Effect of tin on poly( l-lactic acid) pyrolysis

Abstract

Tin 2-ethylhexanoate is an indispensable component of commercially available poly( l-lactic acid) (PLLA). However, the thermal degradation kinetics of PLLA containing Sn have not yet clearly been established; in particular, whether the degradation mechanism is a 1st-order or a random reaction. To clarify the effects of residual Sn on PLLA pyrolysis, PLLA samples with different Sn contents from 20 to 607 ppm were prepared and subjected to pyrolysis analysed with pyrolysis-gas chromatography/mass spectroscopy (Py-GC/MS) and thermogravimetry (TG). The pyrolysis of PLLA Sn-607 (Sn content: 607 ppm) with Py-GC/MS in the temperature range of 40–400 °C selectively produced lactides. In contrast, the pyrolysis of PLLA Sn-20 (Sn content: 20 ppm) was accompanied by the production of cyclic oligomers. The dynamic pyrolysis of PLLA-Sn samples by TG clearly indicated that with an increase in Sn content there was a shift to a lower degradation temperature range and a decrease in activation energy Ea. The kinetic analysis of the dynamic pyrolysis data indicates that the Sn-catalyzed pyrolysis starts through a random degradation behaviour and then shifts to a zero-order weight loss as the main process. Three reactions were put forward as being possible mechanisms of the zero-order weight loss;, one being an unzipping reaction accompanying a random transesterification, the other two being the Sn-catalyzed pseudo-selective and selective lactide elimination reactions from random positions on a polymer chain. The kinetic parameter values obtained could be adequately explained for each degradation process.