GB2038346A - Inhibition coating for propellant charges - Google Patents

Abstract

This invention provides a polyurethane coating for propellant charges wherein the average molecular weight between cross-linked units in the polyurethane is 200-600 and the carbon to oxygen ratio is less than 4. The product combines smokeless combustion, resistance to degradation by nitroglycerine and transparency, which may be prepared at ambient temperatures and applied to a propellant charge by casting, brushing or spraying. Such a polyurethane is prepared by the reaction of an aliphatic polyether polyol component with an aliphatic polyisocyanate component. Suitable polyols have a functionality of 4-8, a molecular weight of 400-1000, and a C to O ratio not exceeding 3, for example a polyoxy propylene glycol adduct of sorbitol, used alone or mixed with polyether triols or diols. Suitable polyisocyanates comprise aliphatic di- or tri- isocyanates with C to O ratios of less than 5, such as hexa methylene di- or tri- isocyanate. The properties of the polyurethane may be modified by the addition of chain extenders and cross-linkers, and fillers.

Description

SPECIFICATION Inhibition system for propellant charges This invention relates to the preparation of ambient temperature cured semi-rigid aliphatic polyurethane resin compositions which produce little or no smoke on pyrolysis for application as inhibitors for propellant charges.

Undesirable combustion around the sides of a propellant charge rather than along the centre of the charge, resulting in irregular ballistics is generally prevented by coating the charge with a curable resin, either by casting the resin around the charge in a mould, spraying or brushing. Casting of a resin around the charge may in some cases be carried out in situ, for example in a rocket motor chamber, in which case the cured resin serves to restrict the charge.

Resins suitable for such an application should preferably combine the following properties: 1. Smokeless combustion to prevent target obscuration in a line of sight guided weapons applications.

2. Mechanical strength without brittleness.

3. Resistance to degradation by long term contact with explosives, especially explosives based on nitrate esters such as nitroglycerine. Migration of nitroglycerine from the propellant can cause loss of adhesion between the propellant and the inhibitor lining compounds.

4. Resistance to hydrolysis and photo-oxidation.

5. Transparency and bubble free curing to facilitate inspection of the charge-inhibitor lining bond.

6. Low viscocity monomers to facilitate handling.

7. Rapid low temperature curing for convenience and to minimise damage to the propellant charge, yet combined with an adequate pot life.

In practice the properties described above are often mutually incompatible. A number of curable liquid resin systems exist, but most investigation has been carried out on the polyurethanes, a broad class of polymeric materials produced by reacting a polyfunctional isocyanate with a polyfunctional compound having active hydrogens in its structure, such as a polyester or polyester which contains hydroxyl groups as the radical having active hydrogen and are usually referred to as polyols. However polyurethanes fall generally into two classes with widely different properties.

Polyurethanes used as elastomes and flexible foams having relatively lightly crosslinked structures prepared by reacting difu nctional polyether of polyester polyols of average molecular weight 1000-3000 with di-isocyanates and suitable chain extenders and crosslinked agents have excellent mechanical properties, but are seriously degraded by long term contact with explosives containing nitroglycerine.

Thermosetting rigid polyurethanes, on the other hand, being more highly crosslinked than elastomers, give much better resistance to solvents and chemical reagents. Highly crosslinked polyurethanes are prepared by reacting polyfunctional polyols of average molecularweight 300-1000 with aromatic di-isocyanates or polyisocyanates. Such polyurethanes are usually cured at elevated temperatures, are highly coloured, and discolour further when exposed to ultraviolet radiation. In addition polyurethanes based on aromatic isocyanates produce copious quantities of black smoke on combustion.

It has now been found that by controlling the degree of crosslinking a polyurethane resin may be produced with the desired combination of properties, ie the transparency, mechanical properties, low temperature curing and smokeless combustion characteristics of the lightly crosslinked elastomers combined with the resistance to nitroglycerine of the thermosetting polyurethanes.

According to the invention, polyurethane for coating a propellant charge is characterised in that it has an average molecular weight between crosslinked unit (Mc) as hereinafter defined, of 200-600 and a carbon to oxygen ratio of less than 4.

Such a polyurethane may be prepared by the reaction of an aliphatic polyol component with an aliphatic polyisocyanate component, preferably under slightly acidic conditions, which favour chain extention, and generally in the presence of a catalyst to direct the reaction and cause it to proceed at a convenient rate at room temperature. The polyol and isocyanate components may consist of a single ingredient or may contain a plurality of ingredients to provide a range of physical properties. A small quantity of aromatic material may be incorporated if necessary to achieve the desired properties, but the overall carbon to oxygen ratio should be kept at less than 4.

