US3305523A - Modification of telechelic-type polymers - Google Patents

Description

United States Patent 3,305,523 MODIFICATION OF TELECHELIC-TYPE POLYMERS Charles II. Burnside, Waco, Tex., assignor to North American Aviation, Inc. No Drawing. Filed Aug. 30, 1962, Ser. No. 221,654 4 Claims. (Cl. 26046.5)

This invention concerns a novel solid propellant polymeric binder. More specifically, this invention pertains to a solid propellant binder having unexpectedly high elongation properties and high solid loading ability.

A typical composite solid propellant is comprised of solid particulate fuel such as minute particles of a metal and a solid particulate oxidizer which serves to oxidize the fuel particles. In order to secure the particles of oxidizer and fuel in an intimate mixture necessary for effective combustion and practical use, a resinous matrix material or binder is required. The binder material normally additionally serves as a fuel component of the solid propellant. This binder is normally a liquid resinous i material and when mixed with high percentages of the solid particles of fuel and oxidizer, forms a viscous material which may be cast in a mold and cured for the desired shaped propellant charge. Physical properties are desired in a solid propellant. These properties are normally in an interrelated combination in that the maximum dilute of all the parameters does not necessarily yield the best solid propellant. One parameter depends upon the value of another in several performance evaluations. The solid propellant should be stable for a long period of time and should not deteriorate chemically or physically during storage. High density of the solid propellant permits the use of a small chamber volume and therefore small chamber weight. The propellant should lend itself readily to production and have desirable fabrication properties such as adequate fluidity during casting or easy to control chemical processes such as curing and a minimal volume change after casting or molding. The mechanical properties and the combustion characteristics such as burning rate should be predictable and uneifected appreciably over a wide range of storage and operating temperatures. This means that the temperature sensitivity should be low.

Additionally the propellant should exhibit good mechanical properties, particularly its tensile, compressive and sheer strength and its rnodulous of elasticity and elongation. Finally the propellant should have high solids loading capability.

The previously set forth desirous properties in a solid propellant have been accomplished through the use of propellants which utilize a carboxy-terminated linear polybutadiene binder. The so-called Flexadyne family of solid propellants was developed specifically to have superior mechanical properties in tension, compression, tear and creep and to maintain a good balance of such properties over the temperature range 75 to 170 F.

One of the components of this family of propellants that gives the group the outstanding properties is the propellant binder polymer utilized. The polymer binder used in these propellants is a carboxy-terminated linear polybutadiene. The excellent properties of the binder are attained when the starting initial molecules of butadiene have the hydrogen atoms at the double bonds in carefully ordered positions in space to obtain a symmetrical cross-linking of the cured polymer. Additionally, the polymer has a properly controlled cistrans stereoisomerism, when the polymerization is stopped at 100 molecules of butadiene per chain, when the number or nature of possible side chains is concerned and when a carboxylic group is attached at either end of each polybutadiene molecule and nowhere else. The resulting symmetrical 3,305,523 Patented Feb. 21, 1967 structure has good elastorneric properties and is achieved in the binder by controlling the chemistry and of the polymerization process itself. This type of polybutadiene polymer is referred to as a telechelic-type polymer and the method of preparation is well known in the art and is described in the Journal of Polymer Science, vol. XLVI, Issue 148, pp. 535-539 (1960). An example of the carboxy-terminated polybutadiene binder used in this group of propellants is Butarez CTL manufactured by the Phillips Petroleum Company. In addition to the carboxy-terminated linear polybutadiene telechelic-type polymers other possible terminal groups can successfully be utilized in the present applications. Such other terminal functional groups would include hydroxy radicals, amine, mercapto and isocyanate radicals. It is understood that any telechelic polymer regardless of the terminal functional groups can be successfully utilized.

