JPH077497B2 - Binder for magnetic recording media - Google Patents

Description

TECHNICAL FIELD The present invention relates to a binder for a magnetic recording medium,
More specifically, the present invention relates to a binder for a magnetic recording medium, which contains a specific thermoplastic polyurethane resin excellent in mechanical properties, heat resistance, durability and dispersibility of magnetic powder.

[Conventional technology and its problems]

In recent years, magnetic recording media such as VTRs, audio tapes, and floppy disks have become widespread, and their required performance has become diverse and strict. Among them, the demand for reliability of recording and reproduction is particularly increasing, and there is a demanded performance that can withstand long-time running in various environments such as low temperature / low humidity to high temperature / high humidity.

Conventionally, polyester resins, polyurethane resins, etc. have been widely used as binders for magnetic recording media, but various studies have been made to meet the above requirements, and nowadays, magnetic layers have good heat resistance and abrasion resistance. In order to improve the durability of the magnetic recording medium, a binder mainly containing a thermoplastic polyurethane resin and polyisocyanate, or a resin having a large number of hydroxyl groups such as a vinyl chloride-vinyl acetate copolymer Binders and the like to which is added have been put to practical use.

These thermoplastic polyurethane resins used as the binder component of the magnetic recording medium include (1) a curing agent (crosslinking agent) such as polyisocyanate as disclosed in JP-A-58-203623. ) To form a network bond to improve the mechanical properties and heat resistance of the binder and to impart heat resistance and wear resistance to the magnetic layer. Thermoplastic polyurethane resin with a relatively low urethane group concentration (containing isocyanate). 15% or less) and (2) excellent mechanical properties such as tensile strength of the coating and heat resistance without reacting the curing agent, and by utilizing this property, heat resistance and abrasion resistance can be imparted to the magnetic layer. It is classified as a thermoplastic polyurethane resin with a high urethane group concentration (isocyanate content of 15% or more).

The thermoplastic polyurethane resin (1) has a low urethane group concentration (isocyanate content of 15% or less), is soft,
A large amount of substantially linear thermoplastic polyurethane resin with a molecular weight of 10,000 to 50,000, especially 15,000 to 35,000 is used.Since it has a relatively low molecular weight, it can be easily made into a magnetic paint, but its mechanical properties and flow starting point Is inferior in heat resistance,
Under the present circumstances, a curing agent is required, and the magnetic layer after curing leaves much room for improvement, particularly in terms of performance in a high temperature environment.

Further, a thermoplastic polyurethane resin having a molecular weight of 50,000 or more extremely deteriorates the dispersibility of magnetic powder and is difficult to be made into a paint.

In the case of (2), a thermoplastic polyurethane resin having a relatively high urethane group concentration (isocyanate content of 15 to 40%), a high hardness, a high flow starting point and a temperature of 100 ° C or higher and a molecular weight of 20,000 to 50,000 is used. Although it has a relatively low molecular weight, it has excellent mechanical properties and a high flow starting point, so although it has excellent heat resistance, wear resistance, etc., it has poor calendering due to factors such as the magnetic layer becoming too hard. The smoothness deteriorates, and undesired phenomena such as so-called "squeaking" occur during tape running. Further, since the attractive force between polyurethane molecules is increased due to the large number of urethane bonds, the solubility in normally used toluene, methyl ethyl ketone, etc. is deteriorated, and the stability of the magnetic coating and the magnetic particles in the magnetic coating film are reduced. In many cases, the dispersibility is significantly reduced.

[Means for solving problems]

Therefore, the present inventors have conducted extensive studies as to a method for improving the above-mentioned defects without impairing the excellent performance of the thermoplastic polyurethane resin, and have a specific molecular weight, a tensile breaking strength, a tensile breaking elongation, and a 100% modulus. By adopting a thermoplastic polyurethane resin having a glass transition temperature, it is possible to easily prepare a magnetic coating material, which is excellent in dispersibility and stability of the coating material and which is excellent in smoothness of the magnetic layer after coating. In addition, even in a system that does not use a curing agent, a magnetic layer with excellent durability can be obtained in a wide temperature and humidity range from low temperature low humidity to high temperature high humidity, and the durability of the magnetic layer by using it together with a low molecular weight polyisocyanate. The present invention has been completed by finding that the above is further improved.

