MAGNETIC RECORDING COATING COMPOSITION

Abstract: A magnetic recording coating composition having a high density of magnetic particles uniformly dispersed in a high performance, thermoset polyurethane resin binder with the dispersion applied as a thin coating onto a rigid substrate. The thermosetting resin binder comprises a blocked isocyanate polymer reacted with a hydroxy ter­ minated or branched polymer. The coating composition has an extended shelf life and can be cured at temperatures below 375°F to produce a smooth, hard, though durable and well dispersed magnetic coating with an improved orienta­ tion ratio that readily accepts buffing, washing, and lubrica­ tion.

Description: MAGNETIC RECORDING COATING COMPOSITION Description Technical Field The invention relates to magnetic recording coating compositions and, more particularly, to a magnetic recording coating composition having finely divided magnetic particles dispersed in a thermosetting resin binder composition applied as a thin film closely adhered to a rigid non-magnetic substrate. Background of the Invention As magnetic recording densities progressively increase and track widths correspondingly decrease magnetic bit cells are of such size that the use of hard non-magnetic particles, which have been commonly used to improve the wear resistance of disk coatings, must be minimized. Further, as new magnetic particles such as cobalt surface diffused or cobalt modified -Fe203 magnetic particles are developed to meet density requirements, high matrix curing temperatures have been found to impair the particle magnetic properties. This degradation may occur as a consequence of cobalt diffusion and reduction of magnetic properties. Also, the coating material must be capable of forming a very thin coating with a smooth surface that will allow a transducer carrying slider assembly to fly only a few thousandths of a micron above the surface. These properties are in addition to the commonly required characteristics of magnetic disk coatings such as being adaptable to normal coating techniques, capable of withstanding repeated head loadings, able to achieve high orientation ratios and capable of good abrasion resistance, impact resistance and corrosion resistance. Summary of the Invention The magnetic recording coating composition of the invention is of the type having finely divided magnetic particles dispersed in a thermosetting resin binder composition applied as a thin film, having a thickness of less than 2 microns, closely adhered to a rigid non-magnetic substrate, said coating composition including 50 to 80 percent by weight of magnetic particles, 20 to 50 percent by weight of binder, the composition, according to the invention, is characterized in that it comprises a blocked isocyanate having at least three reactive sites per chain, a plyol having at least two reactive sites per chain and a molecular weight of 200 to 800 per hydroxy site, and additives not exceeding 30 parts by weight. The composition is preferably prepared by admixing fine magnetic particles, such as magnetic iron oxide, magnetic chromium dioxide, magnetic cobalt modified oxides, alloy particles or other such particulate materials commonly used in the art of producing magnetic film coatings The particles to be embedded in the finished composition range in size from about 0.06 micron to 1.00 micron with the length to width ratios in the range of from 4/1 to about 16/1. Additives used to enhance the properties of the coating include dispersants, catalysts and surfactants. Among the most effective dispersants is polyester polyol which has a carboxy side chain that provides a chelation effect on the magnetic particles. Also being a polyol, this additive becomes part of the cross linked polyurethane binder matrix. If this type polyester polyol is selected as the polyol, no dispersant additive is required. Since newer magnetic particles are more heat sensitive the normal polyurethane reaction or curing temperature of 1900C or less for under an hour rather than the much higher temperature and longer time required by many other coating systems is beneficial. To further enhance this characteristic, catalysts are used to speed the reaction and reduce cure temperature even further. Use of a surfactant enables the binder to retain a greater magnetic particle concentration while forming a smooth, thin coating. After curing and cooling, the magnetic recording surface is polished to produce a super finished surface with an ari thmetic average (AA) roughness of 2.5.10 3 micron to 10.10 micron and a thickness of less than 2 micron. The resulting magnetic recording surface is a very smooth continuous surface without perceptible streaks, lines, craters, voids or asperities. The coating is resistant to abrasion and shedding of the magnetic particles embedded uniformly therein. The surface finish is further highly resistant to conventional cleaning solvents during the washing operation. The finished surface structure retains a small quantity of lubricant in minute sites that effectively lubricates the disk surface, but is neither prone to cause "stiction" or adhesion with a