NATIONAL BUREAU OF STANDARDS REPORT

65^8

THERMAL AND SELF-IGNITION PROPERTIES

OF

SIX EXPLOSIVES

by

J. J. Loftus

and

D. Gross

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

N8S PROJECT MBS REPORT

1002-12-104-29 September 23, 1959 654-8

THERMAL AND SELF-IGNITION PROPERTIES

of

SIX EXPLOSIVES

by

J. J. Loftus

and

D. Gross

for

Department of the Navy

Naval Ordnance Laboratory

White Oak

Ref. No. 60921/5089/58

IMPORTANT NOTICE

NATIONAL BUREAU OF STAN

intended for use yvith in the Go

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Approved for public release by the

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on October 9, 201 5.

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ch permission is not needed,

spared if that agency wishes

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

THERMAL and SELF-IGNITION PROPERTIES

of

SIX EXPLOSIVES

J. J. Loftus and D. Gross

ABSTRACT

Measurements have been made of the specific heat, thermal

conductivity and kinetic properties of six explosives. Based

upon these measurements, estimates are provided of the selfignition hazard associated with the use and bulk storage of

these explosives.

INTRODUCTION

At the request of the Naval Ordnance Laboratory, determination

of the thermal and kinetic properties of a number of propellants

and explosives was intiated. This report summarizes the results

of measurements on six explosives to supplement the results on

three solid propellants reported in NBS Report No. 6111.

From these measurements and an analysis of the self-heating

reaction, an estimation may be made of the self-ignition hazard

associated with the use and bulk storage of such explosives.

These measurements may also be useful for possible intercorrelation

and interpretation of sensitivity data.

MATERIALS

The six explosive materials were furnished by the Naval

Ordnance Laboratory in the shapes required for test. The designations were those supplied by Naval Ordnance Laboratory.

EXPERIMENTAL WORK

A. Kinetic Measurements

Cylindrical specimens 2-in. in diameter by 2-in. long were

prepared by placing together two precut wafers 2-in. in diameter

by 1-in. thick as furnished. Thermocouples were placed within

the specimen which was assembled, placed in a stainless steel

beaker, and mounted within a furnace designed for self-heating

studies. The apparatus and method of analysis of the data have

been described previously/!, 27.

2

At least two tests for each explosive were performed in

order to establish satisfactory reproducibility between duplicate

tests and the results are shown in Figure 1. Straight lines have

been drawn through one set of points in each case and the activation energy E determined from the slopes. Comparative rates of

self-heating at any temperature may be read directly from the

graph by drawing a vertical line at the abscissa point corresponding to the temperature desired. For example, at l65°C (1/T=. 002283 )

,

the explosives RDX, CYCL0T0L and DINA exhibited self-heating at

the rates of 0.015, 0.25? and 7*5 deg C/min, respectively, whereas

any self -heating of explosives Q, QR and TNT was below the limit

of sensitivity of the apparatus used at this temperature.

The values of the kinetic constants determined from the lines

in Figure 1 are listed in Table 1 for the applicable temperature

ranges. Test observations and remarks are summarized below:

MATERIAL

TNT

RDX

Q

QR

DINA

OBSERVATIONS

Specimen melted at about 80°C

Adiabatic control initiated at 190°C

Very rapid combustion upon reaching 265°C; specimen

consumed.

Adiabatic control initiated at l68°C

Short constant temperature phase at about 193°C

Very rapid combustion upon reading 200°C; specimen

consumed.

Adiabatic control initiated at 2

)+7°C

Constant temperature phase in range of 271-276°C

followed by very rapid combustion upon reaching

295°C; specimen consumed.

Adiabatic control initiated at 200°C

Constant temperature phase at about 275°C followed by

very rapid combustion. Specimen consumed except

for approximately 10 gm of gray, crusty residue of

same shape as test specimen (initial weight 180 gm)

which remained following test.

Apparent melting at about J+8°C

Adiabatic control initiated at 126°C

Very rapid* combustion upon reaching 20^-°C; specimen

consumed.

- 3

-

MATERIAL OBSERVATIONS

CYCLOTOL Apparent melting at about 67°C

Adiabatic contol initiated at 151°C

Very rapid combustion upon reaching 204°C;

specimen consumed.

