CA1100631A - Magnetic recording medium and method of preparing the same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A magnetic recording medium is produced using a highly purified lecithin which is selectively adsorbed upon magnetic powder particles so as to coat substantially the entire surface of such particles with a monomolecular layer, thereby providing a magnetic record medium of improved magnetic properties.

Description

Eii3~

BA~ICGROUND OF THE INVENl'ION
Field oE the Invention This invention relates to a magnetic recording medium and to a methocl for its preparation, and is particularly directed to an improved method for clispersing magnetic powder into a resinous binder to achieve improved residual magnetic Elux clensity Br~ improved ratio Rs, and a better packing density.
S~qr~r)Q5s DES~RIPTION OF THE PRIOR AR'r In the manufacture of magnetic recording media, magnetic powder is treated by means of various kincls of dispersants or surfactants in orcler to irnprove the dispersion of the magnetic powder in the magnet-ic coating material. Surfactants such as -fatty acicls, metaLlic salts of fatty acids (metallic soaps,) or the like have been used. However, when these materials are used in a magnetic me.lium, their surEace activity is not sufEicient and thereEore do not improve the dispersion greatly.
In other examples of the prior art, there has been pro-posed a method in which the magnetic powcler is treated preliminarily by lecithin. In this proposal, however, ordinary lecithin as available on the market was usecl, this being a raw lecithin which inclucles a large quantity of impurities.
There has also been proposed a method by N. Kazillo et al. in Japanese Laid Open Specification No. 3()9/1976, a method which uses a lecithin prepared in such a manner that ordinary lecithin is extracted with acetone to remove neutral fats or fatty acids. Even with the acetone extracted lecithin, however, the degree of dispersion o-f magnetic powcler acllieved is insufficient.

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SUMM~Y OF Tl JE INVFNTION
From our experimental work, we have ascertainecl that the typical raw lecithill on the market, and the acetone puri-Eied lecithin ~ ~PS~
contain only about 1077C or 207C pure lecith~spectively. ~e have also determined that since these lecithins contain a . Iarge amount of impurities, they are diE-ficult to apply over the entire surface of mag-netic powder particles as a monomolecular layer. Furthermore, since the surfa.ce activity of such relatively impure materials is insufficient, the dispersion becomes insufficient. In othe:r words, lecithin composi-tions containing such :relatively small alrlounts of pu:re lecithin do not eEfectively cause adsorption of lecithin molecules to the surEace of rmagnetic powder.
The present invention provides a magnetic recording med-ium including a non-magnetic base, a magnetic layer carried by the base and composed oE magnetic powde:r particles dispersed in a resinous binder, the magnetic powder particles being covereà on substantially their entire surfaces with a monomolecular l~yer composed substantially oE lecithin. T]le improved lecithin compositioll which is selectively c~sorbed on the surface of the particles contains a~ least 30~7C pure lecithin and is substantially devoid oE Eats, free fatty acids, cepha ancl proteins.
:~ I`he method o~ preparation basically consists o-f mixing the magnetic powder and an organic solvent with a purifiecl lecithin composition having a purity of greater than 307C in an amount suEficient `
to cover substantially the entire surEace oE the magnetic powde:r particles with a monomolecular layer consisting essentially of lecithin, mixing the thus t:reated magnetic powder particles with a synthetic resin binder to form a magnetic paintl and applying the magnetic paint on a non-magnetic base to form a magnetic layer.
BRIEF DESCRIPTION OF THE DRA~INGS

-Other objects, features and advantaqes of the invention will be readily apparent from the followiny description of certain preferred embodiments thereof, taken in conjunction with the accompanying drawings, although variations and modifications may be effected withou-t departing from the spirit and scope of the novel concepts of the disclosure, and in which:
Figure 1 is a graph showing the absorption spectrum of a choline-reinecke salt in acetone;
Figure 2 is a graph showing the variation in absorbance with the concentration of high purity lecithin used in the present invention;
Figure 3 is a graph showing the variation of absorbance with concentration of choline chloride in acetone solution;
Figure ~ is a graph showing the amount of adsorption of lecithin on gamma ferric oxide as a function of the original concentration;
Figure 5 is a graph showing the relationship between equilibrium concentration and ratio o~ equilibrium concentration to adsorption of lecithin on gamma ferric oxide;
Figure 6 is a diagrammatic illustration of the lecithin molecule;
Figure 7 is a graph plotting adsorption and desorption amounts of lecithin against equilibrium concentration;