That is, the present invention, the number average molecular weight of 5,000 to 40,000 by the analysis by gel permeation chromatography, the breaking strength in the tensile test of the pulling speed 100 mm / min is 200 kg / cm ² or more, breaking elongation 100 ~ 400%, 100% modulus. 150kg / cm ²
Above, the glass transition temperature was 40
The present invention provides a binder for a magnetic recording medium, which contains a thermoplastic polyurethane resin having a temperature of not less than 0 ° C.

The thermoplastic polyurethane resin used in the present invention is a gel permeation chromatography (column Shodex
The number average molecular weight is 5,000 to 40,000 as analyzed by A-80 3804 mixed with Showa Denko KK, and the more preferable molecular weight is 10,000 to 25,000. When the number average molecular weight is less than 5,000, the magnetic layer has poor mechanical properties and adhesion to the substrate. The number average molecular weight is 40,0.
If it exceeds 00, the dispersibility of the magnetic powder is deteriorated, and a great improvement in durability cannot be expected when the low molecular weight polyisocyanate is used together.

Tensile strength of the thermoplastic polyurethane resin used in the present invention is measured by Tensilon UTM-III manufactured by Toyo Sokki Co., Ltd. at a tensile speed of 100 mm / min, a breaking strength of 200 kg / cm ² or more, a breaking elongation of 100 to 400%. , 100% modulus 150 kg / cm ² or more, more preferably breaking strength 250 kg / cm ² or more, breaking elongation 100 to 300%, 100% modulus 230 kg / cm ²
That is all. If the breaking strength is less than 200 kg / cm ² , the elongation at break is less than 100% or more than 400%, and the 100% modulus is less than 150 kg / cm ² , the strength of the magnetic layer is poor, which is disadvantageous in terms of durability.

The physical property values in the present invention are measured using a film having a thickness of 0.15 to 0.22 mm.

The glass transition temperature of the thermoplastic polyurethane resin used in the present invention is measured by a dynamic viscoelasticity measuring device (RHEOVIBRON DDV-III
-EA, manufactured by Toyo Baldwin) measuring frequency 3.5H
z, measured at a heating rate of 3 ° C./min and 40 ° C. or higher. Those having a glass transition temperature of less than 40 ° C have poor heat resistance when the magnetic layer is formed. From the viewpoint of the calendering property of the magnetic layer and the surface smoothness of the magnetic layer, the glass transition temperature is 65.
Desirably below ℃. The glass transition temperature in the present invention is the temperature at which the ratio E ″ / E ′ of the loss elastic modulus E ″ and the storage elastic modulus E ′ becomes maximum in the dynamic viscoelasticity measurement.

The end of the thermoplastic polyurethane resin used in the present invention may be an isocyanate group at both ends,
Both ends may be hydroxyl groups, or one end may be an isocyanate group and the other end may be a hydroxyl group.

Further, from the viewpoint of durability and dispersibility of the magnetic layer, the thermoplastic polyurethane resin of the present invention comprises (I) a polyester polyol having a molecular weight of 500 to 3,000, (II) a diol having 2 to 10 carbon atoms, or bisphenol. A, bisphenol S, hydroquinone, and the like, diols obtained by adding 2 to 4 mol of ethylene oxide and propylene oxide, and (III) a diisocyanate having at least one aromatic ring are preferable.

As the polyester polyol having a molecular weight of 500 to 3,000, ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 2,3-butylene glycol, 1,4-butylene glycol, 2,2-dimethyl-1, 3
-Propanediol, diethylene glycol, 1,5-pentamethylene glycol, 1,6-hexamethylene glycol, cyclohexane-1,4-diol, cyclohexane-1,4-dimethanol, etc. Acid, maleic acid, adipic acid,
Glutaric acid, pimelic acid, suberic acid, azelaic acid,
Sebacic acid, phthalic acid, isophthalic acid, terephthalic acid,
Examples include dibasic acids such as hexahydroterephthalic acid and hexahydroisophthalic acid, esters of these acids, and polyester polyols obtained by polycondensation with acid halides. Further, lactones such as ε-caprolactone are glycols and the like. And polycaprolactone diols which have been subjected to ring-opening addition polymerization in the presence of

Among these, condensed polyester polyols and polycaprolactone polyols of ethylene glycol, 1,4-butylene glycol, 1,6-hexamethylene glycol alone or in a mixture with adipic acid or sebacic acid are preferable.

