High luminosity infrared pyrotechnical composition

[57] ABSTRACT

A high-luminosity pyrotechnical composition comprising, in finely divided state, an illluminant substance of high emissivity in the infrared band, at least one halogenated resin with fluorine atoms constituting at least two-thirds of the total number of halogen atoms and a metal which is highly reductive with the halogen atoms of the resin. The composition is in the form of an agglomerate mixture with the amount of reducing metal being between that stoichiometrically required for the reduction of the halogen atoms and an excess of 20 percent in relation to the stoichiometrical determination. The composition may optionally include a binder and it is press molded to shape the composition as a body. The composition may also contain barium stearate to regulate emission time.

Claims, 1 Drawing Figure

PATENTED m 61873

3.770.525

POWER LEVEL

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3,770,525

HIGH LUMINOSITY INFRARED PYROTECHNICAL COMPOSITION

DETAILED DESCRIPTION

The present invention relates to a high-luminosity pyrotechnical composition which can be used for the production of rockets and similar tracers.

The invention also relates to the process of manufacture of such pyrotechnical compositions.

Pyrotechnical compositions are generally mixtures of combustible substances with metallic oxides or salts selected for the required effect such as a luminous effect obtained when such metallic oxides or salts are raised to incandescence at the instant of combustion of the composition.

Some of such compositions are especially used in tracer devices located in back of rockets and similar vehicles for providing emission of radiance allowing the trajectory of such vehicles to be tracked. It is important that such radiation should be powerful and sustainable 20 as long as possible.

Experience has shown that such tracers exhibit very appreciable power losses in flight in comparison with the results obtained when measurements are taken at the datum point. Efforts have been made to reduce 25 such disadvantages by improvements of a mechanical nature.

In particular, it is known to arrange deflectors on the tracers so as to keep the apparent diameter ofthe flame constant; also it is known to employ an extrusion cowling around the flame so as to create a zone of lesser turbulence thereat.

It is further known to mount a nozzle at the tracer device outlet forming a convergent-divergent assembly which imparts sufficient velocity to the incandescent particles for them to cross the turbulent zone and reach an area where the propellent-gas and air run side by side to form an extension of the flame and thus render it more visible.

An object of the present invention is to eliminate the aforesaid disadvantages but by another means consisting of a new formulation of the pyrotechnical composition which propagates an emissive power much greater in comparison to that obtained from known pyrotechnical compositions.

According to the invention, the high-luminosity pyrotechnical composition is characterized by containing in finely divided state an illuminant substance of high emissivity in the infrared band, at least one fluorinated resin having fluorine atoms which make up at least twothirds of the total of the halogen atoms, and a metal which is highly reductive in the presence of the resin halogens, the composition being in the form of an agglomerate mix in which the amount of the reducing metal is between that required stoichiometrically and an excess of 20 percent over such stoichiometric amount.

Fluorinated resins are known to decompose at a high temperature in the range of 300° to 400° while liberating gases containing the resin halogens. These halogens, and fluorine in particular, are powerful oxidants and react with numerous metals with a strongly exothermic oxido-reduction reaction.

Experience has shown that at the start of pyrolysis of a fluorinated resin mixed with a reducing metal and an illuminant substance, by means of an electrically ignited primer paste capable of radiating a temperature of the order of 330° C, the gases liberated by such resin combine violently with the reducing metal on contact. Through the heat liberated at the onset of such reaction, pyrolysis continues independently inside the pyro5 technical composition until the resin is completey consumed. The combination of resin and metal operates advantageously as a heat generating unit wherein the resin is consumed and at the same time it allows the particles of illuminant substance dispersed through the 10 composition to be raised to the incandescent state.

In a preferred procedure the amount of reducing metal is experimentally calculated with an excess from 10 to 15 percent of the amount required for complete reduction.

The amount of illuminant substance is selected in the range of 2 to 10 percent by weight of the pyrotechnical composition.

According to one preferred embodiment of the invention, the pyrotechnical composition also comprises barium stearate in the ratio of two to 15 percent by weight of the composition. This addition noticeably affects the fundamental reduction reaction time and through the lubricating properties of the stearate promotes the forming of the composition as well as its molding at high temperature.

The pyrotechnical composition can further contain a binding agent composed of a polymerized polyester resin in a ratio not greater than seven percent by weight of the composition.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE of the drawing is a graphical representation of emission for various compositions.

The various constituents of the pyrotechnical compo³⁵ sition will now be described in greater detail.

The fluorinated resin is preferably polytetrafluoroethylene. However, other fluorinated resins can also be used, such as polytrifluorochloroethylene, polyvinylidene fluoride, a copolymer of tetrafluoroethylene and hexafluoropropylene and a copolymer of vinylidene fluoride and hexafluoropropylene.

Whenever possible these resins are used in finely powdered form. In other cases, for instance, with the 45 copolymer of vinylidene fluoride and hexafluoropropylene which is an elastomer, the resin is used in the form of particles, as finely divided as possible.

The resins can be used singly or mixed together.

According to the invention, the reducing metal is chosen from the group consisting of magnesium, silicon, zirconium, aluminium and chromium.

All these metals, selected because of their particular affinity for the fluorine and possibly the chlorine contained in the resin are used in the form of a fine powder.

