Foaming behaviour and cellular structure of LDPE/hectorite nanocomposites

This paper presents the compared analysis of the foaming behaviour and cellular structure of LDPE/hectorite nanocomposites and respective neat LDPE foams. To assess the influence of hectorite on the foaming behaviour and final foam morphology, nanocomposites containing 3 and 7 wt.% of a modified hectorite were first melt-compounded in a twin-screw extruder. Variables such as temperature, pressure and time were optimized to prepare foams in a second stage by a two-step compression-molding process. Crystallinity and crystal structure of the polymer matrix were determined using X-ray scattering (WAXS) and differential scanning calorimetry (DSC). Clay intercalation/exfoliation was analyzed by WAXS and transmission electron microscopy (TEM), with the results indicating that partial exfoliation of the particles was only reached with foaming but not during melt mixing. A quantitative characterization of the cellular structure and morphology of the foamed nanocomposites was done using both scanning (SEM) and transmission electron microscopies. The nanocomposite foams exhibited differences in the crosslinking degree, showing lower gel content values (from 35% of the neat LDPE to as low as 28% for the 7 wt.% hectorite foam), expansion behaviour, cell aspect ratio, with the foamed nanocomposites showing more isometric type of cells, and cell wall texture with regard to the neat LDPE foams. All these differences, analyzed and compared for the three composites, directly affect both the thermal and mechanical responses of the foams and due to that fact are of extreme importance. (c) 2007 Elsevier Ltd. All rights reserved.