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Magnetic recording medium comprising a polyurethane binder having a polar group and containing iron carbide - Patent # 5156916 - PatentGenius
5156916 Magnetic recording medium comprising a polyurethane binder having a polar group and containing iron carbide
Application: 07/693,774
Inventors: Goto; Narito (Hino, JP)
U.S. Class: 428/424.6; 428/425.9; 428/842.9; 428/900
Field Of Search: 428/900; 428/694; 428/425.9; 428/424.6
U.S Patent Documents: 4600521; 4613545; 4632866; 4637959; 4707411; 4784914; 4789599; 4794042
Foreign Patent Documents: 123318
Other References: Patent Abstracts of Japan, vol. 11, No. 378 (P-645) [2825]; Dec. 10, 1987 JPA-62-146,432; Jun. 30, 1987..
Abstract: A magnetic recording medium having a support and provided thereon, a magnetic layer containing a magnetic powder including iron carbide, and a binder comprising a polar group is disclosed. The binder comprises a resin (A) and a copolymer resin formed by copolymerization of a monomer (A) and a copolymeric monomer comprising the polar group, wherein the polar group is selected from a sulfo group, a phospho group, a carboxyl group or derivatives thereof including salts thereof; the resin (A) is preferably a urethane resin having the polar group; the monomer (A) is preferably vinyl chloride.
1. A magnetic recording medium comprising a support having provided thereon, a magnetic layer containing a magnetic powder containing iron carbide, and a binder comprising apolyurethane resin and at least one other resin, each resin having a hydrophilic polar group selected from a sulfo group, a phospho group, a carboxyl group, and derivatives thereof, including salts thereof.
2. The recording medium of claim 1 wherein said iron carbide has a BET specific surface area of 40 m.sup.2 /g or more.
3. The recording medium of claim 1 wherein said other resin comprises a vinyl chloride copolymer.
4. The recording medium of claim 1 wherein said other resin is formed by copolymerization of a first monomer and a copolymeric monomer comprising said hydrophilic polar group.
5. The recording medium of claim 4 wherein said first monomer is vinyl chloride.
6. The recording medium of claim 5 wherein said copolymer resin formed by copolymerization of said vinyl choride and said copolymeric monomer comprises an epoxy group and a hydroxy group.
7. The recording medium of claim 6, wherein said copolymer resin comprises of units of: ##STR10## wherein X represents the copolymeric monomer comprising the sulfo group or the phospho group of the alkaline metal salts.
8. The recording medium of claim 4 wherein a ratio of said polyurethane to said other resin is 20/80 to 80/20, by weight.
9. The recording medium of claim 8, wherein a ratio of said iron carbide to said other magnetic powders is 95/5 to 55/45 by weight.
10. The recording medium of claim 4, wherein a ratio of the monomer having the polar group to total monomers inclusive of polyurethane is 0.01 to 30 mol %.
11. The recording medium of claim 1, wherein said hydrophilic polar group is --SO.sub.3 M, --COOM or --PO(OM').sub.2, wherein M and M' represent independently a hydrogen atom, an alkaline metal, and a substituted or unsubstituted hydrocarbonresidue.
12. The recording medium of claim 11, wherein a monomer from which said resin comprising --SO.sub.3 M is formed is:
wherein M represents an alkaline metal.
13. The recording medium of claim 11, wherein the monomer comprising --COOM is:
wherein M represents an alkaline metal and R represents an alkyl group having 1 to 20 carbon atoms.
14. The recording medium of claim 11, wherein the monomer comprising --PO(OM').sub.2 is:
wherein Y.sup.1 represents a hydrogen atom, M or
CH.sub.2 .dbd.CHCH.sub.2 OCH.sub.2 CH(OH)CH.sub.2 --; Y.sup.2 represents a hydrogen atom, M, or
CH.sub.2 .dbd.CHCONHC(CH.sub.3).sub.2 CH.sub.2 --; X.sup.1 represents ##STR12## resents CH.sub.2 .dbd.CHCH.sub.2 O(CH.sub.2 CH.sub.2 O).sub.m --, OH or OM; M represents an alkaline metal; n and m represent independently an integer of 1 to100.
