Foam, reduction

Quahty control testing of siUcones utilizes a combination of physical and chemical measurements to ensure satisfactory product performance and processibihty. Eor example, in addition to the usual physical properties of cured elastomers, the plasticity of heat-cured mbber and the extmsion rate of TVR elastomers under standard conditions are important to the customer. Where the siUcone appHcation involves surface activity, a use test is frequently the only rehable indicator of performance. Eor example, the performance of an antifoaming agent can be tested by measuring the foam reduction when the sihcone emulsion is added to an agitated standard detergent solution. The product data sheets and technical bulletins from commercial siUcone producers can be consulted for more information. 

Foam reduction, defoaming or deaeration can be achieved by various means. Apart from pressure reduction or mechanical influences (e. g., skimmer or separation grids), special chemical additives are mainly applied to control the foam. In... 

Uses Food emulsifier used in white bread to prolong freshness, improve structure in baking cream to aid dispersion of fat in doughs and sponge mixes in soft caramels for foam reduction and increase in volume Regulatory EU E471, FAOA/VHO compliance... 

Contact angles can change as a result of variation of surfactant concentration as well as surface treatment of soM. The results shown in Fig. 7b relate to foam breaking of aqueous CTAB solutions by particles of ethylene-bis-stearamide (BBS), a waxy solid used iu commercial antifoams [8]. In this case, changes in 6 result entirely from changes in CTAB concentration in the foaming solutions. As in the case of smface treatment discussed before, the percent foam reduction increases with increase in contact angle (decrease in CTAB concentration). We note that the foamability of the CTAB solutions did not vary with surfactant concentration in the absence of the particles. 

Commercial antifoams often rely on the synergistic effects of particles and water-insoluble oil droplets (e.g., hydrocarbon or silicone oil) in reducing foam volume. This is illustrated in Fig. 7b, where it is seen that BBS particles in combination with dodecane are much more effective in causing foam reduction than BBS particles alone. 

Sorbitol is manufactured by the reduction of glucose in aqueous solution using hydrogen with a nickel catalyst. It is used in the manufacture of ascorbic acid (vitamin C), various surface active agents, foodstuffs, pharmaceuticals, cosmetics, dentifrices, adhesives, polyurethane foams, etc. 

Composites can be created ia which the core optimizes desired physical properties such as modulus, whereas the outer layer optimizes surface coasideratioas aot inherent ia the core material. SoHd outer—foam core can provide composites with significant reductions ia specific gravity (0.7). Dry blowiag ageats can be "dusted" onto the peUets orHquid agents iajected iato the first transitioa sectioa of the extmder. 

Miscellaneous chemicals are used to modify the final properties of rigid polyurethane foams. Eor example, halogenated materials are used for flammabihty reduction, diols may be added for toughness or flexibiUty, and terephthalate-based polyester polyols may be used for decreased flammabiUty and smoke generation. Measurements of flammabihty and smoke characteristics are made with laboratory tests and do not necessarily reflect the effects of foams in actual fire situations. 

Moisture. Absorbed and retained moisture, especially as ice, has a significant effect on the stmctural and thermal properties of insulation materials. Most closed-ceU plastic foams have low permeance properties most notably where natural or bonded low permeance surface skins exist (29,30). Design, building, and constmction practices requite adequate vapor retarders, skins, coatings, sealants, etc, in order to prevent the presence of moisture. However, moisture vapor cannot be completely excluded, thus the possibiUty of moisture absorption and retention is always present. The freezing of moisture and mpturing of cells result in permanent reduction of thermal and stmctural performance. 

Industrially, polyurethane flexible foam manufacturers combine a version of the carbamate-forming reaction and the amine—isocyanate reaction to provide both density reduction and elastic modulus increases. The overall scheme involves the reaction of one mole of water with one mole of isocyanate to produce a carbamic acid intermediate. The carbamic acid intermediate spontaneously loses carbon dioxide to yield a primary amine which reacts with a second mole of isocyanate to yield a substituted urea. 

Materials made of siHcon nitride, siHcon oxynitride, or sialon-bonded siHcon carbide have high thermal shock and corrosion resistance and may be used for pump parts, acid spray nozzles, and in aluminum reduction ceUs (156—159). A very porous siHcon carbide foam has been considered for surface combustion burner plates and filter media. It can also be used as a substrate carrying materials such as boron nitride as planar diffusion source for semiconductor doping appHcations. 

The stabihty of a single foam film can be explained by the Gibbs elasticity E which results from the reduction ia equiUbrium surface concentration of adsorbed surfactant molecules when the film is extended (15). This produces an iacrease ia equiUbrium surface tension that acts as a restoring force. The Gibbs elasticity is given by equation 1 where O is surface tension and is surface area of the film. 

The common indices of the physical environment are temperature, pressure, shaft power input, impeller speed, foam level, gas flow rate, liquid feed rates, broth viscosity, turbidity, pH, oxidation-reduction potential, dissolved oxygen, and exit gas concentrations. A wide variety of chemical assays can be performed product concentration, nutrient concentration, and product precursor concentration are important. Indices of respiration were mentioned with regard to oxygen transfer and are particularly useful in tracking fermentation behavior. Computer control schemes for fermentation can focus on high productiv-... 

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