Extrusion of Polystyrene Nanocomposite Foams

N,
=xCI e x P ( - A G , , e , * / ~ )
(1)
*To whom correspondenceshould be addressed. Email:
where NI is the rate of heterogeneous nucleation, fi is the frequency factor of gas molecules joining the nucleus, C, is the concentration of heterogeneous nucleation sites, k is the Boltzmann's constant, T is the temperature, and the nucleation energy AG,,'' is related to the interfacial tension u, wetting angle 0 at the polymer-additive-gas interface, and the pressure difference AP in and out of the cell (bubble) by:
INTRODUCTION
olymeric foams are found in applications ranging from packaging, insulation, cushions, and adsorbents (1). to scaffolds for tissue engineering (2). Recently, microcellular foams generated by using supercritical CO, as the foaming agent have drawn a great deal of attention and interest in industry and academia (3- 13). Compared with traditional foaming agents (hydrocarbons or chlorofluorocarbons), CO, at its supercritical state (T, = 31°C and P, = 73.8 bar, or 1074 psi) exhibits several advantages (14). The high diffusivity makes it possible to dissolve sufficient CO, in a polymer melt quickly. CO, can reduce the viscosity and surface tension of polymer melts, which assists many polymer processing operations. Moreover, CO, is low-cost, non-flammable, environmentally friendly, and chemically benign. Microcellular foams, characterized by cell sizes smaller than 10 pm and cell densities larger than lo9 cells/cm3 (15). can greatly reduce material use without compromising mechanical properties. Typical microcellular plastics exhibit high impact strength or toughness (16-20) and high fatigue life (21, 22). Microcellular polymers are light in weight, and consequently,
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POLYMER ENGINEERING SCIENCE, JUNE 2003, Vol. 43, No. 6
Xiangmin Han, Changchun Zeng, L. James Lee, Kurt W. Koelling, and David L. Tornasko
AGhet*= 16 TI a3f(0)/(3 p2)
in whichf(0) is a function of 8. Although the nucleation mechanism is still not well understood, it is generally known that the size, shape, and distribution of the particles, as well as the surface treatment can affect the nucleation efficiency. Ramesh et aL (38,39) developed a model for heterogeneous nucleation in the blend of polystyrene (PS) and high impact polystyrene (HIPS)based on a micro-void theory that assumes micro-voids exist at the filler-polymer interface. A polystyrene-zinc stearate system was selected by Colton et al. (31-33) to investigate nucleation phenomenon. The homogeneous nucleation theory was extended to a heterogeneous case. They considered the free volume change caused by the dissolved gas and the surface tension change caused by the presence of a third phase and their influence on the Gibbs free energy of nucleation. Park et a l . (40) studied the effect of talc on the cell nucleation of polypropylene foamed by CO, and isopentane. They found that when isopentane is used as the foaming agent, bubble nucleation is dominated by the talc concentration and not affected too much by the isopentane concentration. In contrast, the effect of talc is significant only a t low CO, content and then the CO, content primarily determines the cell nucleation. Recently, Chen et al. (41) stated that the filled polymers 6.e. high density polyethylene (HDPE)/talc, and poly(viny1 chloride) (PVC)/CaCO,) absorb more CO, compared to the unfilled ones since the gas tends to accumulate on the filler-polymer interface and helps create nucleation sites, Furthermore, foams with finer fillers show a higher cell density at a high saturation pressure and a lower cell density at a low saturation pressure. All these studies are based on micron-sized particles. In this work, nano-sized particles, nano-clays, are applied to modify the microcellular foams in a continuous extrusion foaming process. It has been shown that with the addition of a very small amount of nanoclay into the polymer matrix, the nanocomposites exhibit substantial increase in many physical properties, including mechanical strength (e.g. tensile modulus and strength, flexural modulus and strength), thermal stability, flame retardance, and barrier resistance (42471. Smectite clays, such as montmorillonite (MMTI, are of particular interest because they offer a high aspect ratio (100- 1000) and a high surface area. Montmorilonite is hydrophilic in nature, which hinders its dispersion in the organic polymer phase. Ion exchange of the interlayer inorganic cations (Na+,Ca,') with an organic cation renders the hydrophilic clay surface organophilic. The reduction in surface energy improves the wetting characteristics of the clay surface with polymers or monomers. Depending on the dispersion of clay, nanocomposites may possess two idealized structures: intercalated and exfoliated. Intercalation results from limited insertion of polymer chains into the interlayer region and interlayer expansion. Usually the ordered layer