Initiating efficiency

The values have been compiled from various literature sources and hence obtained under a variety of conditirais. Rigorous conclusions based on such data are not possible but they provide a surprisingly good idea of the shape of the relationship. 

The values in Fig. 2.1 show that the detonation velocity of both MF and LA increases with increasing density, as expected. In general, one would further expect that it is desirable to press explosives to densities as close to the theoretical maximum density (TMD) as possible. This is, however, not exactly the case for primary explosives in a detonator where it is more important to have good initiation efficiency rather than high detonation velocity. 

Initiating efficiency, sometimes referred to as initiating power, strength, or priming force, is the ability of a primary explosive to initiate detonation in a secondary explosive adjacent to it. It is usually reported as a minimum amount of primary explosive necessary to cause detonation of the adjacent high explosive with 100% 

The initiating efficiency is not a material constant for a particular primary explosive. It depends on many factors including pressure used for compression, type of ignition, type of confinement, presence of reinforcing cap and its material, type of the secondary explosive, size of the contact surface between primary and secondary explosive, etc. The values of initiation efficiency reported in the literature are therefore difficult to compare due to a variation in these conditions. We have collected initiation efficiencies of some primary explosives with respect to TNT and summarized them in Fig. 2.2. 

These values show variations in the minimal amount based on a combination of these factors which are generally not provided in the references. This makes comparison of various results quite a troublesome task. It is important to understand that a single number reported without further specification (as shown in Fig. 2.2) has very low information value. The effects of the most important factors are therefore addressed in the following sections. 

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The significant thing about these and numerous other side reactions that could be described for specific systems is the fact that they lower the efficiency of the initiator in promoting polymerization. To quantify this concept we define the initiator efficiency f to be the following fraction ... 

The following conditions are stipulated the catalyst decomposition rate constant must be one hour or greater the residence time of the continuous reactor must be sufficient to decompose the catalyst to at least 50% of the feed level the catalyst concentration must be greater than or equal to 0.002 x Q, where the residence time, is expressed in hours. An upper limit on the rate of radical formation was also noted that is, when the rate of radical formation is greater than the addition rate of the primary radicals to the monomers, initiation efficiency is reduced by the recombination of primary radicals. 

Amorphous Silicon. Amorphous alloys made of thin films of hydrogenated siUcon (a-Si H) are an alternative to crystalline siUcon devices. Amorphous siUcon ahoy devices have demonstrated smah-area laboratory device efficiencies above 13%, but a-Si H materials exhibit an inherent dynamic effect cahed the Staebler-Wronski effect in which electron—hole recombination, via photogeneration or junction currents, creates electricahy active defects that reduce the light-to-electricity efficiency of a-Si H devices. Quasi-steady-state efficiencies are typicahy reached outdoors after a few weeks of exposure as photoinduced defect generation is balanced by thermally activated defect annihilation. Commercial single-junction devices have initial efficiencies of ca 7.5%, photoinduced losses of ca 20 rel %, and stabilized efficiencies of ca 6%. These stabilized efficiencies are approximately half those of commercial crystalline shicon PV modules. In the future, initial module efficiencies up to 12.5% and photoinduced losses of ca 10 rel % are projected, suggesting stabilized module aperture-area efficiencies above 11%. 

In equations 8 and 9, is the initiator efficiency, the fraction of initiator radicals that actually initiates chain growth, + k, and is the number of... 

The phenomenon is represented by Figs. 17-40 and 17-41 for Geldart-type A and B solids, respectively (see beginning of Sec. 17). The initial efficiency of a particle size cut is found on the chart, and the parametric hue is followed to the proper overall solids loading. The efficiency for that cut size is then read from the graph. 

It is important to be aware of the filter s properties in different environments. Figure 9.2 shows how, in the case of new filters, separation varies with particle size and filter class. The filter class is based on the average efficiency, and a new filter normally has much lower initial efficiency. In the case of electrostatically charged filters, separation may be significantly higher for new filters. The figure should be seen as an indication of minimum separation during actual operation. 

For MAI, both types I and II have been synthesized, while type 1 has been the major item developed for MPI. The initiation efficiency is assumed as low as approximately 0.3 for an active site of MAI [4-6], but block efficiency is expected to be much higher, because even if a pair of radicals failed initiation, they tend to recombine themselves and then another active site of the same initi-... 

In the polymerization of St initiated with type II MAI composed of polyvinylpyrrolidone (PVP), block efficiency was kept to 80% when feed concentration was above 3 mol/L, but it drastically decreased below 3 mol/ L (Fig. 2) [36,37]. AIBN, the typical low-molecular weight azo initiator, shows a drastic decrease in its initiation efficiency below a critical feed monomer concentration, i.e., 0.5 mol/L. In the case of MAI, it seems that a similar decrease in initiation efficiency occurs at much higher critical monomer concentration due to immobility of macroinitiating radicals. 

Fabric-filter systems, commonly called bag-filter or bag-house systems, are dust-collection systems in which dustladen air is passed through a bag-type filter. The bag collects the dust in layers on its surface and the dust layer itself effectively becomes the filter medium. Because the bag s pores are usually much larger than those of the dust-particle layer that forms, the initial efficiency is very low. However, it improves once an adequate dust-layer forms. Therefore, the potential for dust penetration of the filter media is extremely low except during the initial period after startup, bag change, or during the fabric-cleaning, or blow-down, cycle. 

