A diisocyanate crosslinking agent having a minimum chain length of methylene groups from 25 to 45 which has a coiling-uncoiling ability in a cured solid propellant composition (under stress and relaxing of stress) as a result of the chain length and configuration imparts superior structural strength to the solid propellant composition. A preferred species is 1,37-heptatriacontane diisocyanate, OCN--(CH.sub.2).sub.37 --NCO, which is employed as an effective crosslinking agent in hydroxyl-terminated polybutadiene propellants.

BACKGROUND OF THE INVENTION 
Present day hydroxyl-terminated polybutadienes in solid propellant 
formulations are crosslinked with diisocyanates. Earlier developed 
propellants employed diisocyanates such as toluene 2,4-diisocyanate (also 
referred to as 2,4-tolylene diisocyanate, TDI etc.). Later, it was found 
that higher molecular weight diisocyanates such as hexamethylene 
diisocyanate (HDI), dimeryl diisocyanate (DDI), or isophorone 
diisocyanate, gave better properties because of these multi-functional 
crosslinking agents. The improved properties derived were essential for 
propellant compositions containing high solids loading. The curing cycles 
also resulted in better cures as a result of curing catalysts and curing 
accelerators employed in these types of propellants. 
As the burning rate of the propellant has been enhanced to meet the 
performance requirement of advanced interceptors, particularly for 
upper-stages where ignition and pressurization (while under high stress 
conditions) imposes even more requirements for superior strength for the 
propellant, the need for such superior structural strength for the 
propellant well beyond the present state-of-the art has been a continued 
requirement. 
The present requirement for solid propellant to withstand the acceleration 
loads imposed on them during launch of an advanced, high-acceleration 
interceptor whose mission is to reach a low-altitude intercept point and 
destroy an incoming intercontinental ballistic missile which has 
penetrated the defense has created the need for ultrahigh strength 
mechanical properties well beyond that of the present state-of-the-art 
propellants. 
Studies have been carried out which indicate that current tensile strength 
of 200 psi and strain of maximum stress of 25% would be far too low by a 
factor of three for tensile strength, and a factor of two on strain at 
maximum stress for an unsupported gain. 
The state-of-the-art was improved by employing a perforated, support tube 
which was fitted into the grain's perforation to mechanically reinforce 
the propellant grain. This feature was particularly advantageous where 
pressurization and ignition were required for upper stages which were 
already subjected to extra stresses and forces as a result of 
high-accelerations and maneuvers of the high-acceleration interceptor. 
Achieving tensile strength of 600 psi is unachievable through any 
modification which has been investigated with state-of-the-art 
propellants, even with those which have a considerably lower burning rate 
then that required of interceptor propellants. To further complicate 
matters relative to propellant mechanical properties, the interceptor 
propellants contain high percentages of liquid burning rate accelerators, 
and these liquid burning rate accelerators, which function also as partial 
plasticizers, tend to degrade the mechanical properties. 
An object of this invention is to provide a method of synthesizing solid 
missile propellants which have structural strengths well beyond those of 
the state-of-the-art propellants. 
A further object of this invention is to provide structurally-strong solid 
propellants which have structural strengths well beyond those strengths of 
the state-of-the-art propellants. 
SUMMARY OF THE INVENTION 
The method of synthesizing solid missile propellants to achieve structural 
strengths used beyond that of the state-of-the-art propellants employs 
diisocyanates of increased chain lengths for crosslinking the propellant 
formulations. The increased chain length diisocyanate permits the 
crosslinked hydroxyl-terminated polybutadiene polymers in the solid 
propellant formulations to undergo an uncoiling action as the polymer is 
subjected to stress or a coiling action when the stress is relaxed. A 
preferred, representative diisocyanate, 1,37-heptatriacontaine, has a 
coiled configuration which can uncoil when the polymer is stressed, and 
coil when the stress is relaxed. When the presentative diisocyanate is 
employed as the crosslinking agent for a hydroxyl-terminated polybutadiene 
prepolymer of a higher functionality, the number of crosslink sites and 
the coiled chain length distance between the cure sites are increased. As 
a result, dramatic increases in the mechanical properties capabilities are 
achieved. The simultaneous increase in the number of crosslinks per unit 
volume translates to increased tensile capability while the average coiled 
chain length between cure sites translates to increased propellant 
elongation. Of particular significance is the fact that this produces a 
situation which does not occur in conventional propellants, i.e., 
improvements in the propellant's stress and elongation are accomplished 
without having to trade off one for the other.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A diisocyanate having a minimum chain length of methylene groups from 25 to 
45 which enables the diisocyanate to perform a coiling-uncoiling action 
(under stress and relaxing of stress) as a result of the increased chain 
length and configuration of the diisocyanate crosslinking agent. A 
preferred species is 1,37-heptatriacontane diisocyante, 
OCN--(CN.sub.2).sub.37 --NCO, which is employed as an effective 
crosslinking agent in hydroxyl-terminated polybutadiene propellants. The 
improved mechanical properties of the propellant crosslinked with 
1,37-heptatriacontane diisocyanate results from the 37 methylene groups 
being arranged in a coiled configuration which uncoils when the polymer is 
stressed, and which coils when the stress is relaxed. 
