Testing the structure of magnetic paints with and without superimposed shear

The structure development in dispersions of magnetic barium ferrite particles in cyclohexanone with polyvinylchloride wetting resin was tested by oscillatory rheological measurements and orthogonal superposition of steady and oscillatory shear. The optimum dispersion is achieved at the resin concentration c=c0, which corresponds to a minimum in the viscoelastic modulii. At c<c0 the system is highly elastic, brittle (G′ drops sharply and G′′ goes through a strong maximum at high strain), thixotropic and its relaxation spectrum, H(τ), is dominated by long relaxation times, τ, which indicates the existence of a strong network. At c>c0 the system becomes less elastic, more flexible, less thixotropic and H(τ) is dominated by short τ, which indicates that the structure formation in this case is dominated by small aggregates rather than a network. By superimposing steady and oscillatory shear we show that the steady shear dramatically break the structure and shifts H(τ) to very short τ.