Thermooxidative destruction

It occurs during the chain cracking and radiolysis of hydrocarbons [38], radical polymerization and oligomerization of monomers [39], thermal and thermooxidative destruction of polymers (see Chapter 19) and hydrocarbon oxidation at low dioxygen pressure. 

Macromolecule of PA consists of methylene chains coimected by amide groups, which cause vulnerability of these polymers at thermooxidative destruction. 

Pakhomov and his assistants [36] note, that loss of strength is defined by kinetic destruction of molecular chains in amorphous regions of PA. Low values of activation energy of thermooxidative destruction may be explained by PCA water receptivity and hydrolysis of amide bonds at the time of warming up. So, for dry PCA-E =43 Kcal/mole, and in the presence of moisture - E = (20-30) Kcal/molc. Last conclusions agree with the results of investigation [37] quite of satisfactory. 

Thus, analysis of literary data shows that up to now there is not clear and full idea about the mechanism of PA thermooxidative destruction. And, as a result, there is much indefinite in the question of its stabilization and modification, the more at introducing additives directly into polymer melt [41]. Besides, it should be taken into consideration that majority of these additives possess low thermal stability, and this eliminates their use at the synthesis and production of fibers. 

Tinuvine 770-bis (2, 2, 6, 6 - tetramenthylpiperidile - 4) sebacate, proposed by the firm CIBA [71], and some polymer addititives, including fragments of piperidine cycle are used in industry. As a rule, stabilizers, containing stable radical in their structure, are also inhibitors of thermooxidative destruction. 

Similar effect is probably connected with structural changes in PCA, taking place at addition of compounds XLY-XLYI into polymer and their possible inhibiting effect on PCA thermooxidative destruction. 

Stabilizing activity of HC is observed at thermooxidative destruction (Figurel.2O). Moreover, in this case the following sequence of thermostabilizing activity is characteristic ... 

Curves of mass conservation at temperature rise (Figure 1.24) show that initial PCA (curve 4) is more stable in inert atmosphere, than in the air (curve 2). At the same time quite another picture is observed for dyed samples. Here, the greatest effect of PCA protection is observed at warming up in the air. So, initial PCA loses 60% of original mass at the temperature of 400 C, while the dyed one - 18%. And it should be noted that PCA, dyed by covalently linked dye, possesses higher resistance to thermooxidative destruction, than PCA, dyed by the same dye without forming covalent bond. 

Investigation of the kinetic of mass loss at thermooxidative destruction of modified CDA showed that modification increases its stability to the heating at elevated temperatures (Figure 2.33). Loss of mass of modified CDA in much less in comparison with initial values during prolonged heating of polymer samples in the air (150°-200°C). These conclusions are proved by the results of complex thermogravimetric analysis (TGA). 

From Figure 2.37 it is seen that introduction of dyes increases thermal stability of PVA in the air, moreover it depends not only on the character of the found dye-polymer, but on chemical structure of the dye itself. So, the highest effect appears in phthalocyanine dyes, then follow azodyes and antrachinone dyes have the least effect. At the same time general tendency to improve stability to thermooxidative destruction for samples, containing covalently linked dyes, is displayed here. 

During thermooxidative destruction of PETP-fibre at 280 " C [211] there were found the same products of decomposition as during thermal destruction (in atmosphere of helium) but in much larger quantities. 

A great number of investigations are devoted to the study of thermooxidative destruction by the method of differential thermal analysis (DTA) [212, 213]. During DTA curves analysis it has been found out that resistance to PETP thermooxidation decreases with the increase of its molecular mass [212]. 

With the help of DTA method effect of interesterification and polycondensation catalysts on PETP stability at thermooxidation [214] has been studied. Thermooxidative destruction is accelerated by the presence of metal catalysts. 

So, all works on investigation of the mechanism of thermal and thermooxidative destruction suppose that these processes have radical nature and run by the way of formation and decay of peroxide radicals and hydroperoxides. Together with oxidative decay of aliphatic part, as a result of which polymer chain decomposes with volatile products liberation and formation of new end groups, there are also changes in aromatic part-jointing of polymer takes place. 

