CA1042277A

CA1042277A - Magnetic recording medium and method of making the same

Info

Publication number: CA1042277A
Application number: CA168,110A
Authority: CA
Inventors: Takashi Shimouma; Shigetaka Higuchi; Junetsu Seto
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1972-04-07
Filing date: 1973-04-06
Publication date: 1978-11-14
Also published as: DE2317409C3; GB1431854A; FR2181874B1; FR2181874A1; NL7304945A; DE2317409B2; DE2317409A1; IT983703B

Abstract

ABSTRACT OF THE DISCLOSURE
A method for making a magnetic tape and a magnetic tape which tape comprises a base or substrate which bears a coating layer of a magnetic powder dispersed in a polymeric binder wherein the improvement concerns the provision of a type of magnetic tape having improved anti-abrasive characteristics.
To provide such type tape, there is dispersed in the coating layer thereof in addition to the magnetic powder, a surface active fine powder. Individual particles of this fine powder are coated with a layer of a bound thermoplastic organic polymer. Such a tape exhibit improved Young's Modulus, reduced coefficient of friction on the coating layer, reduced fallen amount of magnetic powder from a tape, reduced magnetic head clogging, improved travelling characteristics, reduced magnetic head abrasion, and the like, singly or in combination.

Description

BACKGROUND OF l~IE INVENTION
Field of the Invention This invention relates generally to a magnetic recording medium, and more particularly to a magnetic recording medium which is high in anti-abrasion characteristic and capable of reducing the abrasion of a magnetic head engaged therewith.
Description of the Prior Art ` A magnetic recording medium, which comprises generally an insulating or substrate material layer and a coating layer, in which magnetic powders dispersed into a binder are coated on -~
the insulating material layer, is used widely with tape re- ~.
corders, video tape recorders, and the like. Since, in use, ;; such a magnetic recording medium engages or contacts a magnetic `~ head, a tape guide, and the like in operation, the coating layer is subject to abrasion and injury which shortens the li~e span of a conventional magnetic recording medium. Furthermore, such abrasion and injury can cause deterioration in quality of a reproduced signal recorded on such a medium, especially in 90-called high density recording and play back where a coating layer with a mirror-like finish is closely contacted with a ~ magnetic head~
;~1 To improve the anti-abrasion characteristics of a ,,; magnetic recording medium, the prior art taught mixing an organic lubricating agent in semi-solid or liquid state into the coating layer binder. This technique is objectionable, ~ however, because, if the amount of such lubricating agent ~
`:;! added to such binder is small, a desired reduction in abrasion ~;;
jj cannot be attained, while, if the amount of such lubricating `~; agent added is increased to useful levels, such lubricating `:~ 30 agent diffuses onto the surface of the coating layer and causes ~-,'''!' " a so-called blooming with the result that the inherent gap `" ID -2-\
.
Z'~7 between the magnetic head and the coating layer is widened which produces deterioration in quality of the reproduced signal.
The prior art has also taught the mixing of solid -powders such as molybdenum disulfide to the binder for use as a ;;
lubricant agent. This technique is also objectionable, however, because if the amount of such lubricating agent added is small, a desired reduction in abrasion cannot be attained, while, if ^~
, the amount of such lubricating agent added is increased to use-ful levels, the amount of the magnetic powders per unit volume 10 of the coating layer (so-called packing density) is reduced -to a level where the magnetic flux density is lowered ob- ;

~` jectionably and quality of recorded signal deteriorates.

- Another solution to the coating layer abrasion problem ; is offered by U.S. Patent No. 3,630,910 where powders of high . hardness, such as, for example, carborundum, chromium trioxide, .; . .
'!''''I or the like, are mixed into a coating layer binder. While this technique can increase the anti-abrasion characteristics of a magnetic recording medium, it can cause excessive and objection-able magnetic head abrasion.
`;~;` In summary, if, by the prior art, an improvement of ,.i,., i , .
~ anti-abrasion characteristics in a conventional magnetic , . . ~ .
~; recording medium is achieved, then a magnetic head engaged ' with such magnetic recording medium is quickly abraded, while, if a magnetic recording medium is made having a reduced tendency , to abrade the magnetic head, then the anti-abrasion characteris-tics of such magnetic recording medium deteriorate. Thus, :;!
~ by the prior art, it is difficult, if not practically impossible, :,i .;
;, to obtain a desirable improvement of anti-abrasion character- -~
, ., ~ :.
istics in a magnetic recording medium without undesirably in creasing the abrasion of a magnetic head utilizing such an : improved medium.

~`` ' ' .,~ . , .
,. . .
D

- .S UMMARY OF THE INVENTI ON
The present invention relates to a magnetic tape comprising a base or substrate which bears a coating layer of a magnetic powder dispersed in a polymeric binder wherein the improvement concerns the provision of a type of magnetic tape having improved anti~abrasive characteristics. To provide such type tape, there is dispersed in the coating layer thereof in addition to the magnetic powder, a surface active fine powder.
Individual particles of this fine powder are coated with a , :~
layer of a bound thermoplastic organic polymer.
The improved anti-abrasive characteristics of the tapes of this invention may be demonstrated in one or more various ways, including, for examples, improved tape Young's Modulus, reduced coefficient of friction on the coating layer, reduced fallen amount of magnetic powder from a tape, reduced magneti. j head clogging, improved travelling characteristics, reduced magnetic head abrasion, and the like, singly or in combination.
In general, such improvements are achieved without sacrifice ; j , .... ..
in tape quality as regards magnetic recording and reproducing i 20 capacity.
;The invention further relates to methods for making such improved type of magnetic tape.
More particularly there is provided a magnetic recording ;
medium comprising a base material and coating layer formed on ;
said base material, said coating layer comprising a binder of thermoplastic polymer, a magnetic powder dispersed in said binder, and a surface active fine powder dispersed in said binder, said surface active fine powder particles being individually coated with a thermoplastic bound polymer and having maximum average particle sizes ranging from about 5 to 50 millimicrons, there being from about 20 to 100 parts by weight of said surface activa powder per 100 parts by weight of said binder.

' `' L D - 4 - `~

. . . . . . . . .. . . . .. .. . . . ..

' ZZ~7 ~
There is also provided a method of making a binder for a magnetic recording medium comprising the steps of dissolving a thermoplastic polymer in a solvent and kneading said solution with a surface active fine powder to form a bound polymer layer around said surface active fine powder at a temperature from about 40 to 80C, with a concentration from about 25 to 90 weight percent of total polymer plus powder against ~ :
said solution, said surface active fine powder having maximum -~ average particle sizes ranging from about 5 to 50 millimicrons, . 10 is contained in said binder from about 20 to 100 parts by weight per 100 parts by weight of said thermoplastic polymer.
There is further provided an anti-abrasive, magnetic / recording medium having spaced, parallel, opposed faces and .
.1 comprising in integral combination a substrate layer and a ,.. . .
s: coating layer, said substrate layer being substantially non-s conductive of electricity and self supporting, and ;'~ comprising an organicpolymer, said coating layer comp~ising :

, a surface active fine powder, a magnetic powder, and an organic : :
. .~ .
., polymer binder, said powders being substantially uniformly .

distributed in said binder, said coating layer containing from ~;~ about 20 to 100 parts by weight of said surface active fine ~ ~:

. powder per 100 parts by weight of said binder, and said . :

coating layer further containing a magnetically effective ~ quantity of said magnetic powder at least sufficient for pre-.. , chosen recording and reproducing purposes, said surface active ~ fine powder being further characterized by - having maximum ; average particle sizes ranging from about 5 to 50 millimicrons, .

