JPH0721849B2 - Binder for magnetic recording media - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a binder for a magnetic recording material, which is excellent in dispersibility of magnetic powder, and which can provide a magnetic layer having excellent wear resistance and heat resistance.

(B) Conventional Technology A magnetic recording material used for a magnetic recording tape, a magnetic disk or the like is usually coated on a substrate with a magnetic paint containing a magnetic powder and a binder for adhering the powder to a substrate such as polyester.
It is obtained by orientation and drying, and is required to have excellent electrical characteristics and durability such as sliding contact resistance with a magnetic head. Therefore, a binder for a magnetic recording material is particularly required to have excellent dispersibility of the magnetic powder in the magnetic coating material and, as a result, to impart a high squareness ratio to the magnetic recording material. It is required to form an excellent magnetic layer.

Conventionally, vinyl chloride-vinyl acetate-vinyl alcohol-based polymers and the like have been used as those having relatively good characteristics, and when the abrasion resistance of the magnetic layer needs to be particularly improved, the polymer and the isocyanate compound are contained. Binders for magnetic recording bodies have been used.

(C) Problems to be solved by the invention However, since magnetic powder tends to be made finer with the increase in density of magnetic recording materials, it is desired to develop a binder having further excellent dispersibility. It had been.

In view of the present state of the binder for a magnetic recording medium, the present invention forms a magnetic layer having excellent dispersibility of magnetic powder when formed into a magnetic coating material and having an abrasion resistance and heat resistance by adding an isocyanate compound. The purpose is to provide a binder for a magnetic recording material to be obtained.

(D) Means for Solving the Problems The present inventors have studied a binder for a magnetic recording medium composed of a copolymer of a vinyl monomer containing a hydroxyl group and vinyl chloride, and further studied a vinyl group containing a phosphate group. The present invention has been completed by finding that the use of a monomer as a component of a copolymer significantly improves the dispersibility of magnetic powder.

That is, the gist of the present invention is a magnetic recording medium characterized by containing a vinyl chloride-based copolymer having a hydroxyl group-containing vinyl monomer and a phosphoric acid group-containing vinyl monomer as constitutional units as a main resin component and an isocyanate compound. It exists in the binder for recording materials.

The vinyl chloride in the vinyl chloride-based copolymer in the present invention imparts appropriate hardness and flexibility to the magnetic layer together with other constitutional units, and if it is too small, the wear resistance of the magnetic recording material will be insufficient. If it is too much, the solvent solubility tends to decrease, so the content in the copolymer is preferably 60 to 95% by weight.

Examples of the hydroxyl group-containing vinyl monomer used as a constitutional unit of the above copolymer include those having a structural formula as a structure of a reaction product of acrylic acid or methacrylic acid and a polyhydric alcohol, or an acrylic acid-based or methacrylic acid-based monomer. Examples of the former include acid-based amides and the like, and specific examples of the former include 2-hydroxyethyl (meth) acrylate (which represents both 2-hydroxyethyl acrylate and 2-hydroxyethyl methacrylate; the same applies hereinafter) and 2-hydroxypropyl. (meth) acrylate, 3-chloro-2-hydroxypropyl (meth) acrylate, polyethylene glycol mono (meth) acrylate _{CH 2 = CR-COOCH 2 CH} 2 OnH (n is 2 to 9 integer represented by the following formulas, R Is hydrogen or a methyl group), and polypropylene glycol mono (meth) acto is represented by the following formula. Related (N is an integer of 2 to 6, R is hydrogen or a methyl group), 2-
Hydroxyethyl-2'-acryloyloxyphthalate (Meth) acrylic acid ester such as
Specific examples of the latter include N-methylol (meth) acrylamide, which are used alone or in appropriate combination as a constitutional unit of a copolymer,
Especially 2-hydroxyethyl (meth) acrylate and 2
-Hydroxypropyl (meth) acrylate is preferably used.

If the amount of use of these hydroxyl group-containing monomers is too large, it tends to cause inconveniences such as a decrease in the solvent solubility of the copolymer, a decrease in the surface smoothness of the magnetic recording material and a decrease in the moisture resistance, while it is too small. And the dispersibility of the magnetic powder tends to decrease, and since the urethane bond due to the reaction with the isocyanate compound is not sufficiently formed, the coating strength tends to be weak and blocking tends to occur. It is preferably contained in an amount of 1 to 30% by weight, more preferably 8 to 20% by weight.

