Study On Ca/Zn Barbiturate Thermal Stabilizers For Poly(Vinyl Chloride)

Zinc barbiturate （ZnL₂） and calcium barbiturate （CaL₂） were synthesizedconveniently by precipitation method in aqueous solution in this thesis. ZnL₂wascharacterized by elemental analysis, DSC, TGA, FTIR; these results indicate thatZnL₂synthesized in aqueous solution is dihydrate and its chemical formulation isC₈H₁₀N₄O₈Zn, namely Zn（H₂L）2·2H₂O. And then ZnL₂and CaL₂were investigated asthermal stabilizers for poly（vinyl chloride）（PVC） by the discoloration test and thedehydrochlorination test at180oC. The related thermal stabilizing mechanisms werealso suggested.Results of thermal stability test of PVC indicate that ZnL₂is one new kind ofhigh-efficiency zinc salt thermal stabilizer, which exhibits excellent initial thermalstabilizing effect and efficiently delays the rapidly thermal degradation of PVC. ZnL₂may replace the labile chlorine atoms by relatively stable structures to interrupt theformation of conjugated double bonds in PVC chains, meanwhile the experimentalresult shows that ZnL₂can absorb HCl at180℃to restrain the self-catalyticdehydrochloration of PVC thermal degradation.CaSt₂/ZnL₂can act as one new type of Ca/Zn composite thermal stabilizer byimpartment of excellent initial color for PVC and prohibition of abruptly thermaldegradation to prolong the stability time, and stabilizing effect varies according to thedifferent ratios of CaSt₂/ZnL₂. The good effect of CaSt₂/ZnL₂can be due to the idealpositive synergistic effect between them.PVC samples stabilized by CaSt₂/ZnL₂show better initial color and relativelylonger thermal stability time with the addition of dibenzoyl methane （DBM） as anauxiliary thermal stabilizer. The auxiliary stabilizing effect can be attributed to therapid C-alkylation reaction between DBM and PVC in the presence of ZnCl₂whichcan act as catalyst of strong Lewis acidity. And this reaction can improve the initialcolor of PVC product through the inhibition of formation and increase of conjugateddouble bonds in PVC chains. Moreover, CaL₂is another new kind of calcium salt thermal stabilizer, which canimpart excellent long-term stability but poor initial color for PVC. CaL₂can offer thechemical groups of barbituric acid which would replace the labile chloride atomsslowly to interrupt the generation of C=C in PVC chains. Experimental results alsodemonstrate that there does not exist synergistic effect between ZnCL2and CaL₂, thusthey cannot be used as Ca/Zn composite thermal stabilizers for PVC, because thedeterioration resulting from ZnCL2, which are generated rapidly and accumulatedexcessively during the thermal stabilizing process of ZnL₂, cannot be weaken timelyand effectively.