Thermal desorption and vapor transport characteristics in an explosive trace detector

Swipe-based explosive trace detectors rely on thermal desorption to vaporize explosive particles collected on a swipe. The vaporized material is carried by air flows from the desorption unit to the inlet of the chemical analyzer, typically an ion mobility spectrometer. We have observed that the amount of explosives detected from a swipe varies with the physical location of explosives collected on the swipe. There are two issues that may contribute to this effect: inhomogeneous or insufficient heating of the swipe during desorption and low velocity air flows that inefficiently transport desorbed vapor during the instruments analysis time. To better characterize this effect, we have simulated the air movements within a generic desorption unit using commercially available computational fluid dynamics software. Simulations are three dimensional, symmetric and solved under steady, laminar flow conditions. The calculated velocity field correlates directly with experimental detector response to the high explosive RDX. Results suggest that the limiting factor in this model thermal desorption unit is the flow-field around the swipe and flow rate into the detector, rather than heat transfer to the swipe itself. Buoyancy effects due to heating dominate the flow-field and produce a vertical bulk fluid motion within the domain that opposes much of the flow drawn into the analyzer.     

Thermal characteristics of desensitizing waxes for explosive compositions

Waxes are added to explosive compositions to provide an explosive binder and lubricant for press-loaded explosives and to desensitize both press-loaded and cast-loaded explosives. The most significant thermal criterion of a good desensitizer is its ability to absorb large quantities of heat at above environmental temperatures. The enthalpy of each wax investigated for explosive incorporation was determined from room temperatures to liquefication by means of a Perkin-Elmer DSC-2. Correlation of enthalpy and desensitization was accomplished by relating the results of a velocity projectile impact test on explosives containing the investigated waxes.Melt and solidification temperatures are also determined from DSC thermograms. The temperature at which a wax ceases to absorb large amounts of heat is the temperature at which liquefication takes place. The temperature at which large quantities of heat are released during a cooling cycle is the solidification temperature. Use of wax having too low a liquefication temperature results in excess exudation from a loaded high explosive charge; too high a liquefication temperature inhibits incorporation of the wax into the molten explosive.     

Thermal stability of Peroxynitrates

Peroxynitrates are thermally unstable intermediates (at ambient temperatures) in the atmospheric degradation of hydrocarbons. In this work, thermal lifetimes of nine peroxynitrates have been measured as a function of temperature and, for two of them, also, as a function of total pressure. In the presence of excess NO, relative concentrations of the peroxynitrates were followed in a 420 I reaction chamber as a function of time by means of longpath IR absorption using a Fourier transform spectrometer. Original data on the unimolecular decomposition rate constants are presented for the peroxynitrates RO₂NO₂ with R = C₆H₁₁, CH₃C(O)CH₂, C₆H₅CH₂, CH₂I, CH₃C(O)OC(H)CH₃, C₆H₅OCH₂, (CH₃)₂NC(O), C₆H₅OC(O), and C₂H₅C(O). Thermal lifetimes at room temperature and atmospheric pressure are very short (in the order of seconds) for substituted methyl peroxynitrates (i.e., R'CH₂O₂NO₂) but rather long for substituted formyl peroxynitrates (i.e., R″C(O)O₂NO₂). Kinetic data from this and previous work from our laboratory are used to derive structure‐stability relationships which allow an estimate of the thermal lifetimes of peroxynitrates from readily available ¹³C n.m.r. shift data. ©1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 127–144, 1999     

Thermal stability of elastomers

Thermal transitions of elastomers are classified and problems concerning their stability are discussed. It is concluded that in the case of general-purpose elastomers no simple correlation exists between the energy of the bonds in the skeleton of a chain and their thermal stability. This also holds for the parameters of the physical structure of the chains. A high tendency to cross-linking, a high concentration of cross-links and their chemical structure give rise to a more perceptible effect.

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Zusammenfassung Thermische Umwandlungen von Elastomeren werden klassifiziert und mit der StabilitÄt von Elastomeren zusammenhÄngende Probleme diskutiert. Es wird gefolgert, da\ bei Elastomeren für allgemeine Verwendungszwecke keine einfache Korrelation zwischen der Energie der Bindungen im Kettengerüst und der thermischen StabilitÄt besteht. Das gilt auch für die Parameter der physikalischen Struktur der Ketten. Der Effekt einer starken Neigung zur Vernetzung, der Netwerkdichte und der chemischen Konstitution der Netzwerkbrücken ist dagegen augenscheinlicher.

