Catalyst System for Polyurethane Propellant Compositions

This invention relates to solid propellant compositions and to a method for their cure. More particularly, this invention concerns itself with an improved catalyst system for curing polyurethane propellant compositions.

In general, solid propellant compositions are composed of a mixture of an inorganic oxidizer disposed in a matrix of plastic resinous or elastomeric material. The matrix provides fuel for the combustion reaction and, in some propellants, acts as a binder for holding the propellant mixture together prior to combustion. Amongst the preferred binder materials are the polyurethanes which have proven to be an excellent fuel and binder component for modern propellants.

The Urethane based propellants are conventionally cured at temperatures of from about 110.degree.-1135.degree. F.. The use of these high temperatures, however, often creates problems of stress and strain in case bonded solid rocket motors. A reduction of the cure temperature to 70.degree.-80.degree. F. would significantly reduce propellant bore strains and bond stresses in case bonded solid rocket motors. The lower cure temperature could also result in better propellant mechanical behavior, because side reactions are also minimized at lower temperatures.

As a consequence of the problems created by high temperature curing, a considerable research effort has evolved in an attempt to develop catalysts which promote the isocyanate-hydroxy reaction at room temperature under the conditions expected in a solid propellant environment. Also, the catalysts have to achieve their effectiveness without seriously affecting propellant processing characteristics, mechanical behavior and storage stability.

One catalyst which has been suggested for use in curing polyurethane propellant is Ferric acetylacetonate, otherwise designated as Fe(AA).sub.3. However, a satisfactory balance between potlife and the time required for full cure is a major problem with such catalysts since the accelerating effect of a higher curing temperature on the urethane reaction cannot be utilized. This is particularly true for a lithium initiated, hydroxy terminated polybutadiene prepolymer cured with a diisocyanate, such as hexamethylene diisocyanate, since both NCO groups of which possess equal reactivity.

At catalyst levels as low as 0.001% Fe(AA).sub.3 the propellant mix will be castable for only 30 minutes in case of hexamethylene diisocyanate and somewhat longer for toluene diisocyanate cured propellants but still requires 7-10 days for full cure. In the absence of catalyst Li HTPB propellants are practically uncurable (weeks at 180.degree. F. are required). Reduction of the catalyst level below 0.001% entails the danger of losing the catalyst in degradative reactions.

With the present invention, however, it has been found that Fe(AA).sub.3 can be effectively utilized as a catalyst for the low temperature cure of polyurethane based propellants, provided it is employed in a system along with a suppressor material and a scavenger material. Such a catalyst system has proven to be highly effective in solving the problems encountered in the curing of polyurethane binders and propellants. The suppressor material can be either a diketone or an acid while zinc oxide has been found to be a most efficient scavenger component.

In accordance with the general concept of this invention, there is provided a catalyst system which strikes a satisfactory balance between potlife, and the time required for a full cure in the cure catalysis of polyurethane based propellants. The catalyst system comprises the catalyst proper Fe(AA).sub.3, a suppressor component which can be acetylacetone and or a .alpha.-hydroxy acid which temporarily inactivates the catalyst, and a zinc oxide scavenger which slowly consumes the suppressor thus reactivating the catalyst.

Accordingly, the primary object of this invention is to provide a novel catalyst system for curing polyurethane based propellants.

Another object of this invention is to provide a propellant catalyst that does not adversely affect the aging, mechanical properties and the processing procedure for polyurethane propellants.

Still another object of this invention is to provide a catalyst system for polyurethane propellants that achieves a satisfaction, balance between potlife and the time required for a full cure.

The above and still other objects and advantages of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description thereof.

Pursuant to the above-defined objects, this invention contemplates a catalyst system for curing a propellant composition which uses a polyurethane, such as a lithium initiated hydroxy-terminated polybutadiene, as a fuel and binder component for the propellant. The key problem in the cure of hydroxy-terminated polybutadiene propellents is the achievement of adequate potlife in the presence of a catalyst, which will effect complete cure in a reasonable time. With a simple catalyst, it is not possible to achieve this objective.

A good catalyst accelerates essentially the urethane reaction, i.e., the cure reaction proper, leaving side reactions like the water-isocyanate reaction comparatively unaffected. At higher levels of catalyst, therefore, the effects of side reactions are minimized and the overall state of cure and hence reproducibility as well as mechanical properties are improved.

The level of contaminants (moisture, etc.) in propellants may vary, depending on exposure to the environment and the characteristics of the propellant components which may not be known with the desired precision. While some of the contaminants may be neutralized by suitable scavengers, it is obviously good practice to minimize the cure time during which these substances can interfere with the urethane reaction. This cure time, unfortunately, must include a lengthy period, referred to as potlife (usually about 8 hours are required) during which the propellant batch must remain liquid in order to be cast into motor chambers. During this time, the propellant viscosity at all shear levels should not rise above 50,000 poise, to assure flaw-free castings.

