NC Ageing; Effects on a High Explosive Formulation Paul Deacon [email protected] Ref.: AWE/CRD11/PD12/0114 UNCLASSIFIED Introduction ‘PBX(A)’ HMX explosive filler Gelatinous binder containing NC Requirement for through life mechanical integrity Molecular mass of NC reduces with age Mechanical strength of a consolidated, high density pressing also reduces with age Molecular mass of NC is quick and relatively easy to measure Mechanical strength can be predicted Ref.: AWE/CRD11/PD12/0114 UNCLASSIFIED 2 UNCLASSIFIED Sources of Evidence Pristine PBX(A) Multiple sources of NC Different molecular mass Cotton linter & wood pulp cellulose Aged PBX(A) Real-time aged at T < 40°C Accelerated aged at 40°C < T < 70°C Aged gelatinous binder Sealed glass vessels Atmospheric control Study chemistry of binder ageing Ref.: AWE/CRD11/PD12/0114 UNCLASSIFIED 3 UNCLASSIFIED NC Ageing Model Arrhenius plot verifies that real time ageing is consistent with accelerated ageing Consistent with single mechanism and a dominant thermolytic process 1000/T (K) 2.85 Nitrocellulose Molecular Mass, Mn ln(k) -13 -15 2.90 2.95 3.00 3.05 3.10 y = -17.226x + 34.038 Ea = 143kJmol-1 2 R = 0.8409 -17 3.15 3.20 3.25 3.30 M n (t ) = 1 1 + kt M n ( 0) -19 -21 Accelerated Aged at 40 < T < 70°C -23 Real Time aged at T < 40°C k = Ae − Ea RT -25 Ref.: AWE/CRD11/PD12/0114 UNCLASSIFIED 4 UNCLASSIFIED Empirical Correlation of Properties NC molecular mass empirically correlated with PBX(A) flexure failure stress Pristine virgin, real-time aged and accelerated aged PBX Minor component of the composition affecting bulk properties PBX(A) Flexure Failure Stress (MPa) 4.0 3.5 3.0 2.5 2.0 1.5 1.0 2 y = -7.13E-11x + 2.94E-05x + 3.98E-02 2 R = 7.72E-01 0.5 0.0 0 50000 100000 150000 200000 250000 Nitrocellulose Molecular Mass, Mn(t) (dalton) Ref.: AWE/CRD11/PD12/0114 UNCLASSIFIED 5 UNCLASSIFIED Testing the Correlation Four PBX(A) variants manufactured to test the empirical correlation Virgin material Control of: 4 Ingredients Manufacturing process Composition Density Testing methods Flexure Failure Stress (MPa) 3.5 Variant 4 Highest NC Molecular Mass Variant 3 3 2.5 Variant 2 2 1.5 Variant 1 Lowest NC Molecular Mass 1 2 y = -7.13E-11x + 2.95E-05x + 3.16E-01 R2 = 9.99E-01 0.5 0 0 50000 100000 150000 200000 250000 Nitrocellulose Molecular Mass (g mol-1) Confirmed NC molecular mass affects PBX mechanical strength Ref.: AWE/CRD11/PD12/0114 UNCLASSIFIED 6 UNCLASSIFIED Predicted Mechanical Properties NC ageing model used to predict reduction in number average molecular mass, Mn Correlated properties used to derive reduction in PBX(A) mechanical strength Trials verification 4.0 Equivalent ages − F= Ae − Ae Ea RT2 Ea RT1 PBX(A) Flexure Failure Stress (MPa) 3.5 3.0 2.5 2.0 1.5 1.0 Derived Flexure Stress Real Time Data 95% Limit 99.9% Limit 0.5 0.0 0 10 20 30 40 50 60 70 80 Equivalent Real Time Age (Years) Ref.: AWE/CRD11/PD12/0114 UNCLASSIFIED 7 UNCLASSIFIED Additional Evidence Non-isothermal ‘thermal ramp’ ageing experiments performed to further verify Arrhenius parameters 30 – 105°C at linear ramp rate (α) 0.5°C day-1 Data Periodic withdrawals Model Fit NC analysed 200000 Mn Ea ~ 148kJmol-1 Ea 1 1 T − = + Ae RT M n (t ) M n (0) α 150000 100000 Equation 50000 0 y = 1/((1/m0)+(x/alpha)*A*f*exp(-(1000* Ea)/(R*x))) Adj. R-Squar 0.98036 Value D m0 D alpha D A D f D Ea D R 300 310 Standard Erro 184000 0 0.5 0 4.72543 13.5688 1E13 0 148.5329 8.46009 8.314 0 320 330 340 350 360 370 380 Temperature (K) Ref.: AWE/CRD11/PD12/0114 UNCLASSIFIED 8 UNCLASSIFIED Conclusions NC is a minor component of HMX-filled PBX(A) NC molecular mass has been shown to correlate with mechanical strength of the pressed PBX(A) Virgin materials using different NCs verifies correlation Arrhenius ageing model predicts molecular mass which allows mechanical strength to be derived Predicted properties are verified with long term storage Trials The NC ageing model has been further verified with a ‘thermal ramp’ ageing experiment Ref.: AWE/CRD11/PD12/0114 UNCLASSIFIED 9
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