; . having a surface area of at least about 40 m2/gr., being sub-; stantially completely insoluble in common organic solvents and in said binder, and having substantially all particles thereof .

each coated with a layer of a bound thermoplastic organic : :
. ~' , , .
~.~. , .
- 5 - ~ :

~ L~4~7~ o polymer, the layer thickness ranging from about 15 to 40 A , -: said magnetic powder being further characterized by -comprising at least one magnetic compound of at least one metal selected from the group consisting of iron and chromium, and having maximum average particle sizes ranging up to about 600 :~ millimicrons with at least about 80 weight percent thereof ::, (based on 100 weight percent total magnetic powder) being in the form of elongated bodies having average length to width ratios in the range from about 2:1 to 20:1, said binder being further characterized by being at least swellable when in a -.
non-thermoset form by common organic solvents to an extent such . :
':';.' :
that a fluid, homogeneous composition thereof may be prepared which is adapted for deposition on a surface with said surface active fine powder and said magnetic powder dispersed therein. . ::
, ! , . . .
; There is further provided a process Eor making an anti- :
, .'', ..... .
abrasive magnetic recording medium comprising the steps of, in .`

sequence, dispersing uniformly a surface active fine powder in . "! : .
a solution of thermoplastic polymer in an inert organic solvent ` ~:
to form a dispersion at a temperature from about 40 to 80C ~
with a concentration from about 25 to 90 weight percent of total `
polymer plus powder against said solution, comprising from about ;`~
20 to 100 parts by weight of said surface active fine powder .~
; per 100 parts by weight of said thermoplastic polymer and having ;~;:
~ a viscosity of not more than about 200 centipoises as measured :
.~: in 10 volume ~ cyclohexanone solution, said surface active fine . powder having maximum average particle sizes ranging from about ~ ~
. 5 to 50 millimicrons, dispersing uniformly into a resulting ~ .
s~id dispersion without substantial gelation of a magnetic :.:
,.; ~. . , . p~wder, coating the so prepared dispersion uniformly onto one :

surface of an insulative substrate, and evaporating said orgnaic :
.. . .
`~:,; solvent from said resulting coating.

~;: ~. .
:,~, , .:
-: - 6 - :~ :
`:```` LD -,.` . .... .
,. ...

~ ~4;~7 : BRIEF DESCRIPTION OF THE DRAWINGS
, :
~ Figure 1 is an enlarged partial vertical cross-section- :.
,"~, al diagrammatic view of one embodiment of a magnetic recording ,' medium according to this invention;
,. .. .
Figure 2 is an illustrative graph showing the relation--' ship between various different binder films of this invention ,.' which films differ from one another as respects quantity of ', dispersed surface active fine powder (here exemplified as .' silica gel) in each binder film and the respective physical ~' `., 10 strength characteristics of Young's Modulus and percent elonga- .' ,'~' tion, Young's Modulus being here expressed as a ratio of the ' Young's Modulus of a given binder film of this invention to ' ' ,`
... .
~' the Young's Modulus of an embodiment of that same film which is not dispersed surface active fine powder;
. .
;, Figure 3 is a diagrammatic view in vertical section of ~:, a single particle of a surface active fine powder and which ~.,:' -bears a polymeric coating, the powder being of the type which ' .
. ,., ~-`, is useful in the practice of the present invention; and '.
. ~,~, .~ - , .
' Figure 4 is another illustrative graph showing the ~ ' relationship between various different members in one class of ' magnetic recording media of this invention which members differ . .
. from one another as respects quantity of dispersed surface , , .
' active fine powder (here exemplified as silica gel) in the ', coating layer of each member and the respective anti-abrasive . , .` characteristics of friction factor and fallen amount of magnetic .' , ' :.:, ', ', powder. -~, , , '' DETAILED DESCRIPTION :
. ,~ .
,. . ~. .
Referring to Figure 1, there is seen an embodiment of ~-',',~ a magnetic recording medium of this invention which is herein , ', 30 designated in its entirety by the numeral 10. Medium 10 ~' '~' comprises in combination an insulating layer or substrate layer ' '' - ~
.. ,: _ 7 -.~,.. _ :
.'~.''`'' , ~ , .
L
... .

: ll-and a coating layer 12. Layers 11 and 12 are integrally :~ bonded together. Layer 12 comprises magnetic powder 13 and : surface active fine powder 15 dispersed in a binder 14. Sub-stantially all particles of the surface active fine powder 15, as well as substantially all of the particles of the magnetic powder 13, are enveloped by, and covered with, polymer layer 16. ;
~ As illustrated in Figure 1, the surface active fine powder 15 ;. is generally selected so as to be smaller than the magnetic ;~
powder 13 in size. For example, a surface active fine powder ~ - :
is generally smaller than about 50 milli-microns which is i typically, smaller than about 1/5 the average length of the ;::- .:
:;, .... ..
.~. shorter axis of a common particle of magne~ic powder, such :.~',~ ' :
as magnetic powder 13 in Figure 1. Such a size differential . : ~
. ~, . - .
. makes it convenient to fill in dispersed form, the polymer .
of a coating layer therewith without substantially changing a ~- :
desired so-called packing density of the magnetic powder 13, in .
such coating layer, with the result that the presence of amounts ;` :-,. :
.; of a powder 15, as taught herein, does not generally lower the . . .
magnetic characteristics of the magnetic recording medium 10.
Further, referring to Figure 3, the binding force between the bound polymer layer 16 and a fine powder 15 is very strong so . .~ .
that the mechanical strength of the coating layer 12 in a . tape of this invention is high which enhances the anti-abrasion ~ :
." :;, .
... : characteristics of the magnetic recording medium 10. `.

A surface active fine powder 15 is generally very small . .

.~ in average particle dimension so that its dispersion character- -.
: -:
istic is typically rather low but its particle surface area is .. :.
: ..................................................................... : .
. large so that its adsorption force is great. As a result, if -.

~`~ a surface active fine powder and a magnetic powder are together ~-. 30 mixed into a polymeric binder by a simple ordinary physical ` :

.. ~ mixing procedure, the surface active fine powder can not be .. : ' ''', -: - 8 - .. ~
` ` D ; ~ ~
.. . .. .. .
'` !; ~

:

zz77 scattered or dispersed sufficiently and in a uniform pattern of non-agglomerated material in the binder. If a dispersion agent or a lubricant agent is present, the surface active fine - powder tends to adsorb same, in the binder and hence the magnetic powder can not be dispersed uniformly in the binder. Thus, it is practically speaking, apparently impossible to disperse the fine powder and the magnetic powder uniformly in a binder by ;
~ physical mixing.
; However, by the present invention, a fluid technique -is utilized to achieve preparations of dispersions wherein mag-. netic powder and surface active fine powder are each uniformly dispersed.
~; A magnetic tape of this invention can employ as the substrate or base layer any conventional organic polymer which is in the form of a stable, reasonably strong film or sheet (herein termed substrate layer), as those s~illed in the art ~; readily appreciate. Preferably, polyesters are employed for such a substrate layer in a wide range of thicknesses.
In the coating layer of a tape of this invention, one can employ as a matrix phase any convenient organic polymeric material which is capable of being swollen or dissolved to an extent that such material can be reduced, prior to coating ~ -~ onto a substrate layer, to a substantially fluid form in com-; ~ bination with a solvent; that is, such material is dissolvable ... .
or swellable in solvent. Furthermore, the magnetic powder and the surface active fine powder (the latter coated with a thin, ~ bonded polymeric layer is herein described) must be uniformly : ~ dispersable therein.
~.. j ~.
;~ As the magnetic powder~ any conventional such material may be employed. Preferably, such powder is comprised of a magnetic material selected from among, at least one of i'~ the group of magnetic oxides of iron and chromium which can be ;.:

..... .
; _ 9 _ j ` D

.. ` ~ .. .. . . .. . ... ~ .

4~77 `-in various crystalline and chemical forms, as those skilled in the art will appreeiate. Preferably, also, such material is in a powdered form having maximum average particle size ranging up i to about 600 millimicrons or even somewhat larger with at least about 80 weight percent thereof (based on 100 weight percent total magnetic powder) being in the form of elongated bodies , ~ .
having average length to width ratios of from about 2:1 to 10~
20:1 or even greater, as those skilled in the art will appreciate.
The surface active fine powder employed in this inven- ;
tion can be any convenient, preferably substantially non-magnetie, material whieh is dispersable in the matrix phase in a particu-late form after being subjected to surface coating with a `
thermoplastic polymeric material. Typieally, suitable surface ; ~-active fine powders are eharaeterized by: ``~
(1) having a maximum average particle size ranging from about 5 to 50 millimicrons r

(2) having a surfaee area of at least about 40 m2/gr., I`

(3) being substantially eompletely insoluble in eommon organie solvents and in said binder, and

(4) having substantially all partieles thereof eaeh eoated with a layer of a bound thermoplastie organic polymer, the `~
layer thiekness ranging from about 15 to 40 A.
Presently preferred surfaee aetive fine powders are `` siliea (siliea gel) and earbon blaek. `
, The bound thermoplastie organie polymer ean be any ,~
.~ sueh polymer whieh bonds to the surfaee of sueh a powder. Pre-~` ferably sueh a polymer is dissolvable in an organic solvent so that the powder partieles ean be dispersed in sueh solution and thereby eoated with the polymer. Examples of typieal suitable ~30 sueh polymexs inelude polyurethane (whieh is presently a `
. . ..
`~ ~referred material) styrene-butadiene copolymer, acrylonitrile- - ~
~ butadiene copolymer, non-vulcanized butadiene rubbers, vinyl ~ -.~ .