The average molecular weight between crosslinked units (Mc) is defined as calculated from parameters of the components using the relationship:

where Mx = molecularweightofingredientx yx = mole fraction of ingredient x fx = functionality of ingredient x yi = mole fraction of monofunctional ingredients For the purposes of the summations, only reactive species are counted, and not the catalyst or any fillers.

As an approximate guide to determining the Mc of a polymeric material prepared according to the invention, Mc may be derived from the physical properties of the material such as stress-strain data or solvent swelling, although it must be noted that Mc determined in this way will frequently differ from Mc calculated from (1) by + 40%, especially when Mc is low (less than about 100).

Mc may be calculated from stress-strain data using the relationship: ART Nle wheres= apparent stress (in Mum~2) on a specimen of density R (in Mgm -3) strained to an extension ratio'.

R = the gas constant T = the absolute temperature.

Mc may be determined by measurements of solvent swelling by constructing graphs of Mc as calculated using equation (1 ) against Wr/Wg (where Wr is the critical weight of the sample, and Wg the weight of the swollen sample) for known materials, and subsequently deriving Mc of an unknown sample by interpolation or extrapolation.

The limitation of Mc and the carbon to oxygen ratio of the invention, produces a polyurethane possessing substantially all of the desired properties outlined above.

The polyol component should generally contain polyether polyols rather than polyester polyols, as they confer greater resistence to hydrolysis on the polyurethane, and have a lower viscosity. To achieve the desired Mc, the polyol component should normally contain a polyol with a functionality of 4-8, a molecular weight of typically 400-1000 and a carbon to oxygen ratio generally not exceeding 3. In a preferred embodiment of the invention, the polyol component contains a polyoxypropylene glycol adduct of sorbitol (f = 6), used either alone or mixed with polyether triols or diols of average molecular weight 300-2000, and carbon to oxygen ratios not exceeding 3 such as polyoxyropylene glycol adducts of ethylene glycol, diethylene glycol, propylene glycol and glycerol, in a quantity corresponding to 0.1 - 0.3 mole equivalents of the sorbitol adduct.

Awide range of physical properties of the polyurethane may be achieved by the addition of low molecular weight chain extenders and crosslinkers, usually diols, diamines ortriols in a quantity corresponding to 0.1 0.3 mole equivalents of the polyether polyol of functionality 4 to 8. Suitable materials include ethylene glycol, diethyiene glycol, ethylene diamine and glycerol.

The polyisocyanate component may consist of aliphatic di- or tri-isocyanates with carbon to oxygen ratios generally not exceeding 5. Suitable isocyanates include hexamethylene diisocyanate and hexamethylene triisocyanate (a biuret modified isocyanate prepared by reacting 3 moles of hexamethylene diisocyanate with 1 mole of water). Other reactants which may be used include diethyl fumarate diisocyanate, and adipo nitrile carbonate. These isocyanates may be used singly or blended to achieve a suitable viscosity.

Organometallic catalysts are most suitable for the reaction, and should preferably be miscible with the polyol and isocyanate components. Suitable catalysts include dibutyl tin oxide, dibutyl tin dilaurate, stannous octoate, zinc octoate and mixtures thereof. Amine catalysts should be avoided as theyfavourthe formation of urea groups and carbon dioxide.

The burn-through rate of the polymer may be reduced by the incorporation into the polyurethane of 5-50% of low carbon to oxygen ratio fillers such as ureaformaldehyde, polyoxymethylenes (= polyacetals), cellulose acetate and oxamide or mixtures thereof. These are most conveniently incorporated into the polyol component using a suitable dispersion technique such as ball milling, or may alternatively be added to the isocyanate component, or to the reaction mixture after mixing the polyol and isocyanate components. The burn-through rate may also be reduced by introducing into the polymer structure isocyanurate as well as urethane groups. Isocyanates are trimerized into isocyanurates in the presence of suitable catalysts such as alkali metal phenolates or alcoholate carboxylates or compounds with ortho (dimethyl-aminoethyl) phenol groupings.The addition of 2, 4, 6, tris dimethyl amino methyl phenol in a quantity corresponding to 1 - 10% by weight of the reaction mixture has been found to be particularly effective.