The phosphoric acid group-containing vinyl monomer in the present invention is a vinyl polymerizable monomer having a phosphoric acid group in the molecule, and specific examples thereof include acid phosphoxyethyl (meth) acrylate and acid phosphoxypropyl (meth). ) Acrylate and 3-chloro-2-acid phosphoroxypropyl (meth) acrylate are preferred examples.

The phosphoric acid group-containing vinyl monomer contributes greatly to the improvement of the dispersibility of the magnetic powder, but if this amount is too large, the solvent solubility of the copolymer decreases and a transparent solution cannot be obtained. In addition, the water resistance becomes poor, and side reactions other than the crosslinking reaction with the isocyanate compound occur, so that the crosslinking effect such as the improvement of abrasion resistance cannot be obtained, and the pot life of the magnetic paint becomes short. . On the other hand, if the amount is too small, the dispersibility of the magnetic powder tends to deteriorate, so the content in the copolymer is preferably 0.05 to 8% by weight, and more preferably 0.1 to 5% by weight.

The binder of the present invention may further contain ethylene, propylene, vinyl acetate or the like as a constitutional unit of a vinyl chloride copolymer, if necessary, and a homopolymer or a copolymer thereof may be used. It may be used together with a vinyl chloride copolymer. In particular, it is preferable to use ethylene as a constitutional unit of the vinyl chloride copolymer from the viewpoint of improving the solvent solubility.

The vinyl chloride copolymer can be obtained by a known polymerization method such as a precipitation polymerization method, a solution polymerization method, a suspension polymerization method or an emulsion polymerization method. In the case of the precipitation polymerization method, methanol or n-hexane, which has good solubility of the phosphoric acid group-containing vinyl monomer and in which the produced polymer is insoluble, is used as a suitable solvent, but the solvent of the produced copolymer is dissolved. Methanol is preferred from the standpoint of productivity and economy. The copolymer can be obtained as a fine powder regardless of the type of solvent used. The degree of copolymerization of the copolymer is preferably in the range of about 150 to 600 from the viewpoint of the mechanical strength of the binder and the characteristics of the magnetic coating material. That is, if the average degree of polymerization is less than 150, it is not practical because the coating surface is weak when it is applied to the substrate as a magnetic coating, and if it exceeds 600, the coating viscosity becomes high and the solution is applied. This is because the workability at the time is poor.

Examples of the isocyanate compound include tolylene diisocyanate, diphenylmethane diisocyanate, dianisidine diisocyanate, tridene diisocyanate, hexamethylene diisocyanate, metaxylylene diisocyanate, and a reaction product of 1 mol of trimethylolpropane and 3 mol of tolylene diisocyanate. The reaction product is commercially available, for example, from Nippon Polyurethane Industry Co., Ltd. under the trade name "Coronate L".

The amount of the isocyanate compound used is too high, the cross-linking density becomes high and the finally obtained coating film becomes hard and brittle,
On the other hand, if the amount is too small, the desired effect cannot be obtained. Therefore, it is usually 0.3 to 30 parts by weight with respect to 100 parts by weight of the above copolymer. To prepare a magnetic coating material with the binder of the present invention, for example, a magnetic powder material such as Co-γ-iron oxide is added to the above copolymer solution dissolved in a mixed solvent of methyl isobutyl ketone and toluene, such as a surfactant. The magnetic coating material is obtained by adding the above-mentioned additive, kneading and dispersing, and mixing an isocyanate compound.

However, the order of addition of the copolymer, the isocyanate compound and the magnetic material powder, the dispersing means and the like in the present invention are not limited at all. As the solvent used when the above copolymer is dissolved to form the binder of the present invention, one or a mixture of two or more of toluene, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone and the like is generally used. The above copolymer is usually 10 parts by weight per 100 parts by weight of the magnetic material powder.
-100 parts by weight, and the magnetic coating is usually 5 to 3 parts by weight.
Prepared to have 0% by weight of the above copolymer.

