Several examples of existing and potential applications of magnetite as a filler have already been given and there are certainly more waiting to be discovered. When considering the use of a novel filler, several practical questions arise, and this section will cover some of those.
Magnetite is a low aspect ratio filler and behaves similarly to other such particulates including calcium carbonate, dolomite, barium sulfate, or glass beads. So, modulus is increased by magnetite and tensile strength is somewhat lower, whereas impact and elongation are better maintained when smaller particles are used (Duifhuis and Weidenfeller 2002).
Long-Term Polymer Stability
There is a widespread perception that iron is detrimental to the thermal stability of polymers. This stems from the use of talc in polyolefins where traces of iron compounds in the talc can catalyze degradation. In reality however, the idea that iron is generally deleterious is not founded in fact. Iron oxides, of which magnetite is one, are used as UV-stabilizing pigments to protect polyolefins (Halliwell 1992; Scott 1997). It turns out that magnetite is not deleterious to stability (Mangnus 2003) as illustrated by aging studies on highly filled natural rubber and EPDM.
The relatively high Mohs hardness of magnetite might raise questions about possible wear on processing equipment. Magnetite actually has lower hardness than glass fiber which is a very common reinforcement. Natural magnetite was proven less abrasive than glass fibers or barium sulfate when all were injection molded at 30 vol% in polypropylene. Like glass fiber, the use of hardened steel in processing equipment is recommended, and control of injection-molding conditions can ensure the magnetite particles are kept away from the walls, thus preventing contact and preventing wear.