NO810949L - CROSS-BONDED FUEL. - Google Patents

Description

The present invention relates to improved cross-linked single base propellant compositions such as nitrate ester-polyether (NEPE) and cross-linked double base (XLDB) propellant compositions which show improved mechanical properties as a result of the use of a biuret-linked polyisocyanate cross-linking agent with high functionality.

In general, XLDB and NEPE propellants are formulated with

a cross-linked polyurethane binder system. For optimal mechanical properties, the binder system contains two cross-linking systems. The XLDB propellants generally consist of a polyester diol and a nitrocellulose that has a hydroxy functionality of more than three hydroxyl groups per molecule. The NEPE propellants consist of a polyetherdiol such as e.g. polyethylene oxide, polytetrahydrofuran or a polymer of a polyetherdiol and either nitrocellulose or cellulose acetate butyrate, also containing more than three hydroxyl groups per molecule. The binders are softened with nitroglycerin or other nitrate esters such as e.g. triethylene glycol dinitrate (TEGDN) or butane triol trinitrate (BTTN) and is filled with particulate solid fuel and oxidizing agents. The hydroxyl-terminated diol and polyol functional groups are then crosslinked into a highly stretchable and tough elastomeric binder using a polyisocyanate crosslinker.

A useful class of polyisocyanate cross-linking agents

are the biuret-bonded polyisocyanates. Biuret-bonded polyisocyanates are prepared by reacting three moles of a di- or polyisocyanate such as e.g. hexamethylene diisocyanate, and one mole of water, and eliminate one mole of CC>2 in the process. A problem with this reaction is that when water is used per se in the reaction mixture, a water-insoluble urea intermediate can be formed which prevents the reaction from going to completion. While some biuret-linked polyisocyanates are prepared via direct addition of water (see eg US-PS 4,062,833), a more general technique is to add the water indirectly via water donors such as a tertiary alcohol, e.g. t-butanol, formic acid, or compounds containing crystal water or in statunascendi (see US-PS 3,358,010 and 3,124,605). In the same way, the reaction can be carried out by using water in a suitable auxiliary solvent,

see e.g. US-PS 4,072,702.

According to the invention, a biuret-bonded polyisocyanate cross-linking agent is provided which can be used as a cross-linking agent in polyurethane binder systems and which provides significant improvements in the mechanical properties of propellants that contain the system, the polyisocyanate cross-linking agent being produced by reacting a polyisocyanate with the formula:

(hereinafter called polyisocyanate (I)) with water in a molar ratio between polyisocyanate (I) and water of approx. 1:0.04 to approx. 1:0.4. Preferably, the biuret-bonded polyisocyanate cross-linking agent has an average functionality of from 3.5 to 6.2.

The biuret-bonded polyisocyanates according to the invention

is prepared by mixing polyisocyanate (I) and water in a suitable container and heating the resulting reaction mixture at atmospheric pressure. Heating of the reaction mixture should not be so severe that water evaporates from it before the reaction with polyisocyanate (I). In general, the reaction temperature will be approx. 80°C. A mild foaming of the reaction mixture occurs due to the evolution of C02. After the main part of the gas evolution is finished, the temperature in the reaction mixture can be increased, e.g. to 100°C, to promote formation of the biuret-bound polyisocyanate in a reasonable time. Violent stirring of the reaction mixture should be maintained throughout the reaction to ensure complete reaction.

Precautions should be taken to prevent external moisture from entering the reaction mixture, e.g. by flushing the reaction mixture with a stream of dry nitrogen. During most of the reaction, the developed C02 supports this.

In the present description, the isocyanate concentration is expressed in milliequivalents of isocyanate per grams of biuret-bonded polyisocyanates (meq NCO/g), to characterize the biuret-bonded polyisocyanates. The isocyanate concentration is determined by reacting the biuret-bound polyisocyanate and an excess of di-N-butylamine and then titrating back with standard HCl using bromocresol green as an indicator.

The intermediate functionality of biuret-linked polyisocyanates is also suggested. As used herein, the term "average functionality" means the average number of isocyanate groups per molecule and the expression as isocyanate groups per molecule (NCO groups/molecule). Medium functionality is measured by a technique that is based on intrinsic gel formation and which involves reacting samples of biuret-bound polyisocyanate with varying amounts of a hydroxy-terminated polyester (made from diethylene glycol and adipic acid) at approx. 50°C in the presence of a urethane catalyst. After the reaction is finished, the closed reaction containers are turned over and the reaction mixtures containing gel are identified. Of the reaction mixtures that form a gel, the lowest concentration ratio between isocyanate and hydroxyl is determined. The test is then repeated using a ratio between isocyanate and hydroxy in a narrower concentration range around the previously isolated concentration. This second sample improves the precision of the measurement.

The average functionality for the polyisocyanate is then calculated as follows:

where: fN£q = average functionality for the biuret-bonded polyisocyanate

f0H= average functionality for the hydroxy-terminated tea

polyester, i.e. 2

eqOH = equivalents of hydroxyl in the gel formed by it

lowest ratio between isocyanate and hydroxyl

eqNCO = equivalent isocyanate in the gel formed at the lowest isocyanate to hydroxyl ratio.

