Article Summary: This article contains the heading for Chapter-2 (Plastic Bonded Insensitive Explosive Development) and Sections: 2.1 (Introduction); and 2.2 (Mechanistic Discussion) of the “Dissertation on the Study of Insensitive Highly Energetic Materials” as part of the Doctoral requirements for Theodore S. Sumrall at The University of Tokyo, March of 1998. Theodore S. Sumrall was awarded a Doctorate Degree from the Department of Chemical Systems Engineering in April of 1998 as a result of his research, development testing and dissertation presentation.
CHAPTER – 2 PLASTIC BONDED INSENSITIVE EXPLOSIVE DEVELOPMENT
As previously stated, prior researchers believed that the physical properties associated with explosives (such as TNT) help contribute to a poor response to impact stimulation. TNT is a very hard material with little strain capability. Stress values for TNT rarely exceed 700 kPa prior to breakage in a JANNAF dog-bone specimen. Plastic Explosives were developed by the British approximately 50 years ago . It was theorized that by blending sensitive molecular explosives (such as PETN or RDX) with a soft, hydrocarbon binder, the molecular explosives might become less susceptible to accidental initiation by what would be considered normal activities such as dropping the energetic material. Plastic Explosives, therefore, have physical properties on the opposite end of the spectrum relative to TNT. While TNT has no strain capability and good stress capability, Plastic Explosives have no stress capability and excellent elasticity (high strain) capability. These Plastic Explosives were very pliable and could be easily deformed by hand.
However, for explosives which required both stress and strain capability (such as shape charges for perforating oil well casing to start oil flow), it was postulated that manufacturing an explosive composition with physical properties similar to solid rocket propellant would allow the energetic material to absorb and evenly distribute shock input (due to impact) while maintaining sufficient physical properties for specific needed applications. These compositions were referred to as “Plastic Bonded Explosives (PBX) because the binder material typically underwent a catalyzed reaction which caused one end of the binder molecule (i.e. HTPB) to react and bond with an adjacent binder molecule. Unfortunately, while PBXs absorb shock at lower levels of shock input, at higher shock levels (such as the adjacent detonation of another energetic material as far as 15 meters distant) the binder becomes in-compressible and assumes properties similar once more to TNT and the advantages associated with a soft, pliable binder are minimized. Therefore, PBX-109 for example, fails the sympathetic detonation test. Additionally, since HTPB is actually a rocket fuel (and is therefore designed to burn) PBX-109 fails the Slow Cook-Off test. Typical PBX compositions also suffer from a number of other inadequacies. Due to the low density and inert nature of HTPB, an associated loss of explosive performance is observed. Also, due to the much higher viscosities associated with typical PBX compositions they cannot be manufactured in traditional TNT type equipment (Figure 2.1) and it becomes necessary for PBX compositions to be manufactured in expensive solid rocket propellant manufacturing equipment. Also, by virtue of the cure chemistry involved (to cross-link the HTPB polymer), a minimum of two days oven cure time is typically required. Significant additional expenditure in terms of processing and curing equipment cause many commercial explosive manufacturers to avoid manufacturing PBX compositions in spite of the improved safety associated with these compositions.
Figure 2.1 25kg Explosives Melt Kettle
The original aspects of this phase of research was as follows:
- This was the first PBX developed (to the researcher’s knowledge*) which incorporated both an oxygenated binder and a solid oxidizer. Theoretical calculations indicated that the replacement of non-oxygenated binder (HTPB based) with oxygenated binder (PPG and PEG based with TA or EOPO plasticizer) and replacement of nitramine with less sensitive oxygen containing salts (such as KN) would result in a decreased sensitivity without loss of performance (Objectives #1 and #2).
- This was the first PBX developed (to the researcher’s knowledge*) which was processable in steam kettles (<2kP viscosity (Objective #4)).
- This was the first PBX developed (to the researcher’s knowledge*) which was cured within 24 hours at moderate temperatures (Objective #5).
- The PBX compositions developed during this phase of research were among the lowest cost compositions available (to the researcher’s knowledge (Objective #10)).
