Structuring of non-Brownian ferrite particles in molten polypropylene: Viscoelastic analysis

This paper focuses on the relation between the structure and the rheological properties of non-Brownian ferrite particles suspended in a molten polypropylene matrix with and without the addition of a dispersant agent. The studied filler levels were 2, 15, and 20 vol. %. Under low deformation, the measurement of the complex shear modulus over time showed that the material structure was not stable. Such evolution was, on the one hand, more visible at concentrations near the percolation threshold (phi(c) approximate to 0.25) and on the other hand, less visible when adding a dispersant agent. Oscillatory time sweep experiments were performed after a preshearing. It was shown that the kinetics of the evolution strongly depends on the applied deformation. Such evolution has also been highlighted by optical microscopy, in which an agglomeration phenomenon was clearly observed after few hours, again less visible for formulations with a dispersant agent. Under high deformations, no structure evolution was noticeable. Our results stress the important role of adding a dispersant into the formulation of highly filled materials. The dispersant agent acts therefore as a stabilizer and a lubricating agent. The origin of such structuring process has been attributed to a favorable entropy gradient from the polymer chains between the particles and the polymer chains in the bulk. (C) 2016 The Society of Rheology.