WO1997015048A1

WO1997015048A1 - Magnetic recording media

Info

Publication number: WO1997015048A1
Application number: PCT/KR1995/000177
Authority: WO
Inventors: Jong Min Park; Hyeon Soo Joh; Jin Sa Kim
Original assignee: Kolon Industries, Inc.
Priority date: 1995-10-18
Filing date: 1995-12-27
Publication date: 1997-04-24
Also published as: KR0163094B1; KR970022999A

Abstract

A magnetic recording media comprising at least two independent polyurethane resins, each having a weight average molecular weight of about 10,000 to 250,000 and an acidity of about 0.1 to 70 and exhibiting high dispersibility and dispersion stability of magnetic component without using chlorine-containing binder component, which is superior in durability and scratch resistance while retaining high magnetic recording performance.

Description

MAGNETIC RECORDING MEDIA

BACKGROUND OF THE INVENTION

Field of the invention

The present invention relates, in general, to magnetic recording media and, more particularly, to magnetic recording media superior in durability and scratch resistance and unharmful to the environment while retaining a high magnetic recording performance.

Description of the Prior Art Typically, a magnetic recording medium is prepared by coating a base film with at least one magnetic layer of a magnetic composition comprising binding material, magnetic component and solvent. The performance of magnetic recording medium is dependent on the magnetic and mechanical properties of magnetic layer including squareness ratio, electromagnetic transformation, wear resistance and durability.

Currently, the demand for coated magnetic recording media, especially, video tapes has been increasing. Users, that is, consumers further require that magnetic recording media be high-quality. In response to the requirement, the manufacturers of coated magnetic recording media have introduced new products into the market. The magnetic recording tape articles are not much different from one another in their electromagnetic transformation. But, there is a great difference in their scratch resistance and durability. For example, when magnetic recording media run on the head of a recording or reproducing machine for long periods, the degrees to which their magnetic layers are peeled off and worn out by the friction between the tapes and the head differ significantly. Because the damage in a magnetic recording layer causes a drop-out phenomenon and false signals, 5 research and development efforts have been directed to the improvement of durability and scratch resistance, above all. These types of research and development efforts have become important as magnetic recording media have developed into more dense forms with use of high frequency

10 signals.

Various attempts have been made to improve the durability of coated magnetic recording media. For example, Japanese Pat. Laid-Open Publication No. Heisei 5-54365 discloses a lower coated layer through which the

15. binding of a magnetic layer to a base film is much improved. Vinyl resin and polyurethane resin are used, as binding components, to improve the initial rigidity in the binder, as disclosed in Japanese Pat. Publication Nos. Sho 43-4634, Sho. 44-18222 and Sho. 53-11444. For binder, an

20 epoxy resin and a linear saturated polyester resin are used, as claimed in Japanese Pat. Publication No. 60-35733 and U.S. Pat. No. 4,238,548, respectively.

The conventional techniques, however, have proven to be problematic in many aspects as will be described below.

25 First, where vinyl resin is employed as a binding component, the magnetic layer becomes viscous under a hot, moist atmosphere, which undesirably increases its friction coefficient. Also, the vinyl resin itself shows an inferior adhesiveness to polyester film, a typical base

30 film, in addition to being poor in compatibility with polyurethane resin. Particularly, where the protection of the environment is currently a worldwide issue of interest, it is undesirable to use vinyl resin because it comprises chlorine, an environmental pollutant.

Then, epoxy resin or linear saturated polyester resin makes the magnetic layer on the base film too rigid to damage the head of the magnetic recording machine or for 5 the media to run with trouble. In addition, the epoxy resin and linear saturated polyester resin themselves have insufficient elasticity so that thickness loss increases, deleteriously affecting the electromagnetic transformation of the magnetic recording media.

10 Besides the above-mentioned problems, vinyl resin, epoxy resin and polyester resin are so problematic in compatibility with other resins used for magnetic recording media that the coating composition comprising them shows bad dispersion. Accordingly, the elongation

15. at rupture of the resulting binder and the toughness of the resulting magnetic layer come to be significantly decreased. Consequently, the magnetic recording media coated with such coating composition has many scratches in the magnetic layer which result from friction and

20 abrasion during running and, thus, additional drop-out phenomenon occurs .

The present applicant made an attempt to enhance the compatibility of the binding components with one another by using soft polyurethane resin and hard polyurethane

25 resin both as binding components as disclosed in Korean Pat. Publication No. 90-7013. According to this patent, the improved compatibility among the binding components brings about a significant advancement in dispersibility of the coating composition and the adhesiveness of the

30 polyurethane resins to the magnetic component and base film is much improved, to enforce the durability and scratch resistance. However, high dense magnetic recording media are required and, as a higher signal frequency range is used, coated magnetic recording media should employ metal powder which has a BET of magnetic component higher than 40 πr/g- To this end, a magnetic coating composition with more improved dispersibility and dispersion stability is required.

