The molecular weight distribution and dynamic mechanical properties of an experimental polymer-bonded explosive, X-0282, maintained at 23, 60, and 74/sup 0/C for 3.75 y were examined, X-0282 is 95.5% 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclo-octane explosive and 4.5% Estane 5703, a segmented poly(ester-urethane). Two mechanical relaxations at about -24 and 42/sup 0/C were found in the X-0282 aged at room temperature for 3.75 years. A third relaxation at about 85/sup 0/C was found in X-0282 aged at 60 and 74/sup 0/C. The relaxation at -24/sup 0/C is associated with the soft segment glass transition of the binder. The relaxation at 42/sup 0/C is associated with the soft segment melting and may also contain a component due to the hard segment glass transition. The relaxation at 85/sup 0/C is probably associated with improved soft segment crystallite perfection. The molecular weight of the poly(ester-urethane) binder decreased significantly with increasing accelerated aging temperature. A simple random chain scission model of the urethane degradation kinetics in the presence of explosive yields an activation energy of 11.6 kcal/mole. This model predicts a use life of about 17.5 years under the worst military operating conditions (continuous operation at 74/sup 0/C)

Caley, L.E.

Hoffman, D.M.

UNT Digital Library

f WASTER rcRi.. a^i. •) vol. n PREPRINT / :crt-S'o3'r Dyii'L'ni.1 Merh'inii"il aru Ki'Le.'ul.'ii' '.vei^i'.i Kfi.'.i.i-enipnt.i on Polyinei ii^rute.i ivq-.!u:;iv<-IV.mi Tuftnv.i.Ly Ac.:ei(i.--ii.-' A b l n 0 ii-."..: J I , 11 h !yji-i ' >^:-in'ft'.i.unc) Hinn-i' L. M'irV. .iol'fman Ixuirmi :.. C'lley 'i'hi;; j-iper w.'in prepare'! I'u;1 prei'enVit;. n the laisL Itaticivil Meetiiii, 11' \.h<z /vmei'i.-.-in Ohemi •••! J .-ic-ty Division ol' Applied Pulyner Jc ie ive: Coating:;, Plas t ies and Adhesive' A t l a n t a , Georgia March >> - April -,, l.wU T V ik a prspniK of v piper iixendsd for publKgliun IP M ifti/niiil ir prirnrdings S n u ihangts ma« be made brfnri puhliutiun ihis preprint is in̂ uY i»ailjl>le *nh ihc un dcrsundm? thai it will nnf hi utrd nr rrnmduccd "ithoul 'hi [hrp'is'nn if the Rulhnr 1OTR; : * •"-Dynamic Mechanical and Molecular Weight Measurements on Polymer Bonded Explosives from Thermally Accelerated Aging Tests II. A Poly(ester-urethane) Binder.* by D. Mark Hoffman and Leonard E. Caley Lawrence Livermore National Laboratory P.O. Box 308 Livermore, CA 91550 ABSTRACT The molecular weight distribution and dynamic mechanical properties ot an experimental polymer bonded explosive, X-0282, maintained at 23, f>0, and 74°C for 3.75 years were examined, X-0282 is 95.5% 1,3,5,7-irtranitro-1,3,5,7-tetraazacyclo-octane explosive and 4.5% Lstane 5703, a segmented poly(ester-urethare). Two mechanical relaxations at about -24 and 42°C were found in the X-0282 aged at room temperature for 3,75 years. A third relaxation at about 85°C was found in X-0282 aged at 60 and 74°C. The relaxation at -24°C is associated with the soft segment glass transition of the binder. The relaxation at 42°C is associated with the soft segment melting and nay also contain a component due to the hard segment glass transition. The relaxation at B5°C is probably associated with improved soft segment crystallite perfection. The molecular weight of the poly(ester-urethane) binder decreased significantly with increasing accelerated aging temperature. A simple random chain scission model of the urethane degradation kinetics in the presence of explosive yields an activation energy of 11.6 kcal/mole. This model predicts a use life of about 17.5 years under the worst military operating conditions (continuous operation at 74°C). *work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract Number W-7405-ENG-48. . , . DISCLAIMER INTRODUCTION Although fluoropolymers have exceptional thermal stability as binders for