Dissertation on Insensitive Highly Energetic Materials (Section 2.4)
Dissertation on Insensitive Highly Energetic Materials (Section 2.4)
2.4 Ingredient Selection and Theory
The overall logic for selection of raw materials is detailed in Figure 2.4-1. Once ingredients were selected, scale up to 450 gram mixes occurred. Ingredients were selected to help ensure that project objectives (Table 1-3) were achieved.
Binder Screening and Selection
Theoretical calculations for non AP containing PBX compositions which utilize an HTPB binder system revealed that significant amounts of un-reacted carbon was being generated in both the burn and detonation reactions. The code predicted that, while the majority of the aluminum was oxidized to Al2O3, the majority of the binder (a hydrocarbon) was un-oxidized.
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Figure 2.3-1 Theoretical Calculation Logic Flow
It was postulated, and supported by thermochemical code output, that replacement of the non-oxygenated HTPB hydrocarbon binder with an oxygenated binder might allow the oxygen to enter into the detonation and/or deflagration reaction. This approach, if successful, would yield a dual benefit. First, oxygen from the binder could be utilized to burn the residual hydrocarbon and secondly, the more oxygenated (and more sensitive) ingredients (such as RDX) could be reduced in content. This would hopefully allow the PBX to pass tests which PBX-109 fails, namely Slow Cook-Off (SCO), Fragment Impact (FI) and Sympathetic Detonation (SD). In the process of oxygenated binder evaluation, binders were considered that not only had relatively high oxygen content, but which would also adequately wet the solids (to ensure low viscosity (Objective #4)) and which had a demonstrated capability of rapid cure (Objective #5). Both of these characteristics would be desirable from production cost standpoints.
Finally, only binder ingredients which were commercially available in large quantities and at a relatively low cost were chosen (Objective #8).
After a thorough evaluation of a number of potential binder candidates, three oxygenated, curable binders were selected for further evaluation, polyethyleneglycol (PEG), polypropyleneglycol (PPG), and ethyleneoxide-propyleneoxide (EOPO). The plasticizer chosen was triacetin (TA). In order to meet rapid cure goals, two types of cure catalysts were chosen for evaluation. The first cure catalyst evaluated was ferric-acetylacetonate (FeAA). The second cure catalyst evaluated was di-butyltin di-laurate (DBTDL) in conjunction with a cure delay/cure stimulation catalytic system consisting of minute concentrations of triphenyl bismuth (TPB) and maleic anhydride (MA) .
A comparison of the theoretical improvement which was predicted to occur by switching from an HTPB binder to a PPG binder for a well characterized explosive (PBX-109) is detailed in Table 2.4-1.
Table 2.4-1
Comparison of Theoretical Performance Improvement with Oxygenated Binders
Characteristic
HTPB Binder
PPG Binder
Density (g/cm3)
1.655
1.7071
Detonation Velocity (m/sec)
6721
7010
Detonation Pressure (MPa)
18435
21666
Percent Un-reacted Carbon
19.5
15.04
Temperature (K)
3682
4052
As shown later, PPG was down selected as the final polymer of choice due to superior processing, curing, and physical property characteristics.
Oxidizer Screening and Selection
Theoretical calculations showed a direct correlation between energetic material density and oxygen content with detonation velocity, detonation pressure and Impulse Density. It was predicted by the TIGER and NASA/Lewis codes that the inclusion of oxidizers such as Ammonium Nitrate (AN), Ammonium Perchlorate (AP), Potassium Nitrate (KN), etc., would improve detonation velocity, detonation pressure and blast pressure impulse.
The addition of oxidizers such as: Ammonium Nitrate (AN); Ammonium Perchlorate (AP); Potassium Nitrate (KN); and etc., have been proven beneficial to the more efficient combustion of fuels. A higher degree of fuel combustion will result in higher temperatures and therefore higher blast pressures. AP has long been the oxidizer of choice for solid rocket propulsion such as the Space Shuttle SRB and NASDA H-II Boosters. However, at the time that this project was initiated, AP availability had decreased (and cost had increased) due to an incident at one of only two major AP producers in the US. Also testing by other researchers revealed a correlation between AP content and Slow Cook-Off (SCO) test failure.
Criteria other than cost and availability which guided oxidizer selection were: high oxygen balance; non-hygroscopic character, and high-moderate density. Potassium Nitrate (KN), for example, met all of the screening criteria. At a crystal density of 2.1 g/cm3, and despite having 67% condensed products, KN reacts to form one half more mole of free O2 than AP as indicated by the following equations.
2KNO3 K2O + N2 + 2 1/2 O2 Equation 2.4-1
2NH4ClO4 N2 + 3H2O + 2HCl +2 O2 Equation 2.4-2
Lead Nitrate (PbN) and Barium Nitrate (BaN) also have high densities (4.53 g/cm3 and 3.24 g/cm3 respectively) are non hygroscopic, and react to form three moles and 2.5 moles of excess O2 respectively according to Equations 2.4-3 and 2.4-4.
Pb(NO3)2 2Pb + N2 + 3O2 Equation 2.4-3
Ba(NO3)2 BaO + N2 + 2 1/2 O2 Equation 2.4-4
KN and PbN were selected as the oxidizers of choice during this phase of research. KN was eventually chosen over PbN due to superior sensitivity and environmental characteristics.
Molecular Explosive Screening and Selection
To achieve performance goals, it was determined that at least some molecular explosives would be required. Molecular explosives are defined as explosives which have the fuel and oxidizer segments linked via chemical bond. Common examples of molecular explosives are TNT, RDX, and HMX. The chemical structure of these three molecular explosives is found in Annex-A. Molecular explosives are therefore unlike other types of explosives, such as emulsion explosives or composite explosives where, although the fuel and oxidizer are in relatively close proximity to each other, they are not linked via chemical bond. Molecular explosive type was screened for incorporation into the insensitive energetic design matrix as outlined in Figure 2.3-1.
As a result of this analysis, from theoretical insensitivity, performance, and cost standpoints, it was determined that the only well characterized, economically attractive, insensitive, high performance explosive which was readily available at the time was nitroguanidine (NQ). Other low sensitivity, high performance explosives existed, but their cost and availability were not comparable to NQ. Tri-Amino Tri-Nitro Benzene (TATB), for example, while highly insensitive has a cost of 7700/kg.
NQ Availability and Selection Rational
Four crystalline configurations of NQ were available at the time this research was being conducted from domestic and international producers. Figure 2.4-2 details the production differences of these four crystalline types. Low Bulk Density Nitroguanidine (LBDNQ), has a very high length to diameter (L/D) ratio. The typical diameter is approximately 5mm however, the length can exceed 100mm (Figure 2.4-3). The LBDNQ is very fibrous with a consistency of cotton and occasionally, the needles are hollow. The bulk density of the LBDNQ is 0.17g/cm3. The LBDNQ, although inexpensive, is processable only in small quantities (
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