CA1048865A

CA1048865A - Multilayer magnetic recording elements and process of preparation

Info

Publication number: CA1048865A
Application number: CA73164476A
Authority: CA
Inventors: Gerard Drapier; Jacques Le Faou
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1972-02-24
Filing date: 1973-02-23
Publication date: 1979-02-20
Also published as: BE795896A; DE2309158C3; DE2309159A1; JPS4898803A; DE2309158A1; GB1417442A; NL7302674A; IT979420B; FR2172838B1; FR2172838A1; NL7302673A; FR2217758A1; JPS5612937B2; FR2217758B1; DE2309158B2; GB1417765A

Abstract

Abstract of the Disclosure Magnetic recording elements which contain at least two magnetic recording layers on the same side of a support and processes for preparing these elements are disclosed. The layer interfaces formed by magnetic recording layers within these elements are substantially free of irregularities which give rise to an objectionable modulation modulation level. This results in elements exhibiting significantly improved recording properties.
The elements can be prepared by applying at least two magnetic recording layers to the same side of a non-magnetizable support while these layers are in the liquid state and then solidifying the layers. A particularly advantageous coating method is disclosed in which a single slide hopper, as illustrated in fig. 3, is employed to simultaneously cost magnetic recording layers on a non-magnetizable support.

Description

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Background of the Invention This invention relates to magnetic recording materials and a process for preparing such materials. In one of its aspects this invention relates to a magnetic recording ele-ment for data storage which element contains at least two magnetic recording layers superimposed on a non-magnetizable support. In still another of its aspects this invention relates to a process for the preparation of such elements.
It is known in the art to record information on re-cording elements coated with one or more magnetic layers.
Such elements are often flexible and can be in the form of - discs, sheets, strips and tapes. The use of recording ele-ments containing dual magnetic recording layers, in compari-son to elements containing a single magnetic recording layer, has the advantage of greatly increased magnetic data storage capacity. Furthermore, a magnetic recording element compris-ing at least two magnetic recording layers having different coercivities makes it possible to design a product having a single optimum bias level for the two layers and to achieve - other design compromises which extend the usefulness of the -~
element.

Description of the Prior Art .
Magnetic recording elements containing at least two magnetic recording layers are known. In coating such products, it has been common practice to apply a dispersion of magnetic particles in a lacquer comprising a binder and a solvent to a non-magnetizable substrate using a sequential coating oper-~ ation, including drying of each layer before the succeeding : - 2 -: ' ' ' ' ' ~48865 magnetic recording layer is applied. This, of course, is a costly and time consuming procedure. Furthermore, this ; practice has resulted in a magnetic recording element which exhibits irregularities at the internal interfaces between layers. Such irregularities give rise to an undesirable modulation noise level.
It is evident, therefore, that the state of the mag-netic recording art will be enhanced by providing a magnetic recording element having at least two magnetic recording layers which element results in normal recordings of better quality in comparison to prior art elements comprising at least two recording layers. Likewise, a means for preparing ~- such elements will represent an advance in the art.
` Accordingly, it is an object of this invention to pro-vide a magnetic recording element which is substantially free ~ of irregularities at layer interfaces within the element which -~ irregularities give rise to an objectionable modulation noise , level.
It is still another object of this invention to pro-vide a method for coating magnetic recording elements in which at least two magnetic recording layers can be simultan-eously applied to a non-magnetizable support without drying each layer separately, as is required with the sequential coating operations generally employed in the prior art.
; 25 Other objects of this invention will become apparent from an examination of the specification and claims that follow.

Summary of the Invention It has now been found that irregularities which norm-ally occur at internal layer interfaces within magnetic 1~4~3865 recording elements containing at least two magnetic record-ing layers and which significantly contribute to the level of modulation noise can be substantially eliminated. It is, therefore, possible to obtain magnetic recording elements which exhibit significantly improved recording properties in comparison to similar multiple layer recording elements pre-sently available. The improved magnetic recording elements of this invention are conveniently prepared by applying at least two magnetic recording layers in superimposed relation-ship to a non-magnetizable support while each of the layers is in a liquid state and then drying the layers on the support.
In this manner, the formation of irregularities at layer in-terfaces which give rise to objectionable modulation noise level are effectively eliminated.
The present invention includes a magnetic recording element comprising (l) at least two magnetic recording layers superimposed on one surface of a support, and (2) a layer interface which is formed by at least one of said recording layers and is free of irregularities which substantially in-; 20 crease modulation noise across said interface.
The present invention also includes a process for pre-paring a magnetic recording element which comprises (1) con-tinuously forming at least two distinct liquid magnetic re-cording layers, (2) applying said liquid layers in superim-posed relationships to the same surface of a non-magnetizable support before drying any of said layers, and (3) thereafter drying said layers on said support. It should be noted that the term "drying", as used herein, includes any suitable method of layer solidification, including for example, solid-ifying the layers by gelation, chemical setting, evaporation,or the like.

