GB2178745A - Insulating material for rocket propellants - Google Patents

Abstract

A material for the production of insulation for propulsive charges of rockets comprises polyurethane as the binder and oxamide as the sole filler. The insulating material may be produced by preparing a preliminary mixture of a polyhydroxyl compound, an organometallic compound as catalyst, and the oxamide, degasifing, adding polyisocyanate and then degasifing again.

Description

SPECIFICATION Insulating material for rocket propellants This invention relates to a hardenable insulating material containing components forming a polyurethane and also containing oxamide and capable of being poured. The material is to be used for an insulating coating provided on a solid propulsive charge for a rocket and containing the polyurethane as the binder and the oxamide as the filler.

In order to ensure that only the desired combustion surfaces of solid propulsive charges for rockets will burn, surfaces not required to burn have to be provided with a firmly adherent insulating layer. Furthermore, the insulation provides the propellant in the combustion chamber with protection from mechanical strain. In the case of face burners, that is propulsive charges where the shell and head surfaces are insulated, an important additional purpose of the insulation is to protect the combustion chamber from the hot gases which form as the propellant is consumed.

The comparatively smoke-free solid propellants used in rocket engineering frequently consists of double-based (DB) propellants. In the case of face burners the smoke accompanying the consumption of propulsive charges containing these propellants mainly emanates from the insulation burning away at the same time.

Particularly in the case of guided rockets and especially with wire-guided types the smoke developed by the exhaust gas has to be reduced to a minimum. On the one hand smoke enables the path of the rocket to be observed, so that the firing position, for example is vulnerable. On the other hand, rockets of this kind often have an incandescent emitter in the tail, such as a pyrotechnic charge which generates IR radiation detected by control apparatus for guidance. It is more difficult to detect radiation however when screened by smoke from the jet of exhaust gas.

The insulating layer is generally made from hardenable synthetic resin compositions, including those with a polyurethane base. It is usually produced by pouring or pressing the synthetic resin composition into the narrow gap formed between a mould and the shell of the propulsive charge. To render polyurethanes capable of being poured the systems known make use of a composition containing a urethane pre-polymer together with a hardening agent (see DE-A 18 09 360, DE-A 24 27 060, DE-A 25 24 843 and CH-B 514 515).

The thermal resistance of the known insulating materials is increased by the adition of fillers.

These usually consist of pulverous or fibrous inert inorganic substances of low thermal conductivity, such as asbestos or lithopone (a mixture of zinc sulphide and barium sulphide) or pulverous endothermically unstable substances such as hydroxides of acid amides. The insulating material described in DE-A 18 09 360, on which the present invention is based, contains oxamide as a filler. In addition to the oxamide the known insulating material used in the prior art nevertheless necessarily includes considerable quantities of inert inorganic fillers, such as asbestos fibres or lithopone. In the known insulating material a urethane pre-polymer is employed, rendering it very tenacious. An example of the hardening agent used for the known insulating material is 4,4'-methylene-bis-(ortho-chloraniline).Dimethyl glycol phthalate is also added to the known insulating material as a softening agent in order to ensure workability.

The known insulating material causes smoke to form despite the use of oxamide, which decomposes endothermically and without residue forming colourless reaction products with a cooling effect, such as mainly water, hydrogen, ammonia and oxides of carbon. Owing to the considerable tenacity of the known insulating material air bubbles present are difficult to remove.

It also has to be pressed into the gap between the mold and the propulsive charge with the use of high pressure hydraulic apparatus. Finally, propulsive charges having the known type of insulating coating causes changes in the ballistic properties of the rocket.

This invention seeks to provide an insulating material for an insulating coating provided on a solid propulsive charge for a rocket in which the aforementioned disadvantages are largely overcome whereby the charge will burn without smoke, prove easy to degasify and easy to pour, adhere firmly to the said propulsive charge, cause no change in the ballistic properties and also satisfy other requiremens arising for insulating coatings of this kind, and wherein particularly high thermal stability is provided.

