JP2005353222A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic recording medium capable of obtaining friction reducing effect, maintaining superior sliding durability, and obtaining the effect for reducing deposit on a magnetic head for a long period of time. <P>SOLUTION: In the magnetic recording medium 10, a magnetic layer 2 formed by applying a magnetic coating includes a prescribed amount of thiophosphorous ester expressed by the following formula (1) and at least one kind of polyvalent carboxylic acid, wherein the formula (1) is (R-S)<SB>3</SB>P, where R is a saturated or unsaturated alkyl group with a carbon number of 7-25 having a straight chain or a branch. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a technique for improving the composition of a magnetic layer in a magnetic recording medium having a structure in which a nonmagnetic lower layer and a magnetic layer are formed by multilayer coating.

Conventionally, as a magnetic recording medium, a magnetic layer is formed by applying and drying a magnetic paint prepared by dispersing ferromagnetic powder, a binder, and various additives together with an organic solvent on a nonmagnetic support. A coating type magnetic recording medium is known.
Such a coating-type magnetic recording medium is used as a recording medium for computers such as a magnetic tape for audio or video, a high-density floppy (registered trademark) disk, a backup data cartridge, and the like. It has become the mainstream of recording media.

  By the way, in order to increase the recording density of the magnetic recording medium, there are methods such as reduction of the recording track width, increase of the linear recording density, and shortening of the recording wavelength. A signal error is more likely to occur as the speed increases.

The following are the main causes of signal errors.
(1) Defects such as scratches existing on the magnetic recording medium. (2) A recording error or a reproduction error due to an increase in spacing due to deposits (so-called head dirt) on the surface of the magnetic head for recording or reproduction. (3) An error due to wear of the magnetic head. (4) Off-track along with dimensional change of magnetic recording medium.

  In the future, development of technology to further improve the reliability of magnetic recording media will be required. Especially in the case of tape-like magnetic recording media, repeated recording and reproduction are possible with the magnetic head and magnetic tape sliding directly. Therefore, there is an increasing demand for improving the reliability of the performance against the magnetic head contamination shown in the above (2) and the wear of the magnetic head shown in (3). It is necessary to study the characteristics of the sliding surface between the magnetic head and the magnetic head.

In order to solve such technical problems, various conventional methods have been proposed in order to obtain a magnetic recording medium having excellent electromagnetic conversion characteristics over a long period of time by preventing wear of the magnetic head to improve running durability. Proposals have been made (see, for example, Patent Documents 1 and 2).
However, according to the techniques proposed in the related art as described above, an effect for reducing “magnetic head contamination” caused by repeated sliding of the magnetic head and the magnetic tape has not yet been obtained.

On the other hand, in order to prevent contamination of the magnetic head, a lubricant is added to the magnetic layer and the non-magnetic layer below it, or applied to the surface of the magnetic layer to reduce the frictional resistance on the surface of the magnetic layer and slide Attempts have been made to improve durability.
In particular, it is known that thiophosphite is a material having an excellent effect of reducing frictional resistance (see, for example, Patent Documents 3 and 4), and it is also known that an effect as a head cleaner can be obtained. (For example, refer to Patent Document 5).
JP 2003-123224 A Japanese Patent Laid-Open No. 11-3516 JP-A-9-128729 JP 2001-84566 A JP-A-11-228994

  However, even in the various magnetic recording media that have been proposed in the past, the “magnetic head contamination” caused by repeated sliding of the magnetic head and the magnetic tape is still a magnetic recording medium whose recording density will further increase in the future. In response to this, it can be said that the study on the reduction is insufficient.

In other words, thiophosphite is an effective material for reducing friction, but it is difficult to maintain the effect for a long time only by using it as a lubricant.
This is because thiophosphite is a highly reactive material and has the property of adsorbing on the surface of the metal oxide.
In particular, when it is contained in the magnetic layer, it is adsorbed to the magnetic powder, and the effective amount of the thiophosphite ester as a lubricant on the surface of the magnetic layer is inevitably reduced, and a sufficient friction reducing effect is obtained for a long time. In addition, it is difficult to maintain the magnetic head, and deposits are generated on the magnetic head due to sliding with the magnetic head.

