Solid propellant with titanate bonding agent

Liquid elastomer-based propellant having incorporated therein organo-titanate compounds are described. The organo-titanates have positive ballistic and physical effects on the propellants, serving to reduce burn rate exponents and overall burn rates, as well as increasing the tensile strength and elasticity of the propellant. Organo-phosphate and pyrophosphate titanates are used as the preferred organo-titanates.

BACKGROUND OF THE INVENTION 
Solid propellants, which are conventionally composed of finely divided 
oxidizer materials dispersed in a resinous binder, are useful for jet 
propulsion devices such as missiles, rockets, and gas generators. 
Desirably, such materials are cast into a metallic combustion chamber 
which is incorporated into the jet propulsion device. See, for example, 
U.S. Pat. No. 3,050,423, which is incorporated by reference herein. 
Because of the extreme stress which the propellant is subjected to during 
the burn and the need to accurately control the rate of burn, the 
formation of a fully satisfactory system has been elusive. In order to 
prevent the physical deterioration of the propellant, it is necessary that 
the mass have high elasticity and tensile strength, since cracking and 
other imperfections may lead to uncontrolled or erratic burning. 
Furthermore, the tendency of the propellant to burn at an accelerating 
rate must be suppressed for dependable operations. These problems are 
described in detail in the aforesaid U.S. patent. 
To prepare a solid propellant having acceptable physical properties, it is 
necessary to carefully control the geometry of the particulates and to 
employ adhesion promoters such as aziridine to form a properly bonded 
charge. Additionally only certain types of resinous materials have been 
found useful in light of the need to control the burning rates of the 
solid propellant, which is particularly difficult under the varied 
pressure conditions experienced in the combustion chamber during the burn. 
BRIEF DESCRIPTION OF THE INVENTION 
In accordance with the instant invention, it has been discovered that the 
incorporation of certain select organo-titanates and zirconates into the 
propellant markedly improves the physical properties and, particularly in 
the case of organo-titanate phosphates and pyrophosphates, enhances the 
ballistic properties. 
The organo-titanate and organo-zirconate may be incorporated into the 
propellant composition by admixing prior to casting or molding. 
Improvement in the dispersibility of the inorganic particles in the 
resinous matrix is evidenced by reduced binder modulus, giving a finished 
product with better bonding of the oxidizer particles to the resin, 
increased tensile strength and elongation. The thus formed propellants 
have a reduced burn rate exponent, enhanced low pressure combustion 
stability, and reduced low pressure extinguishment levels.

DETAILED DESCRIPTION OF THE INVENTION 
Solid propellants are conventionally composed of finely divided inorganic 
oxidizer material, organic resin which may serve as both a fuel and a 
binder, additional powdered metals which provide additional combustible 
material, and minor amounts of other additives such as plasticizers, 
antioxidants, wetting agents, curatives, metal oxides, and reinforcing 
agents. 
Generally speaking, oxidizers are powdered and vary in size broadly from 1 
to 300 microns average particle size, preferably in the range of from 20 
to 200 microns. These materials form the major portion of the total 
composition, generally ranging from 65 to 95% of the total mixture. The 
fuel binder is usually present in minor proportions of the total 
composition, generally ranging from 5 to 35% by weight. Generally 
speaking, it is advantageous to reduce the amount of binder material which 
is present, since such material adds weight to the total charge and its 
gas generation per unit weight is less than that provided by powdered 
metal fuels. The foregoing compositional factors are conventional to the 
art and described in detail in U.S. Pat. No. 3,050,423. 
Certain of the organo-titanates useful in the instant invention have been 
described in the prior art. These include particularly organo-titanate 
phosphates and pyrophosphates. These compounds may be represented by the 
formulas: 
EQU I (R.sup.1 --O).sub.x TiOP(O)(OR.sup.2)(OR.sup.3)!.sub.y 
EQU II (R.sup.1 --O).sub.x TiOP(OH)OP(O)(OR.sup.2)(OR.sup.3)!.sub.y 
##STR1## 
wherein R.sup.1 is a monovalent alkyl, alkenyl, alkynyl or aralkyl group 
having from 1 to 30 carbon atoms or a substituted derivative thereof; 
R.sup.2 and R.sup.3 are independently selected from hydrogen, alkyl, 
alkenyl, aryl, aralkyl or alkaryl groups having from 1 to 30 carbon atoms 
or substituted derivatives thereof, x and y being integers from 1 to 3, 
the total of x and y being equal to 4, and m=2 when n=1, and m=0 when n=2. 