The present invention relates to a magnetic recording medium such as a magnetic tape, a magnetic sheet and a magnetic disk.
Generally, a magnetic recording medium such as a magnetic tape and the like is produced by coating on a support a magnetic composition composed essentially of a magnetic powder, a resin binder and others, followed by drying. In such a magneticrecording medium, especially in a video tape, .gamma.-Fe.sub.2 O.sub.3, Co-.gamma.-Fe.sub.2 O.sub.3 and the like have been conventionally used as a magnetic powder, and it is a recent practice to use magnetic powders with smaller particle size in orderto achieve higher S/N ratios. Further, Ba ferrite and metal powders have been used to meet a requirement for a higher density recording.
On the other hand, as described in Japanese Patent Publication Open to Public Inspection No. 60-124023/1985, a magnetic powder based on iron carbide is proposed. The iron carbide magnetic powder provides a high output at high Hc and an excellentconductivity and light-shielding property. However, a magnetic powder with finer particles (a BET value of over 40 m.sup.2 /g) for a higher density recording results in poor dispersibility of the particles especially of iron carbide, which in turnresults in the inferior electromagnetic conversion characteristics of a magnetic recording medium, and further a durability in repetitive playing also deteriorates due to poor dispersibility.
The object of this invention is to provide a magnetic recording medium which provides a good conductivity and light-shielding property and the electromagnetic conversion characteristics such as a reproduction output as well as the satisfactorytravelling properties such as a travelling durability.
FIGS. 1 and 2 show respectively an enlarged section of a magnetic recording medium;
1 . . . . . Support
2 . . . . . Magnetic layer
3 . . . . . Back coating layer (BC layer)
The present invention relates to a magnetic recording medium where a magnetic layer comprises a magnetic powder containing iron carbide and a binder with a polar group which is selected from the group of a sulfo group, a phospho group, a carboxylgroup, and the derivatives thereof including the salts.
A magnetic recording medium produced in accordance with the invention, containing an iron carbide in a magnetic powder, provides the characteristics peculiar to iron carbide, that is, a high Hc, .sigma. s which is lower than that of a magneticmetal powder but higher than that of a Co-containing iron oxide, and a high reproduction output. Further, a high conductivity and light-shielding effect can be achieved. Iron carbide is chemically stable, and so hard that an addition amount of.alpha.-Al.sub.2 O.sub.3 as an abrasive can be reduced. It is less expensive since it can be produced directly from oxy-iron hydroxide etc. at a lower production cost.
This invention enables a magnetic powder containing iron carbide to disperse well by virtue of a binder having a polar group as mentioned above, even when the magnetic powder is divided into fine particles with a BET value of over 40 m.sup.2 /g.The polar group in the molecule of the binder improves its compatibility with the magnetic powder, which results in improving its dispersibility. As a result, the magnetic recording medium is improved in electromagnetic conversion characteristics suchas a rectangular ratio and an output, and in durability in repetitive play. Good dispersibility and less flocculation of a magnetic powder lead to less dropout in a recording medium.
The magnetic powder of the invention may comprise mainly of iron carbide or of the mixture of iron carbide and other magnetic powders. The preferable ratio of iron carbide to the other magnetic powders is 95:5 to 55:45 (by weight).
Iron carbide contained in a magnetic powder of the invention is represented by FenC (n.gtoreq.2), wherein the typical example is Fe.sub.5 C.sub.2, and the others are Fe.sub.20 O.sub.9, Fe.sub.3 C, Fe.sub.2 C, and the like. There can beincorporated into the magnetic powder the other elements such as Co, Ni, Al, Zr, Cr and Si, besides iron.
