Journal of the Society of Motion Picture and Television Engineers (1950-1954)

factors, and long term storage of di-acetate are not completely satisfactory, several new products are under study. One of these, Du Font's "Mylar," a true plastic with vastly superior strength and stability characteristics is a step in the right direction. This product is now available on a limited basis for armed service use, and magnetic tape made using Mylar base has passed preliminary studies for several special army applications. Pigment The iron oxide used is precipitated out of solution, then subjected to a complicated heat treating cycle to produce controlled and desired magnetic properties. The particle size, approximately \ micron, is very closely controlled to produce an end product that is interchangeable with equivalent products of other manufacturers, and the size uniformity of particles is a very special chemical achievement. Coating Mixture The coating mixture is a magnetic paint, produced from the mixture of iron oxide pigment, a binder mixture and suitable solvents, which have no effect on the base composition. Its function is to hold the magnetic particles firmly in place on the base, with each pigment particle separate from the others. Since the pigment, or iron oxide, is the active ingredient, a high loading in respect to the binder is required. The process of dispersing the pigment in the carrier, or binder, is one of the most difficult from a manufacturing viewpoint and the most important of all production steps. The pigment particles are smaller than those used for paint and, in addition, are highly magnetic, therefore the tendency is to form agglomerates. Steel ball mills, where tremendous sheer stresses are generated by cascading hardened balls, are the best known apparatus for achieving good dispersion. Incomplete dispersion fails to produce the proper mag netic performance, impairs the higl frequency response, and results in overall sensitivity. Application of Coating Mixture to the Base All Reeves Soundcraft tape is coate on specially designed machines on whic the motion of the base through the ma chine rivals the steadiness of a good me tion picture sound recorder. The coat i; ing is done by constant-head hoppei over a rigid support, not over a roller a was the German product. The plasti base receives a precoat before the pig ment coating to provide good anchorag i of the magnetic coating to the base. B I achieving anchorage through the use of ;« precoat, no compromise with the besj all-around physical properties of the coat) ing material is necessary. The precoaf also serves as an adequate barrier layej to minimize curl caused by different moisture absorption rates between th<| base and the coating; but, most imporj tant, it provides an ideal anchorage surf face on the base for the previously men 3 tioned high pigment loaded binder! The coating machine is also equipped with a continuous magnetic monitor i This allows constant checking of trans \ verse and longitudinal uniformity of elec-J trical output during coating. Thisi equipment has a special recording head with a very wide gap, and monitors through the base side before the tape if \ completely dry. The wide-gap record^ head and the base side positioning per-j mit the measurement of actual sensitivity i of the tape, not the thickness of the coat-< ing. This equipment is virtually in-l sensitive to small variations in spacing) between the head and the coating and soj is not affected by variation in base) thickness. Polish and Lubrication Soundcraft tape is surface polished, by) high-speed nylon brushes. Also at this} station the surface is lubricated to mini j mize head wear. This process puts the i 454 April 1953 Journal of the SMPTE Vol. 60