Home Education Fillers for Polymer Applications
Induction heating of magnetite is very effective. An alternating magnetic field is applied and the losses incurred due to hysteresis manifest themselves as heat.
Because induction heating is tied to the magnetic properties of the material, it is only possible below the Curie point. So, in the case of magnetite, induction heating occurs up to ^580 °C and then the temperature levels out. Plastics, being neither magnetic nor electrically conductive, are not induction heatable. It has been shown that addition of magnetite particles as a functional filler imparts induction heatability to polymers and specifically to memory foams (Vialle 2009). As foams are thermally insulating, it is not a simple matter to heat them evenly. By adding magnetite, one can heat effectively throughout the shape memory foam triggering the shape change response. Adding magnetite filler to high-temperature plastic would enable induction-heatable plastic cookware.
The vast majority of plastics and minerals are electrical insulators. Magnetite, in contrast, has significant electrical conductivity and is considered a half metal (Chang et al. 2007). It is a semiconductor and has a small bandgap of just ~0.1 eV due to electron exchange between Fe2+ and Fe3+. The resistivity for very-high-purity material can be as low as 1.73 x 104 p^ cm or 1.0 x 104 p^ cm for thin epitaxial films (Peng et al. 2002). The electrical conductivity of synthetic single crystals has been reported as 250 ^-1 cm-1 at room temperature with a broad maximum centered around 15 °C.
High-purity natural magnetite of differing particle size distributions was added as a filler to polypropylene as well as nylon 6 (Duifhuis and Janssen 2001). It was found that percolation began at 35 vol%, and by a loading of 44 vol%, the surface resistivity was just 1 x 103 ^ square and the volume resistivity approximately 1 x 104 ^ m for the coarser grades, whereas slightly higher values were attained for finer-particle sizes. The paper is especially helpful as it details the processing conditions needed to extrude magnetite up to 80 or even 85 wt% loading (Fig. 2).
As with other conductive particulates, one must reach a critical concentration of particles known as the percolation threshold in order to create a conductive pathway of touching particles. A subsequent paper looked at the same three commercial grades of natural magnetite in PP and nylon 6 (Weidenfeller et al. 2002). The percolation threshold was determined to be 33 vol%, in line with theoretical calculations. At 47 vol% magnetite, a resistivity of 10 k^ m was reported.
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