Thermal degradation dehydration

The use of aliphatic monomers for hyperbranched polyesters has been debated because aliphatic monomers are said to be prone to thermal degradation reactions such as decarboxylation, cyclization, or dehydration [77]. The only commercial hyperbranched polymer is a hydroxy-functional aliphatic polyester, Boltorn, available from Perstorp AB, Sweden. 

Smdies of the thermal degradation of several aromatic acids have been reported. Phthalic acid (80), but not isophthalic acid (81) or terephthalic acid (82), decomposes via dehydration to its anhydride at 140-160 °C. However, (81) and (82) and benzoic acid are thermally stable below 200 °C. Dissociation constants of all 19 isomers of methyl-substimted benzoic acids (83) have been measured in methanol and DMSO. From the pA a values, the substiment effects of the methyl groups were calculated and tentatively divided into polar and steric effects. Also, in the case... 

The most efficient method for the clean hydrolysis of sucrose is by the use of invertase, leading to an equimolar mixture of glucose and fructose (invert sugar). The presence of salts increases the rate of thermal degradation of sucrose.337 The reaction is also possible in the presence of such heterogeneous acidic catalysts as zeolites.338 The hydrolysis of the glycosidic bond is the first step of a number of subsequent reactions that can occur on the glucose and fructose residues, such as dehydrations, combinations with amino acids (Maillard reaction), and many other chemicals or fermentation processes.339... 

In the traditional "wood distillation industry" hardwood was preferred for production of chemicals. Hardwood distillation was formerly an important source for production of acetic acid, methanol, and acetone which were the primary products of this process. The heat required for pyrolysis was generated by burning gas, oil, or coal. In the thermal degradation of wood the volatile components are distillable and can be recovered as liquids after condensation (Fig. 10-2). The solid residue, charcoal, is mainly composed of carbon. At higher temperatures the carbon content is increased because of a more complete dehydration and removal of volatile degradation products. Charcoal is mainly used as combustible material for special purposes. A number of charcoal products are known, including activated carbon for adsorption purposes. 

Fire retardancy of wood involves a complex series of simultaneous chemical reactions, the products of which take part in subsequent reactions. Most FRs used for wood increase the dehydration reactions that occur during thermal degradation so that more char and fewer combustible volatiles are produced. The mechanism by which this happens depends on the particular FR and the thermal-physical environment. The effectiveness of a FR treatment depends upon the performance rating of the treated material when tested in accordance with ASTM E84 (no greater flame spread than 25). 

In the presence of acid additives thermal degradation of cellulose is intensified at lower temperatures, due to the occunence of the dehydration reactions [3], Condensation reactions result in the decrease of volatile products at 450"C combined... 

Wood burns because the cell wall polymers undergo hydrolysis, oxidation, dehydration, and pyrolysis reactions with increasing temperature to give off volatile, flammable gases. The lignin component contributes more to char formation than do the cellulose components, and the charred layer helps insulate the wood from further thermal degradation see Chapter 13). 

The products of caramelization are distributed between volatile and nonvolatile fractions. The composition of the volatile firaction is pretty well characterized, contrary to that of the nonvolatile fraction. Thus, neither is the structure of all compounds formed precisely known, nor are the processes which occur understood in detail (see, for instance, a review by Orsi ). The composition of the volatile fraction from the thermolysis of sucrose is the best recognized. The profound decomposition products from the decomposition in vacuo of sucrose arc water, carbon monoxide, carbon dioxide, formaldehyde, acetaldehyde, methanol, and ethanol. The detailed rates and temperature relationships suggest that, with the possible exception of ethanol, the other products result from secondary reactions of dehydration products. The low-molecular-weight portion of the nonvolatile fraction of the thermal degradation of sucrose contains D-fhictose, D-glucose,... 

The catalysts can also be bonded onto each face of the membrane under pressure and at a temperature (22) usually between the glass transition temperature and the thermal degradation temperature of the membrane (17,23,24). At such temperatures the membrane softens and can flow under pressure, such that the adhesion force of the membrane is at a maximum, and an intimate contact between the catalyst and the membrane can be achieved (17). The heating process is rather short, so that the membrane is not over-dehydrated. A dehydrated membrane gives poor bonding (17). 

Pyrolysis is always accompanied by the evolution of volatile, decomposition products. The proportions and chemical complexity of these depend on the severity of the experimental conditions. However, under diminished pr ure, even at temperatures below 100°, water is liberated, and a major problem in this area of starch chemistry is to distinguish between the processes of dehydration and decomposition. At higher temperatures, the problem of thermal degradation may be complicated by the occurrence of intramolecular rearrangements and second-order interactions. It is also becoming increasingly apparent that the course of pyrolysis is markedly altered by the presence of small proportions of inorganic materials. 

For magnesium stearate, the first mass loss step (4.2%) is associated with dehydration and the second one (88.6%) with thermal degradation of the organic moiety. In the DSC curve, the first and second observed endothermic peaks are associated with the dehydration and thermal degradation processes, respectively. 

The reaction products of the Maillard reaction, such as l-amino-l-deoxy-2-ketose (Amadori product) or 2-amino-2-deoxyaldose (Heyns product), do not contribute to flavor directly but they are important precursors of flavor compounds [48]. These thermally unstable compounds undergo dehydration and deamination reactions to give numerous rearrangement and degradation products. The thermal degradation of such intermediates is responsible for the formation of volatile compounds that impart the characteristic burnt odor and flavor to various food products. For example, at temperatures above 100 C, enolization products (such as l-amino-2,3-enediol and 3-deoxyosone) yield, upon further dehydration, furfural from a pentose and 5-hydroxy methylfurfural and 5-meth-ylfurfural from a hexose [2]. 

Thermogravimetrical analysis (TGA) This technique is widely employed and it measures the amount and rate of change in the weight of a material as a function of temperature under a controlled atmosphere. The measurements are used primarily to evaluate the thermal stability. The technique can characterize materials that exhibit weight loss or gain dne to decomposition, oxidation, or dehydration. As an example, Zhang et al." evaluated the step thermal degradation and the thermal reliability of a silica/n-octadecane MPCM. 

FIGURE 45.10 Droplet temperature T and thermal degradation Aj. in maltose-water system vs. reduced time t/R for two air temperatures (1) to 100°C, and (2) to 80°C. (From Kerkhof, P.J.A.M. and Schoeber, W.J.A.H., Theoretical modeling of the drying behavior of droplets in spray dryers, in Advances in Preconcentration and Dehydration of Foods, A. Spicer (Ed.), Elsevier Applied Science, London, U.K., pp. 349-397, 1974. With permission.)... 

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