Propellants, Explosives, Pyrotechnics 19, 87-89 (1 994) 87

Explosive Properties of the Compositions CC14 + A1 + ZnO M. Syczewski

Division of Highenergetic Materials, Department of Chemistry, Warsaw University of Technology /Politechnika/, ul. Noakowskiego 3, PL-00-664 Warszawa (Poland)

Sprengstoffeigenschaften einer Mischung von CCIdAIIZnO Die erforderlichen Mischungen aus CC14, A1 und ZnO wurden dar-

gestellt und ihre Detonationsfahigkeit untersucht. Die Abhangigkeit der Detonationsgeschwindigkeit (D) von der Ladungsdichte (p) wurde ermittelt. Die Zunahme der Dichte oberhalb 1,8 kg/dm3 fiihrt zu einem schnellen Verschwinden der Detonationsfahigkeit. Bei einer Dichte von 1,3 kg/dm3 wurde die Partikelgeschwindigkeit gemessen. Die auBerordentlich niedrigen Detonationsparameter werden erklan durch die besondere Eigenart der Detonationsprodukte.

Proprietes explosives d'un melange de CCI4/AI/ZnO Le composant CC14 + A1 + ZnO a Cte convenablement prkpart et sa

capacitC de detonation a et6 Ctudite. On a determint une correlation entre la vitesse de detonation (D) et la densite de charge (p). L'aug- mentation de la densite au-dell de 1,8 kg/dm3 entraine une rapide disparition de la capacite de detonation. On a mesure la vitesse des particules pour une densite de 1,3 kg/dm3. On explique les paramktres de detonation extraordinairement bas par le caractkre particulikrement specifique des produits de detonation.

Summary

The compounds CCll + A1 + ZnO were suitably prepared, so that its ability to detonate could be investigated. The detonation velocity (D) dependence on the charge density (p) has been established. The increase of density above 1.8 kg/dm3 leads to quick disappearence of detonation ability. At a density 1.3 kg/dm3 particle velocity was meas- ured. Extraordinarily low parameters of detonation are explained by the special peculiarity of the explosion products.

1. Introduction

The pyrotechnically produced obscurring smoke, based on mixtures of highly chlorinated substances, metals and zinc oxide, is a common feature of the modern military arsenal and has been so for many years. Usually, the smoke producing reaction is that of a slow metal burning in the chlorine rich environment generating volatile metal chlori- des. These chlorides are thrown out into the atmosphere where they condense and nucleate, incorporating atmosphe- ric water to gain an aggregate size that ensures visible obscuration.

In the present investigation, the above mentioned compound's ability to explosive conversion has been exam- ined.

Because explosive properties are better when the compo- sition has a slurry consistency, a liquid substance (CC14) has been used for a highly chlorinated substance. Used zinc oxide absorbed the liquid ingredient, and created dough- like mass with void volume (bubbles) in it.

According to McLain(1) Al is a main heat source in these compositions. Reaction which occurs in the investigated mixtures can be written as follows:

4 A1 + 6 ZnO + 3 CC14 3 2 A1203 + 6 ZnC12 + 3 C (1)

In the thermite type reaction, aluminium powder and zinc oxide furnish initial heat in this process. The reactions of Zn with CC14 produce volatile zinc chloride as the major constituent of the smoke. Such smoke always contains some other products such as CO, A1C13, organic chlorinated substances and others(2-3). Relatively little research has been published, in accessible literature, up to now, describing the explosibility and sensitiveness to detonation of this type of smoke generators. The explosive conversion of such a mix- ture(4) may assure a very quick generating of the smoke screen.

This paper describes the preparation of samples, their ability of explosive conversion as well as the magnitude detonation velocity in definite conditions. Moreover, the parameters of the detonation wave front were estimated through the electromagnetic method.

2. Experimental

Mixtures were prepared as required, directly before use, in order to avoid tetrachloromethane evaporation. At the beginning, the dry ingredients were mixed by hand-flaked aluminium (specific surface area 4500 cmYg) and zinc oxide (sieved through the mesh 100 pm); then tetrachloro- methane was added and carefully stirred (not kneaded) as long as all of the mixture was completely moist. When the mixing was finished the composition was loaded into a tightly closed vessel or it was used for cartridging. The polyvinylchloride cartridge case (tube) was filled up with the composition by hand tamping to suitable density. The cylindrical explosive charges in the polyvinylchloride tube, about 35 mm in diameter (d,) and 160 mm in length, were initiated by a blasting cap No. 8.

0 VCH Verlagsgesellschaft, D-6945 1 Weinheirn, 1994 0721-3 115/94/0204-087 $5.00+.25/0

88 M. Syczewski

goo

800

700

600

m - 400

Propellants, Explosives, Pyrotechnics 19,87-89 (1994)

- .

-

-

-

The preliminary, qualitative estimation of the detonation ability was confirmed on the basis of the press sign after detonation of the explosive charges placed on the steel plate (0.5 mm). For the exact investigations an optimal composi- tion has been chosen - the most sensitive to explosion of the blasting cap.

The detonation velocity (D) was measured on three sec- tors of the explosive charge and the arithmetical mean was taken into consideration if the separate values did not differ more then 10%. The detonation velocity was received as a characteristic value for a given average density of the com- position. The particle velocity was measured in the analo- gous explosive charge (for density = 1.3 kg/dm3) through the electromagnetic methods(5). The expansion produced in the lead bloc was determined by standard method.

3. Results and Discussion

The preliminary investigations proved that the detona- tion ability under the used conditions exists in wide ranges of varying amounts of ingredients used:

A1 from 3% to 70% CC14 from 15% to 55% ZnO from 5% to 65%.

On the basis of the preliminary investigations the optimal composition was chosen. It contains:

A1 - 11.4% ZnO - 39.2% CC14 - 49.4%.