According to this invention there is provided a hardenable insulating material containing components forming a polyurethane and also containing oxamide and capable of being poured, for use as an insulating coating around a solid propulsive charge for a rocket and containing the polyurethane as the binder and the oxamide as the filler, wherein the oxamide forms the only filler and the components forming the polyurethane comprise: (a) at least one polyhydroxyl compound with at least two reactive hydroxyl groups per molecule, (b) preferably at least one polyamine with at least two amino groups per molecule, (c) at least one polyisocyanate with at least two isocyanate groups per molecule, and (d) at least one organometallic compound as a catalyst.

The insulating material according to this invention differs from that of DE-A 18 09 360 in that the only filler used comprises oxamide. It has been found that the inorganic fillers inevitably contained in known insulating material, such as asbestos, cause considerable quantities of smoke to form even if only used in small quantities. The mixture also facilitates the formation of an even mixture, since fibrous substances, in particular, are liable to form lumps, especially in a thickly liquid media such as that present in the insulating material already known.

In the insulating material of the invention the components required for the formation of the polyurethane are not present as pre-polymers, as in the known insulating material, but rather as monomers, together with an organometallic compound by which the addition of the monomers is catalysed to form the polyurethane.

This insulating material according to the invention therefore has a low viscosity, generally below 10,000 cps (100,000 Pa/s). It is therefore not only easy to degasify but can also be poured without difficulty into the narrow gap between the propulsive charge and the mould.

In view of the low viscosity the insulating material according to the invention requires no added softening agent. It has been found that due to the lack of such an agent therein the ballistic properties of the rocket are adequately reproducible. This is no doubt due to the fact that a softening agent contained in the insulating layer will in time migrate into the propulsive charge, thus altering the properties of the latter. The absence of a softener also results in an improvement in the adhesion of the insulating coating to the propulsive charge.

A homogenous mixture of oxamide and monomer is obtained by forming the insulating material in two stages wherein a preliminary mixture is first of all prepared from the oxamide, the hydroxyl compound, possibly the polyamine and the catalyst, the polyisocyanate being added in a second stage in order to obtain the pourable insulating material. The polyamine is preferably provided as the starting material in liquid form, usually melted, in order to produce the preliminary mixture. Each of the two stages is accompanied by degasification in a vacuum and this operation, due to the low viscosity of the preliminary mixture or insulating material, presents no difficulty.

Particular preference is given to those insulating materials in accordance with this invention in which the polyhydroxyl compound comprises a bivalent polyethylene glycol or polypropylene glycol with a mean equivalent weight of 50-250, preferably about 100, the catalyst being an organic lead compound, preferably lead octoate, and the polyisocyanate being tri- or higher valency. The ratio of the isocyanate groups of the polyisocyanate to the hydroxyl groups of the polyhydroxyl compound (NCO/OH ratio) may be 0.85-1.2:1 and the filler comprises 10-30 by wt. of oxamide.

In this insulating material a liquid or molten diamine or polyamine may be prepared as the starting material, preferably comprising 4,4'-methylene-bis(orthochloroaniline) or 3 ,3'-dichloroben- zidine, after which the remaining components are added insuch a way that the pourable insulating material contains 0-5% by wt. of the di-amine or polyamine. The ratio of the isocyanate groups of the polyisocyanate to the hydroxyl groups of the polyhydroxyl compound and also to the amino-groups of the polyamine (NCO/(OH+NH2) ratio) preferably amounts to 0.85-1.2:1.0, the most advantageous ratio being 1.0:1.0.

The polyhydroxyl component of a further insulating material to which particular preference is given according to the invention consists of a polyether containing hydroxyl groups and having an average of 3 hydroxyl groups per molecule and a mean equivalent weight of 80-250, preferably 100-150, while the polyhydroxyl compound may comprise a multivalent polyethylene glycol or polypropylene glycol and the catalyst an organic mercury compound such as phenyl mercury acetate, phenyl mercury ethyl hexoate, phenyl mercury oleate or phenyl mercury propionate, preferably phenyl mercury oleate, the starting material including 20-60 and preferably 35-60% by wt. of oxamide, while a low-molecular di-isocyanate, such as hexamethylene diisocy anate-1 6, trimethyl hexane di-isocyanate-1 ,6, isophoron di-isocyanate, 4,4'-diisocyanatodicyclohexylmethane or 2,6-di-isocyanatohexane acid methyl ester, is employed, preferably hexamethylene di-isocyanate-1,6 and/or trimethyl hexane di-isocyanate-1,6,. The NCO/OH ratio therein is preferably 0.85-1.2:1.0, particular preference being given to a ratio of 1.0:1.0.