:ID - 1 0 . , , ~2~
homopolymers such as polyvinylchloride and vinyl copolymers with vinyl acetate ~which is at present another preferred material), and the li]se, alone or in admixture with one another or other polymers.
It is convenient and presently preferred to use, as the matrix portion of a coating layer, a polymer which is either the same as, or is similar to, the polymer ~sed to coat the particles of surface active fine powder.
Among the polyurethanes, linear polyurethanes are pre-ferable, for example, those obtained by reacting a diisocyanate of this formula OCN ~CH2t NCO, where }l is selected to be an integer of from about 2 through 12 or OCN~X~NCO, where X represents alkylene radical or oxygen, with any one of the `
polyester type polyols of the formula HOOC tCH2tX COOH +
HO(CH2 ~ OH, where x is selected to be an integer of from about 2 through 8 and n is selected to be an integer of from ~ ;
about 2 through 6, polypropylene glycol, or a glycol of the -formula HO ~ (CH2)n - O~mH, where m is an integer selected to be from about 2 through 6 (for example, polytetramethylene glycol), or the like. In general, those on market as the `~
trademarks of "Paraprene", "Estane", "Desmopan", "Elastran", ~`~
"Texicine", "Adiprene", and so on, are suitable.
Styrene-butadiene copolymer, acrylonitril-butadiene copolymer and non-vulcanized butadiene rubber, represented by "Carifrex" TR-1101 ~ (Shell Chemical) (a polybutadiene-polystyrene-block-copolymer rubber), "Hycar" 1432J ~ (Japanese Geon) and "Nipol" BR 1441 ~ , ta non-vulcanized polybutadiene rubber), are now on market.

. . .
In order to manufacture a magnetic recording medium of this invention with powders uniformly dispersed in the coating ::
'' i`., LD ,, .~ ~9LZ~ 7 ; layer, a thermoplastic polymer is first dissolved in an organic solvent and then a surface active fine powder is dispersed therein (which is hereinafter sometimes referred to as the reinforced binder) then a magnetic powder, with, typically, a dispersion agent, a lubricant agent, and the like, as these -additives are conventionally known and used by those sk~illed in this art, are mixed uniformly into the reinforced binder to prepare a magnetic paint which is thereafter coated onto a base substrate material and dried to form a magnetic recording medium ~ of this invention.
.:~
A particularly preferred procedure for making a starting dispersion of surface active fine powder in a solution of ~!' polymer and solvent is described generically and specifically illustrated in Example 1 below, though any convenient dispersion procedure may be used as those skilled in the art will readily 'l appreciate.
.: ~
As indicated, magnetic powders are preliminarily and separately suspended with a dispersion agent into an organic solvent such as, for example, methyl ethyl ketone, ethyl , 20 acetate, cyclohexanone, or the like, and the mixture is kneaded !".
~I by a ball-mill for about 24 hours to disperse the magnetic -~
ii powders uniformly. A reinforced binder, which is generally pro-; vided with addition of a small amount of lubricant, and an electrification resistant agent is added to such kneaded mixture :~ and the resulting system is further kneaded for a period of time, such as about 24 hours, to prepare a magnetic paint. -Thus prepared magnetic paint is coated on a base ;
material to form a magnetic recording medium using any conven- ;rl -' tional coating procedure, such as roller coating, knife coating : . . .
, 30 or the like, as those skilled in the art readily appreciate. ~
.; ::, The amount of added reinforced binder may be determined ,', ': ' " ' .,, , ~ . .
':'. i,. -,,' '~

:` D -... ....
. . .

` ~D42~77 : ~
by multiplying a necessary amount of a conventional binder having no silica gel with the following factor F:
F = (1/PB) (100 + X) (lOO/PB) ~ X/pp) where X represents the weight part of the filled fi~ne powder ;
with respect to 100 weight parts of the reinforced binder, while and pp weight to ~olume ratios of the conventional binder and the filled silica gel fine powder. Thus, irrespective of ~ the filled amount of the silica gel fine powder to be added to - the binder, the volume of the magnetic powder in the coated ~",, `; layer on the magnetic recording medium versus the volume of the binder can be substantially CQnStant.
PREFERRED EMBODIMENTS
. .- .
The present invention is further illustrated by refer-ence to the following Examples. Those skilled in the art will appreciate that other and further embodiments are obvious and within the spi~it and scope of this invention from the teachings of these present examples taken with the accompanying specifica~

:: :tion and drawings. All parts are parts by weight, unless others were indicated.
, 20 ~,~ Example l The following starting materials are employed:
`~; Thermoplastic polymer Polyurethane resin made by Nippon Polyurethane Industry Co., Ltd. available -; commercially under the Trademark r~ Paraprene ........ 100 wt. parts ~; Solvent Mixture of methyl ethyl ketone and ~-cyclohexanone at weight ratio of l:l ...200 wt. parts " ,~ , :

Surface Active Silica gel made by Degussa Co. and Fine Powder available commercially under the Trademark , . .
~ "Aerosil" R-972 .` .:
` - 13 -;`: iD
;. :.: . .

.. . . . . .. ., .. ... , ; ;

Z'77 ; ~
; A series of coating layer binder systems are prepared, ~-.; ~ . , -~ each binder system compirsing the above thermoplastic poly-urethane resin binder of lO0 weight parts dissolved in 200 parts by weight of the above solvent and having dispersed therein --the above silica powder in the individual, respective weight : :
parts of 10, 20, 30, 40, 50, 70 and 100 based on 100 parts by ;

weight of polymer (abbreviated herein PHR).
, ,~ .
~ The following procedure is used:
:;
Thermoplastic polymer and solvent are inserted into a ~
:: ,.:
mixing device, in this example, a kneader, to dissolve the thermoplastic polymer ~o the solvent.
: , In this example, the amount of the thermoplastic - -polymer relative to solvent is selected to be about 50 weight percent based on the total solution weight. ,` ;
Then, the surface active fine powder is added to a thus ~;
,.: . . .
`~ formed solution in a predetermined amount as above indicated. ~
. .~ ! , -~ The mixture which results in each instance is then kneaded in ` a kneader with its lid open.
~ The heat generated by agitation causes vaporization of '' 20 solvent which gradually raises the solids level in the kneader.

In turn, this results l~n a greater average force gradually being applied to the mixture of polymer plus powder in the kneader. In this example, to regulate this procedure, the ..' :
,l kneader was periodically stopped, its lid closed, and water -1 " ." ,'', ~" .
applied to exterior surfaces thereof to adjust the contents to ~`
the kneader to a temperature in the range of from about 40 to 80C. In addition, the concentration of total solids (poly- r' ": ` ' ` mer plus powder) in the mixture in the kneader is adjusted by ;

i , addition of moi~e solvent so that this total solids level is :, . :,:
~ 30 maintained at least in the range of from about 25 to 55 weight .. ! ::~ ,:
percent up to about 90 weight percent, based on total mixture `
'`~: ' ' .'. ,, ' . .
~ ` - 14 -... .
`` ~D :;: ~:
i~`.. ........

., . . . . . . . . " .. . . . . . .