To prepare a polyurethane resin using the materials described above, it is generally necessary to dehydrate the polyol component. The presence of water in the polyol causes hydrolysis of the isocyanatesto give urea and carbon dioxide, which can cause severe bubbling, cratering or blistering in thick sections of the polymer. Fillers may be added to the polyol component prior to dehydration, and thus dried at the same time as the polyol, or alternatively they may be pre-dried and added to the polyol subsequent to dehydration.

Dehydration is achieved by reactiong the polyol under vacuum for example at 1 mm Hg for approximately 4 hours at 100-1 20go. The final water content of the polyol should preferably not exceed 0.02% by weight, though in thin surface coating applications where bubbling is likely to be less of a problem, slightly higher water content may be satisfactory.

If it is desired to use the polyurethane resin in an application which will involve the exposure of a large area of the resin to the atmosphere during curing, it may be necessary to add a suitable reagent such as benzoyl chloride (typically 0.01-0.1% by weight to prevent atmospheric hydrolysis and consequent bubbling.

This may be added after the polyol has cooled to room temperature following dehydration. The catalyst may then be added (typically 0.02-0.2 by weight).

The dehydrated polyol component is then mixed at 20-40=C with a suitable quantity of the isocyanate component, which has been previously degassed for example at 1 mmHg for 10-15 minutes at room temperature, to provide an equivalent ratio range of hydroxyl to isocyanate groupings of from 1.00'0.80 to 1.00.1.10. An excess of isocyanate may be used to allow for bonding to, for example, residual hydroxyl groups in the nitrocellulose of a propellant charge, aand to allow for any secondary crosslinking reactions, such as biuret formation. An excess of isocyanate is also useful in a thin surface coating application of the resin, where it is desirable to minimise the variable effects of moisture absorption from the atmosphere.In this case it may be necessary to add controlled amounts of water after the mixing stage to hydrolyse the excess isocyanate. An excess of polyol may be used if a "tacky" surface is required of the cured polymer, for instance if the polyurethane resin is to be used as a barrier coat adhesive system for binding other inhibition materials to a propellant charge, at the same time reducing the diffusion of nitroglycerine from the propellant into the inhibiting medium.

The mixture is then degassed at for example 1 mmHg for 5 minutes at room temperature, following which it is ready for use. Depending on the composition and in particular the catalyst concentration, a 'pot-life' of 2-4 hours at room temperature can be expected, and adequate mechanical strength for handling may be achieved in about 5-6 hours though several days are usually necessary for complete cure. It is advisable to prepare a trial batch of the mixture to check for deterioration of the components and a suitable gel time before preparing a full-scale batch.

A polyurethane prepared as above will be suitable for a casting application. Inhibiting or barrier adhesive coatings suitable for application by brushing or spraying may be prepared by reducing the viscosity of the liquid mixture by the addition of suitable solvents. Compositions containing 60-80o solids using a 1:1 blend of toluene and ethylene glycol acetate have been found to be particularly suitable.

The preparation and use of a castable polyurethane resin according to the invention will now be described by way of example.

Polyol component The polyol component consisted of a mixture of two polyols, ie a polyether hexol, which was a polyoxypropylene adduct of sorbitol of molecular weight 675, containing a maximum of 0.100 water, and a polyethertriol, which was a polyoxypropylene adduct of glycerol of molecular weight 1000, again with a maximum water content of 0.1To, supplied by Lankro Chemicals Ltd, under the trade names Propylan RF 55 and Propylan G 1000 respectively.

Isocyanate component The isocyanate component consisted of a mixture of two aliphatic isocyanates, ie a tri-isocyanate comprising a biuret adduct of hexamethylene di-isocyanate (hereinafter referred to as HMDIB) with a molecular weight of 478 with the trade name Desmodu r N (1000o) manufactured by Bayer Ag in Germany, and a di-isocyanate comprising a mixture of the 2,2,4 - and 2,4,4 - isomers of trimethylhexamethylene ii-isocyanate with a molecular weight of 210.3 under the trade name of TMDI. manufactured by Veba-Chemie AC IN Germany.

Catalyst The catalyst employed was dibutyltin diacetate, provided by either Albright and Wilsons Chemical Division under the trade name Mellite 2, or Mand T International, Holland.

