The present invention relates to a process for the preparation of a magnetic dispersion for a magnetic coating with magnetic particles having a coercive force of more than 120 kA/m, a relative residual induction Mr/Mm of at least about 0.9 and an orientation ratio of at least 2.7 by means of a single-shaft blade-type kneading apparatus, the shaft rotating in a housing having a housing wall and at the same time oscillating in the axial direction for kneading a mixture of magnetic particles in a thermoplastic binder and a solvent, and the use of an apparatus suitable for the process and a magnetic recording medium produced by the process.
DE-A-39 43 340 discloses a process for the preparation of magnetic coating compositions, a magnetic dispersion having a solids content of from 65 to 95% being kneaded in the kneading zones of a single continuous twin-shaft blade-type kneading apparatus and the gap widths in the dilution/kneading region being smaller than in the kneading zones. An abrupt change in the gap width thus occurs between the kneading zone and the dilution region. In this known process, the circumferential velocity of the blades is said to be within the range from 1 to 50, preferably from 2 to 20, cm/s. Relative residual inductions of from 0.84 to 0.89 are thus achieved in the magnetic tape, in combination with coercive forces of from 644 to 650 oerstedts, corresponding to from 51.2 to 51.7 kA/m, which is completely inadequate for recording media having a high storage density.
In the process of DE-A-39 43 340, an optimum gap width in the kneading zone must be determined for each product by experiments, since otherwise a lower throughput occurs. In addition, operation will always be outside the optimum range owing to unavoidable wear.
It is an object of the present invention to provide an improved process which permits the economical preparation of relatively large amounts of magnetic dispersions which lead to magnetic coatings having better magnetic values and magnetic properties.
We have found that this object is achieved by the features of claim 1 if
a) the radial shear gradient in the gap between the tip of the kneading blade and the housing wall is from about 150 to about 1600, in particular from about 500 to about 1400, l/sec and
b) the circumferential velocity of the kneading blade is from about 20 to about 80, in particular from about 35 to about 60, cm/sec.
The latter is in any case outside the preferred range of DE-A-3 943 340 and also partly well above the maximum value of 50 cm/sec stated there.
In practice, experiments with a twin-screw extruder according to DE-A-3 943 340 with a high shear gradient did not give better magnetic values in the finished magnetic tape.
It was not until, in the single-shaft blade-type kneading apparatus, the further feature, the radial shear gradient between blade circumference and housing wall, was set in the range from about 150 to about 1600, in particular from about 500 to about 1400, l/sec, that particularly good magnetic values and electroacoustic values were obtained for the finished coating, in combination with substantially improved cost-efficiency of the process.
With the single-shaft blade-type kneading apparatus used for the novel process, it is possible to knead all products without modification of the kneading zone. There is no additional testing to discover optimum gaps and lengths of the kneading zone. The development times of a kneaded product are shortened.
Compared with the use of twin-shaft blade-type kneaders, the use of single-shaft blade-type kneaders for kneading magnetic pastes is surprisingly advantageous since, in the former, excessively high temperatures occur in the intermeshing zone between two pairs of blades, in particular when the circumferential velocities of the blades are increased. The high temperatures lead to concentration differences and separation in the kneaded paste and may lead to agglomeration under pressure which in turn is reflected in a nonuniform distribution of magnetic particles in the dispersion and hence in poor magnetic values in the finished magnetic recording medium.
It has proven advantageous if the shearing time below a kneading blade S1-S3 is from about 1/100 to about 5/1000 sec, which can be realized by a high volume flow rate and leads to high productivity. The kneading factor is advantageously from about 2 to about 80, in particular from about 10 to about 80. The kneading factor is dependent on the shear volume in the kneading zone, on the volume flow rate, on the residence time and on the speed (to be explained below) and is thus a characteristic for the throughput of the apparatus used in the process.
The shearing space, i.e. the volume over the length of the kneading zone of the apparatus, should, according to the invention, have a high degree of filling close to 100%, in particular from about 80 to about 100%.
This in turn ensures a high product throughput in combination with excellent magnetic values of the magnetic dispersion produced.
It has furthermore proven advantageous if the ratio of the radial to the axial shear gradient is less than 3.0, in particular from about 1.5 to 2.1.
It is also advantageous if the ratio of the axial to the radial shear surface area is from about 2 to 4. According to the invention, these ratios have an advantageous effect by improving the magnetic values.
The lengths of the individual kneading zones G, H in the apparatus also play a role with respect to the process. It has proven advantageous if the length of the kneading zone corresponds at least to eight times the diameter of the kneading blades.
It is also advantageous if the gaps between the tip of the kneading blade S1-S3 and the closest housing walls has a width of from about 0.007 to about 0.03 times of the external kneading blade diameter.
The use of a single-shaft blade-type kneading apparatus with a housing in which the blade shaft rotates and exerts shear stresses between blade circumference and housing wall and in the axial gaps and in which the blade shaft makes an oscillating movement O in the axial direction is regarded as very advantageous according to the invention, in which kneading apparatus
a) the radial shear gradient in the gap between blade circumference and housing wall is from about 150 to about 1600, in particular from about 500 to about 1400, l/sec and
b) the circumferential velocity of the kneading blade is from about 20 to about 80, in particular from about 35 to about 60, cm/sec.
The special requirements with respect to the preparation of magnetic dispersions from metal pigments having a relatively high coercive force and high relative residual induction under conditions which are economically substantially more advantageous than with the known twin-blade kneaders are thus met, with better tape characteristics of the magnetic tapes produced from such magnetic dispersions for the data and video signal recording sector with high and very high recording densities.
A metal pigment-containing magnetic recording medium produced with a magnetic coating according to the novel process features and comprising a magnetic coating having a coercive force of xe2x89xa7about 130 kA/m, a relative residual induction Mr/Mm of from about 0.9 to 0.92, an orientation ratio of about 2.7 and, after calendering at 70xc2x0 C. and 2350 N, with a carrier signal/noise signal ratio C/N deviation of  greater than xe2x88x922 dB, a 3.5 MHz signal playback level deviation of from +1.0 to 1.7 dB as a characteristic of the success signal of dispersion of the kneading process and a 35.5 MHz signal playback level deviation of  greater than xe2x88x922 dB as a characteristic of the surface quality of the magnetic recording medium, the deviation measurements [dB] each being based on the metal pigment reference magnetic tape C1A-DB from Sony Corporation, has surprisingly good recording and playback properties.
The invention is described below with reference to an exemplary embodiment shown schematically in the drawing.