Retarding Accelerators

Rubber Chemicals. Sodium nitrite is an important raw material in the manufacture of mbber processing chemicals. Accelerators, retarders, antioxidants (qv), and antiozonants (qv) are the types of compounds made using sodium nitrite. Accelerators, eg, thiuram [137-26-8J, greatly increase the rate of vulcaniza tion and lead to marked improvement in mbber quaUty. Retarders, on the other hand (eg, /V-nitrosodiphenylamine [156-10-5]) delay the onset of vulcanization but do not inhibit the subsequent process rate. Antioxidants and antiozonants, sometimes referred to as antidegradants, serve to slow the rate of oxidation by acting as chain stoppers, transfer agents, and peroxide decomposers. A commonly used antioxidant is A/,AT-disubstituted Nphenylenediamine which can employ sodium nitrite in its manufacture (see Rubber chemicals). 

Several of the commonly used rubber chemicals tike accelerators, retarders, antidegradants, etc. are classified as hazardous on account of values of LD50 (lethal dose 50%) [6]. Many of the guanidine... 

The cement usually is mixed dry with the additives. Depending on the application of the cement, a wide variety of additives can be incorporated. These include accelerators, retarders, dispersants, extenders, weighting agents, gels, foamers, and fluid loss additives. 

The requirements of the Helmholtz-Lagrange relation and further constraints on the properties of a lens (for example, a constant distance d between the object and the image which is of great practical importance) do not allow an arbitrary choice of values for the magnification and/or acceleration/retardation of the electrons. Hence, in order to obtain freedom in the selection of certain properties, a lens system must have a certain number of elements. For example, if the electron energy is to be varied, keeping the distance d constant, at least two free parameters are... 

In order to demonstrate the convenient properties of lenses combined with an electrostatic energy analyser, three examples will be discussed first the important case in which an acceleration/retardation lens is placed in front of the analyser which allows an operation mode with a constant pass energy in the analyser second the case in which increased acceptance angles are used, which is important for coincidence experiments and third the case in which 0 eV electrons are handled and analysed with high resolution and high acceptance. 

An experimental set-up using a lens system for the acceleration/retardation of the electrons in conjunction with an electrostatic analyser is shown in Fig. 4.32. The electron spectrometer is a spherical analyser with a mean radius R0 of 101.6 mm combined with a three-aperture zoom lens at the entrance and the exit. Only the purpose and the characteristic properties of the entrance lens will be... 

Figure 4.32 Schematic drawing of an electron spectrometer with acceleration/retardation of the electrons by a three-aperture zoom lens placed at the entrance and exit of a spherical electron spectrometer. The elements of the entrance lens are given by L, (with entrance cone C), L2, and L3 Di and D2 are x-y deflectors for steering the beam and H is a Herzog diaphragm for the compensation of edge effects (see [Her40]). Reprinted from Nucl. Instr. Meth. 222, Parr et ai, 221 (1984) with kind permission of Elsevier Science - NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.

In Fig. 4.33 the relevant potentials and energies are shown for the case of acceleration. Because the electrode L, is grounded, and L3 is at the potential needed to accelerate/retard the incident electrons to the same pass energy in the spectrometer, one derives from equ. (4.40) the following relations for the voltages... 

The issue of proper management of hazardous wastes is one which suffers from much misinformation and confusion. The present chapter reviewed additives and techniques that can be applied to specific solidification problems and immobilization of specific hazardous constituents (e.g., lead and cadmium), including a list of generic additives that can be used to control wastes pH to reduce, oxidize, and co-precipitate constituents and to accelerate/retard set. 

Humic substances are widely recognised to strongly bind to iron and to influence the redox transformations of iron. These substances have been variously reported to accelerate, retard or have no effect on the rate of Fe(II) oxygenation under a variety of pH and dissolved oxygen conditions [82-87]. However, at typical environmental pH, recent studies have found that DOM in natural waters generally accelerates the oxidation reaction [5,88], which has been attributed in part to the high density of carboxylate binding sites in many humic substances [89]. 

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