HTPB prepolymers have about the highest reactivity of any hydroxyl containing compound. Thus, in order to obtain the desired potlife, the slower reacting isocyanates such as TDI, HDI, IPDI, (toluene diisocyanate, hexanedi-isocyanate, isophorone diisocyanate), etc. must be used. Aromatic isocyanates with unhindered NCO groups, or even more so, electronegatively substituted isocyanates, such as tetrafluoro-1,4-phenylene diisocyanate, are too fast, even uncatalyzed, to be applicable.

Even less reactive isocyanates, require low concentrations of the more effective catalysts or the propellant potlife is undesirably short. Slightly higher concentrations of moderately active catalysts which are best in promoting the urethane reaction are also the most active ones so that only very small catalyst concentrations are permissible. This poses an additional problem in that at low concentrations, the catalyst can be completely lost by hydrolysis or other degradative reactions. One method which has been quite successful was the modification (weakening) of catalyst activity by the simultaneous addition of a chelating agent, usually the same that is contained in the metal chelate catalyst, e.g. the combination ferric acetylacetonate-pentanedione, Fe(AA).sub.3 --HAA. With this modification, the catalyst concentration can be increased to safe levels and, moreover, the free chelating agent acts as a stabilizer or suppressor for the catalyst further reducing the danger of loss by degradative reactions.

While the Fe(AA).sub.3 --HAA method has worked satisfactorily in many applications, the over-all required cure times are long and give contaminants too much of an opportunity to interfere in the curing reaction. The addition of a scavenger, however, slowly neutralizes the suppressor and thus restores the activity of the catalyst. This shortens the overall time required for cure and, therefore, gives contaminants less time to interfere. The effect of several chelating agents on catalyst activity is shown in Table I.

The addition of a scavenger component, such as zinc oxide, does not noticeably affect the viscosity build-up during the initial stages of the curing reaction and accordingly, do not shorten pot-life. The use of zinc oxide, as shown in Table II, does substantially shorten the time of cure.

It should be noted that the propellant not containing the suppressor had a useful potlife of less than 1 hour,

Instead of diketones, acids can be used as suppressors because according to the equilibrium: M(AA).sub.3 +HX M(AA).sub.2 +HAA the diketone is liberated, which then acts as the actual suppressor. Because of the higher reactivity of the acids, they can be more readily neutralized by proper scavengers. Thus, the metal chelate-acid-scavenger system is particularly useful if cure is to be effected at ambient temperatures, while the .beta.-diketone is better suited for cure at elevated temperatures, where acids may not yield the required potlife.

Table III shows the modification of catalyst activity by acids, and Tables IV and V, some typical data on viscosity build-up of such systems. Acids can be used in conjunction with metal salts, where the .beta.-diketones are comparatively ineffective suppressors. For example, the catalytic activity of dibutyltindilaurate can be controlled by acids much more effectively than than with a diketone as shown in Table VI. In general, all acids which have a pKa of 6 or lower will yield a satisfactory potlife. The acids tested include acetic, chloroacetic, -chlorooctanoic, .alpha.-bromotetradecanoic, linoleic, mandelic (phenylhydroxyacetic), benzilic (diphenylhydroxyacetic), lactic, pyruvic, phenylacetic, p-nitrophenylacetic and p-toluenesulfonic acid. Of these, the best appear to be the .alpha.-hydroxyacids, mandelic and benzilic acids.

It is of course also possible to use the acid suppressor in combination with a diketone, in this fashion minimizing any possible undesirable side effects of either component. For example, if the required potlife is to be achieved only by a diketone, the time to achieve full cure will be unduly long, while large concentrations of acids are required if satisfactory potlife should result. The latter increases the danger of undesirable side reactions, i.e., noticeable consumption of NCO-groups by the acid.

Table VII illustrates the effect of the zinc-oxide scavenger on a number of acid suppressors and mixed acetylacetone-acid suppressors used in HTPB propellants. It is readily seen that the propellants containing the suppressor reach a harder state of cure, caused by a more complete reaction of hydroxyl and NCO groups, than the corresponding propellants without the scavenger.

In Table VIII, the catalyst-acid-ZnO system is shown for the case of the tin catalyst (dibutyltindilaurate) for which previously there was no known way of tailoring catalyst activity. Again, it should be noted that in the absence of the suppressor, the useful potlife was less than 1 hour. Tables IX, X and XI present more examples where satisfactory potlife followed by good cure has been obtained through the use of the catalyst-suppressor-scavenger combination.

Although the invention has been described by reference to particular embodiments thereof, it is to be understood by those skilled in the art that all the various modifications and alterations that are encompassed within the scope of the appended claims are intended to be included herein .