Dynamic viscosity of strontium ferrite–nylon composite below the melting temperature

Hard-magnetic 3D-printer filaments made of 40 wt. % SrO(Fe2O3)6/PA12 composites made using a twin-screw extruder are being studied to be used for Magnetic Field Assisted Additive Manufacturing (MFAAM). The time dependence of the magnetic properties above the softening temperature of the PA12 matrix but below the melting point was studied using a biaxial Vibrating Sample Magnetometer (VSM). Specifically, the rotation of the magnetic particles in the softened polymer matrix after the application of a rotation field was extracted from the time dependent biaxial VSM signals. Above 132 °C, the strontium ferrite particles can rotate in the nylon matrix. The measured time constant decreases with temperature and magnitude of the rotation field. Model calculations indicate that, for 40 wt. % SrO(Fe2O3)6/PA12 at rotation fields of 500 Oe, the effect of the demagnetizing field on the angle between the magnetic moment and the particle’s easy axis is negligible, allowing one to determine the dynamic “melt” viscosity from the measured transients. The dynamic viscosity decreases from 2 × 105 Pa s at 132 °C to 3.1 × 104 Pa s at 175 °C with a sharp kink observed near 140 °C that correlates with a relaxation from the α′c phase for PA12 observed by others. A yield shear stress was observed for small rotation fields at low temperatures resulting in non-perfect alignment of the magnetic particles. The implications of the measurement results for MFAAM are discussed.