Toward Optical Quality Polylactide Using Fatty Acid Amides as Clarifiers

Polylactide (PLA) represents a sustainable alternative to common optical polymers in lighting applications. However, its application temperature is currently limited, since the material crystallizes and becomes cloudy when exposed to temperatures above 55-65 °C. Here, N,N'-ethylenebis(stearamide) and N,N'-ethylenebis(12-hydroxystearamide) are applied as clarifiers, and their effects are known for foils. This approach is extended to bulky materials (d = 1.5 mm). Optical characterization was complemented by microscopy, differential scanning calorimetry (DSC), and X-ray diffraction (XRD). Additionally, NMR was applied to understand the effects of the fatty acid amides on a molecular level. All methods confirm the nucleating effects of both additives, while the crystallinity and crystal structure remain unchanged. A controlled crystallization at lower temperatures is discovered to promote the effect of the clarifiers by further reducing crystallite size. Finally, the crystallized samples are exposed to 80 °C and found to maintain high optical transmission. A tempering step at low temperatures is suggested to maximize clarifying effects.