(E) Effect of the Invention The binder for a magnetic recording medium of the present invention has the above-mentioned constitution, and particularly, in the vinyl chloride-based copolymer which is the main resin component, the hydroxyl group-containing vinyl monomer and the phosphate group are contained. Since the contained vinyl monomer is contained as a constitutional unit, the obtained magnetic coating material is excellent in the dispersibility of the magnetic powder and thus a magnetic layer having a high squareness ratio is formed, and vinyl chloride is contained in the copolymer. Since it is contained, a magnetic layer having appropriate hardness and flexibility is formed together with other components.

Further, since the isocyanate compound is contained, the hydroxyl group introduced into the copolymer contributes to the crosslinking reaction between the copolymer and the isocyanate compound to form a magnetic layer having excellent abrasion resistance and heat resistance.

(F) Examples The present invention will be described below with reference to Examples. In the following, "parts" and "%" indicate parts by weight and% by weight, respectively.

The measurement of each physical property was carried out by the following methods.

Squareness Ratio A magnetic recording material was prepared by applying a magnetic coating material containing an isocyanate compound onto a polyester film having a thickness of 2.5 μm so that the dry thickness would be 6 μm, producing a magnetic recording medium, and measuring the squareness ratio.

Glossiness The magnetic coating film used for the squareness ratio measurement was measured for reflectance at an incident angle of 60 degrees using a glossmeter. The gloss meter not only shows the surface smoothness of the coating film but also serves as a measure of dispersibility.

Heat adhesion The magnetic recording material was heated at a temperature of 120 ° C for 15 minutes to see if the magnetic layers adhere to each other. In the table, ○ means no adhesion (no blocking occurs). ), And x indicates adhesion.

Gel Fraction A vinyl chloride copolymer film containing no magnetic powder
Toluene-methyl isobutyl ketone at 50 ° C (weight ratio 1:
1) The value (% by weight) obtained by dividing the weight of the film after being immersed in the mixed solvent for a whole day and night by the weight of the film before being immersed, and used as a standard for determining the reaction efficiency of the crosslinking reaction between the copolymer and the isocyanate compound. It will be.

Example 1 8100 g of methanol (first-grade reagent) and α as a polymerization initiator were placed in a stainless steel autoclave with an internal volume of 20 equipped with an initial charging stirrer
After charging 25 g of cumyl peroxyneodecanoate, it was evacuated with an aspirator for 5 minutes to eliminate residual air.

Next, 2600 g of vinyl chloride was charged.

Post-addition charge 3 After removing air from a suspendable stainless steel container (referred to as an addition container), suction a mixed solution of 259 g of 2-hydroxypropyl acrylate dissolved in 500 g of methanol and 15 g of acid phosphoxyethyl methacrylate. After that, 1045 g of vinyl chloride was press-fitted.

This container was shaken to mix and dissolve the contents and then hung on a spring balance and connected to the addition nozzle of the autoclave from the valve at the bottom using a flexible tube.

Polymerization reaction operation The rotation speed of the stirrer of the autoclave was 380 rpm, and warm water was passed through the jacket to raise the internal temperature to 43 ° C.

When the internal temperature reached 43 ° C, the polymerization reaction started, so after that,
The temperature was adjusted so that the internal temperature was 43 ° C.

On the other hand, when the temperature reaches 43 ° C, add the mixed solution of monomers from the addition container.
50g was added, and then 4 every 5 minutes depending on the progress of the polymerization reaction.
6 g × 4 times, 31 g × 8 times every 10 minutes, and 27 g × 49 times every 5 minutes were added.

After 10 minutes from the end of the last addition, the polymerization reaction was stopped by cooling to 25 ° C. The internal pressure of the autoclave is 2.0 kg / cm at the start of the reaction.
² G, and 1.8 Kg / cm ² G at the end of the reaction.

After exhausting unreacted vinyl chloride after cooling, vinyl chloride was sufficiently removed by further passing nitrogen gas, and then methanol slurry of the copolymer was extracted from the autoclave,
After filtration, vacuum drying was performed at 50 ° C. for 24 hours to obtain 1300 g of white powdery granular copolymer A.

The degree of polymerization of the resin is 330, the composition is vinyl chloride 86.5%, 2-
Hydroxypropyl acrylate was 12.9% and acid phosphoxyethyl methacrylate was 0.6%.

The acid phosphoxyethyl methacrylate content was determined by the neutralization titration method with KOH / MeOH.

