Pyrolysis volatile decomposition products

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. 

The extent to which starch and cellulose form volatile decomposition products in the absence of inorganic material may be seen from the results shown in Table IV for the percentage weight of polysaccharide remaining after pyrolysis under vacuum. Although differences between starch and its components are small, cellulose is relatively more stable. The general order of thermal stability appears to be amylose < starch < amylo-pectin < cellulose. 

A more detailed analysis of the composition of the volatile portion by the method of mass spectrometry showed that the products liberated from polyvinyl chloride at 400°C under vacuum in 30 min contain not only hydrogen chloride, but also 26 different aliphatic and aromatic compounds saturated and unsaturated hydrocarbons, diehloroethane, allqrl-and alkylenebenzenes are detected ethylene, propylene, ethane, pentane, hexane, benzene, and toluene predominate quantitatively [30]. It has been found by the methods of chromatography and IR- and UV-spectrom-etry that when the pyrolysis temperature is raised to 450-500°C, substances with three to five condensed aromatic nuclei appear among the volatile decomposition products of polyvinyl chloride [31]. 

The pyrolysis mechanism of PLA is more complex than the simple reaction that gives lactide. Significant amounts of other volatile decomposition products are also generated during the pyrolysis [30-32]. The thermal degradation mechanisms of PLA have been reported by several groups (Figure 23.2) [7-10, 31-37]. 

Lehrle et al. [597] have reviewed the study of polymer pyrolysis by PyGC with special reference to the objective of obtaining results with quantitative significance. Since polymer decomposition is usually quite sensitive to relatively minor changes in pyrolysis conditions, quantitative analysis imposes more stringent control requirements than are necessary in the purely qualitative approach. Also Berezkin [503] has paid attention to various aspects of quantitative analysis by means of PyGC and has pointed out that it is difficult to predict the quantitative composition of the volatile decomposition products formed in pyrolysis on the basis of sample structure and pyrolysis conditions. By quantitative modelling the detailed pyrolysis mechanism... 

Pyrolysis kinetics have been carried out on poly-L-lactone salts using TGA linked to various methods such as NMR spectroscopy, gas chromatography and pyrolysis-gas chromatography-mass spectrometry to identify volatile decomposition products. The effect of the end structures on pyrolysis kinetics was examined and the mechanisms of pyrolytic degradation for both polymers identified. 

An example of the effect of source temperature is seen for TiF40xH 110), for which, at 180°C, the highest m/e corresponds to TiFsOX (i.e., P—HF), whereas at 240°C the thermal decomposition product, TiF20X2, is observed. Compound Cu(NOs)2 shows a parent ion 111, 112) [unlike Sn(NOs)4 (79)], but thermal decomposition occurs even at source temperatures of 100°C resulting in much of the N02 and NO observed. As samples are volatilized from the probe at temperatures of up to 350°C, serious thermal decomposition or polymerization may result 8,113-116). Even with the source at a low temperature, there is still the very hot region in the vicinity of the filament that can cause pyrolysis. 

Figure 9.2 Illustrating the crucial need for cross-linking the preceramic polymer at an early stage in the pyrolysis to prevent loss of material in the form of volatile decomposition or depolymerization products. 

Polysaccharide pyrolysis at 375-520°C is accompanied by a higher rate of weight loss and evolution of a complex mixture of vapor-phase compounds preponderantly of HsO, CO, C02, levoglucosan, furans, lactones, and phenols (Shafizadeh, 1968). The volatile and involatile phase compositions are conditional on the rate of removal of the vapor phase from the heated chamber (Irwin, 1979), inasmuch as the primary decomposition products are themselves secondary reactants. The reaction kinetics is described as pseudo zero order (Tang and Neill, 1964) and zero order initially, followed by pseudo first order and first order (Lipska and Parker, 1966), suggesting an... 

Gas theories. — These attribute the retardant action to modification of the behavior of the volatiles (from the pyrolysis) by gases evolved from the decomposition of the retardant. Two suggested modes of action are (a) prevention of the formation of inflammable mixtures of air and volatile compounds (derived from the cellulosic material), by dilution with noninflammable gases derived from decomposition of the retardant, and (b) inhibition of free-radical chain-reactions in the flame, by introduction of decomposition products (from the retardant) that act as chain breakers. 

Hydrogen sulfide is the main decomposition product seen from the pyrolysis of poly(thiophene-2,5-diyl). Some 2,2 -bithiophene (17.4% of pyrolysate) and only a small proportion of thiophene are generated (less than 5% of pyrolysate). However, the pyrolysis in He also forms char, which is not volatile and cannot be seen in the pyrogram. The bonds that appear to cleave more easily are the S-C bonds and the bonds between the thiophene units (C-C type). Since the hydrogen content of the polymer is low, the formation of SH2 is associated with the formation of char. The elimination of some carbon and sulfur as CS2 or S explains the formation of benzene, thiophene, etc. 

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