B. Specific Heat Measurements

Specimens for these measurements consisted of two pieces each

1-7/8 by 2 by 1/2 in. and each piece was sealed into an individual

polyethylene plastic envelope. The specific heat of each specimen

was measured by substituting it separately for 65.8 g of the initial

177*8 g of Varsol in a half-pint Dewar flask calorimeter, and

measuring the energy equivalents of the calorimeter and contents

electrically. The calorimeter was so constructed that the two

pieces constituting a specimen formed two sides of a channel

through which the Varsol was circulated by a centrifugal pump.

Storage batteries supplied electrical energy to a submerged cartridge heater, measurements being made of the heater current I,

heater resistances and time of power input t. The energy input

was calculated by the equation E = I^Rt/4.l84 calories. From the

value obtained for the calorimeter and 177.8 g of Varsol and the

values obtained when the specimens were tested, the specific heat

of each specimen, including the plastic envelopes, were obtained.

Varsol was used as the heat transfer fluid because of its low heat

capacity, 0.4730 cal/g-deg C as measured over the experimental

temperature range. The specific heat of the specimen was determined

using a va^ue of 0.55 cal/g-deg C for the specific heat of the polyethylene envelope. After the first run, all measurements were made

over the same temperature range. Three tests on each specimen were

performed and the results are summarized in Table 2.

Subsequent to the first tests on both TNT and RDX, some

disintegration of each specimen due to Varsol leakage through the

plastic envelopes whs observed. The other four explosives showed

very little or no disintegration. Repeat tests on the same specimens indicated that the Varsol attack did not appreciably affect

the specific heat determinations.

C. Thermal Conductivity Measurements

Specimens for these measurements consisted of a circular disk

of 6-in. diameter and of uniform thickness of approximately 0.5 in.

The thermal conductivity was determined in the conductive

disc apparatus* consisting of a circular disc of metal sandwiched

between two specimens which in turn are sandwiched between two

water-cooled plates all of the same 6-in. diameter. The stainless

steel conductive disc was uniformly heated at its edge by means

of electrical resistance wire set in an edge groove. The heat

generated at the disc edge tends to flow in the disc radially

toward its center and also from the disc through the two specimens

to two brass cold plates. These plates were maintained at a uniform

temperature by circulating cole} water through copper tubing soldered

to them. Under steady-state conditions, the temperature of the

conductive disc decreases toward its center. By measuring the

temperature of the conductive disc at its center and at a suitable

radius, temperatures Vo and V, respectively, the effective conductance of the specimens can be calculated, if the temperatures of

the cold plates and the conductivity and thickness of the disc metal

are known.

It was decided to use qnly one explosive specimen for these

measurements and therefore a "dummy" specimen was used in place

of a duplicate of the principal specimen. This consisted of a 1-in.

thick semirigid glass-fiber insulating board of stable low thermal

conductivity (0.3^8 mw/cm C at 50°C). A thickness of 0.991 in. was

maintained during use by three small fiber pegs thrust perpendically

through the board. The lower, cold plate of the apparatus, the

dummy specimen, and the conductive disc were fastened together with

rubber cement to form a subassembly.

The apparatus was calibrated by measurements on six specimens

of known thermal conductance in the range 0.3 to A. A rnw/cm^ C. The

conductances of these specimens and of the dummy specimen were previously determined using the guarded hot plate apparatus (ASTM

0177-^5) • A smooth curve drawn through these six points relates

the observed ratio of temperatures V/Vo of the conductive disc to

the value o‘f conductance of the specimen. Departures of the experimental calibration points from this calibration curve did not

exceed one per cent.

Following calibration, measurements were made on the six

explosive specimens. Care was taken to ensure good thermal contact

between the specimen and the working surfaces. During the course

of the measurements, two check readings were made on one of the six

original calibration specimens ( 1.28 cm thick neoprene). The

* A paper giving a rigorous mathematical analysis of the conductive

disc method and details of design and construction is being prepared for publication by H. E. Robinson, D. R. Flynn and T. W.

Watson.

- 5

-

conductance of this specimen as determined in the hot plate was

2.49*+ mw/cm2 C at 48°C while the check values were 2.50 and

2.56 mx/cm^ C, respectively. Therefore, a variation in results

of 2.4 per cent was observed, to which the results on the explosives must be considered subject. The results are given in

Table 3.

The two measurements made on TNT, at 33 and 38°C, indicate

a negative temperature coefficient of conductivity. Because of the

small difference in test temperatures, however, the uncertainty in

this value ( CC = -0,011 per deg C) is probably large.

