LUBRICATION OF MAGNETIC RECORDING MEDIA

Abstract: An aqueous coating composition forming a magnetic coating layer. The composition is improved over prior art magnetic coatings by the inclusion of a colloidal emulsion of a lubricant, such as, for example, a member selected from the group consisting of aliphatic and aromatic stearates, silicone oils, perfluoro alkyl polyethers and fluorinated fluids.

Description: LUBRICATION OF MAGNETIC RECORDING MEDIA Background of the Invention This invention concerns itself with aqueous magnetic coating formulations . It is desirable at times to use aqueous formulations in place of organic forroula- tions for the latter use expensive and toxic flammable solvents which must be captured upon drying of the magnetic formulation to prevent air pollution. Further¬ more, there have been a number of recent developments in improving aqueous formulations, which make them attractive as substitutes for organic based coatings generally. Due to the relative hardness of the electro¬ magnetic head used to read the magnetic information stored on the recording layer, most modern magnetic coatings employ one or more lubricants to reduce friction and thus extend the life of the magnetic head and magnetic media. In the past lubricants have been applied to magnetic layers either by incorporating them directly into the coating composition or by spraying them on the magnetic media before use. In U. S. Patent No. 2,699,408, beeswax was applied to a magnetic coating composition by first emulsifying this material before the addition. Nevertheless, beeswax is not conducive to the formation of a continuous boundary film layer on the surface of a magnetic coating. Instead, it promotes point lubrication, which cannot effectively reduce the friction and scoring generated when magnetic recording media engages the magnetic head. As examples of prior art compositions, reference is made to U. S. Patent No. 4,263,188, which relates to an aqueous magnetic coating composition comprising a resin binder, a wetting agent, dispersant and a lubricant, such as butoxy ethyl stearate. This patent teaches mixing all of the ingredients together and dispersing them on a suitable support. U. S. Patent No. 4,076,861 similarly teaches an aqueous magnetic coating composition using a silicone oil or esters of fatty acids containing 17 or more carbon atoms as lubricants. U. S. Patent No. 4,025,448 teaches a wax composition as a lubricant for a magnetic aqueous coating composition showing that all of the ingredients are simply mixed together before coating. The use of water soluble lubricants in aqueous coating compositions presents its own problems. For example, this class of lubricants can be characterized as being water sensitive, or in other words, hydro- scopic, and are thus not stable when heat is generated within the coating during frictional contact with the magnetic head. As alluded to previously, the use of waxes as a substitute for water soluble lubricants either directly or by emulsification promotes phase separation within the coating layer and thus mitigates against the presence of a continuous film on the surface of the magnetic media. It is thus an object of the present invention to teach a novel aqueous magnetic coating composition. It is yet another object of the present invention to " teach improved lubrication of aqueous magnetic coating compositions . These and other objects will be more fully appreciated when considering the following disclosure. Summary of the Invention The present invention deals with an aqueous coating composition for forming a magnetic recording layer, the improvement comprising the addition of a colloidal emulsion of a lubricant. The lubricant is non-aqueous and thus insoluble in the aqueous compo¬ sition. By using a lubricant, the difficulties outlined above with hydroscopicity are eliminated. The aqueous coating compositions are preferably selected from one or more members of a group consisting of aliphatic and aromatic stearates, silicone oils, perfluoro alkyl polyethers and fluorinated fluids having one of the following structures: • 0 - CF 2 ή 1 0 - CF 2 - F 2 nT wherein n, m = 40-90 and F -£CF(CF 3 )CF 2 0:i n .C2F5 wherein n' = 12-45 and mixtures thereof. It should be understood that no solvent is incorporated within the aqueous coating composition for the lubricants used in the present invention. Instead, a colloidal dispersion of lubricants is previously prepared and added, in a separate step, to the aqueous compositoin. Colloidal dispersion of the lubricant assures the formation of a stable micro-emulsion which diffuses to the surface of the magnetic coating during drying and coalesces to form a thin protective lubri¬ cated layer thereon. - h - Detailed Description of the Invention The aqueous phase of the coating composition can be prepared by dispersing a magnetic oxide particle and dispersing agents in de-ionized water while con- trolling the pH from approximately 8 to 10 by the addition of suitable alkaline materials. It has been found most advantageous to mix the above at high shear while further adding water soluble resins (or emulsions) as well as defoamers. The dispersion is then subjected to milling to break up the aggregates of magnetic oxide materials. It has been found that the weight loading of oxide to water soluble resin can vary in the ratio of 60/40 to 80/20 according to the intended end use of the coating compositio . As stated previously, the present invention primarily deals with the inclusion of non-water soluble lubricants in the aqueous magnetic coating composition by pre-emulsif ing the lubricants prior to their inclusion in the magnetic layer. When stearates are used as the lubricating media, they can be prepared by mixing them in deionized water with ammonium hydroxide while keeping the pH from approximatley 8 to 10 and the temperature from approximately 50-80°C The mixture is maintained at high shear while the stearates undergo partial soaponification. This causes the stearates to form a small colloidal dispersion within the deionized water, which is further broken up into sub-micron size mono-dispersed colloidal particles by, for example, passing the disperson through a Mantan Gaulin homo- genizer at a pressure from 4,000-6,000 psi. Preferred stearates are selected from the group consisting of butyl stearates and butoxy ethyl stearates. Lubricants can also be prepared from silicone oils, such as dimethyl silόxane, and fluorinated oils, such as perfluoro alkyl polyethers, as well as per- fluorinated polyethers. Most advantageous are those fluorinated oils having a molecular weight from approxi¬ mately 1,200-7,000 such as a fluorinated oil sold under the tradename Fo blin by Montedison U.S.A., Inc. Micro-emulsions are prepared from the above lubricants while maintaining a weight ratio of oil to water from approximately 30-50 percent. A mixed emulsifier system comprising an anionic emulsifier, for example, sodium laurayl sulfate, can be employed while the surface tension of the lubricant droplets can be reduced by adding n-decane or cetyl alcohol. For such a system, the following table represents an approximate and typical formulary for emulsification of silicone lubricants (Table l), fluorinated lubricants (Table 2), as well as stearates (Table 3). Table 1 Emulsification of Silicone Lubricants Weight % Water (deionized) 50.0 gms Sodium Lauryl Sulfate 0.1 gms n-decane 0.05 gms Dimethyl siloxane 49.85 gms 100.00 gms total Table 2 Emulsification of Fluorinated Lubricants Weight % Water (deionized) 49.70 gms Sodium Lauryl Sulfate 0.1 gms Carbopol 0.25 gms n-decane 0.05 gms Krytox (a perfluoro alkyl 49.90 gms polyether available from E. I. du Pont de Nemours & Co. ) 100.00 gms total Table 3 Emulsification of Stearates Weight % Water (deionized) * 49.8 gms t j Ammonium Hydroxide 0.2 gms (35% concentration) Stearate (e.g. , butoxy 50.00 gms ethyl stearate) 100.00 gms total The silicone or fluorinated oil can be further 0 prepared by including an anionic emulsifier, while the solution is heated from 50-80°C under constant stirring. The cetyl alcohol or n-decane is mixed with the oil and added slowly to the distilled water/silicone or fluorinated oil mixture, which is stirredvigorously for ^ a short period of time (e.g., about half an hour). The mixture can then be sonicated for several minutes to obtain a micron-emulsion. The micron-emulison can further be broken down by homogenizing from 4,600-6,000 psi as described above. This emulsion was found to be 0 resistant to flocculation. The water-based magnetic formulations generally comprise dispersing agents, magnetic oxides, polymer solutions or latices and lubricants, as well as other additives. For dispersing agents, it has been found that Tamol 165, an ammonium solution of a polyacrylate sold by Rohm & Haas, sodium polyphosphate, and Surfynol 104, a diol of the following structure available from Air Products Co.: CH3 CH 3 CH 3 CH 3 CH 3 -CH-CH 2 -C-CH = -C-CH 2 -CH-CH 3 OH OH have all been found to be adequate. Virtually any oxide could be used in practicing the present invention including, for example, Y -Fe 2 0 , Fe 3 θ , cobolt doped Fe 3 0 4 , Cr 2 0 , as well as various metal particles. A variety of polymer solutions or 5 latices can be included as binders including, for example, polyblends of acrylic copolymer latices, vinyl- based and acrylic latices , polyurethane and acrylic latices and polyurethane urea-formaldehyde or melamine resins . 0 The water-based magnetic compositions can also contain certain additives, such as conductive carbon to reduce resistivity of the coating. Flow control agents can also be included for controlling the rheology of the 5 composition during the coating process. The micro-emulsion of lubricants can be directly incorporated into the above-recited formula¬ tions in an amount which can be varied according to the final intended use of the coatings. Normally, lubri- 0 cants prepared according to the present invention vary from approximately 1 or 2% by weight of the total formulation. The lubricant addition is generally accomplishd by merely mixing the lubricants into the formulation approximately 20-30 minutes before coating. 5 Coating can be made on either flexible or rigid sub¬ strates and dried in air dryers where coalescence of latices and pigments takes place. The lubricants in the form of micro-emulsions are less dense than the mass of the aqueous coating, and they thus tend to coalesce on Q thes surface of the coating, giving a thin film of oil lubricant thereon. The quantities of lubricant found within the magnetic layer as well as lubricant on the surface thereof can be controlled in the composition. For O PI example, as time passes and the external lubricant is consumed, lubricant found within the body of the compo¬ sition can diffuse to the surface of the coating. It has been found that lubricants which are merely over- sprayed on top of the coating cannot perform in such a manner. Also, overspraying of a top coating of lubri¬ cant requires additional coating equipment, and the coating thickness is difficult to control. Excess lubricant on the surface of a magnetic layer can result in increased errors in recording/playback of information noting that the thickness of a lubricant boundary layer must be in the range of from 50-80 angstroms.

