3 General Principlesfor the Synthesis and Modification of Porous Materialsporous structure. Blowing with permanent gases is typically performed batchwise, for instance during injection molding of polymers. If continuous processes, e.g. extrusion, are desired, dissolved hydrocarbons or low-boiling CFCs are preferred. The solubilities of the CFCs can be rather high at elevated pressure, exceeding 50 % (w/w) depending on the polymer and the CFC [ 191. The loading necessary for the blowing of low-density polyethylene, for example, is 5-15 % [19]. After leaving the extruder, the dissolved molecules evaporate, because of the drop in pressure, creating the porous structure. The blowing agent must be selected such that the solubility is substantially lower under the conditions outside the extruder, otherwise insufficient expansion occurs and too much blowing agent remains dissolved in the polymer. Cooling of the polymer after it leaves the extruder preserves the pore system thus formed. For some applications porous beads are desired; these are generated by impregnating nonporous precursor beads with a low-boiling hydrocarbon at elevated pressure and then releasing the pressure to expand the pellets (Fig. 3). The pellets resulting from such an expansion process can be molded by treatment with steam, which leads to surface fusion of the pellets, thus filling the mold and creating the desired shape.Critical in all these blowing processes, as in foaming processes with chemical blowing agents, is the nucleation of the bubbles [18,20,211, because the number of bubbles nucleated, among other factors, determines the size of the resulting pores. Homogeneous nucleation is possible, but nucleation is usually controlled by addition of solids acting as nucleation centers, such as fine particles of silica or silicates. They reduce the surface energy of a bubble formed at their surface and thus facilitate bubble formation. A more detailed description of nucleation is given in Sect. 3.3.2.1.
References see page G5G
