NO144486B - HARDENABLE BINDING AGENT. - Google Patents

Abstract

Curable binder.

Description

This invention relates to a hardenable binder, comprising three components, for use in the production of castable, composite propellants. One of the components is hydroxyl-terminated polybutadiene (HTPB), the second a diisocyanate curing agent and the third a debonding agent.

Castable propellants in which polybutadiene-based binders are used together with, for example, ammonium perchlorate oxidant, are well known. The mechanical and ballistic properties of such composite solid propellants are very dependent on the quality of the adhesive bond between the binder and the oxidant, as discussed in US Patent No. 3,745,074. There are known systems in the industry that have reasonably good adhesion between fuel-binder and oxidant, but with such systems there are other disadvantages.

Such a system uses carboxyl-terminated polybutadiene (CTPB) binders cured with a system comprising aziridines or epoxides or a mixture thereof. Propellants using such binder systems have high elongation over a wide temperature range and a relatively large ability to carry solid particles, referred to as solid charge. Adhesion between binder and oxidant is quite good as the preferred aziridine curing agent tris(2-methyl-aziridinyl-1)phosphine oxide known as MAPO is polymerized around the ammonium perchlorate oxidant particles

to form a strong layer bonded to the bulk of the binder by chemical bonds. However, this system has the disadvantages that it comprises a complicated curing system, the propellants have poor aging properties when exposed to high temperatures and the properties of the polymer binder vary from one batch to another.

Another system that was developed in an attempt to overcome the disadvantages of the above system is the system of HTPB-based composite propellants using hydroxyl-terminated polybutadiene prepolymers. These systems are cured with diisocyanates. With these propellants, it is necessary

you to use a binding agent to provide good adhesion between the binding agent and the oxidant particles. Aziridine polyesters are effective binding agents for these propellants,

and a preferred aziridine polyester is a reaction product between MAPO and diacids. However, this preferred binding agent must be synthesized on site in relatively small amounts

since it has a short shelf life at room temperature and must

is kept at a low temperature to extend its useful life, and a relatively high concentration is required, approx. 2 to 3% of aziridine polyester in the binder, to provide an effective adhesion between the oxidant and the binder, especially when the fixed charge is high.

The present invention is based on that discovery

that a combination of two release agents, an aziridine polyester and an amine polyester, together with HTPB type propellants provide the proper adhesion to maintain the compressive stress capability of the propellant and enable a reduction by a factor of 3 to 4 of the amount of aziridine polyester that is necessary, compared to the known systems. Propellant cylinders discussed here have, for example, increased mechanical properties, including improved elongation at maximum load and at break at comparable tensile strengths and original modules, compared to HTPB propellant systems where only aziridine-polyester binder is used.

According to the invention, a hardenable binder is thus provided for use in the formation of a castable propellant which comprises:

(i) a hydroxyl-terminated butadiene polymer,

(ii) a diisocyanate curing agent, and

(iii) a binder system consisting of an aziridine polyester which is a reaction product of an aziridinyl phosphine oxide and at least one polycarboxylic acid, and an amine polyester which is a reaction product of an alkanolamine and a saturated aliphatic polycarboxylic acid. An antioxidant, such as phenyl-(3-naphthylamine), can also be incorporated together with isodecyl pelargonate which softens.

In the production of a castable propellant using the aforementioned binder, one finely divided perchlorate oxidant is dispersed in di- or trimodal distributions, and other possible ingredients, such as a metal additive. When producing the compound propellant, good dispersion of the ingredients is achieved, and reproducible mechanical and ballistic properties are achieved during processing.

The HTPB prepolymers which are suitable for use in binders according to the present invention are, for example, of the type described in Canadian Patent No. 891,562 and US Patent No. 3,792,003. The prepolymers which are described therein, are hydroxyl-telechelic polybutadienes, especially hydroxyl-terminated polybutadienes which are desirably rich in cis isomers. They are obtained by reacting a monoepoxy compound and the corresponding carboxy polymers. the product is a polymer having hydroxyl groups, including such primary and secondary groups, attached near the end of the polymer molecule, and preferably at the end of the molecule. The monoepoxy compound is an organic compound containing a single epoxy group, including monoepoxy resins. particularly preferred are epoxy compounds of the formula: where each of R and R1 is hydrogen, aryl or an alkyl-preferably lower alkyl group, and more particularly 1,2-alkylene oxides, such as propylene oxide and 1,2-butylene oxide, which form secondary hydroxyl groups when they are reacted with a carboxyl telechelic polymer. The carboxyl-terminated polybutadienes are conveniently those available under the trade name "HC 434" from Thiokol Chemical Corporation and under the trade name "HYCAR-CTB" from B.F. Goodrich and Company. A typical hydroxyl-terminated polybutadiene thus has a molecular weight of approx. 3800 and is assumed to have the following formula:

where x and y each vary between 1 and 67.