In this invention, iron carbide can be used in combination with magnetic oxide powders such as .gamma.-Fe.sub.2 O.sub.3, Co-containing .gamma.-Fe.sub.2 O.sub.3, Co-deposited .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-containing Fe.sub.3O.sub.4, Co-deposited Fe.sub.3 O.sub.4, and CrO.sub.2, and with ferromagnetic metal powders based on Fe, Ni and Co, such as Fe-Al, Fe-Al-Ni, Fe-Al-Co, Fe-Al-Zn, Fe-Ni-Co, Fe-Mn-Zn, Fe-Ni, Fe-Ni-Al, Fe-Ni-Zn, Fe-Co-Ni-Cr, Fe-Co-Ni-P, and Co-Ni. Especially, Fe-based metal powders with Fe of over 80 atm% exhibit excellent electromagnetic properties, and it is advantageous in respect of corrosion resistance and dispersibility to employ metal powders such as Fe-Al, Fe-Al-Ni, Fe-Al-Zn, Fe-Al-Co,Fe-Ni, Fe-Ni-Al, and Fe-Ni-Zn. The magnetic metal powders may contain as additives such elements as Si, Cu, Zn, Al, P, Mn and Cr or their compounds. Hexagonal ferrites such as barium ferrite, and iron nitride are also applicable.
A base polymer used for a binder in the invention comprises an average molecular weight of approximate 10000 to 200000. The examples are urethane resin, copolymer of vinyl chloride and vinyl acetate, copolymer of vinyl chloride and vinylidenechloride, copolymer of vinyl chloride and acrylonitrile, copolymer of butadiene and acrylonitrile, polyamide resin, polyvinylbutyral, cellulose derivatives (cellulose acetate butylate, cellulose diacetate, cellulose triacetate, cellulose propionate,nitrocellulose, etc.), copolymer of styrene and butadiene, polyester resin, synthetic rubbers, phenolic resin, epoxy resin, urea resin, melamine resin, phenoxy resin, silicone resin, reactive acrylicresin, mixture of high molecular weight polyester andisocyanate prepolymer, mixture of polyester polyol and polyisocyanate, urea-formaldehyde resin, mixture of low molecular weight glycol, high molecular weight diol and isocyanate, and mixture of such polymers.
The above-mentioned resins used in the invention preferably contain a hydrophilic polar group represented by --SO.sub.3 M, --COOM, or --PO(OM').sub.2 (M and M' represent each a hydrogen atom and an alkali metal such as lithium, potassium andsodium, or a substituted or unsubstituted hydrocarbon residue).
A vinyl chloride copolymer applicable as a binder in the invention can be obtained by copolymerization of a vinyl chloride monomer with a copolymeric monomer containing an alkali salt of sulfonic acid, caboxylic acid or phosphoric acid and, ifnecessary, together with another copolymeric monomer. The properties of the foregoing vinyl chloride copolymer can be optimized by selecting copolymeric monomers.
The alkali metal of the preceding sulfonate, phosphate and the like is preferably sodium, potassium or lithium, more preferably potassium in respect of solubility, reactivity, yield and the like.
The examples of the copolymeric monomers containing sulfonates are:
The examples of the copolymeric monomers containing carboxylates are:
The examples of the copolymeric monomers containing phosphates are:
wherein M represents an alkali metal; R represents an alkyl group with 1 to 20 carbon atoms; Y.sup.1 represents H, M or
Y.sup.2 represents H, M or
X.sup.1 represents ##STR2## OH or OM; X.sup.2 represents
OH or OM. n and m represent each an integer of 1 to 100.
The preceding copolymeric monomers include the known monomers such as vinyl esters, vinylidene chloride, acrylonitrile, methacrylonitrile, styrene, acrylic acid, methacrylic acid, acrylic esters, methacrylic esters, ethylene, propylene,isobutene, butadiene, isoprene, vinyl ether, allyl ether, allyl ester, acrylamide, methacrylamide, maleic acid, and maleic ester.
