Macrocellullar low-density polymeric foams are found in applications ranging from packaging, insulation, cushions, and adsorbents, to scaffolds for tissue engineering. Recently, microcellular foams, characterized by cell sizes smaller than 10 mm and cell densities larger than 10 9 cells/cm 3, have drawn a great deal of attention and interest in industry and academia. Typical microcellular plastics exhibit high impact strength or toughness and high fatigue life. Microcellular polymers are light in weight and consequently, they have a higher mechanical strength to weight ratio than common structural foams at equivalent densities.
Compared with traditional foaming agents (hydrocarbons or chlorofluorocarbons), carbon dioxide (CO 2) at its supercritical state (T c = 31°C and P c = 73.8 bar, or 1074 psi) exhibits several advantages. The high diffusivity makes it possible to dissolve sufficient CO 2 in a polymer melt quickly. CO 2 can reduce the viscosity and surface tension of polymer melts, which assists many polymer processing operations. Moreover, CO 2 is low-cost, non-flammable, environmentally friendly, and chemically benign.
Comparing to conventional micron-sized filler particles used in the foaming process, nanometer-sized clay particles may offer unique properties. Novel polymer clay nanocomposite foams were prepared using carbon dioxide as the foaming agent. The role of clay on the foaming process was thoroughly investigated. It was found that clay serves as an efficient nucleation agent. Nucleation efficiency increases as the degree of clay dispersion improves. Anchoring a CO 2 philic polymer on clay surface greatly reduces heterogeneous nucleation energy and leads to substantial increase in nucleation efficiency. Combination of clay exfoliation and interfacial properties provide a powerful mean to enhance cell nucleation in foaming ( Figure 4).
The nanocomposites foaming was also conducted in a continuous extrusion foaming process using CO 2 as the foaming agent. Again the strong nucleation effect of clay nanoparticles was observed. 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 mm and increases cell density from 2.7 ´ 10 7 to 2.8 ´ 10 8 cells/cm 3. Once exfoliated, the nanocomposite exhibits the highest cell density (1.5 ´ 10 9 cells/cm 3) and smallest cell size (4.9 mm) at the same particle concentration ( Figure 5).
Compared with polystyrene foams, the nanocomposite foams exhibit higher tensile modulus ( Figure 6), improved fire retardance ( Figure 7), 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.
 Additionally we are also exploring polymer/CNF nanocomposite foams. With the high aspect ratio and nano-scaled dispersion, CNF could considerably decrease the cell size and increase the cell density even with a low nominal particle concentration ( Figure 8). The nano-sized particles serve as the nucleation agent and induce a heterogeneous bubble nucleation during the foaming process. Current study focuses on surface modification of CNF for improved particle dispersion and fine tune of polymer particle CO2 interactions for improved nucleation efficiency. Moreover, comprehensive characterization of the properties of the PS/CNF nanocomposite foams is underway.
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