Solid propellant containing ferrocenyl phosphine derivatives

A solid propellant composition comprising an organic polymer fuel binder, an inorganic perchlorate oxidizer salt, ferrocenyl phosphine or phosphine oxide derivatives and in particular, triferrocenyl phosphine oxide, which function as stable solid combustion modifiers.

The present invention relates to ferrocenyl phosphines or ferrocenyl 
phosphine oxides as a combustion modifier for solid propellant 
compositions comprising an organic polymer fuel binder and an inorganic 
perchlorate oxidizer salt. 
The efficacy of ferrocene, a volatile red organometallic solid, as a 
burning rate accelerator in a solid composite propellant, was discovered 
in the early 1950's. It was found that ferrocene was superior to the 
inorganic compounds, such as iron oxide, copper chromite and the like, 
then in use. Ferrocene, in equivalent amounts, gave much larger increases 
in burning rate and could be used effectively in increasingly higher 
concentrations with concomitant increase in burning rate. 
Unfortunately, propellants containing ferrocene undergo changes in 
composition with time due to volatility of the catalyst compound. This 
results in changes in both mechanical and ballistic properties during 
storage. Rocket motors containing ferrocene-catalyzed propellant grains 
were observed to have red needles of ferrocene sublimed and recrystallized 
on the grain surface. 
Efforts were then turned to development of liquid ferrocene derivative 
catalysts having higher molecular weight and decreased volatility as 
compared with ferrocene. In addition to reducing the ferrocene volatility 
problem, the liquids improve processing properties by reducing the total 
amount of added solids and functioning as a plasticizer. However, two 
serious difficulties were encountered with the liquid ferrocene 
derivatives, crystallization and migration. 
Crystallization of the liquid at low temperatures increases the solids 
content of the propellant above the design concentration and can, thereby, 
adversely affect mechanical properties. 
The liquid also tends to diffuse from the propellant into the rubbery 
materials normally used in making the conventional liners employed with 
solid rocket propellant grains. This results in embrittlement of the 
propellant and undesirable modification of ballistic properties adjacent 
to the interface between the propellant and liner. 
In view of these problems with ferrocene and liquid derivatives thereof, 
efforts have been made to find a solid ferrocene derivative replacement 
for the highly volatile ferrocene. To be successful, such a compound must 
meet several essential criteria. It must produce a substantial increase in 
burning rate of the propellant as compared with the non-catalyzed 
composition. It must be a stable, substantially non-volatile compound. 
It must not adversely affect the physical or ballistic properties of the 
propellant composition in such terms, for example, as weight loss due to 
volatilization or decomposition at the environmental temperatures to which 
the propellant gain will be exposed, including substantially elevated 
temperatures; migration or diffusion; increase in propellant sensitivity 
to friction, impact or heat; production of ballistic unpredictability, 
such as variation in burning rate within the propellant grain; and the 
like. 
A number of solid, relatively stable ferrocene derivatives have been tried 
as propellant burning rate accelerators. These include, for example, 
dimethyl polyferrocenyl methylene (DMPFM), a polymer produced by the 
reaction of ferrocene and acetone; 1,3-diferrocenyl-1-oxo-2 propene; 
1,3-diferrocenyl-1,3-propanedione; and benzoyl ferrocene. DMPFM appeared 
to be one of the more promising solid ferrocene derivatives for use as a 
catalyst because of its stability per se and minimal adverse effects on 
propellant stability. However, it has been found to be inadequately 
effective as a burning rate accelerator. Other solid substantially stable 
ferrocene derivatives, which have been tried as propellant burning rate 
catalysts, produce grains having unacceptable physical and/or ballistic 
properties. 
Recent studies involving diferrocenyl ketone as a propellant burning rate 
catalyst have shown that this compound is not only stable but also 
substantially increases burning rate without adverse effects on the 
stability of the propellant. The use of diferrocenyl ketone in a 
propellant is disclosed and claimed in U.S. patent application Ser. No. 
077,438 filed Sep. 20, 1979, entitled "Solid Propellant Containing 
Differrocenyl Ketone", now U.S. Pat. No. 4,318,760. 
Ferrocenyl phosphine derivatives are a known class of chemical compounds. 