  Therefore, in the present invention, the effect of reducing the surface friction over a long period of time can be obtained, the excellent sliding durability can be maintained, and the effect of reducing the deposits on the magnetic head can be obtained over a long period of time. Therefore, it was decided to provide a magnetic recording medium having excellent durability for use.

In the present invention, on the nonmagnetic support, a lower nonmagnetic layer containing at least nonmagnetic powder and a binder and a magnetic layer containing at least magnetic powder and a binder are laminated to form a magnetic layer. The layer contains 0.1 to 5 parts by weight of a thiophosphite ester represented by the following formula (1) with respect to 100 parts by weight of the magnetic powder, and the polyvalent carboxylic acid is contained in 100 parts by weight of the magnetic powder. A magnetic recording medium containing 1 to 7 parts by weight is provided.
(RS) ₃ P (1)
However, in formula, R is a C7-C25 linear or branched saturated or unsaturated alkyl group.

  According to the present invention, since the lubricating effect on the surface of the magnetic layer is exhibited over a long period of time, an increase in the coefficient of friction is suppressed even when a large number of runs are performed, and magnetic head contamination can be reduced, which is favorable. A highly reliable magnetic recording medium capable of maintaining signal characteristics for a long period of time was obtained.

The magnetic recording medium of the present invention will be specifically described with reference to the drawings, but the present invention is not limited to the examples shown below.
FIG. 1 shows a schematic sectional view of an example of the magnetic recording medium of the present invention.
The magnetic recording medium 10 has a configuration in which a lower nonmagnetic layer 3 and a magnetic layer 2 are formed on a nonmagnetic support 1 by applying a multilayer coating.

As the nonmagnetic support 1, any conventionally known substrate for a coating type magnetic recording medium can be applied. Specific examples of materials include polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyethylene , Polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate, cellulose diacetate, and cellulose butyrate, vinyl resins such as polyvinyl chloride and polyvinylidene chloride, plastics such as polycarbonate, polyimide, and polyamideimide, aluminum alloys, titanium alloys And light metals such as alumina and ceramics such as alumina glass.
The nonmagnetic support 1 may be appropriately subjected to surface treatment such as corona discharge treatment, plasma treatment, undercoating treatment, heat treatment, dust removal treatment, metal vapor deposition treatment, and alkali treatment.

The magnetic layer 2 will be described.
The magnetic layer 2 is composed mainly of a magnetic powder and a binder. In addition, various additives such as a lubricant, an abrasive, a dispersant, an antistatic agent, and an antirust agent are mixed, and an organic solvent is used. It is formed by applying a magnetic paint prepared by kneading and dispersing.

As the magnetic powder, a material having suitable magnetic characteristics (coercive force, magnetization amount) corresponding to the recording / reproducing characteristics of the VTR format and data drive format to which the finally obtained magnetic recording medium is applied is selected.
For example, Fe-based and Fe-Co-based metal powders, barium ferrite, iron carbide, iron oxide, and the like can be mentioned. Co, Ni, Cr, Mn, Mg, Ca, Ba, Sr, Zn, Ti can be used as subelements. , Mo, Ag, Cu, Na, K, Li, Al, Si, Ge, Ga, Y, Nd, La, Ce, Zr, and other metal compounds may coexist.

As the binder to be contained in the magnetic layer 2, conventionally known thermoplastic resins, thermosetting resins, radiation cross-linked resins by electron beams, etc., and mixtures thereof for forming a coating type magnetic layer are used. can do.
Further, a polyisocyanate may be added as a curing agent to the binder to form a crosslinked structure in the molecule.
In this case, examples of the polyisocyanate that is a curing agent include isocyanates such as tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylene diisocyanate, and isophorone diisocyanate, and these isocyanates and trimellilol propane. Any of adducts with polyhydric alcohols or condensation products of isocyanates can be applied.

As the abrasive, it is desirable to apply inorganic particles having a Mohs hardness of 5 or more.
Thereby, a desired roughness can be provided on the surface of the magnetic layer, and a cleaning effect for polishing the sliding magnetic head can be obtained.
Examples of the inorganic particles include Al ₂ O ₃ (Mohs hardness 9), TiO (Mohs hardness 6), TiO ₂ (Mohs hardness 6.5), SiO ₂ (Mohs hardness 7), SnO ₂ (Mohs hardness 6.5). ), Cr ₂ O ₃ (Mohs hardness 9) and α-Fe ₂ O ₃ (Mohs hardness 5.5), which can be used alone or in combination. In particular, inorganic particles having a Mohs hardness of 8 or more are suitable.
When inorganic particles having a Mohs hardness of less than 5 are used, the particles easily fall off from the magnetic layer 2, a sufficient magnetic head polishing effect cannot be obtained, the magnetic head is easily clogged, and running durability is improved. to degrade.
The content of the abrasive is usually preferably in the range of 0.1 to 20 parts by weight, particularly in the range of 1 to 10 parts by weight with respect to 100 parts by weight of the magnetic powder.

