Accelerators, types

There are seven principal classes of accelerators and several miscellaneous products that do not fit into these classes. In addition, many proprietary blends of several accelerators are sold which are designed as cure packages for a specific appHcations. Choosing the best cure system is a responsibiUty of the mbber chemist and requites extensive knowledge of each accelerator type and its appHcabiUty in each elastomer. Table 5 shows a rule of thumb comparison of the scorch/cure rate attributes for the five most widely used classes of accelerators used in the high volume diene-based elastomers. 

Accelerator type Scorch safety Cure rate Cro ss-link length... 

The difference in degree of cure of the blends by different curatives has also been explained on the basis of changes in curative distribution with accelerator types and the effect of cure temperature. The tensile properties of the blend cured by S/ZDEC at 170°C were significantly lower and modulus was higher than those cured by S/MET and S/DPG as shown in Table 11.17. Lowering of cure temperature by 20°C significantly improved these properties. However, the standard deviation in the results increased limiting the potential for any solid conclusion. 

A number of chemicals are used which do not fall into the general categories of the conventional accelerator types used by the rubber industry. 

Accelerator type IORT-NOVAC7 LIAC Laser-driven accelerator... 

Two types of calcium nitrite-based corrosion inhibitors are currently marketed, viz. a set- and strength-accelerating type and a normal-setting type. The former increases the early strength development in concrete. This effect increases with the dosage. Both admixtures are compatible with all types of Portland cements and... 

LNG dispensers are very similar to CNG dispensers in function their primary purpose is to meter and control the flow of LNG to the vehicle. Like CNG dispensers, LNG dispensers do not include a pump. The meters inside LNG dispensers are typically of the Coriolis acceleration type that have low flow restriction... 

Eq. (2.52) is a self-acceleration type kinetic equation for crystallization, but the temperature dependencies of constants are different. In some cases, the temperature dependence of K(T) given by Eq. (2.51) is better expressed by an alternative equation widely used in the theory of crystallization ... 

Reactions (2) of the organic peroxy reactions are not chain branching since both RO2 and RO are active radicals, but at low temperatures they probably speed up the chain propagation by providing a faster alternative to the main route, through peroxy isomerization (Section 1.10). Another accelerating type of reaction is the hydrogen abstraction reactions of NO2, which are initiation reactions. 

Plant Model Accelerator Type Energy, N.of (MeV) Acc Operating power (kW) Aim Status Dose, (kGy) Water treated (mVday) Total cost (/m )... 

Natta and Pasquon [12) were the first to study the effect of the catalyst particle size on the rate of polymerization. For a constant concentration of the monomer, they found that the rate of polymerization changed with time. With ground Ti02 (particle size < 2/i) the rate of propylene polymerization quickly reached a maximum and then decreased gradually to an asymptotic stationary value. In the case of unground particles (size up to 10/i), however, there were no maxima but the rate accelerated to approach the same asymptotic stationary value. The former behavior is referred to as the decay type, whereas the latter is known as the build-up or acceleration type. Figure 9.7 shows typical rate curves of these types and the different zones, termed build-up, decay, and stationary periods, into which they can be classified. 

Figure 9.7 Typical kinetic curves obtained during propylene polymerization by TiClj. (A decay type B build-up or acceleration type I build-up period II decay period III stationary period.)...

As mentioned earlier, the acceleration type behavior is explained by an increase in surface due to breakup of catalyst particles subjected to mechanical pressure of growing polymer chains anchored to the catalyst active centers. The smaller the particle size, the greater the mechanical energy required for further size reduction, and so the particle size—and hence the specific surface area—would reach some asymptotic value. The stationary polymerization rate would correspond to this catalyst particle size. 

Kei et al. [22] have used Langmuir-type adsorption to explain the observed rate behavior both during the initial stage (build-up period) and in the stationary state of the polymerization of propylene with TiCls-AlEta which exhibits a decay-type behavior (curve A in Fig. 9.9) (see Problem 9.8). Similarly the observed rate behavior in the build-up period of the acceleration-type curve (curve B in Fig. 9.9) can be explained. 

The majority of Mdssbauer experiments that are conceived to elucidate problems of chemical structure make use of velocity modulated spectrometers. Most frequently, these are of the constant acceleration type, in which equal time intervals are spent in equal velocity increments. The primary information storage device for such spectrometers is commonly a multichannel analyzer operated in the multiscaler mode. Since the minimum observable line widths (2rnat= hf ) for S7Fe and 1,9 Sn are 0.1946 and 0.6193 mm/s, respectively, and since on the order of at least 5-10 data points on each side of the resonance maximum are... 

Giulietti, A. et al. (2004) Vitamin D deficiency in early life accelerates Type 1 diabetes in non-obese diabetic mice. Diabetologia, 47, 451-462. 

Figure 9.6 Typical kinetic curves observed during propylene polymerization with ground and unground TiCls- Curve A decay type curve B build-up or acceleration type zone I build-up period zone II decay period zone HI stationary period. (After Keii, 1973.)...

TABLE 2.5 Accelerator for Sulfur Vulcanization of Rubbers Accelerator Type and Formula... 

Accelerators are often added to reduce the crosslinking time during curing of novolacs with epoxy resins. Accelerators increase the rate of both equation 22 and equation 23. However, the accelerator type, its concentration, and the... 

Accelerator type Accelerator content (mole-% relative to epoxy groups) ratio k 22/k 23... 

We should stress that this difference only applies to processes in which palladium species have an undefined coordination shell (as in type 1 and type 2 systems). For true ligand-accelerated type 3 processes, which are controlled by a definite stable Ugand in the coordination shell, the difference between electron-rich and electron-poor ( inactivated versus activated ) substrates is indeed not dramatic, which allows for the formulation of broad-scope robust protocols. 

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