Processing procedure for isocyanate cured propellants containing some bismuth compounds

[0003] Bismuth compounds have been used extensively as cure catalysts for isocyanate cured hydroxy terminated propellant binder systems. The most widely used of these bismuth compounds is triphenylbismuth (TPB). Triphenylbismuth is used in very small quantities, usually less than 0.03 percent by weight in the propellant formulation. There are no ballistics additive effects gained when this small amount is used.

[0004] Two bismuth compounds, bismuth salicylate(BS) and bismuth β-resorcylate(Bβ-R), have been identified to be effective as ballistic modifiers in minimum signature propellants(see Table I). These compounds are the salts of their corresponding organic acids, salicylic and β-resorcylic acids, respectively (see Table II). The ballistic modification properties of these bismuth compounds in minimum signature propellants were determined to be comparable to results obtained with similar lead compounds, lead salicylate and lead β-resorcylate. Lead salicylate and lead β-resorcylate are widely used as ballistic modifiers in some minimum signature Army tactical missile propellants.

[0005] Because of the toxic properties of lead compounds, Environmental Protection Agency requirements to eliminate lead from Army propulsion systems have fueled research activities to find alternatives. Bismuth compounds were evaluated as replacements for lead. Initial results were promising. The ballistic properties of the bismuth compounds in similar propellant formulations were comparable to lead compounds. However, researchers suspected and later confirmed that the bismuth compounds would cause a significant decrease in propellant potlife by prematurely catalyzing the propellant hydroxy/isocyanate cure reaction. Because of the significant reduction in propellant potlife caused by the bismuth compounds, research evaluating bismuth compounds as replacements was quickly curtailed.

[0006] Bismuth compounds tend to reduce the potlife of propellants to less than 1-hour, when used in quantities in excess of one tenth percent by weight in propellant formulations. A desirable potlife for castable solid propellants is 4-10 hours. Potlife can be defined as the time it takes for the viscosity of the propellant to increase to forty kilopoise(kp) after the addition of the curing agent. A viscosity of forty kp is generally considered the maximum where proper casting or loading of the propellant into motors can be achieved. Proper casting occurs when no voids or unfilled spaces are created in the propellant due to air being present during the cure reaction. These void formations are undesirable because they cause ballistic and/or mechanical anomalies.

[0007] Table I, Table II, and Table III, all set forth hereinbelow, show typical minimum signature propellant formulation, bismuth ballistic modifiers, and general propellant mixing procedure for minimum signature propellants, respectively. The typical mixing procedure (see Table III) used in formulating minimum signature propellants is to add all liquids, and then all the solids in increments. The mixture is then mixed at 150° F. under vacuum, for some set period of time to rid the mixture of any volatile components. The mixture is then cooled to 90-100° F., before the curing agent is added. When bismuth compounds are added at Step 2, the maximum potlife is less than 1-hour. Sometimes the mixture cures during the first 10-minutes after Step 6. With ballistic modifier addition at Step 2, cooling the mixture to less than 70° F. only slightly increased the potlife to approximately 1-hour. Eliminating the dibutyltin cure catalyst in step 6 only increased the potlife to nearly 2-hours. When the addition of the bismuth was delayed until Step 4, only an additional slight improvement in the potlife to less than 3-hours was achieved. By reviewing these tables one should more fully appreciate the novelty of the improved processing procedure set forth later in Table IV.

[0008] An object of this invention is to provide a procedure for processing isocyanate cured minimum signature propellants containing bismuth compounds as ballistic additives that will result in an adequate potlife for casting propellants into the desired missile configurations. When the proper processing procedure is used, adequate propellant potlife is achieved.

[0009] Bismuth compounds, such as bismuth salicylate and bismuth β-resorcylate, can be used as ballistic modifiers in isocyanate cured hydroxy-terminated propellant binder systems when the processing procedure is changed to delay the catalytic effects of the bismuth on the binder/cure agent reaction. Thus, the mixing procedure set forth in Table IV below depicts the preferred embodiment for obtaining a potlife of greater than 15 hours.

[0010] The FIGURE of Drawing represents potlife values in hours from end of mix plus 7 hours wherein many data points for viscosity are depicted as read by a computer at ambient temperature.

[0011] A representative propellant formulation used in these determinations is as described in Table V. The procedure for processing isocyanate cured minimum signature propellants containing bismuth salicylate and or the equivalent amount of bismuth β-resorcylate to achieve maximum potlife, is changed to give a propellant potlife in excess of 15 hours at 70° F. When the addition of the bismuth compounds is delayed to Step 5 of Table IV, a significant and an unexpected increase in the potlife is achieved. It is believed that at 60° F. the formation of bismuth ions within the liquid propellant matrix is significantly delayed thus delaying the catalytic effect on the cure reaction. When the propellant mixture was maintained at 60° F., the cure reaction did not occur overnight. The results from the potlife determination are shown in the FIGURE of the Drawing. Both the Step and temperature at which the bismuth compounds are added determine the potlife after the addition of the curing agent. In further reference to potlife values shown in FIGURE of the drawing, a plurality of data points are shown (wherein one such data point is depicted as “a”); and line “b” represents the best fit for all data points. The binder (polymer+plasticizer+curing agent) percent can vary to obtain optimum propellant mechanical properties and the desired performance. Also, the concentration of solids (oxidizers, catalysts) in mixture could reduce or increase the potlife since higher solids propellants are normally more viscous at the end of propellant mix cycle.