Rheological characterization of a thermally unstable bioplastic in injection molding conditions.
Résumé
Poly(hydroxy-alcanoates) bioplastics have an unusually small temperature processing window due to their thermal instability, which affects both molecular weight distribution and rheological behavior. The use of a rheometrical device mounted on an injection molding machine has been shown to allow the study of dynamic viscosity variations for very short residence times in the melt (down to a few tens of seconds). Moreover, dynamic viscosity can be obtained over a wide range of shear rates (up to 50000 s−1) with both Bagley and Rabinowitsch corrections. The rheological data obtained for a poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PHBV copolymer shows that the reduction in viscosity associated with degradation is very fast and is detectable even at temperatures close to the melting point. Size Exclusion Chromatography analysis of selected samples shows that, except for residence times below 15s, molecular mass distribution is substantially affected, shifting toward lower masses, and with an increase in oligomer compound content. These oligomers may act as plasticizers of the bioplastic. The results also suggest that important thermomechanical degradation takes place during the plasticization/feeding step of injection molding.