1~48865 Brief Description of the Drawings Fig. 1 of the drawing represents a curve which results when the intensity of the modulation noise in decibels is measured as a function of AC high frequency bias, also in ; 5 decibels, for a magnetic recording element containing two magnetic recording layers and prepared according to the prac-tice of this invention. Fig. 2 of the drawing represents a curve which is obtained when these same measurements are made with a representative prior art magnetic recording element containing two magnetic recording layers which have been coated sequentially with drying between layer applications (solid line curve). For comparison purposes, Fig. 2 also presents a curve (dotted line curve X) representing the re-sults obtained w}th a magnetic recording element of this in-vention. Fig. 3 represents a vertical cross-sectional view showing an embodiment of this invention in which a slide hop-per is employed to simultaneously coat two magnetic recording layers on a non-magnetizable support.

Description of Preferred Embodiments . .
; 20 As indicated previously, irregularities or non-uni-formities which give rise to objectionable modulation noise can occur at layer interfaces within magnetic recording ele-ments. These irregularities may occur as a result of physical disruptions such or surface roughness and/or disruptions in magnetic recording across the interface which may be due to such things as differences in volumetric concentration of magnetic recording material at the interface, changes in mag-netic particle orientation at the interface and changes in ratio of binder to magnetic recording material that may occur , . -: , 10~8865 at the interface. In analyzing a magnetic recording element for such irregularities or non-uniformities, it is possible to isolate a modulation noise signal by recording with a DC
signal in the recording head and measuring the resultant in-tensity as a function of high frequency AC bias. This biasis progressively increased from 0 to such an intensitythat the results are no longer of interest, i.e., a high frequency bias which is as much as one to two times the bias level which would be used for normal recording with the particular mag-10 netic recording element upon which the measurements are beingmade.
To identify irregularities that are responsible for ~ -modulation noise, a magnitude of DC signal of the same order as the magnitude of the signal normally recorded is generally used. A convenient DC signal to use for this purpose is one which is equal to the r.m.s. (root mean square) value of a mid-frequency sine wave that records with one to three per- ~ -cent (1-3~) third harmonic distortion. The exact levels of modulation noise which result will depend upon the level of the DC signal. Measurements of the intensity of the modula-tion noise are made over a suitable length of the magnetic recording element to give a meaningful measurement of the irregularities or non-uniformities wi~hin the magnetic record-ing element. Such measurements are conveniently made over a length of tape which represents several minutes recording time, generally up to about 15 minutes, although 5 to 10 min-utes is generally sufficient. A typical transport which can be used will transport the tape at about 7.5 inches of tape per second.
When modulation noise is examined as a function of ,' . .

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- 1~48865 increasing high frequency AC bias, this, in a sense, measures the cumulative irregularities affecting magnetic recording from the surface of the magnetic recording element down to the effective plane representing the AC bias penetration depth to which it has sufficient intensity to be effective in pro-ducing a recording. Although modulation noise measured in this way is cumulative, it will be recognized that it is weighted more heavily by those irregularities within the zone of optimum recording efficiency within the magnetic re-cording element. This zone is moving progressively throughthe magnetic recording element from top to bottom as the AC
bias level is continuously increasing. In making these mea-surements, the modulation noise level will peak first at that high frequency AC bias level which essentially confines the recording to the top surface of the magnetic layer. This is the maximum modulation noise level for the particular element under investigation with a given high frequency AC bias and will be referred to herein as "the first maxima modulation noise level". The irregularities responsible for this first maxima modulation noise level are those peculiar to the sur-face of the element under investigation. Thereafter, the re-sultant modulation noise level will generally fall with in-creasing high frequency AC bias until the bias level reaches that intensity that the recording extends into the bottom inter-face between the magnetic recording layer and the layer orsupport immediately beneath the magnetic layer. Under these conditions, the irregularities or non-uniformities of that bottom interface will add to the cumulative irregularities above within the magnetic recording media and the resultant will generate an increase in modulation noise, herein referred to as "the second maxima modulation noise level". Fig. 1 of s"

