Modeling Melt Spinning of PLA Fibers

A modified version of a model for fiber spinning was developed for poly(lactic acid) (PLA) based on earlier models that predicted the spinline behavior of poly(ethylene terephthalate) (PET) and Nylon 6.6. Comparisons were made to melt‐spinning data of Takasaki and coworkers for high lactic acid (L)‐content PLA (PLLA) and racemic mixtures of the L‐ and D‐ forms over a range of spin speeds from high‐ to low‐speed conditions. The characteristic necking and “freeze off” under high‐speed conditions is not as severe with PLA as that observed in the earlier model systems, most likely due to the more highly entangled nature of PLA. To address this, the extended pom‐pom model was utilized to model the melt phase. Because of the stereochemistry of PLA, mixtures of the L‐ and D‐ forms result in an additional stereo‐crystal formation with a higher melting temperature than the normal alpha‐crystal formation. In addition to accurately capturing fiber diameter profiles for the PLLA, all the important features of spinning of the racemic mixture (r‐PLA) were also successfully captured with our model. Besides diameter predictions, the model also successfully predicts the elongation to break of the spun fiber by correlating it to the calculated ratio of amorphous to total stress at the freeze off point. In modeling both PLLA and r‐PLA, further insight was gained into the relationship between thermally induced crystallization and flow‐enhanced crystallization.