Preferably, R.sup.1 is an alkyl group containing from 1 to 6 carbon atoms 
and R.sup.2 and R.sup.3 are independently selected from alkyl groups 
having up to 12 carbon atoms or an aryl or alkaryl group having from 6 to 
24 carbon atoms. 
U.S. Pat. No. 4,122,062 describes the organo-titanate phosphates and 
pyrophosphates of formulas I and II generally. As will be shown from the 
data set forth hereinafter, such materials are particularly useful in the 
instant invention. Generally from 0.1 to 2.0 parts of the titanate are 
added, based on the total composition, preferably from 0.2 to 1.0. For 
ammonium perchlorate and aluminum powder, the most preferred is the 
addition of 0.3 to 0.4 wt. %. 
Although titanium IV, (2-propenolato-1)methyl, n-propanolato methyl! 
butanolato-1, tris(dioctyl)phosphato and titanium IV, 
(2-propenolato-1)methyl, n-propanolato methyl! butanolato-1, 
tris(dioctyl)pyrophosphato are the best for increasing the efficiency of 
metal fuel combustion by preventing the agglomeration of molten metal 
particles inside the combustion chamber, titanium IV, 2-propanolato, 
tris(dioctyl)phosphato-0 is the top choice for overall effect on metal 
combustion, reduction of burn rate exponent, bonding agent effects, and 
the ability to function as a wetting agent and viscosity depressant. 
In the sample propellant formulations tested, the addition of titanates 
provided positive rheological benefits. Pseudoplastic and especially 
thixotropic flow behavior was reduced in all cases, bringing about the 
desired Newtonian behavior. In all cases of titanate addition, the CSVC 
(critical solids volume content) increased by at least 150%, thus allowing 
higher solids loading at equal casting viscosity. The practical effect of 
higher solids loading, aside from processing considerations, is the 
increase in specific impulse and propellant density. Tables 1 and 2 
present data indicating the positive effects of titanate addition to solid 
propellants from the standpoint of rheology. 
While it is preferred to admix the organo-titanates along with the other 
components of the composition, the instant invention can also be practiced 
by first treating the solid inorganic particles. The incorporation of the 
organo-titanates into the composition may be done with conventional 
processing equipment. 
Many medium-shear mixers are suitable for the production of solid 
propellants. Among the more common types are the sigma blade, bear claw 
double arm, tangential double arm, vertical two blade planetary, and 
ribbon blender type. The most common type of mixer is the sigma blade. 
Generally, the mixers are of the vacuum hood type with a heating/cooling 
jacket on the mixing bowl. In most HTPB formulations it is not necessary 
to heat the propellant to reduce mix viscosity, but the PBAA, PBAN, and 
CTPB types all require heat. Some propellants, due to shear or cure 
induced exotherm, must be cooled during the mixing process. 
Care must be taken to ensure that the mixer bowl is free of "dead spots". 
This condition can lead to poor wet out of propellant ingredients, and in 
some cases dewetting. Poor wetting may result in a fire or explosion. This 
is especially true in high energy HMX/RDX PEG-NG propellants. Very high 
shear rates should not be used. 
Current propellant binder systems include, but are not limited to, 
polybutadiene acrylic acid (PBAA), polybutadiene acrylic acid 
acrylonitrile (PBAN), carboxyl terminated polybutadiene (CTPB), hydroxyl 
terminated polybutadiene (HTPB), polysulfides, polyether urethanes, 
polyester urethanes, unsaturated polyesters and acrylics, epoxies, and 
nonreactive binders such as polyvinyl chloride (PVC), and nitrocellulose 
(NC) plastisols. 