~he drawin~ represents a curve showin~ a ~irst maxima mod-ulation noise level and a second maxir.la modulation noise level which is characteristic of the dual magnetic layer recording elements of this invention as well as mag-netic recording elements containing only one magnetic record-5 ing layer. This curve represents results obtained when the intensity of the modulation noise is measured as a function of - high frequency AC bias with such elements, as described herein-- before.
As previously indicated, there is shown, in Fig. 1, the modulation noise versus high frequency AC bias curve for a dual magnetic recording layer element prepared according to the teachings of this invention. This curve shows a first maxima modulation noise level Bl at bias intensity Il and a second maxima modulation noise level B2 at bias intensity I2.
Bo is defined as zero dB of noise, and the AC bias intensity producing this noise level, is defined as zero dB of bias.
- Bl is the first maxima modulation noise level and is typically about +8 ds to about +10 dB in most available magnetic record-ing elements, but can be as much as +14 dB or more depending upon the quality of the magnetic recording layers. B2 is the second maxima modulation noise level at the interface between the bottom of the magnetic recording-layer and the non-magnet-ic base. B2 is lower than Bl because irregularities which cause noise level B2 occur at some distance, i.e., the thick-nesses of the magnetic recording layers, from the gap of thereproducing head. B2 is typically about +2 dB to about +4 dB
for most magnetic recording elements, but can be as much as +8 dB or more, the specific value depending upon such things as the quality of the interface and the distance of the inter-face from the reproducing head. Bo is the minimum modulation : ~

