JPH022903B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION When forming a magnetic coating film on the substrate surface of a magnetic recording medium, the present invention is characterized in that an aqueous dispersion or aqueous solution of an acrylic copolymer is dispersed and mixed with magnetic powder to form a dispersion of the magnetic powder. The present invention relates to an aqueous dispersion magnetic coating composition with improved properties.

Ordinary magnetic paints are prepared by dissolving a resin as a binder in an organic solvent and dispersing and mixing magnetic powder in the solution, but a method using such an organic solvent-based magnetic paint has been proposed. According to this, the above-mentioned problems with organic solvents have been solved, but
Because the dispersibility of magnetic powder is insufficient, it is necessary to disperse and mix for a long time in a ball mill in order to stably disperse and mix.

The present inventors have conducted various studies regarding aqueous dispersion magnetic paints, and found that the number of carbon atoms in the alkyl group is 1 to 4.
A colloidal acrylic copolymer containing at least one methacrylic acid alkyl ester and/or acrylic acid alkyl ester and an unsaturated carboxylic acid as essential components, and in which the carboxylic acid component in the copolymer component is partially a water-soluble salt with a base. The present inventors have discovered that extremely good dispersibility can be obtained by using an aqueous dispersion or solution of a polymer as a binder for magnetic powder, and have completed the present invention.

Since the coating composition of the present invention is water-based, the conventional problems of toxicity due to solvent volatilization, dispersion stability of the magnetic coating, dispersibility within the magnetic layer, and water resistance of the magnetic coating can be solved. .

That is, the aqueous dispersion or aqueous solution of the colloidal acrylic copolymer used in the present invention is a methacrylic acid alkyl ester and/or an acrylic acid alkyl ester (hereinafter referred to as the acrylic aqueous polymer) in which the alkyl group has 1 to 4 carbon atoms. It is a copolymer produced by a normal emulsion polymerization reaction containing at least one type of (abbreviated as abbreviated) and an unsaturated carboxylic acid as an essential component, and 25 mol% or more of the carboxylic acid component in the copolymerization component is neutralized with a base. Therefore, it is an aqueous dispersion or solution of a copolymer pulverized to a particle size of 0.1 micron or less.

Alkyl groups include methyl group, ethyl group, n
-propyl group, isopropyl group, n-butyl group,
Examples include isobutyl group and t-butyl group.
The content of methacrylic acid alkyl ester is 30% by weight
It is desirable that the copolymer's softening temperature is lower than that, and the softening temperature of the copolymer is lowered, and the blocking resistance and abrasion resistance of the coating film are deteriorated. Further, as a constituent component of the copolymer, an acrylic acid alkyl ester having 1 to 4 carbon atoms, such as methyl acrylate, ethyl acrylate, butyl acrylate, etc., is used. The carboxylic acid component in the aqueous acrylic polymer is preferably used in an amount of 1 to 30% by weight, more preferably 5 to 15% by weight based on the total amount of monomers.
is within the range of If it is less than 1% by weight, the particles are not sufficiently pulverized, the dispersibility of the magnetic powder is poor, and the water resistance of the coating film is poor. If it exceeds 30% by weight, it tends to gel when mixed with magnetic powder. In addition, the base used to form the basic salt of the carboxylic acid component is ammonia,
Trimethylamine, monoethylamine, diethylamine, triethylamine, 2-methylaminopropanol, morpholine, n-propylamine,
Examples include n-butylamine and monoethanolamine.

The magnetic coating composition of the present invention can be prepared by adding magnetic powder to the acrylic polymer and dispersing and mixing it in a ball mill, or by preparing the magnetic powder in advance in an aqueous system using a copolymer of casein, styrene, and maleic anhydride. An aqueous paste of magnetic powder dispersed with one or more dispersions of a polymer electrolyte such as a combined sodium salt, a polyelectrolyte such as carboxymethylcellulose, or a phosphate such as sodium pyrophosphate is added to the acrylic aqueous polymer. In addition to coalescence, it is made into a magnetic coating composition.
Regarding the mixing ratio of the aqueous acrylic polymer and magnetic powder, it is better to have a smaller amount of the aqueous acrylic polymer resin in order to increase the output sensitivity of the magnetic recording medium. On the other hand, the amount of the aqueous acrylic polymer resin is preferably 20 to 50 parts by weight. Examples of magnetic powder include nickel, chromium, graphite iron oxide, silver, manganese, and cobalt.

In the present invention, in addition to the acrylic aqueous polymer, water-soluble polymer compounds such as polyvinyl alcohol, starch, casein, gum arabic, carboxymethyl cellulose, methyl cellulose,
Hydroxyethylcellulose, sodium alginate, sodium polyacrylate, etc., waxes, silicone-based water repellents, and melamine resin as a water-resistant agent can be used in combination. Furthermore, the coating composition of the present invention is often applied to paper surfaces, cloth surfaces, and the like.

Next, examples of the present invention will be described. "Parts" and "%" in the examples are "by weight".

