Comparative Investigation of the Migration Behavior of Two Stearate Acid Scavengers from Ziegler-Natta Polypropylene into Water during Autoclaving Treatment

Although Ziegler-Natta (Z-N) polyolefins have been widely used as raw materials to produce pharmaceutical or food packaging, the migration of acid scavengers, an additive usually introduced in Z-N polyolefins, from the packaging to its contents has not been reported. In this work, the migration of the two most used acid scavengers, calcium stearate (CaSt₂) and zinc stearate (ZnSt₂), from a Z-N polypropylene random copolymer (PPR) into water during autoclaving at 121 °C were comparatively investigated. It was found that, for PPR plates containing 0.1 wt% CaSt₂ or ZnSt₂ (PPR-0.1CaSt₂, PPR-0.1ZnSt₂, respectively), the concentration of migrated calcium ion into water increased with autoclaving time, while that of zinc ion was much lower at same treatment durations and did not show a significant increase with treatment time. Interestingly, after removing all plates and acidification treatment, a considerable amount of stearic acid was detected in sterilized water for PPR-0.1ZnSt₂, but no such significant stearic acid was detected in sterilized water for PPR-0.1CaSt₂. Based on the structural evolution of the two soaps upon heating, possible mechanisms for the different migration behavior of CaSt₂ and ZnSt₂ from PPR into water during autoclaving treatment were proposed. Our results suggest that the migration issue of acid scavengers is worthy of attention in pharmaceutical packaging materials produced from Z-N polyolefins.