The binders of this invention are formed by emulsion polymerization, solution polymerization, suspension polymerization, bulk polymerization, and the like. In any polymerization method, the known techniques such as use of molecular weightmodifier and polymerization initiator, stepwise or continuous addition of monomers, and the like, can be applied.
An amount of the preceding monomers containing acid salts is preferably 0.01 to 30 mol% in a binder. An excessive amount of these monomers results in poor solubility in a solvent and tends to cause gelation, whereas an insufficient amountprovides the copolymer with less prescribed property.
It is preferable that the preceding vinyl chloride copolymer further contains an epoxy group or a hydroxyl group. A vinyl chloride copolymer in the past (for example, VAGH of U.C.C. Corp.) was comprised of the following copolymeric components:##STR3## (shown as a copolymer unit).
In this copolymer unit, the group CH.sub.3 CO--O-- is considered to scarcely contribute to a cross-linking reaction with a curing agent. It is, therefore, preferable to introduce in place of this group an epoxy group such as ##STR4##
For example, a copolymer consisting of the following units is preferable: ##STR5## wherein X is a monomer component containing an alkali metal salt of a sulfo group or a phospho group.
Particularly useful is a urethane resin, which is preferably used in combination with a vinyl chloride copolymer, an epoxy resin (especially a phenoxy resin), a polyester resin, or a nitrocellulose resin. The resin to be combined with theurethane resin is preferably used in an amount of 10 to 90 parts by weight, preferably 20 to 80 parts per 100 parts by weight of the urethane resin. The proportion exceeding 90 parts makes coating much more fragile, which results in decreasing itsdurability and adhesiveness to a support to a large extent. On the other hand, the proportion less than 10 parts by weight is liable to causing drop-off of a magnetic powder.
Furthermore, an addition of a polyisocyanate curing agent to a magnetic composition containing a binder improves durability of a coating. Such a polyisocyanate curing agent is, for example, bifunctional isocyanates such as tolylenediisocyanate,diphenylmethane-diisocyanate, and hexanediisocyanate, trifunctional isocyanate such as Coronate L (product of Japan Polyurethane Co.) and Desmodur L (product of Bayer AG), a curing agent in conventional use such as a urethane prepolymer having isocyanategroups at both terminals, or any polyisocyanate applicable as a curing agent. Such a polyisocyanate curing agent is used in an amount of 5 to 80 parts by weight per 100 parts by weight of a binder.
A magnetic recording medium of the invention can be formed by providing a magnetic layer 2 on a non-magnetic support 1, which is polyethylene terephthalate etc. If necessary, a back coating (BC) layer 3 may be provided on the other side of thesupport 1, as shown in FIG. 1. An overcoat layer (OC) 4 may be provided on the magnetic layer (2), as shown in FIG. 2.
A magnetic recording medium shown in FIGS. 1 and 2 may or may not comprise a subbing layer (not shown in the drawing) between the support 1 and the magnetic layer 2. The support may be subject to a corona discharge treatment.
Besides a magnetic powder and a binder, a fatty acid and a fatty acid ester as a lubricant are preferably incorporated into a magnetic layer 2. This arrangement enables both of the fatty acid and its ester to demonstrate their characteristicsand compensate the defects coming out in a single use to improve a lubrication effect, which contributes to improving a still picture durability, a travelling stability, a S/N ratio, and the like. The fatty acid is preferably used in a quantity of 0.2to 10 parts by weight per 100 parts by weight of the magnetic powder, more preferably 0.5 to 8.0 parts by weight. The fatty acid less than this range lowers dispersibility of the magnetic powder and is liable to deteriorate a travelling property of themagnetic recording medium, whereas use to excess tends to cause the fatty acid to ooze out and an output to lower. The fatty acid ester is preferably used in a quantity of 0.1 to 10 parts by weight per 100 parts by weight of the magnetic powder, morepreferably 0.2 to 8.5 parts by weight. The ester less than this range tends to cut its effect for a travelling property, whereas use to excess is liable to cause the ester to ooze out and an output to lower.
