Decomposition temperature compared

Figure 4.1 Microstructures of cross sections of group V transition metal - carbon diffusion couples. From top to bottom V-C, Nb-C and Ta-C. Left column C, phase band between the a and the 6 phase right column Absence of the Q phase above the decomposition temperature (compare Table 4.1). Polarized light.

Nair and co-workers [203] reported polycyanate esters of an imide-modified novolac of different maleimide content. The resins underwent a two-stage independent thermal curing through trimerisation of the cyanate groups, as well as addition polymerisation of maleimide moieties. On heating, cyanate esters were transformed into an imido-phenolic-triazine network polymer. The cured resins exhibited a higher initial decomposition temperature compared with the cured maleimide novolacs. However, the thermal stability was found to be inferior to the conventional phenolic-triazine resin. 

Different results are obtained for high-thermal-stability epoxies. In this case the nanocomposite shows a decrease in the onset of decomposition temperature compared to the neat polymer. Hussain et al. showed that the onset of decomposition of an amino-cured epoxy is 420°C, whereas a nanocomposite containing 5 wt% octadecylammonium montmorillonite exhibits, at the same temperature, 420°C, a weight loss of 10%. It is obvious that in this case the weight loss in the nanocomposite is not only a direct effect of volatilization of the surfactant but is attributed to the catalytic effect of onium decomposition. Camino et al. compared the effect of different organo-modified montmorillonites on the thermal stability of a DGEBA cured with methyl tetrahydrophthalic anhydride. They observed that the octadecylammonium montmorillonite nanocomposite has the lowest onset at 5 wt% loss (288°C), compared to bis(2-hydroxyethyl)ammonium montmorillonite... 

The commercial prodnction of PPC is still quite modest at several kilotous annually and its use is limited primarily to binder applications in ceramic sintering processes where it benefits from a lower decomposition temperature compared to other binders [25]. More recently developed cobalt salem catalysts [26] and proprietary catalysts with improved activity and selectivity are able to prodnce tailored PPC polyols with specific molecular weights and hydroxyl group functionalities [27,28]. The polyurethanes synthesized from these polyols are reported to exhibit increased strength and durability caused by the polycarbonate backbone [29]. Besides C02-based PPC, other poly(alkylene carbonate)s are produced in pilot-scale processes [27]. 

Basic oxides of metals such as Co, Mn, Fe, and Cu catalyze the decomposition of chlorate by lowering the decomposition temperature. Consequendy, less fuel is needed and the reaction continues at a lower temperature. Cobalt metal, which forms the basic oxide in situ, lowers the decomposition of pure sodium chlorate from 478 to 280°C while serving as fuel (6,7). Composition of a cobalt-fueled system, compared with an iron-fueled system, is 90 wt % NaClO, 4 wt % Co, and 6 wt % glass fiber vs 86% NaClO, 4% Fe, 6% glass fiber, and 4% BaO. Initiation of the former is at 270°C, compared to 370°C for the iron-fueled candle. Cobalt hydroxide produces a more pronounced lowering of the decomposition temperature than the metal alone, although the water produced by decomposition of the hydroxide to form the oxide is thought to increase chlorine contaminate levels. Alkaline earths and transition-metal ferrates also have catalytic activity and improve chlorine retention (8). 

Determination of the thermal decomposition temperature by thermal gravimetric analysis (tga) defines the upper limits of processing. The tga for cellulose triacetate is shown in Figure 11. Comparing the melt temperature (289°C) from the dsc in Figure 10 to the onset of decomposition in Figure 11 defines the processing temperature window at which the material can successfully be melt extmded or blended. 

Metal chelates afford a better initiating system as compared to other redox systems since the reactions can be carried out at low temperatures, thus avoiding wastage reactions due to chain transfer. Homopolymer formation is also minimum in these systems. It was observed by Misra et al. [66,67] that the maximum percentage of grafting occurs at a temperature much below the decomposition temperature of the various metal chelates indicating that the chelate instead of undergoing spontaneous decomposition receives some assistance either from the solvent or monomer or from both for the facile decomposition at lower temperature. The solvent or monomer assisted decomposition can be described as ... 

When a solid is capable of decomposing by means of several discrete, sequential reactions, the magnitude of each step can be separately evaluated. Thermogravimetric analysis of compound decomposition can also be used to compare the stability of similar compounds. The higher the decomposition temperature of a given compound, the more positive would be the DG value and the greater would be its stability. 

New multinary hydride structures (Li-Mg-B-N-H) were successfully synthesized and characterized using LiBfVLiNLL and MgH2. These hydrides show enhanced reaction kinetics at lower temperatures due to the nanocrystalline behavior of MgH2 having large surface area. Moreover, the catalytic addition drastically reduces the thermal decomposition temperatures of Li-Mg-B-N-H when compared to undoped complex hydrides. 

Ed, the activation energy for thermal initiator decomposition, is in the range 120-150 kJ mol-1 for most of the commonly used initiators (Table 3-13). The Ep and Et values for most monomers are in the ranges 20-40 and 8-20 kJ mol-1, respectively (Tables 3-11 and 3-12). The overall activation energy Er for most polymerizations initiated by thermal initiator decomposition is about 80-90 kJ mol-1. This corresponds to a two- or threefold rate increase for a 10°C temperature increase. The situation is different for other modes of initiation. Thus redox initiation (e.g., Fe2+ with thiosulfate or cumene hydroperoxide) has been discussed as taking place at lower temperatures compared to the thermal polymerizations. One indication of the difference between the two different initiation modes is the differences in activation energies. Redox initiation will have an Ed value of only about 40-60 kJ mol-1, which is about 80 kJ mol-1 less than for the thermal initiator decomposition [Barb et al., 1951], This leads to an Er for redox polymerization of about 40 kJ mol-1, which is about one half the value for nonredox initiators. 

For a purely photochemical polymerization, the initiation step is temperature-independent (Ed = 0) since the energy for initiator decomposition is supplied by light quanta. The overall activation for photochemical polymerization is then only about 20 kJ mol-1. This low value of Er indicates the Rp for photochemical polymerizations will be relatively insensitive to temperature compared to other polymerizations. The effect of temperature on photochemical polymerizations is complicated, however, since most photochemical initiators can also decompose thermally. At higher temperatures the initiators may undergo appreciable thermal decomposition in addition to the photochemical decomposition. In such cases, one must take into account both the thermal and photochemical initiations. The initiation and overall activation energies for a purely thermal self-initiated polymerization are approximately the same as for initiation by the thermal decomposition of an initiator. For the thermal, self-initiated polymerization of styrene the activation energy for initiation is 121 kJ mol-1 and Er is 86 kJ mol-1 [Barr et al., 1978 Hui and Hamielec, 1972]. However, purely thermal polymerizations proceed at very slow rates because of the low probability of the initiation process due to the very low values f 1 (l4 IO6) of the frequency factor. 

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