To prevent the possibility of melting or softening, the cold

plate temperature was maintained at a much lower temperature for

those explosives echibiting low temperature melting. This was

accomplished by using chilled or ice water as the cold plate

coolant. # The specimen conductances were corrected to compensate

for the change in the conductive disc conductivity when its mean

temperature departed from the temperature during calibration (69°C).

D. Calculation of Critical Size

In order to estimate the critical size for ignition of a mass

of self-heating material, the analysis presented by Enig, Shanks

and Southworth/ 3_7 was used. This related the half-thickness of

a material of given thermal and kinetic properties with the temperatures at the center and the surface under critical steady-state

conditions. This analysis has been applied to wood fiberboard and

.cotton linters and fair agreement with experimental results has

been obtained/"~47

The assumption was made that the kinetic properties measured

over higher temperature ranges may be applied to the temperature

range of practical interest for ordinary storage (20 - 100°C).

It was further assumed that the measured thermal properties may

be applied over the whole temperature range. These are broad

assumptions, particularly for those materials for which the kinetic

measurements were made on the liquid phase (DINA, CYCL0T0L and TNT).

Critical radius determinations for a sphere have been made

for each explosive and are listed in Table 4, and shown graphically

in Figure 2. It may be noted from the tables in reference [_ 3

7

that, for a given surface temperature, the critical radius for a

cylinder and the critical half-thickness for a semi-infinite slab

are given very closely by 0.775 Bc and 0.514 Bc respectively,

where Bc is the critical radius for the sphere.
. V
- 6 -

SUMMARY

Measurements of the specific heat, thermal conductivity and

kinetic properties of six explosives have been made and are

summarized in Tables 1 to 3» In the self-heating experiments,

initial self -heating was observed at temperatures ranging from

126°C for DINA to 247°C for Q. With one exception, all materials

were completely consumed during the experiments; a grey, crusty

residue comprising about 5 per cent of the initial weight remained

following the burning of QR. Activation energy values as measured

by this method ranged from 35*5 to 6 5.8 kcal/mole.

A comparison of critical radius determinations, under the

given assumptions, is presented in Table 4 and Figure 2. Size

limitations on the storage of bulk quantities of DINA, CYCLOTOL

and TNT and careful control of temperature and ventilation conditions seem justified on the basis of these calculations. Of

course, consideration must be given to melting or other phase

transformations occurring at relatively low temperatures. In the

case of DINA, CYCLOTOL and TNT (apparent) melting occurred at

temperatures of about 48°C, 67°C and 80°C respectively. Explosives

Q, QR and RDX do not appear likely to present any storage hazard

with respect to self-ignition at ordinary storage temperatures.

ACKNOWLEDGMENT

The specific heat measurements were made by E. S. Newman

and the thermal conductivity measurements by D. R. Flynn.

- 7 -

REFERENCES

1. Raskin, W. H. and Robertson, A. F., "An Adiabatic Apparatus

for the Study of Self-Heating of Poorlv Conducting Materials,

Rev. Sci. Inst. 2£, 541-544 (June 195b),

2. Loftus J. J. and Gross, D. , "Thermal and Self-Ignition Proper

ties of Three Solid Propellants," NBS Rep. 6111.

3. Enig, J. W., Shanks, D. and Southworth, R. W. , "The Numerical

Solution of the Heat Conduction Equation Occurring in the

Theory of Thermal Explosions," NAVORD Rep. 4377 (1956) Astia

Document No. AD-116 o73»

b . Gross, D. and Robertson, A. F. , "Self-Ignition Temperatures

of Materials from Kinetic Reaction Data," J. Research NBS 6l,

413-417 (Nov 1953)
& v

r •

'• - •

•• c
Table 1. Kinetic Measurements

Material

Temperature

Range State

Activation

Energy E

Heat Generation

Coefficient A

°C kcal/mole cal/sec-cm3

TNT 220-260 Liquid 37.0 7.80 x lO^

RDX 170-200 Solid 57.2 3.47 x 1024

Q 250-270 Solid 65.8 7.45 x 1023

QR 200-275 Solid 38.6 5.43 x 101 8

DINA 130-175 Liquid 35.5 2.72 x 1016

CYCLOTOL 155-180 Liquid 43.6 1.03 x 10!9
; ' . .u .

v.. ;
Table 2. Specific Heat Measurements

Material

Sample

Weisht

Envelope

Weieht

Temp.

