Cell wall drainage

Percent shrinkage remains fairly consistent within the cushion formulations, I, II, III and IV. Increased foam shrinkage was observed with back formulations VII and VIE. This could be attributed to better cell wall drainage efficiency, providing more open foam and/or an overall increased carbon dioxide diffusion through the polymer network. 

The formation of cellular products also requires surfactants to facilitate the formation of small bubbles necessary for a fine-cell structure. The most effective surfactants are polyoxyalkylene-polysiloxane copolymers. The length and ethylene oxide/PO (EO/PO) ratio of the pendant polyether chains determine the emulsification and stabilizing properties. In view of the complexity of the interaction of surfactant molecules with the growing polymer chains in foam production, it is essential to design optimal surfactants for each application. Flexible polyurethane foams require surfactants that promote improved cell-wall drainage. This allows the cell walls to become more open during the foaming reaction. Also, the shift away from TDI to MDI in molded HR foams adds new demands on foam surfactants (97). 

The pulmonary lymphatic system contributes to the clearance of fluid and protein from the lung tissue interstitium and helps to prevent fluid accumulation in the lungs [108], The lymphatic endothelium allows micron-sized particles (e.g. lipoproteins, plasma proteins, bacteria and immune cells) to pass freely into the lymph fluid [103], After administration of aerosolised ultrafine particles into rats, particles were found in the alveolar walls and in pulmonary lymph nodes [135], which suggests that drainage into the lymph may contribute to the air-to-blood transport of the inhaled particles. 

Still another factor in cell stability is the drainage of the liquid in the bubble walls which is due to gravity and capillary action. This drainage from both capillary action and gravity can be retarded by an increase in viscosity, especially at the film surface. This is particularly important in primarily thermoset systems which involve simultaneous polymerization and foaming of the liquid components. A balance between the viscosity and gas evolution must be provided in order to obtain not only a stable foam, but also one with the highest foam volume possible. It is obvious that if the viscosity increases too rapidly (as the result of too fast a polymerization) the gas evolution will eventually cease before reaching its desired foam volume, especially for the production of low-density foams. On the other hand, if the viscosity is too low, when most of the foam evolution occurs, foam stabilization may be very difficult and may result in foam collapse (3). 

Foam structure and dynamics. Surface layers surrounding the bubbles in a foam act as a membrane or skin that can stretch and relax in response to the lateral forces acting on it. At first, drainage of liquid taking place at the surface layer is entirely hydrod)niamic, but once spherical bubbles are in contact, flat walls develop between them, and polyhedral cells appear in the foam (Fig. 14.9c). Capillary forces be-... 

For experimental observation of the drainage and stability of liquid films the capillary cell illustrated in Fig. 16 is widely used (Scheludko and Exerowa 1959). First, the cylindrical glass cell is filled with the working liquid (say, water solution) next, a portion of the liquid is sucked out from the cell through the orifice in the glass wall. Thus, in the central part of the cell a liquid film is formed, which is encircled by a Plateau border. By adjustment of the capillary pressure the film radius, / , is controlled. The arrow (see Fig. 16) denotes the direction of illumination and... 

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