i3~l Figure 8 is a graph relating residual magnetic flux density witll concentration o-E the dispersallt;
Figure 9 is a graph showing, ~he relation between the purity of the aclditive ancl the amount which should be added tO -Eorm a monomolecular layer oE lecithin;
Figure 10 is a grapll showing the relationship between residual magnetic -flux density and the purity of lecithin employed; and Figure 11 is a graph showing the relationship between ratio and the purity of the lecithin employed.
r~nQSs DESCRIPTION 0~ THE PREEi'ERRED EMl30DIMENTS
In the present invention, we use a lecithin of higll purity which is at least 3037c, and is preferably at least 507(~. It has been found that using 4 parts per 100 parts magnetic powder (P~-IP) of a lecithin having a purity Oe 307c, more than 2.5 Pl-lP of a lecithin of 50~ purity, and more than 1. 5 PHP in the case of a lecithin having a purity of 917~C~ a monomolecular layer~ composed substantially entirely of lecithin can be applied on substantially the entire surEace of the magnetic powder partic les.
A lecithin having an extremely high purity of more than 30~c, and pre-ferably o-E 57c or more can be obtained by puriEying a commercial lecithin available on the market, or by suitably processing a raw lecithin such as egg lecithin or soybean lecithin, or by using a synthetic lecithin. A suitable method of producing a lecithin of high purity by purifying raw lecithin was disclosed by M. C. Pangborn in Journal oE Biological Chemistry, vol. 188, page 471 (1951). The following examples set forth various methods of producing high purity lec ith in .

. ~ .; , . -EXAMPLE l ~ gg lecithin (raw lecithill) marketed by Aj inomoto Com-pany, Inc., in an amount of 350 grams was extracted with 3 litres of acetone to remove neutral fat and fatty acid material ancl leave a lecithin precipitate containing ~r~ng -~.L~ and proteins ~ s~floQ~p,ds which was thereupon dried. The yield of this precipitate, acetone puri-fied lecithin was 200 grams. Next, this precipitate was extracted with 95~ ethanol. The lecithin precipitate was dissolved in theethanol and separated from impurities such as proteins, cephalin and the like.
The amount of the precipitate was 100 grams. Then, a water solution Oe 50~0 cadmium chloride was added dropwise to the ethanol solution of lecithin to produce a white colloidal precipitate consisting of a complex salt of lecithin and cadmium chloride. This precipitate was ~r \~JASfle~
with acetone, separated by filtration, and vacuum dried at a tempera-ture below 40~C. Next, this complex salt was passed through a separ-atory funnel where it was dissolved in 100 cc of chloroform. The above solution was then mixed with 100 to 150 cc of 307~ ethanol, shaken and permitted to stand. The caclmium chloricle was dissolvecl into the ethanol and the lecithin remained in the chloroform. The ethanol layer is lower In specific gravity than the chloroform layer, so that after standing, the rnixture separated into an ethanol upper layer and a chloroform lower layer. The chloroform layer was then removed from the lower end of the separatory funnel and the chloroform solution was again treated with ethanol, the process being repeatecl three timesO Completion of the extraction was deter mined by the absence of a white precipitate of silver chloridewlen adcling silver nitrate to a portion of the ethanol solution. Next, the chloroform i3~

solution oE lecitllin was driecl by evaporatiorl in a rotary evaporcltor. A
transparent yellow lecithin of high purity was obtained in an amount Oe 15 grams. The purity o-E lecitl~ obtainecl by this puri-Eication was ascertained to be 91~ according to an analysis method to be described later.
To permit easy understanding oE the lecithin puriEication rnethod, the flow chart will be helpful.
raw lecithin (starting material) acetone puri-Eied lecithin neutral eat ancl fatty acid being removed ethanol extraction cephalin and protein eraction being removed.