  Dispersants include fatty acids having 5 to 25 carbon atoms, and metal soaps, fatty acid esters, fatty acid amides, amines, quaternary ammonium salts, phosphoric acid esters, boric acid esters, etc. made of alkali metals or alkaline earth metals thereof. Any known dispersant can be applied.

  Antistatic agents include conductive fine powders such as carbon black and carbon black graft polymer, natural surfactants such as saponin, nonionic surfactants such as alkylene oxide, glycerin, and glycidol, and higher alkylamines. , Quaternary ammonium salts, pyridines, salts of other heterocyclic compounds, cationic surfactants such as phosphonium or sulfonium, anionicity containing acidic groups such as carboxylic acid, phosphoric acid, sulfate ester group, phosphate ester group Examples thereof include amphoteric surfactants such as surfactants, amino acids, aminosulfonic acids, sulfuric acid of amino alcohol, and phosphate esters, and any of known antistatic agents can be applied.

In the magnetic recording medium of the present invention, a thiophosphite represented by the following formula (1) is internally added as a lubricant in the magnetic layer.
(RS) ₃ P (1)
However, in formula, R shall be a C7-C25 linear or branched saturated or unsaturated alkyl group.
Examples of such a thiophosphite include various phosphite compounds such as tridecyl thiophosphite, trilauryl thiophosphite, tripalmityl thiophosphite, and the like.
The addition amount of the thiophosphite is preferably 0.1 to 5 parts by weight with respect to 100 parts by weight of the magnetic powder in the magnetic layer 2.

In the magnetic recording medium of the present invention, a polyvalent carboxylic acid is added to the magnetic layer.
In particular, a polyvalent carboxylic acid having a molecular weight of 300 or less is preferable, and the addition amount is desirably 1 to 7 parts by weight with respect to 100 parts by weight of the magnetic powder.
As polyvalent (divalent or higher) carboxylic acid having a molecular weight of 300 or less, or an anhydride thereof, dicarboxylic acid having two carboxyl groups in the molecule, tricarboxylic acid having three carboxyl groups in the molecule, A tetracarboxylic acid having four carboxyl groups is applicable.
Specifically, examples of the dicarboxylic acid include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, and other saturated aliphatic dicarboxylic acids, maleic acid, fumaric acid, and the like. And aromatic dicarboxylic acids such as unsaturated aliphatic dicarboxylic acid, phthalic acid, isophthalic acid and terephthalic acid.
Examples of the tricarboxylic acid include citric acid, nitrilotriacetic acid, and benzenetricarboxylic acid. Examples of the tetracarboxylic acid include benzenetetracarboxylic acid.
Such a polyvalent carboxylic acid has a strong adsorptive power to the magnetic powder, inorganic powder, and carbon black in the magnetic layer 2, so that the surface coating effect of the powder component can be obtained. Adsorption to the powder component is suppressed, and it is possible to maintain the lubrication effect for a long time.

In addition, the magnetic recording medium of the present invention can be used in combination with the following lubricants.
Specifically, butyl stearate, s-butyl stearate, isopropyl stearate, butyl oleate, amyl stearate, 3-methylbutyl stearate, 2-ethylhexyl stearate, 2-hexyldecyl stearate, butyl palmitate 2-ethylhexyl myristate, a mixture of butyl stearate and butyl palmitate, butoxyethyl stearate, 2-butoxy-1-propyl stearate, dipropylene glycol monobutyl ether esterified with stearic acid, diethylene glycol Various ester compounds such as dipalmitate, hexamethylenediol esterified with myristic acid to form a diester, glycerin oleate and the like can be mentioned.
In addition, when a lubricant is internally added to the magnetic layer 2, the following can be applied in addition to the above example.
For example, C8-22 fatty acid or fatty acid amide, aliphatic alcohol and the like can be mentioned. Silicon oil, graphite, molybdenum disulfide, boron nitride, fluorinated graphite, fluorine alcohol, polyolefin (polyethylene wax, etc.), polyglycol (polyethylene oxide wax, etc.), alkyl phosphate ester, polyphenyl ether, tungsten disulfide Can be used.