explosives, they do not desensitize HNX explosives. Humphrey^ found that Estane 5702-F1, a B. F. Goodrich poly(ester-urethane) had improved insensitivity over fluoroelastomers and plasticized nitrocellulose binders. The question we attempt to answer is whether or not this reduced sensitivity PBX can withstand the 20 year stockpile life required for current military explosives. The dynamic mechanical properties and molecular weight distribution of X-0282, an experimental polymer bonded explosive (PBX), maintained at 23, 60, and 74°l for 3.75 years were examined. Because of a relaxation in the Estane 5703, a poly(ester-urethane) binder similar to Estane 5702-F1, at 42°C associated with soft segment melting and/or the hard segment glass transition, mechanism changes above and below this transition cannot be ignored. Surprisingly, the decrease in molecular weight with accelerating aging temperature obeys the random chain scission model reasonably well yielding an activation energy of 11.6 kcal/mole. The model predicts a use-life of about 17,5 years unuer the worst possible military conditions (?4°C), A considerably longer use-life is predicted for less stringent conditions. EXPERIMENTAL The molecular weight distribution and dynamic mechanical properties of X-0282 maintained at 23, 60, W t for 3.75 years were stuoied using a hater model 100 GPC and a Rheometrics mechanical spectrometer, X-0282 is composed of 95.b weight percent l,3,5,7-tetranitro-l,3,5,7-tetraaz6cyclooctane (HMX) and 4.5 weight percent Estane 5703, a segmented poly(estfcr-urethane) n.anu-factured by B. F. Goodrich Co. Estane 5703 is a segmented polyurethane' of low molecular weight poly(butylene adipate) soft segments and 4,4'diphen-ylmethanediisocyanate-1,4-butanediol hard segments. The resulting polyurethane repeat unit is approximately 0 0 -tiO^CH^OCNH-^CHp^NHCO^CH^OHCh^CC^-}-0 0 -2-where the number of butylene adipate repeat units (n) in the soft segment is 4-6 and the average number of urethane repeat units (m) in the hard segment is 1-3. In the DSC trace in Fig. 1, the soft segment T occurs at -28°C and a broad melting endotherm associated with soft segment melting is observed between 10 and 80°C. From our previous work, the hard segment glass transition should fall in this region, but is obscured by the soft segment melting endotherm. The hard segment melting occurs at about 1 3 0 ° ^ which is outside the military range. RESULTS Dynamic Mechanical Properties of X-0282. The complex molecular architecture and low concentration of the poly(ester-urethane) binder in the PBX makes interpretation of the dynamic mechanical properties of the aged PBXs extremely complex. The shear storage modulus (G 1) and tangent delta are plotted as functions of temperature in Fig. 2 for X-0282 aged at 23, 60 and 74°C for 3.75 years. The room temperature aged PBX has only two relaxa­tions at -23 and 41°C for the 1 Hz, measurement. These are associated with the soft segment T and melting found in the DSC trace in Fig. 1. The PBXs aged at 60 and 74°C exhibit a third relaxation at about 70°C which is most intense in the sample aged at 60°C. Since the soft segments melt between 40 and 80°C, isothermal eging in this range would be equivalent to annealing. Annealing improves the perfection of the higher molecular weight soft segment chains, but some fraction of the lower molecular weight chains will remain molten, The amount of molten soft segments will increase at higher annealing tempera­tures. As a result, aging at supercoolings of 15-25°C below the melting point of the most perfect soft segment crystals will produce many crystallites of moderate perfection. The 60°C aged X-0282 has a large concentration of cyrstallites of moderate perfection which are responsible for the strong relaxation at 65°C and some low molecular weight soft segments which crystallize on cooling to