* Based upon an extensive literature search.
2.2 Mechanistic Discussion
A number of PBX formulations have been developed in the US and abroad. The most widely produced General Purpose (GP) aluminized formulation is termed “PBX-109”. PBX-109 is a much improved insensitive energetic material relative to aluminized TNT based formulas. PBX-109 has fragment velocity and air blast characteristics comparable to the very high performance TNT based explosive (H-6), however, PBX-109 has much improved sensitivity characteristics relative to H-6. The formulational characteristics of PBX-109 and H-6 are detailed in Table 2.2.
Table 2.2 H-6 and PBX-109 Formulation Characteristics
|HTPB Based Binder||0||16|
PBX-109 passes both Fast Cook-Off (FCO) and Projectile Impact (PI) tests where H-6 fails both of these tests. It is theorized that because PBX-109 does not contain any oxygen generating inorganic salts (such as TNT or AP), PBX-109 results in a much milder and cooler burn. Thus when PBX-109 ignites, the energy is released over a much longer period of time. Also, due to the softer nature of the HTPB based binder, PBX-109 is able to absorb the shock associated with a projectile impact and allow this energy to dissipate over a larger area without detonating. Even if the PBX-109 ignites, again, due to the lack of oxygen containing inorganic salts, the burn is relatively mild . The FCO and PI tests are described in more detail in Chapter-4.
However, despite this reduction in sensitivity to both mechanical and thermal shock, PBX-109 is still quite sensitive (>200 Cards) and is subject to accidental detonation due to the high concentration of nitramine (RDX). In fact, PBX-109 is only slightly less sensitive than H-6 with regards to card gap testing at 3.7cm.
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 blast reaction. This approach, if successful, would yield a dual benefit. First, oxygen from an oxygenated binder could be utilized to burn the residual hydrocarbon (which normally does not react in HTPB based PBXs) and secondly, the more sensitive oxygen containing ingredients (such as RDX) could be reduced in content, hopefully, allowing the PBX to pass tests which PBX-109 failed, namely Slow Cook-Off (SCO), Fragment Impact (FI) and Sympathetic Detonation (SD).
Also, additional oxygen could be incorporated into a PBX composition by addition of nitrate or perchlorate salts such as potassium nitrate (KN) or ammonium perchlorate (AP). These salts are typically much less sensitive than nitramines such as RDX. Typically the more energetic AP will not detonate at particle diameters below 10m except at very large diameters. Therefore, the replacement of additional nitramine with oxygenated salts should still permit oxygen to become available for the detonation/combustion reaction while decreasing sensitivity to shock. Also, this would result in a composition cost decrease due to the fact that nitrate and perchlorate salts are less expensive than nitramines.
Also State of the Art (SOTA) PBX formulas were not steam kettle processable. A reformulation (with a solids redistribution) would permit a viscosity reduction and not only a raw material cost reduction but also a tremendous cost reduction in processing costs by virtue of permitting steam kettle processing. Also utilization of more reactive cure agents and catalysts would permit traditional cure times to be decreased 50%.
While the addition of nitrate or perchlorate salts would cause a composition to become more sensitive to an adverse slow cook-off type situation, other ingredients would be added to passivate this response. The ingredient of choice would be nitroguanidine (NQ) which has not only the benefit of severely depressing a burning type reaction but also is a very insensitive high explosive itself, but not sufficiently powerful by itself and also to difficult to initiate.
Figure 2.2 illustrates the mechanics and logic behind this approach.
With the addition of nitrate or perchlorate salts to PBX-109, the composition would now begin to become more characteristic of a solid rocket propellant, similar to that used for large boosters such as the propellant utilized in the Castor IV-A booster motors flown on the Delta-II rockets . Therefore, trade studies were conducted with the Castor IV-A propellant (TP-H8299) as a baseline since TP-H8299 was classified as a Class 1.3 material (i.e. will not detonate above 70 cards) while PBX-109 was a Class 1.1 material.
~ Theodore Sumrall