The present invention concerns the further improvement of mechanical properties of the telechelic polymer binders previously described. Particularly, the invention is directed to the improvement of the mechanical properties of the binder material over a wide range of temperature varying from less than -70 F. to excess of F. The greatly improved mechanical properties are accomplished in the present invention by the extension of the telechelic polymer chains through increasing the chain length and molecular Weight by the addition of from 0.4 to 10 parts by weight of difunctional materials, such as, for example, vinylcyclohexene dioxide. Particularly preferred is from 0.6 to 5 parts by weight of the difunctional additives. The difunctional additive that is utilized depends upon the functional terminal group in the telechelic polymer compound. The difunctional additive must be one that will react with the functional terminal group of the polymer in order for an effective result to be obtained. As will hereinafter be set forth, various difunctional additives may be used depending upon the functional terminal group of the telechelic polymer. Thus, as can be seen, the difunctional additive becomes part of the polymer chains.

constancy of mechanical properties of a solid propellant or erratic temperature change is desirable so that brittleness at low temperature and plastic flow at high temperatures do not limit the use of the rocket propellant to a narrow temperature range. One of the many requirements of a propellant charge is that it be plastic enough or have enough elongation to change dimensions without cracking as required by the rocket motor expansion under pressure. High elongation of the rocket propellant is desirable to accommodate the differential expansion properties between the motor chamber and the propellant. As a result, it is of great significance that the mechanical properties of a solid propellant binder of a telechelic-type can be greatly improved through the addition of the difunctional compounds of this invention. The elongation of the propellants utilizing the binders of this invention have increased 40 to 50 percent over a temperature range of 70 to 170 F. In some instances this represents a doubling of the percent elongation of the propellant.

The telechelic polymers used in the propellant binder as previously described are not only polybutadienes. The invention also pertains to the improvement of the mechanical properties of telechelic polymers which may be of other skeletal structures such as any polymerizable monomer containing an ethylenic linkage.

Non-limiting examples of the monomers that may be utilized in the invention to produce the telechelic polymers include the vinyl halides such as iodoethene, bromoethene, fiuorethene, chloroethene; vinylidene halides including vinylidene chloride, vinylidene iodide, and the like; vinyl esters of carboxylic acids having 1 to 10 carbon atoms including vinyl acetate and the like; and N-substituted acrylamides and methyacrylamide including N methylacrylamide, N-propyl methyacrylamide, N-hexyl aerylamide and the like; allyl and methya-llyl monomers including allylamine, N-methylallylamine, allyl bromide, allyl alcohol, allyl chloride, allyl cyanide, allyl fluoride, allyl glycidyl ether, allyl isodide, allyl mercaptan, allyl sulphide, and the like; isopropenyl monomers including isopropenyl bromide, isopropenyl chloride, isopropenyl fluoride, and the like; vinyl compounds including vinyl alcohol, vinyl bromide, vinyl fluoride, vinyl cyanide, vinyl sulphide, vinyl tribromide, vinyl ether, vinyl napthalene, and the like; vinyl ethers including ethenylenyethene, propenyloxyethene, and the like; acrylates and methacrylates including ethyl acrylate, methyl acrylates, propyl methacrylates, hydroxy propyl methacrylate, hexyl acrylate, and the like; glycidyl acrylates, .glycidyl methacrylates, and the like; and styrene. Additionally, mixtures of the above monomers could be used. For example, the copolymer of butadiene and styrene is applicable.

The functional terminal groups that are present can be selected from hydroxy, amino, mercapto and isocyanate radicals, in addition to any other reactive functional groups. As previously described, the compounds which are added to the telechelic polymer to improve the mechanical properties of the propellants formed can be varied depending upon the functional terminal groups present in the telechelic polymer. When one of the functional terminal groups of the telechelic polymer is a carboxy radical, the difunctional additives may include dialcohols such as dihydroxybenzene, dihydroxynapthaline, dihydroxybutane, and the like; diepoxides such as vinylcyclohexene dioxide, the Epon resins which are the products of epichlorohydrin and bis-phenol-A, diamines such as ethylene diamine, butylene diamine, hexamethylene diamine and the like; diamines such as hydrazine and the like; and lbis-chlorocarbonates such as ethylene bis-chlorocarbonate and the like.

When the functional telechelic groups in the telechelic polymers are hydroxy radicals, the difunctional additives may be selected from the group consisting of diimines; dicarboxylic acid chlorides such as succinyl chloride and the like; dicarboxylic acids such as maleic acid and the like; phosgene; bis-chlocarbonates and the like; and diisocyanates which are formed by treating diamines such as toluene 2,4-diamine, hexamethy aminediamine, and the like with phosgene.

