Thermal oxidative breakdown

A study of the thermal oxidative breakdown of polyethylene under static conditions has revealed that polyamine disulfides are stabilizers, yet to varying degrees (Fig. 1). 

Figure 1 Thermal oxidative breakdown of polyethylene (temperature 200°C P02 = 350 Tor stabilizer concentration 0.5 mass percent). 1-without stabilizer 2-CaO-6 3-polydii-minodiphenylmethane disulfide 4-polydiiminodiphenylsul-fon disulfide 5-polyparaoxydiphenylamine disulfide 6-po-lydimethylaniline disulfide 7-polyaniline disulfide 8-polydiiminodiphenyloxide disulfide 9-polythiosemicarbaz-ide disulfide 10-polyamine disulfide 11-polycarbamide disulfide 12-poly thiocarbamide disulfide 13-polyethylenedi-amine disulfide.

Polythiosemicarbazide disulfide is the most efficient aliphatic polyamine disulfide for inhibiting the thermal oxidative breakdown of polypropylene, while polyimi-noaniline disulfide and polydiiminodiphenyloxide disulfide (Fig. 3) are the most efficient aromatic polyamine disulfides. In contrast to polyethylene, the thermal oxidative breakdown period increases as the concentration increases (Fig. 4), Depending on the concentration, the flow-melt index at 230°C increases at a lower rate than in the case of commercial stabilizer Santanox (Table 2)-... 

Figure 4 The effect of stabilizer concentration on the induction period of polypropylene thermal oxidative breakdown (temperature 200°C P02 = 300 Tor). 1-polyamineani-line disulfide 2-polydiiminodiphenyloxide disulfide 3-polythiosemicarbazide disulfide 4-Santanox 5-hydroru-beanic polydisulfide 6-thiocarbamidepoly disulfide 7-po-lydisulfide.

Table 2 Effect of Polyamine Disulfide Concentration on the Polypropylene Flow-Melt Index During Thermal Oxidative Breakdown...

During processing polypropylene melts under the action of transverse strain there occur strain-chemical conversions which can result in both decrease and increase in their molecular masses the mechanical effect on the rapidity and level of the occurring processes is considerably more prominent than the mere contribution of thermal and thermal-oxidative breakdown. These data necessitate studying the process of polymer destruction. For this purpose it would be most effective to apply the criterion of assessment of the intensity with which destructive processes happen in polymer melts. 

S. A. Trifonov, E. A. Sosnov, and A. A. Malygin, Structure of the surface and thermal-oxidative breakdown of products that has obtained in reaction of polyethylene with vapor of PC13 and VOCl3, J. Appl. Chem. (in Russian) 77(11) 1872-1876 (2004). 

It is now well established that wear of rubber is not a purely mechanical failure process in that it contains a contribution due to thermal-oxidative breakdown of the polymer (226—228). Still, under severe conditions, which apply to all accelerated laboratory tests as well as to many situations encountered in tire wear, abrasion is dominated by tearing processes. 

A significant limitation in the use of polysaccharide polymers in water-based drilling fluids is their temperature stability. The polysaccharide polymers are prone to thermal-oxidative breakdown that reduces their molecular weight. Thomas (33) has observed that the upper temperature limit of starch is about 100 °C and 135 °C for CMC. Darley and Gray (25) have quoted comparable limiting temperatures. Other polysaccharide polymers are restricted to even lower temperatures. Guar gum, for example, is not used at temperatures above 70 °C (25), which limits its use to shallow wells. 

An anodic oxide grown in pure water at 10 pA cm-2 to thicknesses between 4 and 10 nm and subsequently annealed at 700 °C in N2 for 1 hour, showed an interface charge density (1011 eV 1 cnT2) and a dielectric breakdown field strength (11-14 MV cm-1) that are comparable with known values for thermal oxides [Ga2]. While the breakdown field strength of anodic oxides is comparable to thermal... 

Surface layers of silicon oxide are important in semiconductor device fabrication as interlayer dielectrics for capacitors, isolation of conducting layers, or as masking materials. However, anodic oxides, due to their relatively poor electrical properties, breakdown voltage, and leakage current, have not yet found much use in device technology, and cannot compete with thermal oxides obtained at high temperatures of 700 to 900 °C. 

