Preparation of thymolphthalein stearate and appraisement of its efficacy as an antioxidant for polypropylene

Stabilization of polypropylene (PP) against thermal oxidation during its melt processing and service life, due to its susceptibility to oxidation, is of great importance from both scientific and industrial points of view. The present work was devoted to synthesize a new antioxidant for polypropylene through esterification reaction between thymolphthalein and stearic acid. The occurrence of the reaction was confirmed by Fourier transform infrared (FT-IR) spectroscopy, size exclusion chromatography and melting point measurements. The reaction product was proved to be a mixture of thymolphthalein mono- and di-stearate. It was also shown that the esterification of thymolphthalein, in addition to lowering its polarity, lessens its melting point significantly from 246 to 186 °C, hence, helping the synthesized additive to be mixed with the polymer more favorably in comparison to thymolphthalein. Differential scanning calorimetry demonstrated that the new additive improves stability of the polymer in melt state, significantly. Moreover, oven ageing experiments revealed that the additive stabilizes the polymer against oxidation outstandingly in solid state and its efficiency is comparable to that of SONGNOX 1010, which is a powerful commercially used antioxidant for the polymer.