: : :

1~4~'~77 weight and preferably, at ab~ut 80 weight percent, which ~ -prevents the polymer from being excessively deteriorated by heat and/or agitation. If such adjustments of temperature and solvent content are ~ot made, there is a tendency for the ~
polymer not to remain in a dissolved form, with the late~ ~ -result that when a binder system with the powders dispersed -therein is coated onto a base substrate, the polymer may gel which deteriorates the dispersion of magnetic particles, the -surface smoothness of the final coated layer, and, sometimes, even the bonding or contacting characteristics of a system on the substrate. The reason why a temperature is selected within the range of about 40 to 80C in this process is that, if the temperature selected is lower than about 40C, the mixing rate of the fine powders is lowered, while, if the temperature is selected higher than about 80C, the polymer is apt to de-compose excesslvely.
When an 80 weight percent solids mixture of polymer, ;
powder, and solvent is kneaded at a temperature in the range of from about 40 to 80C for about one hour in the hneader, and is thereafter diluted with a solvent having a boiling point higher than about 100C, such as cyclohexanone, to a solids , ... ..
level ranging from about 25 to 55 weight %, (preferably about 40 weight ~), it is observed that the fine powders are rather uniformly, but somewhat incompletely, dispersed, from a microscopic point of view. To complete the dispersion of particles in the system, the system is further kneaded on a roll-mill having three rolls about two or three times. Such roll-mill treatment after such dilution avoids any substrate .. . .
deterioration of polymer, such as might occur if the kneader were used. With a mill roll procedure, if the total solids level is below about 25 weight ~ based on total system weight, . ' .

` - 15 -.:;................................................................... .
. ~ , : .
~`''`` " ~ ' _ I .

22~7 no beneficial mixing effect from the mill rolling is observed under such starting conditions, while if the total solids level .. .. .
is above about 55 weight % (same basis), there may be sufficient heat generated to excessively deteriorate the polymer.
A dispersion of silica in dissolved binder prepared by such a procedure as above descrlbed may be, and preferably is, diluted by further addition of solvent to give the dis-persion the liquidity desired for subsequent coating of the dispersion on a substrate, as those skilled in the art well ~i -appreciate. Since the consistency of individual dispersions characteristically and inherently may vary widely, one conven-. .
;~ ient procedure is to coat a sample of a dispersion on a glass plate, evaporate solvent, and observe surface smoothness.
~;' Comparison to a non-filled, similarly made coating of the same ~-. ~ , polymer is preferred. Surface smoothness of the coated, filled , dispersion is then optimized through solvent dilution thereof ',1 before coating. ;~
Example 2 i; The dispersion of 40 P~IR silica in dissolved polymer . . .
prepared as described in Example 1 is compared with a binder (b) and with a binder (c) in characteristics. The binder (b) is `
prepared by kneading a conventional thermoplastic polymer used ' ' in coating layer binder with the silica used in Example 1 but without using solvent (which is similar to the procedures used in manufacturing vulcanized rubber). The binder (c) is prepared in solution form as in Example 1 but without silica gel and is referred to herein sometimes as a "non-filled binder".
The following Table I shows the results of this comparison. In .. . , ~
this Table, binder (a) refers to the above indicated silica , 30 dispersion from Example 1. -; .::
, ` - 16 -.

. . . . . . ., .. ,. , ,, , : . . . : . ~ .. ~ .. .. .: . . . . .

, Table I , ... . .
, Binder Property sinder (c) sinder (b) Binder (a) : ., , ',' Disso- Methyl ethyl Polymer Gelatinized Palymer ' lution ketone completely' completely '`
~' dissolves dissolves ~', Cyclohexa- Polymer Gelatinized Polymer ,,, none completely completely dissolves dissolves ;, .
Viscosity coefficient 200 cp - 200 cp '`
.,;.~, -' In the above Table I, the coefficient of viscosity :, 1 0 : .
"' in centipoises (cp) is measured for each specimen using a ', cyclohexanone solution which has dissolved therein 10 volume %

~',' of the reinforced binder specimen'(the silica being dispersed therein). ~-When such indicated silica reinforced binder of Example I is used as a binder in a magnetic paint, such reinforced " ~
' binder shows high solubility, and when the magnetic powder is ; dispersed into such binder to make such paint, one must avoid gelling the system because gelation deteriorates the dispersion characteristics of the magnetic powder therein.~' According to the Table I, the reinforced binder (a) is the same as the binder (c) with no silica gel as respects ',, solubility and coefficient of viscosity. Those skilled in , the art may wish to regard a viscosity coefficient of about , 200 cp as a convenient reference or stardard point showing ~'~ absence of gelatinization. Note that the reinforced binder ~' . .
(a) of this invention is equivalent to the non-filled binder (c) " in coefficient of viscosity. Further, it is ascertained that .. ' ;'~
~,, such a silica reinforded binder is generally not greater than about 200 cp in its coefficient of viscosity independently of ; 30 ,`' the amount of fine powders filled or dispersed thereinto.

- The binder (b) with silica gel but no solvent is not .. " , ~' .
' '`~ - 17 - ~

~ 4~7 dissolved but swollen only.
' When 40 PHR of silica gel is dispersed under agitation '' into cyclohexanone solution with the non-filled binder (c) '`~
~ by 10 volume ~, its coefficient of viscosity is found to be '~
. . . .
increased to about 1500 cp. When such a filled binder system is coated on a base substrate material, the smoothness of the ; ;

' surface of the coated layer is found to be deteriorated and ''~'' ' not smooth. -- ' ., ,~,.:, .
Example' 3 The mechanical nature of a silica reinforced binder of ~' this invention prepared according to the procedure of Example 1 is illustrated as follows: In Figure 2 there is seen illustrated test results for a series of silica-containing, reinforced '~` binder films prepared by the teachings of this invention. In ~ ' : ., . : : -making such films, first, a series of reinforced binders is '~
prepared from Example 1. Each such binder is then similarly ~ ' coated on different glass plates, dried to evaporate solvent '`
; peeled therefrom, and tested.
,: .. . :
,~ Curve 1 of Figure 2 shows the inclination over a range of silica contents within which the stress and strain are ; ' ' in proportion to each other and are measured as the ratio of ; ''-a Young's Modulus E, which corresponds to a binder filled with ~ ' silica gel in respective amounts as described on the abscissa, '' t to a Young's Modulus Eo~ which corresponds to a binder contain- ' ing no silica gel. As is apparent from the curve 1, the value ` of the ratio E/Eo increases abruptly in response to the ' '~' increasement of the amount of silica gel filled in the binder. ' ' ' '' When the amount of the silica gel in the binder reaches about 50 PHR, the ratio of E/Eo becomes about seven times as ~ 30 compared with that of the non-filled binder. The improvement '` in Young's Modulus of the binder films correlates well with '' ' the abrasion resistance associated with coating layers in ' ., ~i ;

: `-` lV4Z'~7 ~' magnetic tapes of this invention, as shown by correlation tests.
; As the tensile strength of a binder film increases (see curve 1 of Figure 2), the elongation percentage ~- gradually declines (see Cur~e 2 o~ Figure 2). Elongation percentage, like Young's Modulus, is closely associated with anti-abrasive characteristics in magnetic tape of the inven-tion, as shown by correlation tests. While the binder film tensile strength abruptly increases as shown in Curve 1 in proportion to the amount of silica gel filled into a given polymer, the elongation percentage declines from about 700%
"~ for the case of a non-filled binder as the amount of silica gel filled thereinto increases and becomes lower than about 100% when the amount of silica gel is 100 PHR. From the standpoint of desirable anti-abrasive properties, and because of this decline in elongation percentage with increasing relative weight percentages of silica, it is generally pre-ferred that the amount of silica gel filled (dispersed) into a binder film prepared from a dispersion made as described in Example 1 above fall in the range of from about 20 to 100 PHR.
Example 4 The following starting materials are employed: ;
~`~ Thermoplastic Butadiene copolymer available ;~ Polymer commercially under the Trademark "Hycar" - 1432J from Japanese Geon ... 100 wt. parts n'~,~ Solvent Mixture of cyclohexanone and toluene at weight ratio of 1:1 ~; 30 ... 200 wt. parts -Surface Active Silica gel, made by Degussa Co. and Fine Powder available commercially under the Trademark "Aerosil" 200 , ~ L ~ 19 -;~

~ ZZ7 The above silica of this Example has a smaller particle , size than the silica used in Example 1 but a larger surface area. The silica of this Example is dispersed in the thermoplastic polymer of this Example using the procedure -described in Example ~ to prepare respective dispersions containing 10, 20, 30, 40, 50, 70 and 100 PHR silica. There-after, using the so prepared dispersions, the procedure of Examples 2 and 3 is repeated. The test r~sults obtained are -. . .
similar to those obtained in Examples 2 and 3, except that, apparently because of the higher surface area, there is .. . .
generally a higher degree of reinforcement obtained by using r',~

the silica of the present example. When using a dispersion :
of this Examp]e containing 40 PHR silica, it is ascertained that the ratio of Young's Modulus E/Eo is approximately 6, which is rather a large value. It is also noted that the ;i`
resulting silica dispersions are excellent in solubility , .
characteristics and are not easily gelatinized. ;~

Example 5 To investigate the reasons for the above described in-'.;;:~ ::, creased mechanical strength associated with coating layers ~;

prepared according to this invention, the dispersions obtained ~
... .
by the procedure of Example 1 are subjected to viscoelasticity measurements and their glass transition temperatures observed.