Example 1 0.4 parts of sodium lauryl sulfate, 60 parts of methyl methacrylate, 40 parts of ethyl acrylate, 8 parts of methacrylic acid, 46 parts of water, 0.5 parts of dodecyl mercaptan
51 parts of water and 0.1 part of sodium lauryl sulfate were charged into an ordinary emulsion polymerization reaction vessel equipped with a stirrer, a reflux condenser, and a charging port. Heat to 65°C, then add 1.5 parts of a mixture of 6 parts of water and 0.7 parts of potassium persulfate, and then add 15 parts of the above emulsifying monomer mixture to 70°C.
The mixture was heated to ℃ and polymerized for 30 minutes. After 30 minutes, the remaining 5.2 parts of the above mixture and the remaining emulsifying monomer mixture
139.9 was added in parallel for 60 minutes, and 30 minutes after the addition was completed.
The temperature was maintained at 80℃ for 5 minutes, then 13 parts of water was added, the temperature was maintained at 70℃, a mixture of 80 parts of water and 2.5 parts of 28% ammonia water was added dropwise over 5 minutes, and after further aging at 70℃ for 30 minutes, while stirring. Cool to room temperature, solid content 26.0%, PH
8.5, a viscosity of 1100 cps, and an average particle size of 0.001 micron, an aqueous dispersion of colloidal acrylic acid acrylic copolymer was obtained. 132 parts of the aqueous dispersion of the colloidal acrylic copolymer was preliminarily mixed with 109 parts of water as a dispersant. 731
(manufactured by Kao Atlas) 3.0 parts, as an antifoaming agent
Formaster AP (manufactured by San Nopco) 0.1 part, γ
- 106 parts of iron oxide (MX-450 manufactured by Toda Kogyo Co., Ltd., coercive force 350 oersted) was added, and 4 parts were added in a ball mill.
Spread out time. Mix 218 parts of water-based magnetic paste and apply magnetic paint (solid content 40%, PH 8.0, viscosity 1200 cps) to high-quality paper with a basis weight of 160 g/m ² using an applicator to a dry thickness of 20 microns. The magnetic particles were passed through a magnetic field before they dried to orient the particles, and then dried.
Calendar processing was performed at minute speed. As a result of measuring the magnetic recording material using a DC magnetization characteristic automatic recording device, a coercive force of 320 oersted, a residual magnetic flux of 4.7 Maxwell/cm, and a squareness ratio (residual magnetic flux/maximum magnetic flux) of 0.581 were obtained. In addition, the water resistance of the magnetic recording medium was determined by a water resistance test in which water droplets were dropped on the magnetic surface and wiped off after a few minutes.No change was observed on the magnetic surface, and the moisture and heat blocking resistance of the magnetic surface was 80% at 50℃. RH500g/ ^cm2 , 24
Good results were obtained in terms of time and abrasion resistance (JIS P-8136 Paperboard Abrasion Resistance Test Method).

Example 2 132 parts of the aqueous dispersion of the colloidal acrylic copolymer used in Example 1 was mixed with 106 parts of γ-iron oxide (MX-450 manufactured by Toda Kogyo Co., Ltd., coercive force 350 Oersted) and water. 109 parts, Orotane 731 as a dispersant
(manufactured by Kao Atlas) 3.0 parts, as an antifoaming agent
0.1 part of Formaster AP (manufactured by San Nopco) was added and 218.1 parts of aqueous magnetic paste was mixed and dispersed in a ball mill for 2.0 hours, and the solid content was 39.5.
%, PH8.0, and a viscosity of 980 cps. A magnetic recording medium was prepared using this magnetic paint in the same manner as in Example 1, and as a result of evaluation, the coercive force was 320 oersted, the residual magnetic flux was 5.4 maxwell/cm, the squareness ratio was 0.804, and it had water resistance, moisture and heat blocking resistance, A magnetic coating film with good wear resistance was obtained.

Comparative Example 1 Instead of the aqueous dispersion of colloidal acrylic copolymer used in Example 2, an aqueous dispersion of styrene-butadiene copolymer (trade name: Dow Latex,
1571, manufactured by Dow Chemical Company, solids content 50%) 68.6 parts
While stirring 60 parts of 1.0% ammonia water and 3.4 parts of water, 218.1 parts of the aqueous magnetic paste prepared in advance used in Example 2 was mixed (solid content 39.8%, PH
8.2, viscosity 940 cps) magnetic paint was prepared. A magnetic recording medium was prepared in the same manner as in Example 1, and as a result of evaluation, the squareness ratio was as low as 0.689, and the dispersibility of the magnetic powder was insufficient.

Comparative Example 2 The aqueous dispersion of colloidal acrylic copolymer used in Example 2 was replaced with an aqueous dispersion of ethylene-vinyl acetate copolymer (trade name OM-4200 manufactured by Kuraray Co., Ltd., solid content 56%) 61.2 and 70.8 parts of water were mixed with 218.1 parts of the previously prepared aqueous magnetic paste used in Example 2 to prepare a magnetic paint having a solid content of 40.1 parts, a pH of 7.4, and a viscosity of 4500 cps. A magnetic recording medium was prepared in the same manner as in Example 1, and as a result of evaluation, the squareness ratio was as low as 0.578, and the magnetic properties were insufficient.

Comparative Example 3 Instead of the aqueous dispersion of colloidal acrylic copolymer used in Example 2, 73.8 parts of an aqueous dispersion of vinyl acetate polymer (trade name: Cevian 522, manufactured by Daicel Chemical Industries, Ltd., solid content 46.5%) was used. Example 2: Add 58.2 parts of water to
A magnetic paint having a solid content of 40.2%, a pH of 7.2, and a viscosity of 2800 cps was prepared by mixing it with 218.1 parts of the aqueous magnetic paste prepared in advance. A magnetic recording medium was prepared in the same manner as in Example 1, and as a result of evaluation, the squareness ratio was as low as 0.656, and the magnetic properties were insufficient.

Claims (1)

[Claims] 1 A colloid containing at least one methacrylic acid alkyl ester and/or acrylic acid alkyl ester whose alkyl group has 1 to 4 carbon atoms and an unsaturated carboxylic acid, in which the unsaturated carboxylic acid component is partially made into a water-soluble salt with a base. 1. A magnetic coating composition comprising magnetic powder dispersed in an aqueous dispersion or solution of an acrylic copolymer.