The effect of shear stress on the foaming process has been studied using the Foaming Process Simulator developed previously. The polymer samples were saturated with gas in the test chamber. A rotor was used to apply shear stress to the polymer samples. Foams were obtained by releasing the pressure quickly. Polystyrene. Hled and unfilled, was used as the material. The cell density was analyzed with a scanning electron microscope. It was found that the cell density was s - -cantly increased by introducing shear stress. The higher the shear stress, the more significant the effect. A cell stretch model has been developed to explain the cell nucleation enhancement with shear stress. The nucleation sites are stretched under the shear stress. The stretched nuclei are much easier to expand for cell formation owing to their larger surface areas and non-spherical shapes. The model prediction shows the same tendency of the effect of shear stress observed in the experiment. The key issue with shear stress nucleation is the transformation of mechanical shear energy into surface energy.

This work concerns the effects of the filler size on cell nucleation during the foaming process. The cell density of foams with fillers of two different sizes has been investigated using the foaming process simulator developed previously. It was found that the cell density is strongly affected by the filler size. Foams with a fine filler show a higher cell density at a high saturation pressure but give a lower cell density at a low saturation pressure. At a certain value of the saturation pressure, cell density becomes similar with both fillers. This transition pressure changes with the foaming condition. It goes down with a higher pressure drop rate. The experimental results have been explained with an analysis off iller particle size distribution. The analysis also recommended a way to select filler size if a high cell density is desired in the foaming process.

This work concerns the effects of gas type on the microcellular foam process when using atmospheric gases as a blowing agent. Three gases were investigated, CO2, N2 and argon. Gas absorption, viscosity reduction and cell formation were tested with these gases using the foaming process simulator (2) and an extruder with a capillary die. Significant differences were observed among these gases. The gas absorption of N2 and argon in polymers was much lower than CO2. The diffusion rate also seems to be lower with N2 and argon. The viscosity reduction was also lower with N2 and argon. Cell nucleation density was similar for all the three gases at high saturation pressures, but it was significantly lower with N2 or argon when the saturation pressure is low. The cell size is smaller with N2 and argon at a high saturation pressure, especially with N2. As a result, the foam density is higher with these two gases at the same foaming conditions compared to CO2.