Thermal Oxidations

Oxidation may be initiated by an agent that is able to decompose and produce free radicals. This is so with benzoyl peroxide (Bz202) and azo-bisisobutyronitrile (AIBN). Initiation by benzoyl peroxide produces initial rates which fit, in the experimental range defined by the boundary conditions 0.0446 [RCHO]0 0.224 mole l-1 105 PQl 580 torr 20 T 35° C solvent, benzene. [Bz2O2]0 = 2.95 X 10-3 mole per mole of solution, the equation 

At 25°C, k = 0.85 l1 2 mole-1 2 s-1 2 and the kinetics indicate that the usual mechanism applies (k = k3klln). Since the rate is proportional to the square root of the Bz202 concentration, the first-order decomposition of this peroxide is the only noteworthy source of radicals. The value of the product k3klln is compatible with the photochemical results. The use of a less active initiator, AIBN, at normal temperatures gives kinetics which can only be explained by the intervention of a thermal initiation caused by the reaction of oxygen with benzaldehyde [45]. The results give the data, at 43° C. 

Thermal autoxidation has been studied kinetically up to high degrees of conversion and the observations show the intervention of an appreciable inhibiting effect which is apparently the result of the formation of an inhibiting product at the stage of peracid reaction with aldehyde. The complete scheme thus becomes 

More recently, it has been reported [73] that the effect of phenolic inhibitors on the thermal oxidation of benzaldehyde and substituted benzaldehydes in acetic acid shows that initiation is due to reaction (A) and that there is no evidence for the third-order step, 2 RCHO + 02 (cf. Russian workers refs. 69, 70, and 72). 

The oxidation of benzaldehyde catalyzed by manganese(II) and (III) acetate, cobalt(II) naphthenate, and cerium(IV) naphthenate has been studied by Kresge [46] with acetic acid as the solvent at a temperature of 50°C and an oxygen pressure of about 1 atm. In the case of oxidation in the presence of manganese with an aldehyde concentration of less them 0.5 mole 1 1 and a manganese concentration of less than 10 5moler1, the kinetics of the initial oxidation follow the empirical equation 

Especially above room temperature many polymers degrade in an air atmosphere by oxidation that is not light-induced (heat ageing). A number of polymers already show a deterioration of the mechanical properties after heating for some days at about 100 °C and even at lower temperatures (e.g. polyethylene, polypropylene, poly(oxy methylene) and poly(ethylene sulphide)). 

The rate of oxidation can be determined by measuring the oxygen uptake at a certain temperature. Such measurements have shown that the oxidation at 140 °C of low-density polyethylene increases exponentially after an induction period of 2 h. It can be concluded from this result that the thermal oxidation, like photo-oxidation, is caused by autoxidation, the difference merely being that the radical formation from the hydro peroxide is now activated by heat. 

The primary reaction can be a direct reaction with oxygen 

Polymerization reactions are very complex and polymeric materials are difficult to characterize (Paquette and Kupranycz van Voort, 1987). During thermal oxidation, intra- and intermolecular reactions of alkoxyl, alkyl and peroxyl radicals may lead to the formation of dimers, trimers and large molecular weight polymers with C-O-C and C-O-O-C crosslinks (Nawar, 1984, 1985). Dimers are a major component of non-volatile products formed in oxidized and heated fats/oils (Nawar, 1984 Taub, 1984). As is shown in reaction (11.7), by combining two radicals to form a non-radical dimer, the free radical chain reaction is thus terminated. 

A large variety of compounds can be generated, especially when the fat is rich in polyunsaturated fatty acids (Frankel et al., 1961 Schultz et al., 1962 Artman, 1969 Weiss, 1970 Frankel, 1980, 1984 Nawar, 1984 Hsieh and Kinsella, 1989). However, heating conditions (time, temperature and aeration) and anti-oxidant levels in the oil can modulate the degree of oxidation and type of products generated. A used oil may contain as much as 25-30% polymeric materials at the end of its usable frying capability (Poling et al., 1970 Nolen, 1973). 

Tomanek, A. 1991. Silicones and industry, 42. Munich Wacker-Chemie GmbH. 

Riedel, J. A., and R. Vander Laan. 1990. Ethylene propylene rubbers. In The Vanderbilt rubber handbook, 13th ed., 123-148. Norwalk, CT R. T. Vanderbilt Co., Inc. 

Karpeles, R., and A. V. Grossi. 2001. EPDM rubber technology. In Handbook of elastomers, 2nd ed., ed. A. K. Bhowmick and H. L. Stephens, 845-876. New York Marcel Dekker, Inc. 

