Thermal and Mechanical Hazards of Nitrocellulose and its Mixture with Nitroglycerin

S. Singh1, Q. Kwok1, R. Turcotte1 and M. Paquet2

1Canadian Explosives Research Laboratory, Natural Resources Canada 2General Dynamics Ordnance and Tactical Systems - Canada Valleyfield

7th International Nitrocellulose Symposium, Montréal, Canada, May 31 – June 1, 2016

Outline Introduction

Thermal Study • Accelerating Rate Calorimetry (ARC)

Quantification of Mechanical Hazards • Friction • Impact

Summary

Acknowledgement: Special thanks to Mr. Ian Levac from GD-OTS Canada Valleyfield for the preparation and the shipping of the samples used in this work

Introduction All modes of ignition of energetic materials are essentially thermal in nature:

Fast (millisecond) ignitions are intuitively familiar:

• Striking a match • Lighting a gas BBQ • Using paper (kindling) to start a fire

Slow ignition events (days, years) are less intuitive:

• Most energetic materials are self-heating materials and therefore can be made to spontaneously ignite

Mechanical ignition hazards

• instantaneous friction and/or impact stimuli invoking an unintended reaction (evolution of gas, sound, light…)

Introduction Literature isothermal studies attempt prediction of safe storage periods for NC and NC/NG systems

• Slow ignition (T. Kotoyori 2005):

Autocatalytic induction times from 1 to 10 days (Tiso : 73 to 91°C) • Fast ignition (A.S. Shteinberg 2006): Ignition times from 1 to 15 seconds (Tiso : 212 to 242°C)

Mechanical sensitivity testing addresses probability of initiation (Ip)

• Ip = probability of local ignition • Proper test to simulate process events (hazards) • Unbiased scheme to assign “yes/no” reaction to test • Appropriate statistical analysis method • Is there an acceptable level of risk?

NC Grade N Content (mass %)

C; Type I 13.15 ± 0.05 %

“as rec’d” 25% water-wet Dried in desiccator to constant

mass. Stored and used from desiccator for duration of study

NC/NG (70/30) Prep: GD–OTS Canada Valleyfield NC wetted with alcohol/acetone Acetone desensitized NG added Mixed to a granular paste with suitable viscosity Transferred onto stainless steel plate and left

>48 h to evaporate at room temperature to a given residual solvent Material peeled off and broken into small pieces

“as rec’d” dried

under vacuum Stored in sealed

charge dissipative vials

Samples

Thermal Study - ARC

10 mL spherical Ti vessel

1 mL tube Ti vessel

ARC (Isothermal)

Induction time ()/h0 20 40 60 80 100 120 140 160

T iso/°

C

60

70

80

90

100

110

120

130

= 19.8 h

= 9.5 h

= 4.8 h

= 2.4 h

= 1.9 h

= 0.5 h

= 152.2 h

= 36.4 h

Tiso/°C

= 29.3 h

65

75808595105110115

70

Induction time ()/h0 20 40 60 80 100 120 140 160

T iso/°

C

60

70

80

90

100

110

120

130

= 37.2 h

= 21.4 h

= 12.7 h

= 10.0 h

= 7.4 h = 4.6 h

= 2.3 h = 1.5 h = 0.9 h

Tiso/°C

35 45 55 6579

80

84

86

= 53.5 h

80859095100105110115120125

NC NC/NG (70/30)

103 K/T2.5 2.6 2.7 2.8 2.9 3.0

ln (

/min

)

2

3

4

5

6

7

8

9

NC/NG 70/30 NC alone (grade C)

Ea = 105 ± 5 kJ mol-1

ln(A/min-1) = 27.4 ± 1.5r2 = 0.976

Ea = 95 ± 11 kJ mol-1

ln(A/min-1) = 25 ± 4r2 = 0.898

ARC (Isothermal)

Literature + Isothermal ARC Data

Induction/ignition time/min10-2 10-1 100 101 102 103 104 105

Isot

herm

al T

empe

ratu

re/°C

50

100

150

200

250

Thermal Study - Conclusions Empirical fit can be used as a basis to derive NC shelf-

lives from iterative calculations • More data would be required at lower temperatures

