N23A-T018   TITLE: Reliable Hydroxyl-Terminated PolyButadiene for Rocket Motors

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): General Warfighting Requirements (GWR); Hypersonics

OBJECTIVE: Establish combined modern analytical methods and a resultant predictive model for characterizing Hydroxyl Terminated PolyButadiene (HTPB) polymer binder resin in parallel with chemical cure, mechanical properties, and aging assessments, to support the successful development and use of military grade HTPB in existing and emerging solid rocket motors. The combined analytics and predictive model will reliably correlate binder resin chemical and physical properties with gum stock and formulation quality, provide key characterization parameters leading to updated military specification(s), and improved sourcing of HTPB for successful manufacture of rocket motor, ejection seat, and related propellants.

DESCRIPTION: The DoD has a need for proper analytics coupled with predictive material models for Hydroxyl Terminated PolyButadiene (HTPB) as used in rocket motors and other energetic formulations. Even when meeting military specifications, the binder resin, often and unpredictably, suffers from a too short and variable pot-life, slow and incomplete curing, poor gum stock mechanical properties, and poor aging behavior. As a result, DoD propellant production is often greatly hindered. As the need for more of this DoD-specific HTPB binder resin continues to grow, the fundamental chemical and physical properties of this polymer that lead to desirable rocket motor formulation characteristics remain poorly understood.

One major challenge for HTBP variants used in energetic formulations is that there are several ways to meet outdated military specifications (WS 20700, dated June 1981) that do not define necessary formulation-enabling chemical properties. An HTPB lot can meet specifications as a monodisperse straight-chain, telechelic polymer, or as a complex mixture of variable length, distribution of branches, and reactive sites. Ultimately, this variability leads to inconsistent and unwanted end use behavior. For instance, the variability in branching and the locations of the hydroxyl groups greatly affects how the polymer cross-links, which is magnified by other ingredients when formulating a rocket motor grain. Another example of the problems facing end users of HTPB binder is that processing techniques and the variability in the distribution of reactive sites greatly change the effectiveness of antioxidant protocols when formulating. These are only two examples of the problems associated with the currently available supply of HTPB. OSD and the DPA Title III office recognized the inadequacy of the weapon specification test requirements in determining whether any given lot of HTPB will work sufficiently in a propellant formulation and as such recommended additional tests be performed [Ref 1]. Further, the hydroxyl value of the product obtained from the current, domestic supplier appears to have decreased from historical levels upon attempting to target a lower hydroxyl functionality initially sought for prior program requirements. Concurrent with this change, formulators are finding it even more difficult to consistently produce a propellant that meets their requirements due to the polymer not having sufficient molecular weight or functionality fraction to permit the manufacture of a robust propellant, especially for air-launched applications.

To date, private industry has been unable to solve the technical challenges associated with analytical methods development, detailed understanding of the chemistry, reaction kinetics, safety, reproducibility, and scalability in order to produce a reliable HTPB variant and associated specifications. While there have been prior efforts to develop a greater understanding of the HTPB polymer characteristics [Refs 2-5], to date there has been no definitive model developed using appropriate, modern analytics to correlate polymer structure to performance. Future sources and suppliers of HTPB for energetic formulation use will require this type of analytically driven HTPB predictive model to inform updated military specifications and production practices.

PHASE I: Identify and/or develop combinations of key analytical methods for correlating HTPB polymer system chemical and physical characteristics with gum-stock and formulation end use properties leading toward development of a strong predictive tool. HTPB samples of varying quality should be obtained from the Government and other contractor rocket motor manufactures to assist in Phase I material analytical method development efforts. The quality of the HTPB should not be defined as simply "good" or "bad" but will be assessed by quantifiable data pertaining to pot-life, degree of chemical cure, oxidative susceptibility, mechanical properties, specification, and any other relevant test results that relate binder ingredient characteristics to formulated rocket motor characteristic requirements. Generate needed characterization data otherwise unavailable from Government and other sources. Phase I should culminate in a planned framework approach to building a predictive model that correlates HTPB chemical, physical, and any other relevant properties to resulting gum-stock and formulation characteristics, describing the analytical methods and characteristics to be used and validated in Phase II.

PHASE II: Refine HTPB polymer analytical methods and perform cure studies and aging assessments as needed to fully develop and validate the predictive model. The model should incorporate the capability to predict cured polymer gum stock mechanical properties, cure characteristics including pot-life, and aging characteristics based on a suggested set of analytically generated chemical and physical properties data. Make final model modifications based on validation studies and complete model development. The model will serve to ensure that a fully tested lot of HTPB, using the newly developed and defined set of analytical characterization tests, will meet required gum-stock pot life, cure profile, and aging characteristics.

PHASE III DUAL USE APPLICATIONS: Using the developed HTPB model, work with Government and other contractor entities, as applicable, to develop robust processing and controls to produce HTPB at pilot scale meeting desired material chemical and physical characteristics as previously identified that lead to successful gum stock formulations (i.e., meeting various exemplar rocket motor formulation specification requirements). Additionally, the predictive model and associated data will be used to assist in development a modern DoD specification for HTPB. Outcomes from work under this STTR topic would likely benefit other industrial/commercial uses for HTPB type polymer resins where similar chemical/physical property characteristics are critical to material performance (i.e., rubbers).

REFERENCES:

1.       Funding Opportunity Announcement (FOA) #FA8650-19-S-5010 Call 009 "Hydroxyl Terminated Polybutadiene (HTPB) Production Capability for DOD Munitions Project", 2021.

2.       Vilar, W. D., Menezes, S. M. C., & Akcelrud, L. (1994). Characterization of hydroxyl-terminated polybutadiene. Polymer Bulletin, 33(5), 557�561. doi:10.1007/bf00296164

3.       Vilar, W. D., Menezes, S. M. C., & Akcelrud, L. (1994). Characterization of hydroxyl-terminated polybutadiene. Polymer Bulletin, 33(5), 563�570. doi:10.1007/bf00296165

4.       Regan, P. R., Teo, H. H., Booth, C., Cunliffe, A. V., & Hudd, A. L. (1985). The molecular characteristics of hydroxyl-terminated polybutadiene. British Polymer Journal, 17(1), 22�26. doi:10.1002/pi.4980170106

5.       Prine, N. 2018 �Characterization and Selection of Hydroxyl-Terminated Polybutadiene Polymers for High-Strain Applications�, Honors Thesis, University of Southern Mississippi, Hattiesburg, MS.

 

KEYWORDS: Hydroxyl-Terminated Poly Butadiene; HTPB; predictive model; polymer analytics; material specification; Solid Rocket Motor; SRM; propellants; explosives; energetic materials

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