In order to optimize the preceding effects, a weight ratio of fatty acid to fatty acid ester is preferably 10 to 90 through 90 to 10. A fatty acid has also an effect to promote dispersion. For this reason, it is expected that the fatty acid canreduce an addition of other low molecular weight dispersant, which provides possibility to improve a modulus of a magnetic recording medium.
The fatty acid may be monobasic or dibasic. The fatty acid comprises preferably 6 to 30 carbon atoms, more preferably 12 to 22 carbon atoms. The examples of the fatty acid are:
(17) Glutaric acid
(20) Azelaic acid
(22) 1,12-dodecanedicarboxylic acid
(23) Octanedicarboxylic acid
The examples of the fatty acid ester are:
(13) Lauryl oleate
(14) Octyl oleate
(15) Iosbutyl oleate
(16) Ethyl oleate
(17) Isotridecyl oleate
(18) 2-ethylhexyl stearate
(19) Ethyl stearate
(20) 2-ethylhexyl palmitate
(23) Butyl laurate
(24) Cetyl-2-ethyl hexalate
There may be incorporated into the magnetic layer a lubricant other than the above-mentioned fatty acids and esters thereof (for example, silicone oil, carboxylic acid-modified and ester-modified products), graphite, carbon fluoride, molybdenumdisulfide, tungsten disulfide, fatty acid amide, and .alpha.-olefin oxide. Further, there may be incorporated a non-magnetic abrasive powder such as alumina, chromium oxide, titanium oxide, .alpha.-iron oxide, silicon dioxide, silicon nitride, siliconcarbide, zirconium oxide, zinc oxide, cerium dioxide, magnesium oxide, and boron nitride. The abrasive is preferably added in an amount of less than 20 parts by weight per 100 parts by weight of the magnetic powder, more preferably 3 to 12 parts byweight, and an average particle diameter is preferably 0.6 .mu.m, more preferably less 0.3 .mu.m.
The magnetic layer may further contain an antistatic agent such as graphite and a dispersant such as powder licithin and phosphoric ester, in combination with carbon black.
The examples of the light-shielding carbon black, which can improve a light-shielding effect, are Raven 2000 (specific surface area 190 m.sup.2 /g, particle diameter 18 m.mu.), Raven 2100, Raven 1170 and Raven 1000, all made by Columbian CarbonCorp., and #100, #75, #40, #35 and #30, all made by Mitsubishi Chemical Industries, Ltd.
The examples of a conductive carbon black used in the invention are Conductex 975 (BET value (hereinafter referred to as "BET") 250 m.sup.2 /g, DBP oil absorption (hereinafter referred to as "DBP") 170 ml/100 gr, particle diameter 24 m.mu.),Conductex 900 (BET 125 m.sup.2 /g, particle diameter 27 m.mu.), Conductex 40- 220 (particle diameter 20 m.mu.), Conductex SC (BET 220 m.sup.2 /gr, DBP 115 ml/100 gr, particle diameter 20 m.mu.), all made by Columbian Carbon Corp., Vulcan XC-72 (specificsurface 254 m.sup.2 /g, particle diameter 30 m.mu.) and Vulcan P (BET 143 m.sup.2 /gr, DBP 118 ml/100 gr, particle diameter 20 m.mu.), both made by Cabot Corp., Raven 1040 and Raven 420, Black Pearls 2000 (particle diameter 15 m.mu.), and #44 ofMitsubishi Chemical Industries, Ltd.