(micro-calorimetry) to improve shelf-life predictions • More data would be required to link the fast ignition data

to the slow decomposition regime (10 s to 30 min) • Therefore in the present work, attempts will be made to

develop a pyrolysis technique to obtain such data

The slow decomposition behaviour of the NC/NG mixture and pure NC appears very similar

The NC/NG mixture shows similar induction at temperatures ≈10°C lower than pure NC

v

W Porcelain peg

Sample Porcelain plate

BAM Friction Test

Mechanical Hazard: Friction ABL/ICI

Friction Test

Stainless Steel Plate Sample Deposit

Fixed Stainless Steel Disc

W

v

Pendulum Arm

Heavy Weight

ABL/ICI Hazards quantification via statistical data treatment of measured parameters such as contact geometry, applied pressure, plate velocity, detection of induced reactions (i.e., monitor emissions)… Correlation study on many parameters has shown that D = Pa l <v>2 gives the best overall correlation for a wide range of EMs [Pa = Apparent Pressure, l = Contact Length (in the direction of motion), <v> = Average Relative Velocity]

Main power switch

Photodiode amplifier

Pendulum

CO analyzer

Apparatus Start/Stop

NOx analyzer

PS2 Sliding platform

Friction (ABL/ICI)

Load Pressure monitor

<R>

CO

/ n

L m

m-1

0

500

1000

1500

2000

CO Threshold274 nL mm-1

NOx Threshold1.39 nL mm-1

<D = P l v2> W s-1

106 107

<R>

NO

x /

nL m

m-1

0

2

4

6

8

Dose Threshold4.17 x 106 W s-1

Dose Threshold3.85 x 106 W s-1

Friction: Gas evolution

<R>

CO

/ n

L m

m-1

0

500

1000

1500

2000

CO Threshold459 nL mm-1

NOx Threshold3.44 nL mm-1

<D = P l v2> W s-1106 107

<R>

NO

x /

nL m

m-1

0

2

4

6

8

10

Dose Threshold4.55 x 106 W s-1

Dose Threshold4.63 x 106 W s-1

NC/NG (70/30)

NC

Hazards Quantification - Friction

<D>/W s-1

105 106 107 108

Prob

abili

ty o

f ign

ition

/%

0.001

0.01

0.050.10.2

0.512

5

10

20

30

50

70

80

90

95

9899

CONOx

Regression

<D>/W s-1105 106 107 108

Prob

abili

ty o

f ign

ition

/%

0.001

0.01

0.050.10.20.5

12

5

10

2030

50

7080

90

95

9899

CONOx

Regression

NC/NG (70/30)

NC

Hazards Quantification - Impact

PMax / GPa10-2 10-1 100 101 102

Pro

babi

lity

of I

gniti

on /

%

0.01

0.050.10.20.5

12

5

10

2030

50

7080

90

95

9899

PMax / GPa10-2 10-1 100 101 102

Pro

babi

lity

of I

gniti

on /

%

0.01

0.050.10.20.5

12

5

10

2030

50

7080

90

95

9899

NC

NC/NG 70/30

Hazards Quantification - Impact

Energy Density / J m-2

102 103 104 105 106 107

Pro

babi

lity

of I

gniti

on /

%

0.01

0.050.10.20.5

12

5

10

2030

50

7080

90

95

9899

Energy Density / J m-2

102 103 104 105 106 107

Pro

babi

lity

of I

gniti

on /

%

0.01

0.050.10.20.5

12

5

10

2030

50

7080

90

95

9899

NC

NC/NG70/30

Mechanical Hazards-Conclusions Small scale friction and impact tests can be instrumented and

calibrated to obtain realistic evaluation of Ip for NC and NC/NG • Friction

• Compared to NC, the NC/NG mixture has lower thresholds of CO/NOx production, corresponding to the onset of combustion reactions with a potential to propagate

• Pal<v>2 dose parameter is “unphysical” . [Ignition of the NC and NC/NG in this test is expected to be thermal in origin so “friction temperature” a better dose parameter]

• Impact • Compared to NC, the NC/NG mixture demonstrates higher

probability to initiate given the same dose

What is an acceptable level of risk? These Ip data are useful for insertion into risk assessment models.

© Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources, 2016