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Thermal stability of alcohols

3,3-Dimethylbutanol-2 (3,3-DMB-ol-2) and 2,3-dimethylbutanol-2 (2,3-DMB-ol-2) have been decomposed in comparative-rate single-pulse shock-tube experiments. The mechanisms of the decompositions are The rate expressions are They lead to D(iC₃H₇? H) – D((CH₃)₂(OH) C? H) = 8.3 kJ and D(C₂H₅? H) – D(CH₃(OH) CH? H) = 24.2 kJ. These data, in conjunction with reasonable assumptions, give and The rate expressions for the decomposition of 2,3-DMB-1 and 3,3-DMB-1 are and      

Thermal stability of ketoprofen

The purpose of this investigation is to calculate the kinetic parameters and the kinetic model for the studied process. The results are further used to predict the system’s behaviour in various circumstances. A kinetic study regarding the ketoprofen—involving active substance’s thermal decomposition—was performed under isothermal conditions and in a nitrogen atmosphere, for the temperature steps: 260; 265; 270; 275; and 280 °C. The thermogravimetry/derivative thermogravimetry data were processed by three differential methods: isothermal–isoconversional, Friedman’s isothermal–isoconversional, and isothermal model-fittings. The obtained results are in good accordance with those obtained under non-isothermal conditions of a previous study, and confirm the necessity for the kinetic parameters to be determined, under different thermal conditions, by the adequate calculation methods.     

Thermal stability of polyurethanes

Summary The combined application of thermogravimetry reactiongas chromatography and gel-permeation chromatography permits to follow the heat degradation of polyurethane polymers in inert gas, air and water-saturated environment. The examinations give information on the rate of thermal degradation, the individual volatile degradation components, the critical points of the polymer chains and on the change of their molecular-weight distribution. Gas chromatographic examinations also permit the identification of the chain-extending components of different types of polyurethanes.     

Thermal stability of piroxicam

The application of thermal method is of great importance regarding the pharmaceutical problems such as the control of raw materials, the determination of purity, the qualitative and quantitative analysis of drug formulation, tests of thermal stability and compatibility and the determination of kinetic parameters etc. The purpose of a kinetic investigation is to calculate the kinetic parameters and the determination of the kinetic model for the studied process. The results are further used to predict the system’s behaviour in various circumstances. A kinetic study regarding the piroxicam—active substance’s thermal decomposition was performed under isothermal conditions and nitrogen atmosphere, for the temperature steps: 200, 205, 210, 215 and 220 °C. The TG/DTG data were processed by three differential methods: isothermal—isoconversional, Friedman’s isothermal isoconversional and isothermal model-fitting. The obtained results are in good accord between them, as well as with those obtained under non-isothermal conditions from a previous work and confirm the necessity of the kinetic parameters determining in different thermal conditions, by the adequate calculation methods.     

Thermal stability of high-energy compounds

The influence of molecular structure on the stability of high-energy compounds is considered. The kinetic parameters of the decomposition of various energy-rich groups in monofunctional compounds are established. Data on decomposition of compounds with mixed functional groups are described. The sites of primary breakdown are determined and the mutual influence of functional groups on the stability is considered. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 231–234, February, 2000.     

Thermal stability of syndiotactic polypropene

Thermal stability of syndiotactic polypropene (syn-PP) was investigated by comparison with isotactic polypropene (iso-PP). Heat treatments were carried out in air at 160–220°C for 10–30 min. Under these conditions, molecular weight and molecular weight distribution of syn-PP were almost kept constant, indicating the high stability of syn-PP. Iso-PP was degraded drastically under the same conditions. Thus, it is confirmed that the stereoregularity is one of the dominant factors to determine the stability of PP.     

Thermal stability of transition-metal complexes

This review aims at justifying the relationship between the room-temperature structures of transition-metal complexes and their thermal stabilities. The different factors affecting the thermal stability were also clarified. The survey of a larger number of transition-metal complexes showed various correlations of thermal stability with metal ion, ligand character or counterion.     