The results are recorded in the following Table II: -`''~ "' ' "
. , ,.,: ::
' ', ': ';
: .:. ,:
" .,:
: ,;. , `~ 30 , ;`~ ~,' i -', ~ - 20 -'D
. . . .
. ;.................................................................... .

~042277 Table II
_ .
. .~
Amount silica dispersed Glass Transition :: in polymer (by PHR) temperature (C) ;~. .
O

~ 20 -20 .: 30 -18 -17 :
. 50 -16 The fact that the Young's Modulus of a coating com-position prepared according to the teachings of the invention is :;
increased in proportion to the amount of surface active fine powder dispersed thereinto means that the molecular chain of the thermoplastic polymer adheres to the surface of such powder, while the fact that the glass transition temperature of such ~ polymer/powder mixture increases in proportion to the amount .. of such powder dispersed thereinto shows that (referring to Figure 3) on the surface or outer periphery of substantially .
all particles of surface active fine powder, of which particle ,.. .
. 15 is representative, there is characteristically formed a ~ .
., :
bound polymer layer 16. This polymer/powder composition has a higher glass transition temperature than the non-silica ., containing, thermoplastic polymer by itself.
It is theorized that (and there is no wish or intent to -be bound by theory herein) the thermoplastic polymer (here a polyurethane~ may adhere to the surface of the fine powder ~, by hydrogen bonding in accordance with the following structure:

~l 30 .", '~, '~, ' ..
.~!' - 21 - ..
:`` 'D :: ~;~
... i . . .- .

LZ~7~ /o ' i ~ OH ............... O = C
= \ NH
Si~ Hydrogen bond O ~ ~ ,, ~u , .
i - OH ....... -....... O = C \

The reinforcement mechanism provided by a plurality of particles 15 each having a bound layer 16 all dispersed in a polymer matrix is then fully accounted for by the layer 16 on individual particles 15.
The actual thickness of the bound polymer layer 16 is estimated to range from about 20 to 30 angstroms (A) from -measurement of dielectric constant. The diameter or grain size of an (uncoated) fine powder particle 15 ranges from about 8 to 50 millimicrons (mu), as is typical for silica gels `
commercially available. ` ;
Example 6 ~ ~
, . . ~ - , To demonstrate that the organic polymer comprising the ~ ;
; matrix oE a coating layer in a magnetic tape may be a mixture oE polymers and may be thermoset, the present example is -provided:
~ To disperse silica in thermoplastic resin, the pro-;i cedure of Example 1 is followed. Then, in a dispersion ?``. prepared according to the Example 1 and containing about 40 PHR
; silica, is dissolved by mixing a thermosetting polymer so that the resulting dispersion comprises about 70 weight ~
percent polyurethane identified in Example 1 and about 30 ~ -weight percent thermosetting polymer (based on 100 weight percent total polymer). The composition of the thermosetting i; ;
'i~` polymer so added is as follows: -~

';L," '.i: ~

':
`~```` ~`D :
,.,.;, i ... .. ~

Z7~
Polyurethane polymer Commercially available from Bayer Chemical Co. under the Trademark "Desmophen" 800 ... 1 wt. part Diisocyanate Commercially available from Bayer Chemical Co. under the Trademark "Desmodur" ...1.5 wt. part Following the property measurement procedure indicated in Example 3, the Young's Modulus ratio of E/Eo of the thus ` prepared reinforced coating layer is ascertained to be about 27, which shows that a coating layer of this invention is further improved in Young's Modulus by thermosetting the matrix, as compared with an E/Eo of about 5 for a silica reinforced binder with no thermosetting polymer contained therein. In practice, ; , , .
the anti-abrasive properties for magnetic tape so prepared with such a cross-linked, thermoset matrix phase in the -; coating layer appear to be excellent.

; Example 7 ` The following Example illustrates utilization of the . .. ..
` various dispersions of silica in thermoplastic polymer/solvent systems made according to the teachings of Example 1 to make s`

magnetic tapes of this invention which have anti-abrasive properties:

: The following starting materials are employed:

gamma-Fe2O3 magnetic powder ...100 wt. parts Silica/Polymer Dispersion (dry wt. basis) ........... 20 wt. parts i;~

Dispersion agent; lecithin ... 1 wt. part ";

~;1 Lubricant agent; Olive oil ... 1 wt. part ,~ Electrification resistant agent ; ~Elegan FD made by Nippon ;
;~`, Oils &Fats Co. Ltd.) ... 0.2 wt. parts The preparation procedure is as follows~

The magnetic powder and the dispersion agent are mixed ,, , :
~ ~ Trademark , .~ .
'': . ' ` ' ' `: D 23 -Ç`~

~ ~4z~7~
with a solvent mixture of the type indicated in Example 1 by ball-mill kneading for about 24 hours to produce a uniform dispersion of solids in the organic liquid. ~ `
To each of the dispersions of silica ln polymer/solvent system made according to Example 1 and having the respective ~; -, ~i there indicated silica PHR values, is added the lubricant agent and the electrification resistant agent with mixing to ;~ form a uniform dispersion system thereof.
Each such resulting dispersion system is added to a ~ --~ 10 magnetic powder dispersion and the product combined mixture is ~ further kneaded for about 24 hours to prepare a magnetic ;; paint. Each such paint is then coated onto a substrate base film, and the coated film is allowed to dry, thereby pro- `
~ ducing a desired magnetic tape. The base film used herein is `~ polyethylene terephthalate.
; When each of the magnetic tapes so made is tested in a tape recorder, all of the tapes display good anti-abrasive properties. However, the tape made with less than 20 PHR
. . ::,~ .
silica gel may exhibit an unduly difficult travel, or even `;
,,., :
no ability for continuous travel, which is a behavior similar to that observed with a tape bearing a non-silica containing coating layer made with the same untreated polyurethane resin ~`~ used in Example 1 and which is a behavior that is characteris- -tic of magnetic tapes made with relatively soft coating ,: . , .
`1 layer binders. The problem is that the coefficient of friction with soft (e.g., inadequately reinforced or cross-linked) -`
binders is too high for normal tape transports.
., . .:: .
; Example 8 This Example illustrates operational characteristics ~` 30 of magnetic tape made according to this invention. ~

Referring to Figure 4, the plot designated by reference `

;~ numeral 3 shows the travelling characteristic of various ,'~:" '``.`D - 24 -`;:' : ' ``' ~L~4ZZ77 magnetic tapes made according to Example 7 in relation to their friction coefficient, while theplot designated by reference numeral 4 shows the anti-abrasion characteristics of such magnetic tapes. Here anti-abrasion is determined by the fallen amount of the magnètic powder produced by the frictional defect during travelling of a tape through a test tape recordex. As can be seen from plot 4, as the filled amount of silica gel increases, the fallen amount de~ ~ ases, but when the filled amount of silica gel increases to the vicinity of about 100 PHR, the fallen amount again increases.
The foll~wing Table III shows various characteristics of a magnetic tape of this invention wherein the coating layer contains 40 PHR of silica gel compared with various conventional magnetic tapes.

