Comparative Screening of Stearate-Based Additives as Polylactide Clarifiers

Abstract A variety of stearate-based additives are tested for their clarifying properties in polylactide (PLA) compounds. The tested fatty acid derivatives include metallic soaps, stearate esters and fatty acid amides. PLA exhibits favorable optical properties in its amorphous state, rendering it a viable biobased and biodegradable alternative to conventional transparent polymers. However, its crystallization induces pronounced light scattering, thereby limiting the application temperature of transparent PLA components to their glass transition temperature of 55–60 °C. While the clarifying properties of fatty acid amides in PLA are known, the potential of structurally related stearate-based additives has not yet been comparatively assessed. PLA compounds with 0.5 wt% of stearate-based additives are prepared with standard injection molding procedures, ensuring consistency with industrial processing. Samples are temperature treated at nominal oven temperature of 65 °C to examine crystallization. UV/Vis spectroscopy, differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction are applied to assess optical haze, crystallization kinetics, morphology, and crystal structure. The fatty acid amides show the strongest clarifying effect. Among the non-amide additives, zinc and glycerol distearate exhibit notable effects on haze formation and crystallization behavior. In contrast, sucrose distearate delays PLA crystallization. The results demonstrate that stearate-based additives can substantially modify PLA crystallization kinetics and optical properties, but also show that efficient PLA clarification is strongly dependent on additive chemistry. This comparative screening therefore provides insight into structure-dependent effects of fatty-acid-derived additives in PLA and identifies promising directions for further formulation development.