Intercalated and exfoliated polystyrene/nano‐clay composites were prepared by mechanical blending and in situ polymerization respectively. The composites were then foamed by using CO2 as the foaming agent in an extrusion foaming process. The resulting foam structure is compared with that of pure polystyrene and polystyrene/talc composite. At a screw rotation speed of 10 rpm and a die temperature of 200°C, the addition of a small amount (i.e., 5 wt%) of intercalated nano‐clay greatly reduces cell size from 25.3 to 11.1 μm and increases cell density from 2.7 × 107 to 2.8 × 108 cells/cm3. Once exfoliated, the nanocomposite exhibits the highest cell density (1.5 × 109 cells/cm3) and smallest cell size (4.9 μm) at the same particle concentration. Compared with polystyrene foams, the nanocomposite foams exhibit higher tensile modulus, improved fire retardance, and better barrier property. Combining nanocomposites and the extrusion foaming process provides a new technique for the design and control of cell structure in microcellular foams. 相似文献
A strategy of CO2-assisted extrusion foaming of PMMA-based materials was established to minimize both foam density and porosities dimension. First a highly CO2-philic block copolymer (MAM: PMMA-PBA-PMMA) was added in PMMA in order to improve CO2 saturation before foaming. Then the extruding conditions were optimized to maximize CO2 uptake and prevent coalescence. The extruding temperature reduction led to an increase of pressure in the barrel, favorable to cell size reduction. With the combination of material formulation and extruding strategy, very lightweight homogeneous foams with small porosities have been produced. Lightest PMMA micro foams (ρ = 0.06 g cm−3) are demonstrated with 7 wt% CO2 at 130°C and lightest blend micro foams (ρ = 0.04 g cm−3) are obtained at lower temperature (110°C, 7.7 wt% CO2). If MAM allows a reduction of Tfoaming, it also allows a much better cell homogeneity, an increase in cell density (e.g., from 3.6 107 cells cm−3 to 2 to 6 108 cells cm−3) and an overall decrease in cell size (from 100 to 40 μm). These acrylic foams produced through scCO2-assisted extrusion has a much lower density than those ever produced in batch (ρ ≥ 0.2 g cm−3). 相似文献
ABSTRACTIn the microcellular foam plastic processing, cellular formation stage was being an essential stage since the nucleation and growth of the cell take place within. Based on classical nucleation theory, diminution of the free energy for nucleation, exponentially lead to an increase in the nucleation rate. This can be done by increasing the super-saturation level which achieved by heating the gas-saturated polymer. Hence, the advance is taken out by utilizing the ultrasound wave simultaneously with heating for foaming Polystyrene-scCO2, which, not only to keep the super-saturation degree but also reduce the nucleation barrier. In this work, foaming was conducted under 45 kHz of ultrasound and varying the foaming temperature after saturating polystyrene with scCO2. The results demonstrate, that foaming under ultrasound, the expansion ratio attained up to 1.5 fold, increase along with the heating temperature. Higher cell densities obtained with ultrasound applied at 50°C, however only slight difference can be seen, which about 1010–1011 cell/cm3. From the cell size distribution results, cell distributed around 0.5–3.5 µm, with or without ultrasound applied for 60 and 70°C, Meanwhile at 50°C of foaming, the lowest cell size obtained with the aid of ultrasound in the range of 0.3–2.4 µm. 相似文献
An open-celled structure was produced using polystyrene and supercritical carbon dioxide in a novel batch process. The required processing conditions to achieve open-celled structures were predicted by a theoretical model and confirmed by the experimental data. The theoretical model predicts that at least a saturation pressure of 130 bar and a foaming time between 9 and 58 s are required for this system to produce an open-celled structure. The foaming temperature range has been selected to be higher than the polymer glass transition temperature yet not higher than a temperature limit where the gas starts leaving the system. The experimental results in the batch foaming process verified the model substantially. The SEM pictures showed the presence of pores between the cells, and the mercury porosimetry test results verified the overall open-celled structure. Experimental results also showed that by increasing the saturation pressure and the foaming temperature, there was a drop in the time required for open-celled structure formation. At saturation pressure of 130 bar, foaming temperature of 150 °C and a foaming time of 60 s, open-celled microcellular polystyrene foams were obtained using supercritical CO2 in the batch process. Based on the results, a schematic diagram, depicting the process of foam structure formation from nucleation to bubble coalescence and gas escape from polymer, was proposed. Theoretical calculations showed that by increasing foaming time, cell size was increased and cell density was reduced and the experimental results verified this prediction. 相似文献
Relatioships between the density of foamed rigid PVC/wood‐flour composites and the moisture content of the wood flour, the chemical foaming agent (CFA) content, the content of all‐acrylic foam modifier, and the extruder die temperature were determined by using a response surface model based on a four‐factor central composite design. The experimental results indicated that there is no synergistic effect between teh CFA content and the moisture content of the wood flour. Wood flour moisture could be used effectively as foaming agent in the production of rigid PVC/wood‐flour composite foams. Foam density as low as 0.4 g/cm3 was produced without the use of chemical foaming agents. However, successful foaming of rigid PVC/wood‐flour composite with moisture contained in wood flour strongly depends upon the presence of all‐acrylic foam modifier in the formulation and the extrusion die temperature. The lowest densities were achieved when the all‐acrylic foam modifier concentration was between 7 phr and 10 phr and extruder die temperature was as low as 170°C. 相似文献
Summary: While many experiments have been performed to examine the effects of administering CO2 as a blowing agent in the foaming process, very few studies have investigated the use of N2 for this purpose. In this study, foaming experiments were conducted in extrusion using HDPE as a polymeric material and N2 as a blowing agent. Talc was used as a nucleating agent, and three different pressure‐drop rates were applied to study the effects of pressure‐drop rates on HDPE foams. The experimental results revealed that the void fraction of high‐density foams blown with N2 was not affected by the die temperature, contrasting the situation in low‐density foams. Surprisingly, it was the cell density which determined the void fraction of high‐density foams. It was also found that the use of talc significantly increased the cell density and the void fraction of the foams and minimized the role played by the pressure‐drop rate in cell nucleation.
Effect of N2 content on the cell density of HDPE foams. 相似文献
Summary: Via a batch process in an autoclave, foam processing of intercalated PC/clay nanocomposites, having different amounts of clay, has been conducted using supercritical CO2 as a foaming agent. The cellular structures obtained from various foaming temperature‐CO2 pressure ranges were investigated by SEM. The incorporation with nanoclay‐induced heterogeneous nucleation occurs because of a lower activation energy barrier compared with homogeneous nucleation as revealed by the characterization of the interfacial tension between bubble and matrix. The controlled structure of the PCCN foams changed from microcellular (d ? 20 µm and Nc ? 1.0 × 109 cells · cm?3) to nanocellular (d ? 600 nm and Nc ? 3.0 × 1013 cells · cm?3). The mechanical properties of PCCN foams under compression test were discussed.
TEM micrograph for the structure of the cell wall foamed at 160 °C. 相似文献