Concentration initiator, effect

In order to determine the likely contaminations of chemicals in foods for consnmption, all factors that may impact the final level shonld be considered systematically initial concentrations, effect of each individnal step in food prodnction on final chemical levels, and potential reactions with other agents. 

The effects of the steady-state situation and the effect of the peak load can be described using a model. When the caustic concentration is low, either through initial concentration effects or by mass transfer limitations, the reaction of chlorine with chlorite can occur and chlorine dioxide (Equations 25.3 and 25.8) is formed near the gas-liquid interface. The concentration of chlorite seems quite important and is influenced by temperature (decomposition) and the hypochlorite concentration. A higher chlorite concentration will give, according to the reactions of Equations 25.3 and 25.8, a higher chlorine dioxide content in the presence of chlorine and/or hypochlorous acid. 

Initiator Concentration Effect Lauryl Acrylate Photopolvinerization... 

The probability of collisions between activated chain ends and initiator fragments increases proportionally to the concentration of the initiator in the system. Whereas a high initiator concentration effectively accelerates the polymerization by producing a comparatively large number of active centers, it provides, on the other hand, for chain breakers and prevents the development of long chains. In practice, therefore, some speed of polymerization has to be sacrificed if high-molecu ar-weight material is desired, and the initiator concentration must be kept within limits. 

Choi BG, Park MH, Cho S-H, Joo MK, Oh HJ, Kim EH, Paik K, Han DK, Jeong B (2011) Thermal gelling polyalanine-poloxamine-polyalanine aqueous solution for chondrocytes 3D culture initial concentration effect. Soft Matter 7 456-462... 

An alternative to a fixed-time method is a variable-time method, in which we measure the time required for a reaction to proceed by a fixed amount. In this case the analyte s initial concentration is determined by the elapsed time, Af, with a higher concentration of analyte producing a smaller Af. For this reason variabletime integral methods are appropriate when the relationship between the detector s response and the concentration of analyte is not linear or is unknown. In the one-point variable-time integral method, the time needed to cause a desired change in concentration is measured from the start of the reaction. With the two-point variable-time integral method, the time required to effect a change in concentration is measured. 

Throughout this section we have used mostly p and u to describe the distribution of molecular weights. It should be remembered that these quantities are defined in terms of various concentrations and therefore change as the reactions proceed. Accordingly, the results presented here are most simply applied at the start of the polymerization reaction when the initial concentrations of monomer and initiator can be used to evaluate p or u. The termination constants are known to decrease with the extent of conversion of monomer to polymer, and this effect also complicates the picture at high conversions. Note, also, that chain transfer has been excluded from consideration in this section, as elsewhere in the chapter. We shall consider chain transfer reactions in the next section. 

The molecular weight of a polymer can be controlled through the use of a chain-transfer agent, as well as by initiator concentration and type, monomer concentration, and solvent type and temperature. Chlorinated aUphatic compounds and thiols are particularly effective chain-transfer agents used for regulating the molecular weight of acryUc polymers (94). Chain-transfer constants (C at 60°C) for some typical agents for poly(methyl acrylate) are as follows (87) ... 

The treatments used to recover nickel from its sulfide and lateritic ores differ considerably because of the differing physical characteristics of the two ore types. The sulfide ores, in which the nickel, iron, and copper occur in a physical mixture as distinct minerals, are amenable to initial concentration by mechanical methods, eg, flotation (qv) and magnetic separation (see SEPARATION,MAGNETIC). The lateritic ores are not susceptible to these physical processes of beneficiation, and chemical means must be used to extract the nickel. The nickel concentration processes that have been developed are not as effective for the lateritic ores as for the sulfide ores (see also Metallurgy, extractive Minerals recovery and processing). 