SUMMARY OF THE INVENTION

Therefore, it is a principal object of the present invention to overcome the above problems encountered in prior arts and to provide a magnetic recording medium, comprising at least one magnetic layer which is formed by coating highly dispersible and dispersion-stable magnetic coating composition on a non-magnetic support, which is superior in durability and scratch resistance and not harmful to the environment. Based on the intensive and thorough research by the present inventors, it was surprisingly found that a magnetic coating composition comprising at least two independent polyurethane resins with specific molecular weights and acidities exhibits high dispersibility and dispersion stability of magnetic component without using chlorine-containing binder component, an environmental pollutant and the magnetic recording media comprising the magnetic coating composition is superior in durability and scratch resistance while retaining high magnetic recording performance.

In accordance with the present invention, the above object could be accomplished by a provision of a magnetic recording medium which comprises at least one magnetic layer formed by coating a magnetic coating composition on a non-magnetic support, wherein said magnetic coating composition comprises two independent polyurethane resins, each of which is separately used for primary and secondary dispersion of magnetic component and is selected from the group consisting of:

(a) a polyurethane resin having a weight average molecular weight of about 10,000 to 250,000 and an acidity of about 0.1 to 70; and

(b) a mixture of polyurethane resins having a weight average molecular weight of about 10,000 to 250,000 according to the following formula:

n

Mw = Σ V_j MW_j i=l

where V_; is the weight fraction of each polyurethane resins, Mw_i is the weight average molecular weight of each polyurethane resins, and n is an integer of not less than 2, and an acidity of about 0.1 to 70, said polyurethane resin used for said secondary dispersion having a larger weight average molecular weight and a lower acidity than said polyurethane resin used for said primary dispersion.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, a first polyurethane resin with a weight average molecular weight (Mw) from about 10,000 to 250,000, playing a role of a binder for coating composition, is added to primarily disperse magnetic components. If the weight average molecular weight of the polyurethane resin is below 10,000, magnetic components are dispersed well but the properties of the magnetic coating composition cannot be maintained because of lack of dispersion stability after the dispersion. On the other hand, if the polyurethane resin have a weight average molecular weight of higher than 250,000, it shows low solubility to solvent with a decreased compatibility with dispersing composition. In 5 this case, owing to high viscosity, it is very difficult to disperse the magnetic components.

A second polyurethane resin is added upon secondary dispersion of magnetic components and must have a weight average molecular weight greater than that of the first

10 polyurethane resin, in accordance with the present invention. If the second polyurethane resin has an average molecular weight of no more than that of the first one, the mobility of the added polyurethane resin in the magnetic coating composition becomes increased after

15. completion of the dispersion, so that the magnetic components are likely to be detached from the polyurethane resin with a decrease of dispersion stability. Particularly, it is preferred that the polyurethane resin added for secondary dispersion has a weight average

20 molecular weight greater than that of the polyurethane resin added for first dispersion by at least 500.

In acidity (measured by conventional method and represented by mg of KOH per gram of resin), both of the two polyurethane resins should range from 0.1 to 70. For

25 example, if the polyurethane resins has an acidity lower than 0.1, high dispersion cannot be achieved. On the other hand, if the acidity exceeds 70, the polyurethane resins themselves agglomerate, bringing a significant degradation into compatibility and dispersibility. In

30 contrast with weight average molecular weight, the acidity of the polyurethane resin used for primary dispersion is lower than that of the polyurethane resin used for secondary dispersion. Where the polyurethane resin used for secondary dispersion has an acidity of not less than that of the one used for primary dispersion, the secondary polyurethane resin detaches magnetic components from the primary one, degrading the properties of the magnetic 5 coating composition. It is preferred that the secondary polyurethane resin has an acidity lower than that of the primary polyurethane resin by 0.05 or more.

In accordance with the present invention, two or more species of polyurethane resins may be, in combination,

10 used for the primary and secondary dispersion. In this case, the mixed polyurethane resins has an average molecular weight ranging from about 10,000 to 250,000 and an acidity ranging from about 0.1 to 70.

Usually, polyurethane resin is prepared by reacting

15. a polyhydroxy compound, such as polyol, with a polydiisocyanate compound in a solvent in the presence or absence of an amine chain extender.

