FI95299C - Composition for coating paper - Google Patents

Description

This invention relates generally to a paper coating composition used in the manufacture of coated papers, and more particularly to an aqueous coating composition applied to a paper surface to improve its surface properties such as smoothness, gloss and printability. The present invention also relates to a coated paper having a surface coating layer formed from the above coating composition.

One known coating composition contains a white pigment such as kaolin, a binder, a latex and a dispersant. It is known to add an additive such as sodium alginate or carboxymethylcellulose to control the viscosity, flowability and water retention of such a composition.

However, these known additives have the disadvantage that when used in a sufficient amount to provide the desired water retention, the viscosity of the resulting composition becomes so high and the flowability decreases at the same time that it is impossible to obtain a fast coating. On the other hand, the water retention capacity of the coating composition does not become good enough when the additives are used in such an amount as to provide a suitable viscosity and flowability.

Poor water retention is detrimental because when the coating composition is contacted with the paper to be coated, water tends to separate from the coating composition and penetrates the paper together with a portion of the binder so that the resulting coating has smoothness, printability and the like.

It is an object of the present invention to eliminate the above-mentioned problems of conventional paper coating compositions.

According to the present invention there is provided an aqueous coating composition according to claim 1, comprising - a white pigment - a binder, - an additive which is an acrylic acid / methacrylic acid copolymer and / or a copolymer of the following two monomers: (a)

CH2 = CR1COOH

and (b) an unsaturated ester of the formula ch2 = CR2COOR3, wherein R2 represents hydrogen or a methyl group and R3 represents an alkyl group having 2 to 4 carbon atoms, the additive being present in an amount of 0.001 to 2% by weight of the white pigment , and - polyol.

According to the present invention, there is further provided a coated paper obtained by applying the above composition to the surface of the paper.

The invention is explained in more detail below.

The paper coating composition of the present invention is characterized in that it contains a specific additive, i. an acrylic acid / methacrylic acid copolymer and / or a copolymer of (a) acrylic acid or methacrylic acid and (b) alkyl acrylate or alkyl methacrylate. Monomer (b), i.e. the alkyl group of the alkyl acrylate or methacrylate has 2 to 4 carbon atoms. The molecular weight of the additive is preferably 5,000 to 3,95299 to 500,000. It is important that the additive be present in an amount of 0.001 to 2%, especially 0.01 to 1% by weight of the white pigment. Too small an amount of additive of less than 0.001% by weight is insufficient to achieve the object of the present invention. On the other hand, when the amount of the additive is more than 2% by weight, the viscosity of the resulting coating composition becomes too high or the composition may gel.

In the acrylic acid / methacrylic acid copolymer, the molar ratio of the former to the latter acid is preferably 9: 1 to 3: 2, especially 4: 1 to 13: 7, and the average molecular weight is 10,000 to 100,000. In the copolymer of acid (a) and ester (b), the molar ratio of acid (a) to ester (b) is preferably 19: 1 to 1: 1, especially 4: 1 to 3: 2, and the average molecular weight is 60,000 to 300,000.

Any of the known white pigments conventionally used in the paper coating composition can be used in this invention. Examples of suitable pigments are clay, calcium carbonate, kaolin, talc, titanium oxide, aluminum hydroxide, silicon, zinc oxide, activated white kaolin, acid white kaolin, lacquer, diatomaceous earth, plastic pigments and mixtures thereof. The white pigment content of the coating composition is generally 40 to 70% by weight.

Latex or a solution of synthetic or natural high molecular weight polymers can be used as a binder. Examples of suitable polymers which can be used as a binding agent, be mentioned a styrene / butadieenikopoly-copolymers, styrene / acrylic copolymers, vinyl acetate / acrylic copolymers, ethylene / vinyl acetate copolymers, butadiene / methyl methacrylate copolymers, vinyl acetate / butyl acrylate, styrene / maleiinihappoan-anhydride copolymer, isobutylene / maleic anhydride -¾ 4 95299 polymers, acrylic acid / methyl methacrylate copolymers, water-soluble starch, casein and soy protein. The binder content is preferably 3 to 30%, in particular 7 to 20% by weight of the pigment.

