EP0116795A1 - Combustion inhibitors based on fiber containing oxygenated polyurethane elastomers for double base propellants - Google Patents

Abstract

A combustion inhibitor intended to partially cover the surface of double base propellant blocks, such as those used in the propellants of tactical missiles. The polyurethane elastomer comprises in chemically combined form an aliphatic constituent polyesterpolyol and / or polyetherpolyol containing 2 to 6 hydroxyl groups per molecule, and an aliphatic polyisocyanate. In order not to release combustion gases which would obscure the rear area of the missile and prevent its identification and guidance by the firing station, the inhibitor comprises from 0.1 to 10 parts by weight of thermostable mineral or organic fibers of length between 0.1 mm and 15 mm per 100 parts of inhibitor.

Description

The present invention relates to new combustion inhibitors for dual base propellants comprising a polyurethane elastomer comprising in chemically combined form an aliphatic constituent, polyesterpolyol or polyetherpolyol or a mixture of the two containing 2 to 6 hydroxyl groups per molecule, an aliphatic polyisocyannate constituent , optionally a low molecular weight polyol as chain extender, the inhibitor optionally comprising a gasifiable organic filler and / or an aliphatic plasticizer.

Combustion inhibitors are materials which cover the surface of a block of propellant, apart from the surface of the combustion zone, and which protects the combustion zone from any unintentional ignition which may occur for example under the action of hot gases from combustion.

One of the main functions of combustion inhibitors is therefore to regulate the combustion of a propellant block by making it possible to maintain a regular combustion zone for the duration of the latter.

One of the essential requirements for current and future "tactical" missiles is their ability to be guided from any firing point. This implies that the combustion of the propellant block allowing the movement of the machine does not does not give off gases which would obscure the rear area of the machine and prevent it from being spotted and guided by the shooting station. This brings us to the concept of "discretion" understood in the sense of transparency with respect to guide waves (infrared, visible).

Double base propellants are by nature "discreet", that is to say that their combustion gases do not hinder the guidance of the vehicle.

Inhibitors, on the other hand, are the main cause of the phenomena of opacity which occur during combustion since the surface layers in contact with the propellant block emit fumes which interfere with guidance.

This phenomenon is further accentuated by the fact that the nitroglycerin, present in the double base propellant, tends to migrate into the inhibitor, when the latter has certain affinities with it, which further increases the combustion of the latter and therefore the emission of smoke. The migration of nitroglycerin also has the drawback of peeling off the propellant block inhibitor, which causes detrimental combustion irregularities when fired.

• - résistance à la migration de la nitroglycérine
• - non émission de fumées obscurcissantes lors de la combustion
• - bon collage sur le bloc de propergol

A combustion inhibitor must therefore satisfy in particular the following requirements:

• - resistance to nitroglycerin migration
• - no emission of obscuring fumes during combustion
• - good bonding on the propellant block

A first solution consists in using a material which ablates during combustion by giving off transparent gases. This solution therefore uses "gasifiable" materials under the effect of hot gases. The invention relates to this type of material and this allows it to differentiate itself from the other solution also proposed and which consists in using materials having excellent thermal resistance such as silicones or aromatic polymers and which, unlike precedents do not tarnish and therefore do not give off obscuring gases.

To produce the inhibitors according to the first solution, it is already known that the polymers constituting the material must practically not contain aromatic groups.

French Patent No. 2,275,425 illustrates an embodiment in which the inhibitor material consists of an aliphatic polyurethane based on a polyol and an aliphatic polyisocyanate and a cooling filler.

However, it is necessary, taking into account the affinity of the aliphatic polyurethane for nitroglycerin, to interpose between the inhibitor material and the propellant block a barrier layer based on triisocyanate. Due to moisture cross-linking, this forms a layer with an extremely tight mesh network which prevents the molecules of nitroglycerin from passing through.

In order to remedy this drawback, it has been proposed in patent application No. 2 444 689 to increase the degree of crosslinking of the polyurethane material by using low molecular weight polyols.

The object of the present invention is to provide a significant improvement over the previous inhibitory materials which have just been discussed as regards the migration of plasticizer (in particular nitroglycerin), discretion.

The invention is characterized in that the combustion inhibitor also comprises 0.1 to 10 parts (by weight) of thermostable mineral or organic fibers of length between 0.1 mm and 15 mm per 100 parts of inhibitor.

The presence of refractory fibers is new with respect to this type of dual base propellant inhibitor and is moreover not obvious.

Indeed, the French patent 2,275,425 already mentioned mentions the use, to improve the thermal behavior, of mineral fillers (asbestos, mica, quartz) in powder form.

