Hydroxy-terminated polyether binders for composite rocket propellants

摘要

Propellants based on cross-linked Hydroxy Terminated PolyEther (HTPE) binders arebeing used as alternatives to Hydroxy Terminated PolyButadiene (HTPB) compositions.HTPE propellants have similar mechanical properties to HTPB propellants but they givea less severe response in ‘slow cook-off’ tests for IM compliance. A literature review ispresented on the development and properties of HTPE propellants in an attempt to placethem in relation to recent trends in Insensitive Munitions. To gain a better understandingof the behaviour of HTPE propellants an HTPE pre-polymer and a range of bindernetwork samples with different NCO/OH equivalence ratios, with and withoutplasticizer, have been synthesised and characterised by a range of techniques. Thethermal decomposition of the HTPE binder network and propellant samples were alsostudied. Desmodur N-3200 was used as a curing agent and n-BuNENA as an energeticplasticizer. Similar analyses were performed on analogous HTPB pre-polymer andbinder network samples and the results were compared with those obtained for thecorresponding HTPE samples. Two kinds of HTPE propellant were manufacturedcontaining HTPE pre-polymer, n-BuNENA, 2NDPA and either AP or AP+PSAN asoxidiser. 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 foundthat the HTPE binders soften under the same conditions. This behaviour is possibly dueto chain scission of the soft and hard segments of the HTPE polymer matrix. Thermooxidativeprocesses and reactions of the energetic plasticizer decomposition productsare believed to be the responsible for the scission of the polymeric matrix. From thebinder characterisation and slow cook-off results it is concluded that there is a relationbetween the degree of polymeric matrix scission during slow heating and the violenceof the response at the point of self ignition. This underlies the main difference betweenHTPB and HTPE propellants in slow cook-off. While HTPB compositions becomeharder and more brittle, HTPE propellants become softer and have a lower surface areaat the self ignition point.