WO1986004719A1

WO1986004719A1 - Aqueous thermoset magnetic coating composition and method of making

Info

Publication number: WO1986004719A1
Application number: PCT/US1986/000239
Authority: WO
Inventors: Robert A. Jackson; Stephen A. Joesten; Sikandar Iqbal
Original assignee: Memorex Corporation
Priority date: 1985-02-06
Filing date: 1986-02-04
Publication date: 1986-08-14
Also published as: JPS62501661A; AU5452186A; EP0211897A1

Abstract

Improved, tougher, more stable magnetic recording formulations of the water-based, thermosetting type; e.g., illustrating improved pigment dispersants and improved binder systems cross-linked via surface carboxyl groups.

Description

AQUEOUS THERMOSET MAGNETIC COATING COMPOSITION AND METHOD OF MAKING

BACKGROUND OF THE INVENTION

This invention relates to magnetic coating formulations and related preparation methods; more particularly it relates to those of the water-based, thermosetting kind.

Workers are well aware that it is desirable at times to use aqueous formulations for magnetic coatings (e.g., on a plastic base film in place of organic formulations) . The organics can be expensive, toxic and flammable and must be captured upon drying of the magnetic formulation to prevent air pollution. Furthermore, there have been a number of recent developments in improving magnetic aqueous formulations, which make them attractive as substitutes for organic-based coatings generally.

Notwithstanding the inherent advantages of employing aqueous magnetic coating formulations, such coatings have not, to date, replaced organic formulations commercially. One reason for the lack of success is that prior art magnetic coating layers employ emulsifiers or dispersants, which do not adequately disperse and stabilize the magnetic pigments incorporated in the coatings. As a result, the coatings become hygroscopic and have poor magnetic properties. --Dispersants:

One feature hereof is to teach an improved technique; and related agents, for dispersing magnetic pigments in such coatings. One result of this will be improved stability (over an extended coating-life) and improvement in related magnetic recording/read-back characteristics. More particularly, according to one aspect of this feature, coating particulates are dispersed both electrostatically and sterically (whereas in the prior art, related materials are dispersed only sterically or with poly-electrolytes). And, according another related feature, such dispersion is preferably carried-out with prescribed electrostatic dispersing agents, (certain concentrations thereof) such as a sodium phosphate or related phosphate salts. By contrast, the prior art teaches other techniques and different associated agents for related purposes. For example, U.S. Patent 3,725,285 teaches that a potassium phosphate may be used for related purposes; it also mentions that an emulsifier such as Triton X-1000 (an alkyl aryl polyether alcohol from Rohm and Haas Company) creates foam when used with water-based magnetic recording compositions. U.S. 3,725,285 also mentions that Aerosol C-61, an emulsifier from American Cyanamid Company, will cause the same foaming problem. Another U.S. Patent, U.S. 4,263,188, mentions that a dispersing or wetting agent, Tamol 731 (a sodium salt of carboxylate polyelectrolyte, from Rohm and Haas Company) causes foaming too. In another, somewhat related, feature we teach that one may disperse carbon particles in such a pigment mix sterically, using prescribed acrylic emulsions.

Further, workers realize that it is very difficult to remove air from such high-viscosity pigment mixes.

Common emulsifiers do not form a strong bond with magnetic oxides or conductive carbon and thus diffuse to the surface of a magnetic coating, with time, and cause problems; for example, they attract moisture which increases the coefficient of friction and also hardens the coating.

This invention addresses such problems, teaching the formulation and treatment of water-based magnetic coating composition whose magnetic oxide particles are dispersed, in water, with novel dispersing methods and agents. The polymer solutions (emulsions with additives) are added to the dispersion and coated on a substrate to yield magnetic recording media. That is, the magnetic particles are dispersed in waiter and stabilized sterically and electrostatically for optimum quality performance. --Binders:

Another feature hereof relates to improved polymeric binders for such coatings (e.g., on a plastic or metal substrate), wherein a urethane is cross-linked with an acrylic, to yield an improved thermosetting composition. Workers will perceive decided advantages to such compositions; for instance, the acrylic is much less expensive. Also, we find that, when properly cross-linked, the acrylic can yield a recording tape surface with a very high gloss and good durability (e.g.,. far superior audio characteristics). Now, the prior art teaches using polymer solutions in water as dispersing agents; e.g., using such in combination with an organic solvent like an alcohol. The polymer solutions are normally not completely cross-linked with cross-linking agents; hence they often become hygroscopic and attract moisture. These polymer solutions also increase the viscosity of the composition, making it difficult to process during milling, filtration, and coating. It is desirable to use polymer emulsions (latexes) of high molecular weight with lower viscosity in the composition. The subject novel' binders are selected and cross-linked to give excellent adhesion to a plastic basefilm and impart excellent mechanical properties. These films have resistance to related chemicals, high temperature and humidity.

This novel polymer binder consists of emulsions of acrylics, vinyl acrylics and polyurethanes. The cross-linking of the surface groups on latexes can be achieved ionically or thermally at very low temperatures during a drying operation. Polyaziridine is used for cross-linking carboxylate groups present on the surface of latexes. The polymer binders and cross-linking agents are selected for obtaining an

"interpenetrating elastomeric/plastomeric network" (IEN/IPN) which improves the durability of the thermosetting magnetic composition. Compatible lubricants and flow agents are incorporated to improve the surface properties of the composition on the plastic substrate.

