Thermal initiator decomposition

Many materials need to be dried prior to their analysis to remove residual moisture. Depending on the material, heating to a temperature of 110-140 °C is usually sufficient. Other materials need to be heated to much higher temperatures to initiate thermal decomposition. Both processes can be accomplished using a laboratory oven capable of providing the required temperature. 

Eriction from contact of moving parts, tramp metal, bearings or seals initiating thermal decomposition or igniting flammable vapors. 

Strontium nitrate, Sr(N03)2, is a high-density material with a melhng point of 843 K. The initial thermal decomposition in the low-temperature region produces Sr(N02)2, which decomposes to SrO in the high-temperature region according to ... 

PVDF resins are stable when processed as recommended at temperatures between 190°C and 300° Thermogravimetiic analysis has shown that the initial thermal decomposition starts at approximately 375°C. At temperatures approaching 300°C, the onset of a thermal degradation will be evident as a darkening of the resin. 

Table 18.6 Initial thermal decomposition temperature and corresponding enhancement in NR and its NR/Si02 composites.

In some polymer nanocomposites the initial thermal decomposition will be accelerated due to the presence of alkyl ammonium surfactant on the surface of the clay. For instance in PVC the quaternary ammonium salts may accelerate the degradation of PVC.However, this problem does not exist in clay reinforced natural rubber nanocomposites. 

The decomposition temperature of PLA is normally 230—260°C. Therefore, it is considered to be safe for room temperature applications. PLA is seldom used at elevated temperatures, such as the boiling point of water, because PLA tends to lose its structural properties at temperatures >60°C. Although PLA is unlikely to release toxic substances extensively, residues of plasticizer or oligomers still need further attention. PLA undergoes initial thermal decomposition at temperatures above 200°C by hydrolysis reaction followed by lactide reformation, oxidative main-chain scission, and inter-or intramolecular transesterification reaction (Jamshidi et al., 1988). Thermal decomposition can occur at 200°C without catalysts, but it requires higher temperatures to induce a faster and more prevalent reaction (Achmad et al., 2009). 

As in the case of photochemical initiation, thermal decomposition of the initiator and its consumption in a subsequent chain re-r.-action present the most complicated situation. 

Initial thermal decomposition temperature in TGA thermograms at a heating rate of 10 C/min. Thermal decomposition temperatures at 10% and 60% of the weight-loss, respectively. 

It should be pointed out that there are many methods for initiation of free-radical polymerization, though the most popular approach in industry is the use of chemical initiators such as peroxides and azo compounds. Thermal and radiation initiations are also employed in industry. For chemical initiation, the initiator thermal decomposition is a monomolecular reartion ... 

In spite of the many advantages of the cold injection system, in practice there are also limits to its use. These exist both between different cold injection techniques and in comparison with hot injection techniques (Table 2.23). These include the analysis of thermally labile substances. Because of the low injection temperature a cold injection is expected to be particularly suitable for labile substances. However, during the heating phase, the residence times of the substances in the insert are long enough to initiate thermal decomposition. In this case, only on-column injection can be used because it completely avoids external evaporation of the sample for transfer to the column (see Section 2.2.6.3). A test for thermal decomposition was suggested by Donike with the injection of a mixture of the same quantities of fatty acid TMS esters (CIO to C22 thermolabile) and n-alkanes (C12 to C32 thermally stable). If no thermal decomposition takes place, all the substances appear with the same peak intensity. 

This kinetic stability of carbon compounds has a variety of causes, notably the/ // use of the four valence orbitals sp ) in carbon (leading to the common maximum co-ordination number of four—exceptions, e.g. Me4Li4, (Me3Al)2 in which the co-ordination number rises to S-7 are discussed in Chapter 3), and the high energy of empty antibonding or nonbonding orbitals into which electrons could either be promoted to initiate thermal decomposition or donated in the case of nucleophilic attack. 

From Figure 26(a), it can be seen that 18 electrons in all are required to fill all the bonding and non-bonding molecular orbitals in the complex. These electrons come both from the metal and from the ligands. Kinetic stability of the complex will be achieved if no low-lying orbitals are available into which electrons may readily be promoted to initiate thermal decomposition, or donated, as in nucleophilic attack. It is clear that kinetic stability is unlikely to be attained if bonding or non-bonding M.O.s are empty, that is, if less than a total of 18 electrons is present. This is the molecular orbital explanation of the 18-electron rule. 

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