Orthogonal array design method for optimization experiments of sodium azide microencapsulation with stearic acid

In this study, an orthogonal array design (OAD), OA₉, was employed as a statistical experimental method for mircoencapsulation of sodium azide with stearic acid through a solvent/non-solvent procedure which is based on the coacervation principle. Scanning electron microscopy (SEM) was used to examine the coating morphology. The effect of stearic acid coating on sodium azide decomposition has been studied by means of differential thermal analysis (DTA), thermogravimetry (TG) and differential scanning calorimetry (DSC). Our findings revealed that stearic acid can provide an effective coating shell around sodium azide microparticle and the coating quality is affected by some parameters, such as percent of stabilizer, addition time of non-solvent, volume of non-solvent and stirring speed of the mixture (revolutions per minute, rpm). The effects of these factors on the thermal decomposition temperature of microencapsulated sample were quantitatively evaluated by the analysis of variance (ANOVA). The statistical results showed that sodium azide powder can optimally be coated and stabilized by controlling of stabilizer percent, addition time of non-solvent, and volume of non-solvent. The OAD evaluation of initial experimental data provide optimized amount of the parameters to obtain the most stabilized sample, at which the thermal decomposition temperature of sodium azide is predicted at 436 °C. The prediction is in excellent agreement with experimental result obtained at the same conditions that is 435 °C. These data could be compared to that of the pure stearic acid and sodium azide in which decomposition temperature ranges are 160–300 and 382–397 °C, respectively. Also, the kinetic parameters such as activation energy and frequency factor of the decomposition processes of pure components and microencapsulated sodium azide at optimum condition were obtained from the DSC data by non-isothermal methods proposed by ASTM E696. Our finding showed that the treated NaN₃ has much lower decomposition rate as compared to the pure one.