EP1333016A2 - Semicontinuous casting process for solid propellant pastes using two components - Google Patents

Abstract

Semi-continuous pouring of a propergol paste made from two groups of components uses a first group, comprising 80-99% of the finished solid propergol product containing a liquid pre-polymer, a solid powder charge and a proportion of additives, and a second group, comprising 1-20% of the finished product, containing a reticulating agent and the rest of the additives. Semi-continuous pouring of a propergol paste made from two groups of components uses a first group, comprising 80-99% of the finished solid propergol product containing a liquid pre-polymer, a solid powder charge and a proportion of additives, and a second group, comprising 1-20% of the finished product, containing a reticulating agent and the rest of the additives. The components are mixed in two stages: a low pressure stage in which precisely-dosed ingredients are mixed, and a second high-pressure stage in which the mixture is homogenized. The two groups of components are stored in separate reservoirs (10, 12) and delivered by pumps (11, 13) to a mixing chamber (14) before being passed to a mold.

Description

The present invention is in the field of solid propellant blocks or loads.

More particularly it relates to a process preparation and pouring of a propellant paste, composite type, in a mold to prepare a block or a load of solid propellant. The invention applies in particular to the manufacture of wholesale space launcher loads.

A solid propellant of the composite type comprises a solid or binder polymer matrix, solid fillers pulverulent: oxidizing, possibly reducing and / or energy and various additives feasibility, performance.

The binder is prepared from a resin polymerizable liquid with chemically terminated reagents that are going to be crosslinked by at least one also crosslinking liquid. We introduce in the liquid resin, in an appropriate order, different ingredients of the propellant, then the crosslinker. The resin hardening by baking at a temperature compatible with the presence of materials pyrotechnics, will coat all the ingredients and especially the powdery charges to form a body solid.

The ingredients are chosen to ensure the combustion of the propellant to produce gases. This combustion is generally done in parallel layers: the flame front regression speed is a few millimeters to a few centimeters per second.

One will find for example in DAVENAS "Technology solid propellants ", chapters 10 and 11, Masson 1989 more details on the different ingredients in the composition of composite type propellants.

The process currently used to manufacture such blocks is a discontinuous process, called "batch", consisting in preparing a certain quality of product and to sink a number of loads.

In a first step the different ingredients are introduced in an appropriate order and carefully and mixed for a long time, under conditions of pressures (usually vacuum) and temperatures well accurate. For the next step, this mixture which present in the form of a paste is poured into a mold, with shaping tools. The whole undergoes baking to harden the dough. The mold in some cases constitutes the very envelope of the propellant, envelope which is suitably prepared for carrying out the loading.

The introduction of the crosslinker into the mixture is done towards the end of the kneading step; the dough goes then start to harden: pouring cannot be done that during a limited time, said pot life, during which the mixture remains fluid enough to be poured. he so the casting in the mold must follow the most quickly possible the end of kneading; which implies further to make a compromise between pot life and the cooking time.

For the manufacture of large loads, a few tens to a few hundred tonnes, generally used in space (accelerators next to the launcher) you have to chain several kneaded-poured in the same envelope. The sequence of kneaders and flows must be precise and rigorous. You also have to make a choice economical between the size of the mixers available and that of loading. In all cases the cycle of manufacturing is long and costly because there is a significant immobilization of equipment and personnel during manufacturing.

A solution to the problem of pot life, in the case of the manufacture of small objects, is suggested by a semi-continuous two-component process, described by J.M. TAUZIA in "Some comments on processing energetics materials - Compatibility and Processing Symposium - ADPA - October 23-25, 1989 (at VIRGINIA BEACH).

The process described in this article consists of separate the composition into two groups of components of equivalent viscosity which are then mixed, in a mass ratio close to 1/1, just before casting in the mold. One of the component groups has the polymer to constitute the binder, half of the powder fillers and certain additives; the other group of components includes the crosslinker, too in the form of a polymer, the other half of the fillers and the rest of the additives. These two groups of components are chemically stable when separated.

However, this process presents several disadvantages. Both groups of components have each one half of the powder loads therefore pyrotechnic ingredients, the two groups of components should be prepared and stored in appropriate facilities and meeting standards strict security; thereafter we will say more briefly secure installations.

Furthermore, the homogeneous mixture of these two groups of equally viscous components is difficult to achieve.

Finally, the final composition obtained, in particular by the binder, is different from that of the "batch" process since the two-component process forms the binder from of the mixture of two polymers: the propellant thus obtained must be re-approved for the intended use. It is a long operation and expensive.

The two-component process with a mixture in the 1/1 ratio of two groups of components is therefore not fully satisfactory; and does not seem applicable, in a simple way, to the manufacture of large loads.

The present invention incorporating a technique bicomponent aims to solve problems previously laid.

