Carbon nanotubes in microwave foaming of thermoplastics

Abstract

In this work, we introduce the use of selective microwave heating to solve the problems of conventional heating systems, such as non-uniformity and thickness restriction, in classical thermoplastic foaming methods. To do so, carbon nanotubes (CNTs) are used as microwave absorbers in a transparent polymer media. When irradiated, CNTs are responsible for a local temperature increase in the polymer/gas solution, allowing local polymer softening and increased blowing agent super-saturation, in turn inducing bubble nucleation and growth. The huge number of active sites (CNTs) for both heating and bubble nucleation and the possibility to fine-tune the heating energy provide large uniformity and an extra control over the bubble growth, to generate well-expanded, fine-celled, uniform, relatively thick foams. As a model system, we successfully foamed 1 cm-thick polystyrene/CNT nanocomposites with CO2, and achieved uniform morphologies and large expansion ratios (over 22 folds). For proper comparison, we also utilized conventional foaming methods such as water bath and fast pressure drop, both resulting in denser and less uniform foams. Here, for the first time, a single particle type, CNTs, is used as a “nucleating agent”, a “heating agent” and, as it will be seen in the following, a “collapse-preventing agent”.