Another problem in prior art aqueous-based magnetic coating compositions is the tendency of the magnetic particles and other pigments, such as carbon, to flocculate. The use of improved dispersion techniques, and related agents, as here taught, will not only avoid the flocculation problem, but will, in general, better stabilize the magnetic powder and other pigments incorporated in the composition. It is thus an object of the present invention to provide an improved aqueous magnetic coating composition without the drawbacks of prior art compositions, but exhibiting one or more of the mentioned features. It is yet another object of the present invention to employ improved dispersing techniques and related agents in aqueous magnetic coating compositions to avoid the disadvantages mentioned.

These and other objects of this invention will be more fully appreciated when considering the following disclosure.

SUMMARY OF SOME FEATURES

The subject teachings (examples) deal with an aqueous coating composition for forming a magnetic recording layer, wherein one improvement comprises dispersion with electrostatic and steric means. In the following examples, the magnetic composition will be understood to be coated on plastic or metal substrate, to be comprised of magnetic particles and water-soluble. (or insoluble) binders as major ingredients, and with dispersing agents, flow agents, and lubricants as minor ingredients. With the composition prepared in water instead of organic solvent, several advantages accrue; for example, lower cost, lower toxicity, and less pollution. The quality of the final magnetic coating is largely governed by the quality and stability of the magnetic particle dispersion. This invention will teach not only how to prepare the formulation but how to obtain a stable dispersion. A good stable dispersion will improve surface smoothness. and will lower the coefficient of friction and the porosity of the film, as workers know. Magnetic properties are also improved; for example: orientation ratio, squareness, signal-to-noise ratio, and signal output. One salient objective hereof is to prepare electrostatically and sterically stable dispersions. An advantage of preparing such dispersions in water is that the stability of dispersion-is improved as compared with (organic) solvent-based magnetic compositions. A dispersion rendered by electrostatic/steric stabilization will be found to have long-term stability; thus improved coating processes and magnetic properties are realized.

As a preferred embodiment, the dispersing of such magnetic powders is effected both electrostatically and sterically, with a preferred anionic type electrostatic dispersant, such as sodium polyphosphate (preferably about 1 wt.%).

And, when the pigment includes conductive particles like carbon, these are preferably dispersed sterically using an acrylic emulsion (e.g., like Acrysol WS-24 from Rohm and Haas Company) ; according to a related feature. For instance, a preferred dispersing agent for such thermoset aqueous coating compositions is a polyacrylic emulsion; e.g., an emulsion comprised of 10% polystyrene, 38% n-butyl acrylate and 52% methyl methacrylate (such a terpolymer having an average molecular weight of 20,000 to 40,000). An example of such a polymeric dispersing agent is Acrysol 1-62 from Rohm and Haas Company comprising 10% polystyrene, 38% n-butyl acrylate, and 52% methyl methacrylate, with average molecular weight about 20,000 to 40,000. The electrostatic dispersing agent can be sodium polyphosphate. The preferred aqueous coating compositions will further comprise a water-insoluble polymeric binder having both "hard" and "soft" components. The polymer binder will have carboxylic groups as surface groups on the latex; and such can also be embedded under the surface. Carboxylic groups can also be attached on the backbone of the polymer chain.

A preferred class of polymeric binders consists of polyurethane emulsions with' glass transition (Tg) from -10° to -20°C. One example is QW 4147, an aliphatic polyurethane emulsion from K.J. Quinn Company. Alternatively, the binder may comprise an acrylic copolymer emulsion of styrene (e.g., 50%) and n-butyl acrylate, such as "Rhoplex AC-1024" manufactured by Rohm and Haas Company. Or, instead of polyurethane, one can use a copolymer binder emulsion of vinyl chloride/acrylate (Tg range of -7º to 27°C, such as Geon 460x45 from B.F. Goodrich Company).

These emulsions have surface groups (preferably carboxylic groups) that can be cross-linked by several techniques. Polyaziradine, a trifunctional prepolymer, is preferably used for cross-linking the carboxylic group at lower temperatures. The other techniques generally require high temperature for cross-linking carboxylate groups. A preferred ''hard" component is an acrylate like a copolymer of styrene (e.g., about 50%) and n-butyl acrylate. The "hard" acrylate will exhibit high surface gloss (cf. carbonyl surface groups). In some cases, a vinyl-based polymer may be substituted to some extent. The "hard" component preferably comprises a member selected from the group consisting of acrylic copolymers and terpolymers of acrylonitrile, ethyl acrylate, butyl acrylate, methyl methacrylate and mixtures thereof. More specifically, the "hard" component can comprise a copolymer of approximately 10-20% by weight of acrylonitrile and approximately 80-90% by weight of a member selected from the group consisting of ethyl acrylate and butyl acrylate. Similarly, the "hard" component can comprise a terpolymer of approximately 10-20% by weight of acrylonitrile, approximately 10-25% by weight ethyl acrylate, approximately 60-80% by weight butyl acrylate and approximately 10-20% by weight methyl methacrylate.

