Preparation and Characterization of Microcellular Polystyrene Foams Processed in Supercritical Carbon Dioxide

Abstract

The foaming of polystyrene using supercritical (SC) CO2 has been studied to better understand the microcellular foaming process, as we plan future studies that involve the creation of composite microcellular foams. Rapid decompression of SC CO2-saturated polystyrene at sufficiently high temperatures (above the depressed T g) yields expanded microcellular foams. The resulting foam structures can be controlled by manipulating processing conditions. Experiments varying the foaming temperature while holding other variables constant show that higher temperatures produce larger cells and reduced densities. Structures range from isotropic cells in samples retaining their initial geometry to highly expanded foams recovered in the shape of the foaming vessel and having oriented, anisotropic cells and limited density reduction. Higher saturation pressures lead to higher nucleation densities and hence smaller cells. Decreasing the rate of depressurization permits a longer period of cell growth and therefore larger cell sizes. Foams having a bimodal distribution of cell sizes can be created by reducing the pressure in two stages.

Microcellular foaming of polymethylmethacrylate in a batch supercritical CO2 process: Effect of microstructure on compression behavior

“…As mentioned before, although the foaming experiments were not carried out at the same temperature, no great variation of density is found. On the other hand, this result is in accordance with the small variation in density described in the preparation of microcellular foams from amorphous high-T g polymers, such as polysulfone foams 18,21 or polystyrene, 8 where maximum variations of 20% (with respect to density of unfoamed polymers) are described as a function of foaming time. When density is close to 1 two limiting factors are present: (i) growth is limited by the proximity of the glassy state due to fast cooling, (ii) the transformation of scCO 2 into liquid CO 2 , which is not a foaming agent.…”

“…First, samples were saturated with supercritical CO 2 at 40 C at a maintained pressure of 300 bar during 24 h. Saturation time was selected to assure the complete equilibrium swelling of the polymer by CO 2 . 20,23,31 The depression of the glass transition temperature induced by supercritical CO 2 in several amorphous polymers is well known, 8,25 showing a linear relation T g -pressure, but in PMMA, a retrograde behavior is exhibited. [32][33][34] The T g -Pressure curve is no longer linear and presents a ''retrograde envelope'' between 2 and 6 MPa, i.e.…”

“…In this method, microstructure may be controlled by changing the saturation temperature and depressurization rates. Following this line of investigation, several microcellular polymers have been already obtained, including polystyrene, 6,8 polycarbonate, 9 polystyrene-co-methylmethacrylate, 10 and Polymethylmethacrylate. 11,12 In the second process, a two-step process, the polymer is saturated with scCO 2 at high pressure and low temperature, in a glassy state.…”

Carbon dioxide‐induced crystallization in poly(L‐lactic acid) and its effect on foam morphologies

“…[1][2][3][4][5] The morphologies generated via CO 2 processing are dependent on treatment pressures and temperatures, the physical properties of the polymer and the modifications thus induced as a result of contact with CO 2 . The solubility of CO 2 in a polymer leads to relaxation, chain mobility and hence reduction in the polymer's glass transition temperature (T g ).…”

Microcellular foaming of PP/EPDM/organoclay nanocomposites: the effect of the distribution of nanoclay on foam morphology

“…3 Microcellular foaming technology has been rapidly developed in the past 2 decades, and microcellular parts have been produced via batch, injection molding and extrusion processes. [4][5][6][7][8][9] Despite a variety of production methods, the main foaming mechanism is the same. In the first step, the polymer is saturated with an inert gas at high pressure.…”