If thus 3.05 equivalents of hydroxyl react and form a gel with 1.95 equivalents of isocyanate while a ratio between hydroxyl and isocyanate of .3.1 to. 1.9 doesn't do it then the average functionality:

The average functionality for biuret-bonded polyisocyanates can be adjusted from approx. 3.0 (the average functionality for polyisocyanate (I)) up to approx. 6.2, the point at which the bi-uret cross-linked polyisocyanates show signs of internal cross-linking, gelation and insolubility. This happens by adjusting the molar ratio between polyisocyanate (I) and water from approx. 1:0.04 to approx. 1:0.4. Thus, the average functionality for the biuret-bonded polyisocyanates will increase when the molar ratio between polyisocyanate (I) and water decreases, i.e. by using more water at a fixed polyisocyanate (I) level.

The following examples show the production of biuret-bonded polyisocyanates which can be used according to the invention.

In the examples and in the description, all parts and percentages are by weight unless otherwise stated.

Example A

This example shows the production of a biuret-bound polyisocyanate from "desmodur N-100", a polyfunctional/isocyanate principally comprising a trifunctional isocyanate corresponding to formula I above and with an isocyanate concentration

of 5.149 milliequivalent NCO/g and an average functionality of 3.5 NCO groups/molecule.

1000 g "desmodur N-100" is added to a 2-liter 3-necked round flask equipped with mechanical piping, thermometer and addition opening. The bottle is flushed with dry N2 and heated to 80°C. With vigorous stirring, 6.00 g of distilled water is added to the flask and the temperature is maintained at 80°C for 3 hours.

During this period, elimination of C02 is evident. The temperature is increased to 100°C and held at this level for 5 hours. The resulting biuret-bonded polyisocyanate has an average functionality of 6.1 NCO groups/molecule measured by the previously stated technique and an isocyanate concentration of 4.222 milliequivalent NCO/g.

Example B

By using the same procedure as in example .A

react 1000 g of "desmodur N-100" with 4.29 g of distilled water.

The resulting biuret-bonded polyisocyanate product is analyzed and found to have an isocyanate concentration of 4.417 milliequivalents of NCO/g and an average functionality of 5.3 NCO groups/molecule.

Example C

By using the same procedure as in example A, 273 4 g of "desmodur N-100" are reacted with 5.4 5 g of water. The resulting biuret-bonded polyisocyanate product has an isocyanate concentration of 4.778 milliequivalents of NCO/g and an average functionality of 3.9 NCO groups/molecule.

The biuret-bonded polyisocyanates are used in the propellants according to the invention so that the ratio between isocyanate and hydroxyl equivalents in the propellant is within the range of 0.8:1

to approx. 1.5:1.

The production of the propellants according to the invention generally entails the production of a binder polymer for mixing comprising the binder polymer, a nitrosation stabilizer and possibly an energetic plasticizer. This premix is added to the propellant mixture, heated to a suitable mixing temperature and then the solid components are added. Finally, the biuret-bound polyisocyanate cross-linking agent is added with a cross-linking catalyst such as e.g. dibutyl tin diac.e-tate or triphenyl bismuth and the propellant are mixed to disper-_:;. mix the ingredients evenly. This propellant is then hardened at approx. 49°C.

Example 1

Various biuret-bonded polyisocyanates are prepared using the method according to Example A with the average functionalities shown in the first column of Table I below. These biuret-bonded polyisocyanates are used as cross-linking agents in various SLDB propellant compositions

with the following wording:

These propellants are each cast in a "JANAF" test mold and samples are cured at 49°C for 7 days..6.35 m "JANAF" test pieces are cut from the cured propellants. These samples are tested at 5.08 cm/minute 0 time uniaxial tensile measurements.

Table I below indicates the behavior of the propellants in this test. The second column in Table I indicates the ratio between equivalents of isocyanate and equivalents of hydroxyl (NCO/OH) in the propellant mixtures. Because part of the nitrocellulose stabilizer together with some traces of water consumes part of the isocyanate via side reactions, somewhat more isocyanate is used than would be necessary only for reaction with the hydroxyl functions. Thus, an NCO/OH ratio of 1.2 means that 1.2 equivalents of isocyanate have been used for each equivalent hydroxyl.

To further show the effect on the mechanical properties of the propellants, a diagram was set up for the average functionality of the biuret-bonded polyisocyanate against the propellant module as indicated in the figure, all on the basis of the data given in Table I. As can be seen, medium functionalities below 3.4 NCO groups/molecule cause propellant moduli to drop markedly. The module is very sensitive to slight fluctuations in the effective functionality in this area. Deviations in the degree of cross-linking will therefore strongly influence the reproducibility of this parameter. Reduction in cross-linking can stem from an increased amount of chain termination in the polyester diol and variations in the cross-linking catalyst activity. These changes affect the amount of nitrosation stabilizer or moisture that could be chain terminated. From a reproducibility point of view, higher moduli as a result of intermediate functionalities in biuret-bonded polyisocyanates above 3.4 NCO groups/molecule would be desirable.

Claims (2)

1. Cross-linked propellant composition with a polyurethane binder which is prepared using a polyisocyanate cross-linker, characterized in that the biuret-bound polyisocyanate cross-linker is prepared by reacting a polyisocyanate with the formula: with water in a molar ratio between polyisocyanate and water of from approx. 1:0.04 to approx. 1:0.4. 2. Propellant according to claim 1, characterized in that the biuret-bound polyisocyanate cross-linking agent has an average functionality of approx. 3.5 to approx. 6.2.