The "soft" component is preferably comprised of a polymer emulsion of an aliphatic or aromatic polyurethane

(may be polyester or polyether urethane). A preferred "soft" polymer emulsion comprises a polyurethane having a glass transition temperature of between approximately -10° to -20°C -- cross-linked with the "hard" material (acrylic copolymer or terpolymer). The polymeric binders are principally high molecular weight latex to provide superior mechanical properties. Cross-linking these latexes will further improve solvent-resistance and is intended to yield the mentioned "interpenetrating elastomeric network" during the curing stage (after drying operation). The flow agents are added to enhance rheological properties and give smooth coatings. The lubricants are pre-emulsified and are designed to be stable in the system and to reduce surface friction.

The aqueous coating composition can also employ suitable flow agents, antifoaming agents, wetting agents, lubricants and other known additives. DETAILED DESCRIPTION OF THE INVENTION

Except as otherwise specified, all materials, methods and devices and apparatus of this entire disclosure will be understood as carried out with known expedients according to present good practice.

The subject magnetic (recording) coating compositions will be understood as prepared in water to be coated on a substrate of plastic or metal. The magnetic particles can be γ-Fe₂O₃/Fe₃O₄, as well as other oxides; oxides doped with metals, or pure metal particles. Preferred magnetic powders or oxides will be those selected from the group consisting of γ-Fe₂O₃, Fe₃O₄, or other like oxides doped, or surface-treated, with metal ions such as cobalt, nickel or chromium. These particles are generally acicular (needle-shaped) and have a coercivity of 300-1000 Oersted. Conductivity is controlled by incorporating conductive carbon.

The pigments (magnetic oxide and carbon) have different surface properties and have to be dispersed accordingly. The surface of magnetic oxide particles is mostly hydrophilic, whereas carbon particles have a predominantly hydrophobic surface. Thus, magnetic oxides will be dispersed electrostatically and sterically, whereas carbon will be dispersed only sterically, according to this feature.

Magnetic oxides typically have negative surface charge at high pH and will show -40 to -50 mv "zeta potential". For a superior stable dispersion ("kinetically excellent") we prefer to add electrolytes. Polyphosphate salts will be adsorbed on magnetic oxides, to yield a "zeta potential" of about -70 to 80 mv. Carbon particles, being predominantly hydrophobic, will not adsorb such salts, and thus must be stabilized with polymeric dispersing agents. For optimal electrostatic dispersion, we have discovered that control of the level of a dispersant like sodium polyphosphate (or sodium tri-phosphate, or a related salt of phosphoric acid in some instances) about 1 wt.% (0.8-1.2%) can be quite critical. For instance, a small variation in the amount (e.g., sodium polyphosphate) can cause flocculation.

But the right dispersing methods and agents

(including concentration) can yield surprisingly long-lived stability (shown by improved homogeneity of suspension) and improved recording quality. And, where carbon particles are included, we prefer to disperse them sterically with an acrylic emulsion like Acrysol WS-24 (36%) or Acrysol 1-62 (30%).

The amount of polymeric dispersing agents required is determined by adsorption isotherms for both (oxide and carbon) particles. The resulting dispersion will be found stable (kinetically and thermodynamically) for long periods of time; something essential for good magnetic coatings. The two described dispersion techniques are believed necessary to obtain such a stable dispersion. A preferred dispersing agent for the carbon powder in such thermoset aqueous coating compositions is Acrysol I-62: a polyacrylic emulsion comprised of 10% polystyrene, 38% n-butyl acrylate and 52% methyl methacrylate. This (terpolymer) will have an average molecular weight of 20,000 to 30,000. The wetting agent Surfynol 104 B.C.

(Air Products and Chemicals, Inc.) may be used to control surface tension, as well as to help wet the pigment surfaces. For instance, a solution of Surfynol 104 in butyl cellosolve is very satisfactory. Surfynol 104 is a diol of the following structure:

Ammonium hydroxide or sodium hydroxide can be used to control (raise) the pH of the formulation. A flow agent like RM-5, an aqueous acrylic emulsion from Rohm and Haas Company, is preferred for good rheology and viscosity of the formulation during mixing, milling and coating. The mixture is stirred with a high shear impeller to break-up large aggregates.

Antifoaming agents are necessary to prevent air entrapment in the mixture during high shear mixing. The selected antifoaming agent must withstand the high shear of mixing and milling operations. Silicone-based antifoaming agents are found helpful in reducing foam formation during milling. The amount and kind of antifoaming agent will vary according to the formulation (e.g., the emulsifier present in the emulsion polymers or solutions). The defoamers added during the final formulating stages (after adding all polymers and before coating -- this is a second class of defoamer) are basically organic or non-silicone based defoamers.

A preferred antifoaming agent comprises 25 to 30% mineral spirits and 75-70% non-volatile material. The non-volatiles comprise 70% polyglycol (molecular weight around 10,000) and 30% silicone compound (molecular weight from 150 to 250). Size reduction of aggregates in the mixture is effected by high shear milling using glass, steel, alumina or zirconium beads as the milling media.

