Thermal Oxidation of Polyamides

The heating of polyamides in the presence of oxygen or air leads to considerable changes in their chemical composition and physicochemical properties. This is accompanied by a loss of valuable physico-mechanical properties in the polyamide materials decrease in the breaking strength and breaking elongation for fibers and films, intensification of brittleness. 

The curves cited in Fig. 110 show [26] that polycaproamide fiber loses 0, 14.5, 

When various polyamides [9, 29] (polycaproamide, polyhexamethyl-eneadipamide, polyenanthoamide, and the mixed polyamide obtained by polycondensation of hexamethyleneadipamide and 8-caprolactam, taken in a 40 60 ratio) are heated in the presence of oxygen, gels that swell 

The maximum amounts of water-and chloroform-soluble products are formed after 20 hr of heating, i.e., at the time when the structuring processes are substantially accelerated. 

The chemical structure of the polyamide does not influence the general direction of thermooxidation. Great influence is exerted by the degree of orientation. Thus, for example, in weakly oriented capron film, structuring processes occur at an appreciable rate at only 160°C, while a nonswelling polymer with a dense network is formed at 170°C [29]. 

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The authors propose a mechanism for the thermal oxidation of polyamides, which gives a good explanation for the formation of all these products. Initiation of the chain process of oxidation occurs by stripping of the most labile hydrogen atom by a molecule of oxygen. 

Then a peroxide radical and hydroperoxides are formed at the site of stripping of the hydrogen atom. Water is obtained when the hydroperoxides decompose (see p. 16). In the thermal oxidation of polyamides, the liberation of water may lead to hydrolysis of the polymer and an increase in the number of terminal carboxyl groups. Decarboxylation gives carbon dioxide. In addition to decomposition of hydroperoxides, decomposition of peroxide radicals may also occur ... 

Light-stable polyamides are obtained by adding an aqueous suspension of carbon black containing water-soluble chromium salts, for example, CrF3, before polycondensation [78]. In addition to increasing the photostability of polyamides, chromium also increases their stability to thermal oxidation [68, 41]. The spectra of polyamide samples treated with chromium, cited in Fig. 139, practically do not differ from the spectra of the imtreated samples. The action of chromium apparently reduces to a strengthening of the amide bond, which is important both in photoaging and in thermal oxidation of polyamides [68]. 

Protection from photoaging can be accomplished by the introduction of antioxidants that suppress the processes of photosensitized oxidation. For example, effective stabilizers of polyamide film are the 2,6-di-tert-butyl-4-methylphenyl ester of pyrocatecholphosphorous acid, 2,6-di-tert-butylhydroquinone, and a mixture of potassium iodide with copper naphthenate. All these additives exhibit considerable protective action in the thermal oxidation of polyamides as weU [92]. 

A relevant comparison monitored by chemiluminescence between the progresses in thermal oxidation of polyamide 6.6 and two polyolefins (polyethylene and polypropylene) was reported based on the mechanism of degradation [04M1]. The difference between the stability of these three polymers is characterized by the differences in the induction times of oxidation (Fig. 55). The low bond energies and the presence of high amount of substituted carbon atoms bring about an increasing instability. 

Figure 82 presents the kinetic curves of gas evolution in the thermal oxidation of polsrformaldehyde with additions of radical inhibitors (cone. 1%) in conjunction with polyamide 54 (cone. 1.5%). We can see from the figure that the oxidation of polyformaldehyde in the presence of radical inhibitors is characterized by the presence of considerable induction periods. Radical II is the most effective of the investigated stable radicals. 

Investigations of thermal and thermal-oxidative treated polyamides determined the influence of their chemical structure PA 66 crosslinks faster than PA 6 - with the same number of amide groups - under both inert and oxidative atmosphere. 

Differential scanning calorimetry (DSC) and TGA have been used to evaluate the oxidative and thermal stability of polyamide 6,6 [24, 25]. Eriksson and co-workers [24] evaluated thermal stability in an oxidative environment by isothermal and DSC. They found a decrease in the oxidative stability of the polymer as a result of repeated injection moulding. 

This anhydrous zinc borate is recommended for use in engineering plastics processed at temperatures higher than 300°C which is the upper limit of Firebrake ZB processing temperature. It is reported to be an effective smoke suppressant in chlorofluoropolymers for plenum cable applications.66 Recently it was claimed that this anhydrous zinc borate can replace antimony oxide (or sodium antimonate) completely in high temperature polyamide applications. Like its hydrated analog, this anhydrous zinc borate can also improve CTI, thermal stability, and the color stability of polyamide containing halogen sources.67 68... 

The processing of polymers should occur with dry materials and with control of the atmosphere so that oxidative reactions may be either avoided, to maintain the polymer s molar mass, or exploited to maximize scission events (in order to raise the melt-flow index). The previous sections have considered the oxidative degradation of polymers and its control in some detail. What has not been considered are reactions during processing that do not involve oxidation but may lead to scission of the polymer chain. Examples include the thermal scission of aliphatic esters by an intramolecular abstraction (Scheme 1.51) (Billingham et al., 1987) and acid- or base- catalysed hydrolysis of polymers such as polyesters and polyamides (Scheirs, 2000). If a polymer is not dry, the evolution of steam at the processing temperature can lead to physical defects such as voids. However, there can also be chemical changes such as hydrolysis that can occur under these conditions. 

In practice, the choice of WI is based on the analysis of service, design, economic and other factors. An efficient means of improving wear resistance of the polyamide-steel 45 friction pair turns out to be inhibition of the thermally oxidative and destructive processes in the polymer surface layers to avoid the formation of corrosion-active oxide compounds. It is possible to break this unfavorable cycle by the introduction of antioxidants into the polymer composition, thus disabling macroradicals through the reactions of mechanochemical synthesis, polyamide alloying by functional additives forming separating layers or more thermally stable products, and so on [108]. Application of WI with this aim abates undesirable thermally oxidative processes in polyamide... 

The Tt, Td and of the materials based on CFD increase with the introduction of Cl except for VNHL-20, which shows a reduced CFD Tt, and Td, evidently due to its own low thermal resistance. The inhibitors are likely to assist in binding of formaldehyde formed during CFD destruction and lead to decomposition of the copolymer macromolecular chain. In compositions on a polyamide base. Cl inhibit thermally oxidative processes that accelerate thermal decomposition of the polymer, elevating Tt and Td by 5-10°C. 

Thermal degradation of polyoxymethylene and polyacetals results in the formation of formaldehyde, which is furthermore catalyzed by oxidation of formaldehyde to formic acid. In addition to antioxidants and acid scavengers, formaldehyde scavengers are used in polyacetal formulations. Formaldehyde scavengers of choice are nitrogen compounds, for example, melamine derivatives such as benzoguanamine, urea derivatives such as allantoin, hydrazides, imides, and polyamide. 

The products from the thermal degradation of Noryl GTX poly(phen-ylene oxide)-polyamide in air and in nitrogen have been identified and quantified. Ecotoxicologic testing of the products of pyrolysis with aquatic organisms indicated that in a fire, no greater harm than burned beech wood is to be expected when the fire-fighting water reaches aquatic ecosystems. ... 

F.-C. Chiu, and I.-N. Huang, Phase morphology and enhanced thermal/ mechanical properties of polyamide 46/graphene oxide nanocomposites. Polymer Testing, 31 (7), 953-962,2012. 

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