Effect of processing conditions on the formation of polypropylene/organoclay nanocomposites in a twin screw extruder
Résumé
Polypropylene (PP)/organoclay (Cloisite©20A) nanocomposites are prepared via direct melt intercalation in a co-rotating twin screw extruder. Maleic anhydride (MA)grafted PP (PP-g-MA) is used as a compatibilizer to improve the dispersion of the clay. The formulation used to prepare the nanocomposites is fixed and is equal to 80/15/5 (PP/PP-g-MA/Cloisite©20A), expressed in mass fraction. The objective of the present study is to investigate the effects of processing conditions as well as screw profile upon the formation of PP nanocomposites. The parameters studied are the feed rate and the screw speed, which are varied independently, from 4.5 to 29.0 kg/h and from 100 to 300 rpm, respectively. The state of dispersion is quantified by wide angle X-ray diffraction (WAXD), transmission electron microscopy, and rheological measurements. WAXD results show that the nanocomposites obtained in different conditions have an intercalated structure, with an increase in interlayer spacing. However, this interlayer spacing is globally unaffected by processing parameters. On the opposite, the proportion of exfoliation, estimated by rheological measurements, is depending on operating conditions (screw speed and feed rate). It increases when the feed rate decreases and the screw speed increases. Investigations on the state of dispersion along the screw profile are also presented. They show that no evolution of intercalated structure is observed along the screws and that screw geometry is only efficient in particular extrusion conditions to delaminate clay platelets. Numerical simulations of the twin screw extrusion process, using the software Ludovic©, put in evidence that the total strain is a key factor for characterizing the level of exfoliation in the nanocomposites