Ver State, G. 1986. Ethylene propylene elastomers. In Encyclopedia of polymer science and engineering, vol. 6, 522-564. New York John Wiley Sons. 

Daintith, J. 2005. Oxford dictionary of physics, 5th ed. Oxford, England Oxford University Press. 

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The most common technique for estimating thermal stability is called the Jet Fuel Thermal Oxidation Test (JFTOT). It shows the tendency of the fuel to form deposits on a metallic surface brought to high temperature. The sample passes under a pressure of 34.5 bar through a heated aluminum tube (260°C for Jet Al). After two and one-half hours, the pressure drop across a 17-micron filter placed at the outlet of the heater is measured (ASTM D 3241). 

Copolymer. Acetal copolymers are prepared by copolymerization of 1,3,5-trioxane with small amounts of a comonomer. Carbon-carbon bonds are distributed randomly in the polymer chain. These carbon-carbon bonds help to stabilize the polymer against thermal, oxidative, and acidic attack. 

Properly end-capped acetal resins, substantially free of ionic impurities, are relatively thermally stable. However, the methylene groups in the polymer backbone are sites for peroxidation or hydroperoxidation reactions which ultimately lead to scission and depolymerisation. Thus antioxidants (qv), especially hindered phenols, are included in most commercially available acetal resins for optimal thermal oxidative stabiUty. 

Finishing. AH acetal resins contain various stabilizers introduced by the suppHer in a finishing extmsion (compounding) step. The particular stabilizers used and the exact method of their incorporation are generally not revealed. Thermal oxidative and photooxidative stabilizers have already been mentioned. These must be carefully chosen and tested so that they do not aggravate more degradation (eg, by acidolysis) than they mitigate. 

Thermal Oxidative Stability. ABS undergoes autoxidation and the kinetic features of the oxygen consumption reaction are consistent with an autocatalytic free-radical chain mechanism. Comparisons of the rate of oxidation of ABS with that of polybutadiene and styrene—acrylonitrile copolymer indicate that the polybutadiene component is significantly more sensitive to oxidation than the thermoplastic component (31—33). Oxidation of polybutadiene under these conditions results in embrittlement of the mbber because of cross-linking such embrittlement of the elastomer in ABS results in the loss of impact resistance. Studies have also indicated that oxidation causes detachment of the grafted styrene—acrylonitrile copolymer from the elastomer which contributes to impact deterioration (34). 

Antioxidants have been shown to improve oxidative stabiHty substantially (36,37). The use of mbber-bound stabilizers to permit concentration of the additive in the mbber phase has been reported (38—40). The partitioning behavior of various conventional stabilizers between the mbber and thermoplastic phases in model ABS systems has been described and shown to correlate with solubiHty parameter values (41). Pigments can adversely affect oxidative stabiHty (32). Test methods for assessing thermal oxidative stabiHty include oxygen absorption (31,32,42), thermal analysis (43,44), oven aging (34,45,46), and chemiluminescence (47,48). 

Thermal—Oxidative-Resistance Coatings. The thermal stabihty of coatings produced by either covalendy or noncovalendy incorporating 2,4-dinitroaniline into an inorganic siUcate network and coating it onto a sapphire substrate has been examined (67). Although some increase in the thermal... 

Sihcon dioxide layers can be formed using any of several techniques, including thermal oxidation of siUcon, wet anodization, CVD, or plasma oxidation. Thermal oxidation is the dominant procedure used in IC fabrication. The oxidation process selected depends on the thickness and properties of the desired oxide layer. Thin oxides are formed in dry oxygen, whereas thick (>0.5 jim) oxide layers are formed in a water vapor atmosphere (13). 

Gate oxide dielectrics are a cmcial element in the down-scaling of n- and -channel metal-oxide semiconductor field-effect transistors (MOSEETs) in CMOS technology. Ultrathin dielectric films are required, and the 12.0-nm thick layers are expected to shrink to 6.0 nm by the year 2000 (2). Gate dielectrics have been made by growing thermal oxides, whereas development has turned to the use of oxide/nitride/oxide (ONO) sandwich stmctures, or to oxynitrides, SiO N. Oxynitrides are formed by growing thermal oxides in the presence of a nitrogen source such as ammonia or nitrous oxide, N2O. Oxidation and nitridation are also performed in rapid thermal processors (RTP), which reduce the temperature exposure of a substrate. 

The polymer is exposed to an extensive heat history in this process. Early work on transesterification technology was troubled by thermal—oxidative limitations of the polymer, especially in the presence of the catalyst. More recent work on catalyst systems, more reactive carbonates, and modified processes have improved the process to the point where color and decomposition can be suppressed. One of the key requirements for the transesterification process is the use of clean starting materials. Methods for purification of both BPA and diphenyl carbonate have been developed. 

