Effect of melt processing conditions on the degradation of PLA during single-screw extrusion.

The ester bonds in poly(lactic acid) or PLA, responsible for its biodegradable character, are vulnerable linkages during melt processing techniques such as extrusion. The sensitivity to high temperatures, moisture, and shear stresses causes an undesired molecular weight reduction. The aim of this research is to understand and quantify degradation of PLA during single-screw extrusion. First, the effect of five processing parameters ((1) moisture content, (2) processing temperature, (3) length of compression zone of the screw, (4) die geometry, and (5) screw rotation speed) on the molecular weight of a low D-isomer content PLA is studied by performing a 2⁵ full factorial design. Second, the parameters that significantly affect the molecular weight are studied in more detail at intermediate values. The results highlight that PLA should be processed at low temperature, high screw rotation speed, and limiting the occurring shear stresses. In addition to the main effects, the study emphasizes the importance of interaction effects, since the moisture content also influences the extent of degradation caused by other processing parameters. Drying PLA prevents degradation caused by screw rotation speed and die geometry, and also limits thermal degradation. [ABSTRACT FROM AUTHOR]