Foam structures

A foam can be considered as a type of emulsion in which the inner phase is a gas, and as with emulsions, it seems necessary to have some surfactant component present to give stability. The resemblance is particularly close in the case of foams consisting of nearly spherical bubbles separated by rather thick liquid films such foams have been given the name kugelschaum by Manegold [175]. 

The discussion here is confined to the more common type of foam, the polyederschaum and their interesting geometric aspects. If three bubbles are joined, as in Fig. XIV-13, the three separating films or septa meet to form a small triangular column of liquid (perpendicular to the paper in the figure) 

With three bubbles, the septa must meet at 120° if the system is to be mechanically stable. A fourth bubble could now be added as shown in Fig. XIV-14, but this would not be stable. The slightest imbalance or disturbance would suffice to move the septa around until an arrangement such as in Fig. XIV-14h resulted. Thus a two-dimensional foam consists of a more or less uniform hexagonal type of network. 

The situation becomes more complex in the case of a three-dimensional foam. Since the septa should all be identical, again three should meet at 120° angles to form borders or lines, and four lines should meet at a point, at the tetrahedral angle of 109°28. This was observed to be the case by Matzke [179] in his extensive statistical study of the geometric features of actual foams. 

Two main types of foams may be distinguished (1) spherical foam ( Kugel Schaum ), consisting of gas bubbles separated by thick films of viscous liquid produced in freshly prepared systems. This may be considered as a temporary dilute dispersion of bubbles in the liquid. (2) Polyhedral gas cells produced on aging thin fiat walls are produced with junction points of the interconnecting channels (plateau borders). Due to the interfacial curvature, the pressure is lower and the film is thicker in the plateau border. A capillary suction effect of the liquid occurs from the centre of the film to its periphery. 

The pressure difference between neighbouring cells, Ap, is related to the radius of curvature (r) of the plateau border by, 

Polyederschaum concentrated gas system with high gas voiume arxl thin films 

Kugelschaum (dilute system) with lower gas volume and thick films 

The drainage of excess liquid from the foam column to the underlying solution is, initially, driven by hydrostatic, causing the bubble to become distorted. Foam collapse usually occurs from top to bottom of the column. Films in the polyhedral foam are more susceptible to mpture by shock, temperature gradient or vibration. 

The foam as TLF has a very intriguing structure. If (1) two bubbles of the same radius come into contact with each other, this leads to (2) the formation of contact area and subsequently to (3) formation of one large bubble. 

In stage 2, the energy of the system is higher than that in stage 1, since the system has formed a contact area (dAc). The energy difference between (2) and (1) is y dAc. When the final stage is reached (3), there will be a decrease in the total area by 41% 

FIGURE 8.4 Foam structure of three-bubble (a) and four-bubble (b) aggregates. 

The surface and bulk viscosities not only reduce the draining rate of the lamella but also help in restoration against mechanical, thermal, or chemical shocks. The highest foam stability is associated with appreciable surface viscosity (qs) and yield value. 

The stability of a gas (i.e., N2, C02, air) bubble in a solution depends on its dimensions. A bubble with a radius greater than a critical magnitude will continue to expand indefinitely and degassing of the solution would take place. Bubbles with a radius equal to the critical value would be in equilibrium, while bubbles with a radius less than the critical value would be able to redissolve in the bulk liquid. The magnitude of the critical radius, Rcr, varies with the degree of saturation of the liquid (i.e., the higher the level of supersaturation, the smaller the Rcr). The work, W, required for the formation of the bubble of radius Rcr is given by La Mer  

Two bubbles touch each other and form a contact area. 

When three bubbles come into contact, the equilibrium angle is 120°. The angle of contact relates to systems equilibrium state, which is 120° from simple geometrical considerations. If four bubbles are attached to each other, then the angle will, at equilibrium, be 109°28.  

The pressure difference between neighbouring cells, Ap, is related to the radius 

In a foam column, several transitional structures may be distinguished, as 

Another mechanism of foam instability is due to Ostwald ripening (disproportionation), the driving force for which process is the difference in Laplace pressure between the small and larger foam bubbles. The smaller bubbles have a higher 

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The foregoing discussion leads to the question of whether actual foams do, in fact, satisfy the conditions of zero resultant force on each side, border, and comer without developing local variations in pressure in the liquid interiors of the laminas. Such pressure variations would affect the nature of foam drainage (see below) and might also have the consequence that films within a foam structure would, on draining, more quickly reach a point of instability than do isolated plane films. 

Other interesting Langmuir monolayer systems include spread thermotropic liquid crystals where a foam structure forms on expansion from a collapsed state [23]. Spread monolayers of clay dispersions form a layer of overlapping clay platelets that can be subsequently deposited onto solid substrates [24]. 

The transverse modulus is lower partly because the cell wall is less stiff in this direction, but partly because the foam structure is intrinsically anisotropic because of the cell shape. When wood is loaded across the grain, the cell walls bend (Fig. 26.5b,c). It behaves like a foam (Chapter 25) for which... 

Foam = Structural foam molding T-Form = Thermoforming Blow - Blow molding... 

