Thermal degradation process

Good thermal stability is a requirement for surfactants used in processes to enhance oil recovery. This applies most particularly to steam foam applications where surfactants such as AOS may be exposed to temperatures far above 100°C albeit for short times. Many authors have approached the problem of the thermal stability of a surfactant through a determination of the activation energy of the thermal degradation process. Once the activation energy is known, it can be used to estimate the rate of thermal degradation under various conditions. 

The addition of heat shifts the equilibrium concentrations away from the products and back towards the reactants, the monomers. This is one reason why processing these types of polymers is often more difficult than processing products of chain growth mechanisms. The thermal degradation process can be dramatically accelerated by the presence of the low molecular weight condensation products such as water. Polyester, as an example, can depolymerize rapidly if processed in the presence of absorbed or entrained water. 

Figure 29 Thermal degradation process occurring in PC. Reprinted with permission from reference 69. Copyright 1999, American Chemical Society.

Interrogation of the data shown in Figures 26-28 enables one to construct probable thermal degradation processes, such as those shown in Figure 29. 

Condensation polymers such as polyesters or polyamides undergo more complex thermal degradation processes where the resulting pathway is a combination of different reactions including scission, elimination and cyclization [75]. 

II) Solid-phase reaction zone Nitrogen dioxide and aldehydes are produced in the thermal degradation process. This reaction process occurs endothermically in the solid phase and/or at the burning surface. The interface between the solid phase and the burning surface is composed of a solid/gas and/or soUd/Uquid/gas thin layer. The nitrogen dioxide fraction exothermically oxidizes the aldehydes at the interface layer. Thus, the overall reaction in the solid-phase reaction zone appears to be exothermic. The thickness of the solid-phase reaction zone is very small, and so the temperature is approximately equal to the burning surface temperature, T. ... 

The Isometric ion plots of Figures A and 5 indicate that evolution of benzene from the silicone-epoxy samples occurs in two distinct stages, with the low temperature peak attributable to residual solvent species. Above 200°C, thermal degradation processes involving scission of the Si-phenyl bond occur and account for the increased formation rate of benzene. The other high temperature volatile products are similar to those observed for the novolac epoxy samples, and are attributed to decomposition of the epoxy fraction of samples D and E. 

Table V. Activation Energy of the Thermal Degradation Process. ...

Richards and Slater (75) used a labelled polystyrene to demonstrate the existence of intermolecular chain transfer in the thermal degradation process. Polystyrene-14C, prepared in the normal way, was mixed with an inactive polystyrene specially prepared with weak links so that it degraded at temperatures where the polystyrene-14C was stable when on its own. Appearance of styrene- C monomer in the volatile degradation products proved the existence of intermolecular chain transfer (Reaction 9). 

Further developments of the work include a more accurate study of the mechanisms of desulfurization processes using instrumental improvements. This will enable an easy quantitation of gas yield and a thermochemical approach of elemental processes. We also have been using model polymers to better study the interactions of pyrite and sulfur with the organic matrix during coal pyrolysis, oxidation and combustion (34 and to examine more accurately the specific role of organic sulfur in thermal degradation processes. 

It might be assumed that, as condensed-phase flame retardants function by modifying the normal thermal degradation processes of polymers, they would also function as thermal stabilizers and that thermal antioxidant stabilizers would show flame-retardant properties. However, these statements are rarely the case, and to understand why, it is necessary to compare the mechanistic aspects of flame retardance as discussed earlier with those of thermal degradation and thermal oxidation as well, briefly alluded earlier, and in the case of the latter, the Bolland and Gee mechanism,17 in Scheme 2.1. 

A critical component to any mathematical model of pyrolysis, ignition, or even flame spread is the modeling of small-scale thermal degradation. Traditionally, thermal degradation processes in solids are considered to be analogous to chemical reactions in gases and liquids and are modeled in terms of sets of kinetic rate equations, typically of the form... 

The thermal degradation process can be followed by analysis of the colour generated in a static or dynamic environment. The use of capillary rheometry has also been highlighted (105). 

Thus, upon irradiation of an aqueous reineckate solution with visible radiation between 316 and 750 nm the release of one or more thiocyanate ions is observed with a quantum yield of O = 0.3. The photogenerated thiocyanate can easily be quantified spectrophotometrically at 450 nm as its Fe (SCN)3 complex. This actinometer is quite sensitive to visible light and covers a broad wavelength range. However, the quantum yield does show a wavelength dependence (27), which also varies with the temperature and the pH of the solution. Moreover, because the same thiocyanate release is also observed as a result of the thermal degradation processes (the rate constants depend on the pH of the solution), appropriate control... 

These heat effects may not be captured by use of a dark control sample, if the container is covered with aluminum foil, which blocks the "greenhouse" effect. In cases where significant heating is suspected to be contributing to thermal degradation processes, the container can be left open or vented, or one can use a reduced irradiance level to minimize this effect. Another approach would be to include the dark control in the same container after wrapping the control samples in aluminum foil prior to their placement in the same bottle as the unwrapped test samples. In this case, the environment that the dark controls experience would be much closer to the environment the test samples experience, save for any surface heating caused by the sample color. 

Pyrolysis for 1800 and 3600 s at 350 °C resulted in less oil yield and almost the same amount of wood charcoal. At 450 °C, pyrolysis for 200 s produced more oil than at 80 s, but little difference in char yield. This is probably due to microstnjctural changes (small droplets) in charcoal occurring as the result of the thermal degradation process [5]. 

Reactions 1 and 2 describe reversible photochromism, and 3 and 5 irreversible photochemical degradation processes having rate constants of Pad and pDB - The thermal degradation processes are reactions 4 and 6, with the latter having a negligibly small rate constant as SPs do not degrade appreciably in the absence of UV light. 

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