WO2000007962A2

WO2000007962A2 - Improved composite solid propellants including a hydroxy-functional polybutadiene binder and a rocket motor including same

Info

Publication number: WO2000007962A2
Application number: PCT/US1999/015435
Authority: WO
Inventors: Louis F. Cannizzo; Jeffrey S. Midyett; Richard S. Hamilton
Original assignee: Cordant Technologies, Inc.
Priority date: 1998-07-10
Filing date: 1999-07-09
Publication date: 2000-02-17
Also published as: AU1703300A; WO2000007962A3

Abstract

Solid propellants for rocket motors contain binders which are hydroxy-functional polybutadiene (HFPB). The HFPB is difunctional and the diene linkages are exclusively 1,4-diene linkages, with no vinyl groups. A rocket motor may contain the solid propellant.

Description

IMPROVED COMPOSITE SOLID PROPELLANTS INCLUDING

A HYDROXY-FUNCTIONAL POLYBUTADIENE BINDER

AND A ROCKET MOTOR INCLUDING SAME

BACKGROUND OF THE INVENTION

1. Field of the Invention The invention relates to solid propellants for rocket motors, and more particμlarly propellants including a binder containing hydroxy-functional polybutadiene having 1,4-diene linkages only, without vinyl groups. This invention also relates to a rocket motor assembly including the solid propellant.

2. Description of the Related Art

Known prior patents directed to preparation of polyurethane resin-based binders for solid propellants include Graham et al . , U.S. Patent Nos. 4,110,135 and 4,184,031, the entire disclosures of which are incorporated herein by reference. These patents are particularly directed to the use of organometallic compounds, made from carboxylic acids or compounds convertible thereto, as curing catalysts for controlling the curing rate profile of hydroxy-terminated polybutadiene-based polyurethane propellant binders, and the cure rate profile of polyurethane resin systems generally. In making solid propellants for rocket motors, prior known propellants of the ammonium perchlorate (AP) /aluminum- type have included R45M resin binder, which is hydroxy- terminated polybutadiene binder having a combination of 1,2- diene and 1,4 -diene linkages. R45M is made by Elf Atochem North America, Inc., Philadelphia, PA.

The mechanical properties of solid propellants of the prior art are inadequate for providing high performance solid propellants having an excellent combination of high solids loadings and lower temperature processability, with simplified processing and a reduction in stress and shrinkage induced during cooling.

SUMMARY OF THE INVENTION

An object of the invention is to provide a solid propellant for a rocket motor that is formulated with a binder having a reduced end-of-mix (EOM) viscosity and having improved strain values at given modulus and stress levels.

Another object of the invention is to provide a binder which allows increased solids loadings with improved mechanical properties at such increased solids loadings.

A further object of the invention is to provide a high performance solid propellant having high solids loadings that can be processed at lower temperatures than prior art binders of the same type, thereby simplifying processing and minimizing stress and shrinkage induced during cooling. An additional object of the invention is to provide a rocket motor including the improved solid propellant described.

These and other objects are achieved by providing solid propellants employing hydroxy-functional polybutadiene binders (HFPB) without vinyl groups. The solids can, in general, be 88-92 wt%. The diene linkages are 1,4-diene linkages only, with no vinyl groups being present. The propellant formulations using HFPB have lower EO viscosities and improved strain values at similar modulus and stress levels and allow for higher solids loadings than propellants made with prior art R45M binders. Suitable EOM values are particularly relevant for casting procedures.

Benefits include the ability to use more aggressive grain designs for increased motor performance, due to the improved mechanical properties, and the ability to achieve formulations with higher solids contents thereby providing improved EOM viscosities and the ability to process propellants at lower temperatures. As referred to herein, EOM relates to determination of uncured propellant after mixing has been completed, but before a raising the composition to a temperature sufficient to initiate curing.

AP/aluminum propellants having 88% solids, made with HFPB, have typical end-of-mix (EOM) viscosities of 2-3kP at 57.2°C (135°F) . The strain at failure is typically 70-100%. The HFPB propellant also has satisfactory pot life, burn rate and slope of burn rate. At 91% solids (with no plasticizer) , HFPB propellants have strain at maximum stress values (corrected) of 37%, maximum stress values (corrected) of 1.73 MPa (250 psi) , and EOM viscosities of less than 20kP at 71.2°C (160°F) . A 92% solids propellant made with HFPB has an EOM viscosity of 19kP at 71.2°C (160°F) .