The present invention therefore relates to a method semi-continuous bicomponent for the casting of dough propellant in a mold to make a block of solid propellant by cooking the dough, said dough comprising a liquid prepolymer, a crosslinker liquid, at least one solid powdery charge and various additives, pouring from mixture of two groups of components and said process is characterized in that the first group of components represents about 80% to about 99% of the finished product and essentially comprises the prepolymer, all powdery solid charges and part additives; the second group of components represents about 20% to about 1% of the finished product and includes all of the crosslinker and the other part of the additives. The additives will notably include a part propellant plasticizer. We can still say that the mass ratio of mixing of the two groups of components is between about 80/20 and about 99/1.

Preferably the first group of components represents about 90% to about 99% of the finished product and the second group of components represents approximately 10% about 1% of the finished product. The mass report of mixture of the two groups of components is between around 90/10 and around 99/1.

The propellant binder is either obtained from a carboxy-telechetic polybutadiene or, preferably, hydroxyletéléchétique crosslinked in the latter case with an isocyanate in the form of a monomer, i.e. binder is obtained from an oxetane prepolymer or substituted oxirane; nitramine or nitrate polymer; polyester or polyether.

Powder fillers have at least one oxidizing charge chosen from the group of perchlorates or non-organic nitrates for example nitrate ammonium or ammonium perchlorate. A part of these oxidizing powder fillers can be replaced by energetic charges such as nitramines, for example octogen or hexogen.

Finally, powdery fillers can also understand, to increase the performance of the propellant, reducing charges in powder form metallic, for example aluminum.

Among the various additives usually used for the production of propellants we will put a little apart from the plasticizer and the crosslinking catalyst whose natures and rates can have effects interesting in the present invention, as we will see later.

In particular the choice of plasticizer and its rate in the first group of components avoids problems of settling solid charges during the period of storage of this group of components.

Advantageously, the second group of components only includes liquid crosslinker monomer, without any of the propellant additives that are all incorporated into the first group of components.

• une première étape de mise à la quantité, par un dosage précis des deux groupes de composants dans le rapport massique souhaité, les deux groupes de composants sont alors sommairement mélangés,
• une deuxième étape d'homogénéisation, au cours de laquelle le mélange sommaire précédent est soigneusement mélangé, avant la coulée dans le moule.

According to a first variant of the process, the mixing takes place in two stages:

• a first step of bringing to quantity, by a precise dosage of the two groups of components in the desired mass ratio, the two groups of components are then roughly mixed,
• a second homogenization step, during which the previous summary mixture is carefully mixed, before casting into the mold.

Advantageously, the two mixing steps are decoupled by the passage of the mixture put to the quantity in a buffer tank, before being taken up for the homogenization mixture. The residence time in the buffer tank is as short as possible.

The two mixing steps are done by all appropriate means, but preferably they are by static mixers.

Preferably the mixing of quantity is done at low pressure and the homogenization mixture is done at high pressure.

Decoupling the two stages by passing the mix put into quantity in a buffer tank allows to resume said mixture at high pressure for do the homogenization mixing step and pour the dough with a high flow rate. This high flow is necessary for casting, under conditions satisfactory economic, large loads. The increase in pressure of the mixture brought to the quantity made by a pump placed at the outlet of the buffer tank.

Advantageously, the quantity mixing is made at low pressure between about 0.006MPa and about 1MPa; and the homogenization mixture is done at high pressure between about 2MPa and about 2.5MPa.

Advantageously also the homogenization mixture is done at the cooking temperature of the dough propellant, between about 20 ° C and about 80 ° C. Through baking at around 20 ° C you should simply hear the maintaining the temperature of the propellant paste at the ambient temperature. Cooking temperature may be lowered by increasing the catalyst rate crosslinking among additives. The mixture and the pouring at the same temperature as that of cooking a the advantage of limiting withdrawal problems thermal.

According to a second variant of the process, the mixture of the two groups of components is done in one step. Again the mixing is done by any means appropriate but preferably it is done in a static mixer, fed in mass ratio desired. The static mixer works at high pressure, between about 2MPa and about 2.5MPa. Advantageously, the mixing is done, as before, at the cooking temperature of the propellant paste between about 20 ° C and about 80 ° C. This temperature of cooking can be lowered to the lower limit, by increasing the rate of crosslinking catalyst.

This invention essentially characterized by a particular choice of the two groups of components which will be mixed in an accurate mass ratio solves the problems left hanging by the process bicomponent of the state of the art.

This process of course retains the advantage of the process bicomponent which is to get rid of pot life.

The mixing sequences to prepare the two component groups are completely independent of those of their mixture and their casting which can intervene very long after the preparation of these two groups of components.

Only the first group of components which contains all the powdery charge, so the products pyrotechnics requires careful preparation and stored in secure facilities. The second group of components, which can be reduced to one crosslinker, is prepared and stored under conditions very simple.

The second group of components is essentially liquid and mixes easily with the first group of components presented to it under form of a paste.

This makes it possible to easily adapt the conditions of pressure and flow at the time of large casting loading. So significantly reduce the duration and the cost of the manufacturing cycle.

Finally in the process of the invention crosslinking it being introduced as a liquid monomer the binder obtained will be identical to that of the "batch" process, so the final composition being the same, the propellant does not will not have to be re-certified which reduces considerably the costs.

The invention will be described in more detail using Figures 1 and 2 which very clearly represent schematic examples of installations for setting up work of the two variants of the process.