Next, the lower nonmagnetic layer 3 will be described.
The lower nonmagnetic layer 3 comprises a nonmagnetic powder and a binder as main components, and is mixed with other lubricants and various additives, kneaded and dispersed using an organic solvent, and prepared as a coating for the lower nonmagnetic layer. It is formed by apply | coating.
As the non-magnetic powder, inorganic particles having various shapes such as a needle shape, a spherical shape, and a plate shape can be appropriately used.
As the binder constituting the lower nonmagnetic layer 3, any of those applicable to the magnetic layer 2 described above can be used.
In addition, any known organic solvent, dispersion apparatus, and kneading apparatus for producing a coating for the lower nonmagnetic layer can be used.

The lower nonmagnetic layer 3 may contain inorganic particles having a Mohs hardness of 5 or more. As a result, predetermined fine protrusions can be formed on the surface of the magnetic layer 2 formed on the upper layer, and the running performance can be improved and the cleaning effect of the magnetic head can be obtained.
Examples of the inorganic particles include Al ₂ O ₃ (Mohs hardness 9), TiO (same 6), TiO ₂ (same 6.5), SiO ₂ (same 7), SnO ₂ (same 6.5), Cr ₂ O ₃ (same as 9) and α-Fe ₂ O ₃ (same as 5.5) can be mentioned, and these can be used alone or in combination.

The lower nonmagnetic layer 3 preferably contains the following antistatic agent.
For example, conductive fine powder such as carbon black and carbon black graft polymer, natural surfactant such as saponin, nonionic surfactant such as alkylene oxide, glycerin and glycidol, higher alkylamines, quaternary Ammonium salts, pyridine, salts of other heterocyclic compounds, cationic surfactants such as phosphonium or sulfonium, anionic surfactants containing acidic groups such as carboxylic acid, phosphoric acid, sulfate ester group, phosphate ester group, Examples include amphoteric surfactants such as amino acids, aminosulfonic acids, sulfuric acid of amino alcohol, and phosphate esters.
When conductive fine particles are applied as the antistatic agent, it is preferably 1 to 15 parts by weight with respect to 100 parts by weight of the nonmagnetic powder in the lower nonmagnetic layer 3, and even when a surfactant is used. Similarly, it is preferable to add 1 to 15 parts by weight.

Next, a method for manufacturing the magnetic recording medium of the present invention will be described.
First, a magnetic coating material for forming a magnetic layer and a nonmagnetic coating material for forming a lower nonmagnetic layer are prepared.
These coating materials are prepared by dispersing magnetic powder or non-magnetic powder in a binder, mixing a lubricant, various fillers, additives and the like, and dispersing and kneading using a predetermined solvent.
As the solvent, conventionally known materials can be applied to both magnetic paints and non-magnetic paints, for example, ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, alcohols such as methanol, ethanol, and propanol. Solvent, ester solvents such as methyl acetate, ethyl acetate, butyl acetate, propyl acetate, ethyl lactate, ethylene glycol acetate, ether solvents such as diethylene glycol dimethyl ether, 2-ethoxyethanol, tetrahydrofuran, dioxane, benzene, toluene, xylene, etc. Aromatic hydrocarbon solvents, halogenated hydrocarbon solvents such as methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, chlorobenzene and the like. These can be used alone or in appropriate mixture.

The method for kneading the paint is not particularly limited, and the order of adding each component can be changed as appropriate.
For paint preparation, for example, ordinary kneaders such as sand mill, dyno mill, double cylinder pearl mill, two roll mill, three roll mill, ball mill, high speed impeller disperser, high speed stone mill, high speed impact mill, extruder, Disperse kneaders, high-speed mixers, homogenizers, ultrasonic dispersers, etc. can be applied.

