| Summary: | Propellants based on cross-linked Hydroxy Terminated PolyEther (HTPE) binders are
being used as alternatives to Hydroxy Terminated PolyButadiene (HTPB) compositions.
HTPE propellants have similar mechanical properties to HTPB propellants but they give
a less severe response in ‘slow cook-off’ tests for IM compliance. A literature review is
presented on the development and properties of HTPE propellants in an attempt to place
them in relation to recent trends in Insensitive Munitions. To gain a better understanding
of the behaviour of HTPE propellants an HTPE pre-polymer and a range of binder
network samples with different NCO/OH equivalence ratios, with and without
plasticizer, have been synthesised and characterised by a range of techniques. The
thermal decomposition of the HTPE binder network and propellant samples were also
studied. Desmodur N-3200 was used as a curing agent and n-BuNENA as an energetic
plasticizer. Similar analyses were performed on analogous HTPB pre-polymer and
binder network samples and the results were compared with those obtained for the
corresponding HTPE samples. Two kinds of HTPE propellant were manufactured
containing HTPE pre-polymer, n-BuNENA, 2NDPA and either AP or AP+PSAN as
oxidiser. Also HTPB propellant was prepared. Small cook-off test vehicles (SCTV)
were filled with HTPE and HTPB propellants and slow cook-off tests were performed.
In contrast to HTPB binders, which become harder during slow heating, it was found
that the HTPE binders soften under the same conditions. This behaviour is possibly due
to chain scission of the soft and hard segments of the HTPE polymer matrix. Thermooxidative
processes and reactions of the energetic plasticizer decomposition products
are believed to be the responsible for the scission of the polymeric matrix. From the
binder characterisation and slow cook-off results it is concluded that there is a relation
between the degree of polymeric matrix scission during slow heating and the violence
of the response at the point of self ignition. This underlies the main difference between
HTPB and HTPE propellants in slow cook-off. While HTPB compositions become
harder and more brittle, HTPE propellants become softer and have a lower surface area
at the self ignition point.
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