JPH07118077B2 - Undercoat paint for magnetic recording media - Google Patents

Description

TECHNICAL FIELD The present invention relates to an undercoating paint for forming an intermediate layer provided between a magnetic tape and a base material such as a magnetic tape or a floppy disk.

2. Description of the Related Art A magnetic recording medium is made by applying a magnetic paint on a substrate such as a polyester film. In this case, it was necessary to add carbon black or the like in order to prevent triboelectrification and impart conductivity to the magnetic coating film. In addition, it is specified that the light transmittance is below a certain limit in order to detect the end of video tapes and the detection of index holes in floppy disks. For this reason, it is necessary to add carbon black to the magnetic coating film. It was

The undercoat layer is added for the purpose of increasing the adhesive strength between the magnetic layer and the base material, but attempts have been made to add carbon black to impart conductivity and light-shielding properties to the undercoat layer (for example, , JP-A-50-104003, etc.). In this case,
As a binder, a system soluble in an organic solvent such as methyl ethyl ketone or toluene has been used. Further, as an undercoating paint containing no carbon black, there is an example in which an aqueous dispersion of a water-soluble polymer is used (for example, JP-A-54-65).
No. 04).

Problems to be Solved by the Invention The carbon black-containing undercoating coating material having such a conventional structure has a problem that the solvent resistance when uncured is not sufficient.

Further, an aqueous solution or dispersion of a water-soluble polymer, in particular, a water-soluble polyester resin having a relatively large number of sulfonic acid metal salt substituents has a problem that carbon black is difficult to disperse even though the solvent resistance is good.

The present invention solves such problems, and provides an undercoating coating excellent in conductivity, light-shielding property, surface smoothness, and adhesiveness, which is particularly suitable for a high-density recording medium having a small magnetic coating film thickness. It also enables high productivity such as in-line coating.

Means for Solving the Problems To solve the above problems, the present invention provides a polyester in which 5 to 20 mol% of the dicarboxylic acid component is an ester-forming dicarboxylic acid having an alkali metal sulfonate as a substituent. The present invention relates to an undercoat paint in which carbon black is dispersed in a solution obtained by dissolving a resin in a mixed solvent of cyclohexanone, dioxane and propylene glycol monomethyl ether.

Working With the above-mentioned structure, it is possible to provide a base coating composition having excellent conductivity, light-shielding property, surface smoothness, adhesiveness and the like.

Examples Examples of the present invention will be described below.

In the present invention, a water-soluble polyester resin containing a sulfonic acid metal salt substituent as a binder is used as a binder, and more preferably carbon black is dispersed in a solution prepared by dissolving the above polyester resin in a mixture of specific organic solvents.

That is, as the water-soluble polyester resin, one having the property of being dissolved in hot water or forming colloidal particles is used. Structurally, aromatic and aliphatic dicarboxylic acids as the carboxylic acid component and 5 to 20 mol% of the total amount of the carboxylic acids.
A sulfonic acid metal salt-containing ester-forming monomer corresponding to ethylene glycol alone as a glycol component,
Alternatively, a saturated linear polyester having a basic constituent mainly composed of a combination system with neopentyl glycol is suitable.

In the present invention, the first using a water-soluble polyester resin
The reason is that it is easy to obtain a coating film having excellent solvent resistance against general-purpose solvents such as methyl ethyl ketone, methyl isobutyl ketone, and toluene by only drying. Polyester resins usually have aromatic dicarboxylic acids such as terephthalic acid and isophthalic acid, aliphatic dicarboxylic acids such as succinic acid and adipic acid, and glycols such as ethylene glycol and neopentyl glycol as basic constituent elements. By substituting an ester-forming alkali metal sulfonate compound such as sulfosodium terephthalic acid or sulfosodium isophthalic acid, it is possible to impart water solubility or water dispersibility and solvent resistance to the polyester resin. In this case, the substitution amount is 5 of the total amount of carboxylic acid.
If it is less than mol%, the water solubility and solvent resistance are insufficient,
Further, if the substitution amount is 20 mol% or more, the moisture resistance of the coating film after drying becomes insufficient, which is not suitable.

In the present invention, water is not used to dissolve the polyester resin, but an organic solvent is used because the dispersibility of carbon black is remarkably excellent. As the organic solvent, a mixed system of cyclohexanone, dioxane, and propylene glycol monomethyl ether (hereinafter abbreviated as PGME) is recommended. A water-soluble polyester resin containing a sulfonic acid metal salt substituent in the above range is methyl ethyl ketone. , General-purpose solvents such as methyl isobutyl ketone and toluene do not dissolve, and dissolves in cyclohexanone and dioxane alone or in a mixed system, but the paint obtained by dispersing carbon black has high tackiness and The coatability is not sufficient, and the above polyester resin does not dissolve even if PGME is used as a single system, but by using a mixed system of cyclohexanone and dioxane, the viscosity of the above coating is reduced and And the coating property is significantly improved. The reason why cyclohexanone and dioxane are used together is to optimize the drying property of the coating material by using both of them, which have significantly different boiling points. Therefore, the mixing ratio of cyclohexanone and dioxane may be arbitrary, but PGME is preferably 2% (the same as wt% or less) or more and 25% or less in view of the above effects and resin solubility.

