Practical retarders

The retarders most widely used in practice appear to be hydroxy carboxylic acids or their salts, such as citrates or heptonates, and lignosulphonatcs. Because of the low concentrations required, they are commonly added as solutions. Calcium or sodium lignosulphonatcs are waste materials from the manufacture of wood pulp, when they are formed by the breakdown of lignins. Their molecules are random, three-dimensional polymers, with molecular weights of up to 50000, probably spherical and with many -SOj groups on their surfaces. The molecular structures are not completely 

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At the same time, quinones do not practically retard oxidation of hydrocarbons, since alkyl radicals react very rapidly with dioxygen (see Chapter 4) to give alkylperoxyl radicals, which scarcely react with quinones. Quinones exhibit their inhibiting properties as alkyl radical acceptors only in the oxidation of polymers (see Chapter 19). However, quinones were found to decelerate the oxidation of alcohols very efficiently and for long periods by ensuring cyclic chain termination via the following reactions [31-34] ... 

Recently an analogous mechanism for cyclic chain termination has been established for quinones [47], Quinones, which can act as acceptors of alkyl radicals, do not practically retard the oxidation of hydrocarbons at concentrations of up to 5 x 10 3 mol L 1, because the alkyl radicals react very rapidly with dioxygen. However, the ternary system, /V-phenylquinonc imine (Q) + H202 + acid (HA), efficiently retards the initiated oxidation of methyl oleate and ethylbenzene [47]. This is indicated by the following results obtained for the oxidation of ethylbenzene (343 K, p02 = 98 kPa, Vi = 5.21 x 10-7 mol L 1 s 1). 

A rotary polarization modulator simply consists of an optical element that rotates uniformly at a frequency Q about the transmission axis of light. In practice, retardation plates and polarizers are used. In either case, the Mueller matrix of such a device is found by simply replacing the angle 6 by Q.t in the equations listed in Appendix I. Typical PSGs based on rotary modulators and the associated Stokes vectors, Sp G, that are produced are listed in table 8.2. 

Retarders are usually devices which rotate the polarization plane of radiation or convert linearly polarized radiation into a elliptically or circularly polarized one. Their basic physical function consists in decomposing the electric vector of the linearly polarized radiation into two mutually orthogonally polarized components between which a phase difference retardation) is created. Depending on the physical phenomenon that causes the retardation effect practical retarders based on birefringence and total internal reflection are known and used. 

Where provision has been made for water table adjustment, it is often advisable to raise the water level at times when a crop is not present. This practice retards the destruction of the peat by biological oxidation as well as helping to minimize fires and blowing. Where no such adjustment is possible, it is not unusual for fires to bum slowly for days during dry periods, and in extreme cases destroy the entire peat layer. 

Phenylpyruvic acid can cause mental retardation m infants who are deficient m the enzymes necessary to convert l phenylalanine to l tyrosine This disorder is called phenylketonuria, or PKU disease PKU disease can be detected by a simple test rou tmely administered to newborns It cannot be cured but is controlled by restricting the dietary intake of l phenylalanine In practice this means avoiding foods such as meat that are rich m l phenylalanine... 

Bromine as a Reactive Flame Retardant. Bromine and chlorine are the starting materials for all of the commercial compounds described. Bromine is also used in a somewhat different way to impart flame retardancy. That is, it is used to brominate the resin in interest directly. This is practiced commercially in the case of unsaturated polyesters (59). 

An effective, but not very practical, flame retardant for cotton based on 2,4-diamino-6-(3,3,3-tribromopropyl)-l,3,5-tria2ine [62160-38-7] (DABT) was prepared from ethyl y-tribromobutyrate and biguanide [56-03-1] ... 

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. 

Although phosphine [7803-51-2] was discovered over 200 years ago ia 1783 by the French chemist Gingembre, derivatives of this toxic and pyrophoric gas were not manufactured on an industrial scale until the mid- to late 1970s. Commercial production was only possible after the development of practical, economic processes for phosphine manufacture which were patented in 1961 (1) and 1962 (2). This article describes both of these processes briefly but more focus is given to the preparation of a number of novel phosphine derivatives used in a wide variety of important commercial appHcations, for example, as flame retardants (qv), flotation collectors, biocides, solvent extraction reagents, phase-transfer catalysts, and uv photoinitiators. 

The appearance of streaks with leveling or nonleveling acid and premetallized dye can be subdued by increasing the dye-bath pH from 5.5 to 6—7, at a sacrifice ia dye exhaust, by a dding chemical agents that retard the dye strike or, more effectively, by metering all or a portion of the dye ia a concentrated solution at or near the dyeiag temperature of the fiber (87.8—104.4°C) iastead of at the usual 26.7—48.9°C practiced by the trade (178). 

The transalkylation reaction is essentiaHyisothermal and is reversible. A high ratio of benzene to polyethylbenzene favors the transalkylation reaction to the right and retards the disproportionation reaction to the left. Although alkylation and transalkylation can be carried out in the same reactor, as has been practiced in some processes, higher ethylbenzene yield and purity are achieved with a separate alkylator and transalkylator, operating under different conditions optimized for the respective reactions. 

Antimony trioxide has numerous practical applications (1). Its principal use is as a flame retardant in textiles and plastics (see Flame RETARDANTS Flame retardants in textiles). It is also used as a stabilizer for plastics, as a catalyst, and as an opacifier in glass (qv), ceramics (qv), and vitreous enamels... 

Curing Systems. Polychloroprene can be cured with many combiaations of metallic oxides, organic accelerators, and retarders (114). The G family of polymers, containing residual thiuram disulfide, can be cured with metallic oxides alone, although certain properties, for example compression set, can be enhanced by addition of an organic accelerator. The W, T, and xanthate modified families require addition of an organic accelerator, often ia combination with a cure retarder, for practical cures. 

It is an observed fact that heating PVC at temperatures above 70°C has a number of adverse effects on the properties of the polymer. At processing temperatures used in practice (150-200°C) sufficient degradation may take place during standard processing operations to render the product useless. It has been found that incorporation of certain materials known as stabilisers retards or moderates the degradation reaction so that useful processed materials may be obtained. 

In dry air and in the presence of polymerisation inhibitors methyl and ethyl 2-cyanoacrylates have a storage life of many months. Whilst they may be polymerised by free-radical methods, anionic polymerisation is of greater significance. A very weak base, such as water, can bring about rapid polymerisation and in practice a trace of moisture on a substrate is enough to allow polymerisation to occur within a few seconds of closing the joint and excluding the air. (As with many acrylic monomers air can inhibit or severely retard polymerisation). 

Metallic sodium practically completely retards crecking of heptane and hexadecane over a period of 120 min at 500°C, and in presence of potassium hydroxide with ABC carrier a fraction of decomposed hydrocarbon amounts to 2.1% in case of heptane and to 3.1% for hexadecane. 

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