Oligomer products of diamine condensation with carbonyl compounds, using excess of one of bifunctional compounds have been used lately. These products are used as stabilizers of thermooxidative destruction for different polymers, moreover oligomer stabilizers are more interesting because they are washed out from the polymer at photofading and sweated out, while in use least of all [294]. 

Thermo- and Thermooxidative Destruction of Modified PETP - Fibre... 

Thermooxidative destruction of PETP is accompanied by libration of volatile products, so there is weight loss in polymer sample being tested. That is why, that temperature, at which decrease of polymer weight is observed, characterized its thermooxidative stability. 

From the data of Table 3 it is seen that introduction of hexaazocyclanes into PETP increases temperature of the beginning of PETP decay by 15-70 C, HC-3 and HC-4 being the exception. Such effect is probably connected with structural changes in PETP, taking place at addition of HC-1, HC-2, HC-5, HC-6, HC-7 compounds into polymer, and possible inhibiting effect on thermooxidative destruction of PETP. 

During addition of HC-3 and HC-4 such effect is not observed, probably these additives have inhibiting effect on thermal and thermooxidative destruction, as these additives do not have developed chain of conjugation. Taking into account the fact that polymer materials must work in narrow temperature ranges for a long time, we have studied also kinetics of thermal destruction at isothermal heating. 

Introduction of HC-1 into polymer increases temperature of polymer decay at isothermal heating both in the presence of oxygen and without it. Additive HC-2 increases temperature of PETP decay still more (almost by 30°C higher, then HC-1). Hence, HC-2 protect PETP -fibre from thermal and thermooxidative destruction more effectively. 

Thus, all used additives may be devided into two groups 1) increasing polymer resistance to thermal and thermooxidative destruction 2) decreasing polymer thermal stability. Moreover, effect of additive on thermo- and thermooxidative stability will depend on the length of conjugation chain in modifier s molecule. 

E.M. Plaree, B.L. Bulkin, Mo Yeen Furic, PK - spectroscopy to study destruction of polumers at thermal and thermooxidative destruction of PETP/ZPo/v/w. charact. spectrosc., chromatDgr. and. Phys. Instrum Meth. Symg. Washington, PC (1983), 571-593. 

Schaaf Eckehart, Zimmermann Heinz, Thermogravimetric investigation of thermal and thermooxidative destruction of WTVHFaserfosch and Textiltechn (1974), 25, No 10,434 - 440 (in Russian). 

Birladeanu Constantine, Vasile Cornelia On kinetics of thermal and thermooxidative destruction of PETP//4cZ PoZjto. sin. (1988), No 5, 331-336 (in Russian). 

During heating to 150-220° C polyolefins isolate thermooxidative products of macromolecules with pronounced toxic properties, namely organic acids, ethers, unsaturated hydrocarbons, peroxide and carbonyl compounds (formaldehyde and acetaldehyde), carbon oxide and dioxide, etc. A mixture of such products of the thermooxidative destruction of polyolefins may result in acute chronic poisoning on inhalation. 

As a result of thermooxidative destruction, polyethylene terephthalate liberates the diethyl ether of terephthalic acid, acetaldehyde, methanol and carbon oxide. As investigations have shown, manufacture of PETP films or fibers has brought no change to the state of health of maintenance workers. Moreover, PETP films are used to preserve sterilized foodstuffs and warm up prepared dishes. 

Fluorine containing polyarylketone was synthesized on the basis of 2,3,4,5,6-pentaflourbenzoylbisphenyl esters in Ref [370]. The polymers possesses good mechanical and dielectric properties, has impact strength, solubility and tolerance for thermooxidative destruction. 

The Mechanism of Inhibition Thermooxidation Destruction of PBT by Polymer Azomethines... 

It has been mentioned before that [1] there are potential reactivity centrals at synthesized polyazomethines (PAM). Each of them can take part in radical processes, which are a part of summary thermooxidation destruction of polybutilenterephthalate (PBT), which acts mainly according to chain-radical mechanism. 

Consequently, PAM with triarylmethine fragments in basic chain can inhibit thermooxidation destruction almost on all stage of oxidation of PBT or on account of interaction with radicals, giving molecular or stable products. 

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