The polyol component was prepared by mixing 37.92 parts by weight of the polyether hexol with 14.04 parts by weight of the polyethertriol in a stirred reaction vessel equipped with a thermometer, nitrogen inlet and vacuum take off point. The mixture was covered with a blanket of oxygen-free dry nitrogen, and was stirred while beign heated to approximately 60C when the vacuum was slowly applied. The mixture was heated to 130 C-140-C under a vacuum of better than 1 mmHg and was held in this state for approximately 2 hours. After this period, the mixture was allowed to cool to 70-80-C when the vessel was re-pressurised with nitrogen and 0.03o by weight of the catalyst added.The vacuum was re-applied, and stirring continued for a further hour, when the vacuum was finally released and the stirring ceased. The warm mixture was then poured into warm, dry, glass jars. completely filling them and sealing them to exclude air and other airborne contaminents until the polyol was used.

The isocyanate component was prepared by weighing out the TMDI into a clean, dry glass or polyolefin vessel with a capacity of at least 1.5 times that of the proposed quantity of isocyanate blend. An equal w I g ht of HMDIB was then weighed in, and the mixture stirred by hand. taking care to minimise entrappment of air. The vessel was then placed in a suitalevacuum chamber and evacuated to less than lmmHg for 10-15 minutes or until bubbles ceased to rise in the mixture. The vacuum was then released and the isocyanate mixture stored in similar drv. airtight glass jars ina cool place until use.

Before mixing any of the material for inhibition casting, a trial polymerisation of about 100 gm scale was carried out in order to check that the gel time was satisfactory. Suitable amounts of the polyol and isocyanate components were mixed in a ratio of 1.08:1.00 by weight at 20-25'Cto give approximately 100 gm ofthe mixture, and the mixture was degassed under a vacuum of 1 mmHg for about 5 minutes.

After degassing. the trial blend was left to stand at room temperature, and its state assessed every few minutes with the aid of a glass rod. After 30-45 minutes from mixing, the viscosity increased drastically accompanied by a rise in temperature to 50-60DC, yielding a clear bubble-free block.

A typical apparatus which was used for casting an inhibiting coating around a cylindrical propellant charge is illustrated by way of example in Figure 1 and Figure 2 which illustrate a vertical and a horizontal section respectively through a mould for casting a coating around a cylindrical charge.

With reference to these Figures, the mould was in the form of a hollow aluminium body 1 split longitudinally into two sections, which were held together in use by one or more clamps 2. The bottom of the aluminium body was closed by a silicone rubber bung 3, which had a circular recess in its top surface to accommodate the propellant charge 4 and to locate it concentrically within the body. The inner wall of the body was lined with a tube of polyethylene film 5 as a release aid. The volume between the top surface of the propellant charge and the top of the aluminium body was sufficient to allow the propellant charge to be completely submerged in the liquid resin 7 and at least 5% excess of the liquid resin to be added.

To coat the charge with an inhibiting coating, the surfaces of the propellant charge and the mould which were to be in contact with the resin were thoroughly degreased with cylcohexane and allowed to dry before being assembled as in the Figures.

A suitable quantity of the polyurethene resin was prepared as above, and after vacuum degassing was carefully poured around thepropellant charge, taking care to avoid air entrapment. At the top of the mould, sufficient resin was added to allow for a 5% volum shrinkage during polymerisation. The assembly was left for 5-6 hours to cure sufficiently, when the mould was inverted and the rubber base removed. After checking that the polyurethane so exposed was cured, the clamps were removed, the aluminium body was split, the coated charge removed and the release aid film peeled off. The inhibition coating was left to cure completely over a period of several days.

The properties of this polyurethane composition cured as described above are tabulated below: Density at 25"C (BSI903 Part Al, Balance straddle method = 1.121 Mgm-3 Glass Transition Temperature (ASTM Dl 043-72) = 21"C Tensile properties at unit strain rate and 23"C (Sample: BS Type E Dumbbell, punched from cast sheet) 10% modulus = 69.4 MPa Ultimate Tensile Strength = 20 MPa Ultimate Elongation = 70% Typical properties of other polyurethanes prepared according to the invention are given in the table below by way of example.

