Technological watch

Polylactic acid (PLA) represents a biodegradable thermoplastic polyester and is widely applied in biomedicine and food packaging. The slow crystallization kinetics of PLA, often observed in conventional transformation processes (extrusion, injection, etc.), involves difficulties regarding to thermal and mechanical properties of the final product. Annealing represents a simple and inexpensive technique, allowing to find a window in annealing time and temperature below fusion, with an improvement of the rigidity of PLA.AbstractEffects of annealing temperature (Ta: 80–140°C) and time (ta: 3?30?h) on the crystalline phase transition in poly(lactic acid) (PLA) were studied by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). In the DSC curves, the sample annealed at Ta = 80°C with time interval (ta: 10–30?h) demonstrates a peculiarly small exothermal peak (Texo) around 130°C, just prior to the melting point, corresponding to the disorder?to?order (?’?to??) phase transition, while the sample annealed at temperature (Ta: 90–110°C) shows a double melting behavior considered as the ?’?? phase transition. Towards low temperatures, the glass modulus Eg reported by DMA thermograms, shows an important increase (~30,000?MPa) at Ta = 80°C for ta = 3 h, due to the extremely high self?nucleation density in low?crystallized PLA materials. After a sharp drop to 3600?MPa at Ta = 110°C, a marked improvement of Eg (15,900?MPa) is observed around Ta = 120°C for all samples, regardless of time ta. This interesting effect (improvement of Eg in the range Ta = 100–120°C) can be correlated with the grow of crystallinity in the same domain of Ta, and the ?’?? phase transition Ta (Ta: 90–110°C) determined by the double Tm melting DSC peak, which is confirmed by the increase of Tg for Ta = 90–110°C.