Thermal degradation modeling

Another problem occurs when some fire retardant formulations ate exposed to elevated temperatures (eg, when used as roof tmsses or as roof sheathing) thermal-induced strength reductions can occur in-service. The thermo-chemical factors were discussed by LeVan and Winandy (26), and a kinetic degrade model was developed (27). The treater should be consulted to obtain appropriate in-service modifications for specific fire retardant treatments. 

Studies of thermal degradation on low-molecular weight model compounds have shown that the structure [Eq. (7)1... 

Because of its fundamental role as a precursor of vitamin A and the availability of P-carotene standard in crystalline form, the thermal degradation of P-carotene in model systems has been a subject of intense research. 

Rios, A.O., Borsarelli, C.D., and Mercadante, A.Z., Thermal degradation kinetics of bixin in an aqneons model system, J. Agric. Food Chem., 53, 2307, 2005. 

The thermal degradation of anthocyanins, both in extracts and model systems, was reported to follow first-order reaction kinetics in all studies. The stability of anthocyanins and all pigments found in foods decreased with increases in temperature. 

Kanasawud and Crouzet have studied the mechanism for formation of volatile compounds by thermal degradation of p-carotene and lycopene in aqueous medium (Kanasawud and Crouzet 1990a,b). Such a model system is considered by the authors to be representative of the conditions found during the treatment of vegetable products. In the case of lycopene, two of the compounds identified, 2-methyl-2-hepten-6-one and citral, have already been found in the volatile fraction of tomato and tomato products. New compounds have been identified 5-hexen-2-one, hexane-2,5-dione, and 6-methyl-3,5-heptadien-2-one, possibly formed from transient pseudoionone and geranyl acetate. According to the kinetics of their formation, the authors concluded that most of these products are formed mainly from all-(E) -lycopene and not (Z)-isomers of lycopene, which are also found as minor products in the reaction mixture. 

Observed Rate Constant (Arobs) and Activation Energy ( a) Values Found for Carotenoid Thermal Degradation in Model Systems... 

Zepka, L. Q., C. D. Borsarelli, M. A. A. R da Silva et al. 2009. Thermal degradation kinetics of carotenoids in a cashew apple juice model and its impact on the system color. J. Agric. Food Chem. in press, doi 10. 1021/jf900558a. 

Photochemistry of Model Compounds. Preliminary photochemical studies have been carried out on l,3-diphenoxy-2-propanol (3)8 as a model compound for bisphenol A-epichloro-hydrin condensates 1. The utilization of 3 as a model compound for thermal degradation of 1 has been reported. Irradiation (254 nm) of 3 in acetonitrile (N2 purge) provides two major volatile products, which have been identified as phenol and phenoxyacetone (4), by comparison of retention times (gas chromatography) with known samples. A possible mechanism for... 

Using l,8-diphenyloctatetra-l,3,5,7-ene, (DOT), as a model compound either in dilute, ( 10-5m), hexane or ethanol solutions or incorporated into a film of undegraded PVC confirmed that in the presence of HC1 it underwent a photochemical reaction which resembled that of the polyenes in thermally degraded PVC. The results indicated that the initial rates of reactions proceeding in either solvent showed a second order dependence on HC1 pressure and that the reaction was considerably slower in ethanol than in hexane. Further, when cast in PVC films, the characteristic absorption maxima of DOT were shifted about 16nm to longer wavelengths compared with their absorption in hexane and there... 

A model for the SSP of PET under typical industrial processing conditions has been developed by Ravindrath and Mashelkar [15]. Their calculations are also based on experimental data reported in the literature. The results allow the rough conclusion that the reaction rate decreases by a factor of 6 for the temperature range between 285 and 220 °C, accompanied by a decrease of the thermal degradation by a factor of 40. The fact that suitable SSP conditions can be found to warrant a fast reaction rate and minimal degradation makes this process industrially important. These same authors also state that at an early stage of the reaction the kinetics have a predominant influence, whereas diffusivity plays a major part at a later stage of the reaction. 

Kinetic data on the thermal degradation of ABS and PC/ABS blends are available (130,131). Thermogravimetric analysis suggests that the kinetics of the thermal degradation can be modelled by an auto-catalytic process. 

HDF has been identified as reaction product of a thermal treatment of sugars, especially the 6-desoxy sugar rhamnose [90], Analysis of the free sugars in the LMW-fraction revealed fructose-1,6-biphosphate (FBP) as the predominating carbohydrate in the LMW fraction (5.75 g/kg yeast), but no rhamnose was present [88]. To elucidate the contribution of FBP as precursors of HDF in the yeast fraction, the sugar phosphate was thermally degraded under the same conditions as used for the LMW fraction of yeast. The results revealed FBP, as effective precursor of HDF in aqueous model systems at lower reaction temperatures (100°C Table 19). It should be stressed that additions of the amino acids proline or alanine did not increase the concentrations of HDF from the carbohydrates listed in Table 19 (unpublished results). The data implied that FBP which was the predominant carbohydrate in yeast, is the... 

For the polyurethane composition considered here the maximum (adiabatic) temperature jump is equal to 25 K. This means that at ordinary working temperatures of the mold, i.e., 40 - 80°C, the maximum increase in the material temperature does not reach an unacceptable level of close to 200°C, where thermal degradation of the polymer can begin. We can restrict the maximum temperature growth Tmax by using the mathematical model based on Eqs. (4.10) - (4.13) and then constructing Tmax - vs - to curves for various mold temperatures. These provides a means of choosing the optimum process parameters when the temperature must not exceed Tmax. 

The proposed model was formerly used to describe oxidation kinetics (Bolland, 1946), but can be extended to a general thermal degradation... 

Thermal degradation experiments in neutral atmosphere to determine if polymer thermolysis has (or has not) to be taken into account in the model. 

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