Dr. H. BAUER - RCA Laboratories Princeton, N. J.

DR. HERBERT BAUER is a member of RCA Laboratories where he is conducting research on organic materials related to electronic applications. He is at present responsible for the Laboratories Computer and Video Magnetic Tape Program.

He joined RCA in 1960 after four years with DuPont's Photo Products Department where he was engaged in polymer chemistry research. Dr. Bauer received his PhD in chemistry from the University of Vienna in 1951. After a year as a research assistant at the University of Graz, Austria, he was awarded the National Research Council of Canada Postdoctoral Fellowship.

He joined the staff of the National Research Council in 1953 to conduct research in carbohydrate chemistry. In 1961, Dr. Bauer received an RCA Laboratories "Achievement Award" for research in non-silver halide photographic systems. He is a member of the American Chemical Society, the Austrian Chemical Society, the Chemical Institute of Canada, and Sigma Xi.
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Although magnetic tapes are vital for the operation of computers and video recording systems, they form a weak link in the chain of highly sophisticated equipment. The design of hardware has progressed at a far higher rate than that of the tape, leaving the machines dependent on a marginal product. The evolutionary process of tape development is a slow one, the art is known only to a few, the approaches are controversial - will this change in the future? In support of RCA's substantial business in the television and computer field, as well as in audio products (including audio tape) there is in RCA Laboratories a continued research effort on magnetic tapes. This paper discusses the state of the art of magnetic tape for high-speed applications and some of the problems associated in developing products for future needs.

Only twenty years have passed since the first reels of magnetic tape came off a production line (in Germany) and already industry-wide sales are approaching the $100 million mark. This growth appears even more remarkable for a product, that has not significantly changed since its conception.

What has changed in these twenty years, is the rapidly growing number of applications for magnetic tapes. Today these new applications account for three quarters of the total tape market; for the next few years they promise a 15% to 20% annual growth of the market. In essentially all of these new applications the magnetic tape is used at "high speeds", referring in this case to the relative speed between the magnetic tape and the tape head, regardless of whether the tape slides over a stationary head or the head is moved over a stationary tape.

Most tape applications for data processing and for the storage of video information fall into this category, while the low-speed market is divided between the traditional audio recording field and some special instrumentation applications. There is no sharp separation between the two areas, since in addition to tape speed other operating conditions also influence the stresses on the tape is subjected.

The actual tape speeds in relation to an assumed stationary head range in most modern computer tape stations from 75 to 150 inches per second. They reach 1,600 inches per second or approximately 75 miles per hour in commercial video tape recorders, and this figure is often exceeded in special applications.

Whereas the high tape speeds in computer tape stations are essential in reducing access time to a particular piece of information stored on the tape, the tape speeds in video recorders are dictated by the high frequency of the signals, i.e., by the information density per unit time which has to be recorded.

An advanced commercial computer tape at present is able to store 900 coded characters per square inch and 600,000 per cubic inch of tape.

This means that the information content of approximately 150 single-spaced typewritten pages can be stored in 1 cubic inch of tape.

Furthermore, the 600,000 characters may be recorded or read in less than 10 seconds. A reel of tape priced at $50 may store up to 14 million characters, or 3,500 typewritten pages, at a cost of $3.50 per million characters.

New computer tapes sold by RCA are inspected, repaired and guaranteed to be 100% dropout or error-free. Depending on the conditions of use, they will start to show signs of wear and to cause errors after 10,000 passes over the heads; this point may be reached in some rare applications after a few hours; in others, it will last for years.

The performance of magnetic tape in storing pictorial information is not quite as impressive. Assuming approximately equal picture quality or signal to noise ratio, video magnetic tape is certainly surpassed by photographic film in the storage capacity per unit area; the storage capacity per unit volume of the two media is at present about equal. However, the immediate availability of the stored information and the ease of copying outweigh other factors.

Most difficulties encountered in the present use of magnetic tapes at high speeds can be traced to the unavoidable sliding contact between the tape and the magnetic heads. As yet, no practical solution avoids this close mechanical coupling between tape and tape head.

The necessity for intimate tape-to-head contact arises from the need for short-wavelength recording and the shallow penetration depth of the magnetic impulse into the coating. In a computer tape station, a drop-out or error signal will result from any imperfection in the tape coating which lifts the tape momentarily more than 0.001 inch away from the head, and the computing operation will be stopped immediately.

In video recording systems, where the shortest wavelength on tape is in the order of 0.0002 inch, a separation of the tape from the head by only 0.00005 inch will reduce the recorded signal by approximately 10db. The signal loss will be the same, whether the separation is caused by an irregularity inherent in the tape surface or by any additional layer interposed between the tape and the head.

Problems of friction can rather easily be dealt with in mechanical systems where a selection of structural materials and suitable liquid lubricants is available. They become of a different magnitude, however, when the material composition is dictated by other requirements such as having specific magnetic or electrical properties.

The magnitude of the abrasive forces becomes apparent, if the speeds involved and the lack of lubricants are considered. The abrasion problem is further magnified by the additional, high accelerative forces to which the tape is subjected in certain computer operations. The tape is alternately started and accelerated to its full speed in 4 to 5 msec and stopped again at the same rate, often many times in a single second.