As the low molecular weight diol having 2 to 10 carbon atoms, the glycols used in the production of the polyester polyols may be used alone or as a mixture.

Examples of the diisocyanate having at least one aromatic ring include xylene diisocyanate, toluidine diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, p-phenylene diisocyanate, and m.
-Phenylene diisocyanate, 1,5-naphthylene diisocyanate, 4,4'-biphenyl diisocyanate,
Preferred are 3,3'-dimethylbiphenyl-4,4'-diisocyanate, 3,3'-dimethoxybiphenyl-4,4'-diisocyanate and the like and mixtures thereof.

In producing the thermoplastic polyurethane resin used in the present invention, the polyester polyol (I) and 2 to 2 carbon atoms are used.
The molar ratio with low molecular weight polyols having 10 (II) is
I / II = 1 / 2.5 to 1/20, preferably 1/4 to 1/8, and the molar ratio of OH / NCO of polyol and diisocyanate is 1 / 0.85 to 1 / 1.15, preferably 1.0 / 0.95 to 1.0 / 1.05
Is good. These ranges are preferable from the viewpoint of various properties of the obtained magnetic layer.

Further, in producing the thermoplastic polyurethane resin, it is possible to adopt a conventionally known method, for example,
After sufficiently mixing the reactants, the reaction mixture is poured into a flat plate or a vat to be heated, and then the mixture is cooled and then crushed, or dimethylformamide, toluene, xylene,
A production method such as a solution reaction method in which the reaction is carried out in a single or mixed solvent organic solvent such as benzene, dioxane, tetrahydrofuran, cyclohexanone, methyl ethyl ketone, methyl isobutyl ketone, ethyl acetate, butyl acetate can be employed. At this time, a reaction catalyst may be added to reduce the reaction temperature or the reaction time. Specific examples of the reaction catalyst include, for example, triethylenediamine, tetramethylethylenediamine, amine compounds such as tetramethylhexanediamine and salts thereof, dibutyltin dilaurate, tin octylate,
Examples thereof include organometallic compounds such as lead octylate and manganese octylate, and mixtures thereof. Also,
For the purpose of increasing the stability of the polyurethane resin, antioxidants, ultraviolet absorbers, hydrolysis inhibitors and the like can be blended alone or in combination.

Further, in the present invention, in order to further improve the durability of the polyurethane resin, it is preferable to cure the polyurethane resin by using an appropriate curing agent, and the high energy ray-curable polyfunctional vinyl is used as the curing agent. Examples thereof include monomers and polyisocyanate compounds, but a low molecular weight polyisocyanate compound having two or more isocyanate groups is most suitable.

As the low molecular weight polyisocyanate having two or more isocyanate groups that can be used in the present invention, in addition to the organic diisocyanate used in the production of the thermoplastic polyurethane resin, 3 mol of diisocyanates such as tolylene diisocyanate and hexamethylene diisocyanate are used. And a polyfunctional isocyanate compound such as modified diphenylmethane diisocyanate and polyphenylene methane polyisocyanate. These compounds are commercially available from Nippon Polyurethane Co., Ltd. as "Coronate L" and "Coronate H".
"Death module L", "Death module N", "Death module IL", "Death module" from Sumitomo Bayer Urethane Co., Ltd. under the product names such as "L", "Coronate 2030", "Millionate MR", "Millionate MTL" Module HL ",
Takeda Pharmaceutical Co., Ltd.'s "Takenate D-10" with product names such as "Desmodur R" and "Desmodur RF"
2 "," Takenate D-110N "," Takenate D-202 "
Etc. are each marketed under the trade name.

By adding 3 to 40 parts by weight of the above-mentioned low molecular weight polyisocyanate to 100 parts by weight of the thermoplastic polyurethane resin and curing it, the mechanical strength, abrasion resistance, heat resistance, moist heat resistance, solvent resistance and base of the magnetic layer are improved. The adhesion with the material can be greatly improved.

