Gas dissolution foaming

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]. 

Development of polymer nanoporous foams by gas dissolution foaming requires reaching veiy high pore nucleation densities Ng, number of nuclei/pores per cubic centimeter of the unfoamed material) and reduced coalescence. Coalescence can be avoided, or maintained without significant influence, by an appropriate selection of the polymer matrix and of the foanfing temperature in comparison with the effective glass transition tanperatuie. 

Figure 9.5 Schemes of (a) one-step gas dissolution foaming and (b) two-step gas dissolution foaming. Evolution of the samples from solid to foamed are represented schematically, together with the usual evolution of both temperature (T) and pressure (P) during the foaming process (from the saturation stage to the foaming stage).

Using CO2 gas dissolution foaming processes, several approaches were used to promote the pore nucleation up to the desired levels (typically higher than 10 " -10 nuclei/cm ), depending on the nucleation mechanisms involved ... 

More recently, last evidences about the relationship between an initial stmcturation and a final nanoporous structure have been provided by Forest et al. [105] using ABS terpolymers sheets. They found relationships in shapes and sizes between the nanonodules of different ABS terpolymers and the resulting porous structure obtained by gas dissolution foaming (Figs. 9.35 and 9.36). Therefore, they concluded that the pore nucleation happens inside the butadiene nodules, with the pore growth controlled by the deformation of the stiffer acrylonitrile/styrene/C02 matrix. 

J. Pinto, et al.. Solid skin characterization of PMMA/MAM foams fabricated by gas dissolution foaming over a range of pressures. Defect and Diffusion Forum 326-328 (2012) 434-439. 

Eurthermore, the foaming process can be performed in a batch or continuous system. Batch foaming is characterized by being simple and easy to control. The material is foamed inside an autoclave reactor by a high-pressure gas dissolution... 

FIGURE 49.9 Schematic representation of foaming process (1) dissolution of gas into polymer, (2) cell nucleation, (3) cell growth and expansion, and (4) SEM of hydroxypropyl methyl cellulose acetate succinate (HPMCAS) extrudate foamed with nitrogen gas. 

One of the interesting apphcations where scC02-processing of polymeric materials is beneficial is the method of PGSS (Particles from Gas Saturated Solution). PGSS is a technique able to form polymer/active compound foams, solid particles, or droplets [160]. The principle of the technique is for CO2 to form a gas-saturated solution/suspension which may then be foamed or passed through a nozzle to produce sohd particles or droplets. The technique is suited to thermally labile components, since the process is undertaken at near ambient temperature. Control of particle size has been achieved by the introduction of N2 back-pressure in the collection chamber, as demonstrated by Hao et al. [161] with poly(DL-lactic acid). The PGSS method has been shown to enhance the dissolution characteristics of nifedipine from PEG 400 [162, 163]. 

Finally, we should draw attention to the prevalent use of air-degassed crude oil systems and foam generation by sparging at ambient temperatures and pressures. It is known that solubilities of asphaltenes, resins, PDMS, and PDMS derivatives are likely to be influenced by temperature and dissolution of natural gas. Moreover, sparging represents a poor model for foam generation in gas-oil separators, which involves depressurization and nucleation of bubbles. Use of apparatus designed to replicate the conditions in actual gas-oil separators for basic studies should therefore be encouraged. 

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