Decompositions of the mixture during burning may be represented by the following equation(1):

4.2 A1 + 4.8 ZnO + 3.2 CC14 + 1.6 A1203 + 1.0 AlC13 (2)

The approximate heat of combustion, calculated for this mixture, is about 2390 kJ/kg. The detonation velocity is almost unchanged when the loading density is changed from 1.2 kg/dm3 to 1.75 kg/dm3.

At a density above 1.75 kg/dm3 the detonation velocity drops down suddenly, and at a density about 1.8 kgIdm3 composition does not detonate even where initiated by a blasting cap No 8 with 5 g RDX.

Compound at a density 1.8 kg/dm3 has a porosity of about 45%. In such explosive materials the shock ignition is achieved by “hot-spots’’ mechanism. In this mechanism the local heating in a solid material is achieved by the collapse of individual pores. The relationship between the detona- tion velocity and density (Fig. 1) differs from typical dependences in similar explosive mixtures. As a rule in explosive mixtures, detonation velocity at first increases and then falls with increasing density. According to the hot- spot mechanism, when the total contents of the void volume (bubbles) in the composition is big (small density), the rela- tionship between the detonation velocity and density is sim- ilar to that of high explosives (higher concentration energy - higher D). However, when the content of the bubbles is small (at the right of the maximum D) reciprocal influence

+ 4.8 ZnC12 + 3.2 C

x

r(

Figure 1. Detonation velocity versus density for investigated compo- sitions.

of the reaction in the separate hot-spots is small (hot spots are separated) so not all of the mixture reacts (D decrease). When the reaction in each hot-spot does not influence a reaction in another and the reaction in the separate hot-spot does not fulfil critical conditions in which the reaction self accelerates (according to the theory of Frank-Kamenetski) then detonation does not occur.

In our case, the aboriginal increase in density, has no effect on the detonation velocity and it suggests that an increase in this area D is levelled off by a not full reaction of the composition. In this case the dependence D=f(p) should prove that D<Di (Di - ideal value D) and d,<di (di - a bordering charge diameter). If even at such big d, detona- tion velocity is not ideal, the reaction zone has to be wide (long time reaction)(@. Actually, the measurement of the particle velocity has proved the long duration of the chemi- cal reaction.

The mild accrue of the particle velocity on the shock wave front may prove that wave front is rough (not smooth). (The inertia of the aluminium foil indicator gives considerably smaller accrue time).

Figure 2. Typical record of the particle velocity [horizont. ( 2 ps/div); vert. (200 m/s/div)].

Propellants, Explosives, Pyrotechnics 19, 87-89 (1994)

From records (Fig. 2) the following have been estimated: mass velocity and pressure in the Chapman-Jouguet plain: UcJ = 360 m/s; PcJ = 5 17 MPa; width of the reaction zone: a = 1.73 mm; chemical reaction time: T* = 2.80 ps; approximate value of the pressure and mass velocity on the shock wave front: P,,, = 1170 MPa; U,,, = 791 m/s; exponent polytropic explosion products - kc, = 1.97, as well as D = 1070 m/s. Using experimental relation(7):

- = & K D

Di dc

and substituting D, a and d, the value Di = 1130 m/s was obtained. It shows that conditions of the detonation were not far from ideal.

Extraordinarily small detonation velocity of the investi- gated composition seems to be explained mostly by the big average molecular weight and condensing processes in the detonation products rather than by non ideal detonation. The detonation velocity of the common heterogenous explosives as well as primary explosives, (with the compar- ative explosion heat), even in the smallest density, is mark- edly bigger than in the investigated composition. The aver- age molecular weight of the explosion products of the investigated composition according to the reaction (2) is bigger than 130; for the mentioned primary explosives M<70. According to the relation D - m, detonation velocity of the investigated composition, only on account of it, must be 1.5 times smaller than for some primary explo-

Explosive Properties of the Compositions CC14/Al/ZnO 89

sives. Moreover, value of the expansion produced in the lead block (150 cm3), low value of the pressure in the deto- nation wave, quick reduction pressure in the reacted zone (PB/PH=2.265) and uncommon value relation D/UH=2.97, might be signs of condensing processes in the reacted zone of the detonation wave. Approximate computations of the explosion temperature show that only part of A1203 may exist as a gaseous product.

Some of the above conclusions have, for the present, qualitative form and require more precise investigations, nevertheless, up to now experimental results seem not to deny them.

4. References

J.H. McLain, “Pyrotechnics from the Viewpoint of Solid State Chemistry”, The Franklin Institute Press, 1980, pp. 63-69. H. Ellem, “Military and Civilian Pyrotechnics”, Chemical Pub- lishing Comp., New York, 1968. F.R. Hartley, S.G. Murray, M.R. Williams, and D.B. Jones, “Smoke Generators”, Propellants, Explos., Pyrotech. 7, 12 (1982) (part I); 9, 1 (1984) (part 11); 9, 39 0984) (part 111). M. Syczewski, J. Grochowski, and K. Kowalski, Patent PRL 138810 (1988), WAT, Polska. F.A. Baum, A.P. Orlenko, K.P. Staniukowich, W.P. Czeslyszew, and B.J. Szechter, “Fizika Wzrywa”, Izd. “NAUKA”, Moskwa

R. Chaiken and J. Edwards, “Screening Model of the Heat-pro- ducing Kinetics during the Detonation of Heterogenous Explo- sives”, Symposium H.D.P., Paris, 27-31 August 1978. H. Eyring, R.E. Powell, G.H. Duffy, and R.B. Parlin, “The Stabil- ity of Detonation”, Chem. Rev. 45,69-181 (1949).

1975, pp. 208-258.

(Received July 24, 1992; Ms 50/92)