GB2200362A - An intermediate layer between a rocket solid propellant composition and an insulating layer thereof - Google Patents

Abstract

An intermediate layer between an insulating layer and a propellant composition (i.e. a rocket solid propellant charge) consists of the reaction product of a multivalent isocyanate and a polyol.

Description

INTERMEDIATE LAYER BETWEEN SOLID PROPELLANT CHARGE AND INSULATING LAYER THEREOF This invention relates to an intermediate layer to impart adhesion and prevent the migration of liquid components between a solid propellant composition and an insulating layer thereof.

In order to prevent the formation of a gap between a propellant charge and a combustion-chamber wall (i.e. to prevent back-burning of the propellant charge), to protect the combustion chamber wall from the hot gases which arise upon burn-off of the propellant charge and in order to impart a certain protection against mechanical stress to the propellant charge, such an insulating layer is provided between the combustion-chamber wall and the propellant charge.

It usually consists of a hardenable synthetic resin composition, particularly one based on polyurethane.

To improve the mechanical properties as well as to increase the heat stability of this composition, fillers, such as oxamide, are added to the binder matrix. In the hardened insulating layer impurities and non-hardened binder constituents from the raw materials can be present, and these may act as softeners and may migrate through or exude from the insulating layer.

The propellant charge or composition itself contains liquid components. Thus, double-based propellants, contain, besides nitroglycerine, inert softeners, (or example di-n-propyl adipate, di-n-butyl phthalate or di-ethyl phthalate as well as stabilisers, such as 2-nitrodiphenyl amine for the stabilisation of nitrocellulose.

The migration of the softeners and other liquid components of the propellant composition or, respectively, of the insulating layer leads to a series of undesired variations both in the propellant composition and in the insulating layer. For example, a double-based propellant composition becomes depleted of nitroglycerine, in other words it will suffer an energy loss. Furthermore, it loses stabiliser so that it becomes less safe to use and its useful life is shortened.

As a result of the loss of inert softeners, the mechanical load capacity of the insulating layer diminishes. On the other hand, as a result of ingress of liquid propellant components, the insulating layer swells up and thus further loses mechanical strength.

It can also be chemically decomposed by the incoming compounds -and in certain circumstances it may even become combustible. The migration of nitroglycerine from the charge can result in unfavourable variations in burn-off behaviour, i.e a slower burning speed and a conical burn-off. Also it makes the ignition of the propellant charge more difficult or, indeed, impossible. Finally, migration of liquid components can result in destruction of the propellant charge by embrittlement or to its detachment from the insulation.

In order to prevent the migration of these liquid components, it is known to provide an intermediate layer between the insulating layer and the propellant composition. As this intermediate layer has also to serve the purpose of connecting the propellant composition securely to the insulating layer, it has to possess good adhesive properties.

It is known to produce the intermediate layer form a multivalent isocyanate. In this respect, for example, a di- or triisocyanate in an aprotic organic solvent is applied to the propellant composition, the solvent is evaporated and the isocyanate is hardened in moist air to form a-polyurea (DE-AS 12 OO 184), DE-AS 25 24 843). The propellant composition provided with the intermediate layer is then arranged in an insulating mould and the annular space between the mould and the propellant charge is filled up with an insulating composition, which is hardened to form the insulating layer.

The known intermediate layers made from suitable isocyanates such as triphenyl methane triisocyanate (DD-r), thiophosphoric acid-tris(p-isocyanateophenyl ester) (DD-RF) and biurettriisocyanate (DD-N) formed from three molecules of hexamethylene diisocyanate have a comparatively good barrier action, probably as a result of their tightly cross-lined polyurea network. At the same time, upon the casting-on of the polyurethane insulation they lead to good adhesion between the propellant charge and the insulating layer, but only if the latter is applied within a restricted airing-off and hardening period.

