GB2073764A

GB2073764A - Crosslinked Propellants

Info

Publication number: GB2073764A
Application number: GB8108609A
Authority: GB
Original assignee: Hercules LLC
Current assignee: Hercules LLC
Priority date: 1980-03-20
Filing date: 1981-03-19
Publication date: 1981-10-21
Also published as: JPS56160395A; IT1146449B; IT8120595A0; NO810949L; DE3110795A1; FR2478623A1

Abstract

Disclosed are crosslinked propellant compositions having a polyurethane binder which is prepared using a biuret-linked polyisocyanate crosslinking agent, the polyisocyanate crosslinking agent being prepared by reacting a polyisocyanate having the formula: <IMAGE> with water in a molar ratio of polyisocyanate (I) to water of about 1:0.04 to about 1:0.4.

Description

SPECIFICATION Crosslinked Propellants This invention relates to improved crosslinked single base, such as nitrate ester polyether (NEPE), propellant compositions and crosslinked double base (XLDB) propellant compositions which exhibit improved mechanical properties resulting from the utilization of a high functionality biuret-linked polyisocyanate crosslinking agent.

Generally, XLDB and NEPE propellants are formulated with a crosslinked polyurethane binder system. For optimum mechanical properties the binder system contains two crosslinking systems. The XLDB propellants generally are comprised of a polyester diol and a nitrocellulose which possesses a hydroxyl functionality greater than three hydroxyl groups per molecule. The NEPE propellants are comprised of a polyether diol, such as, for example, polyethylene oxide, polytetrahydrofuran, or a copolymer of a polyether diol and either nitrocellulose or cellulose acetate butyrate, also containing more than three hydroxyl groups per molecule. The binders are plasticized with nitroglycerin or other nitrate esters such as, for example, triethylene glycol dinitrate (TEGDN) or butane triol trinitrate (BTTN) and are filled with particulate solid fuels and oxidizers.The hydroxyl-terminated diol and polyol functional groups are then crosslinked into a highiy extendable and tough elastomeric binder through the use of a polyisocyanate crosslinking agent.

One useful class of polyisocyanate crosslinking agents is the biuret-linked polyisocyanates.

Biuret-linked polyisocyanates are prepared by reacting three moles of a di- or polyisocyanate, such as, for example, hexamethylene diisocyanate, and one mole of water, eliminating one mole of CO2 in the process. One problem encountered in this reaction is that, when water per se is used in the reaction mixture, a water-insoluble urea intermediate may form which prevents the reaction from proceeding to completion. While some biuret-linked polyisocyanates have been prepared via the direct addition of water (see, for example, U.S. Patent No. 4,062,833), a more common technique is to add the water indirectly via water donors such as a tertiary alcohol (such as t-butanol), formic acid, or compounds containing water of crystallization or in statu nascendi (see U.S. Patents Nos. 3,358,010 and 3,124,605).Likewise, the reaction can be run using water in an appropriate auxiliary solvent (see, for example, U.S. Patent No. 4,072,702).

According to the invention, there is provided a biuret-linked polyisocyanate crosslinking agent that may be used as the crosslinking agent in a polyurethane binder system and that provides significant improvements in the mechanical properties of propellants containing the said system, the polyisocyanate cross-linking agent being prepared by reacting a polyisocyanate having the formula:

(hereinafter referred to as polyisocyanate (I)) with water in a molar ratio of polyisocyanate (I) to water of about 1:0.04 to about 1 :0.4. Preferably the biuret-linked polyisocyanate crosslinking agent has an average functionality of from about 3.5 to about 6.2.

The biuret-iinked polyisocyanates of this invention are prepared by mixing polyisocyanate (I) and water in a suitable vessel and heating the resulting reaction mixture at atmospheric pressure. Heating of the reaction mixture should not be so severe as to evaporate the water therefrom prior to reaction with polyisocyanate (I). Generally, the reaction temperature will be about 800C. Mild frothing of the reaction mixture occurs due to the evo!ution of CO2. After the major part of the gas evolution ceases, the temperature of the reaction mixture may be increased (e.g., to 1000C) to promote formation of the biuret-linked polyisocyanate in a reasonable length of time. Vigorous stirring of the reaction mixture should be maintained throughout the course of the reaction in order to assure complete reaction.

