FR2706449A1 - Pyrotechnic composition for delay cords. - Google Patents

Abstract

Pyrotechnic composition intended for the production of delay fuses, of the type consisting of a powder mixture of tungsten, barium chromate and potassium perchlorate, characterised in that it contains an ultrafine silica powder whose particles have a mean diameter of between 7 and 40 millimicrons, according to a mass percentage of the order of 0.5 to 1.5 %, to produce a coating of the powder particles.

Description

The present invention relates to a pyrotechnic composition intended for the

  manufacture of pyrotechnic delays and, more particularly, delay cords. Pyrotechnic delays are well known to those skilled in the art and delays have already been proposed comprising tungsten powder, barium chromate, potassium perchlorate and silica (diatomaceous earth). The role of the latter is to slow down combustion, by capturing calories, and to favor

  the coating of the various grains. Thus, the US-A patent

  4,144,814 describes a composition of this type which can be used in the form of a compressed tablet containing 5% by mass of silica. This composition can be advantageous to use when it is a detonator into which one wishes to introduce a certain delay by interposing a compressed tablet as is the case of the aforementioned patent, but is not suitable when it is a delay line. Indeed, a delay cord consists of a sheath of small diameter (less than 4.1 mm) inside which is placed the pyrotechnic composition having one or more elbows. The compositions described above, although called compositions without CWnez d; Cnn AR y; s7 - nrnrAllî iqtnt- t-nltRI m $ ms lin 1 'amr 3ci A cTfam e nr I n e-P 1 s Anm-l ^ n rt ab s, IT

  the outside of the sheath after the latter burst.

  The risk of bursting is further accentuated in the zones of least resistance of the sheath such as bends and generally causes the combustion to stop. This composition is also not suitable when a very low combustion rate (less than 2.5 mm / s) is sought, especially when used at macromolecular temperatures such as nitrocellulose. This particle size essentially modifies this aspect to facilitate handling and in particular to improve the fluidity of the powder obtained. However, this addition of binder inevitably produces a secondary effect on the reaction phenomenon, since the combustion mode is disturbed. In addition, by participating in the pyrotechnic phenomenon, this binder constitutes a source of gaseous products (H20, C02, etc.) which is added to the already existing release. Risks of bursting of the cord sheath

  are therefore increased and consequently the combustion stops.

  The risk of accelerating combustion also exists, hot gases propagating between cracks: the delay

is no longer reliable.

  The object of the present invention is to provide a pyrotechnic composition which can be used in small diameter delay cords and which provides a low combustion speed, eliminating any risk of bursting of the

combustion sheath and stop.

  The subject of the invention is a pyrotechnic composition intended for producing delay cords, of the type consisting of a mixture of tungsten powders, barium chromate and potassium perchlorate, characterized in that it contains a silica powder. ultrafine, the particles of which have an average diameter of between 7 and 40 millimicrons, according to a percentage by mass of the order of 0.5 to 1.5%, in order to coat the particles of the powders. The silica particles have a diameter

average of 16 millimicrons.

  The composition can comprise 1 part by mass of silica with a mixture containing by mass 33% of tungsten, 60% of barium chromate and 7% of potassium perchlorate. The composition can comprise 1 part by mass of silica with a mixture containing by mass 32% tungsten, 58% barium chromate and 10%

potassium perchlorate.

  The composition according to the invention can be applied to the production of delay cords of 3.1 mm external diameter approximately, having at least one bend at 900. An advantage of the present invention lies in the absence of rupture of the sheath of the cord, what

  whatever the conditions of use.

  Another advantage lies in the conservation of the delay properties in a temperature range from -50 ° C. to + 60 ° C., and for speeds of

  extremely low combustion of the order of 2 to 2.5 mm / s.

  Other advantages will appear on reading

  additional description given by way of example.

  The composition according to the invention comprises the same constituents as the delay composition described in the aforementioned patent. These are tungsten powder, potassium perchlorate, barium chromate and silica. However, to obtain the results of the invention, an ultrafine silica must be used, the particles of which have an average size of 7 to 40 millimicrons and according to a percentage by mass ranging from 0.5 to 1.5% of the total composition. The silica thus used plays the role of a coating material for the other components, which leads to a reduction in internal confinement of the composition by a reduction in its density. It is under these conditions that the results

  particularly surprising are achieved.