When the functional telechelic groups in the telechelic polyers are amino groups, the difunctional additives may be selected from the group consisting of dicarboxylic acids; diamines; diepoxides; and dialdahydes such as malonic aldehyde and succinic aldehyde.

When the telechelic polymers have terminal functional groups that are mercaptans, the difunctional additives utilized may be acid chlorides; bischlorocarbonates; and usually any compound containing two double bonds (diolefins) capable of reacting with the mercaptan, such as ethyleneglycol diacrylate, for example. It is believed the invention will be further understood from the following detailed examples.

When the telechelic polymers have terminal functional groups that are isocyanates, the terminal difunctional additives having difunctional terminal hydroxy may be any compounds or amino groups or one of each functional type.

Example 1 Into a two and one fourth gallon Baker-Perkins dispersion blade mixer was added 1000 grams of Butarez CTL which is a carboxy-terminated linear polybutadiene. Additionally added was 6 grams of vinylcyclohexenedioxide. Water, at 165 F., was circulated through the mixer heating jacket. The material was then blended for fifteen minutes. A 28-inch vacuum was then applied and mixing continued for an additional ten minutes to remove the air present. The contents of the mixer were then poured into a large polyethylene bag. The bag was then sealed and then placed in a metal bucket container, the bag serving two functions. (1) To prevent contact with metal surfaces and (2) to prevent contact with air. The material was then heated for 168 hours at 170 F. A liquid modified polymer of this invention was obtained upon cooling of the material at room temperature and the removal from the sealed bag. The above procedure was repeated utilizing 1000 grams of the Butarez CTL and 10 grams of 1,3-bis[3 (2,3-epoxy-propoxy)propyl]tetra methyl disiloxane. A modified telechelic polymer of this .invention was obtained.

As is in the reaction of most polymeric-type materials, the temperature and length of time of reaction varies considerably. The temperature of reaction may vary from, for example, 15 0 F. to 225 F. with the time of reaction varying according to the temperature utilized with the higher temperatures requiring less reaction time. Thus, reaction times can vary from 145 hours to hours. Basically more important than the temperature and time of reaction ,is the assurance of conversion to the desired product. During the reaction of the difunctional groups with the telechelic polymers, samples of the mixture are taken from time to time during the reaction period. The samples are analyzed, for example, in the case of the carboxy-terminated telechelic polymers to determine the ephr value, that is the carboxyl equivalents. Once the carboxyl equivalents of the mixtures stabilize, there is an indication that the reaction has gone to completion and all of the difunctional groups that are 'going to react have reacted with the carboxy groups present. At this time the reaction is stopped. The same process could be taken when, for example, mercaptan groups are the terminal functional groups of the telechelic polymer. Mercaptan equivalents would be determined during the course of the reaction until a stabilized condition is reached at which time the reaction has gone to completeion and can be stopped.

The invention has peculiar applicability to the field of solid propellants wherein the polymers are used as binders to secure the solid particulate matter utilized.

The solid material which may be dispersed throughout the polymer matrix is usually in finely divided form having a particle size ranging from about 1-500 microns or greater in diameter. When the composition is intended as a solid propellant grain, it is often desirable to employ a combination of two or more different particle size ranges. For example, solid propellants are prepared in which the finer material comprises a fine particle size range of from 1 to about 75 microns and a coarse range of from about 75 to 500. However, particles of any size within the range of 1500 microns may be employed without regard to particle size. This gives desirable burning rates to the propellant. The particle size ranges may be adjusted depending upon the particular binder-fueloxidizer combination employed and the specific impulse desired.

The solid substances with which the polymeric materials are loaded may be inert pigments such as titanium dioxide, lead oxide, ferric oxide, carbon black, powdered metals and alloys, metal fluorides, asbestos fibers, etc.