The illuminant substance is preferably an oxide of a metal chosen from the group consisting of iron, nickel, copper and hafnium.

Those metals and their oxides are expediently employed in the form of exceedingly fine-grained powders.

Market quality barium stearate is also used in powder form.

The polymerized polyester resin, before being put to use, is preferably composed of a liquid unsaturated resin mixed with monomeric styrene. At the moment of use, it is treated with a catalyzer which allows the polymerization to be primed at a moderate temperature.

3,770,525

Beside the catalyzer, it can also be treated with an accelerator which will allow such polymerization at ordinary temperature.

The pyrotechnical composition according to the invention can be obtained by press-molding a mixture of powders of the illuminant substance, the reducing metal, at least one fluorinated resin and the barium stearate.

When one or more of the fluorinated resins cannot be reduced to powder, such resin is put into a solvent beforehand to be dissolved therein and then the mixture of the other powdered constituents of the composition is impregnated with such solution; then, the impregnated mixture is dried.

Preferably, when at least one of the fluorinated resins is polytetrafluoroethylene, molding is effected at a temperature around 320° C by the method known as ‘fritting.’

Before molding, all of the ingredients except the fluorinated resin or resins are dried for an hour-and-a-half atl50°C.

The binding agent for the pyrotechnical composition is here composed of polytetrafluoroethylene.

According to a second equally preferred method, when for reasons of convenience or safety, it is desirable to avoid bringing the mixture to be molded to a high temperature, the molding is carried out at ambient temperature or at a temperature slightly higher, the components of the mixture being the powdered ingredients and a non-polymerized polyester resin; then the molded composition is allowed to stand whereby the polymerization of the resin is achieved.

Before molding, all of the powdered ingredients except the fluorinated resin or resins are dried as before.

The molding mixture is then completed by adding the fluorinated resin to the polyester resin and adding in succession, to the fluorinated resin thus coated, first the reducing metal and barium stearate, and then the illuminant substance.

The pyrotechnical composition binder is here made up of the polyester resin.

Pyrotechnical compositions according to the invention, comprising polytetrafluoroethylene (PFTE), were manufactured according these two methods of press molding, as indicated herein after.

1. Hot Molding

The ingredients other than PTFE were heated in a stove. After heating, all the ingredients were mixed, then loaded into a mold placed in a press. Then, the mixture was raised to a temperature of about 320° C and compressed to a pressure of 1,500 bars.

2. Cold Molding

The ingredients other than PFTE were heated and the mixing of the ingredients was effected as follows:

a. premix of PTFE with liquid polyester resin, b. addition of magnesium and barium stearate to the premix (a),

c. addition of ferric oxide to intermediate mixture (b).

The final resultant mix was dried for at least 12 hours and then placed into a mold on a press bed and finally compressed at 800 bars pressure. Then the molded composition was left for at least 15 days for completion of the polymerization of the polyester resin.

All of the pyrotechnical compositions, molded as just described, were formulated in the same way. They were then placed in casings of corresponding form and the molded compositions were tested in trials. It was found that the total energy supplied by a tracer made according to the invention was approximately twice that furnished by a tracer of equal weight, equal emissive surface and equal combustion time made by the conventional method.

Furthermore, the efficiency of producing pyrotechnical compositions according to the invention with variable amounts of barium stearate was established in trials, and the results are shown by the curves in the drawing wherein the combustion times t in seconds are along the abscissa, and the light-flux power levels in watts per steradian are shown along the ordinate (P axis).

Four series of 10 tests were made with compositions comprising an amount p of barium stearate respectively equal to zero percent, three percent, nine percent and 13 percent and complying with following formulation: Polytetrafluoroethylene 30 g, Magnesium 45 g (40 to 80 micron grain size), Fe₃O-i 7,5 g, Barium stearatep g, Polyester resin 5 g. The mean emission-curves were determined for each of these series.

The emission curve for the first series not containing any barium stearate and not shown in FIG. 1, exhibited a sharp peak for a combustion time of two seconds.

Emission curve A for the second series comprising three percent barium stearate, exhibited two maxima at 1950 w/stearadian and 1,850 w/sr over a doubled combustion time, namely four seconds.

Emission curve B for the third series comprising 9 percent barium stearate exhibited a maximum power level at 1180 w/sr extending over a time of 4.7 seconds whereas the entire combustion time was considerably longer than in the two preceding series, viz., 5.2 seconds.

Emission curve C for the fourth series comprising 13 percent barium stearate differed from the third set by having a slightly sloping, but longer, plateau corresponding to maximum power levels from 900 to 860 w/sr and a substantially longer combustion time of 13.3 seconds.

These tests clearly demonstrate the regulative effect of the barium stearate on the combustion time. An amount of barium stearate between two and four percent according to the invention produces very high maximum power levels of the order of those shown in emission curve A, having at the same time combustion times of adequate duration.

These tests also show that for equal weight of combustion, equal combustion time and equal emission area, the pyrotechnical compositions, according to the invention, offer the outstanding technical advantage of having twice the power obtainable with pyrotechnical compositions of known types.

Furthermore, since rocket tracking stations are usually equipped with infrared ray receivers, the pyrotechnical composition, according to the invention, is eminently adapted thereto, because of the localization of its emissive power, for use in rockets and similar tracers.