Rise

Mean

Temp. Specif.

g g deg C °C cal/

g

TNT 98.7262 2.3383 1.68

3.31

11

23.8

24.5

11

0.2592

0.2742

.2582

RDX 102.0285 2.7555 M

II

II

11

ri

11

0.2687

.2612

.2633

Q 112.6514 2.575-3 II

II

II

11

11

11

0.2732

. 2586

.2501

QR 107.1495 1.5606 II

It

II

it

11

11

0.2575

. 2566

.2595

DINA 99.0307 1.4618 II

II

II

tt

11

11

0.2713

.2732

.2700

CYCL0T0L 104.3517 1.5324 II

II

II

it

11

11

0.2537

.2608

.2614

c Heat

deg C

0. 26*+

0.264

0.261

0,258

0.272

0.259
0
Table

3«

Thermal

Conductivity

Measurements

H >

0-H

x^ -p

EH

OP

"3OO o0a

N '—1 o ON

N

N

O o OO NO ON H ON Un O

pt Pt pt NO Un UN NO o o o o O O O O 0 o o o O O o O 0 o o o O o o O p • o « o • o • o • o • o • o 0

CJ o0 pt CO lr\ ON o cO un a o ON o IfN IfN cO 1—1 o • • 0 • • N cm 1—1

C\l CM CM CM CM

Sh

a

03 SO

o> 00 S E-t

-P

cd S

oO

cO

pt

CO

rO

C\J Un

n

rO

CO

Un

nQ

cO

o\

pt

cO

cO

nO

Pt

nO

COi—I

CO

ON

IN- i—1

UN CM i—1

CM CM CO O CO pt CM UN N • • 0 0 • • • 1 1 cO pt o N m ON

J" IfN UN IfN pt i—i i—1

•H P co NO ON IN UN o tN Q 0 i—i

cO I—1 i—1

CM o H P O • • • • • • • 0 0 o O I—1 N pt CM CO 0 pt UN UN Pt Pt H i—1 o0 • CM N IN- Pt ON UN Pt

tP -p a,N UN ON CM CO

O IN i—1 cti a o • • • • 0 • • O i—l 0 0 pt pt i—1

cO co o NO

O pH eh CM CM CM CM 1—1

pt ON CO Pt NO UN co

O ON CM 00 UN 1—1 N UN > o O O cO CO CM ro 1—1 i—1 i—1 i—1 1—1 i—1 i—1 > 0 * • « • 0 • 1—1 i—1 i—1 i—1 1—1 i—1 i—1 0000 CM CM i—1

CO CM

O i—1 a IN N 00 N N N N o o CNJ CM CM CM CM CM CM •H • a • • « * 0 0 XI i—1 l 1 1 1 i—1 1 1 i—1 i—1

EHi—1

hP 0 <$ O

•H EH EH « PP S Eh

Ph S Q cy

O5 M

O 0 EH EH pc; Q hPOO

Table Critical Size Calculations

Critical Radius of a Sphere

Material 20°C

Surface

48.9°C

Temperature

82. 2°C 100°C

(68°F) (120°F

)

(l80°F) (212°F)

TNT 6.4- x 10? cm 4-.0 x 10^ cm 3000 cm 860 cm

2.1 x 104 ft 1300 ft 97 ft 29 ft

RDX 8.0 x 10? 1.1 x 10?

3.5 x 104 1.8 x 10^ 2700

2.6 x 106 590 90

Q 3.0 x lO^l 2.1 x 10? 1.8 x 10?

6.1 x 10?

2.1 x 10:

1.0 x 1010 6.9 x 10^ 6. 9 x 10*

QR 3.2 x 10§

1.1 x 10?

1.8 x lO^ 1.2 x 10^ 3300

5900 390 110

DINA 9700 700 57 18

320 23 1.9 0.6

CYCLOTOL 5.0 x io?

1.6 X 104 1.9 x 10^ 860 210

620 28 6.8

U8GOMM-NBS-DC

RATE

OF

TEMPERATURE

RISE,

DEG.

C/

MIN.

FIG. I - ADIABATIC SELF- HEATING DATA FOR SIX EXPLOSIVES

CRITICAL

RADIUS

OF

SPHERE,

CM.

SURFACE TEMPERATURES FOR SPHERICAL PILES

OF SIX EXPLOSIVES
.
U.S. DEPARTMENT OF COMMERCE

Krriln'ii'k II. Mueller. Secretary NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

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