A preferred HTPB prepolymer is that sold under the trade name "Poly bd R-45M" by Arco Chemicals Co. and which has the following formula:

where n = 44-60, and the polybutadiene structure is 60% trans-1,4,

20% cis-1,4 and 20% vinyl-1,2.

A particularly preferred HTPB prepolymer is that sold under the trade name "R-45HT" by Arco Chemicals, and this has the same formula as "R-45M", the difference being the value of "n" which is 57-65 when it comes to about "R-45HT".

The prepolymer is used as the main ingredient in the propellant/binder system, and suitably in an amount of approx. 50-85% by weight, preferably in an amount of 60-65% by weight, of the binder mixture. An antioxidant is suitably used together with the prepolymer, and it can e.g. be phenyl-/?-naphthylamine (PBNA), bisphenol "AO-2246" (American Cyanamid) or various compounds having the chemical group p-phenylenediamine.

The preferred antioxidant for the purposes of the present invention is phenyl-/3-naphthylamine in a concentration of approx. 1% by weight of the prepolymer.

When "R-45HT" is used as the HTPB prepolymer, an antioxidant consisting of one or more of the following substances can be used:

A synergistic effect was achieved when a combination of "DTBHQ" and "UOP-36" or "FLEXZONE 7L" was added to the fuel mixture in an amount of 1% of the prepolymer.

The diisocyanate hardener for use in the binder mixture according to the invention is, for example, 2,4-tolylene diisocyanate (TDI), 1,6-hexamethylene diisocyanate (HMDI) or DDI which is a mixture of isomers of diisocyanate containing 36 carbon atoms produced by dimerization of fatty acids with 18 carbon atoms. The preferred diisocyanate is DDI for the purposes of the present propellant/binder systems. The ratio iso-cyanate/hydroxyl (NCO/OH) is adjusted to give the best possible mechanical properties to the resulting propellant, as is known from previous systems. For present purposes, a typical NCO/OH equivalence is in the range of 0.75-0.95.

To facilitate the mixing of the binder mixture with

the solid oxidant when producing the castable propellants discussed here, a plasticizer is appropriately incorporated into the binder mixture in an amount of approx. 15 to 30% based on the total weight of the binder mixture. Compounds which are suitable for plasticizers are well known in the polymer industry, and in the present mixtures can for example be dioctyl adipate (DOA), diethylhexyl azelate (DEHA) or isodecylpélargpnate (IDP). However, the preferred plasticizer is IDP.

The aziridine polyester materials used in accordance with the present invention are prepared by the reaction of tris-(2-methyl-aziridinyl-1)-phosphine oxide, commonly known as MAPO, with a straight-chain diacid of the general formula HOOC(CH2 )xCOOH (x is 2-8) and with another substituted diacid of the general formula HOOCCH(R)(CI^)^CH(R^)COOH in which R and R^ may be the same or different and may be H or OH and y = 0-6, the phosphine oxide being reacted in an amount of 1 mol per carboxyl oxyvalent of the acid.

The amount of the aziridine polyester debinding agent in the binder according to the present invention is between 0.1 and 1% by weight of the total binder mixture.

The other component of the binder combination which is used in the binders according to the present invention is the amine polyester material, which as indicated is a:; reaction product of N-methyldiethanolamine and sebacic acid, hWhich: is known as "polymer Nr-8". Suitable "N-8 Polymers" have the following specifications: ';-■<*>•;-■

Mn (number average molecular weight): 1200-2000, preferably 1500-1800 Acidity: <0.02 equiv./100 g Equivalent OH: 1-1.5 m-equiv./g, preferred 1.1-1.3 Moisture: <0.15% by weight

This binder is used in an amount of approx. 0.1-0.5% by weight of the total binder mixture.

To form the castable propellant, the binder is mixed before curing with finely divided ammonium perchlorate as an oxidant by dimodal or trimodal distribution of fine powder with an average size of 1-400 ym. A preferred particle size distribution of the ammonium perchlorate is 1.7/2.7/1.0 parts of sizes of 400 um, 200 um and 17 um respectively. For use at high temperature, the ammonium perchlorate can be replaced with potassium perchlorate. An anti-caking agent is suitably incorporated, e.g. tricalcium phosphate, in an amount of approx. 1% by weight of the 17 ym perchlorate. If desired, a metallic additive can be incorporated into the propellant mixture in a concentration of 0-20% by weight of the total propellant. This additive can be finely divided aluminum or magnesium powder, preferably with an average particle diameter of 5-50 µm. The other solid material which can suitably be incorporated into the overall mixture is an additive or a catalyst for burning rate, and this can e.g. be iron oxide, copper chromite or an organometallic compound. As a preferred additive or catalyst for the present purpose, ferric oxide is used in an amount of 0.1-1% by weight of the propellant mixture.

