Measurement of the critical pigment volume concentration of particulate magnetic tape

In this paper a relation is derived, which shows that above the critical pigment volume concentration the porosity of a tape, expressed as pore volume divided by the mass of the coating layer, is a linear function of the weight fraction of magnetic pigment. From this equation the CPVC and the density of the binder material can be obtained if the density of the pigment is known. The validity of the equation is demonstrated by measuring the porosity of magnetic tape as a function of pigment loading by means of mercury intrusion porosimetry. Further it was found that above the CPVC the magnetic saturation moment stays constant indicating a constant pigment volume fraction per unit of coating volume.