Smoothing and Cicatrization of Isotactic Polypropylene/Fe₃O₄ Nanocomposites via Magnetic Hyperthermia

We prepare industrially relevant magnetoresponsive thermoplastic nanocomposites capable of being cicatrized and smoothed after additive manufacturing through the application of an oscillatory magnetic field (OMF). The materials are made of an isotactic polypropylene (iPP) matrix filled with magnetite nanoparticles (NPs; 2-22 wt %, 75 nm in diameter) synthesized from steel waste, providing them with a limited ecological impact on top of their ability to be repaired. NPs are found to have no significant impact on the thermal properties of iPP, which allows one to compare directly the magnetothermal effects measured on the different nanocomposites. Beyond the primary temperature increase generated by magnetic hysteresis loss, we show that the OMF irradiation triggers a second heating mechanism from the iPP melting. This phenomenon, which was assigned to NP magnetization and subsequent rotation causing high-frequency mechanical friction, is investigated here in a systematic way. Our results indicate that while the specific power generated by NP friction is (expectedly) proportional to the irradiation time, it is independent of the NP content as long as the temperature is well above the polymer melting point. These observations therefore suggest that the (local) filler-polymer interfacial rheology dictates the amount of heat generated through friction. From an application point of view, 7 wt % of the NPs is found to be enough to induce iPP melting from the magnetothermal effect, which enables the postprocessing of a hot-pressed and 3D-printed specimen through "cicatrization" and "smoothing" experiments. In the former case, rewelding a sample cut into two pieces is found to provide a Young modulus and a yield point similar to those in native hot-pressed samples (exhibiting, however, a lower strain at failure). In the latter case and beyond the improved specimen appearance, smoothing is found to double both the stress and strain at failure of large 3D-printed samples that present, nevertheless, significantly lower properties than those of their hot-pressed counterparts.