When the solids are oxidizing agents, they can be compounds such as metal perchlorates and metal nitrates. The metal perchlorates employed as oxidizing agents or oxygen carriers in the compositions are anhydrous and have the general formula M(ClO wherein M is NH or a metal and x is the valence of M. Since the propellant composition is required to withstand high temperature storage, it is preferable that the melting point and the decomposition temperature of the oxidizer be as high as possible. The perchlorates of the Group I-A, Group- I-B, and Group II-A metals are found to have the re-- quired high temperature stability and are employed in the preparation of propellant compositions by the process of this invention. Hence, the metal perchlorates used in the preparation of the propellant compositions include lithium perchlorate, sodium perchlorate, potassium perchlorate, rubidium perchlorate, and cesium perchlorate which are the perchlorates of the metals of Group I-A of the Periodic Table of Elements; silver perchlorate which is a perchlorate of the Group IB metal; and magnesium perchlorate, calcium perchlorate, strontium perchlorate, and barium perchlorate which are the perchlorates of the Group II-A metals. In addition to the metal perchlorates, the compound ammonium perchlorate finds extensive use in propellant compositions. Examples of the nitrates of the Group IA, and I-B and II-B which are employed in preparing propellant compositions by the process of this invention are compounds such as lithium nitrate, sodium nitrate, potassium nitrate, magnesium nitrate, calcium nitrate, barium nitrate, strontium nitrate, etc; Ammonium nitrate is also used.

The ratio of total solids-to-polymeric binder material in a propellant falls in the range of from about 1:1 to about 9:1 with an optimum ratio of about 85:15.

Other substances which are employed in the preparation of propellants by the process of this invention include minor amounts of burning catalysts, well known in propellant compositions. These are composed of one or a mixture of two or more metal oxide powders in amounts suificient to improve the burning rate of the composition. The amounts usually range from about 0.01 to about 3 weight percent, based on the weight of the oxidizer employed. The particle size of the powders can range from about 10 to about 250 microns in diameter. Non-limiting examples of metals that serve as burning catalysts are copper, vanadium, chromium, silver, molybdenum, sinconium, antimony, manganese, iron, cobalt, and nickel. Examples of metal oxide burning catalysts are ferric oxide, aluminum, copper oxide, chromic oxide, as well as the oxides of the other metals mentioned above.

The fuel particles that are employed in a solid propellant grain are usually a metal or a metal alloy, preferably the fuel contains 1 or more metals of groups I-A, II-A, III-A and groups I-B through VII-B, group III of the Periodic Table. Thus, the metals may contain group I-A elements such as lithium and group IIA metals such as beryllium or magnesium. Illustrative of the group III-A metals is aluminum. The metals of groups I-B through VII-B include copper, silver, zinc, manganese, iron, nickel, platinum and the like. Particularly preferred for inclusion in the polymer matrix are aluminum, beryllium and lithium since these metals are of relatively low molecular weight giving low molecular weight combustion products in addition to having high heats of combustion.

Burning rate depressants and modifiers are also sometimes advantageously added to the solid propellant grain of this invention. These are generally compounds which tend to inhibit burning reaction rates or absorb heat and include specifically carbonyl chloride, oximide, nitroguanidine, guanidine nitrate, and oxalic acid.

As previously described, the improved properties obtained by the modification of a telechelic polymer are particularly applicable in the field of solid propellant compositions where the improved properties are extremely beneficial. The following examples illustrate the manufacture of solid propellants utilizing the modified binders of this invention, and the improved properties obtained therefrom.

Example 11 A propellant composition utilizing a modified telechelic polybutadiene polymer was manufactured. Into a two and one-half Baker-Perkins propellant mixer was carefully weighed 426.48 grams (17.77 weight percent) of modified telechelic carboxy-terminated polybutadiene. The polybutadiene was modified with vinylcyclohexene as described in Example I. Additionally weighed into the mixer was 384 grams (16.00 weight percent) of finelypowdered aluminum metal which acts as the fuel. The mixer was then turned on to disperse the aluminum in the liquid binder material. During this process a vacuum was applied to the mixer. After the aluminum had been dispersed within the binder material, 1584 grams (66.00 weight percent) of the oxidizer which was ammonium perchlorate was weighed and added to the mixer which was then turned on for a short period, approximately two or three minutes to disperse the oxidizer. The vacuum was then applied and mixing was continued until the constituents therein reached the temperature of l60170 F. at which time the mixing was stopped. The temperature is reached due to the circulating of a heating fluid in the jacket that surrounds the mixer. After the mixing had stopped and the cure temperature had been reached,

5.52 grams (.23 weight percent) of a curative which was MAPO (tris[l-(2-methyl)-aziridinyl]phosphine oxide) was added to the mixed constituents without any vacuum being applied. The mixer was then turned on to disperse the curative within the mixture. After the curative had been dispersed, the contents were then poured into a pan 8 by 8 inches and 3 inches deep and cast at a cure temperature of 170 for 48 hours. The curing time can vary and often the mixture is cured at 96 hours at 150 F., it being understood that the cure time decreases with increase in temperature. The resultant solid propellant having the modified carboxy-terminated polybutadiene polymer was obtained. The propellant was then stamped out in the JANAF dogbone configuration for use in determining its physical properties.