1~886~
noise of the magnetic recording media including the two mag-netic recording layers. It occurs at an AC high frequency bias current Io which is normally very close to that level used for recording with the specific element.
Fig. 2 represents a modulation noise versus high fre-quency AC bias curve which results when two distinct superim-posed magnetic gamma ferric oxide recording layers are coated upon a non-magnetizable support using a sequential coating operation, including drying of each layer before application of the succeeding layer. The dotted portion of the curve, in-dicated by X in the drawing, is set forth for comparison purposes and represents the modulation noise versus high fre-quency AC bias curve for a two-layer magnetic gamma ferric oxide recording element in which the two magnetic oxide layers are immediately adjacent one another and are simultaneously applied to the non-magnetic support according to this invention.
Bl, B2, Bo~ Il, and I2 have the same meanings as in Fig. l.
B3 represents the modulation noise level resulting from irreg- -ularities at the interface between the two magnetic oxide re-cording layers at bias intensity I3. This third maxima or peak corresponds to an additional modulation noise contribution and, comparing the curves of Figs. 1 and 2, reflects an in-crease in the average modulation noise from Bo to B~o~ This third maxima or peak may be less sharp than is represented in Fig. 2 of the drawing but, at the very least, there will be a significant and measurable change in the slope of the curve accompanied by a significant increase in average modulation noise level Bo~ B4 represents the modulation noise level for a magnetic recording element of this invention at high fre-quency AC bias I3. I3 is the AC bias where the plane of the interface between the sequentially coated and dried layers g _ ~8865 gives maximum contribution to noise. The increase in noise contributed by such interfaces in comparison to the simul-taneously coated magnetic recording layers, i.e., the dif-ference between B4 and B3 is typically 2 to 4 dB and is often 6 or more dB. The increase in modulation noise at this level of AC bias is a measure of the irregularities at or near the interface between the recording layers and can be used to distinguish between elements of this invention and those con-taining sequentially coated magnetic recording layers which are dried between layer applications. Furthermore, the dif-ference between Bo and Blo for sequentially coated magnetic recording layers is typically observed to be +l dB or more, and may be observed up to +4 dB to +6 dB or more. Inasmuch as this is the resultant modulation noise at or close to the preferred operating level of AC bias, it will be apparent that the introduction of an irregularity resulting from the prior art sequential coating of magnetic recording layers as opposed to coating of magnetic recording layers according to this invention results in a minimum increase in modulation noise level of one decibel and often up to 6 dB.
According to the practice of this invention, there is obtained a multi-layer magnetic recording element in which (1) the increase in modulation noise across the interface (measured at AC bias level I3) is generally about zero and no more than about one decibel and (2) the increase in modulation noise from this element under its condition of normal use (measured at AC bias level Io) is also generally about zero and no more than about 0.5 dB. In view of these low noise levels, it is clear that the elements of this invention contain an inter-face formed by a recording layer with another layer which interface is free of irregularities which substantially in-crease modulation noise. Stated another way, these elements contain a layer interface which is formed by at least one recording layer and is free of irregularities which substan-tially increase modulation noise across that interface.
Referring to Fig. 3, there is shown an apparatus for simultaneously applying liquid layers of coating composition to the surface of a non-magnetic moving support in superim-posed distinct layer relationship in accordance with the prac-tice of this invention. The support which is coated is a continuous web 1 which is moved along a coating path by suit-; able means including a coating roller 2 which rigidly supports web l and smooths it out while also changing its direction of movement. Located in coating position is a slide hopper 3 having cavities 4~ 41~ and 42 into which liquid coating com-- positions S, Sl, and S2, respectively, are pumped at a con-stant rate by metering or constant discharge pumps (not shown) through inlets 5~ 51' and 52' respectively. The coating com-positions pumped into cavities 4 and 4' are liquid magnetic recording coating compositions which are thixotropic, non-Newtonian dispersions comprising finely divided magnetic particles and polymeric binder in a volatile solvent. Due to the high viscosity of such compositions and their rheology, they do not readily flow by gravity on the slide of the hopper and a more fluid coating composition of lower viscosity must be used to form at least one layer which serves as a "lubri-cating layer" or "conveyor belt" on the hopper slide for the more viscous upper layers. This more fluid coating composi-tion S2 of lower viscosity exhibits Newtonian flow character-istics and is pumped into cavity 42 at a constant rate by a 1~48~65 metering or constant discharge pump (not shown) through inlet 52~ Coating compositions S, Sl, and S2 flow from cavities 4, 41~ and 42' respectively, through narrow elongated slots 6, 61, and 62 to form a composite layer made up of three indivi-dual layers in distinct layer relationship where slots 6, 61, and 62 exit the hopper at the common level 7 on the slide.
From this level 7, the composite of three layers formed from coating compositions S, Sl, and S2 flows by gravlty down slide surface 8 in substantially distinct superimposed layer relationship and leaves lip 9 to form a bridge or puddle of -coating compositions. The exposed surface web 1 is rapidly advanced past and into contact with this bridge or puddle, whereupon the layers of liquid coating compositions deposit on the web surface in substantially distinct superimposed layer relationship and are thereafter dried. The relatively fluid sublayer that is in contact with the hopper slide 8 serves to move the upper layers on the slanted plane and makes it possi-ble to simultaneously coat magnetic recording layers according ~- -to the practice of this invention.
The magnetic recording layers which are formed immed-iately adjacent and above the lubricating layer are more highly viscous than the lubricating layer and generally have viscos-ities above about 30 cps. and often ViScOSities in the range of about 30 to about 500 cps. The viscosity of the magnetic recording layer close~ to the lubricating layer is generally at least about 75 cps. and often in the range of about 110 to about 180 cps., while the magnetic recording layer above that recording layer generally has a viscosity above about 70 cps.
and often in the range of about 100 to about 250 cps. The viscosities described herein are measured with a Ferranti-~ - 12 -. ' ~ ' ~.

