Ferrocene Derivatives

Abstract

Compounds useful as burning rate catalysts for solid propellants characterized by low volatility and having the formula ##SPC1## Are disclosed wherein R is O, NR' and S and each R' is hydrogen, lower alkyl, phenyl, cyano and nitro.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to a novel burning rate catalyst and more particularly to a novel burning rate catalyst characterized by very low volatility and which is especially useful for controlling the burning rate of high energy composite type propellants.

Solid propellant compositions are generally classed as being either composite, if they consist of a mixture of a solid oxidant in a matrix of a resinous or elastomeric fuel-binder composition, or homogeneous, if they contain in a single molecule all the required fuel and oxygen necessary for combustion.

The type of propellant composition useful for a given application often is directly dependent upon the burning characteristics required by the particular application. For example, to drive the turbo-pump feed system of the fuel injector of a reaction motor, it is desirable to use a high gas producing propellant characterized by slow burning and low flame temperature; a composition often referred to as a gas generating propellant or simply "gas generator." For such applications, it would be desirable to use nitramine oxidant containing composite propellants because of their generally good burning properties, high energetics and high density which permits higher energy release per unit volume. However, due to their exceptionally low burning rates at the operating temperatures and pressures normally required, the nitramines have not been successfully adapted for gas generator applications.

Burning of a solid propellant is a localized phenomena in that it occurs only on the exposed surface of the propellant grain. The burning surface regresses by burning parallel layers in a direction perpendicular to the surface in accordance with Saint-Robert's or Veille's Law;

r = a + b p.sup.n

Where "r" is the burning rate; "a", "b" and "n" are constants and "P" is pressure. The constant "n" is referred to as the burning rate exponent.

It has long been known that the Veille Law constants can be adjusted favorably by adding certain additives, known as burning rate catalysts, to the composition. These additives tend to accelerate the exothermic reactions at the burning surface thereby acting to increase the burning rate of the composition.

Previously, however, no catalyst has been found which is entirely satisfactory for increasing burning rates of nitramine containing composite propellants. Ferrocene compounds, for example, although long known to be effective burning rate catalysts for many compositions, have not been wholly acceptable for composite propellants because of their undesirably high volatility at moderate storage temperatures. Because of this high volatility, compositions catalyzed by ferrocene compounds possess a continually changing burning characteristic as a function of storage time due to vaporization of the ferrocene compounds from the compositions.

It would be desirable, therefore, to provide a burning rate catalyst which is essentially non-volatile at normal storage temperatures and which may be used to control the burning rate of composite propellants, such as nitramine containing composite propellants.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a relatively non-volatile burning rate catalyst which will accelerate the burning rate of composite propellants without deleteriously affecting their storage stability. It is also an object of this invention to provide a burning rate catalyst which is effective in accelerating the burning rate of nitramine oxidants such as RDX and HMX.

Finally it is an object of this invention to provide a method for producing a novel non-volatile ferrocene burning rate catalyst useful for accelerating the burning rate of composite type propellants.

These and other objects are achieved herein by providing the novel burning rate catalysts of the formula: ##SPC2##

wherein R is selected from the group consisting of 0, NR' and S, each R' being selected from the group consisting of hydrogen, lower alkyl, phenyl, cyano, and nitro.

BRIEF DESCRIPTION OF THE DRAWING

One of the many advantages of the foregoing burning rate catalyst can best be appreciated by reference to a drawing.

The solitary FIGURE is a graph illustrating the change in burning rate as a function of pressure for a polyester RDX type nitramine containing propellant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The novel burning rate catalysts of this invention are provided by condensing ferrocene aldehyde with one or more compounds having a reactive methylene group, such as barbituric acid, thiobarbituric acid, 2-phenylmalonyl guanidine, 2-methylmalonyl guanidine, 2-ethylmalonyl guanidine, 2-cyanomalonyl quanidine and 2-nitromalonyl guanidine.

Examples of ferrocene compounds provided by this invention include 5-ferrocenylmethylene-2-phenylimino-4,6-dihydroxy pyrimidine, 5-ferrocenylmethylene thiobarbituric acid, 5-ferrocenylmethylene-2-methylimino-4,6-dihydroxy pyrimidine, 5-ferrocenylmethylene-2-ethylimino-4,6-dihydroxy pyrimidine, 5-ferrocenylmethylene-2-cyanoimino-4,6-dihydroxy pyrimidine, 5-ferrocenylmethylene-2-nitroimino-4,6-dihydroxy pyrimidine and 5-ferrocenylmethylene barbituric acid.

Because of the very low volatility (Table II) of these compounds, they will not vaporize readily from the composition and hence may be stored almost indefinitely without significantly affecting the burning characteristics of the propellant as a function of storage time; a problem characteristic of the prior art ferrocene burning rate catalysts. While the ferrocene compound of this invention are useful catalysts for a wide variety of propellant compositions, they are particularly useful for accelerating the burning rate of composite type propellants. The drawing is a graph of the log of the burning rate of a polyester-RDX composite propellant as a function of the log of the operating pressure. The slope of the burning rate-pressure curve is the burning rate exponent "n". In the system illustrated, the burning rate exponent of the unmodified propellant (curve 2) is 0.72 whereas that containing 1 percent 5-ferrocenyl-methylene barbituric acid (curve 1) is 0.61. As will be noted, the immediate ferrocene catalyst has a greater effect on the burning rate at lower operating pressures of about 500 psi, than at the higher operating pressures of about 1000 psi. Pressure was determined by burning the propellant in a suitable inert atmosphere in a pressurized bomb and burning rate was determined by microwave detection means whereby microwaves are reflected from the burning surface to a detecting device and the standing wave front so created is measured as a function of time.

