Foamed microcells

The existence of microcells in the structure of oligomeric foams was confirmed by studies on other types of oligomeric foams phenolic polyurethane and urethane-phenolic multicomponent oligomeric foams ... 

As can be seen from the differential curves of the cell volume fraction distribution (Curves 1 in Fig. 2a, b), the microcells occupy a relatively small volume of the foam 5% for y = 40 kg/m and 11% for y = 500 kg/m. They are, however, the most numerous group of cells (Curves 3,... 

Fig. 2). Cells with a radius of 100 A or less make up about 50% of all cells the entire population of microcells (the size of which is equal or less than 1000 A) is by at least three orders of magnitude higher than that of macrocells. Due to the very small size of microcells and their large number, the specific surface S of foams for the light specimen (y = 40 kg/m ) is as high as 100 m /g and for the heavy specimen (y = 500 kg/m ) S = 30 m /g.

Let us now consider the differences between the absolute sizes of microcells reported by Lowe et aL and those found in out experiments. In our first publications and in a monograph ) the minimum size of microcells in phenolic foams has been stated to be 6 tint. The value was obtained for large-cell heavy (7 = 200 kg/m ) foams. In later works ) light phe-... 

In addition to water, formaldehyde the boiling int of which is only about - 20 °C (anhydrous form) is formed in this reaction. The possibility of water vapor formation has been rejected as being the cause of the production of microcells since the temperature evolved in foaming does not exceed 85 °C. 

Differences in the s-forming systems and in the temperature may also explain the variations in the relative number of microcells in the foams. 

Hence, one of the most probable reasons for the formation of open cells, including microcells, due to water vapor is given in The mechanism described there, however, does not explain the formation of closed microcells which we observed in the structure of rigid phenolic foams The formation of these microcells is apparently connected with specific features of foaming and hardening kinetics in oligomeric compositions. 

Hence, the formation of microcells in the structure of phenolic foams is due to the fact that, at a certain str e of foaming (region III), thermodyramic conditions are created for the liberation of gas from the oversaturated mixture (formation of a new portion of bubbles). These bubbles expand according to the same laws as bubbles formed at point A, but they do not attain the same sizes as the macrobubbles. This is ascribed to the fact that at point C the viscosity of the foam system has in-aeased to mch an extent that the ps pressure in ail bubbles (both macro- and microbubbles) is insufficient to cause a further expansion and the foam does not rise further. 

The proposed mechanism nay be generally applied to all foam systems ba d on polyreactive oligomers. Thus, the fornation of microcells in PUR foams and in enolic-based urethane foams nay also be explained by this mechanism. Of course, low-molecular compounds are also formed in these systems as a reailt of polycon-... 

However, the detection of microcells in the structure of several oligomeric foams on the one hand, and the measured values of the specific surface on the other hand, require a re-examination of our notions about the degree of dispersion in these materials. 

As shown above, the minimal of microcells is in the range of 10 jum (R < 1000 A) and S in the range of 10 cm. Thus, recent data on the structure of oligomeric foams, at least those described above, allow assignment of these foams to high or fine dispersion systems. 

A further study of the microcellular structure of oligon ric foams and the development of models taking into account both mararo- and microcells are equally important. 

This idea has been confirmed by Lowe et aL l describing the dependence of water absorption of phenolic foams (7 = 35 kg/m ) on the fraction of closed cells this dependence recalculated for the fraction of open cells is shown in Fig. 9. It is noteworthy that an increase of d< from 10 to 98% corresponds to an increase of the maximum water absorption only from 6 to 8 g/lOOmL On the basis of these data, Lowe et al. concluded that surface chemistry is more important for foamed polymers than the closed cell content They explain the obtained data by the presence of microcells and interstices between macrocells (see Chap. 5.2). 

By varying the volume fraction of polymer, i.e. the apparent denaty, we may, in principle, change and even predetermine the type of macrocell packing and the content of microcells (see Fig. 2), that is to affect moisture and water absorption through macro- and micromorphological parameters of the foamed polymer. 