Methylphenol is converted to 6-/ f2 -butyl-2-methylphenol [2219-82-1] by alkylation with isobutylene under aluminum catalysis. A number of phenoHc anti-oxidants used to stabilize mbber and plastics against thermal oxidative degradation are based on this compound. The condensation of 6-/ f2 -butyl-2-methylphenol with formaldehyde yields 4,4 -methylenebis(2-methyl-6-/ f2 butylphenol) [96-65-17, reaction with sulfur dichloride yields 4,4 -thiobis(2-methyl-6-/ f2 butylphenol) [96-66-2] and reaction with methyl acrylate under base catalysis yields the corresponding hydrocinnamate. Transesterification of the hydrocinnamate with triethylene glycol yields triethylene glycol-bis[3-(3-/ f2 -butyl-5-methyl-4-hydroxyphenyl)propionate] [36443-68-2] (39). 2-Methylphenol is also a component of cresyHc acids, blends of phenol, cresols, and xylenols. CresyHc acids are used as solvents in a number of coating appHcations (see Table 3). 

In the depolymeri2ed scrap mbber (DSR) experimental process, ground scrap mbber tines produce a carbon black dispersion in ok (35). Initially, aromatic oks are blended with the tine cmmb, and the mixture is heated at 250—275°C in an autoclave for 12—24 h. The ok acts as a heat-transfer medium and swelling agent, and the heat and ok cause the mbber to depolymeri2e. As more DSR is produced and mbber is added, less aromatic ok is needed, and eventually virtually 100% of the ok is replaced by DSR. The DSR reduces thermal oxidation of polymers and increases the tack of uncured mbber (36,37). Depolymeri2ed scrap mbber has a heat value of 40 MJ/kg (17,200 Btu/lb) and is blended with No. 2 fuel ok as fuel extender (38). 

The excellence of a properly formed Si02—Si interface and the difficulty of passivating other semiconductor surfaces has been one of the most important factors in the development of the worldwide market for siUcon-based semiconductors. MOSFETs are typically produced on (100) siUcon surfaces. Fewer surface states appear at this Si—Si02 interface, which has the fewest broken bonds. A widely used model for the thermal oxidation of sihcon has been developed (31). Nevertheless, despite many years of extensive research, the Si—Si02 interface is not yet fully understood. 

The corrosion behavior of tantalum is weU-documented (46). Technically, the excellent corrosion resistance of the metal reflects the chemical properties of the thermal oxide always present on the surface of the metal. This very adherent oxide layer makes tantalum one of the most corrosion-resistant metals to many chemicals at temperatures below 150°C. Tantalum is not attacked by most mineral acids, including aqua regia, perchloric acid, nitric acid, and concentrated sulfuric acid below 175°C. Tantalum is inert to most organic compounds organic acids, alcohols, ketones, esters, and phenols do not attack tantalum. 

The corrosion resistance imparted to tantalum by the passivating surface thermal oxide layer makes the metal inert to most ha2ards associated with metals. Tantalum is noncorrosive in biological systems and consequently has a no chronic health ha2ard MSDS rating. 

The first commercial supersonic transport, the Concorde, operates on Jet A1 kerosene but produces unacceptable noise and exhaust emissions. Moreover, it is limited in capacity to 100 passengers and to about 3000 miles in range. At supersonic speed of Mach 2, the surfaces of the aircraft are heated by ram air. These surfaces can raise the temperature of fuel held in the tanks to 80 °C. Since fuel is the coolant for airframe and engine subsystems, fuel to the engine can reach 150°C (26). An HSCT operated at Mach 3 would place much greater thermal stress on fuel. To minimize the formation of thermal oxidation deposits, it is likely that fuel deflvered to the HSCT would have to be deoxygenated. 

In general, the alkah and alkaline-earth metal salts of the B q- and B22-halogenated derivatives have excellent thermal, oxidative, and hydrolytic stabihties. 

Carbon tetrachloride [56-23-5] (tetrachloromethane), CCl, at ordinary temperature and pressure is a heavy, colorless Hquid with a characteristic nonirritant odor it is nonflammable. Carbon tetrachloride contains 92 wt % chlorine. When in contact with a flame or very hot surface, the vapor decomposes to give toxic products, such as phosgene. It is the most toxic of the chloromethanes and the most unstable upon thermal oxidation. The commercial product frequendy contains added stabilizers. Carbon tetrachloride is miscible with many common organic Hquids and is a powerhil solvent for asphalt, benzyl resin (polymerized benzyl chloride), bitumens, chlorinated mbber, ethylceUulose, fats, gums, rosin, and waxes. 

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