Due to the chiral nature of lactic acid, several distinct forms of polylactide exist: poly-L-lactide is the product resulting from polymerization of L,L-lactide. PLLA has a crystallinity of around 37%, a glass transition temperature between 50-80 °C and a melting temperature between 173-178 °C.

compostable resins Polylactic acid Chemical and physical properties
Photo: compostablepackaging.co.uk

Polylactic acid can be processed like most thermoplastics into fiber and film. The melting temperature of PLLA can be increased 40-50 °C and its heat deflection temperature can be increased from approximately 60° C to up to 190 °C by physically blending the polymer with PDLA (poly-D-lactide). PDLA and PLLA form a highly regular stereocomplex with increased crystallinity.

The temperature stability is maximised when a 50:50 blend is used, but even at lower concentrations of 3-10% of PDLA, there is still a substantial improvement. In the latter case, PDLA acts as a nucleating agent, thereby increasing the crystallization rate. Biodegradation of PDLA is slower than for PLA due to the higher crystallinity of PDLA. PDLA has the useful property of being optically transparent.

Bacterial fermentation is used to produce lactic acid from corn starch or sugarcane feedstocks, which is oligomerized and then catalytically dimerized to make the monomer for ring-opening polymerization. It can be easily produced in a high molecular weight form through ring-opening polymerization using most commonly a stannous octoate catalyst, but for laboratory demonstrations tin(II) chloride is often employed.


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