The other carbon black products applicable likewise to this invention are Conductex-SC (BET 220 m.sup.2 /gr, DBP 115 ml/100 g, particle diameter 20 m.mu.), of Columbia Carbon Corp., Vulcan 9 (BET 140 m.sup.2 /g, DBP 114 ml/100 g, particlediameter 19 m.mu.), of Cabot Corp. #80 (BET 117 m.sup.2 /g, DBP 113 ml/100 g, particle diameter 23 m.mu.), of Asahi Carbon Ltd , HS100 (BET 32 m.sup.2 /g, DBP 180 ml/100 g, particle diameter 53 m.mu.) of Denki kagaku Co., and #22B (BET 55 m.sup.2 /g,DBP 131 ml/100 g, particle diameter 40 m.mu.), #20B (BET 56 m.sup.2 /g, DBP 115 ml/100 g, particle diameter 40 m.mu.), and 3500 (BET 47 m.sup.2 g, DBP 187 ml/100 g, particle diameter 40 m.mu.), all made by Mitsubishi Chemical Industries, Ltd. The carbonblack products applicable further to this invention are CF-9, #4000, and MA-600, all made by Mitsubishi Chemical Industries, Ltd., Black Pearls L, Monarck 800, Black Pearls 700, Black Pearls 1000, Black Pearls 880, Black Peals 900, Black Pearls 1300,Black Pearls 2000, and Sterling V, all made by Cabot Corp., and Raven 410, Raven 3200, Raven 430, Raven 450, Raven 825, Raven 1255, Raven 1035, Raven 1000, Raven 5000, and Ketchen Black FC, all made by Columbian Carbon Corp.
An average particle size of a non-magnetic filler used in a back coating layer is preferably 10 m.mu.to 1,000 m.mu.. The filler particles in this size range, which is not excessively fine, will maximize an effect.
The examples of the non-magnetic fillers useful in the invention are silicon dioxide, titanium oxide, aluminum oxide, chromium oxide, silicon carbide, calcium carbide, zinc oxide, .alpha.-Fe.sub.2 O.sub.3, talc, kaolin, calcium sulfate, boronnitride, zinc fluoride, molybdenum dioxide, calcium carbide, and barium sulfate. Some organic powder materials are also useful for the same purpose, for example benzoguanamine resins, melamine resins, and phthalocyanine pigments. It is also possible touse an inorganic powder in combination with an organic powder.
It is especially preferable to use carbon black in combination with a non-magnetic powder. Such combination renders a travelling property of a magnetic recording medium more stable, and a synergism with the non-magnetic particles makes itpossible to improve further a durability of the magnetic recording medium.
The present invention will now be described with reference to the examples. In this connection, it is to be understood that various changes and modifications may be made in respect of the ingredients, the proportions in which they are used, theprocedures in which the embodiments are formed, and the like without deviating from the spirit and scope of the invention.
A magnetic layer was formed on a support consisting of a polyethylene terephthalate film with a thickness of 10 .mu.m in the following manner (hereinafter, parts represent parts by weight, unless a specific remark is issued).
A magnetic coating material was prepared from a prescribed magnetic powder and ingredients shown in Table 1, by dispersing and filtering the ingredients through a 1 .mu.m filter; after adding 5 parts of multifunctional isocyanate the magneticcoating material was coated in thickness of 2.5 .mu.m on the support and finished by supercalendering to form a magnetic layer having the composition shown in Table 1.
Subsequently, a coating material for a back coating layer, which had the undermentioned composition, was coated on the other side of the magnetic layer to form a layer with a dry thickness of 0.4 .mu.m.
______________________________________ Carbon black (mean particle diameter 50 .mu.m) 40 parts Barium sulfate 10 parts Nitrocellulose 25 parts N-2301 (product of Japan Polyurethane Co.) 25 parts Coronate L (product of Japan PolyurethaneCo.) 10 parts Cyclohexanone 400 parts Methyl ethyl ketone 250 parts Toluene 250 parts ______________________________________
As follows are the compositions of vinyl chloride copolymer and polyurethane used for Examples and Comparisons;
______________________________________ Vinyl chloride copolymer containing a sulfonic acid salt average degree of polymerization 314 ##STR6## 94.8 wt % ##STR7## 4.0 wt % ##STR8## 0.4 wt % ##STR9## (X) (containing SO.sub.3 Na) 0.8 wt % Polyurethane containing a sulfonic acid salt Methylenediisocyanate 12.7 wt % Neopentyl glycol 16.2 wt % 1,6-Hexanediol 31.4 wt % Adipic acid 38.7 wt % (containing SO.sub.3 Na) 1.0 wt % ______________________________________
Example 2: same as Example 1.