Precautions should be taken to prevent the entrance of exterior moisture into the reaction mixture, as by purging the reaction mixture with a stream of dry nitrogen. Also, during the major portion of the reaction, the evolved CO2 blankets the reaction mixture thereby aiding in the protection against outside moisture.

Throughout this specification, isocyanate concentration, expressed in milliequivalents of isocyanate per gram of biuret-linked polyisocyanate (meq NCO/g), is used to characterize the biuretlinked polyisocyanates. Isocyanate concentration is determined by reacting the biuret-linked polyisocyanate and an excess of di-N-butylamine and back titrating with standard HCI using brom cresol green as indicator.

The average functionality of the biuret-linked polyisocyanates is also indicated. As used herein, the term "average functionality" means the average number of isocyanate groups per molecule and is expressed as isocyanate groups per molecule (NCO groups/molecule). Average functionality is measured by the intrinsic gelation technique which comprises reacting samples of the biuret-linked polyisocyanate with various quantities of a hydroxy-terminated polyester (made from diethylene glycol and adipic acid) at about 500C in the presence of a urethane catalyst. After the reaction has reached completion, the sealed reaction vessels are inverted and those reaction mixtures containing gels are identified. Of the reaction mixtures which form gels, the one having the lowest concentration ratio of isocyanate to hydroxyl is determined.The test is then repeated using ratios of isocyanate to hydroxyl in a narrower concentration range, bracketing the one concentration isolated previously. This second run improves the precision of the measurement.

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

wnere f\cO=average functionality of the biuret-linked polyisocyanate.

fOH=average functionality of the hydroxy-terminated polyester, i.e., 2.

eqOH=equivalents of hydroxyl in the gel formed at the lowest isocyanate to hydroxyl ratio.

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

Thus, if 3.05 equivalents of hydroxyl react to form a gel with 1.95 equivalents of isocyanate, but a ratio of hydroxyl to isocyanate of 3.1 to 1.9 does not, then the average functionality is:

The average functionality of the biuret-linked polyisocyanates can be adjusted from about 3.0 (the average functionality of polyisocyanate (I)) up to about 6.2, the point at which the biuret-linked polyisocyanates show signs of internal crosslinking, gelation, and insolubility. This is accomplished by adjusting the molar ratio of polyisocyanate (I) to water from about 1:0.04 to about 1 :0.4. Thus, the average functionality of the biuret-linked polyisocyanates will increase as the polyisocyanate (I) to water molar ratio decreases, i.e., using more water at a fixed polyisocyanate (I) level.

The following examples illustrate the preparation of biuret-linked polyisocyanates useful in this invention. In the examples, and throughout this specification, all parts and percentages are by weight unless indicated otherwise.

Example A This example illustrates the preparation of a biuret-linked polyisocyanate from Desmodur N-1 O0 (manufactured by Baychem Corporation), a polyfunctional isocyanate principally comprising trifunctional isocyanate corresponding to formula I above and having an isocyanate concentration of 5.149 meq NCO/g and an average functionality of 3.5 NCO groups/molecule.

1000 g of Desmodur N-1 O0 is added to a 2-liter, 3-necked round bottom flask equipped with a mechanical stirrer, thermometer and addition port. The flask is purged with dry N2 and heated to 800C.

With vigorous stirring, 6.00 g of distilled water is added to the flask and the temperature maintained at 800C for 3 hours. During this time, the elimination of CO2 is obvious. The temperature is increased to 1000C and maintained at that level for 5 hours. The resulting biuret-linked polyisocyanate has an average functionality of about 6.1 NCO groups/molecule as measured by the intrinsic gelation technique and an isocyanate concentration of 4.222 meq NCO/g.

Example B Using the same procedure as Example A, 1000 g of Desmodur N-1 00 is reacted with 4.29 g of distilled water. The resultant biuret-linked polyisocyanate product is analyzed and found to have an isocyanate concentration of 4.417 meq NCO/g and an average functionality of 5.3 NCO groups/molecule.

Example C Using the same procedure as in Example A, 2734 g of Desmodur N-1 00 is reacted with 5.45 g of water. The resulting biuret-linked poiyisocyanate product has an isocyanate concentration of 4.778 meq NCO/g and an average functionality of 3.9 NCO groups/molecule.

