Temperature Dynamics of Carbonizing Cotton Fibers by Microwave Radiation in Air

The dynamics of microwave heating in air of samples made of the cotton fibers pretreated with H₃PO₄ solution is studied using the spectral pyrometry technique. Microwave heating is carried out in an open-type quartz reactor. The thermal spectra of the heated samples are recorded with a small-sized HR2000+ spectrometer in the near-infrared range (650–1080 μm). These spectra allowed determining the temperature of the samples. It is established that the change in the temperature of the sample during heating has three characteristic regions: low-temperature, unstable temperature, and temperature stabilization domain. It is shown that, after 70–75 s from the onset of heating, the carbonization process begins in the local regions of the sample. The effect of generating heat is established. The results of comparative study of the structure and elemental composition of samples before and after microwave heating are reported. It is found that, after 180 s of heating, the fibers become significantly thinner, although they retain integrity along the length. Carbonized fibers contain a significant amount of phosphorus, having the maximum concentration in the outer layers of the sample amounting to 15–20%. Video recording showed that the irradiation process results in developing irregular slit-shaped cavities on the surface of the sample. Inside these cavities, there are electrical discharges and ignitions that contribute to the heating process. The dependence of the intensity of open burning of the sample on the concentration of the impregnating H₃PO₄ solution is established.