Iecithin-cadmium chloride complex salt decomposition oE complex salt chloroEfrm solution oE lecithin 1' high purity lecithirl The higll purity lecithin was mixecl with magnetic powder in an organic solvent consisting oE methyl ethyl kets)ne ~MEK) or cyclohexanone, the mixing and dispersion thereoE being carried out for 7 hours in a ball mill to produce a uniform suspension. The composi-tion Oe this suspension (hereinafter referred to as composition "~") was as follows:

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gamma Fe203 (?Ll sq~ m. /g~`) 300 parts by weight high purity lecithill 4~5 parts by weight ( 1. 5 PT-I P) MEK 300 parts by weight The above composition "~ " was mixed with 50 part:s by weight oE '~VYHI-I" resin sold by Union Carbide, which is a vinyl chlor-icle-vinyl acetate copolyrner and serves as a binding agent, It was also combined with a solution oE 300 parts MEK by weight and milled in a ball mill for 20 hours to obtain a magnetic paint. This magnetic paint was applied to a non-magnetic base composed o-E polyetllylene terepllthalate Eilm (Mylar), driecl, subjected to calendering, ancl s litted to produce a magnetic reco~ape.

A high purity lecithin was obtained by the process set Eorth in Example 1. Composition "A" was Eormecl similarly as in Example 1, except that the amount o-E high purity lecithin was 6 PHP.
~n amount o~ 300 parts by weight oE MEK was applied to the composi-tion and mixed together Eor 1 hour in a ba 11 mill, and thereaEter was allowed to stand for 1 hour. Then, a supernatant solution oE 300 parts by weight was removed to eliminate excess lecithin, i. e., the non-adsorbed lecithin. I'his composition was subjected to the treatments mentioned in Examp:le 1 to produce a magnetic coating, and the coating was applied to a non-magnetic base to obtain a magnetic record~ medium.
The purity oE the high purity lecithin described above was measured by colorimetric method described by M. I-I. Ilack in the Journal oE Biological C hemistry, vol. 169, page 137 ,1947). In this ~31Q~3~

method, a cholin~ component available only in lecithin was separated and the ~hus separated amount is colorometrically determined to determine the amount of leci-thln.
Structurally, lecithin has the struc~tural formula (I) as set forth below which includes a choline chain having formula (2):

CH OCOR ' ~ -H20 ~ OCH2CH2~(cH3) 3 (1) OH OH

OH
HCH2CH2 N (CH3) 3 (2) where ROCO and R' OCO are fatty acid residues from palmitic acid, stearic acid, oleic acid and the like.
The extraction of the choline component and its colori-metric de-termination were performed as follows. First, the lecithin was weighed and put in a 20 cc flask with a stopper into which a one normal ( lN) potassium hydroxide aqueous solution was applied in an amount of 5 cc to perform a hydrolysis reaction for 17 hours at 37C. Thereafter, 4.5N HCl was applied to dis-solve the choline contained in the hydrolysis product and the dissolved choline was filtered to remove precipitates of impurities such as proteins which are insoluble in HCl aqueous solution. The filter cake was treated with additional amounts of 1.5 N HCl for dissolution in an extraction so that the choline compound would not remain in the filter cake. A solution of 30 the completely extracted choline compound was treated wth ammonium reineckate NH4 [Cr (NH3) 2 (SCN) 4] . There was produced a precipitate of choline-reineckate which was removed and cleansed with ethanol. Before the quantitative anal.ysis, the 3~a choline-reineckate precipitation was carried out a-t a temperature of 20 to 25C. Since the precipitate has little solubilit~ in ethanol, the ethanol was used under cool conditlons to minimize the amount of dissolution which occurred. The thus obtained choline-reineckate was dissolved in 5 cc of acetone. This solu-tion hereinafter referred to as solution "B" was pink in color and its absorbance Abs was determined by colorimetry. Figure l shows the absorption characteristic of this reference solution.
As shown in Figure 1, the absorption spectrum has a peak at a wavelength of about 526 nm so that the absorbance of the acetone solution "B" was measured relative to a light having a wavelength of 526 nm.
Figure 2 shows the results obtained from measuring the absorbence of the high purity lecithin according to Example l by means of 1 cm cell. The absorbance Abs in this curve was measured according to the e~uation:
Abs - - log T
where T i5 the light transmittance of the object to be measured.
This measured absorbance was compared wi-th the reference absorbance spectrum to obtain the purity of the lecithin. The reference absorbance was obtained by reacting a commercial choline specimen (choline chloride) to produce a precipitate of choline chloride-reineckate by the same method used in the production solution "B" and the thus produced precipitate was dissol~ed in 5 cc of acetone and its absorbance was measured by the same method. The result is shown in ~igure 3. The absicca represents the amount of choline chloride. The amount of choline chloride can be con-~erted into the amount of lecithin by multiplying a suitable con-version factor. Since the molecular weight of choline chloride is 137.5 and that of lecithin is 793 and assuming that the fatty acid groups are hoth stearic acid groups, a conversion factor of 5.77 is obtained. If the relation between the am~nt of choline chloride and absorbance is converted to the rèlatir4n between the amount of lecithin and absorbance, the weight of lecithin per unit of absorbance was 42.2 mg. The value of weight ' of lecithin per unit of absorbance according to Example 1 was -46.5 mg. so that the purity of this lecithin corresponds to 91%
of the choline specimen. Since the purity of the choline specimen can be regarded as about 100%, the purity of the high purity lecithin can be taken to be about 91%. In this connection, the purity of a raw egg lecithin on the market measured by the same method was 4.3%, and the purity of the lecithin purified by acetone was 7.1%. The purity of a raw soybean lecithin on the market was found to be 9.2% and that of its acetone purified pro-duct was 20%, which were both considerably lower than required.
For this reason, the lecithin content of the compositions of the present invention have purities of at least 30%, and prefer-ably 50% or more.
We will now describe the dispersion effec-t achieved in the present invention. First, we shall consider the adsorptive condition of lecithin on magnetic powder which is the basis of evaluating the dispersion effect. Magnetic powder consisting of gamma Fe2O3 in an amount of 50 grams and high purity lecithin of a predetermined amount were mixed together with MEK in an amount of 100 cc in a ball mill for 7 hours to produce a uniform suspen-sion. After the leci-thin had 6;3~

