Thermal degradation/decomposition

Over the years animal studies have repeatedly shown that perfluorinated inert fluids are nonirritating to the eyes and skin and practically nontoxic by ingestion, inhalation, or intraperitoneal injection (17,22). Thermal degradation can produce toxic decomposition products including perfluoroisobutene which has a reported LC q of 0.5 ppm (6 hr exposure in rats) (31). This decomposition generally requires temperatures above 200°C. 

More recent work reports the onset of thermal degradation at lower temperatures and provides a clearer picture of the role of oxygen (73—75). In the presence of oxygen, backbone oxidation and subsequent cleavage reactions initiate decomposition. In the absence of oxygen, dehydrofluorination eventually occurs, but at significantly higher temperatures. 

Degradatiou. Heating of succinic acid or anhydride yields y-ketopimehc ddactone, cyclohexane-1,4-dione, and a mixture of decomposition products that include acetic acid, propionic acid, acryUc acid, acetaldeide, acrolein, oxaUc acid, cyclopentanone, and furane. In argon atmosphere, thermal degradation of succinic anhydride takes place at 340°C (123). Electrolysis of succinic acid produces ethylene and acetylene. 

Methylene chloride is one of the more stable of the chlorinated hydrocarbon solvents. Its initial thermal degradation temperature is 120°C in dry air (1). This temperature decreases as the moisture content increases. The reaction produces mainly HCl with trace amounts of phosgene. Decomposition under these conditions can be inhibited by the addition of small quantities (0.0001—1.0%) of phenoHc compounds, eg, phenol, hydroquinone, -cresol, resorcinol, thymol, and 1-naphthol (2). Stabilization may also be effected by the addition of small amounts of amines (3) or a mixture of nitromethane and 1,4-dioxane. The latter diminishes attack on aluminum and inhibits kon-catalyzed reactions of methylene chloride (4). The addition of small amounts of epoxides can also inhibit aluminum reactions catalyzed by iron (5). On prolonged contact with water, methylene chloride hydrolyzes very slowly, forming HCl as the primary product. On prolonged heating with water in a sealed vessel at 140—170°C, methylene chloride yields formaldehyde and hydrochloric acid as shown by the following equation (6). 

In contrast to the thermolysis experiments, the main products of the photoinduced decomposition of a-iactams (326) are carbon monoxide and the corresponding imines (69JA1176). This pathway only occurs to a minor extent in some thermal degradations. 

Product Quality Considerations of product quahty may require low holdup time and low-temperature operation to avoid thermal degradation. The low holdup time eliminates some types of evaporators, and some types are also eliminated because of poor heat-transfer charac teristics at low temperature. Product quality may also dic tate special materials of construction to avoid met hc contamination or a catalytic effect on decomposition of the product. Corrosion may also influence evaporator selection, since the advantages of evaporators having high heat-transfer coefficients are more apparent when expensive materials of construction are indicated. Corrosion and erosion are frequently more severe in evaporators than in other types of equipment because of the high hquid and vapor velocities used, the frequent presence of sohds in suspension, and the necessary concentration differences. 

Extensive research has been conducted to determine the thermal-decomposition properties of polymers, the products of their degradation, and the kinetics involved in their reaction during pyrolysis (Ml). Complete comprehension of the mechanism involved in thermal degradation requires, among other facts, knowledge of these three fundamental aspects ... 

Thermal degradation studies of EB-cured terpolymeric fluorocarbon rubber [430] by nonisothermal thermogravimetry in the absence and presence of cross-link promoter TMPTA reveal that thermal stability is improved on radiation and more so in the presence of TMPTA. Initial decomposition temperature, maximum decomposition temperature and the decomposition... 

Thermal operations such as distillation, decomposition, transformation, and rectification often cause thermal degradation. Furthermore, with these processes quantitative catalyst recovery is generally not possible, which results in loss of productivity. 

Increase the oxidation rate of polymers, e.g. metal ions which increase the hydroperoxide decomposition rate. Photodegradation and thermal degradation are enhanced by transition metal ion containing pro-oxidants, such as iron dithiocarbamate (as opposed to nickel dithiocarba-mate, which acts as a photo-antioxidant). 

Dartnell, R. C. et al., Loss Prev., 1971, 5, 53-56 MCA Case History No. 1649 A batch of 8 t of material accumulated in storage at 154°C during 72 h decomposed explosively. Stability tests showed that thermal instability developed when 3-methyl-4-nitrophenol is stored molten at temperatures above 140°C. Decomposition set in after 14 h at 185° or 45 h at 165°, with peak temperatures of 593 and 521°C, respectively. In a closed vessel, a peak pressure of 750 bar was attained, with a maximum rate of increase of 40 kbar/s. Thermal degradation involves an initially slow exothermic free radical polymerisation process, followed by a rapid and violently exothermic decomposition at take-off. 

Mass spectrometry (MS) coupled with pyrolysis has been a key technique in detecting the thermal degradation products of polymers, and thereby elucidating their thermal decomposition pathways [69]. In pyrolysis-MS, a sample is thermally decomposed in a reproducible manner by a pyrolysis source that is interfaced with a mass spectrometer. The volatile products formed can then be analysed either as a mixture by MS or after separation by GC/MS [70]. 

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