In this study, linear homopolypropylene/clay (HPPC) nanocomposite foams with a high expansion ratio of about 18 and a high cell density of about 1.7 × 10 8 cells/cm 3 were produced using an extrusion foaming method with CO 2 as the physical blowing agent. The result was much better than pure HPP foams with expansion ratios of 1.7-2.2 and cell densities of 10 3 -10 5 cells/cm 3 obtained even at the same foaming conditions. The nanoclays had a half-exfoliated structure in the HPP matrix, and their presence dramatically affected the viscoelastic properties of HPP melt and foaming behaviors. It was found that the introduction of a small amount of nanoclay significantly increased the cell morphology of HPP foams at low die temperatures, where the cell wall was very thin and cell distribution was uniform. With an increase in nanoclay content of up to 5 wt %, cell morphology was improved gradually at broader die temperatures. Based on the cell morphology results, a suitable foaming window for clay content and die temperature was established. The mechanisms behind these phenomena are discussed from the perspective of cell nucleation and coalescence. Microstructures were found in the cell walls of HPP and HPPC nanocomposite foams, and they tended to evolve with cell wall thickness, depending on the die temperatures. Scanning electron microscopy (SEM) observation of foams and solvent-etched foams revealed that the microstructures in the cell walls were formed by covering large-sized crystals and that the absence of microstructures was due to the presence of smallsized crystals in the cell walls. A distribution of crystal sizes was observed across the foamed samples, which was affected by the die temperature and the introduction of nanoclay. The possible reasons were elaborated by considerations of temperature gradient. DSC tests indicated that the foaming process induced a lowtemperature peak (T m1 ) and its heat of fusion (∆H m1 ) tended to evolve with the die temperature and the introduction of nanoclay.

Blends of linear-low-density polyethylene (LLDPE), lowdensity polyethylene (LDPE), and high-density polyethylene (HDPE) were foamed and characterized in this research. The goal was to generate clear dual peaks from the expanded polyethylene (EPE) foam beads made from these blends in autoclave processing. Three blends were prepared in a twin-screw mixing extruder at two rotational speeds of 5 and 50 rpm: Blend1 (LLDPE with 20 wt% HDPE), Blend 2 (LLDPE with 20 wt% LDPE), and Blend 3 (LLDPE with 10 wt% HDPE and 10 wt% LDPE). The differential scanning calorimetric (DSC) measurement was taken at two cooling rates: 5 and 508C/min. Although no dual peaks were present, the results showed that blending with HDPE has a more noticeable effect on the DSC curve of LLDPE than blending with LDPE. Also, the rotational speed and cooling rate affected the shape of the DSC curves and the percentage area below the onset point. The DSC characterization of the batch foamed blends revealed multiple peaks at certain temperatures, which may be mainly due to the annealing effect during the gas saturation process.

This article investigates the foaming process of bi-cellular polystyrene foams blown with n-butane and water in extrusion. The bi-cellular foam structure has two types of cells: large cells ranging from 0.1 to 1.2 mm and small cells ranging in size from about 5% to about 50% of the average large cell size, which constitute more than 90% of the total cell volume. A bi-cellular structure has outstanding heat insulation property. In order to generate a bi-cellular structure, a water-blowing technology was used. This technique is environmentally benign and economical since butane and water are used as blowing agents. Despite these advantages, the foaming process and mechanism of bi-cellular foams have not been identified in detail. Therefore, in this article, an attempt has been made to enhance our knowledge and understanding of the foaming behavior of bi-cellular polystyrene foam. The effects of n-butane, water, and silica (a nucleating agent) on the foam cell morphology are presented.