Crystallization Kinetics of Polylactic Acid (PLA) Processed through Solid-State/Melt Extrusion

Polylactic acid (PLA) is a bio-based polymer derived from corn, sugar beets, and other natural sources.  It has recently become of interest because of its biodegradability and similarity in properties to other petroleum-based polymers.  However, the widespread use of PLA is currently hindered by its inherently slow crystallization kinetics relative to other commercial polymers.  The desired semi-crystalline structure is difficult to achieve in a reasonable amount of time when extrusion and other melt-molding techniques are used.

Many different methods have been attempted in enhancing the crystallization kinetics of PLA.  One of the most prominent is nucleating agents.  While effective, this method is not environmentally friendly and can be very costly.  An alternative is processing via solid-state shear pulverization (SSSP), a method developed by Northwestern University.  SSSP has been shown to lead to faster crystallization kinetics in both PLA and polyethylene terephthalate (PETE) without any additives or blend components.

We have recently developed a related extrusion technique called solid-state/melt extrusion (SSME).  SSME combines traditional twin-screw extrusion (TSE) and SSSP.  SSME is designed to combine the advantages of these two processes in an attempt to increase the rate of crystallization for faster, more productive processing.  So far, SSME has been shown to be an effective processing method for polypropylene and polyethylene blends.

This study explores the crystallization behavior and physical properties of PLA processed via SSME.  The effectiveness of processing was assessed by comparing the properties of SSME-processed samples to those of TSE and SSSP.  The rate and level of crystallization was measured by differential scanning calorimetry (DSC), while degradation behavior and mechanical properties were analyzed via thermogravimetric analysis (TGA) and tensile testing, respectively.