Thermal analysis of poly(lactic acid) plasticized by cardanol derivatives

Abstract

This work is aimed to study the suitability of cardanol and its derivatives as plasticizers for poly(lactic acid), PLA. Differential scanning calorimetry (DSC) was used to assess the plasticizing effectiveness of cardanol (CARD) and its derivatives, cardanol acetate and epoxidated cardanol acetate, comparing the results with those obtained with a commercially available plasticizer, poly (ethylene glycol), PEG, with an average molecular weight of 400 g mol−1. Measurement of the glass transition temperature highlighted that, among the tested cardanol derivatives, neat cardanol is the most effective plasticizer for PLA. In fact, the glass transition temperature of PLA plasticized by CARD is only slightly higher than that of PLA plasticized by PEG. This is attributed to the lower compatibility between PLA and CARD compared to PLA and PEG, as estimated by the interaction radius. Therefore, cardanol could represent a technically valid, economic, and largely available plasticizer for PLA. Moreover, DSC, an X-ray diffraction analysis, showed that, compared to PEG, the addition of CARD involves a limited increase of the rate of crystallization, even in this case, due to its lower compatibility with PLA. Dynamic mechanical analysis showed that, below glass transition, PEG is able to reduce the stiffness of PLA by a higher extent. However, as the temperature increases, retention of the shear modulus of PEG-plasticized PLA is much higher than that of CARD-plasticized PLA. As a consequence, above glass transition, the stiffness of CARD-plasticized PLA becomes lower than that of PEG-plasticized PLA. Therefore, despite its lower compatibility with PLA, CARD can impart to plasticized PLA lower modulus compared to PEG, when the plasticizer content is high enough to reduce the glass transition below room temperature.