Effects of Span 60 template and freeze drying on zinc borate produced from zinc nitrate hexahydrate and borax decahydrate

Abstract

Zinc borate is an important additive to polymers and lubricants. The process variables such as reactant concentration, presence of template in precipitating medium and drying method determine the composition and particle size of zinc borates. In the present study, zinc borate precipitate obtained by mixing aqueous zinc nitrate and borax decahydrate solutions was dried either by conventional method or by freeze drying. The products were well characterized by advanced methods. Zinc borate from 1 mol dm−3 reactants had (2.1 ± 0.5)×(2.5 ± 0.5)×(1.3 ± 0.2) μm and (0.5 ± 0.1)×(1.3 ± 0.1)×(0.028 ± 0.01) μm dimensions by conventional and freeze drying respectively. Individual particles smaller in size is obtained since the particles are not agglomerated due to absence of surface tension of liquid water for case of freeze drying. Planar particles agglomerated into 20 to 60 μm crystals in the presence of template Span 60 in 1 mol dm−3 reactants for conventional drying. Nano zinc borate particles with primary particle size of (46 ± 9) nm were obtained by decreasing the reactant concentration to 0.1 mol dm−3. The primary particle size was decreased to (40 ± 3) nm by addition of Span 60 to dilute solutions. However zinc borate nanoparticles obtained from dilute solutions adhered to each other forming agglomerates during conventional drying. Their freeze drying would allow formation of a freely flowing nano powder.