The hydroxy-functional polybutadienes of the invention are difunctional and the diene linkages are 1,4-diene linkages only, with no vinyl groups, that provide lower EOM viscosities and higher strain values than the prior art R45M-containing hydroxy- erminated polybutadiene binder analogs.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the relationship between Haake Viscosity of HFPB compared with R45M for 88% solids propellant. Figure 2 shows pot life at 57.2°C (135°F) of 88% solids

HFPB mix.

Figure 3 shows the burn rate of 88% solids HFPB propellant (strand data) compared to two prior art propellant formulations . Figure 4 is a sectional view of a schematic of a rocket motor assembly suitable for loading with the solid propellant of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The new HFPB of the invention is difunctional (f=2) and contains no vinyl groups. All the diene linkages are 1,4- diene linkages. The HFPB material is produced by olefin metathesis polymerization of octadiene, using no solvent, in the presence of a chain transfer agent and a metal-based catalyst . Any appropriate molecular weight for the HFPB can be selected and produced. The HFPB can be made by an appropriate method, such as those found in Grubbs, U.S. Patent No. 5,880,231 (U.S. Application No. 08/548,445, filed October 26, 1995); Hillmyer et al . , Polymer Preprints (American Chemical Society Journal) Vol. 34 (2), 1993, pp 388-389; Hillmyer et al . , Macromolecules (American Chemical Society Journal) Vol. 26, 1993, pp 872-874, and Vol. 30, 1997, pp.

718-721, the disclosures of which are incorporated herein by reference.

The most significant properties of HFPB, related to use in solid propellants, are lower viscosity, lower glass transition temperature, and greater chain length for the same equivalent weight, when compared to the prior art binder R45M resin. These properties allow higher solids propellants to be made with lower EOM viscosities at the same solids loadings, than in the prior art. The propellants produced have lower glass transition temperatures and increased strain values, when compared with propellants made with binder containing R45M. Table 1 shows a comparison of chemical/physical properties typical of HFPB and prior art R45M binder.

Table 1

The method of producing HFPB is reproducible and controllable and provides material of constant functionality and molecular weight containing no acidic trace impurities. The lower functionality of HFPB, compared to R45M binder, precludes the use of isophorone diisocyanate (IPDI) alone as a curative agent. A higher functionality curative agent is required to obtain the necessary cross-linking required for a thermoset solid propellant. The curative agent used for the HFPB advantageously includes a polyfunctional isocyanate based on IPDI, which is commercially available as Z4470-BA from Bayer Corporation, Pittsburgh, Pennsylvania, combined together with IPDI. Z4470- BA is a solid and is available as a 70% solution in butyl acetate.

Before using Z4470-BA in the curative agent, the Z4470-BA is combined with the IPDI and the butyl acetate is removed under vacuum. The resulting mixture has an equivalent weight of 141 and is used as the curative agent. The Z4470-BA is combined with the IPDI at an equivalent ratio of IPDI : Z4470-BA in a range of from 50:50 to 90:10, preferably from 65:35 to 80:20, more preferably 75:25 and, after processing well, is used as the curative agent for mixing with HFPB. The mixture cured into a hard rubber-like material.

A non-exhaustive example of a typical formulation for the propellant is shown in Table 2, below.

Table 2 - Propellant Formulation

The mix procedure is set forth below in Table 3

Table 3 - Mix procedure

The mix procedure set forth in Table 3 above was performed in a standard vertical propellant pint mixer. Tests of properties of an 88% solids propellant mix, formulated and mixed at 57.2°C (135°F) , as shown in Tables 2 and 3, were carried out and are shown in Table 4, below, for four examples of the invention. Optimum properties were obtained at an NCO/OH ratio of 0.95.

Table 4 - EOM Viscosity and Mechanical Properties

The Haake Viscosity of HFPB and R45M for an 88% solids propellant was determined and is shown in Figure 1. The pot life data is shown in Figure 2. The pot life, i.e., the time to reach a viscosity of 40kP, was 11.5 hours, a value similar to many R45M formulations. The burn data is shown in Figure 3, along with related data for similar formulations using R45M/IPDI and R45M/dimer diisocyanate (DDI) . For the HFPB mix, the burn rate was 8.89 mm/sec (0.35 in. /sec.) at 6.89 MPa (1000 psi) and the slope was 0.42 from 500-3000 psi. For a known commercial propellant, the burn rate was 7.87 mm/sec (0.31 in. /sec.) at 6.89 MPa (1000 psi) (TU-628 motors) and the slope was 0.28. Thus, the HFPB mix gave slightly higher burn rates and slopes. Use of the HFPB-containing curative does not substantially impact the burn rate or slope of the formulation.