Table III
Magnetic Tape Specimen ;
Characteristic 1 2 3 4 ` (this in- (non- (on mar- (on mar-;~~' vention filled) ket) ket) . ; . , , Fallen Amount ;

,, x 10 g :. ,;, : ..
~" Number of -Tape Uses more than - more than several - :~
,~ Until Head 100 100 times , ` Clogging :. , .
; Friction 0.21 0.35 0.23 0.25 ;' i Coefficient , ,~ ~, ,, ; Travelling Travel-,~ Characteris- Good ing be- Good Good ;~ tic comes '~
`~; impossi-ble after 10 times travelling ;, , .~: , ; 30 Head Abraded i Amount 1.5 - 3 -microns/25hr.

In Table III, Specimen (1) is a magnetic tape of this ' , invention employing a coating layer of Example 1 with 40 PHR
,; .:
:" "

~ - 25 -,:

~.~4Z2~
silica powder; Specimen (2) is a magnetic tape employing a coating layer similar to that of Specimen (1) but containing no silica; Specimen (3) is a conventional magnetic tape con-;, . . .
taining in the coating layer a small amount of chromium trioxide as an abrasive; and Specimen (4) is a tape similar -to that of specimen (3) but containing no chromium trioxide.
Further, in the Table III, the designation "Fallen Amount" indicates the total amount of fallen magnetic powder in grams from a magnetic tape with a width of 1/4 inches which is wound on a reel having a diameter of 7 inches and which is driven through an audio recorder five times. Also, in Table III, the designation "Number of Tape Uses Unt~l Head Clogging" indicates the number of cycles that a given length of magnetic tape can make through a home use type tape recorder to repeatedly carry out reproducing without that recorder's magnetic head becoming so clogged with fallen ;`
magnetic powder from that length of magnetic tape that -reproduction from that tape through that head can no longer be carried out; that is an important characteristic for a magnetic tape. Further,in Table III, the designation "Head Abraded Amount" indicates the amount of length that the projected portion of a video recording head is reduced through continuous passage of the coating layer of a magnetic tape thereover per unit of time (here microns per 25 hours).
All of the characteristics in Table III are factors illustrat-ing the anti-abrasive characteristics of a given magnetic tape.
As Table III demonstrates, the Fallen Amount, the Friction Coefficient, and the Head Abraded Amount are all -`;
improved unexpectedly in using a magnetic tape of this invention.

Example 9 The following Example illustrates utilization of the ''` ~'"' ' ~r D

Z~7~
various dispersions of silica in thermoplastic polymer/solvent systems made according to the teachings of Example 4 to make magnetic tapes of this invention which have anti-abrasive properties:
; The following starting materials are employed:
, Co gamma-Fe2O3 magnetic powder ... 100 wt. parts Silica/Polymer Dispersion (dry wt. basis) ... 20 wt. parts ." ~
Dispersion Agent Tallow Diamine Dioleate (TDO) :~
(made by Lion-Armour Co., Ltd.) ... 2 wt. parts ''.: 10 Lubricant Agent* Armid HT (made by Lion-Armour Co., Ltd.) ... 0.1 wt. parts ;

Electrification resistant agent (made by Lion-Armour Co., Ltd.) ... 0.2 wt. parts The preparation procedure used is as indicated in Example 7 and the base film is polyethylene terephthalate.

When each of the magnetic tapes so made is tested in a tape recorder, all of the tapes display good anti-abrasive characteristics. Specifically, when a tape wherein the coating layer contains 40 PHR of silica gel is compared with various conventional magnetic tapes in the manner as earlier described ;
in Example 8 for an Example 1 dispersion, it is ascertained that Head Clogging, Fallen Amount, and Head Abraded Amount are generally equivalent to those for the tape of this invention so evaluated in Example 8. Similar results are obtained when~the coating layer contains amounts of silica gel in the range from about 20 PHR to about 100 P~R.
Example 10 : :
` The ~ollowing Example illustrates preparation of a mag-netic tape of this invention using a 40 PHR dispersion of silica in a thermosetting polymer/solvent system prepared ~ as described in Example 6.
''.''' ~, :
` ~ *Trademark ~;~; .
~ D 27 -~ 42Z77 A magnetic powder dispersion prepared by the procedure of Example 7 using the magnetic powder and dispersion agent of Example 9 is prepared and then mixed with such dispersion of silica in thermosetting polymer dispersion. A magnetic tape of the resulting paint coated on polyethylene terephthalate film is then prepared.
. When this tape is evaluated for anti-abrasive pro- :
perties, it is determined that the product tape has anti-abrasive characteristics approximately equivalent to those of ;, 10 Example 9. In addition, there is an improvement in tape "

;~; travelling characteristic in comparison to the tape of ; ' :'- ' - the type of Example 9.

Example 11 .; -- ,. :, ~
' This Example illustrates magnetic tapes of this invention utilizing chromium oxide magnetic powder.

;` The following starting materials are employed:

~' Chromium oxide (CrO2) `!`'` magnetic powder ... 100 wt. parts ;

Silica/Polymer Dispersion (dry wt. basis) ... 20 wt. parts " ~
`,`~` Dispersion agent,* Duomeen T
(made by Lion-Armour Co., Ltd.) ... 1 wt. part ;
' '!
Olive Oil ... 1 wt. part ;
Using the procedure of Example 1, and employing as the -thermoplastic polymer a polyurethane resin made by Goodrich and available commercially under the trade dèsignation Estane 5740x150, as the surface active fine powder a silica gel made by Degussa and available commercially under the Trademark Aerosil R-972, and as the solvent the mixture of Example 1, dispersions containing respectively, 20, 30 and , r 70 PHR silica are prepared. With each such dispersion, the -* Trademark ~
.. ..
.: :
~ - 28 -''' ~ ~ ,- .

~4ZZ~7 above indicated relative amount of olive oil is blended.
Using the procedure of Example 7, and employing the magnetic powder and dispersion agent above indicated in the present Example, a uniform dispersion of solids in organic : fluid is prepared.
Each member of the above two classes of dispersions is -blended with a member of the other class in the manner taught ... ..
by Example 7 with kneading to produce magnetic paints. Each such paint is then coated onto a substrate base film, and the coated film is allowed to dry, thereby producing a desired magnetic tape. The base film used herein is polyethylene terephthalate.
When each of the magnetic tapes so prepared is evaluated, -it is found that these tapes display high magnetic dènsity ``
and superior electromagnetic conversion characteristics. In -addition, such tapes reduce the amount of magnetic head '!~
abrasion to a level typically in the range of from about 1/3 to ''t`
1/5 that associated with the type of non-silica containing chromium oxide magnetic recording tapes of the prior art, yet -~
,~"~
`i 20 the anti-abrasive-characteristics associated with gamma ~ ~
,., : :
ferric oxide tapes of this invention are not lost.
~` It is theorized (and there is no intention or desire `i to be bound by theory herein) that the improvement in anti-abrasive characteristics of chromium oxide magnetic tapes of this invention prepared as described above is associated with their increased Young's Modulus ratio E/Eo due to the -~
reinforcement effect provided by the surface active fine ~i' powder, while at the same time these tapes also have great elongation percentages, so that these tapes have good anti-abrasion characteristics with ability to absorb the great ~` impacts caused by the magnetic head when in use.
':' '' ~ ~:
,~ 29 ;
~ D

The results of various tests on these so prepared ~
chromium oxide magnetic tapes are given in -the following ;
Table IV:
`.'`'. .
~ Table IV
.~ ., `,'~

; Characteristic Specimen .. . . .

