Studies On Non-Toxic Stabilizers And Their Stabilizing Mechanisms For PVC

As a kind of general plastics, Poly (vinyl chloride) has been largely and broadly used in many industrial applications, especially in building materials application. In recent years, PVC is obtained to develop rapidly. Therefore, the thermal stabilizers for PVC become more important due to poor thermal stability of PVC. The commonly used stabilizers in the industry, which are based on the compounds of the heavy metals such as cadmium, barium and lead salts etc, will be gradually forbidden to use in most countries due to their toxicity. Calcium/zinc and barium/zinc stearates are thought as satisfactory substitutes for them, but their stability are less efficient than that of the stabilizer based on lead salts. In our country, Ca/Zn stabilizers for rigid PVC are mainly dependent on the import. Stability system of Ca/Zn stabilizers, which contain costabilizers, anti-oxidants and lubricants, is based on polyhydric alcohol and metal soaps, hydrotaleites, Calcium-aluminium-hydroxy phosphates and zeolites, respectively. It is very important for traditional PVC industry to prepare high performance stabilizers through the synergistic effectiveness of costabilizers.Based on an understanding of the structural features, instability factors, degradation mechanisms and stability mechanisms of PVC, non-toxic stabilizer based on Ca/Zn stabilizers for PVC and its stabilizing mechanisms are studied in this paper.The following are main conclusions:1.Combination of Cast2 and Znst2have the synergistic effect due to the complex formed between Cast2 and Znst2, which have eutectic point of 97.61 ℃. The possible molecular structure is as follows:The stabilizing mechanisms of the complex are that the ester group in the complex substituted instable chlorine atoms on the main chain of PVC and prohibited the degradation of PVC.2.The color of PVC is deepened during heat processing due to conjugated polyene, which are formed by the progressive elimination of HCl from the chain of PVC. As heating continue, further reactions occur, which lead to degradation of PVC. In order to prohibit the deepening color of PVC during heat processing, MAH is introduced to add to the conjugated double bonds, which belongs to Diels-Alder addition reaction. The growth of the conjugated polyene is prohibited, and the intial colour of PVC is improved. At the same time, the mechanical properties and plasticity of PVC are improved obviously.3.Incorporation of pentaerythrite into Cast2 and Znst2 can prohibit the blackening of PVC. Pentaerythrite was generally thought to complex with ZnCl2 and can prohibit the degradation of PVC. It is found from the fluorescence emission spectra, the FTIR spectra and the X-ray photoelectron spectroscopy that pentaerythrite can complex with ZnSt2 and ZnCl2 and there was no free ZnCl2 in PVC processing. Therefore, incorporation of pentaerythrite can inhibit the degradation of PVC caused by the catalysis of ZnCl2.4.Based on Cast2 /ZnSt2 stabilizers, new Ca/Zn stability system was designed and new zinc soap matched calcium soap was synthesized. Porous Zeolite can absorb HCl to improve the thermal stability of Ca/Zn stabilizers. The effects of Zeolite on thermal stability, the flame retardancy and smoke suppression properties of PVC are studied, and the thermal stability and the flame retardancy and smoke suppression properties are combined commendably to develop non-toxic and functional stabilizers. The filled Zeolite can improve the flame retardancy and smoke suppression properties of PVC. In PVC processing, the Zeolite not only can bring out the partial heat by losing water but also dilute the burnable gas. PVC, whose surface is coverd by the Zeolite, can exhibit the heat insulating efficiency. Effectiveness of flame retardancy is improved due to the composite formed between Zeolite, Carbon and the matrix.5.Effect of organic tin combined with CaSt2 and ZnSt2 on the thermal stability of PVC and the synergistic effect of organic tin with CaSt2 and ZnSt2 were studied. The experimental results show that there is no interaction of organic tin with CaSt2 and ZnSt2, but the incorporation of organic tins, which can react with double bonds in PVC, can prohibit the growth of conjugated polyene at a certain extent and improve the color stability of PVC.6.Hydrotalcite (LDH), which is treated by hyperdispersant, is filled into PVC to improve the thermal stability and impact strength of PVC. The experimental results showed that LDH reacted with HCl, which is produced by the degradation of PVC, isthe ion exchange of HCl with CO32- in the double-layed structure, but the double-layed structure of LDH during reaction are not disrupted.