Claims: WHAT IS CLAIMED IS: 1 • In an aqueous coating composition for forming a magnetic recording layer, the improvement comprising the addition of a colloidal emulsion of a lubricant. 2. The aqueous coating composition of claim 1 wherein said lubricant is insoluble in said aqueous composition. 3. The aqueous coating composition of claim 1 wherein said lubricant is a member selected from the group consisting of aliphatic and aromatic stearates, silicone oils, perfluoro alkyl polyethers and fluorinated fluids having one of the following structures: „__ CF I i 0 - CF - CF 2 r ( 0 - CF 2 - CF 2 wherein n = 40-90 wherein m = 40-90 and F- [CF(CF 3 )CF 03 n 'C 2 F 5 wherein n' = 14-45 and mixtures thereof. 4. The aqueous coating composition of claim 3 wherein said stearate is a member selected from the group consisting of butoxy ethyl stearate and butyl stearate. _ OMPI 5. The aqueous coating composition of claim 3 wherein said silicone oil is a member selected from the group consisting of alkyl siloxanes, vinyl siloxanes and dimethyl siloxanes . 6. The aqueous coating composition of claim 1 wherein said lubricant is present in an amount of approximately 1 to 2% by weight based upon the total weight of the coating composition.

Inventors: IQBAL SIKANDAR, IQBAL, SIKANDAR
Applicants: MEMOREX CORP, MEMOREX CORPORATION
Application year: 1983
Main classification: C09D5
IPC codes: C09D5, G11B5