been adsorbed on the magnetic powcler, the suspension was separated into solid and liquid phases using a centri~ugal separator, and the supernatant solution was removed in a predeterminecl amount. The solvent of tiliS removed portion was vaporized and the amount o~
lecithin was determined by the above-described colorimetric method.
Since this amount o~ lecithin gives the equilibrium concentration ~ Oe adsorption, the amount oE adsorption, r to magnetic powder can be obtained Erom the original concentration, CO~ the. volume Oe lecithi~
solution, V, and the powder weight, w, by means oE the Eollowing equation:

V(CO -C)/W
The measured results oE the amount Oe adsorption at various concentrations Oe high purity lecithin (91~ purity) are given in the Eollowing table.

.

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~ ~ U~ ~, ~ ~ C~ ~ oo C~
V o o o o o ~ ~ x `D

~^
O ~ o o O O O
. n o o o o cs~ o ~ a)r ~ o ~ ~ C~l C~l C~l C~l ~ ~ c~> ~
4 LLJ ~ O O O O O O O C~ O O O
a ~
C~ ~
~.

'v~ ~o C~l In O O Cil ~1 5-( O O C~l `O O
,~ ~ ~ O O O O
. ~ ci4 0 ~0 ,0 h n o o o o o ~ o o o O
~ ~ C~ O 0 m U) O in o U b~ O O O ~ ~ ~ ~ ~i c~ ~ co ,~
X O

~` -12-63~

From the Eoregoing table, witll a high purity o-f lecithi of 913~ purity, when the original concentration CO is in the range vf (). 25 to 1. 0 g/100cc, or the amount o-E lecithin added to the magnetic powder is 2 PflP or less, the equilibrium concentration C is zero, i.e., the added lecithin is all adsorbed to the magnetic powder and no lecithin remains in the solvent. There-Eore, it is conchlcled that the lecithin molecules have strong surface activity and are irreversibly adsorbed.
Figure 4 shows the relation between the amount o-E ad-sorption and the original concentration, CO. From this Eigure, it will be seen thclt there is a linear relationsllip an~l an irreversible ad-sorption is achieved in the range where the original concentration CO
is low. It also should be noted that iE the concentration is increased~
a saturated adsorption occurs.
The monomolecular adsorption step can be expressed by the Langmuir equation as Eollows:

r = rs~cc 1 + KC or c/r = ~ . K + c/rs where rs is the amount oE saturated adsorption and lC is a constant determined by the system. (a~) From the above equation it will be seen that during the process of monomolecular adsorption to the solid surEace, the relation-ship concentration to adsorption becomes linear and the reciprocal of the gradient is the amount of saturated adsorption. Applying the equa-tions to adsorption data of the above-described high purity lecithin-gamma -ferric oxide system of this invention, the adsorptive characteristi -:L3-3~L