Figure 61. Relationship between discharge capacity, initial efficiency, and L, of soft carbon materials.

Figure 61 shows the relationship between the discharge capacity, the initial efficiency, and the L of some soft carbon materials when ethylene carbonate was used as a solvent. Figure 62 shows the re-... 

It is the aim of this chapter to describe the nature, selectivity, and efficiency of initiation. Section 3.2 summarizes the various reactions associated with initiation and defines the terminology used in describing the process. Section 3.3 details the types of initiators, indicating the radicals generated, the byproducts formed (initiator efficiency), and any side reactions (e.g. transfer to initiator). Emphasis is placed on those initiators that see widespread usage. Section 3.4 examines the properties and reactions of the radicals generated, paying particular attention to the specificity of their interaction with monomers and other components of a polymerization system. Section 3.5 describes some of the techniques used in the study of initiation. 

The proportion of radicals which escape the solvent cage to form initiating radicals is termed the initiator efficiency /) which is formally defined as follows... 

In some texts the initiator efficiency (/) is defined simply in terms of the yield of initiator-derived radicals (the fraction of radicals I- that undergo cage escape - Section 3.2.8). This number will always be larger than that obtained by application of eq. 1. 

Thus, the size and the reactivity of the initiator-derived radicals and the medium viscosity (or microviscosity) are important factors in determining the initiator efficiency. Thus, the extent of the cage reaction is likely to increase with... 

Transfer to initiator introduces a new end group into the polymer, lowers the molecular weight of the polymer, reduces the initiator efficiency, and increases the rate of initiator disappearance. Methods of evaluating transfer constants are discussed in Section 6.2.1. 

Suspension polymerizations are often regarded as "mini-bulk" polymerizations since ideally all reaction occurs w ithin individual monomer droplets. Initiators with high monomer and low water solubility are generally used in this application. The general chemistry, initiator efficiencies, and importance of side reactions are similar to that seen in homogeneous media. 

The concentration of monomers in the aqueous phase is usually very low. This means that there is a greater chance that the initiator-derived radicals (I ) will undergo side reactions. Processes such as radical-radical reaction involving the initiator-derived and oligomeric species, primary radical termination, and transfer to initiator can be much more significant than in bulk, solution, or suspension polymerization and initiator efficiencies in emulsion polymerization are often very low. Initiation kinetics in emulsion polymerization are defined in terms of the entry coefficient (p) - a pseudo-first order rate coefficient for particle entry. 

Initiator efficiency increases with reaction temperature (Table 3.4). It is also worth noting that apparent zero-conversion initiator efficiencies depend on the method of measurement. Better scavengers trap more radicals. The data in Table 3,4 suggest that monomers (MMA, S) are not as effective at scavenging radicals as the inhibitors used to measure initiator efficiencies. The finding suggests that in polymerization the initiator-derived radicals have a finite probability of... 

Figure 3.3 Cumulative ( ) and instantaneous ( ) initiator efficiency (/) of AIBN as initiator in S polymerization (50% v/v toluene, 70 °C) as a function of monomer conversion (lines are a polynomial fit to the datapoints).1,32...

Table 3.4 Zero-Conversion Initiator Efficiency (/) for AIBMe under Various...

The high rate of decarboxylation of aliphatic acyloxy radicals is also the prime reason behind low initiator efficiencies (see 3.3.2.1.3). Decarboxylation occurs within the solvent cage and recombination gives alkane or ester byproducts. Cage return for LPO is 18-35% at 80 °C in -octane as compared to only 4% for BPO under similar conditions.144... 

Ideally all reactions should result from unimolecular homolysis of the relatively weak 0-0 bond. However, unimolecular rearrangement and various forms of induced and non-radical decomposition complicate the kinetics of radical generation and reduce the initiator efficiency.46 Peroxide decomposition induced by radicals and redox chemistry is covered in Sections 3.3.2.1.4 and 3.3.2.1.5 respectively. 

The importance of the cage reaction increases according to the viscosity of the reaction medium. This contributes to a decrease in initiator efficiency with conversion. 15 1 155 Stickler and Dumont156 determined the initiator efficiency during bulk MMA polymerization at high conversions ca 80%) to be in the range 0.1-0.2 depending on the polymerization temperature. The main initiator-derived byproduct was phenyl benzoate. 

A slow rate of p-scission also means that the main cage recombination process will be cage return to reform the peroxydicarbonate. Dialkyl peroxides are typically not found amongst the products of peroxydicarbonate decomposition. In these circumstances, cage recombination is unlikely to be a factor in reducing initiator efficiency. 

The low conversion initiator efficiency of di-r-butyl pcroxyoxalatc (0.93-0.97)1-1 is substantially higher than for other peroxyeslers [/-butyl peroxypivalale, 0.63 /-butyl peroxyacetate, 0.53 (60 °C, isopropylbenzene)195]. The dependence of cage recombination on the nature of the reaction medium has been the subject of a number of studies. 12I,1<>0 20CI The yield of DTBP (the main cage product) depends not only on viscosity but also on the precise nature of the solvent. The effect of solvent is to reduce the yield in the order aliphatic>aromatie>protic. It has been proposed199 that this is a consequence of the solvent dependence of p-scission of the f-butoxy radical which increases in the same series (Section 3.4.2.1.1). 

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