japanese Patent Application No. 1 29200/1 978. The dispersant is used in an amount usually of about 0.1 to about 5 parts by weight per 100 parts by weight of the whole pigment component.With use of less than 0.1 parts by weight of the dispersant, the pigments are not fully dispersible in the aqueous medium, whereas use of more than 5 parts by weight of the dispersant, if attempted, will not produce any improved dispersing effect while entailing the likelihood that the coated paper obtained will have impaired properties. More preferably about 0.3 to about 1.5 parts by weight of the dispersant is used per 100 parts by weight of the whole pigment component Examples of useful binders are those already known, such as starch phosphate, starch oxide and like processed starches, carboxymethyl cellulose and like cellulose derivatives, caseins, styrenebutadiene copolymer, methyl methacrylate-butadiene copolymer, polymer or copolymer of acrylate and/or methacrylate, ethylene-vinyl acetate copolymer, etc.The binder is used usually in an ainount of about 5 to about 25 parts by weight per 100 parts by weight of the whole pigment component. When used in an amount of less than 5 parts by weight, the binder will not produce the desired effect whereas use of more than 25 parts by weight of the binder is economically disadvantageous and impairs the moisture retentivity and gloss of the paper obtained. More preferably about 10 to about 20 parts by weight of the binder is used per 100 parts by weight of the whole pigment component.

The present coating compositions may further incorporate known additives such as surfactant, lubricant, defoaming agent, water retaining agent, auxiliary dispersant, levelling agent, gloss imparting agent and the like when desired.

The pigment concentration of the coating compositions is suitably determined as desired. When the above-mentioned copolymer of acrylic acid and maleic acid is used as a dispersant, the slurry which is usually prepared before the preparation of the coating composition can be prepared with a low viscosity in an extremely high concentration of about 70%, although it has been impossible heretofore to prepare such a slurry. With use of the copolymer, therefore, coating compositions are available which have a high concentration but low viscosity and which are easily applicable to papers.

The method of preparing the compositions of this invention is not particularly limited. For example, the composition can be prepared by preparing a mixture of cubic calcium carbonate particles of 0.1 to 1.0 ym in average size and another pigment, adding the mixture to water along with a dispersant to obtain a slurryf and uniformly admixing a binder with the slurry together with the desired additives, if any.

Coated paper is produced in a usual manner, for example, by applying the composition to a coating base with a blade coating machine, air doctor coating machine, roll coating machines or the like, thereafter drying the coating and subjecting the coated paper to other steps as already known in the art.

Examples of useful coating bases are natural pulp paper, synthetic pulp paper, paper made from a mixture of natural and synthetic pulps, synthetic film or sheet, and sheets of various materials. Also usable are such sheets provided with an under coat of usual coating color composition.

The invention is illustrated by the following Examples, in which all parts and percentages are by weight.

The properties listed in the tables are determined by the following methods.

(1) B-viscosity: Apparent viscosity measured by a Brookfield synchroletric viscometer at 300C and 60 r.p.m.

(2) H-viscosity: Apparent viscosity measured by a Hercules Hi-shear viscometer at 300C and 4400 r.p.m.

(3) Whiteness: According to JIS P8 1 23, "Method of Testing Paper and Pulp for Hunter whiteness." (4) Gloss: Specular gloss measured by a gloss meter at an angle of incidence (reflection) of 75 degrees.

(5) Opacity: According to JIS P8 123, "Method of Testing Paper for Opacity." (6) Ink set: Offset ink is kneaded for 3 minutes and applied each in an amount of 0.4 cc to the entire front surfaces of sheets of paper by an RI tester (product of Akira Seisakusho, Japan). The rear surfaces of the sheets are checked for the transfer of ink every 30 seconds after the printing. The period of time elapsed is measured in seconds as an ink set value when the ink has dried to permit no transfer of the ink.