The laboratory prototype of the Dinex electrochemically promoted catalyst unit is shown in Figure 12.12 and the assembled unit schematically in Fig. 12.13. It consists (Fig. 12.14) of a tubular bundle porous (ceramic foam) structure made of CeOa-GcfeOj (CGO) which is an O2" conductor with ionic conductivity significantly higher than YSZ at temperatures below 500°C... 

Recently, many papers have been published on fiber catalysts and foam structures (Figure 9.2). Although, strictly speaking, fibers and foams might not be considered as structured systems, beds of such catalysts exhibit typical features of structured catalysts, namely, low pressure drop, uniform fiow, a good and uniform access to the catalytic surface, and they are definitely nonrandom. Therefore, we have included them in this chapter. 

Figure 9.2 Fiber and foam structures, (a) Knitted silica fibers catalyst. (Reprinted from [7].) (b) Woven active carbon fiber catalyst. (Reprinted from [8].) (c) Aluminum foam. (Reprinted from [9].)...

A substance which assists in holding the foam structure produced by whipping or frothing in the manufacture of latex foam rubber. 

PBT-PC blends show increased melt strength allowing them to be easily processed by blow molding and profile extrusion. The PBT-PC blends have been extruded into sheet and thermoformed into parts. Enhanced melt strength allows PBT-PC blends to be foamed. Structural foam grades for injection molding (10-30% density reduction) are commercially available. 

All the molten-state methods are usable extrusion, injection, blowing, compression, thermoforming, co-injection, machining, welding. Specific grades can be used for foam, structural foam and solvent processing. 

At atmospheric pressure the foam structure depends on the formulation and processing parameters. Foamed sheets and imitation leathers can be manufactured by this process. 

The density of the polymer clearly shows the formation of a foamed polymer. The density values for selected foams together with the polyimide homopolymers are shown in Table 11. The density values for the ODPA/FDA and PM-DA/FDA polyimides were both 1.28 g/cm and 3FDA/PMDA is 1.34, while most of the propylene oxide-based copolymers showed substantially lower values. The densities of the foamed copolymers derived from these copolymers ranged from 1.09 to 1.27 g/cm, which is 85-99 % of that of the polyimide homopolymers. This is consistent with 1-15 % of the film being occupied by voids. From these data (i.e., the comparison of Tables 10 and 11), it appears that the volume fraction of voids or the porosity is substantially less than the volume fraction of propylene oxide in the copolymer (i.e., 70 % or less). Thus the efficiency of foam formation is poor. Conversely, the propylene oxide-based copolymers with PMDA/ODA as the imide component did not show the expected density drop, and the values were essentially identical to that of the homopolymer. In PM-DA/ODA-based systems, molecular ordering and orientation were found to be critical in determining the stability of the foam structure. Where the character-... 

A.T. Florence, T.K. Law, and T.L. Wateley Nonaqueous Foam Structures from Osmotically Swollen W/OAV Emulsion Droplets. J. Colloid Interface Sci. 107,584 (1982). 

DNPT)) were studied using a gas evolution apparatus. The decomposition temperature of ADC decreased with both DNPT and 4,4-oxybis(benzenesulphonyl hydrazide) (OBSH) blending and this affected the structure and properties of the resulting foams. Using a tube mould for an extrudate to vulcanise the NR/EPDM extradate in a hot air oven was found to control the expansion and foam dimensions. The NR compositions affected the foam structure and properties. 16 refs. 

Dallas, Texas, 6th-10th May, 2001, paper 444 INFLUENCING THE FOAM STRUCTURE OF C02-BL0WN POLYPROPYLENE SHEETS Heinz R MichaeU W Aachen,Institut fur Kunststoffverarbeitung (SPE)... 

New York City, 2nd-6th May 1999, p.2084-9. 012 STUDY OF SURROUNDING TEMPERATURE EFFECTS ON EXTRUDED FOAM STRUCTURE Lee S T Lee K Sealed Air Corp. 

Patent Number US 5883145 A 19990316 CROSSLINKED FOAM STRUCTURES OF POLYOLEFINS AND PROCESS FOR MANUFACTURING... 

The density of chemicaUy-blown LDPE foam was altered by varying the amount of blowing agent, degree of crosslinking of the polymer, and the foam expansion temperature. A theory was proposed for the equilibrium density, based on the gas pressures in a Kelvin foam structure, and a rubber-elastic analysis of the biaxial stretching of the cell faces. 20 refs. 

Patent Number EP 702032 A2 19960320 CROSSLINKED FOAM STRUCTURES OF ESSENTIALLY LINEAR POLYOLEFINS AND PROCESS FOR MANUFACTURE... 

Patent Number US 5407965 A 19950418 CROSS-LINKED ETHYLENIC POLYMER FOAM STRUCTURES AND PROCESS FOR MAKING... 

Disclosed is a crossUnked ethylenic polymer foam structure of an ethylenic polymer material of a crosslinked, substantially linear ethylenic polymer. The ethylenic polymer in an uncrossUnked state has (a) a melt flow ratio greater than or equal to 5.63 (b) a molecular weight distribution defined by a given equation and (c) a critical shear rate at onset of surface melt fracture of at least 50% greater than the critical shear rate at the onset of surface melt fracture of a linear ethylenic polymer having about the same melt flow ratio and molecular weight distribution. Further disclosed is a process for making the above foam structure. 

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