Copolymer A was dissolved in a mixed solution of toluene-methyl isobutyl ketone (weight ratio 1: 1) to prepare a 15% copolymer solution, and this solution was then added to 20 parts by weight of the solid content of the copolymer. 80
After adding parts by weight of Co-γ-iron oxide and 8 times the amount of magnetic powder to 1/8 inch diameter stainless steel balls, and mixing and dispersing for 6 hours with a paint conditioner (manufactured by Red Devil Co.) to obtain a magnetic paint, When a magnetic recording medium was prepared and the squareness ratio was measured, a high value of 0.83 was obtained. The glossiness was 97%.

Example 2 Polymerization was carried out according to Example 1 using the same apparatus and the same monomers as those used in Example 1. However, polymerization was carried out by changing the charged amounts of the respective monomers, methanol and the initiator as shown in the weight ratio in Table 1. First powdered copolymer B
Then, the composition of the copolymer was analyzed, and the magnetic recording medium was prepared and the squareness ratio was measured in the same manner as in Example 1. The results are shown in Table 1.

Further, copolymers C, D, E and F having the compositions shown in Table 1 were obtained in the same manner as the copolymer B, and the magnetic recording materials were adjusted. The respective squareness ratios were as shown in Table 1. Also exceeded 0.80 and the glossiness was a high value of 90% or more.

Comparative Example 1 Vinyl chloride / 2 produced in the same manner as Resin A of Example 1
-Hydroxypropyl acrylate copolymer (degree of polymerization
350,2-HPA 13.0% by weight) a, and vinyl chloride / vinyl acetate / vinyl alcohol copolymer (composition ratio 91 in this order)
/ 3/6) b, a magnetic recording medium was prepared according to the same method as in Example 1 and the squareness ratio was measured to be 0.75 and 0.76, respectively.
Met. Further, the glossiness was a low value of 52% and 54%, respectively.

Example 3 Copolymer A obtained in Example 1 and copolymers B to F obtained in Example 2 were dissolved in a toluene-methylisobutylketone (weight ratio 1: 1) mixed solution to obtain 15%. Was adjusted to the above solution. 80 parts by weight of the copolymer solid content in this solution,
After adding 1 part of a 1 / 8-inch diameter stainless steel ball, which is 8 times the amount of Co-γ-iron oxide, and magnetic powder, and mixing and dispersing for 6 hours with a paint conditioner, an isocyanate compound (Coronate L, manufactured by Nippon Polyurethane Industry Co., Ltd.) was used. 0.65 part) was added and stirred to obtain a magnetic coating material. This coating material was applied onto a polyester film, oriented and dried to obtain a magnetic recording material in which the crosslinking reaction was sufficiently advanced. Table 2 shows the squareness ratio, glossiness, and heat adhesion of the magnetic recording medium. Further, the gel fraction showing the cross-linking reactivity between each copolymer and isocyanate has the second
It was as shown in the table.

Comparative Example 2 With respect to the copolymer b used in Comparative Example 1, a magnetic recording material was prepared in the same manner as in Example 1 and the heat adhesion was measured. As a result, blocking occurred.

Claims (6)

[Claims] 1. A magnetic recording comprising a vinyl chloride copolymer having a hydroxyl group-containing vinyl monomer and a phosphoric acid group-containing vinyl monomer as constituent units as a main resin component and an isocyanate compound. Binder for body. 2. The binder for a magnetic recording material according to claim 1, wherein the content of the hydroxyl group-containing vinyl monomer in the vinyl chloride copolymer is 1 to 30% by weight. 3. The binder for a magnetic recording material according to claim 1, wherein the content of the vinyl monomer having a phosphoric acid group in the vinyl chloride copolymer is 0.05 to 8% by weight. 4. The vinyl chloride copolymer has an average degree of polymerization of 15
The binder for magnetic recording bodies according to any one of items 1 to 3, which is 0 to 600. 5. A vinyl chloride-based copolymer having a vinyl chloride content of 60 to 95% by weight, a hydroxyl group-containing vinyl monomer content of 1 to 30% by weight, and a phosphoric acid group-containing vinyl monomer. The binder for magnetic recording bodies according to claim 1 or 4, wherein the content of 0.05 to 8% by weight. 6. The binder for a magnetic recording material according to any one of claims 1 to 5, wherein the phosphoric acid group-containing vinyl monomer is acid phosphoxyethyl methacrylate.