of the high purity leci-thin were determined. Figure 5 shows the relationship and the applicability of the Langmuir formula.
As apparent from Figure 5, a satisfactory linearity is obtained and so a monomolecular adsorption is produced in the concentra-tion range shown in Figure 5.
On the other hand, it is well known that the lecithin molecule can form an oriented monomolecu:Lar layer or BLM
layer (black lipid membrane or bilayer lipid membrane which is composed of two oriented monomolecular layers). It is also known that the lecithin molecule is composed of a hydrophilic group having a dimension of 5 Angstroms and a lipophilic group of 25 Angstroms, as shown in Figure 6. In the drawing of Figure 6, one of the lipophilic hydrocarbon chains is omitted because it is not visible from that position. Accordingly, on the surface of a solid such as gamma ferric oxide, the hydrophilic group is oriented toward the solid surface and the lipophilic group is oriented toward the organic solvent. When the adsorption is carried out in such a manner, the area of solid surface covered by one molecule, or occupied cross sectional area SO is in the ranye from 45 to 110 square Angstroms. In other words, the monomolecular layer of a lecithin molecule is quite compressible and can have a relatively wide range of adsorption density from a sparse condition to a dense condition.

The adsorption condition and the condition of monomolecular layers formed on the surface of gamma ferric oxide particles were investigated according to the amount of saturated mono-molecular adsorption obtained from the measured results of Figure 5. As will be apparent from Figure 5, the gradient or slope of the line is 25.0 and the saturated adsorption is 40 mg per gram of gamma ferric oxide. The molecular weight of lecithin is 793 and the specific surface area S of gamma ferric oxide powder used was 21.1 m /g. Therefore, the occupied cross sectional 3~

area SO of the lecithin - gamma ferric oxide system is calcula-ted as follows:

SO = S/(~ws ,NA) = 21.1 x 10 / 793 x 6.

= 69.4 x 10 l6cm2 where NA is Avogadrols number.
The value of the occupied cross sectional area is quite small within the above-mentioned wide range of occupied cross sectional area so that a relatively dense monomolecular layer is formed on the surface of the magnetic particles. According to Table l, in order to adsorb lecithin onto the surface of gamma ferric oxide particles with the amount of adsorption so as to obtain the ideal effect of surface activity, the amount of lecithin should be 5 PHP or more and the original concentration CO should be 2.5g/100 cc or more, corresponding to a value close to the amount of saturated adsorption.
The amoun-t of adsorption of acetone purified lecithin (:5 PHP) to gamma ferric oxide was measured with the result that equilibrium concentration after the adsorbing process was zero and no lecithin was detected in the solution. This means that all the lecithin was adsorbed to the magnetic powder with the result tha-t its amount of adsorption was calculated to be 3.55 mg/g Fe2O3 which is less than 10% of the value shown in Table 1.
The occupied cros.s sectional area SO of adsorbed molecules to : magnetic powder may be considered the same as that of pure : 30 lecithin so that the actual surface coverage of magnetic powder by lecithin molecules is also less than 10%. In the case of using a raw egg lecithin on the market which has previously been 3~

used as a dispersant, the surEace coverage of pure lecithin was only about 5% of that obtained when the high purity (91%) lecithin o the present invention was used. As a resul-tl the excellent sur-face activity effec-t inherent in leci-thin was not observed.
As noted previously, i-t is advisable to limit the amount of lecithin in the formula-tion so that it does not exceed the amount of saturated adsorption. Otherwise the magnetic character-istic can be adversely affected by excess lecithin which changes the particle surface hydrophilic to cause flocculation.
lQ The existence of irreversible adsorption was ascertained in the following manner. Firs-t, 100 cc of MEK solution containing 3 g high purity (91%) of leci-thin which substantially corresponds to the amount of its saturated adsorption in Figure 4 was mixed with 50 g magnetic powder (Fe2O3) in a ball mill for 7 hours and the actual lecithin concentration of this solution was colorimetrically determined by the aforementioned method. Mext, the solution was diluted by MEK and put in a slowly rotating ball for a 2 hour mixing. After the e~uilibrium condition was reached, the concentration of the supernatant solution was measur~d. By repeating the above process until the concentration was about 1/100 o~ the original concentration when the lecithin concentration at the solution side was regarded substantially as zero~ the desorption behavior of the lecithin was measured.
The solution to be diluted was partially removed in advance by almost the same amount at that supplied in diluent in each diluting operation so that during each slow agitation operation, the ratio of solution to solid was kept substantially constant, and the mixing condition was not affected by dilution. The measured results ii3~

o-f desorption of lecithin in the lecithin - gamma Eerric oxide ~ystem re shown in the following table.

