Morphology, Crystallization and Melting Properties of Isotactic Polypropylene Blended with Lignin

The influence of lignin (L) on the thermal properties and kinetics of crystallization of isotactic polypropylene (PP) is reported in this article. PP blends containing 5 and 15 wt % of L were prepared by mixing the components in a screw mixer. An increase of the thermal degradation temperature of the blends was observed as a function of L content. The crystallization and thermal behavior of the pure PP and of the PP/L blends were analyzed by differential scanning calorimetry (DSC). Isothermal crystallization kinetics were described by means of the Avrami equation, which suggests a three-dimensional growth of crystalline units, developed by heterogeneous nucleation. The isothermal growth rate of PP spherulites was studied using a polarizing optical microscope. The enhancement of PP crystallization rate for the PP/L blends was observed and ascribed to the nucleating action of lignin particles. Non-isothermal crystallization kinetics were applied, according to the results elaborated by Ziabicki and the method modified by Jeziorny. The kinetic crystallizability of the PP is not influenced by the L present in the blend. In the presence of L, PP can simultaneously crystallize in both the and crystalline forms, and the ratio between the and forms was determined by X-ray diffraction analysis. Two melting peaks relative to the two crystalline form of PP were observed for the PP/L blends, for all isothermal crystallization temperatures investigated by means of DSC. The equilibrium melting temperature for -form of pure PP was obtained.