Ignition temperature determination

The comparative evaluation of the heat sensitivity of different explosives can be obtained by ignition temperature determination at a constant heating rate or isothermally. This temperature is also known as the deflagration point or deflagration temperature. 

A heating rate of 5 °C/min is typical for all types of explosives. The exception is the ignition temperature determination of black powder when a heating rate of 20 °C/min is applied. 

Ignition temperatures determined by differential scanning calorimetry (DSC) usually correspond reasonably well to those obtained by a Henkin-McGill study. DSC values tend to be more reproducible from laboratory to laboratory, and from sample to sample. Differences in heating rate, sample size, homogeneity, etc., can cause some variation in values obtained with thermal analysis technique. For any direct comparison of ignition temperatures, it is best to run all of the mixtures of interest under identical experimental conditions, thereby minimizing the number of variables. 

The ignition temperatures determined with the standardized method are the basis for the classification of combustible gases and vapours into T-Classes pursuant to EN 60079-20-1 [20]. 

Any acid-soluble materials present in the sample will react with HF or H2SO4. If the products of these reactions are volatile or decompose at the ignition temperature of 1200 °C, then the change in weight will not be due solely to the volatilization of SiF4. The result is a positive determinate error. 

For processes under development, the most cost-effective means of avoiding potential risk is to eliminate those materials that are inherently unsafe that is, those materials whose physical or physico-chemical properties lead to them being highly reactive or unstable. This is somewhat difficult to achieve for several reasons. First, without a full battery of tests to determine, for example, flammability, upper/lower explosivity limits and their variation with scale, minimum ignition temperatures, and so on, it is almost impossible to tell how a particular chemical will behave in a given process. Second, chemical instability may make a compound attractive to use because its inherent reactivity ensures a reaction proceeds to completion at a rapid enough rate to be useful that is, the reaction is kinetically and thermodynamically favoured. 

The explosive decomposition of the solid has been studied in detail [6], The effect of moisture upon ignitibility and explosive behaviour under confinement was studied. A moisture content of 3% allowed slow burning only, and at 5% ignition did not occur [7], Thermal instability was studied using a pressure vessel test, ignition delay time, TGA and DSC, and decomposition products were identified [8], The presence of acyl chlorides renders dibenzoyl peroxide impact-sensitive [9], There is a further report of a violent explosion during purification of the peroxide by Soxhlet extraction with hot chloroform [10], Residual traces of the peroxide in a polythene feed pipe exploded when it was cut with a handsaw [11]. The heat of decomposition has been determined as 1.39 kJ/g. The recently calculated value of 69° C for critical ignition temperature coincides with that previously recorded. 

The figures for flash points are closed-cup values except where a suffix (o) indicates the (usually higher) open-cup value. The figures for explosive limits (or flammability limits) are % by volume in air at ambient temperature except where indicated otherwise. Where no figure has been found for the upper limit, a query has been inserted. Figures for auto-ignition temperatures are usually those determined in glass (without catalytic effects) except where stated. 

The autoignition temperature (AIT) of a vapor, sometimes called the spontaneous ignition temperature (SIT), is the temperature at which the vapor ignites spontaneously from the energy of the environment. The autoignition temperature is a function of the concentration of vapor, volume of vapor, pressure of the system, presence of catalytic material, and flow conditions. It is essential to experimentally determine AITs at conditions as close as possible to process conditions. 

Also, the spontaneous ignition temperature for liquid or volatile oxidizers can be investigated by testing [157]. Here, a predetermined quantity of sawdust (12 to 50 mesh) is added to a reaction vessel and brought to the desired test temperature. The liquid oxidizer is then cautiously injected with a long hypodermic syringe into the vessel. The extent of reaction is determined from continuous temperature measurements and by visual observations. 

An improved method for determining the AIT of solids has been described, and the effect of catalytically active inorganics on the reactivity and ignition temperature of solid fuels has been studied. Sodium carbonate markedly lowers the ignition temperatures of coal and coke [7], The volume of the vessel (traditionally a 200 ml flask) used to determine AIT has a significant effect on the results. For volumes of... 

The MIE of gas — air or vapour—air mixtures can be determined from the structural formula and the molar heat of combustion of the compounds studied, and equations for the calculation are presented. The method is stated to give more accurate results than conventional methods used to assess flammability of mixtures of gas or vapour with air [1], It is claimed that in oxygen MIEs are about a hundredfold lower than in air [2], A study of the ignition behaviour of dusts, including correlation of electrical and mechanical minimum ignition spark energies and ignition temperature is made [3],... 

Thomas, T. J. et al., Amer. Inst. Aero. Astron. J., 1976, 14, 1334-1335 Ignition temperatures were determined by DTA for the perchlorate salts of ethylamine, isopropylamine, 4-ethylpyridine, poly(ethyleneimine), poly(propyle-neimine), and poly(2- or 4-vinylpyridine). In contrast to the low ignition temperatures (175-200°C) of the polymeric salts, mixtures of the polymeric bases with ammonium perchlorate decompose only above 300°C. 

Heat of combustion, thermal conductivity, surface area and other factors influencing pyrophoricity of aluminium, cobalt, iron, magnesium and nickel powders are discussed [4], The relationship between heat of formation of the metal oxide and particle size of metals in pyrophoric powders is discussed for several metals and alloys including copper [5], Further work on the relationship of surface area and ignition temperature for copper, manganese and silicon [6], and for iron and titanium [7] was reported. The latter also includes a simple calorimetric test to determine ignition temperature. 

Various kinds of information can be expected from the high pressure combustion and flame experiments Reaction kinetics data for conditions of very high collision rates. Results about combustion products obtained at high density and with the quenching action of supercritical water, without or with flame formation. Flame ignition temperatures in the high pressure aqueous phases and the ranges of stability can be determined as well as flame size, shape and perhaps temperature. Stationary diffusion flames at elevated pressures to 10 bar and to 40 bar are described in the literature [12 — 14]. 

Differential thermal analysis (DTA) has provided a wealth of information regarding the thermal behavior of pure solids as well as solid mixtures [10]. Melting points, boiling points, transitions from one crystalline form to another, and decomposition temperatures can be obtained for pure materials. Reaction temperatures can be determined for mixtures, such as ignition temperatures for pyrotechnic and explosive compositions. 

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