They have not, however, been used or suggested for use as propellant 
burning rate catalysts. Like diferrocenyl ketone, many of the ferrocenyl 
phosphine derivatives of the present invention are high melting, 
oxidatively stable, non-volatile solids ideally suited for use as solid 
ferrocene burning rate catalysts. However, triferrocenyl phosphine oxide 
is far superior to diferrocenyl ketone in that higher burning rates for a 
given amount of material are achieved. In addition, propellants containing 
ferrocenyl phosphines are more easily processed than propellants 
containing an equal amount of other solid ferrocenes. 
The present invention now provides a solid propellant composition, 
comprising an organic polymer fuel binder, an inorganic perchlorate 
oxidizer salt, and, as a burning rate accelerator, a solid ferrocenyl 
phosphine or phosphine oxide of the formula 
EQU R.sub.3-n PR'.sub.n 
or 
EQU R.sub.3-n P(O)R'.sub.n 
wherein R is alkyl, cycloalkyl, aryl or substituted aryl, 
R' is ferrocenyl and 
n is 1 to 3. 
The ferrocenyl phosphines or phosphine oxides are oxidatively stable solid 
compounds. When incorporated into a composite propellant comprising a 
synthetic organic polymer fuel, and an inorganic perchlorate salt 
oxidizer, they improve the ballistic performance of the propellant by 
increasing burning rate and reducing the pressure exponent. These desired 
results are achieved without adversely affecting the physical or ballistic 
properties of the propellant. These compounds, which are substantially 
non-volatile and insoluble in the propellant matrix, exhibit no 
appreciable tendency to evaporate, sublime or volatalize, or to diffuse or 
migrate into propellant liner or insulation systems. Neither the 
ferrocenyl phosphine compounds or the propellant composition show phase 
change or decomposition over the useful temperature operating range of the 
propellant. Thus, use of ferrocenyl phosphine derivatives as ballistic 
modifiers achieves the desired improvement in burning rate and pressure 
exponent without undesirable catalyst migration or other adverse effect on 
propellant properties. 
Useful ferrocenyl phosphines and phosphine oxides are set forth in Table 1 
below. Particularly preferred, are those compounds that have a melting 
point above 200.degree. C., such as 1,1.sup.1 -bis(diphenyl 
phosphino)-ferrocene, diferrocenylphenyl phosphine and triferrocenyl 
phosphine oxide. 
TABLE 1 
______________________________________ 
Compound Of Formula 
(A) R.sub.3-n PR'.sub.n or (B) R.sub.3-n P(O)R'.sub.n 
Compound Formula R R' n 
______________________________________ 
1 A CH.sub.3 ferrocenyl 
1 
2 B CH.sub.3 ferrocenyl 
2 
3 A cyclopentyl ferrocenyl 
1 
4 B cyclohexyl ferrocenyl 
1 
5 A phenyl ferrocenyl 
1 
6 A phenyl ferrocenyl 
2 
7 B -- ferrocenyl 
3 
8 B -- ethyl ferrocenyl 
3 
9 A propyl ferrocenyl 
1 
10 A o-tolyl ferrocenyl 
2 
11 A p-nitrophenyl 
ferrocenyl 
2 
______________________________________ 
Preferably R is alkyl of 1 to 6 carbon atoms, cycloalkyl of 3 to 6 carbon 
atoms, naphthyl or phenyl. As an alternate embodiment, substituted 
ferrocenyls may be used for R', as exemplified in compound No. 8 in Table 
1. 
The organic polymer fuel binder useful in the invention can be 
substantially any such binder employed in the art. It can be, for example, 
polybutadiene and its derivatives such as hydroxy- or carboxy-substituted 
polybutadiene, polyurethane, polyethers, polyesters, polybutylenes, and 
the like. The polymer binder may or may not be plasticized with an organic 
plasticizer as is well known in the art. The preferred binders are the 
hydroxy- and carboxy-terminated polybutadienes. Since the use, processing, 
and cure of such binders are well known, they will not be discussed here. 
The inorganic perchlorate oxidizer salt can be, for example, the alkali 
metal, e.g., Na, K, Li; alkaline earth metal, e.g., Ca, Mg; or ammonium 
salts. Ammonium perchlorate is preferred. 
Finely-divided metal fuels, such as Al, Mg, Zr, or the like, may be added 
for high energy, high performance propellants. 
Other additives, conventionally employed in the propellant art, can also be 
incorporated. These include, for example: cure catalysts to shorten cure 
time of the organic polymer; potlife extenders to extend the like of the 
precured composition; ballistic additives to modify burning rate at 
different pressures; and additives to improve physical, shelf life, or 
processing characteristics of the propellant. 
