Chemical stability, effect

Organic metal salts have frequently failed to produce an appreciable chemical stabilization effect, either during dehydrochlorination induction periods or in later decomposition stages. While this does not rule out the occurrence of Frye and Horst substitution reactions, it does suggest that these reactions may not be responsible for the observed retardation of color developments [126-128]. 

Renard, E.V., Pyatibratov, Y.P., Neumoev, N.V., Vhizhov, A.A., Kulikov, I.A., GoTdfarb, Y.Y., Sirotkina, I.G., Semenova, T.I. 1988. Isoparaffinic diluents for tri-n-butyl phosphate. Chemical, radiation-chemical stability, effect on tetravalent plutonium and thorium extraction. Soviet Radiochem. 30(6) 734-745. (Translated from Zhurnal Struktumoi khimii 30(6) 774-787.)... 

With Organotins. In recent years the ability of organotins and metal soaps (see below) to prevent the appearance of color in degrading PVC has been ascribed most frequently to chemical reactions leading to the deactivation of defect sites. Very strong evidence for the operation of such a mechanism, at least in the case of the organotins, has been provided now by the chemical stabilization effects whose discovery (22,23, 24,25) was noted above. However, these effects do not rule out the concurrent operation of other mechanisms in technological situations (22,23). 

The method for calculating effective polarizabilitie.s wa.s developed primarily to obtain values that reflect the stabilizing effect of polarizability on introduction of a charge into a molecule. That this goal was reached was proven by a variety of correlations of data on chemical reactivity in the gas phase with effective polarizability values. We have intentionally chosen reactions in the gas phase as these show the predominant effect of polarizability, uncorrupted by solvent effects. 

Heat stabilizers protect polymers from the chemical degrading effects of heat or uv irradiation. These additives include a wide variety of chemical substances, ranging from purely organic chemicals to metallic soaps to complex organometaUic compounds. By far the most common polymer requiring the use of heat stabilizers is poly(vinyl chloride) (PVC). However, copolymers of PVC, chlorinated poly(vinyl chloride) (CPVC), poly(vinyhdene chloride) (PVDC), and chlorinated polyethylene (CPE), also benefit from this technology. Without the use of heat stabilizers, PVC could not be the widely used polymer that it is, with worldwide production of nearly 16 million metric tons in 1991 alone (see Vinyl polymers). 

Chemical stabilization involves removing the concentration of surface hydroxyls and surface defects, such as metastable three-membered rings, below a critical level so that the surface is not stressed by rehydroxylation in use. Thermal stabilization involves reducing the surface area sufficiently to enable the material to be used at a given temperature without reversible stmctural changes. The mechanisms of thermal and chemical stabilization are interrelated because of the extreme effects that surface hydroxyls and chemisorbed water have on stmctural changes. Full densification of gels, such as the... 

Information pertaining to the hazards of the chemicals used in the process. This should contain at least the following information toxicity, flammability, permissible exposure limits, physical data, reactivity data, corrosivity data, thermal and chemical stability data, and hazardous effects of inadvertent mixing of different materials that could occur. 

High thermostabilizing efficiency of polyamine disulphides relative to chemically cross-linked polyethylene is conditioned by the ability to accept macroradicals at the disulphide bridge and imine group. Besides, the presence of paramagnetic centers causes the adherence of macroradicals providing for an extra stabilizing effect [49]. 

Once an antibiotic producer has been identified, the next stage is to produce sufficient of the antibiotic to evaluate its potential for therapeutic use. Questions, such as, is it toxic to humans , is it effective against disease organisms , does it possess suitable characteristics (for example solubility, chemical stability) for use as a medicine , need answering. Let us assume that a new, potentially useful antibiotic has been discovered. The key questions then become, how can the desired material be produced in the most cost effective way is it possible to produce variants of the antibiotic which have desirable properties, such as greater effectivity against infection, cheaper ways to produce it or increased stability ... 

The biradical catalysts described previously for double-base propints (Ref 80) are also effective for hydrocarbon propints. Table 34 shows how p,p,-biphenylene-bis(diphenylmethyl) compares to n butyl ferrocene as a catalyst in a carboxy-terminated polybutadiene. These catalysts are claimed to overcome all of the processing difficulties, chemical stability and volatility disadvantages attributed to catalysts based on ferrocene and carborane derivatives. Another somewhat similar functioning catalyst, the free radical compd, 2,2-diphenylpicrylhydrazyl,... 

When assessing the robustness of a process, several factors that can adversely affect it include non-selective or side reactions that might produce adverse effects and impurities physical and chemical stability of the materials involved and complexity of the separation train of the processes. 

Chemical surface modifications The first surface modification for the purpose of eliminating EOF and protein adsorption was recommended by Hjerten.28 The attachment of vinyl silanes allowed the polymerization of a variety of molecules to the surface. Most of the chemical modifications used for preparing capillaries for electrophoresis originated from the experience acquired over the years preparing GC and LC stationary phases. Chemical modification should conform to certain requirements, including the prevention of adsorption, the provision of stable and constant EOF over a wide pH range, chemical stability, ease of preparation, and reproduciblity of preparation. The effects of silanization of the inner surface of capillaries on electrophoretic separations have been extensively studied.26-29... 

The optimized drug product may be viewed as a drug-delivery system for the one or more drugs that it contains. The goal of this drug-delivery system is to release the drug(s) to produce the maximum simultaneous safety, effectiveness, and reliability, as depicted in Fig. 9. Various physicochemical product properties that influence the quality features of safety, effectiveness, and reliability are shown in Table 6. Some physicochemical properties can affect two or all three quality features of Fig. 9. For example, consider chemical stability. As a drug decomposes, if the... 

Acute safety quantification Therapeutic index = LD5o/ED5o Clinical effectiveness Chemical stability... 

The most widely used preservative remains benzalkonium chloride, which often is supplemented with disodium edetate. The benzalkonium chloride defined in the USP monograph is the quaternary ammonium compound alkylbenzyldimethylammonium chloride, in which the alkyl portion is composed of a mixture of chain lengths ranging from C8 to C16. This compound s popularity is based, despite its compatibility limitations, on its being the most effective and rapid-acting preservative with excellent chemical stability. It is stable over a wide pH range and does not... 

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