In composite propellant mixtures, it is of course desirable to achieve as high a solid charge as possible, and with the propellant mixtures discussed here, the total solids content can vary between 85 and 90% by weight of the total mixture, and of this, of course, the ammonium perchlorate constitutes the main part and it can be between 68 and 88% by weight of the total driving medium mixture. The other fixed components are adjusted accordingly. Thus, the polymeric binder will amount to approx. 10-15% by weight of the total propellant mixture.

In order to achieve good dispersion of all the ingredients in the preparation of the propellant mixture and also reproducible properties from one batch to another, the preferred process is that the liquid ingredients which are the prepolymer, the plasticizer and the two binding agents are first added to a mixer. The aluminum or other metal powder, when used, and the ferric oxide burn rate additive are then added and the contents mixed for 15 minutes at atmospheric pressure. Then approximately three-quarters of the total amount of ammonium perchlorate is added to the mixture and mixing is continued for a further 10 minutes. A vacuum is applied and the contents of the mixer are heated to 60°C for approx. 60 minutes. The vacuum is then released and the remaining amount of ammonium perchlorate is added and the mixture is continued for a further 5 minutes at atmospheric pressure. The diisocyanate hardener is then added to other ingredients in the mixture and mixing is continued for 5 minutes at atmospheric pressure. Vacuum is then applied again and the mixture is continued for 45 minutes at a temperature of 60°C. The propellant is then cast into molds under vacuum.

The following example illustrates the use of the binder according to the present invention in the production of compound solid propellants. This example is intended to illustrate the present invention. Each of the propellant mixtures given in the example was prepared according to the method described in the previous section.

The mechanical properties tested for the propellant mixtures were tensile strength (er m), elongation at maximum load (em)/ elongation at break (ey) and initial modulus (E). These tests were performed in an Instron apparatus with a cross-head speed of 50.8 mm/min. corresponding to a deformation rate of 0.741 min.<-1> for the ICRPG "manual" test piece.

EXAMPLE

Propellant mixtures were produced with the following amounts of ingredients expressed in weight%:

For testing, three binder components were used in the above-mentioned propellant mixture. The prepolymer used in each binder was the "R4 5-M" type of HTPB binder sold by Arco Chemical Co. and previously described here. Phenyl-p-naphthylamine was incorporated as antioxidant, isodecyl pelargonate as plasticizer and DDI diisocyanate as curing agent. In one binder mixture, the two binders mentioned here were not used, but only the aziridine polyester, for comparison purposes. In the other two mixtures, both debinding agents were incorporated.

The binder mixtures were as follows:

The aziridine polyester of this example was prepared

by reacting 0.15 mol d-tartaric acid and 0.3 mol adipic acid with 1 mol MAPO (aziridine) by the method previously described here. The amine polyester (N-8) in this example had the following properties:

The achieved properties for processing and mechanical properties for the propellant mixtures using the aforementioned binders were: Mechanical properties (originally)

Mechanical properties (56 days at 60°C)

It will be seen from the above results that the processing properties, i.e. the viscosity at the end of mixing and the time of use, for the propellants (II and III) were essentially the same as for the comparison propellant. However, according to the invention, the amount of aziridine polyester used was considerably less than the amount required for the comparison mixture. The propellants also exhibited improved elongations at maximum load and at break, while tensile strength and initial modulus were comparable to the values for the comparison mixture.

Claims (2)

1. Hardenable binder for use in the production of a castable, compound propellant, comprising the following components: (i) a hydroxyl-terminated butadiene polymer, (ii) a diisocyanate curing agent and (iii) a debinding agent, characterized in that the debinding agent consists of 0.1-1% by weight calculated on the total binding agent of an aziridine polyester which is the reaction product of tris-(2-methyl-aziridinyl-1)-phosphine oxide and a diacid selected from the group comprising a straight-chain diacid of the general formula HOOC (C^)xCOOH, where x is 2-8, and a substituted diacid having the general formula HOOCCH(R)(CH2)yCH(R1)C00H, in which R and R± can be the same or different and can be H or OH, and y = 0 - 6, the phosphine oxide being reacted in an amount of 1 mol per carboxyl equivalent of the acid^ and 0.1-0.5% by weight calculated on the total binder of an amine polyester which is the reaction product of N-methyldiethanolamine and sebacic acid. 2. Binder according to claim 1, characterized in that the diacids are d-tartaric acid and adipic acid in the respective amounts of 0.15 and 0.3 mol per moles of aziridine.