In the case of the known intermediate layer materials it is also disadvantageous that their hardening speed is strongly dependent upon the moisture content and the temperature of the air. If the hardening speed is too slow, large amounts of softener components migrate from the propellant into the incompletely hardened intermediate layer and damage this permanently. Equally, in the case of relatively thick polyurea intermediate layers, cretaceous formations frequently occur and bubbles frequently form as a result of CO. which is released upon the reaction of isocyanate and water. In order to achieve a blocking effectiveness which is as high as possible, it is known from experience that a second layer has to be applied to a preliminary intermediate layer which is not too thick and which has already hardened.Moreover, because of the fast reaction of the isocyanate groups, it is necessary to keep to a very strictly limited airing time in order to ensure good adhesion as a result of the formation of chemical bonds between the barrier layer and the insulation.

The object of the invention is to provide an intermediate layer, between a solid propellant composition and an insulating layer thereof which has a reliable hardening speed (not much influenced by ambient conditions) and unto which the insulating layer can be cast within a relatively long time period without significant reduction in the adhesion or barrier effect of said intermediate layer.

In accordance with the invention this is achieved in that the intermediate layer is the reaction product formed from a multivalent isocyanate and a polyol.

In accordance with the invent ion, di or tri isocyanates are preferred as isocyanates. Tripheflyl methane triisocyanate (DD-R), thiophosphoric acid tris(p-isocyanates phenyl ester) (DD-RF) and the biurettriisocyanate (DD-N) formed from three molecules of hexamethylene diisocyanate have provided to be suitable as triisocyanates. However, the triisocyanate which is obtained by reacting three mols of toluylene diisocyanate and one mol of trimethylol propane (which is marketed under the trade name of Desmodur L by Messrs. Bayer AG and is designated hereinunder also as "DD-L") proved to be the most advantageous.

In accordance with the invention, short-chained diols or triols are preferred as polyols, i.e. the chain between the two hydroxyl groups of a diol or between two hydroxyl groups of a triol is not to possess more than 10 carbon atoms. In this way a network is obtained in the reaction product which is sufficiently dense to act as a migration barrier.

1,2-propane diol and bisphenol A as well as trimethylol propane represent particularly suitable polyols. Furthermore, ethylene glycol, 1,4 dimethylol cyclohexane, 1,3-propane diol, diethylene glycol, neopentyl glycol as well as glycerine and 1,2,4-butane triol have proved to be suitable.

The hardened intermediate layer should have a homogeneous molecular structure which is as uniform as possible. The NCO/OH ratio of the isocyanate to the polyol therefore theoretically (preferably) amounts to 1.0 : 1.0. However, since some of the NCO groups react, for example, with the hydroxyl groups of the nitrocellulose of a doubled-base propellant composition and since a side reaction with the water of the air also cannot be completely prevented a fairly large number of free hydroxyl groups always remain on a hardened intermediate layer in accordance with the invent ion. These hydroxyl groups show themselves to be very resistant to customary environmental influences such as light, moisture and dust. Also they are not attacked by degreasing agents or by solvents, such as petroleum ether.They are, therefore, available over an almost unlimited period of time as latent reaction partners for chemical bonding of the insulation, which contain NCO groups, to the barrier layer or the propellant composition.

This ensures excellent adhesion of the insulation layer even after long intermediate storage of the propellant composition provided with the intermediate barrier layer.

In practise, the molar ratio NCO/OHof the isocyanate to polyol may be 0.8:1 up to 3:1.

To accelerate the reaction of the polyisocyanate component, a metallooranic catalyst, more particularly a compound of tin, lead or mercury, such as dibutyl tin dilaurate, lead octoate or phenyl mercury oleate, is preferably used.

To produce the intermediate layer in accordance with the invention, the mixture of the isocyanate component and the polyol component is preferably dissolved in aprotic polar solvents and the resultant solution applied to the propellant composition. In this respect, butyl acetate, methylene chloride, acetone, acetic ester or tetrahydrofurane can be used as aprotic polar solvents.

This solution is preferably applied by spraying.

