Cellular structure of foams

This survey deals with the fundamental morphological parameters of foamed polymers including size, shape and number of cells, closeness of cells, cellular structure anisotropy, cell size distribution, surface area etc. The methods of measurement and calculation of these parameters are discussed. Attempts are made to evaluate the effect and the contribution of each of these parameters to the main physical properties of foamed polymers namely apparent density, strength and thermoconductivity. The cellular structure of foamed polymers is considered as a particular case of porous statistical systems. Future trends and tasks in the study of the morphology and cellular structure-properties relations are discussed. 

At present, there are at least two approaches to the investigation of the cellular structure of foamed polymers. In the first one, which may formally be called a graphical approach, attempts are made to draw conclusions on the macroscopic properties of foamed polymers from morphological parameters such as the geometry and stereometry of cells of various sizes, shapes and types. The second approach, which may be referred to as physicochemical, attempts to explain and predict polymer morphology from the data on the chemical composition of the polymer matrix and the mechanisms of foaming... 

A cellular or porous structure of foam plastics is produced with gas- or foam-forming agents. The quality of the resulting material depends on choosing the right agent,... 

By far the most studied PolyHIPE system is the styrene/divinylbenzene (DVB) material. This was the main subject of Barby and Haq s patent to Unilever in 1982 [128], HIPEs of an aqueous phase in a mixture of styrene, DVB and nonionic surfactant were prepared. Both water-soluble (e.g. potassium persulphate) and oil-soluble (2,2 -azo-bis-isobutyronitrile, AIBN) initiators were employed, and polymerisation was carried out by heating the emulsion in a sealed plastic container, typically for 24 hours at 50°C. This yielded a solid, crosslinked, monolithic polymer material, with the aqueous dispersed phase retained inside the porous microstructure. On exhaustive extraction of the material in a Soxhlet with a lower alcohol, followed by drying in vacuo, a low-density polystyrene foam was produced, with a permanent, macroporous, open-cellular structure of very high porosity (Fig. 11). 

Williams et al. have also investigated the effect of variation of the DVB content of the monomer phase on the cellular structure of the resulting foam [130]. The phase volume and surfactant and initiator concentrations were kept constant while the DVB content was increased from 0 to 100% this caused a drop in average cell size from 15 pm to 6 pm. The increased hydrophobicity of DVB compared to styrene probably results in a more stable emulsion, giving a slower rate of droplet coalescence and smaller average cell size. 

The cellular structure of the quaternary blend systems after foaming at 180°C for 10 s is highlighted in Fig. 33. An excellent homogeneity down to the microscale can be detected for all foamed blend compositions. As already discussed in the previous section, simultaneous foaming of the PPE/PS and the SAN phase in the noncompatibilized blend leads to a bimodal cell size distribution. Besides larger cells induced by the highly expanded SAN phase, smaller cells are formed in the PPE/PS phase (Fig. 33a). 

Nam PH, Maiti P, Okamoto M, Kotaka T, Nakayama T, Takada M, Ohshima M, Usuki A, Hasegawa N, Okamoto H (2002) Foam processing and cellular structure of polypropy-lene/clay nanocomposites. Polym Eng Sci 42 1907-1918... 

Physicochemical studies of the regularities of foaming and formation of cellular structure in foams, studies of rheological and thermodynamic parameters of foaming and of the mechanism of interactions between components, etc. 

The experimentally determined cell diameters depend not only on the type of the starting polymer but also on the composition of the blowing agent and the foaming process. Thus, the critical parameters of the cellular structures of unpressed polystyrene foams of grades PSB and PSB-S are ... 

As noted above, the adequate simulation of the cellular structure of plastic foams is very important for the investigation of the relation between structure and properties of foamed plastics. A solution to this problem would allow to establish not... 

The reaction between isocyanates and water leads to production of gaseous carbon dioxide and an urea group. This reaction is a very convenient source of a gas necessary to generate the cellular structure of polyurethane foams [1-26] ... 

Nam, R H., Maiti, R, Okamoto, M., Kotaka, T., Nakayama, T., Takada, M., Ohshima, M., Usuki, A., Hasegawa, N., and Okamoto, H., Foam processing and cellular structure of polypropylene/clay nanocomposites, Polym. Eng. ScL, 42, 1907-1918 (2002). Navarro-Banon, V., Vega-Baudrit, J., Vaquez, R, and Martm-Martinez, J. M., Interactions in naosilica-polyurethane composites evidenced by plate-plate rheology and DMTA, Macro-mol. Symp., 221, 1-10 (2005). 

The dimensional stability of OPF-filled PU composites at high and low temperature environments were studied by [67]. The dimensional stability of composites prepared with 45-56 pm fibers was within the allowable limits. However higher sizes caused increased deformation due to the deterioration of cellular structure of the PU foam at higher fiber sizes. 

Y. Ema, M. Ikeya, M. Okamoto, Foam processing and cellular structure of polylac-tide-based nanocomposites, Polymer 47 (15) (2006) 5350-5359. 

Figure 1 shows the cellular structure of a typical PP foam, which was manufactured by molding PP foam beads with superheated steam. Each bead consists of closed polyhedral cells, with no visible accumulation of material in the comers. The PP foam beads themselves are melted together at their surfaces and form a secondary cellular structure. 

The cellular structure of synthetic foams is created by so-called blowing agents. Additives are also often required, particularly nucleation agents and stabilisers. Depending on the process and desired foam density, either chemical or physical blowing agents are applied. 

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