Process thermooxidation initiation

Figure 7.10 The sequence of a stage of initiation of process thermooxidation...

If the carboxylic acids on the cellulosic chain are not the major cause of the thermooxidative decay of old cellulosic textiles, one must consider the carbonyl species, particularly the aldehydes on the C2 and Q) carbons. Nikitin (14) noted that the primary autoxidation process is a reaction of molecular oxygen with aldehyde groups, which initiates a chain reaction resulting in more profound changes and decomposition of the molecule . Thus, reduction of the aldehyde groups should lead to improved stability of degraded cellulose. 

Production and use of PVC occur in the presence of air, i. e. in the presence of oxygen. Therefore, it is surprising that the mechanistic details of thermooxidative degradation of PVC are still not fiilly revealed. The major reactions of this process are shown in Scheme 1. As indicated in this Scheme, thermal dehydrochlorination yields HCl and simultaneously sequences of conjugated double bonds (polyenes) in the chain. The reactive polyenes lead to peroxides in a reaction with oxygen followed by the formation of radicals. Subsequent chain reactions result in additional initiation of HCl loss and further oxidative processes (/, 8). 

Labile structures initiating polymer decay [215] are formed during the process of thermooxidation in the air. Thermooxidation rate is defined by the rate of oxygen diffusion into polymer. Constant of destruction rate in the air compared with inert medium increases, and activation energy decreases [216]. However, in some cases active energy increases this is connected with the contribution of physical phenomena of heat and mass transition together with chemical processes into the total kinetics of destruction. 

As described previously, thermooxidative degradation of polyolefins proceeds by a typical free-radical chain mechanism in which hydroperoxides are key intermediates because of their thermally-induced hemolytic decomposition to free radicals, which in turn initiate new oxidation chains. However, since the monomolecular hemolytic decomposition of hydroperoxides into free radicals require relatively high activation energies, this process becomes effective only at temperatures in the range of 120°C and higher. 

Initially, a thermooxidative process takes place, due to the high oxygen content of phenolic resins. Conley and coworkers proposed that the first step, Eq. (24), is the formation of a hydroperoxide (36) which later fragments into phenols containing aldehyde and carboxyl groups.This is followed by a decarboxylation reaction which starts around 300°C and, finally, by a decarbonylation reaction which begins around 500°C. In addition, fragmentation reactions may occur above about 400°C, Scheme 1. 

Some peculiarities of the thermal and thermooxidative decomposition of polyvinyl chloride depend on the conditions of its production. Thus, it is known that samples of the polymer produced by initiating the polymerization of vinyl chloride with ultraviolet irradiation possess higher stability in comparison with samples produced in polymerization under the action of chemical agents [26, 27]. Reversibility of the process of dehydrochlorination in the decomposition of samples of polyvinyl chloride produced by the latex method is noted, while in the process of decomposition of suspension polymer, the phenomenon of reversibilily is not observed [21]. It has been shown that the rate of dehydrochlorination of the latex polymer is significantly higher than that of the suspension polymer under the same conditions [21]. It has been established that the polymerization of vinyl chloride in the presence of oxygen leads to the formation of unstable peroxide groups, which can initiate decomposition of the polymer [28, 29]. It is noted that an extremely substantial influence on the stability of polyvinyl chloride is exerted by the purity of the monomer, as well as the presence of impurities of metals of variable valence [28]. 

After irradiation, the polymer becomes less thermally stable (Figs. 90 and 91). This aftereffect phenomenon can be explained by the formation of free radicals capable of initiating thermal and thermooxidative decomposition during the process of irradiation. Table 12 presents the values of the rate of dehydrochlorination of polyvinyl chloride in the case... 

In a study of the thermooxidative destruction of epoxide resins (hardened with maleic anhydride and polyethylenepolyamine) [12], it was shown that the oxidation process is characterized by the presence of induction periods (Fig. 144), which decreased regularly with increases in the initial oxygen pressure (P) in the system and with increasing temperature (Fig. 145), in accord with the formula... 

Important initiators and accelerators catalysts used in prepolymer synthesis, catalysts used in the curing process, heavy metals, peroxides In polyol, products of reaction of amine catalysts and polyols, nitrous oxide, acids and bases (hydrolysis), traces of solvents of types capable of producing hydroperoxides, products of thermooxidative degradation ... 

At the highest temperatures, thermal and/or oxidative reactions become predominant. The classic experiments by Busse [59, 60] (see Section III.B Volume 2, Sections VI.A. 1 and VIIA.B) on natural rubber show clearly the smooth shift from the mechanical to a thermooxidative process. Similar results have been obtained on mastication of other rubbers as from ethylene-propylene-diene monomer (EPDM) [42, 61, 41] and cw-polyisoprene [4]. The contribution of mechanical energy to initiation of oxidation has been estimated as 30% of the total for the decrease in molecular weight of cis-polyisoprene masticated at 140°C [4]. 

The polyphenylquinoxalines (PPQs) are similar to the PQs but offer better solubility, processability, and thermooxidative stability. These polymers are prepared from the reaction of aromatic few(o-diamines) and bis(pheny -a-diketones) as first reported in 1967. Since their initial disclosure, extensive work has been reported on the chemistry, mechanical, and physical properties of PPQs. A representative synthesis shown in Eq. (3) involves the reaction of 3,3, 4,4 -tetraaminobiphenyl and 4,4 -oxy w(benzil) ... 

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