Synthesis Of Lanthanum Stearate And Its Application In Linear Low Density Polyethylene

Linear low density polyethylene?LLDPE?is a copolymer of ethylene and?-olefins with linear molecular structure.Compared with low density polyethylene?LDPE?,LLDPE has excellent tear resistance,puncture resistance and tensile strength.It is widely used in pipe,film and cable manufacturing.However,the linear low density polyethylene produced by Ziegler-Natta catalyst will retain some chloride ions,which will cause equipment corrosion and resin discoloration and aging.In order to solve this problem,acid remover and antioxidant should be added to the resin.Commonly used acid removers are calcium stearate and zinc stearate.Rare earth stearate as thermal stabilizer of polyvinyl chloride?PVC?materials has the advantages of non-toxicity and long-term stability compared with lead stearate and cadmium stearate.The application of rare earth stearate in LLDPE is rarely reported in literature.Therefore,it is of great significance to study the application of rare earth stearate in LLDPE for improving product quality and developing new differentiated products.Among many rare earth stearates,lanthanum stearate is relatively easy to obtain due to its high abundance,which has the most research and application value.The preparation methods mainly include double decomposition method,melting method and saponification method.In this paper,two synthetic processes of lanthanum stearate were studied:lanthanum hydroxide was synthesized by hydration with lanthanum oxide as raw material,then lanthanum stearate was prepared by neutralization reaction of lanthanum hydroxide and stearic acid;lanthanum stearate was prepared by optimizing the double decomposition process.The melt flow index?MFR?of LLDPE with lanthanum stearate was compared and analyzed through several extrusion tests to judge the processing stability of lanthanum stearate as co-stabilizer.The mechanical properties of LLDPE with lanthanum stearate were investigated by tensile test.One-step synthesis of lanthanum stearate from stearic acid,La₂O₃ and water in water bath was studied.The effects of feeding sequence,ethanol addition,amount of hydrogen peroxide and reaction time on the reaction were investigated.The experimental results showed that the direct reaction of stearic acid,La₂O₃ and water in water bath was slow and a large amount of stearic acid remained.The conversion could be improved by using H₂O_2,but the color of the product was yellowish.Powdered lanthanum stearate with low free acid content was prepared by optimized double decomposition method.The effects of liquid-solid ratio of water to stearic acid and washing times on the reaction were investigated.The optimum technological conditions were as follows:the molar ratio of stearic acid to lanthanum nitrate was 3:1,the mass ratio of water to stearic acid was 12.5:1,the reaction time of acid-base neutralization and double decomposition was 45 minutes,the reaction temperature was 80-90?and the product was filtered and washed three times with hot water at 70-80?.The content of free acid in the product was 0.60%,and the melting point was 121-134?.The application experiment of lanthanum stearate in LLDPE was carried out and compared with the basic sample and the basic sample containing zinc stearate.The melt flow index?MFR?was measured by multiple extrusion to characterize the thermal stability of the material.The mechanical tensile test of extruded LDPE particles was carried out.The results showed that adding lanthanum stearate could stabilize MFR of LLDPE during the first extrusion and avoid resin degradation.When lanthanum stearate was added to LLDPE,the tensile yield stress of the material did not change significantly.When the proportion of lanthanum stearate in LLDPE reached 0.2%,the nominal strain of LLDPE was 14%higher than that of blank sample and 12%higher than that of the basic sample containing zinc stearate with the same proportion.