TABLE 1 Properties of crosslinked polyurethanes based on aliphatic isocyanates Composition of Polyurethane 1 2 3 4 5 6 7 Ingredients mole mole mole mole mole mole mole sorbitol polyoxypropylene glycol adduct 1.0 1.0 0.8 0.8 1.0 1.0 1.0 polyoxypropylene glycol diol - - 0.2 - - - polyoxypropylene glycol triol - - - 0.2 - - glycerol - - - - 2.0 - hexamethylene diisocyanate 3.15 - - - - - 6.6 hexamethylene triisocyanate - 2.1 1.9 1.8 4.2 9.45 2,4,6 tris dimethyl amino methyl phenol (%) - - - - - 12 dibutyl tin dilaurate (%) 0.02 0.02 0.02 0.02 0.02 - benzoyl chloride (%) 0.05 0.05 0.05 0.05 0.05 - colour of moulding ---transparent clear white--- pale transparent yellow white tensile strength (MN)m-2) 40 10 35 41 35 42 42 45 elongation at break (%) 4 56 14 13 2 3 1 smoke on burning in propane gas flame ---no flame--- very slight no smoke slight smoke smoke Climatest 250 hours 34 C 85% rh none to very slight yellowing slight no effect yellowing

Claims (26)

1. A polyurethane for coating a propellant charge, characterized in that it has an average molecular weightbetween crosslinked units, as hereinbefore defined, of 200-600 and a carbon to oxygen ratio of less than 4.

2. A polyurethane according to claim 1 containing isocyanurate as well as urethane groups.

3. Apolyurethane according to claim 1 or claim 2 containing 5-50% by weight of low carbon to oxygen fillers.

4. A polyurethane according to claim 3 in which the fillers include ureaformaldehyde, polyoxymethylenes, cellulose acetate, oxamide or mixtures thereof.

5. A polyurethane substantially as herein described.

6. A process for preparing a polyurethane according to any of claims 1 to 5, involving the reaction of an aliphatic polyol component with an aliphatic polyisocyanate component.

7. A process according to claim 6, in which the polyol component is in excess of the stoichiometric requirement.

8. A process according to claim 6, in which the polyisocyanate component is in excess of the stoichiometric requirement.

9. A process according to claim 6,7 or 8 in which the polyol component contains at least one polyether polyol.

10. A process according to claim 9 in which the polyol component contains at least one polyether polyol with a Functonality of 4 to 8,a molecular weight of 400 - 1000 and a carbon to oxygen ratio not exceeding 3.

11. A process according to claim 10 in which the polyol component contains a polyoxypropylene glycol adduct of sorbitol.

12. A process according to any of claims 10 or 11 in which the polyol component contains polyether triols or diols with average molecular weights 300 - 2000 and carbon to oxygen ratios not exceeding 3, in a quantity corresponding to 0.1 - 0.3 mole equivalents of the polyether polyol of higher functionality.

13. A process according to claim 12 in which the polyether triols ordiols comprise polyoxypropylene glycol adducts of ethylene glycol, diethylene glycol, propylene glycol or glycerol.

14. A process according to any of claims 9 to 13 in which the polyol component contains low molecular weight chain extenders and crosslinkers.

15. A process according to claim 14 in which the chain extenders and cross-linkers are added in a quantity corresponding to 0.1 - 0.3 mole equivalents of the polyether polyol.

16. A process according to claim 15 in which the cross-linkers and chain extenders comprise diols, diamines ortriols.

17. A process according to claim 16 in which the cross-linkers and chain extenders comprise ethylene glycol, diethylene glycol, ethylene dramine or glycerol.

18. A process according to any of claims 6 to 17 in which the polyol component is dehydrated prior to the reaction so as to leave a final water content of not more than 0.02% by weight.

19. A process according to any of claims 6 to 18 in which the polyisocyanate component consists of one or more aliphatic di- or tri- isocyanates with C to 0 ratios of less than 5.

20. A process according to claim 19 in which the polyisocyanate component includes hexamethylene di-isocyanate, hexamethylene tri-isocyanate, diethyl fumarate di-isocyanate, adipo nitrile carbonate, a biuret adduct of trimethyl hexamethylene di-isocyanate or mixtures thereof.

21. A process according to any of claims 6 to 20 in which alkali metal phenolates, alcoholate carbo ylates or compounds with ortho (dimethyl-amincethyl) phenol groupings are added to the reaction mixture.

22. A process according to claim 21 in which 2,4,6 tris-dimethyl amino methyl phono is added to the reaction mixture in a quantity corresponding to 1 - 10% by weight of the reaction mixture.

23. A process according to any of claims 6 to 22 in which the reaction is catalyzed by an organometallic compound.

24. A process according to claim 23 in which the catalyst includes dibutyl tin oxide, dibutyl tin dilaurate, stannous octoate, zinc octoate, or a mixture thereof.