Example 3: same as Example 1, besides that X contains --PO.sub.2 (OH)Na instead of --SO.sub.2 Na. Comparison 1: same as Example 1. Comparison 2: same as Example 1, besides that X in a vinyl chloride copolymer is vinyl chloride and X inpolyurethane is adipic acid. Comparison 3: same as Comparison 2.
A magnetic film having a magnetic layer and BC layer each with a prescribed thickness was produced in a wide strip and rolled up on a reel. The film was cut into a strip of 8 mm width to make the video tapes corresponding to the numbers ofExample and Comparison in Table 1. Respective video tapes were subjected to the following measurements, of which results are shown in Table 1.
______________________________________ Video S/N: Measured on a VHS deck (R-5 was set at OdB) Light transmittance: Used light of a wavelength of 900 nm Travelling durability: Evaluated by a fold and deterioration of a reproduction outputafter carrying out 100 passes. Dropout: Number of dropout in a reproduced image ______________________________________
The results of the measurements in Table 1 show that the magnetic recording material prepared in accordance with the present invention is improved in electromagnetic conversion characteristics as well as travelling durability and conductivity,and dropout is reduced to a large extent.
TABLE 1 __________________________________________________________________________ Example 1 Example 2 Example 3 Comparison 1 Comparison Comparison __________________________________________________________________________ 3 Compo- Ingre- Ferromagnetic Fe.sub.5 C.sub.2 Fe.sub.5 C.sub.2 Fe.sub.5 C.sub.2 Co-.gamma.-Fe.sub.2 O.sub.3 Co-.gamma.-Fe.sub.2 O.sub.3 Fe.sub.5 C.sub.2 sition dients of powder* 100 parts 100 parts 100 parts 100 parts 100 parts 100 parts magneticVinyl chloride With sulfonic With phospho With sulfonic Without Without coating copolymer group group group sulfonic sulfonic group 11 parts 11 parts 11 parts 11 parts 11 parts Polyurethane With sulfonic With phospho With sulfonic WithoutWithout group group group sulfonic sulfonic group 11 parts 11 parts 11 parts 11 parts 11 parts Almina 5 parts Same Carbon black 1 part Lauric acid 1.5 parts Butyl stearate 1 parts Same Same Same Same Cyclohexanone 100 parts Methyl ethylketone 70 parts Toluene 80 parts Prop- Video S/N (dB) +3.5 +4.0 +3.5 +3.0 +1.5 +1.2 erties Rectangular ratio 0.82 0.81 0.82 0.82 0.75 0.73 Travelling durability Good Good Good Good Poor Poor Dropout Good Good Good Poor Poor Poor Surfaceresistance of magnetic 1 .times. 10.sup.8 1 .times. 10.sup.8 1 .times. 10.sup.8 1 .times. 10.sup.11 1 .times. 10.sup.11 5 .times. 10.sup.8 layer (.OMEGA./cm.sup.2) Light transmittance (%) 0.02 0.02 0.02 0.3 0.3 0.02 __________________________________________________________________________ *Ferromagnetic powder Example 1: Hc 700 Oe, .sigma.s 98 emu/g, BET value 40 m.sup.2 /g Example 2: Hc 920 Oe, .sigma.s 100 emu/g, BET value 42 m.sup.2 /g Example 3: Same asExample 1 Comparison 1: Hc 660 Oe, .sigma.s 77 emu/g, BET value 40 m.sup.2 /g Comparison 2: Same as Comparison 1 Comparison 3: Same as Example 1
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