Diffusivity, foaming process

Lin et al. ( 6) measured the emulsion capacity of defatted sunflower seed products. Data in Table VII show that sunflower flour was superior in emulsifying capacity to all other products tested. The emulsions were in the form of fine foams and were stable during subsequent heat treatments. The diffusion-extraction processes employed to remove phenolic compounds dramatically reduced emulsion capacity, although isolating the protein improved emulsion capacity to some extent. 

It is technically difficult to distinguish between diffusion foam collapse and coalescence. That is why the rate of the entire process of internal foam collapse is usually studied. Fig. 6.9 depicts some typical curves for the increase in the average bubble size and the decrease in the specific foam surface. 

The foaming process may also stress the laminate due to the combined effect of temperature and applied pressure on critical areas such as the panel edges and the corners. Defects generated here may in turn increase the gas diffusion inside the panel, reducing the designed lifetime. 

On the contrary, the gas dissolution foaming process, and in particular the high-pressure or supercritical CO2 gas dissolution foaming, allows obtaining micro-and nanoporous polymers. In this technique CO2 is used as a physical blowing agent [39,57-59] this gas is one of the best options for this kind of process because of its excellent characteristics of diffusion in the supercritical state and the mild conditions to reach this state (31°C and 7.3 MPa). Last but not the least, carbon dioxide is a green solvent that can be removed without residue or production of any pollutant compound [60,61]. 

Some applications nonrelated to the properties of the nanoporous materials but to their porous structures are their use as filtration membranes, battery separators (hindering the diffusion of ions in the narrow channels), and catalyst supports (due to their high surface area) as well as gas capture and storage or light harvesting [72]. However, the common factor of all of these applications is the requirement of an open nanoporous structure not only inside the sample but also connected to the exterior of the sample. However, the CO2 foaming process from nanostructured polymers still has not allowed obtaining nanoporous samples with all of these features. Pinto et al. [102] proposed that 25/75 PMMA/MAM nanoporous foams present appropriate inner porous structures for these kinds of applications (bicontinuous nanoporous structures with tunable pore size), but further studies are required to connect effectively this inner porous structure with the exterior of the sample. 

Another key factor for foam processing is the mutual diffusivity of the blowing agent and the polymer. It affects the nucleation (nucleation has been described as a short range diffusion process) and the growth phenomena (gas has to diffuse out of the solution to inflate the bubbles), while from the processing point of view, diffusion... 

Most of these characteristics cannot be simply evaluated in experimental tests because of the difficulties of measuring physical and mechanical properties of polymer/gas solutions in some cases and of the long time required for the experiments (i.e. solubility and diffusivity) in other cases. Therefore, a theoretical prediction of the effect of the operative conditions on the foaming process becomes an important tool for an off-line optimisation of the entire process, and requires the knowledge of the equation of state (EOS) able to correlate pressure, volume and temperature of the material. Several empirical and theoretical equations of state for polymer melts have been proposed by different authors and have been... 

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