25. A polyurethane prepared by a process according to any one of claims 4 to 24.

26. A process for preparing a propellant charge substantially as herein before described with reference to the specific examples, and the accompanying drawings.

26. A process for preparing a polyurethane substantially or herein before described with refernce to the specific examples.

27. A propellant charge coated with a polyurethane according to any of claims 1 - 5 or 25.

28. A propellant charge coated according to claim 27 by a process substantially as herein described with reference to the accompanying drawing.

New claims or amendments to claims filed on 22nd April, 1980.

Superseded claims 1 to 28 New or amended claims:

1. A propellant charge coated with a polyurethane said polyurethane having an average molecular weight between crosslinked units, as hereinbefore defined, of 200-600 and a carbon to oxygen ratio of less than 4.

2. A propellant charge according to claim 1 wherein the polyurethane contains isocyanurate as well as urethane groups.

3. A propellant charge according to claim 1 or claim 2 wherein the polyurethane contains 5-50% by weight of low carbon to oxygen fillers.

4. A propellant charge according to claim 3 in which the fillers include ureaformaldehyde, polyoxymethylenes, cellulose acetate, oxamide or mixtures thereof.

5. A propellant charge substantially as herein described.

6. A process for preparing a propellant charge according to any of claims 1 to 5, wherein the polyurethane is produced by reacting an aliphatic polyol component with an aliphatic polyisocyanate component.

7. A process according to claim 6, in which the polyol component is in excess of the stoichiometric requirement.

8. A process according to claim 6, in which the polyisocyanate component is in excess of the stoichiometric requirement.

9. A process according to claim 6, 7 or 8 in which the polyol component contains at least one polyether polyol.

10. A process according to claim 9 in which the polyol component contains at least one polyether polyol with a Functionality of 4 to 8, a molecular weight of 400 - 1000 and a carbon to oxygen ratio not exceeding 3.

11. A process according to claim 10 in which the polyol component contains a polyoxypropylene glycol duct of sorbitol.

12. A process according to either of claim 10 or 11 in which the polyol component contains polyether triols or diols with average molecular weights 300 - 2000 and carbon to oxygen ratios not exceeding 3, in a quantity corresponding to 0.1 - 0.3 mole equivalents of the polyether polyol of higher functionality.

13. A process according to claim 12 in which the polyethertriols or diols comprise polyoxypropylene glycol adducts of ethylene glycol, diethylene glycol or glycerol.

14. A process according to any of claims 9 to 13 in which the polyol component contains low molecular weight chain extenders and crosslinkers.

15. A process according to claim 14 in which the chain extenders and crosslinkers are added in a quantity corresponding to 0.1 - 0.3 mole equivalents of the polyether polyol.

16. A process according to claim 15 in which the cross-linkers and chain extenders comprise diols, diamines ortriols.

17. A process according to claim 16 in which the cross-linkers and chain extenders comprise ethylene glycol, diethylene glycol, ethylene diamine or glycerol.

18. A process according to any of claims 6 to 17 in which the polyol component is dehydrated prior to the reaction so as to leave a final water content of not more than 0.02% by weight.

19. A process according to any of claims 6 to 18 in which the polyisocyanate component consists of one or more aliphatic di - or tri-isocyanates with C to 0 ratios of less than 5.

20. A process according to claim 19 in which the polyisocyanate component includes hexamethylene di-isocyanate, hexamethylene tri-isocyanate, diethylfumarate di-isocyanate, adiponitrile carbonate, a biuret adduct of trimethylhexamethylene di-isocyanate or mixtures thereof.

21. A process according to any of claims 6 to 20 in which alkali metal phenolates, alcoholate carboxylates or compounds with ortho (dimethylaminoethyl) phenol groupings are added to the reaction- mixture.

22. A process according to claim 21 in which 2,4,6-tris-dimethyl amino methyl phenol is added to the reaction mixture in a quantity corresponding to 1 - 10% by weight of the reaction mixture.

23. A process according to any of claims 6 to 22 in which the reaction is catalyzed by an organometallic compound.

24. A process according to claim 23 in which the catalyst includes dibutyl tin oxide, dibutyl tin dilaurate, stannous octoate, zinc octoate, or a mixture thereof.

25. A propellant charge prepared by a process according to any one of claims 6 to 24.