If necessary, in the binder component, a thermoplastic polyurethane resin, a cellulose derivative such as nitrocellulose, cellulose acetate, or cellulose acetate butyrate, which is usually used as a binder component of a magnetic recording medium; vinyl chloride / vinyl acetate copolymer Polymer, vinyl chloride / vinyl acetate /
Vinyl alcohol copolymer, vinyl chloride / vinyl acetate /
Vinyl chloride resin such as maleic acid copolymer; vinylidene chloride /
Vinylidene chloride resin such as vinyl chloride copolymer, vinylidene chloride / acrylonitrile copolymer; alkyd resin,
Polyester resin such as linear polyester; acrylic resin such as (meth) acrylic acid / acrylonitrile copolymer, methyl (meth) acrylate / acrylonitrile copolymer; acetal resin such as polyvinyl acetal, polyvinyl butyral; phenoxy resin, epoxy resin , Polyamide resin, butadiene / acrylonitrile copolymer,
A styrene / butadiene copolymer or the like may be used alone or in combination.

Ferromagnetic fine powders that can be used in the present invention include γ-Fe ₂ O ₃ , mixed crystals of γ-Fe ₂ O ₃ and Fe ₃ O ₄ , Co-modified iron oxide, CrO ₂ , iron, etc. Other alloy fine powders containing iron as a main component can be preferably used.
The shapes of these ferromagnetic fine powders are needle-like, plate-like,
Any shape such as spherical shape can be used.

[Action]

Although the mechanism relating to the durability of the magnetic recording medium has not been clarified, it can be inferred from the fact that the magnetic recording medium is subjected to a large shear stress due to the contact of the head while the magnetic recording medium is running. It is believed that at least the binder needs to have high mechanical strength. Moreover, it is considered essential that the mechanical strength is stable under the environment in which it is actually used.

Specific thermoplastic polyurethane resin in the present invention, 100% modulus at room temperature compared to conventional binders, greatly improves the mechanical strength typified by breaking strength, and as a measure of the temperature dependence of the mechanical strength It is considered that by improving the glass transition temperature of the resin to be higher than the practical temperature environment of the magnetic recording medium, it is possible to obtain high durability in a wide environment from low temperature to high temperature.

Further, it is considered that when the low molecular weight polyisocyanate, which is a curing agent, is added by lowering the molecular weight as compared with the conventional polyurethane resin, the crosslinking density is increased and the durability is further enhanced.

〔Example〕

The present invention will be specifically described below with reference to synthesis examples and examples, but the present invention is not limited to these examples.

In addition, all "parts" in the examples mean "parts by weight".

Synthesis Example 1 A reactor equipped with a thermometer, a stirrer, and a reflux condenser was equipped with 200 parts of methyl ethyl ketone, 190 parts of cyclohexanone, and a molecular weight of 1,0 consisting of adipic acid and 1,6-hexamethylene glycol.
100 parts of polyester polyol of 00, 2,2-dimethyl-1,
32 parts of 3-propanediol, 8 of 1,4-butylene glycol
Part, 121 parts of 4,4′-diphenylmethane diisocyanate,
Add 0.3 parts of dibutyltin dilaurate as a catalyst,
A thermoplastic polyurethane resin was prepared by reacting at 12 ° C. for 12 hours.

Synthesis Example 2 100 parts of methyl ethyl ketone, 140 parts of cyclohexanone, 100 parts of polycaprolactone polyol having a molecular weight of 1,000, 7.3 parts of ethylene glycol, 2,2-
24 parts of dimethyl-1,3-propanediol, 109 parts of 4,4'-diphenylmethane diisocyanate and 0.3 part of dibutyltin dilaurate as a catalyst were added, and the mixture was reacted at 85 ° C for 10 hours to obtain a thermoplastic polyurethane resin.

Synthetic Example 3 In the same container as in Synthetic Example 1, 116 parts of methyl ethyl ketone, 100 parts of tetrahydrofuran, 100 parts of polyester polyol having a molecular weight of 1,500 consisting of adipic acid and 1,4-butylene glycol, 2,2-dimethyl-1,3-propanediol. 35
Parts, 8.8 parts of 1,4-butylene glycol, 120 parts of 4,4′-diphenylmethane diisocyanate, and 0.3 parts of dibutyltin dilaurate as a catalyst were added and reacted at 70 ° C. for 15 hours to prepare a thermoplastic polyurethane resin.

