Effect of mechanical impact on PBX disks investigated by DMA

One safety aspect during use of polymer (plastic) bonded high explosives (PBX) is their resilience against mechanical impacts by set-back of the HE charge during firing. To investigate this feature PBX disks with 140 mm in diameter and with 14 to 18 mm thickness were manufactured, steel plate covered and, held in a fixture, exposed to hemispherical steel projectile impacts with 90 mm projectile diameter. The frame composition of the PBX is HTPB (hydroxyl-terminated polybutadiene) based binder cured with IPDI (isophorone diisocyanate), HMX, plasticizer DOA (dioctyl adipate) and a phenolic antioxidant. The projectile impact was performed with such a velocity that the steel plate was not perforated but destruction effects on the PBX disks were created. After loading the PBX disk was sectioned into DMA sample stripes with different distances to the impact center (DTIC). The DMA (dynamic mechanical analysis) measurements were performed in torsion and the rectangular samples were about 10 mm wide and 18 to 35 mm long. The temperature dependences of the shear moduli G’, G’’ and the loss factors were determined with a stepwise increase by 1°C from -120°C to +90°C. The nine sinusoidal deformation frequencies were in the range of 0.1 Hz to 30 Hz and applied on each temperature step after an equilibration period. Found was a shift of the glass-to-rubber-transition (GRT) temperature to lower temperatures with decreasing DTIC. With each measurement frequency, a dependence of the GRT temperature Tg with DTIC was found. The dependencies of Tg on deformation frequencies were evaluated with two equations (i) standard Arrhenius (providing f0, Eaf) and (ii) modified Arrhenius (providing f0M, Ea0M and T0M), the last revealed a very clear dependence on DTIC. The changes of the parameters Eaf and Ea0M, T0M are discussed in terms of DTIC. Finally, the ignition scenario is discussed with master curves for G’, G’’ and loss factor.