After a good stable pigment dispersion is obtained, the polymer binders, additives and lubricants may be added to make up the final composition. The polymer binders, selected to optimize the coated tape properties, are mostly water insoluble emulsions of high average molecular weight consisting of hard and soft segments. Binder composition will be formulated in accordance with the intended use

(magnetic media); e.g., to provide necessary durability and toughness The surface charge or groups on emulsion lattices (latexes) are determined by conductometric titration for cross-linking. The (hard, soft) segments will preferably exhibit a carboxylic surface group on the latex (and such can also be embedded under the surface as well as be attached on the backbone of the polymer chain).

In most cases, the "hard" component will comprise an acrylic copolymer; such as styrene (e.g., 50%) plus n-butyl acrylate. Alternatively, a vinyl-based polymer may be substituted in certain cases. The "soft" component will comprise a urethane polymer (or in some cases, a polymer of vinyl chloride or acrylic). For most purposes here, we prefer a polymer emulsion of aliphatic or aromatic polyurethane (e.g., may be polyester or polyether urethane). As particularized below, such hard and soft polymeric segments are cross-linked and arranged so that the poly-blend material physically combines with polyaziridine in an "interpenetrating network" (IPN), enhancing the coating's toughness, durability and impact-resistance (e.g., vs. "Transducer crash"). Such low temperature cross-linking is advantageous because it saves energy; it is enabled preferably with a trifunctional prepolymer type cross-linking agent like polyaziridine (3%).

According to a particular novel aspect, the binder comprises a urethane cross-linked with an acrylic (a,s above noted).

Generally speaking, the aqueous thermoset coating composition will consist of the following functional ingredients in the approximate parts by weight indicated, (when used to formulate magnetic coatings of the type described).

TABLE I COMPOSITION PARTS BY WEIGHT

Magnetic Oxide or Metal Powder 70 -80 Carbon 4 -6

Wetting Agents 0 . 5-1 . 0 Dispersing Agents 1 -3 Antifoaming agents 0 . 1-0 . 5 Flow Agent 0 . 5-2 . 0

Aqueous Emulsion (25-50% solids, i.e. binder) 30-20 Lubricants 1-2

Cross-linking Agent 2-3 The dispersing agents, wetting agents and a pH control agent (ammonium hydroxide or sodium hydroxide) are added to deionized water (DI) in the quantities mentioned. Magnetic oxide and conductive carbon is slowly added and mixed with a high shear blade. Antifoaming agents can be added after all the pigments are completely mixed. The pH of the mixture will be adjusted to about 8 to 10.

This mixture can be further dispersed by high shear milling devices; for example, a Morehouse milling machine. The polymers (aqueous emulsion), flow agents, lubricants, and cross-linking agent are later added.

EXAMPLES

In the following Examples, the present invention is illustrated in greater detail. It will be understood by persons skilled in the art that the components, ratios, and order of operations shown in the Examples can be varied in certain instances without departing from the scope and the spirit of the present invention. The exemplary concentrations of ingredients will be understood as given in parts by weight.

EXAMPLE I

CROSS-LINKED ACRYLIC/URETHANE (WATER-BORNE-FORMULATION #1) PARTS

A - DI Water 43.50 B - γ-Fe₂O₃/Fe₃O₄ Oxide 26.38

C - XC-72R Carbon 2.29

D - Acrysol 1-62 (Acrylic Colloidal Dispersion) 2.04

E - QW4147 (Polyurethane Emulsion) 14.75

F - AC1024 (Acrylic Emulsion) 7.67 G - RM5 (Acrylic Emulsion) 0.96

H - QZ3886 (Polyaziridine Solution) 0.50

I - Silicone Oil Emulsion (lubricant) 0.22

J - Butoxylethyl Stearate (lubricant) 0.88

K - Sodium Polyphosphate (dispersing agent) 0.26 L - Surfynol 104BC (wetting agent) 0.29

M - NH₄OH (Ammonium Hydroxide) for pH 8-10 0.26

100.00

The dispersing agents (0.26% sodium polyphosphate and 2.04% Acrysol 1-62) are added into deionized water. The wetting agent, i.e., 0.29% Surfynol 104BC (50% solution in butyl cellosolve) is added also.

The pH of the mixture is adjusted to about 8-10 by adding ammonium hydroxide.

Conductive carbon (2.29% XC-72R) is slowly added to the mixture and stirred for % hour. The magnetic oxide (26.38%, γ-Fe₂O₃/-Fe₃O₄) is slowly added and stirred vigorously for ½ hour. This mixture of pigments and dispersing agents is further processed through a high shear mill to further assure breaking-up of all aggregates.

The quality of dispersion is checked after each milling sequence with an optical microscope or a scanning electron microscope. (Other techniques for evaluating quality of dispersion may be found helpful, such as with a Disc Centrifuge). The latexes are then added to the dispersion. Polyurethane emulsion (14.75% QW4147) is added as a "soft" segment having carboxylic groups as functional groups. An acrylic emulsion (7.67% Rhoplex AC1024) is added as a "hard" segment having carboxylic group as functional group on the surface. The surface groups of these latexes are determined by conductometric titiration; Rhoplex AC-1024 is approximately 36-40 meg/gms and QW4147 is approximately 195-200 meg/gms carboxylic group on the surface of latexes.

