Depolymerization Properties of Poly Lactic Acid

Although PLA is synthesized from renewable resources such as starch, the production up to the polymeric form requires much energy and many steps (Fig. 9.1] i.e., fermentation and purification of lactic acid, condensation for oligomerization, thermal degradation into lactide, and polymerization [9], if a highly selective depolymerization of PLA to the cyclic monomer, lactide, can be achieved with high efficiency, it will then be possible to reproduce PLA via the shortest and most energy efficient route. 

PLA is prepared by the ring-opening polymerization of lactides, such as L,L-lactide, D,D-lactide, and meso-lactide, which are cyclic dimers of lactic acid, with poly(L-lactide] (PLLA], an especially well-known crystalline polymer, being prepared from L,L-lactide [10-12]. This ring-opening polymerization is an equilibrium reaction, in which the concentration of cyclic monomer is temperature dependent [13]. Therefore, the lactides are regenerated through the thermal depoiymerization of PLA. 

However, the actual thermal degradation of PLA is more complex than the simple reaction that gives lactides. For example. 

In this section, the thermal degradation behaviors of PLA, especially PLLA, and methods to control its degradation are reviewed. 

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