The coating composition of this invention preferably contains a dispersant such as sodium polyacrylate, sodium hexametaphosphate or sodium pyrophosphate.

Because a special copolymer is used as an additive, the coating composition of this invention has excellent water retention, viscosity and flowability, and can produce coated paper with excellent properties such as printability and gloss.

Such properties can be further improved when the copolymer is used in conjunction with a polyol. Examples of polyols include glycerol, alkylene glycols such as ethylene glycol and propylene glycol, and polyalkylene glycols such as polyethylene glycol and polypropylene glycol.

Glycerol is particularly preferably used. The polyol is preferably used in an amount of 1 to 20%, in particular 5 to 15% by weight of the copolymer.

• The coating composition of the present invention can be applied to one or both sides of a paper, such as uncoated paper, sandpaper, or paperboard, to produce coated paper. The coating can be obtained in a manner known per se using, for example, a blade coater, a roller coater, an air scraper coater or a similar coater.

The following examples further illustrate the above invention.

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Preparation of coating liquid: 5 95299

Example 1

A coating liquid having the following composition was prepared:

Ingredients Amount (parts by weight) pigment clay 90 heavy calcium carbonate 10 binder latex 10 oxidized starch 7 dispersant * 2 0.1 1additive * 3 0.1 water 7b * 1 Styrene / butadiene latex (L-1622, manufactured by Asahi Ka-sei Kogyo ) * 2 Sodium polyacrylate (SDA-40N, manufactured by Somar Corp.) * 3 Methacrylic acid / ethyl acrylate copolymer (manufactured by Rohm & Haas), hereinafter Copolymer A.

Example 2 A coating liquid was prepared by replacing the additive of Example 1 with a methacrylic acid / butyl acrylate copolymer (manufactured by Rohm & Haas, hereinafter Copolymer B.)

Example 3 A coating liquid was prepared by replacing the additive of Example 1 with a mixture of 10 parts by weight of a methacrylic acid / ethyl acrylate copolymer (copolymer A) and 1 part by weight of glycerol.

[95299

Example 4 A coating liquid was prepared by replacing the additive of Example 1 with a mixture of 10 parts by weight of a methacrylic acid / butyl acrylate copolymer (copolymer B) and 1 part by weight of glycerol.

Example 5 A coating liquid was prepared by replacing the additive of Example 3 with a mixture of 10 parts by weight of an acrylic acid / methacrylic acid copolymer (copolymer C, average molecular weight 70,000) and 1 part by weight of glycerol.

Example 6 A coating liquid was prepared by replacing the copolymer C of Example 5 with an acrylic acid / methacrylic acid copolymer (copolymer D, average molecular weight 40,000).

Comparative Example 1 A coating liquid was prepared by replacing the additive of Example 1 with carboxymethylcellulose (CMC)

Comparative Example 2 A coating liquid was prepared by replacing the additive of Example 1 with sodium alginate.

Comparative Example 3 «-11 * 1 1" The coating liquid was prepared by replacing the additive of Example 1 with water.

Properties of coating fluids 7 95299

The above coating liquids were tested for their viscosity, flowability and water retention as follows: (1) Type B viscosity (cps):

Measured according to TAPPI standard T648 Su-72 at 60 rpm.

(2) HS (High Share) viscosity (cps):

Measured with a High Share viscometer (manufactured by Seiki K.

K.) at a rotational speed of 4000 rpm.

(3) Flowability: Determined by the following equation (the lower the better):

Fluidity = (viscosity of the coating composition) / (viscosity of the aqueous phase1 of the coating composition) * The composition is first separated into a liquid (aqueous) phase and a solid phase by centrifugation. The viscosity of the liquid phase is then measured.

