JPS5842524B2 - magnetic recording medium - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to magnetic recording media such as magnetic tapes and magnetic tickets.

The magnetic layer of a magnetic recording medium is usually made by applying a magnetic paint to the surface of a substrate and drying it. In recent years, water-based emulsion paints have been proposed as the paint.

This paint is made by dispersing magnetic powder in an aqueous polymer emulsion, and does not pose the fire hazards or health and hygiene problems that conventionally used organic solvent-based paints do.
On the other hand, when a conventional polymer used in conventional organic solvent systems is used as a binder for magnetic powder, an optimal dispersant is required for each individual polymer to ensure the dispersion stability of the paint. Since the selection of this dispersant is extremely difficult, it is not necessarily easy to form a magnetic layer in which the dispersibility of the magnetic powder within the layer is equal to or better than that of an organic solvent.

In light of the above circumstances, the inventors have already used a carboxyl group-containing methacrylic acid ester-acrylic acid ester copolymer as a polymer, dispersed this copolymer in an ammonia aqueous solution, and mixed magnetic powder therein. By using a paint, we succeeded in forming a magnetic layer with excellent dispersibility of magnetic powder.

However, the magnetic layer formed by this method tends to have poor adhesion to the substrate, and in the case of items that are frequently touched by fingers, such as magnetic tickets, there is a risk that the magnetic layer may fall off due to finger pressure.

This invention was made after it was found in subsequent research that the above-mentioned problem could be avoided by using an ethylene-vinyl acetate copolymer together with the above-mentioned copolymer.

The carboxyl group-containing methacrylic ester-acrylic ester copolymer usually contains 20 to 100 parts of a methacrylic ester such as methyl methacrylate or ethyl methacrylate per 100 parts by weight of an acrylic ester such as butyl acrylate or ethyl acrylate. It can be obtained by copolymerization, preferably emulsion polymerization, of 1 to 30 parts by weight of a polymerizable unsaturated carboxylic acid such as acrylic acid or methacrylic acid, and this type of copolymer has excellent affinity with magnetic powder. Contributes to improved dispersibility.

Furthermore, various types of ethylene-vinyl acetate copolymers with different vinyl acetate contents can be widely used, and when used in combination with the above copolymers, they have the ability to improve the adhesion of the magnetic layer to a substrate, especially a paper substrate. The proportion used is usually 20 to 80% by weight, preferably 30% of the total amount.
The amount may be approximately 70 weight φ, and even if such a large amount is used, the dispersibility peculiar to the copolymer will hardly be impaired.

On the other hand, when both of the above-mentioned copolymers are emulsified and dispersed in water and magnetic powder such as γ-Fe2O3 is directly added to this aqueous polymer emulsion, a magnetic powder such as γ-Fe2O3 is added to the aqueous polymer emulsion. Therefore, gelation of the paint tends to occur, making it impossible to obtain the desired magnetic paint with excellent dispersion stability.

Therefore, in this invention, it is necessary to neutralize with an alkali before adding the magnetic powder, and this neutralization suppresses gelation and increases and stabilizes the viscosity of the paint to form a magnetic layer of uniform thickness. Adjust to appropriate viscosity.

The optimal alkali to be used for the above purpose is an ammonia aqueous solution that can be easily removed from the system in the drying process after coating the paint.The amount of ammonia aqueous solution added depends on the degree of neutralization of the paint, It is preferable to select the equivalent ratio obtained by dividing the number of OH groups by the number of carboxyl groups in the copolymer in the paint, and when the degree of neutralization is 0.25 or more,
If the viscosity of the paint becomes high and the value is 1 or higher, the viscosity of the paint tends to decrease, so a neutralization degree of about 0.25 to 1.5 is best.

The magnetic layer can usually be formed by applying the thus prepared magnetic paint onto the surface of a substrate such as a polyester film or paper substrate by a conventional method and drying.

The magnetic layer formed by this method has magnetic powder bonded to a carboxyl group-containing methacrylate ester-acrylic ester copolymer and an ethylene-vinyl acetate copolymer, which improves the dispersion stability of the magnetic paint. Due to the affinity of the carboxyl group-containing methacrylic acid ester-acrylic acid ester copolymer for the magnetic powder, the magnetic powder has excellent dispersibility, and the above-mentioned function of the ethylene-vinyl acetate copolymer makes it suitable for substrates, especially paper substrates. The adhesion to the substrate is improved, and falling off from the substrate due to finger pressure is suppressed.

Next, the present invention will be explained in more detail with reference to Examples.

Note that parts hereinafter refer to parts by weight.

Example: An aqueous dispersion (4) with a solid content concentration of 45 weight φ obtained by emulsifying and dispersing a copolymer consisting of 65 parts of ethyl acrylate, 35 parts of methyl methacrylate, and 10 parts of methacrylic acid, and an ethylene-vinyl acetate copolymer. Combined (vinyl acetate content approx. 30% by weight)
An aqueous dispersion with a solid content concentration of 45% by weight (B
) in the proportions shown in Table 1 below, water and 1 weight ammonia water in the proportions shown in Table 1, respectively, and add 100 parts of γ-Fe203 magnetic powder The mixture was mixed and dispersed in a ball mill to obtain three types of aqueous emulsion magnetic paints.

The degree of neutralization of each paint and the viscosity measured with a Brookfield viscometer are as shown in Table 1.

Each coating material was applied to a paper base of about 175 μm to a dry thickness of about 30 μm, and dried to form a magnetic ticket.

Reference Example In the example, the aqueous dispersion (B) was not used, and the aqueous dispersion (B) was not used.
A magnetic paint was prepared in the same manner as in the example except that the number of parts of N used was 95.1 parts, and the water and 1 weight polyammonium water added to it were 120 parts and 42.4 parts, respectively. I used it to make magnetic tickets.

The results of examining the squareness ratio (Br/Bs) and adhesiveness of each magnetic ticket are shown in Table 2 below.

However, adhesion was determined by observing the state when an adhesive tape was attached to the magnetic layer surface and then peeled off.
A score of 0 indicates that no peeling was observed at the interface even if the paper substrate was peeled off so strongly that the paper substrate was torn.

As is clear from the above table, according to the present invention, the adhesion between the substrate and the magnetic layer can be achieved without impairing the excellent dispersibility of the reference example in which the carboxyl group-containing methacrylic ester-acrylic ester copolymer was used alone. It turns out that sex can be improved.

Claims (1)

[Claims] 1. A magnetic recording medium comprising a carboxyl group-containing methacrylic ester-acrylic ester copolymer and an ethylene-vinyl acetate copolymer as binder components of the magnetic layer.