Introduction to properties, engineering, and prospects of polylactide polymers

The physical and reaction properties of polylactides were characterized from a bulk production perspective. Emphases were placed on determining how initial meso-lactide (major racemization product) affects polylactide (PLA) properties and elucidating the role of the polymerization catalyst in the various bulk reactions. These physical and reaction property results were then combined with pilot plant engineering data to draw inferences about engineering design and resin processing.; The thermal, mechanical, and rheological properties of high molecular weight copolymers of scL- and meso-lactide were characterized. These include the determination of transition temperatures, heat capacities, thermal decomposition behavior, solid and liquid volumetric properties, linear viscoelastic properties, an empirical rheological constitutive equation, and a lactide-PLA mixture viscosity relationship. In addition, PLA physical aging, lactide-PLA vapor-liquid equilibrium, and lactide properties are discussed in detail.; The major bulk PLA reactions are metal-catalyzed esterification type reactions that include reversible polymerization, initiation of PLA chains by transesterification of hydroxy acid impurities and water with lactide, transesterification among polymer chains (interchange reactions), and hydrolysis of PLA. Undesirable side reactions during polymerization include racemization, color formation, and non-hydrolytic chain homolysis. The kinetics of reversible lactide polymerization, transesterification, melt hydrolysis, non-hydrolytic degradation, and racemization during polymerization have been determined and characterized mathematically for the stannous ethylhexanoate system.; A pilot plant was designed for continuous lactide polymerization based on the application of the reversible model to the ideal reactor design equations for two CSTRs in series. The reversible polymerization kinetics developed for stannous ethylhexanoate were verified in continuous operation at both pilot and commercial scale. Continuous devolatilization experiments were conducted with a pilot-scale twin screw extruder. Important devolatilization issues include low monomer vapor pressure at process conditions and the competition of melt depolymerization to reform lactide. PLA resin processing degradation was investigated with injection molding tests. Melt degradation was consistent with the developed kinetic model for stannous ethylhexanoate catalyzed melt hydrolysis at process conditions.; Three technical requirements were identified to facilitate PLA application: (1) melt deactivation of catalyst; (2) a manufacturing process having complete stereochemical control; (3) modification of the base thermal, mechanical, and rheological properties of PLA.