stationary read/ write head or be spun off the disk during rotation at operational speeds. Brief Description of the Drawing The drawing figure is a flow diagram illustrating the steps in formulating the coating composition and applying the coating to a rigid substrate to form a magnetic recording disk that is finished and lubricated. Detailed Description The present magnetic coating includes a polyurethane binder in which magnetic particles are dispersed and which can be applied to a rigid substrate using presently used coating methods. The binder may include catalyst, surfactant and dispersant additives to enhance the properties of the binder, reactivity of the binder component materials or intermediate compounds and enable the binder to function with a high concentration of magnetic particles. In a rigid disk coating application, it is desirable to have an ink or coating material which is stable at room temperature, but that reacts at elevated temperatures to form a thermosetting coating To achieve that condition, the present urethane coating systems are obtained using the socalled "blocked" isocyanates or "splitters". The products have also been referred to as "disguised" and "capped" isocyanates. These blocked isocyanates are reaction products of isocyanates with certain active hydrogen compounds such that the addition product has only limited thermal stability. A typical example is the urethane from a methyl-ethyl ketone Oxime: EMI4.1 wherein R is an aromatic, aliphatic or a combination aromatic/aliphatic hydrocarbon chain. The blocked isocyanate, as shown at the right side of the equation may dissociate at temperatures above 1210C regenerating the isocyanate as indicated to the left of the equa tion. Thus, a polyisocyanate can be reacted with oxime to give a urethane that is stable at room temperature This blocked isocyanate could be dissolved in polyester polyol, acrylic polyol, or epoxy polyol to give a mixture with indefinite storage life at room temperature. Upon heating to 150-204 C, however, the urethane is "deblocked", regenerating the polyisocyanate, which would then react with the polyols to give a polyurethane. The deblocking temperature is dependent on the blocking agent and type of isocyanate. For example, a curing temperature of 3000F is frequently used in curing polyesters with polyisocyanates blocked with phenol or methyl-ethyl ketone Oxime. The lowest cure temperature of P21-OC may be achieved when HCN is used. Blocking compounds used in preparing the blocked isocyanate included phenol, m-cresol, diethyl malonate, ethyl acetoacetate, ethyl cyanoacetate, and caprolactam. The polyisocyanate compounds used in the binder of the present invention have a molecular weight of 150 to 500 per NCO group. Also the isocyanate has at least three reactive sites, NCO groups, per chain so that cross linking and thermosetting properties are established in the resulting polyurethane binder. The polyol compounds which are most readily used to form the binder are polyester, polyacrylic and polyepoxy. These are hydroxy terminated or branched compounds which react with the isocyanate to form the highly cross linked thermosetting polyurethane binder. Although epoxy is not a true polyol, it forms part of the group since through reaction between the secondary hydroxy branch sites, such as in a bisphenol-A epoxy and the isocyanate NCO reactive sites, the resulting compound provides a thermosetting polyurethane having properties identical or closely similar to those of the true polyols. In addition, it has been found that the polyol component should have a molecular weight that is within the range of 200 to 800 per hydroxy reactive site. Longer polymer chains that exceed a molecular weight of 800 per site have proved to be too soft and polyols with polymer chains having a molecular weight of less than 200 per site have been too brittle. Also the polyol compound must have two or more reactive hydroxy sites per chain. If less than two active sites exist per chain, the resulting polyurethane is thermoplastic rather than thermosetting since the required cross linking fails to develop. The equivalent weight value is the weight of the polyisocyanate or the polyol that contains one weight part of -NCO in the case of polyisocyanates, or one weight part of -OH in the case of polyol. The ratio of isocyanate to hydroxyl is one factor controlling the properties of a urethane coating. The physical and/or chemical effects of the NCO/OH value can be related to the type of molecular structure and the molecular weight of the coating. Theoretically, at an NCO/OH ratio of 1.0 the properties associated with molecular weight are optimized. In general, two component urethane coatings are formulated to have NCO/OH ratios in the range of 0.8 to 1.2. The coating further contains one or more additives which operate as a catalyst, dispersant and/or a surfactant and nonreactive solvents. Catalysts are used to promote the reaction between the isocyanate and polyol to reduce both the curing temperature and curing time. Both minimum cure temperature and shortened cure time are important to limit the possible impairment of the