produce the soft, segment relaxation at 42 DC. Aging or annealing X-0282 at 74°C would produce fewer crystallites of higher perfection than at 60°C and a higher concentration of molten soft segment -3-during the accelerated aging test. Thus the 74°C aged X-028? shows a weaker relaxation due to the lower concentration of soft segment crystallites at a higher temperature (80°C) because of their perfection. The higher concentra­tion of molten soft segments succeptable to the ester interchange degradation reaction in the 74°C jged X-0282 may explain the lower melting point of the low molecular weight soft segment component (30°C) found in this sample. In the 23°C accelerated aging test the crystallinity of the soft seg­ments does not improve, which explains the absence of the third relaxation. The relaxations are listed as T (SS) for the glass transition tpmpciature of the soft segments, T^ss) for the low molecular weight soft se'in.ent component melting transition, and 1 f ilSS) for t*n melting trans H u m of t'n-annealed soft segments in Table 1. The shear si.orage modulus at -W, 23, and 74°C is also given in the table. Table 1. Transitions and moduli of thermally arcpln .<teci aging test samples of X-0282. Transition 23°C/3.75 yrs 60°C/3,75 yrs 'i^WJS )i yssrc -23 -21 _:-i EArj(SS) kcal/mole 84 79 yssrc 41 42 TA(SS)°C - 6b Mi Modulus G'(-54cC)HCr9Pa 3.28 3.27 G'(e!30C)xlO~9Pa 1.30 1.70 G'(74°C)xlO~9Pa 0.31 O.bO i._ •1 In previous work MDl-butanediol polyurethane hard sequent;, n- .M.I between F'j and 90 JC depending on hard segment concentrator, soluhi 1 ity m the soft segment, and thermal history, At higher hard segment runci ntratinns hard segment melting occurs between 110 and 17J>°C. Therefore relaxation in X-0282 at the higher accelerated aging temperature1, related to hard segment melting. ! lie tit 11 d rr.t -4-Molecular Weight and Binder Degradation in X-0282. Because of the poly(ester-urethane) ester linkages in both hard and soft segments in this PBX binder, ester interchange could occur in both segments. However, the rate of interchange should be most rapid in the soft segments" 1' 6 because of their lower bond energy ana their being above T (SS). In the presence of the HMX explosive, the molecular weight of the segmented pely(ester-urethane) decreased with increasing accelerated aging temperature as shown in Table II. Since there are multiple segments per chain and the polydis-persity remains constant at about 2.1, deterioration of the d functional ester groups in the soft segments along the polymer backbone is assumed to be random enough for the random chain scission model to apply. Using [q. 2 froir the previous paper to obtain the pseudo-first order rate constant:- k, then plotting In k versus inverse accelerate aging temperature yields M g . 3. According to tq. 3 from the preceding paper, the activation energy of the overall degradation process is II.6 kcal/mule and the frequency factor is ?.l i i(r g/mole-yr. Ihis result is lower than that founo for similar poly(ester-urethanes) in high tumidity studies,'1 but not :nconsislant with activation energies of other uncataly?ed linear aliphatic ester interchange reactions. Since the temperature excursion seen in military operations is I rem - M to 74°C, the use-life of the poly(estcr-urethane) under the v. or si military conditions woulu be only "7.5 years assuming a critical M.iecdlar weight of 5000. Obviously, in no military application will the Pbx see rt,i,tinuous usc­at 74°C for 2C years, and therefore depending on the actual use tetiperalure cycle increased use-life may be expected, furthermore, safety consideration, may preempt use-life requirements, and the insensitivity of poly(e'ter-urelfane) binders for HMX is exceptional. -5-Table 11. Effect of thermally accelerated aqinq on the molecular weight of Estane-5703 in X-0282. " at a Aging Conditions