.

. ~ . -17-. .,.. 1. .. . : . .
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. .

.. . .
... . .
. , ,~

. I ~ ~ _ ' :
. ¢ . ~ r~
d ._ o ~,~, O O ~ O .
~ . . C ~ O ~ O O C> O O

. - Ir~ U~ O-' . , . C ~ ` C? ~ ~o O O
.. . _~ .
~ ~ Q" O ~ ; r ~ o o o .-.
~ ~ I ' FL
- c~ :~ --o O ~ u~ O O bO
- ~Ll o ~ O G~ ~ ~ N ~ ~ ~ ~
, ~ _ ' _ l ~ ~ 3 ^ ¦ . o o o o ¦

I . o o u~ ~ a o o $ c~
3 ~ ~ .
..
u~ o ~
, C~n~ O CC) d N ~; '' : I L' ~ O ~ O o o O t~ o O
' ~ ~ ~, O ~ O O O ~ O
. . _ ' .
~;~ . _ , .

~ .

~.

-In the above table, the equilibrium concentration ~ cor-responds to that of a mixture having an amount oE solvent VO, and the solvent amount VO corresponds to an amount Oe mixture which is addecl to an amount of solvent V2 aeter solvent Vl is removed from the mixture prior to dilution. The amount o~ lecithin R in the total solvent indicates the surm o-E the 1ecit1lin amounts in the removed solvent and the lecithin amount in the added solvent.
Figure 7 illustrates the above results~ indicatillg both the adsorption and desorption steps. As apparently sl10wn by Figure 7, the amount of aclsorption at the desorption step was maintained at substantially 100~ Oe the level Oe saturated adsorptioll, or about 39. 6 mg/g gamma ferric oxide eor all these cleaning operations. There-fore, even witl~ the removal of excess lecithin contained in the solvent, the surface coverage of the magnetic powder is substantially 1007C, The following table, Table 3, shows the measured results obtained with regard to magnetic recording characteristics ot` various samples. Samples - Nos, 1 to 8 were prepared by adding the amount Oe high purity lecithin indicated into the solution "A" with no calender-ing process being applied, Samples 9 to 12 used acetone purified lecithin, and Samples Nos, 13 to 15 used raw egg lecithin, Sample No. 16 had no lecithin and Sample No. 5' is the lecithin o~ Samp1e 5 which had been applied with t ct lendering procest, ,' ,. ~

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__ I _ _ __ _ ~ a~ . ~ c~ c~, ~ ~ ~ ~ ~ C
C~ ~ ~ O 0 3 ~a) C C O O ~
~, ~ ~ - f O O O ~ 1~ 0 COC~ C ~ t-- N O
~ ~ ~ ~ ~ ~ C~ N ~ . ~ ~ ~ `I N
i. .
C~ ~ C U~ C O O O O O O I C O C O O o :L ._ _ !
~ , I ~ O O C ~ O ~ ~ ~ C ~ O C U~ o ~
. U~,~ ~ oo o~ oo o~
_ . - 1 .

1 I I I N ~ ~r ~ " ,_ o o ¢ C .. . _ . _ ,f~ âl oooooooooOOocoooo I, i --~ _ _, ~ ' 1 ~ o o o c o o o o c o c ~ c c o o c a i -- _ ..
.~ ~ o U~ o ~ O o o o C C C C ~ o O
'~ X O , , ~

,~ C
1~ ~

I
, .

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The packing density P and the porosity P were obtained from the following equations:
P(%) = [(Wc/Sm)(~/J~ )/Va] x 100 p(%) = ~1 - Wc/Sm ~[~/~ ] ~ }/Va} x 100 where Va = Sm t Sm = area t = thickness of magnetic coating film Wc = weight of coating film on area Sm ~ -- powder weight per unit coating weight ~ = density of magnetic powder, Sp = 4.66 = density of binder and lecithin system (binder density = 1.35, lecithin density = 1.10) As will be apparent from Table 3, the magnetic recording medium using high purity lecithin has an improved maximum flux density Bm, residual magnetic ~lux density Br, squareness ratio Rs and packing density ~ In Table 3 there is shown an increased rate of Br as compared with the use of raw egg lecithin. The porosity P becomes large in the case of raw lecithin. This is believed to be resulted from the bubbling effect of cephalin -protein fractions as impurities. In this connection, the measured results of magnetic characteristics obtained are shown in Table 4 with the added amounts of cephalin and protein frac-tions being changed, and with no lecithin being present.