It has also been found particularly advantageous for the polyhydroxyl compound used in the insulating material last mentioned to comprise a mixture of two polyethers A and B containing hydroxyl groups, of which there is an average of 3 per molecule, their mean equivalent weights being 80 to 100(A) and 140 to 160 (B), A and B being present in a ratio of betwen 0.0:1.0 and 0.5:0.5, preferably about 0.3:0.7.

The invention is further descried and illustrated in the following examples.

Example 1 26.26 parts of polyethylene glycol with a mean equivalent weight of 100, 0.03 parts of lead octoate and 25.00 parts by weight of oxamide are placed together in this order and mixed, after which they are degasified under a vacuum. To this preliminary mixture is added 48.35 pts. by wt. of tri-osicyanate (Desmodur-N). The mixture is degasified under a vacuum in order to obtain an insulating material capable of being poured.

Example 2 1.73 pts. by wt. of melted 4,4'-methylene-bis-(ortho-chloraniline) are prepared as starting material, given an addition of 24,81 pts. by wt. of polyethylene glycol with an equivalent weight of 100 and stirred to form a homogenous mixture. 0.2 pts. by wt. of lead octoate and 26.00 pts. by wt. of oxamide are then added.

This preliminary mixture is degasified and then given an addition of 47.44 pts. by wt. of triisocyanate (Desmodur-N). The mixture is degasified under a vacuum in order to produce an insulating material capable of being poured.

Example 3 38.20 pts. by wt. of polyether of low viscosity containing hydroxyl groups and having an average of 3 of the latter per molecule (Desmophen 550 U), 0.05 pts. by wt. of phenyl mercury oleate and 40.000 pts. by wt. of oxamide are placed together, in this order, and mixed, after which they are degasified under a vacuum. This preliminary mixture is then given an addition of 29.75 pts. by wt. of hexamethylene di-isocyanate-1 .6. The mixture is degasified under a vacuum in order to produce an insulating compound capable of being poured.

Example 4 26.88 pts. by wt. of a first polyether of low viscosity containing hydroxyl groups, with an average of three such groups (Desmophen 550 U) and 8.10 pts. by wt. of a further polyether of low viscosity containing hydroxyl groups, with an average of three such groups (Baygal K 30), 0.05 pts. by wt. of phenyl mercury oleate and 43.10 pts. by wt. of oxamide are placed together in this order and mixed, after which they are degasified under a vacuum. This preliminary mixture is then given an addition of 21.87 pts. by wt. of hexamethylene diisocyanate-1.6.

The mixture is degasified under a vacuum in order to produce an insulating material capable of being poured.

From each of the insulating materials produced according to Examples 1 to 4 an insulating layer on propulsive charges used for face burners was prepared. For this purpose the propulsive charge is placed in a mould in such a way that an annular gap of about 1 to 3 mm is formed between the mould and the propulsive charge, the insulating material being introduced into the gap. The pot times of the materials produced according to Examples 1 to 4 were found completely adequate. After hardening for several hours at a slightly increased temperature of about 30 the propulsive charge provided with the insulating coating was removed from the mould.

For purposes of comparison an insulating material was prepared from about 42 pts. by wt. of lithopone as the filler and an unsaturated polyester cross-linked with styrene as the binder.

The propulsive charges provided with the insulating layer were consumed by combustion in a smoke-density measuring apparatus in order to measure the smoke occurring as they burnt away. For this purpose a beam of light was caused to pass through the combustion gases, transverse to the direction of the jet exhaust of the rocket, and the absorption or transmission of the ray was measured with a photo-cell. The optical density (OD) was calculated from the resulting transmission values by the following formula:

The attenuation (dB) was then calculated as follows from the optical density: Attenuation

The "OD" comparison" formed the optical density value which was obtained in the burning of the comparison insulating layer containing lithopones.