Figure 1 describes an installation to set up works the invention according to a first mode in two stages decoupled.

Figure 2 describes an installation to set up works the invention according to a second mode in a single step.

These two variants of the process of the invention assume the prior preparation of the two groups of components.

First the second group of components which essentially includes the liquid crosslinker under monomeric form is possibly some additives, is very simple to prepare. It is a mixture with mechanical agitation for example. This group of components represents only a small mass of the finished product, a only one tank will be enough to store it.

However, the first group of components includes liquid prepolymer, all fillers powdery solids and additives must be prepared with the same precautions and care as a propellant. The mixture constituting this group of components is chemical stable since there is no crosslinker; the choice suitable plasticizer avoids settling of solid charges and allows to keep so prolonged mixing in secure facilities appropriate.

This group of components represents the majority of finished product and in the case of a large load (by example 100t), a large quantity of product required prepare by several mixers and store in several tanks taking into account the sizes of the mixers and tanks available.

Figure 1 very schematically shows a installation for implementing the process the invention according to a first variant.

The tanks 1 and 2 respectively containing the first and second groups of components supply a static mixer 3.

On tanks 1 and 2, pistons 8 and 9, driven by a mechanism not shown, feed into the desired mass ratio of the static mixer 3 which makes a summary mixture, but put in the quantity, of two groups of components, it supplies a tank buffer 4. At the outlet of the buffer tank a pump 5 raise the pressure of the mixture to feed a mixer static 6, operating at high pressure, outlet 7 from the mixer directly feeds the casting mold.

For the manufacture of large loads the quantity mixing kit of the first group of components exceeds the capacity of a single tank. Simply have several type 1 tanks on the supply static mixer 3 and by a special system of winnowing to draw the mixture successively in the tanks.

Figure 2 very schematically shows a installation for implementing the method according to a second variant.

The tanks 10 and 12 respectively contain the first and second groups of components. Pumps 11 and 13, operating at high pressure and whose flow rates are in the desired mass ratio, supply a static mixer 14, whose output 15 feeds directly the casting mold.

The dosing and pressurization pumps are chosen in the classic material: they are either peristaltic pumps, i.e. screw pumps (for example STEIBLE brand) or double piston pumps alternative (for example PUTZMEISTER brand) or by pistons directly placed on the tanks of the two component groups.

Static mixers are conduits containing braces forcing the dough passing through to separate and remix. These conduits are dimensioned in diameter, depending on the flow and number of unit elements ensures the quality of the mixture depending on the pressure available. We use 4 unit elements for low pressure mixing and up to 12 unit elements for high mixing pressure. A mixer made up of unitary elements double wall allows by the circulation of a fluid suitable coolant to heat to the temperature of desired casting the propellant paste.

For a propellant whose polymer matrix has base of a hydroxytelechelic polybutadiene resin represents 14% by mass of the finished product, containing 82% by mass of ammonium perchlorate and 4% by mass aluminum the mechanical properties of the propellant manufactured by the process of this invention. The density is 1.72, the Young's modulus is between 5 and 7MPa and the breaking strength is 1.1 MPa: these values are comparable to the values obtained by the "batch" process.

Claims (12)

Semi-continuous two-component process for casting propellant paste into a mold for manufacturing a solid propellant block, said paste comprising a liquid polymer, a crosslinker, at least one solid pulverulent filler and various additives, the casting being carried out from the mixture of two groups of components characterized in that the first group of components represents approximately 80% to approximately 99% of the finished product and essentially comprises the polymer, the solid pulverulent fillers and part of the additives; the second group of components represents approximately 20% to approximately 1% of the finished product and comprises all of the crosslinker and the other part of the additives. Method according to claim 1, characterized in that the first group of components represents approximately 90% to approximately 99% of the finished product and that the second group of components represents approximately 10% to approximately 1% of the finished product. Method according to claim 1 or 2, characterized in that the second group of components only comprises the crosslinker. Method according to claim 1, 2 or 3, characterized in that the mixing takes place in two stages: a first step of adding to the quantity, by a precise dosage of the two groups of components mixed at low pressure, a second stage of high pressure homogenization of the previous mixture. Method according to claim 4, characterized in that the two stages are decoupled by the passage of the mixture brought to the quantity in a buffer tank. Method according to claim 4 or 5, characterized in that the two successive stages are carried out in static mixers at low pressure and at high pressure respectively. A method according to claim 6, characterized in that the quantity mixing; at low pressure is between 0.006MPa and 1MPa. Process according to claim 6, characterized in that the homogenization mixture, at high pressure, takes place between 2MPa and 2.5MPa. Process according to claim 8, characterized in that the homogenization mixture is carried out at the cooking temperature between approximately 20 ° C and approximately 80 ° C. A method according to claim 1, 2 or 3 characterized in that the mixing of the two groups of components takes place in a single step. Method according to Claim 10, characterized in that the mixing of the two groups of components takes place in a static high-pressure mixer. Process according to claim 11, characterized in that the mixing takes place at the cooking temperature, between approximately 20 ° C and approximately 80 ° C.

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