1~48865 Shirley viscometer with cone and plate controlled to measure over a shear scale between 0 and 1200 sec. and at a temper-ature of approximately 25C. Since the magnetic coating com-positions are generally non-Newtonian fluids, their apparent viscosity is a function of the shear rate at which the vis-cosity is measured. Therefore, the viscosities were determined on the Ferranti-Shirley viscometer taking those values attrib-utable to high shear, i.e., 800 sec. 1 to 1200 sec. 1, a shear level characteristically achieved within hoppers of the type described in Fig. 3. The viscosity units are expressed herein in terms of centipoise (cps), but they may also be con-veniently expressed in terms of centipoiseuille (cpl) units which unit is preferred in the International Standard system of units, and is known to be equivalent to 10 centipoise units.
If it is desired to coat a descrete support such as a sheet rather than a continuous web, the apparatus of Fig. 3 ~ can be readily adapted to this purpose by substituting for A, web 1 an endless conveyor belt on which such elements are con-veyed through the coating zone. Furthermore, it should read-ily be appreciated that auxiliary layers, for example subbing layers, conducting layers, or the like, can be coated simul-taneously with the magnetic recording layers.
The lubricating layer described in the above embodiment is formed from a coating composition which is compatible with the coating compositions employed to form the magnetic record-ing layers. Suitable compositions include, for example, those which exhibit Newtonian flow and contain polymers which are conventionally used as binders in magnetic recording layers, but which contain sufficient solvent to make them less viscous.
Suitable compositions contain such polymers as copoly(vinyl-idene chloride acrylonitrile), copoly (vinyl acetate vinyl Il ~ - 12a -1~48865 chloride) partially hydrolyzed and optionally crosslinked with an isocyanate, polyvinylbutyral, terpoly(vinylidene chloride acrylonitrile acrylic acid) and the like, with suit-able solvents, for example, methylethylketone, methyisobutyl-ketone, ethyl acetate, cyclohexanone, and the like. Ingeneral, the lubricating layers exhibit viscosities not great-er than about 20 cps and often viscosities in the range of about 3 to about 15 cps. Viscosities can be measured using any convenient method. However, as previously indicated, those described herein were measured with a Ferranti-Shirley viscometer.
The use of a lubricating layer, as described herein, in a single slide hopper represents a preferred method for preparing elements of this invention. This is particularly 15 true where distinct liquid magnetic recording layers are in juxtaposed face-to-face contact and are simultaneously applied to the substrate. However, any suitable coating technique can be employed provided it achieves the desired application of at least two distinct liquid magnetic recording layers on -the same surface of a non-magnetizable support without sub-stantial drying between layer applications. For example, British Patent 837,095, published June 9, 1960, discloses a method and apparatus for coating layers on a web support in which a plurality of coating liquids are applied to a travel-ing support from stations spaced a short distance apart toform distinct liquid layers. The layers are then dried.
Also, U.S. Patent 3,573,965, issued April 6, 1971, describes a method and apparatus for successively applying two or more liquid coating compositions to a continuously traveling web to form superimposed, distinct layers without setting or ~ - 12b -drying between applications of the coating compositions.
Those skilled in the art having the benefit of this disclosure will readily appreciate that such methods and apparatus can, with appropriate minor modification, be employed to prepare magnetic recording elements comprising at least two superim-posed magnetic recording layers which exhibit substantially no modulation noise increasing non-uniformities or irregular-ities at their interfaces with each other or with other layers.
At least two magnetic recording layers are present in the elements of this invention, but, of course, three or -e~en more such layers can be superimposed on one surface of a non-magnetizabl~ support when practicing this in-vention. The individual magnetic re~ording lay~rs are generally quite thin, often being in the range of about 0.5 to about 6 microns, but the preferred thickness for each individual layer is dependent upon the effective wavelength of the information to be recorded therein, and upon other factors such as the other layers coated in combination with it.

