Study On The Synthesis, Synergism And Application Of Rare-Earth Stearate As Thermal Stabilizer For PVC

When PVC is processed at high temperature, it is degraded by dehydrochlorination, chain scission, and crosslinking of macromolecules. Free hydrogen chloride (HCl) evolves and discoloration of the resin occurs along with important changes in physical and chemical properties. The evolution of HCl takes place by elimination from the polymer backbone; discoloration results from the formation of conjugated polyene sequences of 5 to 30 double bonds. Adding thermal stabilizer in PVC is the best measure to eliminate its degradation when PVC is processed at high temperature. Rare-earth compounds are known to be one of thermal stabilizers with the most developmental potential in 21st century, and one of the most representative rare-earth stabilizers is rare-earth stearate. Synthesis of rare-earth sterarate is always using the traditional double decomposition technique, there are many defects in this technique and single rare-earth stearate still can seldom meet the demand of thermal stabilization and general properties of PVC during molding and applying. To deal with the aforementioned problems, the research plans of this thesis are as follows: 1. On the basis of the literature review, the general situation of PVC industry at home and abroad, the mechanism of thermal degradation of PVC and the function of thermal stabilizers were summarized and analyzed. At the same time a review was made systematically in the field of thermal stabilizers for PVC, especially in the current situation and the applied prospect of the research in rare-earth stabilizers.2. Based on prophase experiments, some shortcomings of traditional double decomposition technique were improved. Four kinds of rare-earth stearates were synthesized by using the improved double decomposition technique, the influencing factors of the synthetic reaction such as reaction temperature, the concentration of RE(NO3)3, the concentration of CH3(CH2)16COOH, the concentration of NaOH, weeping time of NaOH and heat preservation time were investigated. With the data of orthogonal test, the preferable synthetic conditions of rare-earth stearates were found. The results of FT-IR and XRD showed that the bonds between the stearate and rare-earth metal ions in rare-earth stearate are main ionic character and have the stratified crystalline structure. 3. The thermal stabilities of these four rare-earth stearates were tested through cango red test paper and conductivity method, the order of stability of these four rare-earth stearates is Last> Ndst> Yst> Dyst. The synergistic effects of the Last and Ndst with any of the reference stabilizers were also performed by conductivity method. At the same time the mechanisms of stabilization of thermally degraded PVC in presence of rare-earth stearate were proposed by FT-IR, the particular mechanism and the well-defined synergistic action of rare-earth stabilizers were explained by the microcosmic theory.4. Based on the study of single component rare-earth stearate, Last and Ndst were regarded as the main stabilizer, and other reference stabilizers were regarded as auxiliary stabilizers, through repetitive test, the ingredient of the multiplex stabilizers (HREC-1 and HREC-2) were affirmed. The applied research of the HREC stabilizers were performed in service pipe and drainpipe of UPVC, in the end the processing property, mechanical property, coupling effect and the behavior of degradation of the HREC-PVC system were investigated.