JPH0676564B2 - Method for producing undercoat paint for magnetic recording medium - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method for producing an undercoat paint for forming an intermediate layer provided between a magnetic tape or a base material such as a floppy disk and a magnetic layer.

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 the video tape and the index hole of the floppy disk.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 the undercoat layer to impart conductivity and light-shielding properties.
In such a case, as a binder, a system soluble in an organic solvent such as methyl ethyl ketone or toluene has been used. There is also an example in which an aqueous dispersion of a water-soluble polymer is used as the undercoating paint containing no carbon black.

In these conventional configurations, the former does not have sufficient solvent resistance,
Since the latter has the problem that it is difficult to disperse carbon black, an attempt was made to disperse carbon black and then use it as a base coating after dissolving a water-soluble polyester resin with relatively many sulfonic acid metal salt substituents in a specific organic solvent. There is also.

Problems to be Solved by the Invention In such a conventional undercoating paint comprising a water-soluble polyester resin, carbon black and an organic solvent, the water-soluble polyester resin has a relatively poor affinity with the organic solvent, It has been difficult to obtain a paint having good workability and capable of withstanding long-term storage. The present invention solves such problems, and provides a method for producing an excellent undercoating paint having good coatability and capable of withstanding long-term storage.

Means for Solving the Problems The present invention for solving the above problems provides a polyester resin in which 5 to 20% of the dicarboxylic acid component is an ester-forming dicarboxylic acid having a sulfonate alkali metal base as a substituent. Dissolve in a mixed solvent of cyclohexanone and dioxane, to disperse carbon black in the resulting solution to make a coating stock solution, and then to mix propylene glycol monomethyl ether or methyl cellosolve as a diluting solvent with the coating stock solution The present invention relates to a manufacturing method.

Operation By the above method, it is possible to obtain an excellent undercoating paint having excellent conductivity, light-shielding property, surface smoothness, adhesiveness and the like, and having good coatability and capable of withstanding long-term storage.

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 a sulfonic acid metal salt-containing ester-forming monomer corresponding to 5 to 20 mol% of the total amount of the carboxylic acid, and ethylene glycol alone as the glycol component, or neo Saturated linear polyesters having the basic constituents mainly based on the combination system with pentyl glycol are 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 are usually composed of 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. 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, if the substitution amount is 5 mol% or less of the total amount of carboxylic acid, 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. That is, first, a resin is dissolved in a combined system of cyclohexanone and oxane as an organic solvent, and carbon black is dispersed therein by an appropriate means such as a ball mill to prepare a coating solution. Then, prior to coating, propylene glycol monomethyl ether (hereinafter referred to as PGME) or methyl cellosolve and, if necessary, dioxane are mixed and diluted to obtain a finished paint.

In the present invention, the reason for adopting the above two-stage paint manufacturing method is that the above-mentioned stock solution is stable and can withstand long-term storage, but the stock solution itself has high tackiness, and coatability and paint transient are not sufficient. , By mixing a poor solvent such as PGME or methyl cellosolve, the tackiness of the paint is remarkably reduced and the coatability and passability are improved. On the other hand, if a poor solvent such as PGME or methyl cellosolve is mixed, it becomes difficult to store it for a long period of time because the viscosity and the dispersion state change easily. Immediately before the step, a two-step manufacturing method in which a diluting solvent is mixed as in the present invention is used. The mixing ratio of cyclohexanone and dioxane may be arbitrary, but it is preferable that the mixing ratio of PGME in the solvent is 25 (wt)% or less and methyl cellosolve is 40% or less from the viewpoint of resin solubility.

The carbon black used in the present invention is not limited to a kind, but the blending amount is 100% by weight of the polyester resin.
It is recommended to set 5 to 80 parts for each part. In order to prevent frictional electrification, the electric resistance of the coating film should be 10 ⁹ Ω / b or less, and the light transmittance of the magnetic disk should be 2% or less on both sides as specified in JIS C-6290. The aim is to do. For this reason, the light transmittance of the undercoat layer must be 15% or less on one side. In this case, if the blending ratio of carbon black is 5 times or less, the above-mentioned targets of electric resistance and light transmittance cannot be achieved, and if it is 80 times or more, even if the above-mentioned targets can be achieved, carbon It is not suitable because it causes problems in the 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. For example, Ketchen EC
Excellent conductivity can be obtained with a relatively small amount of (EC Co., Ltd. Japan), BP2000, Vulcan XC72 (Cabot Co.) Conductex 975, Conductex SC (Colombian Carbon Co., Ltd.).

Example 1 Isophthalic acid 0.5 mol, terephthalic acid 0.4 mol, sulfosodium isophthalic acid 0.1 mol, ethylene glycol 1.6
Mole, 0.4 mol of neopentyl glycol are catalyzed by zinc acetate, esterification reaction is performed at about 200 ° C, and polycondensation reaction is performed by heating at about 250 ° C 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.

After adding 286 parts each of cyclohexanone and dioxane to 100 parts of the granular granulated product of this resin and heating and stirring to form a solution, BP2000 was added as a highly conductive carbon black 24
Parts were added and dispersed by a ball mill for 160 hours to prepare a coating solution. This stock solution has a viscosity of 102 cp and an overspeed (Toyo Paper No. 2
8.3 kg / cm ² under pressure) was 8.2 g / cm ² min, but it was found to be stable with 128 cp, 7.2 g / cm ² min even when left at room temperature for 1 month.

Next, add PGME 78 parts as a diluting solvent to 696 parts of this stock solution,
The mixture was mixed with a homomixer to obtain a finished paint. This paint has a viscosity of 43 cp, an overspeed of 14 g / cm ² min, good coatability, and a gloss of a coating with a thickness of about 0.5 μm (Nippon Denshoku Industries VG1 type).
45 ° -45 °) was 95, and the surface electric resistance was 1.4 × 10 ⁵ Ω / B, which was good.

[Examples 2 to 3] In Example 1, the results for the case of using methyl cellosolve instead of PGME and the case of using Conductex 975 instead of BP2000 as carbon black are shown in Table 1. It was found that the same good result as in was obtained.

[Comparative Examples 1 to 3] In Examples 1 to 3, Table 2 shows the results when PGME and methyl cellosolve as diluent solvents were added together with cyclohexanone and dioxane from the beginning of dispersion. In this case, the viscosity is high and the process is high immediately after the production, but the viscosity of the paint increases and the deterioration of the viscosity is remarkable when left standing, and even if the redispersion treatment with a homomixer is performed, the viscosity and the viscosity return to the initial state. It was found that the coatability remained unsolved.

EFFECTS OF THE INVENTION As described above, according to the present invention, it is possible to form a coating film having excellent surface smoothness, solvent resistance, adhesiveness, light-shielding property, and free from triboelectrification as an undercoating coating material, In addition, it is possible to manufacture a stable coating material in which the coating viscosity and the transitivity do not change over a long period of time.

Claims (1)

[Claims] 1. A solution obtained by dissolving a polyester resin in which 5 to 20 mol% of the dicarboxylic acid component is an ester-forming dicarboxylic acid having a sulfonic acid alkali metal base as a substituent in a mixed solvent of cyclohexanone and dioxane. A method for producing an undercoating coating material for a magnetic recording medium, which comprises dispersing carbon black in a coating solution to prepare a coating solution, and then mixing propylene glycol monomethyl ether or methyl cellosolve as a diluting solvent with the coating solution.