French Patent 2,290,825 cites a dual base propellant inhibitor composition consisting of an elastomer and a powdery filler material constituting at least 50 _% of the total composition. It is also indicated that, preferably, the particles of the material present in the material inhibiting combustion have less than one micron. It is clear that this document far from proposing the incorporation of fibers to the skilled person would rather dissuade it.

Finally, note the French patent n ° 2 150 552 which describes a composition based on aromatic polyurethane elastomer and from 5 to 65% of fibrous material works according to the concept of non-ablatable material (see the second solution described above).

There is a big difference between the two families of inhibitors. That described in French patent n ° 2 150 552 emits a very high amount of smoke during combustion. The addition of a large amount of fiber is required by the fact that all of the inhibitor must be kept throughout the combustion because otherwise the material ablated under the effect of hot gases would produce smoke. This is confirmed by the last paragraph of French patent 2,150,552.

On the other hand, in the case of inhibitors according to the invention, the fibers do not have the function of preventing the ablation of the inhibitor but on the contrary of allowing the material to gasify on the spot by ensuring better mechanical resistance to the surface matter in the process of gasification.

By aliphatic polyurethanes is meant polyurethanes which do not contain significantly aromatic units. This obviously does not exclude that a small proportion of the patterns is aromatic, but this proportion should not exceed 10%.

These polyurethanes must be oxygenated, and are preferably constituted by the addition reaction between aliphatic polyisocyanates and polyol polyol or polyether polyol comprising 2 to 6 hydroxyl groups per molecule or a mixture of these polyesters or polyether polyol. Preferably in these polyols the ratio between the number of carbon and the number of oxygen is less than 5.

The polyurethane compositions leading to the polyurethane elastomer must preferably be pourable although shaping according to the methods of rubber manufacturers is not excluded for the compositions which require it.

The polyols preferably have a molecular weight of between 400 and 5,000 and advantageously between 500 and 3,000.

Suitable polyesters include those derived from dicarboxylic acids such as adipic, succinic or sebacic acids and low molecular weight glycols such as ethylene glycol, propylene glycol, diethylene glycol, 1,4 butanediol and 1,6 hexanediol.

They also include lactone polymers. These are polymers formed from polyfunctional initiators by successive decyclization of lactone monomers.

n étant inférieur ou égal à 4.In general, the lactones suitable for the invention correspond to the formula:

n being less than or equal to 4.

Among the polyesters of this type, mention may be made of poly-E-caprolactone or poly-γ-butyrolactone.

The adducts of these polyesters can also be used on low molecular weight polyols.

Polyethers which are suitable are in particular polyethylene glycol, polypropylene glycol, polypropylene ethylene glycol, polypropylene glycol, polytetramethylene glycol and the adducts of these polyesters on low molecular weight polyols such as trimethylololpropane, glycerol, pentaerythritol , sorbitol.

It is also possible to add to these compositions well-known chain extenders such as low molecular weight glycols (1,4-butanediol for example) or diamines.

The NCO: OH ratio is preferably equal to 1 or close to 1.

Crosslinking takes place in the presence of well known catalysts such as for example those based on tin such as dibutyl tin diacetate.

For reasons of mechanical properties, it is preferable to use polyether polyols. But it is also very advantageous to use these polyether polyols in admixture with a polylactone, in particular E _- polycaprolactone.

It is also advantageous in known manner to add an aliphatic plasticizer to the polyurethane composition such as, for example, glycerol triacetate or acetyltri-n-butyl citrate.

Up to 40 parts of plasticizer can be added per 100 parts of polyurethane.

Certain gasifiable organic fillers having a melting temperature equal to or higher than that of the polyurethane can be advantageously added.

It will be possible to add up to 300 parts of these charges, for 100 parts of polyurethanes, the only limit being the "flowability" of the composition and the minimum mechanical properties that a combustion inhibitor must have.

Mention may be made, as an indication, of the following fillers: oxamide, ammonium oxalate, ammonium carbonate, polyoxyethylene, polyoxypropylene, polyoxymethylene.

These fillers should preferably be non-hygroscopic because the presence of water interferes with crosslinking.

It is preferable to use oxygenated organic fillers having a ratio of number of carbon atoms: number of oxygen atoms close to 1, so as to favor the oxidation reactions compared to the polymerization reactions and preferably in fine particle size.

The presence of oxygen has, in fact, a positive influence in the absence of slag formation.

Oxamide will advantageously be used because of its very high melting point (approximately 400 ° C.).

The aliphatic polyisocyanate component preferably consists of aliphatic or cycloaliphatic diisocyanate.

As regards the fibers which are the subject of the invention, mention may be made, among those which are suitable, of glass fibers, carbon fibers, silicon carbide fibers, asbestos fibers and polyphenylene terephthalamide fibers sold by the company Dupont de Nemours. under the brand name "Kevlar".