Bulk polymerization has been studied at relatively low temperatures and in toluene and carbon tetrachloride solutions carried to low conversions (12). The effects of temperature and different organic peroxide initiators have been observed. The molecular weight of soluble polymer after 3% conversion is ca — 19,000 and is somewhat dependent on initiator concentration or temperature between 35 and 65 °C. With di-2-methylpentanoyl... 

The reaction involves the nucleophilic attack of a peracid anion on the unionized peracid giving a tetrahedral diperoxy intermediate that then eliminates oxygen giving the parent acids. The observed rate of the reaction depends on the initial concentration of the peracid as expected in a second-order process. The reaction also depends on the stmcture of the peracid (specifically whether the peracid can micellize) (4). MiceUization increases the effective second-order concentration of the peracid because of the proximity of one peracid to another. This effect can be mitigated by the addition of an appropriate surfactant, which when incorporated into the peracid micelle, effectively dilutes the peracid, reducing the rate of decomposition (4,90). 

Insoluble corrosion prodiic ts may be completely impeivious to the corroding liquid and, therefore, completely protective or they may be quite permeable and allow local or general corrosion to proceed unhindered. Films that are nonuniform or discontinuous may tend to localize corrosion in particular areas or to induce accelerated corrosion at certain points by initiating electrolytic effects of the concentration-cell type. Films may tend to retain or absorb moisture and thus, by delaying the time of drying, increase the extent of corrosion resulting from exposure to the atmosphere or to corrosive vapors. 

This equation indicates that the reaction rate is proportional to the square root of the initiator concentration and to the monomer concentration. It is found that the relationship with initiator concentration is commonly borne out in practice (see Figure 2.18) but that deviations may occur with respect to monomer concentration. This may in some cases be attributed to the dependency of / on monomer concentration, particularly at low efficiencies, and to the effects of certain solvents in solution polymerisations. 

An increase in the rate of radical production in emulsion polymerisation will reduce the molecular weight since it will increase the frequency of termination. An increase in the number of particles will, however, reduce the rate of entry of radicals into a specific micelle and increase molecular weight. Thus at constant initiator concentration and temperature an increase in micelles (in effect in soap concentration) will lead to an increase in molecular weight and in rate of conversion. 

Lopata, V.J., Chung, M., Janke, C.J. and Havens, S.J., Electron curing of epoxy resins initiator and concentration effects on curing dose and rheological properties. 28th Int. SAMPE Technical Conference, 901, 1996. 

In the multistep swelling and polymerization method, polymerization time, temperature, and initiator concentration have marginal effects only. The ratio of monovinyl to divinyl monomer in the polymerization mixture, along with the composition of the porogenic systems, is known to be the most important factor in controlling the ultimate macroporous structure. The average molecular... 

Flgure 4 The effect of initiator concentration on the variation of monomer conversion by the polymerization time in the emulsion polymerization of styrene. Styrene-water = 1/3 SDS = 15.4 mM reaction volume = 300 ml stirring rate = 250 rpm temperature = 70°C. 

The type and concentration of oil-soluble initiator are effective both on the polymerization rate and on the average size of the final product. The polymerization rate and the average size of the final product usually increase with the increasing initiator concentration. 

Lu et al. [86] also studied the effect of initiator concentration on the dispersion polymerization of styrene in ethanol medium by using ACPA as the initiator. They observed that there was a period at the extended monomer conversion in which the polymerization rate was independent of the initiator concentration, although it was dependent on the initiator concentration at the initial stage of polymerization. We also had a similar observation, which was obtained by changing the AIBN concentration in the dispersion polymerization of styrene conducted in isopropanol-water medium. Lu et al. [86] proposed that the polymerization rate beyond 50% conversion could be explained by the usual heterogenous polymer kinetics described by the following equation ... 

Figure 1 Effect of monomer concentration on the total conversion percent and grafting efficiency. LR 30 I, acid concentration l%, initiator concentration 0. l%, grafting time 1 h, and reaction temperature 27°C — = total conversion (%) O—O = grafting efficiency (%).

---