Each of the above coating liquids was applied to a paper having a paper weight of 60 g / m2 per square meter to another (4) Water retention capacity:

Measured according to the S&D Warren method (the higher the better).

The test results are shown in Table 1.

Preparation of coated paper: 8,95299 and dried to give a coated paper having a basis weight of 75 g / m 2.

Properties of Coated Paper Coated papers thus prepared were tested for their gloss, brightness, smoothness and surface strength as follows: (1) Gloss:

Measured according to Japanese industry standard JIS PÖ142 (1965).

(2) Brightness:

Measured according to Japanese industry standard JIS P8123 (1961).

(3) Smoothness:

Measured according to Japanese industry standard JIS P8119 (1976).

(4) Surface strength:

Dry anesthesia and wet anesthesia were measured using a weight tester (RI tester, manufactured by Akira Seisakusho K. K.) and evaluated as a 5-level classification (the higher the better).

The test results are also shown in Table 1.

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Claims (15)

An aqueous coating composition comprising: (i) a white pigment; (ii) a binder and (iii) an additive, characterized in that the additive (iii) is at least one substance selected from acrylic acid / methacrylic acid copolymers and copolymers of the following two monomers: · (a) an unsaturated acid of the formula: CH 2 = CR 1 COOH where R 2 is hydrogen or a methyl group and (b) an unsaturated ester of the formula: CH 2 = CR 2 COOR 3, where R 2 represents hydrogen or a methyl group and R 3 represents an alkyl group of 2-4 carbon atoms, Is present in an amount of 0.001-2% based on the weight of the white pigment and that the composition further comprises (iv) a polyol. The coating composition of claim 1, wherein the additive is a copolymer of the acid (a) and the ester (b) with a molar ratio of acid (a) to ester (b) in the range of 19: 1 to 1: 1. The coating composition of claim 1, wherein the additive is a copolymer of acrylic acid and methacrylic acid having a mole ratio of the former acid to the latter acid of 9: 1 to 6: 4. 95299 14 Coating composition according to any preceding claim, wherein the additive has an average molecular weight of 5,000 to 500,000. The coating composition according to claim 4, referring to claim 5, wherein the copolymer of the acid (a) and the ester (b) has an average molecular weight of 60,000 to 300,000. The coating composition according to claim 4, according to claim 3, wherein the acrylic acid / methacrylic acid copolymer has an average molecular weight of 10,000 to 100,000. Coating composition according to any preceding claim, wherein the additive content is 0.01-1% based on the weight of the pigment. Coating composition according to any preceding claim, wherein the white pigment is at least one substance selected from clay, ca.cium carbonate, kaolin, talc, titanium oxide, aluminum hydroxide, silica, zinc oxide, active terra alba, acidic terra alba, substrate pigment, diatomaceous earth and plastic pigment. A coating composition according to any preceding claim, wherein the binder is at least one substance selected from styrene / butadiene copolymers, styrene / acrylic copolymers, vinyl acetate / acrylic copolymers, ethylene / vinyl acetate copolymer, butadiene / methyl methacrylate / methyl methacrylate / butyl acrylate copolymers, styrene / maleic anhydride copolymers, isobutene / maleic anhydride copolymers, acrylic acid / methyl methacrylate copolymers, water-soluble starch, casein and soybean protein. A coating composition according to any preceding claim, wherein the content of binder is 3 to 30% based on the weight of the pigment. A coating composition according to any preceding claim, further comprising a dispersant. A coating composition according to any preceding claim, wherein the polyol is a substance selected from ethylene glycol, propylene glycol, polyethylene glycol, polypropylene glycol and glycerol. The coating composition of any preceding claim wherein the polyol is used in an amount of 1-20% based on the weight of the copolymer. The coating composition of claim 13, wherein the polyol is used in an amount of 5-15% based on the weight of the copolymer. 15. Coated paper obtained by a method comprising coating with a composition according to any preceding claim on the surface of a paper and drying the obtained coated layer. «·. · «