The results are given in the following table: Insulating layer: Attenuation (dB): Comparison 0.0 Example 1: -21.35 Example 2: -21.65 Example 3: -27.65 Example 4: -26.30 It may be seen that the insulating materials according to the invention provide insulating coatings of considerably reduced attentuation, thus causing far less smoke.

Claims (17)

1. A hardenable insulating material containing components forming a polyurethane and also containing oxamide and capable of being poured, for use as an insulating coating around a solid propulsive charge for a rocket and containing the polyurethane as the binder and the oxamide as the filler, wherein the oxamide forms the only filler and the components forming the polyurethane comprise: (a) at least one polyhydroxyl compound with at least two reactive hydroxyl groups per molecule, (b) preferably at least one polyamine with at least two amino groups per molecule, (c) at least one polyisocyanate with at least two isocyanate groups per molecule, and (d) at least one organometallic compound as a catalyst.

2. An insulating material in accordance with Claim 1, comprising at least 10% by weight of oxamide, 15-60% by wt. of the polyhydroxyl compound, 0-15% by wt. of the polyamine, 15-60% by wt. of the polyisocyanate, and not more than 2% by wt. of the catalyst, the ratio of the isocyanate groups of the polyisocyanate to the hydroxyl groups of the polyhydroxyl compound and also to the amino groups of the polyamine being between 0.5 to 1.5 to 1.

3. An insulating material in accordance with Claim 1 or 2, wherein the oxamide content amounts to not more than 60% by wt.

4. An insulating material in accordance with Claims 1 or 2, wherein the polyhydroxyl compound is a bivalent polyethylene glycol and/or a bivalent polypropylene glycol with a mean equivalent weight of 50 to 250 and/or at least one polyether, containing hydroxyl groups, with an average of 3 hydroxyl groups per molecule and with a mean equivalent weight of 80 to 150.

5. An insulating material in accordance with Claim 4, containing two polyethers containing hydroxyl groups with an average of three hydroxyl groups per molecule, one having a mean equivalent weight of 80 to 100 and the other having a mean equivalent weight of 140 to 160, the ratio of the polyether with a mean equivalent weight of 80 to 100 to the polyether having a mean equivalent weight of 140 to 160 being not more than 1:1.

6. An insulating material in accordance with any one of the preceding claims, wherein the polyamine content is not more than 5% by wt.

7. An insulating material in accordance with any one of the preceding claims, wherein the polyamine comprises 4,4'-methylene-bis-(ortho-chloraniline) or 3,3'-dichlorobenzidine.

8. An insulating material in accordance with any one of the preceding claims, wherein the poly-isocyanate comprises a di-isocyanate or a tri-isocyanate.

9. An insulating material in accordance with Claim 8, wherein the di-isocyanate comprises hexamethylene diisocyanate-1,6 or trimethyl hexane di-isocyanate 1.6.

10. An insulating material in accordance with any one of the preceding claims, wherein the ratio of the isocyanate groups of the polyisocyanate to the hydroxyl groups of the polyhydroxy compound and to the amino groups of the polyamine is between 0.85 and 1.2:1.

11. An insulating material in accordance with any one of the preceding claims, wherein the catalyst is an organic lead compound or mercury compound.

12. An insulating material in accordance with Claim 11, wherein the organic lead compound is lead octoate.

13. An insulating material in accordance with Claim 11, wherein the organic mercury compound is phenyl mercury oleate.

14. A process for the production of an insulating material in accordance with any one of the preceding claims, in which process a preliminary mixture of the oxamide and all the polyurethane-forming compounds including the catalyst, with the exception of the polyisocyanate, is prepared first to which preliminary mixture, after degasification under a vacuum, is added the polyisocyanate, after which a further degasification process is applied to produce the pourable insulating material.

15. A process in accordance with claim 14, wherein, to form the preliminary mixture, the polyamine is provided in a liquid state as a starting mixture, after which the oxamide and the other components are added, with the exception of the polyisocyanate.

16. A pourable hardenable insulating material as herein described and exemplified.

17. A process for producing a hardenable pourable insulating material as described herein and exemplified.