Propellant formulations A-D, shown below in Table 5, with reference to changes made in the Table 2 formulation, having 88-92% solids, were tested and the results show the advantageous results using the formulations of the invention. Table 5 - HFPB Formulations Tested

Table 6 - Results from the HFPB Formulations Tested

When no DOA plasticizer is used in the propellant, the propellant/liner bond line will last much longer, providing a longer shelf life for a rocket motor containing the propellant. Plasticizers help in processing and modify the mechanical properties.

If, in comparison, R45M were used instead of HFPB in mixes A and B, use of ODI processing aid and TEPANOL™ as a bonding agent is required, leading to undesirably longer mix times. For mix A, while the mechanical properties are satisfactory with R45M, the EOM viscosity is significantly higher. For mix B, a lower EOM viscosity is obtained using R45M with comparable mechanical properties to HFPB, but much longer processing times are required. Mixes C and D cannot be made with R45M as the viscosity is too high to process in a standard vertical mixer.

Tables 5 and 6 show that for initial formulations using HFPB in a propellant analog (88% solids) advantageously lower EOM viscosities are obtained compared with R45M. In addition, optimal mechanical properties obtained for this formulation appear to be comparable or better than those obtained with R45M in several different production propellants. At 91% solids, the resulting propellant using HFPB had a strain at maximum stress of over 30%. A 92% solids formulation (AP/Al) using HFPB was successfully processed with an EOM viscosity of ≤ 20kP at 71.2°C (160°F) .

Typical known AP/aluminum propellants having 88% solids formulated with R45M resin binder have EOM viscosity of about 5-8kP at 57.2°C (135°F) and a strain at failure of about 50- 70%. At 91% solids (with 2% plasticizer) , propellants made with R45M resin binder have strain at maximum stress values (corrected) of about 26% and maximum stress values (corrected) of about 1.34 MPa (190 psi). The present propellant can be suitably loaded into a rocket motor assembly, such as the one generally designated by reference numeral 40 in Figure 4. The illustrated rocket motor assembly 40 includes a case 42 in which the propellant 44 is loaded to have an annular cross section, i.e., known as a center perforated propellant. (The inventive propellant can also be cast in an end-burning or other configuration.) An igniter 46 is positioned at the forward end of the case 42 within the center of the propellant 44 for ignition of the propellant 44. At the aft end of the case is a converging- diverging nozzle assembly 48 through which combustion products are expelled.

The process can comprise cast pouring the propellant composition into a suitably prepared rocket motor case. Solid propellant rocket motors for stategic, tactical and launch applications provided with the present propellant are contemplated herein. Suitable solid propellant rocket motors therefore include air-to-air missies, the so-called RSRM boosters, Castor IV-type motors and the like. The solid propellant rocket motor can include an insulation liner or the like, such as described in U.S. Patent No. 4,492,779, the complete disclosure of which is incorported by reference.

While the invention has been described above with respect to certain embodiments thereof, it will be appreciated by one skilled in the art that variations and modifications covered by the appended claims may be made without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A solid rocket motor propellant comprising: a fuel component , an an oxidizer, and a binder comprising hydroxy-functional polybutadiene having 1,4 -diene linkages and being free of 1,2 diene linkages .

2. The solid rocket motor propellant of claim 1, wherein said propellant comprises 88-92% of said fuel component and oxidizer.

3. The solid rocket motor propellant of claim 1, wherein said fuel component is aluminum and said oxidizer is ammonium perchlorate .

4. The solid rocket motor propellant of claim 1, further comprising a polyfunctional isocyanate.

5. The solid rocket motor propellant of claim 4, further comprising dioctyl adipate and triphenylbismuth.

6. The solid rocket motor propellant of claim 1, wherein said solid rocket motor propellant is free of a hydroxy-functional polybutadiene binder having 1,2-diene linkages .

7. A rocket motor assembly comprising: a case having a combustion chamber; solid rocket motor propellant loaded in said case, said solid rocket motor propellant comprising a fuel component, an oxidizer, and a binder comprising hydroxy-functional polybutadiene having 1,4-diene linkages and being free of 1,2- diene linkages; and a nozzle assembly in fluid communication with the combustion chamber.

8. The rocket motor assembly of claim 7, wherein said propellant comprises 88,-92% of said fuel component and oxidizer.

9. The rocket motor assembly of claim 7, wherein said fuel component is aluminum and said oxidizer is ammonium perchlorate.

10. The rocket motor assembly of claim 7, further comprising a polyfunctional isocyanate.

11. The rocket motor assembly of claim 10, further comprising dioctyl adipate and triphenylbismuth.

12. The rocket motor assembly of claim 7, wherein said solid rocket motor propellant is free of a hydroxy- functional polybutadiene binder having 1,2 -diene linkages.