Comparative Synthesis Example 1 In Synthesis Example 1, the addition amount was 2,2-dimethyl-1,3-propanediol 15 parts, 1,4-butylene glycol 4.1 parts, 4,
A thermoplastic polyurethane resin was prepared in the same manner except that 72 parts of 4'-diphenylmethane diisocyanate was used.

Comparative Synthesis Example 2 100 parts of a polyester polyol having a molecular weight of 1,000 consisting of a diol obtained by adding 2 moles of propylene oxide to bisphenol A and adipic acid, 22 parts of 2,2-dimethyl-1,3-propanediol, and 1,4-butylene glycol 5.6. Part 4,
A thermoplastic polyurethane resin was prepared in the same manner as in Synthesis Example 1 using 90 parts of 4'-diphenylmethane diisocyanate.

Thermoplastic polyurethane resins obtained in Synthetic Examples 1 to 3 and Comparative Synthetic Examples 1 and 2, and Nipolan N-23 as a commercial product.
01 (Nippon Polyurethane Co., Ltd., polyester type thermoplastic polyurethane resin) number average molecular weight, tensile breaking strength,
The measured values of 100% modulus, elongation at break, and glass transition temperature are shown in Table 1.

Examples 1 to 3 and Comparative Examples 1 to 3 Using the thermoplastic polyurethane resins obtained in Synthetic Examples 1 to 3 and Comparative Synthetic Examples 1 to 2 and commercially available Nipolan N-2301, magnetic disks were prepared by the following method. Was manufactured and its durability was measured.

The results are shown in Table 2.

<Manufacture of magnetic disk and evaluation of durability> 75 parts of polyurethane resin of Synthesis Examples 1 to 3 or Comparative Synthesis Examples 1 and 2 or solution of Nipolan N-2301 (nonvolatile content: 40%)-Vinylite VAGH (manufactured by Union Carbide Company, USA) , Vinyl chloride-vinyl acetate copolymer) 20 parts ・ γ-Fe ₂ O ₃ 100 parts ・ Carbon black 3 parts ・ Lubricant 2 parts ・ Methyl ethyl ketone 200 parts ・ Cyclohexanone 100 parts After kneading the above composition in a ball mill for 24 hours , 10 parts of a curing agent (Nippon Polyurethane Co., Coronate L) was added, and the magnetic coating obtained after kneading for 1 hour was further dried on a polyethylene terephthalate substrate film having a thickness of 70 microns so that the thickness after drying was 2 microns. Was dried and aged. Then, the film is cut into a predetermined size of a magnetic disk, mounted in a drive, and the reproduction output is 50% under the environment of 20 ℃ and 40 ℃
The durability was evaluated by measuring the time until it decreased.

[Effects of the Invention] As shown concretely in Examples, the thermoplastic polyurethane resin of the present invention has an unprecedented excellent effect as a binder for a magnetic recording medium in terms of durability. There is. This effect improves mechanical strength of polyurethane resin, especially tensile breaking strength, 100% modulus, suppresses elongation at break moderately, and magnetic recording glass transition temperature which is a measure of temperature dependence of mechanical properties. This is due to the fact that the temperature of the medium is higher than the temperature environment in which it is considered to be actually used, and further the molecular weight is lower than that of the conventional polyurethane resin, which is a remarkable effect as compared with the conventional thermoplastic polyurethane resin.

Therefore, it is considered that the magnetic recording medium using the thermoplastic polyurethane of the present invention can meet the demand for the emergence of a magnetic recording medium having high reliability in use under various environments.

Claims (1)

[Claims] 1. A number average molecular weight of 5,000 to 40,000 and a tensile speed of 1 as determined by gel permeation chromatography.
Breaking strength in a tensile test of 00 mm / min is 200 kg / cm ² or more, elongation at break 100 to 400%, 100% modulus 150 kg / cm ²
Above, the glass transition temperature was 40
A binder for a magnetic recording medium, which comprises a thermoplastic polyurethane resin having a temperature of ℃ or higher.