Approximately 2-3% of polyaziridine is added, sufficient to react with the desired total number of surface groups. The cross-linking mechanism takes place in two stages. The carboxylic groups are present on the surface and can also be embedded under the surface. It may be found that the carboxylic groups (as determined by conductometric titration; for example with NeoRez 962 from Polyvinyl Chemical Industries) are made up of carboxylic groups on the surface and also on the backbone of the chain. The cross-linking of trifunctional polyaziridine with carboxylic groups starts during the coalescence of latexes after drying operation in the first stage.

The second stage cross-linking (or post-curing). starts when a uniform film is formed and the carboxylic group on the backbone starts reacting with available polyaziridine. Such a cross-linking reaction of carboxylic acid with polyaziridine is described in "Ethylenimine and Other Aziridines" (by O.C. Dermer and G.E. Ham, e.g., see pages 227 and 230 published 1969, by Academic Press) as follows: RCOOH + X(CH₂CH₂HN) → RCOO (CH₂CH₂NH) _xH where R is alkyl group Eq. 1

X = x = 1, 2, 3, etc. (CH₂CH₂NH) = aziridine ring

A secondary reaction which may also occur between carboxylic acid and aziridine is shown below.

where R = alkyl

(CH₂CH₂NH) = aziridine ring

Possible cross-linking reactions which may occur between polymers containing carboxylic acid groups and multifunctional aziridines can be represented by the general Equation (3) . It should be understood by those skilled in the art that cross-linking may occur by a 'combination of reactions represented by Equations (1) and (2) .

R(COOH) R' (CH q. 3 n + ₂CH₂NH)_m → Z E

where n = 2 or more m = 2 or more

R = polyester urethane copolymer of

Mw 12,000-78,000 or other carboxylic acid group containing polymer of appropriate molecular weight

R' = aikyl, aryl, cycloalkyl, etc., and other groups used in preparation of multifunctional aziridines (CH₂CH₂NH) = aziridine ring without indication of substituent groups and/or substitution position Z = cross-linked product

Similar possible cross-linking reactions can occur when "R" polyester urethane copolymer is replaced, by another polymer; for example, a styrene-acrylic copolymer with carboxylic groups present on the surface and also embedded under the surface. The average molecular weight of styrene acrylic copolymer is from 1000 to 100,000.

The cross-linking agent polyaziridine (0.5% of QZ3886 polyaziridine solution from K.J. Quinn) is added to cross-link the carboxylic groups present on the surface during coalescence of the latexes in the drying operation. The flow agent (0.96% of RM-5, acrylic emulsion from Rohm and Haas Company) is added to the formulation, "the lubricants are pre-emulsified as explained in copending U.S. Patent application Serial No. 345,082. The lubricants can be butoxyethyl stearate (0.88%, available from Armak

Company) and silicon oil (0.22% available from Dow Corning. Corporation). A fluorocarbon lubricant can also be employed instead of silicone oil. The fluorocarbon lubricants selected can be perfluoroalkyl polyethers and fluorinated fluids having one of the following structures :

where n, m = 40-90 and

F[ -CF ( CF₃ )CF₂O] _nC₂F₅ where n = 12-45

The butoxyethyl stearate is compatible with such a polymer system and can act as an internal lubricant. The silicone oil (or fluorinated fluids) are incompatible with polymer binder and act as external lubricant, diffusing to the surface during drying of the coatings to thus form a thin layer of lubricant on the surface of coating. Results: The results will be like those in Example II, except that "gloss" will be lower. EXAMPLE II The ingredients in Example II (i.e., Formulation #2 below) are essentially the same as in Example I, except for different "binder polymers" (see 2-E, 2-F) and minor concentration variations. The procedure for making this (#2) formulation for thermσset water based composition is also the same.

CROSS-LINKED ACRYLIC/VINYL CHLORIDE (WATER-BORNE-FORMULATION #2) WEIGHT % A - DI Water 47.38

B - γ -Fe₂O₃/Fe₃O₄ Oxide 28.73

C - XC-72R Carbon 2.50

D - Acrysol 162 (Acrylic Colloidal Dispersion) 2.22 2-E - AC1C-24 (Acrylic Emulsion) 7.66

2-F - Geon 460x45 (Vinyl Chloride Copolymer) 7.81

G - RM5 (Acrylic Emulsion) 1.04

H - QZ3886 (Polyaziridine Solution) 0.53

I - Silicone Oil Emulsion (lubricant) 0.25 J - Butoxylethyl Stearate Emulsion (lubricant) 1.00

K - Sodium Polyphosphate 0.29

L - Surfynol 104BC 0.31

M - NH₄OH (Ammonium Hydroxide) for pH 8-10 0.28 100.00 The formulation #2 (Example II) includes Geon 460x45, a vinyl chloride polymer having glass transition temperature of 26°C. The mechanical properties will be found comparable to those of the polyurethane-based formulations (e.g., #1). Geon 460x45 will render a very durable, glossy film as compared with polyurethane based film, (glossy films are especially suited for audio and video tapes as workers know).

Geon 460x46 (B.F. Goodrich Company) is an alternative vinyl chloride copolymer having Tg of 7°C; it forms a tough film after cross-linking with polyaziridine in the formulation.

It will be found that acrylics/vinyl copolymer- based formulations give glossier films, after surface treatment, as compared with acrylic/polyurethane polyblend compositions. The overall gloss of such water-based compositions will be found much higher than organic (solvent-based) compositions.