- 12c -1~48865 The method of coating multilayer elements from ; thixotropic coating compositions using at least one "lub- -ricating or conveyor belt" layer in a slide hopper, as described in Fig 3, is disclosed and claimed in Le Faou et al Canadian application Serial No. 164,481, filed February 23, 1973, and titled "Process for Coating Thixo-tro~ic Layers"(Canadian Patent 1,018,838, October 11, 1977).
As previously indicated, the magnetic recording elements of this invention contain at least two magnetic recording layers,and at least one of these layers, together with an immediately adjacent layer, form an interface which is free of irregularities which substantially increase modu-lation noise. Such magnetic recording layers are preferably immediately adjacent to one another, preferably contain the - same polymers in their binders and can contain one or more suitable magnetic materials, many of which are well known in the prior art. These materials can be in particulate form and often such particulate magnetic material is dispersed in an organic binder and solvent Typical mag-netic materials include, for example, ferromagnetic iron oxide, both the black oxide or ferrous ferric oxide as well ; as the brown gamma ferric oxide, metal powder of an extremely fine particle size, complex oXides of iron and cobalt, chromium dioxide, various ferrites, magnetic metal alloys, and the like. A particularly desirable material is acicular gamma ferric oxide or ferrous ferric oxide having an acicu-larity ratio above 4 Or 5, and preferably 15 or more which is doped with one or more ions of a polyvalent metal such as cobalt, nickel, zinc, manganese, chromium or the like.
The concentration of dopant ion employed is subject to variation, depending upon such things as size and shape of - the magnetic particles. However, dopant levels in the range of about 1 to about 5~, by weight, particularly with cobalt ion, are generally suitable.
According to one feature of this invention, there is obtained magnetic recording elements in which the upper discrete layers have a very smooth surface and the contact with the magnetic recording or reproducing head is more nearly perfect. Such upper layers can be made very thin ; 40 while retalning this smoothness, s~nce there are no ; slgnificant lrregularitles at their lower interface to ;i - be replicated in their upper surface Such layers can be t .~..1 ,' :
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calendered to take further advantage of this smooth surface.
In a preferred embodiment of this invention that re-cording layer outermost from the non-magnetizable support has a coercivity (H ) of at least 400 to 450 Oersteds and often a coercivity of about 400 to about 1200 or 2000 Oersteds. A
preferred particulate magnetic material which can be present in such an outermost layer is cobalt doped gamma ferric oxide or cobalt doped ferrous ferric oxide having a coercivity up to about 1200 and generally about 500 to about 1100 Oersteds.
Particle sizes in such layers are generally quite small and most often are in the range of about .2 to about .8 microns and desirably in the range of about .2 to about .5 microns.
Particle sizes below about .5 microns are preferred for high frequency recording. Layers which are contiguous or immediate-ly adjacent underlayers with respect to the above outermost magnetic recording layers advantageously have a lower coer-civity, for example, 270 to about 300 Oersteds, although they may have coercivities up to about 500 to 600 Oersteds or more.
Suitable magnetic materials for use in these layers are aci-cular gamma ferric oxide and ferrous ferric oxide, which have lower coercivities than the magnetic materials in the outer-most layers. These oxides can be doped and preferrably cobalt - doped. It should be understood that although this lower layer will have a lower coercivity in comparison to the outermost magnetic recording top layer, the particular ratio of coerciv-ities between these layers will depend to a large degree upon the particular information to be recorded, i.e., the use in-tended for the magnetic recording element. Particle sizes in this underlayer layer are generally somewhat larger than in the magnetic recording layer over it and typically are in range from about .6 to about 1.5 microns.

- , ~ . . . ' The magnetic recording layers can be applied to a wide variety of non-magnetizable supports, including discs, belts, paper or film tapes, and the like. Suitable supports are gen-erally flexible and typically include such materials as cellu-lose nitrate film, cellulose acetate film, polyvinyl acetal film, polystyrene film, polyesters such as polyethylene tereph-thalate film which can be biaxially or asymetrically stretched, polycarbonate film and related films or resinous materials, as : well as paper, metals such as aluminum or brass and the like.
Suitable magnetic particles or pigments are conveniently dis-persed in a solution of a polymeric binder in a volatile solvent for the binder and the dispersion applied as a thin layer to the support as described herein, and the solvent is allowed to evaporate.
Binders that are useful in the practice of this inven-. tion include copolymers of vinyl acetate with vinyl chloride, copolymers of vinylidene chloride with acrylonitrile, copoly-mers of acrylic and/or methacrylic esters, polyvinyl butyral, copolymèrs-of butadiene with styrene, terpolymers of acrylonitrile, vinylidene chloride and maleic or maleamid anhydridés, cro~s- -linked or non-crosslinked, copolymer condensates such as poly-~ amides, polyurethanes, polyesters,such as polyethylene tere-: phthalate and its homologs, as w~ll as mixtures of such binders.
Other binder~ with similar chemical and physical properties are known and can be employed in the practice of th~$ invention.
: In general binders are generally employed in concentrations of-~ up to about 50% and often between about 10% and about 20%, by weight, based on magnetic material.
Suitable solvents which can be empaoyed in the preparation of magnetic ~ispersions in the practice of this ~ invention include organic materials such as methyl ethyl ketone, methyl - 15 -' , , - .