EXAMPLES 1 TO 4 (a) Preparation of aqueous dispersion of pigments Water is added to 100 parts of cubic calcium carbonate 0.15 m in average size and 0.8 part of sodium salt (trade mark: "MX 2008," product of Kao Soap Co., Ltd., Japan, hereinafter referred to merely as "AM") of copolymer (molecular weight: about 4000) of 100 moles of acrylic acid and 70 moles of maleic acid to obtain a calcium carbonate slurry (hereinafter referred to as "slurry A") having a solids concentration of 60%. The AM is used as a dispersant.

Separately water is added to 100 parts of kaolin (trade mark: "UW-90," product of Engelhard Minerals and Chemicals Corporation, U.S.A.) and 0.1 part of AM to prepare a kaolin slurry (hereinafter referred to as "slurry B") having a solids concentration of 60%.

The slurry A, slurry B and water are mixed together in specified ratios to prepare aqueous pigment dispersions each having a specified calcium carbonate/kaolin ratio and a predetermined concentration.

(b) Preparation of coating color composition One of the aqueous pigment dispersions (100 parts, calculated as solids) obtained above, 6 parts of starch phosphate (trade mark: "NISSHOKU MS #4600," product of Japan Maize Products Co., Ltd., Japan) and 9 parts of styrene-butadiene copolymer latex (trade mark: "DL 670," product of Asahi-Dow Ltd., Japan) are uniformly mixed together to prepare a coating composition. The same procedure as above is repeated with use of the other pigment dispersions.

(c) Preparation of coated paper The coating compositions obtained above and having calcium carbonate to kaolin ratios of 5:95 (Example 1), 20:80 (Example 2), 50:50 (Example 3) and 80:20 (Example 4) are applied in an amount of 20 g/m2 to one side of coating bases weighing 70 g/m2 by an applicator bar and then dried in air. The coated papers obtained are stabilized at 200C and 60% RH for 24 hours and further treated by a super calender.

Tables 1 and 2 show the properties of the coating compositions and characteristics of the coated papers respectively. These tables reveal that the compositions of the invention are low in viscosity and that the coated papers prepared with use-of the compositions have outstanding characteristics.

EXAMPLE 5 Water is added to 100 parts of cubic calcium carbonate particles 0.5 ztm in average size and 0.8 parts of the same dispersant as used in Example 1 to obtain a calcium carbonate slurry having a solids concentration of 60%. Subsequently the calcium carbonate slurry, the same slurry B as used in Example 1 and water are mixed together to prepare a coating composition having a calcium carbonate to kaolin ratio of 20:80.

The composition is applied to a coating base and then dried in air in the same manner as in Example 1 to obtain a coated paper, which is similarly stabilized and treated by a super calender.

Tables 1 and 2 show the properties of the composition and characteristics of the paper respectively.

EXAMPLES 6 TO 14 With use of various dispersants and binders, as listed in Table 3, aqueous coating color compositions are prepared which contain 20 parts of cubic calcium carbonate particles 0.15 m in average size and 80 parts of the same kaolin as used in Example 1.

In the same manner as in Example 1, coated papers are prepared with use of the compositions.

Table 4 shows the characteristics of the papers.