Polyurethane resin suitable for the present invention has at least one of the functional groups: -SO_jM, -OSO3M,

20 -COOH, -PO(OM)₂ and -OPO(OM)₂, where M is H, Li, NR₄ where R is H, or an alkyl, alkanol or alkenyl group containing 1-1000 carbon atoms and preferably 1-100 carbon atoms, all of which are known to enhance the dispersibility of magnetic component.

25 Particularly preferred is a polyurethane resin which contains about 10 to 1,000 equivalents of metallic sulfonate compound per 10° g of polyurethane resin or which contains the compounds represented by the following structural formulas: OH

[RⁱO-(CH₂CH₂0)_D]-P-OH

0

R¹0-(CH₂CH₂0)_n 0

P-OH

R¹0-(CH₂CH₂0)_n

OH

R - P—OH

0 OH

OH OH

wherein R¹ is H, a phenyl group, an alkyl containing 40 or less carbon atoms, or an alkyl-phenyl group containing 40 or less carbon atoms; n is an integer of 1 - 50; R' is a phenyl group, an alkyl group containing 40 or less carbon atoms, or an alkyl-phenyl group containing 40 or less carbon atoms; R^J is an aromatic group containing 40 or less carbon atoms or an aromatic group having a substituted alkyl; and R⁴ is a trivalent compound containing 12 or less carbon and having at least two substituted alkyl groups containing 40 or less carbon 5 atoms.

In addition, the polyurethane resin may comprise at least one species selected from the group consisting of piperidine derivative, piperazinone derivative, alkali metal hydroxide containing 1 - 100 carbon atoms, alkaline

10 earth metal hydroxide containing 1 - 100 carbon atoms, and acidic hydrophilic group neutralized with alkyl amine containing 1 - 100 carbon atoms in order to enhance the hydrophilicity and hydrolysis resistance of polyurethane resin.

15. As for the magnetic component, well-known ferromagnetic powder, such as τ-Fe03 , τ-FeO_χ (x=1.33 - 1.55), Fefti , Co-containing Fe₃0₄, Co-modified τ-FeO_χ (x=1.33 - 1.5), Cr0 , ferromagnetic alloy fine powder comprising a major portion (75% or more) of Fe, Ni or Co,

20 or Ba-Fe, may be used. In addition to these elements, the ferromagnetic powder may comprise any one selected from the group consisting of Al, Si, S, Sc, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr and the mixtures

25 thereof.

Such ferromagnetic powder can be prepared into various forms, such as needle, particle and plate, by well-known methods and may have diverse particle sizes. Preferred is one that has a BET of 25 to 55 m²/g, a

30 determinant size of ranging from 150 to 600 Angstrom, and a coercive force ranging from 200 to 2000 Oe.

As usual, the magnetic coating composition, coated on the base film, may be prepared by dispersing the magnetic components and other additives in a binder and adding a curing agent to the mixture. Typically, such magnetic coating composition is coated on a non-magnetic support, which is then oriented and dried, to give a 5 magnetic recording medium.

As mentioned above, a magnetic component with a high BET may be dispersed in at least two polyurethane resins ranging in weight average molecular weight from 10,000 to 250,000 and in acidity from 0.1 to 70, in accordance with

10 the present invention. The same type resins show excellent compatibility with each other by virtue of which the dispersion of the magnetic component is as good as or better than that of preexisting magnetic coating. Application of the present magnetic coating composition

15. to a non-magnetic support can yield a magnetic recording media which has high scratch resistance and durability in addition to being an environment-protective article exclusive of chlorine-containing binder.

A better understanding of the present invention may

20 be obtained in light of the following example which is set forth to illustrate, but should not be construed to limit the present invention.

The physical properties of the video tapes prepared in examples are measured in the following manners.

25 Steal Lasting Time : First, picture information was recorded on a video tape obtained and a stationary picture was taken upon reproducing the picture information. The time taken from the stationary picture till distortion of the picture was measured. The longer the time, the higher

30 the durability and wear resistance.

Scratch Resistance : Using a measuring machine, such as that sold by Steinburg Co. Ltd. , this property of video tape was measured at a speed of 13 m/sec, a weight of 50 g and a tip height of 2.5 mil.

Drop-Out : After being run for 200 hrs. in an incubator maintained at 40 °C and at an RH of 85 %, video tapes were subjected to the recording of 5 step signals at an optimal current. Upon reproducing the signals, a state in which the attenuation amount of video amp was 20 dB and its lasting time was 15 μsec or longer was considered as drop-out. Using a measuring machine, such as that sold by *, drop out was measured, which was expressed by the average drop out number per minute.