The biuret-linked polyisocyanates are used in the propellants of the invention in proportions such that the ratio of equivalents of isocyanate to hydroxyl in the propellant is in the range of from about 0.8:1 to about 1.5:1.

The preparation of the propellants of this invention generally entails the preparation of a binder polymer premix comprising the binder polymer, a nitrosation stabilizer, and, optionaily, an energetic plasticizer. This premix is added to the propellant mix, warmed to the appropriate mix temperature and then the solid ingredients are added. Finally, the biuret-linked polyisocyanate crosslinking agent is added with a crosslinking catalyst such as, for example, dibutyltin diacetate or triphenyl bismuth, and the propellant is mixed to evenly disperse the ingredients. The propellant is then cured at about 490C.

Example 1 Various biuret-linked polyisocyanates are prepared, using the procedure in Example A, having the average functionalities shown in the first column of Table I below. These biuret-linked polyisocyanates are used as the crosslinking agent in several XLDB propellant compositions having the formulation: % By Weight Nitrate ester plasticizers 19.0 Nitrosation stabilizer 0.5 Solids 73.3 Nitrocellulose 0.2 Polyol 6.0 Biuret-linked polyisocyanate crosslinking agent 1.0 These propellants are each cast into a JANAF tensile specimen mold and samples are cured at 490C for seven days. One-quarter inch JANAF tensile specimens are cut from the cured propellants.

These specimens are tested for 2 inch per minute zero-time uniaxial tensile measurements.

Table I below indicates the 2 inch per minute uniaxial tensile behavior of the propellants. The second column in Table I indicates the ratio of equivalents of isocyanate to equivalents of hydroxyl (NCO/OH) in the propellant mixes. Since a portion of the nitrosation stabilizer, together with some traces of water, consumes a portion of the isocyanate via side reactions, slightly more isocyanate is used than would be necessary to react only with the hydroxyl functionalities. Thus, an NCO/OH ratio of 1.2 means that, for every equivalent of hydroxyl, 1.2 equivalents of isocyanate are used.

Table I Mechanical Properties (a) Avg. NCO/OH Functionality Ratio am Em orr Er E 3.2 1.2 64 306 62 308 165 3.3 do 69 247 68 252 303 3.4 do 72 223 72 227 367 3.6 do 76 189 75 200 442 3.8 do 82 156 81 156 508 4.0 do 86 140 85 143 572 4.2 do 84 140 83 140 545 4.9 do 93 109 93 109 664 3.2 1.3 69 234 68 235 186 3.3 do 78 121 77 216 284 3.4 do 85 202 85 204 372 3.6 do 90 184 89 189 445 3.8 do 94 172 94 172 488 4.0 do 98 156 98 156 504 4.2 do 84 140 83 140 545 4.9 do 107 107 107 107 804 4.9 do 107 107 107 107 maximum stress, 804 o'r=stress at rupture, p.s.i.; car,,=maximum stress, p.s.i.; Em=strain at Erstrain at rupture, p.s.i.; E=Young's modulus, p.s.i.

To further illustrate the effect on propellant mechanical properties, a plot of average functionality of the biuret-linked polyisocyanate versus propellant modulus is provided in the Figure, using the data in Table I. As can be seen, average functionalities below 3.4 NCO groups/molecule cause the propellant moduli to drop markedly. Modulus is very sensitive to slight fluctuations in effective functionality in this range. Perturbations in the degree of crosslinking would thereby influence greatly the reproducibility of this parameter. Decreases in crosslinking could result from increased amounts of chain termination in the polyester diol and variances in crosslinking catalyst activity. These changes would affect the amount of nitrosation stabilizer or moisture which could become chain terminated. Thus, from a reproducibility standpoint, the higher moduli resulting from average functionalities of the biuret-linked polyisocyanates greater than 3.4 NCO groups/molecule would be desirable.

Claims

1. In a crosslinked propellant composition having a polyurethane binder which is prepared using a polyisocyanate crosslinking agent, the improvement comprising the biuret-linked polyisocyanate crosslinking agent prepared by reacting a polyisocyanate having the formula:

and water in a molar ratio of polyisocyanate to water of about 1:0.04 to about 1:0.4.

2. The propellant composition of claim 1, wherein the biuret-linked polyisocyanate crosslinking agent has an average functionality of from about 3.5 to about 6.2.

3. The propellant composition of claim 1 substantially as hereinbefore described in any of the Examples.