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

Table I. Magnetic Coating Composition 200 parts* red iron oxide IRN-110 40 parts 40-60 SS type cellulose nitrate 17 parts butyl alcohol 1 20 parts cellosolve 906 parts amyl acetate (commercial grade) 1 36 parts butyl cellosolve 8 parts AA grade castor oil * Parts by weight. tically nonsolvents for the base materials. 8. The amounts of the various ingredients of the formula must be adjusted so that a coating of proper width, thickness and oxide content is obtained. The type of formula which we have found best to meet these requirements is based upon the use of a red iron oxide dispersed in cellulose nitrate as the binding material. The complete composition* is given in Table I. The iron oxide is manufactured by the C. K. Williams Co. The castor oil is Baker's grade AA. The binder, 40-60 SS type cellulose nitrate, is obtained from the Hercules Powder Co. It is delivered wet with butyl alcohol (70% nitrate, 30% alcohol). The butyl alcohol indicated in the formula is the amount that is introduced with the nitrate. Dispersing Method The sound quality of magnetic sound tracks is greatly influenced by the degree of dispersion of the iron oxide particles. Particle flocculation causes increased noise levels and lower signal-tonoise ratios. Consequently, the magnetic-coating composition must be subjected to a dispersing treatment in order to obtain adequate separation of the iron oxide particles. Of the many dispersing methods tried, ball milling has been most effective. Porcelain mills with flint balls have been used. Our procedure for preparing a dispersion is as follows: The cellulose * The Eastman Kodak Company does not intend to sell the coating composition. Table II. Stock Solution for Cellulos* Nitrate Binder 40-60 SS type cellulose nitrate Butyl alcohol Cellosolve Amyl acetate (commercial) 10% 4.3?$ 30% 55.7% nitrate binder is stocked in solution font; using part of the solvent of the dispersior formula. The stock solution which 3j used by Eastman Kodak Co. is given icl Table II. Many other stock solutions are possible using different amounts of solvents The various constituents of the coating^ composition are put into a ball rnillJ The order of addition of ingredients iq of no importance. The ball mill is then] rotated until a satisfactory dispersion is] obtained. The milling time depends.) among other things, on the viscosity ol the mixtures and the size of the mill. For making tracks of 0.100-in. width, one-gallon mills at 60 rpm and 75 F give good dispersions in 24 hr. Milling up to 1 50 hr shows no significant change in sound quality. Very long milling gives poorer sound quality. With quart mills, the same dispersion at the same rpm requires a minimum of 60 hr of milling. The coating composition for 0.100-in. width tracks has a shelf life of at least thirty days. In that time, there is npi| appreciable change in sound quality ofil the coated tracks. Tests are not com j pleted for holding times beyond thirty j days. Because of oxide particles settling " out, all dispersions should be tumbled forji at least 30 min before coating. Narrower J tracks are made from dispersions of ; higher viscosity. The higher viscosity! dispersions have longer shelf lives. 1 Work is now in progress to improve the J dispersions and the keeping properties! of the dispersions. Coating Methods The method used for applying the] magnetic dispersion to film must provide I 492 April 1953 Journal of the SMPTE Vol. 60