Preparation foams

The foam preparation fs ocess is very similar to that of urethane foam preparation. A two-component system is conveniently used, as shown below (1)  

The reaction times are considerably shorter than those encountered with most urethane systems, but the system remains fluid during the rise and the flow is even and controlled. 

According to l.G. Morrison (1), little evidence of striations or flow lines has been observed and from poured foam 8-foot vertical rises in 3 x 15 cavities have been obtained without shear or flow lines. 

Comparative tests between free-rise urethane and pyranyl foams indicate that the total pressures obtained are similar, but that the rate of pressure build-up and pressure dissipation are faster with the pyranyl systems. This property allows short jig-dweU times. Freezer cabinets varying in storage capacity from 7 to 30 ft, have been insulated completely satisfactorily with pyranyl foam (1). 

Like most disperse systems, foams can be obtained by condensation and dispersion methods. The condensation methods for generating foam involve the creation of gas bubbles in the solution by decreasing the external pressure, by increasing temperature or as a result of chemical reaction. Thus, bubble formation may occur through homogeneous nucleation at high supersaturation or heterogeneous nucle-ation (e.g. from catalytic sites) at low supersaturation. 

The most applied technique for generating foam is by a simple dispersion technique (mechanical shaking or whipping). This method is unsatisfactory since it is difficult to accurately control the amount of air incorporated. The most convenient method is to pass a flow of gas (sparging) through an oriflce with a well-deflned radius To- 

The size of the bubbles (produced at an oriflce) r may be roughly estimated from the balance of the buoyancy force Fb with the surface tension force Fg [1], 

As the bubble detaches from the orifice, the dimensions of the bubble will determine the velocity of the rise. The rise of the bubble through the liquid causes a redistribuhon of surfactant on the bubble surface, with the top having a reduced concentration and the polar base having a higher concentration than the equilibrium value. This unequal distribution of surfactant on the bubble surface has an important role in foam stabilisation (due to the surface tension gradients). When the bubble reaches the interface, a thin liquid film is produced on its top. The fife time of this thin film depends on many factors, e.g. surfactant concentration, rate of drainage, surface tension gradient, surface diffusion and external disturbances. 

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CellgeometTy is governed predominantly by the final foam density and the external forces exerted on the cellular stmcture prior to its stabilization in the expanded state. In a foam prepared without such external forces, the cells tend to be spherical or ellipsoidal at gas volumes less than 70—80% of the total volume, and they tend toward the shape of packed regular dodecahedra at greater gas volumes. These shapes have been shown to be consistent with surface chemistry arguments (144,146,147). Photographs of actual foam cells (Fig. 2) show a broad range of variations in shape. 

Foams prepared from phenol—formaldehyde and urea—formaldehyde resins are the only commercial foams that are significantly affected by water (22). Polyurethane foams exhibit a deterioration of properties when subjected to a combination of light, moisture, and heat aging polyester-based foam shows much less hydrolytic stabUity than polyether-based foam (50,199). 

As an example, a foam prepared from III, alumina trihydrate as a filler, benzoyl peroxide as a curing agent, and azobis formamide as a blowing agent, leads to a material with an oxygen index of 48, a long-term stability to at least 150 °C, and a smoke density about one fifth that of a commercial foam [284]. 

In the case of aerated protein icings, however, stabilizers are essential. Where hot water is used, gelling-type stabilizers work best (agar, gelatin, Irish moss extract). In cold foams, prepared with cold water, cellulose-type gums are used to contain water in the foams colloidal precipitates, such as are formed through the reaction of carrageenin and protein, are very helpful. 

The content of the gas is called quality therefore a 70 quality contains 70% gas. Recently, foams with 95% gas have been examined. For such foam types, only foam prepared from 2% of an anionic surfactant with plain water had uniform, fine-bubble structure [782]. 

DOW Chemical, Freeport, Texas, for the foam preparation and testing. 

For PLA/MMT-SBE foams prepared under the condition with low Tf ( 100-110 °C) and high pressure ( 28 MPa), the nanocomposite foams exhibit no significant difference in Nc compared with PLA/MMT-ODA foams. This reasoning is consistent with the large value of W in both systems. 

Several approaches towards monolithic GC columns based on open pore foams prepared in large diameter glass tubes were reported in the early 1970s [26,27, 110]. However, these columns had poor efficiencies, and the foams possessed only limited sample capacities in the gas-solid GC mode. Subsequent experiments with polymerized polymer layer open tubular (PLOT) columns where the capillary had completely been filled with the polymer were assumed to be failures since the resulting stationary phase did not allow the gaseous mobile phase to flow [111]. 