Thermal oxidative stability of poly-p-xylylenes

The air oxidation of poly-p-xylylene films was studied at temperatures between 125 and 200°C. The oxidation kinetics were obtained from neutron activation (NA) oxygen analyses and infrared (IR) Spectroscopy. A correlation between the NA oxygen analyses and mechanical properties indicated that the amount of oxygen incorporated into these polymers before a significant degradation mechanical properties is about 1000 ppm for poly(dichloro-p-xylylene) and 5000 ppm for poly(monochloro-p-xylylene) or poly-p-xylylene. The activation energy for the oxidation of these polymers was about 30 kcal/mole. Long-term-use (100,000 hr) temperatures were also estimated for each of the poly-p-xylylenes studied. The 100,000-hr maximum continuous-use temperature is 112°C for poly(dichloro-p-xylylene), 72°C for poly(monochloro-p-xylylene), and 57°C for poly-p-xylylene.     

Thermal stability of epoxy adhesives

A study was made of the thermal stability of epoxy compounds which were unfilled or contained metallic fillers such as aluminium dust, aluminium flakes, powdered bronze, powdered brass and silver flakes. The properties of the compounds were modified by the use of various hardeners.

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Zusammenfassung Es wurde die thermische StabilitÄt ungefüllter und gefüllter Epoxidkompositionen untersucht. Als metallische Füllstoffe wurden Aluminiumpulver und -flocken, Messing- und Bronzpulver sowie Silberflocken eingesetzt. Die Eigenschaften dieser Kompositionen wurden mit verschiedenen HÄrtern modifiziert.

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Thermal stability of fluorochloroindate glasses

New fluorindate glass compositions have been investigated in order to improve optical transmission as well as thermal properties. Chloride inclusion extends transmission of a fluoride matrix to longer wavelength in infrared region. In the present work thermal parameters of an IZnBS composition, based on InF₃, ZnF₂, BaF₂ and SrF₂, with various amounts of alkaline chlorides were investigated by differential scanning calorimetry. The chloride presence decreased all characteristic temperatures and increased both thermal stability and glass forming ability up to 10% of MCl content, where M=Li, Na, K and Rb. The presence of NaCl promoted glass phase separation. For samples containing same concentration of NaCl, this effect is accentuated for increasing the contents of SrF₂. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal stability of ionene polymers

Two different cationic polymers of the same chemical type and with very similar chemical structures were reacted with a natural bentonite over a wide range of polymer/clay ratios. This study involved the synthesis of cationic aliphatic ammonium polyionenes, specifically 3,6-ionene and 3,6-dodecylionene. Ionenes are ion-containing polymers that contain quaternary nitrogen atoms in the main macromolecular chain as opposed to a pendant chain. The CHN content, basal spacing, and elemental composition of each of the polymer–clay complexes were analyzed by X-ray diffraction, X-ray fluorescence, and thermogravimetry. All the polycations reacted to form interlayer complexes with clay, which displaced more Na⁺ and little Ca²⁺. Sodium and calcium were both present as interlayer cations in the clay and its complexes. The TG/DTG curves show that both polymers underwent thermal degradation in more than one stage. Specifically, 3,6-ionene was found to undergo two stages of decomposition and 3,6-dodecylionene undergo three stages. The behavior of the TG/DTG curves and the activation energy values suggest that 3,6-dodecylionene (E = 174,85 kJ mol^?1) complexes have greater thermal stability than 3,6-ionene (E = 115,52 kJ mol^?1) complexes. The mechanism of degradation suggests a direct interaction with the dodecyl chain containing 12 carbons, which are present in 3,6-dodecylionene but not in 3,6-ionene.     

Thermal stability of substituted poly-xylylenes

The insertion of meta units in the chain of poly-p-xylylene decreases both the onset temperature of oxidative degradation and the rate of decomposition.Functionalization of the polymer with chlorine and sulphonic groups makes the chain more resistant to high temperature treatments, notwithstanding the low stability of the substituent groups.

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Zusammenfassung Die Einführung von Metaeinheiten in die Kette von Poly-p-xylen setzt sowohl die Einsetztemperatur des oxydativen Abbaues als auch die Zersetzungsgeschwindigkeit herab. Die Einführung von Chlor und Sulfogruppen erhöht trotz der geringen Stabilität der Substituenten die Temperaturbeständigkeit der Kette.

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