Video Signal Ratio : After being inserted in a video deck, sold by Victor, Japan, under the trademark designation of "VR-70000", video tapes were recorded with 100 % white signals. The output upon reproduction was measured using a video noise measuring machine and the difference from that of preexisting tape was shown.

EXAMPLE I

The following composition (with a solid content of 43 %) was pre-mixed for 3 hours and subjected to primary dispersion for 6 hours in a sand grinder.

Primary Magnetic Composition Wt. parts

Magnetic Component (with a BET of 34 m²/g) 100 Alpha Alumina 2

Alpha Fe₂0₃ 2

Carbon Black 5

PU Resin (Mw=40000, acidity=0.7) 16 Myristic Acid 0.5

Butyl Stearate 1.0 M i x e d S o l v e n t

(methylethylketone/toluene/eyelohexanone=l/1/1) 168

To the mixture the following composition was added and, then, secondary dispersion was executed for 3 hours in a sand grinder, to prepare a magnetic coating composition.

Secondary Magnetic Composition Wt. parts PU Resin (Mw=40000, acidity=0.7) 7

Myristic Acid 0.5

Butyl Stearate 1.0

Curing Agent (coronate 3041) 3.5 M i x e d S o l v e n t (methylethylketone/toluene/cyclohexanone=l/l/l) 155

The prepared magnetic coating composition was coated on a base film which is, then, oriented, dried, cured, and wound by slitting with T 120. From this, a tape cassette for test was prepared. The resulting tape cassette was tested for steal-lasting time, scratch resistance and drop out and video signal ratio in the aforementioned manner. The results are given as shown in Table 2 below.

EXAMPLE II AND COMPARATIVE EXAMPLE I TO V

Example I was repeated using the resins indicated in Table 1 below as binders for the primary and secondary dispersions. The tests for physical properties of the prepared were carried out the results are given as shown in Table 2 below. TABLE 1

Primary Dispersion Secondary Dispersion

PU1 PU2 PVC PU3 PU4

EXAM. I Mw 40,000 45,000

Wt. part 16 7

Acidity 0.7 0.5

EXAM. II Mw 40,000 20,000 55,000 45,000 wt. part 10 6 2 5

Acidity 1.0 1.0 0.5 0.5

C. I MW 40,000 45,000 wt. part 16 7

Acidity 0.6

C. II Mw 8,000 300,000 wt. part 16 7

Acidity 0.7 0.5

C III Mw 300,000 8,000 wt. part 16 7

Acidity 0.7 0.5

C. IV Mw 40,000 45,000 wt. part 16 7

Acidity 0.5 0.5

C. V Mw 45,000 40,000 wt. part 16 7

Acidity 0.7 0.5

note: PU: Polyurethane Resin, PVC: Polychloride vinyl Resin TABLE 2

Tests

EXAM. Steal Lasting Scratch Drop Out Video Signal No. Time (min) Resist. (min) (ea/min) Ratio (dB)

I > 180 > 50 2 1.9

I > 180 > 50 1 2.4 CI 31 8 6 1.6 C.II 79 16 8 0.6 C.III 27 11 5 - 0.8 CIV 45 12 7 + 0.2 cv 54 21 5 + 0.9

Other features, advantages and embodiments of the present invention disclosed herein will be readily apparent to those exercising ordinary skill after reading the foregoing disclosures. In this regard, while specific embodiments of the invention have been described in considerable detail, variations and modifications of these embodiments can be effected without departing from the spirit and scope of the invention as described and claimed.

Claims

WHAT IS CLAIMED IS:

1. A magnetic recording medium which comprises at least one magnetic layer formed by coating a magnetic coating composition on a non-magnetic support, wherein said magnetic coating composition comprises two independent polyurethane resins, each of which is separately used for primary and secondary dispersion of magnetic component and is selected from the group consisting of: (a) a polyurethane resin having a weight average molecular weight of about 10,000 to 250,000 and an acidity of about 0.1 to 70; and

n

Mw = Σ V_{ Mw_t i=l

where V_j is the weight fraction of each polyurethane resins, Mw^ is the weight average molecular weight of each polyurethane resins, and n is an integer of not less than 2, and an acidity of about 0.1 to 70, said polyurethane resin used for said secondary dispersion having a larger weight average molecular weight and a lower acidity than said polyurethane resin used for said primary dispersion.

2. A magnetic recording medium in accordance with claim 1, wherein the weight average molecular weight of said polyurethane resin used for secondary dispersion is larger than that of said polyurethane resin used for primary dispersion by 500.

3. A magnetic recording medium in accordance with claim 1, wherein the acidity of said polyurethane resin used for secondary dispersion is lower than that of said polyurethane resin used for primary dispersion by 0.05.