The magnetic layer 2 and the lower nonmagnetic layer 3 are formed by applying the prepared magnetic coating material and nonmagnetic coating material on the nonmagnetic support 1 in a multilayer manner and performing a drying treatment.
In addition, as a coating method, a lower layer coating is applied and dried, and a so-called wet-on-dry coating method in which an upper layer coating is applied to the dried lower layer coating and dried, and a lower layer coating in a wet state is applied. There is a so-called wet-on-wet coating method (wet multi-layer coating method) in which an upper coating film is applied over a film. Any method may be applied to the magnetic recording medium of the present invention.

The coating can be applied directly on the nonmagnetic support 1, but may be applied through an adhesive layer as appropriate.
Specifically, as an application method, air doctor coat, blade coat, rod coat, extrusion coat, air knife coat, squeeze coat, impregnation coat, reverse roll coat, transfer roll coat, gravure coat, kiss coat, cast coat, spray coat and Any of spin coating methods can be applied.

As described above, after the nonmagnetic paint and the magnetic paint are applied, these binders are cured and a drying process is performed.
The curing reaction is performed by heating the nonmagnetic paint and the magnetic paint at a predetermined temperature.
Thereby, the lower nonmagnetic layer 3 and the magnetic layer 2 are formed in this order on the nonmagnetic support 1.

Thereafter, a calendar process is performed to smooth the surface. Thereafter, a back coat layer (not shown) is formed on the main surface opposite to the magnetic layer forming surface. The backcoat layer is additionally provided to prevent electrification, improve the slidability between the back surface of the medium and various sliding members, and improve running durability.
Thereafter, the nonmagnetic support is cut into a 1/2 inch width to obtain the desired magnetic tape shape and accommodated in the cartridge.

  For the magnetic recording medium of the present invention, a specific sample magnetic tape was prepared and examined. In addition, this invention is not limited to the example shown below.

First, in Comparative Examples 1 to 7 described below, sample magnetic tapes were prepared by adjusting the content of thiophosphite (C ₁₂ H ₂₅ S) ₃ P in the magnetic layer.
In these, polyvalent carboxylic acid is not contained in the magnetic layer.

[Comparative Example 1]
The first composition is kneaded with an extruder using the materials shown below. Then, the 1st composition and the 2nd composition shown below were added to the stirring tank provided with the disper, and the preliminary mixing was performed. Thereafter, sand mill mixing was further performed, and filter treatment was performed to produce a magnetic coating material for forming a magnetic layer.

(First composition)
Fe-Co-based metal magnetic powder A: 100 parts by weight (major axis length 0.1 μm, Co / Fe = 30 atm%, specific surface area = 47 m ² / g, saturation magnetization = 150 Am ² / kg, coercive force = 184 kA / m)
Vinyl chloride resin A (cyclohexanone solution 30 wt%): 55.6 parts by weight (degree of polymerization 300, Mn = 10000, OSO ₃ K = 0.07 mmol / g, secondary OH = 0.3 mmol / g as a polar group) To do.)
Aluminum oxide powder A: 5 parts by weight (α-Al ₂ O ₃ , average particle size 0.2 μm)

(Second composition)
Polyurethane resin UR8200 (manufactured by Toyobo): 27.8 parts by weight n-butyl stearate: 1 part by weight Methyl ethyl ketone: 121.3 parts by weight Toluene: 121.3 parts by weight Cyclohexanone: 60.7 parts by weight

  Next, the third composition is kneaded with an extruder using the materials shown below. Then, the 3rd composition and the 4th composition shown below were added to the stirring tank provided with the disper, and the preliminary mixing was performed. Thereafter, sand mill mixing was further performed and filter treatment was performed to prepare a coating material for forming the lower non-magnetic layer.

(Third composition)
Acicular iron oxide powder: 100 parts by weight (α-Fe ₂ O ₃ , average major axis length 0.15 μm)
Vinyl chloride resin A: 55.6 parts by weight (resin solution: resin content 30 wt%, cyclohexanone 70 wt%)
Carbon black (Ketjen EC): 10 parts by weight

(Fourth composition)
Polyurethane resin UR8200 (manufactured by Toyobo): 18.5 parts by weight n-butyl stearate: 1 part by weight Methyl ethyl ketone: 108.2 parts by weight Toluene: 108.2 parts by weight Cyclohexanone: 18.5 parts by weight

Next, 4 parts by weight of polyisocyanate (trade name: Coronate L, manufactured by Nippon Polyurethane Co., Ltd.) and myristic acid are used as a curing agent in each of the magnetic coating material and the lower non-magnetic layer coating material prepared as described above. 2 parts by weight were added.
Then, these paints are placed on a polyethylene naphthalate film (PEN film) having a thickness of 6.2 μm which is a nonmagnetic support so that the lower nonmagnetic layer is 2.0 μm and the magnetic layer is 0.2 μm. Simultaneous multilayer coating was applied, and then calendar treatment was performed.