  The use of 1% by mass of silica made up of particles of average size of 16 millimicrons gives complete satisfaction. Thanks to this extreme finesse, we achieve a perfect coating of the mixture

  initial pyrotechnic substances.

  The coating technique is known in itself and is carried out in a conventional manner. It is carried out dry, for example in a mixer sold under the

commercial designation "TURBULA".

  The application of the mixture obtained in delay cords intended for long time delays makes it possible to carry out less confined loads. Degassing during combustion is facilitated, which improves combustion reliability even at low temperatures. In fact, the main pyrotechnic effects (combustion speed and reaction energy) are only modified insignificantly, since the coating material, which is an inert and stable mineral constituent, is introduced into the composition in very small quantities. It does not participate in the combustion reaction and its low employment rate practically does not modify the energy potential of the

composition.

  In terms of implementation, a particularly effective final product is obtained. Indeed, ultrafine silicas are mainly used as anti-caking agents and their properties are remarkable

  to improve the fluidity of powdered products.

  Introduced in the form of coating material in the pyrotechnic compositions, they make it possible to carry out loading in automatic hoppers on relatively simple installations. The industrial production rates then become very important. Table I, given below, shows for two known compositions and two compositions according to the invention, the reaction energies and the combustion rates. The percentages given are percentages by mass. The silica used is sold commercially under the

designation '"AEROSIL".

  COMPOSITION A: 33% tungsten powder,% barium chromate,

7% potassium perchlorate.

  COMPOSITION B: Composition A + 1 part of silica, the particles of which have an average size of

16 millimicrons.

  COMPOSITION C: 32% tungsten powder 58% barium chromate,

% of potassium perchlorate.

  COMPOSITION D: Composition C + 1 part of silica, the particles of which have an average size of

16 millimicrons.

TABLE I

COMBUSTION SPEED ENERGY

  REACTION COMPOSITIONS (mim / s) (cal / g)

A 258 2.23 2.22

B 248 2.25 2.22

C 295 2.19 2.24

D 291 2.20 2.23

  This table shows the negligible influence of ultrafine silica on the pyrotechnic performances of

  pyrotechnic compositions B and D according to the invention.

  Table II, given below, shows for known compositions and compositions according to the invention the effects on reliability by determining the bursts

and combustion stoppages.

  COMPOSITION E: Basic composition 33% tungsten powder, 62% barium chromate,% potassium perchlorate with

  addition of 3 parts of nitrocellulose.

  COMPOSITION F: Basic composition 33.5% of tungsten powder, 61.5% of barium chromate,% of potassium perchlorate with addition of 3 parts of fluorinated copolymer of commercial designation VITON A. The tests are carried out at - 50 C, in 3.1 mm cords with 2 90 ° elbows:

TABLE II

O%% SPEED OF

  COMBUSTION SHUTDOWN COMPOSITIONS

COMBUSTION MENT (mm / s)

A 10 0 2.23

B 0 0 2.25

C 10 0 2.19

D 0 0 2.20

E 100 100 -

F 40 10 3.09

  It can be seen that the bursting percentages reach 10% for the known compositions A and C whereas they do not exist for the compositions B and D according to the invention, although there is no combustion stop

  for similar combustion speeds.

  The results of composition E, which has a conventionally used binder, are very poor. All

  cords burst with stopping combustion.

  The results of composition F which has a conventional binder show a high percentage of bursts, combustion stoppages, and a speed

  relatively high combustion.

Claims (5)

  1. Pyrotechnic composition intended for the production of delay cords, of the type consisting of a mixture of tungsten powders, barium chromate and potassium perchlorate, characterized in that it contains an ultrafine silica powder, the particles of which have an average diameter of between 7 and millimicrons, according to a percentage by mass of the order of 0.5 to 1.5%, to achieve a coating of the particles powders.   2. Pyrotechnic composition according to claim 1, characterized in that the particles of   silica have an average diameter of 16 millimicrons.   3. Pyrotechnic composition according to claim 1 or 2, characterized in that it comprises 1 part by mass of silica to a mixture containing by mass   33% tungsten, 60% barium chromate and 7% potassium perchlorate.   4. Pyrotechnic composition according to claim 1 or 2, characterized in that it comprises 1 part by mass of silica to a mixture containing by mass   32% tungsten, 58% barium chromate and 10% potassium perchlorate.   5. Application of a pyrotechnic composition   according to any one of claims 1 to 4 to the production of delay cords of 3.1 mm in diameter   external approximately, with at least one 90 bend.