The exerted accelerative forces may amount to 150 times the earth's gravitational force and repeatedly extend and relax the whole tape structure. The heat of friction raises the temperature of the tape surface to 100 to 230°F, and has to be dissipated without affecting other properties.

From the above it can be concluded that the main mechanical properties which determine the suitability of a magnetic tape for high-speed uses are those connected with abrasion resistance and surface structure.

On the one hand, the surface of the tape is required to be of almost optical perfection; the surface has to permit a continuous contact of all parts of the tape with the tape head.

On the other hand, a slightly grainy structure of the tape surface is desirable to avoid the tremendous increase in adhesive forces connected with ultrasmooth surfaces.

The magnetic requirements for a computer and video tape do not differ considerably from those of a good quality audio tape. They would, by themselves, be easily attained. Maintaining the difficult balance between output and wavelength response, and wear and surface characteristics will be further discussed when the magnetic material dispersion problems are considered.

The build-up and dissipation of electrostatic charges resulting from sliding the tape over guides and heads presents another problem in some magnetic tape coatings. In computer tape stations with weighing bins, i.e., where the tape is tension-less and free to follow electrostatic attractions, the problem is particularly acute. Magnetic materials, polymeric binders, and support films are high resistivity materials, and ways and means for increasing the conductivity of the finished tape have to be found.

Most of the remaining mechanical properties of high-speed tapes are also intimately linked with the wear performance. High strength of the base material, high flexibility, and elastic recovery of the coating, good adhesion of the coating to the base film, hardness of the coating to prevent deformation and embedding of foreign materials on winding, are just a few. Abrasion products should be absent and if they occur they should be powdery for easy removal. Close tolerances in tape width, straight-ness and smoothness of the slit edges are also of great importance.

A cross-section through a common computer tape (Fig. 1) shows three layers:

the support film,
the magnetic coating, and often
a subcoating

to provide adhesion between them. Magnetic tape coatings consist of a dispersion of a magnetic material, usually small particles of gamma iron oxide in a polymeric binder.

Nitrocellulose and later polyvinyl chloride (PVC) co-polymers have served for a long time as the binder materials in the magnetic tape industry.

PVC as a linear polymer is readily dissolved in common solvents, its high polarity and easy modification by copolymerization make it an excellent material for dispersing oxidic pigments; fair adhesion to common base films is also obtained with it.

Most vinylchloride polymers, however, are hard and brittle, and shrink considerably during solvent removal. In order to make the coatings pliable and to reduce the high shrinkage, high concentrations of plasticizers are incorporated; this renders the coatings soft and low melting which in turn makes them vulnerable to abrasion.

Although the "art" of formulation has optimized thermoplastic coatings, at best high quality audio tapes and perhaps marginal computer tapes have resulted.

The mechanical properties of polymers can be greatly improved by cross-linking linear polymers to form three-dimensional networks. In this manner, materials are made available which range from highly resilient rubbers to hard, yet tough enamels. At present, crosslinked polymers are used in only a few commercial high-speed tapes.

Surveying the large number of available crosslinked polymers, materials that show elastomeric properties appear promising as magnetic tape binders. The three-dimensional regular network structures of elastomers with uniformly distributed crosslinks permit a uniform distribution of applied tensile stresses which gives these materials superior strength characteristics paired with high elongations (Fig. 2). Materials with outstanding properties belong to the class of polyurethane elastomers. They have excellent abrasion resistance and, in fact, have found such diverse uses as covering the leading edges of aircraft wings and heel lifts for ladies' shoes.

The designation "polyurethanes" refers to a class of high-molecular-weight compounds characterized by the recurrence of "urethane" linkages at regular intervals along the polymer chain. The "urethane" linkage is generally formed by the reaction of a difunctional isocyanate with a difunctional alcohol (Fig. 3). While aromatic diisocyanates (such as tolylene diisocyanate) are almost exclusively used for the isocyanate portion, the diol portion can be selected from a wide variety of available materials. This permits an adjustment of the desired physical properties in the final polymer.

To obtain useful products, the diol portion of the polyurethane generally comprises a linear polymer which may be either a polyester or a polyether (R2 in Fig. 3). The molecular weight of the polyol, its chemical nature, the number of methylene or other groups between ester or ether linkages, all determine whether the final product will be a soft elastomer or a hard, impact resistant solid.

Further variation of the polymer properties can be obtained by the nature and number of crosslinks introduced. The crosslinking reaction is usually initiated after the linear urethane pre-polymer has reached a predetermined length.

Polyfunctional alcohols, amines, or water vapor may be used as crosslinking agents. By selection of the proper compound, again the chemist has the possibility to tailor specifically the properties of the final product to his needs.

The degree of dispersion of the magnetic material in the coating not only determines the magnetic characteristics of the tape, but also has a strong influence on the mechanical properties of the tape and on the structure of the tape surface.