Solid ferrocenyl phosphine derivatives are effective for use in a wide 
range of propellant compositions--from high-energy to fuel-rich. The 
amount of the ferrocenyl phosphine or phosphine oxide is in minor 
proportion and may be as high as about 20 percent, preferably about 10 
percent, with a minimum of about 0.1 percent. The specific concentration 
used is largely determined by the desired increase in burning rate. 
The following examples illustrate the efficacy and improved properties of 
the preferred ferrocenyl phosphine derivatives as compared with 
state-of-the-are solid ferrocenes and a liquid ferrocene derivative.

EXAMPLE 1 
A. A solid propellant was prepared comprising 70 percent ammonium 
perchlorate (AP), 16 percent powdered aluminum, 2 percent dioctyl adipate 
plasticizer, 9 percent hydroxyl terminated polybutadiene, 0.5 percent 
bonding agent and cure catalysts, and 2.5 percent diacetyl ferrocene, a 
solid ferrocene burning rate catalyst. This propellant had an end-of-mix 
viscosity of 14 kilopoise, a burning rate of 0.598 in/sec at 1000 psi and 
a pressure exponent of 0.317. 
B. An identical propellant was prepared except for the substitution of 2.5 
percent diferrocenyl ketone for diacetyl ferrocene. This propellant hand 
an end-of-mix viscosity of 12.5 kilopoise, a burning rate of 0.684 in/sec 
at 1000 psi and a pressure exponent of 0.274. 
C. An identical propellant was prepared except for the substitution of 2.5 
percent 1,1.sup.1 -Bis(diphenylphosphino)ferrocene for diferrocenyl 
ketone. This propellant had an end-of-mix viscosity of 7.5 kilopoise, a 
burning rate of 0.537 in/sec at 1000 psi, and a pressure exponent of 
0.311. 
D. An identical propellant was made using 2.5 percent diferrocenylphenyl 
phosphine. This propellant had an end-of-mix viscosity of 6.5 kilopoise, a 
burning rate of 0.657 in/sec at 1000 psi, and a pressure exponent of 
0.338. 
E. An identical propellant was made using 2.5 percent triferrocenyl 
phosphine oxide. This propellant has an end-of-mix viscosity of 10.2 
kilopoise, a burning rate of 0.778 in/sec at 1000 psi and a pressure 
exponent of 0.392. 
F. An identical propellant was made using 2.5 percent Catocene, a commonly 
used liquid ferrocene derivative. This propellant had an end-of-mix 
viscosity of 1.4 kilopoise, a burning rate of 0.731 in/sec at 1000 psi, 
and a pressure exponent of 0.328. 
EXAMPLE 2 
A. A solid propellant was prepared comprising 85.5 percent ammonium 
perchlorate (AP), 11 percent hydroxy-terminated polybutadiene, 1.5 percent 
combustion stabilizer additives, and 2 percent Catocene, a liquid 
ferrocene-derivative burning rate accelerator. This propellant had a 
burning rate of 1.32 in/sec at 1000 psi and a pressure exponent of 0.430. 
EXAMPLE 3 
A. A solid fuel rich propellant comprising 37 percent AP, 23 percent 
carboxy terminated polybutadiene binder, 34 percent polystyrene bead fuel, 
1 percent iron oxide, 2 percent combustion stabilizer additive and 2.75 
percent Catocene liquid ferrocene burning rate catalyst. This propellant 
had a burning rate of 0.930 in/sec at 1000 psi. 
B. An identical propellant was made except for substitution of the 2.75 
percent Catocene by 2.75 percent triferrocenyl phosphine oxide. This 
propellant had a burning rate of 0.941 in/sec at 1000 psi. 
EXAMPLE 4 
Samples of 1,1.sup.1 -Bis(diphenylphosphino)ferrocene (m.p. 180.degree. 
C.), diferrocenylphenyl phosphine (m.p. 194.degree. C.) and triferrocenyl 
phosphine oxide (m.p. 270.degree. C.) were placed in an oven at 
150.degree. F. under a vacuum of less than 10 mm Hg for 24 hours. All 
three materials demonstrated essentially no weight loss or physical 
change, thus demonstrating their non-volatility and thermal stability. 
While the present invention has been described by specific embodiments 
thereof, it should not be limited thereto, since obvious modifications 
will occur to those skilled in the art without departing from the spirit 
of the invention or the scope of the claims.