Processing of nanocomposites based on lamellar clay by twin-screw extrusion

Nanocomposites first appeared in the early 90s, with the pioneering work of Toyota researchers who showed that incorporating a small amount (a few percent by mass) of lamellar clay into a polyamide could greatly improve its mechanical properties, particularly its stiffness. Originally manufactured in solution or by in-situ polymerization, nanocomposites are now preferentially prepared by melt blending, using conventional compounding processes, essentially twin-screw extrusion. However, to obtain the expected properties, the clays need to be exfoliated within the polymer matrix, i.e. dispersed at the level of the individual sheets that make up the tactoids. While this dispersion is easy with polar polymers, such as polyamides, it becomes much more difficult with non-polar matrices, such as polyolefins. Even after modification with organic cations to make the clays more compatible with the matrix, perfect sheet exfoliation is impossible in the absence of a compatibilizer. In most cases, a polyolefin grafted with maleic anhydride is used as the compatibilizer. Although many advances have been made in recent years in the melt preparation of these nanocomposites, choosing the right screw profile and operating conditions to optimize clay exfoliation and sheet dispersion in the matrix remains a challenge.