magnetic qualities of modern magnetic particles such as cobalt modified iron which are magnetically superior to their predecessors, but more prone to degradation when exposed to prolonged elevated temperatures. Catalysts used in this invention can be bases (tertiary amine) type catalysts, such as 2,4,6-tris (dimethylaminomethyl3 phenol, commercially known as DMP 30; triethylamine, N,N-dimethylaminoethanol; 1,4-diazabicyclo (2,2,2) octane; bis (2-dimethylaminoethyl) ether amine or other tertiary amine adducts or metallic compounds, such as stannous octoate; lead octoate, potassium octoate dibutyltin dilaurate or other organo metallic compounds, as well as a combination of tertiary amines with metallic compounds. The dispersing agent additive serves to chelate the magnetic particles such that they are individually suspended within the uncured binder to permit the maximum orientation in the desired tangential direction along the disk surface after application of the ink or coating mixture wherein the magnetic particles are suspended. It is the effectiveness of the dispersing agent in maintaining the separation of magnetic particles, which have a natural attraction for one another, that determines the shelf life of the coating mixture. Since the chemical reaction between isocyanate and polyol can easily be prevented through limiting the temperature to which the coating material is exposed, the real determinant of shelf life is the ability of the coating mixture to produce a high orientation ratio in the ultimately coated disk. One dispersing agent, MULTRON R-221-75 (Mobay) is not only effective as a dispersant additive, but being a polyester polyol also reacts with the isocyanate to form a part of the cross linked polyurethane binder compound following the curing stage of the process. This polyester compound is an effective dispersing agent because of the carboxy group side chain which provides the chelating function. When this polyester compound is used as the polyol which is reacted with the isocyanate, it is unnecessary to provide a separate dispersing agent. Organic and inorganic compounds that contain phosphite or phosphate groups are also effective as dispersant additives. Examples are: titanate coupling agents (Kenrich Petrochemicals), commercially known as KR-112S, KR-212, KR-385, KR-55 and epoxy phosphate esters; the reaction products of epoxy resins and phospholic acid. The surfactant provides wetting, leveling and flow-control functions through reduced surface tension that enables smooth, defect free coatings and makes possible the high concentration of magnetic particles within the binder mixture. To achieve an optimum magnetic coating it is necessary that the magnetic particles achieve a micro scale type dispersion within the binder. Ideally, each particle should be so dispersed as to be independently retained within the binder. This would enable maximum orientation of the particles in the desired direction. Use of the MULTRON R221 polyester disperser identified above provides a super dispersion that can be maintained for at least 6 to 8 weeks and is effective to maintain dispersion through the coating process. Though there are several other coating techniques such as spray coating and roll coating it is probably most common to spin coat the disk substrate. The particles must maintain both the dispersion and a uniform distribution of particles within the coating from the inner to outer coated area. That is, there must be no centrifugal action that separates binder and particles to increase the particle density in the outer area of the disk. In addition the binder must provide sufficient freedom and mobility to the particles to permit orientation from a radial position imparted during the spin coating to the desired tangential position in the finished disk. Also, after the particles have been oriented by a magnetic field, the binder must restrict mobility until the particles are immobilized in the cross linked matrix by the curing step. Several types of volatile, nonreactive solvents can be used in the production of the present composition to reduce the solids content and enable very thin coatings to be applied to the substrate. Examples of various classes of solvents which may be added to the resins and the particulate dispersions are: aromatic solvents - Toluene, xylene, ethyl benzene; Ketones - methyl ethyl ketone, isophorone, methyl amyl ketone, ethyl amyl ketone, methyl isobutyl ketone; and acetate - ethyl acetate, cellosolve acetate carbitol acetate, butyl acetate, butyl cellosolve acetate. The preferred solvents employed in the composition are 60 parts by weight isophorone, 20 parts xylene and 20 parts methyl amyl ketone. It should be noted that the above exemplary list is not exclusive and that other nonreactive solvents may be employed. Using the coating of this invention it is possible to cure the coated and oriented disk by applying a l770C temperature for 45 minutes rather than a much longer time and higher temperature used with prior coatings. Following the curing of the coated disk, the coating must be tolerant of the subsequent process steps of buffing, washing and