Number of Tape Uses good good - - good : Until Head Clogging Occurs . : .
Travelling Char- good d velling velling good comes comes ~
` impos- impos- -- sible sible ~
after after -'~ 20 sever- -times al travel- times ling travel-ling _ ` Head Abraded Amount `~ (microns/hour) 1-2 1-2 1-2 1-2 7-10 The Speciments (1), (2) and (3) in the Table IV are magnetic tapes with each coating layer containing, respective-ly, 70, 30 and 20 PHR, of the silica used in this Example;
Specimen (4) is a magnetic tape similar to that of Specimens (1), (2) and (3) but containing no such silica; and Specimen (5) is an example of a conventional chromium oxide magnetic tape. The characteristics given in Table IV are as described earlier (see Example 8). ;
According to Table IV, when the amount silica in the coating layer ranges from about 30 to 70 PHR, the travelling and head clogging of Specimens (1), (2) and (3) are not impaired, and the Head Abraded Amount is unexpectedly . ~ .,, .: ~ .
reduced to a level of about 1/3 to 1/5 of that of the prior art, so that such Specimens can be regarded as displaying ~` excellent anti-abrasive characteristics.
, ~ ., !
.'`''""~' ' ' ~ 30 `` 1~4~Z7~
Example 12 The following starting materials are employed:
Thermoplastic polymer Mixture of vinyl chloride-vinyl acetate copolymer with ::
butadiene copolymer ~:
Solvent Mixture of cyclohexanone with -: .
toluene at weight ratio of l:l ::. .
by weight ~ .
: Surface Active Eine Powder Carbon black available com- . :
mercially from Mitsubishi ~-. Chemical Industries Ltd. :~
;~ under the trade designation `: .
' "Diablack.. G"
.... ..
. 10 The polymer mixture comprises 25 weight percent VAGH (avail-!, able commercially from Union Carbide Corporation) and 75 :.
:. weight percent of *Hycar 1432J (available commercially from `. ~.
~ Japanese Geon). :.
;. A series of coating layer binder systems are prepared, .~
each such system comprising the above vinyl copolymer .:
.` system having, respectively, 20, 30, 40, 50, 70 and 100 PHR .
carbon black therein, all such systems being prepared by the ;
procedure of Example 1. .
Example 13 . . .
i 20 The dispersion of 40 PHR carbon black in dissolved ; polymer prepared as described in Example 12 is compared with . a Specimen (b) and with a Specimen.(c) using the procedure ., . :.... . .
of Example 2. ..
Specimen (b) is a binder dispersion prepared by -kneading the present thermoplastic resin with the present -, carbon black but without using solvent (which is similar `;~
. to the procedures used in manufacturing vulcanized rubber). :
~: i ., . j Specimen (c) is a binder like Specimen (a) but without any carbon black therein and may be referred to herein as a ; :
~ 30 "non-filled binder". ..
.. ;l The following Table V shows the results of this com- . :~
~ parison. In this Table, Specimen (a) refers to the above ,~ :
~. * Trademark - 31 - -~ .
, ' L, l) .'".. '', ~ '.':' : ,: ,. :

: ~4ZZ77 .. . .
indicated carbon black dispersion from Example 12.
~, .
~i Table V

Specimen Designation Specimen Specimen Specimen Property (c) (b) (a) Polymer gelatinized Polymer ~ ~-~; Toluene completely completely . dissolves dissolves Solubll1ty Cyclohexa- Polymer gelatinized Polymer none completely completely dissolves dissolves ~ -Viscosity 400 cp - 400 cp -~ 10 ' ' ;~ When the indicated carbon black reinforced binder of ~ -Example 12 is used as a binder for a magnetic paint, such ~
.. . .
~`~ binder shows high solubility. One should avoid gelatinizing ~
:.-. :.
such binder in order to avoid deterioration of a dispersion of magnetic powders dispersed therein. According to Table V, , the Specimen (a) is substantially equal to the Specimen (c), which indicates that there is no observed increase in the dispersion viscosity of a carbon black containing dispersion, l such as would serve as a reference standard in the manner of '' 20 E~ample 2 for gelatinization.
,', The prior art teaches that carbon black powder can be `
, ~ added to a resin in the ratio of about 20 weight parts carbon black per 100 weight parts of resin to decrease `` electric resistance in a magnetic recording medium. Such ` prior art appears to employ a carbon black of larger particle --size than that used in this invention. Furthermore, in the present invention, one should avoid using amounts of dis-' persed carbon black in the resin which prevent an increase in s ;, , electrification in a magnetic tape made therewith, otherwise it appears that desirable tape properties deteriorate. Thus, ~;' the amount of carbon black employed should be below about .. ..
:. ~

`''.-~` .D '' "' , . .
- 32 ~

100 PHR in practicing the present invention. In addition, when the amount of carbon black fine powders is increased to a level more than about 100 PHR, there is a tendency for a ' carbon black-filled magnetic paint to gelatinize, which .. ~ ~,;
deteriorates the dlspersion of magnetic powders in the paint and adversely affects the smoothness of the surface of ` ! ~
a coating layer made therefrom.
: , , On the other hand, however, when dispersed (in accord with teachings of this invention) carbon black powder is present in a coating layer at a level of about 20 PHR, there -appears to be almost no improvement over non-carbon filled . .:
systems in product magnetic tapes.
i It is therefore, preferred to employ in magnetic tapes ; of this invention made using carbon black a quantity of carbon black in the range of from about 30 to 100 PHR. Within such ~`
range, product magnetic tapes generally display low electric ~
., ,- . ~
resistance, and excellent anti-abrasion characteristics with low Head Abrasion Amounts.
.' ~, , ,'~ Example 14 ~;` 20 The following starting materials are employed: -~

,~ Gamma-Fe2O3 powder ... 100 wt. parts .. " ~. .
`~ Carbon black/Polymer l Dispersion (dry wt. basis) ... 25 wt. parts ;":, :.
Duomeen T (as dispersion ~i agent) ... 1 wt. part ~;
, ! .:,:, ` Olive Oil (as lubricant agent) ... 1 wt. part ; The carbon black/polymer dispersion used contains 40 ?

PHR of the carbon black type similar to that indicated in , ,i ~ .
Example 13, and the polymer portion thereof is a polyurethane ` type similar to that indicated in Example l; this dispersion ::. .,-Y~ 30 is prepared by the procedure of Example 1. `~

-~ A dispersion of the magnetic iron oxide in organic ;~

~ liquid is prepared in the manner indicated in Example 7, and , ~, .. .
,-, ., D - 33 ~
,~
.:
, :

this dispersion is mixed intimately with the carbon black/
polymer dispersion above to prepare a magnetic paint. This paint is coated onto a polyethylene terephthalate film, and dried to form a magnetic tape of this invention. This tape .
i` is then evaluated for anti-abrasive properties and the results are summarized in Table VI below:
Table VI -:
Specimen Number roperty (1) (2) (3) _ -.::, .
Number of Tape At number lower good Uses until Head - than lO'head (> 100) Clogging OcGurs clogging occurs _ Travelliny char- Travel-acteristic ling im- good good possible Head Abraded ~;, Amount (microns/ - 3 1.5 -' 25 hr.) Electric Resist- > lol2 9 8 -~
ance (ohm -cm) 10 5 x 10 ; In the above Table VI, Specimen (1) comprises a ;~ magnetic recording medium with no carbon black in a poly-.;
I 20 urethane coating layer; Specimen (2) comprises a magnetic ,, recording medium in which 25 PHR carbon black is dispersed, .. " ~.
~ such medium having been prepared using a prior art process .....

`` for making a magnetic paint adapted for avoiding electrifi-.. ; ,.; ; ,, ~j cation of a product magnetic tape coated therewith; and `, Specimen 3 is a sample of tape prepared according to the ., ~ above teachings of this Example.