The synthesis of the 1,37-heptatriacontane diisocyante can be effected by 
starting with the eicosanedioic acid (1,18-octadecane dicarboxylic acid) 
and converting it into the half ester; then into the monoalkyl 
2-oxononatricontanedioate; reducing the latter to the monoalkyl 
monatricontanedioate; and then further conversion into the 
1,37-heptatricontane diisocyante. 
The chemical formulas of the (1) starting, (2a-2d) intermediate compounds, 
and (3) final compound are as follows: 
1. HO.sub.2 C--(CH.sub.2).sub.18 --CO.sub.2 H 
2a. RO.sub.2 C--(CH.sub.2).sub.18 --CO.sub.2 H 
b. RO.sub.2 C--(CH.sub.2).sub.18 --CO--(CH.sub.2).sub.18 --CO.sub.2 H 
c. RO.sub.2 C--(CH.sub.2).sub.37 --CO.sub.2 H 
d. H.sub.2 N--CO--(CH.sub.2).sub.37 --CO---NH.sub.2 
3. ONC--(CH.sub.2l ).sub.37 --NCO 
The descriptive information and data available on the starting material, 
eicosanedioic acid from the Dictionary of Organic Compounds is presented 
in TABLE 1 below. Test data for the final compound when used in a 
propellant composition is shown in Table 2 below. 
Table 2, with title: "Comparison of Ultrahigh-Burning Rate Composite 
Propellants Containing 1,37-Heptatriacontane Diisocyanate", shows the 
superior structural strength achieved as a result of using 
1,37-heptatriacontane in Experimental Propellant B to replace the 
conventional crosslinking agent, isophorone diisocyante, employed in 
Control Propellant A. 
TABLE 1 
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Eicosanedioic Acid (Octadecane-1:18-dicarboxylic acid) 
HO.sub.2 C--[CH.sub.2 ].sub.18 --COOH 
C.sub.20 H.sub.38 O.sub.4 
MW 342 
Cryst. from C.sub.6 H.sub.6 or EtOH. M.p. 122-3.degree. (125.4-126.6.degre 
e.). 
Di-Me ester: C.sub.22 H.sub.42 O.sub.4. MW 370. Platelets from MeOH. 
M.p. 
65.6-66.degree. 
Di-Et ester: C.sub.24 H.sub.46 O.sub.4. MW 398. M.p. 54.5-55.degree.. 
B.P. 230-2.degree./2 mm. 
Dianilide: cryst. from EtOH. M.p. 143-6.degree. 
Di-(p-bromophenyacyl) ester: m.p. 143.5.degree. 
Chuit, Hausser, Helv. Chim. Acta, 1929, 12,856. 
Canonica, Martindli, Bacchetti, Atti accad. nazl. 
Lincei, Rend., Classe sci. fis., mat. e nat., 1952, 
13, 61 (Chem. Abstracts, (1953, 47, 11132). 
Kreuchunas, J. Am. Chem. Soc., 1953, 75, 339. 
Gunthard, Heinemann, Prelog, Helv. Chim. Acta, 1953, 36, 1147. 
Black, Weedon, J. Chem. Soc., 1953, 1785. 
Kananiwa, Isono, Ann. Rept. Fac. Pharm. Kanazawa Univ., 
1952, 2, 30 (Chem. Abstracts, 1955, 49, 1565). 
