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Process development for metal soaps
Zinc stearate (ZnSt2) is an important compound among the metallic soaps. It has many applications e.g., in resins, paints, cosmetics, textile, lubricants and Langmuir-Blodgett films. Double decomposition (precipitation) and fusion processes are widely used techniques in ZnSt2 production. The product purity has been a major concern in most of the ZnSt2 applications such as PVC stabilization, coating of textile goods, additive in cosmetic products. In this study, the production of ZnSt2 using precipitation, fusion and modified fusion processes was investigated based on product purity. Raw materials and ZnSt2 were characterized by using various techniques.In the precipitation process, in order to maximize the solubility of sodium stearate and to minimize the water evaporation, the reaction was carried out at 70°C. 2.5% (w) NaSt was used in the reaction at this temperature. The equivalent, excess Zn and deficient Zn cases were studied to determine the raw materials ratio on product purity. Equivalent raw materials produced highest product purity. From washing experiment, it was seen that Na2SO4 adsorption did not take place on to wet ZnSt2. The washing water to zinc stearate ratio was found to be 40 dm3/kg for effective removal of by products and raw materials unreacted at room temperature. Any further increase in the amount of water did not bring any significant removal results. In the drying of wet ZnSt2 at 100°C, it was determined that it has 85% (w) water. In IR spectra, characteristic ZnSt2 peak was observed at 1540 cm-1. ZnSt2 obtained by this process did not contain any free Na+ and SO4-2 ions as indicated by ICP, EDX and elemental analysis results. Only two of the characteristic peaks of ZnSt2 at 2. values of 6.40 and 19.58 were obtained in XRD pattern of the dried product due to low crystallinity. From SEM micrographs, it was seen that zinc stearate has lamellar structure and particle size changes between 2-4 .m. Melting point of the zinc stearate was found to be about 122°C using optic microscopy with temperature controlled hot stage.In fusion process, reaction was carried out at 140°C in equivalent amounts for different stirring rates 400, 600 and 750 rpm. The increase in mixing rate decreased the delay time occurring at the beginning of the reaction. The conversion was found to be 80% using the developed method from IR spectra. In the comparison of the experimental conversion data with shrinking core model no relation was established. In IR spectra, two peaks observed at 1540 cm-1 and 1700 cm-1 which belong to ZnSt2 COO- stretching and stearic acid C.O stretching vibrations, respectively. All of the characteristic 2. values of zinc stearate were observed for product, which means that the crystallinity of the product is high. From SEM micrographs, it was seen that zinc stearate structure is in the form of layered lamella and particle size change between 4-6 .m. The melting point of zinc stearate samples from fusion process was found to be slightly lower than 122°C by optic microscopy with temperature-controlled hotstage.In the modified fusion process, reaction was carried out at 80°C for 1 h. with equivalent amounts of stearic acid and zinc oxide in the presence of H2O. Sodium stearate 1.5% (w) was added into reaction mixture as a surfactant and its effect was examined. At the end of reaction it was seen from IR spectra that it does not significantly increase the reactants dispersion. The presence of unreacted raw materials was determined in IR spectras and XRD patterns. This result was confirmed by SEM micrographs too. In TGA analysis, thermal decomposition temperature of zinc stearate was found to be 250°C. The use of zinc stearate in n-paraffin wax shifted the thermal decomposition temperature of wax 10°C. Increasing the amount of zinc stearate in n-paraffin increased the decomposition temperature of wax.According to the results of this study, for pure zinc stearate production precipitation process should be preferred in spite of high wash water consumption. The fusion and modified fusion processes needs to be studied further to increase the conversion and decrease the delay time.