Pressure foams

In Eq. (10), is equilibrium volume of foamed polymer, VmeU is volume of thermoplastic melt at experimental temperature Tj Pf is foaming pressure. Other symbols are the same as in previous formulas. 

Fig. 1.4. Scheme of the apparatus for formation and investigation of foams (Foam Pressure Drop... 

The capillary pressure pa in a foam can be significantly increased by the Foam Pressure Drop Technique employed to produce dry foams [32-35]. The principle of this method is that the foam is brought into contact with a porous plate (usually sintered glass filter) under which a reduced pressure p 0 is created, and this pressure difference Ap = p0 - p 0 should not exceed the capillary pressure in filter pores 2acosd / rn (where 6 is the contact... 

The quantity xp is a much better defined characteristic of foam stability (since the pressure in the borders along the height of the foam column remains constant during its destruction). This parameter is also much more sensitive to the kind of surfactant, electrolyte concentration and other additives, compared to the lifetime of the foam in gravitational field, with an averaged pressure value from 0 to pgH. Estimation of the stability of foams from different surfactants by xp and by the Ross-Miles test has been reported in [16]. The results are discusses in Section 7.6.1. The advantages of the Foam Pressure Drop Technique and, respectively, xp as a characteristic of the foam stability, are clearly shown. 

As already mentioned (see Chapter 3), at the instant of foam formation the films and borders are in non-equilibrium state. The films thin mainly due to the capillary pressure, while the borders thin due to gravity or a pressure drop (when the foam is dried by the Foam Pressure Drop Technique [21-23]). The surfactant adsorption layers decrease the flow rate through the borders and films and the process of thinning becomes similar to the flow in thin gaps with solid surfaces. As indicated in Sections 3.2.1 and 5.3 the degree of retardation of the flow depends on the surfactant type and concentration as well as on the film type. A complete immobility at the film and border surfaces usually is not reached. 

In the real polydisperse foam along with coalescence there always acts another process of internal collapse. This is the diffusion decrease in the specific surface which is accompanied by structural rearrangement, i.e. shift of knots and borders, and change in their orientation. This leads to the origination of various local disturbances (Act, Apa, AC, etc.). These local disturbances along with the rupture of individual films cause destruction either of other films and borders or of local volumes or of the whole foam (see Sections 6.5 and 6.6). Finally, various external factors can affect the foam (pressure drop, applied to the liquid phase reduced pressure of the liquid vapour above the foam, leading to evaporation the effect of antifoam droplets a-particle irradiation vibration, etc.). 

Foam stabilising properties of surfactants determined by Foam Pressure... 

These two examples with the homologous series of alkylsulphonates and alkylsulphates indicate the undoubted advantages of Foam Pressure Drop Technique for determining the foam stabilising properties of surfactants. This technique allows to distinguish small differences in the foam stabilising ability of surfactants. 

Though not quantitative, the comparison between the two techniques provides information about the effect of the pressure in the foam liquid phase as well as the effect of the foam film type. The advantages of the Foam Pressure Drop Technique for estimating the foam stabilising ability of the surfactants is indisputable. 

Chapter 1 Formation and Structure of Foams. Pressure in the Liquid l... 

The decreasing foaming pressure can be measured via the internal mold pressure. 

The frothing process was developed by the Du Pont Co. (25). The process has the following advantages isotropic physical properties and lower foaming pressure. The method is preferably used for large-panel production at in-plant production or pour-in-place foaming in field applications, e.g., building panels, chemical tanks, etc. 

Examples of foaming characteristics of resol-type phenolic foams (free-rise) are shown in Table 64 in comparison with rigid urethane foam (3). Rise-profile curves, foaming-pressure curves and temperature curves for phenolic and polyurethane foams are shown in Figure 61. 

Covers polyurethane foam pressure-sensitive adhesive tape used for weather stripping, dust sealing, damping, thermal insulation, sound absorption, and packaging. The specification was validated 6 July 1988. 

Where +1/32 inch (+0.8 mm) has been the standard thickness tolerance for 4 foot (1.22 meter) wide panels produced in double belt lines, the standard for the fluid film process is +0.010 inch (+0.25 mm). At times, an accuracy within +.005 inch (+.127 mm) has been achieved. Such accuracy is possible because of the precision and rigidity of the tunnel construction. The platens are reinforced weldments, stress relieved after welding and then ground and chrome plated. They are separated by precise gage blocks to form the two faces of the tunnel. Deflection is practically non-existant since the foam pressure on the platen is balanced by the fluid pressure within the platen reservoir. The fluid film is accurately controlled to maintain it at a normal working thickness of 0.005 inch (0.127 mm). 

Figure 16 illustrates the independence of steady foam pressure gradient to gas velocity. Model prediction of pressure drop, 1.7 MPa/m (77 psi/ft), is slightly greater than the experimental result, 1.5 MPa/m (68 psi/ft). This discrepancy is understood by comparing the constant liquid velocity (0.028 m/day) used in Figure 16 to the results in Figure 15. The only experimental data point that did not fall on the model-predicted line lies at 0.028 m/day. The data, taken during that particular experiment, appear to have slightly depressed pressure drops.

WSD-M99D57. Polyurethane foam, pressure sensitive adhesive, low permeability. Ford Engineering Material Specification. 

To fully exploit the insulating performances of the open cell PU foams, pressure in the panel has to be kept preferably below 1.0 Pa during its life. To achieve this demanding target, the foam must be 100% open celled and with a very low outgassing rate. 

Figure 2. Foam pressure drops in an oil free short core.

Figure 5. Foam pressure drops from long core experiment (no oil).

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