Antioxidant properties of lignin in polypropylene

This work deals with lignin antioxidant properties in polypropylene. We extracted and characterised 15 lignin samples from various botanical origins (wheat straw and wheat bran) using different processes of extraction and using industrial Kraft lignins fractions (Westaco). These lignins were incorporated in polypropylene (1% weight). We underline the effect of lignin solubility in PP on stabilisation behaviour. The solubility in polypropylene is very low; as a consequence, classically important factors such as phenol content and intrinsic reactivity of the molecule are less important than factors making easier solubility. Solubility in PP could not be measured directly; a morphological criterion (average dimension of lignin aggregates in the blend) has been correlated to antioxidant activity and is thought indirectly to reflect solubility behaviour. The following factors tend to improve the compatibility (blend morphology) and consequently the antioxidant activity: low molecular weight low total OH content (aliphatic + phenolic) The non-linear relation between Kraft lignin activity and its concentration in the blend has been explained by molecular weight polydispersity, and correlated to activities of Kraft fractions. (C) 2003 Elsevier Science Ltd. All rights reserved.