| Zinc stearate is known as one of stabilizing agents for poly vinyl chloride, and it has found wide application. In the paper, process for one-step catalytic synthesis of zinc stearate by reacting stearic acid with zinc oxide, with H2O2 as the catalyst, was investigated. The process is under low temperature and usual air pressure, using simple equipments, with no sub-reaction or pollution. It not only has the advantage of environmental protection and technical innovation , but also has economic feasibility. The process and the effect of zinc stearate on the pyrolytic performance of PVC were studied from following aspects.(1) Finding preferable technical conditions of the process by single factor experiments and orthogonal tests. The results show that reaction temperature, reaction time, liquid-solid ratio, and the amount of catalyst have influence on free acid content from the strongest to the weakest; the preferable technical conditions were confirmed as follows: reaction temperature 80℃, reaction time 60 min, liquid-solid ratio 7, and the amount of catalyst 2.5%. Under these conditions, free acid content in the sample is 0.43% , zinc content in the sample is 11.24%, and the melting point 124℃, which comply with the standard data for high-class zinc stearate.(2) Finding the best technical conditions by unitary orthogonal regression design and the steepest ascending method. According to the results of the unitary orthogonal regression experiment, a unitary regression equation was established to show the relationship of experimental target(free acid content in the zinc stearate sample) and experimental factors. The equation shows that reaction time, interaction between reaction temperature and reaction time, liquid-solid ratio, reaction temperature, interaction of reaction temperature, reaction time and liquid-solid ratio, interaction between reaction temperature and liquid-solid ratio have influence on the experimental target from the strongest to the weakest.Variance analysis shows that reaction time, liquid-solid ratio, and interaction between reaction temperature and reaction time have great influence on the experimental target, while the influence of other factors is weak. The results of the markedness test and the test of goodness of fit show that the regression equation is remarkable and the goodness of fit of the equation isn't remarkable. The mathematic model fits the actual data well.According to the results of the steepest ascending method, the best optimized technical conditions were obtained as follows: reaction temperature 82℃, reaction time 80 min, liquid-solid ratio 8, and the amount of catalyst 2.5%. Under these conditions, free acid content in the sample is 0.22%. The value is far lower than the standard data which are listed in HG/T3667-2000.XRD patterns of the zinc stearate sample show that the sample has a peculiarity of stratified crystalline structure and absolute majority of the sample is zinc stearate, and the rest is zinc oxide. DSC analyze shows that the melting point of the sample is between 120.18℃to 127.93℃, which is same with the melting point of the sample measured by the apparatus.(3) Congo red method was used to study the influence of the amount of the zinc stearate, the thermal temperature of the oil, the ratio of zinc stearate to calcium stearate on the stable time of compound PVC sample, to which zinc stearate was added. The results show that zinc stearate has obvious thermal stabilizer's function for PVC. While more zinc stearate was added, the thermal stable time of compound PVC sample is longer, and the sample has stronger ability of restraining color changes. However, adding too much zinc stearate would weaken the ability. As the temperature of the oil is rising, the thermal stable time of pure PVC sample and compound PVC sample(2% zinc stearate) is decreasing. The ratio of zinc stearate to calcium stearate has great influence on the thermal stable time. When the ratio is 3: 7, thermal stable time of PVC is longest.(4) The pyrolysis course of zinc stearate, pure PVC and compund PVC sample were investigated by pyrolytic kinetics method. The weight loss of zinc stearate sample is 76.57%, and it contains little free acid. The majority of the pyrolytic production is gaseous carbon dioxide, ketones, alkyls, and so on. The rest may be zinc oxide, zinc carbonate and so on. The sample pyrolyzes fastest at 431.44℃. The activation energy of the pyrolysis of zinc stearate sample from 380℃to 430℃is about 126.6 kJ/mol, and the reaction order is about 0.The pyrolysis course of pure PVC sample can be divided into three stages, which are 20～200℃, 200～370℃, and 370～500℃. The weight loss of the sample is 92.47%. At 70.9℃and 176.3℃, the sample loses 1% and 2% of its weight. The sample pyrolyzes fastest at 281.50℃. The activation energy of the pyrolysis of pure PVC sample from 210℃to 280℃is about 137.6 kJ/mol, and the reaction order is about 1.The pyrolysis course of compound PVC sample can be divided into three stages, which are 20～200℃, 200～350℃, and 350～500℃. Zinc stearate has good thermal stability during initial stages of PVC's pyrolysis. The weight loss of the sample is 80.21%, which is 12.26% smaller than that of the pure PVC sample. At 790.9℃, the sample loses 1% of its weight. The T1% of the compound PVC sample is 120℃higher than that of the pure PVC sample, and is 14℃higher than the T2% of the compound PVC sample, which shows that the thermal stability of the compound PVC sample is obviously improved.The largest pyrolytic rate of the compound PVC sample is 130% smaller than that of the pure PVC sample. But the former pyrolyzes fastest at 264.06℃, which is 17℃lower than the temperature at which the latter pyrolyzes fastest. The second pyrolytic stage of the compound PVC sample ends at 350℃, which is 20℃lower than the temperature at which the second pyrolytic stage of the pure PVC sample ends. The difference might be caused by the effect of zinc chloride, which was generated from reacting zinc stearate with PVC, and which could hasten PVC's pyrolysis.The activation energy of the pyrolysis of the compound PVC sample from 220℃to 270℃is about 97.7 kJ/mol, and the reaction order is about 2.6. Comparing to the pure PVC sample, the former is 41% smaller and the latter is 1.6 larger, which shows that zinc stearate has great influence on the pyrolytic mechanism of PVC. |
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