`, As Table VI indicates, the magnetic recording medium ~`1 produced by the Example has reduced head clogging and the ' Head Abraded Amount is reduced to a level of about 1/2 that . !
of the prior art, even with a slight added electrical conductive effect provided by the carbon black.
, '~

`'~ LD , `~ ;LZ2''~'7 .. ;~, The following Table VII shows the travelling nature or characteristic (see Table VI) of the magnetic recording ' medium produced in this Example in relation to its Friction ,~ Factor and its Fallen Amount. As the amount of the carbon ` '~
black dispersed in a coating layer increases, the fallen amount of the magnetic powder decreases and then later increases again in the vicinity of about 100 PHR carbon black.
.. ~.
Table VII

Specimen Number Property 1 2 3 4 ~
,~ Friction Factor 0.40 0.30 0.17 0.20 -; Fallen Amount of -magnetic powder ~
x 10-6 g 300 350 150 170 ~ -, .
,~ Example 15 ~
:.
,`; The following starting materials are employed:

l Chromium oxide magnetic powder ... 100 wt. parts .;! '.`. . .
Silica/Carbon black/Polymer dispersion (dry wt. basis) ... 25 wt. parts Lecithin as dispersion agent ... 2 wt. parts Olive oil as lubricant agent ... 1 wt. part The silica/carbon black/polymer dispension contains ,,'~ 30 PHR carbon black (available commercially under the trade name Diablack G from Mitsubishi Chemical Industries Ltd.) , and 10 PHR silica gel (available commercially under the Trademark Aerosil 200 from Degussa). The polymer portion ,;

~ thereof is a mixture of vinyl chloride/vinyl acetate copolymer ,~ comprising 25 weight parts VAGH (available commercially from Union Carbide Corporation) and butadiene copolymer comprising i 30 75 weight parts of *Hycar 1432J (available commercially from Japanese Geon). This dispersion is prepared by the ' * Trademark '`'"`"'''D 35 - ;

~.~9L2277 procedure indicated in Example 4 above.
A dispersion of the magnetic chromium oxide powder in organic liquid is prepared in the manner indicated in Example 9, and this dispersion is mixed intimately with the silica/ -carbon black/polymer dispersion above to prepare a magnetic paint. This paint is coated onto a polyethylene terephthalate film, and dried to form a magnetic tape of this invention.
The chromium oxide magnetic powder itself is greater in electric conductivity than that of the gamma-Ee2O3 of Example 7, but its surface electric resistance becomes higher ; ;than 101ohm -cm when it is incorporated into the coating layer of a magnetic recording medium, so that it is desirable to lower the surface electric resistance to the order of 109 ohm-cm by some means. In this Example, the amount of dispersed carbon black is lowered over that employed in ; Example VII and the silica gel is added. It is theorized ;~ (and there is no intent or desire to be bound to theory herein) that the high reinforcement effect of the silica and ;~ the electric conductivity and reinforcement effect of the ` 20 carbon black unexpectedly couple with each other. At any ... .
rate, the product magnetic tape has improved anti-abrasion characteristics as shown by the circumstances that (a) its Head Clogging does not offer a problem after the repeating number of transports of the magnetic recording medium exceeds more than 200, (b) surface electric resistance is about 1 x 10 ohm-cm, and (c) its Head Abraded Amount is reduced to 2 microns/hr. (while that of the prior art is in the range of from about 7 to 10 microns/hr.). This combination of silica and carbon black dispersed in a magnetic tape of this invention may be considered to provide a type of synergistic effectiveness in reducing tape anti-abrasive characteristics.

~ .
. .L~ .

':., '' .. - "~ ' '' ' ' ',':

`~ LZZ~7 :~
, ~: It will be apparent that many modifications and variations may be made by those skilled in the art without departing from the spirit and scope of the present invention.
.' . .
;' '',' ',: " '`
''' ' ` ' `' ' .
~:, 1 0 ~- :
~ . , .
~, :
:.;
. ~ .
:` -. :.
, :, '' ' `
i' ', '. ,.
, ;;, . ~
`

'~' 20 `

;~; i ~ , .
., .
... ..
." ~, .. .

~: ~ r'. j .
~'`' ~'' ~ .
`''''1' `` ~.
`. `
~: .: , '. ' : - -' ' ~ '.- :.
~ 37 _ i'' ~ , .,, ~ .
~D `~
~ . -.. .

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A magnetic recording medium comprising a base material and coating layer formed on said base material, said coating layer comprising a binder of thermoplastic polymer, a magnetic powder dispersed in said binder, and a surface active fine powder dispersed in said binder, said surface active fine powder particles being individually coated with a thermoplastic bound polymer and having maximum average particle sizes ranging from about 5 to 50 millimicrons, there being from about 20 to 100 parts by weight of said surface active powder per 100 parts by weight of said binder.

2. A magnetic recording medium as claimed in Claim 1 wherein the glass transition temperature of said coated thermo-plastic bound polymer is higher than that of said thermoplastic polymer of binder.

3. A magnetic recording medium as claimed in Claim 1 wherein said surface active fine powder particles have a surface area of at least about 40m2/gr.

4. A magnetic recording medium as claimed in Claim 1 wherein said thermoplastic polymer is selected from the group consisting of polyurethane, styrene-butadiene copolymer, acrylonitril-butadiene copolymer, non-vulcanized butadiene rubbers, polyvinylchloride, and vinyl chloride-vinyl acetate copolymers.

5. A magnetic recording medium as claimed in Claim 1 wherein said coated thermoplastic bound polymer and said thermo-plastic polymer of binder are each comprised of the same copolymer.

6. A method of making a binder for a magnetic recording medium comprising the steps of dissolving a thermoplastic polymer in a solvent and kneading said solution with a surface active fine powder to form a bound polymer layer around said surface active fine powder at a temperature from about 40 to 80°C, with a concentration from about 25 to 90 weight percent of total polymer plus powder against said solution, said surface active fine powder having maximum average particle sizes ranging from about 5 to 50 millimicrons, is contained in said binder from about 20 to 100 parts by weight per 100 parts by weight of said thermoplastic polymer.

7. A method of making a binder according to claim 6 wherein said surface active find powder is at least one of silica gel or carbon black.

8. An anti-abrasive, magnetic recording medium having spaced, parallel, opposed faces and comprising in integral combination a substrate layer and a coating layer, (A) said substrate layer being substantially non-conductive of electricity and self supporting, and comprising an organic polymer, (B) said coating layer comprising a surface active fine powder, a magnetic powder, and an organic polymeric binder, (1) said powders being substantially uniformly distributed in said binder, (2) said coating layer containing from about 20 to 100 parts by weight of said surface active fine powder per 100 parts by weight of said binder, and (3) said coating layer further containing a magnetically effective quantity of said magnetic powder at least sufficient for prechosen recording and reproducing purposes, (C) said surface active fine powder being further characterized by -(1) having maximum average particle sizes ranging from about 5 to 50 millimicrons, (2) having a surface area of at least about 40 m2/gr., (3) being substantially completely insoluble in common organic solvents and in said binder, and (4) having substantially all particles thereof each coated with a layer of a bound thermoplastic organic polymer, the layer thickness ranging from about 15 to 40 A°, (D) said magnetic powder being further characterized by (1) comprising at least one magnetic compound of at least one metal selected from the group consisting of iron and chromium, and (2) having maximum average particle sizes ranging up to about 600 millimicrons with at least about 80 weight percent thereof (based on 100 weight percent total magnetic powder) being in the form of elongated bodies having average length to width ratios in the range from about 2:1 to 20:1, (E) said binder being further characterized by being at least swellable when in a non-thermoset form by common organic solvents to an extent such that a fluid, homogeneous composition thereof may be prepared which is adapted for deposition on a surface with said surface active fine powder and said magnetic powder dis-persed therein.

9. The medium of claim 8 wherein said organic poly-meric binder comprises a polyurethane.

10. The medium of claim 8 wherein said surface active fine powder comprises at least one member selected from the group consisting of silica gel and carbon black.

11. The medium of claim 8 wherein said organic poly-meric binder is thermoset.

12. The medium of claim 8 wherein the said bound thermoplastic organic polymer on said particles of surface active fine powder is a polyurethane.

13. A process for making an anti-abrasive magnetic recording medium comprising the steps of, in sequence, (A) dispersing uniformly a surface active fine powder in a solution of thermoplastic polymer in an inert organic solvent to form a dispersion at a temperature from about 40 to 80°C
with a concentration from about 25 to 90 weight percent of total polymer plus powder against said solution, comprising from about 20 to 100 parts by weight of said surface active fine powder per 100 parts by weight of said thermoplastic polymer and having a viscosity of not more than about 200 centipoises as measured in 10 volume % cyclohexanone solution, said surface active fine powder having maximum average particle sizes ranging from about 5 to 50 millimicrons, (B) dispersing uniformly into a resulting said dispersion without substantial gelation of a magnetic powder, (C) coating the so prepared dispersion uniformly onto one surface of an insulative substrate, and (D) evaporating said organic solvent from said resulting coating.

14. The process of claim 13 wherein said surface active fine powder is silica gel.

15. The process of claim 13 wherein said thermo-plastic polymer is a polyurethane.