The signal-to-noise ratio is found to be excellent. The output is improved because of the smooth tape surface and improved Theological properties rendered.

Superior hardness, toughness and solvent-resistance is rendered by so cross-linking the surface groups (and thus obtaining interpenetrating elastomeric/plastomeric network (IEN/lPN). Such a network improves durability. The squareness and orientation ratio of resulting magnetic tapes is excellent. A very low coefficient of friction will be found because of the lubricant layer formed on the tape surface.

It will be understood that the preferred embodiments described herein are only exemplary, and that the invention is capable of many modifications and variations in construction, arrangement and use without departing from the spirit of the invention.

Further modifications of the invention are also possible. For example, the means and methods disclosed herein are also applicable to other tape systems, to related flexible disks and the like, as well as to certain coatings for rigid disks.

The above examples of possible variations of the present invention are merely illustrative. Accordingly, the present invention is to be considered as including all possible modifications and variations coming within the scope of the invention as defined by the appended claims.

Claims

What is claimed is:

1. In an aqueous thermosetting coating composition for forming a magnetic recording layer and including pigment suspended in a polymeric binder having a hard component and a soft component, the improvement comprising the use of sufficient dispersant material to disperse the magnetic pigment sterically and electrostatically and yield enhanced stability.

2. The aqueous coating composition of claim 1 wherein the dispersant material includes an anionic electrolyte.

3. The aqueous coating composition of claim 2 wherein the electrolyte comprises a polyphosphate such as sodium polyphosphate.

4. The aqueous coating composition of claim 2 wherein conductive pigment is also present and is dispersed sterically with an acrylic emulsion.

5. The combination as recited in claim 4 wherein the components are bound and cross-linked in a semi-interpenetrating network and are derived from respective different polymeric emulsions, and wherein the binder exhibits carboxyl linkage groups apt for such cross-linkage, at least on the surface thereof.

6. The aqueous coating composition of claim 5 wherein the soft component comprises a urethane or vinyl as a principal constituent and the hard component includes surface carboxyl groups cross-linked with soft material.

7. The aqueous coating composition of claim 6 wherein the hard component is derived from an acrylic emulsion and is substantially fully cross-linked with the soft component to yield said interpenetrating network with associated enhanced durability and recording properties.

8. The aqueous coating composition of claim 7 wherein the soft component is derived from a polymeric emulsion of a polyurethane.

9. The aqueous coating composition of claim 8 wherein the polymer emulsion of polyurethane has a moderate related glass transition temperature, at or somewhat above room temperature.

10. The aqueous coating composition of claim 9 further comprising a flow agent, antifoaming agent, wetting agent, pH control, and lubricant; said constituents yielding enhanced chemical and mechanical stability, smoothness; adhesion and non-hygroscopic character.

11. A method of preparing a thermosetting composition adapted to form a magnetic recording layer and having pigment suspended in a polymeric binder having both a "hard" and a "soft" component, the improvement comprising the use of sufficient polymeric emulsion or polyphosphate dispersant material to disperse the magnetic pigment sterically and electrostatically.

12. The combination as recited in claim 4 wherein at least one dispersant comprises a "moderate molecular weight" polyacrylic emulsion.

13. The combination as recited in claim 12 wherein at least one such polyacrylic emulsion includes styrene and acrylate constituents.

14. The combination as recited in claim 13 wherein at least one such polyacrylic emulsion comprises a terpolymer including methyl methacrylate and butyl acrylate material.

15. The method of claim 11 wherein the dispersant material includes an anionic electrolyte.

16. The method of claim 15 wherein the electrolyte comprises a polyphosphate such as sodium polyphosphate.

17. The method of claim 15 wherein conductive pigment is also present and is dispersed sterically with an acrylic emulsion.

18. The method in claim 17 wherein the components are bound and cross-linked in a semi-interpenetrating network and are derived from respective different polymeric emulsions, and wherein the binder is selected and arranged to exhibit carboxyl linkage groups apt for such cross-linkage, at least on the surface thereof.

19. The method of claim 18 wherein the soft component comprises a urethane or vinyl as a principal constituent and the hard component includes surface carboxyl groups cross-linked with soft material.

20. The method of claim 19 wherein the hard component is derived from an acrylic emulsion and is substantially fully cross-linked with the soft component to yield said interpenetrating network with associated eiihanced durability and recording properties.

21. The method of claim 20 wherein the soft component is derived from a polymeric emulsion of a polyurethane.

22. The method of claim 21 wherein the polymer emulsion of polyurethane has a moderate related glass transition temperature.

23. A method for rendering an aqueous thermosetting magnetic recording medium on a suitable substrate, the medium including fine particles in a polymeric binder, the method including the steps of: A. preparing the binder latex system to comprise a number of water-insoluble polymeric emulsions, having carboxylic linking groups, at least on the surface thereof;

B. preparing dispersants apt for suspending and stabilizing said particles in said binder, both sterically and electrostatically;

C. intermixing the emulsions and dispersants with said particles in an aqueous medium; and thoroughly dispersing the particles therein; D. selecting and preparing a cross-linking agent apt for cross-linking binder carboxylic groups, at least on the binder surface to so combine as to render a prescribed semi-interpenetrating network; and E. coating the resultant medium onto the substrate and drying it there.