Further, as a back coat layer, a coating having the following composition was applied to a thickness of 0.6 μm on the surface opposite to the magnetic layer and dried.
Carbon black (trade name # 80 manufactured by Asahi): 100 parts by weight Polyester polyurethane: 100 parts by weight (trade name N-2304 manufactured by Nippon Polyurethane Co., Ltd.)
Methyl ethyl ketone: 500 parts by weight Toluene: 400 parts by weight Cyclohexanone: 100 parts by weight

  Thereafter, it was slit into a 1/2 inch (12.65 mm) width to obtain a magnetic tape, and after writing a servo signal, it was incorporated into an LTO Ultrium Generation 1 cartridge manufactured by Sony Corporation as a sample.

[Comparative Examples 2 to 7]
0.03-7 parts by weight of thiophosphite (trilauryl thiophosphite) was added to 100 parts by weight of magnetic powder in the magnetic coating. Each addition amount is shown in Table 1 below.
The other conditions were the same as in Comparative Example 1, and a sample was prepared.

[Comparative Example 8], [Examples 1-4]
0.03 to 5 parts by weight of a predetermined amount of thiophosphite ester (trilauryl thiophosphite) is added to 100 parts by weight of magnetic powder, and a polyvalent carboxylic acid (citric acid anhydride) is added. 3 parts by weight was added to 100 parts by weight of the magnetic powder. The respective addition amounts are shown in Table 2 below.
The other conditions were the same as in Comparative Example 1, and a sample was prepared.

[Comparative Example 9], [Examples 5 to 8], [Comparative Example 10]
In the magnetic coating material, 1 part by weight of thiophosphite ester (trilauryl thiophosphite) is added to 100 parts by weight of magnetic powder, and polyvalent carboxylic acid (citric acid anhydride) is added to 100 parts by weight of magnetic powder. On the other hand, a predetermined amount in the range of 0.5 to 9 parts by weight was added. The respective addition amounts are shown in Table 3 below.
The other conditions were the same as in Comparative Example 1, and a sample was prepared.

[Comparative Example 11], [Examples 9 to 13], [Comparative Example 12]
In the magnetic coating, 1 part by weight of thiophosphite ester (trilauryl thiophosphite) is added to 100 parts by weight of magnetic powder, and polyvalent carboxylic acid (nitrilotriacetic acid) is added to 100 parts by weight of magnetic powder. A predetermined amount in the range of 0.5 to 9 parts by weight was added. The respective addition amounts are shown in Table 4 below.
The other conditions were the same as in Comparative Example 1, and a sample was prepared.

  The following evaluation was performed on the samples of Examples 1 to 13 and Comparative Examples 1 to 12 manufactured as described above.

[Magnetic head dirt evaluation]
Using a LTO Ultrium generation 1 drive manufactured by HP, the sample magnetic tape was shuttled over its full length for 48 hours, and the amount of dirt on the magnetic head was evaluated numerically.
The magnetic head contamination was evaluated immediately after the sample magnetic tape was produced and after storage for 14 days under conditions of a temperature of 40 ° C. and a relative humidity of 20%.
As an evaluation method, after the shuttle travels, the adhesive tape is attached to the sliding surface of the magnetic head, and the amount of adhesion to the adhesive tape when peeled off is evaluated in the following five stages.
The degree of contamination of the magnetic head was evaluated by binarizing the adhesive tape surface by image analysis and evaluating the area ratio (%) of the portion where the contamination was attached.
1: Less than 2%.
2: 2% or more and less than 5%.
3: 5% or more and less than 8%.
4: 8% or more and less than 10%.
5: 10% or more.

  Tables 1 to 4 below show the contents of the thiophosphite ester and polycarboxylic acid in each magnetic layer and the evaluation results of the contamination of the magnetic head for the sample magnetic tape described above.