In all cases, the polymeric compound "binds" all propellant ingredients to 
form an aggregate or composite of sufficient strength to withstand the 
thermal and mechanical loads of motor operation and vehicle flight. 
The titanates may be used to advantage in most propellant binders. Positive 
effects are observed in the carboxyl terminated butadienes with a total 
absence of the cure rate problems associated with HTPB binders. 
Where polyurethane systems are employed, it is useful to prepare a two-part 
system consisting of a premix part which contains the majority of the 
ingredients and a curative part which is composed primarily of the 
curative. Such techniques will be readily understood by those skilled in 
the art. 
Other elastomers which may be used as the binder are hydroxy terminated 
butadiene prepolymers such as R45HT made by Arco Chemical Co. and having a 
functionality of about 2.7. These are described in U.S. Pat. No. 
3,932,240. 
The particular organo-titanate selected is dependent to a large degree on 
the physical size of the solid propellant particle being prepared. For 
example, while the pyrophosphates are found to be outstandingly effective 
in reducing the burn rate exponent, in urethane systems they suffer the 
disadvantage of decreasing the cure rate of the catalyst. The 
organo-phosphates, on the other hand, have substantially no effect on the 
cure rate. 
The catalytic effect that the titanates have on the NCO/OH cure reaction of 
the propellant binder system can be controlled, when titanates are used as 
bonding agents, by treating the aluminum or ammonium perchlorate with a 
solvent solution of the titanate and drying the treated particles. This 
procedure requires only enough titanate to produce a monolayer on the 
surface of the solid particles. Since the monolayer is tightly bound to 
the solid particles, and no excess titanate is present, very little effect 
on cure rate of the propellant is observed. Less effective, but still a 
useful approach, is that of blending the titanate and the isocyanate prior 
to their addition to the rubber portion of the propellant binder. 
It has also been discovered that the titanate should be preblended with an 
ester such as isodecyl pelargonate and the mixture allowed to remain at 
room temperature for 24 hours to permit transesterification. 
Pot life control can be enhanced by the addition of minor amounts of 
glycols such as 2,4-pentanediol to the curative part just prior to 
admixture with the premix part of the system. 
In summary, titanates provide the ability to increase the solids loading of 
many propellant formulations, thereby increasing the specific impulse and, 
in most cases, with an increase in density. The practical advantage is 
more power in less space. From a mechanical standpoint, greater 
operational loads can be tolerated, permitting a reduction in weight and 
size of mechanical components. The increased ability to bond to case 
liners and other batches of propellant permits more reliable dual-grain 
designs. All of these factors give the solid rocket motor designer more 
freedom of choice. 
In order to define more clearly the instant invention, attention is 
directed towards the following examples: 
EXAMPLE 1 
A premix part is formed by first admixing an ester plasticizer with the 
titanate and allowing the mixture to stand for 24 hours. Thereafter, a 
hydroxy-terminated butadiene, aluminum powder (3 microns), carbon black, 
catacene, and ammonium perchlorate are added with mixing. A curative part 
is formed by admixing dimer acid diisocyanate with 2,4-pentanedione. The 
amount of the ingredients is selected so as to form a composition having 
the following formulation: 
TABLE A 
______________________________________ 
Ammonium Perchlorate - 200 microns 
55.97 
Ammonium Perchlorate - 500 microns 
23.98 
Aluminum - 3 microns 1.25 
Carbon Black .05 
Catacene .15-1.00 on Total 
Hydroxy-terminated Butadiene (R45-HT) 
12.17 
Isodecyl Pelargonate 3.75 
Dimer Acid Diisocyanate 
2.83 
(excluding Catacene) 100.00 
______________________________________ 
In the foregoing formulations, the following titanates were added at a 
level of 0.3% based on weight of the total formulations: 
______________________________________ 
Code Titanium Coupling Agent 
______________________________________ 
A Titanium IV, 2-propanolato, tris(dioctyl)phosphato-0 
B Titanium IV, 2-propanolato, tris(dioctyl)pyrophosphato-0 
C Titanium IV, bis(dioctyl)phosphato-0, ethylenediolato 
D Titanium IV, (2-propenolato-1)methyl, n-propanolato 
methyl!butanolato-1, tris(dioctyl)phosphato 
E Titanium IV, (2-propenolato-1)methyl, n-propanolato 
methyl!butanolato-1, tris(dioctyl)pyrophosphato 
F Titanium IV bis octanolato, cyclo(dioctyl)- 
pyrophosphato-0,0' 
G Titanium IV bis cyclo(dioctyl)pyrophosphato-0,0' 
______________________________________ 
The data shown in FIG. 1 clearly establish that the titanates of the 
invention reduce the burn rate exponent of the propellant formulation. 
