Water-vapor retarder

Ammonia and water vapor retard the decomposition of the nitrogen iodide, a pressure of 10"3 cm Hg of ammonia being sufficient to stop the reaction... 

Poultry, susceptible to microbiological deterioration, is an excellent substrate for Salmonella. Therefore, the temperature is reduced as rapidly as possible after slaughter. Packagiag at factory level is in soft film, ie, low density polyethylene or plastici2ed PVC, which retards water-vapor loss and permits oxygen entry. 

Sugared cereals are often packaged in aluminum foil or barrier plastic, eg, ethylene vinyl alcohol, laminations to retard water vapor and flavor transmission (see Wheat and other cereal grains). 

When low boiling ingredients such as ethylene glycol are used, a special provision in the form of a partial condenser is needed to return them to the reactor. Otherwise, not only is the balance of the reactants upset and the raw material cost of the resin increased, but also they become part of the pollutant in the waste water and incur additional water treatment costs. Usually, a vertical reflux condenser or a packed column is used as the partial condenser, which is installed between the reactor and the overhead total condenser, as shown in Figure 3. The temperature in the partial condenser is monitored and maintained to effect a fractionation between water, which is to pass through, and the glycol or other materials, which are to be condensed and returned to the reactor. If the fractionation is poor, and water vapor is also condensed and returned, the reaction is retarded and there is a loss of productivity. As the reaction proceeds toward completion, water evolution slows down, and most of the glycol has combined into the resin stmcture. The temperature in the partial condenser may then be raised to faciUtate the removal of water vapor. 

Solutions of these fire retardant formulations are impregnated into wood under fliU cell pressure treatment to obtain dry chemical retentions of 65 to 95 kg/m this type of treatment greatly reduces flame-spread and afterglow. These effects are the result of changed thermal decomposition reactions that favor production of carbon dioxide and water (vapor) as opposed to more flammable components (55). Char oxidation (glowing or smoldering) is also inhibited. 

Here Wx represents the initial rate of heat production for some process (or processes) that is dependent on the partial pressure of the water vapor in the calorimetric system and gives a positive contribution to the initial rate of heat production by wet oxidation (unlike the steam distillation process envisaged for wet oxidation at 225°C, which aids in the retardation of the initial rate of heat production). If this relationship were valid, then a plot of the logarithm of the initial rate Wx against the logarithm of the ratio of the partial pressure of water vapor in the system to the total pressure in the system would be linear. However, as demonstrated by Figure 3, no such simple relationship is found. 

The authors also mention the interesting result that uptake from water vapor at 80 °C was less than that at lower temperatures, as reported by others, and that this difference was not due to the predrying procedure. It was suggested that water would condense on the membrane surface with more difficulty at the higher temperature and that this would retard sorption. This situation is of obvious significance with regard to humidified membranes in fuel cells. Also, as seen in other studies, the water uptake increases with decreasing EW. 

It was observed that the increase of photoconductivity under illumination was retarded by high oxygen pressure that is, the photoconductivity, at a given time and intensity of illumination, has a lower value if the oxygen pressure is high. On the other hand, decay of the induced conductivity was hastened by increased oxygen pressure. Nitrogen, water vapor, or carbon dioxide had no effect. 

Condensed-Phase Mechanisms. The mode of action of phosphorus-based flame retardants in cellulnsic sy stems is probably best understood. Cellulose decomposes by a noncalalyzed route lo tarry depolymerization products, notably levoglucosan, which then decomposes to volatile combustible fragments such as alcohols, aldehydes, ketones, and hydrocarbons. However, when catalyzed by acids, the decomposition of cellulose proceeds primarily as an endothermic dehydration of the carbohydrate to water vapor and char. Phosphoric acid is particularly efficaceous in this catalytic role because of its low volatility (see Phosphoric Acids and Phosphales). Also, when strongly heated, phosphoric acid yields polyphosphoric acid which is even more effective in catalyzing the cellulose dehydration reaction. The flame-retardanl action is believed to proceed by way of initial phosphory lation of the cellulose. 

Some of the salts used as coating materials, particularly carbonates, borates, and ammonium salts, in the course of melting produce a foam on the fiber through the liberation of such gases as carbon dioxide, water vapor, ammonia, and sulfur dioxide. These gases, being incombustible, retard burning by a process similar to that proposed in the gas theory as discussed below. 

Two effects cause the low production capacity of large-grained catalyst. First, large grain size retards transport of the ammonia formed inside the catalyst into the bulk gas stream. This is because the ammonia transport proceeds by slow diffusion through the pore system. The second effect is a consequence of the fact that a single catalyst grain in the oxide state reduces from the outside to the interior of the particle. The water vapor produced inside the catalyst by reduction comes into contact with already reduced catalyst on its way to the outer surface of the catalyst. This induces a severe recrystallization. As an example, if the particle size increases from about 1 to 8 mm, the inner surface decreases from 11 to 16 m2/g to 3 to 8 m2/g74. Therefore the choice of catalyst requires the optimization of 1) catalyst size versus catalyst activity, 2) catalyst size versus pressure drop across the converter and 3) the impact of 1 and 2 on... 

It has extremely low water vapor transmission, even with extruded thin transparent film. Chemical resistance is generally good, although not comparable to PTFE and other fluoroplastics. Chlorinated solvents, in particular, can affect PCTFE, and it can be subjected to stress cracking at elevated temperatures. The presence of chlorine atoms, used to enhance flame retardancy, keeps PCTFE in the non burning category with PTFE. It causes, however, a serious increase in dielectric losses, particularly at high frequencies. 

Reconsider Prob. 14-64. In order to reduce the migration of water vapor, the inner surface of the wall is painted with vapor retarder latex paint whose permeance is 26 X 10"kg/s m Pa. Determine the amount of water vapor that will diffuse through the roof in this case during a 24-h period. 

Aluminum Trihydrate. The utility of aluminum trihydrate as a flame retardant is based on its endothermic dehydration to aluminum oxide and water. In absorbing some of the heat of combustion and lowering the temperature of the substrate near the flame, the hydrate functions as a chemical heat sink. The water vapor provided by such action dilutes the gaseous reactants in the flame until all the water of crystallization is exhausted. 

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