Chemical Reactions - Chemically Active Medium

Immediately during sorption of chemically active media on the plastics surface, chemical reactions with the polymer molecules and/or any additives present (pigments, stabilizers, etc.) take place. Chemical attack results in oxidation, chain cleavage, or crosslinking. Here, double bonds represent the preferred points of attack. Reactions initiated by chemically active aggressive substances always result in irreversible materials changes [243]. 

When a molded material contacts a strongly oxidizing medium, such as nitric acid, sulfuric acid, chlorine, bromine, and ozone, oxidative attack has to be expected. Table 5.68 shows the influence of chlorine on polyethylene resistance. Polymers with double bonds or with tertiary bound hydrogen atoms, such as polypropylene, are particularly prone to oxidation. Degradation due to oxidizing substances causes significant changes in mechanical, electrical, and optical properties. 

Ta b I e 5.6 8 Influence of chlorinated water on resistance In polyethylene at 105 °C (Investigation with water according to ISO 1167, Investigation with chlorinated (3 ppm) water (circulating)) [57] 

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This condition has the following meaning the substance that diffuses away from the surface must entirely take part in the reaction while moving away into the bulk of a chemically active medium. 

Chemical reactions at liquid interfaces exhibit remarkable patterns. Chemical reactions are necessary for structure formation, and hydrodynamic and diffusion effects in the absence of reaction could not generate these patterns. However, different types of reactions led qualitatively to the same result (Avnir et al., 1984). Additionally, surface-driven convection might have a crucial role for the onset of convection patterns in chemically active medium (Muller et al., 1985). 

A systematic investigation of FP was initiated in [1-4], and followed by many others (see references in [5,6]). An important step in the experimental studies of FP was recently made by finding experimental conditions which allow to directly visualize the propagating fronts [7]. FP has much in common with combustion waves, which are much better studied than FP, since in both cases there are exothermic reaction waves propagating through a chemically active medium. FP differs from combustion processes by chemical kinetics, various phase transitions, hydrodynamical effects, and thermophysical properties of the substance. Typically polymerization waves are much slower and less exothermic than combustion waves. 

It is possible to determine the effect on polymers of different substances or media and to differentiate them [3]. When a medium causes a chemical reaction with a polymer, it is called a chemically active medium [2]. These effects are irreversible and the deterioration caused to the polymeric material is very important. Such media are acids, bases, oxidants, and all other substances that can cause chemical reactions such as substitution, addition, and hydrolysis. In some cases the chemical reaction is limited to the polymer surface and produces a protective layer against a deeper deterioration by the corrosive chemical. Such surface reactions are those produced by nitric acid on vulcanized PI and SB rubbers or by sulfuric acid on vulcanized polychloroprene and NR [4-7]. 

Following an idea of Zhabotinsky [4] that chemical oscillating systems could expediently be treated as a black box and that the relevant mathematical semi-phenomenological model has to focus on the basic reactions only neglecting those less important, Vasiliev, Romanovsky and Yakhno [5] suggested a concept of the basic model. These simplified models of an extended active medium could be obtained either by a reduction of pre-existing... 

French) or BROMOFORMO (Spanish) (75-25-2) CHBrj Noncombustible liquid. Violent reaction with chemically active metals, acetone, calcium, strong caustics, potassium, potassium hydroxide, sodium hydroxide. Increases the explosive sensitivity of nitromethane. Incompatible with crown polyethers, sodium-potassium alloys. Forms friction- and shock-sensitive compounds with lithium. Aqueous solution is a medium-strong acid. Liquid attacks some plastics, rubber, and coatings. Corrosive to most metals in the presence of moisture. Thermal decon osition products include highly toxic carbonyl bromide and hydrogen bromide fumes. On small fires, use dry chemical powder (such as Purple-K-Powder), foam, or COj extinguishers. 

AI3-15639 (75-50-3) Extremely flammable liquefied gas [flash point 20°F/-6°C (25% solution in H2O) 41°F/5°C]. A medium-strong organic base. Violent reaction with strong oxidizers, bromine, strong acids, ethylene oxide, halogenated compounds, nitrosating compounds, triethynylaluminum. Incompatible with methyl trichloroacetate. Reacts with mercury, forming shock-sensitive explosive material. Attacks chemically active metals aluminum, copper, tin, zinc, and their alloys. 

Madore and Freedman, 1987). The Belousou-Zhabotinskii chemical reaction, for instance, develops into waves of chemical activity propagating through a receptive liquid medium. 

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