M n % 10" 4 M„ x lCr 6 M w/M n 23°C/3.76 jrs /.16 + 0.05 15.5 +_0.1 2,2 6CL/3.75 yrs 3.64 + O.U-3 8.3 + 0.1 2.3 HXIZ.lb vrs 1.90 + 0.02 ?.P7 + 0.06 ?.0 unaged fc.4 lUKLUSIONS The dynamic mechanical property change which occurred in thermally eel era tea ying experiments with X-Q21U: were caused by annealing phenomena in the poly(titer-urethane) soft segments in conjunction with chain suasion t elevated temperatures. The strength and temperature of 1 he annealing relaxation was dependent en the concentration and perfection c. annealed soft segment crystals. Hit decrease in molecular weight with increasing aUeltrilwi aging temperature followed simple pseudo-first order random chain scission kineti:s. The rate constant for the overall reaction h.vi ar ;;rrhcnuis temperature dependence with an activation energy of 1Kb ncal/ncle. ;. i' result is consistent with degradation ol the polyester soft sr-qmerus. Apparently, the distribution of soft segments in the polymer rackhonr is sufficiently ranacir In Allow the application or the randon •jUSMfjh î oflti. HIM 1 \ | \ | IR I hr. duiunu'nl «J> pri'inHrr" u. an ;u'kiiunl il l *M>ik \pniiMiiril h\ ,111 . imi i i . nl <tti' I w i ld su(v\ *.«M-rnntfni. Vi l toir (hi-1 nitrd Vali- i (.lui'inimnl mil llii I n i m . i h til ( ilifurnia nm urn "1 Ihi'ir t'nlplmi'i's. maki's ;tn\ Hiiriilnlv r\. (lh".\ in implied, (ir a^umt> itni lr|;ul liabilih in tl"ipim*iliilih fin llii" .!!• m i n i , mnipl i lnuvi . or usiTulnov nf ati> iiifurmafiiin. apparatus, pniduii. «i pimcv. din-lirvil. t.t ri-pri'srtws llwl ils tiw would mil inliinui' ptnalrU n*iipd rilihls. Ki-fiTi-ni'v hcriiti (ti aft; spain*' ctiminik'ijl priidm'l.. prmi-svin M'Pin ' M (Mile rtanw. trademark, manufmliirvc, (if tilrii'ivmi1, dii i i iml aiM-*vari|i iiinsltlyli' ur imph lis vndinviiii 'iil. ri'i'iiniliu'llrialiuli. nr laiitnni: h\ iru I mud States (.tiH-rnnwul tit il,r 1 lliH'l'.ili ()l t alifurnta. I hi' *it'»s mill iipiitiiin* nl authors v\pn"-M'il hiTi'i'i du nut n.-i'es\nrilv sink' nr ri'lU'tl ihim' n l ihv I nitid vat*-* l-mrninirni Ihmi-C and shall mil hi- used tin ad.onsinu m prtiduil rn-diirsimi'm purpitsi-s. -6-REFERENCES 1. J. R. Humphrey, LX-14 a New High-Energy Plastic-Bonded Explosive, Lawrence Livermore National Laboratory, Livermore, CA, UCRL-523T0 (1977). 2. G. k. Griffith, private communication, 3. U. M. Hoffman, Org. Coat. Plast. Prepr,, 4|, 894 (1980). 4. C. b. Schollenberger, Adv. Chem. Ser., V/6, 83 (1979). b. 1). w. Brown, R, E. Cowry, and L. E, Smith, Wacromol,, 13, ?4b (1980). 6. C. S. Schollenberger and P. D. Stewart, J. [lastoplast., 3, ?b ( W l ) . 1. J. H. Saunders ana F. Dobinson, Comprehensive Chemical Kinetics, lb, SU7 (1976). ~ o X Id O Q Z liJ 0 20 TEMPERATURE CO Figure 1. A DSC trace of Estane 5102-71, a poly(t£tur-uretht:v-) sinsile.r to Estone 5703j shows the soft segment glass transition, T(SS), at -30°C tnd a wea>., bread Belting endotherm from k^ to 8o°C. -7--100 -60 -20 20 Temperature, °C Fi^L-e S, Shear storage modules (C) and iz.n delta <•! A• C;<H? IBXE fcged 3-75 years at ?3, 60, and 7i*°C as a function of trsij-vr.-iLurf at 1 Hz. show the effects of accelerated aging- Jhe rc.l.ixction U'hnvior caused by accelerated aging is discussed in the text. -12 c -13 Figure 3. An Arrheniu:'. relationship wisa found between the random scission rate constant for degradation of Estane 5701 binder and the accelerated iifling temperature. -14 2.9 3.1 3.3 1/T X 103, °K -9-

Dynamic mechanical and molecular weight measurements on polymer bonded explosives from thermally accelerated aging tests. II. A poly(ester-urethane) binder

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Dynamic mechanical and molecular weight measurements on polymer bonded explosives from thermally accelerated aging tests. II. A poly(ester-urethane) binder

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