Comprehending the Degradation of Poly(Lactic Acid) During Processing: The Effect of Calcium Stearate

ABSTRACT Poly(lactic acid) (PLA) is a promising biopolymer for packaging applications. However, efforts have been focused on improving its processability. Metal stearates are applied in the thermoplastic polymer industry as lubricating agents to enhance the processability of these polymers, although the comprehension regarding their thermostability is still unclear. Herein, a melt‐mixing technique was employed to produce PLA composites modified with calcium stearate. The effects of the additive were evaluated from 0.1 to 0.7 wt.% of calcium stearate, all incorporated into the PLA matrix at 200°C. Melt flow index (MFI) indicated a reduction in the viscosity of the modified materials. However, the significant decrease observed at concentrations above 0.3 wt.% suggested a potential polymer degradation due to the addition of calcium stearate. This phenomenon was related to the scission of the PLA chains in the presence of metal stearates during processing, which was supported by results from gel permeation chromatography (GPC) and thermal analyses. As the calcium stearate content increased, the materials exhibited a decrease in molecular weight, degradation temperature, and degree of crystallinity. In addition, calcium stearate could be incorporated into PLA at 180°C and low frequencies, conditions that avoided polymer degradation, as shown by parallel plate rheometry.