Polymers, thermal decomposition

Hirschler, M.M., Fire Performance of Organic Polymers, Thermal Decomposition, and Chemical Composition, American Chemical Society, Fire and Polymers—Materials and Solutions for Hazard Prevention, ACS Symposium Series 797, Eds. G.L. Nelson and C.A. Wilkie, Washington, DC, pp. 293-306, 2001. 

THE CHEMISTRY OF POLYMER THERMAL DECOMPOSITION - General aspects... 

Thermal decomposition of polymers can be considered as a depolymerization process in only a few cases. Thus, polystyrene and polymethylmethacrylate are examples of polymers that can be thermally degraded with the formation of high yields of the corresponding monomer. However, for most polymers thermal decomposition leads to a complex mixture of products, containing low monomer concentrations. The type and distribution of products derived from the thermal degradation of each polymer depends on a number of factors the polymer itself, the reaction conditions, the type and operation mode of the reactor, etc. 

Figure 3.354. Variation of the iron grinding degree with the activation energy of polymer thermal decomposition 1959].

M. Hirschler, Fire performance of organic polymers, thermal, decomposition, and chemical decomposition. Polymeric Materials Science and Engineering, 83 (2000), 79-80. 

Lange, E.A. and Huh, C. 1994. A Polymer Thermal Decomposition Model and Its Application in Chemical EPR Process Simulation. Paper SPE 27822 presented at the SPE/DGE Improved Gil Recovery Symposium, Tulsa, 17-20 April. DPI 10.2118/27822-MS. 

B.p. — 29X. Monomer used to form polymers (only under rather drastic conditions) or copolymers with C2F4 and vinylidene fluoride, CH2 = CF2. Hexafluoropropene may be prepared by thermal decomposition of CF3CF2CF2C02Na or is prepared commercially by low pressure pyrolysis of C2F4. 

Prolonged exposure to thermal decomposition products causes so-called polymer fume fever, a temporary influenza-like condition. It may be contracted by smoking tobacco that has been contaminated with the polymer. It occurs several hours after exposure and passes within 36—48 hours the temporary effects are not cumulative. 

PVDE is not hazardous under typical processing conditions. If the polymer is accidentaky exposed to temperatures exceeding 350°C, thermal decomposition occurs with evolution of toxic hydrogen fluoride (HE). 

Thermal, Thermooxidative, and Photooxidative Degradation. Polymers of a-olefins have at least one tertiary C-H bond in each monomer unit of polymer chains. As a result, these polymers are susceptible to both thermal and thermooxidative degradation. Reactivity in degradation reactions is especially significant in the case of polyolefins with branched alkyl side groups. For example, thermal decomposition of... 

The dimer of phosphonic acid, diphosphonic acid [36465-90-4] (pyrophosphoms acid), H4P2O3, is formed by the reaction of phosphoms trichloride and phosphonic acid in the ratio of 1 5. It is also formed by the thermal decomposition of phosphonic acid. Unlike the chemistry of phosphoric acid, thermal dehydration does not lead to polymers beyond the dimer extended dehydration leads to a disproportionation to condensed forms of phosphoric acid, such as [2466-09-3] and phosphine. 

Other by-products include acetone, carbonaceous material, and polymers of propylene. Minor contaminants arise from impurities in the feed. Ethylene and butylenes can form traces of ethyl alcohol and 2-butanol. Small amounts of / -propyl alcohol carried through into the refined isopropyl alcohol can originate from cyclopropane [75-19-4] in the propylene feed. Acetone, an oxidation product, also forms from thermal decomposition of the intermediate sulfate esters, eg. 

Polyurethanes. These polymers can be considered safe for human use. However, exposure to dust, generated in finishing operations, should be avoided. Ventilation, dust masks, and eye protection are recommended in foam fabrication operations. Polyurethane or polyisocyanurate dust may present an explosion risk under certain conditions. Airborne concentrations of 25—30 g/m are required before an explosion occurs. Inhalation of thermal decomposition products of polyurethanes should be avoided because carbon monoxide and hydrogen cyanide are among the many products present. 

Acryhc elastomers are normally stable and not reactive with water. The material must be preheated before ignition can occur, and fire conditions offer no hazard beyond that of ordinary combustible material (56). Above 300°C these elastomers may pyrolize to release ethyl acrylate and other alkyl acrylates. Otherwise, thermal decomposition or combustion may produce carbon monoxide, carbon dioxide, and hydrogen chloride, and/or other chloiinated compounds if chlorine containing monomers are present ia the polymer. 

Hypalon raw polymer compounds or cured product may be disposed of in an approved landfill. Incineration is not recommended because of the evolution of toxic gases. Additional information is available from Du Pont concerning these and other potential health hazards when handling Hypalon compounds, finished products, thermal decomposition products, or waste disposal (43). 

The copolymers have been used in the manufacture of extruded pipe, moulded fittings and for other items of chemical plant. They are, however, rarely used in Europe for this purpose because of cost and the low maximum service temperature. Processing conditions are adjusted to give a high amount of crystallinity, for example by the use of moulds at about 90°C. Heated parts of injection cylinders and extruder barrels which come into contact with the molten polymer should be made of special materials which do not cause decomposition of the polymer. Iron, steel and copper must be avoided. The danger of thermal decomposition may be reduced by streamlining the interior of the cylinder or barrel to avoid dead-spots and by careful temperature control. Steam heating is frequently employed. 

However, certain additives can decrease the rate of thermal decomposition [28]. These additives include cyclic sulfates, sulfones, sultones, aliphatic and aromatic anhydrides, and polymers with pendant carboxylic acid functional groups. Most of these materials are latent acids, which decompose on heating in the presence of moisture to form a strong acid, as shown for cyclic sulfate, 9, in Eq. 5. 

Low ionizing potentials or soft ionization methods are necessary to observe the parent ions in the mass spectra of many S-N compounds because of their facile thermal decomposition. Mass spectrometry has been used to investigate the thermal breakdown of S4N4 in connection with the formation of the polymer (SN). On the basis of the appearance potentials of various S Ny fragments, two important steps were identified ... 

From the results obtained by thermal decomposition of both low-molecular weight vicinal dichlorides in the gas phase [74,75] and of the copolymers of vinyl chloride and /rthermal instability of PVC to the individual head-to-head structures. Crawley and McNeill [76] chlorinated m-1,4-polybutadiene in methylene chloride, leading to a head-to-head, and a tail-to-tail PVC. They found, for powder samples under programmed heating conditions, that head-to-head polymers had a lower threshold temperature of degradation than normal PVC, but reached its maximum rate of degradation at higher temperatures. 

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