lubrication. The coating must permit buffing to a smooth surface without having particles torn from the surface. Washing may be done using water detergents or solvents such as isopropyl alcohol. Finally, the surface must adequately retain lubricant to facilitate start-stop action between the coating surface and a cooperating read/write head. Using the coating described above the solvent evaporates prior to the chemical reaction with the reaction and deblocking occurring simultaneously. The surfactant allows the gas bubbles which are initially entrapped and which subsequently escape to form a structure which retains lubricant that is not spun away during disk use. The following are typical examples of the coatings of the present invention wherein the principal coating components and the additives have the weight portions indicated exclusive of associated solvents. The solvent content shown, unless otherwise indicated is the composite solvent concentration including that present in association with the polyisocyanate and polyol components. Examples 1 and 2 show the isocyanate in combination with an epoxy and differing dispersants while Examples 3 and 4 illustrate binder compositions employing different types of polyols. Using a bisphenol-A epoxy, the formation of an oxazolidon ring gives a coating with flexibility and high temperature resistance, but requires high temperature baking (above 2320C) with a special catalyst. The coatings of the present invention are formed by reacting isocyanate with secondary hydroxy. The higher the molecular weight of the epoxy resin, the more hydroxy sites and the harder the resulting coating. Example 1 Parts by weight 42% Ketoxime-blocked aromatic/aliphatic polyisocyanate/isocyanurate copolymer 60.7 Mondur HCB (Mobay) in 70/30 ethylglycol acetate/Xylene solvent 50% Bisphenol-A epoxy resin DER 667 (Dow) 64.9 in 60/20/20 1 sophorone/Xylene/Methyl Amyl Ketone solvent Unsaturated polycarboxylic acid dispersant 0.6 BYK P 104S (Mallinckrodt) Fluorocarbon surfactant 0.1 FC-430 (3M) Organo metal catalyst 0.3 Stannous octoate T-9 (M & Chem) Magnetic cobalt modified iron oxide 100 particles (Pfizer EX 2560) Isophorone solvent 95 Xylene solvent 14.2 Methyl Amyl Ketone solvent 14.2 Referring to the figure, the coating composition of Example 1 is prepared as follows: 1. The magnetic particle dispersion is first prepared by adding 100 parts of cobalt doped magnetic iron oxide particles to 0.6 parts dispersant and a portion of the solvent including 57 parts isophorone, 14.2 parts Xylene and 14.2 parts methyl amyl ketone. This mixture is stirred for a period of 1 to 2 hours. The entire mixture is milled using conventional milling techniques such as ball mill or media mill until free of agglomerates. 2. Thereafter, 64.9 parts of 50% DER 667 mixed with 60.7 parts of 42% Mondur HCB in 19 parts isophorone solvent; added to the mixture of step 1 above and milled. 3. The 0.3 parts catalyst and 0.1 parts surfactant with 19 parts isophorone solvent are added to adjust the final viscosity of the total mixture to approximately 40 CPS at 250C as measured by a Ferranti-Shirley viscosimeter. After adjusting ink viscosity, the mixture is put through a 2 to 8 micrometer absolute filter to remove any possible debris or contaminants. The compositions of Examples 2, 3 and 4 are prepared in a similar fashion. Example 2 Parts by weight Ketoxime blocked polyisocyanate 42.8 Mondur HCB (Mobay) Bisphenol-A epoxy resin 47.5 Epon 1007 (Shell) Polyester polyol 8.1 Multron R-221 (Mobay) Titanate coupling agent 1.6 KR-385 (Kenrich) Fluorocarbon surfactant 0.15 FC 430 (3M) Organo metal catalyst 0.5 Stannous octoate T-9 (M & Chem) Magnetic Particles 180 Solvent 420 Isophorone/Xylene/Methyl Amyl Ketone 60:20:20 The use of polyester systems in urethane coatings involves some trade offs due to limited high functionality of available polyester resins. Unlike epoxy and acrylic resins, the hydroxyl polyesters are terminated at the chain end. The increased weight of polyester resins results in an improvement in coatability and particle dispersability, but decreases coating hardness. To improve the hardness of this type of coating, it is necessary to incorporate a high functionality, rigid polymer segments such as styrene-allyl alcohol of which RJI0l (Monsanto) is an example. Coating hardness is needed, not only for the wear properties of withstanding repeated head loadings, but also for buffing and washing processes. Example 3 Parts by weight Ketoxime blocked polyisocyanate 35.8 Mondur HCB (Mobay) Polyester polyol 20 Multron R-221 (Mobay) Polyacrylic polyol 20 RJlil (Monsanto) Fluorocarbon surfactant 0.3 FC 430 (3M) Organo metal catalyst O.6 Stannous octoate T-9 (M & Chem) Magnetic particles 100 Nonreactant solvent 128.31 Isophorone/Xylene/Methyl Amyl Ketone 60:20:20 Isophorone 24.45 The thermosetting acrylics exist in both the acid and hydroxy types. The hydroxy material is of interest in the formulation of the present urethane-acrylic coatings. Unlike polyester, hydroxy acrylic has a reactive site at the side chain. Example 4 Parts by weight Ketoxime blocked polyisocyanate 61 Mondur HCB (Mobay) Polyacrylic