The type of carbon black used in the present invention is not particularly limited, but as a blending amount, relative to 100 parts of the polyester resin (parts by weight, the same applies hereinafter),
It is recommended that the range is 5 to 80 parts. In order to prevent frictional electrification, the electric resistance of the coating film should be 10 ⁹ Ω / □ or less, and the light transmittance of the magnetic disk should be 2% or less on both sides as specified in JIS C 6290. It is aimed at that. For this reason, the light transmittance of the undercoat layer must be 15% or less on one side. In this case, if the carbon black compounding ratio is 5 parts or less, the above-mentioned targets of electric resistance and light transmittance cannot be achieved, and if it is 80 parts or more, the above-mentioned targets can be achieved. It is not suitable because it causes problems in the carbon dispersion state, paint viscosity, coating smoothness, adhesiveness, etc.

The carbon black used in the present invention is not particularly limited as described above, but a carbon black that easily imparts conductivity is preferable. To give an example, Ketjen EC (Japan EEC), BP2000, Vulcan × C72 (Cabot) C
Onductex 975, Conductex SC (Colombian Carbon Co., Ltd.) etc. provide excellent conductivity with a relatively small amount of compounding.

Hereinafter, specific examples will be described.

Example 1 Isophthalic acid 0.5 mol, terephthalic acid 0.4 mol, sulfosodium isophthalic acid 0.1 mol, ethylene glycol 1.6
Mol, neopentyl glycol 0.4 mol, esterification reaction at about 200 ℃ using zinc acetate as a catalyst, and polycondensation reaction by heating at about 250 ℃ at 1 mmHg or less with antimony trioxide as a catalyst, melt viscosity 9200 poise (Koka type flow tester, 190 ° C), a polyester resin having a flow starting temperature of 130 ° C was produced.

Cyclohexanone 286 per 100 parts of the granular granulated product of this resin
Part, dioxane 286 parts, and PGME 78 parts were added, and the mixture was heated and stirred to form a solution.
Add 24 parts of BP2000 and disperse with a ball mill for 160 hours.
Painted. The viscosity of this paint was 68 CP, and the paint overspeed (Toyo Paper No. 28, under pressure of 3 Kg / cm ² ) was 13 g / cm ² · min. Also, apply this paint on polyester film for about 0.5
It was applied to a thickness of μm, but the coatability was good. This coating has a gloss of 98 (Nippon Denshoku VG1D type 45 ° -45 °), light transmittance of 8.6% (wavelength 940 nm), surface electric resistance of 1.5 × 10 ⁵ Ω / □.
Met. In addition, the adhesion to the base fill by the tape peeling method is good, the adhesion after forming a magnetic coating film on this undercoat layer and the surface condition of the magnetic coating film are also good, and the solvent resistance of the undercoat layer is good. Was also enough.

[Examples 2 to 6] In Example 1, the conditions of the paints and coating films prepared by changing the ratio of cyclohexanone, dioxane, and PGME as solvents and the kind of carbon black were examined.

As shown in the results in Table 1, good results were obtained in the range of PGME2 to 25%, namely viscosity, overspeed, coatability, glossiness and surface electric resistance.

[Comparative Examples 1 to 3] In Example 1, when PGME was not added, and 25
The results of the case of addition of not less than 100% are also shown in Table 1. In the case of the former, it was found that the coating exhibits tackiness and the overspeed becomes slow and streaks tend to occur during coating. Further, in the latter case, it was found that the solubility of the resin is lowered and the passability, coatability and glossiness of the coating are adversely affected.

[Example 7] In Example 1, the solvent resistance and moisture resistance of the undercoat layer were examined when the total amount of dicarboxylic acids was kept constant and the compounding ratio of sulfosodium isophthalic acid was variously changed. The solvent resistance was determined by pressing the gauze containing methyl ethyl ketone against the coating film at a pressure of 100 g and reciprocating at a speed of 10 cm / sec to determine the state of dissolution or peeling of the coating film. Table 2 shows changes in melting, peeling, etc. that occurred when the gauze was run 4 times or less, marked with X, changes with 5-9 times, △, withstood 10 times or more, and ○. Indicated. Regarding the moisture resistance, after observing the surface condition after leaving the coating film in an atmosphere of 60 ° C and 90% RH for 24 hours, no change is observed ○ mark, decrease in surface glossiness is recognized △ mark, adhesiveness The decrease was observed in Table 2 as x mark. From Table 2, it can be seen that the range of 5 to 20 mol% of sulfosodium isophthalic acid component is preferable from the viewpoint of solvent resistance and moisture resistance.

As described above, according to the present invention, since the dispersibility of carbon black is excellent, the surface smoothness of the coating film is good, and the solvent resistance, moisture resistance, adhesiveness, light shielding property are excellent, and the friction It is possible to obtain an undercoating paint that does not cause electrification.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hikofumi Oido 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) Reference JP-A-60-171207 (JP, A) JP-A-58 -124651 (JP, A) JP-A-54-85016 (JP, A)

Claims (1)

[Claims] 1. A polyester resin in which 5 to 20 mol% of the dicarboxylic acid component is an ester-forming dicarboxylic acid having an alkali metal sulfonate as a substituent, a mixture of cyclohexanone and dioxane 75 to 98%, and propylene glycol monomethyl. 3 consisting of 2-25% ether
An undercoat paint for a magnetic recording medium, which comprises carbon black dispersed in a solution dissolved in an original solvent.