Homogeneous foaming systems

Just as the products of polycondensation are greatly varied, so are the reaction conditions used in their production. Some are produced in the melt (many polyamides and polyesters), some initially in the melt but with extensive polymerization continuing in the solid state (polyurethane foams and elastomers), in solution (some polyurethane fibres) or in non-homogeneous liquid systems (some polycarbonates, very high melting polyamides). 

Viscosity measurements at constant temperature were made using a conventional Shirley—Ferranti cone-and-plate viscometer. In some cases complications arose due to phase inversion or to the fracture of low-molecular-weight polymer. One case was a typical one-shot flexible polyether foam system. On the other hand, a polyether prepolymer prepared from 2000 M.W. poly(oxypropylene)glycol and 80 20-TDI, reacted with water, formed a homogeneous system initially, and was suitable for study with this technique. The rate of viscosity increase at several temperatures is shown in Fig. 15. 

The Design Institute for Emergency Relief Systems (DIERS) has developed methods to predict the extent of level swell -. but these are valid only for systems which are not natural foamers. Naturally-foaming systems fill the reactor with a homogeneous two-phase mixture during relief and will always vent as a two-phase mixture. Since only trace quantities of certain substances... 

The homogenize must be placed appropriately ia the system to assure the proper temperature of the incoming product, provide for clarification, and avoid air iacorporation that would cause excessive foaming. The homogenize also maybe used as a pump ia the pasteurization circuit. 

The data required for the emergency vent design includes [191] (1) the thermokinetic and pressure history monitored under near adiabatic conditions, (2) the character of the type of vented system (vapor, gassy, or hybrid), (3) the phase of the vented material (vapor, liquid, or two-phase), and (4) the degree of two-phase disengagement (turbulent, bubbly, or homogeneous). To determine these characteristics, the VSP defines the system as viscous (100 cp) or nonviscous, and also whether or not it has a foaming tendency. 

Reducing the SAN content promotes a finer dispersed and more homogeneous SAN phase, as can be seen by the TEM micrographs in Fig. 26. The altered microstructure shows a strong impact on the cell structure of the foamed blend systems at a PPE/PS ratio of 75/25 and a SAN content of 30 wt%, the cell size decreases,... 

Selective blending the more viscous PPE phase with PS allowed one to tailor the processing window of the miscible PPE/PS blend phase and the microstructure of the immiscible blend system. Following this approach, simultaneous foaming of both blend phases and more homogeneous cell structures could be achieved. Additionally, the overall foam density could be reduced. 

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). 

When the proper inert ingredients are selected and they are compatible with the system and have desirable physical properties, the chemist then considers the behavior of the composition in an actual spray situation. Here he is concerned with (a) homogeneity of spray solution, (b) foaming properties, (c) viscosity of spray solution, and, (d) particle size of spray droplets. 

Foam Volume. The determination of foamability was carried out using the procedure of Hermansson et al. (9.) with modifications. A 1 g sample of the freeze-dried protein fraction was homogenized with 90 ml of citrate-phosphate buffer in a Sorvall Omnimixer at 3000 rpm. The resultant foam and liquid were transferred to a 250 ml graduated cylinder and the mixer cup washed with 10 ml of buffer. The cup was drained for 2 min and the cylinder allowed to stand for 30 min at which time the foam volume was measured. The influence of pH and ionic strength on foam volume was established using the buffer systems previously described. 

The first results about foam electrokinetics have been reported by Sharovamikov [62,63]. An electroosmotic liquid transport is observed in foams from solutions of ionic surfactants (NaDoS, CTAB, PO-3A, etc.) and it is larger than in systems with solid capillaries (specific transport from 1.6-1 O 6 to 210 6 m3 C 1). The maximum electroosmotic pressure depends on the initial pressure in borders and reaches 1 Pa. The addition of dedecanol to the NaDoS solution sharply decreases the electroosmotic transport but increases the electroosmotic pressure. To reduce the influence of border and film non-homogeneity that originates in a static foam under gravity, the electrokinetic studies have been performed in an advancing foam [62]. The specific electroosmotic transport depends on the capillary pressure and reaches a maximum value at pg = 0.5 kPa. The streaming potential (up to 10 mV)... 

Detailed analysis of these approaches is presented by Garrett [19]. As far as these processes are valid for homogeneous defoaming, the impossibility for a complete inhibition of foam formation in many systems, even when the solutions are saturated with antifoams, indicates that these kinetic phenomena are not sufficient for heterogeneous defoaming. Therefore, none of the considered points of view gives a satisfactory explanation of the mechanism of action of antifoams in heterogeneous systems. 

The physicochemical models consistent with foam formation and stabilization are derived from the work on pasteurized and UHT processed miUc-creams advanced by Besner (1997). Foam formation and stabilization can basically be explained as a multistage process, with some significant differences between non-homogenized (pasteurized cream) and homogenized (UHT cream) systems. These differences... 

To date, most industrial silver catalyst systems that were developed are heterogeneous (e.g., Ag-zeolite, lattice silver, nano-silver, and/or foamed silvers) (86). Homogeneous silver-mediated oxidation remains underdeveloped despite recent advances. 

A dispersion Is a system made of discrete objects separated by a homogeneous medium In colloidal dispersions the objects are very small In at least one dimension. Colloidal sizes range from 1 to 100 nm however these limits are somewhat arbitrary, and It Is more useful to define colloids as dispersions where surface forces are large compared to bulk forces. Here we are concerned with systems where the dispersion medium Is a liquid examples are droplets In emulsions or mlcroemulslons (oll/water or water/oll), aggregates of amphiphilic molecules (surfactant micelles), foams, and all the dispersions of solid particles which are used as Intermediates In the manufacture of ceramics. At this stage we are not too concerned with the nature of the constituents, but rather with the structures which they form this Is a geometrical problem, where the system Is characterized by Its surface area A, by the shapes of Its Interfaces (curvatures - b ), and by the distances between opposing surfaces (d — concentration parameter). 

Another system used is "encapsulation". In this system the item is completely surrounded by a homogeneous mass of foam. The item is usually protected by a cocoon of plastic film before the foam is applied. The greatest difficulty in this method is removal of the foam after use. Tear strings of wire are frequently used to facilitate this removal (25). 

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