Synergistic effects of lanthanum-pentaerythritol alkoxide with zinc stearates and with β-diketone on the thermal stability of poly(vinyl chloride)

Lanthanum-pentaerythritol alkoxide (La-PE) was obtained by the traditional preparation method which was a reaction between La2O3 and pentaerythritol. The synergistic effects of La-PE with zinc stearates (ZnSt2) and with stearoyl benzoyl methane (β-diketone) on the thermal stability of poly(vinyl chloride) (PVC) were evaluated by the conductivity test, thermal aging test, and Ultraviolet–visible (UV-VIS) spectroscopy test. The results showed an improvement on the color stability and the long-term thermal stability of PVC, indicating that there was an obvious synergistic effect between La-PE and ZnSt2. In order to explore the mechanism, the capacity of La-PE to react with HCl was tested. The results showed that La-PE had lower capacity to neutralize HCl (80.30 mg HCl/1 g) than ZnSt2 (92.71 mg HCl/1 g). Furthermore, the thermal stability test of PVC stabilized with La-PE/ZnCl2 showed that the thermal stability of PVC with La-PE/ZnCl2 was similar to that of PVC with La-PE/ZnSt2 but better than PVC with pure La-PE. Thus, a possible synergistic mechanism of La-PE and ZnSt2 could be described where zinc species could catalyze the reaction of La-PE and the labile chlorine atoms in PVC chains. Moreover, the hydroxyl group on La-PE could chelate ZnCl2 to inhibit the “zinc burning” phenomenon. However, there was no synergistic effect observed between La-PE and β-diketone. Discoloration in PVC samples that stabilized with La-PE and β-diketone was noticed as early as in the milling process by the open twin-roller at 180 °C, even though it took 110 min for the reaction to change the color to completely black. The results of UV-VIS spectroscopy showed that a strong peak appeared at 360 nm representing the enol form of β-diketone when the dosage of β-diketone exceeded 1 phr. This indicated that the enol form of β-diketone did the work in the thermal stabilizing process and the dosage of β-diketone needed in the PVC stabilizers was very low.