As shown in Table 1, in Comparative Examples 3 to 6 in which thiophosphite is contained in the magnetic layer in an amount of 0.1 to 5 parts by weight with respect to 100 parts by weight of the magnetic powder, the magnetic head immediately after the production of the magnetic tape The evaluation of dirt was sufficiently good for practical use.
In Comparative Example 1 in which the thiophosphite is not contained in the magnetic layer and Comparative Example 2 in which the addition amount is extremely small (0.03 parts by weight), the evaluation of the magnetic head contamination is already practical immediately after the production of the magnetic tape. Was insufficient.
Also in Comparative Example 7 in which an excess amount of thiophosphite was contained in the magnetic layer, the evaluation of the magnetic head contamination was already insufficient practically immediately after the production of the magnetic tape.
From the above, it was found that when the thiophosphite ester is contained as a lubricant in the magnetic layer, the content is preferably 0.1 to 5 parts by weight with respect to 100 parts by weight of the magnetic powder.
However, the samples of Comparative Examples 3 to 6, which were evaluated well immediately after the production of the magnetic tape, were stored for 14 days under the condition of a temperature of 40 ° C. and a relative humidity of 20%, and then were driven to evaluate the contamination of the magnetic head. As a result, the evaluation deteriorated remarkably in any sample. This is because the thiophosphite ester was adsorbed on the magnetic powder with the lapse of time, and the effect as a lubricant was reduced.

As shown in Table 2, the magnetic layer contains 0.1 to 5 parts by weight of thiophosphite with respect to 100 parts by weight of magnetic powder, and citric acid as a polyvalent carboxylic acid is added to 100 parts by weight of magnetic powder. On the other hand, in Examples 1 to 4 containing 3 parts by weight, not only immediately after the production of the magnetic tape but also after being stored under the condition of a temperature of 40 ° C. and a relative humidity of 20%, extremely good evaluation of the magnetic head contamination was obtained. Maintained.
This is because the polyvalent carboxylic acid coats the surface of the magnetic powder, thereby preventing the thiophosphite from adsorbing to the magnetic powder and exerting the effect as a lubricant over a long period of time.
On the other hand, in Comparative Example 8 in which the content of the thiophosphite in the magnetic layer was very small, evaluation of magnetic head contamination was practically insufficient both immediately after the production of the magnetic tape and after storage.

  The amount of polyvalent carboxylic acid added to the magnetic layer was examined using citric acid and nitrilotriacetic acid. As shown in Tables 3 and 4, when 1 to 7 parts by weight is added to 100 parts by weight of the magnetic powder, the evaluation of the magnetic head contamination is good even after storage for 14 days, and the storage characteristics are excellent. It was.

From the above, by including both the thiophosphite ester and the polyvalent carboxylic acid in the magnetic layer, the effect of the thiophosphite ester as a lubricant can be maintained over a long period of time. It has been found that head contamination can be effectively prevented.
In particular, the addition amount of thiophosphite is 0.1 to 5 parts by weight with respect to 100 parts by weight of the magnetic powder, and the addition amount of polyvalent carboxylic acid is 1 to 7 parts by weight with respect to 100 parts by weight of the magnetic powder. It was clarified that the effect of preventing contamination of the magnetic head after storage was high by selecting the range of the portion.

1 is a schematic sectional view of a magnetic recording medium of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Nonmagnetic support body, 2 ... Magnetic layer, 3 ... Lower nonmagnetic layer, 10 ... Magnetic recording medium

Claims (2)

On the nonmagnetic support, a lower nonmagnetic layer containing at least a nonmagnetic powder and a binder,
A magnetic recording medium formed by laminating at least a magnetic layer containing a magnetic powder and a binder,
In the magnetic layer, a thiophosphite ester represented by the following formula (1) and at least one polyvalent carboxylic acid are contained,
The thiophosphite is contained in an amount of 0.1 to 5 parts by weight with respect to 100 parts by weight of the magnetic powder.
The magnetic recording medium according to claim 1, wherein the polyvalent carboxylic acid is contained in an amount of 1 to 7 parts by weight with respect to 100 parts by weight of the magnetic powder.
(RS) ₃ P (1)
However, in formula, R is a C7-C25 linear or branched saturated or unsaturated alkyl group. 2. The magnetic recording medium according to claim 1, wherein the thiophosphite is trilauryl thiophosphite, and the polyvalent carboxylic acid is citric acid or nitrilotriacetic acid.

Images