1C)48865 isobutyl ketone, ethyl acetate, butyl acetate, cyclohexa-none, and the like, as well as mixtures thereof. The magnetic recording layers can contain other additives such as dispers-ing agents in order to facilitate dispersion, lubricants, conductive pigments such as carbon to avoid static, colloidal silica and like materials.
The invention is further illustrated by the following example of its practice:
Example 10Using the apparatus illustrated in Fig. 3, a three layer coating was produced using the compositions and condi-tions set forth in the following Table. The layers are num-- bered in ascending order from the support, layer 1 being coated closest to the support, layer 2 over layer 1, and layer 3 over layer 2.

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1~48865 Coatings were made on polyethylene ~erephthalate base in continuous lengths of 1,000 ft at support velocities of 40 inches per second.
When modulation noise measurements, as described herein, are made on a magnetic recording element prepared according to this example a curve corresponding to Fig. 1 is obtained. In contrast, a similar coating prepared in a sequential manner with drying between applications of the magnetic recording layers corresponds to the curve set forth in Fig. 2 and shows a point of inflection between the first maxima modulation noise and the second maxima modulation noise. It should be noted that this occurs even in those instances where surface treat-ments are used to smooth the layer surfaces between layer applications.
The invention has been described in detail with parti-cular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

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Claims

We claim:

1. A magnetic recording element comprising (1) at least two superimposed magnetic recording layers on one surface of a support and (2) a layer interface which is formed by at least one of said recording layers with an immediately adjacent layer and is free of irregularities which substantially increase modulation noise across said interface.

2. An element according to claim 1 in which each of said magnetic recording layers comprises a particulate magnetic material.

3. An element according to claim 2 in which said magnetic material is dispersed in a polymeric binder.

4. An element according to claim 3 in which the polymer in said binder is the same in each of said layers.

5. An element according to claim 4 in which said magnetic material is a magnetic iron oxide.

6. An element according to claim 1 in which the outermost magnetic recording layer, with respect to said support, has a coercivity in the range of about 400 to about 1,200 oersteds and said outermost layer is immediately adjacent a second magnetic recording underlayer having a coercivity in the range of about 270 to about 600 oersteds.

7. An element according to claim 6 in which said outermost layer comprises gamma ferric oxide which is doped with a polyvalent metal ion.

8. An element according to claim 7 in which said doped gamma ferric oxide is dispersed in a polymeric binder and said immediately adjacent underlayer comprises undoped gamma ferric oxide.

9. An element according to claim 7 wherein said gamma ferric oxide in said outermost layer has a particle size in the range of about .2 to about .5 microns and said immediately adjacent underlayer comprises magnetic parti-cles having a size in the range of about .6 to about 1.5 microns.

10. An element according to claim 6 in which said outermost layer comprises ferrous ferric oxide which is doped with a polyvalent metal ion.

11. A process for preparing a magnetic recording element having at least two magnetic recording layers and exhibiting low modulation noise, said process comprising (1) continuously forming at least two distinct liquid magnetic recording layers, (2) applying said liquid layers in superimposed relationship to the same surface of a non-magnetizable support before drying any of said layers and (3) drying said layers in distinct superimposed relationship on said support, whereby a layer interface formed by at least one of said magnetic recording layers with an immedi-ately adjacent layer is free of irregularities which sub-stantially increase modulation noise across said interface.

12. A process according to claim 11 in which each of said liquid layers comprises particulate magnetic metal oxide dispersed in a binder.

13. A process according to claim 12 in which said oxide is a magnetic iron oxide.

14. A process according to claim 11 in which at least one of said liquid layers comprises magnetic metal particles dispersed in a binder.

15. A process according to claim 11 in which said distinct liquid magnetic recording layers are in juxtaposed face-to-face contact and are simultaneously applied to said non-magnetizable support.

16. A process according to claim 14 in which the distinct magnetic recording layer outermost from said support has a coercivity in the range of about 400 to about 1,200 oersteds and is immediately adjacent to a magnetic recording underlayer which has a coercivity in the range of about 270 to about 600 oersteds.

17. A process according to claim 15 in which said layers are applied to a non-magnetizable polyester support.