While the mechanism for this advantageous result is not fully understood, 
it may be postulated that the organic titanate bonds to the surfaces of 
the fuel and forms a non-combustible coating. Additional experiments have 
shown that the addition of greater amounts of the organo-titanates 
actually can serve to depress the overall burn rate to the point of 
extinguishment. For example, 2% of the pyrophosphate titanates extinguish 
the propellant at pressures below 100 psia, while suppression is realized 
at 0.6 wt. % levels. 
EXAMPLE 2 
A propellant formulation of 15% binder, 19% aluminum, with the balance 
being ammonium perchlorate, was formulated. Small rocket motors were 
fabricated (1.5 in..times.12 in.), which are known to have very short 
residence times in the combustion chamber. Due to the very short 
combustion time available, the propellant produced a specific impulse of 
only 215 pound seconds. 
Following are the results for select titanates: 
TABLE 1 
______________________________________ 
Specific Impulse, 
Titanate lbs. sec. 
______________________________________ 
None 215 
A 228 
B 234 
E 233 
G 215 
______________________________________ 
As can be seen, the pyrophosphate and phosphate materials are much more 
effective than the heterocyclic types. In certain instances the specific 
impulse increased by more than 15 lbs. sec. Since residence times remained 
constant throughout the tests, it is assumed that the titanates prevented 
the agglomeration of the molten aluminum particles into larger ones that 
would result in the lower combustion efficiency of the unmodified 
propellant. 
EXAMPLE 3 
The effect of titanates on propellant mix viscosity is shown in Tables 2 
and 3 below. In each instance, the titanate level was 0.30% of total 
formulation weight. 
TABLE 2 
______________________________________ 
Viscosity 
Titanate 
(kps) 
______________________________________ 
None 13.2 
A 10.2 
B 11.1 
E 11.6 
G 12.3 
______________________________________ 
TABLE 3 
______________________________________ 
Spindle 
Speed* Control A B E G 
______________________________________ 
.3 13.2 10.2 11.1 11.6 12.3 
1.5 13.7 10.3 11.1 11.8 12.9 
3.0 14.0 10.5 11.6 11.7 12.9 
______________________________________ 
*Brookfield LV viscometer with Helipath stand. 
Viscosity in kps. 
The above tables clearly show that the addition of the titanates in each 
instance lowers the viscosity of the formulation. The pyrophosphates and 
the phosphates are the most effective. 
EXAMPLE 4 
This example shows the effect of titanates as bonding agents on propellant 
mechanical properties. As in the previous example, 0.3 wt. % of the 
titanate was used. 
TABLE 4 
______________________________________ 
Young's 
Temp., Bonding Stress.sup.1, 
Strain.sup.2, 
Modulus 
.degree.F. 
Agent PSIG % PSIG 
______________________________________ 
180 None 62 17 455 
A 84 23 435 
B 88 27 450 
E 90 29 458 
G 63 17 450 
77 None 76 18 520 
A 187 77 512 
B 191 83 520 
E 196 85 528 
G 76 20 520 
-45 None 278 20 1940 
A 423 73 2533 
B 428 75 2530 
E 436 81 2590 
G 270 22 1910 
______________________________________ 
.sup.1 At nominal maximum 
.sup.2 Nominal maximum stress 
The above data show the improvement of the organo-titanate phosphates and 
pyrophosphates on the physical properties of the propellants. At 
-45.degree. F., maximum stress and strain as well as Young's Modulus were 
improved. At 180.degree. F. and 77.degree. F., the maximum stress and 
maximum strain were markedly increased.