24. The method as recited in claim 23 wherein the pH of said aqueous medium is adjusted and the dispersants are added, along with surfactants; then the particles are added and intermixed with great agitation, being size-reduced as necessary to render the target dispersion; and wherein the polymer emulsions comprise aliphatic or aromatic polyurethane, having carboxylic functional-groups, along with acrylic emulsion having carboxylic functional groups, at least on the surface; then, sufficient polyaziridine is added to initiate the said cross-linking while the emulsions are drying; then, as needed, all other agents, lubricants, other additives are added and intermixed.

25. In a method of fabricating a flexible magnetic recording medium of the type comprising a backing film carrying one or two aqueous magnetic coatings thereon, wherein each said coating is formed to include magnetic particles suspended within a resinous matrix, the improvement therein comprising: forming said resinous matrix with polyurethane material plus at least one other different resin including carboxyl cross-linker groups, along with dispersant material apt for dispersing said particles electrostatically and sterically; and curing said resin with a multi-functional cross-linking agent apt to induce the polyurethane and other resins to form a semi-interpenetrating network.

26. The method of claim 25 wherein at least one of the other resins is selected and prepared to principally comprise an acrylic exhibiting said carboxyl groups at least on surface portions.

27. The method as recited in claim 26 wherein the binder is prepared by:

A. intermixing at least some of the polyurethane and acrylic and the dispersant material in an aqueous vehicle with the pigment material;

B. processing these until a good stable homogeneous dispersion is obtained;

C . adding the cross-linking, agent and supplementary binder constituents, while facilitating the prescribed cross-linking to yield said network; then

D. finalizing the binder and coating it onto a flexible substrate; orienting it, drying it and calendering it there.

28. The aqueous coating composition of claim 1 wherein the dispersant materials include a polymeric emulsion.

29. The aqueous coating composition of claim 28 wherein the polymeric emulsion includes an acrylic emulsion.

30. The composition of claim 29 wherein the polymeric emulsion comprises Acrysol I-62.

31. The combination as recited in claim 23 wherein the polymer emulsion comprises aliphatic or aromatic polyurethane or a polyester or polyether urethane.

32. A method for rendering an aqueous thermosetting magnetic recording medium on a suitable substrate, the medium including fine particles in a polymeric binder, the method including the steps of: A. preparing the binder latex system to comprise a number of water-insoluble polymeric emulsions ; B . preparing dispersant means apt for suspending and stabilizing said particles in said binder, both sterically and electrostatically; C. intermixing the emulsions and dispersant means with said particles in an aqueous medium; and thoroughly dispersing the particles therein.

33. The method of claim 32 also including the added steps of: selecting and preparing a cross-linking agent apt for cross-linking binder groups, at least on the binder surface to so combine as to render a prescribed semi-interpenetrating network; adding this cross-linking agent to the mixture of emulsions and dispersant means; and coating the resultant medium onto the substrate and drying it there.

34. The method as recited in claim 32 wherein the pH of said aqueous medium is adjusted and the dispersants are added, along with surfactants; then the particles are added and intermixed with great agitation, being size-reduced as necessary to render the target dispersion; and wherein the polymer emulsions comprise aliphatic or aromatic polyurethane, having certain functional groups, along with acrylic emulsion having like functional groups, at least on the surface; then, adding sufficient polyaziridine to initiate the said cross-linking while the emulsions are drying; then, as needed, adding and intermixing all other agents, lubricants, and other additives.

35. In a method of fabricating a flexible magnetic recording medium of the type comprising a backing film carrying one or two aqueous magnetic coatings thereon, wherein each said coating is formed to include magnetic particles suspended within a resinous matrix, the improvement therein comprising: forming said resinous matrix with polyurethane material plus at least one other different resin including certain cross-linker groups, along with dispersant material apt for dispersing said particles electrostatically and sterically; and curing said resin with a multi-functional cross-linking agent apt to induce the polyurethane and other resins to form a semi-interpenetrating network.

36. The method of claim 35 wherein at least one of the other resins is selected and prepared to principally comprise an acrylic exhibiting said cross-linker groups at least on surface portions.

37. The method as recited in claim 36 wherein the binder is prepared by:

B. processing these until a good stable homogeneous dispersion is obtained;

C. adding the cross-linking agent and supplementary binder constituents, while facilitating the prescribed cross-linking to yield said network; then

38. The combination as recited in claim 32 wherein the polymer emulsion comprises aliphatic or aromatic polyurethane or a polyester or polyether urethaner.

39. The method of claim 32 wherein hydrophobic pigment is included and said dispersant means comprises sufficient polymeric dispersant to stabilize the mixture over an extended term, as determined by appropriate adsorption isotherms for the pigment.

40. The method of claim 39 wherein the dispersant means includes sufficient polyphosphate to optimize electrostatic dispersion.

.

41. The method of claim 40 wherein the dispersant wt.% is a sodium polyphosphate or related sort of phosphoric acid controlled to avoid flocculatiori.

42. The method of claim 39 wherein carbon particles are included and the dispersant means includes sufficient acrylic emulsion for adequate steric dispersion.