Morphological structures as isolated or communicating microspheres and their conglomerates (or windows ) in macrocellular walls were observed by scanning electron microscopy by Shutov et al. in phenolic foams. The reviewer termed them microcells These results were corroborated by Lowe, Barnett, Chandley... 

From the data (Fig. 20) of the author it follows that microcells are the most frequently occurring form of gas voids in foamed plastics bas on reactive oligomers. Another characteristic feature of microcells is that their absolute size is practically independent of the volumetric weight of the foam when the latter is varied from 40 to 500 kg/m, the mean size decreases only by a factor of 2 (from 1 to 0.5 microns). A third feature of microcells is their shape the bulk of microcells are spherical whereas most macrocells are oblong (see Chapter 7.3). 

These results reveal that- a plastic foam structure may be considered as a system of thin films and, therefore, support a model of plastic foam morphology namely a matrix system composed of thin polymeric films defining two groups of cells (macro- and microcells). Additional support in favor of this model of plastic foam structure is provided by the studies on the electric properties of plastic foams Among the numerous equations so far advanced for the calculation of the dielectric properties, the expressions which describe the dry foam structure by one of the limiting cases of a matrix system, namely a laminated dielectric structure with layers parallel to the force lines of the electrical field, agree best with the experiments >. 

The internal surface area of plastic foams may be measured not only by direct physico chemical methods but also indirectly by morphological parameters of the structure. Thus, if we assume that the solid phase of the plastic foam contains no microcells and that the macrocells are spherical, the value S. will be given by... 

These results suggest than there is no straightforward relation between V ff and y, Vjff fluctuating about a mean value of 1.26 cm /g. This means that (just as for microcells) the macrocell distribution pattern is independent of the volumetric weight of the foam and, judging from the asymmetrical form of the bar charts, is logaithmically-normal. 

In colloid chemistry all kinds of foams, both liquid and solid, are classified as lower coarse-disperse systems, i.e. such in which the minimum pore diameter is not less than 1 mm. The grouping of plastic foams into coarse disperse systems has until recently, been justified, since the minimum size of the cells that could be observed did not exceed this value. However, microcells have been discovered in polymer foam structures, this requires a revision of the old concepts regarding the dispersity. The real minimum size of microcells is a few hundredth of a micron and their number is well above that of macrocells. These results support the conclusion that plastic foams belong to the group of finely dispersed or colloid systems... 

Y. Fujimoto, et al, Well-controlled biodegradable nanocomposite foams from microcel-lular to nanocellular, Macromolecular Rapid Communications 24 (7) (2003) 457-461. 

Microcell 545 and Microcell 546 are chemical foaming agents with endothermic reaction kinetics. At low doses the formulations produce very fine cell structures. They can also be used as nucleating agents for directly gassed systems or in masterbatch formulations. ... 

The copolymer samples went through five steps in order to generate the microcel-lular foam. First, the samples were brought up to the desired saturation temperature and pressure inside the vessel using the syringe pump and water bath. Second, the polymer samples were allowed to soak at this pressure and temperature for the duration of the desired soak time. Third, the vessel was isolated from the syringe... 

Cell Nucleation After the gas/polymer singlephase solution is formed, the gas-saturated specimen is foamed under a variety of processing conditions. To produce microcellular foamed structures, the gas-samrated samples are subjected to arapid pressure drop and arapid temperature increase (Figure 17.1b) that result in nucleation and growth of billions of gas nuclei [52, 53]. The sudden drop of gas concentration creates a thermodynamic instability in the gas/ polymer solution, which is the main driving force for nucleation of microcells. 

Bandarian, Shojaei, and Rashidi (2011) found out that the iucorporatiou of reactive modified MWNTs, particularly the ones with hydroxyl and carboxyl functional groups, helped in improving the sound absorption properties of flexible PU foams by inducing the formation of microcells of less than 5 pm in the open-cell walls of... 

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