Essentially similar results are obtainable with the other ferrocene compounds disclosed herein and with other oxidants other than the nitramines. For example, desirable burning acceleration can be obtained with ammonium perchlorate or ammonium nitrate containing propellants.

The particular binder selected for the composite propellant is not critical to the functioning of these catalysts and a wide variety may be used in the compositions with similar results. For example, operable binders include, asphalt, the phenolic resins, hydrocarbons such as polyethylene and polystyrene, synthetic rubber such as polybutadiene and modified synthetic rubber such as butadiene acrylic acid copolymers, also vinyl polymers, urea-aldehyde resins, nitrocellulose, glycerine, and polyethyleneglycol adipate.

While it is not understood precisely why the catalysts of this invention accelerate the burning rate, it is believed that they act to reduce the decomposition temperatures of the propellants. For example, at a heating rate 1.degree.C per min, the peak temperature of decomposition (T.sub.dec) of a polyester-RDX propellant (71% RDX, 25 percent polyglycoladipate) was affected by addition of various ferrocene compounds as follows:

TABLE I

71% rdx, 25% polyglycoladipate

Modifier T.sub.dec (.degree.C) unmodified 190.degree.C 1% ferrocene 167.degree.C 1% 5-ferrocenylmethylene barbituric acid 176.degree.C

Having generally described the invention, the following examples are provided herein for purposes of illustration only and are not intended to be binding in any manner.

EXAMPLE

A solution of 3 g (0.024 mole) of barbituric acid in 100 ml water was added to 4.3 g (0.02 mole) of ferrocene-aldehyde in 100 m1 ethyl alcohol. The reaction resulted in the precipitation of a blue-violet colored product which after filtration was washed with water and ether. By recrystallization from ethyl alcohol blue needles were obtained which decomposed >294.degree.C. (Determined by Differential Thermal Analysis).

C II N Fe Anal. calc'd for C.sub.15 H.sub.12 N.sub.2 0.sub.3 Fe: 55.58 3.73 8.64 17.23 Found 55.70 3.87 8.51 16.86

The yield was 6.1g (94%).

The following compounds can also be prepared in essentially the same manner except using the corresponding active methylene containing compound dissolved in a suitable alcohol. Preferably the reactants are used in approximately equi-molar amounts. Among those reactants which may be reacted with ferrocene aldehyde include: 2-phenylmalonyl guanidine, thio-barbituric acid, 2-methyl-malonyl guanidine, 2-ethylmalonyl guanidine, 2-cyanomalonyl guanidine and 2-nitromalonyl guanidine. Condensation of these reactants with ferrocene aldehyde provides respectively; 5-ferrocenylmethylene-2-phenylimino-4,6-dihydroxy pyrimidine, 5-ferrocenylmethylene thiobarbituric acid, 5-ferrocenylmethylene-2-methylimino-4,6-dihydroxy pyrimidine, 5-ferrocenylmethylene-2-ethylimino-4,6- dihydroxy pyrimidine, 5-ferrocenylmethylene-2-cyanoimino-4,6-dihydroxy pyrimidine, and 5-ferrocenylmethylene-2-nitroimino-4,6-dihydroxy pyrimidine.

Table II describes the vapor pressure of barbituryl-ferrocenylmethene as compared with the prior art burning rate catalyst; ferrocene.

TABLE II

Vapor Pressures of Ferrocene Temperature Pressure .degree.C mm Hg 50.0 0.09 63.1 0.27 70.8 0.40 80.5 0.70 92.0 1.47 97.0 2.13

Vapor Pressures of 5-ferrocenylmethylene barbituric acid Temperature Pressure .degree.C mm Hg 40.0 0.016 55.0 0.025 70.0 0.034 85.0 0.043 98.5 0.055

As will be noted from the foregoing Table, the vapor pressure of 5-ferrocenylmethylene barbituric acid at normal storage temperatures is significantly lower than that of ferrocene. This difference in volatility reduces the tendency of the catalyst to vaporize from its propellant compostion thereby reducing the tendency for the burning characteristics of the propellant to change with storage time. Having described the invention, it will be apparent to those skilled in the art that many modifications and changes can be without departing from the scope and spirit thereof.

Claims

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A compound represented by the general formula: ##SPC3##

wherein R is selected from the group consisting of 0, NR' and S, each R' being selected from the group consisting of hydrogen, lower alkyl, phenyl, cyano and nitro.

2. The compound of claim 1 of the formula ##SPC4##

wherein R" is selected from the group consisting of 0 and S.

3. The compound of claim 1 of the formula ##SPC5##

wherein R' is selected from the group consisting of hydrogen, lower alkyl, phenyl, cyano and nitro.

4. The compound of claim 1 which is 5-ferrocenylmethylene barbituric acid.