polyol 132 AT 400 (Rohm & Haas) Polyester polyol dispersant 6.1 Multron R-221 (Mobay) Fluorocarbon surfactant 0.1 FC 430 (3M) Organo metal catalyst 1.0 Stannous octoate T-9 (M & Chem) Magnetic particles 300 Solvent 400 Isophorone/Xylene/Methyl/Amyl Ketone 60:20:20 The coating composition prepared according to the teaching of the invention is applied to an aluminum disk substrate using conventional spin coating techniques. The magnetizable particles are then oriented circumferentially of the disk surface using the well established magnetic field orientation practice following which the coating is cured at an elevated temperature of about 1770C for approximately 45 minutes. After cooling, the disk is polished to produce a super finished surface with an arithmetic average roughness of 2.5.10 3 micron to 10.10 3 micron. The polished disk is then washed using deionized (DI) water and isopropyl alcohol to remove any buffing debris and the finished disk surface is lubricated using a fluorocarbon type lubricant prior to magnetic test. On a disk thus prepared, an 8 MHz (megahertz) signal was recorded using a magnesium ferrite head with a 55 mA recording current and a 3200 rpm rotational speed. The reproduction output was 600 microvolt at the outer periphery and 500 microvolt at the inner periphery. Although this invention has been described with reference to specific examples, it will be apparent that various modifications may be made thereto which fall within the scope of this invention

Claims: CLAIMS 1. A magnetic recording coating composition having finely divided magnetic particles dispersed in a thermosetting resin binder composition applied as a thin film, having a thickness of less than 2 microns, closely adhered to a rigid non-magnetic substrate, said coating composition including 50 to 80 percent by weight of magnetic parti cles, 20 to 50 percent by weight of binder, said compo sition being characterized in that it comprises a blocked isocyanate having at least three reactive sites per chain; a polyol having at least two reactive sites per chain and a molecular weight of 200 to 800 per hydroxy site; and additives not exceeding 30 parts by weight. 2. A magnetic recording coating composition having finely divided magnetic particles dispersed in a thermosetting resin binder composition applied as a thin film, having a thickness of less than 2 microns, closely adhered to a rigid non-magnetic substrate wherein the coating compo sition includes 50 to 80 percent by weight of magnetic particles, 20 to 50 percent by weight of binder, said composition being characterized in that it comprises a blocked polyisocyanate with at least three reactive sites per chain and at least a polyol with at least two reactive sites per chain and a molecular weight of 200 to 800 per hydroxy site having a composite 100 parts by weight, the weight portions of polyisocyanate and polyol components being such that the equivalent weight ratio of NCO/OH is between 0.8 and 1.2 and additives not exceeding 30 parts by weight. 3 The magnetic recording coating composition of claim 2 wherein said polyol is selected from the group of poly ester polyol, polyacrylic polyol and polyepoxy polyol. 4. The magnetic recording coating composition of claim 1, 2 or 3 wherein said additives include an organometal catalyst and a surfactant. 5. The magnetic recording coating composition of claim 4 where said additives further include a dispersing agent. 6. The magnetic recording coating composition of claim 5 wherein said dispersing agent comprises a carboxy type polyester polyol containing a carboxy group side chain. 7. The magnetic recording coating composition of claim 5 wherein said dispersing agent includes organo phosphate ester or inorgano phosphate ester compounds. 8. The magnetic recording coating composition of claim 5 wherein said dispersing agent includes titanate coupling agents. 9. The magnetic recording coating composition of claim I wherein said blocked isocyanate has a molecular weight of 150 to 600 per NCO site. 10. The magnetic recording coating composition of claim 9 wherein said blocked isocyanate is one of an aromatic, an aliphatic or a combination aromatic and aliphatic. 11. The magnetic recording coating composition of claim 10 wherein said blocked isocyanate also includes a trimer ized polyisocyanate adduct. 12. The magnetic recording coating composition of claim 1 or 2 wherein said finely divided metal oxide particles comprise cobalt modified iron oxide particles. 13. The magnetic recording coating composition of claim 12 which further comprises 100 to 800 parts by weight of a nonreactive solvent. 14. The magnetic recording composition coating of claim 13 wherein said nonreactive solvent comprises an organo solvent compound that does not contain an active hydro gen site. 15. The magnetic recording medium of claim 3 wherein said polyol comprises a bisphenol-A epoxy containing a hydro xy group in one of the side and end chains.

Inventors: BLUMENTRITT, BRUCE FREDERICK, SHEN, JOHN CHEN SYAN
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1982
Main classification: G11B5
IPC codes: C09D175, C09D163, G11B5, C09D5