Hydrothermal synthesis of urchin-like alumina for fire-extinguishing powders

Alumina with an urchin-like morphology has been synthesized from Al2(SO4)3/CO(NH2)2/H2O/isopropanol mixture via hydrothermal route followed by calcination. Raman spectra and SEM images have shown the urchin-like structure to be formed of γ-AlOOH phase with the component ratio Al3+/CO(NH2)2/H2O = 1:(2–3):(100–200). The increase in CO(NH2)2 content has resulted in the formation of needle-like crystals of NH4Al(OH)2CO3. Structural transformations of alumina during hydrothermal synthesis have been investigated by SEM and TEM methods. The 5–10 μm urchin-like alumina particles have been found to be composed of 100–200-nm-thick sheets. The alumina samples have been studied by low-temperature nitrogen adsorption method. The adsorption–desorption isotherms and pore size distributions have shown that pore structure of the urchin-like alumina is formed by the slit pores with two open ends. It has been found that after treatment with chlorosilanes the surface of the urchin-like alumina becomes superhydrophobic, the average contact and sliding angle values being 154° and 5°, respectively. Addition of 5 wt% of the hydrophobized urchin-like alumina particles to the model fire-extinguishing powder has made the powder hydrophobic, the average contact angle being increased up to 151°. Introduction of 5 wt% of the hydrophobized urchin-like alumina particles to ammonium dihydrophosphate has resulted in a decrease in the cohesion strength for ammonium dihydrophosphate fire-extinguishing powder from 0.415 to 0.104 kPa. The hydrophobized urchin-like alumina particles can be used as functional additives to the fire-extinguishing powders for the improved flowability and water repellency to be achieved.