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TABLE 2 
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COMISON OF ULTRAHIGH-BURNING RATE COMPOSITE PROPELLANTS CONTAINING 
1,37-HEPTATRICONTANE DIISOCYANATE 
__________________________________________________________________________ 
CONTROL EXPERIMENTAL 
PROPELLANT PROPELLANT 
A B 
INGREDIENTS WEIGHT PERCENT 
WEIGHT PERCENT 
__________________________________________________________________________ 
HYDROXYL-TERMINATED POLYBUTADIENE PREPOLYMER 
5.47 5.40 
BA-114* 0.33 0.327 
TRIMETHYLOLPROPANE 0.067 0.066 
N-HEXYLCARBORANE 9.90 9.82 
ULTRAFINE AMMONIUM PERCHLORATE (1 .mu.m)(4.3 m.sup.2 /g) 
60.4 60.8 
AMMONIUM PERCHLORATE (70 .mu.m) 19.8 19.6 
ALUMINUM POWDER 3.31 3.28 
TRIPHENYLBISMUTHINE 0.033 0.033 
ISOPHORONE DIISOCYANATE 0.721 -- 
1,37-HEPTATRIACONTANE DIISOCYANATE 
-- 0.717 
__________________________________________________________________________ 
PROPERTIES VALUE 
__________________________________________________________________________ 
BURNING RATE (2000 PSI)(CM/S)(IPS) 
24.9/9.8 26.9/10.6 
PRESSURE EXPONENT 0.70-0.80 0.65-0.70 
MAXIMUM STRESS (PSI) 235-245 425-450 
STRAIN @ MAX. STRESS (PSI) 22-25 60-70 
MODULUS (PSI) 1100-1200 1400-1500 
__________________________________________________________________________ 
*REACTION PRODUCT OF 12HYDROXYLSTEARIC ACID AND 
TRIS(2METHYLAZIRIDINYLPHOSPHINE OXIDE 
The coiling-uncoiling capability for the diisocyanate crosslinking agent of 
this invention is considered operative for the chain length of methylene 
group from 25 to 45 and with isocyanate terminal groups for each chain 
length which are reactive with the functional groups of the 
hydroxyl-terminated polybutadiene prepolymer. Thus, as a result of the 
increased chain length, i.e., the increased coiled chain length distance 
between the cure sites, and through the use of an hydroxyl-terminated 
polybutadiene prepolymer of higher functionality to provide an increase in 
the number of crosslink sites, dramatic increases in the mechanical 
properties are achieved. The increased mechanical properties result 
directly from the increase in the number of crosslinks per unit volume 
(which relates to the increased tensile strength capability), while the 
average coiled chain length between cure sites relates to the increased 
propellant elongation. The achievement for the propellant composition is 
an improvement in the stress and elongation values without having to trade 
off one for the other as is the case with conventional propellants. 
The crosslinking compounds of this invention are assigned the formula 
ONC--(R).sub.X --NCO, wherein R equals CH.sub.2 and X equal integers from 
25 through 45. The length of chain is determined by the synthesis outlined 
hereinabove wherein the starting acid is a dicarboxylic acid with the 
--COOH groups at the alpha and omega positions and the chain length of the 
CH.sub.2 groups are based on integers from 12 through 22. 
The crosslinking agent of this invention is used to replace any of the 
diisocyanates which are used in hydroxyl-terminated polybutadienes in 
solid propellant formulations, particularly, where the need exists for a 
propellant with superior structural strength. Inasmuch as it is desirable 
to have increased strengths to a wide range of propellants without a 
sacrifice to the cost or usefulness thereof, the diisocyanate crosslinking 
agent of this invention is useful in any hydroxyl-terminated polybutadiene 
propellants. Other hydroxyl terminated dienes that are crosslinked by 
diisocyanates are also useful as the binder constituent. The binder 
portion of the propellant of this invention can made up to about 25 weight 
percent of the propellant composition. Aluminum metal fuel can be used up 
to about 20 weight percent of the propellant composition. An oxidizer of 
ammonium perchlorate can be used up to about 80 weight percent of the 
propellant composition. The diisocyante crosslinking can be used in an 
amount from about 0.3 to about 0.8 weight percent of the propellant 
compositions. The enumerated major ingredients which should total 100 
percent in a specific composition are noted; however, the usefulness of 
the diisocyanate crosslinking agent of this invention can be extended to 
any modified composition where the composition includes additional 
ingredients such as burning rate catalysts, high energy plasticizers, 
ballistic modifiers, processing aids, and the like. The catalysts and high 
energy plasticizers can include carborane and/or difluoroamino compounds 
which are well established in the high performance propellant art. The 
preparation and manufacturing procedure for the propellant compositions 
which can be crosslinked with the diisocyanate crosslinking agents of this 
invention are well established in the art. The only changes required would 
be that normally required for using a higher molecular weight crosslinking 
agent. These changes would be related to temperature conditions, employing 
dispersing aids, processing aids, or additives to extend potlife as 
required for forming the propellant in its final configuration whether it 
be by casting, extrusion, or other techniques which may be required for 
the final propellant grain.