63~

Amount of Cephalin-Pxotein P Rs Hc Brn Br ~PHP) (%) (%) (Oe) (g uss) (~auss) 5 29.3 79.3 310 1~;90 1340 3 35.4 78.7 310 1550 1220

2 35.9 78.5 310 1550 1220 1 40.9 76.9 310 1~80 1140 It should be noted from Table 4 that the porosities are large while the values of squareness ratio, maximum magnetic flux density, and residual magnetic flux density are low com-pared with those in the cases of high purity lecithin.
Figure 8 shows the relation between concentration of dispersant and the residual magnetic flux density wherein curve 1 was obtained with a high purity (91%) lecithin according to this invention, and curves 2 -to 4 were of acetone purified lecithin, unpurified lecithin, and the cephalin - protein frac-tion~ respectively.
Table 5 shows the measured results obtained from magnetic recording media of samples 17 to 20 prepared after the excess leci.thin had been removed in a solvent according to Example 2.

63~L

, ~
. . .__ ___ _ U~ ~ ~ . . . , . ~ ~ ~ C~
. ~ .
~. ~ ~ ~ l _ ~ ~ _ _ ~ ~L'~ O
C~ ~ O O O O ~ ~
I_ _ ~ OC O 0 aq~O cO~
_ I
_ ~ W ~ ~-- L'~
~ m U'~' t~ .
~_ _ _ . .... 3 ~ L'~ ~ ~ L

3 a . ~ o . ~ r .~ ~ ' ~ O O O O - O
~ ~ ' . . C~ ' L~ L~ U`~ U~ O L/~ I ., " ` . ' ~ . . . I
E .
, ' ~ ~ ~. ~ 0 U~ ,_ ,_ ~ ~ ~ ~

" . ' .
' ' : ,. . ' - ' ' .. . . .
.
2 3 - ~ .

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The dilution condition indicates the dilution multipLying factor, the No. 1 indicating that no dilution was carried out. Sample No. 21 illustrates a case in which no lec ithin was added, and Sample ca~
No. 22 is the ~ where raw lecithin was used.
From Table 5 it will be seen that even in magnetic re-cording media prepared by removing excess lecithin in the solvent, the variation oE characteristics does not appear and every sample shows an increase of B~.
It was further ascertained that when excess lecithin in the soLvent is removed, the wear resistance is improved as com-pared to the case where the lecithin is not removed. I'his was determined by rnoving a magnetic medium ~or a given time urIder wear conditions, and measuring the weight be-fore and after move- --?
ment tO determine the amount oE fallen powder. This measurement was carried out with respect to a magnetic tape prepared in the manner of Example 1 with high purity lecithin being added in an armount oE
1.1 PHP and with a magnetic tape prepared as m Example 1 without the excess lecithin removing process and the amount o-f high purity lecithin being 4. 5 PHP. As apparent ~rom the results of Table 1, when the added amount is l. 1 PHP, the entire amount of lecithin is a~sorbed onto the surface of the magnetic powder while when the addecl amount is 4.5 PHP, the excess lecithin remains in the solvent and excess lecithin remains in the magnetic tape in a binder. In measuring the relative amounts of fallen powder with respect to the two tapes, if the amount oE fallen powder was taken as 1 Eor the lecithin of 1.1 P~IP, then the relative amount for the lecithin of concentration 4.5 PHP is 2.
The gamrna Fe203 used in the above examples was an acicular powder llaving a long axis o~ from 0.7 to 0. 8 microns, and an axis ratio of 8 to 10. It had a saturation magnetization oe 72e~ mu/g and a specieic surface area o~ 21.1 square meters per gram. How~
ever, various kinds o~ magnetic powder can be used such as Fe304, gamma Fe203 or Fe30~L containing cobalt or other elements, iron oxides having intermediate oxidized states between gamma Fe203 and Fe304, iron oxide containing an element such as cobalt and hav-ing an oxidized state between garnma Fe203 and ~e30~,~, or ferro-magnetic powder such as CrO2 (eerromagnetic chromium dioxide) or metals or alloys such as iron, iron-cobalt, iron-cobalt-nicke1, or the likc. In addition, mixtures oE these various materials can be used and these examples are not meant to be limiting.
As the binder agent, there may be used various kinds o~ well known thermoplastic and thermosetting resins such as vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinylidene chloride copolymers, cellulose derivatives SUC]l as nitrocellulose, butadiene-acrylonitrile copolymers, polyester resins, epo~cy resins, polyurethane resins, or mi~tures ~hereoE. Ie necessary, the binder may include a curing agent such as a polyisocyanate compound.
The magnetic layer may also include other conventional additives such as lubricants such as silicone oil, graphite, molybdenum disulfide, fatty acid esters, hydrocarbons, or the lilce. In addition, antistatic agents such as carbon black, quartenary ammonium salts, or the like can also be aclded. The composition may also include abrasive particles such as alumina or Cr20;~ or the like.
~ s the solvent for the composition "A"-eor dilution or for coating purposes, we can use one or more solvents such as MEIC, ~L0~63~