In any case, a method should be selected which does not lead to excessive dissolving out of components from the propellant composition; The solution to be applied preferably has a concentration of polyisocyanate and of polyol of 5 up to 30 percent by weight. The concentration of the metalloorganic catalyst is preferably in the range 0.01 up to 0.5%, of the weight of the solution.

It is expedient to spray the propellant charge with a solvent, such as butyl acetate, prior to application of the intermediate layer solution. In this way the propellant charge is more easily wettable.

The thickness of the intermediate layer preferably amounts to 150 um at most and preferably lies between 80 and 100 um. Too great a thicknessentails a risk of rupturing of the layer.

The intermediate layer in accordance with the invention is particularly suitable for use with insulating layers having a polyurethane basis as well as double-base propellant compositions.

The following example serves to explain the invention further.

A double-base propellant composition that is to be insulated is roughened by means of emery paper or steel wool. The resulting dust is removed and the surface is degreased with a homopolar solvent, preferably petroleum ether. After airing or air drying of the solvent, the intermediate-layer solution is applied by spraying to the propellant composition.

The intermediate-layer solution is composed of the following components: Desmodur L (75% by weight solution in ethyl acetate) 20 parts by weight, trimethylol propane 2.2 parts by weight (,) butyl acetate 50 parts by weight, methylene chloride 50 parts by weight, acetone 21.5 parts by weight, dibutyl tin dilaurate (solvent-naphtha) 0.2 parts by weight.

This solution has a pot life of about 20 minutes.

However, Desmodur L, butyl acetate and methylene chloride on the one hand as well as trimethylol propane, acetone and dibutyl tin dilaurate on the other hand can be stored seaprately for days in a refrigerator.

Upon spraying, which is effected at room temperature, 6 to 7 g of the above solution are needed per 100 cm of propellant composition surface. The intermediate layer is then allowed to harden for at least 5 hours at room conditions. The insulating layer, which is preferably based on polyurethane, can subsequently be cast onto the resultant coated composition at any time over a period of several weeks, without loss of adhesion.

The intermediate layer in accordance with the invention is thus relative unproblematic in production. Nevertheless, it leads to an excellent adhesion and barrier effect.

In trials the same insulating layer based on polyurethane, has been applied to the same double-base propellant composition: (i) without an intermediate barrier layer; (ii) with an intermediate layer only of polyisocyanate; and (iii) with the intermediate layer in accordance with the invention.

The results are reproduced in the following Table 1.

TABLE 1 Migration as a function of the intermediate barrier layer used (T = 40 C) Softener absorption in (mg/cm2) * Time in days Trial Intermediate No. Layer 1 7 14 21 1 without 2.6 7.7 10.8 12.9 2 DD-VL/H2O 0.6-2.4 2.1-6.9 3.2-9.6 3 DD-R/H20 0.2-2.2 7.1 0.9-9.6 1.2-11.6 4 DD-N/H20 0.2-2.1 1.0-5.8 1.3-8.0 1.8-10.6 5 DD-RF/H20 3.0 7.7 10.5 12.6 6 DD-L/H > 0 0.2-0.3 0.3-1.1 0.3-1.3 0.3-1.7 7 DD-L/diethylene glycol 0.1 0.1 0.1 0.1 8 DD/1,2-propane diol 0.1 0.1 0.1 0.1 9 DD-L/dihydroxy methyl cyclohexane 0.2 0.2 0.2 0.2 10 DD/bisphenol A 0.1 0.1 0.1 0.1 11 DD/L/trimethylol propane 0.1 0.1 0.1 0.1 i Increase in weight of the insulation as a result of the components capable of migration which have immigrated from the propellant composition.

Trial No. 1 to 5 : Prior art Trial No. 6 : Improvement through own labours Trial No 7 to 11: Invention DD-L = Triisocyanate (Desmodur L) DD-L/H20 = Triisocyanate hardened with air moisture DD-VL = Diisocyanate (Desmodur VL, 4, 4'-diphenyl methane diisocyanate) The tensile strength of the propellant composition/insulation system for the example 2 to 6 and 11 in accordance with Table 1 is reproduced in the following Table 2.

