Aging of Pentaerythritol Tetranitrate (PETN)
Pentaerythritol tetranitrate (PETN) is a relatively sensitive explosive used in many electroexplosive devices as well as in medicine. Of primary interest to LLNL is its use in items such as exploding bridgewire (EBW) detonators and exploding bridge foil initiators (EFI). In these devices the crystalline powder is pressed into a granular, low-density compact that can be initiated by an exploding wire or foil. The long-term stability of this pressed compact is of interest to weapon stockpile lifetime prediction studies. Key points about potential aging mechanisms can be summarized as follows: (1) There are a number of factors that can contribute to PETN instability. These include particle size, polymorphic phase transitions, crystal structure, impurities, moisture, occlusions, chemical incompatibility and biological (microorganism) action. of these factors the most important for long-term aging of high surface area powders used in detonators appears to be that of particle size growth. (2) There is a great deal of literature on the thermal degradation kinetics of PETN, unfortunately much of it with little bearing on ambient temperature aging during long-term storage. PETN is very stable with respect to thermal decomposition. Low-temperature thermal studies have not revealed evidence of chemical degradation products in archived PETN. Data extrapolated to 30 C predicts a half-life of 12 million years. (3) Moisture seems to lower the activation energy for and accelerate the decomposition of PETN. (4) External drivers affecting stability include temperature, moisture, radiation fields, and stress, while internal drivers include residual solvents, and impurities. Temperature affects kinetic processes of crystal growth such as adsorption, desorption, and diffusion rates of molecules on the surface of PETN crystals. A low-level radiation field may induce unexpected changes in the chemical makeup of PETN and its homologue impurities. Stress at high pressure points caused by crystalline surface irregularities may cause a decrease in surface area over time due to smoothing of contact points and sintering. PETN has been found to contain a number of impurities, including homologues, pentaerythryl ethers with a mixture of hydroxyl and nitrate functional groups, and organic salts, that can alter its physical and chemical properties. (5) The processes of sublimation/recrystallization and surface diffusion are important processes that can cause changes in PETN crystal morphology, particle size distribution and specific surface area. Sublimation feeds a process called Ostwald ripening, which involves the spontaneous growth of larger crystals from those of smaller size. This process proceeds with age regardless of storage conditions unless the crystals are treated to retard growth. The coarsening mechanisms include Ostwald ripening, 'sintering', and surface diffusion. The latter has not been experimentally observed, but a molecular modeling approach has attempted to investigate this phenomenon theoretically. (6) Threshold test fire, particle size distribution, specific surface area, thermal analysis, chemical analysis, powder compact specific surface area and density are approaches that have been used in the attempt to assess changes attributable to aging of PETN, with varying results. The biggest problem lies in the aging mechanism itself, which likely influences small changes in morphology, both internal and external, and particle size distribution changes on a scale too small to reliably monitor. Other aspects, such as subtle changes in density profiles within pressed powder compacts, may be beyond the capability of current technology to assess. Crystal growth significant enough to affect the PETN particle size distribution is expected to impact detonator performance. (7) Factors relevant to stabilizing particle size growth include crystal morphology, powder surface area and particle size distribution; crystal density and internal defects; and powder compact density and density gradients.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 966904
- Report Number(s):
- LLNL-TR-415057; TRN: US200923%%75
- Country of Publication:
- United States
- Language:
- English
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The Role of Pentaerythritol Tetranitrate (PETN) Aging in Determining Detonator Firing Characteristics
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ACTIVATION ENERGY
AGING
AMBIENT TEMPERATURE
CHEMICAL ANALYSIS
CHEMICAL PROPERTIES
CRYSTAL GROWTH
CRYSTAL STRUCTURE
DIFFUSION
DISTRIBUTION
EXPLODING WIRES
IMPURITIES
MOISTURE
MORPHOLOGY
PARTICLE SIZE
PETN
PYROLYSIS
SPECIFIC SURFACE AREA
SURFACE AREA
THERMAL ANALYSIS
THERMAL DEGRADATION