Emulsion derived foams prepared via the concentrated emulsion pathway are characterized by highly interconnected pores, thus offering density values as low as 0.02 g/cm and a relatively narrow size distribution in the pm-range resulting from a thermodynamically stable system. This principle allows for the synthesis of organic as well as inorganic foams that offer a wide range of appHcations [20, 21]. Recently such technique has been applied to form injectable siloxane foams where the emulsified liquid was removed supercritically in order to avoid pore collapse [22]. 

Authors carefully investigated the potential causes of an increase in compressive modulus, which had been previously reported for flexible slabstock foams prepared from the polyols of the study [166]. Flexible foams prepared from a standard formulation containing two different levels of the epoxidized/hydroxylated soybean... 

Figure 7. Foams prepared using natural oil polyols with well balanced catalysis (left) and unbalanced catalysis (right)...

The mechanical properties are also affected by the surfactant concentration in the emulsion precursor. Maxima in both crush strength and Young s modulus were shown at the surfactant concentration for optimum emulsion stability. Foams prepared from 100% styrene were found to have much lower compressive moduli than those containing DVB [130], This was attributed to plasticisa-tion of the polymer by the surfactant. 

Table 6.3 Effect of protein self-assembly, induced by interaction with lecithin, on the stability of foams stabilized by complexes of sodium caseinate (1 % w/v) with soy phospholipids Lipoid S-21 (1(T5 M) (Istarova et al., 2005 Semenova, 2007). Values of Mw and A 2 are presented for the protein with and without surfactant at three pH values. Also shown are photographs of foams recorded 9 minutes following foam preparation. In each of the images the volume of the glass vessel containing die foam is 10 ml. 

Compressive strengths were performed on a number of foam samples in accordance with ASTM 1621. The results obtained were comparable to other polyurethane foams prepared from conventional polyols and diisocyanates. 

Plugs of the foam may also be deployed in low-volume (63), high volume (61), or multiple air samplers (60). Foam preparation is similar to that of the XAD s, and its commercial availability through local sources is a distinct advantage. 

Cunningham et al.199-203 investigated the use of starches and dextrins as extenders for polyurethane foams. Starch generally had little effect on the foam density, while compressive strength moderately decreased with increasing starch content. Hydrophilic foams prepared using starch were suggested to have useful horticultural... 

Saiki, K. Sasaki, K. Carbodiimide-modified polyisocyanurate foams Preparation and flame resistance. 

Foams prepared from peanuts heated at 75 and 100°C varied, but were more stable than those of seeds heated at 50°C for different heating times to 90 min (Figure 11). 

Similar results were obtained for a foam prepared from a NaOL solution. In this case, the effect of foam collapse on the accumulation ratio was even more pronounced. The... 

The first quantitative experiments on shifting the equilibrium in a foam have been conducted with soap solutions [108]. It was found that the foam which underwent decaying was acidic whereas the remaining solution was alkaline. Furthermore, the surface tension of this solution was twice as high as that of the liquid obtained from the foam. The hydrolysis was most pronounced in dilute solutions. When the concentration of sodium oleate was 0.0002%, the ratio between the proportions of the acid and the base in the foam was 2.7, and it was only slightly dependent on the nature of the base. Similar results have been obtained for the foam prepared from sodium laurate solution [3]. 

Another technique of solid foam preparation is based on gas formation in a melted polymerising bulk or in concentrated water suspension of binding materials (cement, gypsum, lime), occurring after physical or chemical processes. It is also possible to incorporate air in a polymerising or solidifying substance bulk. For example, cellular-concrete represents a material in which gas bubbles are uniformly distributed in the bulk. The material produced when suspensions of binding substances are mixed with a foam is called cellular (foam) concrete. If the gas is formed in the concrete bulk as a result of a chemical reaction, for instance, in the reaction of aluminium powder with the liquid phase of the concrete solution, a gas-concrete is produced. 

Figure 2 Foam prepared from liquefied wood in the presence of polycaprolactone.

Eoam densities in the order of 0.1 g cm are routinely used. The stability of the foam is influenced by the components of the chemical system, the viscosity of the foam and the method of foam preparation. The half-life of a foam is the time in which 50 % of the liquid in a given foam volume has been drained from the foam. Foams for textile applications can have half-lives from a few seconds to several horns. 

One of the most promising methods of foam preparation starts from polyreactive oligomers which are obtained by polycondensation of 2-furfuryl alcohol, which is subsequently hardened ... 

Chem. Anlagen Bischo (Reut) Foams Prepared from Phenoplast and Di- and/or polyisocyanates. BDR Pht. 2,542,900 (1977)... 

Toyo Rubber Chem. Antistatic Polyurethane Foam Preparation. Japan Pat. 77,035,399... 

Hile, D. Amirpour, M. Akgerman, A. Pishko, M. Active growth factory dehvery from poly(D,L-lactide-co-glycolide) foams prepared in supercritical CO2. J. Control. Release 2000, 66 (23), 177-185. 

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