Initiator thermal decomposition parameters

Table 6.3 Thermal Decomposition Parameters of Some Initiators...

Two alternative methods have been used in kinetic investigations of thermal decomposition and, indeed, other reactions of solids in one, yield—time measurements are made while the reactant is maintained at a constant (known) temperature [28] while, in the second, the sample is subjected to a controlled rising temperature [76]. Measurements using both techniques have been widely and variously exploited in the determination of kinetic characteristics and parameters. In the more traditional approach, isothermal studies, the maintenance of a precisely constant temperature throughout the reaction period represents an ideal which cannot be achieved in practice, since a finite time is required to heat the material to reaction temperature. Consequently, the initial segment of the a (fractional decomposition)—time plot cannot refer to isothermal conditions, though the effect of such deviation can be minimized by careful design of equipment. 

The rates of radical-forming thermal decomposition of four families of free radical initiators can be predicted from a sum of transition state and reactant state effects. The four families of initiators are trarw-symmetric bisalkyl diazenes,trans-phenyl, alkyl diazenes, peresters and hydrocarbons (carbon-carbon bond homolysis). Transition state effects are calculated by the HMD pi- delocalization energies of the alkyl radicals formed in the reactions. Reactant state effects are estimated from standard steric parameters. For each family of initiators, linear energy relationships have been created for calculating the rates at which members of the family decompose at given temperatures. These numerical relationships should be useful for predicting rates of decomposition for potential new initiators for the free radical polymerization of vinyl monomers under extraordinary conditions. 

Early kinetic experiments on the thermal decomposition of nitro compounds established that for the simplest derivative, nitromethane, the process was first order, but that the reaction was chemically complex owing to further reactions between the products and nitromethane. Cottrell et re-examined the nitromethane pyrolysis and reported values of = 53.2 kcal.mole" and log A = 13 for the Arrhenius parameters of the homogeneous decomposition a radical mechanism was proposed, initiated by C-N cleavage... 

These equations are commonly called pyrolysis relations, in reference to the thermal (as opposed to a possibly chemical or photonic) nature of the initiating step(s) in the condensed phase decomposition process. It can be seen that while the second, simpler pyrolysis expression with constant coefficient As) preserves the important Arrhenius exponential temperature dependent term, it ignores the effect of the initial temperature, condensed phase heat release and thermal radiation parameters present in the more comprehensive zero-order pyrolysis relation. These terms To, Qc, and qr) make a significant difference when it comes to sensitivity parameter and unsteady combustion considerations. It is also important to note the factor of 2, which relates the apparent "surface" activation energy Es to the actual "bulk" activation energy Ec, Es- E /1. Failure to recognize this factor of two hindered progress in some cases as attempts were... 

The thermal decomposition of ammonium salts is an important field of study. Kinetic parameters for the decomposition of NH4NO3 have been determined, it being found that the addition of small amounts of NH4CI reduces both the thermal stability and the initial decomposition temperature of the nitrate. Confirmation of a previously proposed mechanism for the chloride-transition-metal-ion synergistically catalysed thermal decomposition of NH4NO3 has come from visible absorption spectroscopy. Ammine, nitrato-, and chloro-complexes of Cu and Ni° and chloro-complexes of Co i were detected in fused NH4NO3. 

Heating bone tissue to a temperature of about 750 °C ensures a complete biological decontamination [3]. When the temperature increases above this threshold the start of a complex metamorphic transformation of the bone tissue occurs. The thermal decomposition of stoichiometric hydroxyapatite undergoes at temperatures over 800 °C, with the initial formation of oxy-hydroxyapatite and oxy-apatite, followed by the oxy-apatite decomposition into various forms of tricalcium phosphate and/or calcium oxide [4,48]. An endothermic phenomenon can be identified in the range 800-1000 C, assigned to the modification of the hydroxyapatite crystalline lattice parameters, which takes place shortly before the initiation of its decomposition in beta-tricalcium phosphate [(3-TCP, CajCPO ) ] [49]. 

Where, is the area ratio of the total experimental cmwe divided by the total TGA thermogram K is the coefficient of A 7] is the initial experimental temperature (°C), and Tfi the final experimental tenperature (°C). The thermal stability parameter of the TLCP/MWCNT nanocomposites is shown in Table 2. All parameters increased with increasing MWCNT content, indicating that incorporation of MWCNT into the TLCP matrix enhance the thermal stabihty of TLCP/MWCNT nanocomposites. The enhancement of thermal stabihty of the TLCP/MWCNT nanocomposites may be attributed to a physical barrier effect of the MWCNT, which prevented the transport of decomposition in the polymer... 

Activation Parameters. Thermal processes are commonly used to break labile initiator bonds in order to form radicals. The amount of thermal energy necessary varies with the environment, but absolute temperature, T, is usually the dominant factor. The energy barrier, the minimum amount of energy that must be suppHed, is called the activation energy, E. A third important factor, known as the frequency factor, is a measure of bond motion freedom (translational, rotational, and vibrational) in the activated complex or transition state. The relationships of yi, E and T to the initiator decomposition rate (kJ) are expressed by the Arrhenius first-order rate equation (eq. 16) where R is the gas constant, and and E are known as the activation parameters. 

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