Unsaturated fatty acids also seem to undergo oxidative breakdown during cooking. The volatile compounds found in cooked products are generally the same as in the raw product. Frequently there are, however, quantitative differences between the cooked and the raw product. Flowever, not much is known about the thermal fatty acid breakdown, but possibly it involves decomposition of already formed hydroperoxides in the raw product and/or oxidation of already formed volatile compounds. For example, l-octen-3-ol occurs in raw cut mushroom, whereas l-octen-3-one cannot be detected. On the other hand, l-octen-3-one is found in relatively large amounts in cooked mushroom [26]. 

Silicon devices covered with insulating layers of thermal oxide and CVD nitride have been shown to suffer from breakdown problems which limit the applicable voltages [34]. For that reason, very few experimental studies on highspeed CE (requiring high field strengths) have appeared in the literature where silicon substrates were used, in contrast to its successful use in other separation techniques as discussed in Sects. 4 and 5. 

Aldehydes Of the volatiles produced by the breakdown of the alkoxy radicals, aldehydes are the most significant flavor compounds. Aldehydes can be produced by scission of the hpid molecules on either side of the radical. The products formed by these scission reactions depend on the fatty acids present, the hydroperoxide isomers formed, and the stability of the decomposition products. Temperature, time of heating, and degree of autoxidation are variables that affect thermal oxidation (7). 

It seems unlikely then that carbonyl (ketone and aldehyde) formed by thermal breakdown of hydroperoxides are important sensitisers for photo-oxidation of LDPE in normally processed polymers. The evidence is consistent with the theory that allylic hydrx>perx>xlde derived from vlnylidene is the importeuit photo-initiator initially present under these conditions. Vlnylidene disappears as a concomitant of hydroperx>xide photolysis, initiating photo-oxidation in a manner analogous to its function in thermal oxidation. 

The major precursors in meat flavors are die water-soluble components such as carbohydrates, nucleotides, thiamine, peptides, amino acids, and the lipids, and Maillard reaction and lipid oxidation are the main reactions that convert these precursors in aroma volatiles. The thermal decomposition of amino acids and peptides, and the caramelization of sugars normally require temperatures over 150C for aroma generation. Such temperatures are higher than those normally encountered in meat cooking. During cooking of meat, thermal oxidation of lipids results in the formation of many volatile compounds. The oxidative breakdown of acyl lipids involve a free radical mechanism and the formation of... 

Another potential area of application of FTIR spectroscopy is in the determination of the oxidative status or stability of an oil. Autoxidation is a major deteriorative reaction affecting edible fats and oils, and it is of major concern to processors and consumers from the standpoint of oil quality, as the oxidative breakdown products cause marked off flavours in an oil. A wide range of end products are associated with the autoxidative deterioration of fats and oils, the most important being hydroperoxides, alcohols, and aldehydes. Moisture, hydrocarbons, free fatty acids and esters, ketones, lactones, furans, and other minor products may also be produced, with the free fatty acids becoming more important in thermally stressed oils. In addition, there is significant cis to trans isomerisation and conjugation of double bonds in the hydroperoxides formed as an oil oxidises. 

The resistivity and breakdown field of the wet thermal oxide on SiC have been measured to be comparable to those of thermal Si02 on Si, about 1016 Q cm [26,37,40] and as high as 8xl06Vcm [41], respectively. From analysis of Fowler-Nordheim tunnelling currents in the I-V characteristics of 3C-SiC MOS capacitors, the barrier height between thermal oxide and 3C-SiC has been determined as about 3.1 eV [37,40,41],... 

Biswas and Roy [126] also studied the thermal stability characteristics (Table 16.7) of chemically prepared pyrrole (PY) modified poly-N-vinylcarbazole (NVC) composite P(PY-NVC) and reported the percentage weight losses for PPY, PNVC and P(PY-NVC) during thermal degradation. It was observed that the thermal stability of P(PY-NVC) was intermediate between those of individual components. 50% weight loss was recorded at 400°C in the case of PPY, at 450°C for PNVC and at 425°C for P(PY-NVC) respectively. They inferred that the thermo-oxidative breakdown of aromatic linkages of the polymer matrix occurs in the temperature range of 300-550°C in case of P(PY-NVC)... 

---