The combustion inhibitor will advantageously comprise 0.3 to 8 parts (by weight) of fibers of length between 0.1 and 6 mm and preferably 0.5 to 4 parts by weight per 100 parts of inhibitor.

• On dispose concentriquement un bloc de propergol à double base dans un moule cylindrique de manière à ce qu'il y ait un interstice régulier entre la surface du bloc et la paroi interne du moule.

The implementation of the invention can be carried out as follows:

• A double base propellant block is concentrically placed in a cylindrical mold so that there is a regular gap between the surface of the block and the internal wall of the mold.

In a suitable pot, the constituents of the polyurethane, possibly the cooling charge, the plasticizer and the fibers, are mixed by means of a mixer.

Once the mixture is homogeneous, the actual inhibition is carried out, most often by an injection process.

The examples below illustrate the invention.

The tests are carried out with the following propellant composition, expressed as a percentage by weight:

I - Measuring the migration of nitroglycerin

The increase in weight of the inhibitor is measured at regular intervals by immersion of a sample of said inhibitor in nitroglycerin and the increase in weight is followed.

The following inhibitor compositions, expressed in parts by weight, were tested.

Composition 1

• - 1 _% après 24 h
• - 2,2 % après 1 semaine
• - 5 % après 6 semaines

The absorption rate is:

• - 1 _% after 24 h
• - 2.2% after 1 week
• - 5% after 6 weeks

Composition 2

Same composition as 1 except that the carbon fibers are replaced by Kevlar (R) fibers 0.5 mm long sold by Dupont de Nemours.

• - 1,1 % après 24 h
• - 2,5 % après 1 semaine
• - 5,2 % après 6 semaines

The absorption rate is:

• - 1.1% after 24 h
• - 2.5% after 1 week
• - 5.2% after 6 weeks

Composition 3

Same composition as 1 except that the diisocyanate used is 1,6 diisocyanate-2,2 ', 3-trimethyl hexane.

• - 1 % après 24 h
• - 2,8 % après 1 semaine
• - 7,1 % après 1 mois

The absorption rate is:

• - 1% after 24 h
• - 2.8% after 1 week
• - 7.1% after 1 month

Composition 4

• - 1,4 % après 24 h
• - 3 % après 1 semaine
• - 8 % après 6 semaines

The absorption rate is:

• - 1.4% after 24 h
• - 3% after 1 week
• - 8% after 6 weeks

Composition 5 - Comparative test

• - 1,8 % après 24 h
• - 3,2 % après 1 semaine
• - 9 % après 1 mois
• - 14 % après 6 semaines

Composition 1 was tested without fibers:

• - 1.8% after 24 h
• - 3.2% after 1 week
• - 9% after 1 month
• - 14% after 6 weeks

II - Fire measurement

Cylindrical blocks (h = 200 mm, Ø 90 mm) of double base propellants coated with a 5 mm thick inhibitor, obtained as described above, are ignited.

• 1) par un dispositif expérimental qui permet d'évaluer la transparence transversale et longitudinale ; on obtient ainsi une appréciation chiffrée de la discrétion,
• 2) par vérification de l'état de la poche après le tir ; son poids avant et après le tir permet de connaître le taux d'ablation (pourcentage pondéral de matière éjectée sous forme gazeuse au cours du tir).
  

Two series of measurements are made.

• 1) by an experimental device which makes it possible to evaluate the transverse and longitudinal transparency; we thus obtain a quantified appreciation of discretion,
• 2) by checking the condition of the pocket after firing; its weight before and after firing provides information on the ablation rate (weight percentage of material ejected in gaseous form during firing).
  

Claims (3)

1. Combustion inhibitor for a double base propellant comprising a polyurethane elastomer comprising in chemically combined form an aliphatic polyesterpolyol or polyetherpolyol constituent or a mixture of the two, containing 2 to 6 hydroxyl groups per molecule, an aliphatic polyisocyanate constituent, optionally a low polyol molecular weight as chain extender, the inhibitor optionally comprising a gasifiable organic filler and / or an aliphatic plasticizer, characterized in that it additionally comprises 0.1 to 10 parts (by weight) of mineral fibers or organic thermostable with a length between 0.1 mm and 15 mm per 100 parts of inhibitor. 2. Combustion inhibitor for a dual base propellant according to claim 1 characterized in that it comprises 0.5 to 4 parts (by weight) of fibers per 100 parts of inhibitor. 3. Combustion inhibitor for double base propellant according to one of claims 1 or 2 characterized in that the fibers have a length between 0.5 and 6 mm.