Accelerators chemical structure

Functionally, accelerators are classified as primary or secondary. Primary accelerators provide considerable scorch delay, medium fast cure, and good modulus development. Secondary accelerators, on the other hand, are usually scorchy and provide very fast cure. There are a wide variety of accelerators available to the compounder including accelerator blends this number well over 100. In order to rationalize the extensive range of materials it is useful to classify them in terms of their generic chemical structure listed below and shown in Figure 14.4. 

Ic. Cross-Linking of Polymer Chains.—Formation of chemical bonds between linear polymer molecules, commonly referred to as cross-linking, also may lead to the formation of infinite networks. Vulcanization of rubber is the most prominent example of a process of this sort. Through the action of sulfur, accelerators, and other ingredients present in the vulcanization recipe, sulfide cross-linkages are created by a mechanism not fully understood (see Chap. XI). Vulcanized rubbers, being typical network structures, are insoluble in all solvents which do not disrupt the chemical structure, and they do not undergo appreciable plastic, or viscous, flow. 

The next question pertains to substances that will self-react, often with accelerating or explosive rapidity. These substances have various chemical structures that make them susceptible to at least one of three forms of self-reaction ... 

See also in this section "Brisance, Correlation with Chemical Structure "Brisance, Correlation with Properties Other Than Chemical and "Brisance, Determination by Method of Metal Acceleration by Explosives )... 

It should also be feasible to extend further the types of reaction that can be accelerated. For example, large solvent effects have been observed in kinetic studies of many reactions involving anions.46,47,5° 51 In many cases the solvents are aprotic but not truly apolar, in the sense that their molecules have large dipole moments, for example, (CH3)2S=0, CH3CON(CH3)2. Derivatives of polyethylenimine can be made that have substituents mimicking these in chemical structure. For example, acylation of the polymer will produce CH3CO—N=C loci on the macromolecule. Such modified polymers should manifest substantial catalytic effects. 

An optimum processing time of 180 min is required for a complete cycle of 100 kGy dose. A shutdown time of at least 8 h was necessary after every 100 kGy addition to allow sufficient diffusion of oxygen in the PTFE powder. The total time for the whole process from 20 to 500 kGy was approximately 50 h, including the 8 h shutdown intervals after every 100 kGy addition. To achieve 500 kGy, the doses were added to the PTFE powder in 100 kGy steps. These treatment parameters were chosen in order to avoid excess temperature rise, which might favor deactivation of the radical formation, as well as to control agglomerate size and chemical structure via absorbed dose. Further information on the electron accelerator (ELV-2) facility can be found in [11]. 

Resin solidification (curing) occurs by a free radical addition mechanism at the double bonds. That is why no by-products are formed. Curing compositions based on polyester resins contain a large number of different components (resins, initiators, accelerators, monomers, oligomers, fillers, etc), which may have various chemical structures, and may be used in various proportions. 

The constant K in this equation, as in some other cases discussed above, denotes the initial reaction rate. Its temperature dependence is described by the standard Arrhenius equation with activation energy U. The constant co characterizing the self-acceleration does not depend on temperature but does depend on the composition of the reactive medium in particular, such factors as the chemical structure and concentration of the curing agent, and the concentration of the catalyst and other components influence the value of co. 

The degree of light-induced photoyellowing measured on mechanical pulps depends to some extent on the time between the actual irradiation and the brightness measurement. For example, if a sheet is kept for a certain time after accelerated irradiation the brightness increases slowly to reach a new upper limit. If the irradiated and stared sheet is irradiated a second time, the brightness decreases again, i.e. the pulp shows a photochromic effect. The extent of the photochromic effect depends both on the type of mechanical pulp used and on the irradiation time. In this study we have tried to evaluate which factors affect the photochromic effect and to identify the chemical structures responsible. 

In such a way, an ordered supermolecular structure could be spontaneously formed from polymers exhibiting a random coil structure. The fundamental cause of this transformation is naturally the first-order structure of the polymer, i.e. its chemical structure. If a polymer with the proper structural units was prepared, then fiber formation would be accelerated and would especially account for the formation of a highly ordered structure. 

The rate of spontaneous reactivation (Figure 65.1, Reaction 3) can be accelerated by pyridinium oximes that have a chemical structure which fits the structure of the inhibited AChE. The oximes can only be of benefit as long as inhibited AChE is not completely converted to the aged form. 

Finally, a third approach has been proposed with five stages that focus on the chemical structure, DNA-reactivity, epigenetic effects, limited bioassays, and finally, the application of accelerated bioassays. These accelerated bio assays require 40 weeks and apply to the use of sensitive markers for induction of neoplasia in comparison to positive control compounds for important organs in human carcinogenesis. It enables data acquisition of the entire carcinogenesis process directed toward developing mechanistic information. This system would have the potential to replace the chronic bioassay in rodents in some circumstances and could serve a analternative to a chronic bioassay in a second species. ... 

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