43. The method of claim 42 wherein the dispersant means comprises a high molecular weight polyacrylic emulsion plus sufficient surfactant to adequately wet the pigment surface.

44. The method of claim 39 wherein a pH control agent and high-shear-tolerant anti-foamer are included.

45. The method of claim 44 wherein pH is controlled with a hydroxide of sodium or ammonium, and the anti-foamer includes a silicone-based moiety and/or another organic moiety, at least one such moiety being added during the final stages of formulation.

46. The method of claim 45 wherein one anti-foamer comprises a silicone compound and glycol co-constituent plus minor wt.% mineral spirits.

47. The method of claim 12 wherein: the dispersant means is mixed in, then conductive carbon is added and intermixed; and the magnetic pigment is added and intermixed with high shear, then, the latex is added including polyurethane emulsion with certain functional groups and acrylic emulsion including like functional groups; then, sufficient polyaziridine is added to adequately inter-react the requisite functional groups, with cross-linking occurring in two stages: first with surface groups as the latexes coalesce; then internally after curing.

48. The method of claim 16 wherein, the functional groups are carboxylic.

49. The product of the method in claim 32.

50. The product of the method in claim 33.

51. The product of the method in claim 34.

52. The product of the method in claim 35.

53. The product of the method in claim 36.

54. The product of the method in claim 38.

55. The product of the method in claim 39.

56. The product of the method in claim 41.

57. The product of the method in claim 43.

58. The product of the method in claim 45.

59. The product of the method in claim 46.

60. The product of the method in claim 47.

61. The product of the method in claim 48.

62. A high-gloss aqueous thermosetting coating composition for forming a magnetic recording layer including pigment suspended in an acrylic-vinyl co-polymer binder having a hard component and a soft component, plus sufficient dispersant material to disperse the magnetic pigment sterically and electrostatically and yield enhanced long-term stability.

63. The aqueous coating composition of claim 62 wherein the dispersant material includes an anionic electrolyte, and a vinyl polymer having a glass-transition temperature at about room temperature or lower.

64. The aqueous coating composition of claim 63 wherein the electrolyte comprises a polyphosphate such as sodium polyphosphate.

65. The aqueous coating composition of claim 63 wherein conductive pigment is also present and is dispersed sterically with an acrylic emulsion.

66. The combination as recited in claim 65 wherein the components are bound and cross-linked in a semi-interpenetrating network for enhanced durability, hardness and solvent-resistance and are derived from respective different polymeric emulsions, wherein the binder exhibits carboxyl linkage groups apt for such cross-linkage, at least on the surface thereof, and wherein the soft component comprises vinyl chloride, the coating being surface-treated to optimize "gloss" and related surface smoothness characteristics.

67. The aqueous coating composition of claim 66 wherein the soft component comprises said vinyl chloride as a principal constituent and the hard component includes surface carboxyl groups cross-linked with related moieties of the soft material, with sufficient polyaziridine therein for superior film toughness.

68. The aqueous coating composition of claim 67 wherein the hard component is derived from an acrylic emulsion and is substantially fully cross-linked with the soft component to yield said interpenetrating network with associated enhanced durability and recording properties.

69. The aqueous coating composition of claim 68 further comprising a flow agent, antifoaming agent, wetting agent, pH control, and lubricant; said constituents yielding enhanced chemical and mechanical stability, smoothness, adhesion and non-hygroscopic character.

70. A high-gloss thermosetting composition adapted to form a magnetic recording layer and having pigment suspended in a polymeric binder having both a "hard" acrylic and a "soft" vinyl component, plus the use of sufficient polymeric emulsion or polyphosphate dispersant material to disperse the magnetic pigment sterically and electrostatically, the composition being coated on a suitable substrate and surface-treated to optimize smoothness.

71. The composition of claim 70 wherein the soft component comprises vinyl chloride.

72. y A method of preparing a high-gloss thermosetting composition adapted to form a magnetic recording layer and having pigment suspended in a polymeric binder having both a "hard" and a "soft" component, the improvement comprising the use of sufficient polymeric emulsion or polyphosphate dispersant material to disperse the magnetic pigment sterically and electrostatically, and the use of a vinyl moiety as the soft component and an acrylic moiety as the hard component.

73. The method as recited in claim 41 wherein at least one dispersant comprises a "moderate molecular weight" polyacrylic emulsion comprised of styrene.

74. The method of claim 41 wherein the dispersant material includes an anionic electrolyte, and the vinyl comprises vinyl chloride.

75. The method of claim 74 wherein the electrolyte comprises a polyphosphate such as sodium polyphosphate.

76. The method of claim 74 wherein conductive pigment is also present and is dispersed sterically with an acrylic emulsion.

77. The method in claim 76 wherein the components are bound and cross-linked in a semi-interpenetrating network and are derived from respective different polymeric emulsions, and wherein the binder is selected and arranged to exhibit linkage groups apt for such cross-linkage, at least on the surface thereof.

78. The method of claim 72 wherein the soft component comprises a vinyl as a principal constituent and the hard component includes surface carboxyl groups cross-linked with soft material.

79. The method of claim 78 wherein the hard component is derived from an acrylic emulsion and is substantially fully cross-linked with the soft component to yield said interpenetrating network with associated enhanced durability and recording properties.