Inhibition gaseous phase

Si-inorganic system (SI) inhibits gaseous phase combustion, and also affects char formation in the solid phase. This system can be incorporated into polypropylene or nylon 6,6. 

Physical properties of hexachloroethane are Hsted in Table 11. Hexachloroethane is thermally cracked in the gaseous phase at 400—500°C to give tetrachloroethylene, carbon tetrachloride, and chlorine (140). The thermal decomposition may occur by means of radical-chain mechanism involving -C,C1 -C1, or CCl radicals. The decomposition is inhibited by traces of nitric oxide. Powdered 2inc reacts violentiy with hexachloroethane in alcohoHc solutions to give the metal chloride and tetrachloroethylene aluminum gives a less violent reaction (141). Hexachloroethane is unreactive with aqueous alkali and acid at moderate temperatures. However, when heated with soHd caustic above 200°C or with alcohoHc alkaHs at 100°C, decomposition to oxaHc acid takes place. 

McDowell, C. A., and Thomas, J. H., Oxidation of Aldehydes in the Gaseous Phase Part IV. The Mechanism of the Inhibition of the Gaseous Phase Oxidation of Acetaldehyde by Nitrogen Peroxide, Transactions of the Faraday Society, Vol. 46, No. 336, 1950, pp. 1030-1039. 

It is known that a small amount of lead tetraethyl is effective as an anti-knock agent in gasoline engines. This effect is due to the inhibition of the gas-phase reaction by the lead or lead oxide derived from the lead tetraethyl.l o.si] The inhibitory action of lead compounds on hydrocarbon combustion is largely exerted in the gaseous phase of the combustion zone. 

Vinylidene fluoride is taken up rapidly via the pulmonary route in rats, but at equilibrium the mean concentration (by volume) in rats was only 23% of that in the gaseous phase. Metabolism proceeded very slowly and was saturable at exposure concentrations of about 260 mg/m Its maximum rate was 1% that of vinyl chloride and less than 20% that of vinyl fluoride there has been a report of an increase in the urinary excretion of fluoride in exposed rats. No alkylating intermediate was demonstrated after passage through a mouse-liver microsomal system. However, vinylidene fluoride inhibits mixed-function oxidase activity in vitro and, like similar halogenated compounds that are transformed to reactive metabolites, it alters rat intermediary metabolism, leading to acetone exhalation (lARC, 1986). 

The greatest inhibition effect in the gaseous phase is due to partial indirect suppression of the branching step (Equation 4.2). Hence, N20 flames would be less susceptible to poisoning because reaction (Equation 4.11) is a nonbranching reaction. 

The evidence for the action of halogen and halogen antimony compounds in the gaseous phase is well established however halogen-containing flame-retardant systems are often twofold systems providing radical action inhibition in the gaseous phase and, at the same time inhibition in the condensed phase as will be seen in the next section. 

Fig. 2.3. Structure of inhibited polymer films (a) single-layered filled with solid Cl particles (b) with a layer of glued particles (c) with a porous layer impregnated with inhibited liquid (d) with a liquid Cl in the glued layer (e) with a liquid Cl between film layers (f) with Cl in the gaseous phase sealed between layers (g) with a layer foamed by an inhibited gas. (1) polymer base (2) Cl particles (3) porous layer (4) glued inhibited layer (5) gaseous pellet of Cl vapors (6) jelly layer with inhibited liquid...

Fig. 2.5. Production diagrams of single-layer inhibited polymer films (a) joint extrusion of polymers and Cl (b) film saturation with Cl in a gaseous phase ...

Flame retardants exert many different modes of action as a function of the chemical nahue of the polymer-flame-retardant systems and of the interactions between the components. It is considered that inhibition of burning is achieved by modification of either the condensed phase or the dispersed or gaseous phase in a physical and/or chemical mode. In the condensed phase, the following may occur [3] ... 

The initiation of the polymerization with y-radiation from a Co source was studied by Usmanov et al. [460,484,485]. Polymerization was carried out by irradiation of the monomer, in both liquid and gaseous phase, with the use of y-rays at 38 °C. The dose rates were lOrad/s and 0.5Mrad/s. Impurities such as acetylene greatly inhibited polymerization. Oxygen influenced the kinetics, a factor that confirms a free-radical mechanism. Liquids such as difluoroethane, benzene, and carbon tetrachloride reduced the polymerization rate and caused low-molar-mass polymer. On studying the thermal behavior of PVF it was found that the polymers obtained by y-ray initiation in bulk were the most crystallized and had the lowest degree of irregularity in the polymer chain. Nearly no branches were found in contrast to the chemically initiated polymers produced in suspension [482,484]. 

Bromine is believed to perform most of its flame retardant function in the gaseous phase by means of two mechanisms redirection or termination of the chemical reactions involved in combustion, and the evolution of heavy bromine-containing gases which tend to protect the condensed phase by inhibiting access of oxygen and transfer of heat [68]. The chemical reactions may involve halogenation followed by dehydrohalogenation to yield a polymeric residue rich in double bonds which is then converted to carbon. 

For effective fire protection it is necessary to apply compoimds which components inhibit combustion comprehensively in a solid phase it is carried out by transforming the destmction process in a material, in a gaseous phase - preventing the oxidation of the degradation products [1,2]. 

The mechanism of flame suppression is postulated on the reaction of gaseous-phase inhibition by SiCl4 and HCl, which can be produced only at temperatures above 300-500°C, exactly the temperatures realized on the surface of burning polymers ... 

The combustion of methane can be used as a model for the combustion inhibition mechanism in the gaseous phase. This is a radical chain reaction proceeding in a series of step reactions as follows [4] ... 

From simple measurements of the rate of a photocatalytic reaction as a function of the concentration of a given reactant or product, valuable information can be derived. For example, these measurements should allow one to know whether the active species of an adsorbed reactant are dissociated or not (22), whether the various reactants are adsorbed on the same surface sites or on different sites (23), and whether a given product inhibits the reaction by adsorbing on the same sites as those of the reactants. Referring to kinetic models is therefore necessary. The Langmuir-Hinshelwood model, which indicates that the reaction takes place between both reactants at their equilibrium of adsorption, has often been used to interpret kinetic results of photocatalytic reactions in gaseous or liquid phase. A contribution of the Eley-Rideal mechanism (the reaction between one nonadsorbed reactant and one adsorbed reactant) has sometimes been proposed. 

At the low temperatures the activation energy is 4.6 kcal.mole and attributable to the same reaction. The change of the dependence of the rate on light intensity with temperature is interpreted as a change in the mechanism of loss of Cl atoms. At high temperatures the Cl atoms undergo a bimolecular reaction with some gaseous impurity whereas at the lower temperatures three-body gas-phase recombination predominates. Potts and RoIIefson attribute the discrepancy between their results on HCl inhibition and those of Norrish and Ritchie to some impurity introduced by the latter with the HCl. 

An important early method (1929) for the detection of gaseous free radicals was the metallic mirror technique of Paneth. Since then, many compounds have been used to determine the presence and extent of free radical reactions in the gas phase decomposition of organic compounds. The subject of inhibition in gas-phase reactions has been reviewed by Ashmore and by Gowenlock . 

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