Table 1 Calcium carbonate particles Pigment ratio Coating composition Size Calcium Cn cn B-viscosity H-viscosity (,am) shape Kaolin carbonate (%! (cps) (cps) Ex. I 0.15 Cubic 95 5 50 280 14 2 0.115 " 80 20 50 280 15 3 0.15 " 50 50 50 290 17 4 0.1!5 " 20 80 50 300 19 5 0.5 " 80 20 50 260 16 Table 2 Characteristics of coated paper Whiteness Gloss Opacity Ink set Example 1 78 72 91 45 2 82 72 92 35 3 85 73 94 30 4 87 74 95 25 5 82 72 91 37 Table 3 Dispersant Binder (in parts per 100 parts (in parts per 100 parts B-viscosity H-viscosity Ex. of pigments) of pigments) (cps) (cps) 6 AM1) 0.24 Starch phosphate2) 6.0 280 15 Styrene-butadiene3) 9.0 copolymer latex 7 Sodium polyacrylate 0.38 " 300 20 8 Sodium polyacrylate 0.23 " 290 17 AM1) 0.05 9 Ammonium polyacrylate 0.3 " 300 20 Sodium polyacrylate 0.08 10 Sodium hexameta- 0.16 " 280 16 phosphate AM1) 0.1 11 AM1) 0.24 Starch oxide4) 5.0 280 15 Styrene-butadiene3) 9.0 copolymer latex 12 " " Casein 7.0 290 19 Styrene-butadiene3) 9.0 copolymer latex 13 AM1) 0.24 Starch phosphate2) 20.0- 300 20 14 " " Carboxymethyl 5.0 300 20 cellulose Styrene-butadiene3) 9.0 copolymer latex Note: 1) Same as in Example 1.

2) Same as in Example 1.

3) Same as in Example 1.

4) Trade mark: "NISSHOKU MS 3800," product of Japan Maize Products Co., Ltd., Japan.

Table 4 Characteristics of coated paper Whiteness Gloss Opacity Ink set Example 6 82 72 92 35 7 82 71 92 35 8 82 72 92 35 9 82 71 92 35 10 82 72 92 34 11 82 72 92 33 12 82 72 92 35 13 82 72 92 34 14 82 73 93 35 EXAMPLES 15 TO 22 With use of various dispersants and binders as listed in Table 5, aqueous coating color compositions are prepared which contain 50 parts of cubic calcium carbonate particles 0.5 m in average size and 50 parts of the same kaolin as used in Example 1.

Like the compositions in the foregoing examples, these compositions give coated papers of outstanding characteristics.

Table 5 Dispersant Binder (in parts per 100 parts (in parts per 100 parts B-viscosity H-viscosity Ex. of pigments) of pigments) (cps) (cps) 15 Sodium polyacrylate 0.38 Starch oxide1) 5.0 290 35 Styrene-butadiene2) 9.0 copolymer latex 16 " " Casein 7.0 280 32 Styrene-butadiene2) 9.0 copolymer latex 17 " " Starch phosphate3) 20.0 300 37 18 " " Carboxymethyl 5.0 300 35 cellulose Styrene-butadiene2) 9.0 copolymer latex 19 Sodium hexameta- 0.25 Starch oxide 5.0 300 30 phosphate Styrene-butadiene2) 9.0 copolymer latex 20 " " Casein 7.0 280 32 Styrene-butadiene2) 9.0 copolymer latex 21 Sodium hexameta- 0.25 Starch phosphate3) 20.0 300 36 phosphate 22 " " Carboxymethyl 5.0 300 35 cellulose Styrene-butadiene2) 9.0 copolymer latex Note: 1) Same as in Example 11.

2) Same as in Example 1.

3) Same as in Example 1.

Claims (6)

1. An aqueous coating color composition comprising pigments, a dispersant and a binder, from 5 to 95% by weight of the pigments being uniform cubic particles of calcium carbonate of from 0.1 to 1.0 pm in average size.

2. A composition according to claim 1 wherein from 7 to 90% by weight of the pigments are uniform cubic particles of calcium carbonate of from 0.1 to 1.0 jum in average size.

3. A coated paper comprising a base and a coating of pigments formed over the base, from 5 to 95% by weight of the pigments being uniform cubic particles of calcium carbonate of from 0.1 to 1.0 ym in average size.

4. A paper according to claim 3 wherein from 7 to 90% by weight of the pigments are uniform cubic particles of calcium carbonate of from 0.1 to 1.0 ym in average size.

5. An aqueous coating color composition substantially as described herein with reference to the Examples.

6. A coated paper substantially as described herein with reference to the Examples.