Dynamic mechanical signatures of a polyester‐urethane and plastic‐bonded explosives based on this polymer

Abstract

ABSTRACT:The complex shear moduli of the segmented polyurethane Estane 5703p, Livermore explosive (LX)-14, and plastic bonded explosive (PBX)-9501, which use this polymer as a binder, have been investigated. Segmented polyurethanes, such as Estane 5703, contain microphase-separated hard segments in a rubbery matrix of soft segments. LX-14 is composed of 95.5% 1,3,5,7-tetranitroazacyclooctane (HMX) explosive with 4.5% Estane 5703 binder. PBX-9501 is composed of 94.9% HMX, 2.5% Estane 5703p binder, 2.5% nitroplasticizer (NP), and about 0.1% antioxidant Irganox 1010. In the temperature range from Ϫ150 to 120°C, two relaxations were observed as peaks in the loss modulus and tangent delta in Estane 5703p and LX-14. A third relaxation was found in PBX-9501. The low temperature relaxation associated with vitrification of the poly(ester urethane) soft segment occurred in the shear loss modulus (GЉ) at Ϫ29 and Ϫ26°C in Estane and LX-14, respectively, at 1 Hz. In PBX-9501 the Estane soft segment glass transition peak, T g (SS), in the loss modulus occurred at Ϫ40 Ϯ 3°C at 1 Hz. The reduction in soft segment glass transition in PBX-9501 is clear evidence of plasticization of the soft segment by NP. The apparent activation energy of the maximum in the loss modulus for LX-14 and PBX-9501 over the frequency range from 0.1 to 10 Hz was 230 kJ/mole (55 kcal/mole). The hard segment glass transition, T g (HS), was observed as a peak in the loss modulus at about 70°C. In LX-14 the transition was observed at lower temperatures (56 -58°C at 1 Hz) depending on thermal history. There was a low temperature shoulder on the T g (HS) of Estane 5703 associated with soft segment crystallinity. Modulated differential scanning calorimetry (MDSC) was used to verify the T g (HS) in Estane and 50/50 mixtures of Estane with NP. In PBX-9501 the hard segment glass transition occurred between 65 and 72°C. The presence of NP in PBX-9501 gave rise to a new transition, T eu (NP), between 8 and 15°C. This peak is believed to be associated with the eutectic melting of the plasticizer. Returns of fielded PBX-9501 that were 6 and 11 years old were also measured. Small variations in T g (SS) and the rubber plateau modulus were observed in these aged samples, consistent with migration of plasticizer and/or very low levels of chain scission.

Characterization of the viscoelastic behaviour of a plastic-bonded explosive

“…The loss angle tangent at 20 • C did not change during the test. An identical phenomenon has also been observed with PBX 9501 [1]. We therefore propose a number of additional tests ( Table 1, cond.…”

“…for reduced time equal to −8). We have no explanation for this new temperature effect on the behaviour (seen also in PBX 9501 [1]). …”

“…This increase is attributed to several mechanisms as (i) inertial effects during compression and (ii) for brittle-like material as glass and ceramics, a change in the failure mechanism [12] (a macrocrack develops at lower strain rate, a multi fragmentation mechanism could be assumed for higher rate levels). Williamson [1,2] has shown that such increase could also be explained using quasi-static measurements and a time-temperature equivalence for the explosive composition PBX 9501.…”

“…5). The mechanical response is close to the behaviour exhibited by PBX 9501 [1][2][3][4]. The demands of pyrotechnic design and the investigation of aging behaviour both require an in-depth knowledge and modelling of the behaviour of energetic materials.…”

On the thermal expansion hysteresis of a UK PBX

“…Often thermal or dynamic mechanical analysers are used for PBX CTE measurements [2,3]. The samples used for this investigation were originally manufactured as 3-point bend specimens.…”