The self-accelerating decomposition temperature (SADT) of solids of the quasi-autocatalytic decomposition type1

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Abstract

Self-heating or thermally explosible chemicals are divided into two large groups, the thermal decomposition or TD type and the autocatalytic decomposition or AC type. The TD type is further divided into liquids, for each of which the critical temperature for thermal explosion or Tc is calculated by applying the Semenov equation, and, solids, for each of which the Tc is calculated by applying the Frank–Kamenetskii or F–K equation. On the other hand, the AC type is further divided into solids of the quasi-AC type, in each of which the exothermic decomposition reaction occurs almost simultaneously with the endothermic melting, and, liquid or solid high explosives of the true AC type, in each of which the self-accelerating exothermic decomposition reaction begins after an autocatalyst has appeared and accumulated. Neither of the two critical conditions for thermal explosion, i.e., the Semenov equation or the F–K equation, is applicable to the self-heating behaviour of each individual chemical of the AC type for the calculation of the Tc. Instead, the respective self-heating behaviour of them is in accord with the concept of the self-accelerating decomposition temperature (SADT) in principle. The SADT of each individual solid of the quasi-AC type discussed in this paper as well as the critical temperature for self-heating or Tcs of each individual high explosive of the true AC type is determined by the isothermal storage test. In this study, the respective SADTs of five solids of the quasi-AC type were determined using a simple and safe isothermal storage testing device. The SADT thus determined ranged from 28.4°C for 95% 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile) to 86.5°C for 98% 1,1′-azobis(cyclohexanecarbonitrile). These temperature values are in fair agreement with a few corresponding values of the SADT measured by other researchers for them of large quantities.

Section snippets

Relative position that solids of the quasi-AC type occupy amongst the whole family of self-heating chemicals

The self-heating or thermally explosible behaviour of each individual self-heating chemical is related closely to the appearance of the thermogravimetry–differential thermal analysis (TG–DTA) curve which it yields. Thus, one glance at the TG–DTA curve of each individual solid is enough for us to infer the self-heating behaviour to be either the TD type or the quasi-AC type and hence to infer the critical condition for thermal explosion applied to calculate the Tc to be either the Semenov

Structure and performance

A cross-section of the device is presented in Fig. 8. The base of the device is a heater and a thermoregulator constituting an aluminium block bath (the trade name: Dry thermounit DTU-1B and -1C) manufactured by Taitec, Koshigaya, Japan. An aluminium block, 107 mm×107 mm×79.6 mmH, an aluminium lid and a glass closed cell having an inside volume of about 2 ml were newly prepared. The latter cell has fundamentally the same structure as that used for an adiabatic self-heating process recorder [2].

Experimental

(1) First, let us take Lauroyl peroxide (LPO) as an example to illustrate how to select the Ts in the isothermal storage test for a specific solid of the quasi-AC type. A DTA curve of LPO is given in Fig. 9. The melting point of LPO is 48°C, and the EOT appears to be 59°C. The latter temperature is, however, too high and dangerous at which to perform the isothermal storage test for LPO. Consequently, in practice, tests have to be performed at temperatures below the endothermic peak temperature,

Results

A list of the five solids of the quasi-AC type tested is given in Table 3. The respective SADT values of them, each of which was determined in the same manner as done for LPO, are summarized in Table 4 together with a few corresponding values of SADT measured by other researchers for these solids of large quantities, as well as the other reference temperatures, for comparison.

The respective value of SADT determined in this study in general is in fair agreement with the corresponding value of

The value of Ts of the respective solid of the quasi-AC type converges to its lowest, limiting or true melting point on the low temperature side

As mentioned in (7) of Section 3, once the empirical formula (Eq. (2)) has been established for the respective solid of the quasi-AC type, it is possible to calculate an arbitrary value of Ts, as well as the SADT, at which the solid will melt completely and start to self-heat after an arbitrary exposure time. In Fig. 12 are given two values of Ts, 46°C and 44.9°C, on the endothermic peak of LPO. Each of them denotes the temperature at which LPO will melt completely and begin to self-heat 30

Some comments on the isothermal storage testing device

It has been derived above that the proportion which solids of the quasi-AC type occupy amongst the whole family of self-heating chemicals is considerably large.

We have no choice in the present situation but to determine the respective SADT of solids of the quasi-AC type or the respective Tcs of high explosives of the true AC type by experimental measurements, i.e., by performing an isothermal storage test such as the U.S. SADT test, until a new method of calculation is established in the

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This article is taken in part from a paper, `SADT values of solid chemicals of the pseudo-autocatalytic decomposition type', which was presented by T. Kotoyori at an annual meeting of the OECD-IGUS energetic and oxidising substances working group held at the TNO, Rijswijk, Netherlands, 22 May 1996.

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