Ultrasound propagation in polymer melts and its use for the dispersion of layered nanoparticle in polymer during the extrusion process

Abstract

This thesis aims to better understand the propagation of ultrasound in molten polymers in order to optimize their use for dispersing layered nanoparticles during the extrusion process. The study begins by examining the propagation of ultrasound in highly viscous fluids. It analyzes the influence of dissolved gases and viscosity on the propagation of ultrasound in fluids with a viscosity greater than 5 Pa.s. The objective is to determine whether acoustic streaming or cavitation predominantly occurs under these conditions. To achieve this, ultrasound application via an ultrasonic sonotrode in both partially degassed and non-degassed polydimethylsiloxane, at three different viscosities (5, 30, and 300 Pa.s), was studied using an optical camera. A velocity analysis was also performed using the particle image velocimetry (PIV) method. In all fluids, cavitation was observed, being more pronounced in the nondegassed fluids. Acoustic streaming was primarily observed in fluids with viscosities close to 5 and 30 Pa.s. Furthermore, the fluid velocity was higher in the presence of gas. For the fluid with a viscosity close to 300 Pa.s, the presence of gas, however, hindered proper ultrasound wave propagation, preventing the formation of acoustic streaming. The study then focuses on the impact of ultrasound on the dispersion and exfoliation of layered nanoparticles in a polymer matrix during extrusion. To this end, graphene, a graphitic material, as well as clay, were extruded at concentrations ranging from 2 to 22 wt% depending on the particles, in high-density polyethylene (HDPE). A second extrusion was performed on the nanocomposites, with or without ultrasonic assistance. To evaluate the effect of ultrasound on the dispersion of the three particles, various characterizations were carried out, including rheology, imaging, X-ray diffraction, and electrical conductivity. Ultrasound reduced the agglomerate size for all three types of particles, which increased viscosity at low frequency when the percolation threshold was reached. However, this percolation threshold was not affected by ultrasound, nor was the exfoliation of the particles. Therefore, the effect of ultrasound appears to be limited to the deagglomeration of the particles. The initial particle size had an impact on the efficiency of ultrasound: a larger initial size resulted in a more significant reduction in agglomerates compared to a smaller initial size. No effect of the particle type (graphene or clay) on the efficiency of ultrasound was observed. This work provides a foundation for understanding ultrasound propagation in viscous and viscoelastic polymer melts, paving the way for improved ultrasound-assisted extrusion, nanoparticle dispersion, and polymer processing, which future studies can refine by exploring viscosity thresholds, vacuum conditions, and optimized sonotrode coupling.

Date	2 Dec 2025
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Nicole R. Demarquette (Supervisor) & Éric David (Co-supervisor)