cyclohexanone, toluene, tetrahydrofuran, isopropyl alcohol, and butyl acetate. The only requirement is the solvent having a capability of dissolving lecithin.
In the e~amples, the purity of the high purity lecithin was 91%. However r it is also possible to use a lecithin having a purity of 30% or more and to add sufficient amounts so as to substantially cover the whole surface o:E the magnetic powder particles with a monomolecular layer which is substantially entirely lecithin because of the selection adsorption. In this connection, Figure 9 shows the relationship between the degree oE purity and the amount to be added, in which the cross hatched area indicates the range in which the monomolecular layer can be effectively formed. From Figure 9, it will be seen that the ...
monomolecular layer can be formed by using 1.5 PHP where the purity is 90% or more, about 2.5 PHP for a purity of 50%, and 4 PHP or more where the purity is about 30~.
The minimum concentration of high purity lec.ithin should be at least 30% and preferably 50% or more because of the reIationship between the purity of lecithin and its Br and Rs, 24 respectively as shown in Figures 10 and ll.
As described in the foregoing, the monomolecular layer of lecithin can be surely formed on the surface of the magnetic powder, and conse~uently the magnetic and mechanical characteris-tics of the resulting record member can be improved.
It will be evident that a number of changes and variations can be effected without departing from the scope ~f the novel concepts of the present invention.

Claims (9)

WE CLAIM AS OUR INVENTION:

1. A magnetic recording medium comprising:
a non-magnetic base, a magnetic layer carried by said base and composed of magnetic powder dispersed in a resinous binder, said magnetic powder being covered on substantially its entire surface with a mono-molecular layer composed substantially of lecithin.

2. A magnetic recording medium comprising:
a non- magnetic base, and a magnetic layer coated on said base, said magnetic layer including magnetic powder particles dispersed in a resinous binder, said powder particles being substantially completely coated with a layer of lecithin adsorbed thereon, said lecithin being deposited from a composition having a lecithin concentration of at least 30% and being substantially devoid of fats, free fatty acids, cephalin and proteins.

3. A method of preparing a magnetic recording medium comprising the steps of:
(a) mixing magnetic powder in an organic solvent with a purified lecithin composition having a purity of at least 30% in an amount sufficient to cover substantially the entire surface of the magnetic powder particles with a monomolecular layer consisting substantially of lecithin, (b) mixing the thus treated magnetic powder with a synthetic resin binder to form a magnetic paint, and (c) applying said magnetic paint to a non-magnetic base to form a magnetic layer.

4. The method of preparing a magnetic recording medium according to claim 3 which includes the step of removing the non-adsorbed lecithin from the organic solvent after treating said magnetic powder with said lecithin composition.

5. The method of claim 3 in which the lecithin composi-tion has a purity of at least 50%.

6. The method of claim 4 in which the lecithin compo-sition has a purity of at least 50%.

7. The method of claim 3 in which the purified lecithin composition is purified by the decomposition of a lecithin-cadmium chloride complex salt which is a reaction product of an acetone and ethanol extracted lecithin derived from soybean or eggs.

8. The method of claim 3 in which the amount of purified lecithin composition is from 0.5 to 5 parts per hundred parts by weight of magnetic particles.

9. The method of claim 3 in which the amount of purified lecithin composition is in the cross-hatched portion of the diagram of Figure 9 of the drawings.