Example Ill Grams Wt.

Percent Garb cry-terminated linearrpolybutadiene modified with l,3-bis[3(2,3epoxyproxy)propyl] tetra methyl disiloxaue 445 17. 8 Ammonium perchlorate, 1, 650 66. 0 Aluminum 10. 0 MAPO 5 20 A propellant composition utilizing a modified polybutadiene binder was obtained.

Example IV To illustrate the improved properties obtained with the novel polymeric binders of this invention, tests were run comparing the propellant compositions prepared according to Examples II and III with a control propellant which had the following compositions:

Percent by weight Linear carboxy-terminated polybutadiene (unmodified teleohelic polymer) l8 Ammonium perchlorate 66 Aluminum 16 The results obtained are set forth in Table I.

TABLE I Test Temp Control Example I Example II Deg. F

em, percent 26 48 51 S..., p.s.i

so 88 89 E, .s.i 455 275 255 cm, percent 34 G2 G6 Sm, p.s.i- 124 E..., psi. 5 10 345 330 em, percent- 32 51 51 m. p.s.i- 247 241 235 E,.., p.s.i 2, 010 1, 660 2,020

In Table I, e is the elongation at maximum tensile strength, expressed in percent; S is the maximum tensile strength in p.s.i.; and E is the tangent modulus of elasticity. All test specimens were standard JANAF dogbones. All testing was done on an Instron machine at a strain rate of 0.77 in.-in. min. Of considerable import is the fact that the propellant elongation increased from an average of about 30 percent for the control propellant over the entire range of temperatures to an average exceeding 50 percent for the propellant utilizing the modified binder of this invention. As can readily be seen from the table, the percent elongation nearly doubled utilizing the concepts set forth herein. Additionally, as can be seen from the table, the maximum tensile strength increased except for a slight decrease at 70. This is a significant aspect of the effect of the present invention, since the elasticity of the propellant can now be greatly increased without any detrimental affect on the maximum tensile strength. A'sign'ificant advance of the art has been accomplished in providing a propellant binder that will provide superior and improved physical properties over an extreme range of temperatures as seen in Table I.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim:

1. The compound obtained by the reaction of carboxyterminated polybutadiene and 1,3-bis [3(2,3-epoXy-propoxy)propyl] tetra methyl disiloxane.

References Cited by the Examiner UNITED STATES PATENTS 2,848,442 8/1958 Svetlik 260-82.1 3,053,708 9/1962 Hall et al. 149-19 3,055,781 9/1962 Yamamoto 14919 3,070,583 12/1962 Uraneck 26082.1 3,087,844 4/1963 Hudson et al. 14919 3,147,161 9/1964 Abere et a1 14919 FOREIGN PATENTS 226,634 4/ 1959 Australia.

LEON I. BERCOVITZ, Primary Examiner. REUBEN EPSTEIN, C. D. QUARFORTH, Examiners.

L. A. SEBASTIAN, B. R. PADGETT,

Assistant Examiners.

Claims (2)

1. THE COMPOUND OBTAINED BY THE REACTION OF CARBOXYTERMINATED POLYBUTADIENE AND 1, 3-BIS (3, (2, 3-EXPOXY-PROPOXY) PROPYL) TETRA METHYL DISILOXANE.

3. THE METHOD OF IMPROVING THE PHYSICAL STRENGTH PROPERTIES OF CARBOXY-TERMINATED POLYBUTADIENE WHICH COMPRISES REACTING SAID POLYBUTADIENE WITH 1, 3-BIS (3(2, 3EPOXY-PROPOXY) PROPYL) TETRA METHYL DISILOXANE.