An idealized model (Fig. 4) would consist of a tightly packed, well aligned, uniform arrangement of the magnetic particles, each one completely surrounded and bonded by a layer of the binder polymer. In such an arrangement, the magnetic output and resolution would be optimized for the material in question, the magnetic material would act as a reinforcing filler for the polymer and the surface would be flawless and mirror smooth.

In practice, this idealized model is never accomplished. The difficulties arise from four factors:

pigment volume concentration,
particle size,
particle shape, and the
inability of many binder polymers

to wet the oxidic material. In comparison with a high grade enamel
the pigment loading (i.e.7 the ratio of magnetic material to binder) is almost doubled (Fig. 5).

This high content of a filler medium increases the number of voids in the coating due to insufficient binder to fill completely all interstices.

This problem is magnified by both the extremely small particle size and the high acicularity of the magnetic material. Furthermore, binder polymers belonging to the class of urethane elastomers lack almost completely any affinity to the highly polar surfaces of the magnetic material.

As a consequence, many magnetic coatings are porous and contain particle agglomerates. This manifests itself in low cohesive strength and rough surfaces. Research on these deficiencies involved thorough evaluation of the surface chemistry and resulted in a considerable improvement (Fig. 6).

Since crosslinked polymers are insoluble, the usual step of dispersing the magnetic material in the polymer solution is not applicable. The difficulty is overcome by dispersing the magnetic material in prepolymers, short linear sections of polymeric compounds which after coating are chemically linked to each other in the crosslinking or curing reaction. The chemical reaction takes place as soon as the reactive species are combined.

Therefore, the addition process and coating operating have to be closely controlled. The curing reaction is generally accelerated by elevated temperatures, and has to be carried out over extended periods. Ovens have to be designed for the handling of long tape lengths in a continuous process.

The application of very thin layers of highly viscous, thixotropic and unstable dispersions represents a major problem in coating technology.

Coating thickness variations should not exceed 10%. This means that the wet coating thickness has to be maintained within 10% of 0.0006 inch, if a uniform dry coating thickness of 0.0002 inch is required with a formulation having a 3:1 dry-down ratio.

Variations in the thickness and flatness of the base film will necessitate the use of coating techniques which provide automatic compensation.

The complete elimination of all foreign materials such as dust, dirt or gas inclusions is technically not feasible. From presently available data, one can conclude that it is not possible to produce even single reels of high-quality computer tape completely free of dropout-causing imperfections without resorting to some kind of a surface finishing technique such as polishing, calendering, or time consuming hand repair.

At present, one can count an average of 20 to 40 dropouts on computer tapes manufactured under the most carefully controlled conditions.
These defects must be hand repaired before the tape is sold to the ultimate user. Further improvements in process technology will reduce the number of defects to a more acceptable level.

Already, experimental computer tapes and tape stations with storage capacities of 1,500 to 2,000 characters per square inch, 1.5 to 2.0 million per cubic inch, and 25 to 35 million per reel are being designed.

Tape coating thickness is being reduced to 0.0001 inch. Start-stop times of 1.5 msec are being approached. The tape will be enclosed in hermetically sealed cartridges to keep out dirt and dust and to eliminate manual handling. Electronic techniques for reducing the sensitivity towards tape defects are being explored. The cost of tape and tape handling equipment will rise exponentially, but it will be justified by an equal rise in performance.

Research towards improved magnetic tapes for high-speed applications requires a peculiar blend of scientific and technological disciplines.

The successful solution of the outlined problems will depend on the close cooperation between the polymer and surface chemists, the rheologists and the chemical engineers. Close contact should be maintained at all times with the design engineers responsible for future tape systems.

New concepts such as air flotation of the tape at the heads may serve as an interim solution to the wear problem. New magnetic materials with higher magnetic moments and with improved surface characteristics should help reduce some of the other problems. A better understanding of the influences of chemical structure on the mechanical properties of polymers should also result in coatings defying abrasive forces more effectively.

The successful evolution of future computer and video tapes will largely hinge on the application of fundamental knowledge gained in studying the materials problems outlined in this paper.

Dr. N. E. Wolff and Mr. S. M. Bennet have contributed significantly to the magnetic tape research efforts at RCA Laboratories. Dr. N. E. Wolff's helpful suggestions concerning this paper are gratefully acknowledged.

1. Based on RCA-EDP and DuPont Film Dept. estimates.
2. F. Winkel, Technik der Magnetspeicher Springer-Verlag 1960.
3. J. H. Saunders and K. C. Frisch, Poly-urethanes, Chemistry and Technology, Interscience Publishers, 1962.

Fig. 2-A uniformly crosslinked polymer network obtains its strength from the even distribution of the applied forces over a large number of chemical bonds.
Fig. 3-Formation of linear polyurethanes by reaction of diisocyanates with diols.
Fig. 4-Model of an ideally oriented and packed magnetic coating.-Average particle size of magnetic material.
Fig. 5-Comparison of pigment loadings in an enamel and a magnetic tape coating.
Fig. 6-Surfaces of magnetic tape coatings, magnification 170x: (top) poor dispersion; (bottom) good dispersion.