Table 2 Tensile strength EJ (N/mm2) at 20 C a) Tension test after 4 days at +50 C b) Tension test after 60 days at +50 C Blocking layer a b DD-VL/H2O 4.7 4.0 DD-R/H2O n.m.l n.m.

DD-N/H.O 4.8 4.9 DD-RF/H2On.m. n.m.' n.m.

DD-L /H=O 5.1 5.2 DD-L /TMP 5.9 6.1 n.m. = not measureable by reason of too slight adhesion 2 TMP = Trimethylol propane.

It is evident from Table 1 that the migration values of the intermediate layers Nos.7 to 11 in accordance with the invention are distinctly improved compared with the intermediate layers Nos. 1-6 hardened by air moisture.

It can be gathered from Table 2 that the tensile strength and thus the adhesion of the polyol-hardened intermediate layer in accordance with the invention is greater than that of the water-hardened intermediate layers. Furthermore, it emerges from Table 2 that a long-term storage at +50"C does not have a negative influence on the adhesive strength of the intermediate layer in accordance with the invention.

In the case of the intermediate layers in accordance with the invention, if the insulation layer is cast therearound within a period of 6 weeks, then the adhesion is constantly good. On the other hand, with water-hardened DD-VL, DD-N- and DD-Lintermediate layers a distinct loss of adhesion can be observed after only a few days.

Claims (15)

1. An intermediate layer to impart adhesion and prevent migration of liquid components between a solid propellant composition and an insulating layer thereof consists of a reaction product formed from a multivalent isocyanate and a polyol.

2. An intermediate layer as claimed in claim 1, wherein the isocyanate component is a precondensate from three mols of toluylene diisocyanate and one mol of trimethylol propane.

3. An intermediate layer as claimed in claim 1 or 2, wherein the polyol component is a polyhydric alcohol having a chain of 10 carbon atoms at the most between any two hydroxyl groups.

4. An intermediate layer as claimed in claim 3, wherein the polyhydric alcohol may comprise any one of ethylene-, diethylene-, triethylene- glycol, 1,2-, 1,3-propane diol, 2,2-dimethyl-1,3-propane diol (neopentyl glycol), 1,4-dimethylol cyclohexane, diphenyl propane (bisphenol A), glycerine, 1,2,4butane triol or trimethylol propane, di-, tri-, or tetra- glycerine.

5. An intermediate layer as claimed in any preceding claim, wherein the molar ratio of the isocyanate groups of the isocyanate component to the hydroxyl groups of the polyol component is 0.8 : 1 up to 3 : 1.

6. An Intermediate layer a; ssaszeW n any prç claim, wherein the molar ratio of the isocyanate groups of the isocyanate component to the hydroxyl groups of the polyol component is 1:1.

7. A method of producing the intermediate layer as claimed in any preceding claim, wherein a mixture of the polyisocyanate component and the polyol component is dissolved in aprotic polar solvents and the solution is applied to the propellant composition.

8. A method as claimed in claim 7, wherein the solution is applied by spraying.

9. A method as claimed in claim 7 or 8, wherein a metalloorganic catalyst is added to the mixture or to the solution.

10. A method as claimed in claim 9, wherein the metalloorganic catalyst is a tin-, lead- or mercury compound.

11. A method as claimed in claim 9 or 10, wherein the metalloorganic catalyst comprises dibutyl tin dilaurate, lead octoate or phenyl mercury oleate.

12. A method as claimed in any of claims 7 to 11, wherein prior to application of the solution the propellant charge is treated with an organic solvent.

13. -A method as claimed in claim 12, wherein the solvent is butyl acetate.

14. An intermediate layer between a rocket solid propellant composition and an insulating layer therefor substantially as hereinbefore described.

15. A method of producing the intermediate layer as claimed in any of claims 1 to 6 substantially as hereinbefore described-.